| 摘要 |
A frame divides the cover (20) into passage sectors. Passage(s) have opening flaps (34, 35). Remaining sectors are closed with suitably-shaped cover sheets (47-51). Each has a locking device (53), bolting into frame sections (27-32). Preferred features: Cover sheets partially overlap. The frame sections include grooves or channels, into which turned edges of the cover sheets engage. Resilient locking devices (53) can be suspended in corresponding holding recesses (55) in the frame sections. They are rubbery, thickened. Suitable locking recesses are described. Handles on the locking devices, assist handling of the cover sheets. Locking devices are jointed to the cover sheets. A cover sheet includes a suction connection for an extraction hose connector. The frame includes a central ring section (26), fitting inner ends of the cover sheets. A circular outer band (33) joins frame section ends, excepting the sector covered by the flap doors. Flaps are each preferably double, centrally pivoted and connected to two frame sections. Further details of flaps and construction are described. The cover rotates on a central holding tube connected to a lifting unit. This is fixed to the support of a mixing unit, the mixer drive shaft extending through the central holding tube (18). Frames on either side of the flaps have suction slots below the pivots, connected to the extraction system.
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