A small vent damper is fitted to the main damper body and allows air to flow either into or out of the seal interspace. The small vent damper is fitted with its own actuator, or can be linked to the main actuator, such that the vent damper opens as the main damper vane closes, and vice versa.
In the case of a negative pressure system (induced draught system), the small vent damper can be used to allow clean air, at atmospheric pressure, to enter the seal interspace. As long as the pressure of this clean air in the interspace is lighter than the pressure in the ducts, both upstream and downstream of the main damper, then process gas cannot leak past both seal rows.
In the case of a positive pressure system (forced draught system), air can be extracted from the seal interspace and ducted back to a low pressure point. This can be the inlet of the main fan or the inlet of a small auxiliary fan. As long as the pressure of the air in the seal interspace is lower than the pressure in the upstream and downstream ducts, any air leaking past the first seal row will be extracted through the small vent damper and no air will leak past the second seal row.
As with a shut-off damper the seal materials used for the twin seals of an isolation damper can be matched to suit the particular process gases and operating conditions.
For more information on seal options and seal selection criteria, click here.
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