12a 12b etc from time to time is placed below the sealing element 30 in such manner that each flat together with its neighbouring flat temporarily forms a duct in which the cleaning air stream becomes effective the cleaning element 30 does not necessarily extend as shown in fig1 and 3 over a plurality of flats 12 and 12a 12b it is sufficient if it extends over e g the two webs 18 18a of two neighbouring flats 18 18a in such manner that the air tight duct 34 is temporarily formed for design stability reasons however use of a sealing element 30 extending over a plurality of flats is recommended as each flat revolves is placed under the sealing element 30 or the plate 31 respectively and the suction air stream effects an air flow through the duct and produces the desired cleaning effect it should also be mentioned that owing to the total cleaning air stream it is possible to also subject the whole inside room 29 under the hood 28 to slight vacuum in such an advantageous manner that the above mentioned conditions for the air in the surrounding room are achieved the hood 28 can also be connected to a separate suction duct not shown what is important however is the fact that due to the.