Reconsider Prob. 11–18. Using EES (or other) software, plot the overall heat transfer coefficient as a function of the limestone thickness as it varies from 1 mm to 3 mm, and discuss the results.

Data from problem 18

Repeat Prob. 11–17 by assuming a 2-mm-thick layer of limestone (k = 1.3 W/m · K) forms on the outer surface of the inner tube.

Data from problem 117

A long thin-walled double-pipe heat exchanger with tube and shell diameters of 1.0 cm and 2.5 cm, respectively, is used to condense refrigerant-134a by water at 20°C. The refrigerant flows through the tube, with a convection heat transfer coefficient of h_i = 4100 W/m² · K. Water flows through the shell at a rate of 0.3 kg/s. Determine the overall heat transfer coefficient of this heat exchanger.