thus flows through exhaust port 44 to atmosphere when arm 56 rotates counterclockwise damper 48 becomes more fully closed and damper 50 becomes more fully open it is therefore possible by control of motor 46 to vary the air flow from the blower 36 into the reaction vessel 12 using a constant speed blower 36 it would be evident to one skilled in the art that either one or both of dampers 48 and 50 could be eliminated without departing from the spirit of the invention for example if damper 48 were eliminated actuation of damper 50 in the manner described would change the balance of air flow between reaction vessel 12 and exhaust port 44 if damper 50 were replaced by a closed barrier in branch 42 varying the position of damper 48 would control the flow of the air into the reaction vessel 12 as the damper 48 assumed a more and more closed position the blower 36 would operate less efficiently in addition a variable speed and or variable capacity blower 36 may be employed without employing either damper 48 or 50 in that case the blower output would be fed directly into the reaction vessel a feed mechanism 58 feeds raw material into the cylindrical upper parts 16 through a feed port 60 a hinged door 62 which may be spring loaded in the closed position but which is preferably urged to close only by gravity covers the feed port.