Traditionally, supermarket refrigeration system condenser fan and compressor controllers cycle fans and compressors “on” and “off” in an attempt to match system refrigeration capacity to current system refrigeration load levels. Unfortunately, given the discrete capacity steps available from this control methodology, it is rarely possible to deliver the exact amount of compressor and condenser capacity required for a particular level of compressor and condenser loading. Varying degrees of uneven or unstable system operation often result, along higher energy consumption due to control overshoot of desired suction and condensing pressures, and sometimes destructive rapid start and stop cycling of fan and compressor motors.
The application of variable speed drives (VSDs) to condenser fans and compressors motors, controlled using proprietary VSD algorithms, can eliminate these capacity matching problems, and save additional energy as well. The fourth column in Table shows energy consumption when variable frequency drives (VFDs) are used to modulate condenser fan speeds together with both floating condensing pressure and TD(temperature deference) control algorithms. It can be seen that when varying the speed of condenser fans is used as the fan capacity control method, an additional 7.47% saving can be achieved on top of what is achieved using floating pressure and TD control. Therefore, an overall 13.95% energy saving can be achieved when variable speed along with floating condensing pressure control strategies are employed. For condenser fans, almost all of the savings from utilization of VSD (Variable speed drive) control are due to the affinity laws for fans (which tell us fan motor power varies with the cube of fan speed while fan air volume varies linearly with speed) and improved utilization of condenser surface.
VSDs can also be applied to refrigeration compressors as well using proprietary VSD compressor control algorithms. Savings in this case flow primarily from improved suction pressure control and therefore slightly higher on average suction pressures, as well as from modest but still significant compressor volumetric efficiency increases at lower speeds. Analysis of savings from application of VSDs for compressor capacity control can be complex, but savings on the order of 7 to 12% can be anticipated, depending on locale and refrigeration system configuration.