A double-pipe heat exchanger is to be designed to cool 5.5 gal/min of a hot oil from 250°F to 120°F using 11 gal/min of cooling water available at 70°F. The exchanger is to consist of 12-ft-long sections of ¾-in.-diameter copper tubing placed inside 1½-in.-diameter, well-insulated tubing, with water flowing in the annular space between the two tubes. How many sections of copper tubing are required if the flow of the two streams is (a)

counter-current and

(b) co-current? The overall heat transfer coefficient is U_° = 110 BTU/h ft² °F, the heat capacity of the oil is 0.55 BTU/lb °F, and the density of the oil is 52 lb/ft³. Assume that there is no heat transfer to the surrounding.