Variable primary flow can be economically appealing when designing a chilled-water system. We have also seen that the financial benefits depend on the appropriateness of the application and the care with which the system is designed, installed, and operated. As Steven T. Taylor, Principal, Taylor Engineering, aptly states:

"Because of their lower first costs and lower energy costs, variable-primary-flow systems are clearly the right choice for many applications, but not all. They require complex staging control sequences and minimum flow (bypass) controls, so the designer and operator of the system have to be more sophisticated if the system is to be a success. On some projects, the 'fail-safe' nature of primary-secondary systems may offset their energy and first-cost disadvantages."

With that in mind, we offer the following advice to help you determine when a VPF design is practical and how to assure a successful application...

Do not use VPF if:

• System chilled-water temperature is critical, for example, in a "clean-room" or process (computer-chip- making) application.

• Only three-way valves are used; that is, the system flow rate does not vary.

  A design with some three-way valves is permissible. In fact, if you can reach 60 percent of design flow, you will have saved nearly 80 percent of the pumping power.

• It is unlikely that the owner/operator will run the plant as designed.

• Existing chiller controls are old and inaccurate (a 30-year-old chiller with pneumatic controls, for example).

Consider VPF if:

• System flow can be reduced by at least 30 percent of design.

• It provides greater cost savings than a "decoupled" design.

• Operators will understand how the system works and will run it properly.

• The system can tolerate a modest variation in supply water temperature.

• A single chiller is being replaced and the primary flow can be varied. (Previously considered "too small," such applications may be ideal for VPF designs.)

During design and installation:

• Use energy-analysis software to determine the payback period or return-on-investment. Be sure to account for reduced piping and electrical connection costs.

• Remember to design a bypass into the system to provide minimum chiller flow.

• Size each chiller pump to accommodate the pressure drop of the system plus the evaporator.

• Specify high-quality flow meters if you decide to use these devices; they require less calibration and are more accurate. Also pipe the flow meters in accordance with the manufacturer's instructions.

• Invest the extra time needed to determine the control points at which chillers should be turned off. This design task is critical to successfully reduce operating costs...and its complexity will increase with the number (and differing sizes) of chillers.

• Teach the operating staff about their VPF system; how it operates and why it works as it does. Help the building owner design a program for ongoing training.

• Work with businesses that can supply and integrate the entire hydraulic system (chillers, pumps, and controls) during construction and actual operation. The financial benefits of a VPF design can only be obtained if the individual components operate as a system.

During plant operation:

• Maintain evaporator flow between the minimum and maximum limits. The cataloged range for many chillers is 3-11 fps (0.9-3.4 m/s). Ask the chiller manufacturer for the actual values for your machine.

• Keep changes in flow rate through the evaporator within manufacturer-set limits. With sophisticated chiller controls, a 30-percent-per-minute change in the rate of flow should work well in most applications.

• Continue to work with the operating staff and provide additional training as needed.

• Calibrate the flow meters when required (if used).

• Use the automation system to monitor system (chiller plus pump) performance. Document the operating costs.

Continue on to A Parting Thought... or return to the Table of Contents.