Low-Finned Tubes for Condensation

Abstract

Low-finned tubes offer a wide potential for applications in heat exchangers where condensation occurs on the outside of the heat exchanger tubes. Typically, the fin height and the inter-fin spacing is smaller than 1 mm. This leads to an increased surface area, which is usually increased by a factor of 2–5. Additionally to this, fluid dynamic effects come into play, which reduce the condensate layer on the tube and thereby increase the heat transfer coefficient, too. With horizontally arranged smooth tubes a bundle effect can be seen since the condensate layer increases from row to row. This negative effect can be reduced with low-finned tubes where the condensate is withdrawn due to capillary forces and higher heat transfer coefficients can be observed. Pure component condensation with free convection can be described with the theory derived by Nusselt (1916) for smooth tubes. For low-finned tubes a wide range of experimental data is presented along with a newly developed model to predict the outer heat transfer coefficients. The condensation of mixtures differs from the condensation of pure substances, since the thermal resistance in the vapour phase cannot be neglected. Thereby, the heat transfer coefficients are lowered for both smooth and low-finned tubes. An innovative model, which includes the effect of the mole fraction of the mixture components on the heat transfer, is presented. As compared to the well-known film model the new model is based on a fit of the thermodynamic correction factor and can describe the experimentally measured heat transfer coefficients much better.