Handbook of air conditioning and refrigeration / Shan K

where N io � inward fraction of solar radiation absorbed by outdoor glass
N ii � inward fraction of solar radiation absorbed by indoor glass
I o, I i � solar intensity irradiated on outdoor and indoor glass, Btu/h�ft 2 (W/m 2 )
The solar radiation absorbed by the outdoor and indoor panes, in Btu/h�ft 2 (W/m 2 ), is
Absorptance of the outdoor panes � o is
Absorptance of the indoor pane is
�I o � � oI t �I i � � iI t (3.75)
(3.76)
(3.77)
Subscript o indicates outdoor; i, indoor; and 1, 2, 3, and 4, the surfaces of the panes as shown in
Fig. 3.12b.
The airspace heat-transfer coefficient ha,Btu/h�ft2�°F (W/m2 1 � �2 �3 �°C), is the reciprocal of the R
value of the airspace Ra listed in Table 3.4, that is,
ha � (3.78)
The transmittance of solar radiation through both outdoor and indoor panes �oi can be calculated
as
1
Ra (3.79)
Then the heat admitted per square foot (square meter) through a double-glass window Qwoi /As Btu/h�ft2 (W/m2 1 � �2 �3 ), can be calculated as
Q woi
A s
HEAT AND MOISTURE TRANSFER THROUGH BUILDING ENVELOPE 3.35
� o � � 1 � � 2� oi � 3
� i � � 3� o
� oi � � o� i
� SHGC oi I t � U(T o � T i)
1 � � 2 � 3
� �oi It � U� �o �
h� �i �
h� �i h � a � It � U(To � Ti) The SHGC oi for a double-glass window can be calculated as
SHGCoi � �oi � U � �o �
ho �i h � a
(3.80)
(3.81)
Because a glass plate is usually between 0.125 and 0.25 in. (3 and 6 mm) thick, with a thermal
conductivity k of about 0.5 Btu/h�ft�°F (W/m�°C), there is only a small temperature difference
between the inner and outer surfaces of the plate when solar radiation is absorbed. For the sake
of simplicity, it is assumed that the temperature of the plate T g is the same in the direction of heat
flow.
For a double-glazed window, the glass temperature of the outdoor pane T go, °F (°C), can be
calculated as
T go � T o �� �I o � �I i � Q RCi
A s �� 1
h o
� R go
2 �
(3.82)