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The energy efficiency of a screw air compressor can be determined by the single standard of "specific power." To understand specific power, it is essential to have a comprehensive grasp of the compressor's input power.

 

Take a conventional single-stage screw air compressor (model EWA132kw) as an example. Its nameplate parameters usually include:

Flow rate = 24 m³/min, working pressure = 7 bar,

Rated power (i.e., rated output power or rated shaft power): P = 132 kW,

Efficiency: η = 94.7%,

Power factor: COSφ = 0.892,

Service factor = 1.15 (some manufacturers may use 1.2).

 

Based on the above parameters, the following calculations can be made:

 

The rated input power of the unit without considering the service factor and at full load:

P1 = P÷η = 132 kW÷94.7% ≈ 139.39 kW

 

The rated input power considering the service factor (with a 5% margin, not calculated at full value) and at full load:

P2 = (P÷η) × (service factor – 0.05) = (132 kW÷94.7%) × (1.15–0.05) ≈ 153.33 kW

 

The nominal specific power of the compressor at 7 bar, considering the service factor and at full load:

PB1 = P2 ÷ 24 m³/min ≈ 6.39 kW/(m³/min)

 

If it is an air-cooled model, the power consumed by the fan must also be included. Assume the fan of this air compressor has a rated power of 4.5 kW and an efficiency of 85%. The input power consumption of the fan is:

PF = 4.5 kW÷85% ≈ 5.29 kW

 

The total nominal input power of the air-cooled screw compressor (considering the service factor and at full load):

PZ = P2 + PF = 153.33 + 5.29 = 158.62 kW

 

Its nominal specific power at 7 bar is:

PB2 = PZ÷24 m³/min ≈ 6.61 kW/(m³/min)

 

The above outlines the basic calculation method for the specific power of air compressors. Next, we further explore the relationship between power, pressure, and volumetric flow rate.

 

What is the Working Pressure of an Air Compressor?

Working pressure, also known as discharge pressure, refers to the maximum pressure of the gas discharged by the air compressor. Common units include bar or MPa, with the conversion relationship: 1 bar = 0.1 MPa. Note that many users often refer to pressure units as "Kg," where 1 bar ≈ 1 Kg.

 

What is the Volumetric Flow Rate of an Air Compressor?

Volumetric flow rate is commonly referred to as discharge capacity or nameplate flow rate. It indicates the volume of gas discharged per unit time by the air compressor under specified discharge pressure conditions, converted to the intake state. Common units are m³/min or L/min, with the conversion relationship: 1 m³ = 1000 L.

 

What is the Power of an Air Compressor?

The power of an air compressor refers to the nameplate power of its matching drive motor or diesel engine. Common units are kW or HP, with 1 kW ≈ 1.333 HP.

It should be noted that:

1. The power of an air compressor usually refers to the rated power of the matched power source;

2. Units are generally kilowatts (kW) or horsepower (HP).

 

Relationship Between Power, Pressure, and Flow Rate:

When the power remains constant, changes in the compressor’s speed will correspondingly adjust the volumetric flow rate and working pressure.

For example, for a 22 kW air compressor initially set to a working pressure of 7 bar, the corresponding discharge capacity is usually 3.8 m³/min. If the working pressure is adjusted to 8 bar, the speed must be reduced to prevent motor overload, in which case the discharge capacity decreases to 3.6 m³/min. As speed decreases, discharge capacity also reduces, and the same principle applies to other pressure ranges.

http://www.smeatoncompressor.com
SMEATON Fluid Machinery (Shanghai) Co., Ltd.

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