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Thermal Stratification Of A Hot Water Tank

1410 words - 6 pages

For many years, the standard for residential household hot water systems has been an oil, gas, or electrical hot water tank that provides a reliable supply of water at a set temperature. The temperature gradient within the tank was thought to have little significance, as long as the output temperature was constant. It wasn’t until Sharp and Loehrke’s report [36] on the topic was published in 1979 followed by Hollands and Lightstone [37] ten years later that the potential rewards of thermal stratification became apparent. Thermal stratification of a hot water tank entails maintaining the temperature in the upper region at an elevated temperature for use as domestic hot water, while the ...view middle of the document...

In the months following the convention, over 20 attendees died from Legionella infections and 155 were hospitalized [23]. Since then, numerous tests and experiments have been conducted around the world leading to a marked increase in available information concerning Legionnaire’s disease and its prevention. Upon the emergence of thermally stratified solar heated water tanks, the scientific community was not in agreement on which Legionella prevention method was most energy efficient and effective. The aim of this review is to compare the results of experiments, tests, and policies from the past 30 years to determine ideal methods for protecting solar heated water systems from Legionella infestation without using excess energy or endangering the public.

Review of Studies on the Prevention of Legionella Growth

Studies on Legionella detection and prevention began in the years following the major outbreak at the 1976 convention. This work has focused on Legionella contamination in hospitals, as hospital patients often have compromised immune systems and are thus more susceptible to Legionella infection. Tests at hospitals around the world showed surprisingly elevated levels of Legionella contamination in their hot water systems, with contamination percentages above 50% not uncommon [16, 28]. The large and complex nature of hospital hot water systems makes proper cleaning and maintenance difficult, further encouraging the growth of Legionella cultures. As previously mentioned, Legionella thrives in stagnant and sediment-laden water between 25ºC and 45ºC. The area of hot water tanks below the lower element is the most stagnant, the most sediment-laden, and the most likely to lie within the range of 25ºC and 45ºC [7,23]. It was thus no surprise when the highest concentrations of Legionella were found in the bottom of hospitals’ hot water tanks. Research papers on the topic of Legionella detection and prevention began appearing in the scientific community as early as 1987, 11 years after the initial outbreak.

In 1987 a team led by P. Muraca [10] published a paper which explored the efficacy of chlorine, heat, ozone, and UV light for killing Legionella pneumophila using a model plumbing system. The two most effective methods of Legionella eradication studied were found to be heat and UV light. Heat eradication at 77ºC had a slight delay prior to an exponential decay in Legionella cultures (delay explained time required for system to exceed 45ºC), achieving a log 7 reduction after 150 minutes (99.99999% reduction). UV light had a more immediate effect, but was unable to inactivate all Legionella, reaching a limit of log 5 reduction after 20 minutes (99.999% reduction). It should also be noted that the rate of exponential decay of Legionella during the UV trials was far greater than that of heat eradication, reaching a log 4 reduction in the first 10 minutes, whereas the exponential decay during the heat trials reached a maximum of log 1 per 20 minute...

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