Plumbing Engineer October 2021/57
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Specified in a wide range of applications: - Parking garages - Retail/Mixed-use - Hospitality - Healthcare - Industrial Pedestrian to forklift traffic
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Tough, lightweight channels Solid joint connections Chemical-resistant
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cool-down times for copper pipe. The starting temperature was 135 F and the ambient air temperature surrounding the pipe was 65 F to 70 F, an average 67.5 F. Both the 1/2-inch and 3/4-inch uninsulated pipe cooled down to 105 F in less than 25 minutes, the upper end of the high Legionella growth zone. Pipe insulation with a wall thickness of 3/4-inch doubled the cool-down time of 1/2-inch pipe to 40 minutes and almost tripled the cool-down time for 3/4-inch pipe. It will take longer to cool down to 85 F, the lower end of the high Legionella growth zone and still longer to reach ambient temperature. It will take roughly an hour for the uninsu- lated piping to get below 85 F and two to three hours for the insulated piping. While keeping the drop legs hot has the beneficial effect of keeping the water hot for users of the fixtures on each drop leg when the events are clustered close together, it also has the detrimental effect of maintaining the temperature of the water in the drop legs within the high growth range for Legionella for much longer periods of time. But what happens if the drop legs never cool down? Hiller's research examined the cool-down effects on pip- ing that did not have a constant source of heat at the beginning of a drop leg. ASHRAE Guideline 12 2020 recom- mends that if using temperature as the sole method of Legionella control, temperatures need to be continuously maintained above 120 F in all parts of the system. In large buildings, the water heater is generally located in a mechanical room far from the fixtures. To bring the source of hot water closer to the fix- tures, there is often a circulation loop or an electrically heat-traced pipe, either of which provides a constant source of heat at the beginning of the drop legs. This temperature gradient will con- tinue to cause heat to leak into the drop legs due to conduction and convection, effectively keeping a section of pipe on each drop leg within the high growth range for Legionella 24/7. Starting with higher temperatures entering the hot water supply piping increases the temperature gradient and increases the time within the high growth zone. Higher distribution tem- peratures do not reduce the risks; they cause the risky conditions to last longer. Future articles in this series will explore more ways to simultaneously minimize the energy losses in hot water distribution systems and the risks for Legionella growth. We also will exam- ine the effects of several key compo- nents on Legionella growth. O Tim Keane is a consulting engineer with Legionella Risk Management. design, operation and maintenance issues related to Legionella and other waterborne pathogens. Gary Klein, president of Gary Klein & Associates, is intimately involved in energy efficiency and renewable ener- gy, with an emphasis on the water- energy-carbon connection.
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