Energy savings by VFDs

I believe that the ability of VFDs to save energy is exaggerated. VFDs can be helpful to recover losses related to part load operation of refrigeration plants. To estimate energy saved by VFDs a question should be answered. How great are the losses? If a refrigeration plant is poorly designed and/or it has poor operation, energy losses will be significant. A lot of energy can be recovered, although, it is expensive recovery. The Cost of VFD is significant and VFD itself uses an additional 3-5% of energy. A well designed and well operated refrigeration plant will have minimal part load operation losses. VFDs installed for a plant will save little energy and they will have a long payback. Plant design and plant operation are two major factors that determine the effectiveness of VFDs.

Example.

A refrigeration plant has two single stage 500 HP screw compressors. Suction pressure is 0 psig. Discharge pressure is 120-150 psig. VFD for one compressor was installed.

Why was this plant designed as a single stage?

At mentioned suction and discharge pressures, this plant should be designed as two stages or as a single stage with an economizer.

Why are these compressors of equal capacity?

A screw compressor is slightly inefficient when it has capacity 50-100% and it is very inefficient when capacity is lower than 50%. Different compressor can be chosen and part load losses can be reduced. It can be 3 compressors (200 HP, 300 HP, and 500 HP) or it can be 2 compressors (400 HP, 600 HP).

Why is the minimum discharge pressure 120 psig?

Due to high pressure ratio, at higher discharge pressure part load losses are greater than part load losses at lower discharge pressure. Reduction of minimum discharge pressure from 120 psig to 90 psig will save a lot of energy and losses related to part load operation will be reduced. I believe that the energy savings achieved from this step can be compared to the energy savings from a 500 HP compressor VFD. This means that properly designed and properly operated refrigeration plants without VFDs will have the same efficiency as poorly designed and poorly operated plants with VFDs. The cost of a 500 HP VFD (typically more than $100,000) can be saved.