Thermodynamics HW Solutions 735

Thermodynamics HW Solutions 735 - Chapter 9 Natural...

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Unformatted text preview: Chapter 9 Natural Convection 9-41 It is proposed that the side surfaces of a cubic industrial furnace be insulated for $550 in order to reduce the heat loss by 90 percent. The thickness of the insulation and the payback period of the insulation to pay for itself from the energy it saves are 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. Properties The properties of air at 1 atm and the film temperature of ( T s + T )/2 = (110+30)/2 = 70 C are (Table A-15) Hot gases T = 30 C Furnace T s = 110 C = 0.7 2 m 2 m 1- 2 5 K 002915 . K ) 273 70 ( 1 1 7177 . Pr /s m 10 995 . 1 C W/m. 02881 . = + = = = = = f T k Analysis The characteristic length in this case is the height of the furnace, m. 2 = = L L c Then, 10 2 2 5 3-1 2 2 3 10 301 . 3 ) 7177 . ( ) /s m 10 995 . 1 ( ) m 2 )( K 30 110 )( K 002915 . )( m/s 81 . 9 ( Pr ) ( = = =...
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