Sound

Sound is a form of mechanical vibration that propagates in a medium such as air, liquids or solid bodies. In technology, sound is primarily described by changes in pressure and density that move in waves from a source. The propagation and perception of sound plays an important role, particularly in building technology and in the operation of hot water and heating boiler systems, as noise and vibrations may affect both comfort and operational safety.

Types of sound and sound sources

Noise-emitting elements

Noise emissions are sounds that may occur at varying volumes and across a wide frequency range. In a boiler house, they are usually generated by the combustion process, flow noises from pipes and fittings, and all rotating and moving components.

• Gas supply/oil supply
• Flow noises in the gas regulation section
• Oil pump/conveying device
• Combustion air fans/burners
• Motor sound characteristics
• Intake sound characteristics
• Combustion noises
• Flow noise in the lines
• Further components
• Pumps
• Drive motors for valves
• Pipe system
• Flow sound characteristics/combustion sound characteristics in flue gas lines
• Flow noises in throttle valves
• Discharge pipes for the pressure-relief valves

All flowing media causes flow noise characteristics. The key principle to note here is that the faster a medium flows, the higher the noise emission it will generate. In addition, the sound generated is more easily transmitted to the outside by various thin-walled components and amplified by resonating bodies. These may be, for example, the flue gas heat exchanger, the flue gas chamber, the reversing chamber, the air duct or the flue gas line. The volume of water and the comparatively large wall thicknesses on the boiler shell ensure that the boiler shell itself does not emit the combustion sound characteristic into the boiler house.

Technical dimension for quantifying sound

With regard to the acoustic reference value, a distinction must be made between the sound power level and the sound pressure level. The sound power level signifies the energy that is radiated from a source of noise. The sound power level is therefore the characteristic acoustic parameter of a sound source. The sound power cannot be measured directly and is independent of distance.

The sound pressure represents the effect of a sound emission. The sound pressure level serves to directly determine the impact of the noise at the affected area (immission area). The sound pressure level is the sound pressure measured at a point in the room and is consequently perceived as its impact. The sound pressure level can be measured directly and is dependent on distance, since the sound pressure diminishes with increasing distance from the source of noise.

Bosch Industriekessel only provide (total) sound pressure level values for individual components in the technical data. These are expressed in the logarithmic unit decibel (dB). The letter A in the unit dB(A) means that it is an A-weighted sound pressure level. The A-weighting expresses that the frequency ranges are measured differently. The weighting takes into account the characteristics of the human ear. For the total sound pressure level, the sound pressure levels across the different frequencies are added together and an average value is derived from this. The sound pressure level is always specified as a free-field level with the corresponding measurement distance from the sound source.

Sound insulation measures

Sound insulation measures are an essential part of the planning and installation of hot water and heating boilers in order to reduce noise to an acceptable level and ensure compliance with statutory noise protection requirements. They not only serve to minimise noise emissions in the surrounding area, but also contribute to increasing comfort in adjacent rooms.

The main sources of noise in heating boilers are burners, flue gas routing, pumps and vibrations generated during operation.

Sound requirements

Sound requirements are usually defined in a noise assessment report or as specifications issued by the local authority. These are part of the approval process and must be complied with. This is not usually just a matter of avoiding noise in the workplace, but also of the sound emissions that the boiler house as a whole emits into the environment, including beyond the building boundaries, through supply and extract air apertures, and even through the chimney. Determining an overall noise level for the boiler house is a very complex matter and can only be carried out in detail by a noise expert or noise consultant.

Measures for noise reduction within the boiler house

In principle, there are five different approaches to reduce the noise emissions in boiler systems:

• Silencer hood over the burner/fan
• Special noise insulation for the boiler components, the flue gas routing and various throttle valves (e.g. gas pressure regulators)
• Soundproofed design of supply and extract air apertures
• Selection of gas valves and a supply air duct design that allow for good flow
• Reinforcement or stiffening of thin-walled components
• Decoupling of foundation and sound emitter, e.g. by means of silencing pads
• Use of sound-reducing materials in boiler house construction

Measures for noise reduction outside of the boiler house

A special feature is the noise emissions at the chimney outlet, as a significant portion of the noise generated in the combustion chamber is transmitted via the flue system to the chimney. This noise is radiated as airborne noise via the surface of the flue gas system and escapes at the chimney. This noise emission can be effectively reduced through the use of flue gas silencers. This is achieved by flue gas silencers reducing sound propagation in pipes without impeding the transport of the medium. Various selection criteria must be taken into account when choosing the right flue gas silencer:

• Required noise reduction
• Reliable pressure loss
• Available construction volume
• Necessary stability
• Requirements for inspection and cleaning