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Glossary -
Common Terminology Used in the Ducting Industry

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

The reduction of pollutant discharge by means of process modification, shutdown or addition of control equipment.

Abrasive Dust:  
Coarse particles such as sand, metallic compounds, ceramic and glass, that can cause rapid wear on ductwork, air cleaners and fans.

Absolute Humidity:  
The weight of water vapor in a given amount of air space. Absolute humidity is measured by grains per cubic foot.

Absolute Pressure:
The sum of gauge and atmospheric pressure (psia). 

Absolute Temperature:
The temperature measured on the Kelvin scale. 

Absolute Zero:
The lowest temperature theoretically attainable on the Kelvin scale (approximately -273.16° C). 

A substance with the ability to absorb another substance.

A kind of scrubber utilizing the absorption principle.

Acceleration Loss:
The velocity pressure required to accelerate the air from rest to the duct velocity or slot velocity, whichever is higher.

Actual Cubic Feet per Minute of gas volume at the actual conditions of temperature, pressure, moisture, elevation and gas composition. See gas flow rate.

The American Conference of Governmental Industrial Hygienists is a professional society devoted to the development of administrative and technical aspects of worker health protection. The ACGIH issues guidelines and recommendations in the form of Threshold Limit Values (TLVs) which are published annually.

Activated Carbon:
A processed carbon used in filter driers and commonly used in air filters to clean the air. 

A change in gas condition where no heat is added or removed except in the form of work. 

In addition to the adjectival meaning, the term describes any of several substances that collect gaseous pollutants. Used both for measurement and control.

Adsorption, which is often confused with absorption, refers to the adhering of molecules of gases and liquids to the surfaces of porous solids. Adsorption is a surface phenomenon; absorption is an intermingling or interpenetration of two substances.

Particles (solid or liquid) that remain suspended in air for a period of time. Aerosols include mists, smokes, fumes, and dusts.

Fan wheel design with airfoil-shaped blades.

Agglomerating Dust: 
Dust particles that exhibit tendencies for particle growth, such as powdered milk, fertilizer, detergents, etc.

Air Balance:
Term for distributing air through a system to precisely match the required amount. 

Air Change:
The amount of air required to completely replace the air in a room or building; not to be confused with re-circulated air.

Air Changes Per Hour:
The number of times indoor air is theoretically replaced by outdoor air during an hour.

Air Cleaner:
A device to separate contaminants from an air stream. Examples include filters, scrubbers, electrostatic precipitators and cyclones.

Air Cleaning:
Any method of removing various airborne particulates and/or gases from the air. The three types of air cleaning most commonly used are particulate filtration, electrostatic precipitation, and gas sorption. 

Air Conditioning:
Treating air to meet the requirements of a conditioned space by controlling its temperature, humidity, cleanliness and distribution.

Air Contaminant:
An impurity emitted to the outside air. It can be solid, (dust, particulate matter), liquid (vapor, mist) or gas (carbon monoxide, sulfur dioxide).

Air Curtain:
Mechanical air-moving device designed to limit the in-flux of unwanted air at a building opening.

Air Diffuser:
Air distribution outlet or grille designed to direct airflow into desired patterns.

Air Exchange Rate:
he rate at which outside air replaces indoor air in a space. Expressed in one of two ways: the number of changes of outside air per unit of time - air changes per hour (ACH); or the rate at which a volume of outside air enters per unit of time - cubic feet per minute (CFM). 

Air Filter:  
An air cleaning device which removes contaminants from an airstream.

Air Flow:
The distribution or movement of air. Airflow is the result of a difference in pressure (above or below atmospheric) between two points.

Air Handling Unit:
Factory-made encased assembly consisting of a fan or fans and other equipment to circulate, clean, heat, cool, humidify, dehumidify or mix air.

Air Horsepower (ahp):
The power required to move air through a ventilation system against a specific pressure.

Air Infiltration:
The unwanted entrance of air due to leakage, temperature difference, or wind. 

Air Leakage:
Unwanted air intruding into an exhaust system (holes in ducts, missing and ineffective seals, etc.).

Term generally applied to a rotary vane valve that keeps air from entering a dust collector, yet allows collected particulate to continuously discharge from the device.

Air Monitoring:
The continuous sampling for and measuring of pollutants present in the atmosphere.

Air Passages:
Openings through or within walls, through floors and ceilings, and around chimney flues and plumbing chases, that permit air to move out of the conditioned spaces of the building. 

Air Pollution:
The presence in the atmosphere of gases, fumes or particulate matter alone or in combination with each other, in sufficient concentration to disturb the ecological balance; cause objectionable effects, especially sensory offenses; cause transient or chronic illnesses; or impair or destroy property.

Air Quality Standards:
The approximate concentration level of a selected pollutant that is permitted in the atmosphere to minimize detrimental effects.

Air Retarder/Barrier:
A material or structural element that inhibits air flow into and out of a building's envelope or shell. This is a continuous sheet composed of polyethylene, polypropylene, or extruded polystyrene. The sheet is wrapped around the outside of a house during construction to reduce air in-and exfiltration, yet allow water to easily diffuse through it. 

Air, Standard:
Dry air [Oxygen (20.95% by volume), Nitrogen (78.09% by volume) Carbon dioxide (0.03% by volume)] at 70°F and 29.92 in (Hg) barometric pressure. This is substantially equivalent to 0.0757 lb/ft3. Specific heat of dry air = 0.24 btu/lb/F.

Air-to-Media Ratio:
The ratio of air volume (ACFM) to square feet of effective filter media area. Also referred to as the apparent or face velocity through the media (FPM). Typically 3 to 20 fpm for bag medias and 0.5 to 6 fpm for pleated-type media.

Air Toxics:
Any air pollutant for which a national ambient air quality standard (NAAQS) does not exist (i.e., excluding ozone, carbon monoxide, PM-10, sulfur dioxide, nitrogen oxides) that may reasonably be anticipated to cause cancer, developmental effects, reproductive dysfunctions, neurological disorders, heritable gene mutations or other serious or irreversible chronic or acute health effects in humans.

Air Velocity:
Rate of speed of an airstream, expressed in fpm.

Air Vent:
Term for a valve, either manual or automatic, that is used to remove unwanted air from the highest point of a ducting system. 

A substance capable of causing an allergic reaction because of an individual's sensitivity to that substance. 

The height above sea level of a given location. Density corrections for altitude are made using the following formula, where Z is the feet above sea level. Density (Alt) = Density (Std) x [1 – (6.73 x 10-6) Z]5.258.

Air Movement and Control Association.

Ambient temperature:
The temperature surrounding an object.

The highest value reaches by voltage,  current or power during a complete cycle.

A device that reads air velocity such as a wind vane. In fan applications, it is usually a spinning-vane-type instrument used to read low velocities of air flow.

American National Standards Institute; publishes consensus standards on ventilation, e.g. ANSI/AIHA Z9.7-1998 for Recirculation of Air.

Area Source:
Any small source of non-natural air pollution that is not large enough to be classified as a major source or point source.

American Society of Mechanical Engineers.

Aspect Ratio:
The ratio of the width to the length. AR = W/L

American Society of Testing Materials.

Atmospheric Dust Spot Efficiency:
The measurement of a devices ability to remove dust from the air.

Atmospheric Pressure:
Weight of a 1 square inch column of the earth's atmosphere. One atmosphere is approximately 14.7 PSI (at sea level); 407” water gauge. Airflow is the result of a difference in pressure (above or below atmospheric) between two points.

Attainment Area:
An area considered to have air quality as good as or better than the National Ambient Air Quality Standards as defined by the Clean Air Act. An area may an attainment area for one pollutant and a non-attainment area for another pollutant.

Wearing or grinding down by friction. One of the three basic contributing processes of air pollution; the others being vaporization and combustion.

Axial Fan:
Fan where the airflow through the impeller is predominantly parallel to axis of rotation. The impeller is contained in a cylindrical housing.

Axial Flow:
 In-line air movement parallel to the fan or motor shaft.



Backdraft Damper:
Damper used in a system to relieve air pressure in one direction and to prevent airflow in the opposite direction.

BACT (Best Available Control Technology):
An emission limitation based on the maximum degree of emission reduction achievable under Title 1 of the CAAA. EPA will establish BACT standards for serious, severe and extreme nonattainment areas.

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 (cage) support (dust on outside).

An air pollution abatement device that traps gas-borne particulates by forcing the gas through filter bags.

(1) On a fan, the process of adding (or removing) weight on a rotor to move the center of gravity toward the axis of rotation.
(2) In a ventilation system, it’s the process of measuring or calculating the airflow at a branch and altering duct size or a valve to attain desired airflow at that branch.

Barometric Pressure:
A measurement of the pressure of the atmosphere; standard is 29.92” Hg.

A device used to divert the flow of a substance (air or liquid).

Balanced Pressure:

When the both the inside and outside pressure of a container equal each other.


Used to measure atmospheric pressure.

Barometric Damper:
Also called draft damper, is a device installed in a chimney to allow for the adjustment of dilution air.

Measurements from the Local Exhaust Ventilation (LEV) duct network static pressures and airflow measurements that prove all points on the network are within design parameters. This is the reference point is where the system must be returned to when the network gets unbalanced. The information is recorded on the system drawing when the system is installed and started up.

Mechanical device for decreasing friction in a machine in which a moving part bears—that is, slides or rolls while exerting force—on another part. Usually in a bearing the support must allow the moving part one type of motion, for example, rotation, while preventing it from moving in any other way, for example, sidewise. The commonest bearings are found at the rigid supports of rotating shafts where friction is the greatest.

A corrugated cylindrical container which moves with a pressure change.

A rubber like, continuous loop placed between two or more pulleys to transfer a rotary motion.

Bi Fan:

Centrifugal fan with backward inclined blades that move air more efficiently than straight blade fans. BI fans are usually on the clean side of a piece of control equipment as the blades are not especially abrasion resistant.

Two metals with different rates of expansion fastened together. When heated or cooled they will warp and can be made to open or close a switch or valve.

Bio-Hepa Filter:
High-efficiency media air filter designed to be 95% efficient on 0.3 micron and larger particles when clean.

Blast Area:
The fan outlet area less the projected area of the cut-off.

Blast Gate:
Sliding sheet metal valve in a duct used to balance airflow or to isolate a process from a system. Used in ducts to create additional pressure loss in the duct and restrict flow

Particles of dust or fumes that are able to leak through filter media.

A valve with a small opening witch permits a minimum air flow when the valve is closed.

Blind (Blinding):
The blockage of filtration media by dust, fume or liquid covering the filter media and not being discharged by the cleaning mechanism. Once enough material has built up, airflow is severely restricted and the elements have to be cleaned or replaced.

An air handling device for moving air in a distribution system. Similar to a fan.

See Manifold.

The inside diameter of a cylinder.

Boyle's Law:
Boyle's law states that the volume of a given amount of gas varies inversely with its pressure, if temperature is constant. This means that at a constant temperature, the pressure of a gas will increase as the volume of the gas is decreased.

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 tests of the fan (this does not include the drive losses between motor and fan).

Branch Duct:
Duct used to transport contaminants from a collection point to a main duct or air cleaner.

Material building across an opening (such as a screw conveyor) and blocking off that opening.

BTU (British Thermal Unit):
The standard of measurement used for measuring the amount of heat required to raise the temperature of one pound of water by one degree (Fahrenheit). BTUH - The number of BTUs in an hour.

The abbreviation for British thermal units per hour. The BTU/hr. required to raise the temperature of a volume of standard air by a specific number of degrees is calculated by the formula: Btu/hr = Temp. Rise x CFM x 1.085

Building Envelope:
Elements of the building, including all external building materials, windows, and walls, that enclose the internal space.

The passage at one side of or around a regular passage.



Material crusted on a bag that cannot be removed by the cleaning mechanism. Frequently caused by the dust on the bags getting wet.

To determine accurate measurements and reset an instrument to these settings.

Can Velocity:
As related to baghouses, the gas velocity within the collector. Dividing the gas volume by the cross sectional area of the collector determines “can velocity.” Also see interstitial velocity.

The amount of air that a ducting system can transport, typically expressed in CFM (Cubic Feet per Minute).

Capture Velocity:
The air velocity at any point in front of a hood or at a hood opening necessary to overcome opposing air currents and to capture the contaminated air at that point by causing it to flow into the hood.

Carbon Dioxide (CO2):
A colorless, odorless noncombustible gas with the formula CO2 that is present in the atmosphere. It is formed by the combustion of carbon and carbon compounds (such as fossil fuels and biomass), by respiration, which is a slow combustion in animals and plants, and by the gradual oxidation of organic matter in the soil.
Carbon Filter:
Air filter housing activated carbon.

Carbon Monoxide (CO):
A colorless, odorless but poisonous combustible gas with the formula CO. Carbon monoxide is produced in the incomplete combustion of carbon and carbon compounds such as fossil fuels (i.e. coal, petroleum) and their products (e.g. liquefied petroleum gas, gasoline), and biomass. CO is poisonous and symptoms of CO poisoning are similar to those of the flu: headaches, fatigue, shortness of breath, nausea and dizziness. The Consumer Product Safety Commission (CPSC) recommends a yearly, professional inspection.

A physical, chemical or biological agent that is capable of inducing cancer in humans or animals.

Carrying Velocity:
The gas velocity that is necessary to keep contaminants airborne. Usually between 2,000 to 5,000 ft/min in duct work, depending on the nature of the contaminant.

Cartridge Filter:
An air pollution control device that traps gas-borne particulates by forcing the gas through filter element cartridges. The cartridges are typically made with pleated non-woven media. Recently, 100% spun bond synthetic media filters have been developed.

Localized gaseous condition that is found within a liquid stream.
Ceiling Plenum:
Space below the flooring and above the suspended ceiling that accommodates the mechanical and electrical equipment and that is used as part of the air distribution system. The space is kept under negative pressure.

The chief part of fibrous products that are used to make the filter media for many cartridge filters. Sometimes referred to as “paper” media.

Celsius Temperature Scale:
Temperature scale used in metric system. 0 is the freezing point of water, 100 the boiling point.

Centrifugal Collector:
Any of several mechanical systems using centrifugal force to remove particulate from a gas stream.

Chloro- Floro Carbon.

CFM (Cubic Feet per Minute):
The abbreviation for cubic feet per minute, the amount of air, in cubic feet, that flows through a given space in one minute. 1 CFM equals approximately 2 liters per second (l/s).

Chamber Silencer:
Insulated box or housing placed around a fan and motor to attenuate sound.

Charles' Law:
Volume of a given mass of gas at a constant pressure varies according to its temperature.

Chimney Effect:
The tendency of heated air or gas to rise in a duct or other vertical passage, such as in a chimney, small enclosure, or building, due to its lower density compared to the surrounding air or gas.

Coefficient of Entry (Ce):
The hood captures, contains, or receives contaminants generated at an emission source by converting static air pressure to velocity pressure. Coefficient of Entry (Ce) is a measure of the efficiency of a hood's ability to convert static pressure to velocity pressure.

Coefficient of Expansion:
Increase in unit length, area, or volume for one degree rise in temperature.
Coefficient of Performance (COP):
System capacity divided by electrical energy consumed.  

Cold Spot:
On an insulated baghouse, a point where metal goes through insulation, creating an uninsulated area where heat dissipates rapidly.

Collecting Efficiency:
The ability of a dust collector to remove particulate from the exhaust gas. The ratios of particles entering the collection device vs. particles leaving is expressed in percent. inlet loading – outlet loading x 100 inlet loading.

Collection Cell:
Set of alternately charged and grounded plates in an electrostatic precipitator where particle collection occurs.

Used interchangeably with “baghouse,” “cartridge filter” or mechanical collector, as in “cyclone.”

Combustible/Explosive Dust:
Finely divided materials that can cause or support combustion, such as aluminum, flour, corn starch, etc.

Compound Gauge:
Instrument for measuring pressure both above and below atmospheric pressure.    

A factor used by fan manufacturers to correct performance rating in higher pressure ranges to account for the fact that air is a compressible gas that does not follow the perfect gas laws.

Term used to denote increase of pressure using mechanical energy.

Compression Gauge:
An instrument used to measure positive pressure.

Compression Ratio:
The ratio determined by dividing the discharge pressure, in PSI, by the suction pressure in PSI.

A device used to compress air for mechanical or electrical power production, and in air conditioners, heat pumps, and refrigerators to pressurize the refrigerant and enabling it to flow through the system.

Compressor, Reciprocating:
Compressor which uses a piston cylinder mechanism to provide pumping action.
Compressor, Rotary:
Compressor which uses vanes, a mechanisms, or other rotating devices to provide pumping action.
The amount of dust in gas. Usually expressed in terms of grains per ft3, lbs per 1,000 lbs of gas, parts per million or milligrams per cubic meter.

A fluid formed when a gas is cooled .
Liquid or droplets which form when a gas or cooled below its dew point.
The changing of a gas or vapor to a liquid.
Condensible Particulate:
Hot gases or vapors that condense when cooled. UAS uses the term to define the contaminants in a hot process exhaust that are gases when generated and can be cooled to form aerosol particles.

Condensing temperature:
The temperature at witch a substance will condense.

Confined Space:
An enclosure such as a storage tank, process vessel, boiler, silo, tank car, pipeline, tube, duct, sewer, underground utility vault, tunnel, or pit that has limited means of egress and poor natural ventilation and that may contain hazardous contaminants or be oxygen deficient.

Tube or orifice used to restrict flow of a gas or a liquid.

A harmful, irritating, or nuisance material that is foreign to the normal atmosphere.

a-The reduction of pollutant emissions by addition of air cleaning equipment designed to collect all or part of the contaminant generated by a process.
b-Any device to start stop to regulate or protect a system.

A group of relays, switches and circuits used to accurately and automatically operate a device.

Control System:
All of the components required for the automatic control of a process variable.

Control Valve:
Valve which regulates the flow or pressure of a medium which affects a controlled process.

Conveying Velocity:
The air velocity required in a duct system to maintain entrainment of a specific material.

The deterioration of material by chemical or electrochemical reaction resulting from exposure to weathering, moisture, chemical or other agents in the environment in which it is placed.

Corrosive Dust: 
Particles that can cause degradation by chemical action to surfaces they contact, such as salt, soda ash, detergents, etc.

Counter Flow:
Flow in opposite direction.

Critical Temperature:
Temperature at which vapor and liquid have same properties.
Critical Vibration:
Vibration which is noticeable and harmful to structure.

CRST (Cellulosic Fibers, Resin Impregnated, Silicone Treated):
A type of filter media used in high-quality cartridges. The treatments enhance filter life and moisture resistance.

CSA (Canadian Standards Association):
Sets safety standards for motors and other electrical equipment used in Canada.

CTG (Control Techniques Guideline):
Guidance documents issued by EPA which define Reasonably Available Control Technology (RACT) to be applied to existing facilities that emit certain threshold quantities of air pollutants; they contain information both on the economic and technological feasibility of available techniques.
Curve, Fan Performance:
A graphic representation of static or total pressure and fan BHP requirements over an airflow volume range at a stated inlet density and fan speed.

Curve, System:
A graphic representation of the pressure versus flow characteristics of a given system and density.

Cyclone Collector:
A kind of centrifugal collector equipment.



Dalton's Law:
Vapor pressure created in a container by a mixture of gases is equal to sum of individual vapor pressures of the gases con tained in mixture.
A device used to regulate to flow of air.

Term for controls that vary airflow through an air outlet, inlet, or duct. A damper position may be immovable, manually adjustable or part of an automated control system.
Unit used for measuring relative loudness of sounds.

Device used to remove moisture from air.
The process of removing moisture from the air.
Delta P (?P): 
Change in pressure, or pressure drop, that occurs across a piece of control equipment.

 The measure of unit mass equal to its weight, divided by its volume (lbs/ft3); standard air is 0.075 lbs/ft3.
Density Factor:
Ratio of actual air density to density of standard air. The product of the density factor and the density of standard air (0.075 lbs/ft3) will give the actual air density in pounds per cubic foot.

A device that absorbs odors, usually by principle of absorption.
Design Pressure:
Highest or most severe pressure expected during operation.
Condensed atmospheric moisture deposited in small drops on cool surfaces.
Dew Point:
The atmospheric temperature (varying according to pressure and humidity) below which water droplets begin to condense and dew can form.

A flexible material usually made of thin metal, rubber, or plastic.  

Diaphragm Valve: 
Compressed air valve on reverse pulse dust collectors. Valve consists of an elastomer diaphragm in a cast aluminum housing. When pressure is released from the back of the diaphragm, the valve opens and allows the compressed air to flow from the air manifold into the oulse pipe for filter cleaning.

Takes place on particles so small that their direction and velocity are influenced by molecular collisions. These particles do not follow the air stream, but behave more like gases than particulate. They move across the direction of air flow in a random fashion. When a particle does strike a fiber, it is retained by the "van der Waals forces" existing between the particle and the fiber.

Direct Vent:
Pulls outside air for combustion and vents combustion gases directly outside.

Acronym for dioctylphthalate, a chemical used in aerosol form to nondestructively test high efficiency particulate air (HEPA) filters.

Double Bag:
Features a conventional bag on a cage and an inner bag that filters from inside out, with the bottom open for dirty air entrance.  Puts more, but not necessarily effective, cloth area in a given size baghouse.

Draft Hood:
A device built into or installed above a combustion appliance to assure the escape of combustion byproducts, to prevent backdrafting of the appliance, or to neutralize the effects of the stack action of the chimney or vent on the operation of the appliance.

Dry Bulb Temperature:
The actual temperature of a gas, taken with a conventional thermometer.

Dry Collector: 
Dust collectors that use mechanical means (centrifugal force, impingement, filtration) to remove particulate matter from exhaust gas, such as expansion chambers, knockout boxes, centrifugal collectors, cartridge filters and baghouses. Devices not using H2O.

The round or rectangular tube(s),  or conduit, to contain and control air flow.

Duct Fan:
Term for an axial flow fan mounted in a section of duct to move conditioned air.

Duct Silencer: 
Insulated section of ductwork connected to the outlet of a fan to attenuate sound. Silencers are available in various sizes and are designed to achieve the desired level of noise reduction.

Pipes or channels that carry air throughout a building.

Fine, dry particles of matter which may be generated by mechanical action (sawing, grinding, drilling, etc.). Dust may also be generated by operations such as mixing, blending, conveying, etc. of fine powders.

Dust Cake: 
The dust particle build-up on a filter substrate that gives the air cleaner the ability to collect contaminants with high efficiency.

Dust Collector: 
An air-cleaning device used to remove heavy particulate loadings from exhaust systems prior to discharge.

Dust Collector, Cyclone: 
A mechanical device that utilizes the centrifugal force of the inlet gas to remove large particulate matter suspended in the gas.

Dust Permeability:
The mass of dust (grains) per square foot of media divided by the resistance (pressure drop) in inches of water gauge (WG) per unit of filtering velocity, feet per minute (fpm). Not to be compared with cloth permeability.

Double-width, double-inlet fans.

Dynamic Balance:
The mechanical balancing of a rotating part or assembly in motion.



A disk mounted off center on a shaft.

Output of a device, system, or activity, divided by the input necessary to create the output.

Efficiency, Mechanical Total: 
The ratio of fan output to the power applied to the fan. Can be helpful in selecting fan size, type or manufacturer for the same application: ME = TP x CFM 6,356 x BHP.

Efficiency, Static: 
The ratio of fan output less the kinetic energy (outlet-velocity pressure) leaving the fan to the power applied to the fan: SE = SP xCFM 6,356 x BHP.

A discharge or emission of a fluid (liquid or gaseous).

Electrostatic Attraction: 
Mutual attraction caused by static electricity, by which particles tend to draw together or adhere.

Electrostatic Dust: 
Particles that tend to receive and retain a static charge, such as fiberglass, paint pigments, plastics, etc.

Electrostatic Precipitator: 
The two types of electrostatic precipitators are the Cottrell-type (single-stage) and the Penney-type (two-stage). The Cottrell-type combines ionization and collection in the same field while the Penney-type places the ionizer and the collection plates. With either type, particles entering the ionization field receive an electrostatic charge. These charged particles are then collected on the grounded collection plates. The Cottrell single-stage precipitator operates at ionization voltages from 40,000 to 70,000 volts DC. It may also be called a high voltage precipitator and is used to collect particles in application such as utility boilers. The Penney two-stage precipitator uses DC voltages from 11,000 to 14,000 for ionization and is frequently referred to as low voltage precipitator. It is used to collect contaminants in applications such as welding or oil mist and smoke from machining operations.

The heat content per unit mass of a substance.

Entry Loss: 
The loss in pressure caused by air flowing into a system; normally expressed in fractions of velocity pressure.

Environmental Protection Agency.

Equivalent Duct Diameter: 
For rectangular duct with sides “a” and “b” is: D = (4ab/p)0.5

Applied science that investigates the impact of people's physical environment on their health and comfort (e.g., determining the proper chair height for computer operators).

The physical transformation of a liquid to a gas at any temperature below its boiling point.

Evase (Eh-va-say):
A diffuser at the outlet that gradually increases in area to decrease velocity and to convert kinetic energy to static pressure at the outlet and inlet. It is used to ensure smooth and undisturbed airflow in and out of the fan. It converts kinetic energy into pressure energy.

Electrostatic Filter:
A filter for cleaning air that utilizes electricity to attract dust particles to electrically charged plates.

Exhaust Ventilation:
Term for mechanical removal of air from a portion of a building (e.g., piece of equipment, room, or general area).

Expansion Joint:
A device designed to allow movement caused by expansion and contraction.



A thermometric scale in which water boils at 212° and freezes at 32°. °F = (1.8 x °C) + 32°

Fahrenheit Scale:
The scale at which, under standard atmospheric pressure, the boiling point of water is 212 degrees and freez ing point is 32 degrees.
Fail Safe Control:
A device that fails in the open or safe position if and when it ceases to operate.

A power-driven machine that moves a continuous volume of air by converting rotational mechanical energy to an increase in the total pressure of the moving air.

Fan Capacity: 
Performance requirement for which a fan is selected to meet specific system calculations given in terms of ACFM at the fan inlet.

Fan Class: 
Operating limits at which a fan must be physically capable of operating safely.

Fan Laws: 
Theoretical constant relationships between CFM, RPM and BHP for a given fan used in a given fixed system: CFM varies as RPM, SP varies as (RPM)2, BHP varies as (RPM)3

Fan wheel design using forward-curved blades.

A device used to remove solid material from a gas or liquid by means of straining.

Filter Area: 
Quantity of filter media available for collection of contaminants. Expressed in square feet.

Filter Cake: 
The accumulation of dust on a bag before cleaning. This cake assists in the filtering of dust.

Filter Collector: 
A mechanical filtration system for removing particulate matter from a gas stream for measurement, analysis or control.  Also called bag collector.  Filters are designed in a variety of sizes and materials for specific purposes.  Also called bag collector or cartridge.

Fine particulate; aerosol.

Fire Damper:
A damper installed in a duct system that is normally open.  Upon the breakage of a heat sensitive link the damper will close.
Fire Retardant: 
A special treatment that results in the filter media not supporting combustion unless a flame source is present.

Flash Weld:
A resistance type weld in which mating parts are brought together under considerable pressure while a heavy electrical current is passed through the joint to be welded.
Flexible Duct:
A duct that can be routed around obstacles by bending it gradually.
Bending, or contracting and expanding.
Gas or air passage that usually depends on natural convection to cause the combustion gases to flow through it.
A gas or liquid.
Flux lines:
Lines of force of a magnet.
Foot Pound (ft.-lb.): 
Torque rating or requirement equivalent to the force required to move a one-pound weight one foot in distance.

Accumulated pressure, expressed in pounds.

Forced Draft: 
How air is provided in a process such as a combustion process, when air is blown or forced into a process. Known as a “forced draft” system. Also see Induced Draft.

FPM (Feet per minute): 
Commonly defines air velocity (to determine velocity pressure or suitability for material conveying), shaft-bearing speeds (used to determine lubrication requirements) and wheel tip speeds.

Frame Size: 
A set of physical dimensions of motors established by National Electrical Manufacturers Association (NEMA) for interchangeability between manufacturers. Dimensions include:  shaft diameter, shaft height and motor mounting footprint.

The number of cycles per second for any periodic waveform - unit : Hertz.

Friction Loss:
The static pressure loss in a ventilation system due to friction between moving air and the duct wall; expressed as inches w.g. (water gauge) per 100 feet of duct.

Full-Load Speed: 
The speed at which the rated horsepower is developed. This speed is less than synchronous speed and varies with the motor type and manufacturer.

Full-Load Torque: 
The torque required to produce the rated horsepower at full-load speed.

Solid particles generated by condensation from the gaseous state, generally after volatilization from a melted substance (e.g. welding), and often accompanied by a chemical reaction such as oxidation. Gases and vapours are not fumes.



Gage (Gauge): 
A measurement of the thickness of metal used to manufacture ducting components. The LOWER the number, the THICKER the material, i.e. - 18 gage material is much thicker than 22 gage material.

Galvanic Action:
The corroding action of two unlike metals due to electrical current passing between them.
The vapor state of a material.

Normally formless fluids which occupy the space of its enclosure and which can be changed to a liquid or solid state only by the combined effect of increased pressure and decreased temperature. Gasses diffuse.

A material used to form a leak proof seal between 2 parts.
Gasket, Foam:
Joint sealing material made of rubber or plastic foam strips.

An instrument to measure pressure or fluid levels.
Gauge, High Pressure:
Used for pressure ranges from 0 psig to 500 psig 1101.3 kPa to 3 600 kPa.
Gauge, Low Pressure:
Used for pressure ranges from 0 psia to 50 psia 10 kPa to 350 kPa.
Gauge, Manifold:
A chambered device constructed to hold both compound and high-pressure gauges. Valves control flow of fluids through it.
Gauge Port:
A connection in a system to provide for a service technician to install a gauge.
Gauge Pressure:
The pressure differential between atmospheric and that measured in the system.

Gauge Vacuum:
An Instrument used to measure pressures below atmospheric pressure.
General ventilation:
Ventilation system designed to maintain an acceptable indoor air by mixing the contaminated indoor air with fresh outdoor air and by controlling the temperature and humidity of the indoor air.

Unit of weight equal to 1/7000 lb. It is used to indicate the amount of moisture in the air.
Plastic, metal, or rubber doughnut-shaped protectors which line holes where wires or tubing pass through panels.



A family of compounds containing bromine, fluorine, iodine and chlorine used in fighting fires that breakdown in the atmosphere depleting stratospheric ozone.

Substance containing fluorine, chlorine, bromine, and iodine.
A device attached to walls or other structures for support of ductwork.
HAPs (Hazardous Air Pollutants):
Any of the 189 chemicals listed under Title III of the CAAA. All HAP sources will have to comply with GACT or MACT standards.

A ducting arrangement where several ducts are distributed from one common duct.
As applied to pulse-jet baghouses, the pressurized pipe that contains the compressed air supply for pulsing. If it is undersized, insufficient cleaning air will flow through the valve.

Head Friction:
Head required to overcome friction of the interior surface of a conductor and between fluid particles in motion.
Head Pressure:
The discharge pressure in the system.
Head Pressure Control:
A pressure actuated safety control which opens electrical circuit if high-side pressure becomes too high.
Head, Static:
The amount of pressure against which a pump has to work.
Head, Total Static:
Static head from the surface of the supply source to the free discharge surface.

HEPA Filter (High Efficiency Particulate Air Filter):
High-Efficiency Particulate Arrestance Filter - A filter designed to remove 99.97% of specific type particulate material (0.3 micrometers in diameter) from air.

Symbol for mercury. Pressure is often measured in inches of mercury: (1” Hg=13.64” WG).

High Pressure Cleaning Air: 
Air at 80-100 PSIG used for cleaning air in baghouses and cartridge filters.

High-Vacuum Pump:
A pump that can create a vacuum in the 1000 to 1 micron range.
High Volume Sampler: 
Also called a Hi-Vol. A device used in the measurement and analysis of suspended particulate pollution.

Hollow-Tube Gasket:
A sealing device made of rubber or plastic with tubular cross section.

A shaped inlet designed to capture contaminated air and transport it into the exhaust duct system. Hood types include canopy, side draft, slotted, downdraft, etc.

Horizontal Cartridge Unit:
A cartridge-type dust collector where the filters are in a horizontal configuration and the contaminated air typically enters the top of the unit and moves in a downward direction.
(As applied to motors) is an index of the amount of work the machine can perform in a period of time. One HP equals 33,000 ft. lbs. of work per minute, also equal to 0.746 kilowatts. Horsepower can be calculated by: HP = Torque (ft. lbs.) x RPM 5,250.

The process of adding moisture to the air within a space.

A device used to add humidity to a space.
A device designed to regulate humidity input by reacting to changes in the moisture content of the air.

A measure of the moisture content of air; may be expressed as absolute, mixing ratio, saturation deficit, relative, or specific.

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 water vapor in a space, to the saturated pressure of pure water vapor in a space, to the saturated pressure of pure water at the same temperature.

An instrument used to measure the amount of moisture in the air.

Materials having an affinity for water.

Hygroscopic Dust:
Dry particles which attract and retain moisture, such as cement, plaster, cocoa, etc.



A pulley used on some belt drives to provide proper belt tension and to eliminate belt vibration.  
Another term for fan “wheel”.  The rotating portion of the fan designed to increase the energy level of the gas stream.

Impeller Diameter: 
The maximum diameter measured over the impeller blade.

When air flows through a filter, it changes direction as it passes around each fiber.  Larger dust particles, however, cannot follow the abrupt changes in direction because of their inertia.  As a result, they do not follow the air stream and collide with a fiber.

Inches of Water (inch w.g.): 
A unit of pressure.  One inch of water is equal to the pressure exerted by a column of water one inch (25.4mm) high.  Atmospheric pressure at standard conditions is 407 inches w.g.

Inclined Manometer: 
A testing instrument using a liquid column, set at an incline to increase reading accuracy, to measure pressure. Normally used to read velocity pressure.

Induced Draft:
How air is provided in a process, such as a combustion process; where air is drawn or pulled through a process. Also see Forced Draft.

The production of an electric current in a conductor in a changing magnetic field.

Induction motor:
An ac motor that operates on principle of rotating magnetic field.  Rotor has no electrical connection, but receives electrical energy by transformer action from field windings.
Industrial ventilation (IV):
The equipment or operation associated with the supply or exhaust of air, by natural or mechanical means, to control occupational hazards in the air in an industrial setting.
A chemical that prevents corrosion or oxidation.
Tendency of an object to remain in its current state of motion. See WR2.

Inertial Separators: 
Air pollution control equipment that uses the principle of inertia to remove particulate matter from a stream of air or gas. See mechanical and cyclone collectors.

Inlet Diffuser Plenum: 
Air transition chamber between high-velocity ductwork and an air cleaner. The purpose of the inlet plenum is to slow the incoming airstream down to the collector velocity and provide even air distribution across the face of the air cleaner.

Inlet-Vane Damper: 
Round multiblade damper mounted to the inlet of a fan to vary the airflow.

The point of operation at which a fan or system will “hunt” or pulse. Common in FC fans and some other fan types where the point of operation is left of the peak of the static-pressure curve.

Used broadly to denote a device that has measuring, recording, indicating, and/or controlling abilities.
Any method which will retard the flow of heat through a wall. In baghouses, it is commonly a lightweight fiberglass mat.

Insulation, Electric;
A substance which has almost no free electrons.
Insulation, Thermal:
A material with low heat conductivity.
A special case of impingement where a particle is small enough to move with the air stream but, because its size is very small in relation to the fiber, makes contact with a fiber while following the tortuous airflow path of the filter. The contact is not dependent on inertia and the particle is retained on the fiber because of the inherent adhesive forces that exist between the particle and fiber. These forces, called van der Waals forces, enable a fiber to trap a particle without the use of inertia.

Intermittent Cycle:
A cycle which repeats itself.
Interrupted Cycle:
A cycle that does not repeat itself.
Interstate Commerce Commission:
Government body which controls the design and construction of pressure containers.

Interstitial Velocity: 
The apparent velocity of a gas as it passes by a filter bag matrix. It is found by dividing the collector gas volume by the cross sectional area, after the cross sectional of the bags have been subtracted from the collector cross sectional area.

Set of fine wires suspended between grounded plates in electrostatic precipitators where particle charging occurs.

Changes of volume or pressure under conditions of constant temperature.
Isothermal Expansion and Contraction:
An action which takes place without a temperature change.



Connecting point as between two pipes.
Metric unit of heat.

Joule (Thomson Effect):
The change in the temperature of a gas on its expansion through a porous plug from a higher pressure to a lower pressure.



Kelvin Scale:
Thermometer scale on which unit of measurement equals the Celsius degree and according to which absolute zero is 0 degree, the equivalent of -273.16 0 C. Water freezes at 273.16 0 K and boils at 373.16 0 K.

Kilopascal (KPA): 
Metric pressure unit. One-inch water gauge is 0.24836 Kpa.



L-10 Bearing Life:  
The theoretical number of hours after which 90% of the bearings subjected to a given set of conditions will still be in operation. Also known as B-10.

Local Exhaust Ventilation (LEV):
The mechanical removal of contaminated air from the point where the contaminant is being generated or liberated.

A device comprised of multiple blades, which when mounted in an opening, permits the flow of air but inhibits the entrance of undesirable elements.

Low Pressure Cleaning Air: 
Air at less than 30” WG used for cleaning baghouses.

Lower Explosive Limit:
The lower limit of flammability or explosibility of a gas or vapor at ordinary ambient temperature expressed in percent of a gas or a vapor in air by volume.



Magnehelic® Gauge: 
Gauge used for measuring pressure drop across filter media. The gauge displays static pressure in inches of water. This is a registered trademark of the Dwyer Company.

Main or Trunk Duct:
Duct that the branch ducts from multiple sources of contaminant generation enter. The main or trunk duct transports the contaminated air to the air cleaning or exhausting equipment.

Make-up Air (also known as Replacement Air):
Air supplied to a space to replace exhausted air.

A pipe or chamber that branches off into several openings. Sometimes called a blow-pipe. Common uses are dust collection baghouses, where a manifold feeds pressure into separate dust collection bags.
Duct Incorporated Manifolds

A u-shaped device for measuring the static pressure at a point relative to some other point. This pressure difference causes water to rise or fall. The difference in the level of the water columns is equivalent to the pressure differential.

Maximum Continuous Rating:
The point at which the fan is expected to operate.

Mechanical Collector:
Devices that are functionally dependent on the laws of mechanics governing the motion of bodies in space. Can be operated dry or wet. When operated wet, devices are generally called scrubbers. Examples of mechanical collectors are cyclones, selling chambers and various types of impingement collectors.

Mechanical Efficiency:
The ratio of work done by a machine compared to the amount of energy used.
Medium Pressure Cleaning Air: 
Air at 5 to 10 PSIG used for cleaning baghouses.

Metering Device:
A device used to meter the amount of refrigerant entering the evaporator.
A unit of pressure equal to one-millionth of an atmospheric pressure; 0.0000146 PSI.

Micron (µ): 
A unit of length equal to one-millionth of a meter. An average human hair is 70 microns in diameter. In general, particles down to 40 microns can be seen without the aid of magnification.

Minimum Duct Design Velocity: 
Minimum air velocity required to keep the particles in an airstream (duct) in suspension, expressed in Feet Per Minute (FPM).

Minimum Transport Velocity:
The minimum velocity which will transport particles in a duct with little settling (metric-m/s; U.S.-fpm).

An aerosol consisting of liquid particles generated by condensation of a substance from the gaseous to the liquid state.

Suspended liquid droplets generally less than 10 microns, 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.

A device used to convert electrical energy into mechanical energy.

MSDS (Material Safety Data Sheet): 
Compilation of data and information on individual hazardous chemicals produced by the manufacturers and importers of that chemical, as required by OSHA’s Hazard Communication Standard, 29 CFR 1910.1200.

The pressure necessary to rupture a secured cloth specimen, usually expressed in pounds per square inch.



National Association of Corrosion Engineers.

Natural Frequency: 
The frequency at which a component or system resonates.

National Electrical Code.

National Emission Data System.

Needle Valve;
A valve having a slender point fitting into a conical seat, used to regulate accurately the flow of a liquid or gas.

Negative Pressure:
Condition that exists when less air is supplied to a space than is exhausted from the space, so the air pressure within that space is less than that in surrounding areas. Under this condition, if an opening exists, air will flow from surrounding areas into the negatively pressurized space.

NEMA (The National Electrical Manufacturers Association): 
The trade association establishing standards of dimensions, ratings, enclosures, insulation and other design criteria for electric motors.

National Emissions Standards for Hazardous Air Pollutants.

National Fire Protection Association.

National Institute for Occupational Safety and Health.

Noise Criteria: 
A way for an architect to specify the maximum permissible sound power level in each of the eight octave bands NC curves give. In a graphical form, maximum permissible intensity per octave band.

Nuisance Dust:
Innocuous dust not causing a serious health condition.



A comprehensive database that contains information and data on standards interpretation, chemical information, hazardous waste activity 5(a)(1) citations, a health hazard evaluation index, training materials and other information compiled by OSHA on subjects related to occupational safety and health.

Odor Threshold:
The lowest concentration of an air contaminant that can be detected by smell.

Refers to the amount of light that can pass through. Normally refers to the degree of visibility of an exhaust plume. Normal measurement technique used is by EPA method 9.

Operating Cost:
The day-to-day cost of running your air handling equipment, based on daily energy use.

Opposed-Blade Damper:
A type of damper where the blades rotate in the opposite direction.

Organic Compound: 
A compound consisting of carbon, hydrogen and oxygen atoms.

Occupational Safety and Health Administration.

Outdoor Air Supply:
Term for air brought into a building from the outdoors (often through the ventilation system) that has not been previously circulated through the system. Also known as "Make-Up Air".



Pack-less Valve:
A valve that does not use packing to prevent leaks.

Parallel-Blade Damper:
A type of damper where the blades rotate in the same direction.

Parallel Fans:
Two or more fans that draw air from a common source and exhaust into a common duct or plenum. A parallel fan arrangement is generally used to meet volume requirements beyond that of a single fan. Two identical fans in parallel will effectively deliver twice the rated flow of any one of the fans at the same static pressure.

A small, discrete mass of solid or liquid material.

Particulate Matter:
Any solid or liquid material in the atmosphere such as dust, fog, fume, mist, smoke, or sprays. Particulate matter suspended in air is commonly known as an aerosol.

Particulate Matter, Airborne:
Airborne matter is typically in the size range of 0.01 to 100 micrometers.

Fine liquid or solid particles such as dust, smoke, mist, fumes and fog found in air and emissions.    

Permeability, Fabric:
Measured on Frazier porosity meter or Gurley permeometer. Not to be confused with dust permeability. The ability of air to pass through the fabric, expressed in cubic feet of air per minute per square foot of fabric with 0.5” H2O pressure differential.

Photohelic® Gauge:
Gauge used for measuring pressure drop across filter media and for controlling the on/off function of the cleaning system. The gauge displays static pressure in inches of water and provides adjustable contacts to operate the cleaning system. This is a registered trademark of the Dwyer Company.

Piezometer Ring:
A device consisting of a number of pressure taps connected to a common manifold to measure pressure.

Pilot Valve:
The small solenoid valve that is electrically operated to relieve pressure on one side of the diaphragm and cause the operation of a larger diaphragm valve.

Pitot Tube:
A metering device consisting of a double-walled tube with a short, right-angle bend. The periphery of the tube has several holes through which static pressure is measured. The bend end of the tube has a hole through which total pressure is measured when pointed upstream in a moving, gas stream.

Term for a pressure equalizing air compartment connected to a duct or ducts.
Plenum Pulse:
Type of pulsing collector where entire sections of the clean air plenum are isolated and pulsed with either compressed air or air from a high-pressure blower.

A newer standard for measuring the amount of solid or liquid matter suspended in the atmosphere (“particulate matter”). Refers to the amount of particulate matter under 10 micrometers in diameter. The smaller PM10 particles penetrate to the deeper portions of the lungs, affecting sensitive population groups such as children and people with respiratory diseases.

Point of Operation:
The intersection of a fan’s static pressure curve and the system curve to which the fan is being applied. May be designated as velocity pressure divided by static pressure or by a given CFM and SP.

The opening in a cartridge dust collector that provides access to the filters.

Positive Pressure:
Condition that exists when more air is supplied to a space that is exhausted, so the air pressure within that space is greater than that in surrounding areas.  Under this condition, if an opening exists, air will flow from the positively pressurized space into surrounding areas.
Pounds per 100 Pounds of Gas:
A common quantitative definition of pollution concentration.

PPM (Parts per Million):
The number of parts of a given pollutant in a million parts of air. Units are expressed by weight or volume.

Any number of devices using mechanical, electrical or chemical means to collect particulates. Used for measurement, analysis or control. See Electrostatic Precipitator.

See Preseed

Precoating of a dust collector filter (cartridge or bag-type) with material to create a dust cake and ensure peak collection efficiency at system start-up. Usually done when collecting very small particles, such as welding particulate, etc.

The force exerted per unit area.

Pressure, Absolute:
The total pressure measured in relation to a perfect vacuum. The sum of atmospheric and gauge pressures.

Pressure, Atmospheric:
The pressure due to the weight of the atmosphere. It is the pressure indicated by a barometer. Standard atmospheric pressure is 29.92 inches of mercury.

Pressure Drop:
(Differential Pressure, *P) - Pressure drop is a term used to characterize the reduction in air pressure between any two points in the ventilation duct network. The pressure drop occurs as the compressed air travels through filters and ducts. In airflow system, the energy is measured as "pressure drop."

Pressure, Gauge:
Instruments used to measure pressure are called pressure gauges. Generally it gives air pressure relative to atmospheric pressure. It can show a negative pressure (less than atmosphere), or positive (greater than atmosphere).

Pressure, Static (SP):
In flowing air, the total pressure minus velocity pressure.  The portion of the pressure that pushes equally in all directions. 

Pressure, Total (TP):
The sum of the velocity pressure and static pressure of air in a duct.

Pressure, Velocity (VP):
In flowing air, the pressure due to the velocity and density of the air.
Preventative Maintenance:
Regular and systematic inspection, cleaning and replacement of worn parts, materials and systems.  Preventive maintenance helps to prevent parts, material and systems failure by ensuring that parts, materials and systems are in good working order.    

Primary Collector:
A dry or wet collector that is followed by a secondary collector with greater filtering efficiency.

PSI (Pounds per Square Inch):
A measure of pressure .1 psi equals 27.7” water gauge.

PSIA (Pounds per Square Inch, Absolute):
The absolute pressure without reference to another point. Atmospheric pressure is 14.7 PSIA.

PSIG (Pounds per Square Inch Gauge):
The pressure relative to atmosphere. For instance, 10 PSIG equals 24.7 PSIA. This is the more common pressure term.

Pulse Cycle:
As used in conjunction with pulse-jet baghouses the interval of time between pulsing a row of bags and that row being pulsed again.

Pulse Duration:
The length of time a pulse lasts. Generally described as the length of time the electrical signal holds the pilot valve open (20-40 milliseconds), however would more correctly be the description of the length of time the manifold is pressurized (120-180 milliseconds).

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

Pulse-Jet Cleaning:
A cleaning method where a momentary burst of compressed air is introduced through a tube or nozzle to the top cap of a bag. A bubble of air flows down the bag, causing bag walls to collapse behind it. Generic name given to all pulsing collectors.

Pulse Pipe:
Pipe used to transport compressed air from the diaphragm valve to the pulse nozzle in reverse pulse cleaning systems. In some manufacturers’ equipment, the pulse pipe delivers the compressed air directly into the filters being cleaned.

A momentary opening to allow air or fluid to escape.






Radial Blade: 
Fan wheel design with blades positioned in straight radial direction from the hub.

Random Noise:
 A sound that has an average amplitude and constantly changing frequency.

A phenomenon related to negative pressure. When air is drawn out through resistance into a fan inlet, the air is stretched out, or rarefied, and becomes less dense than at the entry to the system. While negligible at low pressures and volumes, high-pressure fan selection must be based on rarefied inlet density.

The phenomenon whereby dust is collected from the air stream and then is returned to the air stream. Occurs when dust is pulsed from a bag and then caught up by an upward moving air stream.

Relative Humidity:
The ratio of the amount of vapor contained in the air to the greatest amount the air could hold at that temperature. Normally expressed as a percentage.

Relief Device:
A device designed to open under a high pressure.

Replacement Air: 
A ventilating term that refers to the replacement of air lost because of exhaust air requirements.

Repressuring Baghouse:
Baghouse using bags that are cleaned by flowing air backwards through the cloth. Same as reverse air baghouse.

In air flow, it is caused by friction of the air against any surface, or by changing the momentum of the gas.

Return Air:
Air that is returned from the primary space to the fan for re-circulation.

Reverse Air Baghouse:
Baghouse using bags that are cleaned by flowing air backwards through the cloth, to cause dust cake release.

Reynolds Number:
A mathematical factor used to express the relation between velocity, viscosity, density and dimensions in a system of flow. Used to define fan proportionality.

Rotary Airlock:
A valve with a rotating wheel generally located on the dust collector discharge hopper or at the end of a screw conveyor. It is designed to allow continuous discharge of the collected dust while maintaining a positive air seal.

The rotating part of most AC motors.

Revolutions per minute.

Runout Duct: 
A section of duct that extends from either a branch or the primary trunk, leading to a terminal.



Saturated Air:
Air containing the maximum amount of water vapor for a given temperature and pressure.

SCFM (Standard Cubic Feet Per Minute): 
The volume that a gas would occupy at standard temperature and pressure conditions (70° F and 14.7 PSIA). See Gas Flow Rate.

Secondary Air:
Air that is mixed with a fuel while it is burning.

Screw Conveyor:
A dust transfer conveyor that uses a screw drive for continuous movement of the material.

The general shape of a centrifugal fan housing. The formed piece to which housing sides are welded.

Scrubber, Gas:
Any device in which a contaminant, solid or gaseous, is removed from a gas stream by liquid droplets. (Types include spray towers, packed towers, cyclone scrubbers, jet scrubbers, orifice scrubbers, venturi scrubbers, impingement scrubbers and mechanical scrubbers.)

Secondary Collector:
A dust collector that is preceded by primary collector(s). The secondary filter normally has a higher filtering efficiency.

The application of a relatively coarse, dry dust to a bag or cartridge before start-up to provide an initial filter cake for immediate high efficiency and to protect bags from blinding. Also, in crystallizing, seeding is used to start the crystallization process.

Seismic Zone 4/100 MPH Wind Load Rating:
Structural rating of a dust collector. The equipment is designed to withstand the effects of an earthquake or wind force as defined in the Uniform Building Code.

Series Fans:
A combination of fans connected such that the outlet of one fan exhausts into the inlet of another. Fans connected in this manner are capable of higher pressures than a single fan and are used to meet pressure requirements greater than single fans.

Service Factor:
The number by which the horsepower rating is multiplied to determine the maximum safe load that a motor may be expected to carry continuously.

Settling Chamber:
A dry collection device that removes particulate matter from the gas stream by slowing down the exhaust gas velocity.

Shaded-Pole Motor:
A special type of single-phase induction motor. Low starting torque, low cost. Usually used on direct-drive fans.

Shaft Seal:
A device to limit gas leakage between the shaft and fan housing.

Shaker Baghouse:
A baghouse using woven cloth bags; cleaning occurs by shaking bags.

SIC Code (Standard Industrial Classification Code): 
A system used to classify industrial manufacturing companies by product and their type of business.

Silica Gel:
A desiccant used for drying refrigerant.

SI Units: 
Systems International d’Unites, International System of Units.  Any one of the units of measure in the international meter-kilogram-second system.

The percentage difference between synchronous and operating speeds.

Slot Velocity:
Linear speed of air through a slot in a hood, expressed in Feet Per Minute (FPM).

Aerosols, gases, and vapours resulting from incomplete combustion.

Solenoid Valve:
Electromechanical valve which energizes a coil that opens a valve allowing the fluid (air or water) to flow through the valve body.

Very finely divided carbon particles clustered together in long chains.

Sound-Pressure Level:
The acoustic pressure at a point in space where the microphone or listener’s ear is situated. Expressed in units of pressure or in decibels.

Any place or object from which pollutants are released.

Source Capture:
A term used to describe applications in which the airborne contaminant is picked up or “captured” directly at the machine or process. Source capture applications involve the use of ductwork to convey the contaminated air to the air-cleaning device. Also sometimes called “direct capture.”

SP (Static Pressure): 
Pressure as measured in all directions within an airhandling system, not including the force or pressure of air movement.

Spark Trap:
Device in a duct system designed to stop or extinguish a spark prior to entering a dust collector. Usually a dropout box in the duct system or inlet plenum that contains baffles to create a torturous air path which causes the spark to quench.

Specific Gravity:
The ratio of density of a substance to the density of a standard, usually water for a liquid or solid, and air for a gas

Specific Humidity:
The weight of water vapor, per unit weight of dry air.

Specific Volume:
The volume of a definite weight of a material.

SRC (Spark-Resistant Construction): 
AMCA standard of guidelines for general methods of fan construction when handling potentially explosive or flammable particles, fumes or vapors.

Steel Structures Painting Council.

A smokestack. Vertical pipe or flue designed to exhaust gases.

Stack Effect:
The overall upward movement of air inside a building that results from heated air rising and escaping through openings in the building super structure, thus causing an indoor pressure level lower than that in the soil gas beneath or surrounding the building foundation.

Standard Air:
Air weighing 0.7488 pound per cubic foot, which is air at 68°F dry bulb and 50% relative humidity at a barometric pressure of 29.92 in. Hg.

Standard Air Density: 
0.075 lbs./ft3. Corresponds approximately to dry air at 68°F and 29.92 in. Hg.

Starting Torque:
The torque produced by a motor as it begins to turn from a standstill and accelerate. Sometimes called locked-rotor torque.

Static Balance:
The mechanical balance of a rotating part or assembly by adding weights to counter-balance gravitational rotating of the part without power driving it.

Static Pressure:
Pressure as measured in all directions within an airhandling system, not including the force or pressure of air movement.

Static Pressure, Cold:
The pressure caused by the resistance to air flow through the system if the gas were at standard conditions or colder, if this is a possibility.

Static Pressure, Fan:
The static pressure for which a fan is to be selected based on system calculations: Fan SP = SP outlet – SP inlet – VP inlet.

Static Pressure, Hot:
The pressure caused by the resistance to air flow through the system at actual conditions. Measured in inches of water (WG).

Streamline Flow:
Fluid flow in which the velocity pressure and fluid density of a given particle remains constant with time.

The change of state from a solid to a vapor state without and intermediate liquid state.

Suction Pressure:
The pressure on the low side of the system.

Superheated Gas:
A gas temperature higher than the evaporation temperature at the exiting pressure.

Supply Duct:
Term for the duct(s) of a ducting system through which air is supplied by the action of the fan or blower.

Surge Limit:
That point near the peak of the pressure curve which corresponds to the minimum flow at which the fan can be operated without instability.

Condensation from the air on surfaces below the due point of the air.

Single-Width, Single-Inlet Centrifugal Fan.

Synchronous Speed:
Rated motor speed expressed in RPM's.  Synchronous speed = 120 x frequency, divided by number of poles.

A series of ducts, conduits, elbows, filters, diffusers, etc., designed to guide the flow of air, gas or vapor to and from one or more locations. A fan provides the energy necessary to overcome the system’s resistance to flow, and causes air or gas to flow through the system.

System Curve:
Graphic presentation of the pressure versus volume flow rate characteristics of a particular system.

System Effect:
The effect on the performance of a fan resulting from the difference between the fan inlet and outlet connections to the actual system, and the standardized connections used in laboratory tests to obtain fan performance ratings.



An instrument that measures the speed of rotation, usually in RPM.

Tape Sampler:
A device used in the measure of both gases and particulates. It allows air sampling to be made automatically at predetermined times.

TEFC (Totally Enclosed, Fan Cooled): 
Totally enclosed motors equipped with a cooling fan to prevent overheating.

Temperature, Absolute:
The temperature at which molecular motion stops. It is the lowest temperature possible. There is no more heat in a substance at this point.
Tensile Strength:
The maximum stress a material can withstand before it breaks; expressed in pounds per square inch.

A component that is at the end of a duct run, such as a suction hood or manifold

Is a heat sensitive resistor that changes resistance with a change in temperature.
A device consisting of 2 different conductors that when heated creates an electrical potential difference.
The study of the mechanics of heat.

Throw, Blow:
Distance an air stream travels from an outlet to a position at which air motion reduces to a velocity of 50 FPM.

Tip Speed: 
Fan wheel velocity at a point corresponding to the outside diameter of the wheel blades; normally expressed in feet per minute (circumference times RPM).

TLV  (Threshold Limit Value):
A list published yearly by the American Conference of Governmental Industrial Hygienists (ACGIH) as a guide for exposure concentrations that a healthy individual normally can tolerate for eight hours a day, five days a week, without harmful effects. Airborne particulate concentrations are generally listed as milligrams per cubic meter of air (mg/m3). Gaseous concentrations are listed as parts per million (ppm) by volume.

A force that produces, or tends to produce, rotation. Commonly measured in feet per pounds or inches per pounds A force of one pound applied to the handle of a crank, the center of which is displaced one foot from the center of the shaft, produces a torque of one foot per pound on the shaft if the force is provided perpendicular to, not along the crank. Torque can be calculated by: Torque (ft.-lbs.) = HP x 5250 RPM.

TP (Total Pressure):
The sum of velocity pressure and static pressure.

Twist Per Inch.

Tons Per Year.

A method of sampling points in a duct where pressure readings will be taken to determine velocity. A traverse divides the duct into equal, evenly distributed areas that are each tested, compensating for errors caused by uneven gas flow in the duct.

TRI (Toxic Release Inventory): 
A listing of pollutants and emission levels from each major source.

Trace and correct faults in a mechanical or electronic system

Trough Hopper:
Dust discharge that has an outlet slot along the entire length of the hopper for transfer of material directly into a screw conveyor or other continuous material transfer system.

The primary section of duct. Duct that the branch ducts from multiple sources of contaminant generation enter. The main or trunk duct transports the contaminated air to the air cleaning or exhausting equipment.

TSCA (Toxic Substances Control Act): 
Administered by the EPA, was passed by Congress to protect human health and the environment by requiring testing and necessary use restrictions to regulate the commerce of certain chemical substances.

Tubeaxial Fan:
Axial fan without guide vanes.

The steel plate that bags and cages are suspended from. Separates the clean air and dirty air sections of the baghouse. Sometimes called cellplate, a term usually used for inside collector baghouses.

Tubular Centrifugal Fan:
Fan with a centrifugal impeller withing a cylindrical housing discharging the gas in an axial direction.

Turbulent Flow:
Airflow in which true velocities at a given point vary erratically in speed and direction.

Turning Vanes:
 Baffles put in a duct to straighten out the air flow.



ULPA Filter:
Ultra Low Penetration Air Filter designed to be 99.997% efficient on 0.3 micron particles when clean.

The condition of a rotor in which its rotation results in centrifugal force being applied to the rotor’s supporting bearings.

System consisting of one or more air cleaners positioned in the overhead space to create a planned air circulation pattern. This method effectively cleans the overall ambient plant air. Unducted systems rely upon constant cleaning and mixing of the entire room volume to remove airborne contaminants. Unducted systems never remove 100% of the contaminants. The objective of this method is to achieve a substantial reduction in steady-state contaminant concentration.

Uniform Flow:
Airflow in which velocities between any two given points remain fairly constant.

Unit System:
A unit that can be removed from a premises without disconnecting any lines or wiring.

Upper Explosive Limit:
The upper limit of flammability or combustibility of a gas or vapor expressed in percent of gas or vapor, in air, by volume.



A pressure below atmospheric.

A device for controlling the passage of fluid (liquid or gas) through a pipe or duct, esp. an automatic device allowing movement in one direction only.

Vaneaxial Fan: 
Axial fan with either inlet or discharge guide vanes or both, includes fixed-pitch, adjustable-pitch and variable-pitch impellers.

Substance in gaseous form.

The change of a substance from the liquid to a gaseous state. One of the three basic contributing processes of air pollution, the others being attrition and combustion.

Vapor Retarder:
A material that retards the movement of water vapor through a building element (walls, ceilings) and prevents insulation and structural wood from becoming damp and metals from corroding. Often applied to insulation batts or separately in the form of treated papers, plastic sheets, and metallic foils.

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 alone. Vapors diffuse.

A component of a heating or ventilation appliance used to conduct fresh air into, or waste air or combustion gases out of, an appliance or interior space.

Vent Damper:
Term for a device mounted in the vent connector that closes the vent when the heating unit is not firing. This traps heat inside the heating system and house rather than letting it draft up and out the vent system.

Vent Pipe:
A tube in which combustion gases from a combustion appliance are vented out of the appliance to the outdoors.

Velocity, Face:
The speed at which air crosses the hood opening (metric-m/s; U.S.-fpm).

A simple instrument for for determining the velocity of gas in a duct. Its operation is similar to an inclined manometer, except that it automatically converts the reading to velocity.

Vena Contracta:
The smallest flow area for flow through a sharp-edged orifice.

Supplying and removing air by natural or mechanical means to and from any space.

A ventilator captures heating or cooling energy from stale indoor air and transfers it to fresh incoming air.

Device used to theoretically increase the efficiency of a compressed air pulse. Designed with converging circular sides to a throat and then diverging sides. Designed such that when a pulse is introduced at the top, a negative pressure zone is created outside the top, and secondary air is induced into the venturi, increasing cleaning energy.

Venturi Scrubber:
A wet type dust collector that can obtain very high efficiency, but requires large horsepower to do so. The gas and dust particles are accelerated in a venturi throat, where finely atomized water is introduced and water/dust collisions take place.

Vertical Cartridge Collector:
A cartridge-type dust collector where the cartridge filters hang vertically. The contaminated air typically enters the bottom of the unit and moves in an upward airflow pattern. Vertical cartridge collectors have been shown in an EPA-sponsored research project to cause particle redisposition during pulse cleaning and are less effective than downward airflow collectors.

Alternating mechanical motion of an elastic system, components of which are amplitude, frequency and phase.

VOCs (Volatile Organic Compounds): 
A group of chemicals that react in the atmosphere with nitrogen oxides in the presence of heat and sunlight to form ozone; does not include methane and other compounds determined by EPA to have negligible photochemical re-activity. Examples of VOCs include gasoline fumes and oil-based paints.

VP (Velocity Pressure): 
The kinetic energy pressure of air in motion. VP is used to get the velocity of a gas stream.
Velocity (SFPM) = 4005 vVP
Velocity (AFPM) = 1096 vVP/?
? = Actual density of the air (lb/ft3 )
VP = Inches of water column



Wet Collector:
Dust collector that uses water to remove particulate matter from the exhaust gas (wet washers, venturis, wet fans).

Woven Cloth Bag:
A bag made of woven material, similar to normal cloth. Used in repressuring and shaker baghousees.






Yield Strength:
Maximum stress to which a ductile material can be subjected before it physically distorts.