5. 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 convective heat transfer coefficient of h; = 5000

W/m² °C. Water flows through the shell at a rate of 0.3 kg/s.

a. Determine the overall heat transfer coefficient of this heat exchanger. (25 pts)

b. What would the overall heat transfer coefficient be if a 2 mm thick layer of

limestone (k = 1.3 W/m °C) forms on the outer surface of the inner tube? (5 pts)