The working principle of a centrifugal blower is similar to that of a centrifugal ventilator, except that the air compression process usually passes through several working impellers (or stages) under the action of centrifugal force.

      The blower has a high-speed rotating rotor. The blades on the rotor drive the air to move at high speed. The centrifugal force makes the air flow along the involute line in the involute-shaped casing to the blower outlet. The high-speed airflow has a certain wind pressure. New air enters from the center of the casing to replenish.

      The working principle of a single-stage high-speed centrifugal fan is: the prime mover drives the impeller to rotate at high speed through the shaft. The airflow enters the high-speed rotating impeller axially from the inlet and becomes radial flow and is accelerated. Then it enters the diffusion chamber, changes the flow direction and slows down. This deceleration effect converts the kinetic energy in the high-speed rotating airflow into pressure energy (potential energy), so that the fan outlet maintains a stable pressure.

      Theoretically, the pressure-flow characteristic curve of a centrifugal blower is a straight line. However, due to the friction resistance and other losses inside the fan, the actual pressure-flow characteristic curve decreases gently with the increase of flow, and the corresponding power-flow curve of the centrifugal fan increases with the increase of flow. When the fan runs at a constant speed, the operating point of the fan will move along the pressure-flow characteristic curve. The operating point of the fan during operation depends not only on its own performance, but also on the characteristics of the system. When the pipe network resistance increases, the pipeline performance curve will become steeper. The basic principle of fan regulation is to obtain the desired operating conditions by changing the performance curve of the fan itself or the external pipe network characteristic curve.

Principle and characteristics of frequency conversion control:

      With the continuous development of science and technology, AC motor speed regulation technology has been widely adopted. Through the new generation of fully controlled electronic components, the frequency converter is used to change the speed of the AC motor to control the fan flow, which can greatly reduce the energy loss caused by the previous mechanical flow control. Energy-saving principle of frequency conversion regulation:

      When the air volume needs to be reduced from Q1 to Q2, if the throttling regulation method is used, the operating point changes from A to B, the wind pressure increases to H2, and the shaft power P2 decreases, but the reduction is not too much. If the variable frequency regulation method is used, the fan operating point changes from A to C. It can be seen that under the condition of meeting the same air volume Q2, the wind pressure H3 will drop significantly, and the power P3 will decrease significantly. The saved power loss △P＝△HQ2 is proportional to the area BH2H3C. From the above analysis, it can be seen that variable frequency regulation is an efficient regulation method. The blower adopts variable frequency regulation, which will not produce additional pressure loss, and the energy-saving effect is significant. The air volume adjustment range is 0% to 100%, which is suitable for occasions with a wide adjustment range and often operating under low load. However, when the fan speed decreases and the air volume decreases, the wind pressure will change greatly. According to the fan proportional law: Q1/Q2＝(n1/n2), H1/H2＝(n1/n2)2, P1/P2＝(n1/n2)3

      The shaft power drops to 1/2, 1/4, and 1/8 of the original, which is why the frequency conversion regulation method can save a lot of electricity. According to the characteristics of frequency conversion regulation, in the sewage treatment process, the aeration tank always maintains a normal liquid level of 5m, and the blower is required to adjust the flow rate in a large range under the condition of constant outlet pressure. When the adjustment depth is large, the wind pressure will drop too much and cannot meet the process requirements. When the adjustment depth is small, its energy-saving advantage cannot be shown, but the device is complicated and the one-time investment is increased. Therefore, under the working condition that the aeration tank of this project needs to maintain a liquid level of 5m, it is obviously inappropriate to use the frequency conversion regulation method.

Inlet guide vane adjustment principle and characteristics:

      The inlet guide vane adjustment device is a set of guide vanes with adjustable angles installed near the air inlet of the blower - the inlet guide vanes, which is used to rotate the airflow before entering the impeller, causing a twisting speed. The guide vanes can rotate around their own axes. Each rotation of the blades means changing the installation angle of the guide vanes, so that the direction of the airflow entering the fan impeller changes accordingly.

The principle of air volume adjustment by inlet guide vanes is:

      When the guide vane installation angle θ＝0°, the guide vane has basically no effect on the inlet airflow, and the airflow will flow into the impeller blades radially. When θ＞0°, the inlet guide vane will deflect the absolute velocity of the airflow inlet by an angle θ in the direction of the circumferential velocity, and at the same time have a certain throttling effect on the velocity of the airflow inlet. This pre-swirl and throttling effect will cause the fan performance curve to drop, thereby changing the operating condition point and realizing fan flow regulation. Energy-saving principle of inlet guide vane regulation.

When the installation angle of the inlet guide vane increases from θ1=0° to θ2 or θ3, the operating point moves from M1 to M2 or M3; the flow rate decreases from Q1 to Q2 or Q3; and the shaft power decreases from P′1 to P′2 or P′3. The area represented by the section line is the power saved by the inlet guide vane compared to throttling regulation. In this project, the depth of the aeration tank is fixed. The blower regulates the flow rate while maintaining a constant outlet pressure, that is, when H=constant and Q=variable, the characteristic curve of the pipe network is approximately a horizontal straight line. The blower uses inlet guide vane regulation, and there is no need to change the pipe network characteristic curve. The pressure-flow performance curve of the fan can be changed by changing the opening and closing angle of the guide vane. The change in flow is achieved by moving the operating point to the new changed fan characteristic curve.

      The centrifugal fan adopts the inlet guide vane adjustment method, which can achieve high efficiency and a wide performance range when running at partial load. Under the condition of keeping the outlet pressure constant, the working flow rate can vary within the range of 50% to 100% of the rated flow rate. The greater the adjustment depth, the more power is saved. If the flow rate is reduced to 60% of the rated flow rate, the inlet guide vane method saves as much as 17% of power compared with the inlet throttling method. In addition, its structure is relatively simple, the operation is reliable, the maintenance and management are convenient, and the initial investment is low. Therefore, the blower in this project adopts the inlet guide vane to adjust the flow rate, which is obviously the best adjustment method.

Comparison of different control methods:

      Although the frequency conversion adjustment of the centrifugal blower has a wide adjustment range and has a significant effect on energy saving, it will be limited by process conditions in the process system, and the adjustment range is only 80% to 100%. When the relative flow rate does not change much, the power consumption of the two adjustment methods of frequency conversion and guide vanes is not much different. Therefore, the energy-saving feature of the frequency conversion adjustment method cannot be shown, which loses the meaning of choosing it. The blower with the guide vane adjustment method can adjust the air volume in a large range (50% to 100%) while keeping the outlet pressure constant to ensure the stability of the dissolved oxygen content in the sewage, which saves energy relatively. Therefore, the high-speed centrifugal fan with the guide vane adjustment method should be selected as the equipment selection for this project. At the same time, in order to better reflect the energy-saving effect, for high-power centrifugal fans, attention should also be paid to the selection of matching motors. For example, the use of 10kV high-voltage motors can also help reduce energy consumption.