Balanced ventilation doors

Apparatus comprising a door in association with a passage, wherein:

• conditions in the passage are such that air flows along the passage from an upstream-side of the door to a downstream-side;
• the passage has side-walls, being side-wall-A and side-wall-B;
• the door is in two door-sections, being door-section-A and door-section-B;
• door-section-A is hinged at hinge-A to side-wall-A, and door-section-B is hinged at hinge-B to side-wall-B;
• the two door-sections are operably pivotable about the respective hinges between respective closed and open positions;
• each door-section is so dimensioned as to extend only partway across the passage in the closed position of the door-sections;
• in the closed position of the door-sections, the door-sections together close off the passage;
• in the open position of the door-sections, the door-sections lie wide apart, creating a clear space between the door-sections;
• the arrangement of the door-sections and the hinges is such that door-section-A opens in the direction towards the upstream-side of the door and door-section-B opens in the direction towards the downstream-side of the door;
• the apparatus includes a linkage, which acts between the two door-sections, the structural arrangement of which is such that the linkage constrains the upstream opening and closing movements of door-section-A to correspond to the downstream opening and closing movements of door-section-B;
• the linkage links the arcuate angular movement through which the door-sections pivot about the respective hinges, relative to each other;
• the arrangement of the linkage is such that, in operation of the door-sections, the linkage constrains the angle through which door-section-A pivots about hinge-A to correspond proportionally to the angle through which door-section-B pivots about hinge-B.