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Condensing process of plate heat exchanger
Sep 30, 2020

When the steam comes into contact with the wall surface at a saturation temperature below the corresponding pressure, condensation occurs on the wall surface. At this time, the steam gives off latent heat of vaporization and condenses into liquid, which becomes a new thermal resistance. According to the different condensation forms of steam on the wall, it can be divided into film condensation (condensation liquid wetting the wall well) and bead condensation (condensation liquid attached to the wall in the form of beads). Since the film-like condensate forms a complete liquid film on the wall of the plate heat exchanger, the latent heat released during the condensation can only be transferred to the wall with lower temperature through the liquid film. In the case of bead condensation, heat transfer occurs on the surface of vapor and liquid bead and between vapor and cold wall, so the heat transfer coefficient of film condensation is lower than that of bead condensation. In the actual process, it is generally film-like condensation, unless the wall surface is pretreated or accelerators are added to the steam. For example, when water vapor condenses and exchanges heat, the wall surface can be considered to be oiling.


During heat transfer, the steam velocity has great influence on condensation heat transfer. When the steam is moving at a certain speed. There will be a force between the vapor and the liquid film. When the vapor and liquid film flow in the same direction. The effect of this force will make the liquid film thin, and promote the liquid film to produce a certain fluctuation, so as to increase the heat transfer. If the steam and the liquid film flow in the opposite direction, the action of the force will impede the flow of the liquid film, the liquid film thickening, heat transfer weakened. However, when the force just exceeds gravity, the liquid film will be driven away from the wall by steam, which will increase the heat transfer coefficient. The flow passage of the spanwise heat exchanger is relatively narrow. The direction of steam flow is suitable from top to bottom, and should be arranged as far as possible to increase heat transfer and facilitate the discharge of condensate.


Heat exchanger in the relative of fluid flow in a different direction, will also affect the coagulation process of upstream channel of the lower temperature difference is big, most of the steam condensation occurred at the bottom of the channel, and downstream, by contrast, in this way, counter-current when steam pressure drop than the downstream is large, the corresponding saturated temperature drop more, so as to affect the condensation heat transfer effect.

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