SYS-APM001-EN
Chilled-Water System Variations
58
Unequal chiller sizing
Many designers seem to default to using the same capacity chillers within a
chilled-water plant.
2,3
There are benefits to sizing chillers to meet the system
loads. One is that when a chiller is brought on line, so is its ancillary equipment,
thus increasing system energy consumption. In general, the smaller the chiller,
the smaller the ancillary equipment. Another is being able to ensure that
chillers are efficiently loaded. Many times this can be accomplished by using
chillers that do not have the same capacity. Examine the use of 60/40 splits (one
chiller at 60 percentage system capacity, the other at 40 percent) or 1/32/3
splits (one chiller at 1/3 of system capacity the other at 2/3). The benefit is that
the system load can be more closely matched with the total chiller capacity,
increasing total system efficiency.
Seriescounterflow application
Another system configuration that can be very energy efficient incorporates the
previously described series application, but does so for both the chilled water
and condenser water. Figure 23 shows such a configuration.
Note that the downstream machine, in this case, produces 40
°
F [4.4
°
C] chilled
water, while the upstream machine produces 50
°
F [10
°
C] chilled water. The
downstream machine receives 85
°
F [29.4
°
C] condenser water, while the
upstream machine receives 95
°
F [35
°
C] condenser water. Therefore, the
pumping requirements are only 1.2 gpm/ton on the chilled water side and 1.5
gpm/ton on the condenser water side--greatly reducing pumping costs.
The configuration has the effect of equalizing each chiller's required lift, as
shown in Figure 23a. The chilled- and condenser-water flow directions are
opposite, or counter, to one another. Thus the name, series-counterflow.
The seriescounterflow configuration
can work with all types of chillers.
Duplex chillers are shown in Figure 23.
A single large chiller may be built as a
duplex machine. The evaporator and
condenser water circuits are common
to both halves of the duplex. Increased
efficiency comes from the separation
of the compressors and refrigerant
circuits. Thus, each half is redundant
for increased system reliability.