Compressor Test Rig Experiment: Procedure, Volumetric Efficiency, and P-V Diagram

Aim: To determine the volumetric efficiency, isothermal efficiency, and mechanical efficiency of a reciprocating air compressor at different delivery pressures.

Apparatus Required

• Reciprocating Air Compressor Test Rig with pressure gauges, air receiver, orifice plate flow meter, and energy meter
• Thermometers (inlet and outlet air temperature)
• Pressure gauges (inlet and delivery)
• Stopwatch

Theory

Volumetric efficiency (ηv): Ratio of actual volume of air inducted to the swept volume of the cylinder. ηv = Qa/(LAN), where Qa = actual air delivery, L = stroke, A = piston area, N = speed. Clearance volume reduces volumetric efficiency.

Isothermal power: P_iso = P₁ × Qa × ln(P₂/P₁) — the minimum theoretical work for compression. Actual input power: from energy meter. Isothermal efficiency = P_iso / P_actual × 100%.

Procedure

1. Start the compressor at no load (delivery valve open to atmosphere). Allow it to reach steady state.
2. Partially close the delivery valve to build up pressure in the air receiver. Set delivery pressure to the first test value (e.g., 2 bar gauge).
3. Record: delivery pressure P₂ (gauge), atmospheric pressure P₁, inlet temperature T₁, delivery temperature T₂, speed N (rpm), energy meter reading.
4. Measure air flow rate Qa using the orifice plate meter (measure pressure differential across orifice, convert to flow rate).
5. Repeat at 3, 4, and 5 bar gauge delivery pressure.
6. Calculate volumetric efficiency, isothermal efficiency, and mechanical efficiency at each pressure.

Calculations

Free Air Delivery (FAD): Correct measured air volume to atmospheric conditions using P₁V₁/T₁ = P₂V₂/T₂
Volumetric efficiency: ηv = FAD / Swept Volume per minute × 100%
Isothermal power: P_iso = P₁ × FAD × ln(P₂/P₁) watts
Actual input power: from energy meter kW
Isothermal efficiency: η_iso = P_iso / P_actual × 100%

Frequently Asked Questions

What is volumetric efficiency of a compressor?
Volumetric efficiency is the ratio of the actual volume of gas drawn in per cycle (at inlet conditions) to the swept volume of the cylinder. It is always less than 100% due to: clearance volume (re-expansion of residual gas), gas heating at inlet, leakage past valves and piston rings, and valve pressure losses. Typical volumetric efficiency for single-stage compressors is 70–85%.

Why is isothermal compression ideal?
Isothermal compression (constant temperature) requires the minimum work input among all compression processes. In practice, compression is polytropic (between isothermal and adiabatic). Isothermal efficiency measures how closely the actual compression approaches the ideal isothermal process — higher isothermal efficiency means better intercooling and lower energy consumption.

What is the effect of clearance volume on compressor performance?
Clearance volume is the volume remaining above the piston at the end of the compression stroke. High clearance volume reduces volumetric efficiency because the residual high-pressure gas must re-expand before new gas can enter the cylinder. As delivery pressure increases, the effect of clearance volume becomes more severe and volumetric efficiency drops significantly.

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