Noise Control for High Plume Exhaust Fans: A Case Study

Abstract

High plume exhaust fans are a notable source of noise for institutions that use fume hoods. These fans are intended toaddress local air quality effects without the need for tall stacks. This is done through the use of high air velocity to propel airemissions upwards. The exhaust is also mixed with dilution air directly at the exhaust point. It is common practice for twoor more units to operate together from a common mixing plenum. The fans exhaust a constant flowrate and compensate forthe change in fume hood airflows by drawing air into the mixing plenum through a bypass damper. The noise emissionlocations are therefore the exhaust point, the bypass intake, fan casing, and the casing of the mixing plenum. Each of theseemission locations becomes significant when large sound level reductions are needed. Large reduction amounts can benecessitated by strict sound level criteria, or close proximity to the points of reception. In the case illustrated here, the pointsof reception were close to the fans. A sound level reduction of approximately 25 dB was needed from the entire system. Itwas therefore necessary to mitigate all of the emission points to achieve an effective overall reduction. Mitigation solutionsproviding this level of reduction are not available from the manufacturer. A supplier focussed on custom noise mitigationwas retained to design, manufacture and install a package of mitigation measures. The final product achieved the necessaryreductions and resulted in a much more pleasant acoustic environment in adjacent areas.