The different types of condensation

Actual condensation can take place in many different ways. If the condensate forms a continuous film, Fig. 4.1, we speak of film condensation. The condensate film can be quiescent, or be in laminar or turbulent flow. The thermal resistance is decisive for the condensation rate, if the molecular kinetic resistance can be neglected. In calculations it is sufficient to just investigate this resistance, as Nusselt [4.2] first did for a flowing laminar film. 

Instead of a film the condensate can also exist in the form of droplets, as shown in Fig. 4.2. This type of condensation is called drop condensation. Whether film or drop condensation prevails depends on whether the wall is completely or incompletely wetted. The decisive factor for this are the forces acting on a liquid droplet, which are illustrated in Fig. 4.3. aLG is the interfacial tension (SI units N/m) of the liquid (index L) against its own vapour (index G), (rSL is the tension of the solid wall (index S) against the liquid and is the interfacial tension of the wall with the vapour, so at equilibrium the contact angle f30 is formed according 

Finite values of this contact angle imply incomplete wetting and droplet formation. If, on the other hand, f30 = 0, the droplets spread out over the entire wall. If the so-called wetting tension is rSG — rSL xLG, an equilibrium according to that in (4.1), can no longer occur. Complete wetting takes place and a film develops. 

In practice, mixtures of vapours whose liquid phases are immiscible often have to be condensed. This results in the formation of a mix of drop and film condensation, Fig. 4.4. In an extended liquid film of one phase, large drops of the other phase develop, some of which reach the wall whilst others are enclosed by the liquid film or float on top of it. 

4 Convective heat and mass transfer. Flows with phase change 

---

The different types of condensing equipment used with the various series are identified by the follotving letters ... 

In the GE model, differences in the characteristics of the corrosion product particulate material are emphasized in detail they are considered to arise from the different types of condensate cleanup systems (powdex or deep-bed) employed in the plants and from the option of forward-pumped heater drain systems. The model includes many empirical coefffcients obtained through laboratory experiments and, above all, by the aid of a large number of data from measurements at operating BWR plants. For this reason, the model is mainly applicable to plants of GE design (Alder et al., 1992). 

---