In an oil cooler, 0.06 kg s¹ of hot oil enters a thin metal pipe of diameter 25 mm. An equal mass flow rate of cooling water flows through the annular space between the pipe and a larger concentric pipe. The oil and water move in opposite direc- tions. The oil enters at 420 K and is to be cooled to 320 K. If the water enters at 290 K, what length of the pipe will be required? Take heat transfer coefficients of 1.6 kW m-2 K-1 on the oil side and 3.6 kW m-2 K-1 on the water side and 2.0 kJ kg K-1 for the heat capacity of the oil and 4.18 kJ kg-1 K-1 for the heat capacity of water.