Chapter 9 Natural Convection 9-38 A steam pipe extended from one end of a plant to the other. It is proposed to insulate the steam pipe for $750. The simple 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 anticipated film temperature of ( T s + T ∞ )/2 = (35+20)/2 = 27.5 ° C are (Table A-15) Steam L = 60 m D =16.03 cm 170 ° C, ε = 0.1 Air T ∞ = 20 ° C Insulation ε = 0.1 1-2 5 K 003328 .0 K ) 273 5 . 27 ( 1 1 7289 .0 Pr /s m 10 584 . 1 C W/m. 0257 .0 = + = = = × = ° = − f T k β υ Analysis Insulation will drop the outer surface temperature to a value close to the ambient temperature. The solution of this problem requires a trial-and-error approach since the determination of the Rayleigh number and thus the Nusselt number depends on the surface temperature which is unknown. We start the solution
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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.