Structure and dehydration principle
The main components of the centrifuge include a drum, a spiral, a differential system, a liquid level baffle, a drive system, and a control system, and its structure is shown in Figure 1. The device accelerates the sedimentation of solid phase particles under the action of centrifugal force by utilizing the density difference between the solid and liquid phases, thereby achieving solid-liquid separation. The specific working process is as follows: the sludge and the flocculant solution enter the mixing chamber inside the drum through the inlet pipe, where mixing and flocculation reactions are carried out; due to the high-speed rotation of the drum and the spiral and the friction resistance generated during the rotation process, the sludge is rapidly accelerated in the chamber to form a cylindrical liquid ring layer; under the strong centrifugal force, the solid particles with higher density settle to the inner wall of the drum; then, using the differential speed between the drum and the spiral, the deposited solid phase is slowly pushed to the conical end, and dehydration and drying are completed after passing through the liquid surface, and finally discharged from the slag discharge port; and the clarified liquid is discharged from the large end of the drum, thus completing the solid-liquid separation process.
Factors affecting centrifuge performance
1.Non-adjustable mechanical factors
1.1 Drum diameter and effective length
The larger the drum diameter and the longer the effective length, the larger the corresponding effective sedimentation area, so the processing capacity is stronger and the residence time of the material in the drum is correspondingly prolonged. At the same time, at the same speed, a larger drum diameter can bring a higher separation factor, thereby improving the separation effect. However, due to the limitation of material strength, the drum diameter cannot be increased indefinitely, because as the diameter increases, the structural strength of the material will be relatively reduced, and the maximum allowable speed will also decrease, resulting in a weakening of the centrifugal force. Generally speaking, the conventional range of drum diameter D is 200 to 1000 mm, and the aspect ratio (L/D) is usually between 3 and 4.
In addition, under the same processing capacity conditions, a centrifuge equipped with a larger drum diameter can operate at a lower speed difference. This is because the spiral equipped with a large diameter drum has a stronger slag discharge capacity, while a small diameter drum needs to increase the speed difference to compensate for the same slag discharge efficiency. The discussion on the speed difference in the following text shows that appropriately reducing the speed difference can not only prolong the dehydration time of the sediment, but also reduce the disturbance of the spiral to the clarification zone, thereby effectively improving the dehydration performance.
1.2 Drum Half Cone Angle
When the sediment settles to the inner wall of the bowl under the action of centrifugal force and is pushed to the discharge port at the conical end, it will be affected by a downward sliding reflux force. The existence of this force is closely related to the size of the semi-cone angle: the larger the semi-cone angle, the stronger the centrifugal force squeezes the sludge, and the greater the pushing torque required by the spiral, which will aggravate the wear of the blades and may even cause sediment reflux, resulting in the inability of the spiral pusher to discharge the slag normally. On the contrary, if the semi-cone angle is too small, the effective sedimentation area will be significantly reduced, thereby reducing the overall separation performance of the centrifuge.
Under the condition of a certain liquid pool depth h, if the cone angle is large (α1 > α2), the length of the drying zone, that is, the distance from the sludge leaving the liquid surface to the sludge discharge port (L12), is shorter, and the dehydration time is correspondingly shorter, resulting in a higher moisture content of the sediment; but at this time, the length of the sedimentation zone (L11) is longer, and the sedimentation area is increased, which is conducive to improving the quality of the filtrate. On the contrary, when the cone angle is small, the drying zone becomes longer, the dehydration time is extended, and the moisture content of the sediment can be reduced, but the clarity of the filtrate will be slightly reduced.
In summary, the semi-cone angle of the drum is a key parameter in the design of the centrifuge. From the perspective of obtaining a good clarification effect, a larger cone angle is appropriate; from the perspective of smooth slag conveying and dehydration efficiency, a smaller cone angle should be selected. Considering that smooth slag discharge is a prerequisite for the normal operation of the centrifuge, the slag conveying requirements must be given priority during design. For materials that are more difficult to separate (such as activated sludge), a semi-cone angle of less than 6° is usually used to reduce the reflux rate of the sediment; for general materials, keeping the semi-cone angle within 10° can ensure smooth slag discharge.
2. Adjustable mechanical factors
2.1 Drum speed
The speed of the drum is usually adjusted by a variable frequency motor or a hydraulic motor. As the speed increases, the centrifugal force generated also increases, which helps to increase the solid content of the mud cake. However, when the speed is too high, it may destroy the flocculation structure in the sludge, which will have an adverse effect on the dewatering effect. In addition, higher speeds place higher requirements on the strength and wear resistance of equipment materials, and will also increase mechanical wear, energy consumption, vibration and noise levels.
2.2 Speed difference
Speed difference is an important parameter that affects the centrifuge’s slag discharge capacity, mud cake thickness and filtrate quality. Properly increasing the speed difference helps to enhance the slag discharge capacity, but it will shorten the residence time of the sediment in the drying zone, resulting in a high moisture content of the mud cake. At the same time, a larger speed difference will aggravate the disturbance of the spiral to the liquid pool in the clarification zone, thereby affecting the cleanliness of the filtrate. On the contrary, reducing the speed difference can increase the thickness of the sediment layer and prolong its dehydration time, thereby reducing the moisture content of the mud cake and reducing the interference of the spiral to the clarification zone, which helps to improve the quality of the filtrate. However, if the speed difference is too small, it will increase the pushing load of the spiral. If the slag discharge capacity is insufficient, it is easy to cause the sediment to be retained in the drum, causing blockage, and even causing the filtrate to carry mud. At this time, it is necessary to adjust it by reducing the feed amount or appropriately increasing the speed difference.
Therefore, the setting of the speed difference should comprehensively consider the material characteristics, processing capacity, separation requirements and structural parameters of the centrifuge. It is generally believed that the optimal dehydration time of the mud cake in the drying zone should be controlled between 4 and 6 seconds. After exceeding this range, the improvement of the dehydration effect tends to be slow and no longer significant.
2.3 Liquid ring layer thickness
The thickness of the liquid ring layer is an important parameter affecting the process performance of the centrifuge. It is directly related to the effective sedimentation volume and the length of the drying zone, thus affecting the overall treatment effect. Usually, the thickness of the liquid ring layer is controlled by manually adjusting the height of the liquid level baffle when the equipment is shut down. During the adjustment process, it is necessary to ensure that all liquid level baffles are of the same height, otherwise it may cause violent vibration during operation. At present, some foreign brands of centrifuges have also realized the automatic adjustment function of the thickness of the liquid ring layer.
When the thickness of the liquid ring layer increases, the sedimentation area will expand accordingly, and the residence time of the material in the drum will be prolonged, which will help improve the quality of the filtrate. But at the same time, the length of the drying zone will be shortened, resulting in a decrease in the solid content of the mud cake. On the contrary, reducing the thickness of the liquid ring layer can increase the dryness of the mud cake, but it will reduce the clarity of the filtrate. Therefore, the height of the liquid level baffle should be reasonably adjusted according to the specific working conditions to achieve the optimal balance between the mud cake thickness and the filtrate quality.
There are many factors that affect the dehydration performance of the centrifuge, and each factor is interrelated and mutually restricted. Therefore, the final treatment effect is the result of the combined action of multiple factors. When selecting a centrifuge, the actual situation of the specific project should be fully considered; and the setting of operating parameters should be based on the final disposal method of the dehydrated mud cake, and the key indicators such as mud cake moisture content, solid recovery rate and economic efficiency of operation should be comprehensively weighed and optimized.
