Handbook of air conditioning and refrigeration / Shan K

3.40 CHAPTER THREE
and fire protection, and life-cycle cost analysis. Energy conservation considerations, including the
control of solar heat with the optimum combination of absorbing and reflective glass and various
shading devices, are covered in the next section.
To reduce the heat loss through glass during winter, one can install double or triple glazing,
storm windows, or low-emission film coating on the surface of the glass. Elmahdy (1996) and de
Abreu et al. (1996) tested the thermal performance of seven insulating glass units. If a clear,
double-glazed insulating glass unit with a silicone foam spacer of 0.5 in. (13 mm) between two
panes is taken as the base unit, the U values of these seven insulating glass units are as follows:
Low-e coating reduces the U value by about 30 percent, and the triple-glazing drops about onethird
compared with a clear, double-glazed insulating glass unit. If the width of the airspace is
decreased to 0.25 in. (6.5 mm), its U value will increase 15 percent. When the airspace is wider
than 0.5 in. (13 mm), regardless of whether a metal or silicone foam spacer is adopted, neither has
any significant effect on U value of the insulation glass unit.
3.11 SHADING OF GLASS
Indoor Shading Devices
Unit Glazing Airspace, in. Spacer
U value,
Btu/h�ft2�°F 1 Clear, double-glazed 0.5 Foam 0.51
2 Clear, double-glazed 0.5 Aluminum 0.51
3 Clear, double-glazed 0.25 Foam 0.58
4 Clear, double-glazed 0.75 Foam 0.51
5 Low-e, double-glazed 0.5 Foam 0.36
6 Clear, Triple-glazed 0.5 Foam 0.32
7 Clear, Triple-glazed 0.25 Foam 0.39
Shading projected over the surface of glass significantly reduces its sunlit area. Many shading
devices increase the reflectance of the incident radiation. There are two types of shading: deliberately
installed shading devices, which include indoor and external shading devices, and shading
from adjacent buildings.
Indoor shading devices not only provide privacy but also are usually effective in reflecting part of
the solar radiation back to the outdoors. They also raise the air temperature of the space between
the shading device and the window glass, which in turn reduces the conductive heat gain in summer.
Indoor shading devices are easier to operate and to maintain and are more flexible in operation
than external shading devices. Three types of indoor shading devices are commonly used: venetian
blinds, draperies, and roller shades.
Venetian Blinds. Most horizontal venetian blinds are made of plastic or aluminum slats, spaced 1
to 2 in. (25 to 50 mm) apart, and some are made of rigid woven cloth. The ratio of slat width to slat
spacing is generally 1.15 to 1.25. For light-colored metallic or plastic slats at a 45° angle, typical
optical properties are � � 0.05, � � 0.55, and � � 0.40. Vertical venetian blinds with wider slats
are widely used in commercial buildings.
Consider a single-glazed window combined with indoor venetian blinds at a slat angle � �45°,
as shown in Fig. 3.13. Let the subscripts g, v, and a represent the glass, the venetian blinds, and the