Engineering design method of the hottest axial flo

2022-10-04
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The engineering design method of axial fan

fan is a general mechanical product with large quantity and wide range; Fan is a fluid machine that compresses and transports gas or air by using the interaction between the rotation of one or more impellers equipped with blades and gas or air; Fan is the general name of turbine compressor, turbine blower and fan

under standard air conditions with inlet pressure and temperature of 101.3kPa and 20 ℃ respectively and relative humidity of 50%, fans with total pressure less than or equal to 30KPa are called fans

fans mainly include centrifugal and axial flow fans

in the axial section, the fan whose airflow flows along the radial direction in the impeller is a centrifugal fan; Centrifugal fan is axial intake and radial exhaust. In the axial section, the fan whose airflow flows along the axial direction in the flow passage of the rotating blade is an axial fan; Axial flow fans are axial intake and exhaust

He added that

in comparison, centrifugal fans have high pressure and low flow; Axial flow fan has small pressure and large flow. Axial flow fans are classified as follows:

1) according to the pressure classification

gb/t/iso 1334.9:1999 "industrial fans vocabulary and type definitions", it is pointed out that:

the pressure ratio of low-pressure fans is lower than 1.02, and the reference Mach number is less than 0.15. When handling standard air, its pressure rise is less than 2KPa

the pressure ratio of medium pressure fan is greater than 1.02 but less than 1.1, the reference Mach number is less than 0.15, and the corresponding pressure rise is 2KPa to 10KPA

the pressure ratio and pressure rise of the high-pressure fan are greater than the above values

the standard further points out that the fan impeller will produce high or low pressure according to its peripheral speed, and defines the pressure range of various fan types, that is, when various fans are at the highest efficiency and speed, the fan pressure is not lower than the value given in the following table. In any case, the defined fan pressure should not exceed 95% of the maximum pressure generated by the fan at the highest speed

2) classified according to the hub ratio

according to the ratio of the hub diameter to the impeller outer diameter, that is, the hub ratio, the axial fan can be divided into low-pressure, medium pressure and high-pressure types, which means that the pressure generated by the axial fan is low, medium or high under a given flow. If the hub ratio is lower than 0.4, it is considered as a low-pressure (or low hub ratio) axial flow fan. If the hub ratio is greater than 0.71, it is considered as a high-pressure (or large hub ratio) axial flow fan. If the hub ratio is between 0.4 and 0.71, it is considered as a medium pressure (or medium hub ratio) axial flow fan

3) classified by purpose

axial flow fans are widely used. According to purpose, they mainly include:

mine axial flow fans: mine main shaft circulation fans (main fans) are used for mine main winding ventilation; Local axial fans (local fans) are used for ventilation in local areas such as mining faces

axial fan of power station: it is used for axial fan supporting boiler in thermal power plant, including forced draft fan, induced draft fan, etc

textile axial fan: used for ventilation in textile workshops

tunnel axial fan: used for tunnel ventilation

fire smoke exhaust axial fan: used for fire smoke exhaust in high-rise buildings

axial flow fan of cooling tower: it is used together with mechanical ventilation cooling tower

axial fan of air cooler: it is matched with the air cooler widely used in petrochemical industry; It is an important part of air cooler

general purpose axial fan: used for ventilation or heating ventilation of factories and buildings

special axial fans: such as those used in naval ships, hovercraft, diesel locomotives, etc

there are other axial fans for other purposes, which will not be described here one by one

4) according to material classification

can be divided into metal and glass fiber reinforced plastic (commonly known as FRP) axial flow fans

glass fiber reinforced plastics (GRP) is the elder of modern composite materials, which has the characteristics of light weight, high strength, fatigue resistance, corrosion resistance, easy molding and so on. FRP fans are widely used in discharging corrosive industrial gases and humid air

5) according to the classification of adjustment methods

the best adjustment method of modern axial fan performance is mainly achieved by adjusting the installation angle of impeller blades and adjusting the impeller speed as a special composite manufacturer. Therefore, there are angle adjustment fans and speed adjustment fans. When the speed is given, the blade installation angle can be adjusted manually one by one or synchronously by some device workers, or synchronously and automatically by some device without stopping the machine. Adjusting the speed of the impeller is to adjust the speed by some device or by stages or stepless speed regulation under the condition of the best installation angle of the blade. The angle adjusting or speed regulating fan is suitable for selection under variable working conditions

6) classification by workspace (or function definition)

air circulator or free fan: this kind of fan works in free space, such as civil floor fan, table fan and ceiling fan

diaphragm mounted axial fan: this fan is installed on the diaphragm or wall to transport air from one larger space or free space to another

jet axial fan: it is used to generate air jet in a space and is not connected with any pipe

pipe axial flow fan: this fan is connected with the pipe () system and works in the pipe; The air flow flows in and out along the axial direction of the fan, so that it can be transported in the pipe

due to different structural types, the pipeline axial flow fan has different references. Understanding and clarifying its different meanings is very important for the design of pipeline axial flow fan. 1.2.1 fan body

axial flow fan body is composed of impeller and guide vane; The guide vanes can be front guide vanes, rear guide vanes, front and rear guide vanes

the axial fan body is also called the stage of the axial fan, and it can also be called the aerodynamic sketch of the axial fan, which is referred to as the aerodynamic sketch for short

there are many types of aerodynamic sketches of axial fans. For ordinary axial fans (compared with meridian accelerated axial fans), there are mainly:

single stage: single impeller (R)

impeller + rear guide vane (r+s)

front guide vane + impeller (p+r)

front guide vane + impeller + rear guide vane (p+r+s)

double stage: impeller + middle guide vane + impeller + rear guide vane (r+c+r+s)

front guide vane + impeller + middle guide vane + impeller + rear guide vane (p+r+c+r+s)

stage I impeller + stage II impeller, Contrarotation (RI +ri)

the above sketch is shown in Figure 1.1 (a) ~ (g)

Figure 1.1 common pneumatic sketch of ordinary axial fan

1.2.2 fan device

axial fan device is the general name of fan body, motor and auxiliary equipment. Auxiliary equipment includes inlet box, collector, hood, diffuser, diffusion tower, muffler, etc., which shall be configured as required

typical axial fan device is shown in Figure 1.2

Figure 1.2 schematic diagram of axial fan device

the functions of each component are briefly described as follows:

impeller

absorbs the energy provided by the motor and does work on the air flow, which is the heart of axial fan. For the pipe axial flow fan, because the pipe cross-section before and after the impeller is unchanged, the axial speed of the air flow does not change, and the function of the impeller is to increase the pressure of the air flow. The impeller is composed of a wheel hub and a certain number of blades. Each blade is like a rotating wing. In order to give better play to the role of blades, the minimum friction loss, air flow separation loss and secondary flow loss should be achieved when conveying air, so as to improve the working efficiency of the impeller

guide vane

when the air flow passes through the impeller, due to the induction effect of blade rotation, the air flow produces the same swirl (induction) speed as the impeller rotation direction. Setting the front guide vane is to let the air flow through it to generate a swirl speed (or pre swirl speed) opposite to the rotation direction of the impeller in advance. This swirl speed counteracts with the swirl speed induced by the blade, so that the air flow forms a single axial flow. Due to the generation of pre rotation speed, the air flow through the leading blade accelerates the decompression process. In addition, by changing the installation angle of the front guide vane, it can also play a role in regulating the flow under variable working conditions. First, the rear guide vane can eliminate the rotation of the air flow generated after flowing through the impeller and ensure the single axial movement of the air flow; The second is to convert the rotational kinetic energy of the air flow into pressure energy and improve the efficiency of the fan. Due to the disappearance of swirl speed, the air flow passing through the rear guide vane will also greatly increase the export proportion of emerging countries, which is a deceleration and pressurization process

the combination of collector and hood is very important to improve and ensure uniform inflow conditions and improve fan efficiency

diffuser

is usually an annular diffuser containing the middle whole fluid, which is an important measure to recover the outlet dynamic pressure and convert it into static pressure, and further improve the static pressure efficiency of axial fan device

1.2.3 inlet and outlet of ducted axial fan

strict regulations are made on the upstream inlet and downstream outlet boundaries of ducted axial fan, which is conducive to the design of fan or fan device and the calibration of its performance

the inlet of the pipe axial fan refers to the inlet flange of the fan; If the inlet is equipped with auxiliary equipment, such as current collector or air inlet box, the more polyurethane is used at the inlet flange of the auxiliary device, the more the fan (device) is imported. The outlet of the pipe axial fan refers to the outlet flange of the fan; If the outlet is equipped with auxiliary equipment, such as diffuser or diffusion tower, the outlet flange of the auxiliary equipment is the outlet of the fan device

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