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Filtration Glossary continued d-f

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Filtration Glossary continued d-f

Machine Coolant Filtration Glossary 

Here is a glossary of Filtration Terms and their meanings.  Please refer to the Machine Coolant Filtration Index for articles and information on these topics.

A - C  |  D  |  E  |  F |  G - M  |  N - R  |  S - Z


Dalton’s Law:  If several types of gas are put into the same container, the total pressure exerted is the sum of the partial pressures that each type of gas would exert if it alone occupied the container. 

Dead Areas:  Places in medium where no filtering occurs.  Also referred to as blind spots.  Opposite of the effective area. 

Dead Hand:  Indicator sometimes used on differential gauge to record maximum differential pressure reached between settings. 

Degradation:  Wearing down, or reduction in the efficiency of a media. 

Degree of Filtration:  The level of cleanliness of a fluid, usually measured in milligrams in a given volume such as quart, liter, gallon, etc.  See Article on How fine to Filter Machine Coolant. 

Dehydration:  Removal of water or hydrocarbon in vapor form from an air or gas, also water from another immiscible liquid.  Differs from entrainment removal in that the dew point of a gas stream will be lowered by vapor removal.  A form of purification. 

Delivery:  Opening through which effluent flows, as open delivery passes. 

AP:  A symbol designating pressure drop.  The difference in pressure between two points, generally measured at the inlet and outlet of a filter, separator/filter, etc.  Normally measured in pounds per square inch gauge (psi) or inches of mercury (In. Hg.). 

Denier Per Filament:  Weight in grams of a single continuous strand of yarn 9,000 meters long.  For each material the dpf is proportional to the cross-sectional area of the filament. 

Density:  Compactness or thickness; ratio of weight of a medium to the weight of an equal volume of fiber.  In general, mass per unit volume, usually expressed in grams per cubic centimeter or pounds per cubic foot, or gallons. 

Depth Type Filtration:  Filtration accomplished by flowing a fluid through a mass filter media providing a tortuous path with many entrapments to stop the contaminates.  Flow may be cross flow such as from the outside to inside and then down the center of an element.  Certain types of solids, or combinations of solids, do not lend to surface filtration where depth type filtration is found to be more suitable. 

Dessicant:  A drying agent or medium used in dehydration of air, gas, or liquids.  Examples:  silica gel, activated alumina, molecular sieve, etc. 

Detergent Oils:  Lubricating oils processing special sludge dispersing properties for use in internal combustion engines.  These properties are usually conferred on the oil by the incorporation of special additives. 

Dew Point:  Temperature at which air is saturated with moisture or in general the temperature at which a gas is saturated with respect to a condensable component. 

Diaphragm:  As used on valves, a diaphragm is a flexible material which permits the movement of internal parts without the loss or redistribution of pressure in the chamber. 

Diatomaceous Earth:  A soft earthy rock composed of the siliceous skeletons of small aquatic plants called diatom (algae).  The particles are pure silica and exhibit a large variety of non-uniform shapes. DE has a very low packing density and will form a highly porous, compression resistant cake on a filter. Frequently used as material for a precoat of a filter media.  Capable of absorbing 1.54 times its own weight in water.  Insoluble in acids except by hydrofluoric, and soluble in strong alkalis. 

Differential Pressure:  The difference in pressure between two given points of a filter, separator/filter, etc. The difference in pressure between two points in a system. Dif­ferential pressure measured across a working filter is a primary indication of the condition of the filter. See Absolute Pressure, Gauge Pressure, Head Pressure, Static Pressure. 

Differential Pressure, Maximum:  The highest pressure differential which an element is required to withstand without structural failure or collapse, or maximum recommended for service. 

Differential Rating:  Differential pressure specified as the maximum an element will withstand without structural failure. 

Diffusion, Movement from a region of high concentration to a region of low concentra­tion. Diffusion is caused by molecular scale collisions. In air filtration diffusion is an impor­tant mechanism responsible for capture of particles smaller than 0.3 (im in size. Parti­cles smaller than O.3 um show a significant degree of Brownian motion caused by colli­sions with air molecules. This motion increas­es the probably of contact with the filter material during transit of the filter. See inertia, interception.  In liquid filtration diffusion is an important mechanism governing the performance of re­verse osmosis separations. Rejection of ionic species by reverse osmosis requires diffusion across a concentration gradient. 

Digested Sludge:  Sludge or thickened mixture of water with sewage solids, the organic matter in which has been decomposed by anaerobic bacteria. 

Dilatant, A fluid property causing the fluid to become more viscous when subjected to stir­ring or other shearing forces. 

Dimensional Stability:  The ability of a material to retain its original dimensions. 

Direct Reading Differential Pressure Gauge:  Type of pressure gauge which automatically shows the differential pressure reading between two points.

Direction of Flow:  Direction in which product flows through element; may be from inside to outside, from outside to inside, or end to end, depending on the design of the element. 

Dirt Holding Capacity:  The volume of contaminate an element can hold before reaching the maximum allowable pressure drop.  Volume will vary depending on the size and design of the element and the density of the solid particles.  Usually reported by weight such as grams or pounds per element.  Also called solids retention or solids holding capacity. 

Discharge:  The liquid that leaves the vessel after passing through the medium.  May be different from filtrate; e.g., wash water discharge. 

Discharge Liquor:  The liquid which has passed through the filter.  Also referred to as effluent, filtrate, mother liquor, solute, strong liquor, product, clean and dry product, etc. 

Discharge Valve:  Valve to allow the effluent stream to flow out of the vessel; may be closed by the operation of a pilot valve, in which case product would not leave the vessel.  Also called a slug valve.  

Discharged Solids:  Solids deposited upon the filter medium during filtration in sufficient thickness to be removed in sheets or sizeable pieces.  Sometimes referred to as cake or residue. 

Discontinuous Phase:  The separated phase or product from the continuous phase. 

Discrete:  Composed of distinct units; separate from the other.  Usually referred to in describing the discontinuous phase. 

Dispersion:  The state of condition of being widely separated. A term describing the particles in a fluid.  See heterodisperse, monodisperse, polydisperse, suspension. 

Disposable:  Describes element which is to be discarded after use and replaced with an identical element.  Same as replaceable.  Opposite of reusable. 

Dissolved Air Flotation,  A separation technique.  Air is dissolved in the liquid under pressure and then the pressure is released.  Tiny air bubbles form in the liquid as the gas comes out of solution.  As the air bubbles rise to the surface of the liquid they carry suspended contaminants with them. 

Distillate:  The condensed product of distillation, as certain petroleum products. 

Distillation:  The process of vaporizing a liquid and collecting the vapor, which is then usually condensed to a liquid.  In industry, this process is widely used to separate the components in a liquid solution.  Since every pure compound boils at a specific temperature it is often possible to boil off one liquid while most of the higher-boiling-point material remains in its liquid state.  It is extremely difficult and sometimes impossible to separate materials in this way when they boil at the same, or near the same, temperature. 

Divider:  Component of a vessel; metal plate used to create chambers such as inlet and outlet chambers. 

Doctor Knife or Blade:  The straight edge that contacts a surface where contaminants have accumulated to skim them off. Scraper for filter cake removal; a knife or blade used to maintain the thickness of cake or precoat by trimming it down at certain intervals.  Frequently used on a drum type filter. 

Dole Value,  A simple flow control device which uses a deformable rubber orifice to maintain constant flow regardless of upstream pressure.  Dole values are used to protect pumps and filters from overload if the system resistance to flow drops too low. 

DOP Test, DOP is an abbreviation for Dioctyl Phthalate.  DOP is a stable oil used to generate a liquid aerosol.  The standard ASTM test calls for vaporization and controlled condensation of the DOP, resulting in a near monodisperse aerosol of droplets 0.3 um in diameter.  The DOP test is commonly used to challenge high efficiency filters like HEPA filters, which by definition are 99.97% efficient at capturing particles 0.3 um in diameter.  0.3 um diameter particles are the most penetrating particles for air filters. 

Downstream:  Portion of the product stream which has already passed through the system, or the portion of a system.   

Drain Valve:  Valve for draining off material which has been separated from a fluid stream.  Also a diaphragm type valve that operates as a part of a system to evacuate a discontinuous phase automatically. 

Drainage Cloth:  Open mesh, coarse yarn fabrics, knit, leno, or simple woven types for support of paper or closely woven fabrics.  They provide depth for drainage and/or support to prevent media breakage or solids buildup beneath the media.  Also called paddock cloth. 

Drop:  The quantity of fluid which falls into one spherical mass; a liquid globule.  May also be described as several to many droplets. 

Droplet:  A minute drop which mates to form larger drops capable of failing by gravity. 

Dual Gravity Valve:  Float operated valve which operates on the interface of two immiscible liquids of different specific gravities and which operates to release one of the liquids. 

Duck:  Covers a wide range of the most durable fabrics made.  Closely woven and heavy, the most common are number duck, army duck, and flat or ounce duck.  The number and ounce ducks are most often used in liquid filtration in the process industries. 

Durometer (Shore):  Measurement of the resiliency of gasket material. 

Dynamic Adsorption:  An adsorption process in which there is constant relative motion between the material being treated and the adsorbent.  In most cases, the gas or liquid being treated is made to flow through a fixed bed of adsorbent.  The drying of annealing gas is an example of dynamic adsorption.  The gas is pumped through an adsorber where it is dried and then, with the moisture removed, it is pumped into the bright annealing furnace. 

Dyne:  The amount of force that causes a mass of one gram to alter its speed by one centimeter per second for each second during which the force acts.


Edge Type:  Applies to liquid filters constructed of metal or paper discs, or specially constructed wire wound cylinders; contaminates are entrapped at the edges of the medium.  Generally used to remove small quantities of very fine particles.  Some have knife or blade cleaners to remove the accumulated solids. 

Effective Area:  The area of a medium that is exposed to flow and usable for its intended purpose; coalescing, filtering, or separating.  Opposite of blind spots or dead area. 

Efficiency:  Degree to which element will perform in removing solids and/or liquids. 

Effluent:  Stream of fluid at the outlet of a filter or separator/filter.  Opposite of affluent or influent. 

Element:  Medium used in a vessel to perform the function of coalescing, filtering, or separating.  Also referred to as a cartridge, repack, etc. 

Element cap:  Component which covers one end of an element and holds the element in place in the vessel.  Sometimes called a yoke. 

Elongation:  The deformation caused by a tensile force, expressed as a percentage of the original length. 

Elutriation:  Washing of the suspended solids in sludges. 

Emulsification:  A dispersion of one substance in the form of minute drops within another substance. 

Emulsion:  A stable suspension of small oil-like particles in water.  The stability of emulsions is usually influenced by pH, ionic concentration and the presence of chemical additives. A dispersion of fine liquid particles in a liquid stream which do not necessarily dissolve in each other but are held in suspension.  Many emulsions may be broken by coalescing if the liquids are immiscible.  Emulsion stabilizers modify the surface tension of the droplets which makes coalescing difficult, if not impossible. 

End Caps:  Components adhered to ends of an element with adhesive or by other means to contain medium in the form designed for the element. 

Entrained Water:  Discrete water droplets carried by a continuous liquid or gas phase when water is immiscible with the liquid.  May be separated from the continuous phase by coalescing and gravity separation.  Usually picked up in a system by condensation or a water washing used in process. 

Entrainment:  Mist, fog, or droplets of a liquid— which is usually considered to be a contaminate when used in the filtration industry. 

Equalizing Line:  A connection to equalize the vacuum or pressure at two separate points. 

Equilibrium,  A condition in a recirculating system when the rate of contaminants entering the fluid is equal to the rate of removal of contaminants. 

Equilibrium Loading:  Maximum loading for a given temperature, pressure, and concentration.  Also used to describe equality of distribution on the surface of a filter media. 

Equivalent Spherical Diameter,  A value commonly reported by particle size measuring equipment.  An instrument may measure the volume or projected area of an irregularity shaped particle, without gathering any information about the particle shape.  This volume or area information is then converted to a number which is the diameter of a sphere having a volume or projected area equal to that of the measured particle.  This number reported by the instrument is the equivalent spherical diameter. 

Eutectic:  Characteristic of a mixture having the lowest possible melting point of two or more constituents which melt completely at a definite temperature to form a single liquid. 

Excelsior:  Wood type frequently used as a coalescing media for light products and a bulk filter.  Selected Aspen type excelsior with selected strand thickness, and averaged length is packed into a cylinder, generally for end to end flow. 

Extractables,  Substances present in a filter that will dissolve in the fluid being filtered.  Extractables are undesirable because they contaminate the fluid.


FDA,  Food and Drug Administration.  FDA guidelines describe how to test or select materials intended for food contact.  Filters to be used in food processing must comply with FDA guidelines. 

Federal Stock Number:  Identification number assigned by the U.S. government to code the specific descriptions of items purchased for their stock; number is preceded by letters FSN. 

Feed:  Material to be filtered.  Also referred to as concentrate, influent, intake, liquor, mud, prefilt, pulp, slimes, or sludge. 

Felt:  A fabric built up of the interlocking of fibers by combination of mechanical work, chemical action, moisture, and heat.  Frequently used as a filter medium, some types of which have been adapted to high temperature applications for air or gas. 

Fiber:  The fundamental unit comprising a textile raw material such as cotton, wool, etc.  Fibers may be elongated, single celled seed hairs like cotton; elongated, multicellular structures like wool; aggregations of elongated cells like flax; short sections of continuous filaments like rayon staple and glass.  Fibers have a length which is many times as great as their diameter.  In order to be spun into a yarn, fiber must possess sufficient length, strength, pliability, and cohesiveness. 

Fiber Migration:  The carryover of fibers from the media used in coalescer, separator, or filter cartridges into the effluent.  More definitive than media migration since fiber migration is qualitative.  U.S. military specifications define a fiber for the purpose of determining fiber migration as any fiber with a length to diameter ratio of 20 or greater and at least 100 microns long, but this definition is not considered as a standard in industry.

Fiber migration is undesirable because it contaminates the fluid.  When fiber migration occur it is usually a transient condition experienced during the first few moments after a new filter is installed.  Fiber migration can be caused by excessive differential pressure, pulsing flows or chemical attack.  See media migration, time-to-cleanup. 

Fiberglass:  A term used to describe a variety of products made of or with glass fibers.  See glass fibers. 

Filament:  A single, continuous strand of indefinite length, such as rayon, nylon, or silk.  Compared to staple fibers such as cotton and wool, a filament possesses extreme length and often may be measured in thousands of yards without a break. 

Fill Wires:  Wires running the short way of the filter cloth as woven.  Often referred to as “shute“ wires.

Film Strength:  The property of a lubricant which permits it to resist being ruptured or broken between two metal surfaces. 

Filter,  (Noun),  A device used to remove suspended particles from a suspending fluid.  (Verb),  To filter something, the act of removing suspended particles from a suspending liquid. 

A term generally applied to a device used to remove solid contaminate from a liquid or gas, or separate one liquid from another liquid gas.  A filter, as referred to in the industry today, is limited to a device which removes solid contaminates only.  If a device is used to remove solid and liquid contaminates it is referred to in general terms as a separator, separator/filter, or entrainment separator.  A filter may be one of a number of such types as replaceable cartridge, cyclone, edge, leaf, battle, plate and frame, precoat, centrifuge.  The term filter is sometimes erroneously used to describe the media used inside the vessel or filter case, but the correct use should be filter element, cartridge, etc. 

Filter Aid:  A substance of low specific gravity which remains in suspension when mixed with the liquid to be filtered.  It should be porous and must be chemically inert to the liquid being filtered.  Increases filtering efficiency and maintains cake porosity. Filter aids are usually powdered but may be loose fibers.  Common filter aids are diatomaceous earth, Fuller’s earth, activated carbon, cellulose, bleaching earth. 

Filter Cloth:  The porous material mounted on a plate or frame which separates the solids from the liquids in filtering.  Also referred to as filter medium, filter plate, or septum. 

Filter/Coalescer:  Single stage, horizontal vessel for coalescing and separating one immiscible fluid from another and the removal of solids.  Generally recommended for use where continuous phase has a light gravity. 

Filter Effect:  Gradual increase of filter resistance. 

Filter Medium:  The porous material mounted on a plate or frame which separates the solids from the liquids in filtering.  Also referred to as filter cloth, filter plate, or septum. 

Filter Plate:  The porous material mounted on a plate or frame which separates the solids from the liquids in filtering.  Also referred to as filter cloth, filter medium, or septum. 

Filter Press,  A type of filter characterized by a framework with many parallel plates holding filter media. 

Filter/Separator:  A term used to describe a unit capable of separating two immiscible liquids while performing the dual function of filtering out solid contaminates.  More commonly referred to as a separator/filter. 

Filterability:  The ease of difficulty of filtering. 

Filters, Bulk Type:  Similar to depth type filters; however, the density is not graded, and therefore the reproducibility of the filtration varies. 

Filters/Depth Type:  See depth type filters. 

Filters/Edge Type:  See edge type filters. 

Filters/Inert Type:  Designed to remove insoluble contaminates; all filters except the absorbent and adsorbent types can be generally considered inert. 

Filter/Surface Type:  Usually constructed of pleated, impregnated paper and/or a combination of paper and other media, such as glass fibers.  Opposite of depth type.  See extended area. 

Filtrate:  The liquid which has passed through the filter; the fluid that has been separated from the solids in the slurry being filtered.  Also referred to as discharge liquor, effluent, mother liquor, solute, or strong liquor. 

Filtration:  The process of removing solid particles from liquid or gas by forcing them through a porous medium.  See the articles on Machine Coolant Filtration. 

Fines:  The portion of a powderlike material composed of particles which are smaller than a specified size. 

Fire Point:  The lowest temperature at which a liquid evolves vapors fast enough to support continuous combustion; usually close to the flash point. 

Flash Distillation:  Operation wherein the liquid is vaporized immediately on entrance to the vessel. 

Flash Point:  The lowest temperature at which a combustible liquid will give off a flammable vapor which will burn momentarily. 

Flat Top (Wire Cloth):  Warp and fill wires crimped in such a manner that all crimps are on underside or screen, thus providing a smooth, flat surface. 

Float Control:  Float operated pilot valve which controls other valves or the opening of an orifice; generally used for the elimination of the discontinuous phase; operates on the interface of two immiscible liquids, or in the case of air or gas would operate on the surface of a liquid. 

Floating Scraper:  Balanced scraper which with minimum pressure follows the contour of a drum. 

Flocculation:  The growing together of minute particles to form larger ones, which are called flocs and are easier to filter.  Also referred to as coagulation. 

Flow Rate:  The rate at which a product is passed through a vessel or system; generally expressed as gallons per minute, barrels per hour, barrels per day, actual or standard cubic feet per minute, hour, day, etc.  Same as rate of flow. 

Fluid:  Term used in filtration and separation to include liquids, air, or gas as a general term. 

Fluid Reconditioner:  Generally a filter system to remove impurities from a liquid to a degree that the liquid may be reused for its original purpose without loss of efficiency or adverse effects.  

Fog:  Condensed water, hydrocarbon, or other liquids which are visible to the naked eye. 

Fractionation Tower:  Tower wherein rising vapors meet descending liquid.  The lower boiling liquids tend to pass on to the condenser, and the higher boiling phases remain as liquids. 

Frazier Permeometer: A porosity testing device.  The normal measurement is the air flow in CFM passed through one square foot of fabric at 1/2” water pressure.  Fabrics with rating of 1 to 10 CFM are considered very tight, whereas, cloths that test at 450 to 500 CFM are extremely porous.  The test is used on woven and non-woven materials. 

Free Board:  Clear space between top of liquid and rim of vessel. 

Freezing Point:  For a pure substance the freezing point or melting point is the temperature at which the liquid and solid are in equilibrium with one another; i.e., at a higher temperature the solid will melt and at a lower temperature the liquid will solidify.  Varies with the composition of the liquid. 

Friction:  The resistance created by the surface of one object moving over the surface of another, due to the interlocking of minute projections from the surfaces; this is called “solid” friction  If a lubricant is introduced between the two moving surfaces “fluid” friction replaces “solid” friction. 

Fuel Monitor:  Vessel which monitors the cleanliness of product streams and shuts off all flow when water or solids contamination reaches predetermined concentration; moisture-sensitive elements trap contaminates and increase differential pressure to activate shut-off diaphragm. 

Fuel Wettable:  Capable of accepting liquid fuels.  Opposite of non-wettable. 

Full Flow:  Product flow through the vessel at the rate for which the vessel is designed to operate.  Also refers to all product passing through a medium.  The opposite of a bypass filter which is designed to filter only a portion of the steam at a higher efficiency on a continuous basis.