Zero gravity, condensation

The capacity of any steam trap will depend on the difference in pressure between its inlet and outlet connections. Under system start-up conditions the steam pressure in the line will at first be only marginally above atmospheric. If the trap discharge line rises to a higher level, or delivers to a pressurized return pipe, no condensate will flow through the trap until the line pressure exceeds the back pressure. It is important that steam traps which can drain by gravity, with zero back pressure, are fitted... 

COEFFICIENTS FOR FILM-TYPE CONDENSATION. The basic equations for the rate of heat transfer in film-type condensation were first derived by Nusselt. " The Nusselt equations are based on the assumption that the vapor and liquid at the outside boundary of the liquid layer are in thermodynamic equilibrium, so that the only resistance to the flow of heat is that offered by the layer of condensate flowing downward in laminar flow under the action of gravity. It is also assumed that the velocity of the liquid at the wall is zero, that the velocity of the liquid at the outside of the film is not influenced by the velocity of the vapor, and that the temperatures of the wall and the vapor are constant. Superheat in the vapor is neglected, the condensate is assumed to leave the tube at the condensing temperature, and the physical properties of the liquid are taken at the mean film temperature. 

An interfacial shear may be very important in so-called shear-controlled condensation because downward interfacial shear reduces the critical Re number for onset of turbulence. In such situations, the correlations must include interfacial shear stress, and the determination of the heat transfer coefficient follows the Nusselt-type analysis for zero interfacial shear [76], According to Butterworth [81], data and analyses involving interfacial shear stress are scarce and not comprehensive enough to cover all important circumstances. The calculations should be performed for the local heat transfer coefficient, thus involving step-by-step procedures in any condenser design. The correlations for local heat transfer coefficients are presented in [81] for cases where interfacial shear swamps any gravitational forces in the film or where both vapor shear and gravity are important. 

The interfacial tension between a pure liquid and its vapor or between two immiscible or partially miscible liquids reflects the difference in the forces of attraction acting on molecules at the interface as a result of differences in the density or chemical compositions of the two phases. It has long been accepted that the existence of condensed phases of matter, especially the liquid state, is a result of van der Waals attractions between molecules. That is especially true for materials that do not possess any chemical structure that could lead to the action of forces of an electrostatic, dipolar, or other related specific character. For the sake of simplicity, consider a liquid whose molecules interact only through van der Waals or dispersion forces. In the bulk of the phase under consideration, all molecules will be surrounded by an essentially uniform force field, so that the net force acting on each will be zero. Molecules located at or near an interface, on the other hand, will experience a distorted field resulting in a net attraction for the surface molecules by the bulk. The unbalanced force of attraction acting on the surface molecules wiU cause the liquid to contract spontaneously to form, in the absence of gravity, a spherical drop. 

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