Stabilizing Chilled Water Distribution

Balancing of chilled water supply and return system has been a major backfire when operating a chilled water airconditioning system. Designer has applied "Reverse-Return" piping technique to equalize pressure at the begin branch line and the end branch line of the chilled water distribution. The technique is also used to equalize pressure on each terminal unit within each branch. After 2-way shut off control valve replaced conventional 3-way bypass control valve, engineer tend to design the chilled water branches with "Direct Return" piping in order to reduce cost and space requirement. "Variable Chilled Water Supply" with either "Primary-Secondary Pump" or "Bypass" has becoming a common design practice. 3 way bypass control valves might be used only at the end of line terminal unit to reduce pressure fluctuation and risk of water hammer.

Serious problems have been found based on imbalance of chilled water supply pressure, especially on large system and long piping distance. End of the line AHU lacked of flow and chilled water supply-return delta T did not comply with the design and normally return chilled water at lower temperature than design. Thus, the system does not reap the supply chilled water supply cooling energy effectively. The airconditioning system does not provide proper temperature and humidity that are too high and jeopardize the performance of airconditioning system.

We have pointed our finger at commissioning and balancing as the solution for the described problems. But less than 1 out of 100 projects that could perform such activity perfectly, due to many limitations, including immediate take over for soft opening, change of functions, new branches, new extensions, add on units, improper control of chilled water pump supply pressure, etc. Further than that, chilled water demand could vary from 10-120% of design flow. Therefore, system pressure has wide range of fluctuation. We can say that, most of the chilled water airconditioning system is "out of control". Meanwhile, there is higher trend of delivering cooling by chilled water, with more district cooling projects. Deliver cooling by chilled water is in fact more efficient than cool air. Since water as the cooling media is more effective, require less energy to transport and less leakage in the system. Advanced airconditioning uses more "Terminal unit" as "Personal airconditioner". New model might includes air purifier and have furniture look.

Stabilizing Chilled Water Distribution

เขียนโดย : เกชา ธีระโกเมน | 15 สิงหาคม 2551

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