Comparison of development status of horizontal screw centrifuges at homeabroad

Preface

Horizontal screw pushing sedimentation centrifuge, referred to as horizontal screw centrifuge, is one of the ideal solid-liquid separation equipment widely used in food, medicine, chemical industry and wastewater treatment. The efficient and reliable performance of the centrifuge is an effective guarantee for product quality, output and recovery. At the same time, the reasonable design, operation and maintenance of the centrifuge is the key to ensure the efficient operation of the centrifuge. In recent years, with the construction of large-scale municipal sewage treatment plants in China, the application of horizontal screw centrifuges in the field of municipal sludge dewatering has increased year by year. Some foreign brands of horizontal screw centrifuges have poured into the Chinese market, and some domestic manufacturers have also launched their own products to enter the market at a lower price. At present, the price of imported equipment is about three times that of domestic equipment, and the price gap is very large. However, the application of imported machines still accounts for a large proportion in China, especially in the field of sludge dewatering in industrial and municipal sewage treatment, especially in some newly-built municipal sewage treatment plants, and imported horizontal screw centrifuges account for a large proportion. In view of this situation, several imported and domestic horizontal screw centrifuges are analyzed one by one in terms of technical performance.

Horizontal screw centrifuge realizes solid-liquid separation by accelerating the settling speed of solid particles under the action of centrifugal force due to the density difference between solid and liquid phases. A cone section is designed in front of the drum. According to the different properties of materials, the materials rotate at a high speed according to the set speed. The materials rotate at the design speed on the inner wall of the drum and form a concentric liquid layer along the drum shell, which is called the liquid ring layer. The solid contained in the material is deposited on the drum wall under the action of centrifugal force, and then the dry material is pushed out of the drum through the operation of screw. The running speed of the drum directly determines the separation factor, while the speed difference of the screw directly affects the moisture content of the solid transported outside the drum. It has a direct impact on the treatment capacity, residence time and solid discharge.

At present, the technology of domestic manufacturers of horizontal screw centrifuges mainly comes from the direct technical cooperation, introduction and production with foreign centrifuge companies, such as Alfa Laval in Sweden and Humboldt in Germany. In addition, the technology of some manufacturers mainly comes from independent development and innovation. Or gradually developed on the basis of introducing, digesting and absorbing foreign technology.

Alfa Laval of Sweden, Westfalia of Germany, flottweg, Humboldt of Germany, pieralisi of Italy, Pakistan industries of Japan and other companies that have entered the Chinese market earlier, as well as Nexon and guinard of France, whose products are mainly concentrated in the field of sludge dewatering in municipal and industrial sewage treatment. The main technical features are basically the same.

(1) Materials at present, the main components of domestic and foreign equipment are made of stainless steel, especially the drum and screw parts are all made of stainless steel. Only a few domestic and foreign manufacturers can adopt suitable materials according to the requirements of different treatment objects and media. In particular, domestic manufacturers adopt high-strength carbon steel for municipal sewage treatment, industrial sewage treatment and non corrosive occasions, which avoids excessive design and greatly reduces the manufacturing cost. More than ten years of practice has proved that it is completely feasible to use carbon steel for corrosion prevention in municipal sewage treatment.

(2) High dry (efficiency) model design the so-called high dry (efficiency) model is a commonly used name in foreign countries, and it is used to be called concentration and dehydration integrated machine in China. The unconcentrated sludge can be dewatered directly. Generally, municipal and industrial sludge with solid content of about 0.4 ~ 0.8% can be treated. At present, some domestic manufacturers have this technology, which mainly comes from the technical cooperation and introduction with foreign companies. Only a few manufacturers are innovative technologies after digestion, absorption and improvement of foreign technologies, but they are different: a few domestic companies have absorbed the BD baffle technology of Alfa Laval company or the pressing screw design of other companies, That is, a special baffle, namely BD baffle, is set at the spiral discharge end of the conical section of the centrifuge, or the spiral design of the conical section (inner cone) is increased, which can make the centrifuge in the ultra deep liquid tank, make the liquid level in the tank run higher than the solid discharge surface, reduce the sludge space volume in the conical section, increase the pressure on the sludge cake, and only transport the lower sediment, while the sludge with high moisture content in the upper part is intercepted outside the pressing conical section, Realize press dehydration (similar to screw press dehydrator) to make the mud drier. The spiral blade of the screw conveyor is designed in the form of gradual change of pitch. The pitch gradually decreases from the big end to the small end of the drum, so that the sediment is compacted by radial centrifugal force and axial two-way extrusion, resulting in extrusion force along the axial direction. Therefore, the capillary pores of the sediment are reduced and the water contained in the sediment is discharged outward, so as to reduce the water content of the sediment. Another design is to set a press plate with Archimedes spiral cross section on the spiral cylinder. When the sediment is pushed from the spiral cylindrical section to the conical section, the spiral press plate gradually compresses the sediment. Due to the wedge-shaped channel formed between the press plate and the drum barrel, the slag pressing pressure increases gradually, reaches the maximum at the outlet, and discharges the filtrate from a wider direction along the tangent direction of the wedge gap [1], but some manufacturers also realize it through the feedback control of the speed difference.

(3) In terms of driving mode, domestic and foreign are basically the same, and there are many different driving modes for imported equipment. The most commonly used driving mode is double motor structure, that is, one motor (usually variable frequency motor) directly drives the drum through the belt to rotate, and the other motor drives the screw through the reducer (differential). In addition to the commonly used double motor system driving mode, there are many driving modes to choose from. Single motor driving mode: that is, one motor drives the drum through the main pulley, and the secondary belt drives the shaft of the differential, resulting in differential speed. When adjusting the differential speed, it needs to be stopped, which belongs to a simple driving mode; Or the differential input shaft is fixed, and the drum is driven by a single motor; There is also a single motor driving mode. For example, Alfa Laval and a few domestic manufacturers use a single motor to drive the main drum to rotate, and the speed difference is generated through the electromagnetic eddy current differential. This is a braking driving mode, which is similar to the braking device and produces a negative speed difference. It has the advantages of convenient control and energy saving, but the disadvantage is that the pushing torque is small. There are many different driving modes for domestic equipment. The most commonly used driving mode is the dual motor structure, that is, one motor (usually variable-frequency motor) directly drives the drum through the belt to rotate, and the other motor drives the screw through the reducer (differential). In addition to the commonly used double motor system driving mode, there are many driving modes to choose from. Single motor driving mode: that is, one motor drives the drum through the main pulley, and the secondary belt drives the shaft of the differential, resulting in differential speed. When adjusting the differential speed, it needs to be stopped, which belongs to a simple driving mode; Or the differential input shaft is fixed, and the drum is driven by a single motor; Another driving method is hydraulic driving, that is, the drum and screw are driven by independent hydraulic system respectively (or the drum is directly driven by variable frequency motor), which has incomparable advantages over other driving methods: greater driving torque (usually up to thousands to tens of thousands of nm, 2 to 4 times that of ordinary motor). The differential structure adopted by centrifuges at home and abroad is basically the same. Generally, it is a two-stage 2K-H, 3k, K-H-V planetary involute gear differential, or a combination of planetary cycloid needle wheel and involute gear differential. Due to the high speed and large transmission torque of the differential, the clearance adjustment requirements of various parts in the assembly process are particularly high, and the tolerance requirements are very precise. Too large or too small clearance is not conducive to the operation of the differential. Centrifuge manufacturers often need special design and processing.

(4) Spiral sealing mode spiral sealing mode mostly adopts mechanical seal or labyrinth seal, which is basically the same in domestic and imported centrifuges.

(5) Wear protection the centrifuges of major manufacturers at home and abroad adopt the inlay process of sintered wear-resistant alloy sheet or ceramic sheet on the outer edge of spiral blade in contact with sludge, which can be replaced easily, but the overall replacement cost is relatively high. Some manufacturers carry out tungsten carbide thermal spraying treatment on the outer edge of spiral blade, and the cost is relatively low.

(6) The high-efficiency dehydration performance of automatic control centrifuge can only be realized through a perfect automatic control system. At present, the mainstream control mode at home and abroad is PLC (programmable logic controller) control. The PLC can be used as the central control system of the centrifuge, and almost all domestic and foreign manufacturers require the PLC to realize the control function of the central control system at the same time. In recent years, some manufacturers began to use industrial computer or DCS control system ten years ago, which can realize more and more powerful control functions than PLC system. Large screen display is adopted, which can display all control parameters and operation status at one time. Menu operation is adopted, with friendly man-machine interface and more humanized operation. The recently launched technology can realize the host computer link through the Internet and carry out data communication with the manufacturer, which is convenient for the manufacturer to carry out remote fault analysis and treatment of the field centrifuge, and even carry out real-time data configuration and update the service of running software through the network.

Through the overall technical comparison, it is suggested that domestic centrifuge units should be used, which is not a very special chemical industry, and domestic centrifuges can be used. Domestic, stable quality, low price, convenient maintenance and fast maintenance. Low maintenance cost, imported, stable quality, high price, inconvenient maintenance and long maintenance time. High maintenance costs.