Structural components of a filter press

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I. Frame The frame is the fundamental component of the filter press; it has thrust plates and pressing heads at its ends, with beams on either side connecting these elements together. The beams serve to support the filter plates, filter frames, and thrust plates. To meet hygiene requirements, the frame needs to be covered with stainless steel. The stainless steel packaging technology can serve as a reference; its quality meets the requirements of European customers. 　　A. Thrust plate: It is connected to the support structure, allowing one end of the filter press to be positioned on the foundation. In a box-type filter press, there is a feed hole in the middle of the thrust plate; there are also four holes at each of the four corners. The holes at the upper corners serve as inlets for washing liquid or pressing gas, while those at the lower corners serve as outlets (either for the underflow or for the filtrate).
B. Compression plate: It is used to compress the filter plates and frames. Wheels on both sides enable the compression plate to roll along the tracks on the beam. 　　C. Beams: These are load-bearing components; depending on the requirements for corrosion resistance in the operating environment, they can be coated with rigid polyvinyl chloride, polypropylene, stainless steel, or new types of anti-corrosion coatings. II. Compression mechanism: Manual compression, mechanical compression, hydraulic compression. 　　A. Manual compression: This involves using a screw-type mechanical jack to push the compression plate in order to compress the filter plate. 　　B. Mechanical compression: The compression mechanism consists of a motor (equipped with an advanced overload protector), a reducer, gear sets, a screw rod, and a fixing nut. During compression, the motor rotates in the forward direction, driving the reducer and gear set, which causes the screw rod to rotate within the fixed nut, thereby pushing the compression plate to press the filter plate and filter frame together. As the clamping force increases, the motor’s load current rises; when it reaches the current level set by the protector, a greater clamping force is achieved, and the motor has its power cut off, causing it to stop rotating. Thanks to the reliable self-locking helix angle of the screw and the fixed nut, the clamping state is maintained throughout the operation. During retraction, the motor rotates in the opposite direction, and when the clamping block on the clamping plate touches the travel switch, the retraction process stops. 　　C. Hydraulic compression: The hydraulic compression mechanism consists of a hydraulic station, cylinders, pistons, piston rods, as well as Hafnian cards that connect the piston rods to the compression plate. The hydraulic station is made up of a motor, oil pump, relief valve (for pressure regulation), directional control valve, pressure gauge, oil circuits, and an oil tank. When the hydraulic pressing machine performs compression, high-pressure oil is supplied by the hydraulic station; the chamber formed by the cylinder and piston is filled with oil. When the pressure exceeds the frictional resistance encountered by the pressing plate, the plate slowly compresses the filter plates. Once the compression force reaches the pressure value set by the relief valve (indicated by the gauge pointer), the filter plates, filter frames (in the plate-and-frame type), or filter plates (in the cartridge type) are compressed, and the relief valve begins to release pressure. At this point, the power supply to the motor is cut off, completing the compression process. During retraction, the directional valve changes direction, allowing pressure oil to enter the rod side of the cylinder; when the oil pressure is sufficient to overcome the frictional resistance of the pressing plate, the plate starts to retract. When hydraulic compression is in automatic pressure maintenance mode, the compression force is controlled by an electric contact pressure gauge. The upper and lower limits of this gauge are set to the values required by the manufacturing process. When the compression force reaches the upper limit indicated by the gauge, the power supply is cut off and the oil pump stops operating. As internal and external leaks in the oil circuit system may cause the compression force to decrease, when it drops to the lower limit indicated by the gauge, the power supply is re-established and the oil pump starts pumping oil again. Once the pressure reaches the upper limit, the power supply is cut off once more and the oil pump stops pumping oil. This cycle is repeated to ensure that the appropriate compression force is maintained throughout the material filtering process. III. Filtration mechanism The filtration mechanism consists of a filter plate, a filter frame, filter cloth, and a pressing diaphragm. The two sides of the filter plate are covered with filter cloth; when a pressing diaphragm is required, a set of filter plates is composed of a diaphragm plate and side plates. On both sides of the base plate of the diaphragm plate, rubber diaphragms are wrapped, and a filter cloth is wrapped around the outside of these diaphragms; the side plates are ordinary filter plates. The material enters each filter chamber through the feed holes on the thrust plate; solid particles are retained in the filter chambers because their size is larger than the pore size of the filtering medium (filter cloth), while the filtrate flows out through the outlet holes located below the filter plate. When the filter cake needs to be dried, in addition to membrane pressing, compressed air or steam can also be introduced through the washing port; the airflow removes the water from the filter cake, thereby reducing its moisture content. 　　(1) Filtration method The ways in which the filtrate flows out are divided into surface filtration and deep filtration. 　　A. Surface flow filtration: A water nozzle is installed at the liquid outlet hole beneath each filter plate, allowing the filtrate to flow out directly from the nozzle. 　　B. Undercurrent filtration: Each filter plate is equipped with liquid outlet holes, and the liquid outlet holes of multiple filter plates are connected to form a single liquid outlet channel, through which the liquid is discharged via the pipes connected to the liquid outlet holes located beneath the thrust plate. 　　(2) Washing method When the filter cake needs to be washed, there are single-direction and double-direction washing with free flow, as well as single-direction and double-direction washing with forced flow. 　　A. In the case of unidirectional flow washing, the washing fluid enters sequentially through the fluid inlet holes in the thrust plate, passes through the filter cloth and then through the filter cake, before exiting from the holes in the non-porous filter plate. At this time, the fluid outlet nozzles of the porous filter plate are closed, while those of the non-porous filter plate are open. 　　B. In the forward-flow two-way washing method, the cleaning fluid washes the surface twice from the two inlet holes located above the thrust plate; that is, it first washes from one side and then from the other. The outlet of the cleaning fluid is in a diagonal direction relative to the inlet, which is why it is also called two-way cross-washing. 　　C. In the unidirectional flow washing process, the washing liquid enters the perforated plate sequentially through the inlet holes on the thrust plate, passes through the filter cloth and then through the filter cake, before exiting from the non-perforated filter plate. 　　D. Cross-flow dual-direction washing involves the washing liquid flowing in through two inlet holes on either side of the baffle, carrying out two washes sequentially; that is, washing is done first from one side and then from the other. The outlet of the washing liquid is in a diagonal direction, which is why it is also referred to as cross-flow dual-direction interlaced washing. 　　(3) Filter cloth: The filter cloth is a primary filtering medium, and its selection and use play a decisive role in the filtering efficiency. When selecting it, it is necessary to take into account factors such as the pH value of the material to be filtered and the size of the solid particles, in order to choose an appropriate filter cloth material and pore size that will ensure low filtering costs and high efficiency. During use, it is important to keep the filter cloth flat and free of folds, as well as to ensure that the pores remain unobstructed