It's been a while since I opened an FAA Commercial study book, but I am not sure if they describe the Air Cycle Machine in detail. The JAA books goes a bit more in depth but without a good teacher a student may not make much sense of, if they're not theoretically minded. More and more JAA theory courses are done as distance courses as well. Self-studies and internet based tests. Classroom hours are cut to make your license cheaper!!
If sky-738 is a real pilot, to me it sounds like one who never really joined the dots and got the big picture. I am not surprised if he's a product of the Indian/Chinese sausage machine trained in the US. 150 hours, I believe, the Commercial Ground School course was where I worked. Question banks readily available to ace the exams.
as we all know that 737NG use ram air for bleed air cool for air conditioner use
sky-738 seem to believe ram air cools the bleed air as means of air conditioning?
my question is ,at that situation , ram air is very hot , so what is the cold air source and how could APU make air cool
I think sky-738 meant to say "bleed air is very hot" and wonders how we can cool the bleed air while stationary on the ground an no air is flowing over the heat exchangers?
The system knowledge is so off the mark that the question is not possible to answer. But don't go to hard on sky-738. He's a victim of the system.