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Condensation heat transfer is the primary mode of heat exchange in devices such as chiller condensers. Heat transfer through gas-liquid phase changes is an important type of convective heat transfer in refrigeration systems; the process in which vapor is cooled and turned into a liquid is known as condensation heat transfer, while the process in which a liquid is heated and turns into vapor is known as boiling heat transfer. During the gas-liquid phase change process, latent heat transfer plays a dominant role; the characteristics of the heat transfer process as well as the surface heat transfer coefficient differ significantly from those of convective heat transfer in single-phase fluids. Equipment such as chiller condensers primarily uses condensation heat exchange. Condensation heat transfer is further divided into two states: film condensation and droplet condensation. There are two forms of condensate: when the condensate can wet the wall surface well, a liquid film is formed on the wall surface, which is known as film condensation ； When the surface tension of the condensate is greater than its adhesion to the wall surface, the condensate cannot wet the wall surface properly, and droplets form on it; this phenomenon is known as bead condensation (droplet condensation). In the case of film condensation, vapor can only condense on the surface of the liquid film. The latent heat released during vapor condensation must pass through this liquid film in order to reach the wall surface, and the thermal resistance of the liquid film becomes the main factor limiting heat transfer in film condensation. Removing the condensate and reducing the thickness of the liquid film are key strategies for enhancing heat transfer in film condensation. When bead condensation occurs, most of the vapor can come into direct contact with the wall surface, and the latent heat released during condensation is transferred directly to the cold wall surface. Compared with heat transfer in the case of film condensation under the same conditions, the surface heat transfer coefficient is several times higher or even an order of magnitude greater; however, bead condensation is difficult to maintain for a long time. The condensation heat transfer in a refrigeration system is essentially film-type condensation heat transfer. http://bbs.eszhileng.com/data/attachment/forum/201505/28/094453g7vz1v1v1pk1h1gv.png