Tom, I may be wrong.This is why I've asked this question,I don't have enaugh technical documentation to check my knowledge about this.
I am a pilot not an engineer.But I've read your answer and I'll think about it while looking further into this.
The way I've been told is this:
cooling cycle:
air flows to the compresssor of an ACM where the air is compressedand temperature is increased.Next ,the air circulates through a secondary heat exchanger for additional cooling.The airflow then passes through the turbineof the ACM where it is expanded and cooled.The cold air flows to a water separatorwhich removes moisturethat has condensed out of the air by operation of ACM.Then ,the processed cold air is mixed with hot air as requested.
Note-the compressore and turbine act over the airflow,and are not actionated by it.This is what I've been told.And I think it's logic,if you push air into a compressor,it will start to rotate,like a fan,but it won't compress the air.It must be actionated somehow to do that.For ex,bya turbine,activated by exaust gases,in the engine comp case.
But there are some years since I pass my exams,so I don't remember much of this subject.
Thks for answering,and don't bow to anyone's knowledge.Just take the best you can,and check it for yourself.At technical knowledge you may be the best there is.Or try to be.It depends only of yourself.
My top instructor,ex factory test pilot for big jets,told me once that you'll never be afraid in whatsoever technical emergency if you know your plane better than your pockets.This is what I've tried to do since than,no matter what plane I'm on.
Sad,but some pilots I know don't consider this very important.They stick with the books,and this sometimes are not so explicit.Sometimes ,the QRH not covers all the emergencies that can appear.
Brgds. Alex