The surface tension of water, a needed quantity in several of the following problems, is related to temperature according to the expression σ = 0.1232 [1 − 0.00146 T] , where σ is in N/m and T is in K. In the English system with σ given in lbf/ft and T in ?F, the surface tension may be calculated from σ = (8.44 × 10−3) [1 = 0.00082 T].
If the cylinder described in Problem 21.3 were initially heated to 500°F, how long would it take for the center of the cylinder to cool to 240°F if it were constructed of
a. copper?
b. brass?
c. nickel?
Problem 21.3
A cylindrical copper heating element 2 ft long and 12 in. in diameter is immersed in water. The system pressure is maintained at 1 atm and the tube surface is held at 280°F . Determine the nucleate-boiling heat-transfer coefficient and the rate of heat dissipation for this system.
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