Condemned to Silence

Modern energy generation plants and decentralized heating systems, as well as combined heat and power (CHP) plants and the like, are increasingly integrated into people's immediate living environments. Their operation can cause disturbing noises, but these can be minimized through the use of exhaust silencers. This requires not only suitable silencer systems but also the necessary know-how for their implementation.
 

Whether oil and gas boilers, combined heat and power (CHP) plants, or biomass systems: operating and combustion noises that are transmitted through the exhaust system and chimney often lead to unwanted noise pollution. In particular, the medium- and high-frequency noises from condensing boilers and fans, or the low-frequency hum of CHP systems, can be disturbing for people in residential or working areas, as well as in the surrounding environment, when the sound levels are too high. Acoustic solutions for exhaust systems help prevent the noise from being radiated into the environment. Depending on the "noise spectrum," resonance, reflection, absorption, or combination silencers are used. Especially in installations or retrofits in existing buildings or complex industrial applications, custom solutions are often required.

Well Prepared

To ensure the correct selection of possible silencers and optimally reduce noise emissions, a situational analysis should first be carried out. This is done in several steps. First, the operating parameters of, for example, a gas-fired fan boiler should be recorded. What are the delivery pressure and exhaust temperature? How humid is the operating mode? And what requirements are there for the silencer in terms of noise protection? Depending on the type of heat generator, the disturbing operational noise typically follows characteristic frequency patterns that need to be identified. Ideally, an on-site assessment of the system should be conducted by experts. Since exhaust systems made of stainless steel are excellent sound conductors, the sound generated by the heating system is typically transmitted through the exhaust system and chimney and radiated at the chimney outlet. To avoid noise disturbances, the assessment level derived from the emission values should not exceed the limit values according to the TA-Lärm (Technical Instructions on Protection Against Noise). Different emissions standards apply depending on the location and time of day.

For example, in Germany, industrial areas are not allowed to exceed 70 dB(A) both day and night. In residential areas, the limit is 50 dB(A) during the day and 35 dB(A) at night. In health resort areas, the limit is even lower: 45 dB(A) during the day and 35 dB(A) at night. In Austria and Switzerland, different regulations may apply. The selection of the correct silencer often requires a sound measurement according to DIN EN 45 635, part 47, which is performed at a distance of one meter from the chimney outlet. Ideally, measurements of ambient noise and exhaust noise should be carried out by specially trained personnel using a sound level meter with the highest accuracy class. An additional test of low-frequency noise components is becoming increasingly important. The data obtained enables experts or engineering firms to calculate the noise levels in adjacent residential areas or determine if the existing noise standards have been exceeded. These results form the basis for the optimal selection of the silencer.

Reflection Silencers

With the appropriate exhaust silencer type, tailored to the respective frequency range, disturbing frequencies can be eliminated, and the sound level can be reduced to the desired level. For example, in combined heat and power (CHP) plants and generators, the motor primarily generates electricity, and usable heat is produced additionally. The motors often generate a low-frequency hum. The specific frequency spectrum and thus the choice of silencer depend largely on the number of cylinders and the speed of the motor. Often, however, in CHP systems or backup power systems (NEA), the noises are in the low-frequency range. In such cases, a reflection silencer is the preferred solution. Here, the sound is reflected several times by integrated intermediate walls. Narrowed pipe sections between the chambers ensure that unwanted sound is trapped in the individual hollow chambers and eventually eliminated by frequency interference. The increased exhaust resistance also has a sound-damping effect, but it leads to higher pressure losses. To use a reflection silencer, there must be sufficient available exhaust overpressure.

Absorption Silencers

In contrast, biomass boilers typically generate little noise during combustion. The noise emissions usually fall into the medium- and high-frequency range and primarily originate from integrated fans. Due to the more aggressive composition of the exhaust gases, absorption silencers with appropriate housing materials made from higher-quality stainless steels are recommended for biomass boilers.

In condensing units, fans are usually installed that often cause a "pulsating hissing" sound, which, together with the flame noise from the combustion chamber, is radiated through the chimney. These are generally medium- and high-frequency noises, which can also be reduced with an absorption silencer. These passive silencers use open-pored, sound-absorbing materials such as mineral wool to dampen sounds, especially in the medium- and high-frequency ranges. Through an internal perforated sheet, the sound waves from the exhaust stream enter the material behind it, which absorbs the sound waves and converts them into heat through friction. Depending on the installation location—boilers, motors or CHP systems, or fans and biomass boilers—and the composition of the exhaust gases, various materials are used to protect the mineral wool from fiber loss due to the exhaust flow. These include high-temperature-resistant industrial fabrics, as well as glass fiber fabrics for protection against dirt, quilted stainless steel wool, and felt.

Special Forms

Absorption silencers are available in many special designs. Modular silencers can be expanded with several absorption elements for higher damping requirements. Exit silencers can be attached to a masonry chimney with minimal modification to an existing system. Silencing cores are designed for retrofitting into chimneys and effectively reduce noise peaks. The installation is easy and can be done at any time. The silencing core is simply suspended from the top into the chimney exit. For higher damping performance, silencers with internal sound insulation elements can be used. If space is limited, especially space-saving silencers can be installed. In these cases, the absorption body runs vertically along the exhaust pipe or at an angle, which results in a much shorter or more compact silencer than usual. Special designs are also necessary for industrial applications, typically involving larger diameters. These include so-called panel silencers with rectangular housings or as pure panel sets for cylindrical installation.

Combined and Resonance Silencers

For newer fan burner types, instead of an absorption silencer, a combined or purely resonance silencer may be necessary. Resonance silencers primarily dampen low-frequency sounds. This type of silencer reduces noise similarly to a reflection silencer. Through a perforated sheet, sound waves are diverted into hollow chambers and canceled out by phase interference. Depending on the specific requirements on-site, several chambers of different lengths may be used in sequence. This allows for individual damping properties tailored to different frequency ranges. Since the chambers are located outside the exhaust pipe and the flue is not blocked, the resonance silencer causes only minimal resistance. Therefore, it is ideal for applications with low exhaust overpressure. If a broader sound reduction is needed, the resonance and absorption principles can also be combined in one silencer. For example, in the first section of the silencer, low-frequency noises are mainly dampened by resonance chambers, and in the second part, medium- and high-frequency sound waves are reduced by an absorption chamber.

Conclusion

Strict noise protection regulations often require the reduction of noise emissions from heating plants and exhaust systems using exhaust silencer systems. There are different types of silencers that can reduce sounds in the medium-, high-, and low-frequency ranges. However, the optimal use of a silencer depends on selecting the appropriate one for the noise source. This often requires in-depth expertise in acoustics, fluid dynamics, design planning, as well as exhaust and sound protection systems. Professionals combine knowledge from these areas and can help with planning and implementation through targeted situational analysis and necessary on-site measurements.