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I'm studying the systems on my first turbine-engine aircraft, specifically, a C208B.

I know that there is "bleed air heat" that mixes with re-circulated cabin-air. I know that the bleed air is compressed, and thus hotter than it would be in an ambient pressure. I also understand that if the compressed air were decompressed it would return to its original temperature or lower due to entropy in the compressed state.

Do these systems utilize heat exchangers much like a car's A/C system except they warm the uncompressed air entering the cabin and cool the compressed bleed air through a heat exchanger? Surely they cannot send actual bleed air directly into the cabin and have it remain hot because it would immediately decompress, right?

How do these systems typically work? How is it different between an uncompressed unpressurized and compressed pressurized cabin? I would think even a pressurized cabin would not retain enough internal pressure to maintain anything close to the heat level of the compressor air.

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    $\begingroup$ See "air cycle machine" on wikipedia for an explanation of how these things work. $\endgroup$ Sep 18, 2021 at 4:29
  • $\begingroup$ Heat though, not air conditioning. $\endgroup$ Sep 18, 2021 at 13:57
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    $\begingroup$ If you expand compressed air via free expansion it will cool down only a bit (ideal gas wouldn't cool at all), not nearly back to the initial temperature—because you are not extracting the energy expended to compress it, and the temperature is proportional to the internal energy. $\endgroup$
    – Jan Hudec
    Sep 19, 2021 at 12:54

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There is lots of detailed information available on how these systems work, but in general once bleed air is tapped off the compressor section the lines split: One line stays hot, and the other runs through a heat exchanger to extract as much heat energy as possible.

From there the cooler, pressurized bleed air is expanded. However, because it was "pre-cooled" in the heat exchanger it does not resume its previous temperature, but instead becomes cold refrigerated air.

Then, (just like your kitchen faucet...) a mixer combines the refrigerated air with the hot air to a tolerable temperature as selected by the crew, and vents it into the cabin.

The temperature control principles are the same whether the aircraft is pressurized or not, but there are additional seals and valves needed to maintain and control pressure.

Regarding the expansion and loss of pressure, the compressor did not just reduce the volume, but added velocity which was turned into more pressure by the stators (energy came from the fuel).

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    $\begingroup$ Does a non-pressurized C208B have a full ACM though? It does not need to pressurize and if it only has heating, not air conditioning, it may just free-expand a bit of bleed air. $\endgroup$
    – Jan Hudec
    Sep 19, 2021 at 12:36
  • $\begingroup$ My answer wasn't specific to the Caravan, because although it was mentioned in the question, I interpreted it as a more generic question about heat exchangers on turbine powered aircraft. But you are correct, the 208 doesn't have the same kind of AC packs as pressurized planes. In fact looking in the manual, it doesn't even have a heat exchanger unless you count the fuel heater that pulls a little heat out of the bleed air. $\endgroup$ Sep 20, 2021 at 1:32

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