Originally Posted by Cron
I pinched the following off a current Rumours thread:
'All AC packs rely on high pressure air from the engines or APU to operate..'
My question is (Keep in mind my knowledge of aircraft is very limited and I'm a bit dim also) that you can buy an A/C unit from your local Electrical store that does not need plumbing - just electricity.
Well, that's not quite true - any a/c needs a heat sink to dispose of the unwanted heat. A window unit has an outdoor "condenser" coil to dump the waste heat to the outdoor air.
In the case of an aircraft - why rob the engines of high pressure air - why not use packs that just require you to rob just the electrical system? I figure this way they can be ground powered as a side benefit as well as being infinitely adjustable as to output.
Put me right someone. Thanks.
The bleed air from the engines supplies the cabin pressurization, but it's way too hot, so the air cycle machines (a.k.a. packs) drop the temperature to the proper level by expanding it through a small turbine. At the time they were designed, older aircrafts' electrical devices were quite heavy, thus unattractive for this task.
In the 787 the electrical system must provide both pressurization and cooling, and it's done with lightweight high-efficiency generators and motors. So whether its bleed air, or generator horsepower though the gearbox, the engine still supplies the energy.