Condensation is the reverse process of evaporation; i.e., the process of conversion of vapor back into liquid. It occurs when a wall (or the vapor near a wall) is cooled to a temperature which is below the saturation temperature corresponding to the vapor pressure. This temperature is commonly called the dew point. A common example of condensation occurs when steam condenses on the walls of the shower in a bathroom (Figure 1.4). Initially the water which condenses nucleates as droplets on the cold wall. As the population of these droplets grow or the rate of condensation increases the droplets coalesce into a film which flows down the wall. The first type of condensation is termed "dropwise" condensation and the second is termed "film" condensation. When the rate of condensation is low (e.g., a noncondensible gas is present) or when the liquid does not "wet" the wall, dropwise condensation occurs. In most engineering components where condensation is a required part of an industrial process film condensation is expected, because of the large mass flux of condensed liquid per unit length of wetted area.
Consider the temperature distribution through the vapor and liquid during condensation as shown in Figure 1.4. The temperature decreases in the vapor as one approaches the wall; appreciably if the vapor is superheated or if noncondensibles are present. There is a slight decrease at the liquid-vapor interface due to the difference in pressure driving the mass transfer. Also, there is a temperature drop in the liquid due to its thermal resistance to heat transfer. If one were to plot the heat transfer coefficient as a function of distance down the wall one finds an interesting behavior. Initially the heat transfer coefficient would decrease because more liquid accumulates as a film and acts as a thermal resistance. However, at some liquid flow-rate the film becomes wavy and eventually turbulent. This surface roughness actually reduces the thermal resistance in the flowing liquid film. At this point and beyond the heat transfer increases as the liquid flow increases, with the Prandtl number having a second-order effect. There are other effects which will be discussed later.