Ashrae Journal - October 2008 - (Page 24) Sizing Pipe Using Life-Cycle Costs By Steven T. Taylor, P Fellow ASHRAE; and Molly McGuire, P Member ASHRAE .E., .E., H VAC piping is typically sized using rules-of-thumb such as maximum friction rate or velocity, or a combination of the two. Figure 1 shows a pipe sizing chart for Schedule 40 steel pipe1 where friction rate is limited to 4 ft per 100 ft (1 m per 30 m) and velocity is limited to 10 fps (3 m/s) as indicated by the red line. Charts like these are generally based on tradition more than objective analysis; in fact, Figure 1 was used for pipe sizing by one of the authors for many years and was passed to him from his boss who got it from his boss. Once pipes are sized, designers must determine total pump head so that a pump can be selected. Head can be determined accurately using network piping software but inputting the required data can be time consuming and prone to errors. It is not uncommon 24 ASHRAE Journal for pump head to be determined using guesstimates based on past experience or minimal calculations. Fortunately, a free spreadsheet2 has been developed as part of the CoolTools™ Chilled Water Plant Design Guide, funded by California utility ashrae.org customers through Energy Design Resources (www.energydesignresources. com). The spreadsheet provides these basic functions: • Pipe sizing based on a balance between first costs and future energy costs, with optional velocity limits for erosion and noise generation; and • Pump head calculations including all fittings, valves, and devices. The spreadsheet is fast and easy to use—calculations for most systems can be done in a few hours. About the Authors Steven T. Taylor, P .E., is a principal and Molly McGuire, P .E., is a mechanical engineer at Taylor Engineering, a consulting engineering firm in Alameda, Calif. October 2008 http://www.energydesignresources.com http://www.energydesignresources.com http://www.ashrae.org
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