1、 The main reason and harm of fouling in plate heat exchanger are that during the use of plate heat exchanger, calcium, magnesium and carbonate in water are decomposed into calcium carbonate and calcium hydroxide precipitates after heat treatment, and the hard scale is formed. Because of the poor thermal conductivity of scale, the heat exchange efficiency of heat exchanger is reduced and the heat energy is wasted seriously. 2、 The cleaning method of fouling of plate heat exchanger 1. Mechanical cleaning (because of the hard scale, it must be brushed with iron) is the simplest cleaning method, but the disadvantages are: ① there are scratches on the plate and it is easier to hang the scale after brushing. ② The workers work in cold water and the working conditions are poor. ③ When cleaning, the heat exchanger must be disassembled, which will cause damage to the plate and rubber strip, resulting in high labor intensity. 2. Chemical cleaning method: pickling is used at present. It is found that formic acid and oxalic acid are the best cleaning solution without corroding the plate of heat exchanger（ 1) Clean with formic acid. Adding buffer and surfactant into formic acid cleaning solution can improve the cleaning effect and reduce the corrosion of the cleaning solution. ① Basic principle of scale removal a dissolution: acid solution is easy to react with calcium, magnesium and carbonate scale to form soluble compounds and dissolve scale. B stripping effect: acid solution can dissolve the oxide on the surface of metal, destroy the combination with scale, so as to peel off the scale attached to the surface of metal oxide and fall off. C gas lift: after acid solution reacts with calcium, magnesium and carbonate scale, a large amount of CO2 is produced. During the overflow process, CO2 gas has a certain lift power for the scale layer which is difficult to dissolve or slow to dissolve, making the scale fall off from the heating surface of heat exchanger. D loosening: for mixed scale containing silicate and sulfate, the residual scale will become loose and easily washed down by flowing acid solution due to the dissolution of oxides of calcium, magnesium, carbonate and iron in acid solution. ② Process requirements of scale cleaning a pickling temperature: increasing the pickling temperature is conducive to improving the scale removal effect. If the temperature is too high, it will aggravate the corrosion of pickling solution on the plate of heat exchanger, and the pickling temperature should be controlled at 60 ℃. B acid washing solution concentration: according to the test, the acid washing solution should be prepared according to the concentration of formic acid 81.0%, water 17.0%, buffer 1.2%, surfactant 0.8%, with excellent cleaning effect. C pickling method and time: pickling method should be combined with static soaking and dynamic circulation. The pickling time is first static soaking for 2 h, then dynamic cycling for 3 h ~ 4 h. in the pickling process, samples should be taken frequently to test the pickling concentration. When the difference between the two adjacent test concentrations is less than 1.2%, the pickling reaction is considered to be over. D passivation treatment: after pickling, most of the scale and metal oxides on the surface of plate heat exchanger are dissolved and peeled off, exposing brand-new metal, which is easy to be corroded. Therefore, after pickling, the plate of heat exchanger should be passivated. ③ The specific steps of cleaning scale a washing: before pickling, the heat exchanger should be washed in an open way, so that there is no mud, scale and other impurities in the heat exchanger, which can not only improve the pickling effect, but also reduce the acid consumption of pickling. B. pour the cleaning solution into the cleaning facility, and then inject it into the heat exchanger. C pickling: soak the heat exchanger filled with acid solution statically for 2 h, and then continuously and dynamically cycle for 3 h to 4 h, during which the positive and negative cleaning is carried out alternately every 0.15 H. After pickling, if the pH value of acid solution is greater than 2, the acid solution can be reused, otherwise, the acid solution should be diluted and neutralized. D alkali washing: after acid washing, the heat exchanger is prepared with trisodium phosphate and softened water in a certain proportion, and the heat exchanger is alkali washed by dynamic circulation to achieve acid-base neutralization, so that the heat exchanger plate is no longer corroded. E water washing: after alkali washing, wash the heat exchanger repeatedly with clean softened water for 0.15h to thoroughly wash the residue in the heat exchanger（ 2) Wash with oxalic acid. Firstly, according to the material and the color of the scale, the oxalic acid can react with the scale and not corrode the plate（ 1) During the test, a does not dismantle the heat exchanger, install water injection valve on the inlet and outlet valves of the secondary network of the heat exchanger, inject 3% ~ 5% oxalic acid solution in the water tank into the heat exchanger, and force the oxalic acid solution to circulate between the heat exchanger and the water tank by using the circulating pump to achieve the purpose of cleaning. B. clean twice, clean for three or four hours every time. After the heat exchanger is disassembled, the plate is clean and fresh, and the test is successful.