| 摘要 |
In a typical installation, stale air is drawn from vents (7) in each room (2) into rising ducts (6), exhausting through extraction cowls (3,4) on the roof (5). The necessary pressure drop is naturally induced by venturi action inherent in the cowl design, but usually induced draught fans, driven by electric motors, are incorporated. Where more than one duct is provided, a motor, or fan, break-down at one extraction point is likely to cause stale air to be drawn into some rooms by a siphoning effect. Such a break-down is detected by comparison of pressure measured by sensors (9,10) in each cowl (3,4) connected (12,13) to a central controller (11) on or under the roof. The controller, connected to the building electrical supply, controls each fan motor through further connections (14,15). On detecting the abnormal pressure condition, the controller trips all the fan motors and the system reverts to operation on natural draught. Each cowl comprises an upper enclosure, containing the motor, supported by vertical legs on a base section centered on the duct end. A vertical-axis centrifugal fan operates in the space between conical surfaces fixed under and over, respectively, these sections.
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