Acounterflow concentric heat tube heat exchanger is used to cool the lubricating oil for a large industrial gas turbine engine. the flow rate of cooling water through the inner tube (di= 2.5 cm) is 0.2 kg/s while the flow rate of oil through the outer annulus (do= 4.5 cm) is 0.1 kg/s. consider the uvalue to be equal to 38.1 w/(m2. k). the oil and water enter at temperatures of 100 and 30oc respectively. how long must the tube be made if the outer temperature of the oil is to be 60oc?