Specifications

40 Catalog WSC/WDC-4

Variable Frequency Drives

Impact of Variable Frequency Drives

The chart below illustrates the relative IPLV efficiencies of various McQuay options for a typical

500-ton selection. The chiller cost increases as the efficiency improves.

Comparative Efficiencies (kW/Ton)

WSC WDC WDC w/VFDWSC w/VFD

.505

.403

.365

.337

IPLV

Notes:

WSC = Single Compressor Centrifugal Chiller

WDC = Dual Compressor Centrifugal Chiller

VFD = Variable Frequency Drive

The IPLV values (defined on page 49) are ARI Certified Ratings based on ARI Standard 550/590-

1998, Standard for Water Chilling Packages Using the Vapor Compression Cycle. Full load is at

44°F chilled water temperature with 2.4 gpm/ton, 85°F entering condenser water temperatures

with 3 gpm/ton. Part load points of 75%, 50%, and 25% employ condenser water temperature

relief (reduction) per the standard.

General

Single and dual compressor units can be equipped with a variable frequency drive (VFD). A VFD

modulates the compressor speed in response to load and evaporator and condenser pressures as

sensed by the microprocessor. Due to the outstanding part load efficiency, and despite the small

power penalty attributed to the VFD, the chiller can achieve outstanding overall efficiency. VFDs

really prove their worth when there is reduced load combined with low compressor lift (lower

condenser water temperatures) dominating the operating hours.

The traditional method of controlling centrifugal compressor capacity is by inlet guide vanes.

Capacity can also be reduced by slowing the compressor speed and reducing the impeller tip

speed, providing sufficient tip speed is retained to meet the discharge pressure requirements. This

method is more efficient than guide vanes by themselves.

In actual practice a combination of the two techniques is used. The microprocessor slows the

compressor (to a fixed minimum percent of full load speed) as much as possible, considering the

need for tip speed to make the required compressor lift. Guide vanes take over to make up the

difference in required capacity reduction. This methodology provides the optimum efficiency

under any operating condition.