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Detailed knowledge of fan basics

Views: 19     Author: BELMONT     Publish Time: 2023-05-05      Origin: https://www.belmont-tech.com


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Detailed knowledge of fan basics

The fan is a mechanical machine that relies on the input mechanical energy to raise the gas pressure and discharges the gas, and the fan is a kind of driven fluid machinery.

 Uses of fans:

1、To deliver (/exhaust) air to the building and replace the polluted air in the room with clean air.

2、plosion-proof exhaust air-exclusion of flammable and explosive gases. This kind of fan has certain requirements for the structure.

3、re smoke exhaust - exclude high temperature smoke and dust when the building is on fire. This kind of fan must pass the national mandatory fire testing certification.

4、eumatic transportation-using wind energy to mix and transport powder or granular objects.

5、haust air containing corrosive gas-Excluding exhaust gas with corrosive gas, mostly used in industry.

6、st removal-using wind energy to remove adhering materials.


Classification of fans

According to the principle of inlet and outlet air:

Axial fan: the direction of the gas entering the impeller and the direction of the gas leaving the impeller in the same horizontal line of the fan, suitable for relatively large air volume, static pressure is not too large occasions.

Centrifugal fan: refers to the inlet direction and the outlet direction of the fan at an angle of 90 degrees (when the gas from the impeller), suitable for higher wind pressure, air volume is also larger occasions, can overcome a large resistance.

Mixed flow fan: refers to the fan between centrifugal fan and axial fan, that is, the angle between the inlet direction and the outlet direction is neither 90 degrees nor in the same horizontal line, which combines the characteristics of centrifugal fan and axial fan.


Classification by pressure:

(1) low pressure centrifugal fan In the standard state, the full pressure of the fan Ptf ≤ 980Pa.

(2) medium pressure centrifugal fan In the standard state, the full pressure of the fan Ptf = 980 ~ 2942Pa.

(3) High pressure centrifugal fan In the standard state, the full pressure of the ventilator Ptf=2942~14710Pa.


Professional terminology:

Air volume: used to indicate the size of air flow. Air volume = cross-sectional area * air speed

Commonly used unit: cubic meters per hour, i.e. CMH, m3/h;

Full pressure: used to determine the magnitude of air resistance. Unit: Pa, Pa


Full pressure = static pressure + dynamic pressure

Static pressure: used to determine the resistance of the air flow, i.e., the along-travel resistance (system resistance);

Dynamic pressure: the resistance generated by the air itself as it flows. Dynamic pressure = 1/2ρv2;

Rotational speed: used to indicate the speed of the fan during operation. Unit: rev/min (r/min), RPM;

Shaft power: the actual energy consumption of the fan. Unit: kilowatt, Kw;

Motor power: is the power of the motor with which the fan is equipped, must be larger than the shaft power. Unit: Kw


The main components of the fan and its role

Belt: used to connect the motor wheel and pulley, transferring power. Belt drive only, direct drive without belts, pulleys, bearings and other driving mechanisms.

Belt pulley: connected to the shaft through the key to play a transfer role. Material: cast iron.

Shaft: connected to the pulley at one end and the impeller at the other end, playing the role of transmission. Carbon steel, fine-turned and tempered.

Bearing: Used to support the shaft (fan shaft) to avoid vibration directly applied to the motor.

Vibration damper (shock pad): to reduce vibration, reduce noise and prolong life. The drive unit and motor of each fan should be evenly connected to the fan body through the vibration damper so as to extend the life of the drive unit and impeller and make the operation more quiet.

Flood cap: Snap on top of the flood water to facilitate installation and avoid rainwater leakage into the room.


Motor: provides power for the fan. It is the main part of the fan.

Check valve: prevent the wind from pouring into the room. There are two kinds of check valves: gravity check valve and electric check valve.

Impeller: The key component that drives the flow of gas. There are many forms, such as axial flow, forward curved centrifugal, backward curved centrifugal, backward curved centrifugal, mixed flow, etc.


Measure the performance of the components:

Shaft: HB220 ~ HB370, HB Rockwell hardness, measure the stiffness of the shaft.

Bearing: L10: 80,000, means 10,000 bearings running at the same time, when 10% of them are damaged, the time recorded is 80,000 hours.

Motor: motor form, power, protection level, temperature rise level, insulation level.

Impeller: balance grade. Balance grade is the index used to measure the residual unevenness produced by the impeller in the process of rotation. Our products all strive to achieve a balance grade of G2.5, which is higher than the G6.3 level of ordinary products.


Motor basics:

Motor power: used to indicate the rated power of the motor. Motor power must be greater than the shaft power of the fan.

Rotational speed and calculation formula: The motor has both single speed and double speed. Unless specifically designated, generally use single-speed motors.


Number of poles and corresponding rotational speed:

Number of poles: the number of pairs of coil windings on the stator (commonly divided into 2, 4, 6, 8 poles)

Corresponding rotational speed: 2 poles --- 2900r/min; 4 poles --- 1450r/min


6 poles --- 960r/min; 8 poles --- 720r/min.

Fans usually choose three-phase AC asynchronous motor (380V/50Hz/3PH) single-phase AC motor (220V/50Hz/1PH) two major categories, mainly three-phase motor.

According to the different uses can be divided into:


Ordinary motor (TEFC - ordinary motor with cooling fan)

Two-speed motor: as the name suggests, there are two speeds of the motor. This kind of motor has two windings inside, and can change the speed by switching in different windings. Normally, it is used for ordinary exhaust air, and in case of emergency, the speed is increased to quickly remove the exhaust air. It can be used for two purposes in one machine.


Explosion-proof motor (EXP) is used when the fan is used for explosion-proof exhaust air.

Variable Frequency Motor (VFD Motor), which can change the speed by changing the power supply frequency and indirectly change the power to achieve the purpose of energy saving. Generally used in conjunction with inverters.


Inner rotor/outer rotor motor

Inner rotor motor: A motor with the stator outside and the rotor rotating inside. Various forms, can meet various occasions, various power requirements.


External rotor motor: motor with stator inside and rotor outside. Small size, lightweight, but less power (7.5kw or less), generally only used in clean room temperature air.


Protection class: expressed by IPXX:

Temperature rise grade: motor as an energy conversion component, in the process of operation itself there is energy loss, there is part of the energy loss caused by its own temperature rise. Therefore, the state provides a temperature level to limit the use of the motor limit temperature (measured by resistance method, can not be measured by hand)


Insulation level: In order to operate reliably, the motor needs to be isolated by insulating material between the charged parts and the shell or between the charged parts, and the service life of the insulating material has a lot to do with the insulation level of the material itself.


Insulation grade B, F, H (high temperature environment).

General motors: temperature rise class B, insulation class F.

Fan selection five elements:

1. air volume, air pressure and other parameters required;

2. installation location, installation form, indoor or outdoor;

3. the composition of the gas to be conveyed

4. the use of working conditions;

5. other requirements such as accessories.

After selection, we should record the air volume, static pressure, shaft power, speed, noise and other parameters in detail.


Ventilator inlet standard condition:

It refers to the pressure of the air at the inlet of the ventilator as a standard atmosphere (760mmHg or 101325Pa), the temperature is 20℃, and the relative humidity is 50% of the gas condition.

Flow rate: It is the volume of gas flowing through the ventilator per unit time, and has nothing to do with air density.

Q= A×V


Common unit of flow rate: m3/h; and m3/s is used in all fan performance calculations.

The full pressure of the ventilator Ptf:

The total pressure of the air flow at a certain point or a certain cross-section is equal to the point on the cross-section

Ptf= Psf + Pdf


Full pressure commonly used units: Pa

Ventilator dynamic pressure Pdf:

The pressure characterized by the kinetic energy of the gas at the exit section of the ventilator, i.e:

Pdf = ρ2 * c22/2

Ρ2: outlet air density c2 export flow rate

Dynamic pressure in common units: Pa

Static pressure of the ventilator Psf:

Ventilator static pressure is defined as the full pressure of the ventilator minus the dynamic pressure of the ventilator. That is:

Psf= Ptf-Pdf, static pressure commonly used units: Pa.


The speed of the ventilator n:

Ventilator speed is the number of rotations per minute, unit (r / min).


Full, static and dynamic pressures in the pipe as shown in the figure:

Efficiency of ventilator:

Detailed knowledge of fan basics



Ventilator full-pressure efficiency ηtf

is equal to the ratio of the full-pressure effective power of the ventilator to the shaft power, i.e:

ηtf = Petf/Psh= Ptfqv/1000/Psh


Mechanical efficiency is the characterization of ventilation fan bearing losses and transmission losses, is the main indicator of the design of the ventilation fan mechanical drive system. When the fan speed does not degree and run in low load conditions, because the mechanical losses remain unchanged, so the mechanical efficiency will also be reduced.


Transmission mode and mechanical efficiency

The choice of motor power:

The power of the motor P is selected according to the following formula:

P ≥ K Psh/=k Ptfqv/1000ηtf

Note: The above table takes Yilida fan as an example.

Example: The customer requires a fan flow rate of 12000m3/h, full pressure of 800Pa, with Yilida SYD450R belt drive fan, the effective power of the full pressure of the fan, the full pressure of the efficiency, with the motor power how much?


Solution:Through the Yilida fan selection software or sample to know the fan shaft power (internal power) Psh = 4.039kw (direct test data);




5.5kw motor is used.

Speed triangle of fan impeller:

 Detailed knowledge of fan basics

Comparison of forward and backward fans:

1. Pressure: When the size of the two impellers, speed and flow rate are the same, the full pressure obtained by the front-bending impeller is greater than the backward.

2. Efficiency: Although the front bend blade to obtain the full pressure of the largest, but mainly the impeller outlet dynamic pressure increase. This dynamic pressure for the full pressure of more than 50%. This part of the dynamic pressure, must be converted into static pressure in the worm gear behind the impeller. Practice has proved that the worm gear efficiency is low. In addition to the loss in the front bend blade flow channel is also larger, so the worm shell and impeller in the flow conditions are to reduce the efficiency of the fan. And the back bend impeller situation is the opposite, so the back bend impeller than the front bend impeller ventilation fan efficiency to be high.

The balance of the fan:

1) reduce vibration;

2) reduce noise;

3) reduce operational stress;

4) increase the life of the bearings

5) reduction of disturbance and fatigue to the operator;

6) Reduce energy loss.