Compressor Intake System Noise Reduction
The intake system will include some or all of
• an intake filter
• an intake silencer
• piping and ducting
• control valves
Most compressors will incorporate an intake filter, which will by itself give some noise attentuation. This may be insufficient, in which case an intake silencer may be needed.
An intake silencer may be reactive or absorbent or both according to the frequency of
the noise. For frequencies above 1:25 Hz, absorptive silencers are required and for
frequencies below 125 Hz, reactive silencers are appropriate. If the 125 Hz band is
straddled, then a combination silencer can be used.
A reactive silencer produces its effect by interference and reflection of the sound waves (an automobile silencer is reactive). A simple silencer would include an expansion chamber with a tail pipe, which may be sufficient by itself if one frequency dominates the spectrum. For multiple frequency suppression the number of chambers has to increase; design details of the method of analysis can be found in specialist publications. The lower the frequency that is to be suppressed, the larger is the vessel required, so if low frequency noise is suspected at an early stage in the design, provision should be made for a reactive silencer and space can be set aside for its installation.
An absorptive silencer produces its effect by suppressing internal reflections with a porous ascoustic material lining the inside of the silencer cavity. When using this type of silencer, precautions must be taken to ensure that the material does not become contaminated by dust, water or oil which will not only reduce its effectiveness but may also be a place where fires start. Under severe vibration the material may disintegrate, discharging debris into the intake of the compressor.
Another type of reactive silencer is the venturi tube, which is particularly effective but expensive. The main advantage of a venturi is that the pressure drop through it is lower than with other kinds of restrictions, so it is particularly suitable for intake silencers where intake pressure losses could seriously affect the compressor efficiency. The flow velocity can be higher than with other silencers. An example of a venturi silencer combined with a filter is shown in Figure 1.