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Chilled-Water System Variations
SYS-APM001-EN Chiller System Design and Control 73
body of water. Flow rates need to be carefully selected to balance the
economic and environmental requirements.
Preferential Loading
Preferential loading is desirable for systems that use heat recovery or free
cooling to allow the equipment used in these schemes to remain more fully
loaded. In a heat recovery system, a more heavily loaded heat recovery
chiller produces more heat that can be recovered for the desired process.
Similarly, if the condenser free cooling method is preferentially loaded in the
sidestream arrangement discussed below, it receives the warmest return
water temperature and thus continues to operate as load and/or condenser
water temperature increases.
Preferential loading may also be beneficial for use with either a high-
efficiency chiller that should be fully loaded whenever possible, or a chiller
using a fuel other than electricity, such as an absorption chiller using waste
steam from a cogeneration plant, or a chiller coupled with an engine that
generates electricity. When used with the latter, the system is able to
preferentially load the alternate fuel chiller when the cost of electricity is high.
Preferential loading - parallel arrangement
If a chiller in a decoupled system is moved to the distribution side of the
bypass line in a primary-secondary system, due to system hydraulics and
temperatures, the chiller is preferentially loaded when it is turned on. As
shown in Figure 44, Chiller 1 always receives the warmest system water and
is preferentially loaded. As previously discussed, chillers on the production
side of the bypass line (Chillers 2 and 3) are loaded to equal percentages. The
parallel preferential arrangement works best if Chiller 1 is capable of creating
the desired system supply water temperature, as it will be sending chilled
water directly to the distribution system.
Figure 44. Parallel preferential loading arrangement
Equal
Percentage
Loading
Preferential
Loading
Chiller 3
Chiller 2
Chiller 1
Bypass Line
Production
Distribution