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Difference between revisions of "Scraper Dischargers"
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This | [[File:Scraper discharges_1.jpg|thumb|200px|right|Scraper discharges]] | ||
'''Scraper discharges''' are used for demanding, discharging high-consistency, fibrous slurry tower applications to ensure process reliability, high efficiency, and low operating costs. | |||
The Discharge Scraper operates in connection with a medium consistency tower bottom discharge pump. The scraper works over the complete bottom area and directs the pulp to the suction opening of the discharge pump. The homogenized bottom area pulp consistency enables a uniform discharge and prevents channeling in the tower. | |||
When combined with an MC pump, the Discharge Scraper produces a stable feed flow to downstream processes such as brownstock washing, oxygen delignification, and the bleaching stages. | |||
==Horizontal Scraper Discharge== | |||
The horizontal scraper discharge centrifuge is a piece of solid-liquid separation equipment with continuous operation and discontinuous discharge. The horizontal scraper discharge centrifuge has a fixed filter bed. Under the action of centrifugal force, the horizontal scraper discharge centrifuge can periodically feed, separate (dehydrate), wash and scraper, and is automatically controlled by the combination of hydraulic and electric. | |||
The horizontal scraper discharge centrifuge is utilized for situations in which solid particle size is greater than 10μm, the mass fraction of solid-phase is 25%～60% and the viscosity of liquid-phase is less than 0.01Pa.s. | |||
===Working principle of horizontal scraper discharge centrifuge=== | |||
After the basket rotating at full speed, the feed valve shall automatically open and the suspension shall enter into the basket through feed pipe. With the help of harrow teeth, the solid-phase medium shall be uniformly distributed inside filter basket, while the liquid-phase medium shall be thrown out through the filter screen and filtration pore on basket wall. With the increase of cake thickness, the harrow teeth rotate relatively. When the harrow teeth rotate to a certain point (the materials layer reaches the allowable maximum thickness), the limit switch is touched and the relay is cut off so that the feed valve shall close to stop feeding. At the same time, the washing valve automatically opens and the washing liquid sprays on the filter cake. After the cake is fully washed, washing valve closes. The centrifuge shall dewater and dry materials. Then the scraper elevates to scraper vertically. The materials which are scrapered down shall be discharged from the centrifuge under the action of gravity. When the materials inside the basket are emptied, the filter screen shall regenerate. After finishing a cycle, the centrifuge shall automatically enter next cycle. For most materials, the left filter cake layer does not need to be washed after scratching at a time, but usually needs to be washed at intervals of 4 ~ 8h. | |||
===Types and operation modes of horizontal scraper discharge centrifuge=== | |||
The horizontal scraper discharge centrifuge can be grouped into common filtration type and siphon filtration type. The horizontal siphon scraper discharge centrifuge has such features as strong separating capacity and easily adjustable filtration velocity, is suitable for the services in which the filter cake is difficult to dewater, the high requirements for the cake washing and for the ultimate moisture content of cake are required. | |||
The horizontal scraper discharge centrifuge control system usually adopts the combination of hydraulic and electrical control to achieve higher automation. | |||
===Discharge mechanism and damping device of horizontal scraper discharge centrifuge=== | |||
The discharge mechanism of horizontal scraper discharge centrifuge is related to the characteristics of filter cake. For the dense filter cake, the narrow scraper shall be selected. While for the most of the loose cakes, the wide scraper generally is adopted. The feed method of scraper can be classified into elevating and rotating scraper. The discharge method includes chute discharge and scroll conveyor discharge. The materials of blade usually are galvanized iron bronze, 1Cr18Ni9Ni, 2Cr13 or spring steel, etc. | |||
The centrifuge vibrates obviously because of the residual unbalance of basket, the interference and uneven material distribution which are caused during feeding and scraper discharging, and the distorted structure of centrifuge cantilever and so on, the damping device is needed for horizontal scraper discharge centrifuge. Most of damping devices adopt rubber or metal spring vibration isolator. The natural frequency of vibration system is 1/4～1/3 of gyro frequency of the machine. | |||
===Horizontal siphon scraper discharge centrifuge=== | |||
The horizontal siphon scraper discharge centrifuge is a kind of scraper discharge centrifuge whose driving force of separation includes both centrifugal force and siphon suction force. Compared with the common horizontal scraper discharge centrifuge, the horizontal siphon scraper centrifuge has higher production capacity and better separation effect. The siphon scraper centrifuge not only can be used for the separation and dehydration of re-alkali, caustic soda and starch, but especially suitable for separating liquid-solid two-phases with small solid particles in suspension, slower filtration velocity and higher requirements for washing cake. | |||
Compared with the common horizontal centrifuge, the siphon scraper centrifuge has following characteristics. | |||
① Compared with the horizontal scraper discharge centrifuge with the same specification, the production capacity increases by 40%～60% and the moisture content of filter cake is lower. | |||
② The filtration velocity can be adjusted by changing the position of siphon suction inlet, so as to get different filtration rate for feeding, separating, washing and dewatering cake and keep the centrifuge operation under the best condition. Horizontal siphon scraper discharge centrifuge has characteristics of smooth operation and less vibration and noise. | |||
③ This equipment not only can be automatically controlled by the combination of hydraulic and electrical control, but also has characteristics of accurate and reliable program operation, and convenient adjustment and maintenance. | |||
===Type selections of horizontal scraper discharge centrifuge=== | |||
① The horizontal siphon scraper discharge centrifuge is suitable for the materials in which the mass fraction of solid phase in suspension is 25%～60%, or the materials with average solid particle size of 15μm and the viscosity of 10Pa.s. For flammable, explosive and poisonous materials, the closed explosion-proof scraper centrifuge should be adopted. | |||
② When the filtration rate of the filtered material is low and the requirement for cake washing and cake moisture content is high, or in order to improve production capacity, siphon scraper centrifuge should be selected. | |||
③ When the horizontal scraper discharge centrifuge is selected, the steady feed quantity and solid-phase concentration in process system should be ensured. Otherwise the larger vibration and noise shall occur to influence the using effect of centrifuge or cause unsafe factors. | |||
④ The horizontal scraper discharge centrifuge has a certain damage to the dewatered solid particles, is not suitable for the solid particles which are not allowed to be broken. | |||
⑤ For the situations in which the residue is easy to harden after dehydration or the filter medium is easy to be blocked while separating the solid-phase, the horizontal scraper discharge centrifuge should not be selected. | |||
⑥ The centrifuge with the specific specifications can be chosen based on the previous experience. A suitable centrifuge generally shall be determined by experiment. | |||