Pool Boiling

POOL BOILING

If a horizontal heating surface is in contact with a boiling liquid, a sequence of events occurs as the temperature difference between the surface and the liquid increases. Figure 3.3 relates heat flux per unit area at the surface, q, to the temperature difference between the surface and boiling water, ΔT. The derived value of the heat transfer coefficient, h = q/ΔT, is also plotted.

When ΔT is small, the degree of superheating of the liquid layers adjacent to the surface is low, and bubble formation, growth, and disengagement, if present, are slow. Liquid disturbance is small, and heat transfer can be estimated from expressions for natural convection given, for example, in equation (3.11). This regime corresponds to section AB of Figure 3.3, over which both q and h increase.

FIGURE 3.3 Variation in heat transfer coefficient and heat flux per unit area.

In section BC of Figure 3.3, vapor formation becomes more vigorous and bubble chains rise from points that progressively increase in number and finally merge. This movement increases liquid circulation, and both q and h rise rapidly. This phase is called nucleate boiling and is the practically important regime. For water, approximate values of q and h may be read from Figure 3.3. At point C, a peak flux occurs and a maximum heat transfer coefficient is obtained. ΔT at this point is known as the critical temperature drop. For water, the value lies between 25 and 32 K. The critical temperature drop for organic liquids is somewhat higher. Beyond C, vapor formation is so rapid that escape is inadequate and a progressively larger fraction of the heating surface becomes covered with a vapor film, the low conductivity of which leads to a decrease in q and h. This represents a transition from nucleate boiling to film boiling. When this transition is complete (D), the vapor entirely covers the surface, film boiling is fully established, and the heat flux again rises.

The low heat transfer coefficient renders film boiling undesirable, and equipment is designed for and operated at temperature differences that are less than the critical temperature drop. If a constant temperature heat source, such as steam or hot liquid, is employed, exceeding the critical temperature drop results simply in a drop in heat flux and process efficiency. If, however, a constant heat input source is used, as in electrical heating, decreasing heat flux as the tran-sition region is entered causes a sudden and possibly damaging increase in the temperature of the heating element. Damage is known as boiling burnout. Under these circumstances, the region CD of Figure 3.3 is not obtained.

Boiling heat transfer coefficients depend on both the physical character of the liquid and the nature of the heating surface. Through the agencies of wet-ting, roughness, and contamination, the latter greatly influences the formation, growth, and disengagement of bubbles in the nucleate boiling regime. There is, at present, no reliable method of estimating the boiling coefficients of heat transfer from the physical properties of the system. Coefficients, as shown for water in Figure 3.3, are large, and higher resistances elsewhere will often limit the rate at which heat can be transferred through a system as a whole.