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Air Cycle Refrigeration

Air-Cycle Refrigeration

a. An air-cycle refrigeration unit uses air as the refrigerant

b. The air in turn is compressed, cooled in a heat exchanger, and

expanded through a turbine to a low temperature where it is capable of

performing cooling

c.Air-cycle equipment is ideally suited for use in aircraft because it is

light in weight and requires less space than the vapour-compression

cycle

d.The air-cycle, however, is not as efficient as the vapour-compression

cycle

The power cycles can be used as refrigeration cycles by simply reversing them. Of these, the reversed Brayton cycle, which is also known as the gas refrigeration cycle, is used to cool aircraft and to obtain very low (cryogenic) temperatures after it is modified with regeneration. The work output of the turbine can be used to reduce the work input requirements to the compressor.

Air Refrigeration Cycle

COPq

w

q

w wRL

net in

L

comp in turb out

, , ,

Simple gas refrigerationcycle

Air-cycle Systems

The most commonly used systems on air-craft are:

The simple cooling system

Bootstrap system

Regenerative cooling system

Simple Cooling System

Simple Cooling System• Ambient air at a velocity equal to the plane speed enters the inlet,

which is designed as a diffuser• The air is slowed down and some part of its kinetic energy is

converted into pressure• This type of compression is called RAM compression• This increase in pressure by ramming action is shown by line ab on TS

diagram• The air is then compressed in the main compressor to the desired

pressure. The process is shown by line bc on TS diagram• After the compressor, this high pressure air is cooled in the heat

exchanger (process cd) where the cooling medium is rammed air• After the heat exchanger, the compressed air enters the turbine and

expands down to the cabin pressure, as shown by line de• The work obtained from the turbine drives the exhaust fan which pulls

rammed air over the heat exchanger

Bootstrap System• Another version of the air-cycle is the bootstrap system shown:

• Like simple cooling system, the ambient air enters the inlet section, gets compressed due to ramming action. Process is shown by line ab on the TS diagram

• Further compression takes place in the main compressor, C1, and

the process is shown by line bc on the TS diagram

• This high pressure, high temperature compressed air is then passed to the primary heat exchanger where it gives some of its heat to the rammed air which is used as a cooling medium. The process is shown by line cd on TS diagram

• The air then enters the secondary compressor, C2, where its

pressure is further raised, and the process is shown by the line de.

Bootstrap System• This compressed high pressure air is then cooled in the secondary

heat exchanger. The cooling medium is again the rammed air. Process is shown by line ef on TS diagram

• Further cooling of air takes place due to its expansion in the turbine to the desired cabin pressure. The process is shown by line fg on TS diagram

• Ram pressure of the moving airplane forces air through the heat exchangers

• The turbine and secondary compressor run on the same shaft, with turbine driving the compressor

Regenerative System• Ambient air enters the inlet section, gets compressed due to

ramming action and the process is shown by line ab on TS diagram

• Further compression takes place in the main compressor an dthe process is represented by line bc on TS diagram

• The high pressure and high temperature air taken from the main compressor is first cooled in the primary heat exchanger by the rammed air, process represented by line cd on TS diagram

• It is then cooled in the regenerative heat exchanger by a part of cold air from the turbine discharge (process de)

• The air then expands through the turbine to the desired cabin pressure represented by process ef on TS diagram

Analysis of Air-Refrigeration Systems• In the compressor, the following relationship applies for reversible

adiabatic compression:

-----------------------(1)

• Where subscript 1 represents the condition before the compressor & subscript 2 represents the condition after isentropic compression, p is the absolute pressure, and K is the ratio of specific heats

• The efficiency of the compressor, is represented by:

-----------------------(2)

• Where is the change in enthalpy in the reversible and adiabatic compression, and is the actual change in enthalpy during the adiabatic compression

Analysis of Air-Refrigeration Systems• Since dry air may be assumed as a perfect gas, then:

-----------------------(3)

• and -----------------------(4)

• Where is the actual temperature leaving the compressor

• The effectiveness of the heat exchanger, E is a measure of how close the temperature of compressed air leaving the heat exchanger approaches the temperature of the entering ambient air

-----------------------(5)

• Where is the temperature of compressed air leaving the heat exchanger and is the entering temperature of the ambient air

Analysis of Air-Refrigeration Systems• In the turbine, the ratio of the air inlet to outlet temperatures for an

isentropic expansion is:

-----------------------(6)

• where subscript 4 represents the turbine discharge conditions after reversible and adiabatic expansion

• The efficiency of the turbine

-----------------------(7)

where is the actual change in enthalpy during the adiabatic expansion, and is the change in enthalpy in the isentropic expansion. Again -------------- (8)

-------------- (9) where is the actual discharge temperature.

 

Problem 1• For a simple air-refrigeration system of an air-craft flying at an

altitude of 1500 m (ambient conditions there is 0.8 bar and -15 °C), the ram air temperature and pressure are 27 °C and 1.05 bar respectively. At the end of isentropic compression, the air is at 100 °C , and is cooled to 40 °C using ram air. The air is then passed to the cooling turbine, where it expands to the cabin pressure. Thetemperature of air at exit from the turbine is -10 °C. If the air leaves the cabin at 27 °C, obtain:

• The maximum pressure in the system

• The COP

• The power input to the compressor for 0.4 kg/s of air flow. Take

and ɣ = 1.4

Warm air inCold air out(To air cabin)

An open-cycleAircraft coolingsystem

Gas refrigerationCycle with regeneration

Air Refrigeration CycleIn the heat exchanger, the bleed air is cooled at constant pressure to point 3 by another stream of air extracted by a fan.

COP = ?

An ideal-gas refrigeration cycle with air as the working fluid is considered. The pressure ratio for the compressor and turbine is 3. The maximum and minimum temperatures in the cycle, the COP, and the rate of refrigeration are to be determined if the mass flow rate of the refrigerant is 0.08 kg/s.

Air Refrigeration Cycle

Assignment: Do problem 10-55/11-61 from Cengel and Boles, 4th/5th Edition.