Analysis of Decanter Centrifuge

With the continuous strengthening of people's environmental awareness and the continuous improvement of environmental policies and regulations, sewage treatment projects and engineering in various industries have increased sharply, and various solid-liquid separation equipment has also been continuously applied in engineering practice. The continuous progress of science and technology, the emergence of new products, technologies, and materials, as well as the increasing economic and technological requirements for service equipment and investment cost indicators in engineering projects, have significantly increased the start-up rate and continuous working time of all equipment in various engineering projects. This has put forward new and higher requirements for Decanter centrifuges in the field of sludge dewatering.


Since the late 1980s, China has been introducing, applying, digesting and absorbing advanced technologies from abroad, while developing similar equipment with independent technology. As an emerging leading equipment in the sludge dewatering process, the adaptability of the Decanter centrifuge to specific process conditions and the reliability of its own work can meet the expected design indicators and process requirements, which have been tested and verified. Based on this situation, a simple analysis of the structure of the Decanter centrifuge is conducted.


1. The structure and working principle of a Decanter centrifuge

The Decanter centrifuge relies on the density difference between solid and liquid phases, and under the action of centrifugal force, accelerates the settling speed of solid particles to achieve solid-liquid separation. There is a conical section designed in front of the drum, which rotates at a set speed according to the different properties of the material. The material rotates at the design speed on the inner wall of the drum, forming a concentric liquid layer along the drum shell, called the liquid ring layer. The solids contained in the material are deposited onto the wall of the drum under the action of centrifugal force, and then the dry material is pushed out of the drum through spiral operation. The operating speed of the drum directly determines the separation factor, while the speed difference of the screw directly affects the solid moisture content transported outside the drum. It has a direct impact on processing capacity, residence time, and solid discharge.


2. The influence of mechanical parameters on processing capacity

2.1 Rotary drum diameter. The diameter of the Decanter centrifuge is 180-920mm, which means that the theoretical volume separation range is 1:130 when the diameter to length ratio is the same

The diameter of a Decanter centrifuge cannot increase indefinitely, because as the diameter increases, the allowable speed will decrease due to the decrease in material strength, resulting in a corresponding decrease in centrifugal force. From an economic perspective, it is not possible to increase the flow rate of a Decanter centrifuge by increasing its diameter.


2.2. The ratio of drum diameter to length. In the past, the aspect ratio remained at 1:2, but now it can reach 1:3 or 1:4 (or even higher). The diameter to length ratio has an impact on the residence time of materials in the clarification zone, as it determines the volumetric flow rate of the centrifuge. The diameter to length ratio does not have a significant impact on the solid flow rate, but it has an impact on the moisture content of the mud cake.


2.3. Rotating cone angle:

The cone angle has a significant impact on the performance of solid flow rate. Due to the effect of centrifugal force, when particles reach the inner wall of the rotary drum and are transported forward by the spiral, they also have a tendency to flow backward. The speed of backward flow is related to the spiral angle, and finally reflected in the cone angle. Both have a direct impact on the conveying performance of solid particles, that is, as the reflux speed increases, the solid conveying speed decreases.


2.4 Machine manufacturing materials:

The robustness of materials is reflected in the speed of the centrifuge, which directly affects the volumetric flow rate and mass flow rate. In addition, the corrosion of the centrifuge is also related to its material structure.

      2.5 BD baffle structure:

     3. The impact of auxiliary equipment on processing capacity

3.1 Hybrid drive system. The use of a drive system with a frequency converter allows for stepless adjustment of the drum speed during operation, and the spiral differential speed also changes accordingly with the drum speed. When the machine is shut down, different speeds can also be achieved by swapping different sets of pulleys.

The use of a hydraulic drive system to drive the screw allows for stepless adjustment of the differential speed during operation, and the hydraulic oil directly corresponds to the torque of the screw drive, that is, it is proportional to the material settling inside the rotary drum.

This interconnection system is a closed-loop control system that can independently control the spiral differential speed according to the requirements of solid conveying settling inside the rotary drum. If the torque increases, the differential speed of the spiral also increases proportionally, while the speed of the rotary drum can remain constant (thereby keeping the solid dryness discharged from the rotary drum constant).

The variable pump impeller technology can adjust the thickness of the liquid layer inside the machine while it is running, so that the Decanter centrifuge can be quickly adjusted without stopping to achieve separation effect.

    3.2. New independent drive system. The structural feature of the new independent drive system is that the drive of the rotary cylinder and the screw are respectively driven by two vertical variable frequency motors.

A new type of gearbox is used to connect the drum and the screw. In addition to withstanding high torque, this gearbox can also independently rotate the screw without the drum rotating.

This drive system can not only maintain the hydraulic system to withstand high torque, but also achieve more precise differential speed and minimize or even eliminate peak starting current and vibration. Specifically, when starting the centrifuge, the screw is first started, which can eliminate any residual substances that may have been left during the previous operation and avoid the possibility of large startup vibrations caused by uneven distribution of residues during the rotary drum startup; At the same time, when shutting down the machine, first reduce the speed of the rotary drum, and then continue to discharge residual substances from the machine through the screw. This system, which drives the drum and screw separately and independently, can also start the screw separately when the machine is blocked, and discharge residual substances that remain inside the machine at low speeds, thereby simplifying the maintenance and repair process.

    4 .Conclusions

Through the above discussion and analysis, it is not difficult to see that the influence of structural design on the processing capacity of decanter centrifuge is mainly reflected in the selection and design of mechanical parameters and corresponding auxiliary equipment. If each problem is considered comprehensively and concretely, it can also be considered comprehensively and concretely. The decanter centrifuge can better serve the solid-liquid separation field of various industries in our country.

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