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Plate heat exchangers are compact and efficient heat exchange devices. They feature high heat exchange efficiency (their heat transfer coefficient is 3 to 5 times higher than that of tubular heat exchangers), low space requirement (1/3 of that of tubular heat exchangers), long service life, low investment costs, easy scale removal, and reliability. In recent years, they have been widely used in industries such as metallurgy, petroleum, pharmaceuticals, shipbuilding, textiles, chemicals, medicine, and food, serving as excellent equipment for applications such as heating, cooling, heat recovery, and rapid sterilization. However, since plate heat exchangers generally operate at high temperatures during heat exchange (especially for steam-water exchange), and have high heat exchange efficiency, they are highly prone to scaling. At the same time, plate heat exchangers have small flow apertures inside; when scaling occurs, these internal channels become narrower or even get blocked, which reduces the heat exchange efficiency of the plate heat exchanger and thus affects the normal operation of production as well as the safety of the equipment. Therefore, plate heat exchangers should be regularly chemically cleaned to remove dirt, thereby ensuring efficient heat exchange and the smooth progress of production. 1 Introduction to plate heat exchangers: A plate heat exchanger is a type of heat exchanger in which thin metal sheets (usually stainless steel) are pressed into plates with specific wave patterns, and these plates are then stacked together using sealing gaskets. It is mainly composed of components such as heat transfer fins, sealing gaskets, clamping bolts, pressing plates, and the overall frame of the device. The hot and cold media flow through their respective channels via adjacent heat exchange plates, with heat exchange taking place between them through a thin layer of heat exchange plates. As a result, it is efficient and energy-saving, features a high heat exchange coefficient, is safe and reliable to use, has a compact structure, small size, and low space requirement. It also allows for flexible combination as well as easy adjustment and maintenance. 2 Preparations before cleaning a plate heat exchanger 2.1 Analysis of scaling in plate heat exchangers Plate heat exchangers can generally be divided into two types: water-to-water heat exchange and steam-to-water heat exchange. In the water-water exchange method, both the hot and cold media are water, and the temperature difference between them is not large, ranging roughly from 70 to 90°C; scaling occurs to a similar extent on both sides. In the steam-water exchange method, the hot medium is steam, which generally does not suffer from scaling, while the cold medium is water at a temperature of around 90°C, making it prone to scaling. Its scale can be roughly divided into water scale and dirt, with water scale being the predominant type. Scale is primarily formed when various salts dissolved in water undergo thermal decomposition, resulting in a decrease in their solubility and subsequent crystallization and deposition on the heat transfer surfaces. These salts are usually carbonates, phosphates, sulfates, and silicates. Such scale crystals are dense and hard, making them difficult to remove. Dirt, on the other hand, is composed of fine particles such as silt, dust, insoluble salt deposits, gelatinous hydroxides, various debris, corrosion products, oil stains, and especially the dead bodies of bacteria and algae along with their sticky secretions. This type of dirt has a larger volume and a softer, more porous texture, making it easier to remove. 2.2 Selection of descaling and cleaning methods and processes for plate heat exchangers: The scale present in plate heat exchangers is mainly in the form of limescale, which is quite hard and firmly bonded to the heat transfer fins; therefore, it is difficult to remove using physical methods. As a result, acid cleaning, which is part of chemical cleaning methods, is chosen for descaling. Based on the scaling condition, degree of aging of the plate heat exchanger, and the requirements of the user, the chemical cleaning of plate heat exchangers can be divided into two methods: disassembly cleaning and non-disassembly cleaning. Dismantling for cleaning and descaling is thorough and effective, but it requires a lot of labor, involves complex procedures, and can easily lead to problems such as leaks in the heat exchanger and damage to its components. Cleaning and descaling without dismantling is not as thorough, but it requires less labor, involves simpler procedures, and is less likely to cause issues like leaks or component damage. When a plate heat exchanger is severely scaled, has low heat transfer efficiency, or even becomes clogged, it must be disassembled for cleaning; when the scaling on the plate heat exchanger is mild or the device is severely aged, cleaning without disassembly can be carried out. During chemical cleaning, a cleaning process that combines cyclic cleaning with immersion cleaning can be employed. Circulating cleaning involves using a circulation pump, a cleaning tank, plastic pipes, and the item to be cleaned to form a closed circulation system; an appropriate amount of cleaning agent is added to this system, and the cleaning is carried out through circulation by the pump. Immersion cleaning, on the other hand, involves waiting until the cleaning agent in the circulation system reaches a certain concentration before turning off the pump and allowing immersion. To maintain the concentration of the cleaning agent, during the circulation process, its concentration in the cleaning tank must be checked every hour, so as to keep it within the safe and effective range of 0.10 to 0.15 mol/L; the cleaning agent may need to be added if necessary. At noon or in the evening, soaking with an added detergent can be used for cleaning. 2.3 Selection of cleaning agents: The heat transfer fins in plate heat exchangers are generally made of austenitic stainless steel, with common grades including AISI304, 304L, AISI316, 316L, and 316Ti. Stainless steel materials need to be cleaned using nitric acid-based cleaning agents. The materials used for gaskets are generally NBR 130-140, RCB 140, and EPDM 150; all of these materials can resist corrosion by acids, alkalis, solvents, alcohols, and other such substances. Based on this, we used a specialized cleaning agent for stainless steel products developed by the Energy Research Institute of the Henan Academy of Sciences (this cleaning agent has passed the technical evaluation organized by the Henan Department of Science and Technology). It is composed of chemical substances such as nitric acid, glycolic acid, corrosion inhibitors, penetrants, and defoamers. This cleaning agent features a high scale removal efficiency, low corrosion rate, and fast cleaning speed for stainless steel products; the optimal cleaning concentration lies within the range of 0.10 to 0.15 mol/L. All the performance parameters of this cleaning agent have been tested and found to meet the requirements specified in HG/T2387-2007 \"Quality Standards for Chemical Cleaning of Industrial Equipment\".