How centrifugal blowers work
Centrifugal blowers work similarly to centrifugal fans, except that the air compression process is usually carried out by several working impellers (or several stages) under centrifugal force. The blower has a rotor that rotates at a high speed. The blades on the rotor drive the high-speed movement of the air. The centrifugal force causes the air to flow in the involute-shaped casing along the involute to the fan outlet, and the high-speed airflow has a certain wind pressure. The new air is replenished by the center of the casing.
The working principle of the single-stage high-speed centrifugal fan is that the prime mover drives the impeller to rotate at a high speed through the shaft. The airflow enters the high-speed rotating impeller and becomes radial flow, which is accelerated, then enters the diffusing chamber and changes the flow direction to decelerate. This deceleration converts the kinetic energy of the high-speed rotating airflow into pressure energy (potential energy), which maintains a stable pressure at the fan outlet.
Theoretically, the pressure-flow characteristic curve of the centrifugal blower is a straight line, but due to the frictional resistance loss inside the fan, the actual pressure and flow characteristic curve gradually decreases with the increase of the flow rate, and the power of the corresponding centrifugal fan - 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 is not only dependent on its own performance, but also depends on the characteristics of the system. When the pipe network resistance increases, the pipeline performance curve will become steeper. The basic principle of fan adjustment is to obtain the required working condition by changing the performance curve of the fan itself or the external pipe network characteristic curve.
Frequency control principle and characteristics
With the continuous development of technology, AC motor speed control technology is widely used. Through the new generation of fully-controlled electronic components, the inverter can change the speed of the AC motor to control the fan flow, which can greatly reduce the energy loss caused by the mechanical regulation of the flow. Energy-saving principle of frequency conversion adjustment:
When the air volume needs to be reduced from Q1 to Q2, if the throttle adjustment method is adopted, the operating point is from A to B, the wind pressure is increased to H2, and the shaft power P2 is decreased, but the reduction is not too much. If the frequency conversion adjustment mode is adopted, the operating point of the fan is from A to C. It can be seen that under the condition of satisfying the same air volume Q2, the wind pressure H3 will drop greatly, and the power P3 will decrease significantly. The power loss saved ΔP = ΔHQ2 is proportional to the area BH2H3C. From the above analysis, the frequency conversion adjustment is an efficient adjustment method. The air blower adopts frequency conversion regulation, which does not generate additional pressure loss, and has remarkable energy saving effect. The air volume range is adjusted from 0% to 100%, which is suitable for the wide adjustment range and often under low load operation. However, when the fan speed decreases and the air volume decreases, the wind pressure will change greatly. The fan proportional law: Q1/Q2=(n1/n2), H1/H2=(n1/n2)2, P1/P2=( N1/n2)3
Blower
It can be seen that when the rotation speed is reduced to half of the original rated speed, the flow rate, pressure and shaft power of the corresponding operating point are reduced to 1/2, 1/4, and 1/8 of the original. The reason for saving electricity. According to the characteristics of frequency conversion regulation, for the sewage treatment process, the aeration tank always maintains a normal liquid level of 5m, and the blower is required to perform a wide range of flow adjustment under the condition of constant outlet pressure. When the adjustment depth is large, The wind pressure is too large to meet the process requirements. When the adjustment depth is small, the advantage of energy saving is not displayed, but the device is complicated and the one-time investment is increased. Therefore, under the condition that the aeration tank of this project needs to maintain a liquid level of 5m, it is obviously inappropriate to adopt the frequency conversion adjustment method.
Imported guide vane adjustment principle and characteristics
The inlet guide vane adjusting device is provided with a set of guide vanes-introducing guide vanes with adjustable angles near the suction inlet of the blower, and the function is to rotate the airflow before entering the impeller, resulting in a twisting speed. The vane can be rotated about its own axis. Each angle of rotation of the vane means that a vane mounting angle is changed, so that the direction of the airflow entering the fan impeller changes accordingly.
Imported guide vane to adjust air volume principle
When the vane mounting angle θ = 0°, the vanes have substantially no effect on the inlet gas flow, and the gas flow will flow radially into the impeller blades. When θ>0°, the inlet guide vanes will deflect the absolute velocity of the airflow inlet in the circumferential speed direction by an angle θ, and at the same time have a certain throttling effect on the velocity of the airflow inlet. This pre-rotation and throttling will result in fan performance. The curve is lowered, so that the operating point changes, and the fan flow adjustment is realized. Energy-saving principle of inlet guide vane adjustment.
When the inlet guide vane installation angle is increased from θ1=0° to θ2 or θ3, the operating condition point is moved from M1 to M2 or M3; the flow rate is reduced from Q1 to Q2 or Q3; the shaft power is reduced from P'1 to P '2 or P'3. The area indicated by the hatching is the power saved by the inlet guide vanes compared to the throttling adjustment. In this project, the depth of the aeration tank is fixed, and the blower performs flow adjustment under the condition that the outlet pressure is kept constant, that is, H=constant, when Q=variable, the characteristic curve of the pipe network approximates a horizontal straight line, and the blower adopts the inlet guide. The leaf adjustment, without changing the pipe network characteristic curve, can change the pressure-flow performance curve of the fan by changing the opening and closing angle of the vane, and the flow rate change is by moving the operating point to the new changed fan characteristic. The method on the curve is implemented.
The centrifugal fan adopts the inlet guide vane adjustment mode, which can obtain high efficiency and wide performance range during 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 rated flow. The greater the adjustment depth, the more labor. If the flow rate is reduced to 60% of the rated flow, the inlet vane mode saves up to 17% more power than the inlet throttling method. In addition, its structure is relatively simple, reliable operation, convenient maintenance and management, and low initial investment. Therefore, the blower in this project uses the inlet guide vanes to regulate the flow, which is obviously the best adjustment method.
Comparison of different control methods
Although the variable frequency centrifugal blower has a wide adjustment range and has significant effect on energy saving, the process system will be limited by the process conditions, and the adjustment range is only 80% to 100%. When the relative flow does not change much, the frequency conversion and the guide The difference in power consumption between the two adjustment methods is not large. Therefore, the frequency conversion adjustment method is adopted, and the energy-saving characteristic is not displayed, which loses the meaning of selecting it. The blower that selects the vane adjustment mode can adjust the air volume (50%-100%) in a large range while maintaining the constant outlet pressure to ensure stable dissolved oxygen content in the sewage and relatively save energy. Therefore, the high-speed centrifugal fan with guide vane adjustment mode 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 choice of supporting motors, such as the use of 10kV high-voltage motors, which also helps to reduce energy consumption.
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