air conditioning

huangyhg

air conditioning
hi guys
i work on b767-300 system project. it's all about the air conditioning system.
we measured the temperature of the air flow coming out to the passenger cabin in a flight from nyc to paris and found that it was too low (like 55f). my question is: what really causes the over cooling of the air? why is the air coming so cold to the passenger cabin?
thanks and i appreciate your inputs
david
eng-tips forums is member supported.
uh, because it came _from_ the passenger cabin shortly before being cooled?   
because bleed air gets really cold at altitude?
mike halloran
pembroke pines, fl, usa
55ºf sounds like a good temperature for an a/c.  the problem sounds like whatever is supposed warm up the air isn't work.
ttfn
the bleed air is typically quite hot coming from the engine.  it passes a heat exchanger then usually goes through a turbine (air cycle unit) that drops the temp very low.  now it passes through another heat exchanger to warm for the cabin (should be pilot controlled temp).  
i suggest looking up the system and it should give you the normal temp range for the cabin air.
i have worked boeing aircraft, but not the 767. with that said, you really need to look at what you are asking.
how many zones were on this particular jet?
were all the zones the same temperature or did you only measure in one area of the aircraft?
was the cabin temperature controlled in the automatic or manual mode?
typically, the air leaving the acm gets mixed with bleed air to raise the outlet temperature to prevent freezing of the water seperator. in the automatic mode, this will be controlled to the lowest zone temperature selected. then the other zones will add heat as required depending on selected temp.
is this a problem the crew agrees they could not control? if so, a thorough debrief and a little troubleshooting (by someone who knows ecs) is all you need.
thanks all for your inputs.
well, not all zones were the same temperature. the over cold air were only in the first class zone, while other zones had normal cooling temperature and it's a problem where the crew couldn't control.
i really wanna know why over cooling is there? is it the heat exchanger or what?
kwan said above that the bleeding air itself is the air that enters the cabin, is that right? i thought that bleeding air is used to heat some ambient air which enters the cabin.
isn't the cabin air compressed before enetring the cabin?
what? so they're whining about how cold their $3000 seats are?
but, one would think that each zone might have its own air warmer, to ensure that different loads can be accommodated.  first class, have the least body density would have a different cooling load than the steerage section in the back.
since the other sections are getting appropriately warmed air, the first class section's air warmer is either not functional or the diverter valve is stuck in the bypass position.
ttfn
this shouldn't be too hard to figure out. since the problem is only in one zone in both auto and manual mode, the trim air valve for that zone would be in question. or, a duct over temp sensor is driving the trim valve closed.
i once had a similar problem where we found that during depot, someone had installed an incorrect orfice plate in the trim air valve. the orfice was suppose to be for something else, but fit ok....too bad it was only one inch! the trim valve appeared to be working ok, and was even changed one time with the mechanic repeating the mistake on reassembly. not to say that is what you have, just a story!!