Commissioning on campus I enjoyed reading the article “Commissioning on Campus” (CSE 04/08, page 27). The real-life scenarios especially piqued my interest. The author is correct in pointing out how the contractor's version of the design drawings would disturb the flow to the cooling towers, but on careful examination of the photo in Figure 1, a problem of equal or greater import ...
Commissioning on campus
I enjoyed reading the article “Commissioning on Campus” (CSE 04/08, page 27). The real-life scenarios especially piqued my interest.
The author is correct in pointing out how the contractor’s version of the design drawings would disturb the flow to the cooling towers, but on careful examination of the photo in Figure 1, a problem of equal or greater import is evident.
If we assume that each tower is supplied by an 8-in. line and the combined flow to the towers requires a 12-in. main, then the tee installed as shown is the wrong size. The tee should have been installed as a 12x12x8-in. tee with the reducer on the downstream side of the run. At the point where the combined flow splits, the cross-sectional area needs to be able to support the flow of two towers. Otherwise, the turbulence and resultant pressure drop would cause significant additional imbalance to the flow. If the installed configuration were allowed to remain, the full flow for two towers would be squeezed into an 8-in. tee. This just adds credence to the fact that engineers need to be diligent when making field observations.
Anthony J. Curiale , CPD, LEED AP Associate Principal Buro Happold Consulting Engineers New York
Author responds:
Anthony Curiale’s point is well taken, but in this particular instance, the lines are generously sized. Total flow is about 492 gpm, and each branch is designed for about 50% of that capacity, so the friction rates are pretty low. In the table that accompanies the article, I contrast the losses at the design flow rate with the limiting condition, which I arbitrarily chose as a flow rate that would create a friction rate in the range of 4-ft water column/100-in. in a 6-in. line.
The reality was that the designer specified and had installed towers with oversized outlets (8 in.) and the plan view drawings indicate that the entire return system should have been run at that size. A schematic shows the tower outlets reducing to 6-in. branches coming into an 8-in. tee with an increaser (or reducer, depending on whether you are an optimist or pessimist) on the run of the tee, but not the branch. So that would imply some sort of reducing tee. I suspect if we had to argue the case, we could have changed everything based on the drawings. But, because it was already there, and because the system could be made to work as installed, we focused on changes where there were more benefits to be realized. Still, it was frustrating to have it all laid out for the best of worlds and not installed that way—as opposed to having a set of documents where a nice arrangement is not so clearly defined.
At any rate, given the relatively low flow rate for the line size and due to space constraints on my article, I did not elaborate on the size of the tee. But, as Mr. Curiale points out, a better practice would have been for the installer to increase the line size and put the tee in at the size of the main leaving it rather than the branch entering it. The only reason it wasn’t an issue was because of the generous sizing of the piping system. I suspect, knowing the designer, that the line sizes were based on some lifecycle cost factors, given the nature of the facility and the associated design life. Large line sizes translate to lower pumping energy requirements in a system that must run around-the-clock. In addition, they make the system more forgiving of field issues such as the one that is discussed in the article.
David Sellers , PESenior EngineerFacility Dynamics EngineeringPortland, Ore.
Clarification
In the Case Study department in the May ( CSE 05/08) issue, “Academic Center schools in electrical distribution” (page 64), CSE incorrectly identified Square D as a division of Schneider Electric North American Operating Division. Square D is a Schneider Electric brand of electrical distribution equipment.
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