A concentric tube heat exchanger for cooling lubricating oil is comprised of a thin-walled inner tube of 25-mm diameter carrying water and an outer tube of 45-mm diameter carrying the oil. The exchanger operates in counterflow with an overall heat transfer coefficient of 60 W/m2 K and the tabulated average properties.
(a) If the outlet temperature of the oil is 60°C, determine the total heat transfer and the outlet temperature of the water.
(b) Determine the length required for the heat exchanger.