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50 Chiller System Design and Control SYS-APM001-EN
System Configurations
Figure 33. Tertiary pumping arrangement
Decoupled system–principle of operation
At the tee connecting the supply and bypass lines, a supply–demand
relationship exists, as shown in Figure 34. Think of the total flow rate from all
operating pump–chiller pairs as supply. Demand is the distribution system
flow required to meet loads. Whenever supply and demand flows are
unequal, water will flow into, or out of, the bypass line. Flow can be sensed
directly or inferred from the bypass-water temperature.
An inadequate supply to meet demand causes return water to flow out of the
bypass leg of the tee and into the distribution system. The mixture of chiller-
supply water and warm system-return water, then, flows into the distribution
loop. Supply-water temperature control is compromised when this happens.
If the bypass-line flow into the supply tee (Figure 34) can be sensed, its
presence can be used to energize another pump-chiller pair. The increase in
supply water flow from the additional pump changes the supply–demand
relationship at the tee, eliminating return-water mixing. As long as return-
water mixing does not occur at the supply tee, no additional chiller capacity is
required. When mixing does occur, an additional chiller may be needed,
depending on the amount of mixing that can be tolerated.
Much of the time, supply exceeds demand and the surplus flows to the return
tee. If a chiller pump is stopped prematurely, the bypass-line flow will again
Differential
Pressure
Transmitter
Variable-Speed
Drive
Check Valve
Chiller 2
Chiller 1
Loads
Loads
Control
Valve
Excess
Inadequate
Supply
(Production)
Demand
Supply
Supply
Return
Bypass
Figure 34. Decoupled system supply
tee
(Distribution)