The amount of air to be moved often defines the size of the fan with due consideration of the space envelope available. The type of fan used is sometimes left to tradition, but should be selected by considering the resistance to flow of air likely to be encountered and the shape of the air path required.
Fans can be categorised into four main types defined by the method of air movement they employ.
Types of Fans and their Uses:
The air flow in axial fans, whose blower wheel is similar to that of a propeller, is conducted to a great extent in parallel to the axis of rotation, in other words in the axial direction. Axial fans with free air delivery at zero static pressure have the lowest power input that rises with increasing counter-pressure. Axial fans for the cooling of electronic equipment are mostly equipped with external housing and an electric motor integrated into the fan hub. This compact construction allows space-saving accommodation of all devices; the flange is equipped with mounting holes.
Many of the cooling problems that occur can be optimally solved by axial and/or diagonal fans. If for example the required cooling air has to be conducted round an angle of 90° or if high pressure is necessary, radial fans are more effective. For your application, ebm-papst offers not only complete radial fans but also motor/blower wheel combinations without external housing.
Mixed Flow / Diagonal Fans
At first glance diagonal fans only differ slightly from axial fans. Intake is axial whereas exhaust is diagonal. Due to the conical shape of the wheel and housing the air is pressurised higher. In direct comparison with axial fans of the same size and comparable performance, these fans are distinguished by the lower operating noise.
Cross flow blowers are used above all, for large-surface air flow in appliances. The air flows through the blade roller twice in the radial direction: At the suction point from the outside to the inside and at the exhaust point from the inside to the outside. Whirls form in the roller due to the vanes which guarantee a steady flow of air through the blower wheel.
Selecting the correct fan for your application
A large amount of the energy consumed by electrical and electronic devices is converted into heat. In selecting the correct fan, therefore,it is important to determine the dissipated energy that must be removed.
The electrical power consumption of the unit to be cooled, often represents a suitable value for this purpose.
Permissible temperature rise
The air flow which the selected fan is required to generate, is determined by the dissipated energy and the permissible rise (ΔT) of the cooling air flow (from entry to exit of the device to be cooled). The max. allow able ΔT depends greatly on the temperature sensitivity of the individual device components. ΔT = 5K means e.g. that the average air flow leaving the device to be cooled may only be 5°C warmer than the ambient temperature (a large volume of air is required for this purpose). A lower air flow rate is sufficient if a higher temperature difference (e.g. ΔT = 20K), can be tolerated.
Required cooling air flow
• In the below diagram a horizontal line is drawn from the dissipated enery to intersect with the selected ΔT line.
• Read down from this point to obtain the required value for the cooling air flow.
Optimum operating range
The required fan, however, must also be able to deliver a suitable static pressure increase Δpf, in order to force the cooling air through the device. A fan must therefore be selected that provides the required air flow performance within its optimum operating range.
If the requirements of an application are fulfilled by more than one fan, the noise level,space requirements, economy and ambient conditions will assist in making the final choice.