Good and bad investments in energy efficiency (2)

Screw compressor VFD

Capacity of compressors should be changed based on the variation of refrigeration loads. Screw compressor has a slide valve to increase and decrease the capacity of this compressor. Unfortunately, unloading of screw compressor reduces its energy efficiency. It means that compressor uses more energy to produce one ton of refrigeration compare to 100% loaded compressor. Look at energy use of partly unloaded screw compressor.

Capacity, %            100      90       80      70      60     50      40       30      20      10

Energy use, %        100      94       88       82     76     70      65       61      58      56

As you can see, this compressor is very inefficient at capacities below 50%.  Ideally, at 50% capacity screw compressor should consume 50% of energy. This ratio is possible to achieve by using compressor VFD. This VFD will gradually change the speed of electrical motor and capacity of the compressor will change accordingly. However, in real life VFD itself consumes 3% of the energy. It means that at 50% capacity compressor with VFD will consume 53% of energy. Slide valve unloading to 50% capacity will give us 70% of the energy use. The difference is 17% and this energy can be saved by installing screw compressor VFD.

Magnitude of energy that can be saved by compressor VFD depends on the average capacity of this compressor and compression ratio. Compression ratio is the ratio of absolute discharge pressure to the absolute suction pressure. Energy efficiency of the screw compressor is lower if it is operating at a lower capacity and at higher compression ratio. It means that screw compressor VFD can recover (save) more energy if it is installed on compressor that operates at low capacity and high compression ratio. Operating hours of the compressor is important as well. However, typically screw compressor with VFD operates as trimming compressor and runs 8760 hours per year.

To estimate real life energy savings and payback on investments in screw compressor VFD, real life average capacity of this compressor and real life compression ratio should be chosen.