Solar Drying: Fundamentals,Applications and Innovations - National ...

Visavale - Principles, Classification and Selection of Solar Dryers
free liquid or wetted surface is present, drying will occur at the saturation temperature,
just as free water at 101.325 kPa vaporizes in a 100 percent steam atmosphere at 100 °C.
On the other hand, when evolved vapor is purged from the dryer environment by using
a second (inert) gas, the temperature at which vaporization occurs will depend on the
concentration of vapor in the surrounding gas.
Figure 1.2. Psychrometric chart: properties of air and water-vapor mixtures
In effect, the liquid must be heated to a temperature at which its vapor pressure
equals or exceeds the partial pressure of vapor in the purge gas, while in the reverse situation
condensation occurs. In most drying operations, water is the liquid evaporated
and air is the normally employed purge gas. For drying purposes, a psychrometric chart
found very useful is that reproduced in Figure 1.2 and can be explained as follows (Perry
2007 and Treybal 1980):
a. In the psychrometric chart the wet-bulb or saturation temperature line gives the
maximum weight of water vapor that 1 kg of dry air can carry at the intersecting drybulb
temperature shown on the abscissa at saturation humidity. The partial pressure
of water in air equals the water vapor pressure at that temperature and the saturation
humidity is defined by
H s = 18
∙ p s
(1.3)
28.9 (P−p s )
b. The percent relative humidity is defined by
H R = 100 ∙ p (1.4)
p s
c. Humid volumes are given by the curves entitled “volume m 3 /kg dry air.” The difference
between dry-air specific volume and humid-air volume at a given temperature is
the volume of water vapor.
d. Enthalpy data are given on the basis of kilojoules per kilogram of dry air. Enthalpy-atsaturation
data are accurate only at the saturation temperature and humidity.
6 Drying of Foods, Vegetables and Fruits