Handbook of air conditioning and refrigeration / Shan K

HEAT AND MOISTURE TRANSFER THROUGH BUILDING ENVELOPE 3.27
FIGURE 3.8 Estimated atmospheric clearness numbers in the United States for nonindustrial localities.
From Eq. (3.53), for the direct radiation radiated onto a tilted surface at an angle � with the horizontal
plane through clear sky I D,Btu/h�ft 2 (W/m 2 ), can be evaluated as
I D � I � � I DN cos � � � I DN (cos� cos� sin � �sin� cos �)
(3.57)
Most of the ultraviolet solar radiation is absorbed by the ozone layer in the upper atmosphere. Direct
solar radiation, through an air mass m � 2, arrives on the earth’s surface at sea level during a
clear day with a spectrum of � 3 percent in the ultraviolet, 44 percent in the visible, and 53 percent
in the infrared.
The diffuse radiation I d,Btu/h�ft 2 (W/m 2 ), is proportional to I DN on cloudless days and can be
approximately calculated as
Id � (3.58)
where C � diffuse radiation factor, as listed in Table 3.6, and Cn � clearness number of sky from
Fig. 3.8. In Eq. (3.58), Fss indicates the shape factor between the surface and the sky, or the fraction
of shortwave radiation transmitted through the sky that reaches the surface. For a vertical surface
Fss � 0.5, for a horizontal surface Fss � 1, and for any tilted surface with an angle �
CIDN Fss 2 C n
(3.59)
The total or global radiation on a horizontal plane IG,Btu/h�ft2 (W/m2 1.0 � cos �
Fss �
2
), recorded by the U.S.