The air moving industry uses a common term to describe certain inlet and outlet conditions that adversely affect fan performance. The term used is “system effect”. Perhaps the term should be “fan installation effect”, because system effect results from the difference in how the fan was tested, compared to how it is installed. To minimize system effects, air must enter or leave a fan uniformly.
First let’s take a look at how fans are tested and cataloged. Most fans available in today’s market bear AMCA Certified Rating Seals. This means that the fan manufacturer followed the test procedures as outlined in AMCA Publication 210 and tested the fan in one of the standardized configurations approved by AMCA. One of the requirements of AMCA, is that directly under the cataloged performance for a given fan model, the fan manufacturer must make a statement as to how that product was tested. Paying attention to these statements is the first step in avoiding system effect problems.
Roof exhaust fans: Performance shown is for Installation Type A: Free inlet, Free outlet. Power rating (BHP) does not include drive losses. Performance ratings do not include the effects of appurtenances in the airstream.
Tube axial fans: Performance shown is for Installation Type B: Free inlet, ducted outlet. Power rating (BHP) does not include drive losses. Performance ratings do not include the effects of appurtenances in the airstream.
Centrifugal fans: Performance shown is for Installation Type B: Free inlet, ducted outlet. Power rating (BHP) does not include drive losses. Performance ratings do not include the effects of appurtenances in the airstream.
It is important to realize that fan manufacturers can only guarantee the fan to perform as tested. In the examples shown, not one of the fans were tested with obstructions, such as elbows, guards or dampers, directly at the fan inlet or outlet. These obstructions cause additional losses that are not included in the fan manufacturer’s tests, and in many cases, are not included in the designers’ usual system resistance calculations. Most designers are well trained in determining the resistance that occurs in the system’s ducts, filters, dampers and elbows that are located some distance from the fan, but they pay little attention to obstructions near the fan. The interaction of the air and the obstruction just prior to the fan causes the additional losses known as system effects.