Explanation Write the expression to calculate the energy balance equation for closed system. E i n − E o u t = Δ E s y s t e m (I). Here, energy transfer into the control volume is E i n , energy transfer exit from the control volume is E o u t and change in internal energy of system is Δ E s y s t e m . Write the expression to calculate the mass of the water. m w a t e r = ρ ν (II). Here, density of the water is ρ and volume of the container is ν . Write the expression for the entropy balance equation of the system. S i n − S o u t + S g e n = Δ S s y s t e m (III). Here, net entropy in is S i n , net entropy out is S o u t , entropy generation is S g e n and change of entropy of the system is Δ S s y s t e m Conclusion: Represent change in total internal energy as Δ U a i r for air and Δ U w a t e r for water. Substitute W e , i n for E i n , Q o u t for E o u t and Δ U w a t e r + Δ U a i r for Δ E s y s t e m in Equation (III). W e , i n − Q o u t = Δ U w a t e r + Δ U a i r W e , i n − Q o u t = [ m c p ( T 2 − T 1 ) ] w a t e r W e , i n Δ t − Q o u t = [ m c p ( T 2 − T 1 ) ] w a t e r (IV). Since the state of air remains unchanged inside the house, the change in internal energy of air is zero. Here, heat loss to the surrounding air is Q o u t , mass of the air is m a i r , mass of the water is m w a t e r , initial temperature is T 1 , final temperature is T 2 , specific heat at constant pressure is c p and time is Δ t . From the Table A-3 “Properties of common liquids, solids and foods” table, the value for the density of water ( ρ ) at room temperature is 0.997 kg / L and specific heat at constant pressure ( c p ) is 4.18 kJ / kg ⋅ ° C Substitute 0.997 kg / L for ρ and ( 50 × 20 L ) for ν in Equation (II). m w a t e r = ( 0.997 kg / L ) ( 50 × 20 L ) = 997 kg Substitute 15 kJ / s for W e , i n , [ ( 50 , 000 kJ / h ) × ( 10 h ) ] for Q o u t , 4.18 kJ / kg ⋅ ° C for c p , 80 ° C for T 1 , 22 ° C for T 2 , 997 kg for m w a t e r in Equation (IV). 15 kJ / s Δ t − [ ( 50 , 000 kJ / h ) × ( 10 h ) ] = [ ( 997 kg ) ( 4

8th Edition

ISBN: 9780073398174

Author: Yunus A. Cengel Dr., Michael A. Boles

Publisher: McGraw-Hill Education

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Chapter 7.13, Problem 202RP

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The duration of the electric heating system is in function during night and the amount of entropy generated during the night.

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Chapter 7.13, Problem 201RP

Chapter 7.13, Problem 203RP

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The duration of the electric heating system is in function during night and the amount of entropy generated during the night.

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The duration of the electric heating system is in function during night and the amount of entropy generated during the night.

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