Plate Heat Exchanger Heat Transfer Technology
Heat Transfer Coefficient of Heat Exchanger The heat transfer coefficient of the heat exchanger can be improved only by simultaneously increasing the surface heat transfer coefficients on both hot and cold sides of plates, reducing the thermal resistance of fouling layers, selecting plates with high thermal conductivity and decreasing plate thickness.
(1) Surface Heat Transfer Coefficient of Plates The corrugations of plate heat exchangers can induce turbulence in fluids at low flow velocity (Reynolds number ≈150), thus achieving a high surface heat transfer coefficient, which is related to the geometric structure of plate corrugations and the flow state of media. Plate patterns include herringbone, flat, spherical and others. Years of research and tests show that herringbone plates with triangular corrugation cross-section (sinusoidal shape delivers high surface heat transfer coefficient, low pressure drop and uniform stress distribution under pressure, yet difficult to process) have high surface heat transfer coefficient. The larger the included angle of corrugations, the higher the medium flow velocity in inter-plate channels, and the higher the surface heat transfer coefficient.
(2) Plate Materials with High Thermal Conductivity Austenitic stainless steel, titanium alloy, copper alloy and other materials are available. Stainless steel features good thermal conductivity with thermal conductivity of about 14.4 W/(m·K), high strength, excellent stamping performance and good oxidation resistance, at a lower price than titanium and copper alloys. It is widely applied in heating projects.
(3) Reduction of Fouling Layer Thermal Resistance The key to reducing fouling thermal resistance of plate heat exchangers is to prevent scaling on plates. The heat transfer coefficient will drop when the scale thickness reaches 1 mm. Therefore, it is necessary to monitor water quality on both hot and cold sides to avoid plate scaling and attachment of impurities. Some heating enterprises add chemicals into heating media to prevent water theft and corrosion of steel components. Attention shall be paid to contamination on plates caused by water quality and sticky agents. Filters shall be adopted if there are viscous impurities in water. Non-viscous chemicals are recommended.
(4) Reduction of Plate Thickness The designed thickness of plates has no relation to corrosion resistance, but relates to pressure bearing capacity of the heat exchanger. Thicker plates improve pressure resistance. When herringbone plates are assembled, adjacent plates are installed reversely and their corrugations contact each other, forming dense and evenly distributed support points. The corner holes and edge sealing structure of plates have been gradually optimized to guarantee good pressure resistance. The pressure bearing capacity of domestic gasketed plate heat exchangers has reached 2.5 MPa. Plate thickness exerts great influence on heat transfer coefficient. A 0.1 mm reduction in plate thickness raises the overall heat transfer coefficient by approximately 600 W/(m·K) for symmetric plate heat exchangers and about 500 W/(m·K) for asymmetric ones. Plates with smaller thickness shall be selected as far as possible.
Mean Temperature Difference Flow patterns of plate heat exchangers include counter flow, parallel flow and mixed flow (both counter and parallel flow). Under identical working conditions, the logarithmic mean temperature difference is the largest in counter flow, the smallest in parallel flow, and intermediate in mixed flow. To increase the logarithmic mean temperature difference, counter flow or mixed flow close to counter flow shall be adopted as much as possible, the temperature of hot-side fluid shall be raised and the temperature of cold-side fluid reduced.
Positions of Inlet and Outlet Pipes For single-pass plate heat exchangers, fluid inlet and outlet pipes shall be arranged on the fixed end plate side for easy maintenance. The larger the temperature difference of media, the stronger the natural convection of fluids and the greater the impact of stagnant zones. Therefore, the inlet and outlet shall be arranged as hot fluid entering from the top and exiting from the bottom, while cold fluid entering from the bottom and exiting from the top. This arrangement reduces the influence of stagnant zones and improves heat transfer efficiency.
