Compressor Volumetric Efficiency (Clearance Re-Expansion)
The clearance (theoretical) volumetric efficiency of a reciprocating compressor -- the fraction of swept volume it draws in as fresh suction vapor, the number compressor-displacement names as a gap. Trapped clearance gas re-expands on the down-stroke before the suction valve opens, cutting the intake: VE = 1 + C - C x (Pd/Ps)^(1/n), with C the clearance ratio (~0.03-0.06), Pd/Ps the absolute compression ratio, and n the re-expansion exponent (~1.11 R-22, 1.16 R-410A). A 0.045-clearance machine at a 300/70 = 4.3 ratio (R-22) is ~88%; a higher head pressure raises the ratio AND the re-expansion loss, so VE and capacity fall -- why a compressor pumps less on a high-ratio day. CLEARANCE VE only; suction superheat, valve/ring leakage, and valve pressure drop lower the actual VE further. Multiply displacement by VE for the pumped suction volume. A recip model; the rated capacity at the operating condition governs.
Formula and source
compression_ratio = discharge_pressure_psia / suction_pressure_psia; volumetric_efficiency = 100 x (1 + C - C x (Pd/Ps)^(1/n)), C = clearance ratio (~0.03-0.06), n = polytropic re-expansion exponent (~1.11 R-22, 1.16 R-410A).
Reciprocating compressor clearance volumetric efficiency (clearance-volume re-expansion; ASHRAE Refrigeration / Dossat Principles of Refrigeration), by name; the compressor's rated capacity at the operating condition governs.
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