Film condensation of steam in the presence of non-condensing gases

Abstract

Considerable progress has been made in recent years towards the understanding of the mechanism of heat transfer by condensation in the presence of non-condensing gases. For the case of laminar film condensation on a plane vertical, surface and for laminar flow of the vapour-gas mixture, boundary layer solutions have been given [22,43,44]. In the present work these are reviewed and modifications to the approximate solutions are suggested. Such limited experimental data (for condensation on vertical flat plates under conditions of free convection) as was available at the outset of the present study, disagreed widely with theory. In the present work further careful measurements were made to test the validity of the theory. The condensing chamber (i.e. the steam chamber) was large in comparison to the condensing surface and care was taken to avoid forced convection effects associated with the vapour supply to the condenser. The vertical test plate was watercooled and the surface temperature and the heat flux measured by thermocouples precisely located in isothermal planes at different depths. Steam was condensed in the presence of air, argon, neon and helium. The pressure in all cases was near to atmospheric. Tests were carried out for a range of gas concentrations and coolant flow rates. The present results, in general, give good support to the boundary layer theory, as do other data [44] for air-steam mixtures at low pressures, published during the course of the present work. The theoretical solutions available at present (for free convection conditions) are not valid for the case where the non-condensing gas has a molecular weight smaller than that of the vapour (i.e. when the convective motion of the vapour-gas mixture near the condensing surface is in the opposite direction to that of the condensate). The present results for steam-helium mixture were used to obtain a semi-empirical equation for this situation.