A cylindrical exhaust duct has a constant inside temperature of 650°F and is insulated on the exterior with a 4-in. layer of rock wool, which has an outer surface temperature of 95°F. The duct has an inner diameter of 3.5 in. and ½-in. thick walls of a ceramic material with a thermal conductivity of 0.78 BTU/h ft °F. Find the rate of heat loss from per linear foot of duct.