Civil Engineering Reference
In-Depth Information
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50% Cooling load 100%
Figure 9.16
Near optimal chilled water supply temperature vs. building cooling load
in chilling systems with variable-speed chilled water pumps.
more sophisticated. Figure 9.15 shows the relations between the chilled water
supply temperature and the power consumption in variable water volume
systems. In variable water volume systems, the optimal set-point is the best
temperature set-point at a certain fixed setting of a chilled water loop. An
optimal set-point exists at a certain load and working condition as illustrated
by Figure 9.15, which changes as the load or working condition changes.
As optimizing the chilled water supply temperature set-point in systems
with variable speed chilled water pumps is not an easy job in practical appli-
cations, some simplified methods have been proposed. For example, Braun
(ASHRAE 2007) proposed a near-optimal control for the chilled water sup-
ply temperature set-point as illustrated in Figure 9.16. The control is based
on the observation that the optimal set-point of a building's central chilling
system decreases as the total building cooling load increases and the relation
between them is approximately linear. Such numerical relation is different for
different buildings and needs to be identified by site-monitoring or tests for
particular buildings. It may be noted that, for chilling systems using cooling
towers, the line moves up when the wet-bulb temperature increases.
9.6.2 Constant water volume systems
In constant water volume systems, the situation is much simpler. The set-
point of the chilled water supply temperature should be set as high as possible
to satisfy the cooling needs of all user terminals as the consumption of the
pumps is almost constant and the optimization is simply to minimize the
power consumption of chillers.
A cooling load following control can be used to determine the optimal
chilled water supply temperature, which can be rather easily implemented in
chilling systems whose BASs are integrated with the digital control stations
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