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Louver Dampers

Flowseal multi louvre dampers are designed and fabricated to meet the requirements of power and industrial plants and to comply fully with the requirements of AMCA publication 850-84. They provide effective and reliable control and or shutoff in hot, noxious, corrosive and dirty gas flow conditions.

Flowseal louvre dampers can be manufactured with either parallel blade or opposed blade configurations. The blades are of a two ply construction and are aerofoil in shape thus providing smooth and laminar flow. In all cases the blades are driven by a series of lever arms and adjustable link rods. All louvre dampers are fitted with spherical maintenance free bearings or maintenance free DU bushes, which have proven most reliable in all conditions.

Flowseal louvre dampers are designed for minimum maintenance and that which is required is simple and can be performed while the system is on line.

Associated Terms:

  • *Louver Dampers / *Louvre
  • Tandem *Louver Damper / *Louvre
  • Circular *Louver Damper / *Louvre
  • Multi bladed Damper
  • Multi leafed Damper
  • Multi Vane Damper
  • Segmented Opening Damper
  • Control Louver Damper
  • Isolation Louver Damper
  • Purge Air Isolation Louver Damper
  • Air Curtain Seal
  • Purge Air Seal
  • Rectangular *Louver Damper / *Louvre
  • Sealing Efficiency
  • >95% Sealing Efficiency
  • >98% Sealing Efficiency
  • >99% Sealing Efficiency
  • AMCA
  • AMCA International
  • Air Movement and Control Association International
  • AMCA Standards

The following styles of louvre dampers are available:

Type NS multi blade Louvre Dampers.?

Sealing efficiency : 95% of full duct area or better.

NS is a no seal damper and is used for control where isolation is not a prerequisite.

Type STS step seat seal multi blade Louvre Dampers.?

Sealing efficiency : 98% of full duct area or better.

Each blade closes onto a landing bar located at all blade tips and along the blade sides. This provides a degree of isolation that is adequate for most applications with no impairment to the control characteristics of the damper.

Type SSF st st flexible seal multi blade Louvre Dampers.?

Sealing efficiency : 99% of full flow or better.

Similar to the type STS described above except that blade edges are fitted with stainless steel flexible seals to improve the sealing efficiency of the damper.

Tandem Louvre Dampers?

The tandem louvre damper is used where total, 100% isolation is required. Tandems are the same as the type SSF damper but has two rows of blades built into one casing with the cavity between the blades being purged at a higher pressure than the maximum system pressure using an air blower.

General Design

Flowseal louvre dampers can be fabricated for any duct size, circular or rectangular and can be fitted to a vertical duct or to a horizontal duct with the blade spindles vertical or horizontal.

The dampers are designed to fulfill specific application requirements and conform to the following minimum specifications.


Materials of construction are selected for their suitability for the conditions of temperature and pressure, and the corrosive and abrasive environment of the system.


Frames are designed to be entirely self supporting in the duct with no external mounting or anchoring required and are normally of the channel type, either welded or of a rolled structural section. The flanges are pre-drilled to customer requirements.

The frames are designed for the system pressure and temperature as well as construction and transportation loadings


Damper blades are of double ply construction and aerofoil in shape. Stub shaft connections are adequately strengthened for the bolt fastenings. Damper blades are designed so that the maximum deflection under operating conditions does not exceed L/360.

Packing glands?

Packing glands are continuously welded to the damper frame at each shaft clearance hole and are filled with packing suitable for ambient as well as operating conditions. The glands are designed in such away that packing may be renewed without removal of bearings or linkages.

Live loaded packing gland seals are available for applications where maintenance is to be kept to a minimum.


Bearings shall be of the spherical maintenance free type or DU Bushes, under no circumstances will ball or roller bearings be considered due to their potential to brinell and seize under normal damper operating conditions. The bearings shall be located sufficiently removed from the heat source to ensure that they are operating within their design temperature rating.

Lever arms and link rods?

The geometry of the lever arms and link rods is to be such that forces are evenly transmitted throughout the arc of travel, and in the case of opposed blade dampers, care is to be taken to prevent toggle action becoming possible. All link rods are to be adjustable. Under no circumstances can any clearances or loose connections be permitted in the link rod geometry as this would adversely affect the operation of the damper. Rod ends are to be of the spherical low maintenance type.

Stub shafts?

Stub shafts will always be manufactured in grade 304 stainless steel or better. The stub shafts are designed to transmit the maximum torque potential of the drive mechanism. All lever and actuator bushes and bosses are to be fitted with parallel keys.

It is imperative that the combined geometry of blade connection, thru gland seal, to outboard mounted bearing and lever arm or actuator connection, is such that the potential for pushing the shaft out of alignment during operation is totally eliminated.


Flowseal louvre dampers can be furnished with electric, pneumatic, hydraulic or manual actuators. The actuators are to be sized to function reliably over the full range of system temperatures and pressures. Actuators may be fitted with positioners, I/P converters, solenoid valves, manual overrides and fail safe open or fail safe closed devices if required. All actuators are to be fitted at the factory and fully tested prior to shipment.

Actuators can be fitted to either shaft and on either side of the Damper.