Energy Efficient Eradication of Legionella in Hot Water Systems

University dissertation from Luleå : Luleå University of Technology

Abstract: Disease related to unsafe water, poor sanitation, and lack of hygiene is some of the most common causes of illness and death all around the world. Since the first detection of Legionella in Philadelphia 1976, Legionella is recognized to cause Legionellosis which is associated with two distinct forms: Legionnaires’ disease and Pontiac fever. The fact that vaccination against Legionella disease is not efficacious enhances the effort towards developing the existence disinfection methods and inventing new technologies. Re-colonization of Legionella in hot water systems may occur within a few days or weeks after disinfection since conventional disinfection methods significantly reduce but do not eliminate pathogens. Understanding the conditions favoring Legionella occurrence in hot and cold systems will aid in developing new treatment technologies that minimize or eliminate human exposure to legionella pathogens. The work introduces the Anti-Bact Heat Exchanger (ABHE) system as a new innovative system inspired by nature. Compared to conventional disinfection methods, the ABHE system proposed to achieve continuous thermal disinfection of bacteria in hot water systems and in simultaneously saving energy and reducing the required costs. Thermodynamic analysis, experimental test and simulation validation of the ABHE by the Engineering Equation Solver (EES)-based model were achieved to define the thermal performance of the ABHE system at given operation conditions. The experimental test shows high potential of recovering heat and thus saving energy by the ABHE system. In addition, pumping power (PP) was relatively small compared to the recovered heat which implies that less energy was required compared to the recovered heat. The effect of working parameters such as temperatures and flow rate on the thermal performance of the ABHE system was furthermore investigated. The study shows that supplied water temperature has similar effects as the disinfection temperature. Namely, increasing supplied water temperature enhances the regeneration ratio (RR) but it requires a large plate heat exchanger (PHE) area and PP. On the contrary, increasing the temperature in use results in a reduced PHE area and PP. Flow rate has the greatest influence on the thermal performance of the ABHE system. Increasing flow rate leads to an increase in the required area of the PHE. The EES-based model investigated the effect of the length and the width of the plates used in the PHE on the RR and the required area of the PHE. Then, the EES-based model was used to optimize the ABHE system in which the PHE area is minimized or the RR of the ABHE system is maximized.

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