Applying variable frequency drives to air units in industrial refrigeration systems.

The refrigeration loads experienced by air units in industrial refrigeration systems can be highly variable and a means for adjusting the capacity of these units is essential in order to maintain appropriate space temperatures. In industrial refrigeration systems, the most common strategy to control the capacity of an air unit is to simply cycle the solenoid valve feeding liquid refrigerant to the coil while the circulating fans on the unit operate continuously. This control strategy is functional but not energy efficient. Alternative strategies that aim to achieve the process cooling objectives while improving the energy performance include: duty cycling the fans and varying fan speed using a frequency drive. This paper discusses the energy benefits of applying variable frequency drives to air units in industrial refrigeration system applications as a means of increasing the energy efficiency of operation when load conditions are variable. Refrigerant-side control strategies overfed evaporator air units are presented.