The most common types of filters are:
Media Filter - The filter tank is filled with a "media" such as gravel or sand, through which the fluid is forced. The open areas between the media granules allows passage of the fluid but not the targeted particles, thereby removing them from the fluid. The media is selected based on the size of the targeted particles.
Disc Filter - A stack of rings with grooves on both sides are compressed together so that the grooves interlock creating a specific sized openings allowing fluid to pass but catching the targeted particles, thereby removing them from the fluid.
Cartridge Filter - A filter body containing a discrete unit called a cartridge that is either removable (for cleaning) or replaceable (throw-away). This cartridge may be constructed with paper, fiber (spun or wound), or any other material which can be built such that the cartridge passes fluid while preventing passage of any particle larger than the targeted size.
Load or Loading
The particles filtered out accumulating within the filter are called the load. As a filter accumulates particles the filter is said to become loaded. The load is often related to the volume of fluid or time required to load. In general, as a filter loads it begins to filter out smaller particles due to the additional "filtering" of the load accumulating within the filter.
Strainer versus Filter
Common vernacular differentiates between "strainers" and "filters". Strainers remove larger debris from a fluid, while filters remove smaller particles.
Backwash and Cleaning
Most filters allow cleaning or backwashing. Cleaning requires disassembly of the filter to wash away the accumulated load. Backwashing is the reversal of flow back through the filter and out a dump line to remove the load from the filter. Commercial filter systems typically feature automatic backwashing to eliminate the need for careful human monitoring. Virtually all irrigation systems should always have automatic backwashing because loading may be highly variable and unpredictable, and also because of the nature of human beings to become complacent and not always do their job.
Automatic backwashing can be controlled or achieved by means of time, pressure differential across the filter, or by flow. Time and pressure differential are by far the most common methods used to trigger a backwashing cycle, and many systems use a combination of both methods.
Pressure differential across the filter is based on the fact that as the filter loads resistance to flow increases, increasing the pressure lost as fluid moves through the filter.
Some filter mechanisms backwash continuously. A "Scanner" is one example of this type. Continuous backwashing is used when loading is fast and continuous, or when the filter must be installed on the suction side of the pump. Normally it is not desirable to ever install a filter on the suction side of any pump, but when required then a continuous backwashing filter should be used to prevent pressure loss and resulting pump cavitation.
IMPORTANT - Follow this link to learn how Disc Filters work. Disc Filters are the method of filtration preferred by Irrigation Craft.
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