In a counter-current heat exchanger, 1.7 kg.s of naphta is cooled from temperature 90C to 20C. The heat capacity of the naphta is 2.5 kJ.kg.K. The naphta flows through the shell space. 2 kg.s of cooling water enter at 15C and flow through a bundle consisting of 37 tubes, the outer diameter of which is 32mm and wall thickness is 3.5 mm. The bundle is 6 m long. Determine the overall heat transfer coefficient.