ICR0207 International Congress of Refrigeration 2003, Washington, D.C. 1 ENERGY USE CHARACTERISTICS OF VARIABLE PRIMARY FLOW CHILLED WATER PUMPING SYSTEMS William P. BAHNFLETH, Eric B. PEYER* Dept. of Architectural Engineering, The Pennsylvania State University, 104 Engineering Unit A, University Park, PA 16802, USA, 814.863.2076, FAX: 814.863-4789, wbahnfleth@psu.edu *Grumman/Butkus Associates, Evanston, IL, USA ABSTRACT The use of variable primary flow pumping in chilled water systems is increasing due to its perceived potential to reduce energy consumption and initial cost. Neither the conditions under which significant energy savings are realized nor the likely magnitude of savings are well documented. To address this deficiency, a program of simulations was conducted that compared variable primary flow system energy use with that of other common system types. Parameters varied included load type, number of chillers in the central plant, temperature difference vs. part load characteristics, and climate. Results of these simulations show that for systems without temperature differential problems substantial reductions in chilled water pumping energy are possible. However, this benefit decreases as the number of chillers increases. The reduction in annual pumping energy due to variable primary flow is generally between 25 and 50 percent. Relative to total plant energy use, this is a reduction of 2 to 5 percent. INTRODUCTION In simplest conceptual terms, a chilled water system for comfort or process cooling is comprised of three subsystems: production, distribution, and use. Thermal loads on the use subsystem are met by cooling capacity created in the production subsystem and delivered by the distribution subsystem. In most systems, chilled water is produced by packaged chillers and utilized in coils and heat exchangers with water-side control by two-way or three-way valves. The distribution system is a network of piping through which the distribution medium, typically water or water-glycol solution, is transported by one or more levels of pumping. The principal objectives of chilled water pumping system selection and design are to provide the required cooling capacity to each load, to promote the efficient use of refrigeration capacity in the plant, and to minimize pump energy consumption subject to whatever budgetary constraints may apply. In the typical design process, such decisions are made on the basis of economic calculations. Accurate energy use prediction is an essential step in the development of the operating cost component of such analyses. (a) (b) Figure 1: Primary-only chilled water systems. (a) Constant flow. (b) Variable flow. The simplest chilled water pumping system has a single, primary, level of pumping that connects plants and loads via a single pumping circuit. When primary-only systems are pumped at constant flow rate, it is typical to have three-way control valves at loads (Figure 1a). When a primary-only system utilizes variable flow pumping, control