mechanism
When the heat transfer film is condensed, the condensation wall surface is covered by the condensate, and the condensation can only be performed on the surface of the film. The latent heat of vaporization must be transferred to the liquid film by heat conduction and convection, and then taken away by the wall surface. The state of movement (laminar or turbulent) has a great effect on heat transfer, forming the main thermal resistance of film-like condensation, which in turn depends on the height of the wall (the length of the liquid film flow) and the temperature difference between the steam and the wall. Generally speaking, the surface heat transfer coefficient of laminar film-like condensation decreases with increasing wall height and temperature difference, while the turbulent film-like condensation has the opposite effect. Film-like condensation occurs in general industrial equipment.
Influencing factors
1.Steam speed
The velocity of water vapor is generally lower than 10m / s, and large velocity will produce obvious viscous stress on the surface of the liquid film. When the steam blows down, it accelerates the liquid film flow, makes it thinner, and enhances heat exchange. However, if the speed is too high, the liquid film will be detached from the wall regardless of whether it is moving downward or upward, and the condensation can be enhanced.
2.Steam contains non-condensable gas
Even small amounts of non-condensable gases in the steam can have a very deleterious effect on condensation heat transfer. When the steam condenses, it brings the non-condensable gas molecules to the vicinity of the liquid film, which gradually accumulates on the surface of the film because it cannot condense, so that the concentration of the non-condensable gas (partial pressure) here is higher than the concentration farther from the wall, thereby increasing The resistance of vapor molecules to the surface of the liquid film. At the same time, because the total pressure remains the same, the partial vapor pressure on the surface of the membrane layer is lower than the distant vapor partial pressure, which in turn reduces the saturation temperature of the vapor on the membrane surface. Therefore, the effective condensation temperature difference is correspondingly reduced, and the surface heat transfer coefficient and heat transfer amount of the condensation heat exchange wall are reduced.