Don’t Blow Your Money Away

By Tom Kuli, Robinson Industries, Inc.

ChemicalProcessing.com

Make the right choice when regulating fan airflow.

Fans that rely on fixed-speed motors in combination with dampers can squander energy if the regulation or restriction of airflow regularly exceeds 20% of design flow. In such cases, variable frequency drives (VFD) may offer a more-energy-efficient option. The critical question is when to employ mechanical methods of airflow regulation and when to employ VFDs. A mechanical method generally incurs lower upfront capital costs but, depending upon the application, a VFD may turn out more cost-effective when considering long-term energy use. So, let’s examine how to determine which option is the most efficient for the application.

VFDs reduce airflow without dampers or other mechanical controls. They allow the operator to adjust the frequency of electric power to the motor and thereby slow down or speed up the fan. VFDs decrease flow noise and stress on equipment at reduced speeds. They often are the ideal solution, provided you can justify the initial capital expenditure.

VFDs may be incorporated into the original fan design or may be retrofitted to an existing fan. However, retrofits may demand important adjustments to the fan or to the coupling between the motor and fan.
The question of airflow regulation comes into play when an application requires that static pressure and air volume outputs drop below their design operating points. Historically dampers were the solution in such situations. Dampers are used with a fixed-speed motor drive. Several types of inlet and outlet dampers exist, as I’ll describe. Some of these options remain viable for applications that require minimal airflow restriction. Before using dampers, however, plant managers need to carefully consider issues relating to energy consumption; stress to fans and fan equipment over time; and how frequently, to what extent, and to what degree of precision airflow must be restricted.

While calculating the energy used by the various options is relatively simple (see sidebar “Assessing annual energy costs,” http://www.chemicalprocessing.com/articles/2008/210.html), it’s, of course, crucial to weigh their effectiveness and energy efficiencies against capital costs to determine the most appropriate choice.

Inlet Dampers
Inlet damper control is widely used to increase operating efficiencies in air movement systems. Most inlet dampers pre-spin incoming air in the same angular direction as the centrifugal fan wheel rotation. This directed air movement reduces power consumption by the fan pressure and airflow, thereby cutting the energy required to operate the fan.

Multiple vanes upstream of the fan wheel inlet provide it with a controlled presentation of air that enables smooth control over a wide range of operation. Inlet dampers create a new fan performance curve for every damper position, losing efficiency as airflow rates decrease (Figure 1).

Figure 1 -- Fan performance curves: A VFD provides markedly higher energy efficiency when airflow is reduced significantly.

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.

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