Guillotine Dampers :

Guillotine dampers are used for isolation service in round or rectangular ducts.  The dampers function is to shut off, not control, flow.  The amount of allowable leakage determines the seal design.  Rigid seats provide adequate closure, with leakage in the range of 0.4 to 1% of flow.  When zero leakage of flue gas is required, a seal air system must be added.  Flexible seals reduce air requirements.

The simplest form of guillotine damper is the single blade.  The one piece blade provides verifiable closure of the duct with minimal leakage at the blade edges.  Seals are provided to control leakage where the blade enters the duct section.  These seals can be air sealed to contain gases in a positive pressure duct.  Leak paths are typically less than 0.1% of duct area with resulting flow leakage of 0.4 to 1.1%.  The blade guides insure smooth operation and are self cleaning in deposit producing environments.

The double blade guillotine supplies a secondary man-safe feature in addition to zero leakage.  The seal air and dual blades provide a thermal barrier; heat and gas are kept out of the isolated chamber.  Leakage control is similar to the basic single blade type.  Self cleaning, rigid seats in the duct require minimal maintenance or inspection.  Seals are used at the blade entry only.

As a general rule, guillotine dampers are not suggested for systems with elevated temperatures (above 500 deg.F), however for each rule there always seems to be an exception.  DDI recommends a our Hot Blade design in this situation.  This design reduces the temperature gradient across the blade thickness during the isolation phase, reducing the stresses on the blade that tend to warp it.