Hey there, why do we need compressor in Air Cycle Machine (ACM) in aircraft (Airbus family)?
I will answer your question but can you answer my ones on my profile? You need an ACM as compressing the air at high altitude has a different effect on the molecules due to the lower melting point- which 'levels' out the particles if you know what I mean!!
It's part of the heat exchanger.
You need to understand the entire cryogenic cycle to see where the compressor is necessary. When the air is compressed the temperature rises. The air is then run through a heat exchanger where much of the heat is transferred to the ram air bypass loop and exhausted. When the air is then expanded back to a lower pressure the temperature drops and that colder air is then used in the condenser to cool the cabin.
Two reasons. First, to heat the air back up so the second heat exchanger can remove more heat from the air. Second, to absorb power from the turbine, which will cool the air exiting the second heat exchanger to the same degree it heated it up before it entered. Heat exchangers only work effectively with a large temperature differential. If you have 350F bleed air going through heat exchangers on the ground in 80F air, the first one will cool the air much more than the second one, and even with an infinite amount of heat exchangers the air would never get cooler than 80F. But with the ACM, you can extract so much heat from the air by cooling it with 80F air that it usually has to be warmed back up again to prevent freezing. A really good illustration of these principles is an air compressor connected to an air drill or grinder. When the compressor is working, it gets hot. Then turn on the drill and put a load on it; the exhaust air is very cold.
