Determine the UA product for the double pipe heat exchanger described in Problem 5.40 for both clean and fouled conditions.

Problem 5.40

A counter flow double pipe heat exchanger has an outer pipe of 4-nom sch 40 commercial steel and an inner tube of 2-std type M copper. Hard water at an average temperature of 120°F flows in the tube at a flow rate of 80 gpm. City water flows in the annulus at a flow rate of 75 gpm and an average temperature of 50°F. The heat exchanger is a single DPHX with a length of 6 ft. After one year of service, the volumetric flow rates in the tube and annulus are estimated to experience a 2.5% reduction due to fouling. The fouling layer is primarily calcium carbonate.