Trane SYS-APM001-EN manual Decoupled system-principle of operation, Tertiary pumping arrangement

System Configurations

Figure 33. Tertiary pumping arrangement

Check Valve

Chiller 2

Chiller 1		Differential
	Variable-Speed
		Pressure
	Drive
		Transmitter

Loads

Control

Valve

Loads

Figure 34. Decoupled system supply

tee

ReturnSupply (Production)

Excess

Supply

Bypass

Inadequate

SupplyDemand

(Distribution)

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

50 Chiller System Design and Control SYS-APM001-EN