NG ACM
 

Basic physics but why does the compressors increase the heat of the bleed air after it had been cooled by ram air and then for it then to be cooled again by secondary heat exchanger?

Thanks

Pin

My theory. There will be a greater loss of latent heat energy in the second heat exchanger if the air comes in as hot as possible. The compressor heats up the air adiabatically, the heat is effectively heat exchanged in the second heat exchanger and the turbine helps dissipating heat and pressure that remains.

I think there are some Boeing engineers on this forum that hopefully will see this thread and put things straight.

Increasing the heat is not the purpose of the compressor rather it is a slightly undesirable by-product of compression. Now compression is what we absolutely need so that the turbine can function more effectively.

Undesireable?! If you remember your thermodynamics the temperature differential is a key factor in the performance of a heat exchanger... Higher delta t, higher heat transfer.

That is absolutely true. But if the compression had not increased the temperature of the air, we might not have had to use a second heat exchanger in the first place.
Some of the energy spent by the compressor gets turned into heat and the rest of it is used in decreasing the volume of air. Now this compressed air is suddenly expanded by the turbine which results in a huge loss of temperature and so the air cools down. The heat exchangers are there to assist the turbine by making the air temperature workable. They are not the primary means of cooling.

Ok, good stuff.

Here's a question why are the ram air doors not left open at cruise allowing more cold air in to cool and reducing the demand on the compressor/turbine then?

The ACM is not required in the cruise. In many aircraft there is a Turbine Bypass Valve that diverts all the air past it so it stops rotating. The heat exchangers function very well on their own, and the intake and exhaust flaps modulate shut as well as these heat exchangers are much too much at altitude.

This is as per the AMM:

"The normal position for the ram air inlet modulation panel in flight with the flaps up is faired (closed). This is to decrease drag."

yes drag.

so ACM's just uses 'floating' or ambient air?

In 737, ACMs use the bleed air for air conditioning and pressurisation. Some aircrafts do use Ram air ventilation, but not 737 NGs.