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The difference between backwashing and self-cleaning filters

JOHNSON SCREEN

The difference between backwashing and self-cleaning filters

Different filtering principles

The filtering principle of self-cleaning filter:

  • The liquid flows in from the inlet, the scraper with compensation is close to the inner surface of the wedge wire filter screen, the liquid flows outward from the inner surface of the filter element through the filter screen, the impurities are intercepted on the inner surface, and the liquid flows out from the outlet.
  • With the passage of time, the impurities gradually increase and the differential pressure increases. Clean the wedge wire filter screen according to the set differential pressure or regularly.
  • The motor drives the scraper to do horizontal rotation to clean the filter screen. When the scraper rotates, the impurities are scraped by the scraper and flow along the filter screen to the bottom of the filter, and the large specific gravity impurities are precipitated to the sewage collection chamber.
  • Each cleaning consists of several strokes. After the surface of the wedge wire filter screen is cleaned, the filtering capacity is restored. During cleaning, filter normally and the flow is not interrupted.
  • After multiple cleaning cycles, when impurities accumulate to a certain amount, open the blowdown valve regularly to discharge the waste liquid containing high concentration impurities. If necessary, it can also be recycled.

Filtration principle of backwash filter:

  • The liquid flows in from the inlet of the filter. Part of the liquid directly enters the filter from the lower end of the filter element, and the other part enters the top of the filter from the middle shunt pipe, and then flows in from the upper end of the wedge wire filter screen. The liquid flows in from the upper and lower openings of the filter screen at the same time, and collects and flows out at the outlet through the inner surface of the wedge wire filter screen.
  • The particle impurities are blocked on the inner surface of the wedge wire filter screen and accumulated into a filter cake, which gradually increases the pressure difference between the inlet and outlet, indicating that the filter cake reaches a certain thickness, and the flow of the filter screen gradually decrease
  • When the set differential pressure or time is reached, the self-cleaning action is started, the reduction motor drives the backwashing rotating arm to align with one or a group of wedge wire filter screen, the upper end of the filter screen is sealed, the lower end is connected to the cleaning suction nozzle, open the backwashing valve, connect the filter screen with the blowdown pipe, and the differential pressure between the outside of the filter screen and the blowdown outlet drives the filtered clear liquid to flow in reverse, wash the filter cake on the inner surface of the filter screen, and discharge it into the blowdown pipe.
  • After the backwashing of one filter element / group is completed, close the backwashing valve and align the cleaning rotating arm with the next filter screen / group for backwashing. After all filter screens are backwashed one by one / group, the whole process is completed.