Sign in to follow this  
Guest Legal Flyer

Air conditioning packs : question for Timothy Metzinger

Recommended Posts

the air conditioning units in boeing are called "packs". I heard from a friend that it is a word derived from the acronym "pac". Can someone give me the full description of it ? I recall there was a message on the forum where it was explained but I can't find it.greetings

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Well my name ain't Timothy, but I'll answer anyway :-)The unit is actually called a PACK (packs would be the plural) and here is its full meaning:Pressurization and Air Conditioning KitRegards,Mark

Share this post


Link to post
Share on other sites

>Well my name ain't Timothy, but I'll answer anyway :-)>The unit is actually called a PACK (packs would be the plural)>and here is its full meaning:>Pressurization and Air Conditioning Kit>Regards,>>Mark>Or, PAC for Pneumatic Air Cycle. Keep in mind that there's no freon or other refrigerant in these things, even though they are air conditioners.

Share this post


Link to post
Share on other sites

Mark and Timothy,thanks for the quick reply !greetings,

Share this post


Link to post
Share on other sites

No freon or refrigerant??I can see how at altitude they could use the outside air to cool things down, but what about when on the ground?!Now I am curious!-Greg

Share this post


Link to post
Share on other sites

Hmmm ... I think that the compression and expansion of copious amounts of plain air (gnerously supplied by two big jet engines) are used to heat or cool depending on your needs. Ever notice how the air coming out of a bicycle pump is warmer that the surrounding air or how a can of compressed air gets very cold when used? Same principal, much smaller scale.>No freon or refrigerant??>>I can see how at altitude they could use the outside air to>cool things down, but what about when on the ground?!>>Now I am curious!>-Greg

Share this post


Link to post
Share on other sites

>Hmmm ... I think that the compression and expansion of>copious amounts of plain air (gnerously supplied by two big>jet engines) are used to heat or cool depending on your needs.> Ever notice how the air coming out of a bicycle pump is>warmer that the surrounding air or how a can of compressed air>gets very cold when used? Same principal, much smaller>scale.>Exactly.At UAL TKO in denver, this stuff is notated as PFM - Pure Freaking Magic, and it used on things wheres the pilots don't need to know how it works, just what it does.If you've ever seen the laymans description of inertial reference - It starts off "The airplane knows where it is, because it knows where it isn't, and it knows where it wasn't just a second ago", it's another PFM system.

Share this post


Link to post
Share on other sites

Can the APU alone supply the PACS on the ground for cool air in a 737 on a hot day? The KC-135 (B707 variant) PACS we used back during the Vietnam war on hot and humid days, even with bleed air from 4 turbines (non turbofans), left the cockpit as a sweat box until we were airborne and above 10,000 ft or so. We didn't have an APU back then and used ground air or a starter cartridge to rotate the number 1 engine.

Share this post


Link to post
Share on other sites

I do believe the APU can supply enough to operate on the ground. See this page: http://www.hilmerby.com/dc9/aircond_panel.html (it's for a DC9 but the concept is the same).Anyway, how the packs work (and I don't mean this to be entirely accurate, I'm trying to remember previous schooling, so this is just to half-arse it) is roughly this:The engine ingests and compresses the air, feeding it into the pneumatic system at high (realitively) pressure (roughly 36 psi or so if memory serves), this heats the air substantially. The reason being is, if you remember back to chemistry and/or physics, tempurature and pressure is related - the higher the temp, the higher the pressure, and visa versa. The air is then sent through heat exhangers (known as intercoolers on turbocharged cars and diesel trucks). This then cools the air while maintaining most of the pressure because, by cooling it, it has a lower pressure, then is immediately augmented by the engines contious feeding of air (poorly worded, I know).If I understand it right, if the cabin needs heating, the heat would be taking at this point, since the air can be well about 200*F (I assume, because in turbocharged cars, a pressure change of 15 psi, depending on the effeciency of the turbocharger or turbine, can be raised well over 100*F above ambient and this operates in the 30 psi range)Further down the line comes a device that rapidly depressurizes the air back down to atmospheric or lower pressure. This makes the air cool very rapidly (remember, if the mass of the air is the same, then the lower the pressure, the cooler the tempurature. Since the mass never changes throughout the pneumatic system, that's why it works here). This is the point where the cold air is taken.It's actually a pretty ingenious system. The reason why refridgerant is used in cars and homes is because there isn't a ready source for continous pressurized air and to create such a source simply for air conditioning would consume a lot of energy BUT since gets have a nice supply of pressurized air, might as well use it!

Share this post


Link to post
Share on other sites

just out of interest, someone may be able to help with this one??I once made a British Airways flight on a 747-400 from Sydney Australia to Heathrow via Bangkok. Both when leaving Sydney and while on the ground at Bangkok, they didn't run the Airconditioning packs until after we were airborne - announced that this was normal procedure for their aeroplane - I can tell you it got very uncomfortable while leaving fromn Bangkok.Any ideas why they didn't run the Packs? Is this normal procedure for BA, or was it likely to be a fault situation?Peter

Share this post


Link to post
Share on other sites

This lot should keep you busy!http://www.boeing-727.com/Data/systems/infoaircon.htmlhttp://www.b737.org.uk/airconditioning.htmif you go to www.smartcockpit.com then you have a 737NG and a 767 AOM ready for you to download!Hope this helps.Richardps: Peter, the reason for this is that in very hot conditions, and at a high take off weight, the crew need the engines to have all the performance that they can give. Thus they shut down the packs to reduce the load on the engines. Normal procedures is to leave the APU running, close both isolation valves, and leav one pack on. At about 2500 feet, the APU is shut down and the packs turned on, the isolation valves are also opened.

Share this post


Link to post
Share on other sites

It's normal for BA to run one pack for takeoff when the temperatures are high. On a 747-400, they close the bleed duct isolation valves and run the #2 pack only using APU bleed. The engines are only used for thrust. Whilst parked or taxying, however, it is normal to use 2 or 3 packs. 2 are not always cool enough on really hot days. One is generally hopeless (unless it is only switched off for takeoff after the cabin has been cooled by all packs... If the cabin has been cooled by all packs, then two are turned off, it usually takes a while for the cabin to heat up again). Packs are often switched off for a minute or two during engine start. Some 747-400 airlines switch all off, some leave one running for engine start.Pack or bleed problems may force the crew (or Maintenance) to switch off the packs during a stopover... or force the crew/engineers to use ground-supplied cool air (or bleed air) until the engines are running. Ground air is not usually as efficient at cooling as all three packs running at once.In your case, it sounds like they had a problem at some stage which forced them to shut down all the packs(?). BTW, how did you ascertain they switched off all the packs? (by temperature alone?).Cheers.Ian.

Share this post


Link to post
Share on other sites

Aircraft airconditioning systems still work on the basic principle of compressing/expanding gasses. In the case of an aircraft, the gas is normal air however.Hot compressed bleed air from the engines or APU is compressed, cooled somewhat by running the air through a radiator (heat exchanger), then expanded, producing a pretty good temperature drop. In flight, the radiator is indeed kept cool by cold, outside air (rammed in by the 300kt "wind"). On the ground, the outside air may not be so cool, but it is usually fan-assisted and helps cool the air in the pack. This is generally enough to keep things cool in the cabin.The NG has a powerful APU which allows both packs to be run on the ground (even in HI mode). On the ground, the speed of the compressor/expander(turbine) in the pack generally determines how much cooling there will be. Packs really scream on hot days (although the passengers will generally not be aware of this because of the cabin wall insulation).As long as there are not too many doors open on the ground during a turnaround (letting warm air in), NG packs will have no trouble keeping a cabin cool. However, packs may take quite some time to cool the cabin if the cabin has been baking in the sun all day without the packs being on. It might take over half an hour to get the cabin cool in such circumstances on a large aircraft such as a 747, so it's vital to get the packs up and running well in advance of the passengers boarding.Hope this helps.Cheers.Ian.

Share this post


Link to post
Share on other sites

I recently had an odd experience as a passenger on a Boeing 717. During taxi and takeoff, the PACs seemed to be directly contolled by the throttles. Every time the engine thrust increased (such as when the aircraft started rolling from a complete stop at a taxiway intersection), the air flow from the nozzels over my head would increase correspondingly. During takeoff, it was kind of funny because the blast of air grew so strong that my wife asked me if the windows were open :-) A couple of minutes after takeoff the airflow went back to to a normal rate and ceased fluctuating. I'd be curious to hear if anyone knows what was going on there? I have not experienced this on other airliners.Andrew

Share this post


Link to post
Share on other sites

Sounds like some control system or regulation valve was not working properly or at least not working to maintain a set pressure. Maybe by design, maybe a fault.Cheers,

Share this post


Link to post
Share on other sites

Don't really know why this would happen, Andrew, although I know what you mean.. I have noticed it, as a passenger, on other aircraft types.On the NG, there is a pack flow control valve which should regulate the flow of bleed air going into a pack even though the bleed duct pressures may be changing with engine rpms. In normal flow mode, my manuals quote a flow rate of about 55lbs of air per minute. I'm wondering if idling NG engines are unable to supply this 55ppm flow rate to the pack under all ambient temperatures? If it can't then the packs may output less air at idle and more at higher thrust.As I understand it, a Pack increases/decreases cabin air temperature by running its turbine slower/faster. When warm temperatures are required, some of the bleed air is diverted around the turbine, missing the turbine (and slowing it down). This may affect the air flow in the cabin... but I can't see how cabin temperatures would change rapidly by taxying an aircraft faster/slower.A simple question... but there may not be a simple answer to it ;-)Cheers.Ian.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this