Normally, an aircraft has two or three ACMs. Each ACM and its components are often referred to as an air conditioning package. The air cycle cooling process uses air instead of phase change materials such as Freon in the gas cycle. There is no condensation or evaporation of the refrigerant involved, and the cool exhaust air from this process is used directly to ventilate the cabin or to cool electronic equipment.
The typical compression, cooling, and expansion seen in each refrigeration cycle is performed in the 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°C 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 a coolant in this air-to-air heat exchanger. Once the hot air has cooled, it is then compressed by a centrifugal compressor.
This compression heats the air (the maximum air temperature at this point is about 250 degrees Celsius) and is sent to a secondary heat exchanger, which again uses the outside air as a coolant. Precooling through the first heat exchanger increases ACM efficiency because it lowers the temperature of the air entering the compressor so that it takes less energy to compress a given mass of air (the energy required to compress a gas by a given amount). Less is needed. The value increases with increasing gas inlet temperature).
At this stage, the temperature of the compressed air is cooled to a little higher than the ambient temperature of the outside air. The compressed and cooled air then passes through an expansion turbine, which removes heat from the air during expansion and cools it down to a temperature below ambient (to -20°C or -30°C). It is possible for the ACM to produce air cooled to below 0°C 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 turn the enclosed centrifugal compressor and an intake fan that draws outside air into the heat exchangers when operating on land. Ram air is used in flight. The power of the air conditioning pack is caused by the pressure drop of the incoming air compared to the cool air coming out of the system. Typical differentials range from about 30 psi or 210 kPa to about 11 psi or 76 kPa.
The next step is air dehumidification. The cooling of the air has condensed the existing water vapor and turned it into fog, which can be removed using a cyclone separator. Historically, the water extracted by the separator was simply dumped out, but newer ACMs spray water into the outside air inlets for each heat exchanger, which gives the cooler more heat capacity and improves efficiency (it also means that the implementation of ACM in an aircraft parked on the asphalt does not leave a pit.).
The air can now be mixed with a small amount of unconditioned engine air in a mixing chamber. This heats the air to the desired temperature and then the air is discharged into the cabin or electronic equipment.
