Damper control

The reduced efficiency accepted, dampers offer a low first cost method of controlling flowrate. They are easily adjusted and additional space is often minimal as they are inserted in the ex­isting duct layout. They are manufactured in all types of mate­rial according to the gas constituents and temperature. They can be positioned either in the inlet or outlet duct, this being de­termined by fan type and characteristics.

Since a damper operates by adding resistance to the system or by “destroying” fan pressure, its only effect upon fan power is to move the operating point nearer to the closed condition. With the wider backward bladed fans, this may have little or no effect on power absorbed as the power characteristic is virtually con­stant (non-overloading) over the working range. With rising pressure a characteristic of closed conditions it also means that the amount of pressure to be dissipated across the damper is ever increasing. The overall efficiency can then be very low.

For the narrower backward bladed fans and for other blade de­signs where the power absorbed reduces significantly at lower flowrates, an outlet damper is a reasonably economical control situation. With wide forward curved bladed, or multivane fans where the pressure characteristic is flat or even reducing to zero flow, the amounts of pressure to be dissipated across the damper are reduced and the fan/damper combination is rea­sonably efficient. It can, therefore, be recommended where system resistance and power absorbed are sufficiently low to justify the use of the multivane.

Parallel blade dampers

The free area through the damper is not substantially reduced until the blades have been turned through a considerable an­gle. The quadrant arm, therefore, has to move through a large arc for a small reduction in fan capacity. This means that such a damper may best be installed on systems requiring flows be­tween 70% and 100% of full capacity. The greater the number of blades, the more movement is necessary for a given flow re­

Duction. Its sensitivity enables predetermined lever settings to give good repeatability of flowrate.

» 50 § 40 30 20 10

0 10 20 30 40 50 60 70 80 90 100 Damper opening %

90 80 70

It is more readily manufactured for rectangular ducts and thus is mainly used on the outlet of centrifugal fans. It may, however, be used on fans fitted with a boxed inlet when a degree of pre-swirl (and powersaving) is achieved. (See Section 6.5.3.)

Opposed blade dampers

These act in the same way as parallel blade dampers, but alter­nate blades are made to turn in the opposite direction. The free area through the damper reduces more proportionally with the blade angle. Flowrate reduction is thus almost directly propor­tional to the angular movement of the damper control arm. Again, this type is normally restricted to the rectangular outlets of centrifugal fans, as the complexity involved in the sealing and leakage of circular units makes this variant too expensive.

0 18 36 45 54 72 9D Blade angle degrees

The dampers are also used when it is necessary to maintain an even distribution of air immediately downstream of the damper, due to the proximity of branch take-offs etc.

Single blade swivel dampers

These are a very simple form of control similar in operation to the parallel bladed type. They can be easily manufactured in circular or rectangular cross-section and thus may be easily po­sitioned on the inlet or outlet of both centrifugal and axial flow fans. Although less movement of the damper arm is needed, sensitivity is also reduced and their use should be restricted to systems requiring flow regulation between 50% and 100% of full flowrate.

It should also be noted that at low flow rates considerable dis­tortion to the velocity profile can result. Under these circum­stances their use adjacent to the inlet of both axial and centrifu­gal fans may be detrimental.

Guillotine dampers

These consist of a single plate which can move from one side of the duct to “cut-off’ the airflow. Most widely used for fan isola­tion, they should only be used for flow regulation after careful consideration. The velocity profile will be considerably distorted to one side and damage to the fan can eventually result. Where a tongue piece is positioned in the outlet of a centrifugal fan, or where the impeller is asymmetrically placed, special care must be taken or there may be a zero effect.

Radial inlet vanes are considered later, for whilst they also act as a damper, their major intention is to promote pre-swirl into the fan inlet. Thus, they materially affect the fan geometry and an additional power saving results.

Acomparison of the flowrate control for various types of damper is shown in Figure 6.3. It must be emphasized that this is ap­proximate only. In fact, it is specific to a particular fan and ducting system. The general trends however may be taken as indicative.

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