27 March 2013

COP (Coefficient of Performance) of screw compressors.

The most of screw compressors are running on a fixed drive speed. The capacity is controlled by a slide valve which creates an internal leak. How this effect influences the efficiency of the compressor unit?

Diagram 1.

The slide valve control is not the best solution for part load. The capacity controlled by drive speed will led to compressor which can be more flexible operated.

Coefficient of Performance (COP) = (Refrigeration Capacity) / (Power Input) is given by full load at 2950 rpm. For the capacity refrigeration of 348 KW divided by the  input power of 222 KW the COP value would be 1,56. The capacity control by creating an internal leak is presented by the green curve, diagram 1.

The red line (diagram 1) indicates the refrigeration capacity versus input power by only drive speed control. The benefits are:

  • Stable COP value,  in this case 1,56
  • Less wear by movable parts
  • No need of hydraulic capacity control
  • Can be modified on existing compressor units
  • More capacity by increased drive speed. In this example the same compressor could handle 430 kW.

Impact of the slide valve control on compressor aggregate is given in Table 1.

Table 1.

The COP reduces if the internal leak increases. The values for position slide valve below 50% (red colour mark) show that the compressor unit requires more input power than it is produced on refrigeration capacity. A flex drive unit, were the rpm controls the capacity, allows more flexible operating conditions, better cooling capacity at lower input energy.

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