Original Date: 03/17/1997
Revision Date: 01/18/2007
Information : Variable Speed Chilled Water Pump
Polaroid let its Core Engineering Services Group investigate the chilled water problems at its new production facility in New Bedford. Built by a mechanical engineering subcontractor, the system worked well under normal conditions, but failed during peak periods in the summer months when pumping demands increased. The heat load calculations confirmed the chillers were properly sized and large enough to handle the production requirements and the comfort cooling requirements of the facility. However, further investigation revealed that 800 feet of the main chilled water piping was improperly sized and causing excessive pressure losses and decreased chilled water flow (Figure 3-1). Polaroid’s engineers calculated that an 18-inch piping system was needed to adequately deliver the chilled water during peak conditions instead of the 12-inch supply and return system installed by the subcontractor.
The subcontractor suggested converting the two existing 12-inch pipes to supply lines and installing one new 18-inch pipe to serve as the return (Figure 3-2). The estimated installation cost for Polaroid was $400,000. The Core Engineering Services Group suggested installing a variable speed booster pump in the existing 12-inch system to handle peak conditions. Polaroid engineers argued that the system design for the 52°F dew point discharge temperature could be met by turning off the chilled water system 65% of the time. During the remaining 35% of the time, the chilled water system would operate on a graduated scale, beginning with minimum cooling needs during moderate ambient conditions (194 hours out of the year) and ending in 100% full load requirements (three hours out of the year). Polaroid engineers reasoned that the small duration of running the booster pump under peak conditions did not justify the payback costs of installing a new 18-inch pipe line. Polaroid management agreed there would not be sufficient payback for the new pipe line investment and concurred with the Polaroid engineers to install only the booster pump.
Installing a variable speed booster pump when capacity demand increases is a cost-effective alternative to retrofitting an entire piping system. By installing the variable speed pump at $77,945, Polaroid saved an avoidance cost of approximately $320,000.
Figure 3-1. New Bedford Chilled Water Line Pressure Drop
Figure 3-2. New Bedford Complex
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