For small fans such as those used on electric motors the noise generated by their centrifugal cooling fans can be controlled by:
- Reducing the fan rotational speed; noise has been shown to increase at a rate of 53 to 64 dB/decade increase of speed. A larger, slower fan will be quieter than a smaller, faster fan delivering the same airflow.
- Reducing the rate at which sound level increases with speed from the power 6 to 5.5 or less.
- Ensuring that the design includes adequate flow-settling distance between the fan blades and any static obstructions to minimise "wake-chopping":
- Turbulence is created in the airflow stream itself. It contributes to broad band noise. Inlet and Outlet disturbances, sharp edges and bends will cause increased turbulence and noise. Obstructions to the airflow must be avoided whenever possible, especially in the critical inlet and outlet areas. When turbulent air enters the fan, noise is generated, usually in discrete tone form, that can be as mush as 10 dB higher and thus cause considerable annoyance.
- For external fans the axial clearance between blades and stator legs or other aerodynamic obstructions should be at least equal to the width of the endshield legs.
- For internal fans, the radial clearance between the blade tips and the stator should be at least one sixth of the blade tip radius.
- All unnecessary flow-obstructions such as casting imperfections, counterbored holes for bolt heads, brackets, lugs, etc., should be removed.
- Maintaining a constant area for air flow through the machine, particularly at the fan inlet, where small modifications can have large effects on air flow.
- Utilising any angular momentum imparted to the air flow by the rotor, by integrating the fan blades with the rotor.
- Matching the pressure head provided by the fan to that required to move the cooling air through the machine with controlled expansion of the air flow leaving the fan.
- Spacing the blades irregularly around the fan disc to improve the subjective quality of the noise radiated by fans.
- Structural vibration can be caused by the components and mechanism within the fan, such as residual unbalance, bearings, rotor to stator eccentricity and motor mounting. Motor mounting noise is difficult to define. It should be remembered that cooling fans are basically motors and should be treated as such when mounted.
Subjects: Noise & Vibration