Fan performance rating tests are conducted in standard configurations which, unfortunately, are not always exactly reproduced in field installations. The connection between a fan inlet and the air system can have a disruptive effect on the airflow into the fan inlet. Non-uniform airflow into the inlet is the most common cause of deficient fan performance.
An elbow or a 90-degree duct turn located at the fan inlet will not allow the air to enter uniformly and will result in turbulent and uneven airflow distribution to the fan impeller. Air has mass, and a moving airstream has momentum. Therefore, the airstream resists changing direction within an elbow (Figure 1).