A fan is a machine used to transport gas, and from an energy point of view, it is a machine that converts the mechanical energy of the prime mover into gas energy. The fan is a habitual abbreviation for gas compression and gas conveying machinery.
1. Definition of fan
To put it simply, a fan is a machine that relies on the input mechanical energy to increase the gas pressure and discharge the gas.
2. The principle of the fan
The gas is treated as an incompressible fluid, and the high and low pressure is used to control the flow and direction of the gas.
3. Fan classification
Classified by pressure
4. Main parameters of the fan
pressure
The pressure of a centrifugal fan refers to the pressure rise (relative to the atmospheric pressure), that is, the increase in the pressure of the gas in the fan or the difference between the gas pressure at the inlet and outlet of the fan. It is divided into static pressure, dynamic pressure, and full pressure. Performance parameters refer to the total pressure (equal to the difference between the total pressure of the fan outlet and the inlet), and its units are commonly used Pa, KPa, mH2O, mmH2O, etc.
flow rate
The volume of gas flowing through the fan per unit of time, also known as air volume. It is often expressed in Q, and the common units are: m3/s, m3/min, m3/h (seconds, minutes, hours). (Sometimes the "mass flow" is also used, that is, the quality of the gas flowing through the fan per unit time, at this time, it is necessary to consider the density of the gas at the inlet of the fan, which is closely affected by the gas composition, local atmospheric pressure, gas temperature, and inlet pressure, and needs to be converted to get the accustomed "gas flow").
Generally, the calculation of the fan flow rate is calculated by the area A of the fan outlet and the wind speed at the fan outlet, which is expressed as:
rotate speed
The speed at which the fan rotor rotates. It is often expressed in n, and its unit is r/min (r is the speed, min is the minute).
power
The amount of power required to drive the fan. It is often expressed in N, and its unit is KW. Generally, due to the small change in pressure and temperature, the fan we commonly use can be calculated according to the air density under the standard state: 1.2kg/m3 without considering the density change caused by the temperature and pressure change of the gas.
pressure
Static pressure Pst: The pressure at which a gas moving in a flat direct channel acts perpendicular to a wall surface in a certain section. Usually measured. It is also referred to as vacuum in some centrifugal fan samples.
Dynamic pressure Pd: The average pressure Pd = ρv2/2 resulting from the gas flow velocity on this section.
Full pressure Pt: The sum of the static pressure and dynamic pressure of the gas on the same section is called the total pressure of the gas, and the difference between the total pressure of the inlet and outlet of the fan is called the total pressure of the fan, that is: Pt=Pst+Pt.
Static pressure ratio: In the hydraulic calculation of pipeline design, the resistance loss of the pipeline should be considered, the greater the wind speed in the pipeline, the greater the resistance loss, and the faster the energy attenuation, so for the fan, the static pressure ratio is a very important value, expressed as η=Pst/Pt.
power
Effective power Pe: The effective energy obtained from the fan per unit time by the gas conveyed by the fan: Pe=Pt×Q/1000[kW]
式中：Pt［Pa］，Q［m3/s］
Shaft power Psh: the energy transmitted by the prime mover to the fan shaft per unit time, the fan shaft power directly connected to the motor is the motor power, if the belt or other transmission methods are used, the influence of the power transfer coefficient should be considered.
Fan efficiency
风机全压效率ηt：风机全压有效功率与风机轴功率之比：ηt=Pet/Psh=Pt×Q/1000/Psh
风机静压效率ηs：风机静压有效功率与风机轴功率之比：ηt=Pes/Psh=Pst×Q/1000/Psh
Fan speed n: unit: r/min or rpm
Function: All performance parameters of the fan will change with the change of speed
Commonly used formula for calculating motor speed: n=120f/p
n is the speed, f is the power frequency, P is the number of poles of the motor (common 2, 4, 6, 8, 10)
The speed of the motor directly connected to the fan is the speed of the motor, and the speed of the fan can be changed by changing the frequency of the power supply. If the belt is conveyed, the speed of the fan can be changed according to the ratio of the diameter of the original and passive pulleys.
The following diagram shows the main parameters for testing wind pressure:
As shown in the figure above, the test value of Pt1 is the full pressure of the air inlet, and Pt2 is the full pressure of the air outlet, then the full pressure of the fan Pt=Pt2-Pt1.
Ps1 is the static pressure of the air inlet, Ps2 is the static pressure of the air outlet, then the static pressure of the fan is Ps=Ps2-Ps1.
The wind velocity is generally Pd=0.5ρv2, so the dynamic pressure can be obtained by measuring the wind velocity v.
The air volume is also calculated by calculating Q=A*v, where A is the area of the fan outlet.
The noise of the fan is also tested, generally placed at a distance of 1 meter from the air outlet, at an angle of 45° below, and then the spectrogram is obtained, and finally the actual noise of the fan is obtained. Of course, the fan noise can also be estimated from the fan flow rate and pressure, which will be discussed in detail later.
Conversion of similar wind turbine performance parameters:
Suppose the parameters of a certain type of fan are as follows:
Flow rate Q, pressure P, power N, speed n, efficiency η.
Fan parameters to be converted:
Flow rate Qm, pressure Pm, power Nm, speed nm, efficiency ηm, the conversion relationship between the two is as follows:
5. Introduction to the structure
Wind turbine naming conventions
The speed at which the fan rotor rotates. It is often expressed in n, and its unit is r/min (r is the speed, min is the minute).
Introduction to the rotation direction of the fan: the fan can be made into two types: from one end of the motor, such as the impeller rotates in the clockwise direction called the cyclone, which is represented by "Shun", and the rotation in the counterclockwise direction is called the reverse fan, which is represented by "reverse".
The outlet position of the fan is expressed by the outlet angle of the casing: "forward" and "reverse" can be made into six angles: 0°, 45°, 90°, 135°, 180° and 225°, and can also be made into others according to the user's requirements.
Fan outlet: The direction of rotation of "left" or "right" is specified, and each has 8 different basic air outlet positions.
The composition of the fan: it is mainly composed of fan blades, collectors, shutters, window opening mechanisms, motors, pulleys, air inlet hoods, inner frames, volutes and other components. When the machine is turned on, the motor drives the fan blade to rotate, and makes the window opening mechanism open the shutter to exhaust the air. The blinds close automatically when the machine is stopped.
impeller
Composition of the impeller: The impeller is the main component of the fan, and the impeller is composed of blades, front and rear discs that connect and fix the blades, and a hub.
According to the relationship between its outlet direction and the rotation direction of the impeller, the blade type of centrifugal fan can be divided into three types: backward type, radial type and forward type.
The bending direction of the rear-facing blades is exactly the same as the natural trajectory of the gas, so there are fewer impacts between the gas and the blades, less energy loss and less noise, and higher efficiency. The bending direction of the forward-facing blade is opposite to the trajectory of the gas, and the gas is forcibly redirectiond, so it has a large noise and energy loss, and is less efficient. The radial blades are characterized by a range of backward and forward blades.
Wind collectors
Composition of the air collector: The collector device is in front of the impeller, which allows the air flow to evenly fill the inlet section of the impeller, and the drag loss when the air flow passes through it is minimal.
Cylindrical: A vortex zone is formed at the inlet of the impeller, which is less effective when the air is taken directly from the atmosphere.
Conical: Better than cylindrical, but it's too short and doesn't work well.
Arc: Better than the previous two.
Cone arc: The best, high-efficiency fans basically use this type of collector.
The coordination between the collector and the impeller is preferably in the form of sleeve clearance. However, the counterpart gap form is generally less used.
In order to reduce the vortex and control the backflow, a wind shield is installed at the air inlet inside the fan.
Chassis
The performance and efficiency of the fan are mainly determined by the impeller, but the shape and size of the volute and the shape of the suction port will also affect it.
The function of the volute is to collect the gas thrown out of the impeller, make it flow to the exhaust port, and in the process of this flow, part of the dynamic pressure energy obtained from the impeller is converted into static pressure energy to form a certain wind pressure.
The shape of the volute: a logarithmic spiral.
Volute outlet diffuser: Because the air flow is deflected to the direction of rotation of the impeller when flowing out of the volute, the diffuser is generally made to expand to the side of the impeller, and its diffusion angle θ is usually 6°~8°
Snail tongue: There is a tongue-like structure at the volute outlet of the centrifugal fan, which is generally called the snail tongue. The worm tongue prevents the gas from circulating through the casing.
Composition of the snail tongue:
1. Sharp tongue: used for high-efficiency fans, fans are noisy.
2. Deep tongue: mostly used for low-speed fans.
3. Short tongue: mostly used for high-speed fans.
4. Flat tongue: used for low-efficiency fans, the fan noise is small.
The gap between the top of the worm tongue and the outer diameter of the impeller has a greater impact on noise. The noise is large when the gap S is small, and the noise is reduced when the gap S is large.
一般取s=(0.05～0.10)D2。
The arc R at the top of the tongue has no obvious effect on the aerodynamic performance of the fan, but has a great impact on noise.
If the arc radius r is small, the noise will increase, and r=(0.03~0.06)D2 is generally taken.
Bearing housings
Principle and characteristics of axial fan: the gas passes through the collector along the axial direction, obtains a certain dynamic pressure and static pressure by the impact of the impeller at the impeller, and then flows into the rear guide vane, the guide vane converts a part of the deflected gas flow energy into static pressure energy, and finally, the gas converts a part of the axial gas kinetic energy into static pressure energy through the diffuser, and then flows out of the diffuser and enters the pipeline. Compared with centrifugal fans, the axial fan is small in size, small in pressure, large in air volume, and easy to install.
Centrifugal fan principle: the working medium flows axially into the impeller, enters the blade flow channel, and transforms into a radial movement perpendicular to the fan shaft;
Under the action of the blades, the medium gains an increase in energy: the static pressure increases, the kinetic energy increases, and when the energy raised is sufficient to overcome the resistance, the medium can be transported.
According to the principle of converting kinetic energy into potential energy, the gas is accelerated by a high-speed rotating impeller, and then decelerates and changes the flow direction to convert kinetic energy into potential energy (pressure). In a centrifugal fan, the gas enters the impeller axially from the (collector), changes to radial as it passes through the impeller, and then enters the diffuser (volute). In the volute, the gas changes the direction of flow causing a deceleration, which converts kinetic energy into pressure energy. The increase in pressure occurs mainly in the impeller and to a lesser extent in the expansion process.
Schematic diagram of the direction of the air outlet of the centrifugal fan:
There are three main impeller forms of centrifugal fans: the impeller of centrifugal fans is much more complex than the impeller of axial fans, and the process requirements are higher.
Fan noise characteristics budget method: fan ratio A sound level LSA refers to the A sound level radiated by the fan at the unit flow rate and unit pressure;
The formula for converting it to A sound level is as follows:
LA=LSA+10lgQVPtf2-19.8 单位：dBA
Las是比A声级(dBA),La是风机A声级(dBA),Ptf是风 also机全压(Pa),QV是风机体积流量(m3/min)。
Generally, for the same structure style or the same series of fans, its ratio A sound level is certain, and the sound level noise of A can be calculated through the above formula, and in most cases you can budget whether this fan is suitable for a certain project, but this is only a budget, and the actual fan noise needs to be subject to the actual measurement.
The Fan Noise Level A Budget Formula, from the "Fan Noise Limits", we can know the specific A sound level LSA of the five types of fans, and the above formulas can be listed in the following table:
Precautions for installation and operation based on wind turbines
Installation clearance of air collector and impeller: installation is carried out in strict accordance with the size of the general drawing, in order to ensure the performance of the fan, especially the gap between the air inlet of the fan and the impeller is in line with the general drawing. For some fans with high gas temperature and large machine size, in order to ensure that the fan runs at high temperature, there is no friction between the air inlet ring and the impeller after the thermal expansion of the casing, and the gap between the air inlet ring and the impeller inlet is not completely uniform.
Fan vibration value parameters: After the fan is started, when the normal number of revolutions is reached, the following checks should be made: the temperature rise of the fan bearing shall not be greater than 40 °C, and the surface temperature shall not be greater than 70 °C.
轴承部位的振动速度有效值≤4.6mm/s。
The temperature of the motor bearing is according to the "Motor Instruction Manual".
The maximum allowable values of wind turbine bearing vibration are:
When the bearing vibration speed is effectively displayed, it is 11mm/s.
When displayed with bearing amplitude, the following values are used:
Maintenance of the fan when it is not in use when it is parked for a long time
(1) Coat the surface of the bearing and other major parts with anti-rust oil to avoid corrosion.
(2) The fan rotor should be manually moved to rotate half a circle (i.e., 180°) every half a month, and the shaft end should be marked before moving, so that the original topmost point is located at the bottom after moving the rotor.
Position of machine opening: