Common Sources Of Noise In Centrifugal Fans

Excessive noise of a fan system can be a serious concern in industries. Higher noise levels caused by the fan systems can disrupt the productivity and cause nuisance to the working environment. The noise generated could be due to the turbulence effect at the fan blades or rotation of the fan shaft or motor. Typically, the noise produced by a centrifugal fan system is dependent on various factors including the fan size, airflow rate and pressure. Fan systems that work at optimum efficiency produce lower noise levels than the fans with inefficient operation. Using over sized fans for industrial applications or operating fans at higher speed produce vibrations, causing higher noise levels. The sources of noise in a centrifugal fan could be due to airborne or structure borne vibration levels. To identify where the issue is being produced and suggest solutions for specific noise issues, it is essential to understand the sources causing the problems.

Fan installations
Any fan installation feature that tends to reduce the efficiency of the fan should be avoided as it results in inefficient operation, high vibration levels and system noise. Obstructions to the airflow at the inlet can create turbulence to the air entering into the fan, resulting in noise of discrete form. Hence, installing bends and dampers close to the fan or placing fan inlet or outlet too close to the wall or bulkhead should be avoided. Also, care should be taken to install the fan away from noise sensitive areas. If it is unavoidable, consider using noise barriers such as silencers to achieve sound attenuation or lower the level of noise. If noise generated by the airflow is too high, consider using a longer silencer with larger open area to get the maximum sound attenuation.

Fan speed
When fans are made to run at a speed greater than its design capacity, the load on the fan system will be higher. This effect of higher speed may cause intense vibration resulting in system imbalance and increased noise.

Fan casings
In a centrifugal fan, housing or casing acts as a protection for moving parts like impeller, hub etc. When fan casings have improper design or multiple sectionals, they produce reverberating noise which could be due to the impeller. Hence the design of the casing has to be optimized by using heavy gauge material or by cladding with insulating material. This will reduce noise levels because the noise generated by the impeller is not transmitted to the outer area.

Duct work
While considering the duct work for sound attenuation, having proper duct design and thickness are very essential. Since abrupt changes in duct design disturb the airflow causing severe turbulence, it is important to avoid sharp bends in close proximity to the fan outlet or inlet. To avoid transmitting the noise and vibration to other parts of the fan, the duct work should possibly be straight and have flexible connections.

Bearings
The noise caused by the bearings is relatively low, which will be more like a dry rumble or a squeal. The possible causes could be improper greasing or loose or improper fitting. In normal fan systems, small bearings rarely produce sound, whereas large bearings used in bigger fan systems generate considerable amount of noise levels.

Shaft sealing
The main purpose of shaft sealing in centrifugal fans is to reduce the leakage of the airflow within the fan system. However, the sealing also prevents the transmission of impeller noise passing to other components of fan system including the casing. When impeller is not properly mounted on the shaft or impeller is fitted loose on the motor or drive shaft it may result in excessive noise. In such cases, using shaft sealing with proper design and material will have an effective control on noise levels.

Airflow control devices
The devices such as dampers and variable inlet guide vanes are typically used to control the airflow at the fan inlet or outlet. When there is a pressure drop in air across the system due to improper installation of the control devices, there will be turbulence in the air at the inlet or outlet resulting in fan noise. This will add to the overall noise of the fan system. Installation of airflow control devices at proper angle would attenuate the sound levels to a considerable extent.

V-Belts
Just like bearings, belts contribute very little to the overall system noise. However, when belts are loose or extremely tight, misaligned or worn out they get misplaced from their position and produce noise due to slipping or vibration. Adjusting the belts to the correct tension or realigning pulleys for proper placing of the belts is recommended to resolve the issue.