Passive Noise Reduction Options for Axial and Centrifugal Fans
Learn more about controlling operating noise.
In applications relating to refrigeration, air conditioning and ventilation technology, users frequently notice that the selected fan installed in the device does not run as quietly as expected. Such fans often generate a level of operating noise considerably higher than that stated in the product documentation, simply because the values stated in the documentation reflect measurements made under optimal test conditions. In the applications themselves, however, the fans are subject to inflow disruptions of varying intensity that result from real life installation conditions. These disruptions generate a non-uniform inflow field causing additional noise which, as perceived by the human ear, often has an annoying or even uncomfortable low frequency tonal quality. Specially designed air inlet grills can reduce these disturbances by essentially straightening the inflow and significantly lowering the level of noise generation.
Fans in air handlers, heat pumps, air conditioners, refrigeration units, electronics cabinets and filtration equipment are installed in many different ways, with varying dimensions and designs. Differences occur in the positioning of the openings required for ventilation, in the heat exchanger surface and in the packing density. When adjacent fans or the walls of a device near the fan inlet create obstacles or asymmetrical constrictions to the air flow, powerful vortices can form that generate high levels of turbulence (Figure 1). This turbulence then hits the rotating blades of the fan, causing major fluctuations in the pressure and speed at the front edge of the blade. This can result in a noticeable increase in broadband air flow noise, but even more significant are large increases in the low-frequency narrowband, tonal frequency noise components.