ASHRAE Journal - June 2010 - 58

VFDs for Large Chillers
This third column in a five-part series on variable frequency drives will cover application of VFDs to large chillers in commercial building air-conditioning systems. By John Dieckmann, Member ASHRAE; Kurtis McKenney; Matthew Guernsey; and James Brodrick, Ph.D., Member ASHRAE

Large Energy Savings Per Installation

sing variable frequency drives (VFDs) for compressor motors in large chillers is a newer application than using VFDs with blowers (discussed in last

month’s column). VFDs are being used with centrifugal compressors and screw compressors, the two major compressor categories for large chillers. Applying a variable speed drive to the compressor provides energy-efficient capacity modulation. While the focus of this series has been on using VFDs for induction motors, high-speed brushless dc motors are an important class of motors. Such motors generally are used to directly drive high-speed centrifugal compressors. Large tonnage chillers generally are capable of capacity modulation, which enables the chiller to run continuously while delivering constant-temperature chilled water as the cooling load varies from maximum to minimum design load. The traditional methods of capacity modulation have been: • For centrifugal compressors, prerotation vanes, also called inlet guide vanes, reduce capacity while avoiding surge, down to around 40%. Hot gas bypass (directing compressor discharge gas to the evaporator) is used to reduce capacity further. • Screw compressors have been equipped with slide valves that simultaneously vary the inlet displacement and the discharge displacement, maintaining a relatively constant built-in volume compression ratio, typically modulating down to about 30% of full load before hot gas bypass is needed for further capacity reduction. • Staged operation of multiple compressors (or multiple chillers) has been
58 ASHRAE Journal

used with both types of chillers, as well as with chillers that use other compressor types, such as scroll or reciprocating. Two metrics commonly used to rate the efficiency of chillers are full-load steady state performance and the integrated part load value (IPLV). The IPLV metric is intended to approximate typical seasonal performance as the load on the chiller varies. Figure 1 shows the air or water temperature entering the condenser versus load that is used to determine the IPLV for air-cooled and water-cooled chillers. In each case, reduced load is assumed to correlate with lower outdoor wet- and dry-bulb temperature, resulting in lower temperature of the cooling medium entering the condenser. The IPLV is a widely accepted metric for comparing the seasonal average performance of chillers. However, in actual applications, chillers operate over a much wider range of loads and condenser temperatures than the IPLV rating load-condenser temperature curves. Reasons for operation at low load percentage and higher condenser temperatures include high ambient wet or dry-bulb temperature coupled with low building occupancy or low solar loading and the general tendency to oversize building cooling equipment. Reasons for operation at high load percentage and
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lower condenser temperatures include high load during chiller start-up after an unoccupied period and operation of one of a set of multiple chillers to handle the building part load. The combination of chiller plant oversizing and the variation of the factors driving the cooling load (some of which can be highly variable, namely weather) can result in almost any combination of chiller load level and condenser temperature. Estimates suggest that more than 90% of water-cooled chiller plant installations include multiple chillers, with two being the most common number.1 In multiple chiller plants, operation of any chiller below 50% load is unusual, with a large percentage of operating time occurring around the 75% load point or higher.1 For multiple chiller plants, equipping each compressor with VFD may provide the greatest energy savings and/or operational flexibility, but it may be more economical (or at least less capital intensive) to apply VFD to the base load chillers or chillers expected to operate at part-load (“swing chillers”). Chillers that operate the most hours with reduced condenser water temperatures (i.e., reduced lift) and/ or reduced load will offer the greatest energy cost savings potential. Several factors account for the improvement in the IPLV provided by variable speed drives. Variable speed operation significantly reduces, if not eliminates altogether, the use of hot gas bypass for continuous operation at low load. In centrifugal chillers, variable speed combined with prerotation vanes enables operation
June 2010

ASHRAE Journal - June 2010

Table of Contents for the Digital Edition of ASHRAE Journal - June 2010