An air cycle machine is the system that provides fresh, temperature-controlled oxygen to an airplane cabin. This complex machine avoids using freon-like air conditioning units and eliminates the moisture associated with those systems. It comes in different types, with each providing a different level of circulation. This machine is the industry standard for providing air to a cabin and has many other benefits, such as its reliability and environmental impact.
This machine begins its complex process at the engines. Bleed air is the excess air from these enormous turbines, and it is fed into a heat exchanger. The bleed air is too hot for comfort at this point, so the natural air temperature in the sky, usually hovering around the freezing mark, is used as a natural form of coolant. This air is sent to a centrifugal compressor in order to compress the air for efficiency purposes, but this also heats the air up again. The air once again passes through a cooling process using the outside temperature as it gets closer to being expelled inside the plane.
The compressed air is expanded and fed into an inlet fan of the air cycle machine. The fan draws the cooled air through the cabin to the individual air vents and then to the general air vents. The temperature of this system can be raised by a control panel that allows uncooled bleed air into the system to raise the temperature to a desired level.
Depending on the size of the cabin, the air cycle machine can be any of four sizes. The smallest and simplest is one consisting of a turbine and a fan. From this point, more compressors and turbines can be added. The most complex machine consists of two turbines, a fan, and a compressor.
This device has been preferred by airlines for many reasons in addition to its ease of use. One of the main reasons is the fact that it is environmentally friendly because it does not use harmful chemicals such as freon. The system does not create moisture and vapor, so it tends to last much longer than traditional air conditioning systems. By utilizing the bleed air properly, it also erases the need for a separate heating system because the engine air creates as much heat as necessary.