Performance
Compressor swept volume and volumetric efficiency
- where
- swept volume (m³/h)
- cylinder bore (mm)
- piston stroke (mm)
- number of cylinders
- shaft speed (rpm)
- actual suction volume flow (m³/h)
- mass flow (kg/s)
- specific volume of suction gas (m³/kg)
- volumetric efficiency (%)
Explain this equation
Works out the swept volume of a reciprocating compressor and compares it with the gas it actually pumps, giving the volumetric efficiency. Use it to check a compressor selection or diagnose worn valves.
The swept volume is the bore area times the stroke (both in metres) times the number of cylinders and the speed; times 60 turns revolutions per minute into per hour. The actual flow is the mass flow times the specific volume of the suction gas, in m³ per second, times 3600 for per hour. Their ratio as a percentage is the volumetric efficiency.
Not all the swept volume is filled with fresh gas: clearance gas re-expands and the suction gas is heated, so actual is always less than swept.
Sense check: four 60 × 50 mm cylinders at 1450 rpm sweep about 49 m³/h. 0.08 kg/s at 0.13 m³/kg is about 37 m³/h, around 76%.
Watch out: volumetric efficiency falls as the pressure ratio rises. Over 100% means a reading or the specific volume is wrong.
Works out the swept volume of a reciprocating compressor and, from the actual mass flow and suction gas specific volume, its volumetric efficiency.
Enter your values to see the result.
Good to know
For scroll and screw compressors take the displacement (m³/h) from the datasheet. Volumetric efficiency falls as the pressure ratio rises, because of re-expansion of clearance gas, suction heating and leakage. Use the specific volume at the actual compressor inlet (including superheat).
Common mistake
Using saturated vapour specific volume when the suction gas is well superheated, or nameplate speed for an inverter compressor.
Limits
Reciprocating geometry only for the swept volume; ηv uses the inputs as given.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.