Thermodynamics HW Solutions 256

Thermodynamics HW Solutions 256 - 0.030 0.030 2. Face...

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Chapter 3 Steady Heat Conduction Analysis The schematic of the wall as well as the different elements used in its construction are shown below. Heat transfer through the air space and through the studs will meet different resistances, and thus we need to analyze the thermal resistance for each path separately. Once the unit thermal resistances and the U -factors for the air space and stud sections are available, the overall average thermal resistance for the entire wall can be determined from R overall = 1/ U overall where U overall = ( Uf area ) air space + ( Uf area ) stud and the value of the area fraction f area is 0.84 for air space and 0.16 for the ferrings and similar structures. Using the available R -values from Tables 3-6 and 3-9 and calculating others, the total R- values for each section of the existing wall is determined in the table below. R- value, m 2 . ° C/W Construction Between furring At furring 1. Outside surface, 24 km/h
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Unformatted text preview: 0.030 0.030 2. Face brick, 100 mm 0.12 0.12 3. Air space, 90-mm, nonreflective 0.16 0.16 4. Concrete block, lightweight, 100-mm 0.27 0.27 5a. Air space, 20 mm, nonreflective 5b. Vertical ferring, 20 mm thick 0.17 --- --- 0.94 6. Gypsum wallboard, 13 0.079 0.079 7. Inside surface, still air 0.12 0.12 Total unit thermal resistance of each section, R 0.949 1.719 The U-factor of each section, U = 1/ R , in W/m 2 . C 1.054 0.582 Area fraction of each section, f area 0.84 0.16 Overall U-factor, U = f area,i U i = 0.84 1.054+0.16 0.582 0.978 W/m 2 . C Overall unit thermal resistance, R = 1/ U 1.02 m 2 . C/W 1 2 3 4 5a 6 7 5b Therefore, the overall unit thermal resistance of the wall is R = 1.02 m 2 . C/W and the overall U-factor is U = 0.978 W/m 2 . C. These values account for the effects of the vertical ferring. 3-99...
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