Dropwise condensation of steam at atmospheric and above atmospheric pressures

Abstract

The results of the initial phases of a study concerned with the mechanism of the dropwise condensation of steam are presented. In the first phase of this work, equivalent heat transfer coefficients at atmospheric pressure for the dropwise condensation of steam on a vertical surface were determined experimentally. The coefficients ranged from 1140 to 37,000 B.t.u./hr-ft 2-°F for heat fluxes of 29,900 up to 167,000 B.t.u./hr-ft 2 with surface temperature differences varying from 45 to 2°F, respectively. Vapor velocities varied from 1·75 to 7·52 ft/sec with the flow directed down the vertical condensing surface. Observations indicated that the vapor velocity across the condensing surface has a significant effect on the equivalent transfer corfficient with the coefficient exhibiting a maximum with increasing vapor velocity. It is believed that this maximum reflects the transition between dropwise and mixed condensation resulting from the greater vapor—liquid interfacial shear stress developed at the higher vapor velocities. Visual observations of the condensation phenomenon were also made at pressures ranging from atmospheric to 200 psig. These observations showed that, as the pressure increases, a transition from dropwise to mixed condensation occurs between 25 to 50 psig. This transition is posited to be associated with the decreased surface tension of the condensed phase at the higher saturation temperatures. A theoretical analysis is presented which is consistent with the experimental observations.