Schemes for cleaning plate heat exchangers

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Heat exchangers are divided into tubular and plate-type heat exchangers. Plate-type heat exchangers, thanks to their advantages such as light weight, small footprint, high heat exchange efficiency, and ease of installation and disassembly, are widely used in industries such as petroleum, chemicals, manufacturing, power generation, metallurgy, and HVAC. However, due to the small gaps between the heat exchange plates, scaling can easily occur, leading to blockages and affecting the heat exchange efficiency; therefore, choosing an appropriate cleaning method is crucial for the maintenance of plate heat exchangers. 1 Introduction to plate heat exchangers: A plate heat exchanger is a type of heat exchanger made by pressing thin metal sheets into plates with a specific corrugated shape, and then assembling them together with sealing gaskets. A plate heat exchanger consists of a fixed pressing plate, upper and lower guide rods, a movable pressing plate, pressing bolts, heat exchange plates, and sealing strips. The refrigerant and the heat transfer fluid flow through the gap channels on both sides of the heat exchange plates to exchange heat; it has many advantages, but it also has some disadvantages. 1.1 Advantages of modular heat exchangers 1) High heat transfer efficiency. The special corrugated structure of the plates enables a high heat transfer coefficient at lower flow rates, typically 3 to 5 times that of conventional shell-and-tube heat exchangers. Plate heat exchangers have narrow flow channels; their plates are wave-shaped, resulting in a complex cross-sectional geometry that causes the flow direction and velocity of the fluid to change continuously. This increases fluid turbulence, enabling turbulent flow at relatively low velocities and thus yielding a high heat transfer coefficient. 2) It takes up little space. The heat exchange area per unit volume of a plate heat exchanger is 2 to 5 times that of a shell and tube heat exchanger. 3) Easy to clean. Compared to tubular heat exchangers, plate heat exchangers allow the plates to be removed by loosening and removing the clamping bolts, which facilitates thorough cleaning and maintenance. 1.2 Disadvantages of plate heat exchangers 1) Poor sealing performance, prone to leakage. The sealing strip is an important auxiliary component in plate heat exchangers, serving primarily to prevent leakage of fluid as it flows between the plates. When selecting the material for such strips, considerations must be given to their temperature resistance, pressure resistance, chemical stability, as well as elasticity. 2) The operating pressure is subject to certain limitations. Limited by the strength of the heat exchange plates and the sealing performance of the sealing strips, the operating pressure generally does not exceed 1 MPa. 3) The gaps between the heat exchange plates are narrow, making them prone to clogging; they are not suitable for fluids containing suspended solids, and their flow resistance is higher than that of shell-and-tube exchangers. 4) The heat exchange plates are thin; repeated disassembly and assembly can lead to deformation and cracking, resulting in leaks. 2 Causes and Effects of Scaling in Plate Heat Exchangers During operation, the main substances that cause scaling between the plates of a plate heat exchanger are scale and dirt. Scale is formed when calcium and magnesium carbonates in water decompose due to heat, resulting in precipitates of calcium carbonate and magnesium hydroxide. These precipitates are quite hard and difficult to remove. Dirt, on the other hand, consists of fine particles such as silt and algae; such deposits are larger in size, softer in texture, and easier to remove. Deposit accumulation reduces the heat exchange capacity of plate heat exchangers, leading to severe waste of thermal energy and affecting the proper operation of the heat exchangers. 3 Selection of Cleaning Methods for Plate Heat Exchangers 3.1 Routine Online Backwashing: During normal operation, loose sediment and algae deposits that accumulate in the gaps between the plates of the plate heat exchanger should be removed through periodic backwashing, as a form of preventive maintenance. This helps to avoid reduced flow rates caused by blockages and slows down the formation of scale. Taking the distillate plate heat exchanger in a certain seawater desalination plant as an example, the steps for online backwashing are as follows. 1) Close the inlet and outlet valves on the seawater side of the plate heat exchanger. 2) Remove the seawater side inlet filter cartridge of the plate heat exchanger. 3) Connect the seawater inlet side drain pipe to the filter cartridge flange of the plate heat exchanger compression plate, and connect the external discharge pipeline. 4) Open the valve on the seawater outlet side to backwash the plates. Precautions for online backwashing: Avoid backwashing at excessive pressure to prevent the gasket from being forced out of the sealing groove; carry out multiple backwash cycles; check the filter cartridge for any damage and replace it promptly if found. 3.2 Chemical cleaning offline – When online backwashing does not yield significant results, it is necessary to carry out chemical cleaning of the plate heat exchanger offline; this method is suitable for types of scale that are easily dissolved, such as carbonate scale. 3.2.1 Selection of cleaning agents Cleaning agents are generally divided into two categories: organic acids and inorganic acids. Organic acids mainly include formic acid, oxalic acid, citric acid, etc.; inorganic acids mainly include hydrochloric acid, nitric acid, hydrofluoric acid, etc. The selection is based on an analysis of the plate heat exchanger’s structure, manufacturing process, plate material, and scale composition. 3.2.2 Not removing the offline chemical cleaning steps – Taking the distillate plate heat exchanger in a seawater desalination plant as an example. Tests conducted by plate heat exchanger manufacturers have shown that 30% citric acid (available in industrial, medical, and food-grade forms) can be used as a cleaning solution (note: 30% citric acid does not cause corrosion to titanium or NBR rubber). It is best to heat the acid solution to 40–50 °C for single-channel cyclic backwashing. 1) Construct a chemical tank, and connect the seawater outlet valve, seawater inlet valve of the plate heat exchanger, the acid-resistant pump, and the chemical tank using PVC pipes to form a circuit. Water under certain pressure is used to continuously perform open flushing of the heat exchanger, thereby removing impurities such as sediment from within it as much as possible. This not only improves the effectiveness of acid cleaning but also reduces the amount of acid required for cleaning. The purpose of water flushing and pressure testing is to remove sediment and loose dirt from the system, and to check for leaks at the temporary connections under simulated cleaning conditions. 2) Connect the inlet pipe for the cleaning solution to the outlet side of the seawater side of the plate heat exchanger, and connect the outlet of the cleaning solution to the inlet side of the seawater side of the plate heat exchanger; flush in the reverse direction. First, soak for 1–2 hours, then start circulating the solution using a pump for 24 hours. If the temperature of the cleaning solution does not reach 40 °C, the circulation time for cleaning must be extended accordingly. According to the chemical cleaning process, during the cycling process, the concentration of the cleaning solution in the tank must be checked every 1 hour, allowing the cleaning agent to be replaced promptly. Keep the concentration of the cleaning solution at 30% at all times, and add cleaning agent based on the measurement data. 3) After cleaning is complete, drain the acid from the system and fill it with fresh water for rinsing, in order to remove any remaining acid and impurities. The water rinsing can be stopped when the pH value of the solution at the outlet is greater than 6 and the difference in turbidity between the inlet and outlet fluids is small. 4) The pickling solution is neutralized and discharged into the chemical tank; after adding caustic soda to adjust the pH value to 6–9, it can be transported by tank truck to the designated location for disposal. 5) Records of the cleaning process should be kept to compare the results before and after cleaning. After cleaning, the plate heat exchanger must undergo a pressure test in accordance with the specifications; only upon passing this test can the heat exchanger be connected to the system. 3.3 Disassembly and cleaning of plates 3.3.1 When disassembly and cleaning of plates are necessary 1) The heat exchange capacity decreases, and the pressure drop increases. When the plate fins are severely fouled, the heat exchange efficiency fails to meet the process requirements, or even blockage occurs, the above two methods are ineffective; in such cases, the heat exchanger must be disassembled and cleaned. 2) Visible leakage occurs, making it impossible to tighten. When leakage occurs between the plates, and leakage persists even after checking the operating pressure of the plate heat exchanger and tightening the compression bolts, it is necessary to disassemble it for repair. 3.3.2 Selection of cleaning agents: When choosing an appropriate cleaning agent, it is necessary to take into full account the material of the plates as well as the composition of the scale. For example, the passivation (protective) layer on the surface of stainless steel plates must not be removed; grease can be eliminated using oil solutions that are capable of emulsifying water; organic or lipid-based contaminants can be removed with sodium hydroxide, at a maximum concentration of 1.5% and at a maximum temperature of 85 °C. A concentration of 1.5% corresponds to adding 3.75 L of a solution with a φ(NaOH) of 30% to 100 L of water; scale can be removed using nitric acid (HNO3), with a maximum concentration of 1.5% and a maximum temperature of 65 ℃. A concentration of 1.5% corresponds to adding 1.75 L of φ(HNO3) at 62% to 100 L of water; nitric acid also helps to form a passivation film on the surface of stainless steel. 3.3.3 Disassembly and cleaning procedures: The disassembly and cleaning process is carried out for plate heat exchangers. Taking a certain seawater desalination plate heat exchanger as an example, the plates of this heat exchanger are made of pure titanium, while the sealing strips are made of nitrile rubber (NBR). The steps are as follows: 1) Disassembly. Cooling and depressurization: Before disassembling the plate heat exchanger, it is necessary to reduce the temperature to below 40 ℃ and depressurize it. The cooling rate must not exceed 10 °C per minute, and the pressure reduction rate must not exceed 10×105 Pa per minute. When removing the heat exchanger, it is necessary to keep 2 or 4 bolts on the diagonal in place, and then remove the remaining bolts. Number the panels during the disassembly process to facilitate assembly. 2) Rinse. After removing the plates from the plate heat exchanger, they are cleaned using a high-pressure cleaner equipped with a brush, or with a soft brush along with cleaning solution or water, in order to remove the loose scale on the plates and to wash away any sediment and algae contaminants. Remove the sealing strip for inspection; if it does not meet the standards, it must be replaced. 3) Soak. Place the panel from which the sealing strip has been removed into the pre-prepared cleaning solution and soak it for 12 hours. If there is a thick layer of dirt or organic matter on the panel, the soaking time can be increased accordingly. Check the concentration of the cleaning solution as well as its calcium hardness; once these values no longer change, remove the panel and rinse it thoroughly with clean water. 4) Passivation. Passivation pre-coating is a conductive film of electrons formed on the metal surface that inhibits the metal’s dissolution process; this film has a very low rate of dissolution in the medium, which enables the anodic dissolution rate of the metal to be kept at a very low level. 5) Assembly. Once the passivation pre-coating is complete, rinse it with water to clean it. Inspect the plates and gaskets carefully; each plate and gasket must be free of defects and contaminants, and any dirt or sediment on the plates must be removed thoroughly. Clean all the plates and gaskets before assembling the panel bundle, as even foreign objects such as sand can cause leaks. If leakage persists in the plate bundle after cleaning and compressing it to its minimum size, it is recommended to replace the gasket. The plates should be reinstalled in their original positions within the plate bundle, in sequence according to their numbers. 6) Treatment of cleaning waste liquid. After cleaning, the waste liquid is added to the pickling tank along with caustic soda to neutralize it to a pH value of 6–9, after which it can be transported by tank truck to a designated location for disposal. 4 Advanced cleaning methods for plate heat exchangers: Through multiple disassemblies and cleanings of the plate heat exchangers in a seawater desalination plant, and by examining the cleaning results, it was found that disassembling the heat exchangers for cleaning yields better results, as the cleaning effect can be directly observed. However, disassembling and reassembling the heat exchangers requires time, labor, and resources; repeated disassemblies can cause deformation and cracking of the plates, and new sealing gaskets are needed, resulting in high costs. New types of non-dismantling cleaning agents can be explored; they not only protect the plates and gaskets from corrosion and aging but also prevent deformation of the plates as a result of disassembly and assembly, saving time and money. 5 Conclusions Plate heat exchangers have their advantages as well as disadvantages; in daily operation, it is necessary to reduce the likelihood of these disadvantages occurring by implementing effective measures: 1) It is essential to maintain strict control over water quality during normal operation. Strictly control the temperature and hardness of the refrigerant and heat medium in plate heat exchangers to extend their service life. 2) Periodic backwashing. The sludge and suspended impurities that enter the plate heat exchanger should be washed away promptly to reduce blockages and the formation of secondary scale. 3) During normal operation, check the temperature and pressure drop from time to time. An increase in pressure drop or a decrease in temperature indicates fouling of the plates, and in such cases the plate heat exchanger needs to be cleaned. Different methods can be employed depending on the situation. If the plate heat exchanger is not leaking but merely has reduced heat exchange efficiency, offline cleaning can be chosen, with cleaning once a year being ideal. If leakage occurs or offline cleaning is ineffective, the cleaning plates need to be disassembled for cleaning.