Composition of a DC fan:
Frame, blade, bearing, PCB control circuit, drive motor
Rotational speed:
Rotational speed refers to the rate at which a fan rotates, typically measured in the number of revolutions per minute, or rpm. The rotational speed is influenced by factors such as the number of windings in the electromechanical coil, wire gauge, outer and inner diameters of the fan blade impeller, blade shape, and the type of bearings used. As the rotational speed increases, so does the airflow.
The magnitude of the rotational speed to some extent reflects the magnitude of the airflow. Under constant conditions, the higher the rotational speed, the greater the noise and vibration will be accordingly. Therefore, provided that the airflow meets the cooling requirements, one should, whenever possible, use fans with lower rotational speeds.
Typical rotational speeds (taking DC axial fans as an example): The 2510 fan operates at 7,000–12,000 rpm; the 3010 fan at 5,000–9,000 rpm; the 4010 fan at 5,000–7,000 rpm; the 5010 fan at 3,500–5,000 rpm; the 6025 fan at 2,600–4,500 rpm; the 7025 fan at 2,400–3,600 rpm; the 8025 fan at 2,000–3,500 rpm; the 9225 fan at 1,600–3,100 rpm; the 12025 fan at 1,500–2,500 rpm; and the 12038 fan at 2,000–3,200 rpm.
The rotational speed of a DC cooling fan can be tested either through the BIOS during computer startup or via monitoring software provided by the motherboard; alternatively, it can also be measured using a tachometer. Note: The first two methods will only yield accurate readings if the fan supports speed measurement functionality.
1. DC fan airflow and static pressure:
There are two methods for measuring airflow and air pressure: one is to use a wind tunnel, and the other is the double-chamber method. However, for most users, such equipment is not readily available. Therefore, the only option is to refer to the data provided by the manufacturer as a guideline—and ultimately, the cooling performance should be the deciding factor.
2. DC fan airflow:
Airflow refers to the product of the planar velocity and the ventilation area of the fan. The ventilation area is calculated by subtracting the projected area at the vortex tongue from the outlet area. Planar velocity is the gas flow velocity across the entire plane, measured in m³/s. Under constant planar velocity, the larger the outer diameter of the fan blade impeller, the greater the ventilation area, and consequently, the higher the airflow. The planar velocity is determined by the rotor’s rotational speed and the wind pressure. With a constant ventilation area, the higher the planar velocity, the greater the airflow. A larger airflow means that more heat can be absorbed from the air; as the air moves and transfers energy, it can carry away even more heat, making the cooling effect more pronounced.
3. DC fan static pressure:
To achieve proper ventilation, the fan must generate a pressure that overcomes the resistance within the ventilation duct. The measured change in pressure is referred to as static pressure—the difference between the maximum static pressure and atmospheric pressure. Static pressure is the pressure exerted by the gas acting parallel to the surface of an object; it is measured through openings oriented perpendicular to that surface. The conversion of kinetic energy required for gas flow into pressure form is called dynamic pressure.
To achieve the desired airflow, both static pressure and dynamic pressure are required. The total pressure is the algebraic sum of static pressure and dynamic pressure. Specifically, the total pressure increase provided by the fan—the difference in total pressure between the fan’s outlet and inlet—is what we refer to as the total pressure. In practical applications, the rated maximum airflow value does not necessarily reflect the actual airflow delivered by the heat sink. A high airflow rate does not automatically indicate strong ventilation performance. This is because, as air flows through the system, it encounters resistance from the heat sink fins or components along its path; this resistance restricts the free flow of air. Consequently, as the airflow increases, the static pressure tends to decrease. Therefore, there must be an optimal operating point—the intersection of the fan’s performance curve and the system’s resistance curve. At this operating point, the slope of the fan’s characteristic curve is at its minimum, and the rate of change of the system’s characteristic curve is at its lowest. Importantly, at this point, the fan’s static efficiency (airflow × static pressure ÷ power consumption) reaches its peak. Of course, in some cases, to reduce system resistance, even a smaller-sized fan can be selected while still achieving the same airflow rate.
Main performance parameters:
The main parameters of a cooling fan include flow rate, pressure, rotational speed, power, and more...