(28 Mar 2011)
INTRODUCTION to FLUID FILTRATION
Fluid filters (as opposed to electronic filters, an unrelated field), are porous materials and other means used to remove particulates and other solid and semi-solid contaminants from a fluid. Non-solid contaminants, such as dissolved gases or other materials can not be filtered; they must be removed by absorbtion, adsorption, distillation, and other means beyond the purview of this monograph. Entrained gases (in bubble form) may be removed by modified filter media.A fluid can be either a gas (air, steam, oxygen, nitrogen, etc.) or a liquid (water, oil, hydraulic fluid, gasoline, etc.).
Porous materials can be cleanable (reusable) or disposable. They can be made of wire mesh or felts or papers or perforated sheets, wound strands, etc.
Reusable mesh or porous metal can be sinter-bonded to prevent movement of fibers or particles relative to one another, thus assuring absolute pore size control and, thus, the flow rate through the medium.
Pore size and shape determines the maximum particle size that can pass the filter. Porous material construction determines the maximum differential pressure which can be imposed upon the medium. Cores and other reinforcements and backup materials placed against the downstream face of the porous medium can enhance the ability to withstand great differential pressures.
Accessories such as bypass valves and differential pressure indicators and alarms are commonly fitted to fluid filters.
Design philosophies must take into account whether it is better to have fluid flowing at all times, regardless of contamination, or to have flow interrupted rather than allow contaminant downstream of the filter medium.
Cleanable (reusable) fluid filters can be readily and throughly cleaned by ultrasonic means;
however care must be taken to assure cleanliness, integrity of the medium, and resultant
maximum pore size (absolute particle size rating).
(03 Mar 10)
THE BUBBLE POINT TEST
(28 Mar 2011)
In a rigid reusable filter medium, such as Pall Corporation's sintered PSS™ Porous Stainless Steel or RIGIMESH® woven wire mesh, the absolute filtration rating can be no lower than the diameter of the largest opening through the medium. Other factors such as tortuosity of path and van derWaal's and other attractive forces might shift the absolute rating slightly but the maximum poresize is a rather definitive limit.
If a filter medium is fully wetted and immersed in a test liquid, gas (usually air) introduced under the medium will remain in place until the static head (pressure) of the gas exceeds that in the largest pore, at which time the gas will escape through that pore, "unwetting" it such that a stream of gas will follow. Cylindrical elements are rotated under the liquid surface to test all portions of the medium.
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