My understanding is that air coming in through the jet turbines is compressed and used to pressurize the cabin. What is the temperature of this compressed air? I realize that the air outside at 35K feet may be something like -70F, but on the other hand, thermodynamics (PV=NRT) as well as conduction from the fiery heat of the jet engine itself, tells me that the air could end up being quite hot. Are the compressors designed to output air ina habitable temperature range, or is some kind of additional cooling or heating required? How are variations in the temperature corrected for?
With all the people crammed into a small space airliners would get very hot in a hurry if not for the introduction of cold air. Remember that the air will be pressurized and rapily cool when it decompresses.
Commenting on the bolded portion above… pressurization air is drawn from the compressor portion of the jet engine, which is well before the combustion chamber. So I can’t imagine there’s any significant heat leakage from combustion to pressurization air. What heating there would be is just compression heating, and that still makes the air too hot to directly dump into the cabin… Wikipedia article about bleed air says that at its hottest it’s over 300 degrees celcius. Ow.
Forward leakage of combustion heat seems trivial compared to just that.
As mentioned in that article, cabin air has to be cooled first. They mention an environmental control system, but it is probably trivial to cool that compressed air through a simple radiator, in the same manner that a common refrigerator uses. However, planes stuck on the ground may have a problem without a powered cooling system. Usually this happens in snow storms, so perhaps it’s not a big problem in cold weather.
Yeah, the article says:
Cool. This basically answers my question. I definitely still wonder exactly how the environmental control system works (basic refrigerator technology to be turned on and off as necessary?) and how things are compensated for at different outside air temperatures and pressures, like, as already mentioned, when the plane is still on the ground, or when the plane is taking off.
They have airconditioning packs with controls for temperature. More modern aircraft might be more sophisticated but the BAe146 has two packs and either manual or automatic control. The controls are an on/off switch for each engine air valve and the APU air valve, an on/off switch for each pack, a switch to select between manual or auto control for each pack, a rotary temperature control knob for each pack when in auto mode, a rocker switch for each pack to drive its output either hot or cold when in manual mode, and a fresh/recirc switch which selects between partially recirculating the cabin air or getting all of the air from the engines.
The controls are on the right side of the overhead panel.
In normal operation in flight all of the engine air switches are on, both packs are on, and the control is auto. We set the controls to have the ducts at around 5ºC but the actual temp varies depending on the phase of flight so it needs to be tweaked occasionally (normally when the cabin crew complain it is too hot or too cold.)
On the ground and for take-off and landing the engine air switches are off and the APU air switch is on to provide air to the packs. On the ground we will sometimes turn pack 1 off to reduce the load on the APU. Pack 1 supplies air to the flight-deck and pack 2 supplies the cabin but with either pack turned off the other will supply both areas.
Can you say what the temperature would rise to in the cabin at cruise altitude if the air conditioning packs were turned off?
What is the most likely reason for the cabin being too hot while at the gate? Money saving? Trying to keep the APU in better shape? Not paying attention and busy with other stuff?
Are there heaters on the plane at all?
The following is specific to the BAe146 but the principle will be the same in other types. If the packs were turned off then you wouldn’t get any air in to the cabin and it would depressurise. If the packs weren’t cooling properly you could have air up to 105ºC in the conditioning ducts but anything above that will shut down the packs. Anything like an over-temp or pack shut down will trigger a warning light and an aural alarm in the flight-deck to alert the crew.
It depends. Is it cold outside or hot? if it is hot outside and there are a lot of passengers in the cabin in may just be that the airconditioning is at max cold and still can’t cool the cabin adequately. If it is cold outside then it may be that no one has told the flight crew it is too hot in the cabin. A big issue is that people have different comfort levels and what is too hot for someone might be just right for someone else.
No. The bleed air coming from the engines or APU is already very hot so additional heating is not required.
Here’s a general overview of the air conditioning system on the CL60X aircraft type:
Bleed air for air conditioning is extracted at the 10th compression stage of the engine turbines, the APU or from a ground air supply. I’ll really just consider the 10th stage bleed air source.
The air pressure from the engine is reduced through the use of valves and venturi tubes. The air passes through a precooler and primary heat exchanger and then through a cooling turbine and secondary heat exchanger before being treated by a water separator which removes water vapour from the air (this prevents corrosion and damage inside the rest of the plane). From there, the air flows to the air conditioning ducts and into the passenger cabin and flight deck.
There are two air conditioning systems - one is generally used for the passenger cabin while the other is used for the flight deck, but both/either can supply either area of the aircraft. Temperature and pressure are monitored by sensors and reported back to the flight crew via EICAS messages.
The precooler and heat exchanger outside temperatures are cooled by ram air. The water collected in the water separator is reevaporated into the cooling side of the heat exchangers (not the air side that ends up in the cabin) which makes the heat exchangers more efficient.
The temperature itself is modulated by a couple of valves that regulate how much air after the precooler is mixed with the cold air from the air conditioning units.
If both air conditioning systems fail, there is an emergency ram air shutoff valve that can supply air directly to the distribution system.
I hope that helps. I’m not actually all that familiar with the system, but I happen to have a general description of it available to me, so there you go.