Ashrae Journal - October 2008 - (Page 8) In “Chiller Plant Optimization Tool” by Terrence Morris and Steve Blaine, P.E., in the July issue, the differential pressure sensor P-1 in Figure 1a, is shown at the discharge of the secondary pumps. Locating this sensor across the most Chiller Plant Optimization remote branch or controlling from the most demanding air handler valve will achieve more savings and may affect the results shown in Figure 5. It is unfortunate that the authors of this article did not also compare the savings of their model primary/secondary chilled water system to an all variable flow system. With some minor changes in the method of control, the reduction in energy consumption with an all variable flow system would be many times the 3% to 4% savings that their optimization program indicates. Depending on the geographical location and the chiller manufacturer and model, all variable flow chilled water systems generally consume 15% to 40% less energy than conventional primary/ secondary systems. This decrease occurs because it allows the chillers to operate in the “Max-Cap” range whenever the outdoor wet-bulb temperature is below design (about 98% of the time). In this range the chiller capacity goes up and the kW/ton goes down. Because the flow through the chillers in conventional primary/secondary systems cannot increase above design, there is no way to use the added capacity that operating in the MaxCap range provides. It’s time to move the primary/secondary design to the HVAC archives and to use more efficient systems. Gil Avery, P.E., Fellow/Life Member ASHRAE, Memphis, Tenn. The Author Responds The model described in the article was based on an existing plant and the goal of the model was to provide a tool that allowed operators to choose the most efficient method of operation. Variable primary flow is a proven approach for many chiller plants but is still fairly unusual in semiconductor facilities. The requirements for high reliability and very precise distribution water temperature control still leads many designers to choose a primary/secondary loop system. That being said, applying the model to a variable primary flow system could point out methods for achieving improved efficiency. The question of balancing the cost of operating the condenser system versus increased efficiency of the evaporators remains the same. Steve Blaine, P.E., Member ASHRAE, Portland, Ore. October 2008 www.info.hotims.com/16018-28 8 ASHRAE Journal http://www.est-aegis.com/aj2 http://www.est-aegis.com/aj2 http://www.info.hotims.com/16018-28
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