11.1 In a fire-tube boiler, hot products of combustion flow- ing through an array of thin-walled tubes are used to boil water flowing over the tubes. At the time of instal- lation, the overall heat transfer coefficient was 400 W/m K. After 1 year of use, the inner and outer tube surfaces are fouled, with corresponding fouling factors of Ri 0.0015 and R = 0.0005 mK/W, respectively. Should the boiler be scheduled for clean- ing of the tube surfaces? =