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TRG-TRC016-EN 47
period two
Chilled-Water System Design
notes
The distribution pump is typically equipped with a variable-speed drive that is
controlled to maintain a certain pressure difference between the supply- and
return-water piping. In response to a reduced cooling load, the two-way valve
modulates closed, restricting the flow of water through the coil. This causes an
increase in system differential pressure, which can be measured and used to
signal a reduction in the speed of the distribution pump.
An alternative is to allow the pump to “ride its pump curve.” As the two-way
valves modulate closed, the increase in system pressure causes the pump to
“ride up” its performance curve (A to B), resulting in a reduction to 50 percent
of design flow in this example. This method, however, generally results in less
energy savings than a pump with a variable-speed drive. Also, proper pump
selection is important and part-load operating conditions must be considered.
In variable-flow systems, ASHRAE/IESNA Standard 90.1–1999 (Section 6.3.4.1)
requires the use of a modulation device, such as a variable-speed drive, on
pump motors larger than 50 hp [37 kW] that have a pump head greater than
100 ft H
2
O [300 kPa].
Varying Distribution Flow
pressure
pressure
difference
difference
riding the pump curve
variable-speed control
head pressure
head pressure
percent flow
percent flow
100
10050
50
0
0
B
A
pump
pump
curve
curve
Figure 53