Trane SYS-APM001-EN manual Guidelines for a successful VPF system, Chiller selection

System Configurations

Figure 39. Example of operating-cost savings for a VPF, single-chiller plant

Analysis results are based on a 50-ton scroll chiller and a 5-hp chilled water pump for two-story office building in St. Louis, Missouri.

Moderate “low ΔT syndrome” by manifolding the chilled water pumps

Manifolding two or more chilled water pumps (or slightly oversizing a single pump) can provide an individual chiller with more than its design chilled water flow… which means that you can fully load the chiller even if the return water temperature is colder than design.

Sometimes described as “overpumping,” this strategy does not cure “low ΔT syndrome”; it merely reduces the adverse effect of low ΔT on system operation.29 (Refer to “Low ΔT syndrome” on page 79). An example of low ΔT syndrome is receiving a depressed (for example, 49°F [9.4°C]) return- water temperature rather than the design (for example, 56°F [13.3°C]) return- water temperature.

Again, the only methods to load the chiller are to decrease the chilled-water leaving temperature or to increase the flow. If the pumping power and speed allows, the operator may be able to increase the chiller’s flow rate and capacity. Do not exceed the pump’s operating envelope.

Guidelines for a successful VPF system

Chiller selection

•Select for the lowest possible minimum evaporator-flow limit (no more than 40–60 percent of system flow)

•Select for the greatest tolerance to large flow-rate changes, while maintaining required temperature setpoint

•Select chillers with approximately equal pressure drops across the evaporator at the design flow rate

68 Chiller System Design and Control SYS-APM001-EN