S K Energy Consulting Ltd.

What is the most efficient way to reduce capacity of the refrigeration plant?
Every refrigeration plant runs at designed conditions only 1% (or less) of the operating hours. The rest of the time (99%) it operates at part-load performance.

To handle reduced load, we have to reduce capacity of compressors, condensers and evaporators. The most efficient way of doing that is by using variable frequency drives (VFD). VFD gradually changes speed of the compressors or fans.

Screw compressors have poor part-load performance, particularly at low suction pressures. If screw compressor with slide valve is loaded 50%, it may draw 70-80% of full- load power. VFD can reduce energy consumption proportionally to refrigeration load and at 50% of refrigeration load compressor will draw 50% of the full-load energy usage. Usually, one compressor with VFD operates as a trimming one for every suction pressure, the rest of the compressors run at full load.

Fans with VFD are even more efficient at part-load performance than compressors with VFD. Due to “cubic law” (fan hp is proportional to the cubic of speed) reduction of fan speed, 50% will give theoretical reduction of the power to 12.5%. Actual fan power energy usage would be slightly higher due to inverter efficiency. All fans (condensers or evaporators) should be ramped together in speed, rather than cycle. Fans operating at reduced speed can give several additional incentives:

reduced motor heat loads in the refrigeration space
reduced fan noise
reduced wear of the belts, sheaves, bearings and motors

In industrial refrigeration it is very important to choose optimum set points and operating strategies. There are several ways to operate refrigeration plants at these settings, but the most efficient one is by using variable frequency drives.

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