AASA Aviation Practice Exam

The coldest air in an air cycle machine comes from the expansion turbine. When high-pressure air expands through the turbine, it does work on the turbine and undergoes a rapid temperature drop (adiabatic expansion). This large temperature decrease is what produces the cold air used for cabin cooling. The heat exchanger cools the air before expansion, but it doesn’t achieve as low a temperature as the expansion stage. The mixer mainly blends cold supply with cabin return air to set the final temperature, rather than generating the coldest air itself.