Part Load Energy Consumption of Screw Compressors in the Refrigeration Industry

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

 Usually a compressor is designed to work in full load (100%) capacity. However in reality most compressors are operating in part load capacity.  

Cooling power vs. electric input power is shown in the table below. Please notice that at 50% load the user is putting more electric energy in than cooling energy is received back.

Screw compressor at part load by 2950 RPM with slide valve control

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

Having a correct and accurate part load indicator is important 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 can easily be upgraded to an indicator system with an output from 0-20mA or 4-20 mA.

Some older compressors work with mechanical indicators like potentiometers to indicate the part load. In the field we often see that the mechanical position is not correct with the given position by the pot meter. The part load signal can indicate an incorrect position and therefore it can create 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 you how to avoid this problem.