A long thin-walled double-pipe heat exchanger with tube and shell diameters of 1.0 cm and 2.5 cm, respectively, is used to condense refrigerant -134a by water at 20°C. The refrigerant flows through the tube with a convection heat transfer coefficient of hi = 4500 W/m2 °C. Water flows through the shell at a rate of 0.5 kg/s. Determine the overall heat transfer coefficient of this heat exchanger.