ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

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 perimeter heat loss factor for slab-on-grade floors, expressed in Btu/h·ft2·°F (W/m2·°K).

fabric filter

filter having a textile-based filter medium.

facade area

area of the facade, including overhanging soffits, cornices, and protruding columns, measured in elevation in a vertical plane parallel to the plane of the face of the building. Nonhorizontal roof surfaces shall be included in the calculation of vertical façade area by measuring the area in a plane parallel to the surface.

face area

total plane area of the portion of a grille, coil, or other item bounded by a line tangent to the outer edges of the openings through which air can pass.

face velocity

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.

facility guide

a basic building systems description and operating plan with general procedures and confirmed facility operating conditions, set points, schedules, and operating procedures to properly operate the facility.

facility management system (FMS)

see EMS, EMCS, BAS, and BMS.

factor of safety

ratio of a normal working condition to the ultimate condition, such as, in strength of materials, the ratio of working stress to ultimate strength.

Fahrenheit temperature

temperature scale used with the I-P system in which, at standard atmospheric pressure, the boiling point of water is 212°F and its freezing point is 32°F, absolute zero is minus –459.69°F.


(1) position or mode of operation a controlled device takes on removal of the control signal and/or power. (2) to return to a position that, on loss of control system power, allows the controlled system to go to a safe mode.

failure response

actions performed by the maintenance department resources that are expended or reserved for dealing with interruptions in the operation or function of a system or equipment under the maintenance program. These responses have two types of classification: repair and service.


(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.

fan air density

density of air corresponding to the total pressure and total temperature at the fan inlet when the fan is operating.

fan airflow rate

the volumetric airflow rate at fan air density.

fan array

multiple fans in parallel between two plenum sections in an air distribution system.

fan blast area

fan scroll outlet area less the area of the cutoff.

fan boundary

interface between the fan and the remainder of the system at a plane perpendicular to the airstream where it enters or leaves the fan. Appurtenances such as inlet boxes, inlet vanes, inlet cones, silencers, screens, rain hoods, dampers, and discharge cones may be included as part of the fan between the inlet and outlet boundaries.

fan brake horsepower (bhp)

the horsepower delivered to the fan’s shaft. Brake horsepower does not include the mechanical drive losses (e.g., belts, gears, etc.).

fan casing

controlled expansion part of the casing of a centrifugal fan that receives fluid forced outward from the impeller or diffuser and leads it to the discharge. Compare to fan shroud.

fan casing

(volute, scroll), part of the casing of a centrifugal fan or compressor that receives fluid forced outward from the impeller or diffuser and leads it to the discharge. Compare to fan shroud.

fan coil unit

factory-made assembly that provides the functions of air circulation, cooling, heating, or cooling and heating.

fan curve
fan discharge area

area where the fan discharge scroll stops. Used in fan system effect calculations.

fan inlet or outlet area

area of the fan or fan-equipment fitting provided for connection to attached ductwork.

fan input power boundary

the interface between the fan and its driver. Drive or coupling losses may be included as a part of the input power.

fan laws

(1) flow rate varies in direct proportion to fan speed (rpm). (2) flow rate varies in direct proportion to pitch diameter of the motor sheave.

fan motor power

the electric power required to drive the fan and any elements in the drive train that are considered a part of the fan.

fan nodal line

line of zero vibration response on the fan such that the blades on opposite sides of the line vibrate in opposite phases.

fan performance curve

graphical representation of static or total pressure and power input over a range of air volume flow rate at a stated inlet density and fan speed(s). It may include static and mechanical efficiency curves.

fan power input

the power required to drive the fan and any elements in the drive train that are considered a part of the fan.

fan power output

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.

fan pressurization test

a means for determining the air leakage of a building using a fan-induced pressure difference.

fan shroud

protective housing that surrounds the fan and which may also direct the flow of air. Compare to fan casing.

fan sound power

total sound power radiated by the fan and transmitted to the duct and the areas surrounding the fan itself.

fan speed

the rotational speed of the impeller. If a fan has more than one impeller, fan speeds are the rotational speeds of each impeller.

fan static efficiency

the fan total efficiency multiplied by the ratio of fan static pressure to fan total pressure.

fan static pressure

the difference between the fan total pressure and the fan velocity pressure. Therefore, it is the difference between static pressure at the fan outlet and total pressure at the fan inlet.

fan system power

the sum of the nominal power demand (nameplate horsepower) of motors of all fans that are required to operate at design conditions to supply air from the heating or cooling source to the conditioned space(s) and return it to the source or exhaust it to the outdoors.

fan total efficiency

the ratio of fan power output to fan power input.

fan total pressure

difference between the total pressure at the fan outlet and the total pressure at the fan inlet.

fan types

there are two main categories of fans used in HVAC&R applications: centrifugal and axial fans. Centrifugal fans types include the following: forward curved (FC), backward curved or backward inclined (BI), airfoil (AF), and radial. Axial fan types include the following: propeller, vaneaxial, and tubeaxial.

fan velocity pressure

the pressure corresponding to the average air velocity at the fan outlet. Measured with pilot tube tranverse of duct. Assumes no change in density or area between the plane of measurement and the fan outlet.

fan wheel

revolving part of a fan or blower. See also impeller.

fan wheel cone

inlet ring, impeller shroud, impeller rim annular plate, or conical ring on the air inlet side of a centrifugal fan to which the impeller blades are fixed.

fanning friction factor

dimensionless number f used in studying fluid friction in pipes, equal to the pipe diameter times the drop in pressure in the fluid due to friction as it passes through the pipe divided by the product of the pipe length and the kinetic energy of the fluid per unit volume.

far field

the portion of the sound field beyond the near field of a sound source in which the sound pressure level decreases by 6 dB for each doubling of distance from the source. Typically free of any reflecting surfaces.

Faraday’s laws of electrolysis

(1) amount of any substance dissolved or deposited in electrolysis is proportional to the total electric charge passed. (2) amounts of different substances dissolved or deposited by the passage of the same electric charge are proportional to their equivalent weights.

feed forward

an element or pathway within a control system that passes a controlling signal from a source in the control system's external environment, often a command signal from an external operator, to a load elsewhere in its external environment. A control system that has only feed-forward behavior responds to its control signal in a predefined way without responding to how the load reacts, it is in contrast with a system that also has feedback. Compare to feedback.


the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or occurrences of the same (i.e., same defined) event/phenomenon (or the continuation/development of the original phenomenon) in the present or future. When an event is part of a chain of cause and effect that forms a circuit or loop, then the event is said to \"feed back\" into itself. Compare to feed forward.


main distribution line (usually three phase) supplying electric energy within an electric service area or subarea.

feedwater economizer

heat exchanger installed in the hot-gas duct between the boiler and the stack to transfer a portion of the heat (that would be lost up the stack) to the feedwater.


(1) commonly used to refer to any opening, usually glazed, in a building envelope, windows. Examples include windows, plastic panels, clerestories, skylights, glass doors that are more than one-half glass, and glass block walls. (2) in an external wall of a building, any area that allows light to pass.

fenestration area

total area of the fenestration measured using the rough opening and including the glazing, sash, and frame. For doors where the glazed vision area is less than 50% of the door area, the fenestration area is the glazed vision area. For all other doors, the fenestration area is the door area. (See door area.)

fenestration elements

(1) framing, mullions, muntins, and dividers. (2)  glazing material, either glass or plastic. (3) external shading devices. (4) internal shading (5) integral (between glass) shading systems or devices.

Fermat’s principle

an electromagnetic wave takes a path that involves the least travel time when propagating between two points.

Fick’s law

rate of diffusion of matter across a plane is proportional to the negative of the rate of change of the concentration of the diffusing substance in the direction perpendicular to the plane.

field in engineering

(1) physical area of engineering activity. Compare to shop. (2) sphere of engineering operation, observation, or intellectual activities.

field of view (FOV)

total angular dimensions within which objects can be imaged, recorded, and displayed by an imaging device when the device is pointed in a fixed direction.

field-installed device (FID)

microprocessor (DDC) control panel, field mounted and connected. Compare to product-integrated control.

figure of merit (FOM)

a quantity used to characterize the performance of a device, system, or method relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application. See also part-load value.


an alphanumeric designation assigned to each symbol for ease in referencing.

fillet weld

weld of approximately triangular cross section, joining two surfaces approximately at right angles in a lap joint, tee joint, corner joint, or socket joint.

film boiling

boiling phenomenon corresponding to the development of a continuous vapor layer on the heating surface that separates this surface from the boiling liquid.

film coefficient

heat transferred between a surface and a fluid in unit time through unit area induced by unit temperature difference.

film-cooling tower

cooling tower with a type of packing over which the water spreads in a thin film.

film-forming condensation

phenomenon encountered when the condensate wets a cold surface to form a continuous film that separates this surface from the vapor.


(1) capacitor and/or inductor placed in a series/parallel combination across a direct current (DC) line to remove the effects of the alternating current (AC) signal or to decrease the ripple voltage in a DC power supply. (2) device to remove gases, liquids, or particles from a mixture of gases, (3) device to remove solid material from a liquid.

filter cell

(also known as filter cartridge, filter unit, or filter element), interchangeable frame or cylinder containing a filter medium.

filter drier (filter dehydrator)

encased desiccant, generally inserted in the liquid line of a refrigerating system, and sometimes in the suction line, to remove entrained moisture, acids, and other contaminants.

filter efficiency (Ef )

contaminant capacity divided by contaminant loading, expressed as a percent.

filter efficiency rating
filter media

portion of a filtrating system (such as close-woven textiles, metal screens, papers, nonwoven fabrics, granular beds, or porous media) that provides the filtration process or liquid solid separation,

filter mixing box

in air-handling units, the section containing the filters and the outside air/return air dampers and mixing plenum.

filter separation efficiency

ratio of the mass of particles extracted to the mass of particles contained in the air before filtering.

filter-media velocity

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).


process of passing a fluid through a porous material in such a manner as to remove suspended matter from the fluid.


thin piece of metal attached to a pipe, tubing, or other surface in order to increase the heat transfer area. See also heat transfer surface.

fin efficiency

ratio of heat actually transferred by a fin to heat transferred if the whole fin were at the temperature of the primary surface to which it is attached.

fin pitch

number of transverse fins per unit length of tube.

fin spacing

distance between two successive transverse fins on a tube.

final filter

(1) a filter positioned in the last filtering position in an air-handling system. (2) filter used to collect the loading dust that has passed through a device during the test procedure.

final resistance

(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).

finned length

on a finned tube, the distance between the two end fins (excluding lengths for return bends and other overall lengths).

finned surface area

total area of fins and prime surface of tube or plate to which fins are attached.

finned tube

heat transfer tube or pipe with extended surface of fins, discs, or ribs.

finned-tube baseboard

a heating terminal unit that primarily consists of one or more finned tubes and a casing cabinet.

finned-tube boiler

a boiler whose heat exchanger consists of only finned tubes.

finned-tube radiator

wall-mounted heater with a plurality of fins bonded to a tube, usually carrying steam or hot water. Also see baseboard radiator.

fire damper

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.

fire point

lowest temperature at which a product ignites and continues to burn for a specified time after a small flame has been applied to its surface under standardized conditions. Compare to flash point.

fire tube boiler

boiler with straight tubes surrounded by water through which tubes pass the products of combustion.


combustion chamber in a furnace or boiler.

firefighter's smoke control station

firefighter's smoke control station (FSCS) includes monitoring and overriding capability over smoke-control systems and equipment provided at designated location(s) within the building for use by the fire department.

firing valve

lubricated, plug-type variable-position valve usually operated with an attached handle or, in the large sizes, by a loose-fitting key or extended-handle wrench.

first air

in a cleanroom, air that issues directly from the HEPA filter before it passes over any work location.

first-hour rating

an estimate of the maximum volume of hot water that a storage-type water heater or an integrated heater can supply within an hour from the time that the water heater is fully heated (i.e., with all thermostats satisfied). This rating is a function of both the storage volume and the recovery rate.

fiscal year

a period used for calculating annual (yearly) financial statements in businesses and other organizations.

fiscal year ending

the end date of a fiscal year.

fit-out project

a project through which furnishings, including partitions, furniture, and tenant equipment (e.g., copy machines, fax machines, personal computers) are delivered.

fitting inlet flow area

measured total inside area determined at the plane(s) of the inlet connection(s). The area shall be based on physical measurements for rigid fittings and physical measurements minus twice the lining thickness for lined fittings.

fitting outlet flow area

total inside area determined at the plane(s) of the outlet connection(s). The area shall be based on physical measurements for rigid fittings and physical measurements minus twice the lining thickness for lined fittings.

fixed error

a term that has the same meaning as systematic error.

fixed-setting control

control in which the setpoint cannot be altered.

flame impingement

condition existing when the flame resulting from the combustion of the fuel comes into contact with any interior surface of the furnace. It has the potential of causing damage to the heat exchanger.

flame safeguard control

system for sensing the presence or absence of flame and for indicating, alarming, or initiating control action.

flammable refrigerant

refrigerant that is ignitable when mixed with air (e.g., ethyl chloride, methyl chloride, and the hydrocarbons). Note: see ASHRAE Standard 34 for flammability categories.


(1) disc rim on the end of a pipe for coupling pipes together, usually by bolts. Flange types include weld neck, slip-on, and grooved end. Flange face can be raised face or flat faced. (2) parts of a channel at a right angle to the central section or web. See also pipe fittings. (3) projection of metal on formed objects.

flanged joint

a joint made by bolting together a pair of flanged ends. The seal is created from a gasket placed between the two flange faces and having the bolts pinching the surface of the gasket.

flanking sound transmission

reduction in sound transmission loss of a wall caused by sound traveling around the wall by other paths (structure borne, leaks, etc.).

flare nut

nut used to hold flared tubing on a flare fitting.

flared joint

(also known as flare fitting), metal-to-metal compression joint in which a conical spread is made on the end of a tube that is compressed by a flare nut against a mating flare.

flaring tool

(also known as flaring block), device for shaping the end of a ductile pipe or tube to increase its diameter to a shape suited for a flare fitting.

flash chamber

in a refrigerating system, a separating tank placed between the expansion device and evaporator to separate and bypass any flash gas formed due to pressure reduction.

flash gas

portion of the liquid refrigerant that is vaporized by sudden reduction of pressure.

flash intercooler

vessel located between compression stages where injected liquid refrigerant vaporizes immediately.

flash point

(1) at the pump suction, the pressure below which the liquid will flash into vapor. See net positive suction head. (2) in a piping system, the pressure below which the liquid will flash into vapor. (3) in a vacuum-cooling chamber, that pressure corresponding to the vapor pressure at the product temperature and below which water vaporizing commences. (4) lowest temperature to which a product must be heated for its vapors to ignite in the presence of a flame when operating under standardized conditions. Compare to fire point.

flash tank

(also known as steam systems), a steel tank that allows any superheated condensate to be vaporized into saturated steam. The tank is usually vented to atmosphere.

flash vaporization

(also known as instantaneous vaporization), partial or total vaporization obtained by sudden reduction of pressure.


the physical transformation of a fluid from its liquid state into its vapor or gaseous state. It is caused by the actual pressure of the fluid being lower than the vapor pressure of the fluid at that operating temperature.

flat plate collector

nonconcentrating solar collector in which the absorbing surface is essentially planar and approximately equal in area to the aperture.

flat weighting

sound pressure level versus frequency, which is not adjusted by any weighting network but is taken just as it is.

flexible duct

ducts constructed of flexible materials, such as polymeric films, metal foils, and impregnated fabrics, and used to connect rigid duct to air devices and terminals.

flexible-shaft centrifugal compressor

impeller and shaft are a one-piece assembly, the latter of a small diameter so that it can flex and spontaneously balance the inertial forces resulting from rotation.

float switch

device in which a float ball, through variations of the level of liquid, operates one or more sets of electrical contacts to activate or deactivate other controls or alarms.

float valve

a valve that operates due to changes in liquid level.

float-and-thermostatic steam trap

see steam trap.

float-and-thermostatic trap (F&T)

steam trap that relies on the density of water to raise a float-and-lever mechanism to operate a valve head. It discharges condensate as it forms and enters the trap body. It includes a thermostatic balance pressure or bimetallic air vent to allow free passage of air on start up and discharges incondensable gases reaching the trap during operation.

floating action

continuous action in which the input variable determines the rate of change of the output variable.

floating control

a combination of a modulating controlled device with a pair of two position outputs. The control signal will activate either one or the other outputs to drive the controlled device towards its open or closed position. When both outputs are off, the controlled device maintains its last position. Also referred to as tristate control.

floating floor

composite flooring system that exhibits a high sound transmission loss value and consisting of a “floating” flooring surface supported from a structural floor through the use of a series of vibration isolation pads.

floc point

temperature observed at the start of the formation of wax or other solid, determined by a standardized test to assess the low-temperature compatibility of certain petroleum products with refrigerants.

flooded evaporator

refrigerant evaporator characterized by no organized flow, in which most of the evaporator surface is in contact with the liquid refrigerant, and from which the refrigerant exits at a vapor quality of one, without significant superheat. Compare to dry expansion evaporator.

floor area

the horizontal area within the inside perimeter of the exterior walls of the building space. Also, the sum of the areas of several floors of the building including basements, mezzanine, intermediate floored tiers, and penthouses of headroom height, measured from the exterior faces of exterior walls or from the centerline of walls separating buildings but excluding covered walkways, open roofed-over areas, porches and similar spaces, pipe trenches, exterior terraces or steps, chimneys, roof overhangs, and similar features.


method of treating material by floating it in a liquid, an ice making method in which ice floats away from the surface on which it has been frozen.


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

flow area

in a heat exchanger with a bundle of tubes, the section that is effectively traversed by the external fluid.

flow coefficient

experimentally determined proportionality constant relating the actual velocity of fluid flow in a pipe, duct, or open channel to the theoretical velocity expected under certain assumptions.

flow control

modulation of a fluid flow rate through a system of piping, ductwork, or variable devices.

flow equalizer

(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.

flow nozzle

tube specially shaped to increase the discharge velocity of the fluid.

flow pattern

configuration of the direction and velocity of a flowing fluid.

flow rate

the mass or volumetric flow of a fluid per unit of time that moves past a given plane.


(1) a device employing a detecting element that determines the flow rate of a volatile refrigerant in the gaseous or liquid phase within a closed conduit by measuring a suitable response of the detecting element. (2) a device for measuring or determining the mass or volumetric flow rate of a fluid in a duct, pipe, or terminal device.


conduit between the furnace or boiler outlet and the integral draft diverter, draft hood, barometric draft regulator, vent terminal, or any other point of draft relief.

flue collar

that portion of an appliance designed for attachment to a chimney, vent connector, or a draft hood.

flue damper

an electrical or mechanical device in the flue outlet or in the inlet of or upstream of the draft control device of an individual, automatically operated, fossil-fuel-fired appliance that is designed to automatically open the flue outlet during appliance operation and to automatically close the flue outlet when the appliance is in a standby condition.

flue gas

(also known as vent gas), all gases in a flue during combustion in the combustion chamber, including reaction products such as excess air, carbon dioxide, carbon monoxide, oxygen, water vapor, nitrogen, and other inerts.

flue gas collector

chamber designed to capture the products of combustion and accompanying excess air.

flue gas vent

see flue.

flue losses

the sum of sensible and latent heat losses above room temperature of the flue gases leaving the furnace or boiler.

flue outlet

opening provided in an appliance for the release of flue gases.


substance, as a liquid or gas, that is capable of flowing and that changes shape when acted on by a force.

fluid filter

fluid filter that uses a fluid spray to remove contaminants from the air

fluid mechanics

mechanics of flowing (deforming) fluids involving changes in momentum, direction of flow, turbulence, pressure, and volume.

fluid transport

the transfer of air, water, or other fluids between components.


science and technology of using the properties of flow of fluids to perform functions of sensing, amplifying, and control in nonelectric circuits and without the use of any mechanical moving parts.

fluidized-bed freezer

trough having a perforated bottom through which an upward flow of cold air suspends the produce, usually of small unit size, and causes it to flow like a fluid.


any of a broad group of organic compounds analogous to hydrocarbons in which all or most of the hydrogen atoms of the hydrocarbon have been replaced by fluorine, some types also contain chlorine, and these are called chlorofluorocarbons. The saturated, aliphatic-type fluorocarbons comprise the series of refrigerants developed by Midgeley in the 1930s. Compare to halocarbon.


(1) (electrical use), amount of some quantity flowing across a given area (often a unit area perpendicular to the flow) per unit time. Note: the quantity may be, for example, mass or volume of a fluid, electromagnetic energy, or number of particles. See also magnetic flux. (2) (mechanical use), substance or mixture that promotes fusion and prevents formation of or eliminates oxides, nitrides, or other undesirable inclusions in the joint area during brazing and soldering operations. (3) (thermal use), See heat flux.


heavy wheel attached to a shaft to reduce rotation fluctuations.

flywheel effect

the continuation of heat transfer after the heat transfer system/equipment has been removed or de-energized. The effect is due to the thermal mass of the space and capacity of that mass to store/release heat. See thermal inertia.

foamed-in-place thermal insulation

insulation formed by introducing into prepared cavities a chemical component and a foaming agent that react to fill the space with a foamed plastic.


formation of a foam or froth of oil refrigerant due to rapid boiling out of the refrigerant dissolved in the oil when the pressure is suddenly reduced. See also boiler foaming.


fine, airborne droplets usually formed by condensation of vapor.

footstep bearing

thrust bearing used to support the lower end of a vertical shaft.

forced draft

combustion air supplied under pressure to the fuel-burning equipment. Compare to induced draft, natural draft.

forced lubrication

(also known as pump lubrication, mechanical lubrication, or forced-feed oiling), lubricating system with the oil fed under pressure by a pump.

forced-air circulation

movement of air by mechanical means.

forced-air distribution system

a heating and/or cooling system that uses motor-driven blowers to distribute heated, cooled, and otherwise treated air for the comfort of individuals or equipment.

forced-air or gravity-type furnace

an electric, gas, or oil-burning appliance designed to supply heat through a system of ducts with air as the heating medium. Forced-air furnaces circulate the heated air by means of a fan or blower, whereas gravity furnaces depend primarily on natural convection.

forced-draft burner

burner that has a fan capable of supplying all necessary air for proper combustion with positive pressure in the firebox.

forced-draft cooling tower

type of mechanical draft tower in which one or more fans are located at the air inlet to force air into the tower.


in an ice plant, a device for cooling water before pouring into ice-making cans, precooler.

forward-curved impeller

centrifugal rotor in which the concave sides of blades face in the direction of rotation.

fossil fuel

fuel derived from a hydrocarbon deposit such as petroleum, coal, or natural gas derived from living matter of a previous geologic time.

four pipe air-conditioning system

multiple piping arrangement in which each unit is fitted with supply and return pipes separately for hot and chilled water.

four wire transmission

(1) in three-phase power transmission, the use of four wires WYE connected. (2) transmission of data in two directions simultaneously over two pairs of wires. Also known as full duplex transmission.

Fourier number

(1) dimensionless number used in calculation of unsteady-state heat transfer, equal to the product of the thermal conductivity and a characteristic time divided by the product of the density, the specific heat at constant pressure, and the distance from the midpoint of the body through which heat is passing to the surface. Symbol is Fo, Fo, or NFo. (2) dimensionless number used in unsteady-state flow calculations, equal to the product of the dynamic viscosity and a characteristic time divided by the product of the fluid density and the square of a characteristic length.


a change in composition of a blend by preferential evaporation of the more volatile component(s) or condensation of the less volatile component(s).

free area

(1) actual open area between the fins of a grille or register. (2) total area through which air can pass in a grille, face, or register.

free delivery

the point of operation where the external static pressure is zero.

free float

refers to a situation where mechanical heating and cooling equipment is off so that the space or zone temperature varies without constraint.

free-field sound field

a sound field free of reflections where the sound declines by 6 dB per doubling of distance out of doors.

freeze concentration (cryoconcentration)

concentrating a solution by partial freezing of water and removing ice crystals.

freeze desalination

freezing saline or brackish water, so that salt-free ice crystals are formed and then removed and melted.

freeze dryer

apparatus or system for drying substances by dehydration in a frozen state. A drum freeze-dryer apparatus having a rotating cylinder in which a product is freeze dried.

freeze drying (lyophilization)

dehydration of a substance by freezing and subsequent sublimation of ice.

freeze out

to separate the constituents of a mixture by freezing one of them.

freeze stat
freeze up

(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.

freeze-drying additive

substance added to a product to be freeze dried in order to facilitate recovery of the frozen mass.

freeze-up control

(1) an insulated room kept below 32°F (0°C). Typical freezer setpoints are generally 10°F (–12°C). (2) device for freezing perishables. See freezing room.

freeze/thaw resistance

property of a material which permits it to be alternately frozen and thawed through many cycles without damage.

freezer burn

damage to frozen produce caused, essentially, by excessive desiccation. Compare to cold injury (low-temperature injury).


Solidification phase change of a liquid or the liquid content of a substance, usually due to cooling

freezing mixture

mixture of salts with crushed ice to lower the ice's melting point.

freezing plateau (thermal arrest)

during the freezing of produce, that part of the graph of temperature against time in which the temperature remains virtually constant. The freezing plateau is induced by the time required to remove latent heat.

freezing point

for a particular pressure, the temperature at which a given substance will solidify or freeze upon removal of heat. Compare to solidification point.

freezing rate

ratio between the distance from the mean surface of a food mass to its thermal center and the time elapsed between the surface reaching 32°F (0°C) and the thermal center reaching 14°F (–10°C).

freezing room

cold chamber usually kept at a temperature of 22°F to 31°F (–5°C to –1°C), with high-volume air circulation.

freezing-point depression

difference between the freezing temperature of a pure solvent and that of a solution.


(1) number of cycles per second through which an alternating electric current passes, in North America, frequency is generally standardized at 60 cycles per second (60 Hz). Most other countries standardize using 50 Hz. (2) the number of times a quantity (such as a sound wave) repeats itself in one second.

frequency range of interest

for general purposes, the frequency range of interest includes one-third octave bands with center frequencies between 31 Hz and 16000 Hz.

frequency response

the normalized motion (vibration) response of a device, such as a fan, to a known excitation, expressed as a function of the frequency of the excitation. The frequency response is usually given graphically by curves showing the relationship of the response to the excitation (and, where applicable, phase shift or phase angle) as a function of frequency.

frequency spectrum

the amplitude of sound or vibration at various frequencies or bands of frequencies.

fresh air makeup

volume of outdoor air introduced into a space.

friction factor

(1) any of several dimensionless numbers used in studying fluid friction in pipes, equal to the Fanning friction factor times some dimensionless constant. (2) coefficient used to calculate friction forces due to fluid flow. (3) quotient of the tangential force exerted by a fluid on a surface (per unit area) by half the product of the density and the square of the velocity.

friction head

height of liquid that represents system resistance caused by the flow.

friction loss

pressure loss due to friction between a flowing fluid and its contact surface.

friction ring (rubbing ring)

component, rotary or fixed, of a shaft seal with a precision-machined contact face of the ring that provides the gastight or liquidtight seal.

frictional resistance

resistance of fluid flow due to friction between the fluid and the contact surface over which it flows.

frost deposit

frost formed on the cold surface (tubes, plates) of a cooling coil.

frost formation

frost formed on any cold part of a refrigerating circuit.

frost heave

lifting of part of a structure due to the expansion of freezing water in the supporting soil.

frost level indicator

pipe loop projecting from a refrigerant vessel indicating approximately the liquid level by the frost formation.

frost point

temperature at which visible frost forms on a surface being chilled.

frost-level indicator

pipe loop projecting from a vessel showing approximately the liquid level by the frost formation.


coating of frost on suction line and crankcase due to liquid refrigerant leaving an evaporator and carried along the suction line into the compressor crankcase.

Froude number

(1) dimensionless number equal to the ratio of the speed of flow of a fluid in an open channel to the speed of very small gravity waves, the latter being equal to the square root of the product of the acceleration of gravity and a characteristic length. (Symbol is Fr2 or NFr 2). See also gravity current. (2) dimensionless number used in studying the motion of a body floating on a fluid with production of surface waves and eddies, equal to the ratio of the square of the relative speed to the product of the acceleration of gravity and a characteristic length of the body. (Symbol is Fr, Fr1, or NFr 1).

frozen earth storage

(also called frozen ground storage or frozen soil storage), underground storage of liquefied gas within a space with walls, floor, and roof consisting of frozen soil or rock.

frozen-food cabinet

refrigerated cabinet for the short-term storage of frozen foods.

frozen-food locker

individual lockable compartment in a collective, multicompartment establishment for the freezing and storage of food.

frozen-food weight factor

conversion factor of 35 lb/ft3 (560 kg/m3) for a net freezer space weight rating. Many commercially available packaged frozen foods weigh less than 35 lb/ft3 (560 kg/m3).


(1) a material that may be used to produce heat or generate power by combustion. (2) a substance that produces useful energy when it undergoes a chemical or nuclear reaction.

fuel input rate

rate at which fuel is supplied to an appliance. The rate may be expressed in Btus per hour, watts, or thousands of Btus per hour (MBH), in cubic feet per hour, liters per second, or thousands of cubic feet per hour, in therms (th) or dekatherms (dth) per hour, or in gallons per hour (GPH).

full-load amperes

(1) current in amperes through a device when at maximum demand. Not to be confused with inrush amperes (at start-up) or locked rotor amperes (work exceeds rating). (2) current that a rotating machine will draw from the power line when the machine is operating at rated voltage, speed, and torque.


solid particles formed by condensation of vapors of solid materials or a discharge gas or byproduct of combustion or chemical process. Very small airborne particles, usually less than one micrometer in size, from burning or melting materials or a chemical process. Note: popularly, the term fumes refers to any kind of air contaminant, many laws and regulations add a qualification that the contaminant have some unwanted action.

fume hood

(generic term) a fume-collection device mounted over a work space, table, or shelf and serving to conduct unwanted gases away from the area enclosed. The least effective fume hood is a canopy hood, open on four sides. The most effective fume hood effective is an enclosed hood with operable front sash and fixed sides and back. Compare to laboratory fume hood.

fume-hood face

plane of minimum area at the front portion of a laboratory fume hood through which air enters when the sash is fully opened or opened to a stop position [usually in the same plane as the sash(es) when present].

fume-hood system

an arrangement consisting of a fume hood, its adjacent room environment, and the air exhaust equipment (such as blowers and ductwork) required to make the hood operable.


in the food industry, exposure of a product to gaseous substances, generally to kill insects, their eggs, and larvae.

functional and performance test protocol

a written collection of test plans and checklists that, when executed in the test process, allow verification of the performance of a system or assembly in a consistent and repeatable manner.

functional performance testing (FPT)

(1) the process of determining the ability of the HVAC system to deliver heating, ventilating, and air-conditioning services in accordance with the final design intent. (2) that full range of checks and tests carried out to determine if all components, subsystems, systems, and interfaces between systems function in accordance with the contract documents. In this context, function includes all modes and sequences of control operation, all interlocks and conditional control responses, and all specified responses to emergency conditions.


(1) enclosed chamber or structure in which heat is produced, as by burning fuel or by converting electrical energy. (2) part of a warm-air heating system in which energy is converted to heat.

fusible plug

safety device for release of pressure by melting a contained substance with a predetermined melting temperature.


chemical term for a change of phase from solid to liquid.