Does anybody know exact structure of cabin pressurization and air delivery system on arliners?
According to data about Boeing... it uses engine bleed air, taken between low (150°C) and high pressure (300°C) compressor stage.
Bleed air is used to power pneumatic system that deliver brake power, heat for deicing wing's and engine's leading edges, fuel heating, fuel and water tanks pressurization and of course cabin pressurization. Bleed air from APU that is conected to the main pneumatic system also starts the main engines.
According to information about B787 it won't use engine bleed air (due to the danger of oil wapors in the airstream) but rather AC electric powered pneumatic motor that will deliver pressure to the cabin. Someone also sais that this will incrise overall fuel efficiency.
Now I was reading on Wikipedia about air cycle machine...
"An air cycle machine (ACM) is the refrigeration unit of the environmental control system used in pressurizedturbine-powered aircraft. Normally an aircraft has two or three of these machines arranged in a system called a "pack". The usual compression, cooling and expansion seen in any refrigeration cycle is accomplished in the ACM by a centrifugal compressor, two air-to-air heat exchangers and an expansion turbine. Bleed air from the engines, an auxiliary power unit, or a ground source, which can be in excess of 150°C and at a pressure of perhaps 32psi [1], is directed into a primary heat exchanger. Outside air at ambient temperature and pressure is used as the coolant in this air-to-air heat exchanger. Once the hot air has been cooled, it is then compressed by the centrifugal compressor. This compression heats the air (the maximum air temperature at this point is about 250°C) and it is sent to the secondary heat exchanger, which again uses outside air as the coolant.
At this point, the temperature of the compressed cooled air is somewhat greater than the ambient temperature of the outside air. The compressed, cooled air then travels through the expansion turbine which extracts work from the air as it expands, cooling it to below ambient temperature (down to -20°C or -30°C). It is possible for the ACM to produce air cooled to less than 0°C even when outside air temperature is high (as might be experienced with the aircraft stationary on the ground in a hot climate).[2]
The work extracted by the expansion turbine is transmitted by a shaft to spin the air pack's centrifugal compressor and an inlet fan which draws in the external air for the heat exchangers during ground running; ram air is used in flight. The power for the air conditioning pack is obtained by the reduction of the pressure of the incoming bleed air relative to that of the cooled air exiting the system. After the air has been cooled down, water vapor in the air condenses, forming fog or high humidity. To get rid of this, the air exiting the expansion turbine is passed through a water separator, which uses centrifugal force to throw the water particles into a coalescer bag that absorbs the moisture.
The air can now be combined in a mixing chamber with a small amount of uncooled engine bleed air. This warms the air to a desired temperature, and then the air is vented into the cabin or to electronic equipment."
...but this reading has no sense for me. I can say that this configuration will be applied on plane that doesn't use engine bleed air. Planes that do, don't have secondary compressor and heat exchange between as it takes pressure air from the engine itself via light air colectors and pipes. And because we have to maintain fuel temp above -37°C (normal operation altitude OAT is -56), and we also have to maintain wings and engines leading edges above water freezing point (I say 0° C), we have a huge heat consumer - so I don't see the point of any extra heat exchanger.
I also can't emagine the AC electric consumption of heating the wings and fuel withought of engine bleed air. I can just predict that above 50kW.
The other thing is that I don't beliewe that you can extract mechanical power from hot air. Car's supercharger (turbine and compresor stage on the same shaft) doesn't use temperature but pressure - meaning it doesn't lower EGT but it only ads backpressure.
And the final question... does airliner AC system maintaining cabin temperature by delivering new warm air to the cabin and releasing old one via cabin air pressure valve or does it use heat exchanger?
I personaly think that there is higher energy heat demand to maintain cabin temperature between 20-24°C than oxygen demand - changing the CO2 rich air. Meaning that it will be waste of air pressure - equal energy if you will maintain tempreature thus constantly applying new warm air.