Hydraulic Filtration Technology: the Basics Explained

In hydraulics, surface filtration and depth filtration are the most common.

Surface filtration filters have geometrically defined gaps; the particles are separated on the surface. In the case of very high dirt accumulation, a filter cake grows on the filter (screen mesh), which acts like an additional filter as its thickness increases. This filter cake should be removed regularly to maintain control over the filtration process.

Depth filtration filters use nonwovens with chaotically arranged fibres, between which the solid particles are separated in depth. Cleaning of the filter material (e.g. glass fibre fleece) is not possible. Therefore, the filters must be replaced when they are completely contaminated. Depth filtration is mainly used where only the fluid is to be reused. Dirty filter inserts are disposed of.

For filtration, filter media are selected whose gaps, or pores, are smaller than the size of the particles to be separated. In addition, the inertia of the dirt particles in fluid flows ensures separation on the filter material. In the case of gases, electrostatics also support filtration. For this purpose, the nonwoven filters used there receive an electrical charge in all their fibres.

Nonwovens made of cellulose, glass fibre, or plastic are not only used for the filtration of gases. They are used just as much for the filtration of fluids as for:

• fabrics made of plastic or metal wires,
• porous solids (mostly sintered metals) and
• bulk materials.

In the future, nanofilters will provide even more refined filtration.

Often, modern filter materials are still chemically, or physically, treated to increase or optimize their natural filter properties.

The quality or efficiency of a filter can be recognised by various specifications or features. These are discussed briefly below.

Filtration ratio and separation efficiency:

The filtration quotient ß is calculated, similarly to an efficiency, as the ratio of the particle count before the filter and the particle count after the filter. The filtration efficiency processes the filtration quotient to show as a percentage how efficiently a filter separates dirt particles from a fluid.

Dirt-holding:

There are two specifications for this feature: absolute dirt holding capacity in grams, and specific dirt holding capacity in milligrams per square centimeter. Both are only used in connection with depth filters.

Pressure drop:

When fluids flow through a filter, there is regularly a pressure drop. There is an initial pressure drop for any filter that is not yet in use. The more dirt is retained, the greater the pressure rise at the filter. This effect is also used by clogging indicators to indicate the optimum time for filter maintenance.

Cleaning:

Robustly constructed surface filters by ARGO-HYTOS can be cleaned up to 3 times. Depth filters, used for medium dirt accumulation and flowed through with nominal volume flow, can be used for at least 1000 operating hours. In principle, replacement is recommended once a year. By the way: 1000 operating hours correspond to a mileage of 60,000 km for a passenger car.

ARGO-HYTOS gear pumps for industrial and mobile hydraulics operate with various oils and lubricants. Even in optimum quality, new oils contain too many contaminants and should not be used without being filtered. The size of these dirt particles is usually only in the range of a few micrometers, which makes them invisible to the human eye. Nevertheless, this is enough to cause long-term damage to plant or machinery.

The first thing that then happens is more frequent plant breakdowns with the need for repairs and expensive downtime. Overall, the service life of the machines decreases, causing further high costs. In contrast, the investment in a suitable filter system is marginal. ARGO HYTOS integrate suitable filters directly into their systems or machines. However, they also offer individual filters for installation or retrofitting in your systems.