question on compressors performance

[saidsaz]

hi all,

here is a basic question that has been a source of confusion ,,let me start discusion for those interested ( and know correct answers) by these two qustions ,,

i always thought that for a constant speed compressor :

1- for any change in suction properties like pressure ,mw ,T SUCTION VOLUME FLOW WILL REMAIN CONSTANT,but power will change propotional to mass flow? is this correct ?

2- in case of throtting suction or discharge volume flow will change(decrease ) cause head will change (increased) as compressor will back up on its curve ? is this correct ?

thnx in advance

 

[ChasBean1]

I think in terms of fans, but I believe the same rules apply. For #2, yes, a throttling (system restriction) will cause the system curve to travel along the RPM line to the left (higher pressure, lower flow).

Volume and mass flow will both change when changing the inlet properties of a compressor or fan. If it is a positive displacement compressor, the volume will remain constant although the mass will change due to varying pressure/fluid density.

I'm flying by the seat of my pants here, but I can't see why power wouldn't still vary by the cube root of the mass flow (as opposed to the volume flow) change. If I double my mass flow, I think power will go up eight-fold, as mass flow and volume flow are linearly related by density. That's my story and I'm sticking with it.... for now.... but don't take this to the bank. If I get a chance I'll think some more about this and repost. -CB

 

[saidsaz]

chasbean1 thnx for ur comments,

small correction here ...
to be exact , power will change ,propotional to the product change of new head and new mass flow , the cube route relation is known fan laws but when changing impeller diameter or speed which are both fixed in my problem ..

i have gone through many literatures on the subject , all were contradicting ,

one opinion is that the for dynamic machines , keep dicusion to dynamic machines, the diameter and speed are only factors fixing volume flow.any change in suction property will reflect on mass flow but not volume flow!!

another opinion , see norman lieberman's "A WORKING GUIDE TO PROCESS EQUIPMENT " PAGE 367-368 " ..THE REDUCED SUCTION PRESSURE INCREASES THE SUCTION VOLUME (ACFM) OF GAS FLOWING TO THE COMPRESSOR ..." ,the example in norman book is a constant speed single stage compressor ...

any machinery or process guru's help is most welcomed !