VPF systems present building owners with several cost-saving benefits that are directly related to the pumps. (See Dispelling A Common Misconception for a look at the effect of VPF on chiller efficiency.) The most obvious cost savings result from eliminating the secondary distribution pump, which in turn avoids the expense incurred with the associated piping connections (material, labor), electrical service, and variable-frequency drive.

Building owners often cite pump-related energy savings as the reason that prompted them to install a VPF system. With the help of a software analysis tool such as System Analyzer™, TRACE®, or DOE-2, you can determine whether the anticipated energy savings justify the use of variable primary flow in a particular application. (See A Comparison Of Pump Operating Costs.)

It may also be easier to apply variable primary flow in an existing chilled-water plant. Unlike the "decoupled" design, the bypass can be positioned at various points in the chilled-water loop and an additional pump is unnecessary.

Caveats. While modern chiller controls have dramatically improved operating stability, the laws of physics still apply. To be successful, a VPF system must comply with the design rules identified below:

• Each chiller has a design operating range that is defined, in part, by minimum and maximum flow rates. The fact that there is a minimum flow rate necessitates a bypass somewhere in the chilled-water loop in case the required system flow falls below the minimum flow required by the chiller.

• A bypass is required whether the primary flow is constant or variable. (Chillers have minimum flow rates in either case.) The position of the smaller bypass required for a VPF system can be the same as the position of the bypass in a "decoupled" system. Some designers prefer, instead, to use three-way valves at the system coils. While this approach ensures minimum chiller flow, it also reduces the pump operating-cost savings by increasing the system flow.

• Measurements of evaporator flow are critical to VPF control. One common practice relies on direct measurement using flow meters. (Be aware that some flow meters require, but seldom receive, frequent calibration.) Another measures water-side pressure drop and estimates evaporator flow based on manufacturer data.

• Determining when to turn off a chiller requires a thorough understanding of system dynamics since flow rates will vary through every operating chiller. The already complex control strategy needed to avoid cycling (restarting the chiller too soon) becomes even more complicated as the number of chillers or chiller capacities increases.

• Fluctuating flow rates can be critical. For applications that require strict chilled-water temperature control, limit fluctuations to less than 10 percent of design chiller flow per minute.

  Relaxing this limit to 30 percent of design flow per minute is permissible in most comfort-cooling applications. If the chiller is properly controlled, the leaving-evaporator water temperature will not deviate from set point for more than a few minutes, if at all.

• Operators must understand how VPF systems work. Training is mandatory.

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