Keeping Passengers Cool on Commercial Aircraft
Aircraft Air Conditioning
The temperature in an airplane cabin is managed, in part, by an Air Cycle Machine (ACM). The ACM acts as the refrigeration unit to provide cool air in pressurized aircraft. An aircraft typically has two or three of these ACM machines arranged in a system called a “pack”. The ACM packs are typically located near the main landing gear.
The cooling process does not require Freon or other chemical refrigerants. Instead, an ACM uses air to cool the air by expanding and contracting the volume of air. This system has some very distinct benefits over a chemical coolant system.
One of the major benefits of this system is that no condensation is created during the cooling process, allowing the cooled air to be used both in the cabin and for controlling the temperature of electronic equipment without the need for a dehumidifier. The working fluid air is freely available, safe and non-toxic.ACM’s are very reliable, have low maintenance costs and down-timeAn air cycle machine can also output heat.
How the ACM Works
You might be wondering how air can be used to cool air. Air is first compressed and then the heat removed. This volume of air is then expanded to a lower temperature than before it was compressed. Work must be removed from the air during the expansion. Work is removed from the air by an expansion turbine. Energy from the air is consumed as the blades are driven around by the expanding air.The energy of the spinning turbine can be used to run other devices, such as generators or fans. Typically, it is used to power a bootstrapped compressor, which raises the compressed (hot) side pressure further without additional external energy, recycling the energy from the expanding air to compress the high pressure air further. The elevation in pressure on the hot side further increases the temperature and makes the ACM produce more useable heat. The cold air from the turbine can be a refrigerant either directly or indirectly by means of a heat exchanger.
The ACM overhaul and repair process is necessary to maintain operation. Over time, internal parts wear and can cause damage to the unit if not properly maintained.
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