Thermodynamics HW Solutions 261

Thermodynamics HW Solutions 261 - C/W wall is better...

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Chapter 3 Steady Heat Conduction 3-150 Two homes are identical, except that their walls are constructed differently. The house that is more energy efficient is to be determined. Assumptions 1 The homes are identical, except that their walls are constructed differently. 2 Heat transfer through the wall is one-dimensional. 3 Thermal properties of the wall and the heat transfer coefficients are constant. Properties The R-values of different materials are given in Table 3-6. Analysis Using the available R -values from Tables 3-6, the total R- value of the masonry wall is determined in the table below. R- value, Construction m 2 . ° C/W 1. Outside surface, 24 km/h (winter) 0.030 2. Concrete block, light weight, 200 mm 2 × 0.27=0.54 3. Air space, nonreflecting, 20 mm 0.17 5. Plasterboard, 20 mm 0.12 6. Inside surface, still air 0.12 5 2 3 4 6 1 Total unit thermal resistance (the R -value) 0.98 m 2 . ° C/W which is less than 2.4 m 2 . ° C/W. Therefore, the standard R -2.4 m 2 . °
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Unformatted text preview: C/W wall is better insulated and thus it is more energy efficient. 3-151 A ceiling consists of a layer of reflective acoustical tiles. The R-value of the ceiling is to be determined for winter conditions. Assumptions 1 Heat transfer through the ceiling is one-dimensional. 3 Thermal properties of the ceiling and the heat transfer coefficients are constant. Properties The R-values of different materials are given in Tables 3-6 and 3-7. Analysis Using the available R-values, the total R-value of the ceiling is determined in the table below. R-value, Construction m 2 . C/W 1. Still air, reflective horizontal surface facing up R = 1/h = 1/4.32 = 0.23 2. Acoustic tile, 19 mm 0.32 3. Still air, horizontal surface, facing down R = 1/h = 1/9.26 = 0.11 Total unit thermal resistance (the R-value) 0.66 m 2 . C/W Highly Reflective foil 19 mm Acoustical tiles Therefore, the R-value of the hanging ceiling is 0.66 m 2 . C/W. 3-104...
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