Apreheater involves the use of condensing steam at 120°c on the inside of a bank of tubes to heat air that enters at 1 atm and 20°c. the air moves at 10 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 to = 80°c? what is the corresponding pressure drop across the tube bank? note: st = sl = 15 mm