Development history and working principle of filters

Filtration technology has been applied in production since ancient China, and paper made from plant fibers existed by 200 BC. In 105 AD, Cai Lun improved the papermaking method. He swings plant fiber pulp onto a dense bamboo curtain during the papermaking process. The water is filtered through the gaps of the bamboo curtain, and a thin layer of wet pulp is left on the surface of the bamboo curtain. After drying, it becomes paper.


The earliest filtration was mostly gravity filtration, but later pressure filtration was used to improve the filtration speed, leading to the emergence of vacuum filtration. The rotary drum vacuum filter invented in the early 20th century achieved continuous filtration operation. Afterwards, various types of continuous filters emerged one after another. Intermittent operation filters (such as plate and frame pressure filters) have been developed due to their ability to achieve automated operation, resulting in an increasing filtration area. In order to obtain filter residue with low moisture content, mechanical pressing filters have been developed.


When the filter is working, the water to be filtered enters through the water inlet, flows through the filter screen, and enters the pipeline required by the user through the outlet for process circulation. The particulate impurities in the water are trapped inside the filter screen. In such continuous circulation, more and more particles are intercepted, and the filtration speed becomes slower and slower. However, the imported sewage continues to enter, and the filter hole becomes smaller and smaller, resulting in a pressure difference between the inlet and outlet. When the difference reaches the set value, the differential pressure transmitter transmits the electrical signal to the controller, and the control system starts the drive motor to drive the shaft to rotate through the transmission component. At the same time, the sewage outlet is opened and discharged from the sewage outlet, After the filter screen is cleaned, the pressure difference drops to the minimum value, and the system returns to the initial filtering state, and the system operates normally. The filter is composed of a shell, a multiple element filter, a backwash mechanism, and a differential pressure controller. The diaphragm inside the shell divides its inner cavity into two parts: upper and lower. The upper cavity is equipped with multiple filter elements, which fully fills the filtering space and significantly reduces the volume of the filter. The lower cavity is equipped with backwashing suction cups. During operation, the turbid liquid enters the lower chamber of the filter through the inlet and then enters the inner chamber of the filter element through the baffle hole. Impurities larger than the gap of the filter element are intercepted, and the clean liquid passes through the gap to reach the upper chamber, and finally is sent out from the outlet. The filter adopts a high-strength wedge-shaped filter screen, which automatically cleans the filter element through pressure difference control and timing control. When impurities accumulate on the surface of the filter element, causing the pressure difference between the inlet and outlet to increase to the set value, or when the timer reaches the preset time, the electric control box sends a signal to drive the backwash mechanism. When the backwashing suction cup port is aligned with the inlet of the filter element, the drain valve opens. At this time, the system releases pressure and discharges water. A negative pressure zone appears between the suction cup and the inner side of the filter element, where the relative pressure is lower than the water pressure on the outer side of the filter element. This forces some clean circulating water to flow into the inner side of the filter element from the outer side. Impurities and particles adsorbed on the inner wall of the filter element flow into the suction cup with water and are discharged from the drain valve. The specially designed filter screen creates a spraying effect inside the filter element, and any impurities will be washed away from the smooth inner wall. When the pressure difference between the inlet and outlet of the filter returns to normal or the timer setting time ends, the entire process does not interrupt the flow of materials, and the backwashing water consumption is low, achieving continuous and automated production. Filters are widely used in the fields of metallurgy, chemical engineering, petroleum, papermaking, pharmaceuticals, food, mining, electricity, and urban water supply. Such as industrial wastewater, filtration of circulating water, regeneration of emulsion, filtration treatment of waste oil, continuous casting water system and blast furnace water system in the metallurgical industry, and high-pressure water descaling system for hot rolling. It is an advanced, efficient, and easy to operate fully automatic filtering device.


The water to be treated by the filter enters the body through the inlet, and impurities in the water deposit on the stainless steel filter mesh, resulting in a pressure difference. Monitor the pressure difference changes at the inlet and outlet through a pressure difference switch. When the pressure difference reaches the set value, the electric controller sends a signal to the hydraulic control valve to drive the motor. After the installation of the equipment, technicians debug it, set the filtration time and cleaning conversion time. The water to be treated enters the body from the water inlet, and the filter begins to work normally. When the preset cleaning time is reached, the electric controller sends signals to the hydraulic control valve and driving motor, triggering the following actions: the electric motor drives the brush to rotate, cleaning the filter element, and controlling the valve to open for drainage. The entire cleaning process only lasts for tens of seconds, When the cleaning is completed, close the control valve, the motor stops rotating, the system returns to its initial state, and begins the next filtering process. The interior of the filter housing is mainly composed of coarse filter screens, fine filter screens, suction pipes, stainless steel brushes or stainless steel suction nozzles, sealing rings, anti-corrosion coatings, rotating shafts, etc.


A simple filter is formed by separating the container into upper and lower chambers using a filtering medium. The suspension is added to the upper chamber, and under pressure, it enters the lower chamber through the filtering medium to form a filtrate. Solid particles are trapped on the surface of the filtering medium to form a filter residue (or filter cake). During the filtration process, the filter residue layer on the surface area of the filter medium gradually thickens, and the resistance of liquid passing through the filter residue layer increases, resulting in a decrease in filtration speed. When the filter chamber is filled with filter residue or the filtering speed is too low, stop filtering, remove the filter residue, and regenerate the filtering medium to complete a filtering cycle.


The liquid must overcome resistance when passing through the filter residue layer and filter medium, so there must be a pressure difference on both sides of the filter medium, which is the driving force for achieving filtration. Increasing the pressure difference can accelerate filtration, but particles that deform under pressure are prone to clogging the pores of the filtering medium at large pressure differences, resulting in slower filtration.


There are three methods for suspension filtration: slag layer filtration, deep filtration, and sieve filtration.


① Filter residue layer filtration: In the initial stage of filtration, the filter medium can only retain large solid particles, while small particles pass through the filter medium with the filtrate. After the formation of the initial filter residue layer, the filter residue layer plays a major role in filtration, where both large and small particles are intercepted, such as in the filtration of a plate and frame filter press.


② Deep filtration: The filtering medium is thick, and the suspension contains fewer solid particles, which are smaller than the pores of the filtering medium. During filtration, particles enter and are adsorbed in the pores, such as through porous plastic tube filters and sand filters.


③ Sieve filtration: The solid particles intercepted by filtration are larger than the pores of the filtering medium, and the solid particles are not adsorbed inside the filtering medium. For example, the rotary drum filter screen filters out coarse impurities in sewage. In the actual filtering process, the three methods often appear simultaneously or sequentially.