Gas Turbine Handbook : Principles and Practices

158 Gas Turbine Handbook: Principles and Practices
Silencer Design and Materials
Silencer design must take into consideration several attributes
that constitute the final design of a silencer. They include but are
not limited to:
• Required noise reduction in conjunction with materials selection,
reliability, and cost
• Allowable size of the silencing system—that is, real estate needed
versus availability
• Allowable pressure drop or losses through the silencing system—
cost of operation
• Aerodynamics—turbulence and flow distribution—longevity of
materials
• Structural and operational design requirements—chemistry of
air and exhaust gases
• Mechanical design of the silencers—thermal cycling, vibration,
corrosion, seismic
The factors that determine the acoustical performance of a passive
duct silencer are based on the following basic design elements:
• Effective (absorptive) length of the silencer
• Thickness of the silencer
• Spacing or air gap between the silencers
• Filler material— acoustical and material properties
• Any covering material used for fiber/fill retention
• Cover sheet perforation— open area ratio, hole diameter, and
thickness
• Velocity of air or gas between the silencers
Note that nowhere is listed the height or number of silencer
panels. The number and height of the silencer panels only affect the
velocity of the gas or air through the silencers. Low velocity means
low pressure drop, high velocity means high pressure drop and possibly
high self flow noise. Self-noise becomes a limiting factor in very
low noise applications. Self-noise is created by the flow of the gas
around the silencer panels thus there is nothing that can reduce this
secondary noise unless there is a secondary low velocity attenuation
system behind the parallel silencers. Typically, the velocity through