Consulting-Specifying Engineer - April 2008 - (Page 32) On the supply side, my calculations indicated that the numbers had the potential to be a bit more significant: The seemingly minor difference in pressure drop of 0.19-ft water column between the piping associated with the two cooling tower cells translates to a potential level difference of 2.25-in. water column in a cooling tower basin where the difference between the operating level and beginning to overflow is 3 in. We will take a closer look at this in the next section. Cooling tower level control Level control issues due to imbalanced supply piping configurations in multiple cell cooling towers are not news to the design community. Multiple cell cooling towers frequently are equipped with equalizer lines that interconnect the basins. These lines provide a path between basins so the water that piles up in the basin with the longer piping run can simply move to the other basin and equalize the levels. But, because the equalizer is a real pipe with real water flowing through it, there will be a pressure drop due to the flow that occurs through it and, as a result, there still is the potential for a level difference between basins. The cooling towers on the Doe library were provided with a 6-in. equalizer line. During my calculations, I discovered that the equalizer as furnished and installed should be able to accommodate any potential level differences that might be generated by the piping differences on the supply side. I did the math because when the towers were started up, there were level control problems. Specifically: • The makeup valve in one cell tended to be open—making up water—while the other cell was losing water through its overflow connection. • On start-up, the pumps pulled the tower level down to the point where both makeup valves opened. But, when the pumps shut down, the basins overflowed. These operating issues translated to unnecessary water and water treatment chemical consumption. Because the provided equalizer was smaller than what was contractually specified for the tower blame was pointed at it and the contractor who supplied the tower. Having eliminated the equalizer line as the potential culprit, the commissioning team was still faced resolving the level control problem. It turns out that there are a lot of issues that could come into play and impact the operating level in the tower basins. 1. The obvious place to start is with the adjustment of makeup valves. For the Doe library towers, the difference between the Input #218 at csemag.com/quickResponse 32 Consulting-Specifying Engineer • APRIL 2008 http://www.mtecorp.com http://www.mtecorp.com http://csemag.com/quickResponse
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