The role of the fins of the finned tube heat exchanger is to enhance heat transfer. When selecting finned tube specifications, the focus should be on enhancing heat transfer. For the enhanced heat transfer of heat exchangers, many theoretical and practical results have already been achieved. For example, by changing the fin material, improving the fin processing technology, increasing the fin ratio, improving the flow state and increasing the turbulence, etc., the height and thickness of the fin are affected by the manufacturing level and process and are limited to a certain Within the scale.
The heat transfer inside and outside the tube is single-phase convective heat transfer, and the heat transfer coefficient increases in proportion to the power exponent of the fluid velocity. Fluid flow is divided into laminar flow and turbulent flow. The main mechanism of laminar and turbulent flow enhancement is to use the increase of the secondary heat transfer surface to destroy the original unenhanced fluid velocity and temperature distribution field. The heat transfer coefficient of laminar flow is relatively low, and the heat transfer coefficient of turbulent flow is relatively high. Usually, a more economical enhancement measure is to change the flow state of laminar flow to turbulent flow. You can reduce the pipe diameter and add disturbances. To achieve this by tapping or increasing the flow speed of the medium, the most economical way in the current process is to use small-diameter heat exchange tubes, such as copper tubes for air conditioning, mostly in sizes of 9.52mm, 12.7mm and 15.88mm.
When the flow state inside and outside the tube is turbulent, a reasonable fin ratio should be used as much as possible. The heat transfer coefficient can be obtained by calculating the heat transfer coefficient of the inner film of the tube, the heat transfer coefficient of the outer film of the tube, and the thermal resistance of the tube wall. For bottlenecks, increase the density of fins and increase the height of the fins on the weaker heat transfer capacity. For the occasion of heat exchange with air, if the air outside the tube has no phase change, the higher the fin ratio, the better. The commonly used steel-aluminum composite fin tube on the air cooler is 25*2-57/2.3/0.3. The best example is that due to the limitation of the processing technology, the density and height of the fins cannot be increased, otherwise there is still potential to be tapped.
Conversely, for the air outside the tube with condensation and phase change, the existence of the water film can increase the heat transfer capacity on the air side, and the difference in heat transfer coefficients inside and outside the tube is not that big. At this time, the focus is on how to remove the condensate generated in time. The condensate stagnating in the middle of the fins will cause a drag on the heat transfer of the fin tubes. It is equivalent to the high temperature and stagnant water outside the tube, which cannot exchange heat with the air. At this time, a larger sheet distance should be used to reduce the tension on the water side, and the selection of fin tube specifications is also particularly important.
The selection of finned tube specifications must also consider the adaptation of the material to temperature. Such as high-temperature flue gas heat exchange, high-frequency welding fins should be used, and the temperature resistance of aluminum rolled fins is not suitable.
Of course, anti-corrosion, service life, contact thermal resistance, and heat exchange effect are all important points in the selection of specifications for finned tube heat exchangers.