What is the air delivery coefficient of a screw air compressor? What are the influencing factors, and what are the methods for calculating its performance?

What is the air supply coefficient of a screw air compressor?

The air delivery coefficient of a screw air compressor is often referred to in the engineering field asGas transmission coefficientorVolumetric efficiencyIt refers to the ratio of the actual gas volume delivered by the compressor to the theoretically calculated gas volume. This metric is a key parameter for assessing the degree to which the compressor’s cylinder volume is utilized. A higher delivery coefficient indicates that, during operation, gas leakage, pressure losses, and heating effects are minimized, bringing the equipment’s actual performance closer to its theoretical design value.

Main factors affecting the gas supply coefficient

In actual operation, the gas supply coefficient is influenced by a combination of various physical and mechanical factors, primarily including the following aspects:

• Gas leakThere are clearances between the screw rotors and between the rotors and the housing. Gas leakage from the high-pressure side to the low-pressure side directly reduces the actual displacement. As operating time increases, wear causes these clearances to widen, leading to a corresponding increase in leakage.
• Inspiratory Resistance and Pressure Loss: As gas flows through the intake filter, the intake valve, and the piping, it encounters resistance. A reduction in suction pressure leads to a decrease in the density of the intake gas, thereby reducing the actual mass of gas drawn in.
• Heating effectDuring the compression process, the frictional heat between the rotor and the casing, as well as the heat generated by gas compression, is transferred to the incoming fresh gas. The resulting thermal expansion of the gas increases its specific volume, thereby reducing the actual volume of gas drawn in.
• Gas Humidity and Condensation: When air containing water vapor is drawn in, some of the water vapor may condense into liquid water during compression and cooling. Liquid water occupies a certain volume and is incompressible, which can indirectly reduce the effective gas flow rate.

Performance Calculation Method for the Gas Supply Coefficient

The core of calculating the gas supply coefficient lies in accurately determining the actual exhaust volume and the theoretical exhaust volume. The specific calculation logic is as follows:

1. Theoretical Displacement Calculation

The theoretical displacement is determined by the compressor’s geometric dimensions and its rotational speed. For twin-screw compressors, the theoretical displacement equals the product of the rotor tooth‑to‑tooth volume, the number of rotor teeth, and the rotor speed. This value is typically established by the equipment manufacturer during the design phase and is indicated on the equipment’s nameplate.

2. Actual Displacement Measurement

The actual displacement must be measured directly at the compressor discharge using a specialized flow‑measurement instrument. During measurement, the discharged gas volume under actual operating conditions shall be converted to its equivalent volume at standard conditions to ensure data comparability.

3. Calculation of the Gas Supply Coefficient

Dividing the actual displacement under standard conditions by the theoretical displacement yields the volumetric efficiency. This ratio is typically expressed as a percentage. If the calculated value is lower than expected, it may indicate internal wear, inadequate sealing, or blockages in the intake system, necessitating targeted maintenance and troubleshooting.

Daily Maintenance Recommendations for Improving the Gas Supply Coefficient

To maintain a high gas supply coefficient and extend the equipment’s service life, routine maintenance is essential:

• Regularly replace consumables.: Replace the intake filter element on schedule to reduce intake resistance; regularly change the lubricating oil and oil filter to ensure proper sealing and cooling of the rotor clearance.
• Monitor operating parametersDuring routine inspections, closely monitor discharge pressure, discharge temperature, and motor current. If a reduction in discharge volume or an abnormal increase in energy consumption is detected, promptly investigate for internal leaks or valve malfunctions.
• Standardize the operating environmentEnsure adequate ventilation in the air compressor room and reduce the ambient temperature, thereby lowering the initial temperature of the intake gas, increasing its density, and ultimately boosting the actual gas delivery rate.