Originally Posted by
hans brinker
Yes, because when you pressurize the temp will go up, so you might need to cool it before it goes into the cabin. I know it is -56deg C outside, but the temperature increase will be bigly...
From NASA (who are a lot smarter than me):
T2 / T1 = (p2 / p1) ^ [(gamma - 1)/gamma]
During the compression process, as the pressure is increased from p1 to p2, the temperature increases from T1 to T2 according to this exponential equation. "Gamma" is just a number that depends on the gas. For air, at standard conditions, it is 1.4. The value of (1 - 1/gamma) is about .286.
So:
Altitude FL400, so p1 = 19KPa, T2 = -56deg C = 220K
Cabin 6,000ft so p2 = 81KPa
T2/220 = (81/19)^0.286
T2 = 220x(81/19)^0.286
T2 = 333 or 60deg C so a little warmer than I prefer my cabin....
Correct but you haven't cooled the outside air. it was -56°c and now it's considerably warmer!
One of the wonders of the traditional air cycle machine is that it is self powering. Take your bleed air at 40psi and 200°C and use it's own energy to cool itself well below ambient.
The electrical power requirements of bleed less systems demonstrate just how much energy you need to pump the volume of air required to not only pressurise but adequately ventilate an aircraft. That energy in the end all comes from fuel. If you are using bleed for some services you may as well use it for all. i don't see splitting pneumatic supplies between bleed and electric driven compressors as being an optimised solution. Neither do any of the airframe manufacturers.