During the design of air heat exchangers, there is a contact thermal resistance between the tubes and fins, which, relative to the total heat exchange thermal resistance, cannot be entirely neglected. Particularly, contact thermal resistance on mechanical expansion tube equipment should not be overlooked, as such equipment is commonly used in low-pressure situations, such as in the air-cooling equipment for intake air in the HVAC industry and supercharged engines. In the assembly process of air heat exchangers, initial disturbances, the micro-hardness of the softer materials on the contacting surfaces, contact area, and contact pressure are all parameters determining the contact thermal resistance. Selecting the correct mandrel size can optimize the degree of contact between the surfaces.

For overall finned tube rolling, there is no issue of thermal resistance at the junction between fins and tubes, nor any concern for tube integrity, regardless of temperature and pressure. However, for certain geometrically structured finned tubes, the胀管 process is the only way to achieve this.