Common Calculations for Fan Selection

Fan Classification and Applications:
Classification by Working Principle
Turbine Fan - A fan that compresses and conveys gas through rotating blades.
Positive Displacement Fan - A machine that compresses and conveys gas by changing the gas volume.
Classification by the Direction of Airflow Movement
Centrifugal Fan - After the airflow enters the fan impeller axially, it is compressed under the action of centrifugal force and mainly flows radially.
Axial Fan - The airflow enters the rotating blade channel axially. Due to the interaction between the blades and the gas, the compressed gas flows approximately along the axial direction on the cylindrical surface.
Mixed Flow Fan - The gas enters the rotating blade passage at an angle to the main shaft and flows approximately along the conical surface.
Cross Flow Fan - The gas passes through the rotating blade passage transversely and is pressurized by the action of the blades.
Classification by the Level of Production Pressure (Calculated by Absolute Pressure)
Ventilator - The exhaust pressure is lower than 112700 Pa;
Blower - The exhaust pressure is between 112700 Pa and 343000 Pa;
Compressor - The exhaust pressure is higher than 343000 Pa;
The corresponding classification of high and low pressure of ventilators is as follows (under standard conditions)
Low-pressure Centrifugal Ventilator: Total Pressure P ≤ 1000 Pa
Medium-pressure Centrifugal Ventilator: Total Pressure P = 1000~5000 Pa
High-pressure Centrifugal Ventilator: Total Pressure P = 5000~30000 Pa
Low-pressure Axial Ventilator: Total Pressure P ≤ 500 Pa
High-pressure Axial Ventilator: Total Pressure P = 500~5000 Pa
Full Name Representation Method of General Ventilators
Composition Representation Method of Type and Variety
Pressure: The pressure of a centrifugal ventilator refers to the pressure rise (relative to the atmospheric pressure), that is, the increase in gas pressure inside the fan or the difference in gas pressure at the inlet and outlet of the fan. It is divided into static pressure, dynamic pressure, and total pressure. The performance parameter refers to the total pressure (equal to the difference between the total pressure at the fan outlet and the inlet), and its common units are Pa, KPa, mH2O, mmH2O, etc.
Flow Rate: The gas volume flowing through the fan per unit time, also known as the air volume. It is commonly represented by Q, and the common units are m3/s, m3/min, m3/h (seconds, minutes, hours). (Sometimes the "mass flow rate" is also used, that is, the mass of gas flowing through the fan per unit time. In this case, the gas density at the fan inlet needs to be considered, which is closely related to the gas composition, local atmospheric pressure, gas temperature, and inlet pressure, and conversion is required to obtain the customary "gas flow rate".
Rotational Speed: The rotational speed of the fan rotor. It is commonly represented by n, and its unit is r/min (r represents the rotational speed, and min represents minutes).
Power: The power required to drive the fan. It is commonly represented by N, and its unit is Kw.
High-temperature Fans and Other Special Fans: Total Pressure P =? Pa, Flow Rate Q =? m3/h, Inlet Gas Density Kg/m3, Transmission Method, Conveyed Medium (air can be omitted), Impeller Rotation Direction, Inlet and Outlet Angles (viewed from the motor end), Working Temperature T =…..℃, Instantaneous Maximum Temperature T =? ℃, Inlet Gas Density □Kg/m3, Local Atmospheric Pressure (or local altitude), Dust Concentration, Fan Damper, Motor Model, Inlet and Outlet Expansion Joints, Integral Base, Hydraulic Coupling (or frequency converter, liquid resistance starter), Lubricating Oil Station, Slow Rotation Device, Actuator, Starting Cabinet, Control Cabinet, etc.
Precautions for High Rotational Speed of Fans (for B, D, and C drives)
1. For Type 4-79: 2900 r/min ≤ No. 5.5; 1450 r/min ≤ No. 10; 960 r/min ≤ No. 17;
2. For Type 4-73 and 4-68: 2900 r/min ≤ No. 6.5; 1450 r/min ≤ No. 15; 960 r/min ≤ No. 20;
Commonly Used Calculation Formulas for Fans (Simplified, Approximate, and Used in General Cases):
1. Shaft Power: N = = N (shaft power) × K (motor reserve coefficient) = Power required by the motor
Note: 0.8 is the fan efficiency, which is a variable, and 0.98 is a mechanical efficiency, also a variable (Type A is 1, Type D and F are 0.

98, and Type C and B are 0.95)
2. Total Pressure of the Fan: (Not Corrected Under Standard Conditions)
P1 = P2 ×
Where: P1 = Total pressure under working conditions (Pa), P2 = Designed standard pressure (or total pressure in the table, Pa), B = Local atmospheric pressure (mmHg), T2 = Temperature of the medium under working conditions in ℃, T1 = Designed temperature in the table or uncorrected designed temperature in ℃, 760 mmHg = Atmospheric pressure at an altitude of 0 m when the air is at 20℃.
2.1 Conversion of Local Atmospheric Pressure Based on Altitude:
(760 mmHg) - (Altitude ÷ 12.75) = Local atmospheric pressure (mmHg)
Note: No correction is required for altitudes below 300 m.
1 mmH2O = 9.8073 Pa, 1 mmHg = 13.5951 mmH2O, 760 mmHg = 10332.3117 mmH2O
2.2 For the fan flow rate, no correction is required at an altitude of 0~1000 m; add 2% to the flow rate at an altitude of 1000~1500 m;
Add 3% to the flow rate at an altitude of 1500~2500 m; add 5% to the flow rate at an altitude above 2500 m.
Specific Speed: ns
nS = 5.54 × n ×
Note: ρ is the gas density (Kg/m3); Formula: P1 = P2 × 1.2/ρ, ρ = 1.2 × (273 + T2)/(273 + 20)
At 20℃ = 1.2, at 50℃ = 1.

089, at 80℃ = 0.996, at 100℃ = 0.943, at 150℃ = 0.813, at 200℃ = 0.743, at 250℃ = 0.672, at 280℃ = 0.636, at 300℃ = 0.614, at 350℃ = 0.564.
Pressure Coefficient:
Pressure Coefficient
ψ = Pressure coefficient, P = Total pressure (Pa), ρ = Gas density (Kg/m3), U = Peripheral speed of the outer edge of the impeller (m/s).
Maximum Torque of the Fan:
550 × Motor Power ÷ Rotational Speed =?. Nm (Generally for large motors or as required by users)
Dynamic Load Coefficient of the Fan:
0.5 for 2900 revolutions, 0.25 for 1450 revolutions, 0.175 for 960 revolutions, 0.0875 for 580 revolutions
Torque of the Damper:
Tmix = (2～2.5) × 10－6 × Q3/2 × P =?. N.m
The Air Exchange Rate per Square Meter of Floor Area in the Standard of "Air Conditioning and Hygienic Engineering" (m3/h? m2)
At the beginning of the fan's operation, the vibration at the bearing part is very small.

However, as the operation time increases, the dust in the fan will unevenly adhere to the impeller, gradually destroying the dynamic balance of the fan and causing the bearing vibration to gradually increase. Once the vibration reaches the maximum allowable value of 11 mm/s for the fan (the maximum allowable values represented by the amplitude values are as follows), the fan must be stopped for repair (removing the dust accumulation and re-performing dynamic balancing). Because it is very dangerous at this time, users must never force the use of the fan. When the fan vibration approaches the dangerous value, the vibration measuring instrument will give an alarm.
The maximum allowable value of the fan bearing vibration is:
(1) When effectively displayed by the bearing vibration speed: 11 mm/s.
(2) When displayed by the bearing amplitude, the values are as follows:
a. When the synchronous speed of the motor is 3000 revolutions per minute: The maximum allowable value is: 0.1 mm (double amplitude)
b. When the synchronous speed of the motor is 1500 revolutions per minute: The maximum allowable value is: 0.2 mm (double amplitude)
c. When the synchronous speed of the motor is 1000 revolutions per minute: The maximum allowable value is: 0.31 mm (double amplitude)
d. When the synchronous speed of the motor is 750 revolutions per minute: The maximum allowable value is: 0.4 mm (double amplitude)
e. When the synchronous speed of the motor is 600 revolutions per minute: The maximum allowable value is: 0.

5 mm (double amplitude)
f. When the synchronous speed of the motor is 500 revolutions per minute: The maximum allowable value is: 0.6 mm (double amplitude)
The normal bearing temperature of the fan is ≤70°C. If it rises to 70°C, the electronically controlled system should (will) give an alarm. At this time, the cause should be found. First, check whether the cooling water is normal? Whether the bearing oil level is normal? If the cause cannot be found for a while and the bearing temperature rises rapidly to 90°C, the electronically controlled system should (will) send an alarm and a stop signal again.
During the start-up, shutdown or operation of the fan, if any abnormal phenomenon is found, it should be checked immediately. For the minor faults found during the inspection, the cause should be identified in a timely manner and eliminated. If major faults are found (such as severe vibration, impact of the fan, or a sharp rise in the bearing temperature, etc.), the fan should be stopped immediately for inspection.
After the fan has been in operation for one month for the first time, the lubricating oil (or grease) should be replaced. In addition to replacing it after each disassembly and repair, under normal circumstances, the lubricating oil (or grease) should be replaced once every 1 to 2 months, and it can also be replaced according to the actual situation.
Proper maintenance is an important guarantee for the safe and reliable operation of the fan and for improving the service life of the fan. Therefore, when using the fan, full attention must be paid.
Maintenance of the Impeller
During the initial operation of the impeller and all regular inspections, whenever there is an opportunity, it is necessary to check whether the impeller has defects such as cracks, wear, and dust accumulation.
Whenever possible, the impeller must be kept clean, and the accumulated dust, rust scale, etc. on it should be brushed off regularly with a wire brush. Because as the operation time increases, these dusts cannot adhere to the impeller evenly, which will damage the balance of the impeller and cause the vibration of the rotor.
Once the impeller has been repaired, it is necessary to perform dynamic balancing on it again. If possible, a portable dynamic balancing instrument can be used for balancing on site. Before performing dynamic balancing, it is necessary to check whether all the set bolts are tightened. Because the impeller has been operating in an unbalanced state for a period of time, these bolts may have become loose.
Maintenance of the Casing and the Air Inlet Chamber
In addition to regularly checking whether there is serious wear inside the casing and the air inlet chamber and removing the serious dust accumulation, no other special maintenance is required for these parts.
Regularly check whether all the fastening bolts are tightened. For fans with compression bolts, compress the disc springs on the feet to the installation height specified in the drawings.
Maintenance of the Bearing Part
Regularly check the oil supply of the bearing lubricating oil. If there is oil leakage in the casing, the bolts of the end cover can be tightened a little. If this still does not work, it may be necessary to replace the sealing packing with a new one.
When the bearing lubricating oil is used normally, it should be replaced at least once within half a year. For the first use, it should be replaced approximately after running for 200 hours. The second oil change should be carried out within 1 to 2 months. After that, the lubricating oil should be checked once a week. If the lubricating oil has not deteriorated, the oil change can be extended to once every 2 to 4 months. When replacing the oil, the lubricating oil of the specified brand (specified in the general drawing) must be used, and the old oil in the oil tank must be completely drained and the tank must be cleaned thoroughly before filling in the new oil.
If the fan bearing is to be replaced, the following points should be noted:
Before installing the new bearing, both the bearing and the bearing housing must be kept very clean. Heat the bearing in oil at a temperature of about 70～80°C and then install it on the shaft. Do not assemble it by force to avoid damaging the shaft.
Maintenance of Other Supporting Equipment
The maintenance of supporting equipment including motors, electric actuators, instruments, meters, etc. is described in detail in their respective operation manuals. These operation manuals are provided by the respective supporting manufacturers, and the manufacturer will randomly pack these manuals and provide them to users.
Maintenance When the Fan is Out of Use
When the fan is out of use and the ambient temperature is lower than 5°C, the remaining water in the equipment and pipelines should be drained to avoid freezing and damaging the equipment and pipelines.
Maintenance Work When the Fan is Stopped and Stored for a Long Time
(1) Apply anti-rust oil to the surfaces of the bearing and other main components to prevent rust.
(2) For the fan rotor, about every half month, manually turn the rotor half a circle (i.e., 180°) by hand. Before turning, make a mark at the shaft end so that the original top point is at the bottom after turning the rotor.
Note: The model of the fan bearing is shown in detail in the general drawing.
I. Severe Vibration of the Fan:
1. The fan shaft is not concentric with the motor shaft.
2. The stiffness of the foundation or the overall support is insufficient.
3. The impeller bolts or rivets are loose and the impeller is deformed.
4. The fit between the impeller hub hole and the shaft is loose.
5. The connecting bolts between the casing, the bearing seat and the support, and between the bearing seat and the bearing cover are loose.
6. There is dust accumulation, dirt on the blades, the blades are worn, the impeller is deformed, and the shaft is bent, causing the rotor to be unbalanced.
7. The installation of the fan inlet and outlet pipelines is poor, resulting in resonance.
II. Excessively High Bearing Temperature Rise:
1. Severe vibration of the bearing housing
2. The quality of the lubricating grease or oil is poor, deteriorated, and contains impurities such as dust, sand, and dirt, or the filling amount is inappropriate.
3. The shaft and the rolling bearing are installed obliquely, and the two bearings in the front and back are not concentric.
4. The outer ring of the rolling bearing rotates. (Friction with the bearing housing)
5. The inner ring of the rolling bearing rotates relative to the main shaft (i.e., the inner ring runs and rubs against the main shaft)
6. The rolling bearing is damaged or the shaft is bent.
7. The cooling water is too little or interrupted (for fans requiring water-cooled bearings).
8. The casing or the air inlet rubs against the impeller.
III. Excessively High Motor Current or Temperature Rise:
1. When starting, the damper in the regulating door or the outlet pipeline is not closed tightly.
2. The input voltage of the motor is low or there is a single-phase power outage in the power supply.
3. The temperature of the medium conveyed by the fan is too low (i.e., the gas density is too high), causing the motor to be overloaded.
4. The system performance does not match the fan performance. The system resistance is small, and the remaining margin is large, causing the fan to operate in the area of low pressure and large flow.