Heat transfer and heating process of finned tube heat exchanger
In general, the fin to tube heat exchanger spacing and fin height mainly affect the fin to tube ratio. The fin to tube ratio is closely related to the film heat transfer coefficient of the medium inside and outside the tube. If the film heat transfer coefficient inside and outside the tube is quite different, the fin to tube with larger fin to tube ratio, such as steam heating air, should be selected to a certain extent. When there is phase change in one side of the medium, the difference of heat transfer coefficient will be large, such as the exchange of hot and cold air. When the hot air decreases below the dew point, Murphy experts suggest that fin tube heat exchanger can be used.
In the case of air and air without phase change, the water and water heat exchange of fin tube heat exchanger is usually suitable for bare tube. Of course, the low fin tube can also be used, because it belongs to the weak heat transfer coefficient at this time, and strengthening either side of it has a certain effect. However, the effect of too large wing ratio is not obvious. The good situation is that the contact area inside and outside the tube is strengthened at the same time, and the threaded tube or grooved tube can be used.
The distance between fins and tubes of heat exchanger is mainly due to the factors of ash deposition, dust deposition, easy cleaning, etc. at the same time, it is required to strictly meet the pressure drop requirements of the equipment. When arranging, the distance between pipes is not easy to be too large. Generally, more than 1mm is suitable for pipe laying. In the process of heat transfer, when the air flows through the finned tube heat exchanger, the positive and negative surfaces of the fins participate in the heat transfer. There is only a small amount of radiation heat transfer in the middle of the two finned tubes, and the heat transfer effect is not obvious.
Finned tube heat exchanger has no fins, no resistance, and air is easy to penetrate. In the process of air heating, the unheated cold air will be neutralized with the heated hot air passing through the middle of the fins, which will reduce the heat exchange effect. Compared with foreign fin tube heat exchangers, the tube spacing is only 0.5mm larger than the outer diameter of the fins. It can be seen that the importance of tube spacing can be seen when the fin tubes are arranged.
The arrangement of finned tube heat exchanger tubes shall be arranged as per the regular triangle as much as possible. When the hot air passes through the first row, it encounters resistance in the second row, and there is a certain rebound wind, so that the whole finned tube 360 ° heat exchange does not have dead angle. Therefore, laser welding stainless steel finned tubes should avoid isosceles triangle arrangement and try not to use square arrangement, unless there are special requirements.
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