Ctm Ambiente, “the silence” of the district heating boilers

In recent times, most of the new energy production systems is constituted by the cogeneration installations, in particular for district heating. They are made by the cogenerator true, that in general is an internal combustion engine, or more rarely a gas turbine, and one or more reserve integration boilers. The problem of noise is clearly present in planning the installation of the cogeneration plants because, for obvious reasons, they are always quite close to the houses that make up the same district heating utilities. However, while all precautions are taken to reduce the noise generated by the CHP, it often does not care enough, or we deal superficially, to bring down the noise coming from the boilers. The latter have a high power of from about 6 MW to over 15 MW. The most noisy outside elements are the burner, with its combustion air fan and the exhaust duct. For the burner and the combustion air fan is quite simple to perform a proper attenuation, it is sufficient to provide the appropriate acoustic baffles in the air intake of the fans and in the ventilation grilles of the building. Another story is the abatement of noise coming out of the exhaust stack.
Here we list a few:
- The noise is characterized by low frequencies
- The fumes are generally high temperature
- The pressure losses must be very low
- The system layout often does not allow adequate space for the insertion of conventional silencers

Lately it is quite frequently the case in which the cause of the overrun the audible limit to the most sensitive receptors is not so much due to the CHP, as the integration and reserve boilers. And in particular to low-frequency noise that escapes from the chimney of the fumes. In figure 1 it is shown the measured spectrum in third octave inside the flue gas duct of a 14 MW boiler. As can be seen the values are very high and most of the acoustic energy is concentrated at frequencies 40 Hz and 50 Hz. In addition to the receptors often are present the characteristics of "pure tone" which is judged to be particularly annoying, and involves the penalty, in case of frequencies below 200 Hz and in the night hours, of 6 dB (A) with respect to noise measured. It is easy to understand how important it is to break down the low-frequency noise emitted from the duct of the boiler flue gas to prevent the system from being stopped for the exceeding the noise limits. Unfortunately, when that happens you usually adopt "buffer solutions" often impromptu, which seldom reach a satisfactory result if not at the price of important changes to the flues, substantial pressure loss and high costs.

The CTM Ambiente has developed and successfully tested a type of silencer suitable for ducts of boiler fumes that solves such problems as it concentrates its effectiveness at low frequencies, the pressure losses are negligible and the changes to the layout of 'plant are not significant. The silencer uses the known principles of acoustic resonance, but unlike conventional silencers, such as those used for internal combustion engines, does not change the flow of fumes and therefore does not cause turbulence and pressure losses. Moreover, the transverse dimensions of the silencer is slightly higher than that of the flue gas duct itself and may be positioned within the same air shaft or the same metallic support structure of the ducts without substantial changes: in fact the reactive silencer replaces a portion of the smoke flue having roughly the same size and weight.

Best practice: the case of the cogeneration plant in Rozzano (MI).
In the cogeneration plant in Rozzano they were installed in 2010 two gas boilers one of 11 MW and another of 14 MW in addition to an existing system which was composed of two co-generators and two smaller boilers. When the new boilers were put on operation, the neighborhood, located in a radius of 100 m, has expressed deep unease because of the noise emitted that it increased by more than 12 dB (A) background noise. Some interventions acoustic reclamation were performed on the return air and an absorptive silencer on the fumes was added, after which the excess over the background noise is fell to 6 dB (A): the noise, due to the fumes of the two new boilers, was concentrated at 40 and 50 Hz frequencies at which the absorption silencers are poorly effective. CTM Ambiente has therefore proposed to adopt for each of the boilers a resonance silencer, or reactive, which exploits the well-known principle of the Helmholtz resonator (Figure 2). In this case, the resonant volume is formed by a gap between the diameter of the smoke duct and the outer diameter of the silencer. The resonant effect is expressed through a series of openings that put in communication the interspace with the duct of smoke: because the fumes do not pass through these openings the load losses through the silencer are negligible. CTM Ambiente has developed a calculation program for determination of the acoustic attenuations, already tested on various applications, mainly on combustion engines, that has been adapted to the needs of the soundproofing of the boilers. By the use of this program it was determined the basic parameters for the sound attenuation required. For each of the boilers were then constructed 3 overlapping silencing modules, one tuned to 40 Hz, another at 50 Hz and the last at 80 Hz. In this way a wide sound spectrum is covered and centered on the fundamental disturbino frequencies. The three modules have formed a single silencer height 5 m which replaced a section of the original pipe. The diameter of the silencer was only 200 mm bigger than that of the original flue gas duct so as it could be placed in the same support lattice structure: 1.400mm diameter for the boiler 14 MW and 1,300 mm for the 11 MW. In the picture you can see the insert operation of one of the silencers in the same structure supporting the existing pipelines that did not need to make any changes.

The attenuation achieved for the two boilers, were in line with theoretical calculations: in Figure 3 shows the results of the measures at the mouth of the two chimneys (red and green graphics) before (solid line) and after (dotted line) the acoustic intervention. As expected, the spectrum of attenuation, particularly high at 40 and 50 Hz also extends to higher frequencies, up to 250 Hz and above. Looking ahead is therefore useless to use the absorption silencers that are provided by default in many applications. The noise level measured with the boilers in operation in the vicinity of housing shows no change compared to background noise, which made it possible to resolve the dispute between the property of the CHP and the surrounding population. The successfully experimented in the solution of Rozzano cogeneration plant may be adopted in prevention: its low impact with respect to the boiler operation parameters, which do not permit significant pressure losses and turbulence at the outlet in the conduits, and its limited weight and overall width, which allows you to not change the layout of central and exploit the vertical section of the pipe with the same structure already provided, can prevent the onset of problems related to excessive noise of boiler flue gas in district heating.