「Why do \"plate heat exchanger units\" develop scale? How to remove scale?

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In plate heat exchanger units, most are heat exchange systems that use water as the heat transfer medium. Certain salts precipitate out of the water as temperature rises and adhere to the surfaces of the plate heat exchanger plates, forming scale. Furthermore, if polyphosphate buffers are added to the cooling water, high water pH can also lead to the formation of scale. In the early stages of scale formation, it is relatively soft; but as the scale layer builds up and the heat transfer conditions deteriorate, the scale becomes harder and adheres firmly to the surface of the heat exchange plates. Furthermore, when the operating conditions of the plate heat exchanger are suitable for crystal formation in the fluid, a scale layer formed by the crystallization of the material can accumulate on the surface of the heat exchange plates; when the fluid contains a high amount of organic substances and the flow rate is low, some of these organic substances will also deposit on the surface of the heat exchange plates, resulting in loose, porous, or gel-like fouling. Methods for cleaning the plate heat exchangers in plate heat exchanger units: 1. Mechanical method: When the plates are slightly blocked or covered with scale, various mechanical forces and energies can be used to remove the dirt from their surface; for example, the plates can be manually cleaned using brushes to achieve a cleaning effect. 2. Flushing method: (1) Counterflow flushing: When scaling is not severe, this method is generally used during operation or during short periods of shutdown; it is possible to avoid disassembling the equipment, but a counterflow bypass line must be pre-installed on the device. (2) High-pressure water jet cleaning: When scaling is severe, using a high-pressure water jet to clean the heat exchanger is effective. Different types of rotating nozzles can be used for various heat exchangers – these nozzles can be rigid or rotatable – and the pressure can be adjusted freely from 10 MPa to 200 MPa. 3. Chemical method: For descaling using chemical methods, it is first necessary to conduct laboratory analysis of the scaling substances to determine their properties, so as to decide which solvent should be used for cleaning. Generally, alkaline cleaning (such as soda ash, caustic soda, trisodium sulfate, etc.) is used for sulfate and silicate scale, while acidic cleaning (such as hydrochloric acid, nitric acid, phosphoric acid, carbonic acid, etc.) is used for carbonate scale. Oil scale coking can be removed by using a cleaning solution prepared from sodium hydroxide, sodium carbonate, laundry detergent, liquid detergent, sodium silicate, and water in specific proportions.