Power plants that regulate fan airflow with mechanical controls may be blowing their money away.
By Jay Henderson, Robinson Industries Inc.
Many power plants regulate fan airflow with mechanical controls, such as dampers. In some applications using fixed-speed motors in combination with dampers may not be the most energy efficient option. Variable frequency drives (VFDs) often are the more efficient way to regulate airflow, especially when air restrictions must be above 20 percent or when wide ranges of airflow are required.
The critical question is when should mechanical airflow regulation methods be used and when should VFDs be used? While mechanical methods are generally less costly in upfront capital costs, VFDs may be cheaper in long-term energy costs, depending on the application. Equations and fan performance curves can help determine which option is the most efficient for the application.
Decisions about airflow regulation are necessary when static pressure and air volume outputs must drop below their design operating points. Historically, dampers coupled with a fixed-speed motor drive were used in such situations. An array of outlet and inlet damper types exists, allowing for several airflow regulation options. Some of these options remain viable for applications requiring minimal airflow restriction. Before dampers are used, however, careful consideration should be given to issues relating to energy consumption; stress to fans and fan equipment over time; and how frequently, to what extent, and to what degree precision airflow must be restricted.
VFDs reduce airflow without dampers or other mechanical controls. They allow the operator to reduce the frequency of electric power to the motor and thereby slow down or speed up the fan. VFDs reduce flow noise and stress on equipment at reduced speeds. They are often the ideal airflow regulation solution, provided the initial capital cost can be justified.
VFDs may be incorporated into the original fan design or may be retrofitted to an existing fan. However, if retrofitted, important adjustments may be required to the fan or to the coupling between the motor and fan; in either case, a fan professional should be consulted.
A trained fan professional can closely examine a system to assess the flow control devices’ initial costs, as well as the devices’ effectiveness and energy efficiencies. The goal is to ensure a successful application that provides reasonable energy savings compared to upfront cost. (See sidebar.)
Outlet Dampers
As the 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 can provide a desired flow reduction. However, pressure increases on the dampers’ upstream side generate increased backpressure on the system. This resistance causes the fan’s operating point to shift negatively on its performance curve (Figure 1). In other words, outlet dampers reduce efficiency.
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Excessive damper use may damage system components. Highly throttled outlet dampers can create severe buffeting and high backpressure that may cause system components to wear prematurely, overheat or even crack, increasing 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.
Inlet Dampers
Inlet damper control is widely used to increase air movement systems’ operating efficiencies as compared to outlet damper flow reduction. Most inlet dampers pre-spin incoming air in the same angular direction as the centrifugal fan wheel rotation. This directed air movement reduces load on the fan and lowers fan pressure and airflow. Therefore, the energy required to operate the fan is reduced.
Multiple vanes upstream of the fan wheel inlet provide a controlled presentation of air to the fan wheel, allowing 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).
The two main types of inlet dampers are louvered inlet dampers and radial inlet dampers. Louvered inlet dampers typically have parallel blades and work well in dirty airstreams. They also are available with opposed blades, but this configuration is not recommended because it does not pre-spin the air. Radial inlet dampers, on the other hand, can pre-spin the air and are typically more efficient than louvered inlet dampers.
Additional types of inlet dampers yield efficiencies similar to radial inlet dampers. Two of the most common are vortex dampers, which require inlet boxes and are available in cantilevered blade and center hub designs, and variable inlet vanes (VIV), which require cone-shaped inlet pieces and are used only in clean air stream applications.
If an air movement system is used infrequently, inlet dampers may come out on top 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 necessary when dampers severely restrict airflow. When airflow is restricted by as much as 70 percent, flow instability or rotating stall (air starvation resulting in high-amplitude pressure pulses) may occur. For example, a steel company that operates coke oven battery scrubber fans with inlet dampers closed 90 percent recently suffered from severe vibration and fan casing cracking. Fan professionals documented the vibration measurements, and pressure pulsation and frequency confirmed a rotating stall condition. After installing a VFD, the company could leave the damper fully open and regulate airflow by adjusting the fan speed. The fan then operated smoothly at all process flow rates and the company realized energy savings estimated at more than $250,000 annually.
Variable Frequency Drives
VFDs generally offer the smoothest flow control over the widest range of volume and pressure. With proven energy saving capabilities, 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. They also reduce common problems associated with outlet and inlet dampers, such as excessive vibration, noise and equipment wear and tear.
As the fan speed decreases with VFDs, the pressure, volume and horsepower all decrease. The curves for both fan performance and brake horsepower shift toward the origin essentially in harmony with the fan law curve (Figure 1). This shift works well for most fixed-resistance systems and offers significant horsepower savings at reduced speeds.
Fan laws, also called affinity laws, may be used to determine the operating points of a centrifugal fan. The third fan law states:
BHP2 = (RPM2/RPM1)3 (BHP1)
Where:
BHP = Brake horsepower
RPM = Revolutions per minute
In other words, cutting a motor’s speed in half will reduce power consumption to one-eighth. For example, a drop from 1,000 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 can further reduce energy demands. Motors typically experience higher currents during start-up than during normal operation, but VFDs allow the motor to be started with a lower current. By eliminating the higher start-up power surge, wear on motor windings and the controller is reduced and the severity of voltage sags that may affect sensitive equipment is lowered.
VFDs also reduce airflow noise at lower speeds and air volumes. Throttling airflow with dampers causes increased noise levels that may negatively affect worker comfort.
Final Selection Criteria
Despite the typical benefits of lower operating and maintenance costs, VFDs are not suitable for all applications. When fitting a VFD to a new or existing fan, some critical fan design features must be considered: (1) natural resonant frequencies over the full operating speed range, including torsional resonant frequencies, (2) couplings, (3) bearings and (4) system static pressure.
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 certain components, particularly impellers, shafts, bearings and foundations. Fan professionals can perform resonant impact testing to determine natural resonant frequencies and they can examine a broad range of speeds when VFDs are being considered. One key natural resonant frequency a fan professional will note is the torsional resonant frequency. This 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 hitting 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 can make various system alterations to shift resonant frequencies outside of the fan’s operating speed range and thereby avoid hitting the first critical speed. The most common way to alter torsional resonant frequencies is to change couplings. Elastomeric block-type couplings or other couplings with high damping characteristics are used frequently. The addition of thicker impeller blades can also alter the system’s resonant frequencies, allowing the fan to operate in the preferred speed range. In addition to mechanical alterations, fans can be programmed not to operate at the VFD “lock out” speeds that are near the first critical speed or other speeds that may excite natural frequencies. A VFD professional should be involved in the programming.
Because sufficient operating speeds are necessary for some bearings, including sleeve bearings, to operate properly, bearings should be carefully selected to match the air movement system. Antifriction bearings typically perform well at any speed.
In systems where duct outlets fitted with closed outlet dampers that open during operation are installed, 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. In these situations, fan professionals will examine the minimum operating speed needed to fulfill pressure requirements and determine if a VFD is the best fit.
Only at maximum speed is power consumption for outlet dampers, inlet dampers and VFDs basically the same. As flow demand decreases, VFDs typically offer the best efficiency. However, inlet dampers may be a viable alternative if system flow demands remain consistently in the 80 to 100 percent range. For most systems, outlet dampers are not a viable means of flow control. A trained fan professional can analyze system operation data to help facility engineers determine the best flow regulation device for their operation.
Author: Jay Henderson is vice president of marketing for Robinson Industries Inc., Zelienople, Pa.
Acknowledgements: Improving Fan System Performance - A Sourcebook for Industry, U.S. Department of Energy and Air Movement and Control Association International Inc., Washington D.C., April 2003.
Figuring Annual Operating Costs
This formula is used to compare the relative energy costs associated with inlet and outlet dampers and VFDs.
Annual Operating Costs: (BHP) (kW/BHP) (days/year) (hours/day) ($/kWh) = $/year
Figures 2 and 3 provide a case study. Company A is adjusting its air movement system to operate at 20 inches static pressure (SP) at a volume of 105,000 cubic feet per minute (CFM).
Outlet Dampers. In Figure 2, Company A partially throttled its outlet dampers to achieve the desired static pressure and volume. The result is 1,000 BHP. The motor speed is fixed at 1,180 RPM. The annual operating cost, therefore, is:
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(1,000 BHP) (0.746 kW/BHP) (350 days/year) (24 hours/day) ($0.05 /kWh) = $313,320
Inlet Dampers. Using partially throttled radial inlet dampers to reach the same static pressure and volume, Company A realized a power rating drop to 700 BHP as shown in Figure 2. This system uses the same fixed-speed 1,180 RPM motor. The annual operating cost is:
(700 BHP) (0.746 kW/BHP) (350 days/year) (24 hours/day) ($0.05 /kWh) = $219,324
Variable Frequency Drives. By using a VFD, Company A can achieve the same operating pressure and volume by reducing the fan speed to 840 RPM. Figure 3 shows that this reduction in speed drops the required horsepower to 450 BHP, which generates an annual operating cost of:
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(450 BHP) (0.746 kW/BHP) (350 days/year) (24 hours/day) ($0.05 /kWh) = $140,994
The annual operating cost results shows that in this case, the facility will save 55 percent in energy costs by choosing a VFD over outlet dampers and nearly 36 percent in energy costs by choosing a VFD over inlet dampers. In addition, Company A may also realize added savings from the reduced equipment wear and tear that a VFD provides compared to dampers.
VFD Benefits in Power Plant Environments
The following illustrates the performance benefits of VFD control as detailed on an actual application involving a gas recirculation fan on a coal-fired generating station boiler.
Test block design performance:
473,000 actual cubic feet per minute (ACFM) and 20.4 inches static pressure (SP) at 665 F (requires 2,070 BHP)
Minimum turndown performance:
237,000 ACFM and 8.5 inches SP, achieved by:
throttled outlet damper (requires 1,580 BHP),
throttled inlet dampers (requires 1,400 BHP), or
VFD control (requires 418 BHP).
VFD benefits:
Direct annual energy savings = $94,919*
Lower noise
Reduced fan rotor stresses
*Assumptions: Fan operates at turndown condition 25 percent of the time and energy cost is $0.05 per kWh