Investment in compressor VFD

These days, to save energy, many companies buy VFDs for screw compressors. Is this a good investment?

Payback on your investment depends of operating conditions of your refrigeration plant.

Screw compressor VFD can be useful to recover losses related to part load performance of this compressor. Magnitude of the losses depends on the operating time, average part load, suction pressure and discharge pressure. Usually, screw compressor with VFD operates as trimming compressor. This compressor will run all year around or 8760 Hrs per year. Determination of average part load is the most difficult part of the estimation. However, it is very important.

Example.

Screw compressor operates at suction pressure 18 psig and discharge pressure 126 psig. Operating time is 8760 Hrs. Compressor electrical motor is 500 HP. VFD cost is $150,000. Losses will be different at different average part loads. If the energy cost is $ 0.1/KWh the losses will be following:

Average load, %     Losses, %    Losses, HP    Losses, $     Payback, years

        30                      23.3              116.5            76,200            1.97

        50                      14.4                72               47,100            3.19

        70                        7.7                38.5            25,200            5.95

From this table we can see that greater unload will lead to greater losses and to shorter payback for compressor VFD. Discharge pressure of 126 psig is close to real life average discharge pressure. Sometimes you can get the estimation for the 181 psig discharge pressure.

 Average load, %     Losses,%    Losses, HP    Losses, $    Payback, years

          30                      33               165               108,000         1.39

          50                      17.7             88.5              57,900          2.59

          70                       9.3              46.5              30,400          4.93

At this condensing pressure losses will be greater and payback will be shorter. However, these are not real life operating conditions. I do not know any refrigeration plant that operates at 181 psig discharge pressure all year round. During 12 months operation, refrigeration load will fluctuate significantly and most likely trimming compressor will operate at average load of 50 %.

Do not forget that VFD itself is not 100 % efficient. Usually, total VFD penalties are 3.0 - 3.5 %.  To estimate energy savings including VFD penalties, losses, % in tables should be reduced by 3.0 - 3.5 % and paybacks will increase.

To get real life VFD payback, the estimation should be based on real life operating conditions, such as the average load, suction pressure and discharge pressure.