The classification, principle and performance of cooling fans

Cooling fans are usually divided into the following three categories:

1 Axial flow type: The direction of the air outlet is the same as that of the axis.

2 Centrifugal: Use centrifugal force to throw the air flow outward along the blades.

3 Mixed flow type: has the above two airflow methods.

Principle of cooling fan

Principle: The working principle of the fan is realized by energy conversion, namely: electric energy → electromagnetic energy → mechanical energy → kinetic energy. The circuit principle is generally divided into multiple forms, and the performance of the fan will be different if the circuit used is different.

The composition of the axial fan:

Fan frame, fan blade, bearing, PCB control circuit, drive motor

Rotating speed:

Rotation speed refers to the speed at which the fan rotates, usually measured by the number of revolutions in 1 minute, namely: rpm. The speed is related to the number of turns of the electromechanical winding, the wire diameter, the outer diameter and bottom diameter of the fan blade impeller, the shape of the blade, and the bearing used. As the speed increases, the air volume increases accordingly.

The speed value represents the air volume to a certain extent. Under certain conditions, the higher the speed, the greater the noise and vibration. Therefore, when the air volume meets the heat dissipation requirements, a low-speed fan should be used as much as possible . General speed (take DC axial fan as an example): 2510 fan 7000~12000rpm; 3010 fan 5000~9000rpm; 4010 fan 5000~7000rpm; 5010 fan 3500~5000rpm; 6025 fan 2600~4500rpm; 7025 fan 2400~3600rpm; 8025 Fan 2000~3500rpm; 9225 fan 1600~3100rpm; 12025 fan 1500~2500rpm; 12038 fan 2000~3200rpm.

   The fan speed can pass the BIOS test when starting the computer, or pass the monitoring software of other motherboards; it can also be tested by the speed tester. Note: The first two methods must be a fan that supports the speed measurement function to be tested.

Air volume and pressure:

There are two test methods for air volume and wind pressure, one is to use a wind tunnel tester, and the other is to use a double-box method. But for the average user, there is no such device. The data provided by the manufacturer can only be used as a reference, and the cooling effect will ultimately depend on.

Air volume: Air volume refers to the product of the plane speed of the fan’s ventilation area. The ventilation area is the exit area minus the projected area at the vortex tongue. The plane velocity is the velocity of the gas moving through the entire plane, and the unit is m3/s. When the plane speed is constant, the larger the outer diameter of the fan blade impeller, the larger the ventilation area, and the larger the air volume. The plane speed is determined by the rotor speed and wind pressure. When the ventilation area is constant, the greater the plane speed, the greater the air volume. The greater the air volume, the greater the heat absorbed by the air, and the more heat can be taken away from the surrounding area when the air moves, and the fan heating effect is more obvious

Wind pressure: In order to perform normal ventilation, it is necessary to overcome the resistance of the fan in the ventilation stroke. The fan must generate pressure to overcome the air supply resistance. The measured pressure change is called static pressure, which is the difference between the maximum static pressure and the atmospheric pressure. It is the pressure that the gas exerts on the surface parallel to the object, and the static pressure is measured through a hole perpendicular to the surface. The conversion of the kinetic energy required in the gas flow into the form of pressure is called dynamic pressure.

   In order to achieve the purpose of air supply, static pressure and dynamic pressure are required. The total pressure is the algebraic sum of static pressure and dynamic pressure. The total pressure refers to the total pressure increase given by the fan, that is, the total pressure difference between the outlet and the inlet of the fan. In practical applications, the nominal maximum air volume value is not the air volume obtained by the actual fan heater. Large air volume does not mean a strong ventilation capacity. When the air flows, the airflow will encounter the obstruction of the fan heat detection sheet or the component in the flow path, and its impedance will restrict the free circulation of the air. That is, when the air volume increases, the wind pressure will decrease. Therefore, there must be an optimal operating point, that is, the intersection of the fan performance curve and the wind resistance curve. At the operating point, the slope of the fan characteristic curve is the smallest, and the change rate of the system characteristic curve is the smallest. Note that the static efficiency of the fan (air volume × wind pressure ÷ power consumption) is the best at this time. Of course, sometimes in order to reduce the system impedance, even a fan with a smaller size can be used to obtain the same air volume.

The main performance parameters of the cooling fan:

The main parameters of the cooling fan include flow, pressure, speed, power, etc…

The bearing system of the fan:

The bearing system of the fan generally recommends the best choice of ball bearings, because the life of the fan heater usually depends on the reliability of its bearings, and the ball bearing system has been proven to have the characteristics of high efficiency and low heat generation. Ball bearings are rolling friction, rolling by metal balls, with a small contact surface and a small friction coefficient; while oil-impregnated bearings have sliding friction, with a large contact surface. After long-term use, the oil will volatilize, the bearing is easy to wear, the friction coefficient is large, and the later noise is louder , Short life. In addition to the large ventilation and high wind pressure, the reliability of a good-quality fan is also very important. The type of bearing used by the fan is very important here. High-speed fans always use ball bearings, while low-speed fans use low-cost oil bearings (Sleeve bearings). Oil-impregnated bearing fans only use one bearing; while ball-bearing fans all need two bearings, a single ball bearing is “1 Ball + 1 Sleeve”, which still has an oil-impregnated bearing. More advanced than single ball is double ball bearing, namely Two Balls. The life of oil-impregnated bearings is generally 10,000 hours, 30,000 hours for single ball bearings, and more than 50,000 hours for double ball bearings (when the ambient temperature is set below 25°C). The oil bearing used in the fan is sintered with copper-based powder. The oil bearing needs to be lubricated to reduce the sliding friction. The lubricating oil is made of lithium-based grease and special engine oil. With long-term operation, the oil in the bearing will evaporate and become dry, the friction coefficient will increase, the fan operation will be affected, abnormal noise may appear, the speed will be slow or even no rotation. The ball bearing replaces sliding friction by rolling friction, has a small friction coefficient and overcomes the shortcomings that the friction coefficient is easy to change, so it has strong running stability and a relatively long life.

Post time: Jan-04-2022