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View Subscription Details: Letter ( 10/31/2016 ) - Plain Text Format Energy Saved by VFDs (2)

VFDs can be installed for refrigeration compressors. These compressors have constant torque loads. With constant torque loads, the torque loading is not a function of speed. As the speed changes, the load torque remains constant and energy use changes linearly with speed. This means that at 50% speed the compressor will use 50% energy. There is no energy saving benefits of using VFDs for reciprocated compressors, unless you want gradual change capacities of these compressors.
Typically, screw compressors use slide valves to reduce capacity. Capacity can be changed gradually but energy efficiency will suffer. This energy efficiency will be reduced gradually (linearly) by unloading screw compressor from 100% to 50%. It will be reduced exponentially when compressor capacity is below 50%.
How much energy can be saved by compressor VFD?
This depends of average compressor load and operating compression ratio.
For example. Assume that the screw compressor has an average load of 50%. At low compression ratio it will use 55% of energy. At high compression ratio it will use 75% of energy. Compressor VFD was installed. The VFD itself uses 3-3.5% of energy. This means that energy savings for the compressor with VFD operating at low compression ratio will be (55% - 50%) - 3% = 2%. For the compressor operating at high compression ratio these savings will be (75% - 50%) - 3% = 22%. At an average compressor capacity below 50% these savings will increase and above 50% these savings will decrease.
Often times, screw compressors VFDs provide very limited energy savings if these compressors operate at low compression ratio.

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