Handbook of air conditioning and refrigeration / Shan K

3.34 CHAPTER THREE
design conditions at 7.5 mph (3.3 m/s) wind speed, the listed U values should be multiplied by a
factor of 0.92. The U values of windows depend on the construction of the window, the emissivity
of the surfaces of glass or plastic sheets, and the air velocity flowing over the outdoor and indoor
surfaces. Q ws indicates the solar heat gain entering the space, in Btu/h (W).
As shown in Fig. 3.11, solar radiation having a total intensity I, Btu/h�ft 2 (W/m 2 ), when it is radiated
on the outer surface of a vertical pane with an angle of incidence � v, the line normal indicating
total shortwave irradiance I t, in Btu/h�ft 2 (W/m 2 ) actually consists of
Heat Gain for Double Glazing
(3.73)
For a double-glazed window, the inward heat flow per square foot of the inner surface of the glass,
as shown in Fig. 3.12b, is
Q RCi
A s
I � I DN � I d � I ref. DN
I t � I cos� v
� N io�I o � N ii�I i � U(T o � T i)
� U � �I o
h o
�� 1
h o
� 1
h a � �I i � T o � T i �
FIGURE 3.12 Heat flow through an insulating glass (double-pane) window. (a) Construction of a typical insulating glass; (b) heat flow
and temperature profiles.
(3.74)