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beakerltn
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Bleed Air And Oxygen

Thu Apr 02, 2015 7:56 pm

I've just wondered, as there's very little oxygen at high altitude, where does the oxygen in the cabin come from during flight? Bleed air at altitude surely contains no oxygen?
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Jetlagged
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RE: Bleed Air And Oxygen

Thu Apr 02, 2015 8:22 pm

The air at altitude contains some oxygen, in the roughly same proportion as at sea level. Engine bleed air is ambient air which has been compressed. This increases the air density, and therefore oxygen density, to near sea level values.
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SAAFNAV
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RE: Bleed Air And Oxygen

Fri Apr 03, 2015 6:03 am

There isn't any less oxygen up there, but the partial pressure is lower than the partial pressure of dissolved blood in the lungs.

As soon as you pressurize the same mix of air, your body can 'process' it again.
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kalvado
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RE: Bleed Air And Oxygen

Fri Apr 03, 2015 1:05 pm

Quoting SAAFNAV (Reply 2):
There isn't any less oxygen up there,

Actually there is. Difference is not that great, but generally oxygen concentration (as oxygen to nitrogen ratio) goes down due to oxygen molecules being heavier. Due to air intermixing in troposphere, difference is much less than it would be in stagnant atmosphere (stagnant case would be 19% at FL400 instead of 21% at sea level)
 
lancelot07
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RE: Bleed Air And Oxygen

Fri Apr 03, 2015 2:38 pm

Quoting beakerltn (Thread starter):
I've just wondered, as there's very little oxygen at high altitude, where does the oxygen in the cabin come from during flight? Bleed air at altitude surely contains no oxygen?

Without oxygen, the engines could not work. They need oxygen for burning (=oxidizing) jet fuel. Without oxygen from the air, you would need rocket engines and bring your oxygen supply with you.
 
nomadd22
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RE: Bleed Air And Oxygen

Fri Apr 03, 2015 2:40 pm

I'm pretty sure air is 21% oxygen at all altitudes. The pressure/partial O2 pressure ratio can change.
One counter intuitive thing for aircraft is that, even though you might be sucking up -80F air to compress for the cabin, you still have to cool it down because compressing it heats it up so much.

[Edited 2015-04-03 07:42:02]
Anon
 
kalvado
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RE: Bleed Air And Oxygen

Fri Apr 03, 2015 3:25 pm

Quoting nomadd22 (Reply 5):
I'm pretty sure air is 21% oxygen at all altitudes

Not really. Things are really funny above 100-150 km, but there is some variation at lower altitudes.
http://www.adlers.com.au/images/oxygengraph.gif is an interesting graph, and that looks realistic.
Reason is that probability of finding molecule up there is given by statistical distribution, exp(-mgh/kBT.
For 12 km and 300 K factor under exponent is 1.32 and 1.51 for N2 and O2 respectively.
And air intermixing works on making things more homogenius
 
Tod
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RE: Bleed Air And Oxygen

Fri Apr 03, 2015 6:42 pm

Quoting nomadd22 (Reply 5):
One counter intuitive thing for aircraft is that, even though you might be sucking up -80F air to compress for the cabin, you still have to cool it down because compressing it heats it up so much.

And then re-heat it back to the desired cabin temp by adding trim air.
 
AA737-823
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RE: Bleed Air And Oxygen

Mon Apr 06, 2015 4:42 am

Quoting Tod (Reply 7):
And then re-heat it back to the desired cabin temp by adding trim air.

Not always.
Several variants of the most popular jetliner in the world don't use trim air!
 
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DocLightning
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RE: Bleed Air And Oxygen

Mon Apr 06, 2015 6:44 am

Quoting nomadd22 (Reply 5):
I'm pretty sure air is 21% oxygen at all altitudes.

It does go slightly down with altitude, but in a real-world case it would still be at least ~19-20% at FL 400 and that's quite breathable if it's compressed to a cabin altitude of 8,000 feet or less.
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Tod
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RE: Bleed Air And Oxygen

Mon Apr 06, 2015 9:04 pm

Quoting AA737-823 (Reply 8):
Several variants of the most popular jetliner in the world don't use trim air!

True enough.
Thanks for the correction.
 
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bikerthai
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RE: Bleed Air And Oxygen

Thu Apr 09, 2015 6:34 pm

. . . only if you fly through the hole in the Ozone Layer  

bt
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flylku
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RE: Bleed Air And Oxygen

Sat Apr 11, 2015 5:30 pm

Quoting AA737-823 (Reply 8):
trim air

Definition please. I know what bleed air is but...
...are we there yet?
 
nomadd22
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RE: Bleed Air And Oxygen

Sat Apr 11, 2015 5:47 pm

Quoting flylku (Reply 12):
Quoting AA737-823 (Reply 8):
trim air

Definition please. I know what bleed air is but...

Cooling packs cool the air too cold in some planes, so you add a little bit of hot "trim air" to the mix to cool it back down. Some designs have well regulated cooling packs and don't need trim air.
Anon
 
nomadd22
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RE: Bleed Air And Oxygen

Sat Apr 11, 2015 10:42 pm

Quoting nomadd22 (Reply 13):
Quoting AA737-823 (Reply 8):
trim air

Definition please. I know what bleed air is but...

Cooling packs cool the air too cold in some planes, so you add a little bit of hot "trim air" to the mix to cool it back down. Some designs have well regulated cooling packs and don't need trim air.

I obviously meant that the trim air was added to warm the mixture for the cabin. Caught again by Anet's inexplicable refusal to let people edit posts more than a few minutes old.
Anon
 
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CCA
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RE: Bleed Air And Oxygen

Sat Apr 18, 2015 2:17 am

Simplified version -Trim air.

Say 4 zones in the cabin A,B,C,D. The temperature is set at

A - 22°C,
B - 23°C,
C - 24°C,
D - 25°C.

Humans generate heat so imagine the air conditioning packs are putting out 5°C to maintain 22°C in the entire aircraft, but the other zones need to be a little warmer so a little trim air (hot) is added to zones B,C & D.
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HAWK21M
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RE: Bleed Air And Oxygen

Sat Apr 18, 2015 9:55 am

Quoting flylku (Reply 12):

Definition please. I know what bleed air is but..

Trim air is hot air mixed with conditioned air to vary the temperature of the conditioned air entering the Flight deck/cabin.
Controlled by trim air valves.
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cobra27
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RE: Bleed Air And Oxygen

Sun Apr 19, 2015 10:25 am

When I was a kid, my father said to me that they carry it onboard, which is obviously false. He was partly right, the emergency oxygen in masks is provided by chemical reaction, but its supply lasts only few minutes.
 
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HAWK21M
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RE: Bleed Air And Oxygen

Sun Apr 19, 2015 10:30 am

Quoting cobra27 (Reply 17):

When I was a kid, my father said to me that they carry it onboard, which is obviously false. He was partly right, the emergency oxygen in masks is provided by chemical reaction, but its supply lasts only few minutes.

On the older types, it was routed to the cabins vide pipelines from an Oxygen Cylinder based below in the Bulk compartment eg B732s.
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BravoOne
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RE: Bleed Air And Oxygen

Sun Apr 19, 2015 8:36 pm

Quoting HAWK21M (Reply 18):
On the older types, it was routed to the cabins vide pipelines from an Oxygen Cylinder based below in the Bulk compartment eg B732s.

I'll have to go out on a limb here and say I have not seen any 737-200 with a gaseous O2 system installed by Boeing. Many BBJs do have this but it's usually something that comes from a completion center during the interior mods. Both the 777 and 787 offer a gaseous system in lieu of the drop down O2 generating mask and even the masks come two different capacities.
 
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HAWK21M
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RE: Bleed Air And Oxygen

Sat Apr 25, 2015 7:55 pm

Quoting BravoOne (Reply 19):
Both the 777 and 787 offer a gaseous system in lieu of the drop down O2 generating mask and even the masks come two different capacities.
http://bloga350.blogspot.com/2013/05...-xwb-equipped-with-new-oxygen.html

Interesting link on another thread....based on the A350.
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CALTECH
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RE: Bleed Air And Oxygen

Sat Apr 25, 2015 11:44 pm

Quoting BravoOne (Reply 19):
I'll have to go out on a limb here and say I have not seen any 737-200 with a gaseous O2 system installed by Boeing.

"Passenger oxygen on 737-1/200's is supplied by two oxygen bottles in the forward hold. The capacity varies with operator but is typically 76.5 cu ft each. Oxygen bottle pressure is indicated on the aft overhead panel."

Passenger oxygen gauge measuring bottle pressure.

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BravoOne
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RE: Bleed Air And Oxygen

Sun Apr 26, 2015 6:26 pm

Quoting CALTECH (Reply 21):
"Passenger oxygen on 737-1/200's is supplied by two oxygen bottles in the forward hold. The capacity varies with operator but is typically 76.5 cu ft each. Oxygen bottle pressure is indicated on the aft overhead panel."

Very good. Was this a corporate or pax airplane. I can find no reference to the system you are showing in My Boeing Fleet?
 
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CALTECH
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RE: Bleed Air And Oxygen

Sun Apr 26, 2015 7:12 pm

Quoting BravoOne (Reply 22):
Very good. Was this a corporate or pax airplane. I can find no reference to the system you are showing in My Boeing Fleet?

Would have to dig up my old 737 manuals, but believe this was in a pax plane. IIRC, there was a fiberglass compartment in the FWD Cargo Compt, on the right side, and the bottle/bottles were there, a smaller Crew O2 and a much larger Pass O2 bottle. At some point in the 737 production run, there was only a Crew O2 bottle in there, and finally they moved Crew O2 into the E&E compartment, though access is from the cargo compartment.
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BravoOne
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RE: Bleed Air And Oxygen

Sun Apr 26, 2015 7:53 pm

Don't mean to beat this to death but was this perhaps and Air Force aircraft? Saw your profile and assumed that maybe you had been on a couple of bases in your life 
 
yeelep
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RE: Bleed Air And Oxygen

Sun Apr 26, 2015 9:06 pm

I can tell you that 737-200 combi's had the same o2 setup as CALTECH described/pictured.
 
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7BOEING7
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RE: Bleed Air And Oxygen

Sun Apr 26, 2015 9:20 pm

Quoting BravoOne (Reply 24):
Don't mean to beat this to death but was this perhaps and Air Force aircraft? Saw your profile and assumed that maybe you had been on a couple of bases in your life

Found on 737-100/-200, 727-100/200, 747-100/-200/-300/-400, 777 -- except in some cases the oxygen pressure is shown on displays (744/777) not a gauge and in later models the switch has a reset position or a circuit breaker can be pulled and reset to reset the system.
 
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CALTECH
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RE: Bleed Air And Oxygen

Sun Apr 26, 2015 9:56 pm

Quoting BravoOne (Reply 24):
Don't mean to beat this to death but was this perhaps and Air Force aircraft? Saw your profile and assumed that maybe you had been on a couple of bases in your life

Been around a bit, long ago, in a place far, far away, but in the fog of time kinda remember servicing the O2 on some of our old 737-100/200s in the FWD Cargo Bin. Could be hallucinating from all the Jet Fuel fumes over the years though.   
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BravoOne
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RE: Bleed Air And Oxygen

Mon Apr 27, 2015 9:36 am

Quoting CALTECH (Reply 27):

Thanks for the insight.
 
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HAWK21M
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RE: Bleed Air And Oxygen

Sun May 24, 2015 12:57 pm

On the B732s there was a facility of refilling the portable O2 bottles from a ground source but was not regular as it was a fire hazard....it was easier to replace the bottles in the bulk compartment.
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