Specifications
IMM WMC-2 Centrifugal Chillers 7
Cooling Towers
The condenser water flow rate must be checked to be sure that it conforms to the system design. A tower bypass
valve, controlled by the unit controller, is required to control the minimum condenser entering temperature.
Unless the system and chiller unit are specifically designed for them, condenser bypass or variable condenser
flow is not recommended, since low condenser flow rates can cause unstable operation and excessive tube
fouling.
Condenser Water Temperature
When the ambient wet bulb temperature is lower than design, the entering condenser water temperature can be
allowed to fall, improving chiller performance.
McQuay chillers will start with entering condenser water temperature as low as 55°F (12.8°C) providing the
chilled water temperature is below the condenser water temperature.
Depending on local climatic conditions, using the lowest possible entering condenser water temperature can be
more costly in total system power consumed than the expected savings in chiller power would suggest due to the
excessive fan power required.
To obtain lower than 55°F (12.8°C) entering condenser water temperature with a tower selected to produce 85°F
(29.4°C) water temperature at design ambient air temperatures, cooling tower fans must continue to operate at
100% capacity at low wet bulb temperatures. As chillers are selected for lower kW per ton, the cooling tower
fan motor power becomes a higher percentage of the peak load chiller power. The offsets of compressor power
and fan power must be examined. On the other hand, the low condenser water temperatures can be easy and
economical to achieve in mild climates with low wet bulb temperatures.
Even with tower fan control, some form of water flow control such as tower bypass must be used and controlled
by the chiller MicroTech II controller.
Figure 3 and Figure 4 illustrate two temperature-actuated tower bypass arrangements. The “Cold Weather”
scheme, Figure 4, provides better startup under cold ambient air temperature conditions. The check valve may
be required to prevent air at the pump inlet.
Figure 3, Tower Bypass, Mild Weather Operation
Figure 4, Tower Bypass, Cold Weather Operation
!
CAUTION
Tower water treatment is essential for continued efficient and reliable unit operation. If
not available in-house, competent water treatment specialists should be contracted.