A. Modes of condensation

Condensate may form from vapor in a number of different ways as illustrated in Figure 1. These ways are as follows:

1. Filmwise condensation: The condensate forms a continuous film on the cooled surface. This is the most important mode of condensation occurring in industrial equipment and is discussed in Section 185.
2. Homogeneous condensation: The vapor condenses out as droplets suspended in the gas phase, thus forming a fog. A necessary condition for this to occur is that the vapor is below saturation temperature, which may be achieved (as illustrated) by increasing the pressure as the vapor flows through a smooth expansion in flow area. In condensers, however, it usually occurs when condensing high-molecular-weight vapors in the presence of noncondensable gas. This topic is dealt with in Section 190.
3. Dropwise condensation: This occurs when the condensate is formed as droplets on a cooled surface instead of as a continuous film. High heat transfer coefficients can be obtained with dropwise condensation, but this Is difficult to maintain continuously in heat exchangers. This topic is discussed in Section 188.
4. Direct contact condensation: This occurs when vapor is brought directly into contact with a cold liquid.
5. Condensation of vapor mixtures forming immiscible liquids: A typical example of this is when a steam-hydrocarbon mixture is condensed. The patterns formed by the liquid phases are complicated and varied as described in Section 187, where this topic is presented.

Figure 1 Illustration of modes of condensation

B. Resistances to condensation