Circular Ventilation Duct Silencer

Product Model	Cylindrical Resistive / Impedance Composite Muffler
Category	PP Machined Products
Reference Price	Price on request

Hot Customizable for Working Conditions 11 Technical Parameters

Product Overview

The round duct muffler is a cylindrical noise-reducing device installed at the outlet of circular corrosion-resistant ducts, fan outlets, and before exhaust stacks. The outer casing is made of flame-retardant PPs sheet material, rolled and welded. The inner wall is lined with a ring-shaped sound-absorbing layer or an internal sound-absorbing tube is installed inside the cylinder. When airflow passes through the internal passage of the cylinder, the sound energy is absorbed by the sound-absorbing material, thereby reducing the fan noise transmitted along the circular duct and radiated to the exhaust stack outlet.

Fan noise contains different frequency components: The resistive muffling section utilizes porous sound-absorbing materials to effectively reduce mid-to-high frequency noise; the reactive muffling section uses expansion chambers and resonance chambers formed by abrupt changes in the duct cross-section, causing specific frequency sound waves to reflect and interfere within the chambers, leading to attenuation, which is effective against low-to-mid frequency noise and pulsating noise. Round mufflers can be designed as resistive single cylinders or as impedance composite types by combining resistive and reactive sections to achieve noise reduction over a wider frequency range, making them a common solution for noise control in round exhaust systems.

Xicheng Environmental can design and manufacture round duct mufflers based on fan airflow, noise frequency spectrum, circular duct, and exhaust stack diameter, determining the cylinder diameter, thickness of the sound-absorbing layer, perforation rate of the internal tube, and muffling length. The round flanges are matched with ducts, and can also be combined with fan outlet reducers and flexible connections to form complete fan exhaust systems, suitable for noise reduction at fan outlets and exhaust stacks in corrosion-resistant ventilation systems.

Working Principle

The principle of the resistive section is sound energy dissipation: the central airflow passage consists of a perforated internal tube, with a ring-shaped space filled with sound-absorbing material outside the tube. Sound waves pass through the perforations into the sound-absorbing layer, causing the air inside the material's pores to vibrate. Due to friction and viscosity, sound energy is converted into thermal energy and dissipated. Mid-to-high frequency sound waves have shorter wavelengths and are easily absorbed into the pores, resulting in significant attenuation. The airflow passage remains unobstructed, and the sound-absorbing material is fixed by a perforated protective tube and protective fabric to prevent it from being carried away by the airflow.

The principle of the reactive section is sound reflection and interference: when the duct is connected to an expansion chamber or resonance chamber with a larger cross-sectional area than the fan duct, sound waves reflect at the abrupt change in cross-section, with some sound energy returning toward the sound source. When the chamber dimensions match specific wavelength sound waves, resonance occurs, canceling out that frequency of noise, leading to attenuation. Reactive sections do not require sound-absorbing materials and are effective against low-frequency and pulsating noise, and they are not affected by oil, moisture, or humidity. By connecting the resistive and reactive sections in series to form an impedance composite muffler, it can cover high, mid, and low frequencies. The PPs outer casing is corrosion-resistant and flame-retardant, ensuring long-term effective noise reduction in corrosive gas environments.

Structural Composition

The round muffler consists of a cylindrical outer casing, an internal sound-absorbing tube (or sound-absorbing plates), sound-absorbing material, a protective structure, reactive chambers, and two end flanges. The resistive section's outer casing is a PPs rolled cylindrical tube, with a perforated PPs internal tube at the center. The annular space between the internal tube and the outer casing is filled with sound-absorbing cotton, and the inner wall of the perforated tube can be lined with a protective fabric to prevent fiber leakage and liquid droplet entry.

In the impedance composite type, an expansion chamber is connected in series at the front or rear of the resistive cylinder. The expansion chamber is a sealed cylindrical tube with an increased diameter. Internally, it is equipped with insert tubes and baffle plates as per acoustic design to form reactive muffling units. Circular flanges are welded at both ends to connect with the ducts and exhaust stacks. Reinforcing rings are installed on the outer wall of the cylinder according to negative pressure and length, and a liquid discharge port is provided at the bottom if necessary to prevent liquid accumulation. Large-diameter mufflers can have an acoustic cone added at the center or be modified into multi-channel designs to increase the effective sound-absorbing perimeter.

Specification Model Table

The following table shows the types and applicable frequency bands of round mufflers, selected based on the fan noise frequency spectrum.

Form	Noise Reduction Principle	Main Effective Frequency Band
Resistive Cylinder	Porous Sound Absorption	Mid-to-High Frequency
Reactive Expansion Chamber	Reflection and Interference	Low-to-Mid Frequency Pulsating
Impedance Composite	Series of Both	Wideband

Product Features

The round duct muffler has good pressure-bearing capacity and combinable frequency bands, making it a common device for noise reduction in circular corrosion-resistant fans and exhaust stacks. Its main features are as follows.

The cylindrical outer casing has uniform stress distribution, suitable for fan outlet pressure-bearing
The resistive section effectively reduces mid-to-high frequency fan noise
The reactive section targets low-to-mid frequency and pulsating noise, and is not affected by moisture
The impedance composite type provides wideband noise reduction, suitable for complex noise frequency spectra
PPs outer casing and perforated internal tube are acid-base resistant and self-extinguishing
Round flanges are matched with circular ducts and exhaust stacks, offering low resistance
Cylinder diameter and length are designed based on airflow and noise, and can be supplied as complete sets

Outer casing material	PPs Flammability Retardant Panels
Section Shape	Round
Quietening Form	Resistance / Impedance Composite
Sound Absorbing Structure	Perforated inner pipe with annular acoustic layer
Resistance structure	Expansion Chamber / Resonant Chamber Optional
Connection Method	Round Flange
Effective Bandwidth	Broadband Impedance Composite
Corrosion resistance performance	Acid and Alkali Salt Resistant
Flame Retardancy	Self-extinguishing
Specifications Design	Based on airflow noise design
Installation Location	Exhaust stack in front of the fan outlet

Application Industries

Anti-corrosion centrifugal fan outlet duct muffling and noise reduction
Exhaust muffler at the front of exhaust stack in waste gas treatment system
Impedance composite muffler for factory boundary noise control
Round exhaust silencer for laboratory rooftop exhaust fan
Noise reduction for outdoor anti-corrosion fan in chemical plating workshop
Low-frequency muffling for prominent fan pulsation noise
Matching muffling section for round exhaust shaft and exhaust stack

Typical Process Locations

Round mufflers are typically installed between the fan outlet and exhaust stack. The fan outlet connects to the muffler via flexible joints and reducer pipes, then discharges into the exhaust stack, attenuating fan noise before it radiates from the stack outlet. When the fan inlet draws air from outdoors or a workshop, a muffler can also be installed at the inlet to reduce noise transmission. For factory boundary noise compliance, mufflers should be placed as close as possible to the exhaust stack outlet or directly as the base muffling section of the stack to minimize regenerated noise from downstream piping.

During installation, flexible joints must be used between the muffler and fan to isolate vibration, preventing fan vibration from transmitting to the thin-walled silencer and causing solid-borne sound transmission and shell booming. Round flanges should be aligned and tightened with gaskets sealed. The weight of the muffler and exhaust stack is supported by independent brackets to avoid pressure on the fan. The internal airflow velocity must be controlled within the recommended range, as high-speed airflow can generate regenerated noise, rendering muffling ineffective. After commissioning, regularly inspect the liquid drain, perforated inner pipe, and acoustic absorption layers. Remove fog from mist-containing gases first to prevent the absorption material from caking due to moisture.

Mainly selected according to pipe section and installation location. Round ducts, fan outlets, and exhaust stacks are matched with cylindrical mufflers, as round casings offer good pressure-bearing capacity and can directly interface with round flanges; rectangular ducts, ceiling installations, and indoor rectangular exhaust systems are matched with square plate mufflers. Both use the same noise reduction principle, with cylindrical mufflers typically employing annular acoustic layers or impedance composite structures, while square mufflers commonly use parallel acoustic panels. Do not rigidly connect a square muffler to a round system; when conversion is needed, add a square-to-round adapter and calculate the resistance.

When the fan noise is primarily in the medium-to-high frequency range, a conventional cylindrical reactive muffler can suffice; when the noise spectrum features prominent low-frequency components, noticeable fan pulsation noise, or when simply extending a reactive muffler does not meet low-frequency requirements, an impedance Composite (impedance Composite ) muffler should be selected, utilizing expansion chambers and resonance chambers to target low-to-medium frequency noise attenuation. Centrifugal fans, positive displacement fans, and high-pressure systems often exhibit significant low-frequency noise. The most prudent approach is to first measure the fan noise octave band spectrum and determine the reactive and resistive section configurations based on the required noise attenuation for each frequency band, avoiding Blindly extending (blindly extending) the reactive section.

Common reasons include: insufficient sound absorption capacity or mismatch between the sound absorption band and the noise spectrum; excessive flow velocity inside the duct generating airflow Recycled noise; vibration of the fan transmitted through the pipeline and supports as solid-borne sound, which is ineffective for mufflers; thin walls of the pipeline and exhaust stack behind the muffler being excited by airflow and transmitting sound; unsealed sound leakage bypasses. System analysis is required: controlling flow velocity, adding vibration isolation soft connections and hangers, soundproofing the exhaust stack, positioning the muffler near the radiation outlet, and redesigning the sound absorption capacity based on actual measured frequency spectra if necessary. Relying solely on adding one muffler to solve all noise issues is not effective.

Porous acoustic absorption materials lose their performance significantly when they absorb moisture and water fills the pores with water films and accumulated water. This is the most common cause of failure in corrosion-resistant exhaust mufflers. When the fan inlet after a spray tower carries water mist, it should first pass through a demister before entering the muffler. The muffler uses a protective fabric to seal the acoustic absorption layer and is equipped with a liquid drain outlet at the bottom. When necessary, moisture-resistant acoustic absorption materials should be selected, or the reactive section should be placed on the mist side. During maintenance, it is found that when the acoustic cotton panels are clogged, sagging, or the protective fabric is damaged, the noise reduction performance can only be restored after opening the maintenance structure to replace the acoustic absorption materials.

The resistance of a muffler is related to the flow velocity in the channel, the perforation rate of the inner tube, the spacing between the plates, and the length. The flow area of the muffler should be checked to ensure the velocity inside the cylinder is controlled within the economical range; if necessary, increase the cylinder diameter and enhance the effective flow area. The perforation rate of the inner tube should not be too low, and the ends of the sound-absorbing plates should be streamlined. No debris or dust should accumulate inside the muffler to block it. If an installed muffler has excessive resistance, first check for blockage by water accumulation or deformation of the inner tube. If the design and selection are too small, replace with a model of larger flow area or convert to a multi-channel structure.

Please provide the fan model, speed, airflow, pressure, and installation location. It is recommended to provide the fan noise octave band spectrum or sound power level, target noise standard (workshop or factory boundary limits), exhaust stack and duct diameter, flange standards, available installation length and space, exhaust gas composition temperature (including mist and dust), and flame retardancy requirements for materials. A spectrum and limit values are required to calculate the noise reduction amount for each frequency band and determine the lengths of the resistive and reactive sections. Providing only diameter and length can only be done based on experience, with no guarantee of noise reduction effectiveness. If complete ducting is required, specify the configuration of flexible connectors, reducers, and rain hoods.

Previous Product Integral Grating Plate Next Product Actuator