Handbook of air conditioning and refrigeration / Shan K

3.12 CHAPTER THREE
Temperature also has an influence on the moisture content of many building materials. At a constant
relative humidity in ambient air, the moisture content of a building material will be lower at a
higher temperature.
When a building material absorbs moisture, heat as heat of sorption is evolved. If liquid water is
absorbed by the material, an amount of heat q l, Btu/lb (kJ/kg) of water absorbed, similar to the
heat of solution, is released. This heat results from the attractive forces between the water molecules
and the molecules of the building material. If water vapor is absorbed, then the heat released
q v, Btu/lb (kJ/kg), is given by
(3.37)
where h fg � latent heat of condensation, Btu/lb (kJ/kg). Heat of sorption of liquid water q l varies
with equilibrium moisture content for a given material. The lower the X and the � of ambient air,
the higher will be the value of q l. For pine, q l may vary from 450 Btu/lb (1047 kJ/kg) for nearly
bone-dry wood to 20 Btu/lb (46.5 kJ/kg) at a moisture content of 20 percent. Many building materials
have very low q l values compared with h fg, such as a q l of 40 Btu/lb (93 kJ/kg) for sand.
Moisture-Solid Relationship
FIGURE 3.2 Sorption isotherms.
q v � q l � h fg
Many building materials have numerous interstices and microcapillaries of radius less than 4 �
10 �6 in. (0.1 �m), which may or may not be interconnected. These interstices and microcapillaries
provide large surface areas to absorb water molecules. Moisture can be bound to the solid surfaces
by retention in the capillaries and interstices, or by dissolution into the cellular structures of fibrous
materials.
When the relative humidity � of ambient air is less than 20 percent, moisture is tightly bound to
individual sites in the monomolecular layer (region A, in Fig. 3.2). Moisture moves by vapor diffusion.
The binding energy is affected by the characteristics of the surface, the chemical structure of
the material, and the properties of water.