Desiccant Air Drying Process

Desiccant dehumidifiers do not cool air to remove moisture. They attract moisture from air by creating an area of low vapor pressure at the surface of the desiccant. The pressure of the water in the air is higher, so the water vapor molecules move from the air to the desiccant and the air is dehumidified. As a wet desiccant is heated, its vapor pressure becomes high and it will give off moisture to the surrounding air. Water vapor moves back and forth from the air to the desiccant, depending on the vapor pressure difference. Figures 39-2a through 39-2c describe the process diagramatically.

Figs. 39-2a, b, and c Diagrammatic Representations of Desiccant Process. Source: Munters Cargocaire and Mason Grant Company

Figs. 39-2a, b, and c Diagrammatic Representations of Desiccant Process. Source: Munters Cargocaire and Mason Grant Company

The process of removing moisture from air with a desiccant is termed sorption. When water vapor is sorbed by a desiccant (sorbent), the water changes phase from vapor to sorbed liquid. This phase change releases energy, primarily the latent heat of condensation, plus a small additional amount (approximately 10 to 20%) due to the attraction between the sorbed water and the desiccant. The total heat released is referred to as the heat of sorption. The release of latent energy results in an increase in the temperature of the desiccant and the surrounding airstream.

The thermodynamics of the sorption process are similar to the reverse of what happens in an evaporative cooler. As moisture is removed from the airstream, its sensible temperature rises. The rise in air temperature is directly proportional to the amount of moisture removed. The dryer the air, the warmer it will be.

The critical element of the sorption process is reversibility. Desiccants are regenerated by being heated to temperatures above that in the sorption process, then placed in a different airstream. The high vapor pressure desiccant surface gives off, or desorbs, moisture to the air to equalize the vapor pressure differential. The dry, hot regenerated desiccant is then cooled to restore its low vapor pressure and is returned to the moist airstream. Typical regeneration temperatures range from 130 to 250°F (54 to 121°C). Thermal energy can be applied with direct firing of fuel or by steam, hot water, or waste heat. Figure 39-3 is a block diagram of the desiccant air conditioning process.


The sorption cycle has three parts:

1. Exposing the desiccant to the airstream to be conditioned when it is cool and dry (low vapor pressure) and can attract moisture.

2. Heating the desiccant to further increase its vapor pressure to a point higher than that of the discharge airstream so that moisture will move off of the surface to the air to equalize the pressure differential.

3. Cooling the desiccant to restore its low vapor pressure so that it can begin attracting moisture once again.


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