Thermodynamics HW Solutions 254

Thermodynamics HW Solutions 254 - 0.17 0.17 2 Face brick 4...

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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 0.80 for air space and 0.20 for the ferrings and similar structures. Using the available R -values from Table 3-6 and calculating others, the total R- values for each section of the existing wall is determined in the table below. R- value, h.ft 2 . ° F/Btu Construction Between furring At furring 1. Outside surface, 15 mph wind
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Unformatted text preview: 0.17 0.17 2. Face brick, 4 in 0.43 0.43 3. Cement mortar, 0.5 in 0.10 0.10 4. Concrete block, 4-in 1.51 1.51 5a. Air space, 3/4-in, nonreflective 5b. Nominal 1 × 3 vertical ferring 2.91 -- -- 0.94 6. Gypsum wallboard, 0.5 in 0.45 0.45 7. Inside surface, still air 0.68 0.68 1 2 3 4 5a 6 7 5b Total unit thermal resistance of each section, R 6.25 4.28 The U-factor of each section, U = 1/ R , in Btu/h.ft 2 . ° F 0.160 0.234 Area fraction of each section, f area 0.80 0.20 Overall U-factor, U = Σ f area,i U i = 0.80 × 0.160+0.20 × 0.234 0.175 Btu/h.ft 2 . ° F Overall unit thermal resistance, R = 1/ U 5.72 h.ft 2 . ° F/Btu Therefore, the overall unit thermal resistance of the wall is R = 5.72 h.ft 2 . ° F/Btu and the overall U-factor is U = 0.118 Btu/h.ft 2 . ° F. These values account for the effects of the vertical ferring. 3-97...
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