When you are asked which material a 5 μm filter element should be made of, the answer is usually not the filtration accuracy itself, but three other things: whether the filtration takes place inside the medium or on its surface, how much contaminant the medium can take up without blocking, and whether the material can withstand the temperature and the chemical environment of the oil. These three points determine the cost of the consumable and the element change frequency.
Glass fibre media: depth filtration, high dirt holding capacity
Glass fibre media consists of multiple layers of fibres randomly interwoven. The fibres form tortuous deep pore channels between them, and the particles are intercepted progressively as they pass through, so the contaminant builds up mainly inside the medium rather than on its surface. The direct benefit of this structure is a high dirt holding capacity: the same element takes up a clearly larger total amount of contaminant than a surface filtration medium does.
The cost of this shows up as a slower differential pressure rise, but the total amount is still considerable, and glass fibre tolerates temperature well, which suits oil that runs hot and needs long continuous operation. The manufacturing risk to note is that loosely fixed fibres can break off and pass downstream, so for insulating oil with strict extractables requirements a product with controlled free silicon and controlled fibre shedding should be selected, and the supplier's extractables data should be checked.
Fibrous media: strong oil absorption, with the temperature and chemical resistance to be assessed separately
In fibrous media the fibres are finer and the structure is more open, and the oil absorption is usually greater than that of glass fibre, which suits oil with a high free oil content where the free oil has to be taken up. The weak point is that the temperature and the chemical resistance are not as good as those of glass fibre: the performance falls once the medium is damp, and if the oil has a high water content or the medium stays in a humid environment for a long time, the life is noticeably shortened.
Surface-coated resin media: surface filtration, high efficiency but a fast differential pressure rise
Surface-coated resin forms a thin micro-porous layer on the surface of the medium, and the particles are held back at the surface instead of entering the depth of the medium. The benefit is that the filtration efficiency stays more consistent with fine particles, and the medium itself does not absorb oil, so there is no oil-starved condition after filtration.
The cost is a low dirt holding capacity and a fast differential pressure rise. The contaminant is concentrated on the surface, and once it blocks, the whole element has to be replaced rather than cleaned and put back into service. Where the contamination load is high, a surface filtration medium is often less economical than a depth filtration medium.
Selection by oil type and contamination type
Insulating oil puts the emphasis on extractables and dielectric cleanliness, so a material with controlled fibre shedding and controlled free silicon comes first; hydraulic oil and machine oil run hot and carry a wide variety of contaminants, so a depth filtration medium with good temperature resistance is usually chosen; lubricating oil often contains metal swarf and oxidation products, and since the metal swarf consists of hard, sharp particles, it takes a depth structure with a large dirt holding capacity to receive it.
It is also necessary to consider whether water has to be removed at the same time as the filtration. Oil carrying water sets stricter requirements on the medium, and because the surface-coated type absorbs neither oil nor water, it has a certain advantage in mixed conditions.
| Material | Filtration mechanism | Dirt holding capacity | Differential pressure rise | Temperature resistance | Suitable for |
|---|---|---|---|---|---|
| Glass fibre | Depth filtration | High | Slow rise | Good | High temperature, continuous operation, insulating oil |
| Fibrous | Depth filtration with adsorption | High, strong oil absorption | Moderate | Average | Free oil present, absorption required |
| Surface-coated resin | Surface filtration | Low | Rapid | Fairly good | Fine particle control, mixed conditions with water |
- Settle three things before choosing the material: the working temperature of the oil, the type of main contaminant, and whether water is to be removed at the same time.
- Insulating oil: check the control data for fibre shedding and free silicon.
- After an element change, confirm that the seals are in place, that there is no bypass, and flush out the residues in the pipework.
- A fast differential pressure rise with little discharge: first check whether the material does not match the contamination type.
The usual sign of a wrongly chosen medium is not insufficient accuracy, but a large difference in life at the same accuracy, and the whole cost difference then shows up in the differential pressure.