Content area
Full text
ABSTRACT
Pump differential pressure can be continuously optimized based on the position of the control valve. Striving to keep one valve almost completely open at all times minimizes the pumping energy. While this may cause some valve hunting, the overall performance of the chilled water system remains good.
INTRODUCTION
ASHRAE 90.1-1999 (ASHRAE 1999) requires that for systems with direct digital control of individual zone boxes reporting to the central building automation system, the static pressure setpoint shall be reset based on the zone requiring the most pressure. The static pressure is reset so that one damper in a zone is nearly wide open. A similar energy-saving method is possible with chilled water systems. Thomas B. Hartman has suggested this control scheme (Hartman 1993, 1996; Hartman et al. 1996). This paper concerns our experience with using differential pressure reset for pump speed control on the secondary chilled water systems at an insurance company's facility.
THE FACILITY
Corporate South is a 3,000,000 ft^sup 2^ (279,000 m^sup 2^) office complex in Bloomington, Illinois. The complex was constructed in two phases. The first phase was completed in 1996 and is approximately 2.1 million ft^sup 2^ (195,000 m^sup 2^). It includes buildings SG and G through M. The second phase is an addition completed in 2001 and is approximately 900,000 ft^sup 2^ (84,000 m^sup 2^). It includes buildings N, P, and R. Seven fan systems supplied by three pump sets provide cooling for phase II. The entire facility is a major corporate headquarters and support area and requires redundancy to provide 7-by-24-hour support to the zone operations centers, data centers, call centers, and field claims offices. The facility also includes 250,000 ft^sup 2^ (23,225 m^sup 2^) of critical data processing and at least an equal area of programming support.
Chilled water for air conditioning is supplied from two remotely located physical plants. These physical plants are located approximately a quarter of a mile from the campus and from each other. The North Plant has two 2080 ton (7300 kW) chillers and a 4,000,000 gallon (15,000 m^sup 3^) thermal storage tank. The south plant has three 2080 ton (7300 kW) chillers and also has a 4,000,000 gallon (15,000 m^sup 3^) thermal storage tank. The chilled water supply temperature is 40[degrees]F...