And It shall come to pass afterward, that I will pour out my spirit upon all flesh; and your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions.
CO Rayner Joel 1960. 1966 (t) Longman Group Limited 1971 Ci) Longman Group UK Limited 1987
© Addison Wesley Longman Limited 1996 The right of Rayner Joel to be identified as author of this Work has been asserted by him in accordance with the Copyright. Designs and Patents Act 1988 All rights reserved; no part of this publication may be reproduced, stored in any retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise without either the prior written permission of the Publishers or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London WIP 9HE. First published as Heat Engines 1960 Second edition 1966 Third edition 1971 Fourth edition 1987 Fifth edition 1996 British Library Cataloguing in Publication Data A catalogue entry for this title is available from the British Library ISBN 0-582-25629-1
Set by 32 in Times Roman 9;>-/12pt Printed in China GCC/O I
Chapter 2, Verse 28
Preface Acknowledgements List of symbols I
10 11 12 13 1t4 15 16
General introduction Systems The laws of thermodynamics Steam and two-phase systems Gases and single-phase systems Thermodynamic reversibility Entropy Combustion Hcat transfer Stcam plant Thc stcam engine Nozzlcs Steam turbines Air and las compressors Ideal,al power cycles Internal combustion engines Bnline and plant trials Refri,eratlon Plychrometry
34 48 54 96 147 155 184 252 276 320 350 369 392 423 503 571 602 620 637
With the new fifth edition, there has again been an opportunity to sift through the text and to rearrange, add to and also remove material. An update has also been carried out where appropriate. The new arrangement roughly separates into two sections. The earlier chapters are concerned with basic engineering thermodynamic theory whereas the later chapters introduce practical applications. Concepts of engineering thermodynamics warrant serious thought and study. Modern industrial society relies almost entirely upon their application to supply its basic energy requirements. Without the various power-generating plants and engines, industrial society would collapse. Failure of electrical generating plant would mean the failure of the electronics industry and all its consequences. There is heavy reliance on the use of fossil fuels which have no immediate replacement. There are sources of renewable energy, such as wind and solar power, but their effect is small compared with nuclear fuels and the fossil fuels, coal, gas and oil. The most serious problem is the rate at which energy is consumed. Much atmospheric damage is caused by exhaust emissions from burning fossil fuels. Better and moderated use of fundamental world resources must be addressed for the future. This book will fit most curriculums that include the study of basic engineering thermodynamics. It is a serious subject, both necessary and rewarding, for perhaps it can help to preserve and enhance the quality of life on earth.
Basic engineering thermodynamics
A pure substance is a single substance or mixture of substances which has the same consistent composition throughout. In other words, it is a homogeneous substance and its molecular structure does not vary. For example, steam and water, or a mixture of steam and water can be considered as pure substances. Each has the same molecular or chemical structure through its mass. Air in its gaseous and liquid form is a pure substance. However, during the liquefaction process of air, which is a mixture of gases, mainly oxygen and nitrogen, the oxygen and nitrogen liquefy at different temperatures. Until all the air becomes liquid, the relative concentrations of oxygen and nitrogen (and other gases) in the liquid that has formed will therefore be different from those of the original air. The relative concentrations will also differ between the condensing vapour and the original air. Thus the air in these circumstances ceases to be a pure substance. The importance of the concept of a pure substance in this work is that the condition, or state, of a pure substance can be completely defined by any two independent properties of the substance. For example, if the pressure and volume of a fixed mass of oxygen is known, then its temperature and such other properties as will be discussed later are also completely known. 1.4
Macroscopic and microscopic analysis
If the properties of a particular mass of a substance, such as its pressure, volume and temperature, are analysed, then the analysis is said to be macroscopic. This is the method of analysis usually used by the engineer and is the type of analysis used therefore in the study of heat engines and engineering thermodynamics. If, however, an analysis is made in which the behaviour of the individual atoms and molecules of a substance are under investigation, then the analysis is said to be microscopic. Some studies in nuclear physics would be of a microscopic nature, such as the atomic structure of a fissionable material like uranium. 1.5
Properties and state
In the macroscopic analysis of a substance any characteristic of the substance which can be observed or measured is called a property of the substance. Examples of properties are pressure, volume and temperature. This type of property which is dependent upon the physical and chemical structure of the substance is called an internal or thermostatic property. Other types of thermostatic properties will be discussed later. If a value can be assigned to a property then it is said to be a point function because its value can be plotted on a graph. Properties which are independent of mass, such as temperature and pressure, are said to be intensive properties. Properties which are dependent upon mass, such as volume and energy in its various forms, are called extensive properties. If a property can be varied at will, quite independently of other properties, then the property is said to be an independent property. The temperature and pressure of a gas, for example, can be varied quite independently of each other and thus, in this case, temperature and pressure are independent properties. It will be found, however, when discussing the formation of a vapour, that the
temperature at which a liquid boils depends upon the pressure at which the formation
of the vapour is occurring. Here, if the pressure is fixed then the temperature becomes dependent upon the pressure. Hence the pressure is an independent property but the temperature is a dependent property. A knowledge of the various thermostatic properties of a substance defines the state of the substance. If a property, or properties, are changed, then the state is changed. Properties are independent of any process which any particular substance may. have passed through from one state to another, being dependent only upon the end states. In fact, a property can be identified if it is observed to be a function of state only. Since, at a particular state, a substance will have certain properties which are fUnctions of that state, then there will be certain relationships which exist between them. These property relationships will be investigated in the text. A property which includes a function of time, used to define a rate at which some interaction can occur, such as the transfer of mass, momentum or energy, is referred to as a transport property. Examples of transport properties are thermal conductivity and viscosity. 1.6 Specific quantity In the discussion of properties it was suggested that those properties which were associated with the mass of a substance are called extensive properties. For convenience, at times, it is useful to discuss the properties of unit mass of a substance. To indicate that this is the case, the word specific is used to prefix the property. Thus, the specific volume of a substance, at some particular state, is the volume occupied by unit mass of the substance at that particular state. Other specific quantities will be discussed in the text. 1.7
Temperature describes the degree of hotness or coldness of a body. The subject of temperature investigation is called thermometry. Many attempts have been made in the past to lay down a scale of temperature. The work has culminated in the generally accepted use of two temperature scales, Fahrenheit and Celsius. The Fahrenheit scale is named after its German inventor, Daniel Gabriel Fahrenheit (1686-1736) of Danzig (now Gdansk, Poland). The Celsius scale (often referred to as the centigrade scale) is named after Anders Celsius (1701-1744), a Swedish astronomer born at Uppsala. The Celsius scale is the temperature scale which is most commonly used worldwide. The Fahrenheit scale is generally becoming progressively phased out. The customary temperature scale adopted for use with the SI system of units is the Celsius scale. For customary use, the lower fixed point is the temperature of the melting of pure ice, commonly referred to as the freezing point. This point is designated 0 0c. The upper fixed point is the temperature at which pure water boils and this is designated 100°C. In the past, this customary temperature scale has been referred to as the centigrade scale. The use of the word centigrade is now discouraged, the accepted reference now being that of the Celsius scale. Of interest, the freezing and boiling points of pure water are designated 32 of and 212 of, respectively, on the Fahrenheit scale.
Basic engineering thermodynamics
It will be shown later, when dealing with the properties of solids, liquids and vapours, that the temperature at which a liquid freezes or boils depends upon the pressure exerted at the surface of the liquid. This temperature increases as the pressure increases in the case of boiling and slightly decreases with increase of pressure in the case of freezing. To standardise the freezing and boiling temperature on a thermometric scale, one must therefore standardise the pressure at which the freezing or boiling occurs. This pressure is taken as 760 mm of mercury which is called the standard atmospheric pressure or the standard atmosphere, being a mean representative pressure of the atmosphere. Figure 1.1 shows the way the customary Celsius scale is divided up. The lower fixed point is 0 °c and the upper fixed point is 100°C; there are 100 Celsius degrees between them. These 100 Celcius degrees are together called the fundamental interval.
In the above discussion it will be noted that the choice of the fixed points was of an arbitrary natu~e. The freezing and boiling points of water were chosen for convenience. Other points on the International Temperature Scale are then chosen and referred to the originally conceived scale. The original choice of fixed points was arbitrary, so the Celsius scale is sometimes called the normal, the empirical, the customary or the practical temperature scale. Since the Celsius scale is only a part of the more extensive thermodynamic, or absolute temperature scale, it is sometimes called a truncated thermodynamic scale. Subsequent work will show that there is the possibility of an absolute zero of temperature which will then suggest an absolute temperature scale. An absolute zero of temperature would be the lowest temperature possible and therefore this would be a more reasonable temperature to adopt as the zero for a temperature scale. The absolute thermodynamic temperature scale is called the Kelvin scale. It was devised by Lord Kelvin, a British scientist, in about 1851. The Kelvin unit of temperature is called the kelvin and is given the symbol K. A temperature, T, on the Kelvin scale is written T K, not T OK. The kelvin has the same magnitude as the Celsius degree for all practical purposes. The absolute zero of temperature appears impossible to reach in practice. However, its identity is defined by giving to the triple point of water a value of 273.16 kelvin (273.16 K). The triple point is defined in Chapter 4. With the absolute zero so defined, the zero of the Celsius thermodynamic scale is defined as 0 °c = 273.15 K.
General Introduction 1.13
If processes are carried out on a substance such that, at the returned to its original state, then the substance is said to have cycle. This is commonly required in many engines. A sequence which must be repeated and repeated. In this way the engine Each repeated sequence of events is called a cycle. 1.14
end, the substance is been taken through a of events takes place continues to operate.
The constant temperature process
This is a process carried out such that the temperature remains constant throughout the process. It is often referred to as an isothermal process. Particular cases of the constant temperature process will be dealt with in the text. 1.15
The constant pressure process
This is a process carried out such that the pressure remains constant throughout the process. It is often referred to as an isobaric or isopiestic process. Particular cases of the constant pressure process will be dealt with in the text.
When a substance is of the same nature throughout its mass, it is said to be in a phase. Matter can exist in three phases; solid, liquid and vapour (or gas). If the matter exists in only one of these forms then it is in a single phase. If two phases exist together then the substance is in the form of a two-phase mixture. Examples of this are when a solid is being melted into a liquid or when a liquid is being transformed into a vapour. In a single phase the substance is said to be homogeneous. If it is two-phase it is said to be heterogeneous. A heterogeneous mixture of three phases can exist. This is covered by the section on the triple point during the discussion on steam (see section 4.4). 1.11
Now that the concept of properties, state and phase have been indicated, it is possible to write down the two-property rule. If two independent properties of a pure substance are defined, then all other properties, or the state of the substance, are also defined. If the state of the substance is known then the phase or mixture of phases of the substance are also known. The idea of the two-property rule was suggested in section 1.3. 1.12
When the state of a substance is changed by means of an operation or operations having been carried out on the substance, then the substance is said to have undergone a process. Typical processes are the expansion and compression of a gas or the conversion of water into steam. A process can be analysed by an investigation into the changes which occur in the properties of a substance, and the energy transfers which may have taken place.
The constant volume process
This is a process carried out such that the volume remains constant throughout the process. It is often referred to as an isometric or isochoric process. Particular cases of the constant volume process will be dealt with in the text.
Energy is defined as that capacity a body or substance possesses which can result in the performance of work. Here, work is defined, as in mechanics, as the result of moving a force through a distance. The presence of energy can only be observed by its effects and these can appear in many different forms. An example where some of the forms in which energy can appear is in the motor car. The petrol put into the petrol tank must contain a potential chemical form of energy because, by burning it in the engine and through various mechanisms, it propels the motor car along the road. Thus work, by definition, is being done because a force is being moved through a distance. As a result of burning the petrol in the engine, the general temperatures of the working substances in the engine, and the engine itself, will be increased and this increase in temperature must initially have been responsible for propelling the motor car. Due to the increase in temperature of the working substances then, since the motor car is moved and work is done, the working substance at the increased temperature must have contained a form of energy resultant from this increased temperature. This energy content resultant from the consideration of the temperature of a substance is called internal energy (see section 1.23). Some of this internal energy in the working substances of the engine will transfer
Consider Fig. 1.4. Here is shown a P- V diagram of the type usually obtained when an expansion takes place in a thermal engine. It is now a curve with original pressure and volume PI and VI, respectively. The final pressure and volume are P2 and V2, respectively. Both the pressure and the volume have changed in this case. What of the area under this graph and will it still give the work done? Figure 1.5 shows the same graph but this time it has been divided up into small rectangles. The area of each small rectangle represents work done as has been shown. The sum of all the areas of these small rectangles would therefore approximate very closely to the area under the graph and hence the work done. The greater the number of rectangles then the more nearly equal are the sum and the actual area, hence the actual work done. If the number of rectangles were made infinitely great, the sum would, in fact, equal the actual area, which would then give the actual work done. Now this is exactly what happens when the area is solved by the use of the integral calculus.
12 Basic engineering thermodynamics
The polytropic process PVn = C, a constant
Changes of state of working substances in thermodynamic systems are often brought about by the expansion or compression of the working substance. Suppose that an experiment is conducted on a mass of-working substance such that an expansion takes place changing the state from state I to state 2. Let the pressure change from PI to P2 and the volume from VI to V2. Assume that arrangements are made to record the pressure and volume as the experiment proceeds. From the results obtained, if values of pressure and volume are plotted on a P-V graph, they produce a smooth curve as shown in Fig. 1.7.
General introduction 18
Basic engineering thermodynamic'!
1.23 Internal energy If a hot body is placed in contact with a cold body then the temperature of the hot body begins to fall while the temperaturc of the cold body begins to rise. To account for this it is said that the hot body givcs up heat, hence its temperature falls, and the cold body receives this heat, hence its temperature rises. Observing this fact, some early invcMtiaators around in the eighteenth century considered that heat must have properties similar to those of a fluid. One such fluid was called caloric. Thus, if a body was heated, caloric was said to have passed from the source of heat supply into the body, hence it became hot. Conversely, if a body cooled, it lost some of its caloric. Since the weiaht of the body was unaffected by being heated or cooled, it was considered that caloric was a weightless fluid and was said to fill the minute spaces or porel of the body. Another such fluid, frigoric, was used to explain the phenomenon of cold. It was said to be composed of minute darta of frOlt. If one's hand is placed on a piece of ice, for example, not only does it feel cold but It oan' feel somewhat painful. It was suggested that the reason for thia WII that the dart-like particles of frigoric were being transferred from the ice Into the hand and thus the hand became cold accompanied by a sense of pain. Yet another fluid which aQOountld ror the heating effect produced by a fire was called phlogiston. To produce its heatiD, eftIet the fire was said to have given up phlogiston to the bodies being heated. AI I IftI&ter.r interelt, when it was found that a gas, which is now known as oxygen, wal dil'lOtly Uloclated with the phenomenon of combustion, it was called dephlogisticated air. The realon for this gas, being the only one associated with the production of nre, WII therefore the only gas without phlogiston; as it received phlogiston. fire resulted and helt WII produced. Nearly all the known phenomenl or heating and cooling could be explained by the introduction of such f1uidlal ~oric, frlgoric and phlogiston. These theories we~ eventually found to be false. Notable among those who showed them to be wrona WII Count Rumford of Munich. Count Rumford was really an American citizen, born nelr BOlton In 1753. His real name was Benjamin Thompson and he had to leave Americi for his part in the rebellion of the British Colonies. He settled in Bavaria where he became the superintendent of an arsenal in Munich. He was rewarded with the title Count Rumford for his services in this respect. During his work in the Irltnal, he noticed that when boring a cannon, the material of the cannon became extremely hot; this did not seem to tie up very well with the caloric theory which was widely accepted at the time. He accordingly conducted an experiment in an attempt to settle this matter. Instead of using a sharp boring tool he used a blunt one which was rubbed against a mass of about 51 kg of gunmetal. After some 960 revolutions the temperature of the gunmetal had risen by about 39°C. A minute quantity of gunmetal had been rubbed off during the experiment. Here then was an experiment in which there had been no hot source to supply heat, by yielding up some of its caloric in heating the gunmetal, yet the gunmetal had, in fact, become extremely hot. It might have been suggested that the caloric, originally in the metallic dust that was rubbed off, had been left behind in the main bulk of gunmetal and was therefore responsible for the temperature rise. The amount of metallic dust was so small and the quantity of heat developed so large that Count
a.mford concluded this was impossible. It also appeared that as long as the rubbing, or friction, of the boring tool continued, heat would continue to be produced, hence the aupply of heat generated in this way was inexhaustible. Since the heat was IInerated as a result of motion, Rumford therefore suggested that heat was in some wlY the result of the motion of the particles which make up a body. There was nothing really new about this idea. It had been the view taken by some philosophers from very early times. After its short accepted life, during the eighteenth century, the supposed fluid nature of heat was dropped. It then became the accepted theory that heat was a manifestation of the degree of agitation of the very minute particles (atoms and molecules, to be discussed later) which make up a body. Part of the old philosophy remains, however, for it is common practice to refer to such things as 'the flow of heat' and 'quantity of heat' which still suggest a fluid nature of heat. It is important, however, to think a little more about the theory that heat is a result of the degree of agitation of the particles which make up a body. If a particle is in motion, it will possess a kinetic energy, which is a function of the velocity at which the particle is moving. It appears, in general, the greater the kinetic energy that can be imparted to the particles which make up a body, the higher the temperature of that body will become. It has now become clear that the store of energy which results from the random motion of the atoms and molecules of a body would be far better referred to as internal energy, leaving the term heat to be used to describe that energy transfer process which results from a temperature difference. At anyone particular state, the atoms and molecules will have a particular overall degree of random motion and, in a pure substance, this degree of random motion will be the same each time the substance returns to that state. The degree of random motion must therefore be a property. Internal energy is a function of the degree of random motion, so it must be a property. Count Rumford's experiment showed that, in his particular case, an increase in internal energy content resulted in an increase in temperature. This is always the case in a single-phase system. Count Rumford's single-phase system was the gunmetal cannon, and note here that the internal energy increase was the result of a blunt boring tool being rubbed against the cannon. The energy transfer in this case was really a work transfer, work having been done against friction. It must be noted, however, that an internal energy increase does not always result in an increase in temperature. It will be shown during the discussion on two-phase systems that when the phase is being changed from one to another, such as water into steam, the temperature will remain constant. Here the internal energy increases at constant temperature; the increase in internal energy is necessary to carry out the degree of separation of the molecules to change the water into steam. The same general situation arises during the change of a solid into a liquid. It has been stated that the internal energy of a substance results from the motion of its atoms or molecules. In a fluid, the atoms and molecules have rather greater motions than solids; in fact, they move about freely (rather more freely in the case of gases). This means that the atoms and molecules will be constantly impinging upon the walls of any container. Now the impact of a particle on a wall means that a force will be imparted to that wall. The constant bombardment of the walls of the containing vessel results in a
Basic engineering thermodynamics
total average force on each wall. When this average force is reduced to that which occurs on unit area of the wall, it is called the pressure on the wall. Again, in the above discussion it has been noted that the internal energy content is the result of the motion of the atoms and molecules which make up a body. Further, it was noted that an increase in internal energy content generally results in an increase in temperature. As the temperature of a body falls, the motion of the atoms and molecules therefore reduces, and the internal energy content also reduces. So it is reasonable to assume that a condition exists in which the atoms and molecules of a body are completely at rest, in which case the internal energy content would then be zero and the temperature would have reached its absolute zero. The idea of an absolute zero of temperature was mentioned in section 1.7. In the case of internal energy, specific internal energy is designated u, the internal energy of any mass, other than unity, is designated U. 1.24 Heat The discussion on internal energy suggested how bodies were once believed to contain heat. This is now not considered as being the case; the internal store of energy is now called internal energy, which is a property. However, it was further suggested that, during an energy transfer process which results from the temperature difference between one body and another, the energy so transferred is called heat. The heat, having been transferred, will then disperse into other forms of energy, such as internal energy or work, the disposal being a function of the system employed. Note that heat is a transient quantity; it describes the energy transfer process through a system boundary resulting from temperature different. If there is no temperature difference, there is no heat transfer. And since the term heat is used to describe a transfer process, heat energy ceases to exist when the process finishes. Thus heat is not a property. Heat energy is given the symbol Q. To indicate a rate of heat transfer, a dot is placed over the symbol, thus
heat transfer/unit time
1.25 Specific heat capacity For unit mass of a particular substance at a temperature t, let there be a change of temperature bt brought about by a transfer of heat bQ. The specific heat capacity, c, of the substance at temperature t is defined by the ratio bQ/bt. Thus c
In the limit, as bt c=_dQ dt
Specific heat capacity can also vary with pressure and volume. This is particularly true of compressible fluids such as gases (see Chapter 5). It is common practice to use an average value of specific heat capacity within a given temperature range. This average value is then used as being constant within the temperature range, so equation [2] can be rewritten c=-Q
heat transfer/unit mass, J/kg change in temperature, K
Note that from equation [3] the basic unit for specific heat capacity is joulesl kilogram kelvin or J/kg K; multiples such as kilojoules/kilogram kelvin (kJ/kg K) may also be used. A particular application of specific heat capacity arises from the use of water as a measuring device in calorimetry. Temperature measurements during a calorimetric experiment are made while the pressure of the water remains constant. A process in which the pressure remains constant is said to be isobaric. The specific heat capacity in this case is therefore said to be the isobaric specific heat capacity and is written cpo The table gives a few examples of average specific heat capacities of some solids and liquids. The specific heat capacity of gases is dealt with separately in Chapter 5. Table of average specific heat capacities Solid
Specific heat capacity (J/kg K)
Specific heat capacity (J/kg K)
Aluminium Brass Cast iron Copper Crown glass
915 375 500 390 670
Lead Nickel Steel Tin Zinc
130 460 450 230 390
Benzene Ether Ethanol Paraffin Mercury
From equation [1]
.Specific heat capacity is generally found to vary with temperature. For example, the specific heat capacity of water falls slightly from a temperature of 0 °C to a minimum of about 35°C and then begins to rise again.
Specific heat capacity (J/kg K) -~--1700 2300 2500 2 130 140
Example 1.5 5 kg of'steel. specif'ic heat capacity 450 J/kg K, is heated from /5 "C to Determine the heat transfer.
5 x 450 x (100 - 15) 5 x 450 x 85 191 250 J 191.25kJ
1.27 The adiabatic process If a process is carried out in a system such that there is no heat transferred into or out of the system (i.e. Q = 0) then the process is said to be adiabatic. Such a process is not really possible in practice, but it can be closely approached. If a system is sufficiently thermally insulated, heat transfer can be considered as negligible and the process or processes within the system can be considered as being adiabatic. Alternatively, if a process is carried out with sufficient rapidity, there will be little time for heat transfer. Thus if a process is rapid enough, it can be considered as being effectively adiabatic. The implications of any particular process being considered as adiabatic will be dealt with in the text. 1.28 Relationship between heat and work Figure 1.11 shows two containers each containing a mass of water m and each having a thermometer inserted such that temperature measurement can be made. In each case, the mass of water is the system. And for this discussion, any other fluid of mass m could also be considered as the system. In (a) it is arranged that an external heater can transfer heat energy Q through the system boundary into the water. In (b) it is arranged that a paddle-wheel is immersed in water such that external paddle or stirring work W is done when the wheel is rotated. In each case it is assumed that there is no energy loss from the system.
It has been shown that internal energy, pressure and volume are properties. During subsequent discussion a particular combination of these properties will oftcn uppear. The combination is in the form u + Pv and, because this combination has a purticulur significance in some processes, it is given a name. The name is enthalpy and is given the symbol h. Thus, h = u + Pv. Note that, since pressure, volume and temperature are properties, their combination is also a property, so enthalpy is a property. Specific enthalpy is designated h. The enthalpy of any mass other than unity is designated H. 1.30
Consider the arrangement in (a). It is common experience to heat water in some containing vessel by means of some external heating device. Let the initial temperature as recorded on the thermometer be f" and after heating, in which heat energy Q is transferred into the water, let the final temperature be f2. Consider, now, the arrangement in (b). The container once again contains a mass of water m but in this case a paddle-wheel is introduced into the water. It is common experience that friction makes things warm. The simple experience of rubbing one's hands together in a brisk manner will show this. In the case under consideration it is possible to rotate the paddle-wheel against the frictional resistance of the water. Assume that the initial temperature of the water is fl and, after doing an amount of work Won the paddle-wheel, the final temperature is f2. Now a similar effect has been produced in both cases (a) and (b) in that a mass of water m starting at a temperature f, has experienced a rise in temperature (t2 - fl). Case (a) used a heat transfer to produce an effect; case (b) used a work transfer to produce the same effect. The conclusion must be that there is a relationship between heat and work. If the unit of energy is the same for both work and heat, since the same effect was produced in each case, the relationship is of the form
of the thermodynamic
The thermodynamic engine is a device in which energy is supplied in the form of heat and some of this energy is transformed into work. It would be ideal if all the energy supplied was transformed into work. Unfortunately, no such complete transformation process exists or, as will be shown in Chapter 6, can possibly exist. The usual process in the engine can be followed by reference to Fig. 1.12.
usually raised above that of the surroundings. In this condition the substance is capable of doing work. For example, it could be enclosed by using a piston in a cylinder; if the piston were free to move, it would be pushed down the cylinder and work would be done as the substance expanded. The substance would lose some of its energy in doing this work. When the substance has performed as much work as is practically possible, it could be removed from the cylinder and rejected to the sink. By returning the piston to its original position and then introducing some more highenergy-containing substance, the process could be repeated. This is what happens in any piston engine. The intake and rejection processes of the working substance are intermittent in this case. In the majority of turbine engines, however, the working substance passes through in a continuous flow. There are two possibilities with regard to the introduction of the energy into the working substance which, in most cases, is either a vapour or a gas. The first possibility is to transfer heat into the substance outside the engine and then to pass the high-energy-containing substance over into the engine. This is the usual process carried out when using steam as the working substance which is formed outside the engine in a boiler and is then passed to the engine. This is a case of a vapour being used as the working substance. The second possibility is to introduce the energy directly into the working substance in the engine. This is the usual process carried out in petrol, oil and gas engines in which the fuel is introduced directly into, and burnt in, the engine cylinders. When this is the case, the engines are called internal combustion engines, IC engines. Each method naturally has its own complexity. More will be said in later chapters. Now a further note about thermal efficiency. It has already been stated that the process in the engine is that of receiving heat, converting some of it into work and then rejecting the remainder. So it appears that, neglecting losses, the difference between the heat received and the heat rejected is equal to the work done, or
Basic engineering thermodynamics
The use of electricity is now so widespread that it is essential to have a knowledge of electrical power. The fact that electrical energy can be converted into mechanical energy can be readily observed in the electric motor. Again, electrical energy can be converted into thermal energy using the common electric heater. Since electrical energy can readily be converted into work, electrical energy input to an electrical circuit is sometimes referred to as electric work transfer. The effort which drives electricity through an electric circuit is called the potential differ~nce, symbol V. This effort is usually supplied by a generator or a battery. The unit of potential difference is called the volt (V). An instrument called a voltmeter is made to measure potential difference. To measure the potential difference of a generator or battery a voltmeter is connected across the terminals of the generator or battery. The quantity of electricity being driven round a circuit is called the current, symbol I. The unit of current is the ampere or amp (A). An instrument called the ammeter is made to measure electric current. In order to measure the current, the ammeter is connected in the circuit such that the current must flow through it. Figure 1.13 illustrates the connections of the voltmeter and ammeter into an electric circuit. Note that the voltmeter is connected across the circuit, thus measuring the potential difference. If any electrical device is connected across a circuit in this manner it is said to be connected in parallel.
The ammeter, on the other hand, is connected actually in the circuit such that the current must flow through it and hence the ammeter will measure the current. If any device is connected actually in an electric circuit, such as the ammeter, it is said to be connected in series. In some generators, and in all batteries, the current delivered to any circuit is always in the same direction. The connections to either the generator or battery are made by means of terminals. Current flowing in one direction is said to be direct current, abbreviated d.c. One of the terminals of either the generator or battery is said to be positive, marked +, and the other is said to be negative, marked -. Direct current is always considered as flowing from the positive to the negative terminal. The generator of direct current electricity is referred to as a d.c. generator. Other generators generate electricity in which the current is continuously changing its direction. Such current is called alternating current, abbreviated a.c. In
this case, neither terminal can be designated as posItive or negative; both are continuously changing in polarity. The type of meters to measure potential difference and current are different in design in this case but they measure potential difference in volts and current in amps, as before. The generator to develop alternating current electricity is referred to as an alternator. Most electric power developed in power stations is a.c., and in the United Kingdom the standard number of current direction changes is 50 per second. Each change from positive to negative and back is called a cycle. Thus, in the above case, the current is said to have 50 cycles per second, which is called the current frequency. Now a frequency of 1 cycle per second is called 1 hertz (Hz). Hence a frequency of 50 cycles per second = 50 cis = 50 Hz. The unit of power in an electric circuit is called the watt, and this is the rate of working in an electrical circuit whose potential difference is I volt with a current flow of 1 amp. Thus for any circuit
All physical things in nature have some form of boundary whose shape in general identifies it as a particular object. Inside its boundary there are various features which have particular characteristics and functions. This internal arrangement is called a system. Outside the boundary of the object are the surroundings, and the reaction between the system and surroundings in general controls the behaviour pattern of the object. A human being and a tree are systems. Heat engines and allied arrangements, which are the concern here, are other systems. It is not necessary that at anyone time a complete object need be under investigation. Only part may be under study and this part may then be considered as the system. In other words, a system can be defined as a particular region which is under study. It is identified by its boundary around which are the surroundings. The boundary need not be fixed. For example, a mass of gas (the system) may expand, so the boundary in this case will modify and interactions will occur with the surroundings at the boundary. If the mass of a system remains constant, the system is a closed system. If, on the other hand, the mass of a system changes, or is continuously changing, the system is an open system. For example, an air compressor is an open system since air is continuously streaming into and out of the machine, in other words, air mass is crossing its boundary. This is called a two-flow open system. Another example is air leaving a compressed air tank. This would be a one-flow open system since air is only leaving the tank and none is entering. In any system, energies such as work and heat could be arranged to cross the boundary. Closed and open systems are illustrated in Fig. 2.1. 2.2
If the volume of a system under study remains constant then this volume is called the control volume. The control volume is bounded by the control surface. A control volume and its surface are illustrated in Fig. 2.2. It is shown as a fixed volume enclosing a steam turbine and condenser. Various masses and energies can be investigated as they cross the control surface into, or out of, the control volume. The control volume is similar in concept to the open system. In the case of the control
The conservation of energy
The concept of energy was discussed in section 1.17. From this discussion it appears that, by designing suitable devices, one form of energy can be transformed into another. In a power station, the potential chemical energy in the fuel produces a hightemperature furnace. Heat energy is transferred from the furnace into the steam being formed, which is passed into a turbine where some of it is converted into work. The work is put into an alternator where some is converted into electrical energy. The electricity generated is then passed out of the station to the public. who use it in various devices to produce heat, light and power. Not all the energy put into the furnaces of the power station ultimately appears as electrical energy. There are many losses through the plant, as indeed there are in any power plant.
Basic enRineering thermodynamics
However, it is found that, in any energy transformation system, if all the energy forms are totalled, including any losses which may have occurred, the sum is always equal to the energy input. Written as an equation, this becomes Initial energy of the system
Energy entering the system
Final energy of the system
Energy leaving the system
Naturally, all the energies must be expressed in the same units. The fact that the total energy in anyone energy system remains constant is called the principle of the conservation of energy. This states that energy can neither be created nor destroyed; it can only be changed in form. As a further point, from work carried out in the field of nuclear physics, it appears there is some relationship between energy and matter. This has been made manifest by the fact that, during a nuclear reaction, some of the energy released can only be accounted for by reference to the loss of nuclear matter which has occurred during the reaction. Thus it appears that matter and energy are related in some way. From this, the conservation of energy should strictly be modified to the conservation of energy and matter. But any matter---energy transformation that occurs outside the nuclear field is extremely small, if it occurs at all. Thus in the absence of nuclear reaction, all energy transformation is discussed using the principle of the conservation of energy.
Energy forms in thermodynamic
Various energy forms can exist in thermodynamic systems. In some systems they may all be present. In other systems only some may be present. The various forms of energy appearing in thermodynamic systems are listed below. The basic unit of energy, in all forms, is the joule (1). Multiples such as the kilojoule (kJ) or the megajoule (MJ) are often used.
Itored within a fluid which results from the internal motion of its atoms and molecules is called its internal energy and was discussed in section 1.23. It is usually designated by the letter U. The internal energy of unit mass of fluid is called the lpeciflc internal energy and is designated by u. 2.4.4 Flow or displacement energy Any volume of fluid entering or leaving a system must displace an equal volume ahead of itself in order to enter or leave the system, as the case may be. The displacing mass must do work on the mass being displaced, since the movement of any mass can only be achieved at the expense of work. Figure 2.3 illustrates a part of a system which can be considered as being the entry or exit of the system. Consider unit mass of fluid entering or leaving the system. Let the fluid be at uniform pressure P and enter or leave the system a distance lover a uniform area A.
Basic engineering thermodynamics
The statement by Sadi Carnot is a positive statement in that it declares when it is possible to produce motive power. The statements which follow are negative because they declare impossibilities.
Rudolf Clausius (1822-1888)
It is impossible for a self-acting machine, unaided by any external agency, to convey heat from a body at a low temperature to one at a higher temperature. The implication of the Clausius statement is that, unless external energy is made available, heat transfer up a gradient of temperature is impossible. The fact that heat transfer can be made to occur up a temperature gradient is made manifest in the refrigerator. However, the refrigerator is not self-acting. It requires external energy in order that it can operate (see Chapter 17). Lord Kelvin (1824-1907)
We cannot transfer heat into work merely by cooling a body already below the temperature of the coldest surrounding objects. Lord Kelvin (William Thomson) implies that when a body reaches the temperature of the coldest surrounding objects no further heat transfer is possible, hence no further work transfer is possible. Max Planck (1858-1947)
It is impossible to construct a system which will operate in a cycle, extract heat from a reservoir, and do an equivalent amount of work on the surroundings. According to Planck, the complete conversion of heat transfer into work transfer is an impossibility. The inference is that there must always be some heat transfer rejection, which is a loss from the system. Kelvin-Planck
It is impossible for a heat engine to produce net work in a complete cycle if it exchanges heat only with bodies at a single fixed temperature. The Kelvin-Planck combination implies that it is not possible to produce work transfer if a heat engine system is connected only to a single heat energy source or reservoir which is at a single fixed temperature. Note that if such a heat engine system were possible it would have perpetual motion! This arrangement is said to have perpetual motion of the second kind. No such arrangement exists (see section 3.3 for perpetual motion of the first kind). In summary, the implications of the second law of thermodynamics are as follows: •
The laws of'thermodynamics
Heat transfer will only occur, and will always naturally occur, when a temperature difference exists, and always naturally down the temperature gradient. If, due to temperature difference, there is heat transfer availability, then work transfer is always possible. However, there is always some heat transfer loss.
Temperature can be elevated but not without the expenditure of external energy. Elevation of temperature cannot occur unaided. There is no possibility of work transfer if only a single heat energy source or reservoir at a fixed temperature is available. No contradiction of the second law of thermodynamics has been demonstrated. If work transfer is supplied to a system, it can all be transformed in heat energy.
Examples of work being transformed into heat energy are seen in the cases of friction and the generation of electricity. But heat energy transfer cannot all be transformed into work transfer. There will always be some loss. Thus work transfer appears to have a higher transfer value than heat transfer. It is important to attenuate this last statement because it is usual that work transfer is only made available by the expenditure of heat transfer. From the second law of thermodynamics it follows that, in order to run all the engines and devices in use at the present time and to maintain and develop modem industrial society, a supply of suitable fuels is absolutely essential. It is by burning and consuming these fuels that the various working substances (e.g. air and steam) have their temperatures raised above the temperature of their surroundings, thus enabling them to release energy by heat transfer in a natural manner according to the second law of thermodynamics. By virtue of the second law of thermodynamics it is essential that all fuels should be used as efficiently as possible in order that fuel stocks may be preserved for as long as possible. It must always be remembered that, once energy has been degraded by heat transfer down a temperature gradient, further energy is only made available at the expense of further fuel. 3.5
The third law of thermodynamics
This law is concerned with the level of availability of energy. Section 1.23 discussed the concept of internal energy. This section suggests that the internal energy of a substance results from the random motion of its atoms and molecules. Furthermore, this motion is also associated with temperature, and from this develops the idea of an absolute zero of temperature when all random motion ceases. Chapter 7 develops the concept of entropy and shows how it is associated with temperature and with the availability of thermal energy. For a substance, if the random translational, rotational and vibrational types of motion of its constituent atoms and molecules are reduced to zero, the substance is considered to become perfectly crystalline and the energies associated with these forms of motion will be reduced to zero. Thus, the energy within the substance is reduced to the ground state. This neglects the energy within the basic atomic structure of the substance, associated with electrons, neutrons and other particles. These considerations led to the development of the third law of thermodynamics: At the absolute zero of temperature substance is zero.
the entropy of a perfect crystal of a
Steam and two-phase systems
Steam and two-phase systems
Before investigating the formation and properties of steam it will be useful to discuss the various forms which matter can take and the relationship between them. Matter can take the forms, solid, liquid, vapour or gas, and many substances can exist in any one of these forms. Consider, for example, a metal. In its natural state, metal is solid. If it is heated, at some temperature the metal will melt and become a liquid. Further transfer of heat, to the now liquid metal, will ultimately transform the liquid into a vapour and finally a gas. If the temperature is now reduced, the gaseous metal will pass back through all the stages it passed through until it finally becomes a solid once again. Each change from one form to another is called a change of phase, and each change of phase is accomplished by the addition or extraction of heat. The temperature at which the changes of phase take place will vary according to the substance being used. A further point to be considered is that a change of phase is accompanied by a change of volume. Generally, the change of volume which accompanies a change from solid to liquid is not very great. On the other hand, the change of volume during the change from liquid to vapour or gas can be very large. The ability of a fluid readily to expand or contract is the requisite feature for successful operation of a thermodynamic engine. Thus both vapours and gases can be used in thermodynamic engines, but the technique for vapours is somewhat different from the technique for gases. The distinction between a vapour and a gas will be made late in this chapter, and Chapter 5 is devoted to the properties of gases. For now the focus is on vapours. A vapour results from a change of phase of a liquid due to a transfer of heat. The bulk of the liquid is generally very much smaller than the bulk of the vapour formed. It follows, therefore, that any liquid which can be easily obtained and handled can be used as the generator of the vaporous working substance for use in an engine. Water is such a liquid. It is in abundant supply, can be easily handled and readily turned into its vaporous phase called steam. Consequently, the change of phase from water into steam deserves closer investigation. But before doing this, it is useful to note that any other liquid undergoing a change of phase into a vapour will follow the
lame general features as the water-steam transformation. The difference lies in the pressures, temperatures and energy quantities at which the various phenomena occur. A system in which a liquid is being transformed into a vapour is a two-phase Iystem. The mixture of liquid and vapour is a two-phase mixture. 4.2
The formation of steam
In the following discussion it will be assumed that the water and ultimately the steam are in some suitable container which can accommodate any changes of state. The container is called a boiler, and the way in which it accommodates the changes will be discussed in Chapter 10. Steam is almost invariably formed at constant pressure, so that it is a good place to begin. If a mass of water is heated then, like all other substances, its temperature increases. There is also a small increase in volume. For a time, these are the only changes which take place. After a while, small bubbles appear, clinging to the side of the containing vessel. They are soon released then float to the surface and disappear. These bubbles are the dissolved gases being driven off; as well as being able to dissolve some solids, water can also dissolve some of the atmospheric gases. Further heating produces further temperature rise but, apart from this, there is no other apparent external change. Soon, however, signs of internal activity appear. Small steam bubbles are formed on and near the heating surface; they rise a little through the water and collapse. Their density is lower than the surrounding water, so they rise through it. The surrounding water is cooler so it extracts some energy from the steam bubble, which immediately collapses. This collapse of steam bubbles is the reason for the singing of a kettle. The temperature continues to rise with the transfer of heat and the bubble activity increases correspondingly. Finally, the water mass is at such a temperature that the steam bubbles are able to rise completely through the water, escaping from its surface. The water mass is now in an extremely turbulent state; it is boiling. A rather more technical term used for boiling is to say that the water is in a state of ebullition. But what of the temperature now? As soon as boiling commenced the temperature ceased to rise, remaining at what is commonly called the boiling point. This is important because, while boiling continues, the temperature will remain constant, independent of the quantity of heat transferred to the water. In fact, so long as there is water present, it is apparently impossible to increase the temperature beyond the boiling point. A name is given to this boiling point; it is called saturation temperature. And what of the nature of the steam being produced? The boiling water is now in great turbulence as a result of the steam bubbles formed, forcing their way up through the water to break through the surface. The turbulence can be increased or decreased by increasing or decreasing heat energy supply. As the steam breaks away from the water surface it will carry with it small droplets of water. The larger droplets will tend to gravitate back to the water surface, but the smaller droplets will continue on their way with the steam. Steam with these small droplets of water in suspension is called wet steam. Steam formed from a water mass will always be wet to a greater or lesser extent; wetness generally depends on the turbulence occurring in the water. It is impossible to obtain dry steam while water is present. Continuing the heating process will produce more and more wet steam until
Steam and two-phase systems
Basic engineering thermodynamics
eventually the whole of the water mass disappears. The temperature, by the way, has remained constant at saturation temperature. The water droplets in suspension make the wet steam visible. Steam itself is a transparent vapour, but the inclusion of water droplets in suspension gives it the white cloudy appearance. What really is being seen is the cumulative effect of the water droplets reflecting light. Further transfer of heat to the wet steam will convert the suspended water droplets into steam and finally a state will be reached when all the water has been turned into steam. The steam is then called dry saturated steam. It has now lost its visible characteristic, it has become completely transparent; this condition marks the end of the constant temperature intermediate phase. Still further transfer of heat to the now dry saturated steam produces a temperature rise and the steam now becomes superheated steam. This is the last phase in the transformation of water into steam. It thus appears there are three distinct stages in the production of steam from water. 4.2.1 Stage 1 Stage I is the warming phase in which the temperature of the water increases up to saturation temperature. The energy required to produce this temperature rise is called the liquid enthalpy. 4.2.2 Stage 2 Stage 2 takes place at constant temperature; it is when the water is transformed into steam. Stage 2 begins with all water at saturation temperature and ends with all dry saturated steam at saturation temperature. Between these two extremes, the steam formed will always be wet steam. The energy required to produce the total change from all water into all steam is called the enthalpy of evaporation. It is sometimes colloquially known as latent heat because no temperature rise is produced in this stage. 4.2.3 Stage 3 Stage 3 begins when all dry saturated steam has been formed at saturation temperature. Further transfer of heat produces superheated steam which is accompanied by a rise in temperature. The amount of energy added in the superheat phase is called the superheat enthalpy. Note that temperature increase (or it could be decrease) only takes place during the transfer of heat when a substance is in a single phase. In this case there has to be all water or all dry steam before the temperature changes. The phenomenon of temperature change happens with all substances but it only occurs in a single phase, be it solid, liquid or vapour. If a two-phase mixture exists (solid-liquid or liquid-vapour), the temperature remains constant until a complete change from one phase to another has been completed. A word here about the three phases of matter - solid, liquid and vapour (or gas) as they are related to atomic or molecular activity. In the solid phase the atoms or molecules of a substance oscillate about a mean position. As the temperature increases, the degree of oscilIation increases until the atoms or molecules are able to overcome interatomic or intermolecular attractions. When this occurs, the substance
becomes fluid and the atoms or molecules are now able to move freely but not independent of the main mass. The substance is now a liquid. The temperature remains constant while the change from solid to liquid occurs; energy increase is necessary to accelerate the atoms or molecules to the velocity required to produce freedom of movement. When the substance is alI liquid, energy increase produces an increase in temperature once again and the atoms or molecules move about faster and faster (see section 1.23 on internal energy). If the atoms or molecules of a liquid move about faster and faster as the temperature increases, the liquid must become turbulent. Eventually, some atoms or molecules must reach escape velocity; they are able to escape from the liquid mass at its free surface because their velocity is sufficient to overcome all internal interatomic or intermolecular attractions. This is the phenomenon of boiling; once again the temperature remains constant while all atoms or molecules absorb enough energy to attain escape velocity. Eventually, all atoms or molecules attain this velocity and the whole mass is now a vapour. From here on, further energy increases will increase the atomic or molecular velocities, causing the temperature to rise.
The previous discussion centred round the fact that steam is usually produced at constant pressure. This pressure can be higher or lower, as the case may be. Boiling wilI occur at saturation temperature, and it is found that saturation temperature depends upon the pressure exerted at the surface of the water. In other words, it depends upon the pressure at which the steam is being formed. Chapter I discussed the fixed points of the conventional thermometer; it explained that the upper fixed point is the boiling point of water, which is taken as 100°C. But this is only the case when atmospheric pressure is 760 mm Hg. If atmospheric pressure is increased, the boiling point (or saturation temperature) increases. Conversely, if the pressure decreases then so does the saturation temperature. Figure 4.1 shows the type of curve obtained when saturation temperature is plotted against absolute pressure. It is calIed the liquid-vapour equilibrium line. It will be noted that the rate of increase of saturation temperature is not as great at the higher pressures as at the lower pressures.
The triple point
Figure 4.1 represents changes in saturation temperature with pressure for steam. Figure 4.2 enlarges this plot for the region of low temperatures and low pressures. The line dividing the liquid and vapour phases is the liquid-vapour equilibrium line; the line dividing the solid and liquid phases is the solid-liquid equilibrium line. The two lines join at point 3, the triple point. The solid-liquid equilibrium is shown as horizontal, indicating there is little change in the solid-liquid (melting) point as a result of change in pressure. Actually, increase in 2 pressure very slightly depresses the freezing point of water up to about 200 MN/m • The liquid-vapour equilibrium line shows the increase in saturation temperature with pressure increase as already discussed.
Steam and two-phase systems
the compression be at constant temperature. Such a compression would appear as FGHJ, for example, moving from right to left. Inspection shows that, no matter how much compression takes place, no liquefaction occurs or can occur. On the other hand, if compression at constant temperature takes place below the critical point, it will appear as KLMN, for example. This shows that compression passes out of the superheat region into the wet region, thus liquefaction is taking place. As long as the temperature is above the critical temperature, no change of phase occurs during compression or expansion; the behaviour is similar to that of a gas. In fact, at temperatures higher than the critical temperature, the vapour becomes a gas. It should again be noted here that, although the above discussion has concentrated on the production of a temperature-enthalpy diagram for steam, similar diagrams can be developed for other vapours. The temperature-enthalpy diagram and others which will follow are called phase diagrams.
Dry saturated steam points C, F and J appear to lie on a smooth curve. If a wide range of pressures are considered and the results plotted at suitable pressure intervals, the complete temperature-enthalpy diagram is obtained (Fig. 4.4). In this diagram it will be noted that the dry saturated steam points have been joined by a smooth curve, called the saturated vapour line. This line and the saturated liquid line enclose an area in which the steam is wet. To the right of the saturated vapour line the steam is superheated. Inside the wet steam area can be drawn lines of constant dryness (shown dotted). These lines are obtained by joining such points as A, B, C, D and E, where the dryness is the same. It is very important to realise that the liquid line and the saturated vapour line may be continued upward until eventually they meet. The point at which the two lines join is called the critical point. This point is found to occur at a pressure of 22.12 MNjm2 and a temperature of 374.15 0c. As the pressure increases toward the critical pressure, the required enthalpy at evaporation is reduced until it finally becomes zero at the critical point. This implies that, at the critical point, the water changes directly into dry saturated steam. The critical point is really the division between behaviour as a vapour and behaviour as a gas. When a gas is compressed it does not liquefy in the normal course of events. This can be explained with reference to the temperature-enthalpy diagram and the critical point. The diagram shows that liquid, in this case water, can only exist at temperatures below the critical point. Consider the case of a compression above the critical point and, for simplicity, let
Volume of steam
As with all other substances, the volume of water and of steam incrcaseH as the temperature increases. In the lower pressure ranges the volume of water is very small compared with the volume of steam it produces. The main change of volume occurs in the evaporation stage. But as the pressure and temperature approach the critical point, the change in volume decreases until, at the critical point, there is no change of volume from the water phase to the dry steam phase. However, the vast majority of applications for steam use a pressure below the critical pressure, so the three stages of formation have to be considered.
Volume of water
Steam tables quote the specific volume of water. At saturation temperature, for a given pressure, the specific volume of water is tabulated as Vfm3jkg. Alternatively, specific volume may be tabulated at a particular temperature against various pressures.
Example 4.6 Determine the specific volume of water at saturation temperature for a pres.fure of 4.0 MN/m2• SOLUTION
Look up the values in steam tables. Pressure (MNjm2)
Saturation temperature CC)
Specific volume vf(m3jkg) -_._._-----~ 0.00 I 252
68 Basic engineering thermodynamics
Steam and two-phase systems
Example 4.7 Determine the specific volume of water at a temperature of 175 DC and a pressure of4.0 MN/m2.
SOLUTION Look up the values in steam tables. Pressure (MN/m2) ..-------
Steam tables quote the specific volume of superheated steam, either with pressure against actual steam temperature or with pressure against degree of superheat. Volume is given in m3jkg.
Example 4.9 Determine the specific volume of steam at a pressure of 2 M N/m2 and with a temperature of325 dc.
0.001 089 0.001 120 0.001 156
Volume of superheated steam
Temperature (0C) 150 175 200
0.001 088 0.001 119 0.001 153
0.001 087 0.001116 0.001 152
SOLUTION Steam tables give pressures and temperatures near to the required state. Pressure (MNjm2)
So the specific volume of water at a temperature of 175 DC and a pressure of 4.0 MN/m2 is 0.001 119m3/kg.
Volume of dry saturated steam
Volume of wet steam
Temperature ("C) 300 325 350
The specific volume of dry saturated steam is tabulated against its corresponding saturation temperature and pressure in steam tables and is designated as vg. Volume is given in m3jkg. 4.17
0.257 7 0.270 3 0.282 5
Specific volume (m3jkg) A
0.1255 0.132 I 0.138 5
0.058 8 0.062 8 0.066 4
This shows that the specific volume of steam at a pressure of 2 MN/m2 and with a temperature of 325 "C = 0.132 1 m3fkg. Saturation temperature at 2 MN/m2 = 212.4 dc. The actual steam temperature is 325 "C, so the steam is superheated. The degree of superheat is 325 - 212.4 = 112.6 K. Some superheat steam tables give the degree of superheat instead of the actual steam temperature.
Consider I kg of wet steam at dryness fraction x. This steam will be made up of x kg of dry saturated steam and (l - x) kg of water in suspension. .'. Volume of wet steam
At the lower pressures the volume Vf is very small compared with the volume vg, and (l - x) is generally small compared with x. Hence the term (l - x) Vf can be sensibly neglected, so Volume of wet steam
Note that from this X=-
vg This equation can sometimes be used to determine the dryness fraction.
Density of steam
If the specific volume of any quality steam is v, then Density
Thus the density of superheated steam in Example 4.9 is given by Density
If the density, p, is known, then Specific volume
Example 4.8 Determine the specific volume of wet steam ()f dryness fraction 0.9 at a pressure of 1.25 MN/m2.
SOLUTION From tables, at 1.25 MN/m2, vg = 0.156 9 m3/kg v = xVg = 0.9 x 0.156 9 = 0.1412 m3jkg
Example 4.10 Steam 0.95 dry at a pressure of 0.7 MN/m2 is supplied to a heater through a pipe of 25 mm internal diameter; the velocity in the pipe is 12 m/s. Water enters the heater at 19°C, the steam is blow into it and the mixture of water and condensate leaves the heater at 90 dc. Calculate (a) the mass of steam entering the heater in kg/h (b) the mass of water entering the heater in kg/h
Steam and two-pha.\'e systems
diagram for a vapour
Figure 4.5 shows another phase diagram for a vapour; this time the axes are pressure and specific volume. It plots a whole series of isotherms (lines of constant temperature). At a temperature less than the critical temperature (T is shown dotted as line EFG. Between the critical temperature isotherm and the saturated vapour line the vapour is superheated. At temperatures above critical (T> Tc), the isotherms gradually lose their discontinuity and eventually become smooth curves such as HJ. The behaviour then becomes that of a gas.
Basic engineering thermodynamics
h = u + Pv u = h - Pv
where u = h= P = v=
specific internal energy, Jjkg specific enthalpy, Jjkg absolute pressure, Njm2 specific volume, m3 jkg
Thus, by using tables of properties and by suitable substitution into equation [2], the specific internal energy of a vapour at a particular state can be determined. Note that some tables actually tabulate specific internal energy.
Example 4.11 1.5 kg of steam originally at a pressure of 1 M N/m2 and temperature 225°C is expanded until the pressure hecomes 0.28 MN/m2. The dryness fraction of the steam is then 0.9. Determine the change of'internal energy which occurs.
Figure 4.8 shows a collector tank that is fed with the steam under test. The entry steam pipe feeds into the top of a perforated cup suspended in the collector tank. A calibrated gauge glass fits into the side of the collector tank. A dry steam exit is provided from the side of the tank at the top and a drain valve is provided at the bottom. In operation, the steam passes into the calorimeter and is rapidly forced to change its direction when it hits the perforated cup. This introduces vortex motion into the steam and the water separates out by centrifugal action. Some drains through the perforated cup, some falls as large droplets and some precipitates on the walls of the tank and will drain down. All will collect at the bottom of the tank, where the level will be recorded by the gauge glass. The dry steam will pass out of the apparatus into a small condenser for collection as condensate. The perforated cup shown in Fig. 4.8 is just one of many devices used to create a vortex in the steam. Several precautions should be taken when using the separating calorimeter. It must be adequately warmed up before starting any measurement, otherwise condensation will occur on the interior of the apparatus, which will introduce an error into the results. As far as is possible, care must be taken to ensure that the steam does not come into contact with the water which has already been separated, otherwise a certain amount of condensation will occur and this too will affect the results. The calorimeter should have adequate thermal insulation to prevent condensation due to heat loss. Theoretically the system should be adiabatic (Q = 0). It is found in practice that not all the water is separated out; some passes out with the assumed dry steam. Consequently, this apparatus can only give a close approximation of the dryness fraction of the steam. From the results obtained, let
Steam and two-phase systems
It can be noted here that a device called a separator is often fitted in series into a wet steam main to help improve the steam quality (i.e. to make the steam dryer). It works in a similar way to the separating calorimeter. One such separator takes the full steam flow and induces a rapid V-turn change of direction to the wet steam. This rapid change of direction precipitates some of the suspended water from the steam. The precipitated water can then be removed through a steam trap after it has drained to the bottom of the separator.
The throttling calorimeter
Figure 4.9 shows the throttling calorimeter. Steam is drawn from the main through a sampling tube placed across the steam main. The tube is perforated by many small holes and its end is sealed. Steam is forced through the small holes to obtain a representative sample across the main. The sampling tube could be placed in any direction across the main. But the steam to be analysed is wet, so the suspended water tends to gravitate to the bottom of a horizontal steam main. A vertical sampling tube will therefore pick off steam from the driest at the top to the wettest at the bottom. Hence the average sample, which then proceeds to the throttling calorimeter, will be more truly representative than for any other orientation of the sampling tube. If the steam main is vertical, the dryness will be near enough constant across the main, so the sampling tube can be placed in any direction.
From the sampling tube the steam under test proceeds, via a stop valve and a pressure gauge, to the throttle orifice of the calorimeter. The stop valve is provided in order that the calorimeter can be isolated from the main when not in use. After throttling, the steam passes into the throttle chamber of the calorimeter where its pressure can be determined using a water manometer and its temperature determined using a thermometer. The steam then passes away to exhaust, either to atmosphere or to a small condenser, where it can be collected as condensate and its mass determined after condensation. The mass of the condensate need not be determined when using the calorimeter alone. It will be noted that the manometer is a water manometer because the pressure after throttling is usually similar to atmospheric pressure. Mercury would be much too dense for this application; it would not be sensitive enough to record the small pressure change that occurs. To operate the throttling calorimeter the stop valve is fully opened, ensuring that the steam does not experience a partial throttle as it passes through the valve. Steam is then allowed to pass through the apparatus for a while in order that pressure and temperature conditions become steady. As a general check on whether the steam is being throttled to superheat condition, remember that the pressure after throttle will not be greatly different from atmospheric pressure. Saturation temperature in this case will be round about 100°C. If the temperature after throttle is somewhat above 100°C, it can be taken that the steam is being superheated. After conditions have become steady, the gauge pressure before throttling is read from the pressure gauge. The temperature and gauge pressure after throttle are recorded from the thermometer and manometer, respectively. The barometric pressure is also recorded. Let 2
Gauge pressure before throttle = P kNjm Barometric height = h mm Hg
Gases and single-phase systems
Gases and single-phase systems
This is an investigation into single-phase systems. The single-phase being considered is that phase above the critical point when a substance is called a gas. The wide use made of gases in the field of engineering makes it necessary to investigate their behaviour when they are heated, cooled, expanded or compressed. The beginning of any investigation, such as the behaviour of gases, is usually made by conducting experiments; from the results obtained laws are determined which govern their behaviour. The first two laws in this chapter were established by experiment. 5.2
An inverted glass pipette A is connected to a glass thistle funnel B by means of a long rubber, or plastic, tube C. Both the pipette and the thistle funnel are mounted vertically such that they can be moved up or down on either side of a vertical scale D. With the tap E open, the apparatus is filled with a suitable quantity of mercury. It is possible to adjust the height of the mercury columns, and hence the volume of gas in the pipette - the system - by moving the thistle funnel up or down. If the tap is then closed, a fixed mass of gas is trapped in the apparatus and modification to the height of the thistle funnel will bring about pressure changes in the gas which will be accompanied by corresponding changes in volume of the gas. The pipette is calibrated to read the volume of gas contained in it; the vertical scale serves to establish the difference in height h of the two mercury columns. The absolute pressure of the gas will be given by the sum of the height h and the barometer reading. In order to satisfy the condition that the temperature should remain constant, a period of time is allowed to elapse after every change of condition before any new readings are taken. After a suitable number of results are obtained, the corresponding values of absolute pressure and volume are plotted on a graph; the curve is shown in Fig. 5.2.
With any mass of gas it is possible to vary the pressure, volume and temperature. In this experiment it is arranged that the temperature of a fixed mass of gas remains constant while corresponding changes in pressure and volume are observed. An apparatus suitable for conducting such an experiment is illustrated in Fig. 5.1.
Taking any point on the curve, I say, the product of its corresponding pressure and volume P and V will equal some number, C say. Investigation of other points, such as 2 and 3, shows that, within the limits of experimental error, the products of their corresponding pressures and volumes also equal this same number, or PI VI
Further experiments at different fixed temperatures, with different fixed masses and with different gases yield the same result, although the constant C will be different with each quantity of gas, each fixed temperature and each type of gas. From the results of this experiment, a general statement may be made: During a change of state of any gas in which the mass and the temperature remain constant, the volume varies inversely as the pressure. Expressed mathematically PV
This is known as Boyle's law, named after its discoverer, Robert Boyle (l627~1691),
From this, then, the law for an adiabatic expansion or compression of a gas is = C, where )I = cpl cv, the ratio of the specific heat capacities at constant pressure and constant volume. The theoretical adiabatic process is sometimes said to be a frictionless adiabatic process. The reason for this is perhaps best understood by attempting to suggest a practical way of carrying out an adiabatic process. If a piece of apparatus for carrying out an expansion or compression could be constructed of a perfect heat insulating material then an adiabatic process would be quite possible. But no perfect heat insulator exists, so perhaps the nearest approach to an adiabatic process is to complete the process very rapidly, in which case there is very little time for heat exchange between the gas and its surroundings. PVY
But, when such a process is carried out, it is found that with both the compression and the expansion, the final temperature is slightly higher than the calculated value. Now since the process is very rapid, the heat transfer required to increase the temperature above the adiabatic temperature could not have transferred from the outside. The answer to this is friction, turbulence and shock within the gas itself. Energy is required to overcome these effects and it will appear as a slightly increased temperature of the gas above its theoretical value. If these effects are neglected then the adiabatic process is said to be frictionless. The average value of )I, the adiabatic index, for air is of the order of 1.4.
Thermodynam ic reversi bi Iity
If a substance passes through the stages of a process in such a manner that, after the process, the substance can be taken back through all the stages in reverse order until it finally reaches its original state, then the process is said to be reversible. After carrying out a reversible process, there would be no evidence anywhere that the process had ever taken place. No such process exists in practice. Within the substance during any process it is probable that eddies will be set up. Also, due to the viscosity of the substance, however slight, there will be some internal friction. It is also very likely that there will be some small irregularity with regard to the distribution of temperature throughout the substance. The degree to which these occur must have some bearing on the final state of the substance after the process. From here, however, it is unreasonable to assume that these various internal phenomena can be repeated in an exactly reversed sequence in order that the reversed process will return the substance to its original state. For these reasons alone, no actual process can be considered as being truly reversible. A point to raise here, however, is that the effect of these internal phenomena is not likely to be great, and from a theoretical standpoint, it is possible to neglect them. This being so, it is necessary to consider what else will affect the concept of reversibility. As an example, consider the expansion of a gas. During any expansion of a gas there will be heat transfer into or out of the gas, with the exception of the adiabatic case in which, by definition, there is no heat transfer (see section 1.27). Now the second law of thermodynamics states that heat transfer will only occur down a temperature gradient as a natural occurrence. During the expansion, assume that there is some heat transfer from the gas to the surroundings. If this is the case, then by the second law of thermodynamics, the surroundings are at a lower temperature than that of the gas. What if an attempt is now made to reverse the process? This now means compressing the gas, which is easy enough. However, it is not possible to return the energy lost by heat transfer to the surroundings because the gas is at a higher temperature than the surroundings. The original pressure and -temperature could not be attained, however, because of the energy loss by heat transfer to the surroundings during the original expansion.
This energy loss cannot be returned because of the limitations imposed by the second law of thermodynamics. Notice that there would also be some heat transfer loss during the reversed process of a compression because, again, the temperature of the gas would be above that of the surroundings. Another point to consider is the effect of pressure imbalance. If a substance at a high pressure expands into surroundings, which are at a lower pressure, then the reversed process of the low-pressure surroundings returning the substance to its original high pressure is impossible without the aid of external energy. Thus if pressure imbalance occurs, the process cannot be reversible. As an extreme example of pressure imbalance, consider the free expansion of air in the Joule's law experiment, discussed in section 5.5. In this case, the compressed air expanded into a vacuum, so no work was transferred. The reverse of this process would be impossible without the aid of external energy. Hence, the original free expansion process is irreversible. It follows that practical thermodynamic processes are irreversible. Now irreversibility evidently involves loss, so it appears that reversibility is bound up with efficiency. A truly reversible process involves no loss and is therefore the most efficient thermodynamic process possible. No external energy is required to return a substance to its original state in a truly reversible process. It is important, therefore, to investigate whether any processes may be considered to be theoreticaIly reversible. 6.2
The adiabatic process
No heat is transferred during an adiabatic process. Thus the effect of the second law of thermodynamics between the substance and its surroundings is eliminated. If the effects of pressure imbalance, internal friction, non-uniform temperature distribution, etc., are neglected, it follows that the adiabatic process is theoretically reversible. For an adiabatic non-flow process it is shown that W = -!:1U. Thus, during an adiabatic expansion, external work is done which equals the decrease in internal energy. If this same amount of work is done on the working substance during the reversed process of adiabatic compression, the work will appear as an increase of internal energy of the substance, and this increase will just equal the loss which occurred during the expansion. Thus the substance will be returned exactly to its original state. The adiabatic process is therefore theoretically reversible. 6.3
Basic engineering thermodynamics
The isothermal process
An isothermal process is carried out at constant temperature. For an isothermal, non-flow process, it was shown that the necessary energy exchange was that Q = W. This means that, during an isothermal expansion, the working substance must receive an amount of heat equal to the external work done. It follows that if, during the reversed process of compression, an amount of heat equal to the work done on the substance is rejected then, neglecting the effects of pressure imbalance, internal friction, non-uniform temperature distribution, etc., the isothermal process is theoretically reversible.
With regard to the transfer of heat into or out of the substance, it must be remembered that an isothermal process is carried out at constant temperature. Assuming that the external surroundings are at this temperature, heat transfer is equally possible in either direction, namely, into or out of the substance. Actually, by the second law of thermodynamics, a temperature difference is required in order to promote a natural heat transfer. Therefore, during an isothermal expansion, it could be considered that the surroundings are at a slightly elevated temperature above the substance, so the necessary condition that the substance shall receive heat would be met. Similarly, it could be considered that the substance has a slightly elevated temperature above the surroundings during an isothermal compression. In this case the necessary condition that the substance should reject heat would be met. Since the temperature difference in each case would be small, the rate of heat transfer would be very slow, so from a practical point of view, the isothermal process is very slow. Actually it is all but impossible as a practical process. However, theoretically it exists and, neglecting the effect of internal friction, etc., it is theoretically reversible.
The polytropic, constant volume and constant pressure processes
In all these cases heat is received or rejected by the substance during the progress of the process, and the temperature changes continuously throughout the process. If the temperature of the surroundings remains constant then, by the second law of thermodynamics, heat transfer between the substance and surroundings is unidirectional, being a function of whether the substance is at a higher or lower temperature than the surroundings. Also, apart from the constant pressure process, in which the pressure of the substance could be made the same as the pressure of the surroundings, in these processes where pressure interaction occurs between the substance and the surroundings, there is pressure imbalance. In these cases, in which there is pressure and temperature difference between the substance and surroundings, the processes are irreversible. These processes could be considered reversible if the temperature and pressure of the surroundings could be made to vary in the same way as the temperature and pressure of the substance. In this way, in a similar manner to the isothermal case, mutual heat transfer in either direction would be possible, and there would be no pressure imbalance. The processes could then be considered reversible. In the main, these conditions are impossible to achieve, although the constant pressure process can be made to approach the reversible condition in a contraflow heat exchanger. Figure 6.1 illustrates the principle of the contraflow heat exchanger. A hot fluid enters at temperature II and is progressively cooled at constant pressure until it leaves the heat exchanger at lower temperature 12. The cool coolant is made to enter at the same end of the heat exchanger at which the cool fluid leaves and passes through the heat exchanger in an opposite direction from the fluid being cooled. The hot coolant thus leaves at the same end at which the hot fluid is entering. The opposite flows account for the name conlraflow.
Now assume that some other engine E can be found which is more efficient than the reversible engine R. Since it is more efficient, engine E will require less heat to perform the same amount of work WR. Let this engine E drive engine R reversed and let them both work between the same source and sink. This is shown in Fig. 6.2(b). Engine E, being more efficient, will require (Q - bQ) units of heat supplied from the source at temperature T\. It will convert WR of this into work and it will reject (Q - bQ) - WR units of heat into the sink at temperature Tz. Now the work WR will drive engine R reversed, which now becomes a heat pump. Thus it will take up (Q - WR) units of heat from the sink at lower temperature Tz. It will convert WR units of work into heat then reject (Q - WR) + WR = Q units of heat into the source at higher temperature TI' Investigation of this system will show that, during the time period considered, there has been a gain of heat to the source of Q - (Q - bQ) = bQ units of heat. Also the sink has lost (Q - W) - [(Q - bQ) - WR] = bQ units of heat. This means that the source at higher temperature Tl is receiving heat from the sink at lower temperature Tz· Now this arrangement is self-acting and has apparently managed to make more heat transfer up the gradient of temperature than has moved down. This would mean that eventually all the heat would be transferred to the source at temperature T\ while the sink at lower temperature Tz would have its energy content reduced to zero! This is contrary to the second law of thermodynamics, so the system is impossible. If, however, the engine E has the same efficiency as the reversible engine R, then Fig. 6.3 shows that the thermodynamic system balances, in which case both the source and sink gain as much heat as they lose. This means that the energy level of both the source and the sink would remain constant. Once started, this system would continue to run indefinitely, so it would have perpetual motion! (See sections 3.3 and 3.4.)
Basic engineering thermodynamics
Engine E is arranged to drive engine R reversed. Engine R will require work WR (WR fl and f3 > fa2 because there must be a temperature difference in order that heat transfer can take place.
The general arrangement of the basic elements of a steam plant are illustrated diagrammatically in Fig. 10.1. Steam is generated in a boiler from which it passes into the steam main. The steam main feeds the steam into a turbine or engine or it may pass into some other plant such as heaters or process machinery. After expanding through the turbine or engine or passing through some other plant, if the plant is working on a 'dead-loss' system, then the exhaust steam passes away to atmosphere. Such is the case with the steam locomotive, which is still in use on some railways in many countries of the world. This system is very inefficient and is rarely adopted in modern plant. It is used in the steam locomotive since, in this case, the plant is mobile and there is not sufficient room for the complex steam reCQvery equipment which can be installed in a power station or factory. If steam recovery plant is installed, the exhaust steam passes into a condenser where it is condensed to water, called condensate. The condensate is extracted from the condenser by the condensate extraction pump from which it passes as feedwater into the feedwater main and back to the boiler. Because the boiler is operating at a high pressure the water pressure must be increased in order to get it into the boiler. This is dealt with by means of a pump called the feed pump. Thus the water returns to the boiler and, neglecting system losses, a steam recovery plant circulates the same water all the time. Actually, there are losses; they are made up in the condenser by means of a make-up water supply. The advantages of steam recovery plant are primarily as follows. Firstly, the pressure in the condenser can be operated well below atmospheric pressure. This means that a greater expansion of the steam can be obtained, which results in more work. Secondly, the water in the circuit can be chemically treated to reduce scale formation in the boiler. The formation of scale in the boiler impedes the transfer of heat from the furnace to the water, so it reduces the boiler efficiency. It may also cause local overheating with resultant damage and, if overheating is serious, it may even cause a burst in the vicinity. The condenser is cooled by circulating cooling water through it. If an abundant supply of water is nearby, such as a river or lake, then this can be used. Trouble may be experienced here due to water pollution. This may take the form of fish or mud
Sa.fie engineering thermodynamics
entering the condenser system. Filters are usually installed to cut down pollution, otherwise the condenser cooling water circuit may become blocked. On the other hand, the river or lake may itself become polluted by hot water returning from the condenser. If the amount of hot water is large it could have an effect on the flora and fauna of the river or lake. If a river or lake is not to hand, or the risk of pollution is too high, it is common to install a cooling tower of wood or concrete. The hot water from the condenser enters the tower approximately midway up and is sprayed to the bottom. Air circulates into the bottom of the tower and passes up through the water spray. Heat transfer occurs between the water and the air, thus cooling the water. The warmed air passes out at the top of the tower. The cooled water is collected at the bottom of the tower from where it is pumped back to the condenser. The same cooling water is circulated through the condenser. It is only necessary to make up any loss. The entire steam plant contains four separate circuits. 10.1.1
The furnace gas circuit
Air is taken into the furnace from the atmosphere to supply the necessary oxygen for combustion. The combustion products pass through the boiler, transferring heat, then pass out to the atmosphere through the flue. Care should be taken to reduce atmospheric pollution from the combustion products to an absolute minimum. Most furnaces are fired by coal, gas or oil. 10.1.2
The steam circuit
Water is passed into the boiler where it is converted into steam. It passes into the plant where it is expanded, giving up some of its energy. It is then condensed in a condenser and passes as condensate to be pumped back into the boiler. 10.1.3
Condenser cooling water circuit
Cool water passes into steam then, at a higher tower. Cool water then pollution to a minimum 10.1.4
the condenser, has heat transferred into it by the condensing temperature, passes out to be cooled in a river, lake or cooling circulates back to the condenser. Care must be taken to reduce if the discharge of condenser cooling water is into a river or lake.
Cooling air circuit
In the case of a cooling tower, cool air passes into the bottom of the tower from the atmosphere and heat is transferred into it from the falling hot water spray. The warm air then passes back to the atmosphere through the top of the tower. In the case of a river or lake the condenser cooling water will mix with river or lake wat~r which will be cooled by heat transfer to the atmosphere. In some steam plant the condensate from the condenser is passed into a tank, called the hot well, which acts as a reservoir for feedwater. From the hot well, feedwater is pumped through the feed pump back into the boiler. In this case, makeup water could be fed into the hot well. 10.2 Boilers A boiler is the device in which steam is generated. Generally, it must consist of a water container and some heating device. There are many designs of boiler but they can be divided into two types: fire-tube boilers and water-tube boilers. Before
describing various boiler designs, it will be useful to discuss the formation of steam in a boiler, and the methods employed to improve its thermal efficiency. Whatever the type of boiler, steam will leave the water at its surface and pass into what is called the steam space. This is the space in the water container directly above the water. Steam formed above the surface of water is always wet and will remain wet so long as there is water present. This is because the steam, rising from the surface of the turbulent boiling water, will carry away with it some minute droplets of water (see section 4.2). The water container must always contain water, so the steam in the steam space is always wet. If wet steam is all that is required, the steam is piped directly from the steam space into the steam main. But if superheated steam is required, the wet steam is removed from the steam space and piped into a superheater. This consists of a long tube or series of tubes which are suspended across the path of the hot gases from the furnace. As the wet steam progresses through the tube or tubes it is gradually dried out and eventually superheated. From the superheater it passes to the steam main. If a control of the degree of superheat is required, as in some of the larger boilers, then an attemperator is fitted. The control of the degree of superheat is obtained by the injection of water or steam into the superheated steam. If an attemperator is fitted, the superheater is generally divided into two parts. The first part is called the primary superheater. Then comes the attemperator followed by the second part of the superheater called the secondary superheater. Now the flue gases will still be hot, having passed through the main boiler then the superheater. The energy in these flue gases can be used to improve the thermal efficiency of the boiler. To achieve this thermal efficiency improvement, the flue gases are firstly passed through an economiser. The economiser is really a heat exchanger in which the feedwater being pumped into the boiler is heated. The feedwater thus arrives in the boiler at a higher temperature than would be the case if no economiser were fitted. Hence, less energy is required to raise the steam, or if the same energy is supplied, then more steam is raised. This results in a higher thermal efficiency. Having passed through the economiser, the flue gases are still moderately hot. Further thermal efficiency improvement can be obtained by passing them through an air heater. This, too, is a heat exchanger in which the air being ducted to the boiler furnace is heated. The air thus arrives at the furnace hotter than if the air heater were not fitted. This results in a higher furnace temperature which thus increases the furnace potential for steam raising. Thermal efficiency improvement results. Still further improvement of boiler thermal efficiency is obtained by the installation of a reheater. The reheater will often appear in the flue gas path before the economiser. In some of the larger steam turbines, e,g. in power stations, steam is removed from the turbine after partial expansion. This steam is fed back to the boiler and then to the reheater. Here it is reheated to a higher temperature and then passed back to the turbine where it completes its expansion in the latter stages. The object of reheating steam in a turbine plant is to preserve the steam quality in the low-pressure stages of the turbine. If there were no reheat, the steam in the lowpressure stages would become too wet. Wet steam has erosive and corrosive effects on the turbine blades. By returning the steam to the boiler, after partial expansion. the quality of the steam is improved and wet steam in the low-pressure stages is
Basic engineering thermodynamics
therefore largely avoided. Reheating also gives greater potential work output from the steam in the low-pressure stages. It may also give a slight improvement in thermal efficiency. In power stations it was usually the practice to cross-connect all boilers and turbines; any boiler could run any turbine. But the installation of reheaters was difficult, so it was rarely adopted. Modern boilers, however, have become much more reliable and new installations have one boiler connected to one turbine, making a single boiler-turbine unit. Reheaters are more easily installed because there is no cross-coupling. The location of the superheater, reheater, economiser and air heater are illustrated in Fig. 10.2. After the air heater, the flue gas passes to the exhaust chimney.
Sttum plunt 10.3
Important parts of a boiler
The following auxiliary equipment is fitted to all boilers: • • •
gauge This will record the gauge pressure of the saturated steam formed in the steam space. A water gauge glass This will record the water level in the boiler. Often two are fitted in case one breaks. A pressure relief valve This is fitted as a safety precaution and is set to blowoff at a particular pressure. Often two are fitted as an added precaution in case one sticks. They are either of the dead-weight or spring-loaded types.
A type of fire-tube boiler is illustrated in Fig. 10.3. Sometimes called un economic boiler, it has a cylindrical outer shell and contains two large-bore flues into which IIrc set the furnaces. The one illustrated has a mechanical stoker and ash remover. Thc hot flue gases pass out of the furnace flues at the back of the boiler into a brickwork setting, which deflects them back to pass through a number of small-bore tubes arranged above the large-bore furnace flues. These small-bore tubes break up the water bulk in the boiler and present a large heating surface to the water. The flue gases pass out of the boiler at the front and into an induced-draught fan, which passes them into the chimney. The general range of sizes of the economic boiler is from small, about 3 m long and 1.6 m diameter, to large, about 6.5 m long and 4 m diameter. Equivalent evaporation ranges from about 900 kg steam per hour to about 14 000 kg steam per hour. Another type of economi~ boiler is illustrated in Fig. 10.4. This type is called a supereconomic boiler. The boiler illustrated is oil-fired through a central large-bore corrugated flue. This gives the flue gas its first pass. At the rear of the boiler the flue gas is deflected down and back to pass through a number of small-bore tubes set in the bottom of the boiler. This gives the flue gas its second pass. At the front of the boiler the flue gas is deflected up and into the boiler again for its third pass, through another set of small-bore tubes set in the sides of the boiler. At the back of the boiler the flue gas then passes to the chimney. With the large number of tubes set in this boiler, there is quite a high heating surface area and the boiler will therefore have a high evaporation rate for its size. The boiler is called supereconomic because of its three gas passes as against the economic two passes. The boiler illustrated is capable of being installed completely as a self-contained working unit, having been assembled at the manufacturer before delivery. It is fully automatic and electronically controlled. The size ranges from an overall length and height of 3.4 m and 2.3 m to an overall length and height of 6.1 m and 3.7 m. The equivalent evaporation ranges from 680 to 8000 kg steam per hour. Such a boiler may also be called a package boiler. It is useful to note that not all fire-tube boilers are horizontal. Some, especially the smaller boilers, stand vertically, with the furnace at the bottom, so they occupy a smaller floor area. This is especially convenient where space is at a premium.
With the increasing demand for higher power output from steam plant it became necessary to develop boilers with higher pressures and steam outputs than could be handled by the shell-type boilers. This led to the development of the water-tube boiler. Consider the pressure. For a constant thickness of material, a tube of smaller diameter can withstand a higher internal pressure than a tube of larger diameter. And the steam output. If the water is contained in a large number of tubes, there is a large heating surface area and each tube contains a small water bulk, so the steam output is greatly increased. By using a large number of tubes, both the pressure and the steam output can be increased.
Basi" engineering thermodynamics
In most water-tube boilers the water circulation is by natural convection, but a few designs employ forced convection. A very large water-tube boiler of modern design is illustrated in Fig. 10.5. It is called a radiant heat boiler. The boiler is fired with pulverised coal. The coalpulverising mills are shown at the bottom. Coal is fed into the mills where it is crushed to powder. Primary air is fed through the mills where it mixes with the powdered coal. The air-coal mixture is then fed through ducting to the boiler burners. The mills are driven by electric motors. The powdered coal and primary air blow through the centf6 of the burners. To encourage and control the burning, secondary air is controlled and fed round the delivery of the coal and primary air supply.
The boiler shown in Fig. 10.5 has twin furnaces. The furnaces are completely water cooled; in fact they consist of water tubes. Most heat energy in this larger type of boiler is transferred by radiation to the vertical water tubes. By the time the flue gas has passed up the boiler and through the superheater, the temperature is not very much higher than the saturation temperature of the steam drum. Under these circumstances there would be little steaming improvement if a convection surface were provided. Thus, convection water tubes, as used in the other types of water-tube boiler, are not included. The superheater is split into primary and secondary sections so as to provide superheat control by the introduction of an attemperator. And note the provision of an economiser and air heater. This boiler has a steam output of 380 000 kg/h. The superheater outlet pressure is II MN/m2 with a final steam temperature of 570 "c. A very large, radiant heat, oil-fired boiler can have a steaming capacity of about 1600 tonne/h (1.6 x 106 kg/h) at a pressure of 16.5 MN/m2 (165 bar) with a delivery steam temperature of 550°C. It will use about 118 tonne (l18 x 103 kg) of fuel oil per hour. One of the difficulties of a pulverised fuel (PF) burning boiler is that the ash in the coal is also pulverised, so it is blown into the furnace and passes up with the flue gas out of the boiler. This dust ash must be removed so that it does not pollute the atmosphere. The dust ash is removed in a precipitator. One type of precipitator gives a vortex motion to the flue gas. The dust is thus flung out of the gas and is collected for disposal. This type of precipitator is called a cyclone precipitator. Another design, the electrostatic precipitator, is operated electrically. The precipitator consists of a bank of plates or wires, some positively charged and some negatively charged. The ash-laden flue gas is passed, relatively slowly, through the plates or wires. During passage through the plates, the ash particles become negatively charged; they move over and cling to the positive plates or wires. The ash is removed by rapping the plates or wires with mechanical rappers. It falls into hoppers from which it is removed. Another effect of the combustion of coal, or the combustion of hydrocarbon fuels such as oil and petrol, is that the combustion can produce undesirable flue and exhaust products which pollute the atmosphere when they are discharged. Such pollutants may have an unfortunate and undesirable effect not only locally but also at some distance from their source. They may be carried long distances in the atmosphere. Two of the main pollutants appear to be the sulphur oxides (written generally as Sax), and the nitrogen oxides (written generally as NOx). They can form acids and compounds which have a corrosive effect on surroundings and buildings, and also have a contributory effect on the formation of what is generally called acid rain. The acid rain appears to have a marked effect on some flora and fauna of forests and lakes. These undesirable effects have led to increasing efforts to scrub the flue gases of pollutants before they are discharged into the atmosphere. Chapter 8 gives further comment on atmospheric pollution. It should be noted that boilers can be fuelled by any substance that will burn. The fuels are mostly coal, fuel oil or gas, which is mostly natural gas. Large water-tube boilers are usually custom-built on site.
Basic engineering thermodynamics
In this type of boiler, water is force-circulated in a single passage through the boiler which consists of a number of tubes in parallel. The pressure in the boiler can be above the critical pressure for steam (approximately 22.12 MN/m2). Thus these boilers will operate at pressures of some 22 MN/m2 to about 34 MN/m2• Steam temperature will be of the order of 600°C. Radiant heat furnaces are employed and general sizes vary from industrial plant up to large power stations. Advantages . claimed with this type of boiler are that the welded construction avoids expansion troubles due to starting up and shutting down. Starting up and shutting down can be accomplished more rapidly. The boiler can be operated at any pressure and temperature over its load range. Steam can be supplied at gradually increased superheat temperature which assists turbine starting. Small, once-through, subcritical boilers are also manufactured. 10.6
Fluidised bed combustion
In the preceding discussion on boilers, the use of coal-burning equipment such as the chain grate and the pulverised fuel burner were indicated and described. A further method of utilising coal is by means of fluidised bed combustion. A diagram of a fluidised bed combustor is shown in Fig. 10.6.
particular velocity and mass flow of air, the bed will begin to behave like 1& nuld: In other words, it becomes a fluidised bed. In fact, a hollow vessel, such liS a ball. would float on its surface in the same way as it would float on the surface of a t1uid. If particles of coal are added to the fluidised bed, they become well mixed throughout the bed. If the temperature of the bed is high enough, the cOlli will chemically combine with the oxygen in the airflow and it will burn. The fluid nuturc of the bed will ensure an even heat transfer through the bed and also to any coolant device immersed in it. Such coolant devices could be required to produce superheated steam in a boiler or hot gas for use in a gas turbine. Hot gas from the combustion process will leave the bed at its top surface. The hot gas can be arranged to proceed through a boiler or a gas turbine or industrial process plant. A device is required for the removal of the coal ash at the base of the bed through the bed grate. A further device, sometimes called an arrester, is located at the top of the com buster to intercept any small ash particles (fly ash), and any particles of bed material which leave the bed with the heated combustion gas. This leaves a cleaner hot gas to proceed to the plant which follows. Particles collected in the arrester are removed from its base. Sorbent material, such as limestone or dolomite, can be arranged to be present in the fluidised bed. As mentioned in section 10.4, the combustion of coal can produce undesirable sulphur or nitrogen oxides. These oxides are attracted to the sorbent material in the bed and, as a result, are much reduced in the flue gas, thus reducing their effect on atmospheric pollution. 10.7
Waste heat boilers - combined heat and power (CHP)
Many engineering processes produce large quantities of energy which can be transferred as a heat by-product. For example, in the manufacture of steel the furnaces produce a large quantity of such energy. Ships using large oil engines produce a considerable amount of energy in the exhaust gas, and large quantities of exhaust gas are available over long periods at sea. A similar situation arises with the use of large industrial gas turbines. In these cases, the energy available for transfer as heat would be wasted unless an attempt were made to recover some. The recovery can be accomplished by employing a waste heat boiler. In the waste heat boiler the hot gases from the furnace, engine or gas turbine are employed in raising steam which can be used to run auxiliary plant such as a steam turbine. They can be used for the generation of electrical power for lighting and heating. In this way the overall efficiency of the plant is improved. The waste heat boiler is usually provided with an auxiliary furnace, normally oilfired or gas-fired, such that it can be operated when waste energy is not available. Waste heat boilers are part of the concept called combined heat and power (CHP). see also Chapter 16. Much more care is now being taken in the use of energy and the reduction of energy losses to a minimum. In power production plant there is considerable anerllY loss in the exhaust system (steam in the case of steam turbines, gas in the case of engines). These losses can be between 65 and 85 per cent of the energy available from the fuel supply. Consequently, much effort is being made to design plant in which both power and heat transfer are made available from the plant.
Basic engineering thermodynamics
For example, it could be arranged that a gas turbine produces power and its exhaust feeds a boiler which produces steam for a steam turbine to produce additional power. Alternatively, a steam turbine could produce power and the exhaust steam from the turbine could heat water by condensing in its passage through a condenser. The hot water from the condenser could be used for heating purposes. As a further example, waste heat recovery is made use of in a motor car. Hot cooling water from the engine is circulated through a heat exchanger, which serves to heat air passing through it on its way to warm the passenger compartment. Such ideas are not new. But with the increasing emphasis on the efficient use of fuel and energy, there is much more interest in the concept of combined heat and power. 10.8 Steam generation by nuclear reaction In nuclear reaction, atoms of uranium or plutonium are bombarded by neutrons, elementary particles present in the nuclei of atoms. As a result of the neutron bombardment, the atoms of uranium or plutonium undergo fission, which means that they are split. The fission releases heat energy and more neutrons, thus the nuclear reaction continues in a chain reaction. This chain reaction is quite unlike the chemical reaction which occurs in fuels such as coal or oil. The device in which a nuclear reaction occurs is called a reactor. There are two principal types of nuclear reactor: thermal reactors and fast reactors. Natural uranium consists of two forms of uranium, called isotopes of uranium. Most of the natural uranium, about 99.3 per cent, consists of the isotope uranium238 (U-238). The remaining 0.7 per cent consists of the isotope uranium-235 (U-235). The isotope U-235 will fission much more easily than the isotope U-238. Uranium fuel for use in thermal reactors is usually arranged to be slightly enriched in U-235 in order to increase the fission potential. In the thermal reactor, the neutrons need to be slowed down in order to produce a higher probability of fission of the U-235. The neutrons are slowed down using a moderator, which can be of either graphite or water. The graphite-moderated reactors are of two general types. In the magnox reactor the fuel is uranium metal clad in a magnesium alloy (Magnox). In the advanced gascooled reactor (AGR) the fuel is uranium dioxide clad in stainless steel. Both types of reactor are cooled by carbon dioxide (COz) gas, which is heated by passing it over the fuel in the reactor core. The hot gas then passes through a steam generator and transfers heat into water in the generator, thus producing steam. The cooled gas then returns to the reactor. The fuel used in the AGR can operate at a higher temperature than the fuel used in the magnox reactor, so it gives a smaller size of reactor for a given energy output. In the pressurised water reactors (PWRs) high-pressure water is used, not only as the moderator but also as the coolant of the reactor. The fuel used is uranium dioxide (UOz) clad in zirconium alloy (Zircaloy). The hot, high-pressure water from the,reactor is pumped through a steam generator and transfers heat into water in the generator, thus producing steam. The cooled water then passes back to the reactor. One type of PWR has the general arrangement shown in Fig. 10.7. And Fig. 10.8 illustrates how four steam generators can be fed from a single reactor in a four-loop
Basic' engineering thermodynamics
During the nuclear reaction in thermal reactors, the isotope U-238 captures some of the neutrons released in the fission of the isotope U-235 and forms a new radioactive element, plutonium (Pu-239), which will fission. By reprocessing the spent fuel from thermal reactors, a new fuel can be produced, comprising some 20-30 per cent plutonium and the remainder uranium. This fuel can be used in fast reactors. In a fast reactor, which can be relatively small, no moderator is required because the fast neutrons do not need to be slowed down. And if the reactor is surrounded by a blanket of uranium containing the isotope U-238, more plutonium is created and the reactor becomes a fast breeder reactor. The high heat energy output from a fast reactor requires a much more efficient coolant than gas or water. The coolant used is usually liquid sodium. The sodium passing through the reactor core will become radioactive, so two sodium circuits are required. The first is contained within the core of the reactor; it cools the core and transfers heat to a second sodium circuit. The second circuit transfers heat to a steam generator. By the use of fast breeder reactors, it is considered that uranium stocks can remain useful as an energy source for some considerable time. There are many nuclear power reactors in use throughout the world. In power production plant the nuclear reactor takes the place of the conventional coal or oil furnace. The whole of a reactor, its equipment and the building in which it is housed, is shielded against radiation and leakage. This is shown diagrammatically in Fig. 10.7. The diagram is not to scale; it is based on the Sizewell B nuclear power station in Suffolk, UK. The dome-topped primary containment building is made of prestressed concrete with a carbon steel liner. It has a wall thickness of 1.3 m with a liner thickness of 6 mm. It contains any leakages or emitted radiation. The containment building houses the reactor, the steam generator and any auxiliary equipment. The primary containment building is surrounded by a steel fabrication, the secondary containment. This is an additional precaution and acts as a leakage collector. The thermal reactor power of the station is 3411 MW with a gross electrical power of 1245 MW. Turbine speed is 3000 rev/min. Steam is produced at a pressure of 69 bar with a temperature of 285°C from feedwater at 227°C. The steam quality is 99.75 per cent dry. Steam is always wet when produced from a water surface. The high dryness fraction in this case is produced by the introduction of swirl-vane moisture separators and a steam dryer at the exit from the steam generator. The primary water coolant is pressurised to 155 bar using a pump, and its outlet temperature from the reactor is 325°C. This temperature is high enough to produce steam in the steam generator but is below the saturation temperature in the primary coolant circuit. Thus it remains as liquid water in this circuit. The illustration shows just one design of a nuclear power station. Many other designs, with varying outputs, are used throughout the world.
Heat transfer required
h2 = specific enthalpy of steam formed, kJ(kg hI = specific liquid enthalpy of feedwater, kJ(kg
Basic engineering thermodynamics
In a jet condenser, the steam to be condensed comes into direct contact with the cooling water, which is usually introduced in the form of a spray from a jet. The steam gives up its enthalpy to the cooling water spray, is condensed and finally leaves as condensate with the cooling water. Both types of condenser may be operated as either a wet or dry condenser. In a wet condenser, any gas which does not dissolve in the condensate (and cooling water for a jet condenser) is removed by the same pump which is dealing with the condensate. In a dry condenser, the free gas and the condensate (and cooling water for a jet condenser) are removed separately. There is a further subdivision according to the relative directions of flow of the condensing steam and the cooling water. The three possibilities are as follows: • • •
Transverse flow The steam flows across the path of the cooling water; this is only possible in the surface condenser. Parallel flow The steam flow is in the same direction as the cooling water. Counterflow Also called contraflow; the steam flows in the opposite direction to the cooling water.
Another subdivision is according to how the condensate in the condenser is removed. Two arrangements are possible: the barometric condenser and the low-level condenser. 10.10.1
The barometric condenser (Fig. 10.9) is mounted on a pipe, usually at least 10.34 m long. The long pipe, called the barometric leg, acts rather like a Fortin barometer. If water were used in a Fortin barometer, the barometric height would be about 10.34 m. But instead of having a Torricellian vacuum on the top of the water in the pipe, there is a positive pressure, less than atmospheric pressure. The height of the water column will therefore be less than 10.34 m; it will be a function of the degree of vacuum that exists. This is illustrated as h in Fig. 10.9. Using this atmospheric leg it is possible for the condensate to drain away by gravity into the atmospheric tank at the bottom. The atmospheric leg dips deeply into the water in the atmospheric tank. The discharge from the atmospheric tank is from a standpipe, whose entry is high up, maintaining a constant high-level discharge. In this way there is no possibility of breaking the vacuum in the condenser. 10.10.2
In the low-level condenser, the condensate is removed using a pump. Low-level condensers are appropriate when there is not enough height available for a barometric condenser. Figure 10.10 shows a transverse-flow surface condenser. Steam is admitted to the top of the condenser and is removed as condensate from the bottom, having been condensed at the surface of the water tubes. Cooling water flows in at the bottom and out at the top of the condenser. Inlet and exit in this case are both at the same end of the condenser, so the water makes two passes through the condenser. Air extraction from the condenser is from the side, as illustrated. The air extraction point is shrouded using a baffle-plate in order to achieve as much separation of the condensate and air as possible. The condenser illustrated is a dry condenser because air and condensate are extracted separately.
Figure 10.11 shows a simple, but effective, jet condenser. It consists of a tall cylinder into which are introduced perforated baffle-plates. They are fixed alternatively on either side of the cylinder and cover just over half the cross-sectional area of the cylinder. Cooling water spray is introduced at the top of the condenser. The perforated baffle· plates help to maintain this spray throughout the condenser. Steam is introduced at the bottom of the condenser. Due to its low density it will begin to rise up in the condenser through the spray curtain. It will thus be condensed; the spray and condensate will fall together to the bottom of the condenser and will be extracted. Air entering the condenser will be warm, so it will rise to the top and will be extracted.
'tIIl,- engineering thermodynamics
CD The steam, after expansion, is passed from the engine or turbine into a condenser. This is shown as process CEo In the condenser the volume of the steam is reduced from Vc to VD. This process takes place at constant condenser pressure Pc and at constant condenser saturation temperature Tc. This process is therefore isothermal.
DA The partially condensed steam at pressure Pc and volume VD is fed from the condenser into the feed pump. This is shown as process ED. In the feed pump the steam is compressed frictionlessly and adiabatically to boiler pressure Po. This is shown as process DA. The compression converts the wet steam at condenser pressure into water at boiler pressure. This water is fed into the boiler, shown as process AF, and the cycle is repeated.
Now the P-V diagram is really composed of two diagrams. There is the engine or turbine diagram FBCE, whose area will give work output; there is also the feed pump diagram EDAF, whose area will give the required work input to run the feed pump. The net work output from the plant, therefore, will be the net area of these two diagrams. This is the area ABCD. Area ABCD is enclosed by two isothermal processes and two adiabatic processes, so this is a Carnot cycle. Its thermal efficiency will be given by (To - Tc)/To, the maximum efficiency possible between these temperature limits (see section 15.2). The T-s diagram of the cycle is shown in Fig. 1O.13(b). •
represents the constant temperature formation of the steam in the boiler.
be represents the frictionless adiabatic (isentropic) expansion of the steam in the engine or turbine.
da represents the frictionless adiabatic (isentropic) compression of the steam in the feed pump back to water at boiler pressure at point a.
represents the condensation of the steam in the condenser.
Now this cycle for operation in a steam plant is practical up to a point. The isothermal expansion of the steam in the boiler and the adiabatic expansion of the steam in the engine or turbine (especially in turbines) is reasonable. The impractical part is in the handling of the steam in the condenser and feed pump. In the condenser, the steam is only partially condensed and condensation must be stopped at point d. Also the feed pump must be capable of handling both wet steam and water. A slight modification to this cycle, however, will produce a cycle which is more practical, although it will have a reduced thermal efficiency. This cycle is the Rankine cycle and is usually accepted as the appropriate ideal cycle for steam plant.
The Rankine cycle
The modification made to the Carnot cycle to produce the Rankine cycle is that, instead of stopping the condensation in the condenser at some intermediate condition, the condensation is completed. This is shown in Fig. IO.14(a). On the T-s
In modern steam turbine plant the pressure ratio through the turbine cl1n be considerable, so any superheated steam supplied will soon become wet after purtiul expansion. Wet steam passing over turbine blades for long time periods will produce some corrosion and erosion of the blades. To avoid this, the superheated steam, after partial expansion in the turbine, is passed back to the boiler to be reheated at constant pressure to a higher temperature. It is then passed back to the turbine; the expansion which follows will be dry and superheated, thus largely eliminating the corrosion and erosion of the turbine blades. Section 10.2 considered the concept of reheat while discussing the improvement of the thermal efficiency of a boiler. Whether or not there will be a change in thermal efficiency of the plant as a whole, as distinct from the boiler on its own, depends upon the reheat temperatures. The higher the reheat temperatures, the more likely there is to be a slight improvement in thermal efficiency of the plant. This will result from the higher potential for enthalpy drop, and hence work output, which can be obtained from the higher-temperature steam.
Example 10.9 Steam is supplied to a turbine at a pressure of 6 M N/m! and al a lemperature of 450°C. It is expanded in the jirst stage to a pressure of I MN/m2. The steam is then passed back to the boiler in which it is reheated at a pressure of 1 M N/m2 to a temperature of 370°C. It is then passed back to the turbine to be expanded in the second stage down to a pressure of 0.2 MN/m2. The steam is then again passed back to the boiler in which it is reheated al a pressure of 0.2 MN/m2 to a pressure of 320°C. II is then passed back to the turbine to be expanded in the third stage down to a pressure of 0.02 M N/m2. The steam is then passed to a condenser to be condensed, but nol undercooled, at a pressure of 0.02 M N/m2 and the condensate is then passed back to the boiler. Assuming isentropic expansions in the turbine and using Fig. 10.21 determine (a) the theoretical power per kilogram of steam per second passing through the turbine (b) the thermal efficiency of the cycle (c) the thermal efficiency of the cycle assuming there is no reheat
In modern steam turbine plant, in which there is a large pressure ratio throUlh the turbine, there is a considerable difference between the superheated steam temperature as supplied from the boiler to the turbine and the condensate temperature as it leilv,. the condenser on its way back to the boiler as feedwater. To increase the feedwater temperature on its way back to the boiler and, as a result, increase the thermal efficiency of the plant, the process of feed heating is introduced. Small quantities of steam are bled at various stages through the turbine; the bled steam passes through a feed heater in which it condenses in a heat transfer process with the feedwater. Thus the temperature of the feedwater is increased. The condensate from the bled steam is pumped into the feedwater main to be returned to the boiler. In large steam turbine plants, several feed heaters are introduced. The process improves the thermal efficiency of the plant. During expansion of the steam through the stages of a turbine, the actual enthalpy drop in anyone stage is less than theoretically obtainable during an isentropic expansion. This is mostly due to the effect of friction between the steam and the turbine blades; there is a similar effect in nozzles (Chapter 12) and gas turbines (Chapter 16). This effect is modelled by the stage efficiency, where S ffi . tage e lClency =
Actual enthalpy drop in stage Isentropic enthalpy drop in stage
The use of stage efficiency is illustrated in Example 10.10.
Example 10.10 Steam is supplied to a turbine at a pressure of 7 M N/m2 and a temperature of 500°C. Steam is bled for feed heating at pressures of 2 MN/m2 and 0.5 MN/m2. The condenser pressure is 0.05 MN/m2 The stage efficiency of each section of the turbine can be taken as 82 per cent. In the feed heaters the feedwater has its liquid enthalpy raised to that of the corresponding bled steam. The bled steam is condensed but not undercooled and, in this state. on leaving the feed heater, is pumped into the feed main as it leaves the feed heater. The sequence is shown in Fig. 10.22. Determine (a) the mass of steam bled to each feed heater in kg/kg of supply steam (b) the thermal efficiency of the arrangement
This illustrates the thermal efficiency improvement heating system.
which results from the use of a feed
A boiler with superheater generates 6000 kg/h of steam at a pressure of 1.5 MN/m2, 0.98 dry at exit from the boiler and at a temperature of 300°C on leaving the superheater. If the feed water temperature is 80°C and the overall efficiency of the combined boiler and superheater is 85 per cent, determine (a) the amount of coal or calorific value 30 000 kJ/kg used per hour (b) the equivalent evaporation from and at 100°C for the combined unit (c) the heating surface required in the superheater if the rate of heat transmission may be taken as 450 000 kJ /m2 of heating surface per hour [(a) 636 kg/h; (b) 7189 kg/h; (c) 3.85 m2] In a steam plant, the steam leaves the boiler at a pressure of 2.0 MN/m2 and at a temperature of 250°C. It is then expanded in a turbine and finally exhausted into a condenser. The pressure and condition of the steam at entry to the condenser are 14 kN/m2 and 0.82 dry, respectively. Assuming the whole of the enthalpy drop is converted into useful work in the turbine, determine (a) the power developed for a steam flow of 15 000 kg/h (b) the thermal efficiency of the engine if the feed temperature is 50°C. Estimate the mass of cooling water circulated in the condenser per kilogram of steam condensed, if the cooling water enters at 18 DCand leaves at 36 °C and the condensate leaves at 50°C. How much energy is lost from the condensate per hour due to its being cooled below the saturation temperature corresponding to the condenser pressure? [(a) 3.058 MW; (b) 27.3%; 26.1 kg/kg steam; 160500 kJ/h]
The steam engine
11.1 General introduction For the steam engine, an honoured place in history is assured. Early developments were made by pioneers such as French physicist Denis Papin (1647-1712) and English engineer Thomas Newcomen (1663-1729), and fundamental improvements came from Scottish engineer James Watt (1736-1819), to mention but a few. Out of these inventions developed the Industrial Revolution and the intensive studies of thermodynamics and other branches of organised science. It has been said that the steam engine owes less to science than science owes to the steam engine. In modern times, steam turbines and internal combustion engines have largely replaced the steam engine due to their higher power outputs, higher efficiencies and smaller bulk for a given power output. However, many steam engines are used in various countries of the world, particularly on railways. It is for this reason, and because the steam engine provides another exercise in the use of steam, that a chapter on the steam engine is included. A steam plant has a low noise level, and atmospheric pollution can be low if a high-efficiency burner system is installed in the boiler unit. There is also a high degree of flexibility in the possible fuels used in the boiler unit. Early steam engine plant produced steam at about atmospheric pressure, mostly in or introduced into a vertical engine cylinder, thus lifting the engine piston. The back of the piston was open to atmospheric pressure. By condensing the steam in the cylinder as a result of the application of cooling water, a vacuum was formed in the cylinder and the atmospheric pressure, acting on the back of the piston, produced a net force to drive the piston downward. Thus work was done by the engine when the piston ascended as the steam was formed, and again when the steam was condensed as the piston descended. This general technique was adopted for many years, and in various forms, until boilers were developed which were capable of producing steam at pressures above atmospheric and until engine valve gear was developed to introduce and exhaust the steam to and from the engine cylinder.
BtUlc ,ngineering thermodynamics
piston rod passes. The crank end of the cylinder is a working end, so it is sealed and the piston rod passes out via a gland. The external end of the piston rod is connected to a crosshead. This crosshead reciprocates in guides and supports the piston rod end. The small end of the connecting-rod is connected to the crosshead. The reciprocating motion of the piston is thus transmitted, via the crosshead, to the connecting-rod. The big end of the connecting-rod is connected to the crank which turns on the crankshaft that runs in the main bearings. A flywheel is fitted to the crankshaft. Control of the steam to and from the cylinder is by means of a valve which is assembled in the valve chest mounted on the side of the cylinder. A common way of operating this valve is to use an eccentric mounted on the crankshaft. The reciprocating motion of the eccentric is transmitted to the valve by a valve operating link caJled the eccentric rod. In order to reduced condensation loss, the cylinder is often surrounded by a steam jacket. High-pressure steam from the main is fed to this steam jacket, which thus helps to maintain a high general cylinder temperature and cuts down working steam condensation loss. Condensate from the jacket is drained through a steam trap. Note the governor fitted to the end of the crankshaft. The governor is connected via a linkage up to the governor valve. The governor serves to maintain nearly constant engine speed at all loads up to full load. The governor in Fig. 11.1 is a throttle governor. This type of governing of a steam engine is explained in section 11.7.
In any steam engine there must be a clearance volume, as illustrated. This clearance volume can be anywhere between 1/10 to 1/50 of the cylinder volume, to give some idea of its size. Admission of the steam is arranged to commence slightly before the piston has reached the inner dead-centre position. This is done so that there will be a maximum pressure build-up for when the piston commences its working stroke, and so that the steam port is sufficiently wide open to give ample steam supply at the commencement of the working stroke. Due to losses in the steam main and throttling in the valve, there is a pressure loss, so the admission line will be below the theoretical line. Throttling in the valve is sometimes called wire drawing because damage can appear as grooves on the edges of the valve over long periods of operation. These grooves are as if a wire had been repeatedly drawn across the edge of the valve. Now the valve, ports, cylinder and piston are exposed alternately to high-pressure. high-temperature working steam and low-pressure, low-temperature exhausting steam. Thus their mean working temperature will lie somewhere between. Upon admission. the working steam will meet working surfaces which are at a lower temperature. thus lome steam will condense on these surfaces. This will result in an increased steam supply to make up for the loss. There is no corresponding increase in work output for thll increased steam supply, so the thermal efficiency tends to be reduced. Due to the lower admission pressure, the expansion line will be lower than the theoretical expansion line. In the earlier part of the expansion. thill pres.ul'l reduction will be accentuated by further condensation on the cylinder wall •. Thi. wil1 be counteracted somewhat in the latter part of the expansion; the Iteam temperature
Basic engineering thermodynamics
becomes lower than the mean cylinder temperature and there is re-evaporation of some of the condensed steam. During the exhaust stroke, the pressure in the cylinder is slightly higher than the external exhaust pressure. This occurs because there must be a net positive pressure from inside to outside the cylinder in order to produce the force necessary to move the steam mass from the cylinder. And due to the higher mean cylinder temperature there is a tendency to evaporate some of the cylinder water film left behind as the result of condensation in the earlier part of the cycle. The exhaust line is therefore higher than the theoretical exhaust line. In order to smooth the operation of the engine, the exhausting of the steam is stopped early at the point of compression. Here the exhaust valve is closed and a quantity of steam is locked up in the cylinder. This steam is called the cushion steam. The pressure of this cushion steam is raised and thus a smooth change from exhaust to admission is made. The cushion steam also tends to reduce steam consumption because, if it did not exist, more steam would be required to raise the pressure from exhaust to admission. The net result of these phenomena is that the actual diagram is much more continuous and has a smaller area than the theoretical diagram. The ratio
The crank-end diagram will, in general, have a smaller area than the h'ld.end diagram. This is because the effective piston area of the crank end III reduOId by the area of the piston rod. Thus the crank-end work is smaller than the he.d.end work. 11.10
Diagram 12345 is the theoretical diagram for an engine exhausting to atmospheric pressure. But if the engine exhausts to a condenser, the diagram becomes 12367. Now the same amount of steam has been used in both cases, but the condenser diagram is larger than the atmospheric diagram by the shaded area 5467 and hence there is a greater output using the condenser. And because there is a greater output using the same amount of steam, the thermal efficiency has been improved. 11.9
The indicator diagram for the double-acting steam engine
For the double-acting steam engine, there will be a separate indicator diagram for each side of the piston. The indicator diagrams already illustrated are for the head end of the engine. Now when the head end is carrying out its working stroke, for the same piston direction, the crank end is exhausting. Also, when the head end is exhausting, the crank end is carrying out its working stroke. The crank-end indicator diagram will therefore appear as illustrated in Fig. 11.11. It is shown superimposed on the head-end diagram. The crank-end diagram is really 180 out of phase with the head-end diagram. Note that the total work done per revolution is the sum of the diagram areas taken separately. 0
The compound steam engine
With the advent of the high-pressure boiler, difficulties arose in the use of the highpressure steam in a single cylinder. If a single cylinder is used with high-pressure steam, then not only has the cylinder to be designed to accommodate the high pressure but also to accommodate the large volume after expansion. This means that the cylinder will be of very heavy construction. Likewise the reciprocating parts must be large. The high-pressure range and the heavy reciprocating parts will create a considerable variation of torque. A heavy flywheel will be required to smooth this out. There is also an increased balancing problem in a single-cylinder engine with heavy reciprocating masses. And due to the large pressure difference between the inlet and exhaust conditions, there will also be a large corresponding temperature difference. This large temperature difference will increase the condensation loss. To overcome these difficulties, the compound steam engine was developed. In the compound steam engine, the steam is expanded through two or more cylinders. The steam is first admitted to the high-pressure cylinder in which it is only partially expanded. In this way the cylinder accommodating the high-pressure steam need not have such a large volume, so it can have a lighter construction. The exhaust steam from the high-pressure cylinder becomes the working steam for the following cylinder. The following cylinder must accommodate a larger steam volume, but the admission pressure is lower so, once again, lighter construction is possible. In each cylinder there will be a lower overall temperature range, so condensation loss is reduced. The lighter cylinder construction means there are lighter reciprocating parts. And in each cylinder there is a lower pressure range. With two or more cylinders and a suitable crank arrangement, it is possible to obtain better balancing than with a single cylinder. Together, these result in a smoother torque output. The smoother torque means that a smaller flywheel is required on a compound engine. If the cranks on a compound engine are not in phase or at 180 then the following cylinder will not be ready for the steam being exhausted from the cylinder before. In this case, a receiver is fitted between cylinders to hold the steam until the following cylinder is ready for it. As the strokes of the cylinders of a compound engine are generally made equal and as the following cylinders accommodate the larger steam volumes, the diameters of the following cylinders are made larger. A two-cylinder compound steam engine is called a double-expansion engine. This is illustrated in Fig. I I.l2(b). A three-cylinder compound steam engine is called a tripleexpansion engine. This is illustrated in Fig. 1I.l2(c). In a double-expansion engine the cylinders are called the high-pressure and low-pressure cylinders. In a tripleexpansion engine the cylinders are called the high-pressure, intermediate-pressure and low-pressure cylinders. 0
expansion would be as shown dotted. The reason for the early releltlle In the lowpressure cylinder is that the extra large cylinder volume which is required for complete expansion does not compensate for the very small extra amount of work that is obtained. This small extra amount of work will be given by the area of the dotted section of the theoretical indicator diagram. The smaller low-pressure cylinder volume, 5-4, more than compensates for the small loss of work. In practice, the highpressure cylinder also has an early release. In choosing the intermediate pressure P?, there are two generally accepted possibilities.
Figure 11.l2(a) shows the hypothetical indicator diagram for a double-expansion engme: • •
The high-pressure cylinder diagram is 1267. The low-pressure cylinder diagram is 76345.
The two diagrams are fitted together for convenience. In practice they are quite separate. The expansion through the engine is assumed continuous and to be of hyperbolic form, PV = C. In the hypothetical diagram it is possible to assume that the expansion is complete in the high-pressure cylinder. This is shown as expansion 2-6. The reason for this is that the cylinder volume, which is 7-6, is not too large and the steam expansion can easily be accommodated. Complete expansion in the highpressure cylinder also makes the high-pressure cylinder volume equal to the cut-off volume of the low-pressure cylinder. But in the low-pressure cylinder there is usually an early release, as shown at 3. In this case the expansion in the low-pressure cylinder is not complete. Complete
Basic engineering thermodynamics If the expansion had been hyperbolic from the pressure of 380 kN/m2 at 0.5 stroke, determine (c) the work done to the position of 0.9 stroke. [(a) 1.104; (b) 1.702 kJ; (c) 1.719 kJ] A single-cylinder, double-acting steam engine is 152 mm bore by 203 mm stroke and the piston rod is 32 mm diameter. During a trial the engine develops a brake power of 13.5 kW and its mechanical efficiency is 8 I per cent. The areas of the indicator diagrams are 1625 mm2 at the head end and 1490 mm2 at the crank end; the length is 75 mm and the indicator calibration is 16 MN m-2/m in both cases. Determine the speed of the engine in rev/min. If the calorific value of the coal used is 25 100 kJ/kg and the overall efficiency from coal to brake power is 7 per cent, calculate the mass of coal used per hour. [418 rev/min; 27.7 kg] A single-cylinder, double-acting steam engine gives a brake power of 75 kW using steam at 1035 kN/m2 with a cut-off at one-third stroke and a back pressure of 27.5 kN/m2. The mechanical efficiency of the engine is 84 per cent and the diagram factor 0.7. If the mean piston speed is 4 m/s and the stroke is 1.2 times the bore, determine the bore and stroke of the engine. [Bore = 388 mm; stroke = 466 mm] A single-cylinder, double-acting steam engine is 250 mm bore by 300 mm stroke and runs at 3.5 rev/so Steam is supplied at 1035 kN/m2, the back pressure is 34 kN/m2 and the diagram factor is 0.81. Determine the indicated power of the engine (a) if cut-off is at 0.25 stroke; (b) if cut-off is at 0.5 stroke. [(a) 48.7 kW; (b) 70.3 kW] A single-cylinder, double-acting steam engine gives an indicated power of 55 kW when running at 5 rev/s; the engine is 0.25 m bore and 0.30 m stroke. Steam is supplied at 860 kN/m2 and the back pressure is 117 kN/m2; cut-off is at 0.37 stroke. Determine the diagram factor. If a condenser is now fitted to the engine so that the back pressure is 34.5 kN/m2, calculate the new indicated power if nothing else changes. [0.72; 63.7 kW] A throttle-governed steam engine developing an indicated power of 37.5 kW uses 1000 kg/h of steam. At no-load the steam consumption is 125 kg/h. Estimate the indicated power of the engine for a steam consumption of 750 kg/h. [26.8 kW] A two-cylinder, double-acting, compound steam engine develops an indicated power of 220 kW at a speed of 4.5 rev/so The stroke in each cylinder is 0.5 m. Expansion is hyperbolic throughout and is complete in the high-pressure cylinder. The diagram factor referred to the low-pressure cylinder is 0.75. Steam is supplied at 12 bar and the engine exhausts at a pressure of 0.28 bar. Total expansion ratio through the engine is 10. Equal power is developed in the two cylinders. Neglecting clearance, determine the engine cylinder diameters. [0.475 m; 0.373 m] A two-cylinder, double-acting, compound steam engine is supplied with steam at a pressure of 1725 kN/m2, the steam exhausts from the engine at a pressure of 41.5 kN/ m2. The low-pressure cylinder has a diameter of 0.45 m and a stroke of 0.4 m. The stroke in both cylinders is the same and the ratio of the cylinder volumes is 2.5: I. The diagram factor referred to the low-pressure cylinder is 0.78. Each cylinder has equal initial piston loads. The engine runs at 4.5 rev/so Expansion in the engine is hyperbolic and the total expansion ratio through the engine is 9. Neglecting clearance, determine (a) the intermediate pressure
the indicated power of the engine the diameter of the high-pressure cylinder [(a) 523 kN/m2; (b) 272 kW; (c) ().2K~ m) A double-acting, compound steam engine has cylinder diameters HP 300 mm, LP 600 mm and the stroke of both cylinders is 400 mm. When running at 160 rcv/min thc engine develops a brake power of 125 kW and its mechanical efficiency is n per ccnt. Steam is supplied at a pressure of 13.8 bar, cut-off in the HP cylinder is at one-third stroke and back pressure is 0.28 bar. Determine the actual mean effective pressure (a) the hypothetical mean effective pressure (b) the overall diagram factor (c) [(a) 266 kN/m2; (b) 373 kN/m2; (c) 0.71) A double-acting steam engine has a stroke of 0.45 m and the bore diameters are HP 0.33 m, LP 0.63 m. Steam is supplied at a pressure of 1200 kN/m2, cut-off in the HP cylinder is at one-quarter stroke and back pressure is 21 kN/m2 Assuming a diagram factor of 0.81 and mechanical efficiency of 82 per cent determine the brake power of the engine when running at 3 rev/so [157.6 kW]
A triple-expansion steam engine has cylinder diameters of 300 mm, 450 mm and 750 mm; the common stroke is 500 mm. Indicator diagrams taken from the cylinders give the tabulated results. Cylinder
Diagram area (mm2) Diagram length (mm) Indicator calibration (kg/mm2/m)
The steam is supplied at a pressure of 1380 kN/m2 and the back pressure is 28 kN/ m2; cut-off in the HP cylinder is at 0.6 stroke. Determine the actual and hypothetical mean effective pressure referred to the LP cylinder and hence find the overall diagram factor. 17.
[317.8 kN/m2; 415 kN/m2; 0.765] A double-acting, compound steam engine is to give an indicated power of 450 kW running at 2.5 rev/so Both HP and LP cylinders have the same stroke and the mean 2 piston speed is 4 m/s. Steam is supplied at 1240 kN/m2, back pressure is 21 kN/m , the total number of expansions is 12 and the overall diagram factor is 0.8. Determine the common stroke and the bore of each cylinder if the bore of the LP cylinder is twice that of the HP cylinder. Determine also the cut-off point in the HP cylinder. [0.8 m; 727 mm; 364 mm;
Basic engineering thermodynamics
Now, the rate of expansion through a nozzle is very high and there is little time for phase change during the passage of the steam through the nozzle. So, considering Fig. l2.4(b), the steam is once again superheated at inlet pressure PI at point a. At point b, due to the rapid expansion, condensation does not commence and the steam continues to behave as a superheated vapour down to point c at some intermediate pressure P3, less than the critical pressure. Point c is on the continuous curve gfc. Remember that a constant pressure line on the enthalpy-entropy chart is curved for superheated steam and straight for wet steam. This non-equilibrium behaviour as a superheated vapour does not continue indefinitely; the restoration of equilibrium quickly occurs at point c, after the throat in the divergent portion of the nozzle. It is accompanied by a small increase of pressure to P4. This is shown as cd, which also shows a small increase of entropy. From point d, equilibrium expansion occurs down to exit pressure P2 at point e. The steam during this type of expansion, in which the phase change is delayed, is said to be supersaturated. It is also said to be supercooled because at point c the steam temperature is lower than the corresponding saturation temperature at P3. This type of expansion is not in equilibrium so it is also said to be metastable. An equilibrium expansion is stable. Slightly higher flow rates are found during supersaturated expansion as against equilibrium expansion. This is because of the higher densities that occur due to supercooling. It was first shown that there is a limit to supersaturated expansion by C.T. Wilson in 1897. This limit approximates to points where the dryness fraction is 0.94 at high pressures to 0.96 at low pressures. The locus of these points produces the Wilson line, as shown in Fig. l2.4(b).
Basic engineering thermodynamics Steam enters a group of convergent-divergent nozzles at a pressure of 3 MN/m2 and a temperature of 300°C. Equilibrium expansion occurs through the nozzles to an exit pressure of 0.5 MN/m2. The exit velocity is 800 m/s. The steam flows at a rate of 14 kg/s. It is assumed that friction loss occurs in the divergent portion of the nozzles only. Using the enthalpy-entropy chart for steam, determine (a) the efficiency of expansion in the divergent portion of the nozzle (b) the total exit area (c) the throa t veloci ty [(a) 0.8; (b) 6125 mm2; (c) 529 m/s]
13.3 Velocity compounding In velocity compounding (Fig. 13.3(a» the steam is expanded in a single row of nozzles, as before. The high-velocity steam leaving the nozzles passes on to the firM! row of moving blades where its velocity is only partially reduced. The sleum leaving the first row of moving blades passes into a row of fixed blades which are mounted in the turbine casing. This row of fixed blades serves to redirect the steam back to the direction of motion such that it is correct for entry into a second row of moving blades which are mounted on the same turbine disc as the first row of moving blades. The steam velocity is again partially reduced in the second row of moving blades. These processes are shown in Fig. l3.3(b). Graphs of pressure and velocity through the turbine are included. Once again, all the pressure drop occurs in the nozzles; the pressure in the turbine remains constant. Only part of the 'velocity of the steam is used up in each row of blades, so a slower turbine results. '·But there is no loss of output because the rows of blades are connected to the same ahaft. This turbine is sometimes called a Curtis turbine; it is quite common in the high-pressure stage of a large turbine. If necessary, further rows of fixed and ~'""n";nn
The turbine just described is a simple turbine and was one of the first to be developed. It is called a de Laval turbine after its inventor. This type of turbine usually rotates at a very high speed, some 300 to 400 rev/so This high speed of rotation will restrict the size of the turbine disc for mechanical reasons such as centrifugal force. Thus, the de Laval turbine is of relatively small size, so it has a small power output. Also, due to the high speed of rotation, a direct drive between the turbine disc and external equipment is not generally possible. For this reason, a reduction gearbox is installed between the turbine disc and the external equipment. A problem in steam turbine development has been to reduce the speed of rotation and at the same time to make full use of the energy in the steam, thus allowing the production of turbines of large size and high power output. Work in this direction has produced many turbine designs, but broadly they can be split into two basic types: impulse turbines and reaction turbines. 13.2
The impulse turbine
The simple de Laval turbine is an impulse turbine. The impulse turbine has two principal characteristics: it requires nozzles and the pressure drop of the steam takes place in the nozzles. The steam enters the turbine with a high velocity; the pressure in the turbine remains constant because the whole of the pressure drop has taken place in the nozzles. And the velocity of the steam is reduced as some of the kinetic energy in the steam is used up in producing work on the turbine shaft. If the whole pressure drop from boiler to condenser pressure takes place in a single row of nozzles, as in the de Laval turbine, then the steam velocity entering the turbine is very high. If some of this velocity is used up in a single row of turbine blading, as in the de Laval turbine, then the speed of rotation is very high. In the impulse turbine this speed may be reduced by three techniques: velocity compounding, pressure compounding and pressure-velocity compounding.
h"", the stoichiometric air, A > I; this is a weak or lean mixture, a fuel-lean mixture.
Generally, in the case of the petrol engine, if A > 1.2, the fuel will not ignite. This is sometimes called the lean misfire limit (LML). Much combustion chamber design effort takes place to produce good combustion with lean mixtures m the lean-burn engine. This attempts to produce a good combustion mixture at the ignition point while preserving a lean overall airmfuel mixture. The improved combustion is accomplished by good combustion chamber
In the petrol engine, the ability of the fuel to perform well during combustion in the engine cylinder is indicated by the octane number of the fuel. Under certain operational conditions, the airmfuel mixture can be prone to pinking. The higher the volume compression ratio of the engine, the more prone to pinking the engine becomes. However, the higher the volume compression ratio, the higher the theoretical thermal efficiency. Certain additives mixed with petrol enable it to operate at the higher compressions. In the main these additives have been small quantities of tetraethyl lead or tetramethyl lead (approximately 0.15 to 0.4 g/l). However, lead is known to have toxic properties, and it leaves an engine in vapour form to become an atmospheric contaminant and pollutant. Thus, petrols are now made for the lead to be reduced or removed, and some petrol is now marketed as being unleaded or green. Efforts are made to improve the combustion properties of unleaded fuel by the introduction of other additives such as benzene, but they can also be toxic. The probable combustion performance of petrol is usually given by the octane number, obtained from an arbitrary scale of OmIOO.The scale is derived from a mix of 2,2,4-trimethylpentane (iso-octane), which is given a value of 100, and n-heptane, which is given the value O. 2,2,4-trimethylpentane has a low pinking characteristic, whereas heptane has a high pinking characteristic. The octane number of a fuel is the percentage of 2,2,4-trimethylpentane in a blend with n-heptane that appears to give the same engine performance as the given fuel. The octane value, or rating, of a fuel with a number greater than 100 is obtained by extrapolation from known data. There are two accepted octane number scales, according to the test method used. They are the research octane number (RON) and the motor octane number (MaN). Usually it is the RON that is quoted. However, the MaN is probably from the more severe test since this test attempts to simulate vehicle operational conditions. RON and MaN numbers of standard petrols are given in the following table. Number of stars
Unleaded petrol is probably in the range 93m96 RON. The performance of diesel fuel oil is indicated by the cetane number. Under certain severe operational conditions, a diesel engine can produce diesel knock. This has a certain similarity to pinking in a petrol engine. A high cetane number suggest the ability of the diesel fuel to resist knock. The cetane number is the percentage of cetane in a blend of cetanc and l-methylnaphthalene having the same ignition and performance quality as It given fuel.
Basic engineering thermodynamics
The gas turbine
The concept of the gas turbine began to be developed in the latter half of the nineteenth century. Ideas on gas turbines had accumulated earlier than this and, in fact, a type of gas turbine was patented in 1791 by John Barber of Nuneaton. The first gas turbine to run under its own power, and simultaneously to deliver external power, appears to have been in 1905 in Paris. But only in recent years have there been great strides in turbine design and use. Gas turbines are very widely used in aircraft. The are increasingly used for the generation of electrical power and there are many installations in ships as propulsion units. Attempts are also being made to develop gas turbines as engines for automobiles. There have been many designs for gas turbines, but the arrangement that has proved most successful in the continuous combustion, constant pressure, gas turbine. Its three basic elements are illustrated in Fig. 16.26(a): an air compressor, a turbine and a combustion chamber (sometimes there are a number of combustion chambers instead of a single large chamber). The method of operation is as follows. Air enters the air compressor in which it is compressed through a pressure compression ratio of some 6: I or 10:I. There are some installations in which the pressure compression ratio is as high as 20: I or even 40: I. The air compressors are usually of the rotary type and are either radial or axial flow. An axial-flow compressor is illustrated. The compressed air is passed from the air compressor into the combustion chamber through a duct. If there are several combustion chambers, the take-off volute from the air compressor will have ducts feeding the combustion chambers equispaced around it. In the combustion chamber, fuel (either a fuel oil, such as gas oil or kerosine, or a gas) is passed into a burner and burnt continuously. Thus the air passing through the combustion chamber has its temperature and volume increased while its pressure remains constant. The combustion products are then passed from the combustion chamber into a turbine in which they are expanded. From the turbine, the combustion products are passed out to exhaust. Figure 16.26(a) shows that the turbine is coupled back to the air compressor by a coupling shaft. On the other side of the turbine there is a coupling by which the turbine can be coupled to drive some external equipment. Part of the turbine output is used to drive the air compressor, so the net output appears for driving external equipment. The air compressor, shaft and turbine assembly is called a spool. Due to the continuous combustion which occurs in the combustion chamber, steps are taken to ensure that temperatures do not become too high. This is usually dealt with by supplying considerable excess air above that required for complete combustion. A special shroud is usually built round the burner in order to meter the air to the combustion space. This ensures there is good burning of the fuel and that further air is mixed with the very hot combustion products further down the combustion chamber. This brings the final combustion product temperature down to something workable before entry to the turbine. Much research and development in metallurgy was necessary to develop metals which could withstand the high temperatures and stresses in gas turbines.
The hybrid burner-ring is shown in Fig. 16.28. The word hybrid refers to the fad that the burners can accommodate both oil and gas. Figure 16.29 shows a schematic diagram of one type of combined-cycle power plant (part of the GUD range by Siemens). It shows the arrangement of a gas turbine in tandem with a three-stage steam turbine. The electric generator lies between the two turbine arrangements. Gas or oil can be used as fuel and the exhaust gas from the gas turbine feeds a three-stage boiler system. Steam from the boiler system feeds the three-stage steam turbine. One system, using the gas turbine described, has an overall thermal efficiency of 57.9 per cent with an electrical output of 254 MW. Figure 16.30 illustrates a block elevation diagram of the practical arrangement of the power plant. Burner flames
Hybrid burner ring
Figure 16.31 shows basic arrangements for some modern power plants fired by fossil fuel. Many arrangements are possible and plants can be tailor-made for specific needs. These plants allow considerable control to be exercised over noxIOus emissions. The gas turbine has a very wide use as an aircraft propulsion unit. Gas turbines are variously designated as turbojet, turbofan, turboprop and turboshaft. The turbojet is the simplest form, consisting of a compressor, combustion chamber(s) and turbine. It is used to produce a high-velocity jet for aircraft propulsion. The turbofan is the most commonly used arrangement in aircraft. Part of the air intake is bypassed around the outside of the combustion chamber arrangement. The remaining air passes through the combustion chamber system. The bypass flow either rejoins the hot flow downstream of the combustion chamber system or is exhau~ted
Internal combustion engines
through an annulus surrounding the hot exhaust. The arrangement with turbofan gives a lower jet velocity, improved lower-speed propulsive efficiency and specific fuel consumption plus a lower noise level. Sometimes the fan section has variable pitch, which helps to reduce fan noise and improves the engine control and flexibility. Figure 16.32 is a cutaway illustration of a modern turbofan engine used for aircraft. The engine illustrated is the Trent 700 manufactured by Rolls-Royce. Air passes into the engine through the turbofan and then divides. Some air passes into the air compressor of the engine but most of it becomes the bypass air. It travels round the outside of the engine, through cowling, ultimately to join the jet exhaust at the rear of the engine. The compressed air from the air compressor in the engine passes into the combustion chamber ring then through the gas turbine to become the jet exhaust and to combine with the bypass air. Some general statistics of the Trent 700 are given in the table. Table of Trent 700 parameters Rotational speed (rev/min) Engine mass (tonne) Overall diameter (m) Length (m) Take-off thrust (kg) Fuel flow rate (kg/h) Cruise fuel consumption (kg/h per kg thrust) Air mass flow rate (kg/s) Overall pressure ratio Air bypass ratio
3300 6 3.66 7.9 34 000-40 800 14 290 0.545 900-1200 39:1 5.3:1
The turboprop and turboshaft engines are similar in arrangement ..They usually have an additional turbine which produces power for external use. In the case of the turboprop unit used on aircraft, the engine will provide power to drive a propeller, and there will be some residual thrust obtained from the exhaust. The combined effect of propeller and jet thrusts is sometimes called the effective power or the total equivalent power. The turboshaft engine is used to power external equipment by tapping the shaft power. It is used in ships, power and pumping stations, hovercraft and helicopters. Some jet engines in aircraft are used with vectored thrust. This means that the jet nozzle assembly can be changed in direction. Such arrangements are used on vertical and short take-off and landing (V/STOL) aircraft. Other jet engines on aircraft will employ reverse thrust. This is a device in the jet exhaust for altering the jet thrust to a more reversed direction, thus assisting in slowing down an aircraft after landing. reciprocating internal Gas turbines are not self-starting machines. In the combustion engine it is necessary only to turn the engine over one compression; the engine will fire and will pick up speed on its own. The gas turbine will not start simply by turning the burner on. It must first be motored up to some minimum speed, called the coming-in speed, before the fuel is turned on. When this speed has been reached, the fuel is turned on, ignited, and the turbine will then pick up speed on its own.
Internal combustion engines
The exhaust from the compressor turbine passes into a second turbine which is completely separate. This is the free power turbine which is coupled to drive external equipment. The advantage of this arrangement is that the free power turbine is completely independent of the air compressor. In this way all units of the plant can be designed to run at their most efficient speed. This is especially so if the free power turbine speed is to be some fixed value such as 3000 rev/min. This is a common speed required in the generation of electrical power. The turbines discussed so far may be put under the general heading of open-cycle or open-circuit gas turbines. This name is given to them because air is taken from the atmosphere and the exhaust products are passed out to atmosphere. Work has also been done on the closed-cycle, or closed-circuit, gas turbines. The basic principle of this type of turbine is shown in Fig. 16.40. It shows the usual turbine-compressor arrangement, but now the air operates in a closed circuit. It is the same air being ~ir~1l1~t~rl
The compressed air passes from the compressor through a heater in which the air is· indirectly heated. The heated air passes into the turbine and after expansion into a cooler, where it is cooled then recirculated back to the compressor. The air in the circuit, even on the low-pressure side of the circuit, is usually compressed above atmospheric conditions. In this condition it has a higher density and has better heat transfer properties. This circuit has the advantage that the heater can use any type of fuel, solid, liquid or gas, because there is no direct mixing with the working air. It is also possible to use a nuclear reactor to provide the necessary energy. Then the circulating gas would probably be changed to some gas such as helium, which is relatively immune to the effects of radioactivity. The disadvantage of this circuit is that it needs a large supply of cooling water for the cooler and the use of indirect heating leads to a reduced thermal efficiency. On small plants the closed circuit has a lower thermal efficiency than the corresponding open circuit. It is probably more advantageous to use the closed cycle in large turbine installations in which many efficiency improvement devices can be installed.
Eng ine and plant trials
When choosing an engine, or any engineering plant for that matter, it is necessary to refer to the relevant performance characteristics. Engine performance characteristics will have been determined during a series of trials then tabulated and, where possible, graphed. Some of the general performance characteristics and their method of determination are now discussed. 17.2
An engine is required to drive external equipment, so it is important to know how much torque the engine will deliver at the various running conditions. The torque, which is usually determined in newton-metres (N m), is measured by coupling a measuring device, called a brake or dynamometer, to the engine output shaft. Four of the most common types of brake are described in the following sections. The rope and Prony brakes are now rarely used; they are included only to illustrate basic principles. 17.3
A typical rope brake arrangement is illustrated in Fig. 17.1. It consists of a rope (sometimes a leather or webbing band) which is wrapped around the flywheel. To one end of the rope is attached a spring balance which is attached to the ground. To the other end of the rope is attached a mass carrier. This mass carrier is allowed to hang freely, although its bottom end is usually connected to the ground by a loosely hanging chain. This is a safety chain in case the rope at any time tends to get caught up on the flywheel, in which case the chain will hold back the mass. Note that the spring balance is always on the side of the flywheel such that, when rotating, the flywheel turns toward the balance. The mass carrier is always on the other side of the flywheel such that the flywheel tends to lift the carrier. For a heavy load on the engine, a large mass is placed on the mass carrier. For light loads, some mass can be removed from the mass carrier. For no load, the rope brake is usually removed from the flywheel.
Basic en/(ineering thermodynamics
A rotor is mounted in a rotor cover which is suitably hydraulically sealed by bearings and glands. The shaft of the rotor passes out from the cover bearings and is mounted in the main bearings which are fixed on the dynamometer bed. The rotor cover is not fixed, but is free to rotate in trunnion bearings which are again mounted on the dynamometer bed. From the side of the rotor cover extends a load-bar on to the end of which is attached a mass hanger. The principle of operation is as follows. The rotor sometimes has holes projecting blades and sometimes cups. The rotor cover may have matching blades or cups. The space between the rotor and the cover is either filled or part-filled with water, according to the design. If the rotor is now rotated, a reaction will be set up with the water and the cover; the cover will tend to rotate in the trunnion bearings with the rotor. The cover rotation is prevented by hanging suitable masses on the mass hanger. Thus a torque reaction is set up and this torque, which is the torque generated by the engine, can be measured. If
mass on hanger, kg distance from centre of dynamometer to hanger, m torque developed by engine, N m
then T= 9.81 Mr The amount of torque absorbed by the dynamometer can be varied by control of the water. In some dynamometers, the water depth is varied: the greater the depth, the greater the load. In others, plates are introduced between the rotor and the cover. The plates are controlled from the outside. If more plate is introduced, there is less interference between the rotor and the cover, so less load is absorbed by the dynamometer. If the plates are withdrawn, more interference will occur, so more load will be absorbed. In some hydraulic dynamometers there is a fixed quantity of water in the cover, but in others the cover is usually filled and means are provided for the free flow of water through the cover. This dissipates friction energy generated between the rotor and the water. 17.6
The electrical dynamometer
This dynamometer is, in many ways, similar to the hydraulic dynamometer. It is of the torque reaction type, but the reaction is not hydraulic; it is magnetic, between the rotating armature of a generator and the magnetic field set up by the field coils mounted in the outer casing of the generator. The outer casing of the generator is not fixed as is usually the case; like the cover of the hydraulic dynamometer, it is mounted in trunnion bearings. As the armature of the generator is rotated by the engine, its reaction with the magnetic field tends to pull the field coils and casing round with it. This rotation is prevented in the same way as in the hydraulic dynamometer - a load-bar with mass hanger fitted to the outside of the casing. Torque is measured as the product of the force required to keep
Basic engineering thermodynamics
then Power output
Neglecting electrical losses, this will generally be very close to the brake power of the engine. 17.8
Indicated power (i.p.)
The indicated power (i.p.) of an engine is the power actually developed in the cylinders. The brake power will, in fact, be less than the indicated power because losses occur from cylinders to shaft, such as friction and running auxiliary equipment (e.g. fuel and oil pumps). In order to determine the indicated power, it is necessary to know the work conditions in the cylinders. The area of a pressure-volume diagram represents work. If the engine cycle can be obtained in the form of a pressure--volume diagram, direct from the cylinders, it would therefore be possible to obtain the work done by the engine as indicated by the cylinder operations. And knowing the number of cycles per second, it will be possible to determine the indicated power output from the cylinders. A scaled-down pressure-volume diagram of an engine cycle can be obtained while the engine is running by using an engine indicator. The engine indicator can appear in various forms: mechanical, part mechanical and part electrical, and electronic. Most modern engine indicators are electronic. 17.9
The engine indicator
Most modern engine indicators are electronic. Suitable transducers and sensors, which monitor such parameters as cylinder pressure, temperature and crank angle, are strategically placed on or in the engine. Signals from the transducers and sensors are transmitted to an electronic processing unit which displays performance information on a visual display unit (VDU). Information can be verbal or by calibrated diagram; it can also be in colour. Verbal information can be cylinder pressure and engine rotational speed, calibrated diagrams can be pressure-volume (Figs 16.4(d) and ]6.5(e» or pressure--crank angle (Fig. ]6.24). The electronic engine indicator can be used for continuous operational performance monitoring. 17.10
Indicated mean effective pressure (/MEP)
The indicated mean effective pressure (IMEP) is that constant pressure which, if it acted over the full length of the stroke, would produce the same amount of work done on the piston as is actually obtained during a complete engine cycle (see also section ]5.1). Figure 17.4 is a P-V diagram of a four-stroke cycle internal combustion engine. Two separate areas are shown. The positive diagram area is the area which produces work output. The negative diagram area is the area which requires work input. The net area gives the theoretical positive cycle work output. In general the negative area is small compared with the positive area, so it is frequently neglected. This is the case with the four-stroke cycle engine.
Basic engineering thermodynamics
percentage of the energy supplied is, of course, the brake thermal efficiency; it is a very low figure because there is considerable energy loss. It is unfortunately true that most of the energy supplied to engine plant is lost. Much effort is made to develop equipment to reclaim some of this waste energy. More details are given in the sections on boilers, the gas turbine and the Stirling and Ericsson cycles. A further method of representing an energy balance is by means of the Sankey diagram.This is a stream or flow diagram in which the width of the stream represents the energy quantity being considered, usually as a percentage of the energy supplied. A Sankey diagram for the piston internal combustion engine is shown in Fig. ]7.6. It starts at the bottom with a stream whose width represents the energy input from the fuel, marked ]00 per cent. Moving up the diagram, the coolant loss stream is led off to the left. The width of this stream represents the percentage loss to the coolant. Next the exhaust loss stream is led off to the left and finally the surroundings loss is led off to the left. The loss streams finally meet as a single loss stream, as shown. Of the original vertical stream, only the b.p. output stream remains at the top of the diagram. The figures on the diagram are percentages of the original energy supplied in the fuel.
Natural heat transfer from high to ambient temperature
Heat transfer from low to high temperature requires external energy
Nautural heat transfer from ambient to low refrigeration temperature 18.1
If a body is to be maintained at a temperature lower than its surrounding or ambient temperature, any heat transfer which will naturally occur down the temperature gradient from the surroundings to the body (second law of thermodynamics) must be transferred back to the surroundings. Unless this is done, the temperature of the body will increase compared to that of its surroundings. Now the transfer of heat from a colder to a hotter body is contrary to the second law of thermodynamics; this implies that external energy is required to effect such a transfer. The external energy can be supplied by a heating device or a compressor (pump); either will produce the necessary increase in temperature, The cyclic process by which natural heat transfer down a temperature gradient is returned up the temperature gradient, using a supply of external energy, is the process of refrigeration. The production of very low temperatures is usually known as cryogenics. In any refrigerator, as the plant is called, an amount of energy will be removed from the cold body by the refrigeration process. This is called the refrigeration effect. The ratio Refrigerating effect External energy supplied is called the coefficient of performance (COP). This definition is similar to that used for efficiency. The term efficiency is not used here because very often COP> I; the term coefficient is preferred for such cases. The various heat transfers associated with the refrigeration process are illustrated in Fig. 18.I. Note that the high temperature is higher than the ambient temperature so that heat transfer can take place. The heat transfer from the high temperature to the low refrigeration temperature takes place in two stages. There is a natural heat transfer to the surroundings from the high temperature to ambient temperature. This is followed by a natural heat transfer from ambient temperature to the low refrigeration temperature. The heat transfer from the low temperature to the high temperature requires external energy and takes place directly.
Low refrigeration temperature Fig. 18.1
Heat transfers during refrigeration
The refrigeration cycle is the reverse of the heat engine cycle. In the heat engine cycle, energy is received at high temperature and rejected at low temperature; work is obtained from the cycle. In the refrigeration cycle, energy is received at low temperature and rejected at high temperature; work (or heat) is required to perform the cycle. Due to the transfer of energy from low to high temperature, the refrigerator is sometimes called a heat pump. 18.2
The working substances which flow through refrigerators are called refrigerants. Refrigerants remalll III the liquid phase at suitable pressures and subzero temperatures « 0 0C); this is a crucial property. It is usual that heat transfer into the liquid refrigerant at low pressure and subzero temperature evaporates the refrigerant. This is called the refrigerating effect. Heat transfer from the refrigerant, at high pressure and temperature, condenses the refrigerant. Since about 1992 the refrigerants industry has experienced something of an upheaval. Before then, probably the most commonly used refrigerants were the chlorofluorocarbons (CFCs). Examples are freon 12 (dichlorodifluoromethane, CCI2F2) and methyl chloride (CH3CI). However, the world is becoming more ecologically conscious, and it has been discovered that the release of CFCs into the atmosphere produces significant ozone depletion in the upper stratosphere. This is mainly caused by increased upper atmospheric loading of the chlorine released by CFCs. The depletion of ozone seems to have produced major effects in the earth's polar regions. Holes have appeared in the upper ozone layer, particularly in the southern hemisphere during spring and summer. The presence of ozone in the upper stratosphere is very important because ozone attenuates the incoming ultraviolet (UV) light from the sun. Ozone depletion may
In large refrigeration plant the evaporator may be suspended in a secondary refrigerant such as brine. The heat exchange then takes place in two stages: between the cold chamber and the secondary refrigerant, which is pumped round the cold chamber, then between the secondary refrigerant and the primary refrigerant in the evaporator of the refrigerator. Again, in large refrigeration plant, the condenser may be water-cooled or have forced-draught air cooling using fans. In small refrigeration plant, such as the domestic refrigerator, the evaporator is suspended directly in the cold chamber and the condenser is suspended in the surrounding atmospheric air. Also, in small refrigeration plant, the throttling process may be accomplished by using a short length of capillary tubing. This produces a fixed low temperature in the evaporator. The control of the cold chamber temperature is obtained by using a thermostat in the cold chamber. When the required temperature is reached in the cold chamber, controls connected to the thermostat, switch off the motor driving the refrigerator. The temperature in the cold chamber then slowly rises and the thermostat switches on the motor; the process is then repeated. If a throttle valve is fitted, there is a control on the evaporator temperature. Figure 18.6 shows the T-s and P-h diagrams of the type of cycle more commonly used in the vapour compression refrigerator. The modifications made to the cycle already illustrated in Fig. 18.5 produce a more effective operation of the plant. Entry to the compressor is at I, where the refrigerant is shown as being dry saturated. Sometimes there is a slight degree of superheat, which increases the refrigcrutina
effect and produces dry compression in the refrigerator, shown as process 1-2. This means there is no loss of mass flow due to evaporation of the liquid refrigerant in the compressor during the induction stroke. If liquid refrigerant washes lubricant from the cylinder waHs and carries it into the other sections of the plant, there may be a reduction of heat transfer. A further improvement can be obtained by undercooling (or subcooling) the refrigerant after condensation, shown as process 4-5. The refrigerant is cooled toward the ambient temperature, producing a wetter vapour at 6, after the throttling process, therefore an improved refrigerating effect. The refrigerating effect per unit time is caHed the duty of the refrigerator. It depends upon the end states of the refrigerant in the evaporator and also the mass flow rate of the refrigerant. Tables of properties for various refrigerants are similar to tables for steam (or water substance, as it is sometimes caHed). The refrigerant tables have their own reference state: commonly the specific enthalpy and specific entropy are considered to be zero at -40°C. Some refrigeration plants have a more complex circuit arrangement than shown in Fig. 18.6. 18.6 Calculations for the vapour compression refrigerator The cycle illustrated in Fig. 18.7 is representative of a typical vapour compression cycle. Tables of properties are available for refrigerants, so the properties of state points 7, 8, 4 and 3 may be looked up in the relevant tables.
During the analysis of the refrigeration process, notice that more energy is rejected at the high temperature than is required to drive the refrigerator. If the temperature during the rejection process is sufficiently high, perhaps the heat transfer during rejection could be usefully used in a warming process. That this heat transfer is greater than the energy required to drive the plant presents an attractive idea. The concept was suggested by Lord Kelvin in 1852. The vapour compressIOn refrigerator, with suitably arranged pressures and temperatures, can be considered as being suitable for a heat pump. Many commercial machines have been manufactured using this process; the evaporator is buried under the soil or suspended in a river or lake. But the heat pump has not gained wide acceptance as a heating system. It is more complex, more difficult to run and more difficult to maintain than its conventional counterparts. However, a decrease in fossil fuel availability could encourage its further development and more widespread use.
Example 18.2 A simple heat pump circulates rejYigerant R401 (SUVA MP52, Du Pont) and is required/or space heating. The heat pump consists of an evaporator, compressor, condenser and throttle regulator. The pump works between the pressure limits 411.2 kN/m2 and 1118.9 kN/m2. The heat transfer from the condenser unit is 100 MJ/h. The R401 is assumed dry saturated at the beginning of compression and has a temperature of 60°C a/ier compression. At the end of the condensation process the refrigerant is liquid but not undercooled. The specific heat capacity of the superheated vapour can be assumed constant. Determine (a) the massflow of R40l in kg/h, assuming no energy loss (b) the dryness jYaction of the R401 at the entry to the evaporator (c) the power of the driving motor, assuming that only 70 per cent of the power of the driving motor appears in the R401 (d) the ratio of the heat transferred jYom the condenser to the power required to drive the motor in the same time
A solution of ammonia and water part-fills the generator. A vertical tube passes through the top of the generator and is immersed in the ammonia-water solution. A heater warms the solution; vapour formed above the surface of the solution forces the level of the solution down, so some solution rises up the vertical tube. The solution level in the generator eventually reaches the bottom of the vertical tube and some vapour passes into the tube. Fresh solution passing into the bottom of the generator again lifts the surface level above the bottom of the vertical tube; the process is then repeated. Thus, alternate small quantities of weak solution of ammonia in water and ammonia rich vapour lift in the vertical tube and pass into the separator. In the separator, solution drains into trap 1. The ammonia vapour passes up out of the separator and on into a condenser; it condenses and the liquid ammonia drains into trap 2. Now, following trap I is the absorber and following trap 2 is the evaporator; connections are as shown III Fig. 18.10. The evaporator-absorber system contains some hydrogen at a partial pressure which IS less than the ammonia pressure on the condenser side of trap 2 and the separator side of trap I. Liquid ammonia from trap 2 drains into the evaporator and evaporates; the partial pressure of this evaporated ammonia plus the partial pressure of the hydrogen balances the ammonia pressure on the other side of the traps. Thus, in the evaporator there is a lower ammonia pressure, so the saturation temperature at which it evaporates IS lower. This IS the refrigeration temperature and the evaporation produces the refrigerating effect. The low-temperature ammonia vapour and the hydrogen eventually appear in the absorber. Here the ammonia is absorbed in the weak solution draining from trap I. The hydrogen remains in the evaporator-absorber system; it is unable to leave because of traps I and 2 and the solution in the bottom of the absorber. The strong ammonia-water solution drains from the absorber and passes back to the generator to complete the circuit. There are no moving parts and there is pressure balance throughout. The heater can be electric or it can be fuelled by liquid fuel or gas. The circuit shown is common in some domestic refrigerators. It has a low coefficient of performance. Larger commercial plants are made which require a mechanical circulating pump. They are sometimes employed where waste heat is available.
Questions A vapour compression refrigerator uses SUVA MP52 (BOC-Du Pont) refrigerant 1. between the pressure limits 110.9 kN/m2 and 860.7 kN/m2. At the beginning of compression the refrigerant is dry saturated and at the end of compression it has a temperature of 52°C. In the condenser the refrigerant is condensed but not undercooled. The mass flow of refrigerant is 4 kg/min. Determine (a) the theoretical coefficient of performance (b) the temperature rise of the cooling water in the condenser if the cooling water flow rate is 960 kg/h (c) the ice produced by the evaporator in kg/h from water at 15°C to ice at 0 0C. Specific enthalpy of fusion of ice = 336 kJ/kg Specific heat capacity of water = 4.187 kJ/kg
Basic engineering thermodynamics
Determine the coefficient of performance (a) the ice produced by the evaporator in kgjh from water at 20 "C to ice at 0 °C (b) the effective swept volume of the compressor in m'jmin (c)
The relevant properties of refrigerant MP52 are given in the table. Pressure
