Understanding some of the common causes for system effect losses can help you more accurately size and select fans.

If system effect situations cannot be avoided, their impact on performance should be estimated and added to the calculated system resistance prior to sizing or selecting the fan.

Beyond the routine system resistance calculations, the location of some common components and their proximity to the fan inlet or outlet can create additional immeasurable losses commonly called “system effects.” If these losses are not eliminated or minimized, the fan speed and horsepower will need to be increased to compensate for the resulting performance deficiencies.

The term “system” refers to the path through which air is pushed or pulled. Because this path can be any combination of ducts, coils or filters, a system can be as simple as exhausting air through an opening or as complex as a multi-zoned system with varying flows and densities. The effect of the system design on the performance capability of a fan is an important consideration.

In the typical process of system design, the performance requirements are calculated and then used to select the appropriate fan. In many cases, however, the effects of the relationship between the system components and the fan are not considered in the calculation or selection process. For example, the resistance of a given size elbow at a given flow can be determined easily using the equivalent-length calculation method. However, if that elbow is located at the fan inlet or outlet, further immeasurable losses will be imposed in addition to the simple loss through the elbow itself. Most importantly, these losses cannot be measured or even detected with field instruments because they are, in fact, a destruction of the fan performance characteristics.

Standardized testing and rating methods for fans are described in AMCA Standard 210, “Test Code for Air Moving Devices.” Specifying fan equipment that is tested and rated in strict accordance with AMCA Standard 210 is the best way to ensure accurate fan performance. However, system effects that alter or limit the ultimate performance remain the most frequent causes of field performance problems.

Airflow-related causes of system-induced performance deficiencies are:
  • Eccentric flow into the fan inlet.
  • Spinning flow into the fan inlet.
Eccentric Flow. Fans perform correctly when air flows straight into the inlet. Air should be drawn into the fan inlet with an evenly distributed velocity profile to allow all portions of the fan wheel to handle an equal air load.

If the air is not drawn into the fan inlet evenly, performance deficiencies will result from the combined effects of turbulence and uneven air distribution. In applications where an elbow is installed at the fan inlet, when the system attempts to change the direction of flow, the air hugs the outside of the inlet elbow entering the fan. This action causes uneven, turbulent airflow into the fan. Another common cause of non-uniform flow into the fan inlet is a poorly designed inlet box. It is important to remember that air has mass.

Spinning Flow. Unintentionally spinning air into the fan inlet can have the same effect on performance as the intentional pre-spin produced by a vortex-type inlet damper. The direction air is flowing when it enters the fan wheel is important. To produce its rated capacity, the fan works on the air by changing its direction and accelerating its velocity. If the air is spinning in the same direction as the wheel rotation, the fan capacity will be diminished. If the air is spinning in the opposite direction of the wheel rotation, the brake horsepower and noise of the fan will increase. The static pressure of the fan might also increase slightly -- but far less than indicated by the increased power consumption.

Evaluating and controlling pre-spinning flow is more difficult than eccentric flow because of the variety of system connections or components that can contribute to pre-spin. Also, spinning often occurs in combination with eccentric flow.

Pre-spinning flow can result from any number of common situations. Two elbows in close proximity to one another can force the air to make consecutive turns in perpendicular planes to form a corkscrew effect. Also, air converging tangentially into the main duct or plenum can create an obvious spinning effect.

Pre-spinning flow also can be induced by such common air cleaning devices as a venturi scrubber or a cyclone. In these cases, it is often the function of the air-cleaning device to create a spinning effect.

This article was provided by the New York Blower Co., headquartered in Willowbrook, Ill., a manufacturer of fans and blowers. For more information, call (800) 208-7918 or (630) 794-5700, visit www.nyb.com, or e-mail nyb@nyb.com.