From the early years of HVAC design, the use of CHW to transfer heat from areas of higher loads (e.g., building loads at air handler coils, or industrial equipment loads at heat exchangers) to a condensing water loop or a refrigeration system for heat rejection has been successful. In a very broad sense, a CHW system consists of the following components:
The CHW portion of the system circulates and flows between the chiller and the building loads through pumping by the CHW pump (although dependent upon the system, usually referred to as the primary pump), and can be operated as constant flow or variable flow. For water-cooled chillers, a condenser water loop is necessary, and always operates when the chiller is energized to operate. This loop also requires a condenser water pump to circulate the CW through the piping between the chiller and the cooling tower or heat rejection device (radiator or closed circuit cooler). The CW system has traditionally been a constant flow (CF) system, but recently designs have included variable flow (VF) in this system as well. Any variable flow application (CHW or CW) increases the intricacy of the design, construction, and operation of a system, but at times of low load and corresponding reduced flow rate requirement, may offer significant pump energy savings. Decisions regarding constant and variable system flows dictate designs typically referred to as primary/secondary (PS) and variable primary (VP) system designs.
Selecting a CF versus a VF system requires many considerations during the design effort. As with any design, the designers of a CHW system should consider various options and equipment through discussions with the owner, and recommend one or more of these options to meet the project goals and performance requirements. Among the many important items to consider regarding these system designs are any system constructability and budgetary constraints, system operability, operations and maintenance costs, and energy consumption costs.
Depending on the size of the building and the related cooling loads necessary to cool and dehumidify the building’s airstreams or other processes where some form of cooling is needed, the CHW system may have more than one of the larger components mentioned (chillers, cooling towers, pumps), and may be independent from nearby surrounding buildings. Or the building may have some combination of CHW distribution piping systems connected to a larger thermal utility network that serves several buildings simultaneously from a large, remote central plant arrangement.
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