Demystifying some of the technical terms in filtration.
Microfiltration is a filtration process using a microporous medias, membrane and depth filters, that retains the suspended solids of a fluid. Sometimes also called Process Filtration this dead-end filtration technology works in a range between 0.1 micron to 40 microns.
1 micron is 0.001mm (1/1000 of a millimetre). A pencil point is about 40 microns or 0.040mm (40/1000 of a millimetre) and one of the smallest things an unaided eye can see.
A size format used in industrial and manufacturing scale applications. Other formats include capsules & disposables for laboratory use, and junior sizes for development use.
A microporous synthetic semi permeable structure manufactured from PES, Nylon, PTFE, and Cellulose that is used to reduce or completely remove microorganisms or bacteria from a fluid. Typically, these are 100 microns thick.
Liquids pass through a tortuous maze of filter fibres that gradually become more compact. As the fluid passes further through this filter bed, smaller and smaller sizes of particulate contamination are removed by a filtration mechanism known as direct interception (or mechanical sieving). Commonly these are manufactured in polypropylene or glass fibre.
The purpose of a prefilter is to reduce overall operating cost by extending the life of the final filter. Typically, depth filters are prefilters to membrane filters.
Mechanisms of filtration
Suspended solids are separated from fluids via three mechanisms: direct interception, inertial impaction, diffusional motion and.
- Direct interception - This type of mechanism is characterized by a particle that follows a fluid streamline and does not deviate from that streamline.
- Inertial impaction - Inertial impaction refers to a particle's inability to follow sudden change in fluid flow direction.
- Diffusional motion - The random motion by small particles suspended in the fluid. It is a zig-zag motion of the particles. The particles move randomly as they are bombarded by molecules in the fluid causing them to become larger from a filtration perspective and easier to remove.
The diameter of the largest hard spherical particle that will pass through a filter under specified test conditions. It is an indication of the largest opening in the filter pore size in a filter medium.
An arbitrary micron value assigned by the filter manufacturer, based upon removal of some percentage of all particles of a given size or larger. It is rarely well defined and not reproducible.
The Beta ratio is a measure of the retention of a filter to solid particles of a given size and is defined as:
β = Number of particles of a given size and larger in the influent / Number of particles of a given size and larger in the effluent
A β = 5,000 can be used as an operational definition of an absolute rating.
Differential pressure is essentially the difference in pressure between two given points. For a filter it is measured between the inlet and outlet of the filter housing that the filter is housed in.
Clean differential pressure (ΔP)
Also known as the initial or design differential pressure but is the value when a filter is newly installed in a system. Typically, this is 2 psi (0.140 bar) for a liquid system.
Changeout differential pressure
The ΔP at which fluid flow is too low to continue using the cartridge filters. Typically, this is 36 psi (2.5 bar)