19-70 In a gas-fired boiler, water is being boiled at 120C by hot air flowing through a 5-m-long, 5-cm-diameter tube submerged in water. Hot air enters the tube at 1atm and 300C at a mean velocity of 7m/s and leaves at 150C. If the surface temperature of the tube is 120C, determine the average convection heat transfer coefficient of the air and the rate of water evaporation, in kg/h 19-71 Reconsider Prob. 19-70. Using appropriate software, evaluate the effect of the tube length on the average convection heat transfer coefficient of air. By varying the tube length from 3 to 18m, plot the average convection heat transfer coefficient as a function of the tube length