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This post was last edited by liaifeng on 2020-3-19 20:17. Heat exchangers are primarily used to facilitate heat exchange between two materials at different temperatures, thereby achieving the purpose of heating or cooling. More than 90% of heat exchange equipment suffers from some degree of fouling, which leads to a series of problems: the formation of fouling increases the thermal resistance to heat transfer; as the scale layer thickens, the effective heat exchange area decreases, thereby worsening the heat exchange performance. The presence of this scale layer reduces the flow area on the heat exchange surface, increasing the resistance to fluid flow and thus raising the power consumption of the driving equipment. Excessive fouling on the heat exchange surface results in excessively high local temperatures, which reduces the mechanical properties of that surface. In severe cases, the fouling can even cause local corrosion or perforation of the heat exchange surface, significantly affecting the service life and safe operation of the heat exchange equipment. Therefore, to ensure efficient production and safety, it is necessary to clean heat exchange equipment regularly, with the cleaning interval getting shorter over time. This, in turn, leads to increased operating costs for the heat exchangers. As a result, it is particularly important to select appropriate cleaning methods based on the different types of heat exchangers and the kinds of dirt present, in order to achieve effective cleaning while also maintaining cost efficiency. The author has summarized the common types of fouling in heat exchangers and the cleaning methods, and provided basic selection principles. 1 Analysis of the types and characteristics of dirt. Understanding the types of dirt and their formation characteristics is a necessary preparation for carrying out the cleaning process. It helps to quickly identify a suitable cleaning solution, enabling efficient and rapid cleaning, reducing waste of labor and resources, and saving on production and maintenance costs. 1.1 The types of fouling vary depending on the heat exchanger design, as well as the fluid media involved, which result in different forms of fouling. According to the classification method proposed by Epstein N, which is based on the main physical and chemical processes that cause dirt deposition, dirt can be divided into types such as crystalline dirt, particulate dirt, chemically reaction-induced dirt, corrosion-induced dirt, microbial sludge dirt, solidified dirt, and other types of dirt. Some also classify them based on the growth pattern of dirt over time, such as linear growth type, decreasing rate type, and hyperbolic type. Since the author focused primarily on the cleaning of heat exchangers, only the characteristics of 6 types of fouling were analyzed. 1. 2 Properties of fouling 1. 2.1 Crystalline fouling Crystalline fouling is a type of deposit formed when certain dissolved inorganic salts present in the flowing medium during the production process reach a supersaturated state, resulting in the precipitation of crystals on the heat exchange surface. Common crystalline contaminants include scale and rust. The solubility of ordinary inorganic salts increases as temperature rises, and these salts tend to crystallize on the cooling surface. The solubility of another type of inorganic salts (those with abnormal solubility, namely slightly soluble and insoluble salts) decreases as temperature rises, and these salts tend to form crystals on the heating surface. Crystalline dirt generally has a high hardness and low viscosity. 1. 2.2 Particle fouling: Particle fouling arises primarily from the accumulation of solid particles suspended in the heat exchanger; larger solid particles settle and accumulate on the horizontal heat exchange surfaces due to gravity, while colloidal particles formed through other mechanisms accumulate on both horizontal and vertical heat exchange surfaces. 1. 2. 3 Chemical reaction fouling: Chemical reaction fouling is a deposit formed as a result of chemical reactions occurring between the fluids in a heat exchanger; the material of the heat exchange surface does not participate in these reactions. The accumulations resulting from the cracking and polymerization reactions of organic substances are typical examples of chemical reaction fouling. Such fouling generally has a high viscosity, and chemical cleaning methods yield good results in removing it. 1. 2. 4 Corrosive scale: The formation of corrosive scale is the result of a reaction between the heat transfer medium and the heat transfer surface. The heat exchange surface is corroded by the heat exchange medium, resulting in an increased roughness that makes it easier for other potential contaminants to adhere to the surface. The degree of corrosion on the heat exchange surface depends on the composition of the medium, its acidity or alkalinity, and the temperature of the fluid flowing in the heat exchanger. 1. 2. 5 Microbial sludge fouling: Heat exchangers often contain microorganisms as well as nutrients that support their growth. The growth, reproduction, and waste products of these microorganisms form an organic film on the walls of the heat exchanger tubes; this film-like deposit is known as microbial sludge fouling. Since the growth of microorganisms is closely related to temperature, controlling the temperature can effectively prevent the formation of such dirt. 1. 2. 6 Solidified fouling: Solidified fouling refers to the fouling that forms on heat exchange surfaces as a result of phase changes occurring in multi-component fluids under conditions of relative subcooling. The relative supercooling here refers primarily to the fact that, compared to the components in the fluid, different components have different temperature points at which they solidify; therefore, the distribution of the temperature field in the heat exchanger is a key factor in the formation of solidified fouling. Although the types of fouling mentioned above, as well as the physical and chemical processes that lead to their formation, are different, the factors responsible for their occurrence can overlap; therefore, scaling in heat exchangers is often the result of multiple processes acting together. The mixed interaction of these processes results in the fouling formed on the heat exchange surface of the heat exchanger being a mixture of various types of fouling.