Internal shading and double glazing
The effect of internal shading devices in reducing solar heat gain is considerable but not so great as the effect of external shades. In tropical and sub-tropical climates, fixed, horizontal, external shades are extensively used and are effective because the sun is high in the sky for most of the day, but in the temperate zones the lower solar altitudes that prevail for the majority of the time make them of much less use. Motorised, automatic, external shades are also used but they need access for maintenance and because of adverse winter weather such maintenance may be expensive. For this reason, internal shading is much adopted and is essential for sunlit windows if air conditioning is to be satisfactory. Table 7.6 indicates the effects of shading windows from solar radiation.
Internal shades are very often of the Venetian blind type and for these to give most benefit, they should be of a white or aluminium colour and should have polished surfaces. They should be adjusted so that they reflect the rays of the sun back to the outside and so that no direct rays pass between the slats. The blinds absorb some radiation, warm up, and convect and radiate heat into the room. The radiant emission is from a surface temperature that is several degrees above room temperature and is acceptable. In comparison, the radiation emitted by the sun is from a surface temperature of 6000°C and causes acute discomfort when received through unshaded windows. Blinds are most effective when fitted between sheets of double glazing.
The comparative effectiveness of various combinations of shades and glazing is expressed in terms of a shading coefficient, defined by ASHRAE (1997b) as the ratio of the solar heat gain coefficent (SHGC) of a glazing system for a particular angle of incidence and incident solar spectrum to the SHGC for clear, single pane glass (standard reference glazing in which t = 0.86, p = 0.08 and a = 0.06 at normal incidence) with the same angle and spectral distribution. Glazing system refers to the combination of glass type and shade. The symbols x, p and a are the transmissivity, reflectivity and absorptivity, respectively, of the glass. It is claimed by ASHRAE (1997b) that, defined in this way, the shading coefficient (SC) is independent of the solar spectral shape and the angle of incidence and that it can be used for single or double glass and for various types of tinted glass.
The shading coefficient for single, clear, 4 mm glass (see Table 7.6) is 1.0 and its transmissivity is 0.87.
Hence a simpler definition by ASHRAE (1997b) merely states that, for a particular glass and blind combination with a solar heat gain coefficient of SHGC, the shading coefficient, SC, is expressed by
SC = SHGC/0.87 (7.13)
This is the sense in which shading coefficient is generally used, expressing the heat gain of a particular combination of glass and shading in comparison with that of an unshaded, 4 mm clear glass window. It has proved mostly acceptable. However, Reilly et al. (1992) claim that this simplified approach may exaggerate the solar heat gain by as much as 35 per cent.
As Table 7.6 shows, the presence of two sheets of glass reduces the net solar transmitted heat by about 15 per cent only. It usually follows from this that the saving in capital and running costs of the air-conditioning system are not enough to pay for the extra cost of double glazing.
The main favourable feature of double glazing is the reduction in noise transmission, but this is effective only if the air gap between the sheets exceeds about 100 mm.
The obvious advantage that a higher relative humidity can be maintained in the room in winter (because the inside surface temperature of double glazing is higher than that of single glazing, thus permitting a higher room dew point) is often spurious; poorly fitting double glazing permits the air in the room to enter the space between the sheets, and objectionable condensation then takes place on the inside of the glass sheets, where it cannot readily be wiped away. On the other hand, a small amount of ventilation through the outer sheet and frame is sometimes deliberately arranged and is most successful in preventing condensation.
Reflective plastic films, if applied properly to the inside surface of clear glazing can be significantly useful, provided that the shading coefficient does not exceed 0.27. External application is not recommended because of potential degradation due to weathering. The use of an applied film with solar control glass is not recommended. The glass manufacturer’s advice should be sought.
Certain types of curtain can also give a measure of solar control. The shading coefficient, according to ASHRAE (1997c), depends on the reflective nature of the fabric, the ratio of the open area between the fibres to the total area, and the fullness of the drape. Values can range from 0.87 to 0.37, with 3 m clear glass, depending on the factors mentioned. The necessary washing of curtains can spoil their effectiveness—particularly when they have had a reflective coating applied to the fibres, which may get washed away after a while.
Posted in Air Conditioning Engineering