Experimental Study Of The CaCO₃Fouling Deposited Characteristics On Heat Transfer Surface

Fouling exists in a wide variety of industrial processes, the fouling on various heattransfer equipment surfaces leads to a significant reduction of heat transfer efficiency, causesa huge waste of resources and high economic cost. It makes the heat transfer equipments runat high risk of safety. Fouling is a study topic in the fields of industry and academia, and hasbeen concerned for a long time. It is a major unresolved problem of heat transfer areas overthe past40years. Thus, it attracts great attentions of many researchers. Out of all types offouling, crystallization fouling is one that probably has the most detrimental effect onindustrial applications. It is valuable to put more efforts on the study of this fouling. Therefore,in this paper, the crystallization fouling of calcium carbonate on the inner tube surface isstudied using experimental method. The intention is to obtain the fouling characteristics, andto rich the knowledge of the crystallization fouling of CaCO3.The content of the study includes: developing the experimental setup, performing thecrystallization fouling of CaCO3; getting the fouling distribution on the tube surfaces byscanning electron microscopy (SEM) method; developing some parameters to describe thecharacteristics of fouling. Because fouling characteristics are diverse, complex and a longgrowth period under actual working conditions, in experimental process, the artificial foulingconditions to shorten the time period of the experiments is adopted.The experimental results shows that the crystals distribute on the tube wall as star pointsin initial stage, as the time increases, the distribution of the crystals becomes denser, andeventually cover the tube surface completely; the effect of the fouling growth characteristicsabove-mentioned on the heat transfer coefficient is that the heat transfer coefficient decreasesfirstly, then increases, then decreases again; the higher concentration of CaCO3is, the shortertime costs in initial stage; the increase rate of the thermal resistance is positive at first time,then becomes negative, becomes positive again at last; the shape of the fouling crystals is inoblique hexahedral calcite and acicular aragonite forms on the stainless steel tube surface, butis in cotton-like form on the glass surface; the numbers of the crystals per unit area increaseswith time; as the concentration is200mg/L, the crystal size ranges from6to15μm,decreases firstly with time and then increases, decreases again; as the concentration is50mg/L, the crystal size ranges from11to21μm, increases firstly with time and then decreases,increases again; per unit area, with increasing concentration the numbers of the acicularcrystals increase, and the numbers of the oblique hexahedral calcite crystal decrease.