Methods for cleaning fouling on heat exchangers

Thread Content

Methods for removing scale from heat exchangers: 1. Common types of heat exchangers, their working principles, and the heat transfer media. A device used to transfer thermal energy between fluids at different temperatures is called a heat exchanger, or simply an exchanger. In a heat exchanger, there must be at least two fluids with different temperatures; one of the fluids has a higher temperature and releases heat ； The other fluid has a lower temperature and absorbs heat. Heat exchangers can be classified into heaters, coolers, condensers, evaporators, reboilers, cryocoolers, superheaters, etc., depending on their purpose. 2. Influence of the chemical composition of the heat transfer medium on scale formation. A heat transfer medium is an auxiliary medium that exchanges heat with the process material; common examples include water, oil, air, etc. Water is the most common heat transfer medium, and its impurities have a significant impact on the formation of scale in heat exchangers. Specifically, these include impurities that are dissolved in water in ionic or molecular form: calcium salts, magnesium salts, and sodium salts. Impurities present in a colloidal state: iron compounds, microorganisms, and sludge from the cooling circulation water; as well as dust from the air and suspended particles in the make-up water, gradually settle in heat exchangers where the flow rate is low. Scale: It is primarily formed by the secretions of microorganisms combined with sediment, corrosion products, and remains of bacteria and algae in water, and it often adheres to the walls of heat exchangers. 3. Physicochemical properties of heat exchanger scale: The layer of deposits that accumulate on the heated surfaces and heat transfer surfaces is commonly referred to as scale. In heat exchangers, especially those with circulating cooling such as compressed cold plates, there are decomposed bicarbonate products and microbial sludge. Carbonate scale is the primary type of scale in circulating cooling water systems and the heat transfer surfaces of heat exchangers. Basic properties of carbonate scale: Carbonate scale appears white or grayish-white in color. If the device is corroded, it will take on the color of the corrosion products. Carbonate scale is hard and brittle, adheres firmly, and is difficult to remove or scrape off. For circulating cooling water, the water quality should be tested regularly to ensure it meets the requirements specified in GB50050 \"Quality Standards for Circulating Cooling Water\". When the water quality does not meet these standards, it should be treated using the methods outlined in GBJ50 \"Design Code for Industrial Circulating Cooling Water Treatment\". This can effectively prevent scale from affecting the heat exchange efficiency of the equipment. 4. The impact of the flow rate of the fluid during heat exchange on scale formation: Increasing the flow rate of the fluid helps to prevent deposits and scaling, but it also increases the pressure drop in the heat exchanger ； The flow direction of the fluid is continuously changed, causing the fluid to constantly strike the walls of the heat exchange tubes, which prevents various impurities in the fluid from accumulating on those walls ； Choosing corrosion-resistant, smooth materials can also slow down the formation of dirt. In practice, we often determine whether a heat exchanger is fouled by checking the pressure difference. The company does the same thing before each equipment maintenance shutdown as well. 5. Influence of the temperature of the medium during heat exchange on scale formation: Changes in the inlet and outlet temperatures of the heat exchanger directly reflect changes in its heat transfer capacity. The flow rates and temperatures at the inlet and outlet of the heat exchanger should be measured regularly. When the heat transfer capacity becomes too low to meet the process requirements, it is necessary to improve this capacity through mechanical or chemical cleaning in order to satisfy and maintain the needs of the process operation. When water is used as the cooling medium, it is advisable to keep the outlet temperature of the water at 50°C, as temperatures above 50°C can cause corrosion of the pipes and severe scaling in the heat exchanger, thereby affecting its heat exchange capacity; therefore, the outlet water temperature should not exceed 65°C. 6. Influence of heat exchanger structure on scale formation. Practical experience has shown that the structural shape of the pipes has a significant impact on scaling. For example, bellows-type pipes not only force the fluid to flow in a counterflow manner multiple times along the tube bundle along a specified path, thereby continuously enhancing the turbulence level of the fluid, but also improve heat transfer efficiency. They possess excellent anti-scaling properties; the mechanism behind this anti-scaling effect is that the high-speed turbulence of the fluid within the flow channels makes it difficult for particles in the fluid to settle and form scale. Even if a small amount of scale does form, the turbulent flow of the medium inside and outside the tubes exerts strong scouring action on the tube walls, resulting in strong anti-scaling capabilities. Furthermore, the bellows experience strain due to the temperature difference stress between the tube side and the shell side, which causes microscopic changes in the curvature of the bellows with its elastic properties, thereby endowing the bellows heat exchanger with scale prevention and removal capabilities. 7. Influence of local environmental conditions on scale formation: When the operating temperature is higher or lower than the ambient temperature, some heat exchangers require insulation or cooling; the integrity of these insulation or cooling layers directly affects the heat transfer performance of the heat exchanger. If the insulation or thermal insulation layer is damaged and the local environmental conditions change, it will also accelerate the accumulation of scale, leading to the formation of a scale layer. 8. The impact of shutdown periods on scale formation: During the annual maintenance shutdowns of heat exchange equipment, if the water accumulated inside is not drained promptly or thoroughly, scale is more likely to form under conditions of relative stillness. Therefore, during equipment shutdowns, especially those that last for an extended period, attention should be paid to draining water from the heat exchanger as well as to its maintenance. 9. Methods and measures to reduce or eliminate scale formation: (1) Reducing or eliminating the conditions that lead to scale formation from a design perspective: The design of heat exchangers involves determining optimal heat transfer areas and relevant structural dimensions through calculations, in order to achieve the desired heat transfer effects. When designing the structure, it may be worthwhile to consider using special designs, such as those that generate turbulence. For important heat exchange equipment, electronic descalers and backwashing systems can also be employed. If water is used as the heat transfer medium, anti-scaling additives and other materials should be used to address the issue of corrosion. In addition, adhering to simple design principles can help reduce or even eliminate the conditions that lead to scaling; for example, dirty and scale-prone fluids should be routed through the tube side, as cleaning the tube side is more convenient. (2) Fluids with low flow rates or high viscosity are suitable for flow in the shell side, as the fluid flows within the shell side surrounded by baffle plates; the continuous changes in flow velocity and direction lead to turbulence, which prevents scale formation. (3) Corrosive fluids should be routed in the tube side to prevent simultaneous corrosion of the tubes and the shell, and the tube side also facilitates maintenance and replacement. (4) The cooled fluid should flow in the shell side, allowing utilization of the shell’s heat dissipation to the outside; meanwhile, it is easier to replace the tubes when scaling occurs on them. (5) Saturated steam should flow in the shell side, as it is relatively clean and not prone to scaling, thus eliminating the need for cleaning. Selection of fluid flow rate: The selection of fluid flow rate involves aspects such as the heat transfer coefficient, flow resistance, and the structure of the heat exchanger. Increasing the flow rate can raise the convective heat transfer coefficient, reduce the formation of fouling, and thereby increase the overall heat transfer coefficient ； But at the same time, it increases flow resistance and raises power consumption ； Choosing a high flow rate reduces the number of tubes; for a given heat exchange area, longer tubes or more passes are required. Too long tubes make cleaning difficult, and increasing the number of passes reduces the average heat transfer temperature difference. Therefore, it is generally necessary to select an appropriate flow rate through various trade-offs. When the temperature difference is small, scaling of the shell-side fluid is not severe, and chemical cleaning can be applied to the shell side, a fixed-tube-sheet heat exchanger should be selected.