Reducing noise from screw compressors

The problem

Screw compressors providing an air supply for the aeration tanks in the blower house of a sewage treatment works were exposing maintenance engineers to A-weighted noise levels of about 100 dB.

Although acoustic enclosures over each of the three blowers were effectively containing casing and drive noise, the air intake was being piped in from a filter outside the enclosure. Therefore noise generated inside the blower itself was passing into the inlet pipework and then escaping into the blower house.

The noise had a strong tonal content appearing with a peak at the lower end of the frequency range, in this case in the 63 Hz octave band.

The solution

As conventional absorptive silencers were inefficient at these frequencies and disproportionately large, the solution was to fit a reactive silencer into each air intake pipe. The strong noise components in the 63 and 125 Hz octave band, indicating strong harmonic content, meant that a twin-chamber design was required.

The chambers were arranged in series with the first designed to resonate, giving maximum attenuation at the fundamental frequency. The second was designed to reduce energy at the principal harmonic frequencies.

The cost

£900 per silencer. (1995)

The result

There was an overall reduction in the noise level of 20 dB; a 37 dB reduction in the 63 Hz frequency band; and a 30 dB reduction in the 125 Hz frequency band.

Sound pressure level at 1 m from the compressor intake:

Source

Photographs courtesy of Thames Water Utilities Ltd. Equipment designed and supplied by Ian Sharland Limited.