Handbook of air conditioning and refrigeration / Shan K

3.36 CHAPTER THREE
Shading Coefficients
The temperature of the indoor pane T gi, °F (°C), is calculated as
T gi � T r � Q RCi � 1
h i
� R gi
2 �
where R go, R gi � R values of outdoor and indoor panes, h�ft 2 �°F/Btu (m 2 �°C/W)
h i � heat-transfer coefficient of inside surface 4 (see Fig. 3.12b) of inside plate,
Btu/h�ft 2 �°F (W/m 2 �°C)
(3.83)
The shading coefficient is defined as the ratio of solar heat gain of a glazing assembly of specific
construction and shading devices at a summer design solar intensity and outdoor and indoor
temperatures, to the solar heat gain of a reference glass at the same solar intensity and outdoor and
indoor temperatures. The reference glass is double-strength sheet glass (DSA) with transmittance
� � 0.86, reflectance � � 0.08, absorptance � � 0.06, and F DSA � 0.87 under summer design
conditions. The shading coefficient SC is an indication of the characteristics of a glazing and the
associated shading devices, and it can be expressed as
solar heat gain of specific type of window glass
SC �
solar heat gain of double-strength sheet glass
�
where SHGCw � solar heat gain coefficient of specific type of window glass
SHGCDSA � solar heat gain coefficient of standard reference double-strength sheet glass
SHGCw �
SHGCDSA SHGCw 0.87 � 1.15 SHGCwi Shading coefficients of various types of glazing and shading devices are presented in Table 3.8.
TABLE 3.8 Shading Coefficients for Window Glass with Indoor Shading Devices
(3.84)
Thickness Solar
Venetian blinds Roller shade Draperies
Type of glass of glass, in. transmittance Glass Med.* Light † Opaque, white Translucent Med. ‡ Light§
Clear
3
32
0.87 to 0.79 0.74 0.67 0.39 0.44 0.62 0.52
Heat-absorbing
3 1
16 or 4 0.46 0.57 0.53 0.30 0.36 0.46 0.44
3
8
0.34
0.24
0.54
0.42
0.52
0.40
0.28
0.28
0.32
0.31 0.38 0.36
Reflective-coated 0.30 0.25 0.23
0.40 0.33 0.29
0.50 0.42 0.38
Insulating glass Outer Inner
Clear out
Clear in
3 1
32,
or 8 0.87 0.87 0.62 0.58 0.35 0.40
Heat-absorbing out
Clear in
1
4 0.46 0.80 0.39 0.36 0.22 0.30
Reflective glass 0.20 0.19 0.18
0.30 0.27 0.26
0.40 0.34 0.33
*Med. indicates medium color.
† Light indicates light color.
‡ Draperies Med. represents draperies of medium color with a fabric openness of 0.10 to 0.25 and yarn reflectance of 0.25 to 0.50.
§ Draperies Light repesents draperies of light color with a fabric openness below 0.10 and yarn reflectance over 0.50.
Source: Adapated with permission from ASHRAE Handbook 1989, Fundamentals.