ME-803
Refrigeration & Air Conditioning
UNIT :-I
SYLLABUS:- Principles and methods of refrigeration, freezing; mixture cooling by
gas reversible expansion, throttling, evaporation, Joule Thomson effect and reverse Carnot
cycle; unit of refrigeration, coefficient of performance, vortex tube & thermoelectric
refrigeration, adiabatic demagnetization; air refrigeration cycles- Joule’s cycle Boot-strap
cycle, reduced ambient cycle and regenerative cooling cycles.
Introduction
Refrigeration is defined as “the process of cooling of bodies or fluids to
temperatures lower than those available in the surroundings at a particular time
and place”. It should be kept in mind that refrigeration is not same as “cooling”,
even though both the terms imply a decrease in temperature. In general, cooling is
a heat transfer process down a temperature gradient, it can be a natural,
spontaneous process or an artificial process. However, refrigeration is not a
spontaneous process, as it requires expenditure of energy (or availability). Thus
cooling of a hot cup of coffee is a spontaneous cooling process (not a refrigeration
process), while converting a glass of water from room temperature to say, a block
of ice, is a refrigeration process (non-spontaneous). “All refrigeration processes
involve cooling, but all cooling processes need not involve refrigeration”.
Refrigeration is a much more difficult process than heating; this is in accordance
with the second laws of thermodynamics. This also explains the fact that people
knew ‘how to heat’, much earlier than they learned ‘how to refrigerate’. All
practical refrigeration processes involve reducing the temperature of a system
from its initial value to the required temperature that is lower than the
surroundings, and then maintaining the system at the required low temperature.
The second part is necessary due to the reason that once the temperature of a
system is reduced, a potential for heat transfer is created between the system and
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
Surroundings, and in the absence of a “perfect insulation” heat transfer from the
surroundings to the system takes place resulting in increase in system temperature.
In addition, the system itself may generate heat (e.g. dueto human beings,
appliances etc.), which needs to be extracted continuously. Thus in practice
refrigeration systems have to first reduce the system temperature and then extract
heat from the systemat such a rate that the temperature of the system remains low.
Theoretically refrigeration can be achieved by severalmethods. All these methods
involve producing temperatures low enough for heat transfer to take place from
the system being refrigerated to the system that is producing refrigeration.
Methods of producing low temperatures
Sensible cooling by cold medium If a substance is available at a temperature
lower than the required refrigeration temperature, then it can be used for sensible
cooling by bringing it in thermal contact with the system to be refrigerated. For
example, a building can be cooled to a temperature lower than the surroundings by
introducing cold air into the building. Cold water or brine is used for cooling
beverages, dairy products and in other industrial processes by absorbing heat from
them. The energy absorbed by the substance providing cooling increases it
temperature, and the heat transferred during this process is given by:
Q= mcp(Δ T)
Where,
m is the mass of the substance providing cooling,
cp is its specific heat and
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
ΔT is the temperature rise undergone by the substance. Since the
temperature of the cold substance increases during the process, to provide
continuous refrigeration, a continuous supply of the
cold substance should be maintained, which may call for an external refrigeration
cycle.
Endothermic mixing of substances
This is one of the oldest methods known tomankind. It is very well-known
that low temperatures can be obtained when certain salts are dissolved in water.
This is due to the fact that dissolving of these salts in water is an endothermic
process, i.e., heat is absorbed from the solution leading to its cooling. For
example,when salts such as sodium nitrate, sodium chloride, calcium chloride
added to water, its temperature falls. By dissolving sodium chloride in water, it is
possible toachieve temperatures as low as –210C, while with calcium chloride a
temperature of –510C could be obtained. However, producing low temperature by
endothermic mixing has several practical limitations. These are: the refrigeration
effect obtained is very small (the refrigeration effect depends on the heat of
solution of the dissolved substance, which is typically small for most of
thecommonly used salts), and recovery of the dissolved salt is often uneconomical
as this calls for evaporation of water from the solution.
Methods of Refrigeration:
a) Natural Method:
The natural method includes the utilization ofice or snow obtained
naturally in cold climate. Ice melts at 00C. So when it is placed in space or
system warmer than 00C, heat is absorbed by the ice and the space
iscooled. The ice then melts into water by absorbing its latent heat at the
rate of 324 kJ/kg. But, now-a-days, refrigeration requirements have
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
become so high that the natural methods are inadequate and therefore
obsolete
b) Mechanical or Artificial Refrigeration
Reversed Carnot engine
A mechanical refrigeration system works on the principle of reversed
Carnot cycle as shown in Work δw is delivered to the refrigerating system,
causing it to remove heat δQ2 from the body or system (at lower
temperature T3) and to deliver it along with work, δw, to another body at
higher temperature, T1, so that,
δQ1 = δw + δQ2
RAC BY:- Raju Kumar Das
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GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
There can be two methods bywhich the temperature T2< T3 may be
attained within the refrigerating system.
1. By lowering the temperature of the working substance in the
refrigerating system to the level of T2. In this case, the heat will be
absorbed due to temperature difference and T3 will decrease as heat
δQ2 flows out.
2. By evaporating some fluid at an appropriate pressure. In this case, a
constant temperature T2 will be maintained and latent heat of fluid will
be absorbed as δQ2
Depending upon the above method used, there are two types
of mechanical refrigerating systems :
i)
ii)
Air systems:Uses air as a working fluid.Air does not undergo any
change of phase, but absorbs heat due to temperature difference.
Chemical Agent Systems: The working fluid changing its phase
while boiling from liquid to vapor state, thereby it absorbs the
latent heat.
Unit of Refrigeration:
Capacity of refrigeration unitis generally defined in ton
of refrigeration. A ton of refrigeration is defined as the quantity of heat to
be removed in order to form one ton (1000 kg) of ice at 0 0C in 24 hrs, from
liquid water at 00C. This is equivalent to 3.5 kJ/s (3.5 kW) or 210 kJ/min.
Reverse Carnot cycle
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GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
Introduction
The Carnot's heat engine is described almost in all courses of physics and
thermodynamics
Carnot engine operates on the reversible Carnot cycle. The direct Carnot's cycle
includes isothermal enlargement of working substance at temperature Т 1, which equals
to the temperature of hotter reservoir of heat; 2) adiabatic enlargement of working
substance, in the process of which it’s temperature lowers to the temperature Т 2, which
equals to the temperature of colder reservoir of heat; 3) isothermal compression of
working substance at temperature Т2;4) adiabatic compression of working substance, in
the process of which it’s temperature rises to the temperature Т1. In the direct cycle the
Carnot's engine gets from the hotter reservoir for one cycle the amount of heat Q1,
transfers the amount of heat Q2(Q2< Q1) to the colder reservoir, and the
difference between Q1–Q2 converts to the work L, which can be used to lifting of
weight. In the reverse cycle the Carnot's engine gets from the colder reservoir for one
cycle the amount of heat Q2, transfers the amount of the heat Q1 to the hotter reservoir
and converts into heat work L, obtained, for example, by of the lowering of a weight.
Q1= Q2+ L.
The efficiency of a heat engine (cycle efficiency) ηis defined by:
2. The proof of Carnot's theorem based on Thomson's postulate Assume there are two
Carnot engine, which efficiency are η1and η2, where η1> η2. The engine 1is running on
the direct Carnot cycle, the engine 2 – the reverse Carnot cycle. The engine 1for one
cycle gets the amount of heat Q1from the hotter reservoir, transfers the amount of heat
Q2to colder reservoir and produces work L. The engine2 gets the amount of heat Q 2'
from the colder reservoir, gets from the engine 1 the amount of work L', transfer to the
hotter reservoir the amount of heat Q1.
Q1 = Q2 + L = Q2' + L'.
If η1> η2, then Q2' > Q2 and L'< L.
RAC BY:- Raju Kumar Das
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GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
The engine 1produces work L. Part of this work is equal to L'is giving to the engine 2, part is equal
toL –L'is spending on lifting of
weight GL – L'= Q2' – Q2.
(see Fig.1)
Fig -1
The result of the two engines is the raising of a weight and the cooling the colder
reservoir (Q2' > Q2). This one contradicts to the Thomson's postulate: "It is impossible, by
means of inanimate material agency, to derive mechanical effect from any portion of matter
by cooling it below the temperature of the coldest of the surrounding objects". Consequently, the
assumption η1> η2 is false. Suggested, η1< η2. When the engine 1 is running on reverse Carnot
cycle and the engine 2 is running on the direct Carnot cycle, we get again a result that contradicts
the Thomson's postulate. Efficiency of the engine 1can be neither less nor more efficient engine 2.
The efficiency of the Carnot engine is independent of the working substance.
3. The proof of the Carnot's theorem based on "antipostulate"
Assume there are two Carnot engine, which efficiency are η1and η2, where η1<η2. The engine 1
is running on the direct Carnot cycle, the engine 2– the reverse Carnot cycle. The engine
1for one cycle gets the amount of heat Q1from the hotter reservoir, transfers the amount of heat Q2
to colder reservoir and produces work L. The engine2gets the amount of heat Q2' from the colder
reservoir, gets of the engine 1 theamount of work L,
from an external source – the amount of work L' – L, transfer to the hotter reservoir the amount of
heat Q1. Q1 = Q2 + L = Q2' + L'.
If η1< η2, then
Q2> Q2' and L < L'.
The engine 2absorbs work L, which is produced by engine 1,and the work L'– L,
which is obtained by lowering of weight G (see Fig. 2). L'– L= Q2 – Q2'.
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
The result of the two engines working is lowering of a weight and heating of colder
reservoir (Q2> Q2'). This contradicts to the "antipostulate": "It is impossible to use the
mechanical effect to the heating the coldest of surrounding objects". Consequently, the
assumption η1< η2is false. Suggested that η1> η2. When the engine 1is running on
reverse Carnot cycle and the engine 2
is running on the direct Carnot cycle, we get again a result that contradicts the
"antipostulate". Efficiency of the engine 1can be neither less nor more efficient engine2.
The efficiency of the Carnot engine is independent of the working substance.
APPLICATION OF VORTEX TUBE TO REFRIGERATION CYCLES
The relative inefficiency of the vortex tube as a stand-alone cooling device has
thus far limited its use. The coefficient of performance (COP) of the vortex tube as a
refrigerator, cooling provided per work required to compress the fluid, is very low (less
than 0.1 near roomtemperature) relative to a domestic refrigeration cycle. However, its
simplicity makes it an extremely compact, reliable, affordable, and flexible alternative
in some special applications. A more exciting and potentially much broader use for the
vortex tube exists if it can be integrated into a refrigeration system as an alternative to
the conventional throttling valve in order to accomplish the required expansion process
in a less irreversible manner. The vortex tube model developed in the previous section
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GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
will be employed to evaluate the utility of the vortex tube within vapor compression and
Joule-Thomson refrigeration systems. The results of these analyses must be qualified by
the statement that the vortex tube model has been developed for and verified against
data taken at room temperature under single-phase operating conditions. The
performance of a vortex tube may be substantially different in the two-phase and nonideal conditions encountered within a refrigeration system but the data available for
these operating conditions are scarce and often conflicting. The analysis remains
a valuable tool provided that the underlying physical model is accurate and it clearly
points out those situations where the potential exists for significant improvements in
cycle performance
Vapor Compression Cycle
Vapor Compression Cycle Figure 5 illustrates a conventional vapor compression
refrigeration cycle with a throttling valve and Figure 6 illustrates the corresponding ideal
thermodynamic cycle using a typical synthetic refrigerant, R134a. The process of expanding the twophase fluid through the valve, from state (3) to state (4), follows a line of constant enthalpy. In the
situation where the valve is replaced by a vortex tube, the corresponding expansion for the fluid
extracted from the cold side of the tube would be limited to an isentropic process, forreasons described
earlier. The limiting exit states would therefore be (4c) and (4h), as indicated in Figure 6. Clearly,
under these assumptions the vortex tube is not capable of producing a temperature separation effect
under the vapor dome – the hot and cold fluids leave at the same temperature and at most we might
expect a difference in quality based on centrifugal forces. This fact has been experimentally observed
in those few instances
where a vortex tube has been operated in the saturated region, for example by Takahama et al. (1979)
using steam. Therefore, there is no benefit associated with the use of a vortex tube within a pure
refrigerant, vapor compression refrigeration cycle. The isenthalpic and isentropic temperature change
between two isobars are not identical when the refrigeration cycle extends into the super-critical region
orutilizes non-azeotropic mixtures and in these situations it is possible that the application of a vortex
tube may be beneficial. Carbon dioxide is a naturally occurring substance that has recently received
renewed attention as a potential alternative to synthetic refrigerants (Lorentzen, 1994) because it is
non-flammable, non-toxic, readily available, and compatible with most materials. However the
theoretical COP of a carbon dioxide refrigeration system is not as high as that of most synthetic
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GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
refrigerants, in large part due to throttling losses (these inefficiencies explain more than 40% of the
total loss according to Robinson et al., 1996). The potential for increasing the performance of a carbon
dioxide cycle via modification of the expansion mechanism is therefore high. Li et al. (2000)
theoretically investigated a carbon dioxide cycle with an expansion valve, a work extracting expansion
device, and a vortex tube and reported that use of a vortex tube could improve the performance relative
to the valve. This analysis made no attempt to model the vortex tube beyond invoking the 1stand
2ndlaw of thermodynamics. If the physical model described above is used to model the vortex tube in
this situation then the predicted benefit in cycle performance disappears, again because the temperature
separation effect goes rapidly to zero as the vapor dome is approached.
Joule-Thomson Cycle
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GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
Joule-Thomson (JT) cryogenic refrigerators rely on expansion through a
throttling valve yet do not necessarily operate in the vapor dome. These
refrigeration devices are used in low cost applications or in situations where
reliability is of paramount importance such as tactical cryocoolers for
infrared detectors,
refrigeration for electronics, cryotherapy probes, and cryocoolers for spaceborne detectors. Figure 7 illustrates schematically the Joule-Thomson
cryogenic refrigeration cycle and Figure 8 illustrates the cycle qualitatively
on a T-s diagram. Notice that the JT cycle relies on the fact that a
temperature drop is produced during the isenthalpic expansion through the
valve (from state 2 to state 3). This allows a small warming of the refrigerant
as it accepts the refrigeration load (from state 3 tostate 4) before entering the
recuperative heatexchanger. In order to be viable, the JT cycle must operate
in a region where the Joule-Thomson coefficient, the partial derivative of
temperature with respect to pressure at constant enthalpy, is positive. This
limitation explains why typical JT systems operate with very high pressure
ratio and require large, high effectiveness recuperative heat exchangers
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GIIT Gwl
ME-803
Refrigeration & Air Conditioning
UNIT :-I
The vortex tube produces a large temperature drop in the cold exit fluid regardless
of the Joule-Thomson coefficient. For example, the isenthalpic temperature drop
associated withexpanding room temperature air from 3 atm to 1 atm is only 0.2 °C yet
Figure 3 shows that a temperature drop of 27°C can be achieved at these conditions
using a vortex tube, an increase of two orders of magnitude. Figure 9 illustrates the
simplest possibility for implementing a cryogenic refrigeration cycle using a vortex
tuberather than the expansion valve. The fluid leaving the cold exit is used to accept the
refrigeration load and the fluid leaving the hot exit bypasses the recuperative heat
exchanger. Figure 10 illustrates the temperature entropy diagram for this vortex tube
cycle; the state points from the original JT cycle are retained for illustration. The
temperature entering the recuperative heat exchanger, state (1), remains the same but the
temperature of the leaving gas, state (2), is increased as a result of unbalancing the heat
exchanger. The temperature drop associated with the fluid passing through the cold side
of the vortex tube is much larger than it would be had it undergone an isenthalpic
expansion (the temperature of state (3) is reduced), but some part of the flow has been
sacrificed to accomplish this effect. The cold fluid is heated to the same refrigeration
temperature, state (4), prior to entering the recuperative heat exchanger.
A model of the refrigeration cycle shown in Figure 9 has been developed using
the vortex tube model already described. For a given mass flow rate (m&) and pressure
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ME-803
Refrigeration & Air Conditioning
UNIT :-I
ratio (PR), the refrigeration load(Qload) can be predicted as a function of refrigeration
temperature (Tload) given the characteristics of the heat exchanger (a total conductance
UA), the vortex tube, and a cold gas mass fraction (y). Figure 11 illustrates the
refrigeration load as a function of the cold gas mass fraction using air as the working
fluid for one set of operating conditions (Trej= 300 K, PR = 5, m & = 10 g/s, UA = 10
W/K, and Tload= 250 K). When the cold gas mass fraction is equal to unity, the cycle is
equivalent to a JT cycle – all of the gas passes through the vortex tube, undergoes an
isenthalpic process, passes through the refrigeration load, and returns through the
recuperative heat exchanger. Notice that under these operating conditions a JT cycle is
not viable, the refrigeration load is negative because the pressure ratio is low and the
heat exchanger small. As the cold gas mass fraction is reduced the refrigeration power
increases and reaches a maximum value of 94 W at an optimal cold gas mass fraction
near 0.5. As the cold gas mass fraction is reduced, three interrelated effects occur; there
are two negative effects – unbalancing the heat exchanger and reducing the mass flow
rate through the refrigeration load that are countered by a large positive effect increasing the temperature drop from state (2) to state (3). The optimal value of cold gas
mass fraction balances these effects and is therefore somewhat larger than the cold gas
mass fraction that maximizes the temperature drop through the vortex tube, near 0.25 as
shown in Figures 3 and 4.
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ME-803
Refrigeration & Air Conditioning
UNIT :-I
Figure 12 illustrates the refrigeration load for the vortex tube cycle
using the optimal cold gas mass ratio as a function of the load temperature
and at several pressure ratios for one set of operating conditions (air, T rej=
300 K, m&= 10 g/s, and UA = 10 W/K). Also shown in Figure 12 is the
performance of a JT cycle at identical operating conditions. Notice that the
vortex tube cycle significantly outperforms the JT cycle; this is particularly
true as the pressure ratio or the heat exchanger size is reduced. This
investigation clearly illustrates that the addition of a vortex tube has the
potential to significantly increase the performance of a single-stage JT
system and motivates further study into these devices and their use in
cryogenicrefrigeration systlarge increases in the efficiency of JT systems
have been made possible through the use of mixed gas refrigerants (Little,
1998) and this technology can be augmented by the use of phase separators
as originally proposed by Kleemenko (1959). There is an obvious parallel
between these phase separators and the natural separation mechanism that
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ME-803
Refrigeration & Air Conditioning
UNIT :-I
occurs within the vortex tube due to centrifugal effects (Marshall, 1977).
There exists then the possibility of performing augmented expansion
simultaneously with some phase separation within a vortex tube in a mixed
gas cycle. Complex, multi-stage cycles can be considered with the potential
to significantly out-perform the currently available JT technology without
adding significantly to cost or detracting from reliability.
Air Cycle Refrigeration
Introduction
Refrigeration is a world wide multi-billion pound business, with applications
ranging from food processing to air conditioning. This industry is under massive
pressure to develop new ideas, technologies and methods of work that will meet
the demands of an ever changing world.
One major driver for the industry is the impending legislation resulting from the
Kyoto and Montreal Protocols, which will outlaw the use of the most common
chlorofluorocarbon (CFC) and hydrofluorocarbon (HFC) refrigerant gases
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ME-803
Refrigeration & Air Conditioning
UNIT :-I
currently being used in the world today, in an attempt to reduce global warming
and its effects.
CFCs and HFCs are toxic and have large global warming and ozone depletion
potentials; consequently an environmentally-friendly alternative is urgently
required. A solution to this problem has been designed in the form of an air
cycle refrigeration system for blast freezing applications
which uses air as the refrigerant in place of CFCs and HFCs.
Design
Air cycle refrigeration manipulates the fact that when air expands, there is an
associated drop in its temperature.
Figure 1 shows a simple cycle diagram for the air cycle system.
Before the air can be expanded to produce a cooling effect, it must
first be compressed; this is achieved by passing the air through two
stages of turbomachinerycompressors. The compression of the air
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UNIT :-I
results inits temperature rising, so to provide very cold air at the end
of the cycle, it is necessary to remove heat from the air; this is
achieved by passing the air through heat exchangersimmediately
after each stage of compression. The final stage of the cycle is to
expand the air using a turbine, yielding an instantaneous drop in the
temperature of the air to around -60oC, dependent upon necessary
cooling requirements.
Advantage
The air cycle refrigeration system for blast freezing applications offers the
following main benefits:
• The system is immune to the impending phase out of CFCs and is
environmentally-friendly.
•It is effectively leak-free, eliminating the need for costly refills and
potential fines associated with refrigerant leaks
•Air cycle refrigeration systems offer better performance at off-design
conditions than existing systems
•The lower temperatures offered by the system lead to shorter freezing
times and hence improved
Process throughput, whilst also reducing moisture loss and improving food
quality Air cycle refrigeration is the subject of ongoing research at Queen’s
University, Belfast, by Dr. S. W. T. Spence et al.
Aircraft cooling systems
In an aircraft, cooling systems are required to keep the cabin temperatures at a
comfortable level. Even though the outsidetemperatures are very low at high altitudes, still
cooling of cabin is required due to:
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Refrigeration & Air Conditioning
UNIT :-I
i.
ii.
iii.
Large internal heat generation dueto occupants, equipment etc.
Heat generation due to skin friction caused by the fast moving aircraft
At high altitudes, the outside pressure will be sub-atmospheric. When air at this
low pressure is compressed and supplied to the cabin at pressures close to
atmospheric, the temperature increases significantly. For example, when outside
air at a pressure of 0.2 bar and temperature of 223 K (at 10000 m altitude) is
compressed to 1 bar, its temperature increases to about 353 K. If the cabin is
maintained at 0.8 bar, the temperature will be about 332 K. This effect is called
as ram effect. This effect adds heat to the cabin, which needs to be taken out by
the cooling system.
iv.
Solar radiation For low speed aircraft flying at low altitudes, cooling system
may not be required, however, for high speed aircraft flying at high altitudes, a cooling
system is a must. Even though the COP of air cycle refrigeration is very low compared to
vapour compression refrigeration systems, it is still found to be most suitable for aircraft
Refrigeration systems as:
i.
Air is cheap, safe, non-toxic and non-flammable. Leakage of air is not a problem
ii.
Cold air can directly be used for cooling thus eliminating the low
temperature heat exchanger (open systems) leading to lower weight
iii.
The aircraft engine already consistsof a high speed turbo-compressor,
hence separate compressor for cooling system is not required. This reduces the weight per
kW cooling considerably. Typically, less than 50% of an equivalent vapour compression
system
iv.
Design of the complete system is much simpler due to low
pressures.Maintenance required is also less.
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UNIT :-I
Figure 9.5 shows the schematic of a simpleaircraft refrigeration
system and the operating cycle on T-s diagram. This is an open
system. As shown in the T-s diagram, the outside low pressure and
low temperature air (state 1) is compressed due to ram effect to ram
pressure (state 2). During thisprocess its temperature increases from
1 to 2. This air is compressed in the main compressor to state 3, and
is cooled to state 4 in the air cooler. Its pressure is reduced to cabin
pressure in the turbine (state 5), as a result its temperature drops
from 4 to 5. The cold air at state 5 is supplied to the cabin. It picks
up heat as it flows through the cabin providing useful cooling effect.
The power output of the turbine is used to drive the fan, which
maintains the required air flow over the air cooler. This simple
system is good for ground cooling (when the aircraft is not moving)
as fan can continueto maintain airflow over the air cooler. By
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applying steady flow energy equation to the ramming process, the
temperature rise at the end of the ram effect can be shown to be:
Bootstrap system:
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shows the schematic of a bootstrap system, which is a
modification of the
simple system. As shown in the figure, this system consists of
two heat exchangers (air cooler and aftercooler), in stead of
one air cooler ofthe simple system. It also incorporates a
secondary compressor, which is driven by the turbine of the
cooling system. This system is suitable for high speedaircraft,
where in the velocity of the aircraft provides the necessary
airflow for the heat exchangers, as a result a separate fan is
not required. As shown in the cycle diagram, ambient air state
1 is pressurized to state 2 due to the ram effect. This air is
further compressed to state 3 in the main compressor. The air
is then cooled to state 4 in the air cooler. The heat rejected in
the air cooler is absorbed by the ram air at state 2. The air
from the air cooler is further compressed from state 4 to state
5 in the secondary compressor. It is then cooled to state 6 in
the after cooler, expanded to cabinpressure in the cooling
turbine and is supplied to the cabin at a low temperature T
7 Since the system does not consist of a separate fan for driving the
air through the heat exchangers, it is not suitable for ground cooling.
However, in general ground cooling is normally done by an external
air conditioning system as it is not efficient torun the aircraft engine
just to provide cooling when it is grounded. Other modifications
over the simple system are: regenerative system and reduced
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ambient system. In a regenerative system, a part of the cold air from
the cooling turbine is used for precooling the air entering the
turbine.As a result much lower temperatures are obtained at the exit
of the cooling turbine, however, this is at the expense of additional
weight and design complexity. The cooling turbine drives a fan
similar to the simple system. The regenerative system is good for
both ground cooling as well as high speed aircrafts. The re
duced ambient system is well-suited for supersonic aircrafts and
rockets.
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Dry Air Rated Temperature (DART): The concept of Dry Air Rated
Temperature
is used to compare different aircraft refrigeration cycles. Dry Air Rated
Temperature is defined as the temperature of the air at the exit of the cooling
turbine in the absence of moisture condensation. For condensation not to
occur during expansion in turbine, the dew point temperature and hence
moisture content of the air should be very low, i.e., the air should be very
dry. The aircraft refrigeration systems are rated based on the mass flow rate
of air at the design DART. The cooling capacity is then given by:
where .mis the mass flow rate of air, TDARTand Ti are the dry air rated
temperature and cabin temperature, respectively. A comparison between
different aircraft refrigeration systems based on DART at different Mach
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UNIT :-I
numbers shows that: i.DART increases monotonically with Mach number for
all the systems except the reduced ambient system
ii.The simple system is adequate at low Mach numbers
iii.At high Mach numbers either bootstrap system or regenerative system
should be used
iv.Reduced ambient temperature system is best suited for very high Mach
number, supersonic aircrafts
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl