An odd thought on centrifugal compressors.

Main Question or Discussion Point

Hi everyone I'm new.

I had an odd thought when thinking about centrifugal compressors.

Take a normal straight veined compressor. Normally it just sucks the air straight in and has to accelerate it effectively from zero to however fast it ends up going.

But what of you modified the inlet a bit. Instead of being just a hole you configured it so it spun the air as it got sucked in. In a similar manner to what a cyclone separator inlet does. If you make it spin in the same direction as the impeller, then wouldn't it mean that instead of accelerating the air from zero, it would just have to add to its already rotating velocity and so require less power to achieve the final velocity?

Well I have thoroughly confused myself trying to work this out. So I thought if anybody could someone here can.

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Hi Windman,

Welcome to PF...

Not sure about gas compressor examples, but what you describe is found on some centrifugal pumps and is called an inducer;

http://www.mcnallyinstitute.com/10-html/10-1.html

"An axial flow impeller called an Inducer (it works like a booster pump) can be placed ahead of the regular pump impeller, on the same shaft, to increase the suction pressure and lessen the chance of cavitation. In some instances this can allow the pump to operate at a higher speed with a given NPSH. The inducer will contribute less than 5% of the total pump head, and although low in efficiency the total efficiency of the pump is not reduced significantly. The total reduction in NPSH required can be as much as 50%."

As mentioned in the link above, the advantage in pumping liquids is reduced NPSH-R and consequent reduction in cavitation.

Another example is found in many water turbines;

http://www.statemaster.com/encyclopedia/Francis-turbine

"The Francis turbine was developed by James B. Francis. It is an inward flow reaction turbine that combines radial and axial flow concepts."

Again, these are liquid applications - not gas - so not sure about the advantage for your compressor idea.

.