Pressure Ratio in Multi-Stage Reciprocating Compressor

[nirlt]

As a general observation in multistage reciprocating compressor (API 618), pressure ratio of initial stage (low pressure stages) is higher compare to final stage (high pressure stage). e.g. for 2-stage compressor, pressure ratio for 1st stage is 1.9 and pressure ratio of 2nd stage is 1.5.
Why?

 

[BigInch]

Of course. Doubling the pressure of a low pressure stage, say 50 psig x 2 = 100 psig, vs doubling the pressure of a higher pessure stage, say 200 psig x 2 = 400 psig would require maybe 400/100, or 4 times the strength in materials to be provided. As pressure ratio goes up, so does temperature as well and the gas passing through a first stage is already hot, generally making each sucessive stage a bit more inefficient, so a given amount of power does that much relatively less work for each higher stage to the point where decreasing the ratio makes sense.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[zdas04]

I do it because of BigInch's second point. Most recips have fin-fan coolers that are rated at an approach to ambient temperature. So if it is 100[°]F outside and the gas is at 80[°]F in the pipeline and your cooler is designed for a 20[°]F approach to ambient then if you do 4 ratios in both stages with k=1.28 you get:

T(in) = 80[°]F
T(first disch) = 271[°]F
T(interstage out) = 120[°]F
T(second disch) = 325[°]F

That last number is higher than most people will allow. Shifting the load just a bit to the first stage fixes that.



David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

There is an optimum work division between stages as well.

http://www.tech.plym.ac.uk/sme/THER205-web/recips.pdf

This doc also mentions hitting the temperature wall.

If it ain't broke, don't fix it. If it's not safe ... make it that way.