Thermodynamics HW Solutions 66

Thermodynamics HW Solutions 66 - energy balance relation...

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Chapter 1 Basics of Heat Transfer 1-126 The heating of a passive solar house at night is to be assisted by solar heated water. The length of time that the electric heating system would run that night with or without solar heating are to be determined. Assumptions 1 Water is an incompressible substance with constant specific heats. 2 The energy stored in the glass containers themselves is negligible relative to the energy stored in water. 3 The house is maintained at 22°C at all times. Properties The density and specific heat of water at room temperature are ρ = 1 kg/L and C = 4.18 kJ/kg·°C (Table A-9). Analysis ( a ) The total mass of water is () ( ) kg 1000 L 20 50 kg/L 1 = × = = V m w ρ Taking the contents of the house, including the water as our system, the
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Unformatted text preview: energy balance relation can be written as 22 C 50,000 kJ/h water 80 C ( ) ( ) air water out , energies etc. potential, kinetic, internal, in Change system mass and work, heat, by nsfer energy tra Net U U U Q W E E E in e out in + = = = 43 42 1 43 42 1 & [ ( )] , W t Q m C T T e in out = 2 1 water Substituting, (15 kJ/s) t- (50,000 kJ/h)(10 h) = (1000 kg)(4.18 kJ/kg C)(22 - 80) C It gives t = 17,170 s = 4.77 h ( b ) If the house incorporated no solar heating, the 1st law relation above would simplify further to & , W t Q e in out = Substituting, (15 kJ/s) t- (50,000 kJ/h)(10 h) = 0 It gives t = 33,330 s = 9.26 h 1-66...
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