Specifications

24 Catalog WSC/WDC-4
Systems with refrigerants that operate at a positive pressure, such as R-134a, have smaller
impellers and gas lines since these refrigerants require lower gas flow rates. R-123 requires
approximately six times the gas flow rate in cfm per ton than R-134a. At ARI standard conditions,
18.1 cfm (8.54 l/sec) of R-123 is required per ton of refrigeration. Contrast this to R-134a that
requires only 3.2 cfm (1.5 l/sec) per ton. This means that for a given capacity, the cross-sectional
area of the impeller inlet "wheel eye" as well as the suction and discharge lines will be six times
larger for R-123 than for R-134a at equivalent pressure drops. The wheel eye diameter is the
major factor in determining the overall impeller diameter and geometry.
Designers of centrifugal equipment must also consider the tip speed requirement. To produce the
required pressure difference or lift, a centrifugal impeller must achieve a given tip speed. Tip
speed is the velocity of the "tip" of the impeller relative to its surroundings. Imagine an observer
standing on the impeller. The observer sees his surroundings pass by him at a certain velocity.
This velocity is the impeller tip speed, usually expressed in feet per second (meters per second).
An analogy can be drawn to a car driving down a road. The tip speed of the tire is equal to the
speed of the car.
Since all the refrigerants that have been discussed require tip speeds in the range of 670 to 700
ft/sec (204 to 213 m/sec), we see that the impeller angular velocity (rpm) is largely affected by its
diameter. It was pointed out earlier that negative pressure impellers must be larger than those in
positive pressure machines due to the drastic differences in required gas flow rates. Larger
diameter impellers must rotate at slower rpm than smaller diameter impellers. Referring again to
the car example demonstrates that different combinations of diameter and rpm produce the same
tip speed. Imagine a freeway carrying vehicles with different size tires all traveling at 55 mph.
The tip speed of all of the tires is fixed at 55 mph even though the small tires of a utility trailer
rotate at a much higher rpm than the tires of a tractor-trailer.
The relationship of diameter and tip speed can be shown by the following equation:
[] .)(/2.229)( inDiameterxfpsTipSpeedRPM =
[] .)(/1910)/( cmDiameterxsmTipSpeedRPM =
Again, this indicates that for a given speed requirement, a smaller diameter impeller in a
compressor will operate at a higher rpm than a larger diameter impeller. Again:
Stress is proportional to the tip speed
2
: Impellers with similar tip speeds have similar stress.
Quiet, stable capacity from 10% to 100% without hot gas bypass
Compressor capacity on McQuay chillers is maximized at full load and modulated to 10% load by
interlocked inlet guide vanes and the movable discharge geometry. This McQuay design
innovation has real owner benefits. Most centrifugal compressors do not unload this well and
waste energy at low load conditions by unnecessary compressor cycling or by the use of
inefficient hot gas bypass.
No leakage at the capacity control mechanism
An oil pressure operated, guide vane activating piston is internally mounted and powered,
eliminating leakage from external linkage and seals. The vanes are positioned in response to
variation in leaving chiller water temperature. A built-in compensating control allows automatic
override of normal operation to close the vanes for low suction pressure or current limiting duty.