Technical Reference
Liquid Filtration Pressure Drop The difference between the inlet pressure and the outlet pressure is called pressure drop or differential pressure. It’s symbolized by ∆P. ∆P is an irrecoverable loss of total pressure caused by the filter, and is mostly due to frictional drag on the fibers in the media. ∆P may increase as the particulate rating or efficiency of the filter gets better. ∆P also increases as the filter is being loaded with contaminant.
mean there are smaller pores in the media; smaller media pores cause more flow resistance, in turn causing higher pressure drop. 2. Dirt, Contaminant As dirt gets caught in the media, it eventually begins to build up and fill the pore openings. As the pore openings shrink, the differential pressure, also referred to as pressure drop, increases.This is called restriction.
Major Factors Contribute to Pressure Drop 1. Filter Media
Typically there is a restriction limit for the system the filter has been applied to.The amount of restriction a filter can have before the system performance becomes affected is called the filter terminal pressure drop. This will usually be the point at which the filter capacity will be stated.
Media is the main factor influencing pressure drop; indeed, it causes pressure drop.That’s why having a low-friction, high-flowing media is so important. The natural cellulose or paper fibers typically used in filtration are large, rough, and as irregular as nature made them. Donaldson developed a synthetic media with smooth, rounded fibers, consistently shaped so that we can control the fiber size and distribution pattern throughout the media mat, and allow the smoothest, least inhibited fluid flow. Our synthetic media is named Synteq. Synteq fibers offer the least amount of resistance to fluid passing through the media. Consistency of fiber shape allows the maximum amount of contaminant catching surface area and specific pore size control.
This photo from our scanning electron microscope shows actual dirt particles building up in the media pores.
3. Flow Higher flows create higher pressure drop. With fast moving fluid, there will be more friction causing higher pressure drop across the media. 4. Fluid Viscosity Measured in centistokes (cSt) or Saybolt Seconds Universal (SSU or SUS), fluid viscosity is the resistance of a fluid to flow. As fluid viscosity increases, the cSt rating increases. Higher fluid viscosities also mean higher pressure drop because the thicker oil has a tougher time passing through the layer of media fibers. Cold start fluid is a good example of highly viscous fluid. Filter media, amount of contamination, flow rate, and fluid viscosity are all factors in the importance of sizing the filter for the system requirements. Filters that are too small won’t be able to handle the system flow rate and will create excessive pressure drop from the start.The results could be filter operation in the by-pass mode, filter failure, component malfunction, or catastrophic system failures. Filters that are too large for the system can be too costly. Oversized filters require more system fluid and higher cost replacement elements. Finding the optimal filter size is important.
Donaldson Synteq synthetic filter media is comprised of smooth rounded fibers provide low resistance to fluid flow.
Cellulose filter media photo from scanning electron microscope magnified hundreds of times.
The result is media with predictable filtration efficiencies – removing specified contaminants and maximum dirt holding capacity. Natural cellulose fibers are larger than synthetic fibers and jagged in shape, so controlling size of the pores in the media mat is difficult and there is less open volume. In most applications this results in higher ∆P as compared to synthetic filters. Higher beta ratings
54 • Engine Liquid Filtration
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