Adouble-wall heat exchanger is constructed of two semi-circular copper ducts held together by exterior straps. each tube has a wall thickness of 3 mm and inner radius of 20 mm. the tube outer surfaces are well-insulated. if hot and cold water at mear temperatures of 330 k and 290 k, respectively flow through the tubes both at 0.2 kg/s, what is the rate of heat transfer per length of tube? assume heat transfer is only across the planar surfaces in contact.