An Evaluation of Models for Condensation Heat Transfer on Low-finned Tubes

Several theoretical models for calculating heat-transfer coefficients during condensation on horizontal low-finned tubes have been examined. In the present paper these are discussed and compared with recent experimental data considered to be reliable. It is evident that the effect of both gravity and surface tension-induced pressure gradient must be accounted for when calculating the condensate film thickness, and hence the heat flux on the various parts of the fin and tube surface. Of critical importance also is capillary retention, which causes condensate to be held between the fins on the lower part of the surface. Except when the thermal conductivity of the tube material is high, it is also necessary to take account of the temperature drop arising from conduction in the fin. The experimental data used for the comparisons cover a range of fin geometric variables (height, thickness and space between fins) and tube diameter as well as different tube materials and condensing fluids. In one of the two most successful models the condensate flow and heat transfer over the various surfaces are analysed in a quite rigorous manner, but this model necessarily incorporates certain simplifying assumptions to obtain a fourth order differential equation for the condensate film thickness profile along the fin surface. The second successful theory uses dimensional analysis to treat surface tension-induced condensate drainage. In this case recourse to experimental data is needed to determine two constants. These are both of order unity and the final result is in the form of algebraic equations which may readily be used in design and optimization.