View Full Version : 737 Acm

22nd May 2004, 11:13
Anyone knows how are the components of the ACM-air cycle machine -on the 737 actionated? The ACM has a turbine and a compressor that are cooling the air required to mantain a comfortable temp in the plane.
A hotter setting for the temp will decrease the demand on the ACM.
Will this have some impact on fuel usage?
As I know there is no mechanical connection between the engine and the packs,so my guess is that the compressor and the turbine in the ACM are electrical?
I know the turbofan is operated with blead air.

22nd May 2004, 17:58
My understanding is as follows, but please feel free to correct me!!

The ACM is just driven by bleed air passing over its turbine.. its RPM is therefore dependant partly on the temperature selected (colder, faster).

I thought the mass flow controller keeps the demand on bleed air fairly constant, so changing temp shouldn't significantly change the fuel usage. The changing temp would only affect whether more air goes over the ACM or bypasses it as trim air.

I would suspect that turbofan operation would affect economy more significantly because the air used to drive the turbofan (in the pack) is dumped into the Ram air duct for the pack and not used for pressurisation.

(Please note I am assuming the 'Pack valve' acts as a mass flow controller, with the 'Air mix valves' acting as temperature controllers. The turbofan valve is upstream of the pack valve.)

22nd May 2004, 18:09
the ACM consists of one compressor which compress the hot air from the engine,than ,after a second heat exchanger the air enters into a turbine which cools the air.I don't know if this components are actionated by airflow from the engine,but if so,their speed must influence also the air drawn from the engine.

22nd May 2004, 18:21
I will say I am not a commercial pilot, just a trainee, however my understanding is as follows.

The ACM speed is influenced by the amount of air passing over its turbine.
The air after passing the pack control valve can be sent one of two ways:
i) To the mix manifold without cooling
ii) Over the ACM for cooling.

It is the pack control valve that controlls the pack usage of bleed air. This will therefore increase bleed usage if the pack is put in high flow. It is the air mix valve that is controlled by the temperature controller - this routes the air over the ACM or direct to the mix manifold depending on the temp required. So temperature control appears to have no direct control over bleed air usage, just the path it follows.

Please feel free to correct me!!

22nd May 2004, 18:47
well,tom , I knew that the turbine role it's to cool the air.It's not like the engine turbine,which is driven by the airflow and is conected to the fan.
The turbine from the pack must rotate and cool the airflow.It is not a free turbine.Also the compressor.
It's like talking about a propeler,if it's rotated by the forward movement of the plane or by the engine and thus moving the air backwards. Of course,it's actionated by the engine.
The temperature selector regulates the airflow through the pack ACM,as you said by means of a mix valve.
The airflow is as demanded,if you select high flow,the compressor will go faster,sucking more air from engine bleed.

22nd May 2004, 19:02
Hi Alex,

I think the way I have been taught about ACM operation for my ATPLs is very different to the way you have been taught for the 737.
As your are a 737 pilot I have to bow to your knowledge, and will allow someone else to answer.

All the best,

22nd May 2004, 19:45
Tom, I may be wrong.This is why I've asked this question,I don't have enaugh technical documentation to check my knowledge about this.
I am a pilot not an engineer.But I've read your answer and I'll think about it while looking further into this.
The way I've been told is this:
cooling cycle:
air flows to the compresssor of an ACM where the air is compressedand temperature is increased.Next ,the air circulates through a secondary heat exchanger for additional cooling.The airflow then passes through the turbineof the ACM where it is expanded and cooled.The cold air flows to a water separatorwhich removes moisturethat has condensed out of the air by operation of ACM.Then ,the processed cold air is mixed with hot air as requested.
Note-the compressore and turbine act over the airflow,and are not actionated by it.This is what I've been told.And I think it's logic,if you push air into a compressor,it will start to rotate,like a fan,but it won't compress the air.It must be actionated somehow to do that.For ex,bya turbine,activated by exaust gases,in the engine comp case.
But there are some years since I pass my exams,so I don't remember much of this subject.
Thks for answering,and don't bow to anyone's knowledge.Just take the best you can,and check it for yourself.At technical knowledge you may be the best there is.Or try to be.It depends only of yourself.
My top instructor,ex factory test pilot for big jets,told me once that you'll never be afraid in whatsoever technical emergency if you know your plane better than your pockets.This is what I've tried to do since than,no matter what plane I'm on.
Sad,but some pilots I know don't consider this very important.They stick with the books,and this sometimes are not so explicit.Sometimes ,the QRH not covers all the emergencies that can appear.
Brgds. Alex

22nd May 2004, 20:23
Hi Alex,

My understanding is different regarding a typical ACM.
The sequence I have been taught is the same i.e.
Primary heat exchanger, ACM compressor, Secondary heat exchanger, ACM turbine, water seperator.

I was taught that the ACM is very similar to a turbocharger, and has no external input of power. I was taught the airflow itself drives the ACM, and that the ACM (well a turbocompressor ACM) is a single shaft turbocompressor with a straight shaft (no gearing) linking the compressor to the turbine.

The compressor by definition is a compressor, and therefore is driven by the turbine, it merely compresses the air entering the compressor. I was taught the airflow over the turbine of the ACM drives the compressor of the ACM. The air does work in driving the ACM turbine and therefore looses energy..ie pressure, and therefore the temp drops. Therefore no external input of power.

I have had a look at the 737 manual and it looks to be a turbocompressor ACM. Other types of ACM obviously work differently such as the brake turbine or turbofan ACM.

All the best,

22nd May 2004, 20:59
I was wondering about this myself. That is, where does the cold air come from while sitting on the ground etc. My question is, why is a separate compressor used? Couldn't bleed air right off the APU/Engines go through the cooler, separater etc, thereby eliminating the extra compressor?

Gotta love V1P1/T1 = V2P2/T2 :)

gas path
22nd May 2004, 21:01
Ok basic acm operation:
bleed air from the engines or apu or indeed a ground source is fed to the packs by the PFCV's (pack flow control valves). From here it passes through the primary section of a dual heat exchanger where the cooling process begins. The air is then compressed in the compressor section of the ACM and cooled again as it passes through the secondary section of the dual heat exchanger, then the temperature of the air is greatly reduced as the air expands across the turbine (in the ACM) which being mechanically coupled drives the compressor. (bootstrap cycle!), but, as not all the heat needs to be removed from the air a turbine bypass valve controlled by a pack temp controller bypasses some of the air that would pass across the turbine, this will reduce the need to later add trim air to the zones.
The bleed air will start off with a temerature (depending on aircraft and engine type) of around 350f (180c) and the ACM will easily drag this down to typically 0c, some systems can get ice rattling around in the mix manifold (plenum) if the control system loses the plot!!. Some packs have the addition of a fan driven by the comp/turbine of the acm that is for use on the ground to increase the airflow across the heat exchangers. Other refinements may include moisture removed by the water separators being sprayed into the inlet to the heat exchangers to increase cooling through evaporation. The packs, controlled by the temp. contollers will control to the cabin zone with the least heat requirement and hot trim air will be added to the ducts feeding those zones that require more heat to get the cabin (zone) temperature to the selected level.
Nice diagramms here for the 737 737 site (http://http://www.b737.org.uk/airconditioning.htm)