Thermodynamics HW Solutions 758

Thermodynamics HW Solutions 758 - A wall = 0.8 ft 2 and A...

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Chapter 9 Natural Convection 9-91E The R -value of the common double door windows that are double pane with 1/4-in of air space and have aluminum frames is to be compared to the R -value of R -13 wall. It is also to be determined if more heat is transferred through the windows or the walls. 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-factor of the window is given in Table 9-6 to be 4.55 × 0.176 = 0.801 Btu/h.ft 2 . ° F. Analysis The R -value of the windows is simply the inverse of its U -factor, and is determined to be F/Btu ft h 1.25 F ft Btu/h 801 . 0 1 1 2 2 window ° = ° = = U R which is less than 13. Therefore, the R -value of a double pane window is much less than the R -value of an R -13 wall. Now consider a 1-ft 2 section of a wall. The solid wall and the window areas of this section are
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Unformatted text preview: A wall = 0.8 ft 2 and A window = 0.2 ft 2 . Then the rates of heat transfer through the two sections are determined to be Btu/h 0615 . F/Btu . h.ft (13 F) ( ) ft (0.8 wall value, R ) ( 2 2 wall wall wall T T T T A T T A U Q o i o i wall = = = = & Btu/h 160 . F/Btu . h.ft (1.25 F) ( ) ft (0.2 value R ) ( 2 2 window window window window T T T T A T T A U Q o i o i = = = = & Therefore, the rate of heat transfer through a double pane window is much more than the rate of heat transfer through an R-13 wall. Discussion The ratio of heat transfer through the wall and through the window is Air R-13 Wall Aluminum frames 60 . 2 Btu/h 0.0615 Btu/h 160 . wall window = = Q Q & & Therefore, 2.6 times more heat is lost through the windows than through the walls although the windows occupy only 20% of the wall area. 9-81...
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This note was uploaded on 01/24/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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