Air Pollution Control Useful Definitions

Evaporative Gas Cooling

Droplet Size, Sauter Mean: A way of describing a droplet distribution ratio between the diameter and the surface area. It is defined as a single droplet that has the same diameter to surface area ratio as all the diameters in the entire droplet distribution have to the surface are of all the droplets in the entire droplet distribution. NOTE: For a single Sauter Mean droplet size, there is an infinite number of combinations of droplet distributions and the Sauter Mean droplet diameter is essentially useless for calculation purposes in an evaporative gas cooling application without an actual droplet distribution at the operating parameters of the nozzle.

Droplet Size, Effective: A way of describing a droplet distribution with respect to its ability to evaporative within a certain time and within given operating parameters. It is defined as a theoretical single droplet size that has the same diameter as an entire droplet distribution that evaporates at the given parameters if all of the droplets in that distribution were of the same diameter. NOTE: For the Effective droplet size to be useful, it must be used in an numerical evaporation model that is correlated to actual nozzle performance within the desired operational parameters and to the actual droplet distribution.

GCT: Gas conditioning tower. An expanded chamber used for slowing down gases and increase their residence time sufficiently for full droplet evaporation.

Ladder Logic: A method for programming a PLC utilizing a diagramming language similar to an electrical ladder diagram.

PLC: Programmable Logic Controller. An electronic computing device used for controlling a process or an apparatus related to a process. Normally composed of a CPU (Central Processing Unit) and I/O cards (Input and Output interface processors), and controlled by way of a customized program according to one of many programming languages such as ladder logic or statement list.

Spray Lance: An apparatus made up of piping that holds a spray nozzle at one end into the gas stream and with a supply connection at the other end.

Valve Rack: A frame-mounted valve assembly used for controlling the water (and air, in the case of an air-atomized system) injected through nozzles.

 

Gas Filtration and Dedusting

Abrasion, flex: Cloth wear in a creased area of a fabric filter bag caused by excessive bending of the filtration material on a stationary surface, usually the bag support cage.

Abrasion, surface: Cloth wear in a surface area of a fabric filter bag caused by contact of the filtration material with a stationary surface, such as a filter casing wall, or with abrasive material in the gas stream.

Acrylic: A synthetic polymerized fiber that contains at least 85% acrylo-nitrile.

Air Horsepower: The theoretical horsepower required to drive a fan if there were no losses in the fan, that is, if its efficiency were 100%

Air-to-cloth ratio: The amount of air or process gas entering the baghouse divided by the sq. ft of cloth in the baghouse (cp. Filtration Velocity).

Aspect Ratio: The ratio of the length of a chamber to its width.

Aspect Ratio of an Elbow: The width (W) along the axis of the bend divided by depth (D)

Blast Gate: Sliding damper

Bleed-through: Particles of dust or fumes that migrate through the bag. (see leak-through).

Blinding: Blocking of the porous filter media that results in a reduction in gas flow and/or an increase in pressure drop across it. If the medium cannot be cleaned readily and the pores reopened, this condition is often referred to as "permanent blinding" or plugging".

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).

Bridging: Material blockage across an opening such as a hopper outlet or between bags.

Cake: Particle formation developed on the surface of the filter medium during the filtration process.

Can Velocity: The theoretical speed of dust laden air as it passes upward between the filter bags. The formula for calculating can velocity is as follows: The volume of the gas flow divided by the cross-sectional area of the baghouse (length x width), less the cross-sectional areas of the bags (the sum of the cross-sections of all the bags).

Capture Velocity: The air velocity at any point in front of a ducted opening necessary to cause the contaminated air at that point to flow into the duct.

Cold Spot: On an insulated baghouse, a point where a part of the baghouse casing creates a heat sink through the insulation, causing condensation of gases on an internal surface exposed to the gases.

Concentration: Content of a substance within a volume, typically either a gas or a particulate.

Density: The ratio of the mass of a substance to its volume. The mass of a substance for a unit volume. When synonymous with mass, density is stated as weight per unit volume.

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 lb/cu ft) will give the actual air density in lbs per cu ft. d x 0.075 = actual density of air, lbs per cu ft.

Diaphragm Valve: A compressed air operated valve that is used to pulse bags.

Dust Collector: An air cleaning device to remove particulate from exhaust systems before discharge to atmosphere.

Dust Loading: The quantity of dust in the gas stream, usually expressed as grains of dust per cubic foot of air.

Entry Loss: Loss in pressure caused by air flowing into a duct or hood usually expressed as inches Water Gauge.

Fabric: A collective term applied to cloth no matter how constructed and regardless of the kind of fiber used. In the commonest sense, it refers to a woven cloth.

Felt: Fabric structures constructed by the interlocking action of the fibers themselves, without spinning, weaving, or knitting.

Felted Bag: Type of bag frequently used on pulse-jet dust collectors. Features a thick mat of fibers, sometimes supported by woven backing or scrim. A felted bag without scrim is called self-supporting.

Filter Cake: The accumulation of dust on a bag. Often assists in the filtration process. Also see cake.

Filter Media: The permeable barrier employed in the filtration process to separate particles from the fluid stream.

Filtration: A process by which particles are separated from a fluid or gas stream.

Filtration Velocity: The velocity of the gases as they travel through the filtration surface, normally a European standard, defined by the total gas volume divided by the filtration surface area, normally stated in meters per minute (cp. Air-to-cloth ratio).

Flange-to-Flange: The dust collection apparatus from inlet flange to outlet flange.

Fumes: Small solid particles formed by the condensation of vapors.

Gas-to-Cloth-Ratio: The amount of process gas entering the fabric filter dust collector divided by the amount of cloth area filtering the dust from the air. Normally the gas flow is given in CFM and the cloth in square feet.

Gases: Formless fluids which tend to occupy an entire space uniformly at ordinary temperatures and pressures.

Glazing: High-pressure pressing of the filter medium at elevated temperatures; fuses surface fibers to the body of the filter medium.

Grain: A dust-weight unit commonly used in air pollution control. Equal to 1/7000 lb.

Grain Loading: Grains of dust per unit of gas volume (typically, a cubic foot).

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 the standard substance usually referred to. For gases, dry air, at the same temperature and pressure as the gas, is often taken as the standard substance.

Header: A pressurized pipe that distributes the fluid within it to various points along its length. In dust collection, a pipe that distributes the compressed air for a pulse jet baghouse or the inlet duct that distributes the gases into each compartment.

Hood: A shaped inlet designed to capture contaminated air and conduct it into the exhaust dust system.

Humidity, Absolute: The weight of water vapor per unit volume, pounds per cubic foot or grams per cubic centimeter.

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.

Hydrophilic Fibers: Fibers that do not readily absorb water.

Inch of Water: A unit of pressure equal to the pressure exerted by a column of liquid water on inch high at a standard temperature.

Interstices: Openings between the fibers in a woven or felted filter bag.

Leak-through: Dust or fume that leaks through a bad bag seal or a hole or crack in the tubesheet.

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

Minimum Design Duct Velocity: Minimum air velocity required to move the particulates in the air stream, fpm.

Mists: Small droplets of materials that are ordinarily liquid at normal temperature and pressure.

Needled Felt: A felt constructed by the use of barbed needles moving up and down, pushing and pulling the fibers to form an interlocking of adjacent fibers.

Permeability: A measure of fabric porosity or openness, usually expressed in CFM at a 0.5" W.C. pressure differential.

Plenum: Pressure equalizing chamber.

Porosity: Sometimes (erroneously) used as a synonym for permeability. Originally a designation for the amount of air in a fabric, i.e., blankets.

Pre-coat: Material added to air stream on initial process startup to aid in establishing filter cake on bags.

Pressure Drop: Resistance to air flow; may refer to pressure differential across the cloth, across the baghouse or the pressure drop across the entire 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. (The tendency to either burst or collapse the pipe).

Pressure, Total: The algebraic sum of the velocity pressure and the static pressure (with due regard to sign).

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.

Pulse Cycle: The interval of time between one pulsing of a row of bags and the next pulsing of the same row.

Pulse Interval: Time between pulsing one row of bags and pulsing the next row.

Pulse Jet: Generic name given to all pulsing collectors.

Radiation, Thermal Radiation: The transmission of energy by means of electromagnetic waves of very long wave length. Radiant energy of any wave length may, when absorbed, becomes thermal energy and results in an increase in the temperature of the absorbing body.

Re-entrainment: The phenomenon that describes the act of a particle re-entering the gas flow after it has been collected. In electrostatic precipitation, the particle can re-enter the gas stream after it has been collected on the plate through the action of rapping or by way of high gas velocity. In fabric filtration, it is the act of re-depositing of dust on the filter bag surface after it has been pulsed off. Re-entrainment is usually caused by turbulence in the hopper or high can velocity.

Replacement Air: A ventilation term used to indicate the volume of controlled outdoor air supplied to a building to replace air being exhausted.

Reverse-Air Baghouse: A gas cleaning apparatus employing the gases from the exhaust of another compartment blown in reverse across the bag from the clean side, thus releasing the collected particulate.

Scrim: A very loosely woven fabric onto which felt is needled.

Seeding: The application of a relatively coarse, dry dust to a bag before startup to provide an initial filter cake for immediate high efficiency and to protect bags from blinding. (see Pre-coat)

Shaker Baghouse: A gas cleaning apparatus implementing a shaking mechanism to clean the fabric filter bags.

Singeing: Passing of the filter medium over an open flame, thereby removing the protruding surface fibers in order to reduce the chance of blinding the bags on start-up.

Smoke: An air suspension (aerosol) of particles, usually but not necessarily solid, often originating in a solid nucleus, formed from combustion or sublimation.

Standard Conditions, Air: Dry air at 70 F and 29.92 in (Hg) barometer. This is substantially equivalent to 0.075 lb/cu ft. Specific heat of dry air = 0.24 BTU/lb/F.

Surface Treatment: Immersing the filter medium in a solution to provide the fibers with a coating that will lubricate and thereby help the surface release the filter cake and/or reduce self-abrasion; the most common coatings are Teflon and Silicone-graphite.

Temperature, Effective: An arbitrary index which combines into a single value the effect of temperature, humidity and air movement on the sensation of warmth or cold felt by the human body. The numerical value is that of the temperature of still, saturated air which would induce an identical sensation.

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.

Tube-Sheet: The steel plate that bags are suspended from in a baghouse.

Turnkey: Complete baghouse System including all dust pickups, ducting, dust discharge auxiliaries and all equipment that is part of the dust collection system.

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