Ethanol plants that regulate fan airflow with mechanical controls, such as dampers, may be blowing their money away. Fixed-speed motors with dampers may be inefficient energy consumers, especially when compared with variable frequency drives (VFDs).
 

 VFDs efficiently regulate airflow when air restrictions are greater than 20 percent or wide ranges of airflow are required. While mechanical methods generally require lower upfront capital costs, VFDs may provide better long-term energy cost effectiveness. A few simple calculations and a fan performance curve allow one to determine which option is most efficient, depending on the application.
 
 VFDs reduce airflow without dampers or other mechanical controls, allowing the operator to adjust the frequency of electric power to the motor, thereby controlling fan speed. They also reduce flow noise and stress on equipment at reduced speeds. VFDs may be incorporated into the original fan design or retrofitted to an existing fan. However, retrofits may require adjustments to the fan or the coupling between the fan and motor.
 
 Airflow must be regulated when an application requires static pressure and air volume outputs drop below their design operating points. Historically, dampers, which use a fixed-speed motor drive, were used in this situation.
 
 However, before using dampers, ethanol plant managers need to carefully consider issues related to energy consumption, and stress to fans and fan equipment. They must also determine how frequently, to what extent and to what degree of precision airflow must be restricted.
 
 A trained fan professional can closely examine a system to assess the initial cost, effectiveness and energy efficiencies. The goal is to ensure a successful application that can provide reasonable energy savings compared to the upfront cost.
 
 Inlet/Outlet 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. The directed air movement reduces power consumption by the fan pressure and airflow, thereby reducing the energy required to operate the fan.
 
 <center><img src="/images/upload/20071101024204.jpg">
 Figure 1</center>

 
 Multiple vanes upstream of the fan wheel inlet provide a controlled presentation of air to the fan wheel, providing 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).
 
 If an air movement system is infrequently used, inlet dampers may appear best in an initial cost/potential energy savings comparison between airflow regulation devices. They are especially effective when restricting airflow by less than 20 percent.
 
 Some caution is needed when dampers severely restrict airflow. Flow instability or rotating stall (i.e., air starvation resulting in high-amplitude pressure pulses) can result when airflow is restricted by as much as 70 percent.
 
 As their name suggests, outlet dampers control flow by restricting the airstream's path at the outlet. They are rarely used on large industrial fans due to their high inefficiencies and potential to damage system components.
 
 Partially closing, or throttling, parallel or opposed damper blades provides a desired reduction of flow. However, pressure increases on the upstream side of the dampers, generating increased 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 high backpressure that may cause system components to prematurely wear, overheat or crack, causing increased operating and maintenance costs. Dampers are also 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 volumes and pressures. They are 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.
 

 Pressure, volume and horsepower decrease as the fan speed decreases with VFDs. The curves for fan performance and brake horsepower (bhp) shift toward the origin, essentially in harmony with the fan law curve. This shift works well for most fixed-resistance systems and offers significant horsepower savings at reduced speeds.
 
 Fan laws may be used to determine the operating points of a centrifugal fan. The third fan law states:
 
 BHP2 = (RPM2 / RPM1)3 (BHP1)

 
 Therefore, cutting a motor's speed in half will reduce power consumption to one-eighth. For example, a drop from 1,000 revolutions per minute (rpm) at 1,000 bhp to 500 rpm results in power reduction to 125 bhp. Much less energy is needed to run the motor at the lower speed.
 
 VFDs' soft-start capabilities offer further opportunities for reducing energy demands. Motors typically experience higher currents during start-up than normal operation. VFDs allow the motor to be started with a lower current, reducing wear on motor windings and the controller, and lowering the severity of voltage sags that may affect sensitive equipment.
 
 Final Selection Criteria
 Despite the typical benefits of reduced electricity demand and maintenance costs, VFDs are not suitable for all applications. Further, there are critical fan design considerations when fitting a VFD to a new or existing fan.
 
 The various components that make up air movement systems have natural resonant frequencies. If excited during operation, these frequencies may cause vibratory stress and fatigue, resulting in noise and possibly the cracking or destruction of impellers, shafts, bearings and foundations. Fan professionals perform resonant impact testing to determine natural resonant frequencies. They also examine a broad range of speeds when VFDs are considered for an installation. A key frequency, torsional resonant frequency is calculated using data collected from components in the air-movement system, including the fan, coupling and motor assemblies.
 
 Fans are typically designed to have normal operating speeds above or below any natural resonant frequencies, including torsional frequencies. If the normal operating speed is above these frequencies, VFDs run the risk of reaching them as the rotating speed is decreased. The first fan speed at which a shaft lateral resonant frequency is reached is called the first critical speed.
 
 Fan professionals make various system alterations to shift resonant frequencies outside of the fan's operating speed range and thereby avoid reaching the first critical speed. The most common choice for altering torsional resonant frequencies is to change couplings. Modification to the fan rotor can also alter resonant frequencies in the system, enabling the preferred operating speed range. In addition to mechanical alterations, fan professionals have the option of programming the VFD to "lock out" speeds that are near the first critical speed or that may excite natural frequencies. A VFD professional should be involved in this process.
 
 In systems utilizing duct outlets fitted with closed outlet dampers that open during operation, sufficient static pressure is required for the dampers to function properly. If pressure levels are too low when the fan's rotational speed decreases, the dampers will not open.
 

 Only one point exists at which the power efficiencies for outlet dampers, inlet dampers and VFDs are essentially the same—maximum speed. With maximum flow, power consumption is relatively the same for all three systems. As flow demand decreases, VFDs typically offer the best efficiency. However, inlet dampers may be a viable alternative if system flow demands consistently remain in the 80 percent to 100 percent range. For most systems, outlet dampers are not a viable means of flow control. A trained fan professional will analyze this data to help ethanol producing facility engineers determine the best flow regulation device for their operation.
 
 Jay Henderson is vice president of marketing and Tom Kuli is a chief engineer for Zelienople, Pa.-based Robinson Industries, Inc. Reach them at (724) 452-6121.
 
 The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of Ethanol Producer Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).

The Right Choice in Regulating Airflow

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The Right Choice in Regulating Airflow