smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

Pack Light 737

Sun Jan 06, 2013 1:22 pm

PACK LIGHT 737 NG
In regards to the PACK LIGHT annunciation on the 737 NG, I need some clarification as I believe I am completely confusing myself.
In the situation that the PACK Light illuminates as a result of PACK overheat (so does not extinguish after Master Caution reset) As per the QRH you select a warmer temperature on ALL the TEMP selectors in order so that AFTER Trip Reset you reduce the load on the PACK (as would be the case on a HOT day and the pack was working too hard to cool it down)
I have read that BOTH air mix valves are driven to full cold and TEMP selectors are disabled.
My questions/what I need clarifying is

When it says BOTH air mix valves. Does each of the TWO Packs have two air mix valves each or are these TWO air mix valves common between BOTH Packs. What is confusing me is that if it is a case that each PACK has TWO air mix valves and in this case due to an overheat the PACK shuts down and its two air mix valves drive full cold WHY then could I not still use the TEMP Selectors for control over the other pack that is still operating.
 
yeelep
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Joined: Mon Apr 04, 2011 7:53 pm

RE: Pack Light 737

Sun Jan 06, 2013 9:25 pm

Quoting smartt1982 (Thread starter):
When it says BOTH air mix valves. Does each of the TWO Packs have two air mix valves each or are these TWO air mix valves common between BOTH Packs.

Each pack has one Air-Mix valve, and they are not common between both packs.
 
AA737-823
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Joined: Wed Mar 01, 2000 11:10 am

RE: Pack Light 737

Sun Jan 13, 2013 10:50 pm

Greetings from 737 maintenance land.

Quoting yeelep (Reply 1):
Each pack has one Air-Mix valve, and they are not common between both packs.

What he said.
Otherwise, I find YOUR question to be CONFUSING. Perhaps it's your overuse of CAPS. Perhaps NOT.
 

But I'm confused between your saying that you must select a warmer temperature, but yet drive both mix valves to full cold. HUH???
 
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Jetlagged
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RE: Pack Light 737

Mon Jan 14, 2013 12:36 am

Quoting AA737-823 (Reply 2):
But I'm confused between your saying that you must select a warmer temperature, but yet drive both mix valves to full cold. HUH???

The pack controller will automatically drive the air mix valve to full cold if the pack trips. The crew should select a warmer temperature before resetting the pack(s).
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
yeelep
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Joined: Mon Apr 04, 2011 7:53 pm

RE: Pack Light 737

Mon Jan 14, 2013 3:41 pm

He also posted on a 737 specific forum. It turns out his question was about the -800, so no air-mix valves involved.
 
AA737-823
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Joined: Wed Mar 01, 2000 11:10 am

RE: Pack Light 737

Tue Jan 15, 2013 12:27 am

Quoting Jetlagged (Reply 3):
The pack controller will automatically drive the air mix valve to full cold if the pack trips. The crew should select a warmer temperature before resetting the pack(s).

Aah, got it. That's consistent with my knowledge on the subject!

Quoting yeelep (Reply 4):
He also posted on a 737 specific forum. It turns out his question was about the -800, so no air-mix valves involved.

Um, what? To my knowledge (working on -700s, which are very similar, although the -800 adds trim valves to provide for the three cabin zones), the -800 still has mix valves.... am I wrong about that?
 
yeelep
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Joined: Mon Apr 04, 2011 7:53 pm

RE: Pack Light 737

Tue Jan 15, 2013 3:38 am

Where the -700 has a air-mix valve and low limit valve, the -800 uses a TCV (temperature control valve) and standby TCV. The air-mix valve has a cold and hot valve. On the -800 the cold valve is completely eliminated and the hot valve is basically the TCV, but the air re-enters the system at the ACM turbine discharge (mix-muff) instead of the mix chamber. The StbyTCV is in parallel with the TCV and under normal operation prevents icing in the condenser, although the method of control is different than the low limit valve.
 
smartt1982
Topic Author
Posts: 226
Joined: Sat Nov 24, 2007 11:17 pm

RE: Pack Light 737

Wed Jan 16, 2013 4:50 pm

Quoting yeelep (Reply 6):
but the air re-enters the system at the ACM turbine discharge (mix-muff) instead of the mix chamber.

Where is the air re-entering from?

Apologies for not clarifing it was the 800 series in my initial post.

Thanks yeelep for the tech advice, both here and on the other forum.

Thanks again
 
yeelep
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Joined: Mon Apr 04, 2011 7:53 pm

RE: Pack Light 737

Wed Jan 16, 2013 7:07 pm

Quoting smartt1982 (Reply 7):
Where is the air re-entering from?

Short answer, from the FCSOV.

Long answer:
On the -700, the air entering the pack goes through the FCSOV (flow control and shutoff valve) then divides. The majority of the air goes through the primary heat exchanger, the cold side of the air-mix valve, the ACM (air cycle machine) compressor, the secondary heat exchanger, ACM turbine, mix-muff, water separator, conditioned air check valve to the mix chamber and on to the distribution manifold and ducts..
The air that travels the other route goes through the hot side of the air-mix valve to the mix chamber.
There is also the low limit valve that tees into the duct after the cold side of the air-mix valve and goes to the mix-muff at the exit of the ACM turbine to prevent icing.

On the -800 the air entering the pack goes through the FCSOV then divides into four different ducts that I'll number 1-4. Number 1 goes through the primary heat exchanger, the ACM compressor, secondary heat exchanger, hot side of the reheater and condenser, water separator, cold side of the reheater, ACM turbine, mix-muff, cold side of the condensor, conditioned air check valve and on to the distribution manifold and ducts.
Number 2 and 3 are in parallel with a TCV in one and a standby TCV in the other, they then join together and go to the mix-muff at the ACM turbine exit.
Number 4 is for the trim air system that is supplied by both packs. Each packs number 4 duct first has a check valve to prevent any back flow back into either pack. Then they merge into a single duct and go through the trim air PRSOV (pressure regulating and shut off valve), then divide into three ducts with a trim air valve, one each for the flight compartment, forward cabin and aft cabin zones and then join the distribution ducting.

So basically the 737 has two different pack systems, a two zone and a three zone type. The -100, -200, -300, -500, -600, -700 are two zone. The -400, -800, -900, -900ER are three zone.

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