Factors that govern selection of an exhaust silencer are:
1. Available space.
3. Noise reduction required.
4. Allowable back pressure.
7. Proximity to other components or personnel.
As a general rule, an increase in silencer size will improve the silencing level. An increase in noise suppression for a given silencer volume will normally increase exhaust restriction and thus increase back pressure.
A round silencer is usually preferable, as it has a greater tendency to contain the sound, rather than allowing it to escape through the flat surface. A double wrap tubing and double wall surface aids in containing the sound as well.
Generally speaking, silencers providing two levels of silencing are available: industrial silencers which provide approximately 15 to 20 decibels of suppression, and residential/critical silencers which provide approximately 20 to 30 decibels of suppression.
The exhaust inlet and outlet are commonly on the ends of the silencer, however the inlet and outlet can be on the sides, which often simplifies the exhaust piping installation.
Provision should be made to prevent rainwater from entering the exhaust outlet opening. Counterbalanced flapper type rain caps have proved successful in many varied applications. Because of the high pressure drop caused by conical shaped covers and other common ventilating covers, they are seldom practical.
From an aesthetic point of view, care should be given to the location of the exhaust outlet point, considering the tendency over a period of time for exhaust gas carbon deposits to accumulate on any nearby structures.
Because water vapour is formed in the combustion process of diesel fuel, it is necessary to incorporate into the exhaust piping design a condensate trap and drain valve. The condensate trap should be located as close as is practical to the engine.
In multi-engine installations, special consideration must be given when manifolding or joining the exhaust runs from the engines into one common exhaust run. A problem may arise if one or more engines are operating and the exhaust gas finds its way back into a non-operating engine. The use of check valve devices within the exhaust pipe runs should be avoided owing to their strong tendency to freeze and become inoperative.
Provision should be made for relative movement between the exhaust piping and the engine so that no damaging stresses will be imposed on the exhaust system components, because of engine mount flexibility or thermal growth.
Because the exhaust gas temperature leaving the engine is generally in the 500° C range, the piping may be insulated, as steam pipes are, to minimise the heat radiated to the room.
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