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42 | Sound pressure level from noise in the boiler system
6
Sound pressure level from noise in
the boiler system
6.1
Sound emissions from the boiler system
The noise in the installation room caused by a boiler
system and the noise transmitted to the surrounding
area are subject to regional regulations which must be
taken into account when designing a boiler system.
The overall sound emissions from a boiler system are
influenced by a range of sources. The various noises
include:
• Machine noise (e.g. burner, fan, pumps, drive motors
for valves)
• Flow and combustion noises caused by hot flue gas
created during combustion, which is routed by the
boiler through the flue system to the chimney. From
an acoustic point of view, the heat source itself is not
a source of sound, but acts as a resonating body for
noise that comes from the combustion reactions
within the combustion chamber.
There can be a range of other sound sources (structure-
borne noise due to rotational movement of machines,
flow noises in valves, etc.) which must also be taken into
account.
6.2
Noise in the installation room
Individual sound pressure levels can be specified for
machine noises that are the main cause of sound impact
in the installation room. The individual sound pressure
level of a machine can only be specified for "free field"
conditions at a distance of 1 m (without any influence
from other sound emitters). When calculating the overall
sound pressure level in the installation room, take the
reciprocal influence of the various sound sources and
local conditions (e.g. sound absorption characteristics
of the installation room wall) into account.
Machine noise can be reduced by encasing the
machines, e.g. with a burner silencer hood or a silencer
housing for the fan.
6 720 807 794 (2013/04)
6.3
Noise at the chimney outlet
A significant proportion of the noise development in the
combustion chamber is transmitted along the flue
system to the chimney. This sound is emitted via the
surface of the flue system as airborne noise and escapes
at the chimney. The noise from a boiler system
predominantly contains low frequency sound.
These sound emissions can be effectively reduced with
a flue gas silencer. To design a flue gas silencer (to
maintain the prescribed sound immissions levels), the
frequency spectrum of the sound at the chimney outlet
from the boiler system must be known.
The graph in Fig. 25 shows the average sound pressure
level of a boiler system, measured at the chimney outlet
at a distance of 1 m and an angle of 45°, with no flue gas
silencer in the flue system. As the combustion system
(e.g. due to the burner construction or the flow profile
that occurs in the combustion chamber) and flue system
(e.g. due to the numbers of bends, length and diameter
of the flue) have a considerable influence on the values
that occur, only averages can be given here for the sound
pressure level. The sound levels calculated in the flue
pipe, directly downstream of the boiler, are up to 15 %
higher than the sound levels at the chimney head.
Giving the levels directly in the flue pipe and directly
downstream of the boiler is not helpful as the influences
mentioned above as well as sound reflection and
resonance (e.g. standing waves) mean a correct
calculation is not possible, or is very difficult to achieve.
Furthermore, a flue gas silencer is designed for the
boiler system using the sound levels that occur at the
chimney head.
Due to the complexity of the subject of sound, we
recommend involving an acoustician or sound expert to
design the flue gas silencer.
If possible, the sound levels that actually occur in the
boiler system should be calculated first. These values
can be used to design a flue gas silencer which can be
retrofitted to the boiler system. The pressure loss of the
silencer (approx. 1 mbar to 3 mbar) should initially be
taken into account when sizing the burner.
UNIMAT
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