To measure the energy use and economic benefits of variable primary flow chilled water systems, ARTI conducted a study that compared variable primary flow chilled water system energy use with that of other common system types including: constant flow/primary-only chilled water systems; constant primary flow/variable secondary flow chilled water systems; and primary/secondary chilled water systems with a check valve installed in the decoupler.

According to the ARTI study results, primary-only chilled water systems reduced the total annual plant energy by 3 to 8 percent, first cost by 4 to 8 percent, and life cycle cost by 3 to 5 percent relative to conventional constant primary flow/variable secondary flow chilled water systems. Several factors significantly influenced energy savings and economic benefits of the variable primary flow system relative to other system alternatives. These included the number of chillers, climate, and chilled water temperature differential.

"In view of both the state-of-the-art review and parametric study results obtained in this project, it can be concluded that variable primary flow is a feasible and potentially beneficial approach to chilled water pumping system design," notes the study. "However, the magnitude of energy and economic benefits varies considerably with the application and is obtained at the cost of more complex and possibly less stable system control. The literature on effective application of variable primary flow is growing and should promote its appropriate and effective use in the future."

To download a copy of the final report, visit the ARTI Web site at www.arti-21cr.org.

Publication date: 05/10/2004