Thermodynamics HW Solutions 780

Thermodynamics HW Solutions 780 - Chapter 9 Natural...

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Unformatted text preview: Chapter 9 Natural Convection 9-107 An electric hot water heater is located in a small room. A hot water tank insulation kit is available for $30. The payback period of this insulation to pay for itself from the energy it saves is to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The local atmospheric pressure is 1 atm. 4 Any heat transfer from the top and bottom surfaces of the tank is disregarded. 5 The thermal resistance of the metal sheet is negligible. T s = 40 C = 0.7 Water heater H = 2 m 3 cm 40 cm 3 cm Room 20 C Properties The properties of air at 1 atm and the film temperature of ( T s + T )/2 = (40+20)/2 = 30 C are (Table A-15) 1- 2 5 K 0033 . K ) 273 30 ( 1 1 7282 . Pr /s m 10 608 . 1 C W/m. 02588 . = + = = = = = f T k Analysis The characteristic length in this case is the height of the heater, L c = L = 2 m. Then, 10 2 2 5 3-1 2 2 3 10 459 . 1 ) 7282 . ( ) /s m 10 608 . 1 ( ) m...
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This note was uploaded on 01/24/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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