Refrigeration Systems
Technical college/ Baghdad
4th Year
Week No. :23
Theoretical hours: 2
Practical hours: 2
Units: 6
Air cycle refrigeration systems
First 10 minutes: review the last lecture.
Then explain the new lecture, solve an example.
Last 10 minutes review the lecture and ask if there are any questions.
The specific objectives of the lesson:
This lesson discusses various gas cycle refrigeration systems based on air, namely:
1. Bell-Coleman Refrigerator.
2. Aircraft refrigeration cycles namely:
a. Simple system.
b. Bootstrap system.
c. Regenerative system.
3. Discusses the assumptions made in the analyses of air cycle systems.
4. Discusses the cycles on T-s diagrams.
5. Discusses various cycle calculations.
Aircraft cooling systems
In an aircraft, cooling systems are required to keep the cabin temperatures at a comfortable
level. Even though the outside temperatures are very low at high altitudes, still cooling of cabin is
required due to:
1. Large internal heat generation due to occupants, equipment etc.
2. Heat generation due to skin friction caused by the fast moving aircraft.
3. 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.
4. 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:
1. Air is cheap, safe, non-toxic and non-flammable. Leakage of air is not a problem.
2. Cold air can directly be used for cooling thus eliminating the low temperature heat exchanger (open
systems) leading to lower weight.
3. The aircraft engine already consists of 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 vapours compression system.
4. Design of the complete system is much simpler due to low pressures. Maintenance required is also
less.
Bell-Coleman Cycle:
The Bell-Coleman Cycle is also known as the Air-Standard Refrigeration Cycle or Reverse Brayton
Cycle. This refrigeration cycle involves isentropic compression, followed by isobaric heat rejection,
then isentropic expansion (usually by a turbo expander), and finally isobaric heat intake.
This cycle is commonly used in jet aircraft, using engine bleed air for compression and venting to
the atmosphere. It is also commonly used in commercial air liquefaction plants.
(a) Air refrigeration system
(b) Air refrigeration system
(c) Air refrigeration system
The components of the air refrigeration system are shown in Fig. (a). In this system, air is taken
into the compressor from atmosphere and compressed. The hot compressed air is cooled in heat
exchanger upto the atmospheric temperature (in ideal conditions). The cooled air is then expanded in
an expander. The temperature of the air coming out from the expander is below the atmospheric
temperature due to isentropic expansion. The low temperature air coming out from the expander
enters into the evaporator and absorbs the heat. The cycle is repeated again. The working of air
refrigeration cycle is represented on p-v and T-s diagrams in Fig. (b) and (c).
2
Process 1-2 represents the isentropic compression of air by the compressor.
Process 2-3 represents the discharge of high pressure air from the compressor into the heat
exchanger. The reduction in volume of air from v3 to v5 is due to the cooling of air in the
heat exchanger.
Process 3-4 represents the isentropic expansion of air in the expander.
Process 4-1 represents the absorption of heat from the evaporator at constant pressure.
Process 6-1 represents the suction of air into the compressor.
Analysis of Bell-Coleman Cycle:
Assumptions:
1) The compression and expansion processes are reversible adiabatic processes.
2) There is a perfect inter-cooling in the heat exchanger.
3) There are no pressure losses in the system.
3
Advantages of Bell-Coleman Cycle:
a) Air is a cheaper refrigerant and available easily compared to other refrigerants.
b) There is no danger of fire or toxic effects due to leakage.
c) The total weight of the system per ton of refrigerating capacity is less.
Disadvantages of Bell-Coleman Cycle:
(a) The quantity of air required per ton refrigerating capacity is far greater than other systems.
(b) The COP is low and hence maintenance cost is high.
(c) The danger of frosting at the expander valves is more as the air taken into the system always
contains moisture.
4
5
6