Thermodynamics HW Solutions 255

# Thermodynamics HW Solutions 255 - Chapter 3 Steady Heat...

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Chapter 3 Steady Heat Conduction 3-142 The winter R -value and the U -factor of a flat ceiling with an air space are to be determined for the cases of air space with reflective and nonreflective surfaces. Assumptions 1 Steady operating conditions exist. 2 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 are given in Table 3-6 for different materials, and in Table 3-9 for air layers. Analysis The schematic of the ceiling 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 )
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