Sliding Sleeve Dampers
The sliding sleeve damper is frequently used for gravity ventilators.
In construction it closely fits the under circumference of the ventilator airshaft and operates vertically. It occupies almost no space in the airshaft and offers no resistance or turbulence to the airstream in any position. Its normal position is open, and with a fusible link it can be used as an automatic-opening damper where there is need for such a control unit.
Sliding sleeve dampers have one principal advantage. Because of their vertical sides, there is no tendency for dust or dirt to accumulate.
The sliding sleeve damper is operated by chain only. Radial spiders keep the damper cylindrical to the operating chain attached to the hub of the spider, passing upward through a pulley fastened below the finial and then down past the spider hub, where a spring clip is located. The spring clip enables the damper to be positioned at any point between open and closed.
Sliding Cone Dampers
As shown in Figure 6-28, a sliding cone damper consists of a cone with apex down and with a flared outer rim that seats on the upper edge of the airshaft when closed. This directs the airstream to the outlet openings of the ventilator, lessens turbulence in the ventilator head, and consequently increases capacity.
From the apex of the damper, the operating chain passes upward to a pulley located below the finial, then downward through a slot in the cone to a pulley located on the inside of the airshaft, and then through the base to the building below.
When used in connection with the fusible link, this damper becomes an automatically closing, fire-retarding unit.
Butterfly Dampers
The two butterfly dampers used in roof ventilators are (1) the single-disc damper and (2) the divided-disc damper. They receive their name from the butterfly-wing appearance of the metal disc used to open or close the air passage.
The single-disc butterfly damper seats tightly against the ring channel, preventing air leakage when closed. When operated by hand chain, these dampers are normally counterweighted to close but can be supplied weighted to open when required.
The divided-disc butterfly damper is used for sizes that prevent the use of a single disc. The divided halves (or wings) of the disc swing upward, pivoted on two rods whose ends bear in the damper ring channel on which the damper edges seat tightly.
Butterfly dampers, single- and divided-disc, are the only dampers that can be successfully used in ventilators constructed of asbestos- protected steel. In this material, the damper ring is omitted and the damper pivot rods are mounted on brackets secured to the airshaft wall.