Selection of cleaning agent
At present, the choice of cleaning agent is pickling, which includes organic acid and inorganic acid. Organic acids mainly include oxalic acid, formic acid, etc. Inorganic acids mainly include hydrochloric acid, nitric acid, etc. According to the analysis of scale formation, process, material and scale composition of plate heat exchanger:
(1) The plate heat exchanger has small circulation area and complex internal structure, so it is not easy to discharge the cleaning liquid if it produces precipitation.
(2) The plate heat exchanger is made of nitinol alloy and hydrochloric acid is used as cleaning fluid. It is easy to produce strong corrosion to the plate and shorten the service life of the plate heat exchanger. Through repeated tests, it was found that formic acid was the best cleaning solution. Adding buffer and surfactant into formic acid cleaning solution, the cleaning effect is better, and the corrosion of cleaning solution to plate can be reduced.
2. The basic principle of removing scale in plate heat exchanger
(1) Dissolution: the acid solution is easy to react with calcium, magnesium and carbonate salt water scale to generate soluble compounds, which make the scale dissolve.
(2) Stripping effect: acid solution can dissolve oxide on metal surface, destroy the combination with scale. Thus, the scale attached to the metal oxide surface is peeled off. And fall off.
3. Process requirements for scale cleaning
(1) Concentration of pickling solution: according to the repeated tests, the pickling solution should be prepared at the concentration of 81% formic acid, 17% water, 1.2% buffer and 0.8% surfactant, with excellent cleaning effect.
(2) Pickling temperature: raising 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 plate heat exchanger. Through repeated tests, it is found that the pickling temperature should be controlled at 60 ℃.
(3) Passivation treatment: after pickling, most of the scale and metal oxide on the surface of plate heat exchanger are dissolved and fall off, exposing brand-new metal, which is very easy to be corroded. Therefore, after pickling, passivation treatment shall be carried out on the plate of plate heat exchanger.