9 the suction hoods 21a 21b are one piece components made of an extruded aluminum profile such a seal may be a sealing gasket 24 made for example of rubber as illustrated in fig3 as shown in fig4 the air removal ducts 15a and 16a join opposite ends of the suction hood 21a whereas the air removal conduits 15b and 16b extend from opposite ends of the suction hood 21b the conduits 15a 15b as well 16a and 16b are joined together and a respective further air removal duct 15c 16c extends therefrom to enter an air chamber 25 which is joined by the inlet suction side of the fan 13 as shown in fig5 the flow passage cross section of the suction hoods 21a 21b and the conduits 15a c and 16a c is substantially the same the air streams designated with the letters a f as best seen when considering fig1 5 together flow in a closed circuit in recapitulation the air stream exits from the pressure side of the fan 13 passes through the chamber 12 and the wide slotted nozzle 14 into the chamber 19 therefrom through the chamber 18 into the feed chute 9 then through the screens 11a 11b into the suction hoods 21a 21b and then through the air removal conduits 15a 15b 15c 16a 16b and 16c into the collecting chamber 25 and then into the intake side of the fan 13 between the pressure side of the fan 13 and the entrance of the chamber 12 a truncated pyramid shaped guide element 26 is provided which widens in the flow.