737NG air conditioning not very efficient?

[Tapshi]

I remember reading somewhere that the air cycle machine cools the air down to as low as 2 degree Celsius. The bleed air is added to get the right pack temperature. If this is true, why can't the air cycle machine just produce the right temperature. Wouldn't that be more efficient?
I'm still new to the aircraft. So just wondering. Please correct me if I am wrong somewhere!

[de facto]

I remember reading somewhere that the air cycle machine cools the air down to as low as 2 degree Celsius.

The ACM can cool bleed air to below water freezing level hence a 2c water seperator anti ice system.

The ACM is what it is,an extra cooling system ,not a temperature controller.

[code0]

for air conditioning and pressurization, Bleed air from various stages of the engine is taken via a FLOW CONTROL VALVE.

This hot air then split into two lines, one goes to the "Pack" for cooling

The other goes to the TRIM AIR VALVE ----> HOT AIR

Cooled air from the Packs are then connected to the MIXER UNIT to mix with the recirculated air from the cabin. ----> COOLER AIR

Now as for you question, why this can not be done by simply regulating the temperature by the packs is a very practically challenging one. As the cabin is divided into CKPT/FWD/AFT the temp needed in those zones could be varied!

And Pack is simply a machine that cools the hot bleed air down! but the temperature regulator system is the smart one to get the correct temp and supply to correct zone.

stand to be corrected though!

[ImbracableCrunk]

"why can't the air cycle machine just produce the right temperature?"

It does for at least one zone. It then adds trim air for zones that want warmer temps.

If all three zones want 18 degrees, the packs will cool the air to 18 degrees. Not to 2 and then warming them back up with trim air.

[Tapshi]

But from what I've read, bleed air from the engines is used as trim air downstream of the packs but some of this bleed air also goes to the pack where this air is mixed with the cooled air from the ACM, within the pack itself, downstream of the water separator.

This doubt cropped up when I saw that my refrigerator had no 'trim air'. But it still produced the commanded temp. So why not so in the air conditioning system

[Sciolistes]

I don't think the fridge will produce the requested air temperature. From what I have seen, the unit will operate until just below the required temperature and then switch of until just above the required temperature.

I think it is a far more elegant and practical design solution to produce cold air and mix it with hot are to obtain the correct temperature, especially considering operational redundancy, the volume and different temperature needs in different parts of the aircraft.

[itsresidualmate]

The air conditioning pack produces the requested temperature, the ACM is just a component of the pack. The ACM is a turbine/compressor. Some hot air bypasses the ACM, some hot air enters the compressor, is cooled by an air to air heat exchanger, then is expanded through the turbine thus cooling it.
All air that goes through the ACM is cooled. The only way to vary it as you suggest would be to slow the turbine down; more wear, parts breaking, unnecessary complexity. Simpler just to bypass some hot air.
To give you an example, if you selected full hot on a system that routed all air through the ACM, you would need to stop the ACM turning to ensure no cooling took place.

[itsresidualmate]

To clarify, someone mentioned the ACM producing the correct temp for one cabin zone, don't forget its the PACK that produces the correct temp not the ACM. Further downstream a trim air valve can add hot air for a different cabin zone, but the ACM only ever produces freezing air!

[code0]

Actually it happens in Airbus, when you fully close the TRIM AIR VALVE!

Then the air with same temperature is diverted to all the zone FROM THE PACKS

In that instance only as you mentioned the ACM provides the right temperature (ACM directly provides the temperature control!)


But if you AFT CBN is cold! you have to supply hotter air by opening the TRIM Air Valve because it is always easy to get a little hot! air stream from the engine and mix with pack air to supply!

hope this helps!

[itsresidualmate]

No, sorry it doesn't it! The trim air valve adds hot air to pack output to alter temperature in a different cabin zone, but within the pack there is also a temp control valve that bypasses the ACM to provide desired pack temp. The ACM outputs freezing air, nothing else. It cannot control temperature.

[Yeelep]

One thing that everybody is disregarding is the importance the ACM plays in condensing water so it can be removed by the water separator. Also the OP compared the PACK operation in relation to a refrigerator. A fridge will cycle on and off to maintain a small temp. range. If you did this in a 737 you would lose your source of pressurization every time the aircon cycled off. Not a good thing.

[Tapshi]

Thanks a ton guys. I guess I understand it better now!
Regards
Tapshi

[code0]

@itsresidualmate, that is what I was talking about! if you close the TRIM VALVE the air is directly taken from the pack regardless to how much cooler that pack supplies!

[code0]

@itsresidualmate : I just checked this further, the Airbus says, in case of a TRIM VALVE motor failure, the packs directly supply to the MIXER unit and to the Cockpit and Cabin. Further, PACK1 for CKPT and PACK2 to FWD/AFT CBN

[itsresidualmate]

Hi codeo, don't forget what you're talking about is the PACK, that contains an air cycle machine and a hot air bypass valve that mixes cold ACM output with hot engine bleed to get required temp. The ACM can only output freezing air, there is no control of it.
The trim air valve you mention is downstream of the pack to give an option of different temperatures in two different cabin zones, but you need the pack to work properly for this i.e. ACM mixing its cold output with bypass air!

[code0]

Yes itsresidualmate, what I mentioned was A320s the BY-PASS valve in the pack regulates the temperature too. Hence in case of a TRIM AIR VALVE failure, the Pack directly supply the air. And yes you are right, that I forgot to mention it is the PACK rather the ACM, as simply ACM can not do that

Code0

[Yeelep]

Just so everybody is on the same page (not a Airbus code0)

The -800, -900 Packs will output the lowest zone temperature selected, whether it is the flight deck, fwd or aft cabin. That air then goes to the mix manifold (the flight deck tees off the L/H Pack output just before the manifold). After the air exits the manifold for whichever zone its going to, hot air is added through the trim air valves to raise the temperature to that selected for the two zones that need it. If the trim air is turned off, the L/H Pack supplies the selected temp. for the flight deck and the R/H Pack supplies the average temp. selected for the fwd and aft cabin.

Of course if its a -600? or -700, there are only two zones. No temp. or trim air valves, only a Air-Mix valve in each pack to control the Pack output temp.. L/H Pack for the flight deck, R/H Pack for the cabin.

[EW73]

Not quite the same as the -700s I associate with...

The L/H pack does indeed feed the flight deck, but also feeds into the mix manifold, there, together with the R/H pack, supplies the cabin areas.

The Trim Air system is the same system as the Classic B747 has been running forever, just on a smaller scale!

EW73

[Yeelep]

Your'e right, I was just being a little simplistic in my explanation. To be more specific, the L/H pack will output the temperature to attain the desired temp. in the flight deck. The air that doesn't go to the flight deck from the L/H Pack ends up in the mix manifold with the R/H pack output and then to the cabin. The R/H Pack outputs the temp. needed to attain the desired cabin temp., compensating for whatever the the L/H pack temp. output is.