Why is bleed air taken from some stage of the compressor used for de-icing? Why not e.g. exhaust gas?

Question

In the answers to this question, it is said that

The de-icing system on most turbine aircraft (including MD-82 involved in that accident) uses bleed air from the engines, that is it extracts some air from behind the (low pressure stage of the) compressor. This air is therefore not ejected from the nozzle and not producing thrust, so the thrust is reduced.

My question is:

Why is bleed air taken from some stage of the compressor used? Why not e.g. exhaust gas?

For de-icing, the temperature of the used medium must be above 0°C (melting ice), while the environmental temperature usually is far below. So the engine has to invest energy to generate bleed air with reasonable temperature, the air cools down to still >0°C during de-icing, and is then vented out to the environment. There, it expands and cools down far below the environmental temperature. This expansion means that energy is wasted, and also, when the bleed air is cooled down / expanded before inserting it into the de-icing system (I don't know if this is done), energy would be lost. In addition, air/pressure is lost in the compressor stage, which makes the combustion less effective. (Again, I don't know how much air is taken, and how big the effect is)

On the other side, the exhaust gas of the engine is very hot due the combustion and could be used without the need of extra compression. So, using some exhaust gas, one would not waste so much energy. If it's too dirty, heat exchangers could be used to heat fresh air.

I would think of this reasons

• Bleed air is anyway used for many purposes in an aircraft, so this is more economic than a completely separated system
• De-icing is not used for long time during flight, so again no need for a dedicated system

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EDIT:

I'd like to expand the question to explain what makes me curious. In the comment, the correctness of this sentence is challenged:

There, it expands and cools down far below the environmental temperature.

This is a simple thermodynamic process. The air is compressed adiabatically, i.e. without adding heat to it. The heat comes from the thermal energy of the air, now also compressed to a lower volume. De-icing cools down the air, and when releasing the air to the environment, it expands to the orignal pressure. As thermal energy has been removed during de-icing, the temperature drops below environmental temperature.

Here is the math behind:

The relation of pressure and temperature in this case is:

$$ p_1^{1-\gamma}\cdot T_1^\gamma=p_2^{1-\gamma}\cdot T_2^\gamma\qquad \gamma \approx 1.4$$

Let's assume a pilot switches on de-icing during flight at 11km altitude. There, environmental pressure is 0.25bar (atmospheres) and temperature is -50°C (223K). It was also said here in the answers that it's possible that the bleed air has about 200°C (473K). The formula now gives a bleed air pressure of 3.47 bar, so a pressure ratio of about 14. The air is now cooled down while the pressure is maintained by the engine. I assume de-icing will be effective for bleed air temperatures above 0°C. So if the air is released at this temperature, the temperature will fall down to -144°C (128K). Another number: If releasing at 100°C, the temperature will drop to -97°C (175K).

(Of course, the air will mix with the environmental air immediately)

In principle, one can play with the numbers, increase/decrease altitude / temperatures and discuss how adiabatically this (de)compression processes are.

Anyway, this is a big air conditioner, using the thermal energy for de-icing and wasting the cooled air. If one only needs hot air, something coming from the exhaust system would always be more efficient.

This is not really efficient. May be the bleed air behind the de-icing system can still be used for other purposes, as it still has the pressure?

Answer 1

Using air from the exhaust section either for anti-ice directly, or using a heat exchanger, would create many problems.

• The air must first be used to burn fuel before reaching a turbine bleed location. Air taken from the compressor section hasn't required any fuel to be burned (other than to compress that air of course).
• The air will be much, much hotter than air from the compressor. Turbines, especially in the high pressure section, are made of specialized alloys and can be additionally actively cooled to prevent them from melting. This means more specialized, and therefore more expensive and/or heavy material must be used for the whole anti-ice system.
• Components must also be designed for the case of this bleed air leaking. Hotter turbine air would be a huge problem for this.
• The air will have the combustion byproducts in it. This can coat the insides of the anti-ice system, reducing its effectiveness, and possibly requiring it to be disassembled and cleaned periodically.
• If there is some type of engine fire, the fire typically goes downstream, which would send it into this anti-ice system as well.
• In the case of using a heat exchanger, this would be yet another component to fit into spaces that are already very full, and yet more weight on the airplane.

The air from the compressor is already plenty hot. Take a look at the diagram in this answer. Notice that all the compressor bleed air goes through the precooler before going to cowl or wing anti-ice. This means that the compressor air is already hotter than is required. So taking air from the turbine provides no benefit for creating the problems listed above.

Also, taking air from the compressor does reduce engine efficiency, but you are going to have to get that energy from somewhere.

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To address your points about the thermodynamics:

Assuming your numbers are correct, what is wrong with that scenario? Releasing air at -97°C that was initially at -50°C means that you extracted 47°C of energy from it to use for anti-ice, using the systems already available on the aircraft (bleed air, precooler). Kind of like a heat pump, but just using air? Wouldn't it be less efficient to burn fuel to heat this air, which would then leave the aircraft at a higher temperature, and therefore throwing away more energy?

In principle, one can play with the numbers, increase/decrease altitude/temperatures and discuss how adiabatically this (de)compression processes are.

(Emphasis added) I'm not convinced that your assumption that these processes are adiabatic is a good basis for your model, since that is an idealization (though your initial bleed numbers do seem to match here and here). An important point is the way the air is used. A typical installation uses piccolo tubes. Small holes along the tubes release the air onto the leading edge. This whole leading edge cavity then vents to the environment. (This doesn't seem to match your assumption that the air is cooled at constant pressure) In order for this air to be useful any further, you would have to somehow return this air to for reuse. This adds more complication, when there is already an abundant supply of compressed bleed air available.

Another option would be to somehow maintain the pressure behind the surfaces with ice protection. This may be feasible for the engine inlet, but not for wing leading edges that include moving slats.

Even assuming you can somehow recover this air for use, that would be another system and more complication added to the aircraft. And you can't rely on this system, because anti-ice is not always in use. You have to provide a pretty large gain in efficiency to justify adding weight and complexity to the airplane.

Maybe you would like to see proof that most the work done on the air is used in the anti-ice process, and adding heat from combustion is not necessary. We may need to get someone like Peter Kämpf in here to run some numbers.

Answer 2

Your first reason is the major one: Bleed air is abundant (jet engines generate far more than they "need" for operation), already hot (typically well over 100C), and produced at a relatively high pressure - that's why it's tapped for things like anti-icing systems, cabin air conditioning, and starting other engines.
It takes little effort in design to install another valve in the bleed air system to direct the flow to anti-icing systems.

Capturing the turbine exhaust (hot combustion gasses) would require its own special hardware (valves, ducts, etc.), and measures to cool the combustion gasses to a temperature the anti-ice system can handle (heat exchangers), which means more complexity and more weight, so if an aircraft already has a bleed-air system available it makes sense to use it for the de-ice/anti-ice system rather than add another equally-complex arrangement to capture combustion gasses.

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For "bleedless" aircraft (ones that don't have the ability to pull bleed air from the engine's compressor) alternate de-icing systems are used (e.g. the 787 uses electric heating elements).
These could also be used on aircraft with bleed systems, but they have their own disadvantages in terms of maintenance, weight, and complexity. (Bleed air anti-icing systems are comparatively simple: a few valves and tubes with holes drilled in them to let the hot air out where it's needed. Maintenance and upkeep are fairly trivial.)

Answer 3

I agree with you that the bleed air ends up colder than ambient, but not that this "means that energy is wasted".

On the contrary. Once the portion of air we're using is back at ambient pressure, it will be colder than before the engine took it in. That means that heat energy has been removed from it. Where did that energy end up? The only place for it to go is in the wing surfaces that we wanted to heat up. (This assumes that the piping and so forth is thermally insulated so well that we can neglect unwanted heat losses there).

The net result is that the amount of heat energy deposited in the metal of the wings is larger than the mechanical energy we used to compress the air in the first place. Which sounds pretty efficient to me.

On the other hand, engineering for the de-icing air to be warmer than strictly necessary after we're done with it, that would be a waste.