A/C Packs
 

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.

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.

In an electrically powered A/C, electricity is used to compressed a gas. In the case of an aircraft, compressed gas is already available. Since what you need is a compressed gas to be cool, it is more efficient to just use the gas available. Energy conversions are wasteful. So it would not be a good idea to have the engines run generators (mechanical to electric conversion) then run an electric compressor (electric to mechanical conversion). It's a very elegant system. Also, the system is more simple because the electric motor (item that can break) is not present.

Currently I'm building my propane powered A/C in my basement, since I got interested in thermodynamics. Ok I have strange hobbies.

The new 787 somehow does not use air from the engines. I think many people are curious as to how they will get an energy saving this way. I certainly am.

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.

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.

Thanks chaps, now I see it.

Not all apu's (power packs) use compressed air to start engines. The new S92 uses hydrolics off the apu to start the engines.