An Analysis of Heat Transfer in Assemblies of Finned Heat-Exchange Elements

Together with the widely used heat exchangers with an extended heat transfer surface, uniquely designed heat-exchange assemblies are also available. This paper presents the results of thermohydraulic tests of finned tube assemblies brazed from KP 20 elements of four sizes. The elements are a 0.4 mm thick plate coated with 0.025 mm thick copper film. One or two 17 mm long tapered necks are drawn in the plates. A set of cones inserted into one another forms a brazed finned tube with a toothed inner surface. To check the resistance of the brazed joint to water temperature gradients, 301 cycles of experiments were carried out to determine the temperature of the water supplied to the assembly. Each cycle differed by the water temperature at the outlet from the upper and lower tubes of the assembly and the temperature of the water and the fin metal. After the experiments, the assemblies retained their shape, dimensions, and tightness. It has been found out that the experimental results agree with the predictions by the regulatory recommended practice if a correction for the small number of rows in the tube bundle is used. The tested assemblies demonstrated high strength characteristics. High fin thickness-averaged heat conductivity at a level of 75 W/(m K) was confirmed. It has been demonstrated that the heat transfer rate increases by a factor of 2.15 as compared to the predictions for smooth tubes with a moderate increase in the hydraulic resistance caused by process protrusions on the tube inner surface. The expressions for thermohydraulic design calculations of heat exchangers consisting of similar tubes were corrected. The calculations were made using expressions from the regulations RD 24.035.05-89 “Thermal and Hydraulic Design Calculation of Heat-Exchange Equipment for Nuclear Power Plants” with amendments taking into account the high degree of finning of the tested tubes and the asymmetry of the edges of the fins relative to the axis of the pressure bearing tubes as well as the small number rows in the tube bundles.