On a car the a/c compressor runs off the engine.
In a house the a/c compressor is run by an electric motor.

(you can figure out my next question, can't you?)

How is the compressor on a airliner (737, 747,etc) powered?
I've never noticed a big v-belt on the engine before.

If it is powered by a electric motor, if so how is the generator powered? Is there some type of power take off from jet engines?

Thanks,

Paul

It is my understanding, from watching something on the History Channel on the history of HVAC (It was more interesting than it sounds, really) that jet engines compress the air coming into them, and that this compressed air is taken from the engines before fuel is mixed in and burned. Hopefully someone else can elaborate a bit more.

I thought the pilot just opened a window

Ok ok I know

Engine bypass air.

WTF does that mean.

I was on a plane last week an it hit me that the real cool stuff doesn't start until we get in the air with the engines really humming.

On all the planes I worked on, they had ducting that would divert engine intake air to a turbine that was used for the hvac system.

Also, consider the temp of the intake air at altitude - real a/c

So the turbine would only be spinning with enough speed to power the compressor when the plane is in the air.

Correct?

On the ground, air is bled off the APU (auxillary power unit - small jet engine) ducted into the heat exchangers and then fanned into the cabin...Once the engines are running, (APU off) air is bled off the 13th and 6th stages of the compressor (on each engine) then run through the heat exchangers and fanned into the cabin...The reason it's not so cool is because the bleed air is around 450 degrees and so it just cannot cool fast enough before it goes into the cabin...Ambient air also plays into the equation...The hotter the day, the hotter the bleed air, the longer it takes to cool it down...However, the air speed through the duct system is constant, so it does not get to linger or re-circulate through the exchangers again...

Once in the air, outside RAM air comes in to help and very quickly when you figure 35,000 feet is around 40 below 0...Also, the tube (airframe) becomes cooler instantly at that temp...

Except for the Concorde...That airplane growns 11.12 inches in flight due to heat expansion...Every flight...Remember, metal has memory...What happens to a paper clip after you bend it 100 times...

2-OH!

Well in flight. How about no compressor at all

The air outside the plane is cold enough to cool the cabin. They more then likely need to add heat to it before it enters the cabin. This is kind of the same concept large buildings use for cooling requirments in the winter season so they don't need to run a chiller.

Cheers

Sounds good, but remember at 35,000 feet the air is pretty thin. The stewardesses may like the idea of unconscious passengers, but I don't think that would fly with the flying public!

lol


m

It takes air off the compressor section. Depending on what stage it is coming off of it could be upwards of 490 Deg. F( if over that you should get a pack trip) It then goes into a precooler at the pylon. From there it goes into two big heat exchangers and an air cycle mechiene ( think a big turbo) and past the 35 degree valve then into a water seperator. The 35 deg. valve injects hot air back in so the water seps dont freeze.After that it goes into the distribution bay at the back of the forward bag bin then off to cool little Timmy on his way to Grandmas. The out flow valve at the rear closes to start presureization. but as you climb the pres actually decreases from ground pres. The cabin altitude will climb to aprox 10000 feet while the airframe is at 30000+ when you come down it increases the cabin pres to field elevation. That is basicaly it. Hope this helps.

And if you board a plane in Norfolk, VA in the summertime, it blows snow

The new CF 7E7 is gonna eliminate bleed air ducting.
They'll use a hellforstout APU to do the work conventionally done by bleed air.

This makes the airplane MUCH simpler to build and, with the CF fuselage, they can raise the humidity in the cabin to a much more comfortable level.

I would be willing to bet this idea gets scrapped. The airlines will not want to run an APU in flight just to run the packs when they have good engines they can pull air off for virtually no cost

Bleed air ain't free.
Cheers, Elliot

Except for the cabin altitude part....35000 FT at 8.2 psi differential pressure is around 5500 FT cabin altitude.
I spent a lot of hours in the air controlling the pressurization

It's a done deal....but I will bet 50 bucks if you insist

Martin,
That's why you get the big bucks.
cheers, Elliot
PS Congrats.

Dominic,

Flight Engineer?? Or something else.

Me, P3 Orion FE 4600hours. No PIC, time damn the bad luck, kept trying to get those guys to eat the chow hall cooking. Never could get them to do it.

In a three man cockpit or Flight Station for you gentle types, The three have names, do you know what they are?


BOSS(capt) BUBBA (2P)



BOY (FE)

HA HA HA HA OH OH HO HO

Erik

I am not sure the question was answered. Why is the A/C so bleeping hot on the ground. Well sir that is easy.

Most AC systems I am familiar use some sort of condensor and an evaporator and a pump, whether it is run from 14th stage bleed air or 6th or an EDC (engine driven compressor) The bleed air from an APU (Auxilliary power unit)(little tiny jet engine that provides ground electrical power and A/C, and a whole lot a noise) drives a turbine type device (looks like a really big turbocharger) which intakes outside air at a high rate of speed. This is sent into a heat exchanger that is very large so the air slows down across it's interior surfaces, it is cooled to near ambient temp then sped up into a moisture separator of some sort, then mixed with out side air (on the ground) then distributed into the cabin.

The reason you do not get much a/c out inside the plane is the air outside is warm and the heat exchanger is "exchanging" as much heat as it can. Many times you just can't keep up with what the aircraft is absorbing radiantly during the day. Also many older aircraft have water separators that need to be thawed and dried between flights so the flight station may have the A/C in a drying mode (hot) to rid the system of moisture. At least in the old bucket of bolts I flew that was the case. But anyhow, some airports in hot Climates have AC carts on the ramp, they are the ones with the big 12 Inch hose (no pun intended) sticking into the side of the plane usually near the nose landing gear. Aft or somewhere near there.

As for In flight A/C that can be characterized as in flight heat, depending on the day you may see -30C OAT (outside air temp) or colder depending on altitude, you are getting heated , not cooled. One thing that is very important is how much of the air is recirculated, when the out flow valve is closed to maintain cabin pressure, not much air is flowing into the aircraft from the outside, the bleed air (from the engines) is driving compressors to keep the aircraft pressurized. But not at a very high rate. So if you fart, everyone will get a little. HA HA HA Yeah I know I am little touched.

Hope this clears the air (I crack myself up),

Erik Madsen US NAVY DD214'd 1995

Heh Martin, Howabout a main mount/Brake/MLG Change (hit a caribou) in Thule Greenland outside in a survival suit because the tow tug is down for PM. Talk about a PITA. Yeah Baby!

Why couldn't this be a easy answer?
I thought the question was easy.

How in the hell am I going to explain this to a 3rd grader?

Thanks guys,

Paul

Martin,
Go to upsjobs.com and send us a resume. Wouldn't you love to work on a 100series 747. Or a DC8? I'm flying an MD11 and nobody knows shit about the airplane. If nothing works right we just de-power the airplane, then reboot everything. 19 months till retirement.
Cheers, Elliot

The air comes out the side of the jet motor and is really hot. Then it goes through a machine that makes it a little bit cooler by using the wind that the airplane flies through. Then the air goes through another machine that squeezes it really tight and then lets it go real fast and when it comes out of the second machine the air is really cold. Then there is a mixing machine that mixes the really cold air with the really hot air and that machine can make the air any temperature you want it to be. That oughta confuse the little bugger. Good luck.
Cheers, Elliot

Thanks Elliot,

Thats a little better.

Erik,
I spent 4000 hours as an FE in C-141B's (USAF) and B-727's (in the civilian world).
I'm still in the Air Force but have one of those indoor jobs that requires no heavy lifting...But I may return to be an F.E. before I retire, I still have about 8 good years left to serve. Currently Stationed at Langley AFB, VA. (BTW: the weather is awsome this weekend...just wish I had the 914 together)
Later,
Dominic

Desk Job,

I hope you did'nt fall off a wing during preflight or anything. LOL
Hope you get back to flying, Is it a paygrade problem, too high to fly we used to call it.

Erik