Cleaning methods for scaling of plate heat exchangers

        1. The selection of cleaning agents is currently based on acid washing, which includes both organic and inorganic acids. Organic acids mainly include oxalic acid, formic acid, etc. Inorganic acids mainly include hydrochloric acid, nitric acid, etc. Based on the analysis of scaling and process, material, and scale composition of the heat exchanger, it is concluded that: 1) The flow area of the heat exchanger is small, and the internal structure is complex. If the cleaning solution settles, it is difficult to discharge.2) The material of the heat exchanger is nickel titanium alloy, and hydrochloric acid is used as the cleaning solution Easy to cause strong corrosion to the plates, shortening the service life of the heat exchanger. Through repeated experiments, it has been found that choosing formic acid as the cleaning solution has a good effect. Adding buffering agents and surfactants to formic acid cleaning solution can improve the cleaning effect and reduce the corrosion of the plate by the cleaning solution. Chemical experiments on scale samples have shown that formic acid can effectively remove scale. Through acid immersion tests, it was found that formic acid can effectively remove scale attached to the plates, and its corrosion effect on the heat exchanger plates is also minimal.

        2. The basic principle of removing scale: 1) Dissolution: Acid solution easily reacts with calcium, magnesium, and carbonate scale to generate soluble compounds, which dissolve the scale. 2) Stripping effect: Acid solution can dissolve oxides on the surface of metals The combination of destruction and scale. Thus, the scale adhering to the surface of the metal oxide is peeled off and detached.3) Gas lifting effect: After the acid solution reacts with calcium, magnesium, and carbonate scale, a large amount of carbon dioxide is produced. Carbon dioxide (CO2) has a certain lifting force during the overflow process for difficult to dissolve or slowly dissolving scale layers, causing scale to fall off the heated surface of the heat exchanger. 4) Loosening effect: For mixed scale containing silicates and sulfates, the residual scale will become loose due to the dissolution of calcium, magnesium, carbonate, and iron oxides in the acid solution, making it easy to be washed down by the flowing acid solution.

        3. Process requirements for cleaning scale: 1) Pickling temperature: Increasing the pickling temperature is beneficial for improving the descaling effect. If the temperature is too high, it will intensify the corrosion of the heat exchanger plate by the pickling solution. Through repeated experiments, it has been found that the pickling temperature should be controlled at 60C. 2) Acid pickling solution concentration: According to repeated experiments, the acid pickling solution should be prepared at a concentration of 81% formic acid, 17% water, 1.2% buffering agent, and 0.8% surfactant to achieve excellent cleaning effect. 3) Acid washing method and time: The acid washing method should be carried out by combining static soaking and dynamic cycling.The pickling time is static soaking for 2 hours, followed by dynamic cycling for 3-4 hours. During the pickling process, samples should be taken frequently to test the pickling concentration. When the difference in concentration between adjacent tests is less than 0.2%, the pickling reaction can be considered complete. 4) Passivation treatment: After acid pickling, most of the scale and metal oxides on the surface of the plate heat exchanger are dissolved and peeled off, exposing new metal that is highly susceptible to corrosion. Therefore, after acid pickling, the heat exchanger plates are passivated.

        4. Specific steps for cleaning scale: 1) Rinse: Before pickling, perform an open rinse on the heat exchanger to ensure that there are no impurities such as mud or scale inside. This can improve the pickling effect and reduce the acid consumption. 2) Pour the cleaning solution into the cleaning equipment and then inject it into the heat exchanger. 3) Acid washing: Soak the heat exchanger filled with acid solution in a static state for 2 hours. Then continuously cycle it dynamically for 3-4 hours. During this period, clean it alternately in both positive and negative directions every 0.5 hours. After acid washing, if the pH value of the acid solution is greater than 2, the acid solution can be reused. Otherwise, the acid solution should be diluted and neutralized before being discharged. 4) Alkali washing: After the acid washing is completed, prepare NaOH, Na, PO, and softened water in a certain proportion,Using dynamic cycling to perform alkaline washing on the heat exchanger, achieving acid-base neutralization and preventing corrosion of the heat exchanger plates. 5) Water washing: After alkaline washing, use clean softened water. Rinse the heat exchanger repeatedly for 0.5 hours to thoroughly remove any residue inside. 6) Record: During the cleaning process, the time of each step should be strictly recorded to check the cleaning effect. In short, after cleaning, the heat exchanger should undergo a pressure test and can only be used after passing the test. 3. Measures to prevent scaling of plate heat exchangers: 1) Strictly control the water quality during operation. The water in the system and the softened water in the softening tank must undergo strict water quality testing before being injected into the pipeline network.

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