Do Airconditioning Packs produce oxygen?

[Thick E]

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.

[ballyboley]

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.

[WingnutAlan]

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.

[Piper19]

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.

[Silver Spur]

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.

[SNS3Guppy]

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.

[777den]

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%)

[ChristiaanJ]

2. more Ozone ( there is naturally more ozone at high altitudes)
Correct me if I'm wrong, but I thought the compression and resulting heating tended to decompose most if not all the ozone, even at altitude.

CJ

[dartagnan]

Ozone is O3, after mixing with some carbon(C+O), it becomes O2...

[dixi188]

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.

[h3dxb]

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.
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

[cockney steve]

and given that you're high up above most polution, it's some of the cleanest air you'll breathe, too.

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.

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:rolleyes: 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. :hmm:

[NSEU]

Airco packs don't heat the air nor do they pressurize the air,

Having said that, typical air cycle machine operation is based on compression, cooling and expansion. There will be a degree of heat and pressure change. The packs will operate quite well without trim air.... assuming all cabin zones have the same demand for heat/cold. It's how some systems operate in the backup mode.

Rgds.
NSEU

[TURIN]

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.

How can fuel get into the bleed air?

I can see how oil (warm but not burnt) can get into the cabin. But fuel??:confused:

[Old Fella]

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.

[Short_Circuit]

Do Airconditioning Packs produce oxygen?

No ................ :8

[SNS3Guppy]

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.


It is true, actually. No, recirculated air isn't cleaner than bleed air...which is compressed air from the atmosphere...high above most pollutants. Recirculated air is nothing more than bleed air which has entered the cabin and is recirculated by fans before being exhausted through the aircraft outflow valves.

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.


How do you propose to pressurize the cabin, then?


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.


We're talking about turbine powered pressurized aircraft here...not volkswagen bugs.

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.

[muduckace]

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.

A completely false statement as bearing leakage from the C1 and subsequent bearings in the compressor section, if leaking oil will introduce oil into the bleed secton used for trim air. This air is hot enough to induce vapors from oil in most cases (air volitility is a factor too). I can not count the times I have replaced ACM socks to satisfy the many "abnormal cabin smell" PIREPS I have read. There is no other influence than oil into the beed system that normally is the influential factor.

[Old Fella]

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.

[NSEU]

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)

[toolowtoofast]

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)

which shouldn't be a problem at 35000'. if it is, you've got more than a smell in the conditioned air to worry about....

[muduckace]

Wish you old guys were still driving in my world. A C-1 leak will blow into the compressor. Most of the PIREPS I deal with are admittantly a result of APU fouled bleed air, once it gets into the ducts it gives reason for our average soft skinned drivers to big deal.

You gents are a rare breed in this world I live in, have had the pleasure to fly with a few of your era in the early part of my 15 year career.

Hell I had a jackass RTO yesterday for a pilot induced RTO as a result of an ATS run-away that did not exceed thrust limits.

There are more pilots out there these days are way more concerned with SOP's that they end up spending more time concerned with liability than commonsense.

[4greens!]

Some people in this thread would be better informed if they read this (dont be put of by the Learmount word )Toxic cabin air is more poisonous than reckoned - Learmount (http://www.flightglobal.com/blogs/learmount/2009/04/toxic-cabin-air-is-more-poison.html)

[SNS3Guppy]

A completely false statement as bearing leakage from the C1 and subsequent bearings in the compressor section, if leaking oil will introduce oil into the bleed secton used for trim air.


No, a completely true statement. (I'm a practicing A&P/Inspector too, incidentally). The cabin bleed air is indeed among the freshest a passenger will ever breathe.

As for where contamination might be introduced, C1 in one engine isn't possible in another. Bearings, specific terminology, types, locations, and sources vary with the engine. However, introduction of oil or other contaminants into the airstream is a rare and unusual thing in bleed air used for cabin pressurization and air conditioning.

As for the highly scientific comments by one poster who "smelled" exhaust...this is really quite irrelevant, particularly at altitude where I'm quite sure he smelled no exhuast, oil, or other suggestions of contamination.

On the ground when operating around other aircraft, of course one is going to smell exhaust or other scents in the air...bleed air is air from outside the airplane being drawn in. If there's a forest fire nearby, you might smell that. If you're starting an engine with a tailwind, you may smell your own exhaust as it's re-ingested. This is exactly what you'd smell if you were standing on the ramp or taxiway...it's the air you'd be breathing anyway.

At altitude you're breathing air which often is far above most pollution and contaminants, and is among the cleanest of any air you'll ever breathe.

[TURIN]

The cabin bleed air is indeed among the freshest a passenger will ever breathe.

There are a few RJ/146 pilots who will take issue with that statement. :uhoh:

Agreed though. Most airframe/engine combinations do not produce toxic or even non toxic but noticable fumes in the cabin.

The days when we used to go through a full case of 24qts Mobil JetII on one a/c turnround are long gone. Most of the big twins nowadays will happily go multisectors with no engine oil uplift required. :ok:

[filalb]

I would like to read your opinions about the following:

Take an airliner at cruise speed and FL350 for instance.

What is the consequence for the engine in terms of bleed air consumption/power loss if the pilot decides to decrease the cabin temperature?

Will it take more bleed air for the ACM? Less bleed air (already hot)? Or will it remain unchanged?

Regards

[TURIN]

At a rough guess I would say no consequences at all.

The aircraft will still need the same overall flow rate to maintain cabin pressure so all that will happen is the trim air valves will close a bit reducing the amount of hot air into the cabin and the pack flow control valves will compensate to maintain the cabin altitiude, I think. Could be wrong though, I often am. :O

[SNS3Guppy]

What is the consequence for the engine in terms of bleed air consumption/power loss if the pilot decides to decrease the cabin temperature?

Will it take more bleed air for the ACM? Less bleed air (already hot)? Or will it remain unchanged?



That really depends on the temperature change requested, and the system in use. It also depends on the pack configuration.

Small temperature changes can be effected by changing the ratio of warm "trim air" to a given zone in the airplane. This doesn't change any of the flow through the packs at all, but simply changes the ratio of cold pack air to the warm air being taken from the bleed air manifold. Total airflow doesn't change.

A larger change means that the bypass valve in the pack is opened or closed more, altering the amount of airflow which either goes through the pack, or around it. The pack doesn't use more air, but just uses it differently. The pack's job is to extract the proper amount of energy to release the air into the air conditioned ducts at the proper temperature.

In order for the pack to extract energy, cooling airflow is required through the pack heat exchangers, and this cooling air comes with a drag penalty. An increase in cooling drag means an adjustment required in thrust, which means slightly higher fuel consumption. Small changes, very little noticable amount. Bigger changes, a bigger ammount.

Some operators, particularly freight operators, will shut off a pack (or two) in flight in order to save on fuel. This reduces the amount of bleed air required, which makes the engines more efficient, which means a lower fuel burn.

Actually adjusting the temperature up or down by a few degrees makes little difference in the engine operation. The airflow into the bleed air manifold doesn't increase or decrease significantly at all with changes in cabin temperature, because it's all a matter of how the air is used to change the temperature...not how much air is used. It's about the ratio of warm to cold. The total amount doesn't really vary, just the amount of warm or cold...and the engines don't know the difference (it's decided downstream from them...think about it like this: the engines send X amount of airflow, and that's all...the environmental system decides what to do with it, and how to present it to the occupants of the aircraft). The only thing which makes a substantial difference in bleed demand so far as the environmental system goes, is how many packs are operating.

[muduckace]

Actually adjusting the temperature up or down by a few degrees makes little difference in the engine operation. The airflow into the bleed air manifold doesn't increase or decrease significantly at all with changes in cabin temperature, because it's all a matter of how the air is used to change the temperature...not how much air is used. It's about the ratio of warm to cold. The total amount doesn't really vary, just the amount of warm or cold...and the engines don't know the difference (it's decided downstream from them...think about it like this: the engines send X amount of airflow, and that's all...the environmental system decides what to do with it, and how to present it to the occupants of the aircraft). The only thing which makes a substantial difference in bleed demand so far as the environmental system goes, is how many packs are operating.

MMM, not completely true.

At altitude the packs operate off of more efficient ram air, sure it takes some stress off of the bleed air needed to run the ACM's but given the OAT the trim Air "direct bleed" is dumping hot air in mass volumes into the aircraft to maintain desired temperature...

Thus..

A warmer demand required at altitude = more bleed flow = degraded engine performance/efficiency.

I may be corrected on other aircraft but am speaking of the DC/MD-10 and-11. I believe you may be confusing the turbine bypass valve in most ACM's that regulate ACM temp with trim air valves.

[Jimmy Do Little]

There are more pilots out there these days are way more concerned with SOP's that they end up spending more time concerned with liability than commonsense.
So very true!

[TURIN]

A warmer demand required at altitude = more bleed flow = degraded engine performance/efficiency.

Surely the overall flow rate, whether that be through trim air or pack valve/bypass etc will remain constant at constant altitude/cabin pressure.

If the hot air flow rate increases to raise cabin temp then the pack flow rate will reduce accordingly, it will not matter if the cold air is coming from the ACM or from bypass air through the heat exchangers only.

It's been a while since I worked on the '10' but I would have thought it will be the same for all types. :ok:

[Jet II]

Thus..

A warmer demand required at altitude = more bleed flow = degraded engine performance/efficiency.

I may be corrected on other aircraft but am speaking of the DC/MD-10 and-11.

You may be correct on the old DC10/MD11 but modern aircraft have the pack flow rate set by the pack flow controller to a set schedule depending on various parameters - temp selection is not one of those parameters.

[Swedish Steve]

At altitude the packs operate off of more efficient ram air, sure it takes some stress off of the bleed air needed to run the ACM's but given the OAT the trim Air "direct bleed" is dumping hot air in mass volumes into the aircraft to maintain desired temperature...

Thus..

A warmer demand required at altitude = more bleed flow = degraded engine performance/efficiency.

You are getting confused here.
ALL the air that goes into the cabin comes from the engine compressors. This air is cooled down on its way to the cabin. In the cruise, this cooling is provided by the ram air going through the heat exchangers. On the ground the cooling is by the ACMs. The ram air flow does NOT enter the cabin.
The temp demand from the flight deck will change the ratio of bleed air passing through the heat exchangers, but the overall bleed demand from the engines is more or less fixed. (Some aircraft have a pack high flow selector which will increase it a bit)