Specifications

Catalog WSC/WDC-4 27
Bearing design, and consequently bearing life, is determined largely by the above criteria. Rpm,
by itself as an absolute, is only one half of the equation in the design process. One can also see
that higher rpm and smaller, lighter parts actually reduce the load and wear on bearings.
It is the surface velocity in conjunction with the load to be supported that determines bearing life
and therefore bearing selection. Referring to the analogy of the tractor trailer versus the utility
trailer, one sees that even though the utility trailer tires operate at a much higher rpm, the tractor
trailer wheel bearings must be much more massive due to the much heavier dynamic loading.
Shaft rotating speed has little effect on bearing wear.
The smaller rotating mass of a machine will improve the life of the bearing. Before the shaft
begins to spin, it rests on the bearing surface. Once the shaft starts rotating, an oil film develops
between the shaft and the bearing that supports the shaft. The low mass of a positive pressure
machine not only exerts a smaller static load on the bearings, but the fast spin-up enabled by the
low inertia of the modern gear drive compressor permits the supportive oil film to build up more
quickly.
Liquid refrigerant injection into compressor discharge-Unique
Although this sounds complex, this feature is quite simple. Most of the noise in all centrifugal
compressors results from high gas velocity in the discharge line.
The McQuay liquid injection system injects liquid refrigerant into the discharge gas through a
radial array of ports. This refrigerant mist absorbs sound energy (much like a foggy day) and the
flash gas cools the discharge gas leaving the compressor. The net result is significant noise
reduction.
In addition, by removing superheat from the discharge gas, the condenser becomes more efficient,
improving unit efficiency. Sound Levels -- One Of The Quietest Centrifugal Chillers In The
Industry.
McQuay centrifugal chillers are one of the quietest units available in the marketplace due to
several unique design features.
Quiet full load sound levels and QUIETER part load sound levels-Unique
The highest noise levels for McQuay chillers are at full load. As McQuay chillers unload, noise
levels reduce. Other chillers on the market are typically the opposite, with higher sound levels at
part load. Be certain to compare noise levels at several load conditions.
Moveable Discharge Geometry-
Figure 11, Movable Diffuser Geometry
The other feature to reduce
noise and increase stability at
low loads is the McQuay
movable discharge geometry.
Less refrigerant is circulated as
the chiller capacity reduces.
The left drawing shows the
operation at full load of a unit
with a fixed compressor
discharge cross section. At full
load, a large quantity of gas is
discharged with a fairly uniform
discharge velocity as indicated
by the arrows.
The middle drawing shows a fixed compressor discharge at reduced capacity. Note that the velocity is
not uniform and the refrigerant tends to reenter the impeller. This is caused by low velocity in the
discharge area and the high pressure in the condenser, resulting in unstable surge operation and with
noise and vibration generated.
Part Load
Gas Flow with
Movable Geometry
ImpellerImpeller
Full Load Stall or Surge at Part Load
Gas Flow at Discharge of
Impeller with Fixed Geometry
Impeller