System effect is the difference between published performance and installed performance. Fans are performance tested in a controlled environment where installation conditions are ideal. Results are then published in catalogs and computer selection programs. When products are installed in a real-world environment, very often installation conditions are not ideal. While system should be avoided whenever possible, sometimes it is unavoidable due to time or space constraints.
Why is understanding system effect important? Because fan performance may be affected. A decrease in fan performance is the most common system effect issue. To overcome the performance deficiency, fan speed must be increased, which means more energy will be required to achieve the desired performance.
To ensure you do not have system effect, airflow entering or leaving the fan should always be uniform, meaning the air should flow smoothly and be evenly distributed. When a change in direction occurs, the airflow is no longer smooth. Like a bend in a river, a higher amount of the fluid goes to the outer edges and takes time to return to the smooth pattern. Airflow through ductwork behaves in the same manner.
The ideal installation of a fan has straight, smooth ductwork directing the air into the inlet. The most common system effect occurs when the inlet conditions of the fan are not ideal or straight. Elbows or dampers at or close to the inlet cause airflow to become turbulent, which may result in system effect. A soluion would be to add some distance between the fan and the damper and/or elbow with a curb extension.
Outlet conditons are as important as inlet conditons. It is critical for airflow leaving the fan to be smooth with a straight discharge duct. When transitions or elbows are installed at the outlet, or no outlet ductwork is installed, fan performance decreases.
For most fan applications at least two and one-half duct diameters of straight ductwork is required at the inlet and/or outlet to achieve cataloged performance. For applications where the velocity is over 2,500 feet per minute, add one duct diameter for each addtional 1,000 feet per minute of velocity. Duct diameter is roughly the same as impeller diameter. For example, a 12-inch impeller diameter fan will require 30-inches of straight duct on both the inlet and outlet of the fans.