Polytropic and Adiabatic Efficiency 1

[wzal]

Hi, Everyone,

I was working on a rotovary vane compressor for HVAC and asked by our customer how much the polytropic efficiency is.

I checked my books (most of them refrigeration) and found for intermittent cycle compressors (Recip, vane, screw), people use adiabatic efficiency. For centrifugal compressor, polytropic.

Could anyone tell me why?

How to convert from one to the other?

Thanks,

wzal

 

[jay165]

Asked and answered:

thread 798-84662

 

[Montemayor]

wzal:

Since the hyperlink on thread 798-84662 produced no results, I'll give it a shot.

A positive displacement compressor follows the adiabatic compression process very closely, while the centrifugal machine follows what is called a polytropic process. Your rotary vane compressor is closer to being a positive displacement machine. I gather you're not up on thermodynamics and you're not an engineer, so it would be very tiresome and I could really bore you with a lot of stuff that would just confuse you if I were to start explaining both processes. Therefore, suffice it to say that it's either a chicken or a duck - you can't have a "chuck".

A polytropic process differs from an adiabatic process in that the change of state during compression does not take place at constant entropy. All real compressors are, to some degree, polytropic machines - but your rotary vane is nearer to an adiabatic and is conventionally treated as such.

The theoretical energy required by a polytropic machine divided by the actual input energy required is called the polytropic efficiency. It is analogous to the adiabatic efficiency. I don't understand what you would do if you were able to convert one to the other, as relates to your rotary vane compressor. If you have the compressor's adiabatic efficiency, give that to your customer and tell him that defines his machine better - not a polytropic figure.

This thread really belongs in our new gas compression forum.

Art Montemayor
Spring, TX

 

[25362]

Thread798-84662 is operative.