Water enters a counter flow, double-pipe heat exchanger at a rate of 150 lb_m/min and is heated from 60°F to 140°F by an oil with a specific heat of 0.45 Btu/lb_m °F. The oil enters at 240°F and leaves at 80°F. The overall heat-transfer coefficient is 150 Btu/h ft² °F.

a. What heat-transfer area is required?
b. What area is required if all conditions remain the same except that a shell-and-tube heat exchanger is used with the water making one shell pass and the oil making two tube passes?
c. What exit water temperature would result if, for the exchanger of part (a), the water flow rate were decreased to 120 lb_m/min?