Mitigating System Effect to Optimize Fan Performance and Efficiency

The only way to overcome system effect and achieve specified airflow volume is to increase fan speed. Increasing fan speed, however, results in increased energy consumption (just a 10-percent increase in fan speed will result in a 33-percent increase in energy consumption) and costs and greater stress on system components. Additionally, it may prevent the motor, electrical conduit, starter, and disconnect from achieving the necessary brake horsepower.

Figure 1 shows the impact system effect has on system performance.

In addition to hindering fan performance, system effect can increase noise and vibration and lead to premature impeller or bearing failure. The associated performance testing, engineering analysis, field service, long-term maintenance, and lost production from unplanned downtime and extended startup can be costly.

With any installation, system effect and all other system-associated losses must be considered to understand how a fan will perform relative to its laboratory tests once installed. During the fan-selection process, the combined impact of those losses should be taken into account. For optimal system performance and cost-effective operation for years to come, best practices for fan ducting and installation need to be followed.

FIGURE 1. Impact of system effect on system performance. Source: AMCA “System Effect” online educational module

Laboratory vs. Real-World Conditions

To recognize how system effect impacts air-system performance, it is important to understand the conditions under which fans are tested in a laboratory. Housed fans typically are tested with an open inlet that includes a bell mouth to eliminate entry losses and achieve uniform airflow across the inlet. The discharge includes a straight run of ductwork that produces fully developed airflow (free of swirl or turbulence) prior to the air entering the test chamber. Uniform, fully developed airflow enables a fan to move air efficiently and quietly through a duct system without causing excessive vibration. In the field, however, inlet and outlet conditions rarely mimic those of laboratory-test setups, as the installation and ducting are influenced by the existing infrastructure and space limitations.

Common Causes of System Effect

The most common causes of system effect include uneven or spinning airflow at a fan’s inlet, obstructions to airflow at the inlet or outlet, improperly configured ductwork at the inlet or outlet, and/or failure to correct for losses caused by fan accessories.

System effect can be avoided by accounting for all factors, including the shape of the transition points between the fan and existing ducts, ductwork configuration close to the fan, and accessories. For optimum air performance, airflow at the fan’s inlet needs to be uniform, symmetrical, and free of swirl. Similarly, airflow must be able to diffuse and fully develop across the fan’s outlet. Even minor improvements to airflow stability can reduce system effect and, in turn, increase fan performance and operating efficiency.

An inlet vane damper is a modulating device that affects fan performance. As a damper is closed, air begins to pre-spin into the fan; the fan wheel no longer can move as much air, and flow, pressure, and brake horsepower all decrease. Even when a damper is fully open, the vanes interfere with normal flow and reduce fan performance. If the losses are not accounted for, the fan will have to run at a speed higher than the one specified during fan selection. Fans should be tested with accessories that determine the losses and the fan speed required to overcome them.

Reducing System Effect at a Fan’s Inlet

A lack of uniform airflow entering a fan’s inlet is one of the greatest and most common causes of system effect. Often, these losses are the result of elbows and isolation dampers being installed too close to a fan’s inlet.

Depending on the application, a variety of strategies can be employed to improve airflow at a fan’s inlet. Take, for example, the 90-degree round elbow located at the inlet of a fan shown in Figure 2. Air entering the fan wheel is not uniform, loading on various parts of the fan wheel instead of at the center as designed for optimal performance. Some air is circulating back into the elbow, creating additional losses. In this case, the addition of turning vanes in the elbow will help direct air toward the center of the fan wheel.

Figure 3 shows the effect of a rectangular inlet box mounted directly to the inlet of a fan. Again, the fan wheel is not being uniformly loaded, resulting in performance loss. With an inlet box, the cavity, or dead area, below the outlet is where air will get hung up, creating additional losses. Improving the shape of the inlet box or adding straightening vanes can help to redirect the flow of air into the wheel.

Figure 4 shows air entering a fan from the side as opposed to straight through the inlet. The air is spinning in the opposite direction of the fan wheel. Consequently, the fan is having to work harder, resulting in greater energy consumption and stress on fan components.

Reducing System Effect at a Fan’s Outlet

Similar to inlet flow, outlet flow is impacted significantly by the placement and distancing of ductwork and dampers. The effective length of ductwork at a fan’s outlet is one of the most important factors in fan and system efficiency. In most cases, the profile of the air coming out of a fan is asymmetrical, causing turbulence and a lack of static-pressure regain.

For symmetrical and uniform flow to be achieved, outlet ducting must be long enough to allow airflow to diffuse and fully develop. This is called 100-percent effective duct length. As a rule of thumb, the effective length of outlet ducting should be no less than 2.5 duct diameters when duct velocity is 2,500 fpm (13 m/s) or less. For every additional 1,000 fpm (5 m/s), one duct diameter should be added.

Figure 5 shows the velocity profile of air as it exits a fan. Air is forced against the outside of the scroll, resulting in uneven flow at the outlet. An effective run of ductwork allows for a uniform velocity profile. Note that at approximately 50 percent of effective duct length the fan achieves approximately 80 percent of its pressure regain.

FIGURE 2. Non-uniform airflow into a fan inlet induced by a 90-degree, three-piece section elbow—no turning vanes. Source: AMCA Publication 201-02 (R2011), Fans and Systems

FIGURE 3. Non-uniform airflow into a fan inlet induced by a rectangular inlet duct. Source: AMCA Publication 201-02 (R2011), Fans and Systems

FIGURE 4. Example of a forced inlet vortex. Source: AMCA Publication 201-02 (R2011), Fans and Systems

In addition to effective duct length, the placement and direction of elbows is significant at a fan’s outlet. An elbow installed too close to the outlet will result in a significant loss of airflow. If the elbow turns in the opposite direction of the fan’s rotation, the loss will be even greater. When a design requires the installation of an elbow, a minimum of two to three duct lengths is recommended to allow the velocity profile of air exiting the fan to develop across the ductwork.

Conclusion

By and large, most fan-performance deficiencies are the result of improper system design. This is because fans are simple, standard mechanical devices, while systems are complex and unique, with many installation variables that can adversely impact performance.

When designing a fan system for optimal operation, remember to allow enough room for needed accessories and appropriate ducting connecting the fan to the larger system. The long-term cost savings will be worth the extra effort upfront. Additionally, because fan installations are so customized, it is important to partner with a knowledgeable and experienced vendor to ensure optimum system performance, efficiency, and longevity.

FIGURE 5. System-effect curves for outlet ducts—centrifugal fans. Source: AMCA Publication 201-02 (R2011), Fans and Systems

This process fan exhausting to atmosphere has to overcome system effect resulting from the use of a cutoff sheet in conjunction with a rectangular duct and a 90-degree-elbow turn without any run of ductwork.

After a neighbor complained about the noise generated by this backward-inclined fan, which exhausted fumes from a tank at a rate of 50,000 cu ft (3,750 lb) per minute, the owner sought to redirect the noise by reversing the flow of air so that the air came back toward the fan. It was a self-defeating proposition: The air turbulence caused by the 180-degree turn at the outlet and the speed at which the fan had to run to overcome the system effect actually increased the noise.

A fan and collector purchased at a salvage auction. The owner used a 50-gal. drum as part of the duct system between the fan and collector. In addition to a base that is unstable, there is no length of ductwork at the fan outlet to establish uniform airflow.

Pressure blowers used in a combustion-air system. The 90-degree elbows are turning air in the opposite direction it exits the fans. The system effect could have been avoided with upblast fans.

Instead of a rectangular-to-round transition from the fan inlet, a transition plate is being used in this application, resulting in the bottom-loading of air on the fan wheel.