How System Design Can Affect the Performance of a Centrifugal Fan?

Centrifugal fans and blowers have become a preferred choice for most of the industrial applications because of their relatively simpler design, durability and higher efficiency. Since a typical centrifugal fan has the capability to increase the speed of air stream coming through it, these fans are extensively used in industrial processes where large movement of air or gas is necessary. To get the desired high pressure and volume outputs, industries use different types of centrifugal fans with varied blade configurations in series and parallel arrangements. Yet, these fans may not achieve the desired airflow movement which is required for industrial process because of their reduced performance.

A centrifugal fan may have performance deficiencies depending on the system design as there could be losses due to multiple arrangements of system components and their proximity to the fan inlet and outlet. Fan performance is generally tested to standards by considering ideal inlet and discharge airflow conditions. However, there could be many disturbances to the fan inlet and discharge conditions caused by the system effect, making the fan to operate inefficiently. When a centrifugal fan is subjected to system induced losses, it will not perform as per the standard tested conditions. There will be a deficiency in performance resulting in poor fan operation.

Some reasons for poor performance of a centrifugal fan could be

Non-uniform inlet flow
Airflow to the fan inlet should be smooth and straight in an uniform manner without any disturbance. If the air is not drawn in an evenly distributed form, there will uneven load on the fan wheels and the effect would be irregular, turbulent flow of air-stream through the system. Improper inlet box design or installation of ducts or elbows directly at the inlet may cause non-uniform airflow into the fan inlet, resulting in reduced fan performance.

Spinning air at the fan inlet
Another factor that impacts the performance of a fan system is the direction of the airflow entering into the fan wheel. Air entering into the fan can have a spinning effect due to improper positioning of system components at the fan inlet. If the entering air is spinning in the same direction of the fan wheel rotation, then the fan capacity will be diminished. In case if the airflow is spinning in opposite direction to the fan wheel rotation, there will be increase in the horsepower requirement and system noise. In both the cases, the fan will have unstable operation conditions and reduced performance.

Improper duct work at the outlet
Duct work at the fan outlet has a great effect on fan performance. To achieve better fan efficiency, the discharge duct should be straight and have proper length & uniform cross section. The duct must be near the fan outlet for smooth and evenly distributed airflow. It should be of the same size as the outlet opening with no turns, bends, elbows & branches connected adjacent to the fan outlet. Otherwise, the air discharged at the outlet will not have uniform velocity and there will be performance drop at the discharge conditions. In case if duct fitting is equipped with turns and bends, then they should be in the same direction as the wheel rotation. If turns are made in the opposite direction of the wheel rotation, the pressure loss will be higher resulting in poor fan performance.

Obstructions at inlet or outlet
To avoid poor airflow through centrifugal fan system, there should be minimal obstructions at the fan inlet and outlet. The most common obstructions that can cause system effect are, placing a fan inlet or discharge too close to walls or bulkhead, damper locations at the inlet and outlet, location of weatherhoods, poorly designed guards and other defective installations. Anything that obstructs the airflow at the inlet or outlet will have a system effect and result in poor fan performance.

To achieve good performance, fan installations with system effects should be avoided. To reduce the effects of system losses and minimize performance problems, it is important to follow proper design in fan installation. Fan systems that are not designed for minimal system effect, will require higher speed and increased horsepower capability to compensate for the performance deficiencies. 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.