The tube in a heat exchanger has a 2-in. inner diameter and a 3-in. outer diameter. the thermal conductivity of the tube material is 0.54 btu/h∙ft∙℉, while the inner surface heat transfer coefficient is 50 btu/h∙ft2∙℉ and the outer surface heat transfer coefficient is 10 btu/h∙ft2∙℉. determine the overall heat transfer coefficients based on the outer and inner surfaces.

Apreheater involves the use of condensing steam at 100°c on the inside of a bank of tubes to heat air that enters at 1 atm and 25°c. the air moves at 5 m/s in cross flow over the tubes. each tube is 1 m long and has an outside diameter of 10 mm. what is the minimum value of nl needed to achieve an outlet temperature of t 75 c? what is the corresponding pressure drop across the tube bank?