737 air conditioner question
hi,everybody.
as we all know that 737NG use ram air for bleed air cool for air conditioner use .but my question is when a very hot day on ground ,as we use APU for air,it still can provide very cold air to make us feel comfortable.
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.