5.8 A specialized tube designed to enhance heat transfer between a condensing refrigerant and water is shown in Figure P5.8. The inner tube is 3 8-std type L copper and the outer tube is 2-std type L copper. Rectangular straight fins made of copper (k = 400 W/m K) with a thickness of 2 mm are attached to the outside of the inner tube to provide enhanced heat transfer surfaces (fins). The fin array, fits snugly into the inside diameter of the outer tube. The refrigerant inside the inner tube, Refrigerant-134a, is condensing at a constant pressure of 11.6 bar.

Cold water flows at a rate of 0.2 gpm through all the channels created by the fins at an average temperature of 15°C. Because of the phase change of the refrigerant, the convective heat transfer coefficient in the inside tube can be assumed to be very large. It can also be assumed that the tube is long enough such that entry effects can be considered negligible.

For the case where the heat exchanger is brand new (i.e., no fouling), determine the following:

a. UA product of the heat exchanger per unit length (W/m K)

b. Heat removal rate from the refrigerant per unit length of tube (W/m)