Plate heat exchanger repair: cleaning and protection of plates_
959 Keeping the plates clean is one of the important conditions for maintaining a high heat transfer coefficient. Between the plates, the medium moves along the narrow and tortuous flow channel. Even if a not too thick scale is produced, it will cause changes in the flow channel, which will significantly affect the movement of the fluid, increase the pressure drop and increase the heat transfer coefficient For example, a sugar factory uses a plate heat exchanger for heating. Because the secondary water is not well treated, the flow channel is blocked in only two months during use and cannot continue to operate. Plate heat exchanger maintenance can start from the following aspects to clean and protect the plates: 1. The chemical cleaning method is to circulate a chemical solution through the heat exchanger to dissolve the dirt on the surface of the plate. discharge. This method does not need to disassemble the heat exchanger, simplifies the cleaning process, and also reduces the cleaning labor. Because the plate corrugation can promote the violent turbulence of the cleaning liquid, which is conducive to the dissolution of the scale layer, the chemical cleaning method is an ideal method. 2. Mechanical (physical) cleaning method. This method is to manually wash the plates with a brush, so as to achieve the purpose of removing dirt on the surface of the plates. Although this method is relatively straightforward, it is not easy to clean the harder and thicker scale layer. 3. Comprehensive cleaning method. For the case where the dirt layer is relatively hard and thick, it is difficult to clean it simply by using one of the above methods. The comprehensive method is to first soften the scale layer by chemical cleaning method, and then remove the scale layer by mechanical (physical) cleaning method to keep the plate clean. 4. Precautions during cleaning ①The solution must maintain a certain flow rate during chemical cleaning, generally 0.8～1.2m/s. Its purpose is to increase the turbulence of the solution. ②Different chemical cleaning fluids should be used for different dirt. In addition to the frequently used diluted soda ash solution, 5% nitric acid solution can be used for scale. For the scale generated in the production of soda ash, 5% hydrochloric acid solution can be used. But do not use chemical lotions that corrode the plates. ③ During mechanical (physical) cleaning, it is not allowed to use carbon steel brushes to scrub the stainless steel plates, so as not to accelerate the corrosion of the plates. At the same time, the surface of the plate cannot be scratched or deformed. ④The cleaned plates should be rinsed with clean water and wiped dry, and the plates should be prevented from being deformed when placed. Due to the continuous expansion of the application field of plate heat exchangers, different media and processes have different requirements for temperature resistance, pressure resistance and corrosion resistance. It is obviously impossible to use a 'universal' gasket to adapt to all situations. Instead, gaskets of different materials should be used for different situations. The commonly used gasket cross-sectional shape is a six-sided row. Commonly used gasket types include natural rubber, neoprene, EPDM, silicone rubber, fluororubber, asbestos fiberboard, etc.