arrangement shown in fig4 the assembly of the nozzles 98 is replaced with a shroud 104 which includes on its upper portion a horizontal disc 106 and on its lower portion a conical plate 108 the disc 106 and plate 108 are spaced apart to provide an interior chamber 110 between them the outside edges of disc 106 and plate 108 are adjacent to the casing and louvers 116 and are spaced slightly apart to provide an annular discharge slot 112 through which the gas is applied the chamber 110 is supplied with gas through ports 114 when the valve 92 is open the plate 108 inclines upwardly as it extends toward the casing 14 and is preferably oriented at an incline that matches the upward incline of the louvers 116 this allows gas flowing along the upper surface of the plate 108 and through the discharge slot 112 to flow in a direction to readily pass directly through the louver openings 116 a to enhance removal of materials that may plug one or more of the louver openings the arrangement shown in fig4 operates in substantially the same manner described previously the principal difference is that rather than being discharged at discrete locations defined by the nozzles 98 the gas is applied substantially continuously around the.