cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

Today's Pop Quiz - 11/20/02

Thu Nov 21, 2002 7:08 am

B737-300/500/600/700

Pirep:
Left Air mix valve moves ok in auto, but will not drive at all in Manual.
You check it out, and it works as advertised in auto, but doesn't move in manual. It is a hard failure (Don't u wish it was always like that!!)

Question?
Do u have a wiring problem, or could you have a bad mix valve?
 
NKP S2
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Joined: Mon Dec 20, 1999 3:16 am

RE: Today's Pop Quiz - 11/20/02

Thu Nov 21, 2002 9:15 am

Well first; Thanks for giving me a hard fault instead of an intermittent one Smile -- IIRC, there's only one actuator motor on the mix valve. Gotta be the wiring for the manual temp control circuit... or maybe bad contacts dedicated to the manual circuit in the pack switch or temp selector? Couldn't hurt to check the C/B for the manual side first I guess though. -- I've got a question as well: I've worked all 737 series up to and including the -400, but no "higher": I take it that the 500 and up ( including NG ) use the older system with a mix valve(s) rather than -400's 'Zone Control' system with trim-air?
 
Mr Spaceman
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RE: Today's Pop Quiz - 11/20/02

Thu Nov 21, 2002 9:41 am

Hi guys.

>Cdfmxtech, I don't know the answer, however, I would like to make sure I understand what part of a 737 the question is about.

Is the Left Air mix valve part of an air mix manifold that mixes fresh outside air via bleed air from the engines (after passing through ozone converters and heat exchangers for the A/C packs) with filtered cabin air that's being recirculated?

I hope I got that right.

Chris  Smile
"Just a minute while I re-invent myself"
 
LMP737
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Joined: Wed May 08, 2002 4:06 pm

RE: Today's Pop Quiz - 11/20/02

Thu Nov 21, 2002 2:47 pm

Maybe because this is just before my bed time but are you referring to the recirc fans? If you are there are only two switch positions, OFF and AUTO.

Please let me know if I need some sleep.  Smile
Never take financial advice from co-workers.
 
NKP S2
Posts: 1665
Joined: Mon Dec 20, 1999 3:16 am

RE: Today's Pop Quiz - 11/20/02

Thu Nov 21, 2002 3:06 pm

Mr Spaceman: Not quite...but not a bad guess either Smile The air mix valve(s) is located in the respective A/C pack bay and controls the flow of hot bleed air and cold conditioned air to be sent to a mixing chamber and main mix manifold. ( the mix valve itself is basically a tandem valve with the butterflies 180 deg. out from each other ) The "hot" air does not go through the heat exchangers on it's path to the mix valve- it's pretty much raw bleed air taken just downstream of the pack valve/flow control valve. The "cold" side is the air that is cooled by the pack via heat exchangers and Air Cycle Machine. All this temperature regulated ( mixed ) air ( basically ) comes together ( from both the left and right pack ) in the main distribution manifold- it is there where the recirulation inlet(s) are for the recirc. fan(s). I hope this answers your question without sounding too confusing.
 
cdfmxtech
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Joined: Wed Jul 05, 2000 11:37 am

RE: Today's Pop Quiz - 11/20/02

Thu Nov 21, 2002 3:14 pm

NKP S2
Correct. The control for the mix valve manual and automatic is through a common wire. So it is most likely a wiring/switch problem upstream of the valve c/p.
Regarding your question:
The B737-500 uses the mix valve. On the 737NG, I'm pretty sure the -600 uses the mix valve, and I know that anything larger than the -700 uses the Trim Air temp ctrl sys (Zones) and the different pack (condenser/reheater cycles). The condenser/reheater pack and trim air sys are bascially the same as the -400 as I understand.
------------------------
Mr Spaceman
You're a bit confused.

I'll explain as best I can
Some Basic B737-300/500 pack components:
Pack valve (Flow Control and Shutoff valve)
Mix valve: Valve has 2 sides, one hot and one cold. They are 180 degrees out of phase, meaning that when the hot side is open, the cold side is closed.
Heat Exchangers
Air Cycle machine
Flighdeck selector panel: Includes temp selector switch. Manual mode manually positions the mix valve. In auto mode, a cabin temp controller uses sensed temperature and desired temperature to position the mix valve.

Hot bleed air flows thru the pack valve when the pack switch is turned on. When the pack is first turned on, the mix valve is in the full cold position, which means that all air will go thru the cooling cycle. This hot air from the cold side of the valve goes through the ram air cooled primary heat exchanger to remove some of the heat from the air. This air then goes to the compressor side of the Air Cycle machine to provide the initial drive (The turbine side will drive the ACM later). This compressed air, now hotter will leave the ACM and got thru the secondary heat exchanger. The Air is then cooled again. This air now goes to the turbine section of the ACM. The air is now expanded which is how you get the cool air.

The flightdeck selector panel for the temp control has a switch that has 2 modes: Auto and manual. I will just focus on manual now. When in manual mode, the switch is spring loaded to the center position which provides no signal to the mix valve. Position the switch to cold and hold it, and the valve will drive to the cold positoon (allowing the majority of bleed air to go to thru the cooling pack cycle). When you drive the valve to the hot side, you change the position of the valve. The cold side will start to close and the hot side will start to open. When the hot side starts to open, it will allow some of that bleed air to bypass the cooling cycle. This air outputs to mix with the output of the cooling pack (which is now a less because there is less air available to drive the ACM), thus the air is now a little warmer.

Alot of this stuff might be hard to visualize, but maybe you can. Hope it helps.
 
cdfmxtech
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Joined: Wed Jul 05, 2000 11:37 am

RE: Today's Pop Quiz - 11/20/02

Thu Nov 21, 2002 3:19 pm

LMP737

Recirc Fans
Get some sleep bro...it's probably been a long day!! Big grin
I'm kidding with you, please don't take it personal.....








But, no really get some sleep!
 
dc10hound
Posts: 460
Joined: Fri Dec 15, 2000 4:18 pm

RE: Today's Pop Quiz - 11/20/02

Thu Nov 21, 2002 10:09 pm

Correct. The control for the mix valve manual and automatic is through a common wire. So it is most likely a wiring/switch problem upstream of the valve c/p.

Cdf

The problem must be in the auto/manual switch/selector, because if it were a broken/shorted wire, the valve would not work in both manual and auto, right?


"Eagles soar. But weasels never get sucked into jet intakes.."
 
NKP S2
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Joined: Mon Dec 20, 1999 3:16 am

RE: Today's Pop Quiz - 11/20/02

Fri Nov 22, 2002 2:32 am

I think that yes, it could be in the temp selector ( auto/manual ), and an open/short downstream from there would affect both auto and manual, but I imagine it could still be an open upstream of there in the manual circuit on back to the C/B for the manual control where the juice comes from.
 
dc10hound
Posts: 460
Joined: Fri Dec 15, 2000 4:18 pm

RE: Today's Pop Quiz - 11/20/02

Fri Nov 22, 2002 3:21 am

I think that yes, it could be in the temp selector ( auto/manual ), and an open/short downstream from there would affect both auto and manual, but I imagine it could still be an open upstream of there in the manual circuit on back to the C/B for the manual control where the juice comes from.

...and we won't know for sure unless we find the schematic, will we?  Big grin
"Eagles soar. But weasels never get sucked into jet intakes.."
 
NKP S2
Posts: 1665
Joined: Mon Dec 20, 1999 3:16 am

RE: Today's Pop Quiz - 11/20/02

Fri Nov 22, 2002 4:00 am

I'm off to the salt mines soon...I'll check the WDM just for kicks, and see if we can get this pack off MEL Smile
 
dc10hound
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RE: Today's Pop Quiz - 11/20/02

Fri Nov 22, 2002 8:58 am

I'm off to the salt mines soon...I'll check the WDM just for kicks, and see if we can get this pack off MEL

That's cool NKP,  Smile

Just remember to check the SSM when you have that WDM reference in your hand. Just in case one or the other is wrong or has a conflict...

But that can never, never happen...

....right?

-geo
"Eagles soar. But weasels never get sucked into jet intakes.."
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: Today's Pop Quiz - 11/20/02

Fri Nov 22, 2002 2:04 pm

The auto control wiring splices in with the manual control wiring after the auto wiring leaves the Cabin temp controller. A wiring issue btwn the switch and this splice could cause this, but that is why I just made the blanket statement of wiring. Didn't wanna get too specific as far as the diagnosis went.
But it would most likely be a wiring problem or switch problem. I just wanted to hear that more than likely, it IS NOT a mix valve problem.

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