Trim air 737ng
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Trim air 737ng
Hi
Im having trouble understanding what exactly trim air does. I know its always on (bar abnormal situations, but what does it actually do?
Regards
Aslan
Im having trouble understanding what exactly trim air does. I know its always on (bar abnormal situations, but what does it actually do?
Regards
Aslan
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The 737 800 and 900 has three temperature zones (I think the 400 is the same, but I've never flown it), but like the other models, has only two packs. So, the packs produce air conditioned air at the coldest of the temperatures selected in the three zones. The warm trim air is then added to the ducts to the two areas with higher temperature demands.
It is not fitted to the 600 or 700, which are also NG models.
It is not fitted to the 600 or 700, which are also NG models.
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Rephrasing: The packs put air into a common manifold, which is then distributed into the different cabin zones.
If one zone is particularly hot, perhaps due to lots of people in it, the packs will adjust their output temperature downwards to keep that zone cool.
However, since the packs are only outputting one particular temperature, the other zones (which are fed by the common manifold) will be over-cooled. This is why hot trim air is added to these other zones (Trim air is added in precise quantities to individual zones).
If one zone is particularly hot, perhaps due to lots of people in it, the packs will adjust their output temperature downwards to keep that zone cool.
However, since the packs are only outputting one particular temperature, the other zones (which are fed by the common manifold) will be over-cooled. This is why hot trim air is added to these other zones (Trim air is added in precise quantities to individual zones).
As was explained to me in simple terms.
The packs on the -800/900 push out a constant temperature air which is a little cooler than crew and passengers would normally find comfortable. Via the mix manifold this air is fed to the three zones. As this air is cool it needs warming up to reach a comfortable level which is where the (hot) trim air plays it's part. The -400 has essentially the same system. In my FCOM page 2.31.7 shows a diagram.
I was told it is easier and more efficient for these packs to run at a constant output temperature, and then add warm air as needed, as opposed to providing air that is too warm and adding cold air, or modulating to provide conditioned air.
The -300/500/700 series (and probably the -100/200/600) use a different pack which modulates the output temperature within the pack itself to provide a comfortable level of conditioned air.
I am sure an engineer will come along shortly and be able to provide a more detailed answer.
Hope that helps.
The packs on the -800/900 push out a constant temperature air which is a little cooler than crew and passengers would normally find comfortable. Via the mix manifold this air is fed to the three zones. As this air is cool it needs warming up to reach a comfortable level which is where the (hot) trim air plays it's part. The -400 has essentially the same system. In my FCOM page 2.31.7 shows a diagram.
I was told it is easier and more efficient for these packs to run at a constant output temperature, and then add warm air as needed, as opposed to providing air that is too warm and adding cold air, or modulating to provide conditioned air.
The -300/500/700 series (and probably the -100/200/600) use a different pack which modulates the output temperature within the pack itself to provide a comfortable level of conditioned air.
I am sure an engineer will come along shortly and be able to provide a more detailed answer.
Hope that helps.
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I like post #2, 'cause that's how it is....
Then I sorta like post #4, but not as easy to understand...
Then there's post #5...well, how does that go again?
EW73
PS...The B747 Classics have been using this system for all models "forever", as far as I am aware.
Then I sorta like post #4, but not as easy to understand...
Then there's post #5...well, how does that go again?
EW73
PS...The B747 Classics have been using this system for all models "forever", as far as I am aware.
Last edited by EW73; 3rd Nov 2010 at 09:19.
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The packs on the -800/900 push out a constant temperature air which is a little cooler than crew and passengers would normally find comfortable.
However, I've noticed the individual zone trim air valve positions on some aircraft types (not sure about the 737), and one will usually be closed. You don't want to mix hot trim air with cold pack air if it is not necessary. It would be like turning on your home airconditioning to too cold, then switching on an electric fire.
"0.00" is a completely closed trim air valve (no additional heat added to a zone). "1.00" is fully open (max trim heat applied to a zone).
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Refer to the simplified schematic of the -400 below. The -800 is identical.
The packs definately output air at the lowest selected zone temperature on the ECS panel. That air then travels to the mix manifold and from there through the individual supply ducts to the respective three zones. For the two remaining zones that have a higher temp selected than the coldest, trim air is mixed into the supply duct for each of those zones to bring the supply duct air temperature up to the selected temperature for each of those zones. As NSEU posted one of the trim air valves will be closed, and the valve that is closed will be in the supply duct for the zone with the lowest selected temperature.
As you can see from the schematic trim air is hot air from the pnuematic duct that has not passed through the aircon pack. It passes through the trim air PRSOV and is directed into the respective supply ducts after the mix manifold via the trim air valves.
GB
The packs definately output air at the lowest selected zone temperature on the ECS panel. That air then travels to the mix manifold and from there through the individual supply ducts to the respective three zones. For the two remaining zones that have a higher temp selected than the coldest, trim air is mixed into the supply duct for each of those zones to bring the supply duct air temperature up to the selected temperature for each of those zones. As NSEU posted one of the trim air valves will be closed, and the valve that is closed will be in the supply duct for the zone with the lowest selected temperature.
As you can see from the schematic trim air is hot air from the pnuematic duct that has not passed through the aircon pack. It passes through the trim air PRSOV and is directed into the respective supply ducts after the mix manifold via the trim air valves.
GB
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NSEU....
Like I said, read post #2...because that's exactly what your screen data is showing you.
The data shows zone D as being the zone requiring the coolest conditioned air, (which is also supplied to all the other zones as well), so all those other zones (which all require some conditioned air temp somewhat higher than zone D), add varying amounts of hot trim air, to slightly warm their supply air to satisfy their particular needs.
Nothing different here, except for the fancy screen.
EW73
Like I said, read post #2...because that's exactly what your screen data is showing you.
The data shows zone D as being the zone requiring the coolest conditioned air, (which is also supplied to all the other zones as well), so all those other zones (which all require some conditioned air temp somewhat higher than zone D), add varying amounts of hot trim air, to slightly warm their supply air to satisfy their particular needs.
Nothing different here, except for the fancy screen.
EW73
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Like I said, read post #2...because that's exactly what your screen data is showing you.
What if the actual temperature in the zone with the coldest selected temperature is already colder than your selected temperature?
Cheers
NSEU
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Easy.....the next zone that then becomes the warmest, thereby needing the coolest supply air will control the pack output temperature, and all the other zones will slightly adjust their trim air supply to suit!
This process will continue as the zones vary in their actual verses selected temperature settings.
The answer to your question is that if the temperature is colder in that (coldest) zone that is selected, the supply air temp will warm to suit, and take all the others with it, until/if it no longer becomes the lowest supply air temp.
EW73
This process will continue as the zones vary in their actual verses selected temperature settings.
The answer to your question is that if the temperature is colder in that (coldest) zone that is selected, the supply air temp will warm to suit, and take all the others with it, until/if it no longer becomes the lowest supply air temp.
EW73
Last edited by EW73; 4th Nov 2010 at 09:48.
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Historic background to the two systems
Correct me I'm wrong but I think a little discussion of the background to the two different systems goes right back to the original 100/200 series. The pack system on the 3/5/600's is substantially the same (albeit improved) design and part of that goes right back to the old 727! Once Boeing came up with the idea of extending the 300 into the 400 they realised that the original 2 zone system was not going to work. They based the three zone system on that used on the 757. This system has since been updated and used on the 800 and 900 plus BBJ2.
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And can I add that the 12 o'clock position is not the "AUTO" position? It's the middle of the auto range. . . freakin' half-assed instructors. . . grumble grumble.
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Don't forget your cold air plenum is also feeding the avionics racks and electrical power dstribution panels - they need cold air to prevent overheating.