ASHRAE Terminology

time varying loads acting on the blades of a fan due to nonconformities of the airflow. Note: spatial nonuniformities of airflow, which are steady in time, give rise to harmonic excitation at frequencies which are integer multiples of the rotation rate of the fan. Time excitations of the airflow give rise to random excitation. Turbulence of the airflow gives rise to random excitation.

ventilation airflow divided by room volume. It indicates how many times, during one hour, the air volume from a space is replaced with outdoor air.

a diffuser, grille, register, or transfer grille used for controlling air patterns or air distribution or for preventing line of sight through an opening while allowing airflow through. See air terminal device or diffuser.

transportation of a specified airflow to or from the treated space, by ducts or plenums. Air-treatment devices can be added to the distribution system for the purpose of treating the air (e.g., cleaning, heating, cooling, humidifying or dehumidifying, etc.)

rapid falling of cold air that occurs when a variable-air volume (VAV) box or other device reduces airflow and the supply air leaves the diffuser at very low velocity.

the uncontrolled inward airflow through openings in the building envelope caused by the pressure effects of wind, the effect of differences in indoor and outdoor air density, or both (cfm) [m3/s].

adjusting airflow rates through air distribution system devices, such as fans and diffusers, by manually adjusting the position of dampers, splitter vanes, extractors, etc., or by using automatic control devices, such as constant-air-volume or variable-air-volume (VAV) boxes.

(1) an air-distribution assembly consisting of inlet duct connection(s) and outlet duct connection(s) whose purpose fulfills (either manually or automatically) one or more of the following functions: controls the rate of the airflow, controls the velocity or pressure and/or temperature of the air, mixes primary streams of different temperatures or humidities, or mixes within the device primary air with air from the treated space. (2) an air terminal unit may be composed of automatic or manual dampers, filters valves, heating or cooling coils, sound attenuation, nozzles, or fan assemblies. See also diffuser terminal.

value used in dust collector calculations to measure air velocity through the bag filter media. Note: calculated by dividing active volumetric airflow by the effective area of the cloth media.

value used in dust collector calculations to measure air velocity through non-cloth bag filter media (e.g., paper cartridges). Note: calculated by dividing active volumetric airflow by the effective area of the cloth media.

(1) movement of air usually within boundaries (such as ducts). (2) the volume of air per unit time.

the volume of standard air per unit of time that moves past a given plane, expressed in cubic feet per minute (cfm) or liters per second (L/s).

deterrent (due to friction, change of direction, etc.) to the passage of air within an air-distribution system and/or equipment.

material or construction that adequately impedes transmission of air under specified conditions. Compare to structural barrier, water vapor retarder.

the effective area of an air terminal device equal to the measured airflow rate divided by the velocity reading of a particular instrument used in a prescribed manner.

device to sense and measure velocity of airflow at a point.

the methodical proportioning of air and hydronic flows through the system mains, branches, and terminal devices using acceptable procedures to achieve the specified airflow or hydronic flow within testing, design, and installation limitations.

adjustable blade device to control airflow.

overall leakage of the building characterized by the airflow rate at a given pressure difference across the envelope of the building. Compare to air infiltration).

series of air-directing vanes (usually adjustable) that are used to direct and/or control the combustion airflow through the burner.

air terminal unit using a method of volume modulation whereby airflow is varied by distributing that volume required to meet the load, the balance of primary air being diverted to the return.

a terminal unit, typically having more than one outlet, that uses a method of volume modulation whereby airflow is varied by distributing the volume required to meet the space requirements, the balance of supply/exhaust air being diverted away from the space.

nonducted IDUs intended to be installed flush mounted with the ceiling. These IDUs can have configurations of indoor airflow coming from one, two, or four directions, or from a circular direction.

device to protect air openings in fire-rated ceiling assemblies that operates to interrupt airflow automatically in the event of fire in order to restrict passage of heat and flame.

the dimension of the finned surface as measured from the entering air face to the leaving air face in the direction of airflow.

a thermodynamic factor that must be applied to determine fan total efficient from fan airflow rate, fan total pressure, and fan power input. It may be considered to be the ratio of the mean airflow rate through the fan to the airflow rate at fan air density. It is also the ratio of the fan total pressure that would be developed with an incompressible fluid to the fan total pressure that is developed with a compressible fluid.

(1) natural ventilation in which the airflow mainly results from wind pressure effects on the building facades and where stack effects in the building are of less importance. (2) type of ventilating with air supply and exhaust points at opposite sides of ventilated space.

element inserted into an air-distribution system or element of an air-distribution system permitting modification of the air resistance of the system and consequently changing the airflow rate or shutting off the airflow.

required airflow when the system is operating under assumed maximum conditions of design, including diversity.

device that injects steam directly into the airflow.

actual discharge airflow rate divided by the theoretical discharge airflow rate at a given pressure difference.

one designed with airflow essentially in a vertical path, discharging air at or near the bottom of the furnace.

(1) current of air, when referring to pressure difference that causes a current of air or gases to flow through a flue, chimney, heater, or space. (2) current of air, when referring to localized effect (generally, the unwanted local cooling of the body caused by air movement) caused by one or more factors of high air velocity, low ambient temperature, or direction of airflow whereby more heat is withdrawn from a person’s skin than is normally dissipated.

systems that primarily use a combination of mechanical equipment, airflow, water flow, and electrical energy to provide heating, ventilating, air conditioning, filtration, humidification, and dehumidification for the purpose of human comfort or process control of temperature and humidity.

(1) leakage of indoor air out of a building through similar types of openings. Like natural ventilation, it is driven by natural and/or artificial pressure differences. (2) uncontrolled outward air leakage from conditioned spaces through unintentional openings in ceilings, floors, and walls to unconditioned spaces or the outdoors caused by pressure differences across these openings due to wind, inside-outside temperature differences (stack effect), and imbalances between supply and exhaust airflow rates.

sound power that is transmitted from an exhaust terminal inlet back to the room (counter to the airflow).

terminal unit for regulating exhaust or return airflow.

airflow leaving a treated space. Compare with return air.

the rate of air movement at the face of the device (airflow rate divided by face area), expressed in m/s (fpm) to three significant figures.

(1) a machine used to create flow within a fluid, typically a gas, such as air. (2) any device with two or more blades or vanes attached to a rotating shaft used to produce an airflow for the purpose of comfort, ventilation, exhaust, heating, cooling, or any other gaseous transport. The opening(s) may or may not have an element or elements for connection to ductwork. (3) fan consists of a rotating arrangement of vanes or blades that act on the air (impeller). Usually it is contained within some form of housing or case. This housing or case may direct the airflow or increase safety by preventing objects from contacting the fan blades. Most fans are powered by electric motors, but other sources of power may be used, including hydraulic motors and internal combustion engines. Fans produce airflows with high volume and low pressure, as opposed to compressors, which produce high pressures at a comparatively low volume. A fan blade will often rotate when exposed to an air stream, and devices that take advantage of this, such as anemometers and wind turbines, often have designs similar to that of a fan. (4) see also impeller.

the volumetric airflow rate at fan air density.

the useful power delivered to air by the fan, it is proportional to the product of the fan airflow rate, the fan total pressure, and the compressibility coefficient.

the mean rate of air movement through the filter media (airflow divided by net effective filtering area). The term is not applicable to plate-type electronic air cleaners. Media velocity is measured in m/s (fpm).

(1) the resistance at which a filter, or filter media, should be replaced. (2) the resistance to airflow of the air-cleaning device at which the test is terminated and results calculated, expressed in Pa (in. of water).

device that interrupts airflow automatically through part of an air system to restrict passage of flame. Installed in a fire-rated wall or floor and closes automatically in the event of fire to maintain the integrity of the fire-rated separation.

continuous motion of a fluid in pipes, ducts, channels, or through openings. See also airflow, backflow, slip.

(1) a perforated plate installed downstream of a fan in an air-handling unit designed to diffuse airflow equally across a downstream coil. Also called a diffuser plate. (2) in a multiple cooling tower installation, a pipe installed below and interconnecting the cold basin of multiple cooling towers to equalize the basin water levels and prevent the sump suction in one tower from drawing air or creating a vortex. (3) in refrigeration, a line or lines installed between compressors to equalize oil levels. (4) the bypass pipe in a primary/secondary flow system that hydronically decouples the primary from the secondary pumping system. See cross connection.

(1) failure of a refrigerating unit to operate normally due to formation of ice at the expansion device. (2) frost formation that impedes airflow through a coil.

the rate of sensible heat removal by the cooling coil for a given set of operating conditions. This value varies as a function of performance parameters such as EWB, ODB, EDB, and airflow rate. (Also see sensible heat.)

the total rate of both sensible heat and latent heat removal by the cooling coil for a given set of operating conditions. This value varies as a function of performance parameters such as EWB, ODB, EDB, and airflow rate. Also see sensible heat and latent heat.

product that uses only airflow through a medium to remove contaminants from the air.

air temperature of the internally induced airflow. The mixed airstream resultant temperature of the mix of room air temperature stream with supply airstream.

air that is drawn into a terminal by means of induction and discharged through the terminal outlet

type of mechanical draft tower in which one or more fans are located in the air outlet to induce airflow through the air inlets.

the pressure loss of the device operating at a specified airflow rate with no dust load, expressed in Pa (in. of water).

having airflow nominally in one direction.

airflow within the fume hood is described as lazy when smoke remains on the work surface without smoothly flowing to the back baffle.

device that can be used to manually adjust the airflow rate by manual operation. Also see damper, valve.

a modulating damper or set of dampers used to control the outdoor airflow to the system in excess of minimum ventilation outdoor air for free cooling (airside economizer). Also called economizer outdoor air damper. May also serve to provide the minimum outside airflow control.

element inserted in the ductwork that facilitates the determination of air temperature, air humidity, airflow rate, and/or pressure.

fan whose characteristics combine both centrifugal and axial airflow.

diffuser with features of an air terminal unit and with an integral airflow control device.

airflow resulting from the difference between atmospheric density and some lower density, chimney effect.

the total area in the device on which dust collects. For devices using fibrous media, it is the net upstream area of the media exposed to airflow measured in the plane or general surface of the media. Net effective area excludes the area blocked by sealants, flanges, or supports. In electronic air cleaners, it is the total exposed surface of those electrodes available for dust precipitation, including the ionizing section but excluding supports, holes, and insulators. Net effective filtering area is measured in m2 (ft2) to three significant figures.

the exhaust flow rate for a hood minus any internal discharge makeup airflow rate.

one without uniform airflow patterns and velocities.

the outdoor air fraction is the ratio of the volumetric flow rate of outdoor air brought in by the air handler to the total supply airflow rate.

volumetric outside airflow rate expressed as a percentage of total supply airflow rate.

a theoretical airflow distribution pattern within a ventilated space where the supply air is instantaneously and uniformly mixed with the air in the space such that the concentration of all constituents in the air, and the age of air, are spatially uniform.

fan assembly consisting of a single inlet impeller mounted perpendicular to airflow that pressurizes a plenum chamber in an air-distribution system. The impeller is typically an airfoil or backward-inclined design.

the relative position on the cooling unit characteristic curve corresponding to a particular airflow rate. It is controlled during a test by adjusting the position of the throttling device, by changing flow nozzles or auxiliary fan characteristics, or by any combination of these.

control system in which the airflow through the air terminal varies with system pressure.

control system in which the airflow through the air terminal is independent of system pressure. Also known as “pressure-compensated.”

air temperature of the primary airflow (e.g., supply airflow from an air-handling unit).

a procedure employed for verifying the accuracy of the measuring techniques (temperature, airflow rates, duct heat calibration) employed in determination of the room heating effect. This qualification procedure is periodically employed in determining the room heating effect.

the airflow rate in m3/s (cfm) at which the device is tested.

the operating pressure loss at the airflow rate at which a disposable device (e.g., filter) should be replaced or renewed, as recommended by the manufacturer, expressed in Pa (in. of water).

airflow within the hood when smoke released in the hood moves forward, toward the front of the hood. This term does not apply to the forward motion of the roll inside the hood that occurs in the upper cavity of the hood above the hood opening or to the cyclonic motion that occurs behind a closed horizontal sash.

a tubular windscreen to be attached to a standard microphone and designed to minimize its sensitivity to airflow noise.

terminal where primary airflow is modulated and mixed with induced air by a continuously operated integral fan to provide a relatively constant volume of discharge.

air-terminal unit assembly having one ducted air inlet and a device for regulating the airflow rate so it is independent of inlet pressure changes.

an engineered system that uses mechanical fans to produce airflows and pressure differences across barriers to limit smoke movement.

(1) movement of air into and out of buildings, chimneys, flue gas stacks, or other containers and is driven by buoyancy. Buoyancy occurs due to a difference in indoor to outdoor air density resulting from temperature and moisture differences. The result is either a positive or negative buoyancy force. The greater the thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect. The stack effect is also referred to as the chimney effect, and it helps drive natural ventilation and infiltration. (2) movement of air or other gas in a vertical enclosure (e.g., duct, chimney, building), induced by the density difference between the air or other gas in the enclosure and the ambient atmosphere. Note: stack effect is a significant concern in heating-system design for tall buildings in cold climates. Sometimes referred to as chimney effect. (3) pressure difference caused by the difference in density between indoor and outdoor air due to an indoor/outdoor temperature difference. (4) the vertical airflow within buildings caused by temperature differences between the building's interior and exterior.

(1) external static pressure loss for forced-air systems using packaged air-handling equipment, the static pressure loss resulting from airflow through the supply and return ductwork and other elements external to the packaged air-handling unit. Compare to internal static pressure loss and total static pressure loss. (2) internal static pressure loss for forced-air systems, the static pressure loss resulting from airflow through the internal elements of the packaged air-handling unit. These internal elements may include such items as filters, coils, dampers, mixing sections, etc. Compare to internal static pressure loss and total static pressure loss. (3) total static pressure loss for forced-air systems, the sum of the external static pressure loss and the internal static pressure loss. See internal static pressure loss and external static pressure loss. The term is not applicable to an individual fan or a fan located within a builtup air-handling unit. Fans are rated by the term fan total static pressure, which is not the same as fan total pressure. Compare to fan total pressure.

layers of air at different temperatures or different velocities flowing through a duct or plenum system and affect fan and/or pump performance in the distribution system.

vapor or gas seal integral to the structural materials of an enclosure. See water vapor retarder, airflow retarder.

(1) air delivered by mechanical or natural ventilation to a space, composed of any combination of outdoor air, recirculated air, or transfer air. (2) air entering a space from an air-conditioning, heating, or ventilating apparatus for the purpose of comfort conditioning. Supply air is generally filtered, fan forced, and either heated, cooled, humidified, or dehumidified as necessary to maintain specified conditions. Only the quantity of outdoor air within the supply airflow may be used as replacement air.

device that relies on the cooling effect of the airflow to change the temperature of a heated body in proportion to the air speed. Types include hot-wire anemometer, heated-bulb thermometer, heated-thermocouple anemometer, and heated-thermistor anemometer.

airflow created by a convective heat source that rises due to natural thermal buoyancy. Plume formation and growth are dependent on the intensity of the heat source and on the degree of stratification of the ambient air.

combination of varying both airflow and temperature in response to space load, for the purpose of resetting temperature to maintain greater low-load airflow to the space than in a true variable volume system.

a method of controlling airflow produced by a fan by means of adjusting the speed or revolutions per minute (rpm) of the fan.

rate of airflow to or from a space, expressed in terms of volume per unit of time, either through the ventilation system or infiltration through building envelope. These are commonly expressed as cubic feet per minute (CFM) in USCS units or liters per second (l/s) in SI units.

(1) a separately controlled heated or cooled space. (2) one occupied space or several occupied spaces with similar occupancy category, occupant density, zone air distribution effectiveness, and zone primary airflow per unit area. (3) space or group of spaces within a building for which the heating, cooling, or lighting requirements are sufficiently similar that desired conditions can be maintained throughout by a single controlling device.