Handbook of air conditioning and refrigeration / Shan K

29.26 CHAPTER TWENTY-NINE
Desiccants can be classified as adsorbents, which absorbs moisture without accompanying physical
and chemical changes, and absorbents, which absorb moisture accompanied by physical or
chemical changes. Three kinds of desiccants are widely used in desiccant-based air conditioning
systems: silica gel, lithium chloride, and molecular sieves.
Silica Gels. These are solid desiccants and adsorbents. Structurally, they contain numerous pores and
capillaries in which water is condensed and contained. Silica gel has a high capacity to absorb moisture
3 and releases it at a higher temperature. They are low in cost and available in sizes from �16-in.
(4.8-mm) beads to powderlike grains. Recently, titanium silica gel is used for its stable property.
Lithium Chloride (LiCl). This is an absorbent. It is in dry form when each LiCl molecule holds
two water molecules. If each LiCl molecule holds more than two water molecules, it becomes a liquid
and continues to absorb moisture. LiCl has a high capacity to absorb and to hold moisture.
Lithium chloride is widely used in rotary wheel dehumidifiers.
Molecular Sieves. These are actually synthetic zeolites, a solid desiccant and an adsorbent in the
form of crystalline aluminosilicates produced by a thermal process. Molecular sieves show physical
stability and high moisture-releasing capacity at high regenerating temperatures of 248 to 428°F
(120 to 220°C), and they are recommended in direct gas-fired applications.
FIGURE 29.5 Sorption isotherms of some desiccants. (Source: ASHRAE Handbook 1989, Fundamentals.
Reprinted with permission.)