Primary/secondary/tertiary loops

A primary/secondary hot-water and chilled-water loop can increase energy efficiency in a number of ways. One way is to design the primary loop to accommodate the modular boiler/chiller arrangement that is recommended in the "right-sizing" approach in Chapter Three. In such a multiple-plant configuration, staging the boiler and chillers on and off allows flexibility in meeting load. This design includes a variable volume circulator, electric isolation valves, and DDC. Care must be exercised so that flow rate within the boilers and chillers does not go below minimum during their operation. Also see an alternative design approach that employs "all-variable speed chiller plants" as promoted by Hartman (2001). [Hartman, 2001] [Mayhew, 1993; Ciborowski and Pluemer, 1991]

The second way to increase energy efficiency is to replace with a variable volume pump any bypass valve that controls differential pressure in the secondary loop. The goal is to have a true primary/secondary piping loop arrangement that is hydraulically pressure independent, which allows different flow rates in each loop in order to minimize pump energy. Each secondary loop utilizes a variable frequency drive and two-way valves at each coil except the end coil, which uses a modulating three-way valve. "Thermal inertia problems" are eliminated when there is low demand for hot/chilled water because of the differential pressure control of this arrangement. [Mayhew, 1993; Ciborowski and Pluemer, 1991]

A third way that primary/secondary and tertiary loops save energy is that they can be operated at different temperature drops. Designing an increased temperature drop means that the pumping horsepower can decrease for a given amount of cooling/heating, saving energy. For instance, a primary loop may be sized for a 10°F temperature drop to allow production of low-temperature chilled water. However, secondary loops could be designed for a range of temperature drops from 12°F to 15°F or more, creating a medium-temperature chilled water loop. The secondary and tertiary loops operating at different temperatures can match their respective connected loads with optimized pumping power. [Mayhew, 1993; Ciborowski and Pluemer, 1991]


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