# How does the pressure change in the Air Cycle Machine?

In this picture:

#3 says:

"Air expanding to ambient pressure turns expansion turbine, is greatly cooled, and drives compressor."

I understand all of that except for the "air expanding to ambient pressure" part.

Ambient means relating to immediate surroundings of something so the pressure can change. I understand that if it decreases in size like at #1 it would get hotter or if it increases in size it would become colder, but how can it be ambient if all the air is expanding to the same size when at #3 the column that the air passes through doesn't change shape to change the pressure?

Basically, how can it be ambient pressure if the column doesn't change size?

• Not sure if I understand the question. If we replace "ambient pressure" with something more specific like "15 psi", does the diagram make sense to you or not? – Daniel K Jun 2 '17 at 23:47
• Two things: 1) this is just a schematic diagram, and does not reflect the dimensions of the actual plumbing used in an aircraft, and 2) pressure can change while the 'column' area remains constant, if the flow velocity is adjusted accordingly. – Sanchises Jun 6 '17 at 10:29

Strictly speaking you're right, the authors used a brief and easy to visualise but slightly inaccurate phrasing. The blue bit is open to the cabin, and at the end of it will have cabin ambient pressure. At the beginning of it the pressure will be slightly higher, but very much lower than before the turbine.

The pipe diametre does not need to change for the pressure to vary. Air streams from high pressure to low pressure, upstream pressure will always be higher than downstream pressure, by an amount to overcome the friction forces.

The underlying principle is explained by the Ideal Gas Law aka Pv=nrt. https://en.wikipedia.org/wiki/Ideal_gas_law

When the compressed air, which has been cooled expands to an ambient pressure (like 15 or perhaps less PSI at altitude) the air cools in temperature. It is mixed with cabin air, which creates a warmer mixture, but still cooler than the cabin.

You ask how it can be ambient if the column doesn't change, but the column does change. The pressure in the intercooler is much higher than ambient, and has been heated by virtue of compression. In the intercooler it gets cooled, and when then expanded back to "ambient" it further cools, so that it is cooler than ambient.

If it appears that I didn't understand your question, perhaps you could restate the question.