Chapter 3 Steady Heat Conduction3-22E The inner and outer surfaces of the walls of an electrically heated house remain at specified temperatures during a winter day. The amount of heat lost from the house that day and its its cost are to be determined. Assumptions1Heat transfer through the walls is steady since the surface temperatures of the walls remain constant at the specified values during the time period considered. 2 Heat transfer is one-dimensional since any significant temperature gradients will exist in the direction from the indoors to the outdoors.3Thermal conductivity of the walls is constant. PropertiesThe thermal conductivity of the brick wall is given to be k= 0.40 Btu/h⋅ft⋅°F. Analysis We consider heat loss through the walls only. The total heat transfer area is A=×+×=240 9 30 91260()2ftThe rate of heat loss during the daytime is &(.)(55)QkATTLday2Btu / h.ft. F)(1260 ftFftBtu=−=°−°=120404515040/ hThe rate of heat loss during nighttime is Btu/h080,10ft1C)3555()ftF)(1260Btu/h.ft.
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