Thermodynamics HW Solutions 172

Thermodynamics HW Solutions 172 - Chapter 3 Steady Heat...

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Chapter 3 Steady Heat Conduction 3-33E Two of the walls of a house have no windows while the other two walls have 4 windows each. The ratio of heat transfer through the walls with and without windows is to be determined. Assumptions 1 Heat transfer through the walls and the windows is steady and one-dimensional. 2 Thermal conductivities are constant. 3 Any direct radiation gain or loss through the windows is negligible. 4 Heat transfer coefficients are constant and uniform over the entire surface. Properties The thermal conductivity of the glass is given to be k glass = 0.45 Btu/h.ft °F. The R-value of the wall is given to be 19 h.ft 2 °F/Btu. Analysis The thermal resistances through the wall without windows are 2 m 480 ft) ft)(40 12 ( = = A F/Btu h 0411417 . 0 00052 . 0 03958 . 0 0010417 . 0 F/Btu h 00052 . 0 ) ft 480 ( F) . Btu/h.ft 4 ( 1 1 F/Btu h 03958 . 0 ) m 480 ( F/Btu hft 19 F/Btu h 0010417 . 0 ) ft 480 ( F) . Btu/h.ft 2 ( 1 1 1 , 2 2 2 2 wall 2 2 ° = + + = + + = ° = ° = = ° = ° = = ° = ° = = o wall i total o o i i R R R R A h R kA L R
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