One of the most important considerations in fan design is that the fan must impart a uniform velocity over it′s entire area.
There are several simple relationships between fan capacity, pressure, speed, and power, which are referred to as the fan laws. The first three fan laws are the most useful and are stated as follows:
- The capacity is directly proportional to the fan speed.
- The pressure (static, total, or velocity) is proportional to the square of the fan speed.
- The power required is proportional to the cube of the fan speed.
Of the many ways to dissipate heat in electronic components, forced convection cooling is the most effective. These Engineering notes will discuss several areas of importance in determining the correct fan or blower for any specific application. Once the decision is made to use forced convection cooling, several points must be considered before a fan can be specified.
Forced convection heat transfer can be effected in two ways:
- When evacuating a cabinet (fan on the exhaust side), the air distribution inside the cabinet is flexible. Cooling ports can be placed at any position in the cabinet to insure proper cooling in desired locations. Heat from the fan itself is not dissipated into the cabinet. However, filtering the fan on the exhaust side is extremely difficult.
- Pressurizing the cabinet is the preferred method, since incoming air can be readily filtered. With the cabinet under pressure, any cracks or crevices will have a small amount of leakage from the cabinet and dust will not seep in. The fan is handling cooler, denser air, and it will have a slightly higher pressure capability. Fan life and reliability are increased because the fan ambient temperature is lower. The disadvantage of pressurization is that heat generated by the fan is dissipated into the cabinet.
Follow these basic 5 steps when designing for a cooling fan.
- Locate the fan to pressurize the enclosure assuring lowest fan operating temperature.
- If air flow is to be vertical through the equipment to be cooled, place fan at the bottom so that it works to aid natural convection.
- Place largest heat source toward air exit so that it will have maximum heating effect on air cooling low power areas of the system.
- If air filters and RFI screens are required in a given application, their air impedance must be considered in air mover selection.
- Use the largest filter possible, to:
- Increase dust capacity
- Reduce pressure drop.
- Blow air into cabinet to keep dust out, i.e. pressurize the cabinet.
- Use the largest filter possible, to:
- Avoid as many obstructions in the packaging of the equipment at fan outlet and general air path since more resistance to flow means a larger, more power consuming air moving device. Leave a fan radius between obstructions and the fan inlet and exhaust.
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 fan 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.