Discharge silencers are essential to good system performance on all rotary blower systems. The belief that the discharge creates less noise than the inlet is erroneous because the discharge pulsations and noise are normally contained in a closed system. The rotary positive blower does not compress the air as it moves from the blower inlet to the blower discharge. Compression takes place when line pressure backs up into the open port — thus
compressed air is then pushed out into the line. This action takes place in a very short period of time and produces steep wave forms that can be destructive to piping and other equipment unless properly treated.
Blower displacement and speed are the major parameters in designing discharge silencers that perform their assigned function and will live throughout the useful life of the blower.
Normally for good silencing the discharge silencer should have an internal volume to blower displacement ratio of 18 to 1. Blower speed dictates the model used. When the blower speed is below the transition speed the multi-chamber type is recommended to properly treat the noise emitted from the blower discharge.
When the blower speed is above the transition speed, the intensity of the high frequencies is increased, requiring a modified design incorporating acoustic materials to absorb these frequencies. Normally acceptable discharge silencer pressure drop is in the range of 3 to 12 inches of water.