Compressor Volumetric Efficiency (Clearance Re-Expansion)
The clearance (theoretical) volumetric efficiency of a reciprocating compressor.
Example
You enter
- Clearance ratio (~0.03-0.06) 0.045
- Suction pressure (psia) 70
- Discharge pressure (psia) 300
- Polytropic exponent n (~1.11 R-22) 1.11
You get
- Compression ratio 4.29:1
- Clearance volumetric efficiency 87.8
Checked against published worked examples from: IGNOU (egyankosh.ac.in).
Details, formula, and sources
ScopeThe 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 or tablecompression_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).
Source and editionReciprocating 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.
Free accessThe clearance re-expansion relation is public thermodynamics; the clearance ratio, the suction and discharge pressures, and the polytropic exponent come from the compressor and the operating condition.
What governsEstimate. AHJ and licensed professional govern.
API field namesclearance_ratio, suction_pressure_psia, discharge_pressure_psia, polytropic_exponent, compression_ratio, volumetric_efficiency_pct
- Clearance re-expansion VE = 1 + C - C x (Pd/Ps)^(1/n); C ~0.03-0.06, n ~1.11 R-22 / 1.16 R-410A; a higher compression ratio lowers VEASHRAE Refrigeration / Dossat
- Clearance VE only the actual VE is lower -- suction superheating, valve/ring leakage, and valve pressure drop each subtract more; actual pumped volume = displacement x VE; the rated capacity governscompressor engineering practice