Water entering a shell-and-tube heat exchanger at 30C is to be heated to 70C by an oil. The oil enters at 100C and leaves at 70C. The heat exchanger is arranged with the water making one shell pass and the oil making four tube passes. If the water flow rate is 70 kg/min and the heat exchanger area is 25 m2 , determine: a) the logarithmic temperature difference and b) the overall heat transfer coefficient of this heat exchanger. USE the LMTD method to solve the problem. Write down all your assumptions.