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Cooling Of Cabin Air In Engine  
User currently offlineJawed From United States of America, joined Sep 2006, 482 posts, RR: 0
Posted (7 years 7 months 2 weeks 1 day 13 hours ago) and read 4085 times:

I read the the cabin air in most passenger jets comes from the heavily compressed (and hot air) at the front of the engines, so it first has to be cooled down before being passed into the cabine.

So I'm wondering: what is this cooling equipment? And where is it located? And what happens if it fails? Doesn't that mean the passengers will be BBQ'ed alive by the hot air?

18 replies: All unread, jump to last
 
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 1, posted (7 years 7 months 2 weeks 1 day 12 hours ago) and read 4075 times:

This air is cooled in the airplane's PACS (Pressurization and Air Conditioning System). Since I am not a mechanic, I really can't describe the intricate details of their operation, however, there are many previous threads relating to Pacs.

User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 2, posted (7 years 7 months 2 weeks 1 day 12 hours ago) and read 4071 times:

The mechanism of cooling it is really pretty simple and energy efficient. After all, the plane is operating most of the time in minus-fifty temperatures.

There are little radiator-like heat exchangers out on the engine pylons that cool the hotter bleed air. After all, other than for anti-icing, we want the pressure, not the temperature.

The air conditioning packs also use heat exchangers, where raw airflow extracts heat from the compressed air and they use expansion of the air to cool it. The schematic of the system might get complicated but the cooling principles are simple science.

At the end of the processing it is common for hotter, untreated air to be added back into the ducting for temperature control.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineEMBQA From United States of America, joined Oct 2003, 9364 posts, RR: 11
Reply 3, posted (7 years 7 months 2 weeks 1 day 12 hours ago) and read 4067 times:

Quoting Jawed (Thread starter):
What happens if it fails? Doesn't that mean the passengers will be BBQ'ed alive by the hot air?

No......The hot bleed air never actually mixes with the cabin air. The bleed air passes through the ACM and Heat Exchangers were the heat energy is transfer to the cabin air. To cool, the air passes through venturies which causes a drop in pressure and thus a drop in temprature.



"It's not the size of the dog in the fight, but the size of the fight in the dog"
User currently offlineN8076U From United States of America, joined Jun 2006, 425 posts, RR: 9
Reply 4, posted (7 years 7 months 2 weeks 1 day 11 hours ago) and read 4047 times:

Quoting EMBQA (Reply 3):
No......The hot bleed air never actually mixes with the cabin air.

Actually, the bleed air from the engines IS what ends up in the cabin.

Chris



Don't blame me, I don't work here...
User currently offlineDarkBlue From United States of America, joined Sep 2003, 233 posts, RR: 10
Reply 5, posted (7 years 7 months 2 weeks 1 day 11 hours ago) and read 4038 times:

Quoting EMBQA (Reply 3):
No......The hot bleed air never actually mixes with the cabin air. The bleed air passes through the ACM and Heat Exchangers were the heat energy is transfer to the cabin air. To cool, the air passes through venturies which causes a drop in pressure and thus a drop in temprature.

Umm, assuming I follow your explaination correctly, I think you have it backwards. The air you breath in the cabin is in fact the same air that was pulled from the hot compressor stages. Without repeating too much what SlamClick has already explained, this air is cooled down using much cooler air from the bypass duct (behind the fan). The amount of fan air used is adjusted to maintain a defined bleed temperature level. I'm sure different people call this heat exchanger in the pylon different things, but I've always known it as the precooler.

Quoting Jawed (Thread starter):
So I'm wondering: what is this cooling equipment? And where is it located? And what happens if it fails? Doesn't that mean the passengers will be BBQ'ed alive by the hot air?

The system is design so that if the precooler fails, the shut off valve will close, stopping the flow of hot engine air into the aircraft.


User currently offlineEx52tech From United States of America, joined Dec 2006, 559 posts, RR: 1
Reply 6, posted (7 years 7 months 2 weeks 1 day 11 hours ago) and read 4034 times:

Quoting EMBQA (Reply 3):
No......The hot bleed air never actually mixes with the cabin air

Not true......Just to give one example, I have on two different occasions had to burn the skydrol out of the pneumatic system on a DC-10, when it got in there via the APU after the #2 hyd. reservoir sprung a leak. The fluid got into the APU intake, and thus into the pneumatic system and the cabin through the air cycle machines. You accomplish this burn out by removing the air filter covers on the air cycle machines, and run the engines, and the APU, with their bleed air turned on.



"Saddest thing I ever witnessed....an airplane being scrapped"
User currently offlineSfomb67 From United States of America, joined Dec 2005, 417 posts, RR: 0
Reply 7, posted (7 years 7 months 2 weeks 1 day 9 hours ago) and read 4003 times:

Boy, is this getting complicated, or what. The air conditioning/pressurization system is probably one of the most misunderstood systems on an airliner. From Boeing: http://www.boeing.com/commercial/cabinair/index.html


Not as easy as originally perceived
User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 8, posted (7 years 7 months 2 weeks 1 day 8 hours ago) and read 3999 times:

Quoting Jawed (Thread starter):
what is this cooling equipment? And where is it located? And what happens if it fails? Doesn't that mean the passengers will be BBQ'ed alive by the hot air?

The Air Conditioning system.
The Packs are located mainly Below the Fuselage between the Wings.
The Hot air is temperature regulated.If Packs dont function there is no Pressurisation & Aircraft has to descent to a comfortable Altitude.
regds
MEL



Think of the brighter side!
User currently offlineValcory From United States of America, joined Dec 2006, 89 posts, RR: 0
Reply 9, posted (7 years 7 months 1 day 15 hours ago) and read 3777 times:

Quoting N231YE (Reply 1):
This air is cooled in the airplane's PACS (Pressurization and Air Conditioning System). Since I am not a mechanic, I really can't describe the intricate details of their operation,

This is for a 767 757 also work the same way.Preconditioned bleed air enters the cooling pac through a flow control valve and is ducted to the primary heat exchanger.After passing through the heat exchanger where bleed air is cooled by ram air,the bleed air enters the compressor section of the air cycle machine (ACM) and is compressed to a higher pressure and temperature.The bleed air is cooled again by ram air in the secondary heat exchanger and after passing through the high pressure water separator components(reheater,condensor and water extractor) enters the turbine section of the ACM. The air expand through the turbine section,in expanding generates power to drive the compressor and cooling fans impeller.The energy removed during the expansion causes teperature reduction resulting in very low turbine discharge air temperature.The PACK discharge air is delivered through check valves to a mix manifold where the air mixes with the other pack before delivery to flight deck and cabin zones.


User currently offlineMiles_mechanic From Canada, joined Sep 2001, 136 posts, RR: 0
Reply 10, posted (7 years 7 months 1 day 13 hours ago) and read 3756 times:

Hello everyone

I am a mechanic working on smaller airplanes in the commuter category, and so have some experience on them, and on them it is actually the air out of the engine that ends up in the cabin to pressurize the cabin and to control the temperature of the cabin. There is a heat exchanger in the engine compartment on the one airplane to cool the bleed air, and the other has a couple of heat exchangers. On the Cessna Conquest, because of it being a high altitude turboprop, it has an emergency pressurization setting where you can put raw hot bleed air into the cabin so you can make a decent down to a lower altitude where you can shut the system off. We had a problem with that system once and the valve opened and wouldn't shut off, so the pilots had to fly it back home, we went into the airplane 30 minutes after they got back and the seat legs were still warm. The pilots were so hot they almost couldn't take it. Guess that was not the greatest way to supply emergency control, but at least it was only a one time thing.

Miles.


User currently offlineTod From Denmark, joined Aug 2004, 1724 posts, RR: 3
Reply 11, posted (7 years 7 months 23 hours ago) and read 3703 times:

Quoting Valcory (Reply 9):
to a mix manifold where the air mixes with the other pack before delivery to flight deck and cabin zones.

And in many cases, in addition to the air from the packs, supplemental air fans pump recirc air into the mix manifold too.

Tod


User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 12, posted (7 years 7 months 23 hours ago) and read 3701 times:

Quoting SlamClick (Reply 2):
At the end of the processing it is common for hotter, untreated air to be added back into the ducting for temperature control.

Trim Air


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 13, posted (7 years 7 months 22 hours ago) and read 3694 times:

A couple of basic principles:

Heat exchangers
take a cold airflow and a hot airflow and even out the temperature between them. They are basically the same as car radiators, only gas-gas rather than gas-fluid. There's also a pressure loss in every flow going through a heat exchanger.

Compressors
do work on the air, increasing the pressure and thus the temperature

Turbines
use the air to do work, decreasing the pressure and thus the temperature.

(1) You take hot, pressurised air from the engines. You then put it through a heat exchanger, creating less hot and pressurized air.

(2) Then you pass the same air into an air cycle machine. In the ACM,

(3) A compressor compresses the air, creating hotter air under even higher pressure.

(4) The air is then passed through another heat exchanger, again creating cooler air.

(5) You then feed the air through a turbine connected to the compressor, thus driving the compressor and creating lower pressure, cold air.

(6) This air is mixed with trim air straight off the engine bleed system, or possibly extracted after the first heat exchanger, to create air with the right temperature for cabin climate control, avionics cooling etc.

Valves in various points of the system control the volume and temperature of air coming into and going out of the cabin, thus regulating temperature and cabin pressure.

If you didn't compress the air, you could never get air cooler than the ambient temperature. The smaller the temperature difference between the two airflows through a heat exchanger, the larger the heat exchanger must be. This creates more drag and is thus Bad.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineAAR90 From United States of America, joined Jan 2000, 3465 posts, RR: 47
Reply 14, posted (7 years 7 months 22 hours ago) and read 3683 times:

Back to the original questions:

Quoting Jawed (Thread starter):
So I'm wondering: what is this cooling equipment? And where is it located?

As previously noted, they are called PACS (or if you prefer PACKS) normally located in the lower fuselage (DC8/10 under the nose, most others between the wings, some in aft section behind the cabin).

Quote:
And what happens if it fails? Doesn't that mean the passengers will be BBQ'ed alive by the hot air?

There are temperature and pressure controls that keep things within limits. If something exceeds a limit the incoming bleed air is secured and the system shuts down. Normally happens automatically, but if that safety feature fails, the pilots can manually shutdown the systems... you might get a bit warm, but you won't "BBQ."  Wink



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineTristarSteve From Sweden, joined Nov 2005, 3977 posts, RR: 34
Reply 15, posted (7 years 6 months 4 weeks 1 day 9 hours ago) and read 3609 times:

Just thought you might be interested in a few words on the B777 pack. It is the most complicated system I have encountered, but works a treat.
The ACM has a compressor, two turbines and a fan on a single shaft.
On the ground the normal air flow is
Primary heat exchanger
ACM compressor
Secondary heat exchanger
reheater core
Condensor core
Water collector
Reheater
ACM turbine 1
Condensor
ACM turbine 2
And out into the mix manifold.

There are three valves, an Economy cooling valve, a second stage Turbine Bypass valve, and a Low limit valve, and an economy cooling check valve.
In flight these valves all control so at cruise the airflow is

Primary heat exchanger
Secondary heat exchanger and out to the mix manifold.
In cruise the ACM is not running. There is enough cooling from the heat exchangers, and the airflow over them is controlled by the Ram air inlet doors which are nearly closed.


User currently offlineOryx From Germany, joined Nov 2005, 126 posts, RR: 0
Reply 16, posted (7 years 6 months 4 weeks 1 day ago) and read 3575 times:

AFAIK there are two principles out there: Bleed air is taken from thernengine and either conditioned and routed into the cabin or just used torndrive the pacs.rnAt an B744 it's the later case where bleed air drives a turbine, whichrndrives a compressor which compresses ambient air to (and above) cabinrnpressure.

User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 17, posted (7 years 6 months 4 weeks 1 day ago) and read 3562 times:

Quoting Oryx (Reply 16):

I am curious about your keystrokes. There seem to be five instances of the spacebar giving you the characters "rn" rather than a space. Any reason why this might be so?



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineLiedetectors From United States of America, joined Jul 2005, 360 posts, RR: 0
Reply 18, posted (7 years 6 months 4 weeks 1 day ago) and read 3556 times:

If you all want to burn out the ECS pack, then you can direct the bleed air to the pack.

since the bleed air can have temperatures in excess of 600 degF, you are going to need to cool it down. ECS packs can generally handle hot air but not for long periods of time. before the bleed air makes it into the cabin it is cooled with fan air in the precooler. the precooler is nothing more then a heat exchanger with vains of bleed air blowing across vains of fan air. the precooled air is then sent to the ECS pack and WAI systems.



If it was said by us, then it must be true.
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