Study On Characteristics Of Fouling Development And A Wastewater Evaporator With De-fouling Function

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.