Method and apparatus for operating a multiple hearth furnace

摘要

A multiple hearth furnace having a drying zone, a combustion zone and a cooling zone includes a recirculation loop that recycles exhaust gas from the drying zone to the cooling zone. In some embodiments, a first control loop including a temperature measurement device that measures temperature in the combustion zone controls fan speed of a recirculation fan that drives the recirculation loop. A second control loop monitors recirculation fan temperature and overrides the first control loop if the recirculation fan temperature exceeds a predetermined maximum. A third control loop controls air flow into the furnace.