Thermodynamics HW Solutions 760

Thermodynamics HW Solutions 760 - A window is the window...

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Chapter 9 Natural Convection 9-93 The existing wood framed single pane windows of an older house in Wichita are to be replaced by double-door type vinyl framed double pane windows with an air space of 6.4 mm. The amount of money the new windows will save the home owner per month is to be determined. 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. 4 Infiltration heat losses are not considered. Properties The U-factors of the windows are 5.57 W/m 2 . ° C for the old single pane windows, and 3.20 W/m 2 . ° C. for the new double pane windows (Table 9-6). Analysis The rate of heat transfer through the window can be determined from ) ( window overall window o i T T A U Q = where T i and T o are the indoor and outdoor air temperatures, respectively, U overall is the U-factor (the overall heat transfer coefficient) of the window, and
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Unformatted text preview: A window is the window area. Noting that the heaters will turn on only when the outdoor temperature drops below 18 C, the rates of heat transfer due to electric heating for the old and new windows are determined to be Single pane Double pane W 729 C 7.1) )(18 m C)(12 . W/m (5.57 2 2 old window, = = Q & W 419 C 7.1) )(18 m C)(12 . W/m (3.20 2 2 new window, = = Q & W 310 419 729 new window, old window, saved = = = Q Q Q & & & Then the electrical energy and cost savings per month becomes th $15.62/mon = = = = = = ) ($0.07/kWh kWh/month) 223 ( energy) of cost nit savings)(U Energy ( savings Cost kWh/month 223 h/month) 24 kW)(30 310 . ( savings Energy saved t Q & Discussion We would obtain the same result if we used the actual indoor temperature (probably 22 C) for T i instead of the balance point temperature of 18 C. 9-83...
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