154 the stream of exhaust air defined by slots 150 and the portion of the peripheral extent of inlet end surface 156a with which it engages are of arcuate configuration when viewed in cross section as viewed in fig1 the concave outer surface 148a of wall portion 148 is seen as converging towards the stream of exhaust air discharged from slots 150 at an angle of about 135 u00b0 and as being generally aligned with or following the contour of the inner surface of housing second part 144 whereby these elements cooperate to direct dust laden air from shroud exhaust port 120 into inlet end 156 of convergent divergent flow passage 154 at an angle relative to the axially extending stream of reduced pressure air created by nozzle 136 and member 138 this angle may be varied but it must be greater than 90 u00b0 and less than 180 u00b0 now referring to fig1 14 it will be apparent that member 138 is removably snap fit mounted within housing first part 140 by means of a pair of resiliently deformable detents 140e 140e formed integrally with the housing first part and a pair of arcuate abutment surfaces 138a 138a which extend radially between an outer guide surface 138b and a pair of inner guide surfaces 138c 138c of the member arranged adjacent inlet end 156 of its flow passage 154 one end of each inner.