Water at 20 °c flows at 3 m/s in a tube with a diameter of 50 mm. a) what is the pressure drop over 30 meters if the walls of the tube are smooth? water at 20 °c flow at 3 m/s in the annulus of a concentric tube heat exchanger. the inner wall of the annulus has a diameter of 25 mm and the outer wall a diameter of 50 mm. what is the pressure drop over 30 meters if: b) the walls are taken as smooth? c) the walls are made of commercial steel?