Detailed explanation of condensation heat transfer process of plate heat exchanger
When the steam contacts with the wall which is lower than the saturation temperature under the corresponding pressure, condensation will occur on the wall. At this time, the steam will release the latent heat of vaporization and condense into liquid, which becomes a new thermal resistance. According to the different condensation forms of steam on the wall, it can be divided into membrane condensation (the condensation liquid is very good to wet the wall) and bead condensation (the condensation liquid adheres to the wall in the form of liquid beads). Because the film condensate forms a complete liquid film on the wall of plate heat exchanger, the latent heat released during condensation can only be transmitted to the wall with lower temperature through the liquid film; when the bead condensation, the heat transfer is carried out between the steam and the liquid bead surface and between the steam and the cold wall, so the film condensation has a lower heat transfer coefficient than the bead condensation. In the actual process, it is generally film condensation, unless the wall is pretreated or accelerator is added in the steam, such as oil can be considered on the wall when the water vapor condensation heat exchange.
In the process of heat transfer, the steam velocity has a great influence on condensation heat transfer. When the steam moves at a certain speed, there will be a force between the steam and the liquid film. When the flow direction of steam and liquid film is the same, the effect of this force will make the liquid film thin, and make the liquid film produce a certain fluctuation, so that the heat transfer increases. If the flow direction of steam is opposite to that of liquid film, the effect of force will hinder the flow of liquid film, thicken the liquid film and weaken the heat transfer. However, when the force just exceeds the gravity, the liquid film will be driven away from the wall by the steam, which will increase the heat transfer coefficient. The flow path of the plate heat exchanger is narrow, the effect of the steam flow rate is significant, the flow direction of the steam is suitable from top to bottom, and it should be arranged in a single way as far as possible to increase the heat transfer and facilitate the discharge of the condensate.
The different relative flow direction of the fluid in the heat exchanger will also affect the condensation process. The temperature difference in the lower part of the channel is large when the reverse flow occurs, most of the steam condensation occurs in the lower part of the channel, while the reverse flow occurs, so the pressure drop of the steam in the reverse flow is larger than that in the forward flow, and the corresponding saturation temperature drops more, thus affecting the condensation heat exchange effect.