Variable frequency drives (VFD) have been used in supermarkets with increasing success for at least 15 years. The appeal of VFDs is based on two effects they can achieve for a storeowner: energy efficiency and condenser noise abatement, an important measure in maintaining good relations with the store's neighbors.
VFDs are used to continuously adjust the speed of AC motors that are used to drive refrigeration compressors, air handler fans, and condenser fans. When used with a refrigeration compressor, variable speed can serve to exactly match the refrigeration capacity with the load. When used with a fan motor — whether in an outside condenser or in an HVAC system — again the VFD works to supply just the amount of motor speed to meet demands. In both cases, energy usage is minimized. In the case of the condenser fans, there is considerable reduction in objectionable noise.
Refrigeration and HVAC systems are designed for full-load conditions. Most of the time, though, the load is not peak, and full motor capacity isn’t required. In these average conditions, motors in traditionally designed systems (without VFDs) are either constantly running at a higher speed than necessary, or they are cycling on and off frequently. Running at too high a speed wastes energy, and frequent cycling on and off accelerates wear and shortens the useful lifetime of the motors, contactors, and other components. It also eliminates many opportunities for reducing energy costs. VFDs can help in both of these areas, providing better product and personnel environments and cutting operating cost.
When used to vary the speed of a refrigeration compressor and networked to a Danfoss AKC 55 rack controller, an AKD drive offers a virtually infinite range of capacities from the refrigeration rack. This means that rack capacity can be controlled to exactly match the load on the refrigeration system. Without the use of a VFD, the only way to vary the rack's capacity is to turn compressors on in different combinations; but each time a compressor is started, considerable energy is expended to overcome inertia and bring the motor up to speed. With a VFD on one of the rack's compressors, and with the rack sized properly, the speed variation on that one compressor will provide all the capacity change necessary to meet 90% of the load variations of the rack.
VFDs applied to condenser fans also have the benefit of reducing noise. Before VFDs were available, local ordinances or neighborhood complaints often made necessary costly acoustical barriers to shield a store's surroundings from the noise of its condenser fans at full speed. Sometimes owners were even forced to relocate condenser units at great expense. By installing a VFD on the condenser, the fans can be run at a lower speed and both the high speed noise and the frequent starting noise are greatly reduced.
AKD variable frequency drives are also used in supermarkets to regulate the speed of the HVAC fan motors. With a variable frequency drive, the fan speed (and the energy consumption) can be reduced to a minimum whenever cooling or heating is not required. When there is no heating or cooling on, the fan runs just fast enough so that air is circulating.
Store owners can expect payback from energy savings in under two years when installing VFDs on condenser and HVAC fans. There are measurable energy savings from refrigeration compressor installations of VFDs, but the greatest effect in those applications is in added machinery life, reduced maintenance costs, and a better storage environment for refrigerated products. Because the refrigeration capacity supplied is always matched to the load in cases and boxes, variations in temperature are much reduced, lengthening healthful product life. d
Authors Max Robinson and David Hebel, work with Danfoss Refrigeration and can be reached at 410/931-8250.