Which is more suitable for sewage treatment process, centrifugal fan or roots fan?

In the sewage treatment process, the applicability of centrifugal fan and Roots fan should be comprehensively judged in combination with specific process requirements (such as aeration mode, air volume and air pressure requirements) and operating conditions (such as load fluctuation and energy consumption cost). The core differences between them are reflected in pressure characteristics, air volume adjustment mode and energy consumption adaptability. The following is a comparative analysis from key dimensions, and the applicable scenarios are defined:

I. Comparison of core performance and operation characteristics

Comparative dimension Roots blower (positive displacement blower) centrifugal blower (turbine blower)

The pressure characteristic has a wide pressure range (usually 5.88~98kPa), and the pressure has nothing to do with the air volume (as long as the motor power is sufficient, the pressure automatically adapts to the system resistance), which is suitable for high resistance working conditions (such as the aeration head is blocked and the pool body is deep). The pressure range is narrow (usually ≤58.8kPa), and the pressure is inversely proportional to the air volume (the air volume decreases significantly when the pressure increases), which is suitable for low resistance and stable working conditions.

Air volume characteristics: The air volume is basically constant (only slightly changes with the rotating speed), with the characteristics of "constant air volume and variable pressure", and the air volume is stable when the load fluctuates. Wide range of air volume adjustment (adjustable by means of frequency conversion, imported guide vanes, etc.), "variable air volume, variable pressure" characteristics, suitable for scenes with large load fluctuation.

The adjustment mode is mainly through "frequency conversion speed regulation" or "bypass ventilation":

-Frequency conversion speed regulation: good energy-saving effect (the air volume is proportional to the rotational speed, and the power is proportional to the cubic of the rotational speed);

-By-pass ventilation: high energy consumption (the fan is running at full load and the excess air volume is vented), which is only suitable for small-scale regulation. Mainly through "frequency conversion speed regulation", "inlet guide vane adjustment" and "outlet throttle adjustment":

-frequency conversion/inlet guide vane: high regulation efficiency and obvious energy consumption advantage at low load;

-Outlet throttling: large energy loss, less use.

Energy consumption adaptability: The energy consumption is better under the conditions of low air volume and high pressure; However, when running at full load, if the actual demand air volume is lower than the rated air volume, the energy consumption will be wasted obviously (such as bypass regulation). Energy consumption is better under high air volume and low pressure conditions; When running at low load (such as frequency conversion to 50% air volume), the energy consumption is only about 12.5% of the full load (cubic characteristic), and the energy saving advantage is outstanding.

The structure of maintenance and service life is simple (double rotors are engaged), wearing parts are few (mainly bearings and seals), the maintenance period is long (usually once every 3 to 6 months), and the service life is 8 to 10 years. The structure is complex (impeller, volute, bearing system), so it is necessary to clean the impeller regularly (to prevent dust accumulation and corrosion), and the maintenance frequency is slightly higher (usually once every 1~3 months), with a service life of 5~8 years.

Noise and vibration operation noise is high (85~110dB), so it is necessary to provide a sound insulation cover; Small vibration (high rotor balance accuracy). The running noise is low (70~90dB), and no additional sound insulation is needed; However, "surge" is easy to occur at low load (sudden drop of air volume and intensified vibration), so it is necessary to avoid running in the surge area.

Second, the applicable scenarios in the sewage treatment process

In sewage treatment, the fan core is used for aeration (activated sludge process, biological contact oxidation process, etc.), which should be selected according to the type of aeration tank, water depth, load fluctuation, etc.

1. Roots blower: more suitable for "high pressure and constant air volume" working conditions.

Applicable scenarios:

Deep-water aeration tank (water depth > 6m): the system resistance is large (water depth pressure+aeration head resistance needs to be overcome), and the "high pressure adaptability" of Roots blower can supply gas stably;

Intermittent aeration process (such as SBR sequencing batch reactor): constant air volume is needed in the aeration stage (to ensure the stability of DO concentration), and the "constant air volume characteristics" of Roots blower are more matched;

Small sewage treatment station (treatment capacity < 1000 m/d): the fluctuation of process load is small, and the roots blower has simple structure, low maintenance cost and easier operation;

Scenes where the aeration head is easily blocked (such as using microporous aeration disc): the system resistance will increase with the degree of blockage, and the roots blower can automatically adapt to the pressure change to avoid the sudden drop of air volume.

Not applicable scenario:

Large flow and low pressure conditions (such as shallow aeration tank, water depth < 4m): Roots blower consumes more energy than centrifugal blower, and its operating cost is high;

Scenes with continuous aeration and large load fluctuation (such as urban sewage treatment plants): the air volume needs to be adjusted frequently, and the energy consumption of roots blower (especially bypass adjustment) is obviously wasted.

2. Centrifugal fan: more suitable for "large flow and variable load" working conditions.

Applicable scenarios:

Large-scale urban sewage treatment plant (treatment capacity > 5000 m/d): the aeration tank has a large volume and high air demand (usually > 100 m/min), and the centrifugal fan's "large flow and low energy consumption" can reduce the operating cost;

Processes with continuous aeration and large load fluctuation (such as A/O and oxidation ditch): influent COD and NH3-N fluctuate greatly, and DO needs to be controlled by adjusting the air volume (such as increasing the air when the influent load is high and decreasing the air when it is low). The "frequency conversion regulation" of centrifugal fans has obvious energy-saving advantages;

Shallow aeration tank (water depth < 4m): the system resistance is small, the centrifugal fan runs stably at low pressure, and the energy consumption is 15% ~ 30% lower than that of Roots blower;

Noise-sensitive scenes (such as residential areas around the factory): Centrifugal fans have low noise and do not need additional sound insulation measures, which meets the requirements of environmental protection.

Not applicable scenario:

High-pressure working conditions (such as deep-water aeration and serious blockage of aeration head): it is easy to reduce the air volume and even trigger surge due to insufficient pressure;

Small sewage treatment station: centrifugal fan has high initial investment (30%~50% higher than roots fan with the same air volume), complicated maintenance and low cost performance.

Third, the three key indicators of type selection decision

Wind pressure demand: first calculate the total resistance of the system (= water depth pressure+aeration head resistance+pipeline resistance). If the total resistance is greater than ＞50kPa, roots blower is preferred; < ＜50kPa, centrifugal fan is preferred;

Air volume and load fluctuation: if the air volume demand is more than 100 m/min and the load fluctuation is more than 30%, the centrifugal fan (frequency conversion) is more energy-saving; Roots blower is more economical if the air volume is less than ＜50m³/min and the load is stable.

Operating cost: for large-scale projects with long-term continuous operation (annual operation time > 8,000 h), the energy consumption account-energy saving benefits of centrifugal fans under variable load can usually offset the initial investment difference; For small projects or intermittent operation projects, Roots blower's "low initial investment+low maintenance cost" is better.

summary

Roots blower selection: scenes with small flow, high pressure, stable load and intermittent operation (such as small sewage station and deep-water SBR pool);

Choose centrifugal fan: the scene with large flow, low pressure, large load fluctuation and continuous operation (such as large urban sewage plant and shallow oxidation ditch).

In the actual selection, it is necessary to combine the manufacturer's performance curve (to prevent centrifugal fans from entering the surge zone), equipment price, local energy consumption cost and other details, and if necessary, compare the life cycle costs of the two fans through "energy consumption simulation calculation".