The choice of blower control mode in sewage treatment process?

In order to further reduce the energy consumption of sewage treatment facilities, the principle and characteristics of the single-stage high-speed centrifugal blower frequency conversion and guide vane adjustment air volume are analyzed. According to the requirements of the sewage treatment process on the air pressure and air volume of the fan, it is proposed that the air blower should adopt the imported guide vane method to adjust the air volume.

In the urban sewage treatment process, the activated sludge method has the characteristics of low investment, high treatment efficiency, and mature operating experience, so it is widely used. The aeration system usually uses blast aeration. In actual operation, the sewage water quality, water volume, environment and other factors are always changing. The aeration system should be able to adjust the air supply in time according to the changes in the dissolved oxygen content of the aeration tank to ensure the treatment effect and not waste energy. Therefore, in the project design stage, both the owner and the design unit attach great importance to the selection of blowers and the selection of control methods.

In the preliminary design of the sewage treatment plant project, the air blowing equipment adopts a single-stage high-speed centrifugal fan, and the scheme of frequency conversion control to adjust the air volume. In order to meet the requirements of the sewage treatment process, maximize energy conservation and reduce construction investment, after inspection and several experts have been organized to conduct technical and economic analysis and demonstration, it is believed that the imported guide vane is a blower for the characteristics of the sewage treatment blast and aeration process of this project. Reasonable control method.

Analysis and selection of centrifugal fan control mode

Centrifugal fan is currently the most widely used fan and the main object of fan energy saving. It is learned from the investigation that the main problem existing in the operation of wind turbines is serious energy waste. According to the statistics of relevant national departments, the electricity consumption of fans and pumps accounts for about 40% of the total electricity consumption in the country[1]. The main reason for the large energy consumption of fans is that the fans in operation adopt a large number of adjustment methods such as baffles and valves. Although this method is simple and easy to implement, it will produce a lot of energy loss during the adjustment process. Therefore, the blower that needs to adjust the air volume frequently in the sewage treatment project should choose the appropriate adjustment method to reduce the energy consumption.
The working principle and characteristics of

1. The working principle and characteristics of centrifugal fan

The working principle of a single-stage high-speed centrifugal fan is: the prime mover drives the impeller to rotate at a high speed through the shaft, and the airflow enters the high-speed rotating impeller from the inlet axis and becomes a radial flow which is accelerated, and then enters the diffuser cavity, changing the flow direction and decelerating. This deceleration effect converts the kinetic energy in the high-speed rotating airflow into pressure energy (potential energy), so that the outlet of the fan maintains a stable pressure.

Theoretically, the pressure-flow characteristic curve of a centrifugal blower is a straight line, but due to losses such as frictional resistance inside the fan, the actual pressure and flow characteristic curve decreases gently with the increase in flow rate. The corresponding centrifugal fan’s power- The flow curve rises as the flow rate increases. When the fan is running 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 resistance of the pipe network increases, the pipeline performance curve will become steeper.

The basic principle of fan adjustment is to obtain the required working conditions by changing the performance curve of the fan itself or the characteristic curve of the external pipe network.

2. Frequency conversion control principle and characteristics

With the continuous development of technology, AC motor speed control technology is widely used. Through a new generation of fully-controlled electronic components, the use of a frequency converter to change the speed of the AC motor to control the flow of the fan can greatly reduce the energy loss caused by the previous mechanical control of the flow.
The energy-saving principle of frequency conversion adjustment: frequency conversion adjustment is an efficient adjustment method. The blower adopts frequency conversion adjustment, which will not produce additional pressure loss, and has a significant energy saving effect. The adjustment range of air volume is 0% to 100%, which is suitable for occasions with a wide adjustment range and often running under low load. However, when the fan speed drops and the air volume decreases, the wind pressure will change greatly. According to the fan ratio law:
Q1/Q2=(n1/n2),  H1/H2=(n1/n2)2,  P1/P2=(n1/n2)3
It can be seen that when the speed is reduced to half of the original rated speed, the flow, pressure, and shaft power of the corresponding operating point will be reduced to 1/2, 1/4, and 1/8 of the original. This is that the frequency conversion adjustment method can be greatly improved. Reasons for power saving.
According to the characteristics of frequency conversion adjustment, in the sewage treatment process, the aeration tank always maintains the normal level of 5 m (see Figure 1), and the blower is required to perform a wide range of flow adjustment under the condition of constant outlet pressure. When it is larger, the wind pressure will drop too much, which cannot meet the process requirements. When the adjustment depth is small, its energy-saving advantage is not shown, but the device is complicated and the one-time investment is increased (in this project, the use of frequency conversion adjustment for the blower increases the one-time investment by 200,000 yuan compared with the guide vane adjustment). Therefore, it is obviously inappropriate to adopt the frequency conversion adjustment method under the working condition that the aeration tank of this project needs to maintain a liquid level of 5m.

3. Inlet guide vane adjustment principle and characteristics

Inlet guide vane adjusting device is to install a set of adjustable turning angle guide vanes near the suction inlet of the blower-the inlet guide vane, its function is to make the airflow rotate before entering the impeller, resulting in twisting speed. The guide vane can rotate around its own axis, and each angle of the blade rotation means that the installation angle of the guide vane is changed, so that the direction of the airflow entering the fan wheel is changed accordingly.
The principle of inlet guide vane regulating air volume is: when the guide vane installation angle θ=0°, the guide vane basically has no effect on the inlet air flow, and the air flow will flow into the impeller blade in the radial direction. When θ>0°, the inlet guide vane will deflect the absolute speed of the air inlet along the circumferential speed by angle θ, and at the same time have a certain throttling effect on the speed of the air inlet. This pre-rotation and throttling effect will lead to the performance of the fan The curve drops, so that the operating conditions are changed, and the fan flow adjustment is realized. The depth of the aeration tank is fixed. The blower adjusts the flow under the condition of keeping the outlet pressure constant, that is, when H=constant and Q=variable, the characteristic curve of the pipe network is similar to a horizontal straight line. The blower adopts inlet guide vanes to adjust without resorting To change the characteristic curve of the pipe network, the pressure-flow performance curve of the fan can be changed by changing the opening and closing angle of the guide vane. The flow change is achieved by moving the operating point to the new changed fan characteristic curve. of
The centrifugal fan adopts the inlet guide vane adjustment method, which can obtain high efficiency and a wide performance range under partial load operation. Under the condition of keeping the outlet pressure constant, the working flow can be changed within the range of 50% to 100% of the rated flow . The greater the adjustment depth, the more energy-saving. If the flow is reduced to 60% of the rated flow, the inlet guide vane method saves power by up to 17% compared with the inlet throttling method. In addition, it has a relatively simple structure, reliable operation, convenient maintenance and management, and low initial investment.

4. Comparison of different control methods

Shows the relationship between air volume and shaft power in different control modes. Although the centrifugal blower adjusted by frequency conversion has a wide adjustment range and has significant effects on energy saving, the process system used in this project will be restricted by process conditions, and the adjustment range is only 80% to 100%. When the relative flow rate changes little , The power consumption difference between the frequency conversion and the guide vane adjustment methods is not big. Therefore, the frequency conversion adjustment method is adopted, and its energy-saving characteristics cannot be displayed, which loses the meaning of choosing it. The blower that chooses the guide vane adjustment method can adjust the air volume (50% to 100%) in a large range under the condition of keeping the outlet pressure constant to ensure the stable dissolved oxygen content in the sewage and relatively save energy. Therefore, a high-speed centrifugal fan with a 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 10 kV high-voltage motors will also help reduce energy consumption.

Conclusion

Through the analysis of the principles and characteristics of the frequency conversion and guide vane adjustment methods, it is clear that in the sewage treatment process using blast aeration, the selection of the blower control method should not only consider energy saving, but must meet the requirements of the aeration process for air volume, Under the premise of wind pressure requirements, comprehensively consider the range of flow rate, fan power, technical complexity, reliability and investment of the regulating device, carry out technical and economic analysis, and make a reasonable choice

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