I found on aerosavvy.com/aircraft-pressurization the paragraph below, which talks about bleed air on airliners:

Most modern airliners use bleed air from the compressor section of the engines to pressurize the cabin. This very hot air must be cooled to a comfortable temperature before it’s directed into the cabin.

If it is cooled too much it wouldn't be under high pressure, right?

The cabin needs high pressure air because the air at 30,000ft is at too low of a pressure, so they need to simulate a 6000-8000 feet pressure. This bleed air is very hot high pressure air, I understand you have to cool it because it is way too hot for anything to operate with it.

But if you cool it, doesn't it lose pressure, so it would be low pressure air which is not what we want? I don't know where I am getting confused at.

  • $\begingroup$ The difference in pressure between sea level and 8000 feet is a whole 7.7" hg. At 40,000ft the outside air pressure drops to 5.5"hg (it is 29.9" hg at sea level). 1" hg == 0.49psi. Point being, the pressure's we are talking about here are very low. $\endgroup$
    – Ron Beyer
    May 4, 2017 at 3:34
  • 4
    $\begingroup$ Cooling a volume of hot bleed air will reduce its pressure, but the cool air that results will still be at a much higher pressure than required for the cabin. $\endgroup$
    – user21565
    May 4, 2017 at 4:37
  • 1
    $\begingroup$ @Mother Grinning Bird - your comment might make a good answer. $\endgroup$
    – Koyovis
    May 4, 2017 at 6:32
  • $\begingroup$ In the old days, when smoking was allowed on cabins, the pressurization relief valves, usually located near the rear of the fuselage, would be stained, and the paint would be stained, from the release of tobacco smoke from the aircraft. Also, there are some aircraft with are pressurized with air without benefit of an air cycle machine, however there is still thermal mixing, and ram air is still utilized with an intercooler. $\endgroup$
    – mongo
    May 4, 2017 at 13:53

2 Answers 2


That's one of the reasons why bleed air is compressed again before cooling.

In Air Cycle Machines, high-pressure bleed air from the engines is first passed through a compressor, further squeezing the already hot gas. It is then routed through a heat exchanger or two to remove heat. The now cooler but still highly compressed air then passes through an expansion chamber into a larger chamber. The combined effects of driving the turbine and expanding into a larger chamber dramatically cools the air (usually down close freezing; water traps are critical in the system to prevent freeze-up). (Source)

Simple diagram:

enter image description here

enter image description here

The outflow valve is what regulates the pressure, it makes sure the right amount of air leaves the cabin.

The automatic controller normally maintains the proper cabin pressure altitude by constantly adjusting the outflow valve position so that the cabin altitude is as low as practical without exceeding the maximum pressure differential limit on the fuselage.


The cooling process used is isobaric, not isovolumetric. It maintains a constant pressure by increasing the density of the air by IGL:

$P = \rho *R*T$

In order to maintain a constant pressure throughout the heat exchanger, the outlet air density gets larger as the temperature drops during cooling.

This causes a drop in the volumetric flow rate through the heat exchanger but also maintains a constant mass flow rate.


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