Reciprocating Compressors Volumetric Efficiency – Corrections to Ev for Gas Characteristics

In practice, corrections have to be made to Ev to account for valve losses, non-ideal re-expansion of gas in the clearance volume, internal leakage, compressibility, and other effects. Each manufacturer has his own set of empirical corrections. Therefore, several corrections for Ev are used in the industry.

Several of these corrections were compared for natural gas (S.G. = 0.72) and for propane. They were all within about 5% for pressure ratios between two and four. However, for hydrogen-rich gases with molecular weights below 10, the corrections varied as much as 17% at a pressure ratio of four. However, the maximum pressure ratio for hydrogen-rich gases is typically about three (to keep the discharge temperature below 300°F). At a pressure ratio of 3, the deviation is less than 10%. The following equation yields results that are approximately the average of the other formulas when used for more ordinary lubricated-cylinder compressor applications. Also, see the following discussion for limitations and corrections.

where:
Ev = volumetric efficiency, in percent
Zs = compressibility at suction conditions
Zd = compressibility at discharge conditions
F = 2.4 for mol. wt. < 10 = 1.4 for mol. wt. 10 to 30 = 1.0 for mol. wt. > 30.1
r = pressure ratio

Equation 300-3 is similar to the formula in the GPSA Handbook. The main difference is that F in Equation 300-3 adjusts for internal gas slippage which varies inversely with molecular weight.

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