Bleed Air Heater Therodynamics
 

It goes like this....
Compressor draws in outside air.
Compressed (high pressure) air warms up.
Bleed valve allows hot/high pressure air into cabin.
Hot/high pressure air is released, returning to atmospheric pressure in cabin, thus cooling down....
So, why is the air hotter than when it started its journey if it has returned to its original pressure? Mechanical friction?

Hovering

I'm no scientist, but I would assume once the molecuels are excited(heated) they take some time to cool(slow) back down, thus the air stays warm until it get to us in the cabin.... It both actions, heating and cooling, were instant, then I would see how the air would return to its orginal temp, but it's not.

RH

my theory - the heat in the bleed air (once uncompressed) is heat picked up from the hot engine casing.

It does eventually - but only after it has transferred its heat to the surrounding air. Usually, a small volume of very hot bleed air is used to mix with a larger volume of ambient temp air being passed through a plenum chamber. This heated ambient air is then blown into the cabin

I agree; when the air is being bled to the mixer unit it is still under high pressure and thus still hot. This hot air is then mixed with the cooler, ambient, air to achieve more manageable temperatures.

Sorry but I disagree with all of you! As hovering says, if the air is compressed and then expanded why does it not end up at the same temperature? It would not matter whether it was mixed with cold air -the cold air is at roughly atmospheric pressure so the hot air must have expanded before its mixed, at which point it will instantly cool - it won't take any time to cool, same as it doesn't take any time to heat up when its compressed (just as well otherwise your diesel car wouldn't run!). There is no delay because there is no movement of energy as would occur with a constant-pressure heating from, say, an electric fire. (I think the word I am looking for is "Adiabatic")

HP - in the 332L the air decompresses at the heater valve which is away from the engine casing.

In fact hovering has it correct when he says "friction". It is the extra work done to the air by the compressor that raises the air's temperature by a greater amount than would result from the compression. So when the air expands again its hotter than when it started.

Think of it in terms of conservation of energy. Unless the compressor is 100% efficient (which nothing is!) the extra energy required to drive the compressor above that used to compress the air has to go somewhere - and of course it goes to heating up the air by friction.

Another example for 332L2 / 225 pilots with the hydroelectric generator is the hydraulic heater. This maintains the temperature of the LH hydraulic system within a certain range simply by bleeding high pressure fluid through a restriction back to the reservoir. The fluid is incompressible and the heating effect comes purely from friction through the restriction, plus perhaps because the pump is having to work harder.

And another one - why does the main gearbox oil heat up? there is hopefully no metal-to-metal contact, no combustion etc. The heat is purely the result of thrashing the oil around and squeezing it through bearings etc - ie friction in a fluid

HC

HC (and hovering) you're right that friction (and contact with hot surfaces) raises the temperature, and these are the main mechanisms by which gearbox oil (an essentially incompressible fluid) heats up. These processes undoubtedly have a role to play in the compression heater or 'Roots Blower', but they are not the main one which is compression.

P1*V1/T1 = P2*V2/T2 is the combination of Boyle's and Charles' Law which guides you here. If the pressure and volume were the same on either side of the roots blower, then there would be no temperature rise. It's only where the compressed air is maintained at a higher pressure that the air is 'charged' ie. hotter.

the heat has to be passed on, you instilled energy into the air molecules but air is then a poor conductor of heat as we all know so expanding it again isnt enough, it has to touch something ie: other cooler air molecules or solid objects, you are blowing more hot air into the cabin than can be cooled so its cosey.
Sorry blabbering... squeeze it, it gets hot, expand it but it still contains the heat regardless until it is passed.
Isnt that heat called layton heat? (spelling):confused:

QR

More 4x required for you I think.....Latent heat is the heat associated with phase changes from solid to liquid to gas and vice versa.

If you compress a gas efficiently (say by slowly moving a piston up inside a perfectly insulated cylinder) and then decompress it again (by moving the piston back again) you will be back exactly where you started in terms of pressure, temperature and volume. The only reason why your heater works is because of the energy losses in the compressor due to air friction. And to some extent, as Extinct-Fiat says, due to contact with hot surfaces, though the casing near the front of the engine may not be much above the temp of the compressed air.

By compressing the air energy has been added to the system. Some, but not all of the energy is extracted through expansion or lost via heat transfer as it transits the heater ducts. The energy that remains manifests itself in a higher temperature when the air is brought back to ambient pressure. Bottom line - there's more energy in the exiting air than in that which enters the engine inlet.

-Stan-

Surely the point is that 'work' has been done on the air and energy transfered to it (from the fuel), so why do we expect it to resume its same state as precompression? The compression isn't taking place 'for free'.

Helicomparator
Yea i realised i typed quicker than i thought, i stand corrected :rolleyes:
I like to keep it simple, anything that is expanded rapidly is cooled, the air from the compressor isnt cooled that quick so doesnt return to normal given it was full of heat energy,think of a gas bottle already cool in its self.. masive expansion and so ice forms, tips of a jet plane, thats why they form so easily when doing high G' manouvers, Adiabatic cooling? 2nd time lucky :confused: :rolleyes:
Slgrossman has the right way of putting it. :ok:

I think 212 is getting close - as he says work is done on the air.

However if you take my previous example of the piston and cylinder, although you do work on the air to move the piston to compress, you could then connect the piston to some machine and have the compressed air do work on the machine as it expands, getting all your energy back. But in the case of the heater, you have to do more work than the minimum required to compress the air in order to overcome the inefficiencies / friction, and it is that bit of the work only, that results in hot air in the cockpit.

Another example - car on a hilly road. On the way up hill, the engine has to work hard to increase the potential energy of the car. On the way back down the other side, the potential energy is released and means the engine has to add no power to keep the car moving. But overall, the engine still has to produce power to overcome friction (tyres, bearings, air etc). That additional energy ultimately goes to heating up those tyres, bearings, the air etc.

So 212, the compression could take place "for free" if the process was 100%efficient and if the energy released in expansion was captured. But of course its not, so the air ends up hotter.

HC

I don't think the speed at which the air is compressed or decompressed plays a factor, other than if compression or decompression were happen slowly, there would be increased opportunity for heat transfer from the walls of the air containment system, which may be a heating or cooling factor, depending on where in the system the air is. This can be realized with the time it takes the system to warm up. The source air from the compressor should have a fairly stable temperature once the engine is running, but it takes some time for the warm compressed air to heat up the pipes delivering it to the cockpit. If you where to turn on the heater in the summer, you would have almost instant high heat, which is not being transferred to the air from the delivery system istelf...
So, where is the extra heat, left over after decompression back to atmosperic pressure, coming from?
(yes, it is all about conservation of energy, and I am sticking with mechanical friction for now, but I still don't 'KNOW').

Does it affect the price of fish either way?

HC thanks, to amplify my comments a bit further:
it's an 'open system'; the difference in total energy between a fluid exiting the system, and the fluid entering the system, is equal to either a) the work done on the fluid (compression powered by the gas generator in this case) resulting in a temperature rise, or b), the 'heat' extracted from the fluid within the system, resulting in a temperature decrease.

(Words in '...' are Thermodynamic terms)

I could say "it's not rocket science", but perhaps in this case that's a bad example!!:\

Edited to add: Naturally the simplistic equations stated do not take into account losses occuring within the system, e.g. heat transfer to the turbines and casing, but illustrate the basic principle of an open system.

212man has it right. To put it another way, I think it is the first law of thermodynamics which says energy can be neither created or destroyed, merely changed in form. To compress a gas you need to imput energy (don’t you work up a sweat pumping up a tyre with a hand pump). When that gas is allowed to expand (back to ambient in the case of bleed air heater) that gas retains the heat (energy) required to originally compress that gas (ignoring any losses). Allied to this is that the N1 turbine/compressor system is about 98% efficient. That is, the turbine recovers 98% (approx) of the energy required by the compressor.

A measure of the "work done to the air" is confirmed by the loss of power experienced when using bleed air. That energy has to be demonstrated somewhere.

I'm a rookie at this whole turbine engine thing (arguably at this whole flying helicopters thing in general!) and have a question that relates to this... My (limited) understanding is that the limiting effect of taking bleed air off the engine is to remove cooling air (hot as it is) from the system resulting in higher operating temperatures (t4 / tot). Is this the action that causes "power loss" becuase the engine will reach it's temperature limitation while still at a lower throttle setting, or is the power loss due to the loss of some air otherwise used for combustion, or some other factor I'm not considering.
I guess I'll play around with the heater tomorrow morning (if it's cool enough) that ought to help my grasp a little...

Now that the thread has settled a bit, Let me toss in my 2 cents:

The heat comes from the eye-watering inefficiency of the engine, where it has to literally smash the air into submission to make it go to higher pressure, and the extra work performed on the air heats is far more than the pressure change alone. Were the engine 100% efficient, a bleed air heater could not work.

The gas is compressed and therefore heated by the work performed on it. This heat is very real, and is much greater than the theoretical heat gained by the pure pressure work expended on the gas. Why? Because the compressor is not terribly efficient, at about 35% efficiency, it takes three times as much work to compress the gas as you get out of it when you release it. Therefore, when it cools as it goes back to normal pressure, it is still plenty hot. Where did that heat come from? The work that was "wasted" in the inefficient compressor.

A normal gas turbine is about 36% efficient overall (energy in the fuel goes in, Horse power comes out.) Here is the proof - a very efficient turbine burns .40 lb fuel per hp per hour. 1 HP has 550 ft-lb/sec or 2 million ft-lb/Hr heat value. 778 ft-lb = 1BTU, so a Horse Power is equal to 2600 BTU/Hr. A good turbine engine needs .40 lb of fuel per hour per horse power, and fuel has 18,000 BTU/Lb stored in it. That means a turbine engine needs 7200 BTU/Hr to make 1 HP. So it eats 7200 BTU, and produces 2600 BTU of work, net efficiency is 36%

Truly an amazing day.... I fully agree with a posting from Nick!

HC

Have to agree with Nick here.;) ;)
But hey, turn on the heater, cabin warms up. Turn it off, cabin cools down. What more do you want?

Thank you Nick for your excellent answer. It was very well written.

36% is actually not too shabby on the scale of engine efficiencies - the best transport diesels run at 45%. What sort of pressure ratio does (say) an Allison 250 run at? I remember from the dim and distant past that each stage generates something like 1.2 Bar, and there are 10 stages. That gives 12 bar in the combustion chamber...

Mart

Sounds like a load of hot air to me...:E

so we have covered bleed air heating , how does bleed air cooling work ?

Graviman,
You are right, 36% is actually good, relative to what is possible. The 250 series has a single centrifugal compressor which looks like the pump in a washing machine or the blower in a vacuum cleaner- a conical shaped wheel that has spiral fins on it, so the air is swept up and spun into an ever decreasing space (since the fins get shorter and shorter as they get to the top.) The wheel spins so quickly that the air does not escape due to its own inertia. The 250 gets a 9:1 pressure increase in this one stage!

http://www.acec-ark.com/Techinfo/Int...entrifugal.JPG

widgeon,
The bleed air cooler works just like a refrigerator or air conditioner. The working gas (air for us, freon for the AC) is squeezed and made hotter by that squeezing, then that hot pressurized gas is run through a cooler/heat exchanger while still pressurized. This is really like a car radiator. The fluid's heat is removed (and dumped outside) and then the gas is rapidly expanded so the cooling is appreciable. Often, a turbine (run by bleed air) inside the system is used to increase the pressure of the gas further, so the cooling is more efficient.
Here is a schematic of one such system:
http://www.tpub.com/ase2/73.htm

And there was i naively thinking 4:1 was the highest a centrifugal stage could achieve - them heli engine boys is pretty smart.

Mart