Clog-proof Technique And Heat Exchange Characteristic Research Of The Direct Untreated Sewage Source Heat Pump System

Municipal sewage contains so abundant low level heat that it will mean ahuge value for development and utilization when used as a cold and heat source ofheat pumps for heating and air-conditioning system of buildings.The water quality of municipal sewage is extremely poor and the thermal andflow properties of it are quite different from those of clear water. In contrast, thewater quality of secondary effluent from sewage treatment plants (discharge water)is close to that of clear water. However, sewage treatment plants are mostly locatedin suburbs far away from the city buildings, so it is exactly untreated sewage allover the city sewers that will mean the huge value for development and utilization.Traditional heat pump air conditioning units have strict requirements on waterquality. Therefore, measures of intermediate heat-transfer are often taken in thepresent heat pump system based on untreated wastewater to avoid sewage flowingdirectly into the evaporator or condenser of the system which is the so-called"indirect system" and not only introduces extra intermediate heat-transfer devicesbut also increases system energy loss. So the development of heat pump systemnamely the so-called "direct system" allowing sewage to enter the units directlyshows a great allure on the academic and engineering fields from the origin of thetechnology of sewage-source heat pumps.Considering the specialty of the water quality of sewage, direct sewage-sourceheat pump system must solve the following key problems. Firstly, higherrequirement is needed for efficiency and reliability of filtering and anti-blocking.Secondly, the feature and discipline of flow, heat-transfer and fouling heatresistance of sewage must be recognized. Thirdly, the heat-transfer performance ofsewage evaporator and condenser should be recognized as well. At last, reasonablematch and performance of the two machines of direct heat system need to be takeninto consideration. This paper mainly introduces the research work on thementioned techniques as follows.The primary precondition of safe and enduring heat-transfer for direct sewagesource heat pump set is to guarantee that large-scale contaminant of all shapescannot enter the heat exchanger of the set. Corresponding to the universally employed sewage clog-proof machine in indirect sewage source heat pump systems,we have performed engineering practice and test, according to which we makefollowing conclusions. Firstly, filamentous contaminant is the main incentive ofblocking filter surface. Secondly, referring to the traditional method, it is tough tomeasure the filtration and regeneration ability of the clog-proof machine’s filtersurface only with the diameter of filter hole given. Thirdly, the condition of internalleakage mixing water exists inevitably. The paper establishes a design index systemof clog-proof machine including filter pore diameter, recoil area ratio, rotatingspeed, resistance and so on, and winding length, recoil efficiency, internal leakagerate and heat-transfer guaranteed rate are the core of the index system. The paperalso defines the optimal filter pore size, recoil square ratio and cleaning period ofthe clog-proof device by engineering tests, all three of which provide reasonablefoundation for scientific and rational design of clog-proof device for engineeringapplication.An experiment table focusing on city untreated sewage properties research hasbeen designed and constructed in Harbin Taiping sewage treatment plant as to bementioned in the paper. Experiments have been executed on the fouling heatresistance performance, flow resistance characteristics and flow and heat-transfercharacteristics of urban untreated sewage turbulent flow in a variety of pipes.Through considerable experimental statistics, we can reach such conclusions asfouling heat resistance growth disciplines for kinds of common heat-transfer pipes,the function between the stable value of fouling thermal resistance and the sewageflow rate in the pipe, an expression for resistance coefficient of sewage turbulentflow and finally a heat transfer correlation formula. The experiment resultsdemonstrate fouling growth model as a progressive function of time. Specifically,experiments verify that with same pipe diameter and flow rate, the flow resistancecoefficient of wastewater is approximately1.08~1.12times that of clear water,whereas the convective heat transfer coefficient is about75~82percent of that ofclear water. These fundamental statistics and empirical formulas provide morereliable basis for heat-transfer device development and system design of sewagesource heat pump systems.Based on the sewage flow, heat transfer and fouling resistance properties andthe characteristic of difficulty in heat transfer enhancement at the sewage side ofheat transfer tube, a distributed parameter model of the sewage evaporator and condenser is established, and simulation programs have been developed andemployed to do amounts of numerical study which obtains the scope of sewageevaporator and condenser heat transfer coefficients and the law of influence exertedby sewage temperature and flow changes on their heat transfer properties. It is alsofound that compared with the two machines of the conventional heat pump, thesewage evaporator and condenser have following prominent characteristics. Fourprocesses are appropriate for wastewater inside the machines, and steamsuperheating segment in condenser takes up a great proportion. In addition, theimpact of superheating on condensation heat transfer is significant. All theachievements lay the foundation for system evaporator and condenser design andsystem performance analysis.Through direct sewage source heat pump system modeling and numericalsimulation, the efficiency range of direct system and the law of sewage temperature’sinfluence on performance are given. Then we set the standard designed workingcondition of sewage source heat pump units and do deep analysis on the relationshipbetween the matching ratio of sewage evaporator and condenser area and heat output,cooling capacity and power output of the units. For the greater range of suitable arearadio and power output radio of sewage source heat pump unit, we put forward theconcept that different area ratio will be adopted while designing heat pump units indifferent regions. Finally the characteristics of heat pump units and principles fordesign and selection are introduced under non-standard working conditions.By researching, this paper has mainly solved problems faced by direct sewagesource heat pump system such as the clog-proof reliability, sewage flow and heattransfer calculation method, heat transfer properties of sewage evaporator andcondenser and the matching of the two devices, all of which lay theoreticalfoundation for the development of direct sewage source heat pump system.