A counter flow double pipe heat exchanger 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 140 Btu/hr-ft2-°F. Design conditions call for a total heat transfer duty (heat transfer rate between the two fluids) of 20,000 Btu/hr. Determine the required length of this heat exchanger (ft).