Don’t Blow Your Money Away

Make the right choice when regulating fan airflow.

By Tom Kuli, Robinson Industries, Inc.

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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.

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