Types of heat exchanger scaling

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Scaling can be defined as the deposition of unwanted substances on heat transfer surfaces. Scaling is an inevitable result of heat transfer between two flowing streams on a metal wall. The degree of fouling varies greatly depending on the properties of the fluid being processed. Due to the deposition of unwanted materials, there is additional heat transfer resistance, which results in a decrease in the overall heat transfer coefficient. Due to the reduced overall heat transfer coefficient, more heat transfer area is required to meet the given heat load. The performance of the heat exchanger declines as fouling reduces the flow channels, thereby increasing the pressure drop. Worse still, the throughput is reduced due to partial blockage of the flow path. 01 Negative effects of scaling: Increased capital costs, as more heat transfer area is required to address the scaling issue ； The pumping cost is high ； The thermodynamic efficiency of condensation and refrigeration cycles is low ； The maintenance costs are high ； Overall loss ； Downtime costs ； 02 Types of fouling: Sedimentary fouling occurs due to the presence of dissolved substances such as CaCO3, Caso, and salts in water, which lead to the formation of scale on the heat transfer surfaces. These salts exhibit inverse solubility properties. Particle fouling is the accumulation on heat transfer surfaces of suspended particles present in the fluid. It is called sedimentary dirt. One example is the rust particles contained in the cooling water. Chemical reaction scaling: It is the formation of deposits through chemical reactions between various components in the fouling flow. Examples of chemical reaction fouling include the coking and cracking of hydrocarbons, polymerization, etc. Corrosion fouling occurs when the heat transfer surface itself reacts to form corrosion products. It will contaminate the heat transfer surface. Biological fouling occurs due to the attachment of microorganisms or larger organisms to the heat transfer surfaces. Solidification fouling occurs when a liquid solidifies on a supercooled heating surface. An example of solidified dirt is freezing. 03 Factors affecting scaling: Fluid velocity, fluid temperature, structural material, and surface finish of the material. A higher fluid velocity can minimize scaling. The ideal velocity of the liquid is 1.5–2.1 m/sec inside the pipe, and 1.0–1.5 m/sec outside the pipe. 04 When using dirty fluids inside pipes, the following strategy can be followed to prevent scaling: use pipes with a large diameter ； Maintain high speed ； Sufficient margin is left for pressure drop ； Retain available spare tube bundles or spare heat exchangers ； Use two shells in parallel ； Use metal wire finned tube inserts ； Use online cleaning. 05 When the dirty fluid is on the shell side, the following strategies can be adopted: Use a U-tube or floating head design ； Use a square or rotated square tube layout ； Minimize dead zones through optimal baffle design ； Maintain high speed on the shell side.