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78 Chiller System Design and Control SYS-APM001-EN
Chilled-Water System Variations
series. The left half of Figure 50 shows a modularized configuration where
series chiller modules are placed in parallel with each other, so that any
upstream chiller’s valves could be “paired” with virtually any downstream
chiller by opening the appropriate valves. The condenser side in a
counterflow arrangement is shown in the right half of Figure 50. The
advantages of this system for large chilled water systems include highest
efficiency, scalability as the project grows, and high redundancy without a
significant investment in extra equipment.
Figure 50. Series arrangement of evaporators and condensers
Unequal Chiller Sizing
Many designers seem to default to using the same capacity chillers within a
chilled-water plant.
25, 26
There are benefits to using unequally-sized chillers
to meet the system loads. One is that when a chiller is brought online, 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 percent of system capacity,
the other at 40 percent) or 1/3–2/3 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 by
eliminating the operation of chillers and ancillary equipment for more hours
of the year. One caveat for variable-primary-flow systems is that if the
pressure drops are different across the unequally sized chillers, they will load
even more unequally without the use of pressure-reducing valves that
require extra pump energy, as shown in Table 15 on page 61.
Circuit 1
Circuit 2 Circuit 1
Circuit 2
37.0°F
[2.8°C]
55.0°F
[12.8°C]
98.9°F
[37.2°C]
3 Chiller Modules
85.0°F
[29.4°C]
Evaporators Condensers
Circuit 1 Circuit 2
Circuit 1 Circuit 2
45.1°F
[7.3°C]
3 Chiller Modules
91.3°F
[32.9°C]
Circuit 2 Circuit 1
Circuit 2 Circuit 1
Circuit 1 Circuit 2
Circuit 1 Circuit 2
Circuit 1 Circuit 2
Circuit 2 Circuit 1
Circuit 2 Circuit 1
Circuit 2 Circuit 1