Specifications

Catalog WSC/WDC-4 25
Figure 8, HFC 134a Impeller Compared to R-123 Impeller
Left
: Impeller from a McQuay single stage
300 ton (1050 kW) compressor; diameter =
6.3 in. (16 cm), weight = 3.0 lb (1.4 kg)
Right
: One of three impellers from a 300
ton negative pressure compressor; diameter
= 26 in. (66 cm), weight = 27 lb. 12.2 kg)
Single Stage Simplicity = Savings
Compressor efficiency is not a function of multiple impellers. Maintenance of optimum
efficiency at peak and, more importantly, at part load, is a function of the total compressor and
chiller design including:
• Motor efficiency
• Refrigerant type
• Condenser and evaporator surfaces
• Compressor mechanical friction
• Impeller and vane design
• Refrigerant flow passages
Of these, the least considered performance factor on actual versus theoretical performance is the
refrigerant flow passages between the discharge of one impeller and the inlet to the next impeller
on multi-stage machine design. The energy loss in a single passage will be greater or equal to the
loss in the suction passage between the evaporator outlet and the first stage impeller inlet,
depending upon the compactness of the total compressor design. Single stage impeller design can
eliminate that additional loss, and provides an opportunity for maximum system efficiency.
The primary advantage to multi-stage centrifugal operation, in the pressure and volume ranges
characteristic of typical air conditioning systems, is the expansion of impeller head coefficients at
reduced volumetric flows or cooling loads. The McQuay backward inclined single stage impeller,
combined with unique movable diffuser geometry at the impeller discharge, provides a stable
operating range superior to multi-stage systems. Thus, selection of McQuay chillers permits
operation from 100% to 10% capacity (to 5% on WDC dual compressor chillers) without surging
and at maximum efficiency, i.e. no hot gas bypass.
Optimum compressor efficiency is designed into each McQuay impeller. The McQuay designed
impeller not only minimizes pressure loss at the inlet and maximizes compression efficiency, but
also breaks up pure tone sound to operate at competitively low sound power levels. A simple
short diffuser and a volute design passing compressed gas directly into the condenser maintain
the compressor efficiency.