Handbook of air conditioning and refrigeration / Shan K

14.4 CHAPTER FOURTEEN
Equilibrium Chart
When LiBr is dissolved in water, the boiling point of the solution at a given pressure is raised.
However, if the temperature of the solution remains constant, the dissolved LiBr reduces the vapor
pressure of the solution.
When a solution is saturated, equilibrium is established. The number of molecules across the
interface from liquid to vapor per unit time is equal to the number of molecules from vapor into
liquid. If the number of liquid molecules per unit volume is reduced due to the presence of a solute
then the number of vapor molecules per unit volume is also reduced. Consequently, the vapor pressure
of the solution is decreased.
The properties of an aqueous lithium bromide solution, including vapor pressure, temperature, and the
mass fraction at equilibrium, may be illustrated on an equilibrium chart based on the Dühring plot, as
shown in Fig. 14.1. The ordinate of the equilibrium chart is the saturated vapor pressure of water in
log-scale millimeters of mercury absolute (mm Hg abs) and the corresponding saturated temperature
(°F). The scale is plotted on an inclined line. The abscissa of the chart is the temperature of the solution
(°F). Mass fraction or concentration lines are inclined lines and are not parallel to each other.
At the bottom of the concentration lines, there is a crystallization line or saturation line. If the
temperature of a solution of constant mass fraction of LiBr drops below this line—or if the mass
fraction of LiBr of a solution of constant temperature is higher than the saturated condition—the
part of LiBr salt exceeding the saturated condition tends to form solid crystals.
FIGURE 14.1 Equilibrium chart for aqueous lithium-bromide (LiBr) solution. (Source: Carrier Corporation.
Reprinted with permission.)