Rotary Screw Compressor Selection

The working principle of rotary screw air compressor is that twin-screw rotation will cause the change of air volume, and the key parameters for selecting a proper compressor include pressure, air flow and power.

Pressure
Pressure can be revealed in different units, and the following paragraph is a brief introduction to commonly used unit for rotary screw compressor.
1. Operating pressure is usually called as exhaust pressure, and it is the maximum pressure of exhaust air.
2. The most frequently used unit is bar or Mpa, and the relationship between these two units is 1 bar = 0.1 Mpa.
3. Usually, user call the pressure unit as KG, and 1 bar equals to 1 kg.

Air Flow
1. Air flow refers to the inlet air volume under the required exhaust pressure, and this volume is converted from the air volume discharge by the compressor within unit time.
2. The unit for air flow is m3/min or L/min, and 1m3 = 1000L.
3. The most commonly used unit is m3/min.

Power
The power of air compressor is the power of equipped actuating motor or diesel engine, and its unit is kW or HP where 1kW is equivalent to about 1.333HP.

Pressure Selection
When buying an air compressor, the pressure should be confirmed first. Due to the pressure loss from installation site to application site of compressed air, the working pressure should be added with 1-2 bar. The diameter of pipeline and the quantity of turning points also have an influence over the pressure loss, and bigger diameter results in lower pressure loss while smaller diameter leads to higher pressure loss.

Thus, if the distance between compressor and gas equipment that uses compressed air is too long, pipeline with big diameter is highly recommended. In addition, if possible, the compressor should be installed near the target machine to shorten the distance.

Air Flow Selection
Proper model selection is important for the user and the equipment. If the air flow is too big, energy will be wasted, and if the air flow is too small, the compressor will work under overload state for a long time, or the problems of low pressure and insufficient supply of compressed air will occur.
1. When determining the air flow, the air volume required by target machine should be calculated, and then this value is multiplied by 1.2.
2. For a new project, customer can select the compressor according to the air flow value supplied by the designing institute.
3. The air flow of the gas equipment should be provided by the supplier, and this value is also helpful for the model selection.
4. For the rebuilding of compressor station, the original value and actual working condition can be taken into account as a reference.

Relations of Power, Pressure and Air Flow
When the power is fixed, the change of rotating speed will lead to the change of pressure and air flow. For example, for a compressor with power of 22 kW, if the pressure is 7 bar, the air flow is 3.8m3/min. If the pressure is changed to 8 bar, the air flow will be 3.6m3/min calculated by the technical curve of motor. The rotation speed is lowered, and the air flow is reduced accordingly.

When it comes to the choosing of power, the working pressure and air flow should be satisfied first, and the power just needs to meet the requirement of the drive motor.

Based on the relations above, the model selection procedure is divided into three steps, the first step is pressure determination, then air flow determination, and power determination is the last.

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