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Efficiency = Actual Mass displaced by the compressor divided by the theoretical mass displaced.

If we continuously increase the pressure in the compressor tank, will the efficiency stay roughly constant or is there another relationship between Pressure in the tank and efficiency of the compressor?

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I am not familiar with measuring efficiency based on mass the way you described it. However, there is a thing called volumetric efficiency which is the volume of air actually drawn into the cylinder divided by the piston volume. And yes, this value will drop as the discharge pressure increases. At some pressure, the efficiency will become zero, which means the compressor cannot compress air to any higher pressure than that. Here's a page with some formulas, http://petrowiki.org/Reciprocating_compressor (look for volumetric efficiency halfway down the page).

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  • $\begingroup$ Thanks for your answer Daniel. It's very helpful. As far as I'm aware Volumetric Efficiency (Zeta) is also equal to actual mass drawn in divided by the theoretical mass drawn in. If this is true, then would it follow the same efficiency drop over an increase in pressure? $\endgroup$ Feb 11 '17 at 23:03
  • $\begingroup$ Yes, I think to deal with mass you are just multiplying by (density / density) = 1, assuming the theoretical and actual density is the same, so the quantity you specify should be the same as volumetric efficiency. And yes, that would definitely drop with an increase in discharge pressure. As some pressure ratio, the efficiency goes to zero and the compressor is pumping zero mass. $\endgroup$
    – Daniel K
    Feb 12 '17 at 4:12

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