Windows heat transfer through

Most chapters contain a real world application, end-of-chapler optional section called Topic of Special Interest where interesting applications of heat transfer are discussed such as Thermal Comfort in Chapter 1, A Brief Review of Differential Equations in Chapter 2, Heat Transfer through the Walls and Roofs in Chapter 3, and Heat Transfer through Windows in Chapter 9. 

Special Topic Heat Transfer through Windows... 

FIRE SIMULATOR predicts the effects of fire growth in a 1-room, 2-vent compartment with sprinkler and detector. It predicts temperature and smoke properties (Oj/CO/COj concentrations and optical densities), heat transfer through room walls and ceilings, sprinkler/heat and smoke detector activation time, heating history of sprinkler/heat detector links, smoke detector response, sprinkler activation, ceiling jet temperature and velocity history (at specified radius from the flre i, sprinkler suppression rate of fire, time to flashover, post-flashover burning rates and duration, doors and windows which open and close, forced ventilation, post-flashover ventilation-limited combustion, lower flammability limit, smoke emissivity, and generation rates of CO/CO, pro iri i post-flashover. 

A window consisting of a single piece of clear glass can also he treated with R-value analysis. As with the wall, there is convective and radiative heat transfer at the two surfaces and conductive heat transfer through the glass. The resistance of the window is due to the two surface resistances and to the conductive resistance of the glass, For typical window glass, R = 0.003 (W/ni -°C)" (0.02 (Btu/h-ft -°F) ) so the total resistance of the window is = (0.12 + 0.003 + 0.04) (W/m -- C) ... 

Consider a 0.8-m-high and 1.5-m-wide glass vyindow with a thickness of 8 mm I and a thermal conductivity of /r = 0.78 W/m K, Determine the steady rate of heat transfer through this glass v/indow and the temperature of Its inner surface for a day during which the room is maintained at 20 C while the temperature df the outdoors Is - lO C. Take the heat transfer coefficients on the inner and outer surfaces of the window to be /), = 10 W/m C and fr = 40 W/m "C, which includes the effects of radiation. 

SOLUTION Heat loss through a window glass is considered. The rate of heat transfer through the window and the inner surface temperature are to be determined. 

Assumptions I Heat transfer through the window Is steady since the surface temperatures remain constant at the specified values. 2 Heat transfer through the wall Is one-dimensional since any significant temperature gradients exist in the direction from the indoors to the outdoors. 3 Thermal conductivity is constant. 

Then the steady rate of heat transfer through the window becomes... 

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. 

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

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. 

Assumptions 1 Steady operating conditions exist. 2 Heat transfer through the window is one-dimensional. 3 Thermal properties of the windows and the heat transfer coefficients are constant. 

Assumptions 1 Steady operating conditions exist. 2 Heat transfer through the window is one-dimensional. 

Consider a l.2-m-high and 2-m-wide glass window with a thickness of 6 nun, thermal conductivity k = 0.78 W/m C, and emissivity e = 0.9. The room and the walls that face the window are maintained at 25°C, and the average temperature of the inner surface of the window is measured to be 5°C. If the temperature of Ihe outdoors is -5 C, determine (a) the convection heat transfer coefficient on Ihe inner surface of the window, (b) the rate of total heat transfer through the window, and (c) the combined natural convection and radiation beat transfer coefficient on the outer... 

Considering that about 70 percent of total heat transfer through a window is due to ccntcr-of-glass seclion, estimate the percentage decrease in total heat transfer when triple-pane window is used in place of double-pane window. 

A liorizonlal 1.5-iii-wide, 4.5-m-long double-pane window consists of two. sheets of glass separated by a 3.5-cni gap filled with water. If the glass surface temperatures at the bottom and the lop arc measured to be bO C and 40°Ci respectively, die rate of heat transfer through die window is... 

Calculate (a) the overall coefficient U for heat transfer through a vertical glass window from a room at 70 T to still air at 0°F. Assume that a single pane of glass is 5 in. thick and 4 ft high, (b) Calculate U for a thermopane window with a -j-in. air space between the two panes. For this glass k = 0.4 Btu/h-ft-°F. 

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