Air is flowing through a smooth thin-walled 4-in- diameter copper tube that is submerged i...

Air is flowing through a smooth thin-walled 4-in- diameter copper tube that is submerged in water. The water maintains a constant temperature of 60°F and a convection heat transfer coefficient of 176 Btu/h·ft²·R. If air (1 atm) enters the copper tube at a mean temperature of 90°F with an average velocity of 8 ft/s, determine the necessary copper tube length so that the outlet mean temperature of the air is 70°F.