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Appendix A DEFINITIONS3 Aerosol; An assemblage of small particles, solid or liquid, suspended in air. The diameter of the particles may vary from 100 microns down to 0.01 micron or less, e.g., dust, fog, smoke. Air Cleaner; A device designed for the purpose of removing atmospheric airborne impurities such as dusts, gases, vapors, fumes, and smokes. (Air cleaners include air washers, air filters, electrostatic precipitators, and charcoal filters.) Air, Dry; Air containing no water vapor. Air Filter; An air cleaning device to remove light particulate loadings from normal atmospheric air before introduction into the building. Usual range; Loadings up to 3 grains per thousand cubic feet (0.003 grains per cubic foot). Note; Atmospheric air in heavy industrial areas and in-plant air in many industries has higher loadings than this, and dust collectors are then indicated for proper air cleaning. Air Horsepower; The theoretical horsepower required to drive a fan if there were no losses in the fanâthat is, if its efficiency were 100 percent. Air, Standard; Dry air at 70° F and 29.92 inches (Hg) barometer. This is substantially equivalent to 0.075 Ib/cu ft. Specific heat of dry air = 0.24 Btu/lb°F. Air-to-Cloth Ratio; The volumetric rate of capacity of a fabric filter; the volume of air (gas), in cubic feet per minute, per square foot of filter media (fabric) . Anemometer; An instrument for measuring the velocity of air or gas. Aspect Ratio of an Elbow; The width (W) divided by depth (D) in the plane of the bend. AR = W (see Figure 3-9) D Aspiration; The movement of air by suction. a Compiled in part from Industrial Ventilation; A Manual of Recommended Practice, 25th Edition, American Conference of Government Industrial Hygienists, Lansing, 1978 and "Fundamentals of Fabric Collectors and Glossary of Terms," Publication No. F-2, Industrial Gas Cleaning Institute, Inc., Stamford, 1972. 89
90 Atmospheric Pressure; The pressure of the atmosphere as measured by means of the barometer at the location specified. Backwash; A method of fabric cleaning where direction of filter flow is reversed, accompanied by flexing of the fabric and breaking of the dust cake. Also known as backpressure, repressure, collapse-clean, etc. Bag; The customary form of filter element. Also known as tube, stocking, etc. Can be unsupported (dust on inside) or used on the outside of a grid support (dust on the outside). Blast Gate; A sliding plate installed in a supply or exhaust duct at right angles to the duct for the purpose of regulating airflow. Blinding (Blinded); The loading, or accumulation, of filter cake to the point where capacity rate is diminished. Also termed "plugged." bph; bushels per hour, bpm; bushels per minute, bps; bushels per second. Brake Horsepower; The horsepower actually required to drive a fan. This includes the energy losses in the fan and can be determined only by actual test of the fan. (This does not include the drive losses between motor and fan.) Capture Velocity; The air velocity necessary to entrain dust particles, cfm; cubic feet per minute. Convection; The motion resulting in a fluid from the differences in density and the action of gravity. In heat transmission this meaning has been extended to include both forced and natural motion or circulation. Cyclone; Mechanical dust collector using inertial principles for dust separation. Daroper; An adjustable plate installed in a duct for the purpose of regulating airflow. 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, when weight can be used without confusion, as synonymous with mass, density is the weight of a unit volume of a substance. Density Factor; The ratio of actual air density to density of standard air. The product of the density factor and the density of standard air (0.075 Ib/cu ft) will give the actual air density in Ibs. per cu ft; d x 0.075 = actual density of air, Ibs per cu ft.
91 Dust; Defined for the purpose of this study as particles of grain and foreign material that will pass through a 200-mesh (74- ) screen. Dust Collector; An air cleaning device to remove heavy particulate loadings from exhaust systems before discharge to outdoors. Usual range; Loadings of 0.003 grains per cubic foot and higher. See Cyclone, Fabric Filter. 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 medium divided by the resistance (pressure drop), inches WG per unit of filtering velocity, fpm. Not to be compared with cloth permeability. Entry Loss; Loss in pressure caused by air flowing into a duct or hood (inches H2O) . Envelope; A common form of filter element. Fabric; A planar structure produced by interlacing yarns, fibers or filaments. May include felts and woven materials. Fabric Filter; A dust collector using a fabric for separation of dust from air. Filter Drag; Pressure drop, inches WG per cubic foot of air per minute, per square foot of filter medium. Analogous to the resistance of an element in an electrical circuit. The ratio of filter pressure to filter velocity. Filter Medium; The substrate support for the filter cake; the fabric upon which the filter cake is built. fpm; feet per minute. Grain; A measure of weight, 1/7000 pounds 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 at 39.2° F is usually the standard substance. For gases, dry air, at the same temperature and pressure as the gas, is often taken as the standard substance. Hood; A shaped inlet designed to capture contaminated air and conduct it into the exhaust duct system. Humidity, Absolute; The weight of water vapor per unit volume, pounds per cubic foot or grams per cubic centimeter.
92 Humidity, Relative; The ratio of the actual partial pressure of the water vapor in a space to the saturation pressure of pure water at the same temperature. (The amount of water contained in air at a given temperature as a percentage of the total amount of water it can contain at that temperature.) 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 Ib per sq ft. Lower Explosive Limit; (See also Minimum Explosible Concentration.) The lower limit of flammability or explosibility of a gas or vapor at ordinary ambient temperatures expressed in percent of the gas or vapor in air by volume. This limit is assumed constant for temperature up to 250°F. Magnehelic Gauge; A pressure differential gauge that, when used in filter-type dust collectors, measures the static pressure difference between the dust-laden air intake (lower) chamber of a collector and the clean air outlet (upper) chamber. It is important to know that as the static pressure difference increases, the airflow through the cloth filter is being restricted. Manometer; An instrument for measuring pressure; essentially 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. Micron; One micron is approximately 1/25,000 of an inch (0.00004 inches) (human hair diameter ranges from 50-75 microns). Minimum Design Duct Velocity; Minimum air velocity required to move the particulates in the air stream, fpm. Minimum Explosible Concentration; The smallest amount of particulate material, which when dispersed in a volume of air, will permit combustion of the dispersion to occur (oz/ft^, g/m^). 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 a 0.5 inches H2o pressure differential. Photohelic Gauge; Same as magnehelic gauge (see above), but has electrical contacts and can be wired to alarm on control panel. Pitot Tube; A means of measuring velocity pressure. A device consisting of two tubesâone 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 connected across a differential pressure-measuring device, the static pressure is compensated automatically and the velocity pressure only is registered.
93 Plenum Chamber; A pressure-equalizing chamber connected to one or more ducts. Porosity, Fabric; Term often used interchangeably with permeability. Actual percentage of voids per unit volumeâtherefore, the term is improperly used where permeability is intended. Pressure, Atmospheric; The pressure due to the weight of the atmosphere, as indicated by a barometer. Standard atmospheric pressure is 29.92 inches of mercury; equivalents in other units are 760 mm of mercury, 14.7 psia, and 407 inches of water. Pressure, Gauge; Pressure measured from atmospheric pressure as a base. Gauge pressure may be indicated by a manometers 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 short blast 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). Pressure, Velocity; The pressure required to accelerate air from a state of rest to the particular velocity required. It is the pressure that would be exerted against a flat surface placed in a moving air stream at right angles to the direction of motion. ps i; pounds per square inch. 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. Reverse Jet Cleaning; A cleaning method using a traveling ring traversing the exterior of the filter bag. High-pressure air is blown backward through the fabric through small holes or slots in contact with the cloth.
94 Shaking (Cleaning); A common, mechanical method of removing dust from filter elements. Backwash, or other supplemental methods, are often used with shaking. 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 percent 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, Wet-Bulb; Thermodynamic wet-bulb temperature is the temperature at which liquid or solid water, by evaporating into air, can bring the air to saturation adiabatically at the same temperature. Wet-bulb temperature (without qualification) is the temperature indicated by a wet-bulb psychrometer constructed and used according to specifications. Transport (Conveying) Velocity; See Minimum Design Duct Velocity. Volume, Specific; The volume of a substance per unit mass; the reciprocal of density; usually given in cubic feet per pound, etc. Water Gauge (WG) ; A term used after numerical pressure figures, that means the figure is calculated in terms of inches of water as obtained from the differential water levels of the two legs of a manometer. (Example; 2.8 inches WG).
