Trane SYS-APM001-EN manual System Configurations Distribution

Figure 30. Distribution loop

Return	Supply
Bypass Line
Loads
Distribution

Figure 31. Example pump curve

Pump Curve

B

A

Head

Flow

System Configurations

Distribution

Distribution pumps take water from the supply water tee (point B in

Figure 29), push it through all the distribution piping and load terminals, and then on to the return water tee (point A in Figure 29). This pump can (and should) allow variable flow.

By itself, the distribution system is easy to understand. Figure 30 shows a simplified distribution system consisting of multiple cooling coils, each controlled by a valve that regulates the flow in its respective coil. In this case, the flow control valves should not be three-way because a constant flow is not desired. Instead, two-way modulating valves are used. As the aggregate loads change system flow, a constant speed pump would “ride” its flow-rate versus head-pressure relationship. This means that in response to the change of flow required, the pump will find a new equilibrium point along its operating curve (move from point A to point B in Figure 31).

Alternatively, multiple pumps or variable-speed pumps can be used to limit the dynamic pumping head, similar to VAV fan control. Properly designed, part-load pumping power can approach the theoretical cubic relationship to flow, thus reducing energy consumption significantly. Today, most decoupled systems use a variable-speed drive on the distribution pump, and it may be required by the applicable energy code.

A common strategy for operating the variable speed pump is to adjust the speed of the pump’s motor to create a sufficient differential pressure, ΔP, at one or more critical points in the system, as shown in Figure 33. This pressure difference tends to decrease when the air-handler control valves open in response to increasing loads. To restore the ΔP across the system, the pump controller increases the speed of the pump. Conversely, when the air-handler control valves close in response to decreased coil loads, the pump controller slows the pump speed to maintain the target ΔP.

Distribution-loop benefits of decoupled system arrangement

The distribution system benefits from the ability to accommodate load diversity, the fact that system flow is variable, and (in a properly operating system) the fact that return water is maintained at temperatures near design. The last assumption is discussed further in “Low ΔT syndrome” on page 79.

Load diversity. Not all chilled-water loads peak simultaneously. Therefore, the quantity of water that flows at any one time is reduced from the “sum of the peaks” load that would be required in a constant-flow distribution loop. This presents the possibility of reducing chiller, pump, and pipe sizes significantly.

Variable flow. Because two-way control valves are used on the cooling coils, only the water that the loads actually use is pumped. Most of the time, this means a significantly reduced flow rate, accompanied by an even more significant reduction in pumping energy.

48 Chiller System Design and Control SYS-APM001-EN