Specifications

Catalog WSC/WDC-4 57
operate during the day, when cooling tower water temperatures are high, a proper selection
becomes more difficult because the two refrigerant pressure differentials are significantly
different.
A three-way condenser water control valve is always required.
Variable Speed Pumping
Variable speed pumping involves changing system water flow relative to cooling load changes.
McQuay centrifugal chillers are designed for this duty with two limitations.
First, the rate of change in the water flow needs to be slow, not greater than 10% of the change per
minute. The chiller needs time to sense a load change and respond.
Second, the water velocity in the vessels must be 3 to 10 fps (0.91 and 3.0 m/sec). Below 3 fps
(0.91 m/sec), laminar flow occurs which reduces heat transfer. Above 10 fps (3.0 m/sec),
excessively high pressure drops and tube erosion occur. These flow limits can be determined from
the McQuay selection program.
We recommend variable flow only in the evaporator because there is virtually no change in chiller
efficiency compared to constant flow. In other words, there is no chiller energy penalty. Although
variable speed pumping can be done in the condenser loop, it is usually unwise. The intent of
variable flow is to reduce pump horsepower. However, reducing condenser water flow increases
the chiller’s condensing pressure, increasing the lift that the compressor must overcome which, in
turn, increases the compressor’s energy use. Consequently, pump energy savings can be lost
because the chiller operating power is significantly increased.
Low condenser flow can cause premature tube fouling and subsequent increased compressor
power consumption. Increased cleaning and/or chemical use can also result.
Vibration Mounting
Every McQuay chiller is run tested and compressor vibration is measured and limited to a
maximum rate of 0.14 inches per second, which is considerably more stringent than other
available compressors. Consequently, floor-mounted spring isolators are not usually required.
Rubber mounting pads are shipped with each unit. It is wise to continue to use piping flexible
connectors to reduce sound transmitted into the pipe and to allow for expansion and contraction.
Operating/Standby Limits
Equipment room operating temperature (check factory for VFD applications): 32°F to 104°F.
Equipment room standby temperature: 32°F to 122°F.
Maximum entering condenser water temperature, startup: design plus 5 degrees F.
Maximum oil cooler/VFD entering water temperature: 90°F.
Minimum leaving chilled water temperature without anti-freeze: 38°F.
System Water Volume
It is important to have adequate water volume in the system to provide an opportunity for the
chiller to sense a load change, adjust to the change and stabilize. As the expected load change
becomes more rapid, a greater water volume is needed. The system water volume is the total
amount of water in the evaporator, air handling products and associated piping. If the water
volume is too low, operational problems can occur including rapid compressor cycling, rapid
loading and unloading of compressors, erratic refrigerant flow in the chiller, and loss of
temperature control.
For normal comfort cooling applications where the cooling load changes relatively slowly, we
recommend a minimum system volume of three minutes times the flow rate (gpm). For example,
if the design chiller flow rate is 1000 gpm, we recommend a minimum system volume of 3000
gallons (1000 gpm x 3 minutes).
For process applications where the cooling load can change rapidly, additional system water
volume is needed. A process example would be a quench tank where the load would be stable