Consulting-Specifying Engineer - April 2008 - (Page 30) Location Critical issue Pressure drop for each branch if flow was equal (ft/water column) Design flow—246 gpm per branch Limiting condition (max flow for line size) — 825 gpm per branch Potential difference in flow for each branch Potential difference in level, inches (feet converted to inches) Return (water back from chiller) Water distribution to tower cells Run of tee 0.11 1.15 37 Branch of tee 0.20 2.18 144 Supply (water to chiller) Basin level control Run of tee 0.22 2.43 37 2.21 25.61 Branch of tee 0.41 4.57 144 Design limiting condition Table 1: Losses through the piping network on the library project with the project’s design flow and at the limiting flow rate for the line size2. Source: David Sellers branch, which is equal to the pressure drop due to flow in the constrained direction. Two questions remain: • How much of a difference in flow will it take for things to come into balance? • Will this difference in flow result in operational or performance issues with the tower? For the cooling tower under discussion, the answer to the “how much of a difference in flow” question is related to the line size and the flow in the line. Specifically, if the flow rate in the line is at the low end of its application range, the losses will be lower and the issues created by the non-symmetrical arrangement will be minimal. On the other hand, if the flow is at the upper limit of the application range for the line, the problems can be significant. The answer to the possibility of “operational or performance issues” question concerns location. If the non-symmetrical piping is on the return line—the water coming back from the chillers to the tower—then it will impact the uniformity of flow distribution to the hot basins in the cooling tower cells. This in turn will directly impact tower performance and indirectly impact the level in the cold basin. In contrast, if the non-symmetric piping is on the supply line—the water leaving the towers to go to the chillers—then it will impact the level in the basins. The flow/loss relationship is not linear—rather the losses generally vary with the square of the flow. In other words, doubling the flow in a piping network with unequal pressure drops through the various branches, which are causing a 1-in. difference in cooling tower cold-water-basin level at the original flow rate, has the potential to create a 4-in. difference in level, all other things being equal. If the piping branches serving the tower basins were identical from a pressure drop standpoint, then there would be no difference in flow through them and thus no potential for generating a level difference. On the return side, the non-symmetry is likely a non-issue for the library project (see Table 1). This is good because, as often is the case with new construction, the speed of the field staff exceeded the speed of the paper chain that transmitted my observations to the designer. Figure 3: The piping for the Doe library project. Source: David Sellers By the time my concerns reached the design team, the nonsymmetric piping arrangement (see Figure 3) had been installed. Thus, we decided to take a “wait and see” attitude. In accordance with a maxim often stated of Jay Santos, PE, as part of his training courses, our field engineers verified that “nature doesn’t lie” during the early phases of the commissioning project when their functional testing showed that the flow distribution in the hot water basins was not significantly impacted by the piping arrangement under any operating conditions. 30 Consulting-Specifying Engineer • APRIL 2008
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