Fan Design for Electronic Cabinet Cooling
January 1, 2007
Electronic cabinet cooling applications present many challenges in the design and operation of a fan system. Once the design concept of the cabinet has been developed, the type of fan required to ventilate the cabinet must be evaluated.
In many applications, the decision will generally revolve around an axial (propeller) or centrifugal fan. While generating the airflow required to ventilate the cabinet is the goal, the key factors involved in selecting the fan type are space considerations and pressure requirements. In today’s designs, space within the cabinet is at a premium. In some cases, propeller fans will require less space because they can be directly mounted in the cabinet wall. However, propeller fans are low-pressure devices and might not be applicable if the internal resistances to airflow are too high. In this case, a centrifugal fan should be considered.
Other factors that should be considered in fan selection include sound, efficiency and cost.
Airflow and PressureThe amount of heat to be removed in watts (W) and the resistance the fan is working against must be known to size the fan. Figure 1 summarizes the required airflow for a known amount of heat to be rejected. By knowing the temperature differential, or delta T (ΔT), as it is sometimes called, that needs to be maintained, the amount of airflow required to remove excess heat can be easily determined. In this case, ΔT is determined by subtracting the ambient temperature from the internal cabinet temperature.
The static pressure required by the fan can be determined from testing or past experience. However, it should be noted that all components within the cabinet provide resistance to airflow and must be accounted for in determining the system resistance. Failure to determine system resistance will lead to errors in airflow as the fan will not operate at its designed performance level. If ducting is applied within the cabinetry, commonly used duct design procedures can be employed to determine the system’s static pressure requirements.
Other FactorsOnce the actual airflow and pressure performance have been determined, other factors such as space requirements, noise, efficiency and cost can be evaluated.
Space within the cabinet often is at a premium. Motorized impellers can be used to minimize the fan’s space requirements. These devices combine the motor and impeller rather than using a separate fan and motor coupled together. They are available in both axial and centrifugal arrangements and require substantially less space than their fan and conventional motor counterparts. In addition, motorized impellers are controllable across much of their operating range; thus, the performance point can be “dialed in” as needed rather than operating at full load at all times. Examples of motorized impellers are shown in figure 2.
Motorized impellers are available with alternating current (AC), direct current (DC) and electronically commutated (EC) motors. In most applications, AC motors are acceptable; however, DC impellers are used in environments where low electrical noise requirements exist. High efficiency EC motors can maintain their efficiencies when operating at a partial load.
Factors that alter the performance of a fan system are known as system effect factors. The Air Movement and Control Association International Inc. (AMCA International) provides guidelines associated with system effect, including factors associated with centrifugal fans commonly encountered in electronic cabinet cooling applications.1
One common factor affecting fan performance is a restricted inlet to the fan. For a centrifugal fan, a minimum of one inlet diameter of free area opposite the fan inlet is recommended. When an obstruction is present, it will act to increase the resistance (static pressure) of the system. If the obstruction is not accounted for in determining the design pressure requirements, the fan will not operate at its design point, and the airflow available to ventilate the system will be reduced.
Figure 3 highlights the impact of an obstructed inlet on airflow. The design system curve represents the calculated system resistance curve; the in-use system curve represents the actual system resistance due to an obstruction at the inlet. Point 1 indicates the design operating point; however, with the obstruction to the inlet, the actual operating point is at point 2. Thus, the actual airflow is reduced by the difference between the airflow at point 1 and point 2. If the obstruction was known at the design phase, the fan selection could be altered so that the actual airflow would more closely match the design requirement.
Another common factor affecting fan performance pertains to a centrifugal fan designed into a plenum that is located within the cabinet. The walls of the plenum should be located at a distance greater than 1.6 times the diameter of the fan. At lower ratios, the fan performance will be reduced because this condition does not allow for airflow to develop properly as it leaves the fan blade. This condition acts to increase the pressure and will reduce the airflow delivered by the fan similar to the effect of an obstructed inlet.
When possible, the cabinet should be pressurized rather than evacuated to minimize infiltration through seams and cracks. Drawing air out of the cabinet through evacuation can lead to dust and dirt infiltrating the cabinet. Also, cooling air should enter the cabinet at the lowest possible point, and it should exit the cabinet at the highest heat-generating component within the cabinet. The forced cooling air flows upward and works with the buoyancy of heated air within the cabinet. Finally, the distance between the intake and exhaust should be maximized to reduce the potential of short circuiting the heated air.
Considering the fan system during the design phase of a cabinet can prevent many issues associated with reduced or poor airflow. By properly evaluating the load requirements and other design factors, an appropriate fan selection can be made, and the cooling requirements of electronic cabinets can be met.