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84 TRG-TRC016-EN
notes
period four
Chiller-Plant Control
In a primary-secondary system, the direction and quantity of flow in the bypass
pipe is an excellent indicator of when to turn a chiller on or off . As discussed in
Period Two, the water flow in the bypass pipe can be measured directly using a
flow meter, or indirectly by measuring system water temperatures and applying
flow-mixing equations. The rules applied to the bypass flow to determine when
to turn a chiller on and off are:
n When there is a deficit flow, a chiller may be added.
n When there is excess flow greater than that of the next chiller to be turned
off, plus a 10 to 15 percent safety factor, that chiller may be turned off.
n If neither of the above conditions exists, do nothing.
As an alternative to measuring flow in a primary-secondary system with four or
less chillers, system supply and chiller-plant return-water temperatures may be
used to decide when to turn a chiller on or off. This is similar to the logic applied
to constant-flow systems. It is simple and has a low installed cost, but it is less
accurate than flow determination, especially as the number of chillers
increases.
“Low ∆T syndrome” can also affect the operation of primary-secondary
systems. Unlike constant-flow systems, the primary-secondary system will
maintain the required system flow and supply-water temperature, and therefore
maintain occupant comfort. However, it accomplishes this by turning on
additional chillers before all operating chillers are fully loaded. This may reduce
overall system efficiency.
Although some have proposed solutions such as putting a valve in the bypass
line, lowering the supply-water temperature, or controlling the system
differently, these are only band-aids that mask the actual problem and often
cause other operational difficulties. Fixing the root cause of low ∆T syndrome in
the distribution system is the best course of action for proper and efficient
system operation.
b
y
p
a
s
s
p
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p
e
b
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p
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load indicators
Flow
flow
flow
met er
met er
chiller
chiller
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plant
plant
controller
controller
Figure 92