Don’t Blow Your Money Away

Make the right choice when regulating fan airflow.

By Tom Kuli, Robinson Industries, Inc.

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The two main types of inlet dampers are louvered and radial.

Louvered inlet dampers typically have parallel blades and work well in dirty airstreams. They also are available with opposed blades — but this configuration isn’t recommended because it doesn’t pre-spin the air.
Radial inlet dampers, due to their ability to pre-spin air more effectively, typically are more efficient than louvered ones.

Other types of inlet dampers include: vortex dampers, which require inlet boxes and are available in cantilevered blade and center hub designs; and variable inlet vanes, which require cone-shaped inlet pieces and are used only with clean air streams. Both boast efficiencies comparable to those of radial inlet dampers.
If an air movement system is infrequently used, inlet dampers may come out on top in an initial-cost/potential-energy-savings comparison of airflow regulation devices. They are especially effective when restricting airflow by less than 20%.

Some caution is in order when dampers severely limit airflow. Restricting airflow by as much as 70% may lead to flow instability or rotating stall, i.e., air starvation resulting in high-amplitude pressure pulses. Take the recent case of a steel mill operating coke-oven-battery scrubber fans with inlet dampers 90% closed. Fan professionals documented severe vibration and cracking of fan casings. Measurements of vibration, pressure pulsation and frequency confirmed a rotating stall condition. After installing a VFD, the mill could leave the damper fully open and regulate airflow by adjusting the fan speed. The fan then smoothly operated at all process flow rates. The mill realized energy savings estimated at more than $250,000 annually.

Outlet Dampers
These control flow by restricting the airstream’s path at the outlet. They are rarely used on large industrial fans due to their poor efficiencies (Figure 1) and potential to damage system components.

Partially closing, or throttling, parallel or opposed damper blades provides the desired reduction of flow. However, pressure increases on the upstream side of the dampers boost backpressure on the system. This resistance causes the fan’s operating point to shift negatively on its performance curve.

Excessive damper use may damage system components. Highly throttled outlet dampers can create severe buffeting and very high backpressure that may cause system components to prematurely wear, overheat or even crack, leading to increased operating and maintenance costs. Such dampers also are prone to corrosion in dirty airstreams. In addition, particulate matter buildup on damper blades and thermal distortion of damper elements can impede the ability to adjust the blades for throttling.

Variable Frequency Drives
VFDs generally offer the smoothest flow control over the widest range of volume and pressure. With proven energy-saving capabilities, they’re a popular choice for air movement systems that operate for long periods at reduced flow and pressure. In addition, VFDs are compact and easily added to most existing motors, and they reduce common fouling problems associated with outlet and inlet dampers, such as excessive vibration, noise and equipment wear and tear.

As the fan speed (rpm) decreases with VFDs, the pressure, volume and horsepower all decline. The curves for both fan performance and brake horsepower (BHP) move essentially in harmony with the fan law curve (Figure 1). This shift works well for most fixed-resistance systems. It offers significant horsepower savings at reduced speeds, as shown by the third Fan Law: BHP2 = (rpm2/rpm1)3(BHP1)

In other words, halving a motor’s speed will reduce power consumption to one-eighth. For example, a drop from 1,000 rpm at 1,000 BHP to 500 rpm results in a power reduction to 125 BHP. Much less energy is needed to run the motor at the lower speed.

The soft-start capabilities available with VFDs offer further opportunities for decreasing energy demands. Motors typically experience higher currents during startup than during normal operation. VFDs allow the motor to be started with a lower current. By eliminating the higher startup power surge, VFDs reduce wear on motor windings and the controller, and lower the severity of voltage sags that may affect sensitive equipment.

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