【作者】 沈朝；

【导师】 姚杨； 姜益强；

【作者基本信息】 哈尔滨工业大学 ， 供热、供燃气、通风及空调工程， 2013， 博士

【摘要】 利用污水源热泵系统(Wastewater Source Heat Pump, WWSHP)对污水废热进行回收具有重要的节能环保意义。由于污水水质较差，在废热回收过程中污水换热器表面会形成一层污垢，从而导致热泵系统运行性能下降，因此需要周期性除污来保证换热设备的正常运行。目前常见的方法有两种：一种是人工拆卸清洗，另一种是设计大尺寸或高流速的换热器来抵消污垢的影响。前者浪费人工既脏又累，后者则未能从根本上克服污垢，增加成本和运行费用。针对污水换热器结垢问题，本文对污垢堆积特性进行了机理研究来寻求有效的抑垢除垢方法；针对洗浴废水这类水质的污水搭建了废热回收实验台，来研究污垢对实际热回收系统的影响以及系统运行特性；提出了一种适用于污水源热泵系统的具有除污功能的壳管式污水蒸发器来保证污水废热回收系统的高效性和可靠性。本文的主要研究内容如下：本文首先从机理上分析了污垢堆积过程，搭建了污垢堆积特性测试实验台，研究了污水换热过程中各因素对污垢形成的影响。实验结果表明，污水中直径较小的粒子是形成污垢的主要成分。污水流速越大，渐近污垢热阻越小，沉积在换热器表面的粒子平均直径也越小。污水低流速运行时，在污垢形成初期，污垢反而增强换热，此时的污垢热阻定义为“负污垢热阻”。但污水在高流速运行时，这种现象并不出现。当污水换热器安装在污水泵吸入口时，形成的渐近污垢热阻以及沉积粒子的平均直径比安装在污水泵出口时要高。污垢成分分析测试结果表明，污垢中主要成分为无机物。为了研究实际工程中污垢对污水换热器的影响，在某洗浴中心搭建了利用浸泡式蒸发器（Immersed Evaporator, IE）进行洗浴废水废热回收的热泵系统。研究了洗浴废水这类水质的污水中形成的污垢对系统的影响，以寻求合适的污垢去除方法。本文对不同运行工况下的现场实测数据进行了分析。结果表明，应用水泵使浸泡式蒸发器内污水上下循环流动可减弱甚至消除温度在垂直方向的不均匀分布现象，并能改善污水源热泵的性能和压缩机的吸气压力。通过对污水源热泵系统一个月的性能监测发现，由于换热器表面污垢的生长，系统COP逐渐减小，因此对污水换热器进行定期清洗是十分必要的。同时，污垢生长导致污水管道的最大运输能力下降，且第一个月下降程度最大，在随后的四个月下降不明显。与几种传统热水加热系统的成本以及运行费用的对比结果表明，该废热回收热水系统具有明显的经济性。针对换热器污垢问题，提出了WWSHP系统的关键部件——具有除污功能的干式壳管式污水蒸发器（Dry-Expansion Shell-and-Tube Evaporator, DESTE）。在该洗浴中心搭建了利用DESTE换热器进行废热回收的WWSHP系统实验台，对DESTE的运行特性进行了实验研究。连续一个月的测试数据表明，一个月后系统蒸发温度、蒸发器换热能力、系统COP均因受到换热器表面污垢的影响而降低。一个月后利用DESTE换热器的除污装置对换热器进行了除污，并对除污前后的系统性能参数进行了测试和对比分析。对比结果表明，除污后制冷剂的平均蒸发温度、换热能力、COP在很大程度上有了提高，基本恢复到洁净时的运行状态。这些测试结果表明该换热器具有高效的除污功能。将传统的IE换热器和本文提出的DESTE换热器在WWSHP系统（IE-WWSHP和DESTE-WWSHP）中的运行和换热特性进行了对比分析，包括不同的热水设定温度、污水排放流量和形式，热水排放模式(连续/间断)等。并从初投资、运行费和环保方面对两种换热器进行分析。结果表明，DESTE换热器可以取代传统IE换热器。DESTE-WWSHP系统中的污水储水箱内污水温度分布比IE-WWSHP系统均匀；DESTE-WWSHP系统的热水加热能力比传统IE-WWSHP系统高；DESTE换热器污水侧换热系数、DESTE紧凑度均比IE换热器的高，因此体积比IE换热器小很多。分析表明，在除污方面，DESTE-WWSHP较IE-WWSHP系统节省劳动力和电能，同时避免难闻气体的泄漏造成的空气污染。为进一步了解DESTE换热过程，对其流动与换热机理进行了分析，根据质量守恒、动量守恒和能量守恒，建立了该干式壳管式污水蒸发器的换热模型。在验证了模型的有效性后，利用该模型对不同制冷剂和污水流速下的换热管内制冷剂的压力、空泡率、比焓以及温度分布进行了模拟。对比分析了除污前后两侧流体的温度分布以及制冷剂侧换热系数的变化。模拟研究了污垢的形成过程对污水换热器换热特性的影响。确定了最小污水流量、最优污水流量以及最佳除污污垢热阻。本课题研究是“十一五”国家科技支撑计划项目“水源地源热泵高效应用关键技术研究与示范”（2006BAJ01A06）的一部分。本文工作对深入认识污垢的堆积机理以及污水的流动换热特性具有重要意义，为寻找有效的抑垢除垢方法提供理论指导，本文提出的除污型污水换热器可有效解决污水废热回收过程中的污垢问题，为污水源热泵系统的推广提供有益参考和借鉴。更多还原

【Abstract】 Using heat pump to recover heat from wastewater (called Wastewater Source HeatPump, WWSHP) is an environment-friendly and energy-saving technology, whichin turn is a promising research field. However, during the process of waste heatrecovery, foulant suspending in wastewater will deposit on the heat transfer surface,forming fouling layer which affects the heat transfer seriously. Therefore, aperiodical cleaning on the heat exchanger is required to ensure the WWSHP runs atan acceptable efficiency. Currently, two cleaing motheds are presented: one is toremove the fouling manually; the other is designing the heat exchanger with oversize or high wastewater flow rate to counteract the bad impact of fouling. The firstone is a troublesome and dirty job, requiring a high pay for cleaing workers; thesecond onecouldn’tsolve the fouling issue essentially, but increases the operationcost. To solve the fouling issue, the mechanism of fouling development wasresearched in this thesis, with purpose of seeking for an effective cleaning mothed.The quanlity of waste bath water is different from other wastewater, thus a heatpump system recovering heat from waste bath water was built to study the impact offouling on the system. Whilst, the performance of this heat recovery system wastested； A novel shell-and-tube heat exchanger with de-fouling function wasdeveloped, which can be used in WWSHP system. All research included in thisthesis are summarized as follows:The mechanism of fouling development was analyzed, referring sufficientliteratures. And then, an experimental apparatus to research process of particaldeposition on heat transfer surface was built. A series of test to investigate theeffects of operation parameters on the fouling deposition were carried out based onthis apparatus. Results suggested that the small particles in the wastewater were themain source for the fouling deposition on the heat transfer surface. With theincreasing of wastewater flow rate, the asymptotic fouling thermal resistancedropped, and the average particle diameter of foulant deposited on the surface of thetubes decreased as well.The phenomenon of “negative fouling thermal resistance”at the beginning of fouling development, which means fouling deposition couldenhance the heat transfer, appeared at the low flow rate of wastewater. But thisphenomenon disappeared at high flow rate. The asymptotic fouling thermalresistance and the average particle diameter of foulant were higher when the heatexchanger was installed at the suction-inlet of wastewater pump than that at the shoot-outlet. Ash fouling is the main ingredient of the overall foulant deposited onheat transfer surface.To research the fouling developed in the special wastewater of waste bath water andfind a suitable fouling cleaning method, an experimental WWSHP integrated with aconventional immersed evaporator (IE) was built in a sauna center which recoveredheat from waste bath water. Analysis about the operating performance of thisWWSHP system based on the collected field data in various operating conditions isreported. Test results suggested that circulating wastewater from the bottom to thetop of the wastewater storage tank (WST) could weaken even out the verticalwastewater temperature distribution inside the WST and improve the COP andcompressor suction pressure of the WWSHP accordingly. The daily averaged COPof the WWSHP was monitored for over an entire month. It was shown that themeasured COP gradually reduced, suggesting the need for regular cleaning of theheat exchangers used in a WWSHP system. In addition, the recorded maximumtransporting capacity of the wastewater pipe also reduced due to the bio-foulingbuild-up. Finally, an economic analysis is presented, comparing the WWSHPsystem with some conventional water heating systems.To solve the issue of fouling existed in wastewater heat exchangers, a novelDry-Expansion Shell-and-Tube Evaporator (DESTE) with de-fouling function wasdeveloped in this thesis which can be used in WWSHP system. An experimentalWWSHP integrated with DESTE was built in the sauna center to investigate itsoperation characteristics. One-month long test to examine the effect of bio-foulingon the operating performance of the WWSHP was conducted. The test datasuggested that with the growing of bio-fouling on the evaporator surface during thetesting period, the daily averaged refrigerant evaporating temperature, the dailyaveraged heat exchange capacity and the daily averaged COP of the WWSHPdecreased gradually due to the development of fouling. However, after cleaning bythe de-fouling device of DESTE at the end of the one-month long testing period, theaveraged refrigerant evaporating temperature, the averaged heat exchange capacityand the averaged COP were improved greatly and restored to the status in cleancondition basically. These comparison results clearly demonstrated that the novelevaporator with de-fouling function was effective in easily removing the bio-foulingbuild-up at a low cost.To compare the performance of the DESTE-WWSHP with the conventionalimmersed evaporator (IE) based wastewater source heat pump (IE-WWSHP), anexperimental platform was built by installing a DESTE and an IE unit in parallel ina same WWSHP system. Several operating parameters were investigated, including water heating capacity, coefficient of performance (COP), hot water temperaturesettings, wastewater discharge rate/pattern, and hot water discharge mode(continuous or intermittent). Further analysis on initial and operation cost andemvironment protection was given. Results showed that the DESTE-WWSHPdelivered comparable heat transfer performance to the IE-WWSHP, and thus, couldserve as a replacement for the latter apparatus. Specifically, the water temperature inthe water storage tank was more uniformly distributed in the DESTE-WWSHP thanthe IE-WWSHP; the DESTE-WWSHP exhibited higher water heating capacity thanthe IE-WWSHP at the continuous hot water discharge mode. The DESTE has ahigher heat transfer coefficient on the wastewater side than that of the IE unit. Atthe same heating capacity, the DESTE was far more compact and thus its volumewas smaller than the IE unit. Additional benefits offered by this DESTE-WWSHPincluded reducing labor and power costs for cleaning heat exchanger, anddecreasing odor nuisance also can be found.The mechanism of flow and heat transfer of the DESTE was analyzed, and then theheat transfer process was modeled based on conservation equations of mass,momentum and energy. Based on the model verified through comparingexperimental data with calculated data, the distributions of pressure, void fraction,specific enthalpy and temperature of refrigerant were simulated at different flowrates of refrigerant and wastewater respectively. The temperature variation ofrefrigerant and wastewater along the flow-line and the heat transfer coefficient ofrefrigerant were investigated before and after cleaning. At last, the effects of foulingdevelopment on performance of the DESTE were investigated. The minimumwastewater flow rate, the optimal wastewater flow rate and the optimal foulingthermal resistance of de-fouling were obtained from the results.This research is one sub-project of Study on the key technique and demonstration ofhigh-efficiency water/ground source heat pump system, supported by NationalEleventh Five-Year Technological Supporting Project from China Ministry ofScience and Technology (MOST) and coded as2006BAJ01A06. The research inthis thesis is also useful for understanding the mechanism of fouling developmentand the heat transfer inside the wastewater heat exchanger. It can provide atheoretical guide for making a strategy to restrict and remove fouling. The novelwastewater evaporator (DESTE) presented in this research can solve the foulingissue encountered by WWSHP users, which can be taken as a reference for extendthe application of WWSHP systems.更多还原