Handbook of air conditioning and refrigeration / Shan K

3.38 CHAPTER THREE
40° north latitude. The detailed optical properties of their surfaces are as follows:
The R value of the 0.25-in. (6-mm) thickness indoor glass is R g � 0.035 h�ft 2 �°F/Btu (0.0063
m 2 �°C/W), and that of the enclosed airspace is R a � 1.75 h�ft 2 �°F/Btu (0.32 m 2 �°C/W).
At 4 P.M. on July 21, the outdoor temperature at this location is 100°F (37.8°C), the indoor temperature
is 76°F (24.4°C), and the total solar intensity at a direction normal to this west window is
248 Btu/h�ft 2 (782 W/m 2 ). The outdoor surface heat-transfer coefficient h o � 4.44 Btu/h�ft 2 �°F
(25.2 W/m 2 �°C), and the heat-transfer coefficient of the inner surface of the indoor glass h i � 2.21
Btu/h�ft 2 �°F (12.5 W/m 2 �°C). Calculate the following:
1. The inward heat flow of this window
2. The temperatures of the outdoor and indoor glasses
3. The shading coefficient of this double-glass window
4. The total heat gain admitted through this window
Solution
1. From Eq. (3.76), the absorption coefficient of the outdoor glass can be calculated as
And from Eq. (3.77), the absorption coefficient of the indoor glass is
From Eq. (3.75), the heat absorbed by the outdoor glass is
Also, the heat absorbed by the indoor glass is calculated as
And then, from Eq. (3.22), the overall heat-transfer coefficient of this double-glass window is
U �
�
� � 0.80 � 1 � 0.08 � 1 � 0.12 e 1 � 0.84
� 2 � 0.08 � 2 � 0.12 e 2 � 0.84
� i � 0.16 � 3 � 0.68 � 3 � 0.16 e 3 � 0.15
� 4 � 0.08 � 4 � 0.76 e 4 � 0.84
� o � � 1 � � 2� o� 3
� 0.12 �
� i � � 3� o
1 � � 2� 3
�I o � � o I t � 0.189 � 248 � 46.9 Btu / h�ft 2
�I i � a iI t � 0.135 � 248 � 33.5 Btu / h�ft 2
1
1 � � 2� 3
�
1/h i � R g � R a � R g � 1/h o
0.12 � 0.8 � 0.68
1 � 0.08 � 0.68
0.16 � 0.8
1 � 0.08 � 0.68
� 0.189
� 0.135
1
1/2.21 � 0.035 � 1.75 � 0.035 � 1/4.44 � 0.400 Btu / h�ft2 ��F