Specifications

Catalog WSC/WDC-4 23

Unit Design Features

Compressor Design

Gear-Drive Offers Greater Operating Efficiency Than Direct Drive

Centrifugal compressor efficiency is a function of impeller design and application to the

refrigeration system. The increased heat transfer surface and efficiency of modern heat

exchangers have changed compressor head and impeller tip speed requirements. Direct-drive

designs limit the manufacturer’s ability, within a single compressor size, to select impellers at or

near peak impeller efficiency. While a unit selected at poor impeller efficiency might produce the

required performance at peak load, its operating characteristics over the entire range of part load

performance are sharply curtailed, resulting in increased annual operating costs.

McQuay gear-drive centrifugal chillers provide a variety of tip speed ratios to permit selection of

impellers for maximum efficiency over their entire part load to full load range and are also ideal

for 50 Hz application. Mechanical gear losses are limited by design standards to less than one-

half of 1%. The impeller efficiency obtained by alternate gear selections can increase chiller

efficiency by as much as 7%.

As energy costs continue to rise, the economic advantages of gear drive to obtain maximum

efficiencies will be even more advantageous. The efficiency of either direct-drive or gear-drive

compressor can be improved through the use of variable frequency drives to reduce compressor

speed at low load/low head conditions.

Extended Motor Life

McQuay’s modern compact compressor design provides many operating advantages that improve

its overall reliability and durability. One such advantage is prolonged motor life. A motor draws

locked rotor current until it reaches break-away torque at approximately 80% of its running speed.

While drawing locked rotor current, the stresses on the motor are over six times that of full load.

The McQuay compressors absolutely minimize this stress through the unique gear drive and light

weight drive train that allows a 500-ton (1750 kW) compressor to reach running speed in less than

three seconds. The owner benefits from a longer motor life.

The REAL FACTS on Speed, RPM and Tip Speed in Centrifugal Compressors

The question: "How fast does it spin?" is common when discussing compressors. There is a

widespread concept that the impeller rotating speed (rpm) is the determining factor in the life,

reliability, and efficiency of the compressor. This is absolutely false. An engineering examination

will show that rpm, as an absolute, is not considered in the design of rotating mechanical

components. It is the combination of velocity of the outside edge of the impeller (tip speed),

mass, and physical size that define the design criteria for these components. Shaft, bearing, and

impeller design is based on parameters such as surface velocity, diameter, weight, rotational and

torsional critical speed, as well as the type of material and lubrication system used.

Stress on an impeller is proportional to the square of the tip speed. Rotational speed is only

part of the equation along with impeller diameter.

In centrifugal compressor design, two parameters, impeller diameter and impeller tip speed, must

be determined. Impeller diameter is determined by the required volume flow rate supplied to the

inlet of the impeller. Refrigerants which operate at a negative pressure such, as HCFC-123 have

high cfm/ton (m

/kW) flow rates and require larger diameter impellers and refrigerant lines to

keep pressure drop to reasonable levels.