What powers the ACM?

[FL420]

On the A320 and B757 what actually turns the compressor and turbine of the pack's ACM?

Is it high pressure air from the engine or APU compressor impinging on the compressor or turbine wheel of the ACM?

If not, is the ACM run by an AC motor while the hot bleed air from the engine or APU is just used to provide the air to be compressed and then expanded for cooling by the ACM?

If it's hot air turning the ACM, what are the CB's for the pressurization/air conditioning system on the B757 cockpit overhead panel for?

TIA

 

[CA1900]

On the A320 and B757 what actually turns the compressor and turbine of the pack's ACM?

Is it high pressure air from the engine or APU compressor impinging on the compressor or turbine wheel of the ACM?

Yes.

It's whatever bleed air source is supplying compressed air to the ACM. At the gate, it could be the APU or a pneumatic cart (unusual). When the engines are running, it's normally bleed air from the engines. (If you see the large-diameter yellow hose hooked up at the gate, the ACM isn't being used at all. It's all being done by a ground unit.)

If not, is the ACM run by an AC motor while the hot bleed air from the engine or APU is just used to provide the air to be compressed and then expanded for cooling by the ACM?

No, the hot bleed air does both -- it supplies the warm air to be cooled down, and that's also what turns the turbine. An ACM basically takes warm air and heats it up in a compressor to run through a heat exchanger, because it will be able to transfer more heat out of the air if it's at a higher temperature differential from the outside air.

Once it passes through the heat exchanger, it goes through an expansion turbine that cools the air back down, and provides the energy to drive the compressor on the other side. The turbine is self-sustaining, because the air is cooled back down about the same amount as it was warmed up in the compressor section. It's because of the heat exchanger in the middle that it ends up ice cold.

The energy to turn the turbine was already provided in the form of compressed air. At the end of the cycle, the air has less energy (heat) than when it went in, which is exactly what we want -- that's why it's cooler.

If it's hot air turning the ACM, what are the CB's for the pressurization/air conditioning system on the B757 cockpit overhead panel for?

I haven't flown a 757, but I would assume they're for the temperature and other controls of the system, as on other airplanes. Jets regulate the temperature by mixing it with air that has bypassed the ACM, and is thus warmer than the ACM air.

Hope that helps.

 

[Say Again Over]

For example, on the Airbus prior to engine shut down we push a button called "APU Bleed" (providing the APU is running). Along with supplying the line with bleed air, it will open the cross bleed valve (if in AUTO) for two pack operation.

 

[FL420]

Thanks for the replies guys. I guess I either never learned or had forgotten that the air-air heat exchanger between the compressor and expansion turbine required high-powered fans to draw cooling air over the heat exchanger during low airflow operations found on the ground or sometimes in the air in the dirty configuration.

 

[XJWRENCH]

The pressure of the air has a lot to do with it not really the temp. The combined gas law. Temp is proportional to press of a gas. As you pressurize a gas, the temp goes up. If the engineers could figure out how to pressurize a gas without temp increasing, they would do it. That is why normally air is passed through a heat exch twice, one before it enters the ACM and again before it enters the expansion turbine. You need the air to be pressurized in the compressor not heated. The lower the temp of the air entering the exp turbine, the more eff. the ACM is. The action of the air cooling down is actaully the rapid decrease in pressure.( ie take a look at a propane tank on a cool humid summer night when you are grilling, it gets frosty) I guess my point is you need Pressure not Heat in an ACM. Downstream of the ACM hot bypass air is mixed in to get a more ' Room temperature ' conditioned air.