Handbook of air conditioning and refrigeration / Shan K

TABLE 3.2 Surface Heat-Transfer Coefficients h, Btu/h�ft 2 �°F
h � h c � h r. Table 3.2 lists the h values for various surface types at �T sa � 10°F (5.6°C) and
�T sa � 1°F (0.56°C) during summer and winter design conditions. The values are based upon the
following:
● T rad � T a, where T a is the air temperature, °F (°C).
● �T sa indicates temperature difference between surface and air, °F (°C).
● Emissivity of the surface � � 0.9 and � � 0.2.
3.4 MOISTURE TRANSFER
Sorption Isotherm
HEAT AND MOISTURE TRANSFER THROUGH BUILDING ENVELOPE 3.11
Surface emissivity �
0.90 0.20
Indoor surface Indoor surface
Direction of
�Tsa � 10°F
Outdoor
�Tsa � 10°F
Description
Forced convection
heat flow Summer Winter �Tsa � 1°F surface Summer Winter �Tsa � 1°F
�30 fpm 2.21 2.11 2.16 1.46 1.43 1.44
50 fpm 2.37 2.27 2.32 1.62 1.59 1.60
660 fpm (7.5 mph) 4.44
1320 fpm (15 mph)
Free convection
7.08
Horizontal surface Upward 1.36 1.27 1.17 0.62 0.60 0.44
Vertical surface 1.42 1.33 1.15 0.68 0.66 0.30
Horizontal surface Downward 1.03
Note: Assume space temperature T r � 74°F year-round and T r � T rad; here T rad indicates the mean radiant temperature of the surroundings.
Moisture is water in the vapor, liquid, and solid states. Building materials exposed to excessive
moisture may degrade or deteriorate as a result of physical changes, chemical changes, and biological
processes. Moisture accumulated inside the insulating layer also increases the rate of heat
transfer through the building envelope. Moisture transfer between the building envelope and the
conditioned space air has a significant influence on the cooling load calculations in areas with hot
and humid climates.
Moisture content X, which is dimensionless or else in percentage, is defined as the ratio of the mass
of moisture contained in a solid to the mass of the bone-dry solid. An absorption isotherm is a
constant-temperature curve for a material in which moisture content is plotted against an increased
ambient relative humidity during an equilibrium state; i.e., the rate of condensation of water vapor
on the surface of the material is equal to the rate of evaporation of water vapor from the material. A
desorption isotherm is also a constant-temperature curve for a material. It is a plot of moisture content
versus a decreased ambient relative humidity during equilibrium state.
Many building materials show different absorption and desorption isotherms. The difference in
moisture content at a specific relative humidity between the absorption and desorption isotherms is
called hysteresis. Figure 3.2 shows absorption and desorption isotherms of a building material.