Fan Fan

It is defined as a fan with a lower air pressure. 1,000 (mm-water) is called a fan. The fan with air pressure ranging from 1,000 (mm-water) up to but not reaching 10 (m-water) (0.1 MPa) is called a blower (blower). Both types are collectively called fans.

There are many types of fans. according to the flow rate and pressure of the fluid being transported And according to the purpose of use as shown in the table, 1 can be divided into major types: centrifugal, which works by providing centrifugal force to create current in the direction perpendicular to the axis, axial flow, which creates a fluid stream (air) in the same direction as the shaft, cross flow, which has properties between the two above, and other types. However, in order to be able to install and connect to various pipes easily, centrifugal fans sometimes look similar to axial flow from the outside. In general, axial flow fans are suitable for low pressure - high flow rates. Centrifugal is suitable for high pressure.

Table 1 Types of Fans

However, axial flow fans that can handle a reasonable amount of pressure and centrifugal fans that can handle a reasonable flow rate are available. Multi-blade fans, sometimes called sirocco fans, are most commonly used in air conditioning and ventilation applications.

Classification of fans can be divided into major categories. According to the characteristics of air movement, 2 is as follows.

(1) Centrifugal flow or radial fans

(2) Axial flow fans

(1) Centrifugal flow or radial fans

Centrifugal fans or fans which have radial air flow It consists of rotating blades inside the fan house. The blade set consists of small blades assembled together to form a wheel. The air pressure is raised inside the fan housing. This value can be increased by increasing the length of the propeller. This will cause the centrifugal force within the system to increase. Air flows into the inlet pipe in a direction parallel to the axis of the impeller. and flows out in the direction perpendicular to the axis of the propeller shaft in the outlet pipe. This type of fan is classified according to the shape of the blades as 3, which is

(1.1) Straight blade or Radial fans

This type of fan has a minimum number of blades of approximately 6 to 20. The blades and blades are located in the radial plane from the shaft. The impeller rotates at a minimum speed of approximately 500-3000 rpm, so it is suitable for applications that require a small flow volume. and has a high air pressure value

(1.2) Forward curved blade fans

This type of fan has forward curved blades. In the same direction as the rotor blades rotate, there will be approximately 20 – 60 blades. The blades are similar to a squirrel cage. The blade shaft is small and rotates at a higher rotational speed than a straight blade fan. The operation of this type of fan is the quietest. The disadvantages are that there is a possibility of motor overload and unstable fan operation. Therefore, they should not be used in applications or systems with constantly changing air flow rates. This type of fan will provide the highest air pressure and air flow rate.

(1.3) Backward curved blade fans

This type of fan has blades that tilt backward. In the direction opposite to the direction of rotation of the impeller. There will be approximately 10 –50 blades and it is a fan with a high rotational speed. Does not cause excessive noise. There is no appearance of the motor running over capacity. and there is no unstable working period. Suitable for use with ventilation and the air used must be clean. Because the pressure and air volume can be easily controlled. This type of fan will be more expensive than other types of the same size.

Figure 1 shows the air flow through a centrifugal fan housing.

Figure 2 shows a forward curved blade type centrifugal fan.

Figure 3 shows a backward curved blade type centrifugal fan.

(2) Axial flow fans

In this type of fan, air flows parallel to the axis of the blades. and perpendicular to the plane of rotation of the propeller. The propeller assembly is mounted on the drive shaft of the motor. which is inside the fan Allows the motor to dissipate heat with the air being driven. This type of fan is cheap. The fan operation is noisy and the fan operation period is unstable. Therefore suitable for ventilation work, small size, easy to move. Can be divided into 2. Characteristics are:

(2.1) Fans that produce spiral wind (Tube axial fans)

(2.2) Fans that let air flow in a straight line (Vane axial fans)

(2.1) Fans that produce spiral wind (Tube axial fans)

This type of axial flow fan It has a structure consisting of a set of propellers which rotate inside a cylindrical tube. The air that is driven through the blades spins in a spiral. It has a turbulent flow characteristic. This type of fan provides moderate air pressure.

Figure 4 shows a tube axial fan.

(2.2) Fans that produce air in a straight line (Vane axial fans)

This type of axial flow fan There will be fins to control the air flow. driven It is installed inside the fan housing. The outlet pipe area is behind the propeller set. To help the flow of air that is driven It has the most linear direction. This reduces the turbulent air flow characteristics. and reduce energy loss due to turbulent air flow within the system. This makes the efficiency and price higher than tube axial fans.

Figure 5 shows Vane axial fans.

2. Fan features and performance
(CHARACTERISTICS AND PERFORMANCE OF FAN)

while the fan is working It will cause the air to move with the difference in pressure that occurs. As air moves outward, the distance increases. will cause the pressure to decrease If the pressure values ​​in various ranges are taken Let's draw a graph comparing the air flow rate obtained in that pressure range. If the pressure value is the total pressure value of the system When the total pressure drop of the system is subtracted from the velocity pressure value. Another graph is obtained which shows the static pressure of the system. We can use this graph to select the appropriate operating point for that type of fan.

Figure 6 shows finding the optimum fan operating point from the characteristic curve.
of a 27-inch backward-curved centrifugal fan system at 1,080 rpm

The airflow specified by a fan manufacturer is normally obtained under standard test conditions, such as 15 °C, ambient atmospheric pressure of 1 bar, and an elevation equivalent to mean sea level. Actual installation conditions may differ, so operating performance may not match those specifications.

Fan performance changes with environmental conditions such as temperature, rotational speed and air density. Fan testing standards have therefore been developed, and tests have established the following performance curves for different fan types.

• • Forward-curved centrifugal fans (Forward curved blade fans)

Figure 7 shows the performance of a forward-curved centrifugal fan

From the picture 7 shows that When the percentage of open volume is higher, the power input to the shaft increases. of the fan has a higher value as well. This causes the fan motor to overload while the resistance of the system is reduced. Therefore, this type of fan should not be used in systems with constantly changing air flow rates. The optimum range for operation of this type of fan is The open volume percentage range is approximately 30 – 50 %, which will allow the fan to operate at its maximum efficiency. The static pressure curve has an unstable operating range for the fan, i.e., the percentage of open volume does not exceed 40 %. Therefore, the open volume for air entering the fan housing should not be used during this range.

• • Backward curved blade fans

 Figure 8 shows the performance graph of a backward curved blade centrifugal fan.

From Figure 8 it can be seen that the appropriate range. For the operation of this type of fan, the percentage range of the volume that is open for air to enter the fan housing is approximately 50 – 65 %, which will make the operation of this type of fan most efficient. Fan efficiency is also highest when the power required to drive the fan shaft is high. This type of fan does not have the characteristic of the motor overloading and there is no period of unstable fan operation.

• • Straight curved blade fans

Performance graph of this type of fan It is similar to the performance curve of a forward curved blade fan. That is, the fan's power curve will become higher and higher. Even though the pressure of the system is reduced But the speed of air flowing through this type of fan housing is lower than that of a forward curved blade fan.

• • Axial flow fans

Figure 9 shows the performance graph of an axial air fan.

From Figure 9, it can be seen that the static head and total head curves of this type of fan decrease and increase. In the range of percentage of volume open for air to enter the fan housing, the value is approximately 30 – 50 %. If this type of fan is operated in this range, it will cause instability within the system. And the appropriate range for the fan's operation is The open volume percentage range is approximately 55 – 75 %, which will allow the fan to operate with maximum efficiency. Can drive large amounts of air and using not too much power to drive the sled The fan operation curve is relatively flat. Especially within the operating range where the percentage of open volume is approximately 40 %, that is, the power used to drive the fan within that range is relatively constant.

Variation law and similarity law for fans and pumps

The fluid flow rate is proportional to the fan speed. Pressure loss in air ducts and water pipes connected to the fan. It varies according to the square of the speed of the flow (speed), that is, it varies according to the square of the flow rate. Therefore, if the rotational speed changes The pressure is proportional to the square of the rotational speed. and the shaft drive power is proportional to the third power of the rotational speed. This relationship is called the law of variation. This can be expressed with the following formula.

In the above formula, V1, V2 represents flow rate, n1, n2 represents rotational speed, P1, P2 represents pressure, W1, W2 represents shaft drive power.

Additionally, when running similar fans under similar conditions (i.e. peak efficiency) and assuming the fan efficiency is the same, The flow within all pumps is similar. The relationship between diameter D, rotational speed n and flow rate, pressure and shaft power can be calculated using the following formula, which is called the fan similarity law.

The law of variation and the law of similarity tell us that if we have a fan that is too powerful (It's too big.) It's not just about reducing the size of the air pipe to create more resistance. But if we change the size of the fan (propeller) or the rotational speed It can also effectively reduce energy use. This method is an important energy conservation strategy in selecting or modifying fluid-operated equipment.

3. Fan operation control (OPERATION CONTROL OF FAN)

The system for adjusting the fan flow rate to suit the usage load is called a variable air volume system (VAV: Variable Air Volume).
How to control variable flow rates

1. Output damper-inlet valve control.
2. Speed control
3. rotor blade control
4. Stationary blade control
5. Controlling the number of machines (In the case of running the machine in parallel or in series)
6. Bypass control

Figure 10 Several flow rate control methods.

(1) How to choose a fan drive system?

In the case where there is no need to adjust the air volume Running the machine at a constant speed is sufficient. which generally uses a 3 single phase squirrel cage induction motor. For medium and large fans It uses a synchronous motor that has a higher power factor and efficiency than an induction motor. In this case, there is only a problem at startup. In the figure 11 shows the speed-torque characteristics. Under the condition that the inlet valve is closed But the outlet valve and the bypass valve are open. But in an axial flow fan, the variable blade angle is used. From this picture, it can be seen that the torque at startup is not a problem.

Turbo fans generally produce a large amount of air. And the diameter of the propeller is also large. Therefore, compared to other loads, the moment of inertia will be much higher. If starting the 3 squirrel cage phase induction motor by means of supplying full voltage (full-voltage starting) Finally, heat equivalent to the driving energy stored in the moment of inertia is generated in the rotor windings. Therefore, it is necessary to consider whether an increase in the temperature of the squirrel-cage windings will be a problem.
In the event that a large amount of starting current cannot be supplied Or in the case of having to start the machine frequently It uses a rotor-wound induction motor. But if you run a squirrel cage induction motor with variable speed using an inverter. It will make the problem during starting the machine go away. It is also very effective in conserving energy. Therefore, in the latter Therefore, variable speed drive using an inverter is used to start the machine. and in normal operation Will switch to using electricity from the electricity authority to distribute for further use.

To adjust the air volume This is achieved through mechanical means such as variable pitch airfoil systems and through speed control. Using the mechanism is cheap but if used over a wide range it will cause the air delivery efficiency to decrease in the later stages. From having a goal of saving electricity As a result, high-quality inverter-based control systems are used in various types of ventilation machines, dust collectors, and tunnel ventilation machines.
Driving medium and large fans and blowers at variable speeds with alternating current. It was used to good effect during the oil crisis in the 1970 era by initiating the use of thyristor inverters. Today, inverters that are low in price, small in size but have high capabilities are It has been produced through developments in power electronics and variable speed operation that takes advantage of the advantages of squirrel cage induction motors. It tends to become widespread quickly. Starting with the fan and blower. medium and large sizes, which are efficient in energy conservation Tall to small

Figure 11 Speed-Torque Characteristics Under the condition that the inlet valve is closed Then open the outlet valve and bypass valve.

4. Guidelines for energy conservation in fans
(GUIDELINE FOR ENERGY CONSERVATION IN FAN)

Points that need to be checked for energy conservation in fan systems are as follows

- Is the fan pumping out more air than is required?
- Is the fan running at high efficiency?
- In the case of a change in the air volume Does the fan maintain good performance even at low air volumes?
- Is the air volume control method highly efficient and consistent with the factory's needs?
- Regarding the resistance of the pipe Is the wind speed too high? Is there unnecessary resistance, such as too much bent pipe?

Important points in energy conservation in fans

1. Reducing the amount of air - pressure to suit what the load requires.
2. Controlling the operation of the engine to suit the change in the amount required by the load.

Problems specific to the fan include:

1. Choosing a fan with high performance and characteristics suitable for use.
2. There is no dust accumulation on the hull and blades that reduces the air flow.
3. Reduce leakage from Labyrinth seal of shaft seal and balance disk.

(1) Energy conservation project uses fans in line with the load.

(1.1) When the required air volume is reduced and there is little change.

If you plan for too much and want to reduce the amount of wind due to a reduction in production capacity If using the output damper to adjust the engine, there will be a high loss of power output. The solution in this case is to reduce the diameter of the propeller and change to a propeller with a smaller size. Reduce the number of floors in multi-level blowers. Adjust the blade angle of the axial flow fan.

(1.2) When the required air volume changes

- Dim the output damper. (The output power shifts only along the shaft output power curve.)
- Inlet vane control (output power consumption is lower than damper control)
- Control the number of machines (In the case of running several fans in parallel If the number of machines running is reduced in accordance with the amount of wind will make each fan less efficient)
- Control the variable pitch of the moving blade of the axial flow fan (highly efficient control over a wide air volume range)
- Control rotational speed

(2) Method of analysis for energy conservation.

(2.1) Check the performance curve – In the case of an older fan, check to see if there is a fan with higher performance.
(2.2) In the case where the fan's capacity is too high Are there any dampers or valves in the pipe?
(2.3) Are there curved pipes before and after the fan that increases pressure loss?
(2.4) Is the air velocity in the pipe too high causing the resistance of the pipe to be high?
(2.5) Is there air leaking from the pipe or flange?
(2.6) The air filter is clogged. Is there dust in the ducts, in the hull, and in the blades?
(2.7) Is there any waste, such as supplying air when it's not needed? Run the engine while the exit door is closed, etc.

(3) Energy saving by controlling fan speed.

To adjust the amount of air and compression required, originally the method was to adjust the damper. But nowadays, speed control is often used instead of dampers. To control the air volume and pressure This allows for substantial conservation of energy. The figure 12 is a figure that represents the above principle. In the case of using the method of adjusting the damper to make it narrower It will affect the wind resistance curve. Working point of the fan Which intersection point will be moved to while using the speed change method? The air delivery resistance curve remains unchanged. The fan operating point is shifted to the intersection between the desired flow rate and the resistance curve. These principles It is the same as in the case of pumps. But for that pump Because it's a real head Therefore there is a minimum speed point. But in a fan there is no minimum speed point. Resulting in high efficiency in energy conservation.

Figure 12 compares damper control with speed control for adjusting the flow rate.