Trying to understand air cycle systems, particularly on aircraft.

My understanding is when the engines are running, bleed air is taken from the compressor stage and is regulated by a control valve. Bleed air is too hot to be efficiently used in the air-cycle machine, so it is first sent through a primary heat exchanger where it is cooled by cold air taken from the intake stage of the engine. The air is then piped into the air cycle machine/ACM.

My question is: if bleed air is too hot and must go through a primary heat exchanger first, why not just use the cool air taken from the intake stage and eliminate the first primary heat exchanger loop? Is it a question of pressure? Or is there not enough air available at the intake? Thanks for any info.

Wow! Talk about an off topic detailed question!!! But I like it

Out of my wheel house but I’ll add a post. Bleed air, so we’re talking about turbine systems I assume. I assume it’s a pressure issue- a/c system needs to work at higher altitude, so intake air isn’t high enough pressure. Compressing air for pressurization adds heat.

Looking forward to someone knowledgeable responding.

This message has been edited. Last edited by: ECSquirrel,

Thanks for your response. Yes, bleed air refers to air taken from the compression stage of a jet turbine engine. It's used for a bunch of stuff depending on the aircraft and systems.

This is the basis of my rudimentary understanding of the air cycle system FYI

https://youtu.be/UfgbXAWpj9M?si=xW-uFdwCBpeMWRJJ

Bleed air is used for 2 reasons. Primarily because it pressurizes the airplane. Just using cool outside air through an intake scoop would not pressurize the cabin. The second reason is that at lower speeds or on the ground (zero speed) the outside air would not flow through to do any good.

Pressurization is accomplished by controlling the outflow of air from the cabin. Without any outflow, the incoming air would eventually pressurize the cabin to some crazy high lethal pressure, though it would blow apart the airframe well before that point. So there are outflow valves which open more, or less, to let just the right amount of air leak out so that the pressure is kept at the desired point. Inflowing air is only slightly controlled for pressure or flow rate, and not in consideration of cabin pressurization. By the design of the PACs the air is always at a suitable pressure.

Think of it like blowing up a balloon that has a pinhole leak. If you match the incoming air to the amount leaking out the balloon will stay inflated to the same size (and pressure).

The ACM, or air-cycle-machine, is also known as the Pneumatic Air Conditioner or PAC. The design is really pretty slick.

Bleed air is high pressure and high temperature. We know if we let air pressure decrease then the temperature will decrease too. And if we cool it then the pressure will decrease. We can also extract energy by utilizing the pressure to do some work.

The high pressure hot air comes in and goes through a heat exchanger, making it cooler but still moderately pressurized. Then that air goes through a compressor, which heats it up and pressurizes it. Then that air goes through another heat exchanger. The larger the temperature difference between the bleed air and the outside air in the heat exchanger, the better the heat exchanger cools it. So that is why the bleed air is re-compressed before the second heat exchanger.

The energy for that compression stage is taken from the air after the second heat exchanger. An expansion turbine is spun by that now doubly-cooled air, which extracts energy to spin the compressor stage. So this is a third stage of cooling and reducing pressure.

The PAC is plumbed so that some hotter air is pulled off earlier, and a mixing valve allows some of it to mix back into the final air if it needs to be warmed up. At high altitudes it definitely needs some warming up!

One of the aircraft I flew had PACs which took internal condensation from the cooling process and sprayed it onto the outside of the heat exchanger to further aid cooling. That aircraft was designed and built in Brazil where cooling was the goal, not heating! When turning the control knob clockwise, it commanded MORE COLD, unlike the typical cooler to the left, warmer to the right control.

Many thanks!