Implementation of active noise control in a multi-modal spray dryer exhaust stack

Tonal noise emitted from large-diameter spray dryer exhaust stacks used in the dairy industry can give rise to complaints from nearby communities. In many cases, the tone at the fan blade passing frequency is characterized by a frequency above the first mode cut on frequency of the exhaust stack and both its amplitude and the frequency are time varying. The variation in amplitude is a result of turbulence and temperature variations in the duct which cause angular variations in the nodal plane of modes with diametrical nodes. This in turn results in large fluctuations in sound pressure with time at any specified location in the duct, thus presenting a significant challenge for an ANC system with fixed control source and error sensor locations. In many food processing industries, the use of sound absorptive materials in silencers is not acceptable and, particularly when the fan speed is variable, it is difficult to achieve an acceptable passive solution at a reasonable cost. Here, the design and implementation of an active noise control system for tonal noise propagating above the cut-on frequency of the first higher order mode in large size cylindrical industrial exhaust stack is discussed, where the frequency and amplitude vary significantly and relatively rapidly with time. Physical system design principles and control algorithm optimization for a practical active noise control system are presented. Finally, real time control results which were achieved by a prototype installation on a large-diameter, in-service exhaust stack are given. Significant noise reductions were achieved in the community.