How to select aeration fan for sewage treatment?

Several problems should be paid attention to in the selection of sewage treatment aeration fan:

1. Calculate parameters according to the actual situation

The sewage treatment aeration fan is selected in the sewage plant. The manufacturer's product samples provide performance parameters in the standard air inlet state, but the fan is not in the standard state when it is actually used. When the environmental conditions of the fan (such as temperature, atmospheric pressure, altitude, etc.) are different, the performance of the fan will also change. The performance parameters of the product samples cannot be directly used in design selection. On the contrary, according to the actual use state, the performance requirements of the fan must be converted to the fan in the standard air intake state.

2. Analysis of influencing factors of outlet pressure

The exhaust pressure of the volume blower is not determined by the blower itself, but by the device after the gas is discharged from the blower, namely the so-called "back pressure". The sewage treatment aeration fan has the characteristics of forced gas transmission.

The exhaust pressure shown on the nameplate of the blower is the rated exhaust pressure of the blower. In fact, the blower can work at all pressures lower than the rated exhaust pressure. As long as the strength and exhaust temperature allow, the rated exhaust pressure can be exceeded.

In the case of the sewage treatment plant, the absolute pressure (back pressure) generated by the exhaust system is the sum of the pressure loss value of the pipeline system, the depth of the aeration tank and the ambient atmospheric pressure. For some reasons, for example, the pressure loss of the pipeline system increases and the back pressure rises due to the blockage of the aeration head or the pipeline, and the pressure of the fan will also increase accordingly. In addition, the pressure loss of the pipeline system is reduced, the back pressure is continuously reduced, and the pressure of the fan is also reduced due to reasons such as the rupture of the aeration head or pipeline leakage.

In conclusion, when determining the pressure of the sewage treatment aeration fan, as long as the fan can reach the pressure under the standard state, it is equal to the sum of the atmospheric pressure under the use state, the depth of the aeration tank and the pipeline loss.

3. Blower airflow factor

When calculating the oxygen demand for sewage treatment, the result is that the mass flow of oxygen required under standard conditions qm (kg/min) is converted to the volume flow of air required under standard conditions qv1 (m3/min). If the use status of the blower is not standard (such as in plateau areas), the air density and water content will also change.

Therefore, it is necessary to calculate the volume flow that can provide the same mass flow, that is, the conversion flow qv2. When used in plateau areas, the ambient atmospheric pressure will also change, and the pressure will increase accordingly, the leakage flow qvb of the blower will increase, the air volume flow supplied by the blower will decrease, and the oxygen supply will also be insufficient.

Therefore, in the design, when the use conditions change, taking into account the influence of various factors, the actual air flow supplied by the fan must meet the use requirements, and calculate the conversion flow qv2 and outflow flow qvb2.

4. Pay attention to the difference between winter and summer

The fan selection shall pay attention to the variation law of fan supply flow. That is to say, the volume flow of the same blower is unchanged in winter and summer, but the flow depends on the quality of air density, that is, the oxygen supply will be different.

Under the standard state and use state, the volume flow of the blower remains unchanged, but the air density and water content change. When the temperature decreases in winter, the oxygen supply from the blower to the air pool increases, and when the temperature increases in summer, it decreases. For example, a sewage treatment plant uses a blower in the above calculation example to calculate the actual oxygen supply of the blower according to the change of the ambient temperature. The change rule of the year is that in the actual operation process, the system oxygen demand also changes in summer due to the changes of the parameters such as the inlet water, water quality, and water temperature. The water temperature in winter decreases, and the oxygen demand of the aeration tank decreases. This is because the temperature drops in winter, the air density increases, the dry air quality supplied by the fan is larger than the standard state, and the oxygen supply increases. From the actual measurement of operation, the dissolved oxygen in the exposure pool in winter is 1~3 mg/L higher than that in summer.

Therefore, in the process of production and operation, the equipment must be adjusted in time for these changes so that the oxygen filling capacity of the fan can meet the oxygen demand in actual operation. For sewage treatment aeration fan, it is economical and practical to use frequency converter to change the fan speed to adjust the air supply flow.