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What should be the pitch and distance of the finned tube heat exchanger tubes?
Dec 07, 2018

Under normal circumstances, the pitch and sheet height of the fin-and-tube heat exchanger mainly affect the winging ratio, and the finning ratio has a great relationship with the film heat transfer coefficient of the medium inside and outside the tube. If the heat transfer coefficient of the inner and outer membranes of the tube is quite different, a fin tube with a relatively large 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 large, such as the exchange of hot and cold air. When the hot air is reduced below the dew point, a finned tube heat exchanger can be used. In the case of heat exchange without phase change between air and air, or heat exchange between water and water, it is usually suitable for bare tubes. It is of course also possible to use a low finned tube, since this is a weak heat transfer coefficient, and strengthening either side has a certain effect. However, the excessive winging ratio is not obvious, and the best inner and outer contact areas of the pipe are simultaneously strengthened, and a threaded pipe or a grooved pipe can be used.

The fin spacing is mainly considering factors such as dust accumulation, dusting, easy cleaning, etc., and it is necessary to strictly meet the requirements of equipment pressure drop. When arranging, the spacing between the tube and the tube is not too large, and generally more than 1 mm is suitable for the tube. In the process of heat exchange, when the air flows through the finned tube heat exchanger, the two sides of the fin are mainly engaged in heat exchange. There is only a small amount of radiation heat transfer in the middle part of the two finned tubes, and the heat transfer effect is not obvious. This part has no fins and no resistance, and the air is easy to penetrate. In the process of air heating, the cold air without heat exchange will form a neutralization with the heated hot air passing through the middle of the fin, which will reduce the heat exchange effect. Compared with the foreign finned tube heat exchanger, the tube The spacing is only 0.5 mm larger than the outer diameter of the fins, and the importance of the tube spacing when the finned tubes are arranged is visible.

The tube arrangement of the finned tube heat exchanger should be arranged as far as possible in an equilateral triangle. After the hot air in the first row, the resistance is encountered in the second row, and there is a certain rebound wind, so that there is no dead angle in the 360° heat exchange of the entire finned tube. Therefore, you should avoid the arrangement of the isosceles triangles and try not to use squares 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 finned tubes. The ratio of the narrow gap flow surface to the windward surface should be calculated when the fin tube is arranged, so that the wind speed is calculated according to the oncoming wind speed. The air mass flow rate is obtained, and the coefficient of friction is obtained for the dynamic viscosity of the air at different temperatures.