Do Airconditioning Packs produce oxygen?
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Do Airconditioning Packs produce oxygen?
I was asked this question recently and recalling the Air Cycle Machine diagrams I said 'No, they just condition the air i.e filter it and cool/heat it as necessary'.
However, it got me thinking. Even at high altitudes the outflow valve is always partially open. On a longhaul flight, if the packs were not actually producing oxygen then surely by the time that you got to where you were going, you would either have extraxcted the oxygen overboard or the crew and pax would have used the available oxygen in the recirculated air.
I know its a complete dullard question but I don't see any mention in the FCOMs as to actually adding oxygen by way of airconditioning.
However, it got me thinking. Even at high altitudes the outflow valve is always partially open. On a longhaul flight, if the packs were not actually producing oxygen then surely by the time that you got to where you were going, you would either have extraxcted the oxygen overboard or the crew and pax would have used the available oxygen in the recirculated air.
I know its a complete dullard question but I don't see any mention in the FCOMs as to actually adding oxygen by way of airconditioning.
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I dont know the exact figure, but the number of "air changes" the packs/outflow produce is well in excess of what you would get in the average commercial building i.e. they will still completly replace all of the air in the cabin maybe 20 times an hour, so there is probably far far less o2 in the average stuffy conference room full of people than there is at 41,000ft! Obviously the makeup of the air doens't change, i.e. there is still 21% o2 in the air - the packs simply make up for the fact there is less of it and increase the density of air in the cabin by pressurising it.
Last edited by ballyboley; 3rd May 2009 at 15:03.
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Nope
ThickE
When the airliner is at cruising altitude the a/c pack heats the surrounding air and compresses it from abient to cabin pressure. This process ensures that the density of the air, and therefore the density of the oxygen, is the same as it would normally be at cabin pressure. You are right: if the cabin air wasn't allowed to escape the oxygen level would drop and worse, the CO2 level would rise. Even at max recirculation the cabin air is replaced regularly.
There is no system on any aircraft I know of which adds oxygen to cabin air.
Hope this helps.
Ex aircraft mechanic.
When the airliner is at cruising altitude the a/c pack heats the surrounding air and compresses it from abient to cabin pressure. This process ensures that the density of the air, and therefore the density of the oxygen, is the same as it would normally be at cabin pressure. You are right: if the cabin air wasn't allowed to escape the oxygen level would drop and worse, the CO2 level would rise. Even at max recirculation the cabin air is replaced regularly.
There is no system on any aircraft I know of which adds oxygen to cabin air.
Hope this helps.
Ex aircraft mechanic.
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Airco packs don't heat the air nor do they pressurize the air, the only thing they do is cool the air to about 2 degrees C, and you adjust temperature by letting more or less hot compressor air mix with it.
The air comes from the engine compressor thus is at about 190 degrees C and only needs cooling. The engine compressor also makes that the air is already pressurized, final adjustments to keep normal cabin pressure of delta 6-8 PSI is done with the outflow valve.
Oxygen is always 21 percent of the air, but if you pressurize it, your density increases thus more molecules of air per cubic metre, and thus more oxygen for you to breath per cubic metre.
small note before anyone jumps on it:the compressor in a cooling pack is to increase efficiency, not to compress the air to a normal level.
The air comes from the engine compressor thus is at about 190 degrees C and only needs cooling. The engine compressor also makes that the air is already pressurized, final adjustments to keep normal cabin pressure of delta 6-8 PSI is done with the outflow valve.
Oxygen is always 21 percent of the air, but if you pressurize it, your density increases thus more molecules of air per cubic metre, and thus more oxygen for you to breath per cubic metre.
small note before anyone jumps on it:the compressor in a cooling pack is to increase efficiency, not to compress the air to a normal level.
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On most airliner, during cruise, typically, the cabin pressure will be around 8000'. So the oxygen level will be the same as if we are sitting on a ground with 8000' elevation.
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Thick E,
The same percentage of oxygen exists in the atmosphere at high altitude vs. low altitude. The difference is that the air is more dense at lower elevations, and the atmosphere has greater pressure...the oxygen in the atmosphere has more pressure ("partial pressure") with which to pass through the membranes in our lungs and into our blood stream.
As we climb higher and higher, the air is less and less dense, and is under a lower and lower pressure as well. The same percentage of the atmosphere is still oxygen (approx 21%)...there's just less atmosphere, and less pressure.
In order to ensure enough pressure for us to breathe properly and respirate, we pressurize airplanes. We do this by compressing the atmosphere in the aircraft engines, and piping it into the cabin as "bleed air," or air which is bled from the engine compressors.
When the air is compressed, it becomes very hot. It leaves the engine compressor around 600 degrees, and is cooled along the way by various means. It's still too hot for use in the cabin, so we use air conditioning machines, called air cycle machines, to adjust the temperature. Part of the air conditioning process involves cooling the bleed air we use for pressurization, and part of it involves using warm bleed air to then fine tune the cooled air to the desired temperature. The fine tuning is done outside the air cycle machine by several methods at the same time, including bypassing the air cycle machine with warm air, and injecting warm air downstream ("trim air").
To regulate the pressure in the cabin, one or more "outflow valves" open and close to slow the rate at which the air can leave the cabin. Depending on the aircraft size, the entire cabin airflow is replaced every twenty seconds to every minute or two...as air is constantly flowing into the cabin, and leaking out through the outflow valve. The amount of pressurization that's held is varied by changing the size and opening of the outflow valve.
What this means for you in the passenger cabin is that you're constantly receiving fresh air from outside the aircraft. The air you exhale is quickly vented overboard, and a constant supply of fresh air is being piped aboard. You scarcely notice the change, but just as air is being piped into the cabin, it's also being vented back out. Simply put, you won't run out of oxygen in the pressurized cabin air because it's constantly a new supply...and given that you're high up above most polution, it's some of the cleanest air you'll breathe, too.
The same percentage of oxygen exists in the atmosphere at high altitude vs. low altitude. The difference is that the air is more dense at lower elevations, and the atmosphere has greater pressure...the oxygen in the atmosphere has more pressure ("partial pressure") with which to pass through the membranes in our lungs and into our blood stream.
As we climb higher and higher, the air is less and less dense, and is under a lower and lower pressure as well. The same percentage of the atmosphere is still oxygen (approx 21%)...there's just less atmosphere, and less pressure.
In order to ensure enough pressure for us to breathe properly and respirate, we pressurize airplanes. We do this by compressing the atmosphere in the aircraft engines, and piping it into the cabin as "bleed air," or air which is bled from the engine compressors.
When the air is compressed, it becomes very hot. It leaves the engine compressor around 600 degrees, and is cooled along the way by various means. It's still too hot for use in the cabin, so we use air conditioning machines, called air cycle machines, to adjust the temperature. Part of the air conditioning process involves cooling the bleed air we use for pressurization, and part of it involves using warm bleed air to then fine tune the cooled air to the desired temperature. The fine tuning is done outside the air cycle machine by several methods at the same time, including bypassing the air cycle machine with warm air, and injecting warm air downstream ("trim air").
To regulate the pressure in the cabin, one or more "outflow valves" open and close to slow the rate at which the air can leave the cabin. Depending on the aircraft size, the entire cabin airflow is replaced every twenty seconds to every minute or two...as air is constantly flowing into the cabin, and leaking out through the outflow valve. The amount of pressurization that's held is varied by changing the size and opening of the outflow valve.
What this means for you in the passenger cabin is that you're constantly receiving fresh air from outside the aircraft. The air you exhale is quickly vented overboard, and a constant supply of fresh air is being piped aboard. You scarcely notice the change, but just as air is being piped into the cabin, it's also being vented back out. Simply put, you won't run out of oxygen in the pressurized cabin air because it's constantly a new supply...and given that you're high up above most polution, it's some of the cleanest air you'll breathe, too.
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The major differences in 8000 ft cabin air vs. 8000 ground air are :
1. less pollution ( you are above most pollution)
2. more Ozone ( there is naturally more ozone at high altitudes)
3. dryer ( since you are heating -60 deg outside air the Relative humidity is almost 0%)
1. less pollution ( you are above most pollution)
2. more Ozone ( there is naturally more ozone at high altitudes)
3. dryer ( since you are heating -60 deg outside air the Relative humidity is almost 0%)
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Originally Posted by 777den
2. more Ozone ( there is naturally more ozone at high altitudes)
CJ
My brain has come up with some numbers I must have learnt many years ago.
The certification requirements of the airflow into the cabin of a commercial airliner are :- 1 pound (mass) of air per minute per occupant in normal operation, and
0.5 pound of air per minute per occupant in standby/emergency operation.
On the BAC 1-11 this gave a flow of 60 pounds per minute for each pack. (Although the maximum occupancy was 125)
The current certification requirements may not be the same though.
Hope this helps.
The certification requirements of the airflow into the cabin of a commercial airliner are :- 1 pound (mass) of air per minute per occupant in normal operation, and
0.5 pound of air per minute per occupant in standby/emergency operation.
On the BAC 1-11 this gave a flow of 60 pounds per minute for each pack. (Although the maximum occupancy was 125)
The current certification requirements may not be the same though.
Hope this helps.
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Packs do not produce oxygen itself, but A/C's cockpits have 100% fresh air provided from the packs. The air is taken before the mixing chamber. Pax get 60%, rest is 40% filtered recycled air.
Modern A/C's have also Ozone Converter.
Modern A/C's have also Ozone Converter.
Upstream of each flow control valve there is an ozone converter installed,
which comprises a ceramic core to convert most of the ozone in the bleed air into oxygen. But this is not extra oxygen.
rgds
which comprises a ceramic core to convert most of the ozone in the bleed air into oxygen. But this is not extra oxygen.
rgds
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and given that you're high up above most polution, it's some of the cleanest air you'll breathe, too.
There has been much discussion about toxic cabin-air, in the Forum. IMHO ALL heating of Cabin-air should be done via a sealed heat-exchanger.
Slight thread drift
The old air-cooled VW beetle derived heat from muffles round the exhaust manifolds, which were massively over-engineered . The cooling-fan system ensured that the ducting was always under positive-pressure, thereby excluding toxic fumes ....the weakness in the system, being the source of clean, uncontaminated air, to feed the fan. 
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Airco packs don't heat the air nor do they pressurize the air,
Rgds.
NSEU
That would be true, if all cabin air was "fresh" air frim outside......even recirc. cabin air is potentially cleaner than bleed air that has been through the engine and picked up leakages of partially burnt oil/fuel fumes.
I can see how oil (warm but not burnt) can get into the cabin. But fuel??
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Fuel in bleed air
If unburnt fuel is getting into the bleed air system the engine has big problems. Bleed air is drawn from the compressor section of the engine, which is not supplied any fuel. AsTurin says, it is possible for oil to get into the bleed air stream if an oil seal leaks. The same can happen in the ACM. However, I would rather be breathing conditioned bleed air rather than just recirculated cabin air any day. That is why recirculated air is also changed regularly. It is recirculated on its way eventually to the outflow valves, not recycled. There is a big difference.
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That would be true, if all cabin air was "fresh" air frim outside......even recirc. cabin air is potentially cleaner than bleed air that has been through the engine and picked up leakages of partially burnt oil/fuel fumes.
Bleed air doesn't pick up "leakages of partially burnt oil/fuel fumes." As others have noted, bleed air comes from the compressor section. As any "partial burning" takes place downstream from the compressor...you're not going to get any burned oil or "fumes" in the bleed air. Labrynth seals are used to contain oil at each bearing point in the engine, and these are pressurized seals to prevent oil from entering the airstream. Oil consumption in turbine engines is extremely low, and leakage into the airstream through the engine is little or none...certainly nothing of any concern considering the high cubic feet per minute volume of airflow through the engines, and through the bleed system.
So yes...bleed air and consequently cabin air is some of the cleanest air you'll ever breathe.
There has been much discussion about toxic cabin-air, in the Forum. IMHO ALL heating of Cabin-air should be done via a sealed heat-exchanger.
Slight thread drift The old air-cooled VW beetle derived heat from muffles round the exhaust manifolds, which were massively over-engineered . The cooling-fan system ensured that the ducting was always under positive-pressure, thereby excluding toxic fumes ....the weakness in the system, being the source of clean, uncontaminated air, to feed the fan.
The VW bug used an air cooled engine, and used the same type of heating system for the vehicle as most air light airplanes; a muffler shroud. The muffler shroud doesn't prevent "toxic fumes," as any leakage from the exhaust into the shroud will introduce carbon monoxide into the car...just as it does in a light airplane.
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SNS3Guppy
Bleed air doesn't pick up "leakages of partially burnt oil/fuel fumes." As others have noted, bleed air comes from the compressor section. As any "partial burning" takes place downstream from the compressor...you're not going to get any burned oil or "fumes" in the bleed air.
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Bleed air.
As Guppy stated, turbine engine oil seals for the shafts are air pressurized labyrinth seals and oil consumption is very low in normal operation. About the only way oil can get into the bleed air system is, on a turbo-prop installation, if the propeller suffers an oil leak it is possible for oil to enter the compressor of the core engine. This however is a rare occurrence.
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Having smelled fuel in the cabin on engine start many times (after the doors have closed), I'm sure there is a way for fuel to get into the bleed air system (even if it's only from re-ingestion from other engines in strong tailwinds)