More application in the drying air heater apparatus, the heated air is mainly used. Drying medium temperature around 150 ℃, commonly finned steam heater. There SRZ (around steel pipe) and SRL (steel pipe and aluminum fins), and other varieties of more than 30 series over inside. Fin spacing 2.3-8mm, good heat transfer performance, resistance is not. But after a long run, you need to clean between the fins fouling. Finned air circulation fan heater is installed in the drying cabinet body. Piece tube air heaters finned steam heater pass into steam, condensation heat transfer coefficient is large (7000-12000W / ㎡.K), so the overall heat transfer coefficient is mainly controlled by the air-side tube. Available empirical data drawn charts, according to the geometry of finned tubes, air average temperature, fin tube bundles equivalent diameter and air flow through the mass flow rate at the minimum section pursuit take the total heat transfer coefficient, the lack of data usually takes 50- 150 W / ㎡.K.