07_Properties_GB_2008

07_Properties_GB_2008 - Thermal Properties of Materials...

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Thermal Properties of Materials Arch 140, Lecture #7 - Tuesday, Feb 12, 2008
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CONDUCTION T hot T cold Δ T d A Q Q = heat flow (Btu/hr) d = thickness (in) Δ T = temperature (ºF) A = area (ft 2 ) k = conductivity Btu-in hr- ft 2 -ºF k
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CONDUCTION T hot T cold Δ T d A Q Q = x A x Δ T k d Q = heat flow (Btu/hr) d = thickness (in) Δ T = temperature (ºF) A = area (ft 2 ) k = conductivity Btu-in hr- ft 2 -ºF k
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Q = x A x Δ T k d CONDUCTION Btu 1 * Btu-in * ft 2 * °F hr in hr-ft 2 -°F = d = thickness (in) Δ T = temperature (ºF) A = area (ft 2 ) k = conductivity (Btu-in/hr- ft 2 -ºF)
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Q = x A x Δ T k d CONDUCTION Btu 1 * Btu-in * ft 2 * °F hr in hr-ft 2 -°F = d = thickness (in) Δ T = temperature (ºF) A = area (ft 2 ) k = conductivity (Btu-in/hr- ft 2 -ºF)
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A * 1/rd * Δ T hr-ft 2 -ºF/Btu-in r resistivity A * 1/R * Δ T hr-ft 2 -ºF/Btu R resistance A * C * Δ T Btu/hr-ft 2 -ºF C conductance A * k/d * Δ T Btu-in/hr-ft 2 -ºF k conductivity Heat flow,Q= Units Symbol Property “___ivity” = property of a material (independent of amount) “___ance” = property of an object (depends on amount) C = 1/R = k/d = 1/dr or R = d/k Alternative Measures of Conductive Properties
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Conductive Properties of Typical Building Materials 4.26 Water (still) 0.17 Air (still) 1500. Aluminum 310. Steel 1.72 8” concrete block 12. Stone 0.6 3” common brick 5.0 Brick 0.05 ¼” glass 5.4 Glass 4.3 2x4 wood stud 0.8 Wood (soft) 19.0 5-1/2” insulation 0.3 Insulation (hr-ft2-°F/Btu) Resistance, R Component (Btu-in/hr-ft2-°F) Conductivity, k Material See Reader, Appendix 5 for property values NOTE: R = d/k
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Example • 4" thick wall of concrete, k = 12 (Btu-in / hr-ft 2 -ºF) • Find R = d/k = 4 (in) / 12 (Btu-in / hr-ft 2 -ºF) = 0.33 hr-ft 2 -ºF/Btu
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• 4" thick wall of concrete, k = 12 (Btu-in / hr-ft 2 -ºF) • Find R = d/k = 4 (in) / 12 (Btu-in / hr-ft 2 -ºF) = 0.33 hr-ft 2 -ºF/Btu • If the wall was 10 ft 2 and had a 5ºF temperature difference across it, how much heat would flow? Q
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07_Properties_GB_2008 - Thermal Properties of Materials...

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