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caspar777

understanding trim air behaviour

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My understanding of the air conditioning system is that very hot air from engines is cooled down by packs, and used as air conditioning through out the aircraft. The trim air system then uses the hot air from engines to carefully inject hot air to meet a desired temp between 18-28 degrees. 

When selecting AIR on EFIS, I did not really understand how the behaviour related to my understanding. Firstly, while taxiing I noticed the trim air valves were randomly opening and closing (even though set as ON). Why is this (Air system was setup normally, with engines and packs all on)?

II thought maybe the trim air valves open and close to do the heat monitoring (stop flow when temp is met, allow flow when too cold). However, I then turned the heating down in the cabin and flight deck, to see if the trim air valves would cut flow, and they didn't (remained green). So then I realised the trim air flow remains on at all time but just changes level depending on on temp request. So, this does not explain the random opening and closing of trim valves during taxi. Note all demand hydraulics was set to auto. 

Thanks if anyone can clear this confusion up.

 

 

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Trim Air is not taken directly from the engines, it is taken from the Pressurised Air Conditioning Kits (aka the 'packs'). Airliners tap bleed air off (typically) a pair of the compressor stages of the engines (actually from three different stages on the 777's engines since they are bigger and more complex). But whether taking air from two or three points, this is done from several points on the engine rather than just one alone so that it doesn't interfere with the engine's ability to be producing thrust efficiently. The bleed air comes automatically off the compressor stage tap point which is developing the highest pressure when the aircraft engines are at low RPM settings, but it automatically switches to the stage with the lowest pressure when at high RPM settings; this in order to minimise the loss of thrust from taking air from the engine which would otherwise be producing thrust since modern engines are of course high bypass and produce most of their thrust from the fans, not the rear exhaust jet.

That bleed air then goes through a heat exchanger, which works very similar to the radiator in your car in that some cold air bled from the engine fan is ducted over the heat echanger to cool the bleed air down.

This slightly cooled down air then goes to the packs which are often located in or near the wings and can be recognised by the leading edge having a ram air intake of some sort, often near the wing root (although this is not its only function, that intake also sends cold air to a secondary heat exchanger). Then this air is further cooled by a Cold Air Unit (basically a fridge), then the air is sent through a secondary heat exchanger, then it is sent to a spiral-shaped tube which functions as a water extractor by applying centrifugal force to the air to get the water out (this is what you see dripping from the drains under the aeroplane when it is on the stand), then the air is sent through a filter. After all of this processing, this cooled and filter air is mixed with air directly taken from the ram air intake (about 50/50), and this mixture is what is sent to the cabin air vents although it may also pass through another filter on its way there. 

All of the above goes some way to explaining why things are automatically opening and closing on occasion but since there are several zones on an aircraft in terms of air conditioning, typically three for something like a 737 or A320: the cockpit, the forward cabin, the rear cabin (often more than three on larger aircraft), and since they don't all lose or gain heat at the same rate, the demands can vary for different sections of the aeroplane so any valves or other bits and bats which control the environment on board the aeroplane may flip on and off depending on all kinds of things, speed of the aircraft, thrust settings of the engine, whether the APU is on, whether or not the plane is full, empty, half-full or whatever, how many seats there are in that zone, whether passengers are closing or opening the gasper controls for air flow above their heads and so on.

If you are worried about keeping your passengers warm, make an announcement that 'there is no cause for alarm' which of course will instantly make everyone have cause for alarm, and that'll get them sweating lol.

Edited by Chock

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On 6/12/2018 at 10:55 AM, caspar777 said:

When selecting AIR on EFIS, I did not really understand how the behaviour related to my understanding. Firstly, while taxiing I noticed the trim air valves were randomly opening and closing (even though set as ON). Why is this (Air system was setup normally, with engines and packs all on)?

Sounds like you’re maybe confusing the function of the trim air pressure regulating and shutoff valves (PRSOV) with the zone modulating valves.  The primary function of the trim air PRSOV is to maintain pneumatic pressure to the trim air zone modulating valves. Its goal is to maintain cabin pressure + 5 psi.  These are also the valves you’re controlling with the switches on the overhead panel as well as what you see on the synoptic display. The zone modulating valves are what control the amount of hot air that goes to each cabin zone, there is one valve per zone but only the flight deck valve is displayed on the synoptic.

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On 6/13/2018 at 1:55 AM, caspar777 said:

My understanding of the air conditioning system is that very hot air from engines is cooled down by packs, and used as air conditioning through out the aircraft. The trim air system then uses the hot air from engines to carefully inject hot air to meet a desired temp between 18-28 degrees. 

When selecting AIR on EFIS, I did not really understand how the behaviour related to my understanding. Firstly, while taxiing I noticed the trim air valves were randomly opening and closing (even though set as ON). Why is this (Air system was setup normally, with engines and packs all on)?

II thought maybe the trim air valves open and close to do the heat monitoring (stop flow when temp is met, allow flow when too cold). However, I then turned the heating down in the cabin and flight deck, to see if the trim air valves would cut flow, and they didn't (remained green). So then I realised the trim air flow remains on at all time but just changes level depending on on temp request. So, this does not explain the random opening and closing of trim valves during taxi. Note all demand hydraulics was set to auto. 

Thanks if anyone can clear this confusion up.

 

 

 

Because air coming out from the packs can be extremely cold, I.e. sub zero temperature.

and just like what Alan had explained that each air conditioning zone generate heat at a different rate. Therefore you may find for example, the trim valve is always open in the cockpit as well as in First Class and business class. However the position in EY may be different. 

 

As as long as you don’t get any EICAS msg and the temp feels right and there is no complian from the ISM. I don’t think most of us would even bother to look at it closely. 

 

Good observation by the way. 

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On 6/12/2018 at 9:23 PM, Chock said:

Pressurised Air Conditioning Kits (aka the 'packs’)

Pack is short for package. It’s not an acronym. 

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