Sound power measurements for hot exhaust stacks

Rawlinson, David and Joseph, Philip (2006) Sound power measurements for hot exhaust stacks. In Proceedings of Inter-Noise 2006. International Institute of Noise Control Engineering..

Abstract

The measurement of the noise emitted by hot gases, issuing from exhaust stacks, has been of particular interest to the power industry for many years. A method adopted by the ISVR uses two separate probes, positioned axially in the duct, with condenser microphones fitted to the probes outside the exhaust duct away from the hot flow gases. The paper describes the measurement method as refined and tested in the laboratory. Measurements are then reported for an exhaust stack, with typical exhaust gas temperatures of 250ºCand velocities of greater than 50 m/s. The laboratory tests used two adjoining reverberation rooms with a test duct positioned between the two rooms. The tests were undertaken at ambient conditions.The in-duct measurements of the sound power were compared to measurements taken in the receiving room using the method described by ISO 3744. The two measurements were found to be in close agreement. Errors incurred by the techniques have been demonstrated in this low-turbulence experiment to be within ± 3dB. It was shown that separating the sensors axially (with respect to the duct) gave less rejection of the unwanted signal caused by the turbulent flow compared with that with the sensors orientated radially. The effects of temperature on the probe’s frequency response are predicted and incorporated into the analysis of instack data. When the technique was used in the field there was good repeatability of the in-duct exhaust stack measurements when measured on different days, using slightly different design refinements of the probes and different transducers. The sound pressure level was found to be greatest near the duct wall although the variation in level with radial position was small. Variations inplant operating conditions gave changes in in-duct sound power levels that were expected.

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