SCHOOL OF MECHANICAL
DEPARTMENT OF MECHANICAL ENGINEERING
LESSON NOTES
U4MEA13 APPLIED ENGINEERING
THERMODYNAMICS
VELTECH Dr.RR & Dr.SR TECHNICAL UNIVERSITY
1
U4MEA13
APPLIED ENGINEERING THERMODYNAMICS
LTPC
3104
OBJECTIVE:
The student must acquire the knowledge capability of analyzing and solving any concept
or problem associated with heat energy dynamics and utilization.
UNIT I: GAS AND VAPOUR POWER CYCLES
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Otto, Diesel and Dual cycles, Air standard efficiency, Mean effective pressure, Comparison
of Otto, Diesel and Dual cycles, Ideal and Actual Brayton cycle.
Rankine cycle, Reheating and Regeneration cycles, Binary vapour cycles, Combined gasvapour power cycles, cogeneration.
UNIT II: REFRIGERATION
9
Air refrigeration cycle, Vapour Compression Refrigeration cycle, Sub cooling and Super
heating, Performance calculations, Vapour Absorption Refrigeration – Ammonia water,
Lithium Bromide water systems (Description only), Comparison between Vapour
Compression and Vapour Absorption Refrigeration systems, Desirable properties of
Refrigerants.
UNIT III: PSYCHROMETRY AND AIR – CONDITIONING
9
Psychrometric properties, Use of psychrometric chart, Psychrometric process – Sensible heat
exchange process, Latent heat exchange process, Adiabatic mixing, Evaporative cooling,
Property calculations of air-vapour mixtures.
Principles of air-conditioning, Types of air conditioning systems – summer, winter, year
round air conditioners, Concept of RSHF, GSHF, ESHF, Simple problems.
UNIT IV: AIR COMPRESSORS
9
Classification and working principle, Work of compression with and with-out clearance,
volumetric, iso-thermal and isentopic efficiencies of reciprocating air-compressors, Multistage compression and intercooling, Work of Multi-stage compressor.
Rotary compressors, Concept of positive displacement, Roots blower, Vane type blower,
Screw compressor, Axial flow and centrifugal compressors (Description only)
UNIT V: FUELS AND COMBUSTION
9
Solid, Liquid and Gaseous fuels, Combustion process, Enthalpy of formation, Enthalpy and
internal energy of combustion, Higher and lower heating values, Adiabatic combustion
temperature, First law analysis of Reacting systems.
Combustion equation, Stochiometric air fuel ratio, Excess air, Composition of combustion
products, Analysis of combustion products, Air-fuel ratio from analysis of combustion
products.
TOTAL: 45+15(Tutorial) = 60 periods
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TEXT BOOKS
1. Nag, P. K, Engineering Thermodynamics, 6 th Edition, Tata McGraw Hill, New
Delhi, 1995
2. Yunus, N. J., Cengel, A., and Michael Boles, A., Thermodynamics - An
Engineering Approach, 2nd Edition, McGraw Hill India, 1994
REFERENCE BOOKS
1. Yadav R., “Thermodynamics and Heat Engines”, Vol 1, Central Publishing
House (1995)
2. Jones J.B and Dugan R.E., “Engineering Thermodynamics”, Prentice Hall of
India (1998)
3. Roy Choudry T., “Basic Engineering Thermodynamics”, Second Edition, Tata
McGraw Hill (2000)
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UNIT I
GAS AND VAPOUR POWER CYCLES
CONSTANT VOLUME OR OTTO CYCLE
This cycle is so named as it was conceived by ‘Otto’. On this cycle, petrol, gas and many
types of oil engines work. It is the standard of comparison for internal combustion engines.
Figs. 13.5 (a) and (b) shows the theoretical p-V diagram and T-s diagrams of this cycle
respectively. _ The point 1 represents that cylinder is full of air with volume V1, pressure p1
and absolute temperature T1. _ Line 1-2 represents the adiabatic compression of air due to
which p1, V1 and T1 change to p2, V2 and T2, respectively. _ Line 2-3 shows the supply of
heat to the air at constant volume so that p2 and T2 change to p3 and T3 (V3 being the same
as V2). _ Line 3-4 represents the adiabatic expansion of the air. During expansion p3, V3 and
T3 change to a final value of p4, V4 or V1 and T4, respectively. _ Line 4-1 shows the
rejection of heat by air at constant volume till original state (point 1) reaches.
Consider 1 kg of air (working substance) :
Heat supplied at constant volume = cv(T3 – T2).
Heat rejected at constant volume = cv (T4 – T1).
But, work done
= Heat supplied – Heat rejected
= cv (T3 – T2) – cv (T4 – T1)
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CONSTANT PRESSURE OR DIESEL CYCLE
This cycle was introduced by Dr. R. Diesel in 1897. It differs from Otto cycle in that heat is
supplied at constant pressure instead of at constant volume. Fig. 13.15 (a and b) shows the pv and T-s diagrams of this cycle respectively.
This cycle comprises of the following operations :
(i) 1-2......Adiabatic compression.
(ii) 2-3......Addition of heat at constant pressure.
(iii) 3-4......Adiabatic expansion.
(iv) 4-1......Rejection of heat at constant volume.
Point 1 represents that the cylinder is full of air. Let p1, V1 and T1 be the corresponding
pressure, volume and absolute temperature. The piston then compresses the air adiabatically
(i.e., pVγ = constant) till the values become p2, V2 and T2 respectively (at the end of the
stroke) at point 2. Heat is then added from a hot body at a constant pressure. During this
addition of heat let volume increases from V2 to V3 and temperature T2 to T3, corresponding
to point 3. This point (3) is called the point of cut-off. The air then expands adiabatically to
the conditions p4, V4 and T4 respectively corresponding to point 4. Finally, the air rejects the
heat to the cold body at constant volume till the point 1 where it returns to its original state.
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DUAL COMBUSTION CYCLE
This cycle (also called the limited pressure cycle or mixed cycle) is a combination of Otto and
Diesel cycles, in a way, that heat is added partly at constant volume and partly at constant
pressure ; the advantage of which is that more time is available to fuel (which is injected into
the engine cylinder before the end of compression stroke) for combustion. Because of lagging
characteristics of fuel this cycle is invariably used for diesel and hot spot ignition engines.
The dual combustion cycle (Fig. 13.19) consists of the following operations :
(i) 1-2—Adiabatic compression
(ii) 2-3—Addition of heat at constant volume
(iii) 3-4—Addition of heat at constant pressure
(iv) 4-5—Adiabatic expansion
(v) 5-1—Rejection of heat at constant volume.
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COMPARISON OF OTTO, DIESEL AND DUAL COMBUSTION CYCLES
Following are the important variable factors which are used as a basis for comparison of
the cycles :
_ Compression ratio.
_ Maximum pressure
_ Heat supplied
_ Heat rejected
_ Net work
Some of the above mentioned variables are fixed when the performance of Otto, Diesel and
dual combustion cycles is to be compared.
Efficiency Versus Compression Ratio
Fig. 13.26 shows the comparison for the air standard efficiencies of the Otto, Diesel and
Dual combustion cycles at various compression ratios and with given cut-off ratio for the
Diesel and Dual combustion cycles. It is evident from the Fig. 13.26 that the air standard
efficiencies increase with the increase in the compression ratio. For a given compression
ratio Otto cycle is the most efficient while the Diesel cycle is the least efficient.
(ηotto > ηdual > ηdiesel).
Note. The maximum compression ratio for the petrol engine is limited by detonation. In their
respective ratio ranges, the Diesel cycle is more efficient than the Otto cycle.
For the Same Compression Ratio and the Same Heat Input
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A comparison of the cycles (Otto, Diesel and Dual) on the p-v and T-s diagrams for the same
compression ratio and heat supplied is shown in the Fig. 13.27.
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— For the maximum pressure the points 3 and 3′ must lie on a constant pressure line.
— On T-s diagram the heat rejected from the Diesel cycle is represented by the area under
the line 4 to 1 and this area is less than the Otto cycle area under the curve 4′ to 1 ;
hence the Diesel cycle is more efficient than the Otto cycle for the condition of maximum
pressure and heat supplied.
GAS TURBINE CYCLE—BRAYTON CYCLE
Ideal Brayton Cycle
Brayton cycle is a constant pressure cycle for a perfect gas. It is also called Joule cycle.The
heat transfers are achieved in reversible constant pressure heat exchangers. An ideal gas
turbine plant would perform the processes that make up a Brayton cycle. The cycle is shown
in the Fig. 13.33 (a) and it is represented on p-v and T-s diagrams as shown in Figs. 13.33 (b)
and (c).
The various operations are as follows :
Operation 1-2. The air is compressed isentropically from the lower pressure p1 to the upper
pressure p2, the temperature rising from T1 to T2. No heat flow occurs.
Operation 2-3. Heat flows into the system increasing the volume from V2 to V3 and
temperature from T2 to T3 whilst the pressure remains constant at p2. Heat received = mcp
(T3 – T2).
Operation 3-4. The air is expanded isentropically from p2 to p1, the temperature falling
from T3 to T4. No heat flow occurs.
Operation 4-1. Heat is rejected from the system as the volume decreases from V4 to V1 and
the temperature from T4 to T1 whilst the pressure remains constant at p1.
Heat rejected = mcp(T4 – T1).
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The eqn. (13.16) shows that the efficiency of the ideal joule cycle increases with the pressure
ratio. The absolute limit of upper pressure is determined by the limiting temperature of the
material of the turbine at the point at which this temperature is reached by the compression
process alone, no further heating of the gas in the combustion chamber would be permissible
and the work of expansion would ideally just balance the work of compression so that no
excess work would be available for external use.
13.10.2. Pressure Ratio for Maximum Work
Now we shall prove that the pressure ratio for maximum work is a function of the limiting
temperature ratio.
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Open Cycle Gas Turbine—Actual Brayton Cycle
Refer Fig. 13.35. The fundamental gas turbine unit is one operating on the open cycle in
which a rotary compressor and a turbine are mounted on a common shaft. Air is drawn into
the compressor and after compression passes to a combustion chamber. Energy is supplied in
the combustion chamber by spraying fuel into the air stream, and the resulting hot gases
expand through the turbine to the atmosphere. In order to achieve net work output from the
unit, the turbine must develop more gross work output than is required to drive the
compressor and to overcome mechanical losses in the drive. The products of combustion
coming out from the turbine are exhausted to the atmosphere as they cannot be used any
more. The working fluids (air and fuel) must be replaced continuously as they are exhausted
into the atmosphere.
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Note. With the variation in temperature, the value of the specific heat of a real gas varies, and
also in the open cycle, the specific heat of the gases in the combustion chamber and in turbine
is different from that in the compressor because fuel has been added and a chemical change
has taken place. Curves showing the variation of cp with temperature and air/fuel ratio can be
used, and a suitable mean value of cp and hence γ can be found out. It is usual in gas turbine
practice to assume fixed mean value of cp and γ for the expansion process, and fixed mean
values of cp and γ for the compression process. In an open cycle gas turbine unit the mass
flow of gases in turbine is greater than that in compressor due to mass of fuel burned, but it is
possible to neglect mass of fuel, since the air/ fuel ratios used are large. Also, in many cases,
air is bled from the compressor for cooling purposes, or in the case of air-craft at high
altitudes, bled air is used for de-icing and cabin air-conditioning. This amount of air bled is
approximately the same as the mass of fuel injected therein.
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RANKINE CYCLE
Rankine cycle is the theoretical cycle on which the steam turbine (or engine) works.
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REGENERATIVE CYCLE
In the Rankine cycle it is observed that the condensate which is fairly at low temperature has
an irreversible mixing with hot boiler water and this results in decrease of cycle efficiency.
Methods are, therefore, adopted to heat the feed water from the hot well of condenser
irreversibly by interchange of heat within the system and thus improving the cycle efficiency.
This heating method is called regenerative feed heat and the cycle is called regenerative
cycle. The principle of regeneration can be practically utilised by extracting steam from the
turbine at several locations and supplying it to the regenerative heaters. The resulting cycle is
known as regenerative or bleeding cycle. The heating arrangement comprises of : (i) For
medium capacity turbines—not more than 3 heaters ; (ii) For high pressure high capacity
turbines—not more than 5 to 7 heaters ; and (iii) For turbines of super critical parameters 8 to
9 heaters. The most advantageous condensate heating temperature is selected depending on
the turbine throttle conditions and this determines the number of heaters to be used. The final
condensate heating temperature is kept 50 to 60°C below the boiler saturated steam
temperature so as to prevent evaporation of water in the feed mains following a drop in the
boiler drum pressure. The conditions of steam bled for each heater are so selected that the
temperature of saturated steam will be 4 to 10°C higher than the final condensate
temperature. Fig. 12.15 (a) shows a diagrammatic layout of a condensing steam power plant
in which a surface condenser is used to condense all the steam that is not extracted for feed
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water heating. The turbine is double extracting and the boiler is equipped with a superheater.
The cycle diagram (T-s) would appear as shown in Fig. 12.15 (b). This arrangement
constitutes a regenerative cycle.
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REHEAT CYCLE
For attaining greater thermal efficiencies when the initial pressure of steam was raised
beyond 42 bar it was found that resulting condition of steam after, expansion was
increasingly wetter and exceeded in the safe limit of 12 per cent condensation. It, therefore,
became necessary to reheat the steam after part of expansion was over so that the resulting
condition after complete expansion fell within the region of permissible wetness. The
reheating or resuperheating of steam is now universally used when high pressure and
temperature steam conditions such as 100 to 250 bar and 500°C to 600°C are employed for
throttle. For plants of still higher pressures and temperatures, a double reheating may be
used. In actual practice reheat improves the cycle efficiency by about 5% for a 85/15 bar
cycle. A second reheat will give a much less gain while the initial cost involved would be so
high as to prohibit use of two stage reheat except in case of very high initial throttle
conditions. The cost of reheat equipment consisting of boiler, piping and controls may be 5%
to 10% more than that of the conventional boilers and this additional expenditure is justified
only if gain in thermal efficiency is sufficient to promise a return of this investment. Usually
a plant with a base load capacity of 50000 kW and initial steam pressure of 42 bar would
economically justify the extra cost of reheating. The improvement in thermal efficiency due
to reheat is greatly dependent upon the reheat pressure with respect to the original pressure
of steam. Fig. 12.23 shows the reheat pressure selection on cycle efficiency.
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Fig. 12.24 shows a schematic diagram of a theoretical single-stage reheat cycle. The
corresponding representation of ideal reheating process on T-s and h-s chart is shown in
Figs. 12.25 (a and b).
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Refer to Fig. 12.25. 5-1 shows the formation of steam in the boiler. The steam as at state
point 1 (i.e., pressure p1 and temperature T1) enters the turbine and expands isentropically to
a certain pressure p2 and temperature T2. From this state point 2 the whole of steam is drawn
out of the turbine and is reheated in a reheater to a temperature T3. (Although there is an
optimum pressure at which the steam should be removed for reheating, if the highest return is
to be obtained, yet, for simplicity, the whole steam is removed from the high pressure
exhaust, where the pressure is about one-fifth of boiler pressure, and after undergoing a 10%
pressure drop, in circulating through the heater, it is returned to intermediate pressure or low
pressure turbine). This reheated steam is then readmitted to the turbine where it is expanded
to condenser pressure isentropically.
Note. Superheating of steam. The primary object of superheating steam and supplying it to
the primemovers is to avoid too much wetness at the end of expansion. Use of inadequate
degree of superheat in steam engines would cause greater condensation in the engine
cylinder ; while in case of turbines the moisture content of steam would result in undue blade
erosion. The maximum wetness in the final condition of steam that may be tolerated without
any appreciable harm to the turbine blades is about 12 per cent. Broadly each 1 per cent of
moisture in steam reduces the efficiency of that part of the turbine in which wet steam passes
by 1 per cent to 1.5 per cent and in engines about 2 per cent.
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Advantages of superheated steam :
(i) Superheating reduces the initial condensation losses in steam engines.
(ii) Use of superheated steam results in improving the plant efficiency by effecting a saving in
cost of fuel. This saving may be of the order of 6% to 7% due to first 38°C of superheat and
4% to 5% for next 38°C and so on. This saving results due to the fact that the heat content
and consequently the capacity to do work in superheated steam is increased and the quantity
of steam required for a given output of power is reduced. Although additional heat has to be
added in the boiler there is reduction in the work to be done by the feed pump, the condenser
pump and other accessories due to reduction in quantity of steam used. It is estimated that the
quantity of steam may be reduced by 10% to 15% for first 38°C of superheat and somewhat
less for the next 38°C of superheat in the case of condensing turbines.
(iii) When a superheater is used in a boiler it helps in reducing the stack temperatures by
extracting heat from the flue gases before these are passed out of chimney.
Thermal efficiency with ‘Reheating’ (neglecting pump work) :
Heat supplied = (h1 – hf4 ) + (h3 – h2)
Heat rejected = h4 – hf4
Work done by the turbine = Heat supplied – heat rejected
= (h1 – hf4 ) + (h3 – h2) – (h4 – hf4 )
= (h1 – h2) + (h3 – h4)
Thus, theoretical thermal efficiency of reheat cycle is
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If we use steam as the working medium the temperature rise is accompanied by rise in
pressure and at critical temperature of 374.15°C the pressure is as high as 225 bar which will
create many difficulties in design, operation and control. It would be desirable to use some
fluid other than steam which has more desirable thermodynamic properties than water. An
ideal fluid for this purpose should have a very high critical temperature combined with low
pressure. Mercury, diphenyl oxide and similar compounds, aluminium bromide and zinc
ammonium chloride are fluids which possess the required properties in varying degrees.
Mercury is the only working fluid which has been successfully used in practice. It has high
critical temperature (588.4°C) and correspondingly low critical pressure (21 bar abs.). The
mercury alone cannot be used as its saturation temperature at atmospheric pressure is high
(357°C). Hence binary vapour cycle is generally used to increase the overall efficiency of the
plant. Two fluids (mercury and water) are used in cascade in the binary cycle for production
of power. The few more properties required for an ideal binary fluid used in high temperature
limit are listed below :
1. It should have high critical temperature at reasonably low pressure.
2. It should have high heat of vaporisation to keep the weight of fluid in the cycle to
minimum.
3. Freezing temperature should be below room temperature.
4. It should have chemical stability through the working cycle.
5. It must be non-corrosive to the metals normally used in power plants.
6. It must have an ability to wet the metal surfaces to promote the heat transfer.
7. The vapour pressure at a desirable condensation temperature should be nearly tmospheric
which will eliminate requirement of power for maintenance of vacuum in the condenser.
8. After expansion through the primemover the vapour should be nearly saturated so that
a desirable heat transfer co-efficient can be obtained which will reduce the size of the
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condenser required.
9. It must be available in large quantities at reasonable cost.
10. It should not be toxic and, therefore, dangerous to human life. Although mercury does not
have all the required properties, it is more favourable than any other fluid investigated. It is
most stable under all operating conditions. Although, mercury does not cause any corrosion
to metals, but it is extremely dangerous to human life, therefore, elaborate precautions must
be taken to prevent the escape of vapour. The major disadvantage associated with mercury is
that it does not wet surface of the metal and forms a serious resistance to heat flow. This
difficulty can be considerably reduced by adding magnesium and titanium (2 parts in 100000
parts) in mercury.
Thermal properties of mercury :
Mercury fufills practically all the desirable thermodynamic properties stated above.
1. Its freezing point is – 3.3°C and boiling point is – 354.4°C at atmospheric pressure.
2. The pressure required when the temperature of vapour is 540°C is only 12.5 bar (app.)
and, therefore, heavy construction is not required to get high initial temperature.
3. Its liquid saturation curve is very steep, approaching the isentropic of the Carnot cycle.
4. It has no corrosive or erosive effects upon metals commonly used in practice.
5. Its critical temperature is so far removed from any possible upper temperature limit
with existing metals as to cause no trouble.
Some undesirable properties of mercury are listed below :
1. Since the latent heat of mercury is quite low over a wide range of desirable condensation
temperatures, therefore, several kg of mercury must be circulated per kg of water evaporated
in binary cycle.
2. The cost is a considerable item as the quantity required is 8 to 10 times the quantity of
water circulated in binary system.
3. Mercury vapour in larger quantities is poisonous, therefore, the system must be perfect
and tight. Fig. 12.35 shows the schematic line diagram of binary vapour cycle using mercury
and water as working fluids. The processes are represented on T-s diagram as shown in Fig.
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COMBINED GAS–VAPOR POWER CYCLES
The continued quest for higher thermal efficiencies has resulted in rather innovative
modifications to conventional power plants. The binary vapour cycle discussed later is one
such modification. A more popular modification involves a gas power cycle topping a vapor
power cycle, which is called the combined gas–vapor cycle, or just the combined cycle. The
combined cycle of greatest interest is the gas-turbine (Brayton) cycle topping a steamturbine
(Rankine) cycle, which has a higher thermal efficiency than either of the cycles executed
individually. Gas-turbine cycles typically operate at considerably higher temperatures than
steam cycles. The maximum fluid temperature at the turbine inlet is about 620°C (1150°F)
for modern steam power plants, but over 1425°C (2600°F) for gas-turbine power plants. It is
over 1500°C at the burner exit of turbojet engines. The use of higher temperatures in gas
turbines is made possible by recent developments in cooling the turbine blades and coating
the blades with high-temperature-resistant materials such as ceramics. Because of the higher
average temperature at which heat is supplied, gas-turbine
cycles have a greater potential for higher thermal efficiencies. However, the gas-turbine
cycles have one inherent disadvantage: The gas leaves the gas turbine at very high
temperatures (usually above 500°C), which erases any potential gains in the thermal
efficiency. The situation can be improved somewhat by using regeneration, but the
improvement is limited. It makes engineering sense to take advantage of the very desirable
characteristics of the gas-turbine cycle at high temperatures and to use the hightemperature
exhaust gases as the energy source for the bottoming cycle such as a steam power cycle. The
result is a combined gas–steam cycle, as shown in Fig. 10–24. In this cycle, energy is
recovered from the exhaust gases by transferring it to the steam in a heat exchanger that
serves as the boiler. In general, more than one gas turbine is needed to supply sufficient heat
to the steam. Also, the steam cycle may involve regeneration as well as reheating. Energy for
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the reheating process can be supplied by burning some additional
fuel in the oxygen-rich exhaust gases. Recent developments in gas-turbine technology have
made the combined gas–steam cycle economically very attractive. The combined cycle
increases the efficiency without increasing the initial cost greatly. Consequently, many new
power plants operate on combined cycles, and many more existing steam- or gas-turbine
plants are being converted to combined-cycle power plants. Thermal efficiencies well over 40
percent are reported as a result of conversion.
COGENERATION
In all the cycles discussed so far, the sole purpose was to convert a portion of the heat
transferred to the working fluid to work, which is the most valuable form of energy. The
remaining portion of the heat is rejected to rivers, lakes, oceans, or the atmosphere as waste
heat, because its quality (or grade) is too low to be of any practical use. Wasting a large
amount of heat is a price we have to pay to produce work, because electrical or mechanical
work is the only form of energy on which many engineering devices (such as a fan) can
operate. Many systems or devices, however, require energy input in the form of heat, called
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process heat. Some industries that rely heavily on process heat are chemical, pulp and paper,
oil production and refining, steel making, food processing, and textile industries. Process heat
in these industries is usually supplied by steam at 5 to 7 atm and 150 to 200°C (300 to
400°F). Energy is usually transferred to the steam by burning coal, oil, natural gas, or another
fuel in a furnace. Now let us examine the operation of a process-heating plant closely.
Disregarding any heat losses in the piping, all the heat transferred to the steam in the boiler is
used in the process-heating units, as shown in Fig. 10–20. Therefore, process heating seems
like a perfect operation with practically no waste of energy. From the second-law point of
view, however, things do not look so perfect. The temperature in furnaces is typically very
high (around 1400°C), and thus the energy in the furnace is of very high quality. This highquality energy is transferred to water to produce steam at about 200°C or below (a highly
irreversible process). Associated with this irreversibility is, of course, a loss in exergy or
work potential. It is simply not wise to use high-quality energy to accomplish a task that
could be accomplished with low-quality energy.
Industries that use large amounts of process heat also consume a large amount of electric
power. Therefore, it makes economical as well as engineering sense to use the alreadyexisting work potential to produce power instead of letting it go to waste. The result is a plant
that produces electricity while meeting the process-heat requirements of certain industrial
processes. Such a plant is called a cogeneration plant. In general, cogeneration
is the production of more than one useful form of energy (such as process heat and electric
power) from the same energy source. Either a steam-turbine (Rankine) cycle or a gas-turbine
(Brayton) cycle or even a combined cycle (discussed later) can be used as the power cycle in
a cogeneration plant. The schematic of an ideal steam-turbine cogeneration plant is shown in
Fig. 10–21. Let us say this plant is to supply process heat Qp at 500 kPa at a rate of 100 kW.
To meet this demand, steam is expanded in the turbine to a pressure of 500 kPa, producing
power at a rate of, say, 20 kW. The flow rate of the steam can be adjusted such that steam
leaves the processheating section as a saturated liquid at 500 kPa. Steam is then pumped to
the boiler pressure and is heated in the boiler to state 3. The pump work is usually very small
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and can be neglected. Disregarding any heat losses, the rate of heat input in the boiler is
determined from an energy balance to be 120 kW. Probably the most striking feature of the
ideal steam-turbine cogeneration plant shown in Fig. 10–21 is the absence of a condenser.
Thus no heat is rejected from this plant as waste heat. In other words, all the energy
transferred to the steam in the boiler is utilized as either process heat or electric power. Thus
it is appropriate to define a utilization factor _u for a cogeneration plant as
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UNIT II
REFRIGERATION
AIR REFRIGERATION SYSTEM
Introduction
Air cycle refrigeration is one of the earliest methods of cooling developed. It became obsolete
for several years because of its low co-efficient of performance (C.O.P.) and high operating
costs. It has, however, been applied to aircraft refrigeration systems, where with low
equipment weight, it can utilise a portion of the cabin air according to the supercharger
capacity. The main characteristic feature of air refrigeration system, is that throughout the
cycle the refrigerant remains in gaseous state.
The air refrigeration system can be divided in two systems :
(i) Closed system (ii) Open system.
In closed (or dense air) system the air refrigerant is contained within the piping or
components parts of the system at all times and refrigerator with usually pressures above
atmospheric pressure.
In the open system the refrigerator is replaced by the actual space to be cooled with the air
expanded to atmospheric pressure, circulated through the cold room and then compressed to
the cooler pressure. The pressure of operation in this system is inherently limited to operation
at atmospheric pressure in the refrigerator.
A closed system claims the following advantages over open system : (i) In a closed system
the suction to compressor may be at high pressure. The sizes of expander and compressor can
be kept within reasonable limits by using dense air ; (ii) In open air system, the air picks up
moisture from the products kept in the refrigerated chamber ; the moisture may freeze during
expansion and is likely to choke the valves whereas it does not happen in closed system and
(iii) In open system, the expansion of the refrigerant can be carried only upto atmospheric
pressure prevailing in the cold chamber but for a closed system there is no such restriction.
Reversed Carnot Cycle
If a machine working on reversed Carnot cycle is driven from an external source, it will work
or function as a refrigerator. The production of such a machine has not been possible
practically because the adiabatic portion of the stroke would need a high speed while during
isothermal portion of stroke a very low speed will be necessary. This variation of speed
during the stroke, however is not practicable. p-V and T-s diagrams of reversed Carnot cycle
are shown in Figs. 14.1 (a) and (b). Starting from point l, the clearance space of the cylinder
is full of air, the air is then expanded adiabatically to point p during which its temperature
falls from T1 to T2, the cylinder is put in contact with a cold body at temperature T2. The air
is then expanded isothermally to the point n, as a result of which heat is extracted from the
cold body at temperature T2. Now the cold body is removed ; from n to m air undergoes
adiabatic compression with the assistance of some external power and temperature rises to
T1. A hot body at temperature T1 is put in contact with the cylinder. Finally the air is
compressed isothermally during which process heat is rejected to the hot body.
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Reversed Brayton Cycle
Fig. 14.4 shows a schematic diagram of an air refrigeration system working on reversed
Brayton cycle. Elements of this systems are :
1. Compressor
2. Cooler (Heat exchanger)
3. Expander
4. Refrigerator.
In this system, work gained from expander is employed for compression of air, consequently
less external work is needed for operation of the system. In practice it may or may not be
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done e.g., in some aircraft refrigeration systems which employ air refrigeration cycle the
expansion work may be used for driving other devices.
This system uses reversed Brayton cycle which is described below :
Figs. 14.5 (a) and (b) shows p-V and T-s diagrams for a reversed Brayton cycle. Here it is
assumed that (i) absorption and rejection of heat are constant pressure processes and (ii)
Compression and expansion are isentropic processes.
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Note. The reversed Brayton cycle is same as the Bell-Coleman cycle. Conventionally BellColeman cycle refers to a closed cycle with expansion and compression taking place in
reciprocating expander and compressor respectively, and heat rejection and heat absorption
taking place in condenser and evaporator respectively. With the development of efficient
centrifugal compressors and gas turbines, the processes of compression and expansion can be
carried out in centrifugal compressors and gas turbines respectively. Thus the shortcoming
encountered with conventional reciprocating expander and compressor is overcome.
Reversed Brayton cycle finds its application for air-conditioning of aeroplanes where air is
used as refrigerant.
Merits and Demerits of Air refrigeration System
Merits
1. Since air is non-flammable, therefore there is no risk of fire as in the machine using
NH3 as the refrigerant.
2. It is cheaper as air is easily available as compared to the other refrigerants.
3. As compared to the other refrigeration systems the weight of air refrigeration system
per tonne of refrigeration is quite low, because of this reason this system is employed in
aircrafts.
Demerits
1. The C.O.P. of this system is very low in comparison to other systems.
2. The weight of air required to be circulated is more compared with refrigerants used in
other systems. This is due to the fact that heat is carried by air in the form of sensible heat.
VAPOUR COMPRESSION SYSTEM
Simple Vapour Compression Cycle
In a simple vapour compression system fundamental processes are completed in one cycle.
These are :
1. Compression 2. Condensation 3. Expansion 4. Vapourisation.
The flow diagram of such a cycle is shown in Fig. 14.9.
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The vapour at low temperature and pressure (state ‘2’) enters the “compressor” where it is
compressed isentropically and subsequently its temperature and pressure increase
considerably (state ‘3’). This vapour after leaving the compressor enters the ‘‘condenser”
where it is condensed into high pressure liquid (state ‘4’) and is collected in a “receiver tank”.
From receiver tank it passes through the “expansion valve”, here it is throttled down to a
lower pressure and has a low temperature (state ‘1’). After finding its way through expansion
“valve” it finally passes on to “evaporator” where it extracts heat from the surroundings or
circulating fluid being refrigerated and vapourises to low pressure vapour (state ‘2’).
Merits and demerits of vapour compression system over Air refrigeration system :
Merits :
1. C.O.P. is quite high as the working of the cycle is very near to that of reversed Carnot
cycle.
2. When used on ground level the running cost of vapour-compression refrigeration system
is only 1/5th of air refrigeration system.
3. For the same refrigerating effect the size of the evaporator is smaller.
4. The required temperature of the evaporator can be achieved simply by adjusting the
throttle valve of the same unit.
Demerits :
1. Initial cost is high.
2. The major disadvantages are inflammability, leakage of vapours and toxity. These have
been overcome to a great extent by improvement in design.
Functions of Parts of a Simple Vapour Compression System
Here follows the brief description of various parts of a simple vapour compression system
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shown in Fig. 14.9.
1. Compressor. The function of a compressor is to remove the vapour from the evaporator,
and to raise its temperature and pressure to a point such that it (vapour) can be condensed
with available condensing media.
2. Discharge line (or hot gas line). A hot gas or discharge line delivers the high-pressure,
high-temperature vapour from the discharge of the compressor to the condenser.
3. Condenser. The function of a condenser is to provide a heat transfer surface through
which heat passes from the hot refrigerant vapour to the condensing medium.
4. Receiver tank. A receiver tank is used to provide storage for a condensed liquid so that a
constant supply of liquid is available to the evaporator as required.
5. Liquid line. A liquid line carries the liquid refrigerant from the receiver tank to the
refrigerant flow control.
6. Expansion valve (refrigerant flow control). Its function is to meter the proper amount of
refrigerant to the evaporator and to reduce the pressure of liquid entering the evaporator so
that liquid will vapourize in the evaporator at the desired low temperature and take out
sufficient amount of heat.
7. Evaporator. An evaporator provides a heat transfer surface through which heat can pass
from the refrigerated space into the vapourizing refrigerant.
8. Suction line. The suction line conveys the low pressure vapour from the evaporator to the
suction inlet of the compressor.
Vapour Compression Cycle on Temperature-Entropy (T-s) Diagram
We shall consider the following three cases :
1. When the vapour is dry and saturated at the end of compression. Fig. 14.10
represents the vapour compression cycle, on T-s diagram the points 1, 2, 3 and 4 correspond
to the state points 1, 2, 3 and 4 in Fig. 14.9.
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VAPOUR ABSORPTION SYSTEM
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Introduction
In a vapour absorption system the refrigerant is absorbed on leaving the evaporator, the
absorbing medium being a solid or liquid. In order that the sequence of events should be
continuous it is necessary for the refrigerant to be separated from the absorbent and
subsequently condensed before being returned to the evaporator. The separation is
accomplished by the application of direct heat in a ‘generator’. The solubility of the
refrigerant and absorbent must be suitable and the plant which uses ammonia as the
refrigerant and water as absorbent will be described.
Simple Vapour Absorption System
Refer Fig. 14.21 for a simple absorption system. The solubility of ammonia in water at low
temperatures and pressures is higher than it is at higher temperatures and pressures. The
ammonia vapour leaving the evaporator at point 2 is readily absorbed in the low temperature
hot solution in the absorber. This process is accompanied by the rejection of heat. The
ammonia in water solution is pumped to the higher pressure and is heated in the generator.
Due to reduced solubility of ammonia in water at the higher pressure and temperature, the
vapour is removed from the solution. The vapour then passes to the condenser and the
weakened ammonia in water solution is returned to the absorber.
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In this system the work done on compression is less than in vapour compression cycle (since
pumping a liquid requires much less work than compressing a vapour between the same
pressures) but a heat input to the generator is required. The heat may be supplied by any
convenient form e.g. steam or gas heating.
Practical Vapour Absorption System
Refer Fig. 14.22. Although a simple vapour absorption system can provide refrigeration yet
its operating efficiency is low. The following accessories are fitted to make the system more
practical and improve the performance and working of the plant.
1. Heat exchanger. 2. Analyser. 3. Rectifier.
1. Heat exchanger. A heat exchanger is located between the generator and the absorber.
The strong solution which is pumped from the absorber to the generator must be heated ; and
the weak solution from the generator to the absorber must be cooled. This is accomplished by
a heat exchanger and consequently cost of heating the generator and cost of cooling the
absorber are reduced.
2. Analyser. An analyser consists of a series of trays mounted above the generator. Its main
function is to remove partly some of the unwanted water particles associated with ammonia
vapour going to condenser. If these water vapours are permitted to enter condenser they may
enter the expansion valve and freeze ; as a result the pipe line may get choked.
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3. Rectifier. A rectifier is a water-cooled heat exchanger which condenses water vapour and
some ammonia and sends back to the generator. Thus final reduction or elemination of the
percentage of water vapour takes place in a rectifier. The co-efficient of performance (C.O.P.)
of this system is given by :
Ammonia Water vapour absorption refrigeration system:
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1)
Evaporator: It is in the evaporator where the refrigerant pure ammonia (NH3) in
liquid state produces the cooling effect. It absorbs the heat from the substance to be cooled
and gets evaporated. From here, the ammonia passes to the absorber in the gaseous state.
2)
Absorber: In the absorber the weak solution of ammonia-water is already present.
The water, used as the absorbent in the solution, is unsaturated and it has the capacity to
absorb more ammonia gas. As the ammonia from evaporator enters the absorber, it is readily
absorbed by water and the strong solution of ammonia-water is formed. During the process of
absorption heat is liberated which can reduce the ammonia absorption capacity of water;
hence the absorber is cooled by the cooling water. Due to absorption of ammonia, strong
solution of ammonia-water is formed in the absorber.
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3) Pump: The strong solution of ammonia and water is pumped by the pump at high
pressure to the generator.
4) Generator: The strong solution of ammonia refrigerant and water absorbent are heated
by the external source of heat such as steam or hot water. It can also be heated by other
sources like natural gas, electric heater, waste exhaust heat etc. Due to heating the
refrigerant ammonia gets vaporized and it leaves the generator. However, since water has
strong affinity for ammonia and its vaporization point is quite low some water particles also
get carried away with ammonia refrigerant, so it is important to pass this refrigerant
through analyzer.
5) Analyzer: One of the major disadvantages of the ammonia-water vapor absorption
refrigeration system is that the water in the solution has quite low vaporizing temperature,
hence when ammonia refrigerant gets vaporized in the generator some water also gets
vaporized. Thus the ammonia refrigerant leaving the generator carries appreciable amount
of water vapor. If this water vapor is allowed to be carried to the evaporator, the capacity of
the refrigeration system would reduce. The water vapor from ammonia refrigerant is
removed by analyzer and the rectifier. The analyzer is a sort of the distillation column that is
located at the top of the generator. The analyzer consists of number of plates positioned
horizontally. When the ammonia refrigerant along with the water vapor particles enters the
analyzer, the solution is cooled. Since water has higher saturation temperature, water vapor
gets condensed into the water particles that drip down into the generator. The ammonia
refrigerant in the gaseous state continues to rise up and it moves to the rectifier.
6) Rectifier or the reflex condenser: The rectifier is a sort of the heat exchanger cooled by
the water, which is also used for cooling the condenser. Due to cooling the remaining water
vapor mixed with the ammonia refrigerant also gets condensed along with some particles of
ammonia. This weak solution of water and ammonia drains down to the analyzer and then
to the generator.
7) Condenser and expansion valve: The pure ammonia refrigerant in the vapor state and at
high pressure then enters the condenser where it is cooled by water. The refrigerant
ammonia gets converted into the liquid state and it then passes through the expansion valve
where its temperature and pressure falls down suddenly. Ammonia refrigerant finally enters
the evaporator, where it produces the cooling effect. This cycle keeps on repeating
continuously. Meanwhile, when ammonia gets vaporized in the generator, weak solution of
ammonia and water is left in it. This solution is expanded in the expansion valve and passed
back to the absorber and its cycle repeats.
Lithium Bromide-Water vapour absorption refrigeration system:
1)
This refrigeration system is used for large tonnage capacity. In this system, lithiumbromide is acting as the absorbent and water is acting as refrigerant. Thus in the absorber the
lithium bromide absorbent absorbs the water refrigerant and solution of water and lithium
bromide is formed. This solution is pumped by the pump to the generator where the solution
is heated. The water refrigerant gets vaporized and moves to the condenser where it is heated
while lithium bromide flows back to the absorber where it further absorbs water coming from
the evaporator. The water-lithium bromide vapor absorption system is used in a number of air
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conditioning applications. This system is useful for the applications where the temperature
required is more than 32 degree F.
Special Features of Water-Lithium Bromide Solution
Here are some special features of the water and lithium bromide in absorption refrigeration
system:
1) As such lithium bromide has great affinity for water vapor, however, when the waterlithium bromide solution is formed, they are not completely soluble with each other under
all the operating conditions of the absorption refrigeration system. Hence, when the
water-lithium bromide absorption refrigeration system is being designed, the designer
must take care that such conditions would not be created where the crystallization and
precipitation of lithium bromide would occur.
2) The water used as the refrigerant in the absorption refrigeration system means the
operating pressures in the condenser and the evaporator would be very low. Even the
difference of pressure between the condenser and the evaporator are very low, and this
can be achieved even without installing the expansion valve in the system, since the
drop in pressure occurs due to friction in the refrigeration piping and also in the spray
nozzles.
3) The capacity of any absorption refrigeration system depends on the ability of the
absorbent to absorb the refrigerant, which in turn depends on the concentration of the
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absorbent. To increase the capacity of the system, the concentration of absorbent
should be increased, which would enable absorption of more refrigerant. Some of the
most common methods used to change the concentration of the absorbent are:
controlling the flow of the steam or hot water to the generator, controlling the flow of
water used for condensing in the condenser, and re-concentrating the absorbent leaving
the generator and entering the absorber.
Parts of the Water-Lithium Bromide Absorption Refrigeration and their Working
Let us see various parts of the water-lithium bromide absorption refrigeration and their
working (please refer the figure above):
1) Evaporator: Water as the refrigerant enters the evaporator at very low pressure and
temperature. Since very low pressure is maintained inside the evaporator the water exists
in the partial liquid state and partial vapor state. This water refrigerant absorbs the heat
from the substance to be chilled and gets fully evaporated. It then enters the absorber.
2) Absorber: In the absorber concentrated solution of lithium bromide is already
available. Since water is highly soluble in lithium bromide, solution of water-lithium
bromide is formed. This solution is pumped by the pump to the generator.
3) Generator: The heat is supplied to the refrigerant water and absorbent lithium
bromide solution in the generator from the steam or hot water. Due to heating water gets
vaporized and it moves to the condenser, where it gets cooled. As water refrigerant
moves further in the refrigeration piping and though nozzles, it pressure reduces and so
also the temperature. This water refrigerant then enters the evaporator where it produces
the cooling effect. This cycle is repeated continuously. Lithium bromide on the other
hand, leaves the generator and reenters the absorber for absorbing water refrigerant. As
seen in the image above, the condenser water is used to cool the water refrigerant in the
condenser and the water-Li Br solution in the absorber. Steam is used for heating waterLi Br solution in the generator. To change the capacity of this water-Li Br absorption
refrigeration system the concentration of Li Br can be changed.
REFRIGERANTS
A ‘refrigerant’ is defined as any substance that absorbs heat through expansion or
vaporisation and loses it through condensation in a refrigeration system. The term
‘refrigerant’ in the broadest sense is also applied to such secondary cooling mediums as cold
water or brine, solutions. Usually refrigerants include only those working mediums which
pass through the cycle of evaporation, recovery, compression, condensation and liquification.
These substances absorb heat at one place at low temperature level and reject the same at
some other place having higher temperature and pressure. The rejection of heat takes place at
the cost of some mechanical work. Thus circulating cold mediums and cooling mediums
(such as ice and solid carbondioxide) are not primary refrigerants. In the early days only four
refrigerants, Air, ammonia (NH3), Carbon dioxide (CO2), Sulphur dioxide (SO2), possessing
chemical, physical and thermodynamic properties permitting their efficient application and
service in the practical design of refrigeration equipment were used. All the refrigerants
change from liquid state to vapour state during the process.
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Desirable properties of an ideal refrigerant
An ideal refrigerant should possess the following properties :
1. Thermodynamic properties :
(i) Low boiling point
(ii) Low freezing point
(iii) Positive pressures (but not very high) in condenser and evaporator.
(iv) High saturation temperature
(v) High latent heat of vapourisation.
2. Chemical Properties :
(i) Non-toxicity
(ii) Non-flammable and non-explosive
(iii) Non-corrosiveness
(iv) Chemical stability in reacting
(v) No effect on the quality of stored (food and other) products like flowers, with other
materials i.e., furs and fabrics.
(vi) Non-irritating and odourless.
3. Physical Properties :
(i) Low specific volume of vapour
(ii) Low specific heat
(iii) High thermal conductivity
(iv) Low viscosity
(v) High electrical insulation.
4. Other Properties :
(i) Ease of leakage location
(ii) Availability and low cost
(iii) Ease of handling
(iv) High C.O.P.
(v) Low power consumption per tonne of refrigeration.
(vi) Low pressure ratio and pressure difference.
Some important properties (mentioned above) are discussed below :
Freezing point. As the refrigerant must operate in the cycle above its freezing point, it is
evident that the same for the refrigerant must be lower than system temperatures. It is found
that except in the case of water for which the freezing point is 0°C, other refrigerants have
reasonably low values. Water, therefore, can be used only in air-conditioning applications
which are above 0°C.
Condenser and evaporator pressures. The evaporating pressure should be as near
atmospheric as possible. If it is too low, it would result in a large volume of the suction
vapour. If it is too high, overall high pressures including condenser pressure would result
necessitating stronger equipment and consequently higher cost. A positive pressure is
required in order to eliminate the possibility of the entry of air and moisture into the system.
The normal boiling point of the refrigerant should, therefore, be lower than the refrigerant
temperature.
Critical temperature and pressure. Generally, for high C.O.P. the critical temperature
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should be very high so that the condenser temperature line on p-h diagram is far removed
from the critical point. This ensures reasonable refrigerating effect as it is very small with the
state of liquid before expansion near the critical point. The critical pressure should be low so
as to give low condensing pressure.
Latent heat of vapourisation. It should be as large as possible to reduce the weight of the
refrigerant to be circulated in the system. This reduces initial cost of the refrigerant. The size
of the system will also be small and hence low initial cost.
Toxicity. Taking into consideration comparative hazard to life due to gases and
vapoursunderwriters Laboratories have divided the compounds into six groups. Group six
contains compounds with a very low degree of toxicity. It includes R12, R114, R13, etc.
Group one, at the other end of the scale, includes the most toxic substances such as SO2.
Ammonia is not used in comfort air-conditioning and in domestic refrigeration because of
inflammability and toxicity.
Inflammability. Hydrocarbons (e.g. methane, ethane etc.) are highly explosive and
inflammable. Fluorocarbons are neither explosive nor inflammable. Ammonia is explosive in
a mixture with air in concentration of 16 to 25% by volume of ammonia.
Volume of suction vapour. The size of the compressor depends on the volume of suction
vapour per unit (say per tonne) of refrigeration. Reciprocating compressors are used with
refrigerants with high pressures and small volumes of the suction vapour. Centrifugal or
turbocompressors are used with refrigerants with low pressures and large volumes of the
suction vapour. A high volume flow rate for a given capacity is required for centrifugal
compressors to permit flow passages of sufficient width to minimise drag and obtain high
efficiency.
Thermal conductivity. For a high heat transfer co-efficient a high thermal conductivity is
desirable. R22 has better heat transfer characteristics than R12 ; R21 is still better, R13 has
poor heat transfer characteristics.
Viscosity. For a high heat transfer co-efficient a low viscosity is desirable.
Leak tendency. The refrigerants should have low leak tendency. The greatest drawback of
fluorocarbons is the fact that they are odourless. This, at times, results in a complete loss of
costly gas from leaks without being detected. An ammonia leak can be very easily detected
by pungent odour.
Refrigerant cost. The cost factor is only relevant to the extent of the price of the initial
charge of the refrigerant which is very small compared to the total cost of the plant and its
installation. The cost of losses due to leakage is also important. In small-capacity units
requiring only a small charge of the refrigerant, the cost of refrigerant is immaterial. The
cheapest refrigerant is Ammonia. R12 is slightly cheaper than R22. R12 and R22 have
replaced ammonia in the dairy and frozen food industry (and even in cold storages) because
of the tendency of ammonia to attack some food products.
Co-efficient of performance and power per tonne. Practically all common refrigerants
have approximately same C.O.P. and power requirement.
Action with oil. No chemical reaction between refrigerant and lubricating oil of the
compressor should take place. Miscibility of the oil is quite important as some oil should be
carried out of the compressor crankcase with the hot refrigerant vapour to lubricate the
pistons and discharge valves properly.
Reaction with materials of construction. While selecting a material to contain the
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refrigerant this material should be given a due consideration. Some metals are attacked by the
refrigerants ; e.g. ammonia reacts with copper, brass or other cuprous alloys in the presence
of water, therefore in ammonia systems the common metals used are iron and steel. Freon
group does not react with steel, copper, brass, zinc, tin and aluminium but is corrosive to
magnesium and aluminium having magnesium more than 2%. Freon group refrigerants tend
to dissolve natural rubber in packing and gaskets but synthetic rubber such as neoprene are
entirely suitable. The hydrogenerated hydrocarbons may react with zinc but not with copper,
aluminium, iron and steel.
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UNIT III
PSYCHROMETRY AND AIR – CONDITIONING
Psychrometric properties, Use of psychrometric chart, Psychrometric process – Sensible heat
exchange process, Latent heat exchange process, Adiabatic mixing, Evaporative cooling,
Property calculations of air-vapour mixtures.
Principles of air-conditioning, Types of air conditioning systems – summer, winter, year
round air conditioners, Concept of RSHF, GSHF, ESHF, Simple problems.
CONCEPT OF PSYCHROMETRY AND PSYCHROMETRICS
Air comprises of fixed gases principally, nitrogen and oxygen with an admixture of water
vapour in varying amounts. In atmospheric air water is always present and its relative weight
averages less than 1% of the weight of atmospheric air in temperate climates and less than
3% by weight under the most extreme natural climatic conditions, it is nevertheless one of
most important factors in human comfort and has significant effects on many materials. Its
effect on human activities is in fact altogether disproportionate to its relative weights. The art
of measuring the moisture content of air is termed “psychrometry”. The science which
investigates the thermal properties of moist air, considers the measurement and control of the
moisture content of air, and studies the effect of atmospheric moisture on material and
human comfort may properly be termed “psychrometrics’’.
DEFINITIONS
Some of the more important definitions are given below :
1. Dry air. The international joint committee on Psychrometric Data has adopted the
following exact composition of air expressed in mole fractions (Volumetric) Oxygen 0.2095,
Nitrogen 0.7809, Argon 0.0093, Carbon dioxide 0.0003. Traces of rare gases are neglected.
Molecular weight of air for all air conditioning calculations will be taken as 28.97. Hence the
gas constant, Rair = 0.287 kJ/kg K Dry air is never found in practice. Air always contains
some moisture. Hence the common designation “air” usually means moist air. The term ‘dry
air’ is used to indicate the water free contents of air having any degree of moisture.
2. Saturated air. Moist air is said to be saturated when its condition is such that it can coexist in natural equilibrium with an associated condensed moisture phase presenting a flat
surface to it. For a given temperature, a given quantity of air can be saturated with a fixed
quantity of moisture. At higher temperatures, it requires a larger quantity of moisture to
saturate it. At saturation, vapour pressure of moisture in air corresponds to the saturation
pressure given in steam tables corresponding to the given temperature of air.
3. Dry-bulb temperature (DBT). It is the temperature of air as registered by an ordinary
thermometer (tdb).
4. Wet-bulb temperature (WBT). It is the temperature registered by a thermometer when
the bulb is covered by a wetted wick and is exposed to a current of rapidly moving air (twb).
5. Adiabatic saturation temperature. It is the temperature at which the water or ice can
saturate air by evaporating adiabatically into it. It is numerically equivalent to the measured
wet bulb temperature (as corrected, if necessary for radiation and conduction) (twb).
6. Wet bulb depression. It is the difference between dry-bulb and wet bulb temperatures (tdb
– twb).
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7. Dew point temperature (DPT). It is the temperature to which air must be cooled at
constant pressure in order to cause condensation of any of its water vapour. It is equal to
steam table saturation temperature corresponding to the actual partial pressure of water
vapour in the air (tdp).
8. Dew point depression. It is the difference between the dry bulb and dew point
temperatures (tdb – tdp).
9. Specific humidity (Humidity ratio). It is the ratio of the mass of water vapour per unit
mass of dry air in the mixture of vapour and air, it is generally expressed as grams of water
per kg of dry air. For a given barometric pressure it is a function of dew point temperature
alone.
10. Relative humidity (RH), (φ). It is the ratio of the partial pressure of water vapour in the
mixture to the saturated partial pressure at the dry bulb temperature, expressed as
percentage.
11. Sensible heat. It is the heat that changes the temperature of a substance when added
to or abstracted from it.
12. Latent heat. It is the heat that does not affect the temperature but changes the state
of substance when added to or abstracted from it.
13. Enthalpy. It is the combination energy which represents the sum of internal and flow
energy in a steady flow process. It is determined from an arbitrary datum point for the air
mixture and is expressed as kJ per kg of dry air (h).
Note. When air is saturated DBT, WBT, DPT are equal.
PSYCHROMETRIC RELATIONS
Pressure
Dalton’s law of partial pressure is employed to determine the pressure of a mixture of gases.
This law states that the total pressure of a mixture of gases is equal to the sum of partial
pressures which the component gases would exert if each existed alone in the mixture volume
at the mixture temperature. Precise measurements made during the last few years indicate
that this law as well as Boyle’s and Charle’s laws are only approximately correct. Modern
tables of atmospheric air properties are based on the correct versions. For calculating partial
pressure of water vapour in the air many equations have been proposed, probably Dr.
Carrier’s equation is most widely used.
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thermal equilibrium exists with respect to water, air and water vapour, and consequently the
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air is saturated. The equilibrium temperature is called the adiabatic saturation temperature
or the thermodynamic wet bulb temperature. The make-up water is introduced at this
temperature to make the water level constant. The ‘adiabatic’ cooling process is shown in
Fig. 10.2 for the vapour in the air-vapour mixture. Although the total pressure of the mixture
is constant, the partial pressure of the vapour increases, and in the saturated state corresponds
to the adiabatic saturation temperature. The vapour is initially at DBT tdb1 and is cooled
adiabatically to DBT tdb2 which is equal to the adiabatic saturation twb2 . The adiabatic
saturation temperature and wet bulb temperatures are taken to be equal for all practical
purposes. The wet bulb temperature lies between the dry bulb temperature and dew point
temperature.
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PSYCHROMETRIC CHARTS
The psychrometric charts are prepared to represent graphically all the necessary moist air
properties used for air conditioning calculations. The values are based on actual
measurements verified for thermodynamic consistency. For psychrometric charts the most
convenient co-ordinates are dry bulb temperature of air vapour mixture as the abcissa and
moisture content (kg/kg of dry air) or water vapour pressure as the ordinate. Depending upon
whether the humidity contents is abcissa or ordinate with temperature co-ordinate, the charts
are generally classified as Mollier chart and Carrier chart. Carrier chart having tdb as the
abcissa and W as the ordinate finds a wide application.
The chart is constructed as under :
1. The dry bulb temperature (ºC) of unit mass of dry air for different humidity contents or
humidity ratios are indicated by vertical lines drawn parallel to the ordinate.
2. The mass of water vapour in kg (or grams) per kg of dry air is drawn parallel to the abcissa
for different values of dry bulb temperature. It is the major vertical scale of the chart.
3. Pressure of water vapour in mm of mercury is shown in the scale at left and is the absolute
pressure of steam.
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4. Dew point temperatures are temperatures corresponding to the boiling points of water at
low pressures of water vapour and are shown in the scale on the upper curved line. The dew
points for different low pressures are read on diagonal co-ordinates.
5. Constant relative humidity lines in per cent are indicated by marking off vertical distances
between the saturation line or the upper curved line and the base of the chart. The relative
humidity curve depicts quantity (kg) of moisture actually present in the air as a percentage of
the total amount possible at various dry bulb temperatures and masses of vapour.
6. Enthalpy or total heat at saturation temperature in kJ/kg of dry air is shown by a diagonal
system of co-ordinates. The scale on the diagonal line is separate from the body of the chart
and is indicated above the saturation line.
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7. Wet bulb temperatures are shown on the diagonal co-ordinates coinciding with heat
coordinates. The scale of wet bulb temperatures is shown on the saturation curve. The
diagonals run downwards to the right at an angle of 30º to the horizontal.
8. The volume of air vapour mixture per kg of dry air (specific volume) is also indicated by a
set of diagonal co-ordinates but at an angle of 60º with the horizontal. The other properties of
air vapour mixtures can be determined by using formulae (already discussed).
In relation to the psychrometric chart, these terms can quickly indicate many things about
the condition of air, for example :
1. If dry bulb and wet bulb temperatures are known, the relative humidity can be read from
the chart.
2. If the dry bulb and relative humidity are known, the wet bulb temperature can be
determined.
3. If wet bulb temperature and relative humidity are known, the dry bulb temperature can
be found.
4. If wet bulb and dry bulb temperatures are known, the dew point can be found.
5. If wet bulb and relative humidity are known, dew point can be read from the chart.
6. If dry-bulb and relative humidity are known, dew point can be found.
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7. The quantity (kg) of moisture in air can be determined from any of the following
combinations :
(i) Dry bulb temperature and relative humidity ;
(ii) Dry bulb temperature and dew point ;
(iii) Wet bulb temperature and relative humidity ;
(iv) Wet bulb temperature and dew point temperature ;
(v) Dry bulb temperature and wet bulb temperature ; and
(vi) Dew point temperature alone.
Figs. 10.4 and 10.5 show the skeleton psychrometric chart and lines on carrier chart
respectively.
PSYCHROMETRIC PROCESSES
In order to condition air to the conditions of human comfort or of the optimum control of an
industrial process required, certain processes are to be carried out on the outside air available.
The processes affecting the psychrometric properties of air are called psychrometric
processes.
These processes involve mixing of air streams, heating, cooling, humidifying,
dehumidifying, adiabatic saturation and mostly the combinations of these.
The important psychrometric processes are enumerated and explained in the following text
1. Mixing of air streams
2. Sensible heating
3. Sensible cooling
4. Cooling and dehumidification
5. Cooling and humidification
6. Heating and dehumidification
7. Heating and humidification.
Mixing of Air Streams
Refer Figs. 10.6 and 10.7. Mixing of several air streams is the process which is very
frequently used in air conditioning. This mixing normally takes place without the addition or
rejection of
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Process 1-2 : It denotes the cases in which the temperature of the heated spray water
is less than the air DBT.
Process 1-3 : It denotes the cases in which the temperature is equal to the air DBT.
Process 1-4 : It denotes the cases in which a spray temperature is greater than air DBT. As in
the case of adiabatic saturation, the degree to which the process approaches saturation can be
expressed in terms of the by-pass factor or a saturating efficiency.
If the water rate relative to the air quantity is smaller, the water temperature will drop
significantly during the process. The resultant process will be a curved line such as the
dashed 1-4 where 4 represents the leaving water temperature.
Note. It is possible to accomplish heating and humidification by evaporation from an open
pan of heated water, or by direct injection of heated water or steam. The latter is more
common. The process line for it is of little value because the process is essentially an
instantaneous mixing of steam and the air. The final state point of the air can be found,
however by making a humidity and enthalpy balance for the process. The solution of such a
problem usually involves cut-and-try procedure.
AIR CONDITIONING SYSTEMS
Air conditioning systems require basic arrangement for getting refrigeration effect through
ooling coil followed by subsequent humidification/dehumidification and heating etc. in order
to provide air conditioned space with air at desired temperature and humidity. Air
conditioning systems require different arrangements depending upon the atmospheric air
condition and comfort condition requirement. Such as summer air conditioning systems and
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inter air conditioning systems are different. These systems have different arrangement if
outdoor conditions are hot and humid, hot and dry etc. Summer air conditioning system for
hot and dry outdoor condition is given in Fig. 18.20. Here the comfort conditions may require
delivery of air to air-conditioned space at about 25ºC DBT and 60% relative humidity where
the outdoor conditions may be up to 40–44º C DBT and 20% relative humidity in Indian
conditions. Generic arrangement has air blower which blows air across the air filter between
(1) and (2). Air coming out from filter passes over cooling coils and is subsequently sent for
humidification between states (3) and (4). Large size water particles carried by air are
retained by water eliminator. Air finally coming out at state (5) is sent to air conditioned
space. Here psychrometric representation is made considering negligible change in humidity
in water eliminator.
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UNIT IV
AIR COMPRESSORS
Classification and working principle, Work of compression with and with-out clearance,
volumetric, iso-thermal and isentopic efficiencies of reciprocating air-compressors, Multistage compression and intercooling, Work of Multi-stage compressor.
Rotary compressors, Concept of positive displacement, Roots blower, Vane type blower,
Screw compressor, Axial flow and centrifugal compressors (Description only)
INTRODUCTION
Compressors are work absorbing devices which are used for increasing pressure of fluid at
the expense of work done on fluid. The compressors used for compressing air are called air
compressors. Compressors are invariably used for all applications requiring high pressure air.
Some of popular applications of compressor are, for driving pneumatic tools and air operated
equipments, spray painting, compressed air engine, supercharging in internal combustion
engines, material handling (for transfer of material), surface cleaning, refrigeration and air
conditioning, chemical industry etc. Compressors are supplied with low pressure air (or any
fluid) at inlet which comes out as high pressure air (or any fluid) at outlet, Fig. 16.1. Work
required for increasing pressure of air is available from the prime mover driving the
compressor. Generally, electric motor, internal combustion engine or steam engine, turbine
etc. are used as prime movers. Compressors are similar to fans and blowers but differ in terms
of pressure ratios. Fan is said to have pressure ratio up to 1.1 and blowers have pressure ratio
between 1.1 and 4 while compressors have pressure ratios
more than 4.
Compressors can be classified in the following different ways.
(a) Based on principle of operation: Based on the principle of operation compressors can be
classified as,
(i) Positive displacement compressors
(ii) Non-positive displacement compressors
In positive displacement compressors the compression is realized by displacement of solid
boundary and preventing fluid by solid boundary from flowing back in the direction of
pressure gradient. Due to solid wall displacement these are capable of providing quite large
pressure ratios. Positive displacement compressors can be further classified based on the type
of mechanism used for compression. These can be (i) Reciprocating type positive
displacement compressors (ii) Rotary type positive displacement compressors
Reciprocating compressors generally, employ piston-cylinder arrangement where
displacement of piston in cylinder causes rise in pressure. Reciprocating compressors are
capable of giving large pressure ratios but the mass handling capacity is limited or small.
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Reciprocating compressors may also be single acting compressor or double acting
compressor. Single acting compressor has one delivery stroke per revolution while in double
acting there are two delivery strokes per revolution of crank shaft. Rotary compressors
employing positive displacement have a rotary part whose boundary causes positive
displacement of fluid and thereby compression. Rotary compressors of this type are available
in the names as given below;
(i) Roots blower
(ii) Vaned type compressors
Rotary compressors of above type are capable of running at higher speed and can handle
large mass flow rate than reciprocating compressors of positive displacement type. Nonpositive displacement compressors, also called as steady flow compressors use dynamic
action of solid boundary for realizing pressure rise. Here fluid is not contained in definite
volume and subsequent volume reduction does not occur as in case of positive displacement
compressors. Nonpositive displacement compressor may be of ‘axial flow type’ or
‘centrifugal type’ depending upon type of flow in compressor.
(b) Based on number of stages: Compressors may also be classified on the basis of number
of stages. Generally, the number of stages depend upon the maximum delivery pressure.
Compressors can be single stage or multistage. Normally maximum compression ratio of 5
is realized in single stage compressors. For compression ratio more than 5 the multi-stage
compressors are used. Typical values of maximum delivery pressures generally available
from different types of compressor are,
(i) Single stage compressor, for delivery pressure up to 5 bar
(ii) Two stage compressor, for delivery pressure between 5 and 35 bar
(iii) Three stage compressor, for delivery pressure between 35 and 85 bar
(iv) Four stage compressor, for delivery pressure more than 85 bar
(c) Based on capacity of compressors: Compressors can also be classified depending upon
the capacity of compressor or air delivered per unit time. Typical values of capacity for
different compressors are given as;
(i) Low capacity compressors, having air delivery capacity of 0.15 m3/s or less
(ii) Medium capacity compressors, having air delivery capacity between 0.15 and 5 m3/s.
(iii) High capacity compressors, having air delivery capacity more than 5 m3/s.
(d) Based on highest pressure developed: Depending upon the maximum pressure available
from compressor they can be classified as low pressure, medium pressure, high pressure and
super high pressure compressors. Typical values of maximum pressure developed for
different compressors are as under;
(i) Low pressure compressor, having maximum pressure up to 1 bar
(ii) Medium pressure compressor, having maximum pressure from 1 to 8 bar
(iii) High pressure compressor, having maximum pressure from 8 to 10 bar
(iv) Super high pressure compressor, having maximum pressure more than 10 bar.
RECIPROCATING COMPRESSORS
Reciprocating compressor has piston cylinder arrangement as shown in Fig. 16.2.
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Reciprocating compressor has piston, cylinder, inlet valve, exit valve, connecting rod, crank,
piston pin, crank pin and crank shaft. Inlet valve and exit valves may be of spring loaded type
which get opened and closed due to pressure differential across them. Let us consider piston
to be at top dead centre (TDC) and move towards bottom dead centre (BDC). Due to this
piston movement from TDC to BDC suction pressure is created causing opening of inlet
valve. With this opening of inlet valve and suction pressure the atmospheric air enters the
cylinder. Air gets into cylinder during this stroke and is subsequently compressed in next
stroke with both inlet valve and exit valve closed. Both inlet valve and exit valves are of plate
type and spring loaded so as to operate automatically as and when sufficient pressure
difference is available to cause deflection in spring of valve plates to open them. After piston
reaching BDC it reverses its motion and compresses the air inducted in previous stroke.
Compression is continued till the pressure of air inside becomes sufficient to cause deflection
in exit valve. At the moment when exit valve plate gets lifted the exhaust of compressed air
takes place. This piston again reaches TDC from where downward piston movement is again
accompanied by suction. This is how reciprocating compressor keeps on working as flow
device. In order to counter for the heating of piston-cylinder arrangement during compression
the provision of cooling the cylinder is there in the form of cooling jackets in the body.
Reciprocating compressor described above has suction, compression and discharge as three
prominent processes getting completed in two strokes of piston or one revolution of crank
shaft.
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The isothermal efficiency of a compressor should be close to 100% which means that actual
compression should occur following a process close to isothermal process. For this the
mechanism be derived to maintain constant temperature during compression process.
Different arrangements which can be used are:
(i) Faster heat dissipation from inside of compressor to outside by use of fins over cylinder.
Fins facilitate quick heat transfer from air being compressed to atmosphere so that
temperature rise during compression can be minimized.
(ii) Water jacket may be provided around compressor cylinder so that heat can be picked by
cooling water circulating through water jacket. Cooling water circulation around compressor
regulates rise in temperature to great extent.
(iii) The water may also be injected at the end of compression process in order to cool the air
being compressed. This water injection near the end of compression process requires special
arrangement in compressor and also the air gets mixed with water and needs to be separated
out before being used. Water injection also contaminates the lubricant film on inner surface
of cylinder and may initiate corrosion etc. The water injection is not popularly used.
(iv) In case of multistage compression in different compressors operating serially, the air
leaving one compressor may be cooled up to ambient state or somewhat high temperature
before being injected into subsequent compressor. This cooling of fluid being compressed
between two consecutive compressors is called intercooling and is frequently used in case of
multistage compressors.
Considering clearance volume: With clearance volume the cycle is represented on Fig. 16.3
(b)The work done for compression of air polytropically can be given by the area enclosed in
cycle 1–2–3–
4. Clearance volume in compressors varies from 1.5% to 35% depending upon type of
compressor.
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ROTARY COMPRESSORS
Rotary compressors are those compressors in which rotating action is used for compression
of fluid. Rotary air compressors have capability of running at high speeds up to 40,000 rpm
and can be directly coupled to any prime mover such as electric motor, turbine etc. due to
compact design, no balancing problem and less no. of sliding parts. Comparative study of
rotary compressor with reciprocating compressor shows that rotary compressors can be used
for delivering large quantity of air but the maximum pressure at delivery is less compared to
reciprocating compressors. Generally, rotary compressors can yield delivery pressure up to
10 bar and free air delivery of 3000 m3/min. Rotary compressors are less bulky, and offer
uniform discharge compared to reciprocating compressor even in the absence of big size
receiver. Lubrication requirement and wear and tear is less due to rotary motion of parts in
rotary compressors compared to reciprocating compressors. Rotary compressors may work
on the principle of positive displacement and dynamic action both. Rotary compressors
having positive displacement may be of following types:
(i) Roots blower
(ii) Screw type or Helical type compressor
(iii) Vane type compressor
Rotary compressors employing dynamic action may be of centrifugal type or axial type
epending upon the direction of flow. These centrifugal type or axial compressors may also be
termed as nonpositive displacement type steady flow compressors.
(i) Roots blower: Roots blower is a positive displacement type rotary compressor. It has two
rotors having two or three lobes having epicycloid and hypocycloid or involute profiles such
that they remain in proper contact. Figure 16.13 shows two lobe rotors in a roots blower. To
prevent wear and tear two rotors have clearance in between. Out of two rotors one is driven
by prime mover while other one is driven by first rotor. When two rotors rotate then their
typical geometry divides the region inside casing into two regions i.e. high pressure region
and low pressure region. Although there occurs slight leakage across the mating parts which
can only be minimised not eliminated completely.
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Figure 16.13 b shows the general arrangement in roots blower. It has inlet at section 1–1 and
exit at 2–2. Air at atmospheric pressure enters the casing and is trapped between rotor A and
the casing. When the rotor rotate then air trapped in volume space V is displaced towards
high pressure region due to rotation of rotor. Exit end is connected to receiver in which air is
gradually transferred and the pressure inside receiver increases due to cumulative effect of air
being transferred from atmospheric pressure region to receiver region. In one revolution this
positive displacement of air trapped between rotor and casing from inlet end to receiver end
shall occur four times in case of two lobe rotor as shown. While in case of three lobes rotor
this transfer shall occur six times. Every time when V volume of air is displaced without
being compressed to the receiver side high pressure region, then the high pressure air rushes
back from receiver and mixes irreversibly with this air until the pressure gets equalized. Thus,
gradually air pressure builds up and say this pressure becomes p2. For inlet air pressure being
p1.
(ii) Screw type or Helical type compressor: Screw type compressor is very much similar to
roots blower.
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These may have two spiral lobed rotors, out of which one may be called male rotor having 3–
4 lobes and other female rotor having 4–6 lobes which intermesh with small clearance.
Meshing is such that lobes jutting out of male rotor get placed in matching hollow portion in
female rotors. Initially, before this intermeshing the hollows remain filled with gaseous fluid
at inlet port. As rotation begins the surface in contact move parallel to the axis of rotors
toward the outlet end gradually compressing the fluid till the trapped volume reaches up to
outlet port for getting discharged out at designed pressure. Since the number of lobes are
different so the rotors operate at different speed. The material of casing may be cast iron or
cast steel while rotors may be of steel and generally internally cooled by circulation of
lubrication oil. Surface of lobes are smooth and the shaft is sealed by carbon rings at oil
pressure. Two rotors are brought into synchronization by the screw gears. Thrust upon rotors
is taken care of by oil lubricated thrust bearings. These compressors are capable of handling
gas flows ranging from 200 to 20000 m3/h under discharge pressures of 3 bar gauge in single
stage and up to 13 bar gauge in two stages. Even with increase in number of stages pressures
up to 100 bar absolute have been obtained with stage pressure ratio of 2. Mechanical
efficiency of these compressors is quite high and their isothermal efficiencies are even more
than vane blowers and may be compared with centrifugal and axial compressors. But these
are very noisy, sensitive to dust and fragile due to small clearances.
(iii) Vane type compressor: Schematic of vane type compressor is shown in Fig. 16.15. It has
cylindrical casing having an eccentrically mounted rotor inside it. The rotor has number of
slots in it with rectangular vanes of spring loaded type mounted in slots. These vanes are
generally non metallic and made of fibre or carbon composites or any other wear resistant
material. These vanes remain in continuous contact with casing such that leakage across the
vane-casing interface is minimum or absent. It has one end as inlet end and other as the
delivery end connected to receiver. Upon rotation the eccentric rotor has the vanes having
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differential projection out of rotor depending upon their position. Air is trapped between each
set of two consecutive blades in front of inlet passage and is positively displaced to the
delivery end after compressing the volume V1 initially to V2, V3 and V4. When compressed
volume comes in front of delivery passage and further rotation results in the situation when
partly compressed air is forced to enter the receiver as their is no other way out. This
cumulative transfer of partly compressed air in receiver causes irreversible compression
resulting in gradual pressure rise. The p-V representation shown in Fig. 16.15 (b) indicates
that the total pressure rise is due to the combined effect of reversible pressure rise inside
casing and irreversible pressure rise inside receiver. Generally, the contribution of reversible
pressure rise and irreversible pressure rise is in proportion of 50 : 50.
Vane compressors are available for capacity up to 150 m3/min and pressure ratios up to 8
and efficiency up to 75%. For higher pressure ratios the efficiency of vane compressors is
more than that of roots blower but the vane compressors have maximum speed up to 2500
rpm as compared to 7500 rpm in case of roots blower. Vane compressors have large power
requirement as compared to roots blower
CENTRIFUGAL COMPRESSORS
Centrifugal compressor is a radial flow machine compressing the fluid due to the dynamic
action of impeller. Centrifugal compressors have impeller mounted on driving shaft, diffuser
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and volute casing as shown in Fig. 16.16. Centrifugal compressors have air inlet at the centre
of impeller. The portion of impeller in front of inlet passage is called impeller eye.
Impeller is a type of disc having radial blades mounted upon it. Compressor casing has a
diffuser ring surrounding impeller and the air enters the impeller eye and leaves from
impeller tip to enter diffuser ring. Volute casing surrounds the diffuser ring. Volute casing
has cross section area increasing gradually up to the exit of compressor. These impellers of
centrifugal compressors may also be of double sided type such that air can enter from two
sides (both) of impeller. Thus double sided impeller shall have double impeller eye compared
to single impeller eye as shown in Fig. 16.17.
Air enters the impeller eye axially and flows radially outwards after having entered
compressor. Radial flow of air inside compressor is due to impeller (blades) rotating about its
axis. These impeller blades impart momentum to the air entering, thereby rising its pressure
and temperature. Subsequently the high pressure fluid leaving impeller enters the diffuser
ring where the velocity of air is lowered with further increase in pressure of air. Thus in
diffuser ring the kinetic energy of air is transformed into pressure head. High pressure air
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leaving diffuser is carried by volute casing to the exit of compressor. Due to increased cross
section area of volute casing some velocity is further reduced causing rise in its pressure,
although this is very small. Total pressure rise in compressor may be due to ‘impeller action’
and ‘diffuser action’ both. Generally, about half of total pressure rise is available in impeller
and remaining half in diffuser. Pressure and velocity variation in centrifugal compressor is
shown in Fig. 16.18.
Centrifugal compressors are used in aircrafts, blowers, superchargers, etc. where large
quantity of air is to be supplied at smaller pressure ratios. Generally, pressure ratio up to 4 is
achieved in single stage centrifugal compressors while in multistage compressors the pressure
ratio up to 12 can be achieved. These compressors run at speed of 20,000–30,000 rpm.
AXIAL FLOW COMPRESSORS
Axial flow compressors have the fixed blades and moving blades mounted along the axis of
compressor. Air enters axially and leaves axially. It has primarily two components i.e. rotor
and casing. The rotor has blades mounted on it constituting moving blade ring. Blades are
also mounted on the inner side of casing thereby constituting stages as fixed blade ring
followed by moving blade ring followed by fixed blade ring, moving blade ring and so on.
Due to the reduction in volume the volume space for compressed air may be gradually
reduced. Gradual reduction in volume can be done by flaring the rotor while keeping stator
diameter uniform or by flaring the stator while keeping rotor diameter constant as shown in
fig. 16.21. The pressure of fluid entering the axial flow compressor increases upon passing
through the fixed and moving blades. This flow of fluid over moving blades is accompanied
by enthalpy rise while the fixed blades merely deflect the fluid so as to facilitate smooth entry
into moving blades. Absolute velocity of air increases along axis of rotor due to work input
from the prime mover. Relative velocity of air decreases during its flow through rotor. Blades
have aerofoil section so as to have minimum losses due to turbulence, boundary layer
formation and separation, eddy formation etc.
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UNIT V
FUELS AND COMBUSTION
Solid, Liquid and Gaseous fuels, Combustion process, Enthalpy of formation, Enthalpy and
internal energy of combustion, Higher and lower heating values, Adiabatic combustion
temperature, First law analysis of Reacting systems.
Combustion equation, Stochiometric air fuel ratio, Excess air, Composition of combustion
products, Analysis of combustion products, Air-fuel ratio from analysis of combustion
products.
CLASSIFICATION OF FUELS
Fuels can be classified according to whether :
1. They occur in nature called primary fuels or are prepared called secondary fuels ;
2. They are in solid, liquid or gaseous state
SOLID FUELS
Coal. Its main constituents are carbon, hydrogen, oxygen, nitrogen, sulphur, moisture and
ash. Coal passes through different stages during its formation from vegetation. These stages
are enumerated and discussed below :
Plant debris—Peat—Lignite—Brown coal—sub-bituminous coal—Bituminous coal—
Semibituminous coal—Semi-anthracite coal—Anthracite coal—Graphite.
Peat. It is the first stage in the formation of coal from wood. It contains huge amount of
moisture and therefore it is dried for about 1 to 2 months before it is put to use. It is used as a
domestic fuel in Europe and for power generation in Russia. In India it does not come in the
categories of good fuels.
Lignites and brown coals. These are intermediate stages between peat and coal. They have a
woody or often a clay like appearance associated with high moisture, high ash and low heat
contents. Lignites are usually amorphous in character and impose transport difficulties as
they break easily. They burn with a smoky flame. Some of this type are suitable for local use
only.
Bituminous coal. It burns with long yellow and smoky flames and has high percentages
of volatile matter. The average calorific value of bituminous coal is about 31350 kJ/kg. It
may be of two types, namely caking or noncaking.
Semi-bituminous coal. It is softer than the anthracite. It burns with a very small amount of
smoke. It contains 15 to 20 per cent volatile matter and has a tendency to break into small
sizes during storage or transportation.
Semi-anthracite. It has less fixed carbon and less lustre as compared to true anthracite and
gives out longer and more luminous flames when burnt.
Anthracite. It is very hard coal and has a shining black lustre. It ignites slowly unless the
furnace temperature is high. It is non-caking and has high percentage of fixed carbon. It burns
either with very short blue flames or without flames. The calorific value of this fuel is high to
the tune of 35500 kJ/kg and as such is very suitable for steam generation.
Wood charcoal. It is obtained by destructive distillation of wood. During the process the
volatile matter and water are expelled. The physical properties of the residue (charcoal),
however depends upon the rate of heating and temperature.
Coke. It consists of carbon, mineral matter with about 2% sulphur and small quantities of
hydrogen, nitrogen and phosphorus. It is solid residue left after the destructive distillation of
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certain kinds of coals. It is smokeless and clear fuel and can be produced by several
processes. It is mainly used in blast furnace to produce heat and at the same time to reduce
the iron ore.
Briquettes. These are prepared from fine coal or coke by compressing the material under
high pressure.
LIQUID FUELS
The chief source of liquid fuels is petroleum which is obtained from wells under the earth’s
crust. These fuels have proved more advantageous in comparison to sold fuels in the
following respects.
Advantages :
1. Require less space for storage.
2. Higher calorific value.
3. Easy control of consumption.
4. Staff economy.
5. Absence of danger from spontaneous combustion.
6. Easy handling and transportation.
7. Cleanliness.
8. No ash problem.
9. Non-deterioration of the oil in storage.
Petroleum. There are different opinions regarding the origin of petroleum. However, now it
is accepted that petroleum has originated probably from organic matter like fish and plant life
etc., by bacterial action or by their distillation under pressure and heat. It consists of a
mixture of gases, liquids and solid hydrocarbons with small amounts of nitrogen and sulphur
compounds. In India, the main sources of Petroleum are Assam and Gujarat. Heavy fuel oil or
crude oil is imported and then refined at different refineries. The refining of crude oil supplies
the most important product called petrol. Petrol can also be made by polymerization of
refinery gases. Other liquid fuels are kerosene, fuels oils, colloidal fuels and alcohol.
GASEOUS FUELS
Natural gas. The main constituents of natural gas are methane (CH4) and ethane (C2H6). It
has calorific value nearly 21000 kJ/m3. Natural gas is used alternately or simultaneously with
oil for internal combustion engines.
Coal gas. Mainly consists of hydrogen, carbon monoxide and hydrocarbons. It is prepared by
carbonisation of coal. It finds its use in boilers and sometimes used for commercial purposes.
Coke-oven gas. It is obtained during the production of coke by heating the bituminous coal.
The volatile content of coal is driven off by heating and major portion of this gas is utilised in
heating the ovens. This gas must be thoroughly filtered before using in gas engines.
Blast furnance gas. It is obtained from smelting operation in which air is forced through
layers of coke and iron ore, the example being that of pig iron manufacture where this gas is
produced as by product and contains about 20% carbon monoxide (CO). After filtering it may
be blended with richer gas or used in gas engines directly. The heating value of this gas is
very low.
Producer gas. It results from the partial oxidation of coal, coke or peat when they are burnt
with an insufficient quantity of air. It is produced in specially designed retorts. It has low
heating value and in general is suitable for large installations. It is also used in steel industry
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for
firing open hearth furnaces.
Water or illuminating gas. It is produced by blowing steam into white hot coke or coal. The
decomposition of steam takes place liberating free hydrogen, and oxygen in the steam
combines with carbon to form carbon monoxide according to the reaction :
C + H2O → CO + H2
The gas composition varies as the hydrogen content if the coal is used.
Sewer gas. It is obtained from sewage disposal vats in which fermentation and decay occur.
It consists of mainly marsh gas (CH4) and is collected at large disposal plants. It works as a
fuel for gas engines which in turn drive the plant pumps and agitators. Gaseous fuels are
becoming popular because of following advantages they possess.
Advantages :
1. Better control of combustion.
2. Much less excess air is needed for complete combustion.
3. Economy in fuel and more efficiency of furnace operation.
4. Easy maintenance of oxidizing or reducing atmosphere.
5. Cleanliness.
6. No problem of storage if the supply is available from public supply line.
7. The distribution of gaseous fuels even over a wide area is easy through the pipe lines and
as such handling of the fuel is altogether eliminated.
8. Gaseous fuels give economy of heat and produce higher temperatures (as they can be
preheated in regenerative furnances and thus heat from hot flue gases can be recovered).
THEORETICAL AND ACTUAL COMBUSTION PROCESSES
It is often instructive to study the combustion of a fuel by assuming that the combustion is
complete. A combustion process is complete if all the carbon in the fuel burns to CO2, all the
hydrogen burns to H2O, and all the sulfur (if any) burns to SO2. That is, all the combustible
components of a fuel are burned to completion during a complete combustion process (Fig.
15–8). Conversely, the combustion process is incomplete if the combustion products contain
any unburned fuel or components such as C, H2, CO, or OH. Insufficient oxygen is an
obvious reason for incomplete combustion, but it is not the only one. Incomplete combustion
occurs even when more oxygen is present in the combustion chamber than is needed for
complete combustion. This may be attributed to insufficient mixing in the combustion
chamber during the limited time that the fuel and the oxygen are in contact. Another cause of
incomplete combustion is dissociation, which becomes important at high temperatures.
Oxygen has a much greater tendency to combine with hydrogen than it does with carbon.
Therefore, the hydrogen in the fuel normally burns to completion, forming H2O, even when
there is less oxygen than needed for complete combustion. Some of the carbon, however,
ends up as CO or just as plain C particles (soot) in the products. The minimum amount of air
needed for the complete combustion of a fuel is called the stoichiometric or theoretical air.
Thus, when a fuel is completely burned with theoretical air, no uncombined oxygen is present
in the product gases. The theoretical air is also referred to as the chemically correct amount of
air, or 100 percent theoretical air. A combustion process with less than the theoretical air is
bound to be incomplete. The ideal combustion process during which a fuel is burned
completely with theoretical air is called the stoichiometric or theoretical combustion of that
fuel (Fig. 15–9).
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For example, the theoretical combustion of methane is Notice that the products of the
theoretical combustion contain no unburned methane and no C, H2, CO, OH, or free O2. In
actual combustion processes, it is common practice to use more air than the stoichiometric
amount to increase the chances of complete combustion or to control the temperature of the
combustion chamber. The amount of air in excess of the stoichiometric amount is called
excess air. The amount of excess air is usually expressed in terms of the stoichiometric air as
percent excess air or percent theoretical air. For example, 50 percent excess air is
equivalent to 150 percent theoretical air, and 200 percent excess air is equivalent to 300
percent theoretical air. Of course, the stoichiometric air can be expressed as 0 percent excess
air or 100 percent theoretical air. Amounts of air less than the stoichiometric amount are
called deficiency of air and are often expressed as percent deficiency of air. For example,
90 percent theoretical air is equivalent to 10 percent deficiency of air. The amount of air used
in combustion processes is also expressed in terms of the equivalence ratio, which is the
ratio of the actual fuel–air ratio
to the stoichiometric fuel–air ratio. Predicting the composition of the products is relatively
easy when the combustion process is assumed to be complete and the exact amounts of the
fuel and air used are known. All one needs to do in this case is simply apply the mass balance
to each element that appears in the combustion equation, without needing to take any
measurements. Things are not so simple, however, when one is dealing with actual
combustion processes. For one thing, actual combustion processes are hardly ever complete,
even in the presence of excess air. Therefore, it is impossible to predict the composition of
the products on the basis of the mass balance alone. Then the only alternative we have is to
measure the amount of each component in the products directly. A commonly used device to
analyze the composition of combustion gases is the Orsat gas analyzer. In this device, a
sample of the combustion gases is collected and cooled to room temperature and pressure, at
which point its
volume is measured. The sample is then brought into contact with a chemical that absorbs the
CO2. The remaining gases are returned to the room temperature and pressure, and the new
volume they occupy is measured. The ratio of the reduction in volume to the original volume
is the volume fraction of the CO2, which is equivalent to the mole fraction if ideal-gas
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behaviour is assumed (Fig. 15–10). The volume fractions of the other gases are determined
by repeating this procedure. In Orsat analysis the gas sample is collected over water and is
maintained saturated at all times. Therefore, the vapor pressure of water remains constant
during the entire test. For this reason the presence of water vapor in the test chamber is
ignored and data are reported on a dry basis. However, the amount of H2O formed during
combustion is easily determined by balancing the combustion equation.
ENTHALPY OF FORMATION AND ENTHALPY OF COMBUSTION
The molecules of a system possess energy in various forms such as sensible and latent energy
(associated with a change of state), chemical energy (associated with the molecular structure),
and nuclear energy (associated with the atomic structure), as illustrated in Fig. 15–14. In this
text we do not intend to deal with nuclear energy. We also ignored chemical energy until now
since the systems considered in previous chapters involved no changes in their chemical
structure, and thus no changes in chemical energy. Consequently, all we needed to deal with
were the sensible and latent energies. During a chemical reaction, some chemical bonds that
bind the atoms into molecules are broken, and new ones are formed. The chemical energy
associated with these bonds, in general, is different for the reactants and the products.
Therefore, a process that involves chemical reactions involves changes in chemical energies,
which must be accounted for in an energy balance (Fig. 15–15). Assuming the atoms of each
reactant remain intact (no nuclear reactions) and disregarding any changes in kinetic and
potential energies, the energy change of a system during a chemical reaction is due to a
change in state and a change in chemical composition. That is, (15–4) Therefore, when the
products formed during a chemical reaction exit the reaction chamber at the inlet state of the
reactants, we have _Estate _ 0 and the energy change of the system in this case is due to the
changes in its chemical composition only. In thermodynamics we are concerned with the
changes in the energy of a system during a process, and not the energy values at the particular
states. Therefore, we can choose any state as the reference state and assign a value of zero to
the internal energy or enthalpy of a substance at that state. When a process involves no
changes in chemical composition, the reference state chosen has no effect on the results.
When the process involves chemical reactions, however, the composition of the system at the
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end of a process is no longer the same as that at the beginning of the process. In this case it
becomes necessary to have a common reference state for all substances. The chosen reference
state is 25°C (77°F) and 1 atm, which is known as the standard reference state. Property
values at the standard reference state are indicated by a superscript (°) (such as h° and u°).
When analyzing reacting systems, we must use property values relative to the standard
reference state. However, it is not necessary to prepare a new set of property tables for this
purpose. We can use the existing tables by subtracting the property values at the standard
reference state from the values at the specified state. The ideal-gas enthalpy of N2 at 500 K
relative to the standard reference state, for example, is h –500 K _h–°_ 14,581 _ 8669 _ 5912
kJ/kmol. Consider the formation of CO2 from its elements, carbon and oxygen, during a
steady-flow combustion process (Fig. 15–16). Both the carbon and the oxygen enter the
combustion chamber at 25°C and 1 atm. The CO2 formed during this process also leaves the
combustion chamber at 25°C and 1 atm. The combustion of carbon is an exothermic reaction
(a reaction dur-ing which chemical energy is released in the form of heat). Therefore, some
heat is transferred from the combustion chamber to the surroundings during this process,
which is 393,520 kJ/kmol CO2 formed. (When one is dealing with chemical reactions, it is
more convenient to work with quantities per unit mole than per unit time, even for steadyflow processes.) The process described above involves no work interactions. Therefore, from
the steady-flow energy balance relation, the heat transfer during this process must be equal to
the difference between the enthalpy of the products and the enthalpy of the reactants.
That is, (15–5) Since both the reactants and the products are at the same state, the enthalpy
change during this process is solely due to the changes in the chemical composition of the
system. This enthalpy change is different for different reactions, and it is very desirable to
have a property to represent the changes in chemical energy during a reaction. This property
is the enthalpy of reaction hR, which is defined as the difference between the enthalpy of the
products at a specified state and the enthalpy of the reactants at the same state for a complete
reaction. For combustion processes, the enthalpy of reaction is usually referred to as the
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enthalpy of combustion hC, which represents the amount of heat released during a steadyflow combustion process when 1 kmol (or 1 kg) of fuel is burned completely at a specified
temperature and pressure (Fig. 15–17). It is expressed as
which is _393,520 kJ/kmol for carbon at the standard reference state. The enthalpy of
combustion of a particular fuel is different at different temperatures and pressures. The
enthalpy of combustion is obviously a very useful property for analyzing the combustion
processes of fuels. However, there are so many different fuels and fuel mixtures that it is not
practical to list hC values for all possible cases. Besides, the enthalpy of combustion is not of
much use when the combustion is incomplete. Therefore a more practical approach would be
to have a more fundamental property to represent the chemical energy of an element or a
compound at some reference state. This property is the enthalpy of formation h –f , which
can be viewed as the enthalpy of a substance at a specified state due to its chemical
composition. To establish a starting point, we assign the enthalpy of formation of all stable
elements (such as O2, N2, H2, and C) a value of zero at the standard reference state of 25°C
and 1 atm. That is,
H –f _ 0 for all stable elements. (This is no different from assigning the internal energy of
saturated liquid water a value of zero at 0.01°C.) Perhaps we should clarify what we mean by
stable. The stable form of an element is simply the chemically stable form of that element at
25°C and 1 atm. Nitrogen, for example, exists in diatomic form (N2) at 25°C and 1 atm.
Therefore, the stable form of nitrogen at the standard reference state is diatomic nitrogen N2,
not monatomic nitrogen N. If an element exists in more than one stable form at 25°C and 1
atm, one of the forms should be specified as the stable form. For carbon, for example, the
stable form is assumed to be graphite, not diamond. Now reconsider the formation of CO2 (a
compound) from its elements C and O2 at 25°C and 1 atm during a steady-flow process. The
enthalpy change during this process was determined to be _393,520 kJ/kmol. However,
Hreact _ 0 since both reactants are elements at the standard reference state, and the products
consist of 1 kmol of CO2 at the same state. Therefore, the enthalpy of formation of CO2 at
the standard reference state is _393,520 kJ/kmol (Fig. 15–18). That is, The negative sign is
due to the fact that the enthalpy of 1 kmol of CO2 at 25°C and 1 atm is 393,520 kJ less than
the enthalpy of 1 kmol of C and 1 kmol of O2 at the same state. In other words, 393,520 kJ of
chemical energy is released (leaving the system as heat) when C and O2 combine to form 1
kmol of CO2. Therefore, a negative enthalpy of formation for a compound indicates that heat
is released during the formation of that compound from its stable elements. A positive value
indicates heat is absorbed. You will notice that two h – °f values are given for H2O in Table
A–26, one for liquid water and the other for water vapor. This is because both phases of H2O
are encountered at 25°C, and the effect of pressure on the enthalpy of formation is small.
(Note that under equilibrium conditions, water exists only as a liquid at 25°C and 1 atm.) The
difference between the two enthalpies of formation is equal to the hfg of water at 25°C, which
is 2441.7 kJ/kg or 44,000 kJ/kmol. Another term commonly used in conjunction with the
combustion of fuels is the heating value of the fuel, which is defined as the amount of heat
released when a fuel is burned completely in a steady-flow process and the products are
returned to the state of the reactants. In other words, the heating value of a fuel is equal to the
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absolute value of the enthalpy of combustion of the fuel. That is, The heating value depends
on the phase of the H2O in the products. The heating value is called the higher heating
value (HHV) when the H2O in the products is in the liquid form, and it is called the lower
heating value (LHV) when the H2O in the products is in the vapor form (Fig. 15–19). The
two heating values are related by
where m is the mass of H2O in the products per unit mass of fuel and hfg is the enthalpy of
aporization of water at the specified temperature. The heating value or enthalpy of
combustion of a fuel can be determined from a knowledge of the enthalpy of formation for
the compound involved.
FIRST-LAW ANALYSIS OF REACTING SYSTEMS
chemically reacting systems involve changes in their chemical energy, and thus it is more
onvenient to rewrite the energy balance relations so that the changes in chemical energies are
explicitly expressed. We do this first for steady-flow systems and then for closed systems.
Steady-Flow Systems
Before writing the energy balance relation, we need to express the enthalpy of a component
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in a form suitable for use for reacting systems. That is, we need to express the enthalpy such
that it is relative to the standard reference
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Once the reactants and their states are specified, the enthalpy of the reactants Hreact can be
easily determined. The calculation of the enthalpy of the products Hprod is not so
straightforward, however, because the temperature of the products is not known prior to the
calculations. Therefore, the determination of the adiabatic flame temperature requires the use
of an iterative technique unless equations for the sensible enthalpy changes of the combustion
products are available. A temperature is assumed for the product gases, and the Hprod is
determined for this temperature. If it is not equal to Hreact, calculations are repeated with
another temperature. The adiabatic flame temperature is then determined from these two
results by interpolation. When the oxidant is air, the product gases mostly consist of N2, and
a good first guess for the adiabatic flame temperature is obtained by treating the entire
product gases as N2. In combustion chambers, the highest temperature to which a material
can be exposed is limited by metallurgical considerations. Therefore, the adiabatic flame
temperature is an important consideration in the design of combustion chambers, gas
turbines, and nozzles. The maximum temperatures that occur in these devices are
considerably lower than the adiabatic flame temperature, however, since the combustion is
usually incomplete, some heat
loss takes place, and some combustion gases dissociate at high temperatures (Fig. 15–26).
The maximum temperature in a combustion chamber can be controlled by adjusting the
amount of excess air, which serves as a coolant. Note that the adiabatic flame temperature of
a fuel is not unique. Its value depends on (1) the state of the reactants, (2) the degree of
completion of the
reaction, and (3) the amount of air used. For a specified fuel at a specified state burned with
air at a specified state, the adiabatic flame temperature attains its maximum value when
complete combustion occurs with the theoretical amount of air.
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ANALYSIS OF EXHAUST AND FLUE GAS
The combustion products are mainly gaseous. When a sample is taken for analysis it is
usually cooled down to a temperature which is below the saturation temperature of the steam
present. The steam content is therefore not included in the analysis, which is then quoted as
the analysis of the dry products. Since the products are gaseous, it is usual to quote the
analysis by volume. An analysis which includes the steam in the exhaust is called a wet
analysis.
Practical analysis of combustion products :
The most common means of analysis of the combustion products is the Orsat apparatus
which is described below :
Construction. An Orsat’s apparatus consists of the following :
(i) A burette
(ii) A gas cleaner
(iii) Four absorption pipettes 1, 2, 3, 4.
The pipettes are interconnected by means of a manifold fitted with cocks S1, S2, S3 and S4
and contain different chemicals to absorb carbon dioxide (CO2), carbonmonoxide (CO) and
oxygen (O2). Each pipette is also fitted with a number of small glass tubes which provide a
greater amount of surface. These tubes are wetted by the absorbing agents and are exposed to
the gas under analysis. The measuring burrette is surrounded by a water jacket to prevent,
changes in temperature and density of the gas. The pipettes 1, 2, 3, 4 contain the following
chemicals :
Pipette 1 : Contains ‘KOH’ (caustic soda) to absorb CO2 (carbon dioxide)
Pipette 2 : Contains an alkaline solution of ‘pyrogallic acid’ to absorb O2 (oxygen)
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Pipette 3, 4 : Contain an acid solution of ‘cuprous chloride’ to absorb CO (carbonmonoxide)
Furthermore the apparatus has a levelling bottle and a three way cock to connect the
apparatus either to gases or to atmosphere. Procedure. 100 cm3 of gas whose analysis is to
be made is drawn into the bottle by lowering the levelling bottle. The stop cock S4 is then
opened and the whole flue gas is forced to pipette 1. The gas remains in this pipette for
sometime and most of the carbondioxide is absorbed. The levelling bottle is then lowered to
allow the chemical to come to its original level. The volume of gas thus absorbed is read on
the scale of the measuring bottle. The flue gas is then forced through the pipette 1 for a
number of times to ensure that the whole of the CO2 is absorbed. Further, the remaining flue
gas is then forced to the pipette 2 which contains pyrogallic acid to absorb whole of O2. The
reading on the measuring burette will be the sum of volume of CO2 and O2. The oxygen
content can then be found out by subtraction. Finally, as before, the sample of gas is forced
through the pipettes 3 and 4 to absorb carbonmonoxide completely. The amount of nitrogen
in the sample can be determined by subtracting from total volume of gas the sum of CO2, CO
and O2 contents. Orsat apparatus gives an analysis of the dry products of combustion. Steps
may have been taken to remove the steam from the sample by condensing, but as the sample
is collected over water it becomes saturated with water. The resulting analysis is nevertheless
a true analysis of the dry products. This is because the volume readings are taken at a
constant temperature and pressure, and the partial pressure of the vapour is constant. This
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means that the sum of the
partial pressures of the remaining constituents is constant. The vapour then occupies the same
proportion of the total volume at each measurement. Hence the vapour does not affect the
result of the analysis.
Note. Quantitatively the dry product analysis can be used to calculate A/F ratio. This method
of obtaining the A/F ratio is not so reliable as direct measurement of air consumption and fuel
consumption of the engine. More caution is required when analysing the products of
consumption of a solid fuel since some of the products do not appear in the flue gases (e.g.
ash and unburnt carbon). The residual solid must be analysed as well in order to determine
the carbon content, if any. With an engine using petrol or diesel fuel the exhaust may include
sunburnt particles of carbon and this quantity will not appear in the analysis. The exhaust
from internal combustion engines may contain also some CH4 and H2 due to incomplete
combustion. Another piece of equipment called the Heldane apparatus measures the CH4
content as well as CO2, O2 and CO.
AIR-FUEL RATIO FROM ANALYSIS OF PRODUCTS
When analysis of combustion products is known air-fuel ratio can be calculated by the
following methods :
1. Fuel composition known
(i) Carbon balance method (ii) Hydrogen balance method
(iii) Carbon-hydrogen balance method.
2. Fuel composition unknown
(i) Carbon-hydrogen balance method.
1. Fuel composition known
(i) Carbon balance method. When the fuel composition is known, the carbon balance method
is quite accurate if combustion takes place with excess air and when free (solid) carbon is not
present in the products. It may be noted that the Orsat analysis will not determine the
quantity of solid carbon in the products.
(ii) Hydrogen balance method. This method is used when solid carbon is suspected to be
present.
(iii) Carbon-hydrogen balance method. This method may be employed when there is some
uncertainty about the nitrogen percentage reported by the Orsat analysis.
2. Fuel composition unknown
When the fuel composition is not known the carbon-hydrogen balance method has to be
employed.
HOW TO CONVERT VOLUMETRIC ANALYSIS TO WEIGHT ANALYSIS ?
The conversion of volumetric analysis to weight analysis involves the following steps :
1. Multiply the volume of each constituent by its molecular weight.
2. Add all these weights and then divide each weight by the total of all and express it as
percentage.
HOW TO CONVERT WEIGHT ANALYSIS TO VOLUMETRIC ANALYSIS ?
1. Divide the weight of each constituent by its molecular weight.
2. Add up these volumes and divide each volume by the total of all and express it as a
percentage.
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WEIGHT OF CARBON IN FLUE GASES
The weight of carbon contained in one kg of flue or exhaust gas can be calculated from the
amounts of CO2 and CO contained in it.
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