Chapter 3 Steady Heat ConductionSpecial Topic: Heat Transfer Through the Walls and Roofs 3-134C The R-value of a wall is the thermal resistance of the wall per unit surface area. It is the same as the unit thermal resistance of the wall. It is the inverse of the U-factor of the wall, R = 1/U. 3-135C The effective emissivity for a plane-parallel air space is the “equivalent” emissivity of one surface for use in the relation that results in the same rate of radiation heat transfer between the two surfaces across the air space. It is determined from )(4142effectiveradTTAQs−=σε&11112εεεeffective1=+−where ε1and ε2are the emissivities of the surfaces of the air space. When the effective emissivity is known, the radiation heat transfer through the air space is determined from the relation above. &Qrad3-136C The unit thermal resistances (R-value) of both 40-mm and 90-mm vertical air spaces are given to be the same, which implies that more than doubling the thickness of air space in a wall has no effect on
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