Heat exchanger development for waste water heat recovery
Degree GrantorUniversity of Canterbury
Degree NameMaster of Mechanical Engineering
Hot water plays an import role in modem life. The consumption of hot water represents a significant part of the nation's energy consumption. One way of reducing the energy consumption involved, and hence the cost of that energy, is to reclaim heat from the waste warm water that is discharged to the sewer each day. The potential for economic waste water heat recovery depends on both the quantity available and whether the quality fits the requirement of the heating load. To recover heat from waste water in residential and commercial buildings is hard to achieve in quality because of its low temperature range. Nevertheless, efforts to recycle this waste energy could result in significant energy savings. The objective of this research was to develop a multiple panel thermosyphon heat exchanger for a waste water heat recovery system. The advantage of the system proposed in this work is that it not only provides useful energy transfer during simultaneous flow of cold supply and warm drain water but also has the ability to store recovered energy at the bottom of a hot water storage tank for later use. While this concept is not new, the design of the heat exchanger proposed for the present study is significantly different from those used previously. Component experiments were carried out to determine the performance characteristics of a single thermosyphon panel. By changing the inclination angle of the single panel heat exchanger and varying its working condition, it was found that the inclination angle of 10° could be identified as the minimum inclination angle at which good performance was still obtained. The close values of the overall heat transfer coefficients between top surface of the panel insulated and both top and bottom surfaces of the panel uninsulated shows that the overall heat transfer coefficient of the single panel was dominated by the bottom surface of the panel. Even if in a worst case the top surface of the panel might be possibly covered by the deposits from the waste water, it would not affect much on the heat transfer performance of the panel. Measurements of hot water usage and waste water temperature and flow rates were obtained for a potential application of the proposed exchanger (the dishwasher for the kitchen in the University Halls of Residence). A model of a multi-panel thermosyphon heat exchanger was also developed to predict the energy savings that would be expected if such a heat exchanger was used in this situation. The result indicated that an overall electricity of 7500 kWh could be saved annually from the dishwasher system by employing a four-panel thermosyphon heat exchanger.