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Glossary of Air Handling for Terms

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

A.C.: an electric current which continually reverses its direction of flow in a regular fashion. The vast majority of electricity supply systems use A.C. The other system of electric current transmission is Direct Current (D.C.).

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

absorber: A kind of scrubber utilizing the absorption principle.

ACFM: the actual gas volumetric flow rate (Actual Cubic Feet per Minute).

acid: chemical substance whose water solutions exhibit a pH less than 7.

adsorbent: In addition to the adjectival meaning, the term describes any of several substances that collect gaseous pollutants. Used both for measurement and control.
aeration: the process or method of bringing about intimate contact between air and a liquid.
air changes per hour (ACH): The movement of a volume of air in a given period of time. If a building has one air change per hour, it means that all of the air in the building will be replaced in a one-hour period.

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

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

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, standard: Dry air at 70° F and 29.92 inches Hg barometric and is approximately equivalent to 0.075 lb/ft

air velocity: Rate of speed of an airstream, expressed in fpm.

altitude: 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

anemometer: 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 at registers and grills.

atmospheric pressure: One atmosphere is approximately 14.7 PSI; 407” water gauge. Airflow is the result of a difference in pressure (above or below atmospheric) between two points.

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.


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.
balancing: (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.

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.

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 damper in a duct used to balance airflow or to isolate a process from a system.

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.

British thermal unit (BTU): heat required to raise the temperature of 1 lb. of water by 1° F. 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


capacitor start motor: type of single-phase induction motor with a capacitor connected in series with the starting winding. High–starting and breakdown torque, medium starting current. Used in hard-starting applications; compressors, pumps, etc.

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 monoxide: a colorless, odorless gas that is toxic because of its tendency to reduce the oxygen-carrying characteristic of blood.

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.

celsius: a thermometric scale in which water boils at 100° and freezes at 0°, same as centigrade. °C = 0.5556 x (°F – 32°) 5/9 = 0.5556

CFM: volumetric flow rate expressed as Cubic Feet (of any gaseous mixture) per Minute. See Also: gas flow.

chemical abstracts service (CAS): registry Number is a numeric designation assigned by the American Chemical Society’s Chemical Abstracts Service, which uniquely identifies a specific chemical compound.

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

combustible/explosive dust: finely divided materials that can cause or support combustion, such as aluminum, flour, corn starch, etc.

combustion products: (1) Primarily gaseous matter such as carbon oxides, nitrogen, oxygen and water vapor, resulting from the combustion of fossil fuels. (2) In the context of emission control, the gaseous products resulting from burning any kind of material containing carbon in a free state or combined state. Also referred to as “combustion contaminants.”

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.


D.C.: direct current: an electric current which flows in one direction only. Other system of electric current transmission is alternating current (A.C.).

damper: change in pressure, or pressure drop, that occurs across a piece of control equipment.

dbA: sound-pressure level corrected to the “A” weighing network.

decibel: the logarithmic ratio between some known reference and some quantity of electrical or acoustic signal power.

delta P (?P): change in pressure, or pressure drop, that occurs across a piece of control equipment.

dew point: the temperature at which the equilibrium vapor pressure of a liquid is equal to the existing partial pressure of the respective vapor. (For air containing water vapor, it is the temperature at which liquid water begins to condense for a given state of humidity and pressure as the temperature is reduced. For flue gas containing water vapor and SO3, it is the set of conditions at which liquid sulfuric acid begins to condense as the temperature is reduced.)

dry bulb temperature: the actual temperature of a gas, taken with a conventional thermometer.

DSCFM (dry standard cubic feet per minute): see gas flow rate.

dust collector: an air-cleaning device used to remove heavy particulate loadings from exhaust systems prior to discharge.


effluent: any gas or liquid emerging from a pipe or similar outlet; usually refers to waste products from chemical or industrial plants as stack gases or liquid mixtures.

efficiency (air): the relative ability of an air cleaner to collect contaminants, usually expressed in percent. There are many methods used to measure air cleaner efficiency. Results achieved with different methods and contaminant samples may not be directly comparable. Both the basis and test method must be fully stated if efficiency figures are to be meaningful.

electrostatic dust:
particles that tend to receive and retain a static charge, such as fiberglass, paint pigments, plastics, etc.

elevation: the distance of the subject site above or below sea level.

emission: release of pollutants into the air, water, or ground from a source.

equivalent duct diameter: for rectangular duct with sides “a” and “b” is: D = (4ab/p)0.5

exhaust: the gases emitting from a plating or other process.

exhaust stack temperature: the temperature of the exhaust gas, measured in the discharge stack.

exhaust volume: the amount of exhaust gas (air, products of combustion and water vapor) leaving the exhaust stack; usually measured in ACFM.


fahrenheit: a thermometric scalre in which water boils at 212° and freezes at 32°. °F = (1.8 x °C) + 32°

fan: 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

FC: fan wheel design using forward-curved blades.

fan performance curve: 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.

fan static pressure: the static pressure for which a fan is to be selected based on system calculations: Fan SP = SP outlet – SP inlet – VP inlet

federal implementation plan (FIP): under current law, a federally implemented plan to achieve attainment of an air quality standard, used when a state is unable to develop an adequate plan. Under the Senate bill, a plan containing control measures developed and promulgated by EPA in order to fill gaps in a State Implementation Plan (SIP).

filter area: quantity of filter media available for collection of contaminants. Expressed in square feet.

filtration: a means of separation where constituents are separated usually by physical methods.

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.

FRP: abbreviation for fiberglass-reinforced-plastic.

fume scrubber: a device that uses a liquid spray to remove aerosol and gaseous pollutants from an air stream. The gases are removed either by absorption or chemical reaction. Solid and liquid particulates are removed through contact with the spray. Scrubbers are used for both the measurement and control of pollution.


gas flow rate (cubic feet per minute: CFM): the volume of process gas at any point of the plant exhaust system measured in terms of minutes. There are several units of measurement:

gassing: the evolution of gasses from one or more of the electrodes during electrolysis

HEPA filter (high efficiency particulate air filter): capable of removing at least 99.97% by count of a standard 0.3 micron challenge particulate (DOP test).


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

hood: 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.

humidity: measure of the amount of water vapor in air.

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.

HVAC: heating, ventilating and air conditioning.

hydrophilic: tendency to absorb water

hydrophobic: tendency to repel water


impeller: 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.

inch of water: a unit of pressure equal to the pressure exerted by a column of water, one inch high at a standard temperature. (407” WG = 14.7 PSI)

inches wg (inches of water gauge): See: inch of water.

indicator (pH): a substance that changes color when the pH of the medium is changed. In the case of most useful indicators, the pH range within which the color changes is narrow.


kelvin: absolute temperature in the SI system scale.


LAER (lowest achievable emission rate): the rate of emissions that reflects either the most stringent emission limit contained in the implementation plan of any state (unless it is proved that such limitations are not achievable, or the most stringent emission limit achieved in practice, whichever is most stringent).

laminar flow: gas or fluid in parallel layers with some sliding motion between the layers.

liquid flowrate: the amount of water or “scrubbing liquid” introduced into a wet collector.

louver: 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 nox burner: one of several combustion technologies used to reduce emission of Nox.

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.

make-up air: See: replacement air.

manometer: 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.

membrane: a microporous structure that acts as a highly efficient filter that allows passage of water, but rejects suspended solids and colloidals; depending on membrane type, ions and small molecules might or might not be rejected.

microbar: a unit of pressure equal to one-millionth of an atmospheric pressure; 0.0000146 PSI.

mist: 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.

motor starter: an electrical control device used to start a motor and to protect the device from current overload condition. A motor starter should be selected to match the operating voltage and horsepower requirements of the motor.

MPH: miles per hour.

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.


NOX (nitrogen oxides): chemical compounds containing nitrogen and oxygen. Reacts with volatile organic compounds in the presence of heat and sunlight to form ozone. It is also a major precursor to acid rain. Nationwide, approximately 45% of Nox emissions come from mobile sources, 35% from electric utilities, and 15% from industrial fuel combustion.


opacity: 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.
opposed-blade damper: a type of damper where the blades rotate in the opposite direction.

OSI (ounces per square inch): a unit of pressure equal to one 1/16 PSI or 1.733 inches of water.

ozone: a compound consisting of three oxygen atoms, that is the primary constituent of smog. It is formed through chemical reactions in the atmosphere involving volatile organic compounds, nitrogen oxides and sunlight. Ozones can initiate damage to the lungs as well as damage to trees, crops and materials. There is a natural layer of ozone in the upper atmosphere that shields the earth from harmful ultraviolet radiation.


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.

particle: a small, discrete mass of solid or liquid material.

particulate: a particle of solid or liquid matter.

particulate matter: any solid liquid material in the atmosphere.

PEL (permissible exposure limits): limits developed by OSHA to indicate the maximum airborne concentration of a contaminant to which an employee may be exposed.

permit: an authorization, license or equivalent control document issued by EPA or an approved state agency to implement the requirements of an environmental regulation such as a permit to operate a facility that may generate harmful emissions.

pH: a unit of measure depicting the hydrogen concentration of a solution: Scale 1 to 14. Where 7 is neutral; <7 acidic; >7 basic.

pH adjustment: the act of changing the pH of an aqueous solution by adding acid or caustic.

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.

plenum: pressure equalizing chamber.

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

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: the differential pressure between two points in a system. The resistance to flow between the two points.

pressure, static: the potential pressure exerted in all directions by fluid at rest. For a fluid in motion, it is measured in a direction normally 90° to the direction of flow. Usually expressed in inches water gauge when dealing with air.

pressure, velocity: the kinetic pressure in the directional flow necessary to cause a fluid at rest to flow at a given velocity. Usually expressed in inches water gauge.

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.


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.

rarefication: 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.

relative humidity: the ratio of existing water vapor to that of saturated air at the same dry-bulb temperature.

replacement air: a ventilating term that refers to the replacement of air lost because of exhaust air requirements.

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.

rotor: the rotating part of most AC motors.

RPM: revolutions per minute.


saturated air: air containing the maximum amount of water vapor for a given temperature and pressure.

SCFM: the gas volumetric flow rate, corrected to 70° F (standard temperature) and standard pressure (1atm) by calculation (Standard Cubic Feet per Minute).

scrubber: a device that uses a liquid spray to remove aerosol and gaseous pollutants from an air stream. The gases are removed either by absorption or chemical reaction. Solid and liquid particulates are removed through contact with the spray. Scrubbers are used for both the measurement and control of pollution.

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.

SIP (state implementation plan): documents prepared by states, and submitted to EPA for approval, which identify actions and programs to be undertaken by the state and its subdivisions to implement their responsibilities under the Clean Air Act.

slot velocity: linear speed of air through a slot in a hood, expressed in Feet Per Minute (FPM).

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.”

specific gravity: the ratio of the weight or mass of a given volume of any substance to that of an equal volume of some other substance taken as a standard. The ratio of the density of any gas to the density of dry air at the same temperature and pressure is the specific gravity of the gas.

ssplit-phase motor: the most common type of single-phase induction motor. Moderate starting torque, high-starting current, high breakdown torque. Used on easy-starting equipment, such as belt-drive fans.

squirrel-cage winding: a permanently short-circuited winding, usually uninsulated and chiefly used in induction motors, having its conductors uniformly distributed around the periphery of the machine and joined by continuous end rings.

stack: a smokestack or exhaust stack. Vertical pipe or flue designed to exhaust gases.

standard air density: 0.075 lbs./ft3. Corresponds approximately to dry air at 70°F and 29.92 in. Hg.

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 (sp): the potential pressure exerted in all directions by fluid at rest. For a fluid in motion, it is measured in a direction normally 90° to the direction of flow. Usually expressed in inches water gauge when dealing with air.

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

STEL (short-term exposure limit): the employee’s 15-minute time weighted average exposure which cannot be exceeded at any time. STEL is set by OSHA for each pollutant and expressed in terms of ppm or mg/m

SWSI: single-width, single-inlet centrifugal fan.

synchronous speed: rated motor speed expressed in RPM. Synchronous speed = 120 x frequency, divided by number of poles.

system curve (air): graphic presentation of the pressure versus volume flow rate characteristics of a particular system.


TEFC (totally enclosed, fan-cooled): totally enclosed motors equipped with a cooling fan to prevent overheating.

threshold limit values (TLV): represents the air concentrations of chemical substances to which it is believed that workers may be exposed daily without adverse effect.

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 registered trademark for an exposure limit developed by the American Conference of Governmental Industrial Hygienists (ACGIH). A listing of TLVs may be found in the ACGIH’s Documentation of the Threshold Limit Values and Biological Exposure indices for 1988-1989.

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

TP: Total pressure. The sum of velocity pressure plus static pressure.

transformer: an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. Transformers provide a change in voltage, current, phase or other electric characteristic during the transfer and hence are used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits.

transmitter: a wireless or radio transmitter; the set that transmits or sends a wireless

trees: branched or irregular projections formed on a cathode during electrodeposition especially at edges and other high current density areas.

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.

tubbeaxial fan: axial fan without guide vanes.

TWA (time weighted average): employee’s average airborne exposure which cannot be exceeded in any eight-hour work shift. TWA is set by OSHA and expressed in mg/m.


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

ultrafiltration: the process that uses membranes to achieve separation of various constituents; a typical ultrafiltration membrane allows water, ions, and small molecules to pass through while rejecting large molecules and suspended solids.

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

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.


velocity pressure: the kinetic pressure in the directional flow necessary to cause a fluid at rest to flow at a given velocity. Usually expressed in inches water gauge.

volt: the standard unit of potential difference or electromotive force, named after Alessandro Volta. One volt is the potential difference across a conductor when a current of one ampere dissipates one watt of power.


watt: a unit of power. In electrical terms, the product of voltage and amperage. 746 watts are equal to one horsepower.

wet-bulb temperature: the temperature of a gas stream taken with a wetted thermometer. It is approximately equal to the adiabatic saturation temperature of the gas.

WR2: the unit designation of fan wheel rotational inertia in lb-ft2 also know as WK2.

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3430 East Broadway Rd.
Phoenix, Arizona 85040
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