How filters work and types

Filters can be divided into surface type, depth type and magnetic filters according to the filter material. Their filtration of solid pollutants is accomplished through direct interception and adsorption.

1. surface filter

In surface filters, almost all of the filtered particulate contaminants are intercepted on the upstream side of the filter element surface. The filter element material has uniform calibrated pores, which can filter out solid particles larger than the calibrated pores. Filters belonging to this category include line gap type, mesh type and chip type.

Mesh filter. The filter element 1 is composed of a metal mesh wound around the support frame 2. It relies on the tiny mesh of the metal mesh to block the passage of impurities in the oil. The mesh filter can generally filter out impurity particles with d>0.08~0.18mm, and has low pressure loss. At 0.01Mpa. The mesh filter without a shell is usually installed at the suction port of the hydraulic pump and is used as a coarse filter to ensure that the pump is not damaged by large particles of dirt. This filter is characterized by simple structure, large flow capacity, and easy cleaning, but low filtration accuracy.

The line gap filter consists of a shell 3, a filter element 4 and a frame 5. Use a thin copper wire to flatten a small section of the copper wire at regular intervals, and then wrap it around the frame, thus forming many small filtering gaps. The line gap filter can filter out d>0.03~0.1mm impurity particles with a pressure loss of about 0.07MPa~0.35MPa. It is often used in low-pressure pipelines. This filter has a simple structure and high filtration accuracy, but the material of the filter element Low strength and difficult to clean.

2. depth filter

The filter element of the depth filter is made of porous and permeable material with tortuous channels inside. Pollution particles larger than the pore size are directly intercepted on the outer surface upstream of the oil. When smaller particles enter the internal channel of the filter element, they deviate from the flow stream due to surface tension (molecular adsorption force, electrostatic force, etc.) and are adsorbed. on the inner wall of the filter channel. Therefore, the filtration principle of the depth filter has both direct interception and adsorption. This filter element material includes paper core, sintered metal, felt and various fibers.

Sintered filter. The filter element 3 is made of sintered metal powder, and uses micropores between metal particles to block the passage of impurities in the oil. By changing the particle size of the metal powder, filter elements with different filtration accuracy can be produced. The sintered filter can filter out d>0.01~0.1mm impurity particles with a pressure loss of about 0.03MPa~0.2MPa. Used in pressure oil line or return oil line. It is characterized by simple manufacturing, high filtration accuracy, and the filter element can withstand high pressure, but the metal particles are easy to fall off and are difficult to clean after being clogged.

The paper core filter has the same structure as the line gap filter, but the filter element is a paper core made of plain or corrugated phenolic resin or wood pulp microporous filter paper. In order to increase the filtering area, the paper core is often made into a pleated shape. The paper core filter can filter out d>0.03~0.05mm impurity particles with a pressure loss of about 0.08MPa~0.4MPa. Paper core filters are used in situations that require high oil precision. They are characterized by high filtration precision, but the filter element cannot be cleaned after it is clogged, and the paper core must be replaced, so it is a disposable filter element.

3． magnetic filter

Magnetic filters mainly rely on the magnetic field force of magnetic materials to attract iron filings and magnetic abrasives. The filter element is made of permanent magnets and can absorb iron filings, iron powder or magnetic grinding powder in the oil. It is often combined with other types of filter elements to form a composite filter, which is especially suitable for machine tool hydraulic systems that process steel parts.