| 摘要 |
The shutter consists of a rectangular-section housing (1), e.g. of metal, containing a flap (2) which turns on a pivot (3) and engages with a seat inside the housing. The flap is made from two offset parallel sections (7,8) situated on either side of the pivot (3), with the seat also in two sections (9,10) which are offset in a corresponding manner. The edges (72,82) of the flap surfaces and the corresponding edges (91,101) of the seat sections are angled so that they fit together when they meet. The two sections of the flap are preferably made from different materials to combine their properties for the resistance of heat and preventing the spread of fire.
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