The coldest air of an air cycle machine of a jet aircraft air conditioning system is found in the

• A. Expansion turbine
• B. Compressor
• C. Mixer
• D. Heat exchanger

Answer: (A)

In an air cycle machine, the cooling effect comes mainly from expanding compressed air through a turbine. When the air expands in the expansion turbine, it does work on the turbine and undergoes a near-adiabatic expansion, causing a large drop in temperature. This makes the air exiting the expansion turbine the coldest in the cycle, before it is tempered by heat exchange and mixing for cabin supply. The compressor heats the air, and the heat exchanger and mixer adjust temps, but neither reaches the extreme cooling achieved by the expansion. So the coldest air is at the outlet of the expansion turbine.