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Rotary filters are used to separate liquids from solids. A typical application would be separating orange pulp and seeds from orange juice or vegetables from the brine used to cook them.
Filters are used to separate solids from liquids. In many cases this is done to purify the liquid with the filtered solids discarded. Other processes may use filters to eliminate the liquid so the solids can be used.
Filters can be divided into 2 types:Absolute filters are ultrafine sieves or screens. The filter media has holes smaller than the smallest particle to be separated. The mixture is fed to the filter and the liquid passes through with the solids remaining on the filter surface. An absolute filter guarantees that no particles will carry through to the discharge. Absolute filters are readily available in pore sizes to 0.22 microns and smaller for sterilization by filtering out bacteria.Depth filters rely on passing the liquid through a maze of twisty little passages. These passages are larger than the particle size. Theoretically, particles can pass. In practice the many changes of direction and velocity as the particle makes its way through the filter tends to trap them in the filter medium.
The filter itself is a rotating drum with screens or perforations. The drum is contained in a tank. Product may be entered into the center of the drum. As the drum rotates liquid drains via the perforations and in collected in the tank where it is discharged. Concentrated solids are carried to the discharge end of the drum. This may be by gravity if the drum is inclined. Another alternative is to add a helical flight to the inside of the drum which conveys the product to the discharge. In some cases, both gravity and helix will be used in combination.
Some solids may stick to the inside of the drum and an scraper may be used to scrape it loose so it can make its way to the discharge. If the drum has an internal helix, this scraper may not be practical. Another option is to use spray jets mounted outside of the drum to spray through the screen to dislodge sticky solids.
Some designs reverse this process. The design, a rotary screened or perforated drum in a tank is similar. The difference is that the product is introduced to the outside of the drum rather than the inside. Product enters at a right angle to the drum and is distributed evenly over its length. The distribution takes place at about the 10 o’clock position on the drum, with the drum rotating clockwise. Liquid passes through the screen and falls into the drum where it is collected in a pan or allowed to fall through and exit the bottom of the drum into a collecting tank. Solids are carried over the top of the drum and, at about the 2 o’clock position, scraped off into a tray to be carried away.
Clean in place and/or backflushing system to clean accumulated solids in the screens will be helpful in reducing cleanup times.
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