Yet another fluid-circulation arrangement requires the installation of a two-way valve at each air-handling unit to modulate flow through the coil. In contrast to the constant-flow three-way valve system, the two-way valve delivers a variable volume of water to the coils, based on load. This means the volume of fluid provided to the building loop by the chiller(s) varies.
Most chillers can’t be operated at a variable flow rate, so the piping must be arranged to bypass that volume not required in the loop, back to the return main. Since a constant-pressure differential must be maintained between the building supply and return mains (the two-way valves at each branch operate best at a specific fluid pressure, so the system is designed to accommodate this—similar to the maintenance of constant pressure in a VAV air-distribution system), the bypass valve modulates flow to keep a differential pressure sensor in the most distant part of the loop from the pump reading constant.
As the building load increases, the flow to the building loop increases, and so the flow through the bypass leg diminishes. (As the bypass flow increases, the load is diminishing.) If the bypass leg is sized for one chiller, when the bypass flow approaches 100 percent it’s time to take one chiller off-line, and vice versa for an increasing building load: When bypass flow drops below 10 percent, another chiller is brought automatically on-line.
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Excerpt from Energy Conservation Projects.