quickmover
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How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 5:16 am

This may sound like a dumb question, but I've always wondered how airliners get oxygen at high altitudes?

thank you.
 
quickmover
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RE: How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 5:22 am

I mean oxygen for the passengers.
 
Newark777
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RE: How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 5:24 am

Well, there sure is enough oxygen to feed those giant engines, right?  Wink

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julesmusician
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RE: How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 5:28 am

Pressurized air for the cabin comes from the compressor stages in the aircraft's jet engines. Moving through the compressor, the outside air gets very hot as it becomes pressurized. The portion drawn off for the passenger cabin is first cooled by heat exchangers in the engine struts and then, after flowing through ducting in the wing, is further cooled by the main air conditioning units under the floor of the cabin.

The cooled air then flows to a chamber where it is mixed with an approximately equal amount of highly filtered air from the passenger cabin. The combined outside and filtered air is ducted to the cabin and distributed through overhead outlets.

Inside the cabin, the air flows in a circular pattern and exits through floor grilles on either side of the cabin or, on some airplanes, through overhead intakes. The exiting air goes below the cabin floor into the lower lobe of the fuselage. The airflow is continuous and quickly dilutes odors while also maintaining a comfortable cabin temperature.

About half of the air exiting the cabin is immediately exhausted from the airplane through an outflow valve in the lower lobe, which also controls the cabin pressure. The other half is drawn by fans through special filters under the cabin floor, and then is mixed with the outside air coming in from the engine compressors.

These high efficiency filters are similar to those used to keep the air clean in hospitals. Such filters are very effective at trapping microscopic particles as small as bacteria and viruses. It is estimated that between 94 and 99.9 percent of the airborne microbes reaching these filters are captured.

Key Characteristics and Overall Effectiveness
There are several characteristics of the cabin air system that deserve special emphasis:

* Air circulation is continuous. Air is always flowing into and out of the cabin.
* The cabin has a high air-change rate. All of the air in the cabin is replaced by the incoming mixture of outside air and filtered air during intervals of only two to three minutes, depending on airplane size. That's 20 to 30 air changes per hour.
* Outside-air mixing replenishes the cabin air constantly. The outside-air content keeps carbon dioxide and other contaminants well within standard limits and replaces oxygen far faster than the rate at which it is consumed. Replenishment also assures that the recirculated portion of the air does not endlessly recirculate but is rapidly diluted and replaced with outside air.

Source: http://www.boeing.com/commercial/cabinair/

[Edited 2005-11-09 21:31:35]
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critter
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RE: How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 5:36 am

Whether it be for the passengers or to support the combustion of the engines the concept is the same. There is still oxygen present at 30K feet, it is just sparse. The air at that altitude is thin, and the oxygen is spread out. To receive oxygen for the passengers, air is bleed off of the engines (which compress the thin air into a very high density) the aircrafts pressure control system then maintains the proper pressure to give the effect of being at a lower altitude.

The thin air is however very desirable for flight because the thinner the air, the less resistance on the fuselage, also the engines regulate fuel usage to match barometric altitude. The result is the higher you go the more efficient the flight becomes. It is limited by the aircrafts structural ability to withstand pressure differential and also the engines ability to compress the air efficiently to maintain combustion.

critter
 
Tod
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RE: How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 7:44 am

Quoting Julesmusician (Reply 3):
The cooled air then flows to a chamber where it is mixed with an approximately equal amount of highly filtered air from the passenger cabin.

Also, the cooled air is mixed with engine bleed hot air to bring it up to the desired cabin temp.

Quoting Julesmusician (Reply 3):
The other half is drawn by fans through special filters under the cabin floor

Often there air additional recirc fans and filters above the ceiling.

Quoting Julesmusician (Reply 3):
Outside-air mixing replenishes the cabin air constantly

The FAA requires a minimum of .55 pounds of fresh air per minute for each occupant.
Ref: 14CFR25.831

Tod
 
kaddyuk
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RE: How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 9:55 am

Emergency Oxygen is provided by either one of two possible systems, An oxygen resovoir (Like on a B747-400) or by means of Chemical Oxy Generator (Like on an Airbus A340).

Pilot Oxy runs on a seperate system...

Always remember "If its green, keep it clean..."
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SlamClick
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RE: How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 11:21 am

Oxygen makes up about 21% of the earth's atmosphere from sea level all the way to the edge of space. So at altitudes like 30-40 thousand feet all you need to do is compress it to near sea level pressures and the oxygen is there. It is not like there is no oxygen at altitude - there is less air. Compress the air and the oxygen level is the same.
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HAWK21M
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RE: How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 7:37 pm

Quoting Quickmover (Thread starter):
This may sound like a dumb question

No question is dumb if itsgoing to provide you with an answer others don't know about  Smile

Quoting Julesmusician (Reply 3):

Oxygen in Air is provided by the AC packs that cool Air from the Compressors & mixed with Recirculated air to be distributed to the Cabins as per Temperature & pressure desired.
Emergency Oxgen is provided by Chemical Oxygen Generators located in each PSU unit on the Overhead of the Pax seat.
In addition there are Portable O2 Cylinders on board.

regds
MEL
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474218
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RE: How Do Airliners Get Oxygen At 30k Ft?

Thu Nov 10, 2005 11:54 pm

Quoting Julesmusician (Reply 3):
cooled by heat exchangers in the engine struts

What aircraft has its heat exchangers in the engine struts?
 
erj-145mech
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RE: How Do Airliners Get Oxygen At 30k Ft?

Fri Nov 11, 2005 12:08 am

Quoting 474218 (Reply 9):
What aircraft has its heat exchangers in the engine struts?

They're not struts, but the engined pylons on the Embraer 134/145 series airplanes have heat exchangers mounted insided. They're removed and cleaned every 4000 hours.
 
ac21365
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RE: How Do Airliners Get Oxygen At 30k Ft?

Fri Nov 11, 2005 11:50 pm

Correct me if I'm wrong: I thought larger airliners only compressed the cabin to ~850mb or so. It would make sense to do it that way, since compressing air from 100mb-250mb back to 1000mb or so would make it MUCH warmer than 850mb air, causing more energy expendature via air conditioning.
 
SlamClick
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RE: How Do Airliners Get Oxygen At 30k Ft?

Sat Nov 12, 2005 12:10 am

Quoting AC21365 (Reply 11):
Correct me if I'm wrong: I thought larger airliners only compressed the cabin to ~850mb or so

You are correct, actually more like 750-800mb. It is not the heat that is the reason though, since, logically the adiabatic rate should raise the cabin air only to the approximate value of the equivalent altitude - say 6000-8000 feet which would be rather chilly. Rather, it is the structural demands of such a high pressure differential on such a large pressure vessel.

Quoting 474218 (Reply 9):
What aircraft has its heat exchangers in the engine struts?

Most jet airliners that I am aware of have some sort of precooler near the engine pneumatic bleeds. What they want is the pressure, not the temperature. As a gee-whiz number, the compressor section of a jet engine may heat the intake air as high as 600 degrees F before the fuel is introduced. We certainly don't want that in the cabin.

Most would have a heat exchanger, especially when high-pressure bleed air is being used by the system. It is just a scoop to take ambient air and direct it over a cooling core like a car radiator. The pneumatic air passing through is thus cooled.

The air cycle machines that are the air conditioning units normally have two more, larger heat exchangers as part of their design. The final adjustment usually comes with a regulator valve that can admit un-cooled air downstream of the ACMs, just before it enters the cabin duct system.
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320tech
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RE: How Do Airliners Get Oxygen At 30k Ft?

Sat Nov 12, 2005 2:55 pm

The A320 has precoolers in the pylons (1 in each). If the bleed air temp exceeds 230 degrees C (or so, can't remember off hand), the fan air valve opens, which allows bypass air through the precooler. Air is delivered to the packs at around 200 degrees C.
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fr8mech
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RE: How Do Airliners Get Oxygen At 30k Ft?

Sun Nov 13, 2005 1:34 am

Quoting 474218 (Reply 9):
What aircraft has its heat exchangers in the engine struts?

The distinction needs to be made between heat exchangers and bleed air precoolers, a precooler is a heat exchanger.

As stated, bleed air temperature is well above anything usable in the airframe. The air temperature is regulated to something usable. The only number I recall at this moment is a max. temp of 450F or 490F on the B747 classic. At this point the air source is automatically shut down by the PRSOV.

The precooler uses fan air in order to cool the bleed air to something below 490F and above about 300F. Remember, bleed air is also used in the anti-ice systems, thus must remain hot. The precooler is either in the pylon or mounted on the engine itself.

Search for a picture of an MD11 or DC10 engine/pylon and you will see louvers on the pylon (I forget which side). This is the precooler exhaust.
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