I am sizing a RV. I need to determ 1

[Bill3752]

I am sizing a RV. I need to determine the rate through a centrifugal compressor. I have the performance curves, which show a design rate of 1500 acfm suction. At times we use the compressor to process material having a higher molecular weight. Should I assume that capacity through the compressor is the same 1500 acfm (assuming the inlet P/T stay the same)?

 

[don1980]

When operated at a fixed speed, a compressor discharge pressure increases with increasing MW (e.g. it's very hard to compress H2). So, during a failure scenario you'll reach the RV set pressure sooner if the failure occurs while processing the higher MW stream.

Determine the orifice size for the required relief flowrate while processing the low MW stream and the high MW stream, and then size the RV for the larger of the two.

 

[Bill3752]

Don, thanks for the feedback. Actually, I am attempting to do what you suggested. To restate my question, at the same inlet and outlet pressure and temperature, I am trying to compute the rate at relief for the higher molecular weight gas. I only have the curves for the lower MW gas.

 

[georgeverghese]

If the curve shows polytropic head vs flow, then it will apply for a lower mol wt gas also. Also note that the relief rate should be computed using the highest normal suction pressure you could see at this machine, while the corresponding discharge pressure will correspond to the set pressure of the RV + permitted overpressure. This will result, in most cases, a relief volumetric rate which is lower than the design acfm. The calculation for this relief rate can trip up most process engineers who are attempting this for the first time, so ask for help to review if so.

 

[Bill3752]

Don and George, I think I have this one figured out. While I don't have the polytropic head (Hp), I do know the Horsepower. Given that and other data for the normal gas conditions, I was able to back calculate the Hp and efficiency. Then using an estimated efficiency and new gas conditions, I was able to estimate the ACFM and mass rate.

George, your comment about allowed over pressure is correct, since the pressure ratio is an important variable in the rate/HP calcs.

Thanks

 

[georgeverghese]

One way to estimate compressor poly eff is to use the actual discharge temp in the calcs, assuming there is a good value for actual Cp/Cv for this operating pressure / temp range.