Alternative Silencers

Cleanable round reactive silencers

© Fraunhofer IBP
© Fraunhofer IBP

The cleanable round reactive silencer (RRSD) consists of ring-shaped chambers around the stack with an opening to the stack that is covered with a perforated sheet metal. Figure 1 shows photographs of installed silencers on top of exhaust stacks and Figure 2 during production without outer cover. Each chamber of the RRSD can be tuned to a desired frequency range.

The cleaning of the silencer is accomplished by attaching pipes to each chamber supplying cleaning fluid which can leave the chambers through the perforate sheet metal rings. By combination of different chambers, as shown in the picture below for a silencer with six chambers, a broadband attenuation is made possible for the low frequency end.

The operation principle of the silencer is a combination of lambda/4- and Helmholtz resonators acting as acoustic filters. The prediction of insertion loss for these silencers relies on well established transmission line theory which uses the input impedance of the chambers. A layout program has been developed to allow licensees an easy layout and design of such silencers. The program also accounts for the influence of temperature and flow velocity on the attenuation.

Features of the RRSD:

  • excellent low frequency performance
  • non-fibrous and cleanable
  • negligible pressure loss

Angular Stack Silencer

© Fraunhofer IBP
© Fraunhofer IBP

The integration of silencers into an exhaust stack offers the advantage to use a considerable length, which is usually necessary to avoid air pollution, as an attenuator of the noise. This allows to do without or minimize installations blocking the flow. A second advantage is the reduction of required space, compared to conventional solutions with a compact silencer between noise source and stack.

The novel silencer, installed in more than 30 industrial plants since 1996, consists of panel resonators which are fitted into the stack e.g. forming a hexagonal or octagonal shape in cross-section as shown in Figure 1 . The sheets are fixed on Y- or M-shaped metal stripes in such a manner that they remain even and without any tension. The space between the metal sheets and the stack is filled with a porous foam or mineral wool.

The attenuation of the silencer is based on the eigenfrequencies of the plate in a highly damped baffle resonator, which may be tuned to very low frequencies. A broadband attenuation is achievable by a variation of the plate and absorber thicknesses on opposite or adjacent locations.

A prototype of 5 m length was produced in order to verify the performance of the new silencer. The first installation of an Angular Stack Silencer, having a total length of 20 m including the 5 m prototype stack, was carried out in a mineral wool production line. Figures 2 and 3 show a photograph of the prototype and the complete silencer during installation.

Measurements where carried out to verify the performance of these silencers, and the result for the prototype is shown in the upper line of Table 1 . The lower line in Table 1 shows the attenuation measured for the complete silencer converted to the reference length of 5 m and verifies the acoustic effectiveness.

The silencer achieved in this manner requires only a small increase in stack diameter in order to keep the flow velocity constant. For mid and high frequencies additional conventional porous silencers are combined with the Angular Stack Silencer allowing the acoustic treatment of the whole frequency range.

If necessary, it is possible to expand the acoustical performance to the mid and high frequency range, using thin plates (≥ 0,025 mm) in front of a small porous absorbent layer(≥ 5 mm). Thus, it is possible to shift the acoustical performance of the panel resonator up to the 4 kHz range. For large stack diameters and/or high transmission loss requirements, the stack cross-section area has to be divided into smaller cross-section areas e.g. by thin panel resonator splitters. A crosswise divided cross-section area layout of an Angular Stack Silencer is shown in Figure 4.

Of course, the panel resonator splitters may be used as conventional splitters inside ducts or housings. To predict the insertion loss of the Angular Stack Silencer a calculation program has been developed (Figure 5). The prediction of the performance of such a silencer relies on calculations of the input impedance of the vibrating plates and on measurements of the absorption coefficient of various plate and foam configurations carried out at an impedance test site of the IBP which has a cross-section of 1.2 x 1.6 m2, allowing the measurement in real dimensions.

Frequency Hz 50 63 80 100 125 160 200 250
Attenuation of test stack dB 5 6 7 8 9 6 6 3
Attenuation of installed stack dB 6 11 8 8 9 7 7 8

Table1: Measured attenuation of a 5 m long Angular Stack Silencer

Since all chimneys, including the Angular Stack Silencer, are acoustically tested before delivery, a pool of data is obtained for comparison of measurement and calculation without flow. Inspections of the panel resonator behaviour at temperature were made at a special temperature test stand (max 500 °C) at the IBP. The flow noise and the dependency of insertion loss and flow velocity has been tested at a standard test site. In this manner, a high layout reliability has been achieved.

Features of the Angular Stack Sileners excellent low frequency performance:  

  • closed and cleanable surface
  • low pressure loss
  • for stack-diameters greater 800 mm