Design and Manufacturing of ACM Tester
Normally an aircraft has two or three of these ACM. Each ACM and its components are often referred to as an air conditioning pack. The air cycle cooling process uses air instead of a phase-changing material such as Freon in the gas cycle. No condensation or evaporation of a refrigerant is involved, and the cooled air output from the process is used directly for cabin ventilation or for cooling electronic equipment.
The usual compression, cooling, and expansion seen in any refrigeration cycle is accomplished in ACM by a centrifugal compressor, two air-to-air heat exchangers, and an expansion turbine.
Air from the engines, an auxiliary power unit, or a ground source that can be over 150 degrees Celsius and at a pressure of 32 psi (220 kPa) is directed to a primary heat exchanger. Outside air at ambient temperature and pressure is used as the coolant in this air-to-air heat exchanger. Once the hot air has been cooled, it is then compressed by a centrifugal compressor. This compression heats the air (the maximum air temperature at this point is around 250 degrees Celsius) and it is sent to a secondary heat exchanger which again uses outside air as the coolant. Pre-cooling through the first heat exchanger increases the efficiency of the ACM, as it reduces the inlet air temperature to the compressor, so that less work is required to compress a given mass of air (the energy required to compress a gas by a given amount increases with increasing gas inlet temperature).
At this stage, the temperature of the compressed air is cooled to slightly above the ambient temperature of the outside air. The compressed and cooled air then passes through an expansion turbine, which extracts heat from the air as it expands, cooling it to a temperature below ambient (down to -20 degrees Celsius or -30 degrees Celsius). It is possible for the ACM to produce air cooled to below 0 degrees Celsius, even when the outside air temperature is high (as may be experienced with a stationary aircraft on the ground in hot weather). The work extracted by the expansion turbine is transmitted by a shaft to rotate the enclosed centrifugal compressor and an inlet fan that draws external air into the heat exchangers when operating on the ground. Ram air is used in flight. The power of the air conditioning pack comes from the pressure drop of the inlet air relative to the cool air exiting the system. Typical differentials are from around 30 psi or 210 kPa to around 11 psi or 76 kPa.
The next stage is dehumidification of the air. Cooling the air has caused the water vapor in it to condense into fog, which can be removed using a cyclone separator. Historically, the water extracted by the separator was simply dumped overboard, but newer ACMs spray the water into the outside air inlets for each heat exchanger, which gives the cooler more heat capacity and improves efficiency. (It also means that running an ACM on an aircraft parked on the tarmac does not leave a puddle.)
Now the air can be mixed with a small amount of unconditioned engine air in a mixing chamber. This warms the air to the desired temperature and then the air is discharged into the cabin or electronic equipment.