In a fire-tube boiler, hot products of combustion flowing through an array of thin-walled tubes are used to boil water flowing over the tubes. at the time of installation, the overall heat transfer coefficient was 400w/m2*k. after 1 year of use, the inner and outer tube surfaces are fouled, with corresponding fouling factors of r”ƒ,i= 0.0015 and r” ƒ,o= 0.0005 m2*k/w, respectively. determine the value of the overall heat transfer coefficient u.