Originally Posted by
TrainDriverSparky
I'm curious. What are packs and bleeds??
Originally Posted by
MADPhil
Not that I'm an expert or a staffer but I think that bleeds are the compressed outside air that is stolen from the engines (bled) to ventilated the cabin and packs are the heating/cooling units that are used to make it more comfortable.
MADPhil has very nearly got it. 'Bleeds' are indeed hot compressed air that is bled off the compressor section of an engine for used thermal anti-ice for wing leading edges (I believe these are now electric on the 787) and engine intakes. Air is taken from two sources to ensure adequate pressure, one is normally a high pressure bleed port and the other a mid pressure. The compressed air is also used to supply hot compressed air to the air conditioning 'packs'. The bleed air is partially cooled before it enters the pack by a heat exchanger, it is then re compressed, partiallycooled again and passed through an expansion turbine that makes it very cold indeed, water is then removed to the maximum extent possible before it enters the distribution ducts. The temperature coming out of the pack can be varied at source and then again further down stream by being 'topped up' with hot 'trim air' that fine tunes the temperature. The air in the distribution ducts also pressurises the water tanks so that the taps work. Bleed air is also used to pressurise the hydraulic tanks to prevent foaming of the fluid.
The 787 does away with the bleed for most uses by use of electric compressors instead.