Non-adjustable stemming machine coefficient
Drum diameter and effective length
The larger the diameter of the drum, the longer the effective length, the larger the effective settlement area and the greater the handling capacity, and the longer the material stays in the drum. At the same speed, the greater the separation factor, the better the separation effect. However, due to the limitation of materials, it is impossible to increase the diameter of centrifuge drum indefinitely, because with the increase of diameter, the maximum allowable speed will decrease with the decrease of material firmness, so the centrifugal force will also decrease accordingly. Generally, the drum diameter is between 200 ~ 1000mm, and the length-diameter ratio is between 3 ~ 4. At present, centrifugal sludge dehydrator is developing in the direction of high speed and large length-diameter ratio. This kind of ball press is more suitable for the treatment of low concentration sludge and has better biscuit degree. In addition, in the case of the same throughput, the centrifuge with large drum diameter can run at a lower differential speed, because the spiral conveying capacity of the large drum diameter is greater, and to achieve the same conveying capacity, the centrifuge with small drum diameter must be realized by improving the differential speed.
B drum half cone angle
When the sediment settled on the inner side of the centrifuge drum is pushed to the discharge port along the tapered end of the drum, the sediment is subjected to a backflow force sliding downward due to centrifugal force. The half cone angle of drum is an important parameter in centrifuge design. In terms of clarification effect, the cone angle should be as large as possible; Considering the slag conveying and dewatering effect, the cone angle is required to be as small as possible. Because slag conveying is a necessary condition for the normal operation of centrifuge, the optimal design must first meet the slag conveying condition. For materials that are difficult to separate, such as activated sludge, the half cone angle is generally less than 6 degrees to reduce the reflux speed of sediment. For ordinary materials, the half cone angle is within 10 degree, which can ensure the smooth transportation of sediment.
C tone
Pitch, that is, the distance between two adjacent spiral blades, is a very important structural parameter, which directly affects the success or failure of slag conveying. When the screw diameter is fixed, the greater the pitch and the greater the screw angle, the greater the chance of material blockage between screw blades. At the same time, large pitch will reduce the number of revolutions of spiral blades, lead to uneven distribution of materials at the cone end of the drum and increase the vibration of the machine. Therefore, for materials that are difficult to separate, such as activated sludge, it is difficult to transport slag, and the pitch is smaller, generally 1/5 ~ 1/6 of the drum diameter, so as to facilitate transportation. For easily separated materials, the pitch should be larger, generally 1/2 ~ 1/5 of the drum diameter, so as to improve the capacity of transporting sediment.
D spiral type
Screw is the main component of centrifugal sludge dewatering machine. Its function is to transport the sediment deposited in the drum and discharge it smoothly. It is not only a discharging device, but also determines the production capacity, service life and separation effect. According to the different relative motion modes of liquid and solid in the drum, the types of spiral are divided into countercurrent and parallel flow. The feed cavity of the counter-current centrifuge is located in the middle of the spiral, that is, near the boundary between the drying zone and the settling zone, to ensure that the liquid phase has enough settling distance, but the solid phase can only stay for the time required to pass through the cone, so it needs higher centrifugal force; When the material enters the drum from here, the settled solid particles in this area will float up again due to disturbance, and turbulence and additional vortex will be generated, which will reduce the separation effect. Because the feed inlet of the parallel flow centrifuge is at the end of the drum, the countercurrent turbulence is avoided and the sediment is not disturbed. The whole length of the centrifuge plays a settling role, which enlarges the settling area and increases the residence time of the suspension in the centrifuge, thus improving the separation effect. Because prolonged and undisturbed sedimentation can effectively reduce the dosage of flocculant, greatly improve the flow state of fluid in the machine, and increase the centrifugal force by increasing the diameter of the drum, it can significantly reduce the rotating speed, save power consumption, reduce noise and prolong the life of the machine. Centrifuges with parallel spiral structure are especially suitable for difficult-to-separate materials with small solid-liquid density difference, poor solid-phase sedimentation performance and low solid content. The filtrate of the parallel flow centrifuge is discharged through the skimming tube, and the solid particles that are not separated when the filtrate passes through the skimming tube will be separated and deposited in the skimming tube, which will block the passage of the skimming tube for a long time and need to be cleaned regularly.
In recent years, with the increasing demand for sludge dewatering, an efficient spiral structure came into being. For example, the BD baffle technology of Alfa Laval Company in Sweden, that is, a special baffle is set at the spiral discharge end of the cone section of the centrifuge, which can make the centrifuge in an ultra-deep liquid pool state to increase the pressure on the mud cake, only transport the sediment at the lower part, and keep the sludge with high water content at the outer side of the cone section, thus realizing squeezing dehydration and waterproof casing, and making the sludge drier. NOXON Company of Sweden adopts the patented technology of inclined plate settling principle to design the impeller blades of centrifuge to the best inclined state. The parameters such as blade inclination, pitch and blade spacing were optimized, which improved the treatment capacity and reduced the dosage of flocculant and the moisture content of mud cake.
2 adjustable mechanical factors
Drum speed
The adjustment of drum speed is usually realized by variable frequency motor or hydraulic motor. The greater the rotating speed, the greater the centrifugal force, which is helpful to improve the solid content of mud cake. However, if the rotating speed is too high, the sludge flocs will be destroyed and the dewatering effect will be reduced. At the same time, higher rotating speed requires higher materials, which will increase the wear, power consumption, vibration and noise level of the machine.
Differential speed (differential ratio)
Differential speed directly affects slag discharge, cake degree and filtrate quality, which is one of the important parameters that need to be adjusted according to the operation situation in the operation of centrifugal sludge dehydrator. Increasing the differential speed is beneficial to improve the slag discharge capacity, but the dehydration time of sediment will be shortened and the water content of mud cake after dehydration will be high. At the same time, excessive differential speed will increase the disturbance of the screw to the liquid pool in the clarification area, and the quality of the filtrate is relatively poor (commonly known as "back mixing"). Reducing the differential speed will increase the thickness of sediment, prolong the dehydration time of sediment and reduce the moisture content of dehydrated mud cake. At the same time, the screw has little disturbance to the materials in the clarification area, and the quality of filtrate is relatively better, but it will increase the load of screw pushing. It is necessary to prevent the sediment in the centrifuge from blocking due to the decrease of slag discharge, and to prevent the filtrate from carrying a lot of mud. At this time, it is necessary to reduce the feed speed or increase the differential speed. Some models of equipment have the function of automatically accelerating slag discharge. When the set torque reaches a certain limit, the equipment will automatically reduce the dosage of mud and drugs, improve the differential speed and quickly push out the accumulated mud ring layer. When the torque is reduced to a certain value, the flow and differential will automatically return to normal. This is an effective measure to protect the equipment. However, in the long-term operation, this situation should be avoided frequently, because it is easy to make the equipment in an unstable flow and unstable pressure difference state, and the fluctuation in the process will affect the treatment effect and reduce the treatment capacity. Therefore, the differential speed should be determined according to the material properties, processing capacity, processing requirements and centrifuge structural parameters. That is to say, it is necessary to find the best treatment capacity and the range of grade difference required by the treatment effect on the spot according to the situation, and achieve the highest treatment capacity as possible under the condition of satisfying the mud cake degree. Simply put, it is the contradiction between processing ability and processing effect. To improve the processing capacity, it is necessary to improve the differential speed ratio, but it may reduce the biscuit degree; In order to improve the degree of mud cake, it is necessary to reduce the degree of discrimination, thus reducing the throughput. Therefore, the field debugging work is to find the best operating parameters of the equipment that meet the actual sludge properties in their respective fields, so as to achieve the dual purposes of the highest operating efficiency of the equipment and the best treatment effect. There is no simple data to calculate, so we can only rely on long-term actual debugging to accumulate experience and adjust it in time according to changes. At the same time, in a certain range, the control of difference and the control of flocculant dosage are complementary. When the mud cake degree is required to reach a certain level, when the difference degree is reduced, the dosage of flocculant can be saved at the same time. Simply put, it is to increase the processing pressure of equipment and reduce the use pressure of flocculant. Therefore, the appropriate use of minimum discrimination can reduce the dosage of flocculant to some extent. As the saying goes, it is called "medicine is saved when the equipment runs well, and medicine is wasted when the equipment runs badly". The quality of the equipment depends not only on the design and machining accuracy of the equipment itself, but also on the control of the operating parameters of the equipment.
For centrifuges with automatic differential adjustment function, the parameter setting of differential should be determined in combination with long-term use, and corrected at any time according to possible changes.
C liquid ring thickness
The thickness of liquid ring layer is an important parameter for equipment optimization, which directly affects the effective settling volume of centrifuge and the length of drying area (shore area), and then affects the treatment effect of sludge dehydration. Generally, it is realized by manually adjusting the height of the liquid level baffle when stopping. When adjusting, the height of each liquid level baffle should be consistent, otherwise the centrifuge will vibrate violently during operation, and some products of foreign manufacturers can automatically adjust the thickness of the liquid ring layer.
With the increase of liquid ring layer thickness, the settling area increases, the residence time of materials in the machine increases correspondingly, and the quality of filtrate improves, but at the same time, the length of drying area (shore area) in the machine is shortened, which leads to the decrease of mud cake degree. On the contrary, reducing the thickness of liquid ring layer can obtain higher solid content of mud cake, but it will sacrifice the quality of filtrate. Therefore, the height of liquid level baffle should be adjusted reasonably to achieve the best combination of mud cake degree and filtrate quality.
In general, many equipment suppliers will adjust the liquid level baffle in advance before leaving the factory. However, due to the difference of different site conditions, if the operation state is not ideal, engineers from equipment manufacturers can be invited to adjust the liquid level baffle to make it more in line with actual needs.
2.3 process factors
Because the centrifuge uses the density difference between solid and liquid to realize solid-liquid separation, the greater the specific gravity of sludge particles, the easier it is to separate. Generally speaking, the primary sludge of urban sewage treatment plants is easy to dehydrate, while the residual sludge is difficult to dehydrate, while the dehydration performance of mixed sludge is somewhere between the two. Sludge produced by different sewage quality and sludge obtained by different water treatment processes will be very different, so there will be different performances in sludge dehydration. In order to improve the dewatering performance of sludge, an appropriate amount of organic polymer flocculant, such as polyacrylamide (PAM), should be added evenly before mechanical dewatering to reduce the specific resistance of sludge and make it easier to dewater after solid-liquid separation. The type of flocculant must be suitable for the characteristics of sludge, equipment type and operating conditions. In many cases, the reagent that performed well in the beaker test of flocculant selection did not perform better in practical application. The important reason is that although the characteristics of chemicals conform to the characteristics of sludge to a certain extent, they cannot fully meet the operating conditions of equipment. According to the actual operation, after the dosage of flocculant (sludge dehydrating agent) reaches a certain level, the dosage of flocculant has little effect on the solid content of centrifugal demineralized cake, but has a great influence on the quality of filtrate. Therefore, in the process of sludge dewatering, it is completely unnecessary to continue to increase the dosage of flocculant under the condition of meeting the requirements of mud cake degree and supernatant quality, which is also the main reason for the waste of flocculant on site. In addition, with the increase of flocculant dosage, the quality of supernatant is better. But in many cases, it is not worth the loss to pursue the quality of supernatant excessively and add more flocculants. It is not cost-effective to increase the sludge recovery rate by several percentage points and consume more flocculants (just like spending 100 yuan to buy 10 yuan's goods). As long as the solid content of the supernatant is controlled within a certain index range.
In general, the sludge concentration that the equipment can adapt to has a certain range. Too low or too high sludge concentration will consume more flocculant. Under the condition of normal operation of sludge concentration, the dosage of flocculant is roughly proportional to the solid content of sludge to be treated. Therefore, when the sludge flow is constant, the dosage of flocculant should be adjusted according to the sludge concentration. Many times, due to the change of sludge concentration, the dosage of flocculant was not adjusted in time, which led to poor field operation performance or increased drug consumption.
If the concentration of sludge is increased but the dosage of flocculant is not increased, the treatment effect will be affected, the biscuit degree of sludge will be reduced and the supernatant will be turbid. On the other hand, if the sludge concentration is reduced, the dosage of flocculant will not be reduced, resulting in the waste of flocculant, but the treatment effect is not obvious.
In addition, if the actual dosage is insufficient due to poor dissolution of flocculant or the supply of effective components in the liquid medicine is insufficient due to low concentration of flocculant, it will be difficult to form mud cakes with corresponding dryness, which will affect the quality of supernatant; However, if the concentration of flocculant is too high, the active groups on the polymer chain of flocculant will shield and wrap each other, making it difficult for effective components to give full play to their efficacy, thus causing drug waste; Because the dosage of flocculant is too large, the redispersion of floc will also destroy the stability of floc and the flocculation effect is not good.
Too much flocculant not only causes waste, but also can not significantly improve the treatment effect. In the treatment of municipal sludge, the concentration of organic polymer flocculant solution is generally 1 ‰ ~ 5 ‰, and the dosage of flocculant is generally 3 ~ 5 kg/TDS. The flocculant dosage of some industrial sludge may reach or exceed 10kg/TDS, which depends on the properties of sludge and the performance of sludge dehydrator. Due to the different performance of dehydrator equipment, sludge with the same properties will have different flocculant consumption performance when using the same type of flocculant.
There are many factors that affect the dewatering effect of centrifugal sludge dehydrator, and each factor affects each other, so the treatment effect is the result of the comprehensive action of the above factors. The selection of centrifuge should be combined with the actual engineering situation, and the adjustment of operation parameters should be comprehensively considered from the factors such as the optimal water content of mud cake, solid recovery rate, economy and so on.