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SYS-APM001-EN Chiller System Design and Control
87
System Controls
Chilled-Water System Control
Chilled water reset—raising and lowering
Many chilled-water plants use chilled water reset, that is, the chiller’s leaving-
water temperature setpoint, in an effort to reduce chiller energy consumption.
This can either be accomplished by the chiller controller or by the system
controller.
Raising the chilled-water temperature reduces chiller energy consumption. In a
constant-volume pumping system, this may reduce overall system energy
consumption as long as humidity control is not lost. Humidity control may be
lost if, as chilled-water temperature is increased, the air temperature leaving the
coil increases to a point where it no longer performs adequate
dehumidification.
In a variable-volume pumping system, however, raising the chilled-water
temperature increases pump energy, often substantially, and typically increases
total system energy. Before considering increased chilled-water temperature,
the system operator should calculate the increased pumping energy and
compare it with the chiller energy savings. It should be noted that ASHRAE/
IESNA Standard 90.1–2007
20
requires chilled-water reset for constant-volume
systems—with some exceptions—but exempts variable-volume systems from
this requirement for the reasons discussed.
An often overlooked method of decreasing system energy consumption is to
reduce the chilled-water temperature, thereby decreasing pumping energy but
increasing chiller energy. This strategy is possible with adequate chiller capacity
and lift capability. Reducing chilled-water temperature may also improve
dehumidification in the building. Another result of reducing the chilled-water
temperature is increased chiller capacity during times when the condenser
water temperature is cooler than design. This allows more time before another
chiller and its ancillary equipment are started.
Be aware that any change in chilled-water setpoint requires changes to be made
to the system chiller-sequencing algorithms to ensure that system capacity is
met. This added complication may not be warranted.
Chilled-water pump control
In constant flow systems, the pumps are either on or off, providing relatively
constant flow when turned on. In practice, some flow variation will occur as
system pressure drop changes. In a variable-flow system, pump control is most
often performed by maintaining a pressure differential at a selected point in the
system. For example, a variable-speed drive will increase its speed if the sensed
pressure differential is too low, or slow down if the pressure differential is too