smartt1982
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737: 1 Pack + 2 Bleeds. Why Not?

Sat Jun 23, 2012 7:05 pm

What is the reason not being able to run one pack from 2 x engine bleeds. Is it that one pack cannot simply cope with the amount of bleed air coming from 2 sources of bleed?

Also, why is it that there is a lot more backup for the air conditioning of the CONT CAB/cockpit ie bigger source of air and temp control. What was Boeings thinking behind this, pilots up the front are flying the plane so we will make sure they always have suitable hair.

Of course no problem with it but just curios, why such emphasis on air going to the cockpit.

Is it the same for newer Boeings?
 
David L
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RE: 737: 1 Pack + 2 Bleeds. Why Not?

Sat Jun 23, 2012 7:48 pm

Quoting smartt1982 (Thread starter):
why such emphasis on air going to the cockpit

I suspect you nailed it with this:

Quoting smartt1982 (Thread starter):
pilots up the front are flying the plane

On the other hand...

Quoting smartt1982 (Thread starter):
so we will make sure they always have suitable hair

In my experience they haven't been entirely successful on that front.
 
tb727
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RE: 737: 1 Pack + 2 Bleeds. Why Not?

Sat Jun 23, 2012 7:53 pm

Quoting smartt1982 (Thread starter):
What is the reason not being able to run one pack from 2 x engine bleeds. Is it that one pack cannot simply cope with the amount of bleed air coming from 2 sources of bleed?

You'll probably fry the pack, it's just too much air. You also can't run 2 packs off 1 engine bleed, at least on the 727, not enough air in that case. You can run both packs off the APU bleed though.
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tdscanuck
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RE: 737: 1 Pack + 2 Bleeds. Why Not?

Sat Jun 23, 2012 8:44 pm

Quoting smartt1982 (Thread starter):
What is the reason not being able to run one pack from 2 x engine bleeds. Is it that one pack cannot simply cope with the amount of bleed air coming from 2 sources of bleed?

In addition to the capacity problem, the bleeds will never be at exactly the same pressure so one will always overpower the other if they're ganged together.

Quoting smartt1982 (Thread starter):
Also, why is it that there is a lot more backup for the air conditioning of the CONT CAB/cockpit ie bigger source of air and temp control.

You can't meet the smoke containment or smoke clearing regulations if you feed the flight deck with the same supply as the cabin. You need the flight deck to have cleaner (generally no recirculation), high volume (lots of flow), and higher pressure (make sure flows go out of the flight deck, not in) air.

Also, the cooling for the flight electronics is supplied by flight deck air drawn through the panels by cooling fans. You need to send enough extra air to the flight deck to meet the cooling demand plus all the other needs.

Quoting smartt1982 (Thread starter):
Is it the same for newer Boeings?

Basically, yes, although the 787 doesn't have a bleed system so it doesn't have the same pack limitations.

Tom.
 
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Starlionblue
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RE: 737: 1 Pack + 2 Bleeds. Why Not?

Sun Jun 24, 2012 1:27 am

Quoting David L (Reply 1):
Quoting smartt1982 (Thread starter):
so we will make sure they always have suitable hair

In my experience they haven't been entirely successful on that front.

It is a problem if you have to wear a hat.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Tristarsteve
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RE: 737: 1 Pack + 2 Bleeds. Why Not?

Sun Jun 24, 2012 10:02 am

Quoting tdscanuck (Reply 3):
In addition to the capacity problem, the bleeds will never be at exactly the same pressure so one will always overpower the other if they're ganged together.

The reason for not mixing bleed sources is that Boeing are considering the failure of the bleed NRV in the engine. If the other bleed source is higher pressure then this bleed air can be fed into the engine.
If all the NRVs are working normally, there is no problem mixing bleeds. (but you have no way of knowing)
 
yeelep
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RE: 737: 1 Pack + 2 Bleeds. Why Not?

Sun Jun 24, 2012 6:48 pm

Steve, I agree with your reasoning. However I am not familiar with the acronym NRV. Are you referring to the 5th stage check valves? I'm guessing NRV stands for something like non reversion valve.

Quoting tb727 (Reply 2):
You'll probably fry the pack, it's just too much air.

You won't destroy the pack using dual engine bleed source. Each engines bleed pressure is controlled through the pressure regulator and shutoff valve (PRSOV) to limit output pressure to around 42 PSI. The volume of air is limited by the pressure and the diameter of duct leading to the pack.
 
737tdi
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RE: 737: 1 Pack + 2 Bleeds. Why Not?

Mon Jun 25, 2012 7:56 am

Something y'all may not realize???? There have been some very catastrophic engine failures due to dual bleed malfunctions (737NG). Our engine run checklists were changed and all of us run/taxi designees were told to stress the importance of the dual bleed light on engine run/taxi operations.

I.E.: While taxing at higher then idle throttle, or engine high power runs a failure of the bleed air system on one engine when the bleed air switch/engine bleed switches are improperly selected will cause a melt down (my words) of the opposite engine. I have pics. but I can't show them. Turbines completely fried, melted, fubar.

By the way, this only happens in ground ops.. It won't happen at all if you follow the checklist!!!!!!!!!!. Just a little nugget of info..

737tdi
 
tb727
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RE: 737: 1 Pack + 2 Bleeds. Why Not?

Mon Jun 25, 2012 10:01 pm

Quoting yeelep (Reply 6):
You won't destroy the pack using dual engine bleed source. Each engines bleed pressure is controlled through the pressure regulator and shutoff valve (PRSOV) to limit output pressure to around 42 PSI. The volume of air is limited by the pressure and the diameter of duct leading to the pack.

Ahh yes, that's true, I was thinking of a different problem I had with a 13th stage bleed valve.
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HAWK21M
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RE: 737: 1 Pack + 2 Bleeds. Why Not?

Tue Jun 26, 2012 9:09 am

Quoting yeelep (Reply 6):
I am not familiar with the acronym NRV

Non return valve or Check valve.....allows flow in one direction only.
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