Part load energy consumption of screw compressors

The capacity of most fixed-drive speed compressors is controlled by creating an internal leak in the compressor. The design is calculated on a working condition of roughly -35/+35°C.

Usually a compressor is designed to work at full load (100%) capacity. In reality, however, most compressors operate at part load capacity.









Cooling power vs. electrical input power is shown in the table below. Please notice that at 50% load, the user expends more electrical energy and receives less cooling energy in return.

Take a look at the cooling capacity between 80 and 100%. The highest cooling capacity is created in that range.

Having a correct and accurate part load indicator is important in order to avoid losses.

Older compressors suited with mechanical indicators can be easy modified to a electronic contactless system.

Standard compressors like the Howden WRV- Sabroe 128-163 –VMY 325 – Stall S range / miniscrew and the KAB/Grasso range are easy to upgrade to an indicator system with an output from 0-20mA or 4-20 mA.

Some older compressors work with mechanical indicators, such as potentiometers, to indicate the part load. In the field, we often see that the mechanical position does not match the position given by the pot meter. The part load signal can indicate an incorrect position and lead to a loss.

With the PLC and compressor controllers, the position of the slide capacity valve becomes important. The output is also important.  Most work with a 4-20 mA signal.

Be aware of the fact that when your compressor is working in part load capacity, you are not only wasting energy but also losing cooling power. In the next month we will explain how you can avoid this problem.