Posts: 8
Joined: Wed Dec 20, 2006 8:25 pm

ATR-72-500 Air Conditioning

Tue Jun 19, 2007 8:34 pm

Hello, I have several questions about air conditioning:

First Id like to recall tecnical background: ATR-72-500 has two packs, the left pack supplying air to flight deck and cabin and the right pack to cabin only. Packs discharge temperature is regulated by the automatic controller which adjust the temperature control valve adding hot air to cool air from the compresor/turbine section of the pack. My questions are:
1) What is the purpose of the hot by-pass valve and tic valve?
2) If I want to set cockpit temperature to lower level than cabin temperature, the left pack temp will be lower than the right pack temp, wont it? But there is an aspect that I dont understand: as the left pack also supplies air to cabin, than the cabin receives air of two different temp, cooler from the left pack and requested air temp from the right pack. In this case the cabin temp would be below requested level. Theorically, in this situation, right pack automatic temp controller should rise the right pack outlet temp to balance the impact of cooler air from the left pack. Is my intuition correct? It may be a silly question for you, guys but I cant find explicite answer in my systems manual and I try to understand deeply every aspect of a ATR-72 aircraft
3) Why the pack discharge temp is regulated by adding hot air to cooler air from the compresor/turbine section of the pack and why the discharge temp. isnt regulated only by compresor/turbin section of the pack? I know that on aircrafts like B744 adding trim air makes sense because the cabin has several section and for each section there can be set different temp, but on an ATR...?
Thanks guys for your response, sorry for my bad English.
Posts: 4955
Joined: Wed Mar 01, 2000 11:10 am

RE: ATR-72-500 Air Conditioning

Wed Jun 20, 2007 6:20 pm

No prob, your English is very good.
It's very late, but I will attempt an abbreviated answer to question number three.
All packs work (more or less) the same. You take very hot bleed air, cool it, compress it, and then expand it. This makes cold air. That way, you can cool yourself on the ground in an ATR (your airplane) or on a 747 on the ground (my airplane). But when things get chilly, as in wintertime ground ops here in Alaska, or airborne flight year-round, that air needs to be warmer. At that point, hot bleed air is RE-introduced to the cold air, to nudge the "finished product" back up to more comfortable temperatures.
This is the way a pack works, regardless of electronic controls or mechanical, ATR or A380.
In other words, the output temperature of the compressor/turbine section is NOT controllable. If it's too cold, you simply add hot trim air.

Hope that helped!

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