View Full Version : Trim air 737ng
aslan1982
2nd Nov 2010, 17:02
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
Whippersnapper
2nd Nov 2010, 18:31
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.
STBYRUD
2nd Nov 2010, 21:28
...because they only have two temperature zones, and not three like the larger models :)
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).
Capt Chambo
3rd Nov 2010, 01:27
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.
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.
b744FPEK
3rd Nov 2010, 13:30
trim air =hot air
mix manifold air =cold air =the coldest zone need
there are 3 zones .
so .for the other two zones ,add some hot air (trim air )
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.
Sounds like an inefficient way of doing it. Yes, the output of the packs going into the mix manifold on some aircraft is quite cold. This air has to mix with the existing hot air in the cabin to provide a comfortable temperature.
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.
http://www.iinet.net.au/
[email protected]/Pack3Snapshot1.jpg
"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).
Gas Bags
4th Nov 2010, 01:31
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.
http://i1012.photobucket.com/albums/af246/GasBags/P1010071.jpg?t=1288833468
GB
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.
Post #2 says "So, the packs produce air conditioned air at the coldest of the temperatures selected in the three zones. ", which is not exactly true of the 747-400.
What if the actual temperature in the zone with the coldest selected temperature is already colder than your selected temperature? ;)
Cheers
NSEU
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
How on earth did we get past #2?:ugh:
catiamonkey
4th Nov 2010, 09:48
NSEU is right, should say demanded.
Cars with more than one zone (passenger and driver) work the exact same way.
Vertical Speed
4th Nov 2010, 11:22
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.
atpcliff
4th Sep 2012, 12:38
The 747-400 freighter has 7 temperature zones. I believe the passenger version only has 6 zones.
cliff
KGRB
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.
ImbracableCrunk
4th Sep 2012, 19:24
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.
Yeelep
5th Sep 2012, 05:25
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.Not in the case of the 737NG. Air from the flight deck and E/E compartment is used for cooling, none of it comes directly from the pack or distribution system.
McBruce
5th Sep 2012, 20:34
While on this subject it reminded me of a previous question I tried to find an answer to concerning the -800.
The gasper air valves in the cockpit, is this mix manifold air or air coming from the cockpit feed from the left pack?
de facto
6th Sep 2012, 06:39
The flight deck Air comes from the Left pack via a separate duct than for the pax so does not use recirculated air...it does not enter the mix manifold.
Fresher air for us.:ok:
Dougie_diesel
10th Sep 2012, 03:18
Here's what Mr. Boeing has to say on the matter.....
Zone Temperature Control
This part of the A/C system increases the temperature of the conditioned air that flows into the occupied areas of the airplane. It also gives pressure regulation and on/off control for the trim air part of the system. These are the primary components:
Trim air pressure regulating and shutoff valve
Zone trim air modulating valves
Temperature sensors.
The system calculates the necessary pack outlet temperatures to satisfy the cooling needs to the flight compartment and the mix manifold. The system also calculates the heating necessary for each temperature control zone. These are the temperature control zones:
Flight compartment zone
Passenger cabin zones (2).
Air from the pneumatic system adds heat to a zone that needs warmer air. The trim air pressure regulating and shutoff valve gives on/off control and keeps trim air pressure at a necessary limit.
See the trim air pressure regulation and shutoff control section for more information about the trim air pressure regulation and shutoff control systems. (SECTION 21-60 (javascript:parent.controller.internalLinkByFragmentKey('n90 371314b9a46599fad7f98c2e74f2')))
The zone trim air modulating valves control the heat added to the conditioned air for each zone.
See the zone temperature control section for more information about the zone temperature control systems. (SECTION 21-60 (javascript:parent.controller.internalLinkByFragmentKey('n90 371314b9a46599fad7f98c2e74f2')))
Zone Temperature Control
This part of the A/C system increases the temperature of the conditioned air that flows into the occupied areas of the airplane. It also gives pressure regulation and on/off control for the trim air part of the system. These are the primary components:
Trim air pressure regulating and shutoff valve
Zone trim air modulating valves
Temperature sensors.
The system calculates the necessary pack outlet temperatures to satisfy the cooling needs to the flight compartment and the mix manifold. The system also calculates the heating necessary for each temperature control zone. These are the temperature control zones:
Flight compartment zone
Passenger cabin zones (2).
Air from the pneumatic system adds heat to a zone that needs warmer air. The trim air pressure regulating and shutoff valve gives on/off control and keeps trim air pressure at a necessary limit.
See the trim air pressure regulation and shutoff control section for more information about the trim air pressure regulation and shutoff control systems. (SECTION 21-60 (javascript:parent.controller.internalLinkByFragmentKey('n90 371314b9a46599fad7f98c2e74f2')))
The zone trim air modulating valves control the heat added to the conditioned air for each zone.
See the zone temperature control section for more information about the zone temperature control systems. (SECTION 21-60 (javascript:parent.controller.internalLinkByFragmentKey('n90 371314b9a46599fad7f98c2e74f2')))