Industrial fans are rotating devices that circulate air and provide ventilation in industrial plants. Whether small or large, the design of industrial fans is such that they produce noise usually above the recommended decibel level in a manufacturing setting. One recent technology that has been helping to mitigate this noise level is the industrial fan silencer.

Industrial fan silencers provide noise attenuation when placed at the inlet or outlet of an industrial fan. But just like other industrial devices, the desirable performance of fan silencers primarily depends on whether they are correctly sized for a particular application.

This article gets back to the basics of industrial fan silencers. It will cover essential design criteria and calculations for selecting an industrial fan silencer. But before getting right into it, engineers and technicians must first understand how these devices work.

How industrial fan silencers work

To appreciate the role of industrial fan silencers today, consider a typical industrial fan used as a piece of ventilating equipment in a food processing plant. Suppose a technician is positioned at 5 ft from the fan's outlet. The engineer will typically hear a noise at levels of up to 95 dB, above the recommended noise level (85 dB) at that distance.

A silencer typically features fibrous absorption material. When placed at an industrial fan's inlet (or outlet), it absorbs the acoustic energy into the fibrous material and converts it into heat energy. Silencers provide desirable attenuation in the mid- and high-frequency ranges between 500 and 8,000 Hz.

Sizing an industrial fan silencer: An overview

To correctly size a fan silencer, engineers must ensure that the silencer flow area is sufficient to accommodate the maximum airflow from the fan without imposing excessive restriction (primarily pressure drop). Therefore, engineers typically start by specifying the maximum allowable pressure drop before proceeding with the following steps.

1. Calculate maximum velocity to achieve allowable pressure drop.

Where:

V = air or gas velocity (ft/min)

ΔP = maximum allowable pressure drop (inches of water)

c = silencer pressure drop coefficient

P = operating pressure, psig

T = air temperature (°F)

2. Determine the airflow area required

Where:

A = flow area required (ft²)

V = actual airflow rate in cubic feet per minute (actual CFM or ACFM)

However, if standard airflow rate in cubic feet per minute (SCFM) is given, the actual airflow rate (ACFM) can be calculated using:

3. Select a silencer size with a flow area greater than the one calculated above.

Industrial fan silencers come in different sizes, depending on the manufacturer and application requirements. Therefore, engineers should contact the manufacturer or check manufacturers’ catalogs to choose a silencer that satisfies the area requirement.

4. Determine actual air velocity based on the area of silencer selected

Where:

A is the flow area of the silencer size selected, ft²

5. Determine the actual pressure drop in inches of water based on the silencer size selected.

Note: Absorptive silencers are usually sized for 4,000 ft/min to 8,000 ft/min velocity to prevent excessive self-generated noise and aerodynamic noise. In addition, “air” is assumed as the flowing gas in this article. For other gases, density and other correlations may be required.

Performance parameters for industrial fan silencers

In addition to the sizing calculations, engineers also need to consider some performance criteria when selecting industrial fan silencers for a particular application. The parameters include:

• Required acoustic insertion loss

The insertion loss is the decrease in the sound pressure levels that can be expected when the silencer is placed at the inlet or outlet of the fan. The graph shows a typical noise attenuation curve with the insertion loss at each frequency. It can be used as a guideline to evaluate the noise levels of the industrial fan after the silencer has been installed.

Typical noise attenuation curve. Source: Temitayo Oketola

• Allowable pressure loss

Engineers should select fan silencers with a pressure drop that does not exceed the maximum allowable pressure drop.

• Generated noise

Generated noise describes the noise produced when air passes through the silencer at a given velocity and direction. A lower noise level value indicates better silencer performance.

Silencer types

The common silencer types include:

• Rectangular silencers

Rectangular silencers are designed to join rectangular ductwork at a relatively low cost. They offer high sound attenuation and low-pressure losses. In addition, these silencers can be stacked into large banks, eliminating expensive transitions common in other types of silencers.

• Elbow silencers

Elbow silencers are desirable for applications with limited space and where there are moderate pressure drops and very high insertion loss that rectangular silencers cannot be used. These silencers feature elbow baffle passages that minimize pressure loss. In addition, they are configured to suit most duct sizes without the use of transitions, like the rectangular silencers.

• Axial fan silencers

They are designed to be coupled to an axial fan and are engineered to provide sound attenuation at the source and improve aerodynamic performance at the inlet and outlet of the fan.

• Circular silencers

Circular silencers are designed for maximum attenuation with minimum pressure drop. They are desirable when round ductwork is utilized and eliminate the need for square-to-round transitions that cause undesirable pressure loss.

[Learn more about industrial silencers with Engineering360]

Conclusion

While this article presents helpful information about sizing and selecting industrial fan silencers, there are several other factors to be considered when sizing silencers. Besides, different manufacturers have different guidelines and catalogs to help engineers select silencers for different applications. Therefore, engineers are advised to reach out to silencer manufacturers to discuss their application requirements.