Question 13.11: Adiabatic Saturation Process on Psychrometric Chart If an ad......

As indicated in Figure 13.12, this process is a constant wet-bulb temperature process, not a constant-enthalpy process because water supplied to maintain the sump water level carries enthalpy with it. From Equation 13.14, the humidity ratio at the exit can be found to be

W = 0.622 \times \left\lgroup \frac{p_{vap}}{p_{tot}  –  p_{vap}} \right\rgroup                   (13.14)

Equation 13.40, IP can now be used to find the inlet humidity ratio:

W_{1} = \frac{(1093  –  0.556  T_{wb2}) W_{2}   –  0.240  \times (T_{db1}  –  T_{wb2})}{1093 +  0.444  T_{db1}  –  T_{wb2}}                    (13.40)

To find the inlet RH, Equation 13.14 can be solved for the water vapor pressure:

from which p_{vap,1} = 0.216  psia

from which p_{vap,1} = 0.216  psia.
At 82°F, the saturation pressure is 0.542 psia from Table A3. Finally, from the definition of RH, we have

Comments
It is a common human experience to have an evaporating liquid cool one’s skin. Adiabatic saturation is the basic process involved in evaporative cooling of airstreams. It provides an economical method for reducing the dry-bulb temperature of air in climates with low ambient air humidity. Evaporative cooling is  essentially an incomplete adiabatic saturation process. In actual practice, the exit airstream is often not entirely saturated or at the thermodynamic wet-bulb temperature.
This same phenomenon is used in mechanical equipment for buildings in several ways, for example, space cooling can be achieved in dry climates by passing air over or through a wet medium or spray. Sensible heat is removed from the airstream by evaporating some of the liquid.
Evaporative cooling equipment is discussed in Section 20.8. Cooling towers (discussed in Section 17.7) are used to reject heat from chillers using the same principle.