| Urban sewage, river water, ground water and seawater are generally acknowledged as ideal cool and heat sources of heat pump system. But these waters quality can't conform to correlative standard of heat-transfer equipment, and urban sewage quality is the worst. There are obstruction and pollution problems in sewage source heat pump (SSHP) system project, flow and heat-transfer characteristics are unknown, high thermal resistance resulted by fouling in heat exchanger restricts the application of SSHP. The paper studies the fouling forming mechanism, and then applies solid-liquid fluidized bed technique for fouling prevention and removing of SSHP system. The results of theory and experiment show that solid-liquid fluidized bed technique can effectively solve the fouling problem in SSHP system and have wide application foreground in SSHP system. The main contents of the paper are as follows:(1) According to fouling characteristics of SSHP system, fouling forming process is divided into five processes, adhesion mechanism into physical-chemistry mechanism and microbe mechanism and fouling of SSHP system into two layers: mucous membrane and its growth. Based on Kern-Seaton model, taking different stress of fouling in different fouling layer and microbe physiology into account, the paper explains the reason high velocity can prevent and remove fouling and high velocity can't absolutely remove fouling, indicates key way of removing fouling is that adhesion stress is less than peeling stress.(2) The destroyed stress of fouling of SSHP system is discussed, shearing stress model of solid-liquid fluidized bed heat-exchanger is analyzed, and collision stress model is modified. It shows that shearing stress increases with particle hold-up and circulating velocity, collision stress increases with particle density and particle radial velocity, and increasing shearing stress can't absolutely remove fouling. The paper explains prevent and remove fouling mechanisms of solid-liquid fluidized bed technique by using fatigue theory, and initially investigates mechanism of strengthening heat transfer.(3) Main mechanisms of prevent and remove fouling in solid-liquid fluidized bed heat-exchanger is summarized as follows: incompact fouling is removed from tube surface by increasing shearing stress, compact fouling is removed from tube surface by increasing shearing stress and stochastic collision of particle, more compact fouling is destroyed by discontinuous stochastic collision of particle and then is removed from tube surface, and increasing shearing stress and stochastic collision of particle can prevent fouling forming on tube surface. (4) A new solid-liquid separator is invented, the separator can realize the absolute separation and recirculation use of particle, but it doesn't consume extra energy.(5) An experimental system of the solid-liquid cycling fluidized bed heat-exchanger (SLCFBE) is built, and the pressure drop and convection coefficient of urban sewage are analyzed by this system. Compared with the conventional shell and tube heat-exchanger, the solid-liquid fluidized bed heat exchanger (SLCFBE) of urban sewage with particle in the liquid can achieve higher convection coefficient, control fouling and on-line clean the system effectively, and the effect of steel ball is better than that of sand, it shows prospect of wide application.(6) Sewage source heat pump system with solid-liquid fluidized bed technique is studied; intake water devices of rotary plate and rotary plate are invented by using auto-backflushing, which solve the obstruction of heat-exchange equipment because of large scale sewerage in urban sewage. Combining solid-liquid fluidized bed technique with auto-intake water device; a high efficient system of utilization of cooling and heating source of once-through water (urban sewage, river water, ground water, seawater, etc) is presented, which can solve the fouling problem and low convection coefficient.
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