Technology Research On Heat Recovery Of Low Temperature Industrial Wastewater

Industrial waste heat resources are abundant in our country, which may cause the heat pollution and the energy waste. If they could be recycled and utilized fully, then the energy utilization efficiency would be improved significantly, which may play an important role in energy-saving and pollutant reduction. The heat pipe heat exchanger has the advantages of high heat transmission efficiency, simple structure, safety, reliability and so on. Therefore it has been widely used in the recovery and utilization of the industrial waste heat resources. But the research shows that the scaling problems of the heat exchanger, which are related to impurity substance of waste heat, result in the decrease of heat transfer efficiency and economic problems. In order to keep higher heat transfer performance during the operation, it is particularly urgent and important to optimize the heat exchanger structure and develop online cleaning method.In this paper, an experimental platform of heat pipe heat exchanger for recovering low temperature industrial waste heat was set up. And a special on-line mechanical cleaning device was devised, which could clean the heat pipe outer wall surface automatically. Heat transfer performance of the heat pipe heat exchanger was investigated with the evaluation indexes which consisted of six aspects: heat transfer rate, heat transfer coefficient, effectiveness, exergy efficiency, entranspy efficiency and entranspy dissipation number. Optimum operating method was proposed and descale effect of the on-line cleaning device was analyzed. The main conclusions were obtained as follows:The heat transfer rate and heat transfer cofficient increased with the increase of cold water volume flow rates at constant setting waste water volume flow. Meanwhile, heat transfer rate, heat transfer coefficient, exergy efficiency, entranspy efficiency and entranspy dissipation number increase with the increasing waste water volume flow rate at a constant cold water volme flow rate, but effectiveness decreased with the same operating conditions. Under the experimental conditions, the optimum cold and waste water mass flow rates were chosen as 3.000m3/h, 1.200m3/h, respectively.The heat transfer rate and heat transfer coefficient initially dropped and then gradually tended to stabilize with the increase in running time. They varied from 7.267 k W, 151.904W/(m2·°C) to 6.308 k W, 129.621W/(m2·°C), and decreased by 13.20% and 14.67% respectively. Effectiveness, exergy efficiency and entranspy efficiency decreased continuously, which were from 0.143, 41.3% and 9.56% down to 0.118, 37.5% and 8.56%, decreased by 17.48%, 9.20% and 10.46% respectively. However, entranspy dissipation number rised steadily. Severe scaling problems appeared in the heat pipe outer wall surface, which affected the heat transfer performance.With the running of on-line cleaning device, the heat transfer rate, heat transfer coefficient, effectiveness, exergy efficiency and entranspy efficiency increased gradually and tended to be stable eventually. In the steady state, the performance parameters such as heat transfer rate was 6.795 k W, heat transfer coefficient 134.304W/(m2·°C), effectiveness 0.085, exergy efficiency 40.69% and entranspy efficiency 24.2%, in comparison with the initial values, which increased by 6.11%, 9.49%, 7.19%, 7.93% and 5.22%, repectively. The on-line cleaning device had a large effect on scale removal of the heat pipe heat exchanger.