Volume Ratio Effect

[planck121]

I am trying to understand the reasoning behind Volume ratio effects in multistage centrifugal compressors. I believe if the gas density changes so does the polyhead curve on these compressors. But I am practically trying to understand in simple terms what actually do volume ratio affects stand for...Do they represent a change in the suction volume for subsequent stages. Does low or high flow go into the other stages depending on changes in gas properties as lets say compared to the first stage....

thank you

 

[BigInch]

It helps you take in at suction the appropriate amount of gas to match the maximum amount of work the compressor can do on it and reach the discharge conditions at the time. There is no point trying to draw in 10 lb/min, if the passages into the compressor will only allow you to draw a volume of gas that has less than, or more than 10 lbs. Since gas is highly compressible, its weight, especially at suction conditions, can vary considerably with both pressure and temperature. A certain suction volume at 10 degrees and 20 psig will contain twice the weight, if the suction pressure changed to 40 psig, so theoretically you could cut the suction volume space in half and still get the same gas weight intake to the compressor with the same cylinder stroke distance.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[tgmcg]

Are you familiar with velocity triangles? Volume reduction ratio affects the velocity component in the flow direction tangent to each blade, which is then vectorially added to the velocity component tangent to the impeller OD. As the volume ratio increases (discharge volume reduces), the velocity component opposite the circumferential component will decrease and you will get more head from the impeller for a given impeller inlet flowrate.