Ottawa Health Sciences Co Generation Facility /
Trans-Alta Energy Corporation
situation:
Aercoustics was retained by Trans-Alta to mitigate excessive low frequency noise from high speed operation of two wet surface air condensing (WetSAC) fans at the Ottawa Health Sciences Co Generation Facility (OHSC). These fans are axial and 26 feet in diameter with flow rates of 1,000,000 cubic feet per minute. The 15-foot exhaust stack is constructed of a stiff, lightweight quarter-inch fiberglass and shroud-shaped for aerodynamics.
Complaints of low frequency noise from the the OHSC generally came from the neighbouring community between 7PM and 11PM under receptor downwind and temperature inversion conditions. The indoor low frequency noise level was the same as outdoors. The noise was a distant rumble or vibration that caused rattling in floors, walls and windows.
assessment:
Aercoustics carried out an extensive research and cost benefit analysis of several noise control strategies and selected active noise cancellation (ANC) which promised to be the most practical and cost effective.
An active noise control demonstration was needed to determine its feasibility in mitigating WetSAC low frequency noise emissions. Low frequency noise in the 24 Hz range was reduced by at least 6dB with the use of a horn loaded, compressor driven, electro-pneumatic transducer and signal controller. Since the demonstration system proved to be costly and maintenance intensive as a permanent installation, a study into achieving the same performance from a more practical and cost-effective alternative was conducted.
solution:
A permanent ANC system was engineered and installed. Several innovative and unique design features in the installation combine fundamental engineering principles of electrical, mechanical, acoustical and audio engineering with signal processing, instrumentation design and manufacturing quality control. These resulted in custom designed and fabricated high precision loudspeakers and microphones (electrodynamic transducers). They were manufactured and designed to operate year round in the very humid WebSAC area across a temperature range of -40 to +40°C. The axial acoustic centres of the source and anti-noise source are nearly identical, a condition necessary to maximize noise cancellation.
results:
Aercoustics’ ANC system effectively mitigated the environmental low frequency noise emissions. The significance is also supported by long-term cost savings and superior acoustic performance over conventional noise control methods.
Sound measurements around the plant indicate a 11 dB reduction at 23.8 Hz – about twice the reduction possible through traditional methods. This not only surpasses the 6 dB target but also confirms expectations of high performance as a result of aligning the acoustic centres of the anti-noise and offending noise sources. With a global cancellation pattern, there are no regions where the anti-source compounds the offensive noise.
Low frequency noise from the fan is about four times louder without the ANC system. The system has operated reliably since August 1994, and there have been no noise complaints since its installation.
The project also won the 1995 Award of Excellence from the Association of Consulting Engineers of Canada.
testimonial:
“TransAlta is pleased with the system operation to date as it appears that all environmental concerns regarding viable system operations have been addressed. We are especially impressed with the reliability shown by the system hardware during the past winter, in particular the microphones and loudspeakers.”
“We should note that complaints from our neighbours have ceased since the system was installed in August …”
“TransAlta and its neighbours look forward to a long term, trouble free operation of the active noise cancellation system and we would also like to thank Aercoustics for their creativity and persistence in providing this solution to our problem.”
K.F. McAndrew, Director, Plant Operations
TransAlta Energy Corporation
Also, an independent auditor validated the performance of the system 14 years after commissioning.