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Commissioning Of Central Chilled Water Plants

I was asked to make a presentation on the topic of commissioning central chilled water plants at this year’s ASHRAE Annual Meeting in June. When brainstorming about what might be “special” about commissioning chiller plants compared to other systems, there wasn’t much from a process standpoint. That is, the commissioning process is the same whether you are commissioning HVAC, plumbing, electrical, security, or any other type of facility “system.”

One of the most important aspects of commissioning chiller plants, however, is the need to pay attention to the integrated “systems” associated with a new or modified chiller plant. Defining systems and the limits thereof is an important part of any commissioning plan, but it tends to be especially critical for chiller plants.

Individual pieces of equipment are not systems, e.g., chillers, cooling towers, pumps, heat exchangers, thermal storage tanks, etc. Central plant systems would include the chilled water system, the condenser water system, the chemical treatment systems, refrigerant detection and emergency ventilation system, etc., all of which are made up of multiple components. However, do the central plant systems stop at the chiller plant walls?

No, they don’t. The purpose of most central plants is to serve users, typically distributed cooling coils throughout a building or an entire campus of buildings. Therefore, the central chilled water system needs to include the individual users, and the Design Intent Document is the place to document the performance requirements of the central systems as defined to meet the needs of the users. There is not much sense in designing, building, and commissioning a central plant if the commissioned performance does not match the requirements of the users.

If the central plant is planned to serve existing building systems, this process of defining user requirements can be quite painstaking. It requires knowledge of each coil’s performance characteristics and the sequence for controlling its valve(s). It also requires knowing each building’s distribution system controls, i.e., are there any temperature or pressure control bypass loops, variable speed drive tertiary pumps, etc.? In the case of most campus settings, the answer to these questions varies from building-to-building and, often, coil-to-coil, depending on the vintage of the building system installations and the professionals who designed them.

Will the new plant be designed and controlled to serve the existing operational parameters of the buildings, or will the building systems need to be modified in order to work properly with the new plant configurations and controls? If the latter, the individual building system retrofits should be included in the central plant commissioning work. For example, if the central plant chillers are selected for a high-efficiency 14-18°F T, it is probably critical to the operation of the entire system that the coils served be capable of operating at those higher T’s. That may require providing lower chilled water supply temperatures and rebalancing (decreasing) the water flow to each coil originally designed for a lower chilled water supply temperatures and rebalancing (decreasing) the water flow to each coil originally designed for a lower T.

Clearly, if the central plant is being designed to serve an entirely new campus of buildings, the background work is less onerous but the importance of coordinating the building system designs with the central plant design cannot be underestimated. For example, if water-side free cooling is included, the year-round, relatively constant load chilled water coils in each building (i.e., those serving interior spaces without economizer control, such as computer room air conditioning units or process equipment) need to be designed for the warmer chilled water supply temperatures expected under the free cooling mode during the winter.

All of these performance requirements are perfect for inclusion in the Design Intent Document prepared prior to the commencement of any serious plant equipment sizing, selection, and layout. Central chiller plants cannot be designed and constructed in a vacuum, because a chiller plant is useless as a stand-alone monument. It needs to meet the needs of the building users in order to be considered successful. Once a comprehensive, customized Design Intent Document is prepared, the commissioning process proceeds exactly as it would for any other type of facility “system.”

 

COMMISSIONING: GETTING IT RIGHT

Engineered Systems, August, 2001

Rebecca Ellis, PE, LEED AP, CCP, CxA
President
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318
rteesmag@QSEng.com