Specifications

Catalog WSC/WDC-4 7
Unmatched Unloading
McQuay’s Distinction Series chillers offer unloading to 10% of full load for a WSC single
compressor chiller and 5% for a WDC dual compressor units, without
using inefficient hot gas
bypass. This unloading capability, unique in the industry, provides improved stability of the
chilled water temperature and less harmful cycling of compressors. There are significant reasons
for this beneficial operating characteristic, including:
Moveable Diffuser
McQuay has pioneered the use of moveable discharge geometry to lower the surge point of
centrifugal compressors. The point at which the compressor enters a stall or surge condition
generally limits compressor unloading. At low loads, low gas velocity through a fixed discharge
area results in low gas velocities and the gas can stall or
surge in the impeller. When in a stall condition, the
refrigerant gas is unable to enter the volute due to its low
velocity and remains stalled in the impeller. In a surge
condition the gas rapidly reverses direction in the impeller
causing excessive vibration and heat. McQuay compressors
reduce the discharge area as load decreases to maintain gas
velocity and greatly reduce the tendency to stall or surge.
See page 27 for a complete description.
Thermal Expansion Valve
There are three refrigerant control devices used in the
industry, expansion valves, fixed orifices, and float
systems. Of the three, only expansion valves, (electronic
up to 300 tons and thermal over 300 tons) as used by
McQuay, offer good refrigerant management throughout
the entire chiller operating range. Expansion valves help
the McQuay chillers achieve their industry leading
capacity reduction capability.
Quiet Operation
McQuay chillers have two unique features to limit sound generation. One is the unique liquid
injection system and the other is that McQuay chillers get quieter as they unload.
Liquid Injection
A small amount of liquid refrigerant is taken from the
condenser and injected into the compressor discharge area.
The liquid droplets absorb sound energy and reduce the
compressor’s overall sound level. The droplets evaporate
and reduce discharge superheat.
Moveable diffuser closing off impeller discharge area
Radial ports inject liquid refrigerant into the
discharge gas as it enters the volute
.
Pilot expansion valve
Main expansion valve