Thermal contact conductance of expanded tube joints is dependent upon the interfacial pressure between the mating parts. A review of the literature indicates that very little work has been done towards relating the joint stresses to the dimensional parameters for mechanically expanded plate finned-tube heat exchangers, commonly called fin coils. This paper presents an experimental testing technique to investigate the fin-to-tube mechanical bond based upon the axial pull strength of the joint.

Typical coils from two manufacturers were used in this investigation. They were fabricated from 3/8 in. copper tubes and plate aluminum fins. The coils tested had fin spacings of 6, 7, 8, 12, and 20 fins per inch, fin thicknesses of 0.008 and 0.0055 in. tube thicknesses of 0.015 and 0.016 in. and diametrical interferences of 0.001, 0.006, and 0.010 in.

The maximum force attained during a tube pullout was found to be descriptive of the bond. Plots of this force versus fin number and net interference are presented that illustrate the sensitivity of this force to the fin thickness, fin number, and net interference. These plots show that the fin thickness has the predominant effect on the pullout force and, in turn, the bond between the fin collar and the tube.

Units: I-P