Trane SYS-APM001-EN manual Advantages of variable primary flow, Operational savings of VPF designs

System Configurations

Operational savings of VPF designs

Bahnfleth and Peyer15 discuss the operational savings of VPF designs. For many common systems, however, the primary pump power on which they base their assessment may be too high.

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•The bypass can be positioned either upstream or downstream of the cooling coils.

•A control valve in the bypass ensures that the amount of flow through the operating chiller(s) never falls below the minimum limit, but remains closed most of the time.

Notice that the VPF design adds a modulating control valve in the bypass line. At low loads, the bypass valve delivers the water necessary to maintain the minimum evaporator-flow limit of each operating chiller. By contrast, the bypass line in a primary-secondary system ensures constant chiller flow at all times.

A less obvious difference between variable and constant primary flow lies in system operation. In a primary-secondary system, a chiller and its primary pump typically operate in tandem. The VPF design can separate pump control (delivering enough water) from chiller sequencing (making the water cold enough).

Like the secondary pump in a primary-secondary system, the pumps in a typical VPF system operate to maintain a target differential pressure, ΔP, at a specific point in the system (Figure 37). 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.

Meanwhile, the plant controller stages the chillers on and off to match cooling capacity with system load. If the air handlers operate properly, the difference between the return- and supply-water temperatures, ΔT, remains nearly constant. Therefore, increasing the water flow through the chiller evaporators increases the load on the operating chillers.

Advantages of variable primary flow

The desire to make or save money lies at the heart of many of our decisions. In the context of HVAC design, decisions made to save money often involve a trade-off between acquisition expense and operating cost. If you can realize savings on both fronts, so much the better.

Perhaps this explains the increased interest in chilled water systems with VPF. VPF designs use fewer pumps and fewer piping connections than primary– secondary systems, which means fewer electrical lines and a smaller footprint for the plant. These factors reduce the initial cost of the chilled- water system, although the savings may be partially offset by additional costs for flow-monitoring and bypass flow (bypass line and control valve). VPF designs may also require more programming for system control than other designs.

As for operating costs, how much a VPF design saves depends on the pressure drops and efficiency of the pumps (see sidebar). A VPF design displaces the small, inefficient, low-head primary pumps used in primary–

Chiller System Design and Control SYS-APM001-EN