HVAC in airplanes

[cesarvi03]

Dear Forum Members:

Could anybody of you explain me why when the AC system turns on in an airplane it comes into it in the form of fog?

Your technical support is highly appreciated.


Rgds,



Cesar Villavicencio

[koldenhott]

Dear Forum Members:

Could anybody of you explain me why when the AC system turns on in an airplane it comes into it in the form of fog?

Your technical support is highly appreciated.


Rgds,



Cesar Villavicencio

Willis Carrier, while waiting for a train, observed the fog roll in and
observed the relationship of air temperature and water vapor in
air...

...and thus psychrometrics was born in the early 1900's!

[INTECHBILL]

Depending on the size of the plane - Many have (2) systems the primary being a vortex system that is operated by a compressor and high pressure air -- We used this system at the copper mine as a suit while working above the furnaces , it creates much more condensation than the average system do to the forced air within the exchangers - So your fog your seeing is water vapor and it does carry a lot of bacteria and germs within the plane - the latest cure was the airborne spray I'm sure you've see it - one of the biggest problems with a pressurized cabin is that all air transfers are controlled by the pilot and your usually breathing the persons bad breath next to you the whole trip and this is really nice whenever they ate a pound of garlic the night before.

[BaldLoonie]

Willis Carrier, while waiting for a train,

What's that got to do with an airplane?

[qwerty hvac]

What's that got to do with an airplane?

A TRANE is an airplane without wings.

[koldenhott]

What's that got to do with an airplane?

DOH! (head slap!)

Ol' f*rt like me still thinking old technology like trains instead of new tech planes...

...actually mentioning Carrier was a way for all to think about psychrometry behind the fog coming out of plane's vent...

[oroy54]

I don't really know. I would guess R-134A would be the juice. There are compressors that pressure the cabin. To pressure up the cabin there always heat of compression. You feel and smell that also. The compressors keep on pumping during the flight.

[timebuilder]

1) Airplane pressurization

Air is diverted from the compressor section in the jet turbine (prior to being mixed with fuel) and is run through a heat exchanger to cool it, (or through the air pac in lager planes) and that is then ducted through a set of valves into the cabin. The key to maintaining a given pressure is an outflow valve, along with its backup. A dial in the cockpit is used to set the altitude of the airplane, and a differential is calculated so as to not over stress the fuselage, which can only take a small amount of pressure within it. The outflow valves react to the stetting of the cabin pressure control, and bleed enough air out of the airplane to maintain pressure. The airplane is constantly receiving fresh air from the engine compressors, and constantly ejecting air through the outflow valves. For descent, the pressurization is set to the altitude of the airport, and pressure is reduced until that altitude is reached. On landing, a dump valve opens to fully de-pressurize the cabin.

2) Air Conditioning

Small planes and some jets use a typical vapor compression system, usually run off the engine or driven by an electric motor. Because of the age of most airplanes, this is usually R12. The electric motor allows the jet to be connected to a ground power unit and cool the plane while awaiting passengers. Larger jets often have a small jet turbine used only for electric power and running the climate system on the ground.

(Simplified explanation) Larger jets use another air bleed off the engine compressors similar to the pressurization feed, which drives the "air pac," or "air cycle machine," a section under the belly that uses the air bleed to spin a compressor that compresses the cabin air, which heats it, and then the pressurized cabin air is cooled through a heat exchanger, and allowed to expand back into the cabin, much cooler because of the expansion and the heat lost in the exchanger. If you are seeing fog, the condensate section is not draining properly, or the ductwork in the airplane has moisture in it.

I'm curious why you asked. Insight?

[timebuilder]

One more thing. In the new Boeing 787, the bleeds are being eliminated in favor of conventional, electric compressors.

[Bubbleheadski]

One more thing. In the new Boeing 787, the bleeds are being eliminated in favor of conventional, electric compressors.

you never cease to amaze me! I am humbled to in your presence!

[timebuilder]

You can take the man out of the airplane, but you can't take the airplane out of the man....

[oroy54]

The OP had 1 post. Oh well.

But my thought. An airplane on the ground is subject to any ambient temperature.

The air temperature 25,000 ft up drops well below zero.
You might think it would takes heaters to keep the cabin warm.

It still takes A/C.

Do you know why????

[bja105]

The OP had 1 post. Oh well.

But my thought. An airplane on the ground is subject to any ambient temperature.

The air temperature 25,000 ft up drops well below zero.
You might think it would takes heaters to keep the cabin warm.

It still takes A/C.

Do you know why????

Maybe friction, moving through what little air there is at 600 MPH must make some heat. Maybe also no clouds and little atmosphere between the airplane and the sun? I'm guessing.