Chapter 9 Natural Convection 9-25 Some cans move slowly in a hot water container made of sheet metal. It is proposed to insulate the side and bottom surfaces of the container for $350. The simple payback period of the 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. 3 Heat loss from the top surface is disregarded. Properties 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 assume the surface temperature to be 26 ° C. The properties of air at the anticipated film temperature of (26+20)/2=23 ° C are (Table A-15) 1-2 5 K 00338 .0 K
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