Windows, double-paned

Windows provide another opportunity to cut your energy costs. Caulk old windows that might be leaky to prevent drafts, and choose double-paned windows if you re building an addition or replacing old windows. 

Heat transfer by natural convection across an enclosed space, called an enclosure or, sometimes, a cavity, occurs in many real situations, see [34] to [67]. For example, the heat transfet between the panes of glass in a double pane window, the heat transfer between the collector plate and the glass cover in a solar collector and in many electronic and electrical systems basically involves natural convective flow across an enclosure. 

The inner and outer glasses of a 1,2-m X 1,2-m double-pane window arc at I5°C and 9°C, respectively. If ihe 6-mm space between tlte two glasses is fdled with still air, determine the rate of heal transfer through Ihe window. 

EXAMPLE3-3 Heat Loss through Double-Pane Windows... 

Consider a 0.8-m-high and 1.5-m-wide double-pane window consisting of two 4-mm-thick layers of glass (k = 0.78 W/m K) separated by a lO-mm-wide stagnant air space k = 0.026 W/m K). Determine the steady rate of heat... 

Examine natural conveclion from finned surfaces, and determine the optimum fin spacing, B Analyze natural convection inside enclosures such as double-pane windows, and... 

Gases are nearly transparent to radiatioo, and thus heat transfer through a gas layer is by simultaneous convection (or conduction, if the gas is quiescent) and radiation. Natural convection heat transfer coefficients are typically very low compared to those for forced convection. Therefore, radiation is usually disregarded in forced convection problems, but it must be considered in natural convection problems that involve a gas. This is especially the case for surfaces with high emissivities. For example, about half of the heat transfer through the air. space of a double-pane window is by radiation, The total rate of heat transfer is determined by adding the convection and radiation components,... 

The vertical 0.8-m-high, 2-m-wide double-pane wlndov/ shown in Fig. 9-29 consists of two sheets of glass separated by a 2-cm air gap at atmospheric pressure. If the glass surface temperatures across the air gap are measured to be 12 C and 2°C, determine the rate of heat transfer through the window. 

SOLUTION Two glasses of a double-pane window are maintained at specified temperatures. The rate of heat transfer through the window is to be determined. Assumptions 1 Steady operating conditions exist. 2 Air Is an ideal gas. 3 Radiation heat transfer Is not considered. 

The thermal resistance network for heat transfer through the center section of a double-pane window (the resistances of the glasses are neglected). 

The thermal conductivity of air at room temperature is -0.025 W/m "C, which is one-thirtieth that of glass. Therefore, the thermal resistance of 1-cm-thick still air is equivalent to the thermal resistance of a 30-cin-thick glass layer. Disregarding the thermal re.sislances of glass layers, the thermal resistance and (/-factor of a double-pane window can be expressed as (Fig. 9-36)... 

Roughly half of the heat transfer through the air space of a double-pane window is by radiation and the other half is by conduction (or convection, if there is any air motion). Therefore, there are two ways to minitnize and thus the rate of heat transfer ihtough a double-pane window ... 

Therefore, it is usually mote economical to coat only one of the facing surfaces. Note from Fig. 9-37 that coating one of the interior surfaces of a double-pane window with a material having an etnissivity of 0.1 reduces the rate of heat transfer through the center section of the window by half. 

Another way of reducing conduction heat transfer through a double-pane window is to use a less-conducting fluid such as argon or krypton to ItU the gap between the glasses instead of air. The gap in this case needs to be well sealed to prevent the gas from leaking outside. Of course, another alternative is to evacuate the gap between the glasses completely, but it is not practical to do so. 

The thicker the air space in multiple-glazed units, the lower the /-factor, for a thickness of up to 13 mm in) of air space. For a specified number of glazings, the window with thicker air layers will have a lower /-factor. For a specified overall thickness of glazing, the higher the number of glazings, the lower the /-factor. Therefore, a triple-pane window with air spaces of 6.4 mm (two such air spaces) will have a lower /-value than a double-pane window with an air space of 12.7 mm. 

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