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About Dampers : LOUVER DAMPER

Louver Dampers:

The louver type of damper consists of several blades mounted parallel across a duct, with centrally pivoted shafts extending out through a frame and driven by a linkage.  Louver dampers are versatile, able in theory to handle any application in the power plant.  Some of their advantages:

  • Fit anywhere in ducting, at any altitude.

  • Compact, no bonnet, little external clearance needed.

  • Lightweight

  • Drive and linkage more readily accessible

  • Control function simple with opposed blades.

  • Thin metal blade construction gives quick response to thermal transients.

  • Fast opening and closing.

  • Low leakage to outside environment.

  • Good modulation and control characteristics.

  • Actuation power requirements are low.

  • Normally does not require a support structure.

Louver dampers have basic disadvantages, however.  Here’s a partial list:

  • Large leakage perimeter.

  • Leakage through seals goes downstream inside the duct.

  • Comparatively high pressure drop because of blade and seal obstruction of flow.

  • Obstructions such as seals, shafts, stops, and fasteners tend to catch ash and scrubber slop.

  • Inherent flimsiness of long thin blades tends to promote flutter.

  • Blades and seals, always in the flow, tend to corrode and erode more.

  • Blades can buckle and wrap, causing leakage and lockup.

  • Bearing troubles are possible.

  • Inadequate drive power and blade strength to crush through concretions and force a seal.

  • Larger flange-to-flange required to contain blade(s) in full open position.

  • Sealing problems in dirty applications.

  • More moving parts and thus increased maintenance

The drive for a louver are simple, needing only 90° of motion.  The torque requirement can vary widely over damper life, if corrosion and thermal effects are severe.  Electric motors, air cylinders, and oil cylinders are able to actuate louver dampers with little difficulty.

Louver dampers are best applied to balance or control flow.  The normal leak path of 0.7% of duct area produces a three to five percent leak rate in average applications.

As a general rule, a damper should be used in the middle third of the control range for best results.  This gives equal percentage in either direction should the need arise.

A variety of options are available.  Metallic spring seals, sealed (dust tight) blade ends, shear seals, and many materials are available. 

 

Leakage performance of a louver depends on ratio of flowing to shutoff pressure, design temperature, number of blades, and blade edge treatment.  Since the first two are system related, the last two are the usual areas of improvement.  Reduction of number of blades reduces leak path to the limit that a single blade has only perimeter leakage.

OPPOSED BLADES LOUVER DAMPERS are the most effective type of damper for controlling the flow of gases with the least disturbance to flow in ducts. Each pair of blades is carefully fitted to ensure precise operation in opposing directions, and thus creating highly controllable passages for the gas.

The frame is made from a formed or fabricated channel and is designed to withstand any system upset conditions. The blades are designed to adequately handle all loads, up to and including upset conditions with a maximum center span deflection determined by the engineering department so as to ensure smooth and trouble free operation. The drive shafts are supported in maintenance-free bearings. An adjustable floating packing gland seals the shaft passage through the damper frame. A special non-binding, fully adjustable external linkage is used to operate the damper. A wide variety of operators can be specified -electric, pneumatic, hydraulic or manual.

Double Louver Dampers:

The Double Louver Damper is used to combine flow control with the capability of 100 % isolation.

The damper is configured with two rows of blades in a common frame. 100% isolation is achieved by closing the blades and pressurizing the space between the two rows with clean air from ambient at a pressure greater than the upstream pressure in the duct. The clean air barrier prevents leakage of flue gas into the isolated area and also ventilates the downstream area with clean air.

Tandem Louver Dampers:

Where a double louver damper forms a seal air barrier between seperate rows/banks of blades, a tandem damper has only on row of blades and the air chamber is formed between the upstream and downstream section of each blade.

The single set of shafts allows the number of blades to be minimized. Even wide blades can be contained within the minimum duct space required by a double louver.

In addition to eliminating the need for a second set of shafts, the wider blades allowed can reduce the number of blades in the damper. The smaller number of shafts substantially reduces the bearing and packing maintenance and operating torque.

This compact structure allows much more freedom of duct design than a conventional double louver.

Damper Don © 2009

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