Courtesy: ESDĪs previously noted, variable-primary flow helps chillers fill in the gap between 10% to 30% load conditions. Variable-primary and variable-secondary systems can: Based on design and operation trends in the commercial sector over the past 10 years, variable-primary and variable-secondary systems (as well as tertiary systems for some applications) are recommended to simplify operations and adjust for future system variations. Independent secondary systems can fully use the CUP’s built-in thermal mass before initiating the chillers from the primary loop. The fluctuated flow range in primary loop helps the facility avoids the low-return water syndrome.įigure 2: Shown is a convential constant primary and variable-secondary system. A central plant with 7,000 to 10,000 refrigeration tons in commercial applications usually has complicated occupancy schedules and a large geometry complex. The industry has not yet widely recognized the practice of variable-primary and variable-secondary systems. Proper control of the distribution flow during the partial load condition between 10% and 40% should be carefully analyzed. Comparing the load and flow values reveals the range gap where load and flow will not fully follow a linear relationship.
In the case of centrifugal chillers with VFDs, a load can continue to drop to between 10% to 15% of design load. Flow needs to be maintained between 40% to 50% of the design flow to maintain a proper heat transfer rate. However, chillers need to maintain a minimum of 1.5 to 2 feet per second velocity to avoid laminar flow conditions in evaporator coils to maintain proper heat transfer factor. This allows the chillers to vary their load more easily from 10% to 100%. Modern CUPs are usually designed with centrifugal chillers with variable frequency drives. Figure 1: This illustration shows a typical variable-primary system with bypass control valve.
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