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Selection of tube spacing and fin spacing of finned tube heat exchanger

 

Under normal circumstances, the spacing and height of the finned tube heat exchanger mainly affect the fining ratio, which has a great relationship with the film heat transfer coefficient of the medium inside and outside the tube. If the heat transfer coefficients of the inner and outer films of the tube are quite different, a finned tube with a larger fin ratio, such as steam heating air, should be selected.


When there is a phase change in one side of the medium, the difference in heat transfer coefficient will be greater, such as the exchange of hot and cold air. When the hot air drops below the dew point, a finned tube heat exchanger can be used.


In the case of heat exchange between air and air without phase change, or heat exchange between water and water, a bare tube is usually more suitable. Of course, a low-finned tube can also be used, because at this time it is a weak heat supply coefficient, and strengthening either side of it will have a certain effect.


However, the effect of too large finning ratio is not obvious, and the best internal and external contact area of the tube is strengthened at the same time, and a threaded tube or a fluted tube can be used. The pitch of the fins is mainly to consider factors such as dust accumulation, dust formation, easy cleaning, etc., and at the same time, it must strictly comply with the equipment’s pressure drop requirements. When arranging, the distance between the tube and the tube is not easy to be too large, generally more than 1mm is suitable for tube layout.


In the process of heat exchange, when air flows through the finned tube heat exchanger, the front and back sides of the fin are mainly involved in heat exchange. There is only a small amount of radiant heat transfer in the middle of the two finned tubes, and the heat transfer effect is not obvious. Since this part has no fins and no resistance, the air is easy to penetrate.


In the process of air heating, the unheated cold air will neutralize the heated hot air passing through the middle of the fins, which reduces the heat exchange effect. Compared with foreign finned tube heat exchangers, tubes The spacing is only 0.5mm larger than the outer diameter of the fins, which shows the importance of the tube spacing when the fin tubes are arranged. The tube arrangement of the finned tube heat exchanger should be arranged in an equilateral triangle as far as possible.


The hot air passing through the first row encounters resistance in the second row, and there is a certain rebound wind, so that the 360° heat exchange of the entire fin tube does not have a dead angle. Therefore, the isosceles triangle arrangement should be avoided, and the square arrangement should not be used as much as possible, unless there are special requirements.


The pressure drop on the air side is a very important parameter in the design, which has a great relationship with the arrangement of the fin tube. When designing the fin tube arrangement, the ratio of the narrow gap flow surface to the windward surface should be calculated, so as to calculate according to the front wind speed. The air mass flow rate is calculated, and the friction coefficient is calculated for the dynamic viscosity of the air at different temperatures.

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