35 is located at a downstream end 46 of each of the holes 36 with the holes extending with the parallel upper and lower edges to an upstream end 47 just beyond the mouth of the duct 13 thus the material flowing onto the accelerator plate 35 and its nose 42 is halted at that location thus causing the material to flow outwardly through the holes 36 a and 36 b as the material begins to flow outwardly it is engaged by the surfaces 40 and 41 as best shown in fig3 the surfaces are symmetrical so that only one of the surfaces will be described thus the surface 40 includes an end 40 a commencing at the trailing edge 39 b and from that end 40 a it is flat extending to approximately a mid point 40 c from that mid point it is curved rearwardly toward a rearward most point 40 b from that point 40 b the surface 40 curves forwardly to a point 40 d after which it reaches the plane p 1 thus it will be noted that each of a series of lines 40 d 40 e 40 f are angularly retarded relative to points p 3 p 4 p 5 lying in the plane p 2 it will be appreciated that all of these points or lines are purely imaginary and are used merely for explanation of the shape of the surface thus the point p 3 lies at a common radial distance from the center 19 a with the point 40 d with each of.