final sub set 94 of holes have the largest diameter e g 1 5 mm in this final sub set 94 the holes are arranged so that as one moves in the direction 98 by rotating the duct 60 there is an increasing number of holes in each row that in use will be arranged in alignment with the outlet 55 of the housing 50 this modified form of outlet 65 a of the duct 60 has an advantage in that it maintains the strength of the duct 60 better than a slot 65 and can be provided around a greater portion of the total circumference of the duct 60 furthermore the outlet 65 of the duct 60 can be manufactured precisely more particularly the holes of the outlet 65 a can be manufactured more precisely than the slot 65 in fig2 in fig5 also possible positions of the outlet 55 of the housing 50 are shown with respect to the outlet 65 a of the duct 60 as shown in full lines the outlet 55 of the housing 50 is arranged with respect to the holes of the outlet 65 a of the duct 60 in such a way that the flow through the duct 60 and the outlet 55 of the housing 50 is almost 100 of the flow through a free outlet 55 while in the position shown in dashed lines the outlet 55 of the housing 50 is arranged in such a way that the flow through the duct 60 and the outlet 55 of the housing 50 is only a fraction of the flow through a free outlet 55 although fig2 and 3 show a different inlet and a different outlet either of these modifications can be used.