5.10 A counterflow DPHX is being used to cool hot oil from 320°F to 285°F using cold water. The water, which flows through the inner tube, enters the heat exchanger at 70°F and leaves at 175°F. The inner tube is ¾-std type L copper.

The overall heat transfer coefficient based on the outside diameter of the inner tube is 90 Btu/h·ft2·°F. Design conditions call for a total heat transfer between the two fluids of 15,000 Btu/h. Determine the required length of this heat exchanger (ft).