Horsepower for vacuum operations

[zdas04]

I just ran a case in 3 different vendor programs, the Cooper-Bessemer equation, and a couple of different adiabatic horsepower equations that has me baffled.

With a constant discharge pressure of 80 psig (at 6,000 ft, so 92 psia), a constant suction temperature of 70F, and a constant flow rate of 150,000 ft³/d. I changed the suction from +5 psig (17 psia) to -5 psig (6 psia) and the horsepower requirements went down. Compression ratios went from 5.4 to 15.3.

It is obvious to me that all of this arithmetic is intended for positive psig to positive psig (in fact one of the compressor manufacturer's had a disclaimer that their horsepower calculation had "significant error" with a vacuum suction).

Does anyone have an equation that is reliable for a vacuum suction?


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The Plural of "anecdote" is not "data"

 

[zdas04]

I mis-copied my notes and thought P_s stood for "suction pressure" when it really stood for "Standard pressure". The equations all work out with a lower suction pressure requiring more hp when you type them in correctly.

That doesn't explain the compressor manufacturers though.

David