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View Subscription Details: Letter ( 08/31/2016 ) - Plain Text Format Energy Saved by VFDs (1)

Today many refrigeration plants use VFDs to save energy. How can energy be saved by VFDs?
There are two applications of VFDs: variable torque and constant torque. In industrial refrigeration, pumps and fans have variable torque loads and compressors have constant torque loads. In this article we will review VFDs with variable torque. In this application, power varies directly with speed cubed. What does this mean?
For example. To keep required condensing pressure 20 HP condenser fan cycles. Assume that this fan on average operates 50% of the time. This means that the average power use of this fan is 10 HP. When we add VFD to this fan, the average speed will be 50% of nominal speed and energy will be saved. According to affinity law at 50% speed this fan will use 0.5 x 0.5 x 0.5 = 0.125 or 12.5% of total power or 2.5 HP.
Annual energy savings of this fan with VFD will be (10 - 2.5) x 0.7457 x 8760 hrs = 48,992 kWh
Theoretically VFDs can be used for condenser pumps as well. However, condenser pump in addition to the water flow should provide required head. At lower pump speed, pump head will be reduced and the evaporative condenser will have partly wetted surface causing the condenser efficiency to suffer.
Similarly, we should evaluate performance of evaporator fans with VFDs. At lower fan speeds the air throw will be reduced and temperature in the refrigerated room can suffer. Evaporator fans VFDs should not be use for the refrigerated rooms where evaporator fans air throw is critical.

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