YUHONG GROUP - Impact of Fin Spacing and Arrangement on Finned Tube Heat Transfer Performance

1. The fin spacing has a greater impact on the fin surface heat transfer coefficient. Under the premise of the existing structural dimensions, when the fin spacing is within 10mm, the surface heat transfer coefficient decreases as the fin spacing decreases, and when the fin spacing is more than 10mm, the surface heat transfer between the fins basically does not affect each other.
2. After reducing the fin spacing, the finned tube surface area increases, but the surface heat transfer coefficient decreases, comprehensively, reducing the fin spacing can reduce the fin surface heat transfer thermal resistance to a certain extent, and its effect is gradually weakened with the increase in ambient temperature.
3. After increasing the fin spacing, the fin surface heat transfer coefficient increases to a limited extent, and the fin external surface area decreases due to the increased spacing, which is unfavourable to the finned tube heat dissipation from a comprehensive point of view.
4. The finned tube external surface area and fin surface heat transfer coefficient are the two main factors that determine the convective heat transfer of the finned tube. In determining the fin spacing of the finned tube heat exchanger, the two should be considered comprehensively on the finned tube heat transfer performance.
In the field of flue gas waste heat recovery of various types of waste heat boilers and economisers of conventional power station boilers, spiral finned tube is the most widely used type of enhanced heat transfer tube, mainly due to its easy production and obvious enhanced heat transfer effect. The traditional spiral finned tube consists of a base tube with continuous spiral fins. In order to further improve the finning ratio and to make manufacturing easier, a serrated spiral finned tube with serrated fins was developed on the basis of the continuous spiral finned tube.

Compared with the continuous spiral finned tube, the serrated spiral finned tube has the following important advantages:
1. Eliminate the drawing process of fin material, easy to manufacture
2. The fins disturb the fluid more strongly, and the heat transfer coefficient is higher.
3. Fin height can be made higher, finning ratio is greater.
4. The airflow is easy to penetrate into the root of the fin, and the efficiency of the fin is improved.

Based on the above reasons, heat transfer equipment with serrated spiral finned tube is lighter in weight and lower in cost, so it has been applied in large-scale heat transfer equipment such as combined cycle waste heat boiler, coal economiser of power station boiler, heat pipe air preheater, refining and chemical heating furnace, and so on.

Tube bundle arrangement structure:
According to the comparative study of the downstream and staggered row of serrated spiral finned tube bundles, under the same conditions, the heat transfer coefficient of the fin side of the staggered row of tube bundles is about 3 times of that of the downstream tube bundles, and the resistance is about 1.5 times of that of the downstream tube bundles. After the staggered arrangement, the increase in resistance of the finned tube bundle can be compensated by the reduction of the number of tube rows in the bundle under the same heat transfer capacity.
As a result, staggered finned tube bundles have been more widely used in the field of engineering heat transfer. However, in some cases, for example, when the tube bundle is working in an ash-containing gas stream, for the sake of easy ash cleaning, slagging prevention and other considerations are still used in the paralleled tube bundle. In the combined cycle waste heat boiler, the flue gas outside the tube contains little ash, so basically all the convection heat exchanger tube bundle staggered arrangement.