A thin-walled tube of 20-mm diameter passes hot fluidat a

Chapter 9, Problem 9.68

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A thin-walled tube of 20-mm diameter passes hot fluidat a mean temperature of 45?C in an experimental flowloop. The tube is mounted horizontally in quiescent airat a temperature of 15?C. To satisfy the stringent tem-perature control requirements of the experiment, it wasdecided to wind thin electrical heating tape on the outersurface of the tube to prevent heat loss from the hotfluid to the ambient air.(a) Neglecting radiation heat loss, calculate the heatflux qethat must be supplied by the electrical tapeto ensure a uniform fluid temperature.(b) Assuming the emissivity of the tape is 0.95 andthe surroundings are also at 15?C, calculate therequired heat flux.(c) The heat loss may be reduced by wrapping theheating tape in a layer of insulation. For 85% mag-nesia insulation (k?0.050 W/m?K) having a sur-face emissivity of ?0.60, compute and plot therequired heat flux qeas a function of insulationthickness in the range from 0 to 20 mm. For thisrange, compute and plot the convection and radia-tion heat rates per unit tube length as a function ofinsulation thickness.

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