Dampers and Air Control

Posted on Tuesday, November 21, 2017
The output of a fan into a system is the flow-rate and static pressure at the intersection of the fan’s performance curve and the system’s characteristic curve.

At a given speed the fan will produce a certain amount of airflow and static pressure at this intersection. What happens if we need to reduce that flow or if we need different flows during the operation of the fan?

For most cases speed control is the most efficient method of controlling the duty point of a fan, but there are occasions where a reduction in airflow is required, but the static pressure of the system is not reduced. In this case one of the better ways to make a fan produce less air is to use inlet dampers to control the flow. There may also be a limited number of high temperature applications where outlet dampers may still be an appropriate control method. Knowing how these dampers work can help you in the selection process.

Inlet Volume Control

The IVC is a damper built into the inlet cones used on Chicago Blower’s airfoil and backward inclined centrifugal fans. For fans that do not use an inlet cone, such as pressure blowers, radial blade fans, and axial fans, a bolt-on inlet vane damper (also called a VIV, IVD, or inlet vanes) can be used with a similar effect. Much like the rudder on a boat, the angle of the vanes forces the air off to one side in the same direction as the wheel rotation. In essence, the air is being spun in the same direction of the wheel. The prespun air moves away from the spinning wheel blades as it enters the fan causing the wheel to develop less pressure than before. Both airflow and horsepower are reduced (see the below fan curve).

As the volume decreases, due to closing the damper, the velocity in the system drops proportionately. This means that the airflow-static pressure point and overall brake horsepower decreases, as the vanes are closed, following the system’s characteristic curve down to close to zero. The “A” curve represents 100% open, “B” represents 60%, “C” represents 30%, and finally, “D” represents fully closed.

Outlet Dampers

An additional control method is to use an outlet damper. Outlet Dampers are normally rectangular box dampers with parallel or opposed vanes. These dampers are bolted to the fan’s discharge flange. Outlet dampers control the airflow in an entirely different manner than the IVC dampers. As the vanes are closed in the damper, they block the discharge which adds pressure to the system and changes the system’s characteristic curve. In other words, they block the flow coming out of the fan. The graph below helps to explain the change in pressure caused by an Outlet Damper. As the Outlet Damper is closed, the system characteristic curve changes by moving up the fan performance curve from point “a” to point “d”.

More static pressure is being added to the system, which reduces the airflow output of the fan. Since the damper only changes the amount of pressure is in the system, the fan will follow its original brake horsepower curve. The Outlet Damper also allows for control after the air has passed through the fan. A parallel blade Outlet Damper can be used to throw the air off to one side if there is a split or turn in the ductwork directly passed the fan discharge. Opposed blade Outlet Dampers are used when it is necessary to maintain an even distribution of air immediately downstream from
the damper.

Inlet vs. Outlet Damper

Why would you choose one damper over the other? For the most part, the IVC is the better option. The IVC is able to vary the airflow of the fan, while at the same time, saving energy consumption. Remember, since air is steered by the IVC blades in the direction of the rotating wheel, the wheel is performing less work and saving brake horsepower. Dampering the fan using an IVC also moves the peak pressure point further to the left. This prevents the fan from going into stall as the damper is closed. As illustrated above, the Outlet Damper can go left of peak because the system point is traveling up the performance curve. When airstream temperatures exceed 650°F, the outlet damper is the better choice because the bearings and rotating parts for the IVC, that are located within that hot airstream, would fail.

Whatever the application is, dampers are an inexpensive way to control airflow. For more information on damper types and damper selection for your project, please contact us via the form below.