Bleeds and Packs

[meggriffin]

Hi Guys,

Just having a bit of trouble getting my head around the whole Bleeds and Packs thing. If bleeds provide pressurisation then why do we need packs and vice versa.

Regards,

Meg

[fredgrav]

In very brief: BMC's (Bleed Monitoring Computers) provide automatic engine bleed air control; air is taken from Engine IP valve (from HP to meet high demands), APU or an external source. Bleed air is sent to packs, cooled down (ACM, heat exchangers) and supplied for pressurization, air conditioning, avionics ventilation, water tank pressurization, hydraulic reservoir pressurization, anti-ice and engine start ... without packs, compressor air would be too hot for being delivered to the users (150-200° C) ... that's it ! ;)

[FLEXPWR]

Hi,

To put it in very basic terms, bleed air provides indeed pressurization, but comes out of compressor stages at temperatures exceeding 200 degrees C.
Now, since noboby wants to have chargrilled passengers before we even start to take off, the air conditionning "packs", will regulate the flow of air entering the aircraft, and cool it to survivable levels.

Within the pack is what is called an ACM (air cycle machine, although some other terms may be used), where the air coming from the bleed valves is compressed (therefore the heat is increasing again...wait, we're almost there!) and then cooled through a heat exchanger (just like a car radiator), this cooling is not sufficient on its own, but the temperature will greatly reduce on the turbine (the "opposite" side of the compressor in the ACM), where the air expands again, now routed to the air conditioning duct.

The temperature right at the outlet of the turbine side of the ACM is very cold (usually maintained just above freezing to prevent ice formation). Since this air is too cold to deliver to the cabin, some of the hot air from the bleed is picked up and rerouted to bypass the pack, to mix with the cold air.

Now you have 2 regulating valves playing the major role in this arragement: the pack valve, that will regulate the amount of air (flow) going to the cabin, and the temperature regulating valve, which mixes some hot air from the bleed into the cold air of the turbine outlet.

There are a number of other tweaks to this type of system, but I hope this intro will help. You can think of it in the same way a refrigerator works: the freon is compressed, therefore heats up (you can feel it at the back of the fridge on the radiator), cools a bit on the radiator before being expanded as it enter the "inside" of the fridge, and your beer stays cool! :}

Any additions welcome,

Flex

[Kirks gusset]

In simple terms, the engine bleed air is just a hot high pressure air supply, if this were just pumped into the cabin two things would happen, one we would all melt, and two the pressure would build up and blow the hull to bits.
The bleed air supply is regulated by control valves and then supplied to the packs which control temperature and presurisation within the limits demanded for passenger health and comfort.
Packs won't work with no bleed air, however, we have to have the ability to turn the bleeds off for both safety and performance considerations.

A full and logical explanation can be found
at:SmartCockpit - Airline training guides, Aviation, Operations, Safety (http://www.smartcockpit.com/pdf/plane/boeing/B737/systems/0011/)

Don't get bogged down with too much detail on this.

[CAT1 REVERSION]

The new 787 is doing away with engine bleed air alltogether....In favour will be electric air compressors - much more efficient and less smelly!!!

Bleed air - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Bleed_air)

:ok:

[Mad (Flt) Scientist]

I'd certainly quibble with the idea that independent compression is "much more" efficient that using engine bleed, given how efficient engine compressors are.

There are some very misleading comparison numbers chucked around on this topic - you have to compare apples with apples, and a lot of times that isnt done.

[PappyJ]

You can think of it in the same way a refrigerator works: the freon is compressed, therefore heats up (you can feel it at the back of the fridge on the radiator), cools a bit on the radiator before being expanded as it enter the "inside" of the fridge, and your beer stays cool! http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/badteeth.gif


A+ answer. :D

[FE Hoppy]

Pressurisation is one use of bleed air. There are many other uses of bleed air. Bleed air is any pneumatic source tapped from the engine. This often is from the high pressure compressor at one or more places.

Examples of uses for bleed air could be

Environmental Control
Wing and/or Engine anti - Ice
Engine start
Water pressurisation
Operation of flight controls


So Bleed Air has many uses.

Under the heading Environmental Control we could include air-conditioning and pressurisation.
For air-conditioning we need some means of controlling the temperature of the bleed air. This could be done by means of an air-conditioning pack consisting of an air cycle machine. This can condition the air before we allow it to enter the aircraft.
For pressurisation we need to control the amount of this air that we allow both into, and more importantly out of the aircraft. In controlling the flow of air we control the pressure inside the aircraft.

There are other solutions available. Imagine a space craft for example where no "bleed air" is available. You still need to control the environment inside the craft.

[jetslut]

Gents,
I think Megg is probably looking for the fundamentals here, so bear with me while I tell 'em.
Are we sitting comfortably?

Bleed Air is the compressed (and therefore heated) air that is tapped (bled) from the compressor section of the engine. This air may originate from up to 3 different stages (each having it's own pressure and tempurature) depending on engine type. This air is controlled by a Pressure Reducing and Shut Off Valve (PRSOV) which does exactly that, - presents the bleed air at temps and pressures the Pressurisation / Cabin Conditioning Control machinery can deal with.

The 'Pack' should be thought as as an independent air-conditioning maschine using the Bootstrap theory (compressor - intercooler - turbine) to provide temperateure control AND pressurisation.

Generically, a pressure controller controls cabin pressure/cabin Alt (airflow mass/press into the cabin ((also outflow valve)) and a Cabin Conditioning controller provides the comfort (temperature) aspect, but this is all achieved with a single source of air (bleed).

Before everyone jumps on my back about trim air and 35f, 65f et al stats,
I've tried to keep it simple for Megg.

[Otto Throttle]

Meggs,

I still haven't seen a simple Janet and John answer like the sort that I sometimes need, so;

Bleeds are the basically the mechanisms used to take very hot air from the engine for a variety of uses, including pressurisation.

Packs are the pieces of equipment that bleed air must pass through to bring it to a sensible temperature before it can be used for cabin/hold pressurisation.

In essence, bleeds are stage 1 and packs are stage 2 of the pressurisation process.

[GAZIN]

'In essence, bleeds are stage 1 and packs are stage 2 of the pressurisation process.'

Otto Throttle. Bleeds & packs are one way of providing 'Air Conditioning', pressurisation does not always have to follow, on the ground or below 10,000ft for example you don't need to pressurise the cabin.
'Pressurisation' is achieved by controlling the rate that conditioned air leaves the aircraft. It relies on a flow of air into the cabin but does not have any control on that supply of air, only the release of it.

[ivor toolbox]

Top marks in school go to Mr Flexpwr...

To add a little more; think of pressurisation like blowing up a childs balloon;
to get the same in our aeroplane we first bung up all the leaky holes (I know some aeroplanes leak more than others eh?); then to make sure our balloon doesn't doesn't burst; we regulate a leak (using what are called 'Outflow valves'), allowing the air that has come from the air conditioning
packs, to escape once it has done its' job of warming or cooling down
everyone inside.

ttfn

[Exup]

For a simple answer read Otto's post.

[Basil]

electric air compressors - much more efficient and less smelly
Wouldn't bet on it.
You lose the weight of the bleed equipment but gain the weight of the compressors.
I haven't seen the compressor design but, unless oil contamination is impossible, then that risk remains.
Seem to recollect on SCUBA compressors careful maintenance was truly VITAL although I'm sure there will be little design similarity.
New technology - ooh, err missus!

[capt. solipsist]

To recap: Bleed air is used by a number of other acft systems. One of them is the airconditioning system. And by now, we have discussed almost to the bone how the airconditioning system works. :)

On the other hand, reduced in very simple terms, the press'n system is a hole cover. :} Pressurization control is simply controlling the cover to a hole in the acft's fuselage so it doesn't explode like an over-inflated balloon. Mostly, this is done by some contraption they call auto press'n controller, but at some non-normal cases, we have to earn our keep and actually put some thought and effort on how to properly set it manually. Good thing there's a checklist for that. :ok:

[rawdataILS]

I'd certainly quibble with the idea that independent compression is "much more" efficient that using engine bleed, given how efficient engine compressors are.


it has very little to do with the efficiency of the engine's compressor. the problem with bleed air is that the pressure and temperature are much greater than you actually want to pressurise the cabin so you have to reduce both. this is an inefficient process, you lose lots of energy. an electric compressor will only compress the air as much as is required, so the engine will only have to provide the minimum required amount of shaft power to run the generator which in turn powers the air-con compressor.

Also theres likely to be another benefit - the thermodynamic efficiency of the engine cycle may be slightly improved if its fully optimised for its primary role of providing thrust, with only shaft power taken from it, rather than having a massive (and variable) bleed from the compressor air flow.

lastly, with an electric pressurisation system, you dont spend ten hours breathing the oily nasties coming out of the engine compressor bearings. (the ones 'they' will only admit exist once the last conventionally pressurised aircraft has left the european register in 25 years time...)

[FE Hoppy]

lastly, with an electric pressurisation system, you dont spend ten hours breathing the oily nasties coming out of the engine compressor bearings. (the ones 'they' will only admit exist once the last conventionally pressurised aircraft has left the european register in 25 years time...)

Every compressor needs bearings, they all need lubricating so your electric pressurisation system will suffer the same risk of oily nasties!

[TerminalTrotter]

I suspect that being able to run your compressor at well below 200C will have a significant effect on the production of said nasties, and this has probably had a bearing on the choice of this system.

TT

[nicolai]

Every compressor needs bearings, they all need lubricating so your electric pressurisation system will suffer the same risk of oily nasties!
You may not need to put such high-performance, and therefore toxic, oil in the electric air compressor as you have to put into the bearings of a turbine engine. This changes the risk level.

[rawdataILS]

yep, its all about the operating temperature, type of lubricating oil required, the type of seals used in the compressor vs those used in a engine compressor. The whole machine is just different and can be designed with a focus on providing air for the cabin, rather than the combustion chamber.

[PrestonPilot]

Does air for the cabin come from the LP or HP compressor? Thanks

[gusting_45]

Depends on phase of flight. At low or idle thrust the HP bleed ps should open as LP bleed air will be insufficient.

[misd-agin]

Even simpler - bleed air is too hot. Packs cool it down.


Temperature and pressure from bleed air varies depending upon engine speed.


Packs can regulate the output air to an acceptable temperature and pressure.

[SMOC]

Don't forget the pre cooler cools the air first and the pylon bleed valve (PRSOV) regulates the pressure.