To keep it simple, this document will use the term “filter” to represent barrier mechanisms used to separate solid particles from a flowing liquid. However, the actual definitions of filters, strainers and screens are a matter of perspective.
Membrane specialist – Filters remove particles smaller than 1 micron down to individual ions. Strainers are devices that remove suspended solids from 1 micron to baseballs. Screens are what grandmother put in her open windows in the summertime.
Self-cleaning filter specialist – Filters remove suspended particles from sub-micron to 1,000 micron. Strainers are devices that remove nuts, bolts, baling wire and small body parts. Screens remove turtles, fish, tree limbs and rowboats.
Basket strainer specialist – Filters and strainers are two sides to the same coin. Screens are simply big brothers to filters/strainers.
Electronics specialist – Filters separate out specific electromagnetic waves. Strainers overexert and screens are LCD, LED or CRT.
Media – The media used to separate particles from a liquid stream vary in many ways. Large water supply inlets may use bar screens having 1/2? x 4? steel bars set on edge from 3/8?–2? apart to prevent large objects from passing. Membrane systems use semi-permeable thin sheet or tubular membranes that might allow water molecules to migrate through under high pressure, but stop chloride ions (0.0004 micron) from passing. Many devices use non-woven cloth or woven cloth, wire or plastic elements as a barrier to particles of a certain minimum size. Perforated plate is often used as well. Granular media such as sand has been used for nearly 4,000 years as a means of removing solid particles from drinking water. Various other media also have been used, from hair clumps to rolls of toilet paper.
Orientation – Most separating devices have a cylindrical body and are mounted parallel to the ground (horizontal) or perpendicular to it (vertical). Some can be oriented in any fashion, while others are mounted in a fixed way.
Materials of construction – Solids can be separated from fluids of many kinds. Some liquids are caustic while others are acidic. High TDS liquids like sea water require different materials than low TDS freshwater. Some chemicals make the fluid corrosive while other fluids might contain explosive gases. Care must be taken in selecting materials for use with potable (drinking) water. The construction materials must match the application.
Instrumentation – Filters might have simple dial pressure gauges or they might incorporate pressure or differential pressure transducers that send analog signals to a central control system. Others use a simple differential pressure switch that closes upon sensing a pre-set pressure difference between the inlet and outlet of the filter. Flow or conductivity measuring devices are sometimes incorporated into the instrumentation mix.
Controls – The simplest filter control is a human being that manually cleans the filter element on a fixed time schedule or when instrumentation indicates service is needed. Other filtration systems use electro-mechanical controllers to put the filter through a fixed routine, while other systems incorporate a Programmable Logic Controller (PLC) to make decisions on the course-of-action to take based on specific instrument inputs.
Methods of cleaning
Manual – This type of cleaning often requires that the filter be taken off-line, depressurized and opened. It always involves labor.
- Commodity – Some separation media must be replaced after each use. Such media includes bags, cartridges, paper rolls and diatomaceous earth. Media must be purchased, replaced and discarded as needed.
- Reusable – These media must be removed by hand and cleaned before being reinstalled inside the filter. Hand cleaning might require dry brushing, wet brushing, air or water pressure washing, steam cleaning, caustic soaking or acid soaking.