Thermodynamics HW Solutions 164

Thermodynamics HW Solutions 164 - Chapter 3 Steady Heat...

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Chapter 3 Steady Heat Conduction 3-22E 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. Assumptions 1 Heat 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. 3 Thermal conductivity of the walls is constant. Properties The 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 9 1260 () 2 f t The rate of heat loss during the daytime is & (. ) (55 ) Qk A TT L day 2 Btu / h.ft. F)(1260 ft F ft Btu = −° = 12 040 45 1 5040 / h The rate of heat loss during nighttime is Btu/h 080 , 10 ft 1 C ) 35 55 ( ) ft F)(1260 Btu/h.ft.
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