Specifications

32 Centrifugal Chillers IMM WMC-2
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, improper
motor cooling, shortened equipment life and other undesirable consequences.
Some of the things the designer should consider when looking at water volume are the minimum cooling load,
the minimum chiller plant capacity during the low load period and the desired cycle time for the compressors.
Assuming that there are no sudden load changes and that the chiller plant has reasonable turndown, a rule of
thumb of “gallons of water volume equal to two to three times the chilled water gpm flow rate” is often used.
For process applications where the cooling load can change rapidly, additional system water volume is needed.
A process example would be a quenching tank. The load would be very stable until the hot material is immersed
in the water tank. Then, the load would increase drastically. For this type of application, system volume may
need to be increased.
Since there are many other factors that can influence performance, systems may successfully operate below
these suggestions. However, as the water volume decreases below these suggestions, the possibility of problems
increases.
Variable Speed Pumping
Variable water flow involves inversely changing the water flow through the evaporator as the load changes.
McQuay chillers are designed for this duty provided that the rate of change in water flow is slow and the
minimum and maximum flow rates for the vessel, as shown in Figure 11 on page 16 are not exceeded.
The recommended maximum change in water flow is 10 percent of the allowable flow change per minute. Flow
is usually not reduced below 50 percent of design flow (provided vessel minimum flow rates are not exceeded).
For example, a 150-ton chiller might have chilled water flow of 360 gpm, reduced to 50 percent, would be 180
gpm. However, the minimum flow rate is 216 gpm, so the flow change would be 360 gpm minus 216 gpm, or
144 gpm. This means that the allowable flow rate change would be 10 percent of 144 or 14.4 gpm per minute.
MicroTech II Control
Figure 18, MicroTech II Control Panel
WMC chillers are equipped with the McQuay MicroTech II
control system consisting of:
• Operator interface touchscreen panel (shown at the left). It
contains a 15-inch Super VGA color screen.
• Control Panel containing the MicroTech II unit controller, two
MicroTech II compressor controllers with connections to the
compressor-mounted controls, and various switches and field
connection terminals.
Operating instructions for the MicroTech II controller are
contained in Operating Manual OM WMC-3.