Water at an average temperature of 110°C and an average velocity of 3.5 m/s flows through a 5-m-long stainless steel tube (k = 14.2 W/m·K) in a boiler. The inner and outer diameters of the tube are D_i = 1.0 cm and D_o = 1.4 cm, respectively. If the convection heat transfer coefficient at the outer surface of the tube where boiling is taking place is h_o = 8400 W/m²·K, determine the overall heat transfer coefficient U_i of this boiler based on the inner surface area of the tube.