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7. EXHAUST SILENCING
Excessive noise is objectionable in most locations.
Since a large part of the generating set noise is
produced in the engine's pulsating exhaust, this
noise can be reduced to an acceptable level by
using an exhaust silencer. The required degree of
silencing depends on the location and may be
regulated by law. For example, the noise of an
engine is objectionable in a hospital area but
generally is not as objectionable in an isolated
pumping station.
7.1 Exhaust Silencer Selection
The silencer reduces noise in the exhaust system by
dissipating energy in chambers and baffle tubes
and by eliminating wave reflection that causes
resonance. The silencer is selected according to
the degree of attenuation required by the site
conditions and regulations. The size of silencer and
exhaust piping should hold exhaust back pressure
within limits recommended by the engine
manufacturer.
Silencers are rated according to their degree of
silencing by such terms as "low degree" or
"industrial", "moderate" or "residential" and "high
degree" or "critical".
• Low-Degree or Industrial Silencing - Suitable
for industrial areas where background noise
level is relatively high or for remote areas
where partly muffled noise is permissible.
• Moderate-Degree or Residential Silencing -
Reduces exhaust noise to an acceptable level
in localities where moderately effective
silencing is required - such as semi-
residential areas where a moderate
background noise is always present.
• High-Degree or Critical Silencing - Provides
maximum silencing for residential, hospital,
school, hotel, store, apartment building and
other areas where background noise level is
low and generating set noise must be kept to a
minimum.
Silencers normally are available in two
configurations - (a) end inlet, end outlet, or (b) side
inlet, end outlet. Having the choice of these two
configurations provides flexibility of installation,
such as horizontal or vertical, above engine, on
outside wall, etc. The side-inlet type permits 90°
change of direction without using an elbow. Both
silencer configurations should contain drain fittings
in locations that assure draining the silencer in
whatever attitude it is installed.
The silencer may be located close to the engine,
with exhaust piping leading from the silencer to the
outside; or it may be located outdoors on the wall
or roof. Locating the silencer close to the engine
affords best overall noise attenuation because of
minimum piping to the silencer. Servicing and
draining of the silencer is likely to be more
convenient with the silencer indoors.
However, mounting the silencer outside has the
advantage that the silencer need not be insulated
(though it should be surrounded by a protective
screen). The job of insulating piping within the
room is simpler when the silencer is outside, and the
insulation then can aid noise attenuation.
Since silencers are large and heavy, consider their
dimensions and weight when you are planning the
exhaust system. The silencer must be adequately
supported so its weight is not applied to the
engine's exhaust manifold or turbocharger. The
silencer must fit into the space available without
requiring extra bends in the exhaust piping, which
would cause high exhaust back pressure. A side-
inlet silencer may be installed horizontally above
the engine without requiring a great amount of
headroom.
Silencers or exhaust piping within reach of
personnel should be protected by guards or
insulation. Indoors, it is preferable to insulate the
silencer and piping because the insulation not only
protects personnel, but it reduces heat radiation to
the room and further reduces exhaust system noise.
Silencers mounted horizontally should be set at a
slight angle away from the engine outlet with a
drain fitting at the lowest point to allow the disposal
of any accumulated moisture.