Types of aircraft cooling systems
Aircraft air refrigeration systems are required due to heat transfer from many external and internal heat sources (like solar radiation and avionics) which increase the cabin air temperature. With the technological developments in high-speed passenger and jet aircrafts, the air refrigeration systems are proving to be most efficient, compact and simple. Various types of aircraft air refrigeration systems used these days are:
- Simple air cooling system
- Simple air evaporative cooling system
- Boot strap air cooling system
- Boot strap air evaporative cooling system
- Reduced ambient air cooling system
- Regenerative air cooling system.
Simple air cooling system:
Process 1-2= Isentropic ramming of air
Process 2-3i=Isentropic compression in main compressor
Process 2-3=Actual compression in main compressor
Process 3-4=Constant pressure heat rejection in heat exchanger
Process 4-5i=Isentropic expansion in cooling turbine
Process 4-5= Actual expansion in cooling turbine
Process 5-6=Constant pressure heat addition in cabin
- Ambient air gets rammed before the main compressor. The air required for refrigeration system is then bled off from this compressor.
- It is then sent to the heat exchanger where this high pressure and high temperature air is cooled using ram air.
- Air is further cooled in the cooling turbine due to its expansion. The turbine work drives the cooling fan which draws ram air through the heat exchanger.
- The cooled air from turbine is then sent to the aircraft cabin.
Boot strap type aircraft refrigeration system:
Fig. Boot Strap Aircraft Air Refrigeration System
- The ambient air undergoes ramming before the compressor. Assuming isentropic ramming (process 1-2 in figure).
- After compression (2-3), the air bled from the main compressor is cooled by ram air in the first heat exchanger or intercooler (3-4).
- This air is then compressed further in the secondary or auxiliary compressor (4-5) and is led to the second heat exchanger, again cooled by ram air (5-6).
- The air is then expanded in the cooling turbine (6-7) which drives the secondary compressor. This cooled air is then finally sent to the cabin.
- This system is preferred for high speed aircrafts and hence the pressure of air after ramming is comparatively high. So a cooling fan is not required since ram air will now automatically flow over the heat exchanger tubes.
- Multistage compression with intercooler decreases the compression work for this system and thus in turn improves the C.O.P.