Many existing chilled water plants are designed using primary-secondary pumping where one set of pumps circulates water at the chilled water plant and then another set of pumps circulates water to the building or buildings. This is common in larger buildings or campuses where the many spaces or buildings are served by one system, and allows the flow to be variable to the zones. Many newer chillers allow variable flow through the chiller with increasingly smaller minimum flow rates, thus saving energy. Many older chillers will not allow variable flow or have high minimum flow rates, requiring more flow and more energy. Separating the pumping to the zones allows the flow to be variable and the pressure that a single pump needs to overcome to be less.
These existing primary-secondary plants may find their chilled water supply temperatures to the zones continue to rise above the leaving temperature of the individual chillers. This is a situation commonly known as being “water logged”. This is indicative of the secondary chilled water flow being greater than the primary water flow at the chiller. When this happens, control of the leaving water temperature is lost because the plant leaving chilled water temperature becomes a mixed temperature of the leaving primary chilled water temperature and the returning secondary temperature. This may be a result a number of variables such as: three-way control valves on the secondary system bypassing too much supply water to return; coils selected for a small temperature rise on the secondary side; or improper chiller sequencing. Solutions to solve this problem can be varied but involve finding ways to either lower the secondary chilled water or raise the primary flow rate. Consider trending your chilled water supply and return temperatures on both the primary loop of the chillers and the secondary loop of the building to see if the temperatures are consistent over time. Water logging of the system will lead to increased energy use, as the zone itself is not getting the optimum water temperature that the plant is producing.
If you think this may be occurring at various times of the year in your plant, or if you would like assistance with an upcoming project, please contact Joel R. Altland, PE at jra@ba-inc.com or (717) 845-7654 and we can help you correct the situation. After all…We Make Buildings Work!