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Glossary of Fabric Filter Terms (Air Filtration)


ABRASION – FLEX: Where the cloth has abraded in a creased area by repeated bending.

ABRASION – SURFACE: Where the cloth surface has been abraded by rubbing, scuffing, erosion.

ABSOLUTE ZERO: The zero from which absolute temperature is reckoned. Minus 460°F., approximately.

ACETATE: A manufactured fiber in which the fiber forming substance is cellulose acetate.

ACRYLIC: A man-made polymerized fiber which contains at least 85% acrylonitrile.

AERODYNAMIC DIAMETER, KINETIC DIAMETER: The diameter of a sphere of unit density having the same terminal settling velocity as a particle in question, regardless of its geometric size, shape, and true density. For certain particles of unknown shape, size, and density, aerodynamic diameter may be obtained from measures of settling velocity, impaction behavior, etc.

AEROSOL: An assemblage of small particles, solid or liquid, suspended in air      gas.

AGGLOMERATE (n.), AGGREGATE (n.). A particle consisting of two or more unitary structures (e.g., a doublet, triplet,..etc.).

AGGLOMERATION (n.) See coagulation.

AIR, DRY: In psychrometry, air containing no water vapor.

AIR, STANDARD: Air with a density of 0.075 lb. per cubic foot. This is substantially equivalent to dry air at 70°F. and 29.92 in. (Hg) barometer.

AIR-TO—CLOTH-RATIO: The volumetric rate of capacity of a fabric filter; the volume of air (gas) cubic feet per minute, per square foot of filter media (fabric).

ANEMOMETER: An instrument for measuring the velocity of air or gas.

ATMOSPHERIC PRESSURE: The pressure of the atmosphere as measured by means of the barometer at the location specified.

BACKWASH: A method of fabric cleaning in which direction of gas flow is reversed, accompanied by flexing of the fabric and breaking of the dust deposit; also called backpressure, repressure, collapse-clean.

BAG: The customary form of filter element. Also tube, stocking, etc. Can be unsupported (dust on inside) or used on the outside of a grid support (dust on outside).

BATCH CLEANED: Usually refers to a process used in heat cleaning fiber glass cloth in roll form by exposing it at 500°F. to 600°F. for prolonged periods to burn off the starches or binders.

BLAST GATE: A sliding plate installed in a supply or exhaust duct at right angles to the duct for the purpose of regulating air flow.

BLINDING (BLINDED): The loading, or accumulation, of filter cake to the point where capacity rate is diminished. Also termed “plugged”.

BRITISH THERMAL UNIT (btu): The amount of heat required to raise one pound of water one degree Fahrenheit.

BROKEN TWILL: Modified twill weave where the diagonal twill line is shifted in a regular pattern.

BULKED YARN: Multi-filament yarn which has been processed by high pressure air passing through the yarn and relaxing it into gentle loops, bends, etc.

CALENDERINO: The application of either hot or cold pressure rolls to smooth or polish a fabric, thereby reducing the thickness of the cloth and decreasing air permeability.

CANTON FLANNEL: Usually a twill weave fabric with the filling float heavily napped.

CHAIN WEAVE: A 2/2 broken twill weave, arranged 2 threads right and 2 left.

CLOTH: In general, a pliant fabric; – woven, knitted, felted, or otherwise formed of any textile fiber, wire, or other suitable material. Usually understood to mean a woven textile fabric.

CLOTH WEIGHT: Is usually expressed in ounces per square yard or ounces per square foot. However, cotton sateen is often specified at a certain number of linear yards per pound of designated width. For example, a 54″ – 1.05 sateen weighs 1.05 linear(‘ yards per pound in a 54″ width.

COAGULATION (n.) The process of inter particle contact and cohesion.

C’ ,SENSATION: The process of changing a vapor into liquid by the extraction of heat.

CORONIZING: A heat cleaning process for fiber glass fabric to burn off the starches (used in processing) usually at temperatures of 1000°F. for short duration.

CORROSION: Deterioration or physical degradation due to chemical action.

COTTON NUMBER: Staple yarns are generally sized on the cotton system. Example: an 18 singles yarn is of such size that 18 hanks (each hank contains 840 yards) weighs one pound.

COUNT: The number of warp yarns (ends) and filling yarns (picks) per inch. Also called thread count.

COUNT MEDIAN SIZE: A measurement of particle size for samples of particulate matter, consisting of that diameter of particle such that one half of the number of particles is larger and half is smaller.

COVER: A description term for the appearance of woven goods. A well covered cloth is the opposite of an open, or “reedy” cloth.

CRIMP: The corrugations in a yarn from passing over and under other yarns at right angles.

CROWFOOT SATIN: A 3/1 broken twill arranged 2 threads right, then 2 threads left, etc. Also called 4 shaft satin, or broken crow weave.

DAMPER: An adjustable plate installed in a duct for the purpose of regulating air flow.

DEHUMIDIFY: To reduce by any process the quantity of water vapor.

DENIER: The number, in grams, of a quantity of yarn, measuring 9000 meters in length. Example: A 200 denier yarn measuring 9000 meters weighs 200 grams. A 200/80 yarn indicates a 200 denier yarn composed of 80 filaments. Usually used for continuous multi-filament yarns of silk, rayon, Orlon, Dacron, Dynel, Nylon, etc.

DENSITY: The ratio of the mass of a specimen of a substance to the volume of the specimen. The mass of a unit volume of a substance. Dry air at 70°F. and 29.92″ Hg has a density of 0.075 pounds per cubic foot.

DIMENSIONAL STABILITY: Ability of the fabric to retain finished length and width, under stress, in hot or moist atmosphere.

DRILL: Same as twill except the diagonal twill line usually runs from lower right to upper left. A 2/1 LH twill, or 3/1 twill.

DUST: Term applied to solid particles predominantly larger than colloidal and capable of temporary suspension in air or other gases. Dusts do not tend to flocculate except under electrostatic forces; they do not diffuse but settle under the influence of gravity. Derivation from larger masses through the application of physical force is usually implied.

DUST COLLECTOR: A device to remove solid aerosol particles from a gas stream.

DUST LOADING: The weight of solid particulate suspended in an air (gas) stream,usually expressed in terms of grains per cubic foot, grams per cubic meter or pounds per thousand pounds of gas.

DUST PERMEABILITY: Defined as the mass of dust (grains) per square foot of media divided by the resistance (pressure drop) inches w.g. per unit of filtering velocity, fpm. Not to be compared with cloth permeability.

END:* An individual yarn or cord; a warp yarn running lengthwise of the fabric.

ENTRY LOSS: Loss in total pressure caused by air (gas), flowing into a duct or hood (usually expressed in inches w.g.).

ENVELOPE: A common form of filter element.

ERMON: Wearing away due to mechanical action.

EXTENSIBILITY: The stretching characteristic of fabric under specific conditions of load, etc.

FABRIC: A planar structure produced by interlacing yarns, fibers or filaments.

KNITTED FABRICS: are produced by interlooping strands of yarn, etc.

WOVEN FABRICS: are produced by interlacing strands at more or less right angles.

BONDED FABRICS: are a web of fibers held together with a cementing medium which does not form a continuous sheet of adhesive material.

FELTED FABRICS are structures built up by the interlocking action of the fibers themselves, without spinning, weaving or knitting.

FERET’S DIAMETER: The normal distance between two parallel tangents to the extreme points on the particle measured in a consistent manner.

FIBER: The fundamental unit comprising a textile raw material such as cotton, wool, etc.

FILAMENT: A continuous fiber.

FILL: Crosswise threads woven by loom.

FULL COUNT: Number of fill threads per inch of cloth.

FILLING YARN: Yarns in a fabric running across the width of a fabric; i.e., at right angles to the warp.

FILTER DRAG: Pressure drop, inches w.g. per cubic foot of air per minute, per square foot of filter media. Analogous to tee resistance of an element in an electrical circuit. The ratio of filter pressure to filter velocity.

FILTER MEDIUM: The substrate support for the filter deposit; the fabric upon which the filter deposit is built.

FILTER VELOCITY: The velocity, feet per minute, at which the air (gas) passes through the filter media, or rather the velocity of approach to the media. The filter capacity rate.

FILTRATION RATE: The volume of air (gas), cubic feet per minute, passing through one square foot of filter media.

FINISHED: A fabric which has been processed after weaving, i.e., other than in the greige.

FLAME RETARDANT: A finish designed to repel the combustibility of a fabric, either of a durable or non-durable type.

FLOAT: The position of a yarn that passes over two or more yarns passing in the opposite direction. Example: in standard cotton sateen, yarns “float” four, and pass under one. In other words 4/1.

FLOCCULATION: Synonymous with coagulation.

FLUOROCARBON: Fiber formed of long chain carbon molecules, available bonds saturated with fluorine.

FOG: Suspended liquid droplets generated by condensation from the gaseous to the liquid state, or by breaking up a liquid into a dispersed state, such as by splashing, foaming and atomizing. (See mist)

FULLED: A woven fabric treated to raise fiber ends (like napping) so that the thready, woven look is partially or completely obscured.

FUME: Properly, the solid particles generated by condensation from the gaseous state, generally after volatilization from melted substances, and often accompanied by a chemical reaction such as oxidation. Fumes fluctuate and sometimes coalesce. Popularly, the term is used in reference to any or all types of contaminant, and in many laws or regulations with the added qualification that the contaminant have some unwanted action.

GAS: One of the three states of aggregation of matter, having neither independent shape nor volume and tending to expand indefinitely.

GLASS (FIBER-GLASS): A manufactured fiber in which the fiber forming substance is glass.

“GRAB” TENSILE: The tensile strength, in pounds per inch, of a textile sample cut 4″ x 6″ and pulled in two lengthwise by two 1″ square clamp jaws set 3″ apart and pulled at a constant specified speed.

GRAIN: 1/7000 pound or approximately 65 milligrams.

GRAVITY, SPECIFIC: The ratio of the mass of a unit volume of a substance to the mass of the same volume of a standard substance at a standard temperature. Water is usually taken as a standard substance, Sp.g..1.0. For gases, dry air at the same temperature and pressure as the gas may be taken as the standard substance.

GREEN’S DIAMETER: The average diameter of a hypothetical particle that is in some way representative of the particles in the sample.

GREIGE CLOTH: Cloth as it comes off the loom, or so-called “loom finish”.

GRID CLOTH: The cloth used in supporting the silver in making a supported, needled felt.

HAND OR HANDLE: The “feel” of the cloth – as soft, harsh, smooth, rough, silken-like, boardy, etc.

HARNESS: The frame used to raise or lower those warp yarns necessary to produce a specific weave at the same time permitting the filling to be passed through by the shuttle.

HEAD END: A piece of fabric taken from the end of a roll of cloth.

HEAD SET: A finishing process for that particular fiber, in fabric form, to stabilize it against further shrinkage at predetermined temperatures.

HEAT, SPECIFIC: The heat absorbed (or given up) by a unit mass of a substance when its temperature is increased (or decreased) by one degree. The common unit is the btu per degree Fahrenheit. For gases, both specific heat at constant pressure (c p) and specific heat at constant colume (cv) are frequently used.

HOOD SUCTION: The entry loss plus the velocity pressure in the connecting duct.

HUMIDITY, ABSOLUTE: The weight of water vapor carried by a unit weight of dry air or gas. Pounds of water vapor per pound of dry air; grains of water vapor per pounds dry air.

HUMIDITY, RELATIVE: The ratio of the absolute humidity in a gas to the absolute humidity of a saturated gas at the same temperature.

HYDROPHILIC FIBERS: Those fibers which are water absorbent.

IMPACTION: A forcible contact of particles of matter, a term often used synony­mously with impingement.

INCH OF WATER: A unit of pressure equal to the pressure exerted by a column of liquid water one inch high at a standard temperature. The standard temperature is ordinarily taken as 70°F. One inch of water at 70°F.=5.196 lb per sq. ft.

INTERLACING: The points of contact between the warp and filling yarns in a fabric.

INTERSTICES: The openings between the interfacings of the warp and filling yarns; i.e., the voids.

ISOKINETIC: A term describing a condition of sampling, in which the flow of gas into the sampling device (at the opening or face of the inlet) has the same flow rate and direction as the ambient atmosphere being sampled.

K FACTOR: The specific resistance of the dust cake, inches water gauge per pound of dust per square foot of filter area per feet per minute filtering velocity.

LOOM FINISH: Same as greige cloth.

MANOMETER: An instrument for measuring pressure; a U-tube partially filled with a liquid, usually water, mercury or a light oil, so constructed that the amount of displacement of the liquid indicates the pressure being exerted on the instrument.

MARTIN’S DIAMETER: The distance between opposite sides of the particle, measured in a consistent direction, such that the diameter bisects the projected area.

MASS MEDIAN SIZE: A measurement of particle size for samples of particulate matter, consisting of that diameter such that the mass of all larger particles is equal to the mass of all smaller particles.

MICRON: A unit of length, the thousandth part of 1 mm or the millionth of a meter, (approximately 1/25,000 of an inch).

MILDEW RESIST FINISH: An organic or inorganic finish to repel the growth of fungi on natural fibers.

MIST: Liquid, usually water in the form of particles suspended in the atmosphere at or near the surface of the earth; small water droplets floating or falling, approaching the form of rain, and sometimes distinguished from fog as being more transparent or as having particles perceptibly moving downward.

MODACRYLIC: A man-made fiber which contains less than 85% acrylonitrile (at least 35% ).

MOL: A weight of a substance numerically equal to its molecular weight. If the weight is in pounds, the unit is “Pound Mol”. For dry air at 70°F., and a pressure of one atmosphere, a pound mol occupies 386 cubic feet.

MONOFILAMENT: A continuous fiber of sufficient size to serve as yarn in normal textile operations.

MULLEN BURST: The pressure necessary to rupture a secured fabric specimen, usually expressed in pounds per square inch.

MULTIFILAMENT: (Multifil) A yarn bundle composed of a number of filaments.

NAPPED: A process to raise fiber or filament ends (for better coverage and more surface area) accomplished by passing”the cloth over a large revolving cage or drum of small power-driven rolls covered with card clothing (similar to a wire brush).

NEEDLED FELT: A felt made by the placement of loose fiber in a systematic alignment, with barbed needles moving up and down, pushing and pulling the fibers to form an interlocking of adjacent fibers.

NON-WOVEN FELT: A felt made either by needling, matting of fibers or compressed with a bonding agent for permanency.

NYLON: A manufactured fiber in which the fiber forming substance is any long-chain synthetic polyamide having recurring amide groups.

OLEFIN: A manufactured fiber in which the fiber forming substance is any long-chain synthetic polymer composed of at least 85% by weight of ethylene, propylene, or other olefin units.

PARTICLE: An aerosol particle may consist of a single continuous unit of solid or liquid containing many molecules held together by intermolecular forces and primarily larger than molecular dimensions (> 0.001 pm). A particle may also be considered to consist of two or more such unit structures held together by interparticle adhesive forces such that it behaves as a single unit in suspension or upon deposit.

PARTICLE SIZE, PARTICLE DIAMETER: Some consistent measure of the spatial extent of the matter constituting the particle, such as a measured diameter, area, or volume; or some consistent measure of the behavior of the particle as a function of the spatial extent. If particles are spherical or cubical, particle size corresponds to a single measurable geometric length. If particles are irregularly shaped, diameter is defined by consistent arbitrary measures. Other regular geometric shapes may be defined in terms of dimension ratios.

PARTICLE SIZE DISTRIBUTION: The frequency with which particles of various sizes are distributed in a given sample.

PERMEABILITY, FABRIC: Measured on Frazier porosity meter, or Gurley permeometer, etc. Not to be confused with dust permeability. The ability of air (gas) to pass through the fabric, expressed in cubic feet of air per minute per square foot of fabric with an 0.5″ H2O pressure differential.

PICK: An individual filling yarn running the width of a woven fabric at right angles to the warp. In England it is termed woof, or weft.

PICK GLASS: A magnifying glass used in counting the warp and filling yarn in the fabric.

PITOT TUBE: A means of measuring velocity pressure. A device consisting of two tubes – one serving to measure the total or impact pressure existing in an air stream, the other to measure the static pressure only. When both tubes are connect across a differential pressure measuring device, the static pressure is compensated k automatically and the velocity pressure only is registered.

PLAIN WEAVE: Each warp yarn passing alternately over each filling yarn. The simplest weave, 1/1 construction. Also called taffeta weave.

PLENUM CHAMBER: An air compartment maintained under pressure, and connected to one or more ducts. A pressure equalizing chamber.

PLY: Two or more yarns joined together by twisting.

POLYESTER: A manufactured fiber in which the fiber forming substance is any long-chain synthetic polymer composed of at least 85% by weight of an ester of dihydric alcohol and terephthalic acid.

POROSITY, FABRIC: Term often used interchangeably with permeability. Actually percentage of voids per unit volume – therefore, the term is improperly used where permeability is intended.

PRESHRUNK: Usually a hot aqueous immersion of the cloth to eliminate its tendency to shrink in the further wet performances.

PRESSURE, ATMOSPHERIC: The pressure due to the weight of the atmosphere, as indicated by a barometer. Standard atmospheric pressure is 29.92* of mercury. Equivalents in other units are 760 mm of mercury, 14.7 psi, and 407″ water column.

PRESSURE, GAUGE: Pressure measured from atmospheric pressure as a base. Gauge pressure may be indicated by a manometer which has one leg connected to the pressure source and the other exposed to atmospheric pressure.

PRESSURE JET CLEANING: A bag cleaning method where a momentary burst of compressed air is introduced through a tube or nozzle attached to the top cap of a bag. A bubble of air flows down the bag, causing bag walls to collapse behind it.

PRESSURE, RESISTANCE: Resistance pressure (RP) is the pressure required to overcome the resistance of the system. It includes the resistance of straight runs of pipe,entrance to headers, bends, elbows, orifice loss, and cleaning device. It is indicated by the difference of total pressure between two points in the duct system.

PRESSURE, STATIC: The potential pressure exerted in all directions by a fluid at rest. For a fluid in motion, it is measured in a direction normal to the direction of flow. Usually expressed in inches water gauge, when dealing with air.

PRESSURE, TOTAL: The algebraic sum of the velocity pressure and the static pressure (with due regard to sign). In gas-handling systems these pressures are usually expressed in inches water gauge. The sum of the static pressure and the velocity pressure.

PRESSURE, VELOCITY: The kinetic pressure in the direction of flow necessary to cause a fluid at rest to flow at a given velocity. Usually expressed in inches water gauge.

PROJECTED (AREA) DIAMETER: The diameter of a circle of area equal to the profile of the projected area of the particle.

PULSE JET: A system of bag cleaning using a momentary burst of compressed air in the discharge nozzle of a filter bag, which stops filter flow and inflates the bag in the opposite direction.

RAVEL STRIP TENSILE: The tensile strength, in pounds per inch of a 6″ long textile sample cut just over one inch wide, (with yarns peeled off each side down to exactly one inch wide) pulled in two lengthwise between jaws set 3″ apart and pulled at a constant specified speed.

RAYON: A manufactured fiber composed of regenerated cellulose.

REED MARKS: The indentations between 2, 3, or 4 ends, usually eliminated in finishing.

REPEAT: The number of threads in a weave before the weave repeats or starts over again. The number of ends and picks in the repeat may be equal or unequal, but in every case the repeat must be in a rectangular form.

RESISTANCE: Analogous to electrical resistance – the pressure drop across the filter media and dust cake – expressed in inches water gage.

REVERSE JET CLEANING: A cleaning method (Hersey) using a traveling ring traversing the exterior of the filter bag. High pressure air is blown backwards through the fabric through small holes or slots in contact with the cloth.

SANFORIZED: A patented process where the cloth is “puckered” in the warp direction to eliminate shrinkage in laundering.

SARAN: Any long-chain synthetic polymer composed of at least 85% vinylidene -chloride units.

SATEEN: Cotton cloth made with a 4/1 satin weave, either as warp sateen or filling sateen.

SATIN WEAVE: A fabric usually characterized by smoothness and luster. Generally made warp face with a great many more ends than picks. The surface consists almost entirely of warp (or filling) floats in construction 4/1 to 7/1. The intersection points do not fall in regular lines, but are shifted in a regular or irregular manner.

SCOUR: A soap and hot water wash to “off loom” fabric.

SELVAGE: The binding lengthwise edge’of a woven fabric.

SETTLING VELOCITY: The terminal rate of fall of a particle through a fluid as induced by gravity or other external force; the rate at which frictional drag balances the accelerating force (or the external force).

SHAKING (CLEANING): A common, mechanical method of removing dust from filter elements. Backwash, or other supplemental methods, are often used with shaking. Air-shaking is a bag cleaning means wherein bags are shaken in a random fashion by high velocity air stream rather than by mechanical devices.

SINGEING: The burning off of the protruding hairs from the warp and filling yarns of the fabric.

SINGLES: The term used to imply only one yarn.

SIZING: A protective coating applied to yarn to insure safe handling; i.e., abrasion-resistance during weaving.

SLEAZY: Lacking in firmness or substance; thin, flimsy.

SLIPPAGE: The movement or shifting of yarns in a fabric from their normal position. SLOB: A heavy accumulation of fiber or lint carried on a yarn and interlocked during weaving.

SMOG: A term derived from smoke and fog, applied to extensive atmospheric contamination by aerosols, these aerosols arising partly through natural processes and partly from the activities of human subjects. Now sometimes used loosely for any contamination of the air.

SMOKE: Small gas-borne particles resulting from incomplete combustion, consisting predominantly of carbon and other combustible material, and present in sufficient quantity to be observable independently of the presence of other solids.

SONIC (SOUND): A fabric cleaning method using acoustic energy to vibrate the filter elements. Used alone, or as a supplement to shaking, or backwash cleaning.

SOOT: Particles of carbon impregnated with “tar,” formed in the incomplete combustion of carbonaceous material.

SPUN FABRIC: Fabric woven from staple (spun) fiber – same as staple.

STANDARD ATMOSPHERE: The pressure exerted by a column of mercury 29.92″ high at 70°F approximately 14.7 psi.

STAPLE FIBER: Man-made fibers cut to specific length – 1-1/2″, 2″, 2-1/4″ etc. -natural fibers of a length characteristic of fiber, animal fibers being the longest.

STOKES’ EQUIVALENT DIAMETER: The diameter of a hypothetical sphere having the same terminal settling velocity as the particle in question and having the same density as the particle material, whatever the size and shape.

S” TWIST: The yarn spirals conform in slope to the center portion of the letter 4:

TAFFETA: Closely woven plain weave (1/1) fabrics with the warp yarns greatly outnumbering the filling yarns.

TEMPERATURE, ABSOLUTE: Temperature expressed in degrees above absolute zero.

TEMPERATURE, DEW-POINT: The temperature at which the condensation of water vapor in a space begins for a given state of humidity and pressure as the temperature of the vapor is reduced. The temperature corresponding to saturation (100 per cent relative humidity) for a given absolute humidity at constant pressure.

TEMPERATURE, DRY-BULB: The temperature of a gas or mixture of gases indicated by an accurate thermometer after correction for radiation.

TEMPERATURE SCALES: Temperature scales, Centigrade and Fahrenheit derive their degree values by dividing the difference between the ice point and steam points of water as follows: Centigrade 100 and Fahrenheit 180. The value of a Fahrenheit degree is therefore 5/9 of a Centigrade degree. The Fahrenheit scale is generally used in air handling practice. The Rankine scale, sometimes called Fahrenheit absolute, has its zero at the lowest attainable temperature, exactly 459.67 degrees below the zero of the Fahrenheit scald. To convert Fahrenheit to Rankine temperature (generally designated R), add 459.67 degrees, (460 is sufficiently accurate).

TEMPERATURE, WET-BULB: Wet bulb temperature is a measure of the moisture content of air (gas). It is the temperature indicated by a wet bulb psychrometer.

TENACITY: Ultimate tensile strength of a fiber filament, yarn, etc. expressed in grams per denier (g.p.d.).

TENSILE STRENGTH: The ability of yarn or fabric to resist breaking by direct tension. Ultimate breaking strength in psi.

TENTER FRAME: (Pin tenter) A machine for drying cloth under tension. Tentering. Also called framing.

TEXTILE: That which is or may be woven. Comes from the Latin “Texere”, to weave. Hence any kind of fabric.

THREAD COUNT” The number of ends and picks per inch of a woven cloth. For example 64×60 (ends count first).

THROW: Process of doubling or twisting fibers into a yarn of the desired size and twist.

TOW: A large number of filaments collected in a loose rope-like form, without definite twist.

T.P.I.: Twist per inch. (Turns per inch.)

TWILL WEAVE: Warp yarns floating over or under at least two consecutive picks from lower to upper right, with the point of intersection moving one yarn outward and upward or downward on succeeding picks, causing diagonal lines in the cloth.

TWIST: The number of complete spiral turns per inch in a yarn, in a right or left direction; i.e., “S” or “Z” respectively.

VAPOR: The gaseous form of substances which are normally in the solid or liquid state and which can be changed to these states either by increasing the pressure or decreasing the temperature.

VELOCITY_ HEAD: Same as velocity pressure. (See Pressure, Velocity.)

VELOCITY OF APPROACH: The velocity of air (gas), feet per minute, normal to the face of the filter media.

VELOCITY TRAVERSE: A method of determining the average air velocity in a duct. A duct, round or rectangular, is divided into numerous sections of equal area. The velocity is determined in each area and the mean is taken of the sum.

VOLUME, SPECIFIC: The volume of a substance per unit mass; the reciprocal of density; usually given in cubic feet pe.. pound, etc.

WARP: Lengthwise threads in loom or cloth.

WARP BEAM: Large spool-like or barrel-like device on which the warp threads are wound.

WARP COUNT: Number of warp threads per inch of width.

WARP SATEEN: The face of the cloth having the warp yarns floating over the filling yarns and being greater in number than the filling yarns (per inch).

WEAVE: The pattern of weaving; i.e., plain, twill, satin, etc.

WEFT: Same as filling, the crosswise threads (yarns).

WOOF: Same as filling or weft.

WOOLEN SYSTEM: A “system” of yarn manufacturing for spinning wool fiber into yarn, usually more open and not aligned as parallel as the cotton system.

WORSTED SYSTEM: A system of yarn manufacturing suited for medium and longer wools. Includes additional processing steps resulting in the most uniform yarn. The resulting yarn is compact and level.

WOVEN FELT: Predominantly a woven woolen fabric heavily fulled or shrunk with the weave being completely hidden due to the entanglement of the woolen fibers.

YARN: Twisted fibers or filaments in a continuous strand suitable for weaving, etc. Ply yarn is formed by twisting two or more single yarns together. Ply yarns are in turn twisted together to form cord.

YARN SIZE (DENIER, OR COUNT): A relative measure of fineness or coarseness of yarn. The smaller the number in spun yarns, the coarser the yarn. The higher the denier of a filament yarn, the coarser (heavier) the yarn.

“Z” TWIST: The yarn spirals conform in slope to the center portion of the letter “Z”.