10 Product Manual PM WSC/WDC
Since the impeller shaft must be sized to support the static, rotational and torsional loads applied by the impeller,
as impellers become larger, shafts must also become proportionally larger. These factors also come into play in
the design or selection of a bearing. The primary criteria used in bearing design are:
1. The load per unit of bearing area.
2. The relative velocity of the two bearing surfaces.
3. The bearing dimensions.
4. The viscosity of the lubricating oil.
Notice that item 2 returns to the phenomenon of tip speed. Surface velocity is simply the tip speed of the inner
bearing surface or shaft with respect to the outer bearing surface as illustrated below.
A bearing is basically two infinite surfaces passing over one another with a velocity equal to the surface
velocity.
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. These two characteristics drastically reduce wear on the
compressor at the time it is most likely to occur. The same phenomenon, although less extreme, also holds true
during coast-down. The quicker, the better.
The table at the right compares refrigerants
in common use today in centrifugal
compressors. Note that required compressor
tip speeds are all within eight percent of each
other.
All McQuay centrifugal chillers use
refrigerant HFC-134a. The machine design
characteristics of this refrigerant (and its
predecessor, R-12) such as small moving parts, low mass, low inertia, quick spin-up and coast-down, and
simplicity of design, have continuously proven themselves since the first chiller was introduced in 1962. The
small and lightweight rotating parts lend themselves to easy servicing of the compressor and its associated
parts and piping.
Refrigerant
HCFC
123
HFC
134a
HCFC
22
Condenser Press. (psig @ 100°F) 6.10 124.1 195.9
Evaporator Press. (psig @ 40°F) (Inches
of Mercury Vacuum)
(18.1) 35.0 68.5
Refrig. Circulated (lbs/min./ton) 3.08 3.00 2.78
Gas Flow (cfm/ton) 18.15 3.17 1.83
Tip Speed (ft./sec.) 656 682 707
Ozone Depletion Potential (ODP) 0.02 0.00 0.05
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