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Basic knowledge of cooling fan

Basic knowledge of cooling fan
A、Classification of cooling fans
Cooling fans are usually divided into the following three categories:
1. Axial flow: the flow outlet direction is the same as the axial direction
2. Centrifugal force: the centrifugal force is used to throw the airflow outward along the blade.
3. Mixed flow: It has the above two flow modes
 
B、the principle of the cooling fan
The fan works according to the principle of energy conversion to achieve, namely electrical energy → electromagnetic energy → mechanical energy → kinetic energy.
The circuit principle is generally divided into a variety of forms, the use of different circuits, the performance of the fan will be different.
 
C、Technical specifications of cooling fan
1. Speed: Speed refers to the speed of fan rotation, which is usually measured by the number of turns in one minute, namely RPM.
The speed is related to the number of turns of electromechanical winding, line diameter, the outer diameter and bottom diameter of the fan blade wheel, the shape of the blade and the bearing used, etc., the speed increases and the air flow increases accordingly.
The value of speed represents the volume of air to a certain extent. When the condition is fixed, the higher the speed, the greater the noise and vibration will be. Therefore, under the condition that the volume of air meets the requirements of heat dissipation, low-speed fans should be used as far as possible.
General speed (take DC axial flow 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.
 
2. 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 amount of 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.
 
3. Wind pressure: In order to carry out normal ventilation, it is necessary to overcome the resistance in the ventilation stroke of the fan. The fan must generate pressure to overcome the air supply resistance. The measured change in wind pressure is called static pressure, which is the difference between the maximum static pressure and the atmospheric pressure. 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.
 
Converting the kinetic energy required in gas flow into 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 increase in the total pressure given by the fan, that is, the difference between the total pressure 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 heat sink. Large air volume does not mean strong ventilation capacity. When the air flows, the airflow will encounter the interference of the heat dissipation fins or components in the flow path, and the 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 × air pressure ÷ power consumption) is the best at this time. Of course, sometimes in order to reduce the system impedance, even a smaller size fan can be used to obtain the same air volume.
 
Other main performance parameters of cooling fan:
project unit Description
Air volume CFM Air volume refers to the total volume of air sent or sucked in by the air-cooled phoenix fan per minute. If it is calculated in cubic feet, the unit is CFM; if it is calculated in cubic meters, it is CMM. The air unit often used by cooling fans is CFM.
Wind pressure mm-H2O In order to carry out normal ventilation, it is necessary to overcome the resistance in the ventilation stroke of the fan. 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.
Speed PRM The rotation speed of the fan is usually measured by the number of revolutions in 1 minute.
Noise dBA The sound produced by the fan running at a normal speed and the resistance of the airflow pressure of the outer frame bracket.
Starting voltage V The minimum voltage that the fan can start when the power is suddenly turned on The smaller the starting voltage, the wider the voltage operating range.
Rated voltage V Generally 3V,5V,9V,12V,24V,48V,110V,220V,480V.
Working current A The current output when the motor is working. The smaller the working current, the smaller the motor heating and the more reliable
Power W Product of working current and rated voltage

 

D、Bearing system of cooling fan
The bearing system of the fan generally recommends the use of ball bearings, because the life of the cooling fan 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 and 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 and low-speed fans use low-cost oil bearings (Sleeve bearings). Oil-bearing fans only use one bearing; while ball-bearing fans all need two bearings, a single ball bearing is "1Ball+1Sleeve", which still has an oil-bearing part. 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, and the friction coefficient will increase, the fan operation will be affected, abnormal noise may appear, the number of rotations 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.