Filter
Top Products
System Controls
SYS-APM001-EN Chiller System Design and Control 93
These three energy consumers must be balanced to minimize overall energy
use. This makes varying condenser water flow complex, but the strategy
below has been implemented on projects.
Control of the condenser water pumps and cooling tower fans is based on
chiller load, which is calculated using the chilled-water flow rate and
temperature differential. When the chiller load is less than 80 percent, the
control system varies the speeds of both the cooling tower fan and
condenser-water pump in proportion to chiller load, until the minimum
allowable condenser-water flow rate is reached. As chiller load decreases
further, the condenser-water pump remains at the minimum speed, while the
cooling tower fan speed continues to decrease.
Figure 55. Effect of chiller load on water pumps and cooling tower fans
The minimum allowable condenser water flow rate is the highest of the
following:
• Minimum water flow rate required through the chiller condenser (consult
chiller manufacturer)
• Minimum water flow rate required through the cooling tower fill (consult
cooling tower manufacturer)
• Minimum water flow rate required to properly cool the pump motor
(consult pump manufacturer)
• Minimum water flow rate that produces enough pressure to overcome
the static lift from the base of the cooling tower to the top of the tower
Decoupled condenser-water system
To overcome possible problems with variable-water flow across a cooling
tower, some designers decouple the condenser-water system as shown in
Figure 56. This arrangement allows energy optimization through a reduction
of pumping power without excessively complicating the system. Because
cooling towers work best with full water flow over the transfer media, a
pump speed
chiller load
fan speed
0
20
40
60
80
100
10 20 30 40 50 60 70 80 90 100
speed, %
chiller load, %
20
40
60
80
100
chiller load, %
The June 2006 ASHRAE Journal article,
“Prescription for Chiller Plants,” (Baker,
Roe, and Schwedler) details a chilled-
water plant with variable-speed
condenser water pumps.