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

(thermal conductance), time rate of steady-state heat flow through unit area of a material or construction, induced by a unit temperature difference between the body surfaces. Units of C are Btu/h·ft2 or °FW/m2·K. Note that the C-factor does not include soil or air films.


[C-scale or dB(C)], sound levels at different frequencies modified by a weighting network as defined in ANSI S1.4. Adjusts the levels of a frequency spectrum in a modest way, similar to the way the human ear does when exposed to high levels of sound. These results are close to unweighted or linear sound spectrums.


a term applied to any spaces in the aircraft occupied by passengers or crew members.

cabin altitude

the effective altitude to which the aircraft cabin is pressurized.

cabin pressure control system (CPCS)

part of the environmental control system that regulates cabin altitude.


parts design and manufacturing method utilizing a computer database where drawings are not needed. Synonymous with computer integrated manufacturing.

calculated variable

(1) variable that cannot actually be measured directly but one which can be calculated by measuring other variables (e.g., measure wet-bulb temperature and measure dry-bulb temperature to determine enthalpy). (2) variable that is calculated from one or more inputs.


the act of comparing an instrument of unknown accuracy with a standard of known accuracy to detect, correlate, report, or eliminate by adjustment any variation in the accuracy of the tested instrument.


comparison of the particular instrument with a primary standard, a secondary standard of higher accuracy than the instrument to be calibrated, or a known input source.


heat required to raise the temperature of 1 gram of water 1°C, specifically, from 4°C to 5°C. The Fifth International Conference on the Properties of Steam (1956) defined the International Table calorie as 4.1868 J. Mean calorie = 1/100 part of the heat required to raise 1 gram of water from 0°C to 100°C. Kilocalorie = 1000 calories.


An apparatus for measuring heat quantities generated in or emitted by materials in processes (e.g. may be used for determining refrigerant flow rate by the principle of known heat input or output, known physical characteristics of the transfer media and observed thermal differences).


the arched curvature of the propeller blade.

camber depth

the perpendicular distance from the chord of the blade’s cross section to the point of maximum camber.


covered area that extends from a wall of a building, protecting an entrance or a loading dock.

canopy hood

a wall-mounted or free-standing kitchen hood. Hoods will overhang the appliance(s) on all open sides, which forces replacement air to be drawn across the open sides of the cooking equipment, thus increasing the effectiveness of the hood to capture and contain effluent generated by the cooking operations.

cantilever valve

see reed valve.


property of an electric device or capacitor that permits storage of electric energy in an electrostatic field and the release of that energy at a later time.


in an alternating electrical system, a device that will store an electric charge used to change a power factor.


(1) measure of the maximum amount of energy or material that may be stored in a given system. See also nameplate rating, air-conditioner capacity. (2) the rate of heat removal by the refrigerant used in the compressor or condensing unit in a refrigerating system. This rate equals the product of the refrigerant mass flow rate and the difference in the specific enthalpies of the refrigerant vapor at its thermodynamic state entering the compressor or condensing unit and refrigerant liquid at the thermodynamic state entering the mass flow control device. (3) the rate that heat is removed or added to a system. (4) maximum load for which a machine, apparatus, device, or system is designed or constructed.

capacity factor

(of a machine, equipment, or thermal storage), ratio of the average load required, in the period of time considered, to the capacity in mass, volume, or energy terms. Reciprocal of storage factor.

capacity of refrigerating system

the cooling effect produced by the change in enthalpy between the refrigerant liquid entering the expansion valve and the vapor leaving the evaporator, generally measured in Btu per hour (kW or tons of refrigeration).

capacity, air conditioner, total

available capacity of an air conditioner for removing sensible and latent heat from the space to be conditioned (Btu/h).

capacity, latent cooling

the rate, expressed in watts (Btu/h), at which the equipment removes latent heat from the air passing through it under specified conditions of operation.

capacity, sensible cooling

the rate, expressed in watts (Btu/h), at which the equipment removes sensible heat from the air passing through it under specified conditions of operation.

capacity, total cooling

the rate, expressed in watts (Btu/h), at which the equipment removes heat from the air passing through it under specified conditions of operation.


action by which the surface of a liquid in contact with a solid (as in a small bore tube) is raised or lowered proportional to surface wetting.

capillary air washer

enclosure with an assembly of cells packed with fibrous materials over which water is sprayed and through which air is passed to clean it.

capillary tube

(1) refrigerating capillary tube is a tube of small bore used for the simultaneous purposes of metering the refrigerant and of accomplishing the expansion process between condenser and evaporator in refrigerating systems. (2) small-bore tube used for metering by controlling length and bore size. In refrigeration, a tube of small internal diameter used as a refrigerant pressure and flow control between high and low sides. (3) tube used to transmit pressure from the sensitive bulb of some temperature controls to the operating element.

carbon dioxide (CO2)

A naturally occurring gas, CO2 is also a by-product of burning fossil fuels (such as oil, gas, and coal), burning biomass, land-use changes (LUC), and industrial processes (e.g., cement production). It is the principal anthropogenic greenhouse gas (GHG) that affects the Earth’s radiative balance. It is the reference gas against which other GHGs are measured and therefore has a global warming potential (GWP) of 1.

carbon dioxide equivalent (CO2e) emission

The amount of carbon dioxide (CO2) emission that would cause the same integrated radiative forcing or temperature change, over a given time horizon, as an emitted amount of a greenhouse gas (GHG) or a mixture of GHGs. There are a number of ways to compute such equivalent emissions and choose appropriate time horizons. Most typically, the CO2-equivalent (CO2e) emission is obtained by multiplying the emission of a GHG by its global warming potential (GWP) for a 100-year time horizon. For a mix of GHGs it is obtained by summing the CO2e emissions of each gas. CO2e emission is a common scale for comparing emissions of different GHGs but does not imply equivalence of the corresponding climate change responses. There is generally no connection between CO2e emissions and resulting CO2e concentrations.

carbon dioxide removal

Anthropogenic activities removing CO2 from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products

carbon filter, gas

activated carbon as the air-cleaning agent

carbon steel pipe

pipe that owes its properties chiefly to the carbon content of the steel.


absorption of injected CO2 into a liquid, usually preceded or accompanied by liquid cooling.


apparatus for injecting CO2 into water for preparing carbonated beverages.


formation of carbonaceous deposits, which may be produced by decomposition of lubricating oil or other organic materials.

Carnot cycle

An ideal reversible thermodynamic cycle comprising two isothermal processes and two isentropic processes. It is often used to indicate the maximum amount of mechanical energy that can be gained by conversion of a given amount of heat.

Carnot-cycle efficiency

conversion of heat to work, which is limited by the temperature at which conversion occurs as (T1 – T2)/T1, where T1 is the higher absolute temperature where heat is absorbed, and T2 is the lower absolute temperature where heat is rejected.

carrier frequency

in a periodic carrier, the reciprocal of its period. Note: the frequency of a periodic pulse carrier often is called the pulse repetition frequency in a signal transmission system.

carrier frequency

the frequency that is used to modulate the input signal for amplification.

cascade control

complex control system in which the set value of one or more controllers is altered by one or more controlling equipment devices.

cascade refrigerating system

one having two or more refrigerant circuits, each with a pressure-imposing element, condenser, and evaporator, where the evaporator of one circuit cools the condenser of another (lower temperature) circuit.


enclosure normally housing fans, coils, filters, or other components and generally made of metal lined where necessary with material for thermal insulation and/or acoustic attenuation.

casing radiated sound power level

(1) sound power that radiates from a fan located within a housing, section, or casing. (2) sound power that radiates from the air terminal unit casing.

cast-iron sectional boiler

assembly of individual, hollow, cast-iron sections connected with push nipples, external headers, or internal seals.


Any substance of which a small amount relative to the reactants notably affects the rate of a chemical reaction without itself being consumed or undergoing a chemical change. Most catalysts accelerate reactions, but a few (negative catalysts, or inhibitors) retard them.

catalyst air cleaner titanium dioxide (PCO), gas phase

process uses light energy and titanium oxide to catalyze the oxidation of contaminants in air or water. The light energy causes a chemical reaction that produces reactive oxygen species (ROS), which are highly reactive and break down bioaerosol into carbon dioxide and water.

catalyst air cleaners, gas phase

Materials used to change gases to a more desirable gas. Acids, through donated protons (H+), are common reaction catalysts, especially in organic chemistry (catalysts take part in the reaction but are not consumed). Many metals like platinum, iron, or nickel also have catalytic activity.


negative electrode in an electrolytic system at which reduction occurs (e.g., Fe++, Cu++, Ca++, Mg++). Compare to anode.

cathode ray tube (CRT)

(1) electronic vacuum tube containing a screen on which information may be shown by modulated beam of electrons (a beam of cathode rays). (2) electronic storage tube. (3) picture tube. (4) oscilloscope tube.

cathodic protection

technique to minimize corrosion of a metal surface by coating the cathodic surface of an electrochemical cell.


positively charged ion of an electrolyte that migrates toward the cathode influenced by an electric potential gradient.


(1) formation by mechanical forces of vapor in liquids, specifically, the formation of vapor cavities in the interior or on the solid boundaries of liquids in motion, where the pressure is reduced to a critical value without a change in ambient temperature. (2) formation of cavities on a surface of a solid by liquid moving over it with velocity high enough to induce erosion of the surface when the cavity collapses. (3) in pumps, cavitation occurs when the pressure of the fluid is below the vapor pressure of the fluid at that temperature. Cavitation has been described as having marbles or small stones inside the impeller casing. Cavitation over an extended period of time will erode the impeller and cause pump failure.

CBR (chemical, biological, and/or radiological)

generally used with respect to airborne contaminants.


(1) overhead interior lining or surface of a room. (2) an upper exposure level that should not be exceeded such as the permissible exposure level ceiling (PEL-C) or threshold limit value ceiling (TLV-C).

ceiling air return

air removed from the space more than 4.5 ft (1.4 m) above the floor.

ceiling cassettes

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.

ceiling damper

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.

ceiling diffuser

see diffuser.

ceiling fan

a nonportable (permanently installed) device suspended from a ceiling or overhead structure for circulating air via the rotation of fan blades.

ceiling outlet

see diffuser.


(in a cooling tower) smallest tower subdivision that can function as an independent heat exchange unit. It is bounded by exterior walls or partitions. Each cell may have one or more fans or stacks and one or more distribution systems.

cellular elastomeric thermal insulation

insulation composed principally of natural or synthetic elastomers or both, processed to form a flexible, semirigid or rigid foam, having a predominately closed-cell structure. Insulation is usually expressed in k value (Btu/h·ft·°F [w/m·°K]).

cellular filter

air filter of juxtaposed square or rectangular elements which can be easily dismantled for cleaning or replacement.

cellular polystyrene thermal insulation board

insulation composed of cellular polystyrene in the form of boards, produced by heat and pressure from expansion of foamable polystyrene beads within a mold (bead board) or by in situ foaming of molten polystyrene in an extrusion mode (extruded board).

cellular polyurethane thermal insulation

insulation composed principally of the catalyzed reaction product of polyisicyanate and polyhydroxy compounds, usually processed with fluorocarbon gas to form a rigid foam having a predominately closed-cell structure.

Celsius temperature

The temperature used in the SI system that is related to the absolute temperature in Kelvin given by the formula: t (°C) = T (K) – 273.15.

centaxial fan

in-line duct fan with centrifugal blades which can develop static pressures higher than normal duct fans. Also identified as a tubular centrifugal fan.

centigrade temperature
central control

ability to control all functions from one central location, thereby enabling the operator to request and respond to all commands from one physical or network location.

centrifugal compressor

a nonpositive displacement compressor that depends, in part, on centrifugal forces for pressure rise. A turbocompressor.

centrifugal exhauster

factory-assembled fan consisting of one or several centrifugal wheels connected to a motor and enclosed in a housing. A back-draft damper can be provided.

centrifugal fan

fan in which the air enters the impeller axially and leaves it substantially in a radial direction. Fan rotor or wheel within a scroll-type casing (shroud) that includes supports for either belt drive or direct connection. Centrifugal-fan-types are as follows: forward curve, backwardly inclined (backward curved), airfoil, or radial blade design. Fans can be provided as single width, single inlet (SWSI) or double width, double inlet (DWDI) configurations and are limited to 16 predefined arrangement types based on discharge location and rotation.

centrifugal freeze drying

process in which a liquid product is vacuum frozen while being centrifuged in order to avoid foaming.

centrifugal pump

pump having a stationary element (casing) and a rotary element (impeller) fitted with vanes or blades arranged in a circular pattern around an inlet opening at the center. The casing surrounds the impeller and usually has the form of a scroll or volute. Centrifugal pump types are inline or base mounted. Pump arrangements are end suction, horizontal, or vertical split case.


device for separating substances of different densities by centrifugal force.

certificate of readiness

a document stating that all equipment, systems, and controls have been correctly installed, operated as specified, tested, adjusted, and balanced, and are verified as ready for functional performance testing and other acceptance procedures.

certified standard instrument

an instrument calibrated by the manufacturer or other reliable agency and certified as traceable to the National Institute of Standards and Technology (NIST).

challenge (air) stream

Test contaminant(s) of interest diluted to the specific concentration(s) of the test prior to filtration.

change of state

(1) change from one of the three phases, solid, liquid, or gas, to another. (2) occurrence in a remote system causing the contact of an alarm or status device to move from one of two possible positions to the other (e.g., into alarm, causing the contact of an alarm device to close or return to normal, causing the contact to open).


(1) change from heating to cooling or vice versa. (2) change from one set of controls to another.

changeover temperature

outdoor temperature the designer selects as the point of changeover from cooling to heating by the HVAC system.

charge capacity

the amount of heat that can be transferred into the storage device at a specified rate for a specific set of values for the initial temperature of the storage device, the temperature rise of the exiting fluid, and the mass flow rate of fluid through the storage system.

charge neutralizer

a device that brings the charge distribution of the aerosol to a Boltzman charge distribution. This represents the charge distribution of the ambient aerosol.

charge test time

the duration of a single transient test in which energy is added to the storage device.

charging apparatus

a device that allows the accurate vacuum transfer of small volumes of gaseous refrigerants to the sealed tube (or metal test cell) containing precharged lubricant. This apparatus consists of a manifold (metal or glass), vacuum pump, pressure gage, high vacuum gage, refrigerant cylinder, valves, and filling ports. The function of this apparatus is to evacuate the tube, degas the lubricant, add refrigerant along with the test materials, and seal it. It is calibrated so that the required mass of refrigerant is added very accurately by following the change in pressure on the pressure gage as refrigerant is added to the tube.

charging connection

device to enable a refrigerating system to be charged with refrigerant. Also, the tube or hose through which charging is accomplished.

charging valve

valve used to charge or add refrigerant to a system or add oil to a compressor crankcase.

Charles’ law

at constant pressure, the volume of a fixed mass or quantity of gas varies directly with the absolute temperature. Also known as Gay-Lussac’s law.

check valve

An automatic valve that prevents return flow of fluid.


verification checklists that are developed and used during all phases of the commissioning process to verify that the Owner’s Project Requirements are being achieved. Checklists include general verification, testing, training, and other specific requirements.

chemically active or toxic

materials corrosive or in-themselves toxic or productive of poisonous gases or fluids. Flammable or explosive materials easily ignited, including materials known to be fire producers or explosives.


to apply refrigeration moderately to products without freezing them.

chill factor

the apparent temperature felt on exposed skin as a function of air temperature and wind speed. Chill factor is expressed in time (e.g., 11 seconds) to express how long it will take exposed skin to freeze. Compare to wind chill, which is expressed as a temperature.

chilled beam systems

chilled ceiling systems have a very low profile (often flush with the ceiling) and work by means of convection heat transfer and induced air movement in the room in which they are placed. Chilled ceilings lack the ability to control the humidity or provide ventilation and must be paired with a ventilation system in order to maintain latent heat gains.

chilled cargo

cargo maintained at an assigned temperature above its freezing point.

chilled ceiling (radiant ceiling)

Ceiling panels that are made up of elements that connect together and cool primarily through radiation. The cooling medium is usually water.

chilled water

water used as a cooling medium (particularly in air-conditioning systems or in processes) at below ambient temperature.


(1) direct-expansion chillers are complete refrigerating systems consisting of a compressor, condenser, and evaporator with all operating and safety controls. Compressor types include the following: reciprocating, centrifugal, or screw design. Compare to absorption chiller. (2) refrigerating machine used to transfer heat between fluids. Chillers are either direct expansion with a compressor or absorption type.


Cooling of a substance without freezing it


one or more passageways, vertical or nearly so, for conveying flue gases to the outside atmosphere. Compare to flue or vent.

chimney effect

rising of air or gas in a duct or other vertical passage, like a building, caused by lowered density air at the top of the passage.


(1) generally, any of several compounds composed of carbon, fluorine, and chlorine, used chiefly as refrigerants and as blowing agents in plastic foams. Compare to fluorocarbon, halocarbon. (2) a fully halogenated (no hydrogen remaining) halocarbon containing chlorine, fluorine, and carbon atoms.


the straight line distance between the leading and trailing edges of a blade.

chronic toxicity

adverse health effect(s) from long-term, repeated exposures. This information is used, in part, to establish TLV-TWA, PEL, or consistent indices.

circuit breaker

a device designed to open and close a circuit by nonautomatic means and to open the circuit automatically at a predetermined overcurrent without damage to itself (when properly applied within its rating).

circulating fan

free flow propeller fan designed to circulate the air in a room without any air duct.

circulation-type evaporator

flooded evaporator comprising a low-pressure receiver, in which the unvaporized refrigerant returns to the evaporator inlet by gravity or by means of a pump or an ejector.


a small pump, typically fractional horsepower.

class of construction

for the building envelope, a subcategory of roof, above-grade wall, below-grade wall, floor, slab-on-grade floor, opaque door, vertical fenestration, or skylight.

class of refrigerating system

formerly in extensive use but now becoming obsolete as a result of code change to classification. Systems are classifications according to the degree of probability that a leakage of refrigerant will enter occupied areas. The two classifications are high probability system or low probability system. See ASHRAE Standard 15.

clean space

a defined area in which the concentration of airborne particles is controlled within specified limits, air that has been treated to remove pollutants, particulates, and odors.

clean-water pump

a device that is designed for use in pumping water with a maximum nonabsorbent free solid content of 0.016 lb/ft3 (0.26 kg/m3) and a maximum dissolved solid content of 3.1 lb/ft3 (50 kg/m3), provided that the total gas content of the water does not exceed the saturation volume, and disregarding any additives necessary to prevent the water from freezing at a minimum of 14°F (–10°C).


specially constructed, enclosed area environmentally controlled with respect to airborne particulates, temperature, humidity, air pressure, air-pressure flow patterns, air motion, vibration, viable organisms, and lighting.

clear ice

(also known as crystal ice), block ice obtained by slow freezing (brine at about –5°C) and by stirring the water during freezing, mainly by means of air injection, subsequently removing the core of unfrozen water in the block, where impurities are concentrated, by suction, at the end of the process.

clear zone

when outlets are placed within or near the test zone, a clear zone is defined as the space around the outlet within which long-term occupancy is not recommended.


distance between the item requiring maintenance and the closest interfering surface.


(1) short section of rolled steel angle used to connect two intersecting steel members. (2) strip of sheet formed by roll forming into a profile that is used to secure the sheet metal rolled jointing flanges added to rectangular ducts.


a unit used to express the thermal insulation provided by garments and clothing ensembles, where 1 clo = 0.88 ft2·h·°F/Btu (0.155 m2·°C/W).

clo unit

unit of measurement of the insulation or thermal resistance of clothing.

close nipple

nipple with a length twice the length of a standard pipe thread.

closed crankcase compressor

one in which the crankcase is completely sealed from the atmosphere but connected with the low-pressure side of the system.

closed cycle

Thermodynamic cycle having a recirculating working fluid independent of the atmosphere.

closed process

series of changes of state in a system at the termination of which the system is reverted to its original state.

closed system

heating or refrigerating piping system in which circulating water or brine is completely enclosed, under pressure above atmospheric, and shut off from the atmosphere, except that the expansion/compression tank could be open to the atmosphere. See also water system.

closed-loop control

(1) (also known as feedback control) control system in which the effect of the control action on the controlled variable is sensed and used by the controller to provide a new output (feedback control). Compare to open-loop control. (2) signal path that includes a forward path, a feedback path, and a summing point and that forms a closed circuit.

clothing/ensemble insulation

the resistance to sensible heat transfer provided by a clothing ensemble. Expressed in clo units. Note: the definition of clothing insulation relates to heat transfer from the whole body and, thus, also includes the uncovered parts of the body, such as head and hands.

cloud point

temperature at which a clear liquid becomes hazy or cloudy due to the formation of crystals or particles when tested under standardized conditions.

Coanda effect

characteristic of an airstream that causes it to cling to the surface along which it flows. The velocity of the airstream as it passes along the surface generates low pressures. This action causes surrounding air to be aspirated.

coaxial cable (coax)

specially constructed single or multiconductor cable which provides shielding from electrostatic fields.

coaxial condenser

water-cooled condenser in which water and refrigerant flow in parallel paths but in opposite directions.

cock valve

generally a plug valve, usually for regulating the flow of a fluid, and requiring a wrench for operating.

code official

a coefficient is a factor in a mathematical product.

coefficient of compressibility (compressibility factor)

coefficient required to correct the perfect gas equation when applied to real gases.

coefficient of discharge

ratio of the net area at vena contracta of air flowing through an orifice to the total free area of the opening.

coefficient of expansion

The change in length per unit length or the change in volume per unit volume, per degree change in temperature.

coefficient of friction

a number that, when multiplied into the number expressing the pressure between two bodies, gives the resulting friction.

coefficient of performance (COP)

(1) ratio of the rate of net heat output to the total energy input expressed in consistent units and under designated rating conditions. (2) ratio of the refrigerating capacity to the work absorbed by the compressor per unit time.

coefficient of performance, heat pump heating

the ratio of the rate of heat delivered to the rate of energy input, in consistent units, for a complete heat pump system, including the compressor and, if applicable, auxiliary heat, under designated operating conditions.

coefficient of restitution

a fraction that, when multiplied into the relative velocity of two colliding bodies just before impact, gives their relative velocity just afterward.

coefficient of variation (CV)

standard deviation of a group of measurements divided by the mean.


sequential production of either electrical or mechanical power and useful thermal energy (heating or cooling) from a single energy form.

cognizant authority

an agency or organization that has the expertise and jurisdiction to establish and regulate concentration limits for airborne contaminants, or an agency or organization that is recognized as authoritative and has the scope and expertise to establish guidelines, limit values, or concentrations levels for airborne contaminants.


cooling or heating element made of pipe or tube that may or may not be finned and formed into helical or serpentine shape.

coil deck

insulated, horizontal partition between refrigerated space and bunker.

coil depth

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

coil face area

product of the height and length of the coil finned area.

coil height

dimension of the vertical face of the coil as installed, including only the height over tubes and fins exposed to the flow of air. Note: some steam coils have vertical tubes.

coil length

dimension of the face of the coil in the direction of the bare tubes, finned tubes, or both, exposed to the flow of air.

coil recovery loop

finned-tube water coils with interconnecting piping placed in supply and exhaust airstreams and filled with a circulated liquid heat transfer fluid.

coil width

dimension of the face of the coil perpendicular to the direction of the tubes. Does not include the casing. Note: height may be substituted for width if the condenser has a vertical coil orientation.

coil, indoor

the heat exchanger that removes heat from (cooling) or adds heat to (heating) the airstream being conditioned.

coil, outdoor

the heat exchanger that rejects heat to (cooling) or absorbs heat from (heating) a source external to the conditioned space. In the cooling mode, the coil operates as a condenser. In the heating mode, the coil operates as an evaporator.

coincident demand

the metered demand of a device, circuit, or building that occurs at the same time as the peak demand of the building or facility or at the same time as some other peak of interest, such as a utility’s system load. This should properly be expressed so as to indicate the peak of interest (e.g., “demand coincident with the building peak.”)


solid substance remaining after the partial burning of coal in an oven distillation or in a retort.

Colburn heat transfer equation

dimensionless heat transfer equation used in calculating natural convection movement of heat from vertical surfaces or horizontal cylinders to fluids (gases or liquids) flowing past these surfaces. The symbol is jH.

Colburn mass transfer equation

dimensionless mass transfer equation consisting of the Sherwood number divided by the Reynolds number and the Schmidt number to the 1/3 power. The symbol is jD.

cold box

The thermally insulated section of a cryogenic plant that contains the components operating at low temperatures (heat exchangers, distillation columns, pipes, etc.).

cold chain

series of actions and equipment applied to maintain a product within a specified low-temperature range from harvest/production to consumption.

cold injury (low-temperature injury)

storage at a temperature below which physiological disorder in produce will manifest itself. This temperature will vary with the produce.

cold room (cold chamber)

insulated structure served by a refrigerating system.

cold shrink fitting

process for assembling two precision-machined parts by cooling the inner member so that it can be inserted into the outer member, the members fit tightly together when both are at the same temperature.

cold storage

technology or systems used in the process of preserving perishables by refrigeration in systems usually operating below 45°F (7°C). Compare to cool storage, ice storage.

cold store

refrigerated warehouse.

cold technology

any technology which involves and deals with processes, systems, and equipment related to refrigeration and cold science.

cold trap

apparatus in which the walls are cooled in order to condense and trap vapors, can be used to reduce pressure.

cold-air distribution system

system that uses a primary air supply with a temperature range of approximately 33°F to 50°F (1°C to 15°C). Note: typically used with ice storage systems. Compare to cold-water distribution system.

cold-room door dike

projection on the door that extends into the refrigerated compartment(s) and that functions primarily as a barrier to minimize heat flow to the interior of the cabinet.

cold-room flexible door

two-way, push-through-type door made of thick plastic sheeting.

cold-room sliding door

single or multiple door that is movable laterally, usually in guides.

cold-room swinging door

door pivoting on a vertical axis and that can be either pulled or pushed open.

cold-room, flush-fitting door

door that does not protrude beyond the face of the wall.

cold-storage cooler

insulated room usually maintained below 40°F (5°C) but not below 30°F (1°C).

cold-storage disease (storage disorder)

injury to produce occurring during storage.

cold-storage locker

cold-storage establishment containing food-storage boxes or lockers for individual users.

cold-storage room

cold room designed to receive and store produce sometimes already cooled down to approximately the desired storage temperature.

cold-store facility

(also known as cold store complex or cold store combine), warehouses and food processing plants grouped with a central refrigerating installation.

cold-water distribution system

system that uses a primary chilled-water supply with a temperature range of approximately 34°F to 40°F (1°C to 10°C). Note: typically used with ice storage systems. Compare to cold-air distribution system.


piece of metal that is added to shaped sheet metal components (e.g., tapers, transitions) to provide parallel ends to facilitate jointing with adjacent components.

collecting electrodes

for plate-type electronic air cleaners, the metal plates on which dust is deposited, including those in the ionizing section.

collector cover glazing

the material covering the aperture to provide thermal and environmental protection.

collector time constant

the time required for the fluid leaving a solar collector to attain 63.2% of its steady-state change following a step change in irradiance.

collector-loop heater

a heater installed within the collector loop when testing the solar domestic-water heating system with a nonirradiated array.

collimation angle

the angle within which the radiation beams from the source depart from the line drawn from the source to the receiver.


the appearance of a lubricant when viewed by transmitted light.

color rendition

effect of a light source on the color appearance of an object in comparison with the color appearance observed under a reference light source, usually daylight.

color temperature

temperature of a perfect radiator (blackbody) that would emit the same relative intensity at two wavelengths (usually red and green lights) as the relative intensity radiated by the subject surface.

column friction loss

the friction loss through a column of pipe with a line shaft through it such that fluid flows in an annulus. Column friction loss is dependent upon both capacity and the length and diameter of column and shaft used.

combination control

control device in which one or more control variables are being monitored (such as a combination high- and low-pressure control for a refrigerant system).

combination space-heating and water-heating appliance

a unit that is designed to provide space heating and potable water heating from a single, primary energy source.

combined appliance

an assembly consisting of a heat pump or air conditioner, a desuperheater, a water heater, and if required, a potable water pump, the assembly provides space conditioning and domestic hot water.

combined cycle gas turbine

device that uses waste heat boilers to capture exhaust energy for steam generation.

combined heat and power (CHP)

Combined heat and power (CHP), also known as cogeneration, is the simultaneous production of electricity and heat from a single fuel source, such as natural gas, biomass, biogas, coal, waste heat, or oil. (

combined mode

an operating mode that occurs during either the cooling season or the heating season when the appliance operates to meet a water heating load along with a space conditioning load. With respect to seasonal performance calculations, this mode occurs when both a water heating load and a space conditioning load occurs simultaneously.

combined panel

a complete ceiling panel that is designed and can be independently installed and operated for both sensible cooling and sensible heating of an indoor space through heat transfer between the thermally effective panel surfaces and the occupants and/or the indoor space by thermal radiation and natural convection.

combined performance factor cooling season (CPFCS)

the seasonal coefficient of performance of the combined appliance when used to meet both the space-cooling and domestic water-heating loads that occur during the space-cooling season. The quantity is dimensionless.

combined radiative and convective surface coefficient

constant of proportionality relating the rate of combined convective and radiative heat transfer at a surface to the temperature difference across the air film on that surface.

combined section of an air-handling unit

section within which two or more functions are combined.

combined space-heating and water-heating mode

an operating mode where the heat pump is space heating and the desuperheater is heating domestic water.

combined surface coefficient
combining-volumes principle

when gases take part in chemical reactions, the volume of the reacting gases and those of the products (if gaseous) are in the ratio of small whole numbers, provided that all measurements are made at the same temperature and pressure. Also known as Gay-Lussac's law.

combining-weights law

if weights of elements that combine with each other are called their combining weights, then elements always combine in ratio of their combining weights.

combustible gas or vapor detector

instrument for determining concentration of combustible gas or vapor.


chemical process of oxidation that occurs at a rate fast enough to produce heat and usually light either as a glow or flame.

combustion air

air required to provide for the complete combustion of fuel and usually consisting of primary air, secondary air, and excess air.

combustion chamber

enclosure, with or without lines or baffles, into which fuel or gaseous derivatives of fuel are discharged so that combustion can occur.

combustion control

(1) adjustment of the fuel rate and air/fuel mixture ratio in response to heating load and flue gas air condition over the full range of the burner capacity from some preset minimum to 100%. (2) device or series of devices that control the flow of fuel and combustion air in the desired ratio to provide efficient combustion.

combustion detector

part of primary safety control which is responsive directly to flame properties.

combustion gas tests

sampling of combustion products to determine the percentage of constituents and their temperature.

combustion products

effluents from the combustion of a fuel, including the inerts but excluding excess air.

combustion volume

space provided for the burning of fuel.

comfort air conditioning

treating air to control its temperature, relative humidity, cleanliness, and distribution to meet the comfort requirements of the occupants of the conditioned space.

comfort chart

chart showing operative temperatures with dry-bulb temperatures, relative humidities, and air motion by which the effects of the various conditions on human comfort may be compared.

comfort condition

environmental condition in a space such that the majority of the occupants should, on a statistical basis, be comfortable.

comfort cooling

refrigeration for comfort, as opposed to refrigeration for storage or manufacture.

comfort index

index combining the properties of an environment for evaluating the sensation of comfort of occupants, equal to 15 plus 0.4 times the sum of the dry-bulb and wet-bulb temperatures in degrees Fahrenheit. See also thermal comfort.

comfort zone

(1) operative temperature. See temperature. (2) range of effective temperatures under which most of a group of people feel comfortable.

commercial refrigerator

(1) refrigerated enclosure containing goods which are accessible to the exterior through a door. (2) types of refrigerators used commercially, including reach-ins, walk-ins, and refrigerated display cases (all types being either service or self-service, which are used by business establishments).

commercial system

heating, cooling, or refrigerating system used in a commercial or business place.

Commissioning (Cx) Plan

a document that outlines the organization, goals, schedule, allocation of resources, and documentation requirements of the Cx Process.

Commissioning (Cx) Process

a quality-focused process for enhancing the delivery of a project. The process focuses on verifying and documenting that the facility and all of its systems and assemblies are planned, designated, installed, tested, operated, and maintained to meet the Owner’s Project Requirements (OPR).

commissioning authority

an entity, identified by the owner, who leads, plans, schedules, and coordinates the commissioning team to implement the commissioning process.

commissioning process

a quality-focused process for enhancing the delivery of a project. The process focuses upon verifying and documenting that the facility and all of its systems and assemblies are planned, designed, installed, tested, operated, and maintained to meet the Owner’s Project Requirements. (See Owner’s Project Requirements).

commissioning process progress report

a written document that details activities completed as part of the commissioning process and significant findings from those activities. The commissioning process progress report is continuously updated during the course of a project. The progress report is incorporated into the commissioning plan as an ongoing appendix.

commissioning team

the individuals who, through coordinated actions, are responsible for implementing the commissioning process.

community energy system
companion flange

pipe flange to connect with another flange or with a flanged valve or fitting. It is attached to the pipe by threads, welding, or other methods and differs from a flange (which is an integral part of a pipe or fitting).

compartment water cooler

a water cooler that, in addition to the primary function of cooling and dispensing potable water, includes a refrigerated compartment with or without provisions for making ice.

complex ester

an ester lubricant prepared from a polyol and both mono- and dicarboxylic acids, either together or sequentially.


act of complying with the rules or requirements of a standard.


smallest functional element of an installation.

component direct evaporative cooler

a self-contained cabinet without a fan whose primary functions are (1) the conversion of the sensible heat of unsaturated air passing through the cabinet to latent heat by the process of evaporating recirculating or nonrecirculating water directly exposed to this air and (2) the movement of this air through the cabinet that allows a portion of this water to evaporate.

component of ventilation or air conditioning

single, functional element forming a part of a ventilation or an air-conditioning installation.

component substances law

every material consists of one substance or is a mixture of two or more substances, each of which exhibits a specific set of properties independent of the other substances.


ratio of components in a blend, normally expressed as a mass percent.

compound compression

Compression in two or more stages, as where the discharge of one compressor is connected with the suction of another, or compression by a single compressor having separate cylinders, or rotors or impellers, for each stage.

compound compressor

compressor in which compression is accomplished by stages, as in two or more cylinders.

compound gage

pressure gage that indicates pressures above and below atmospheric pressure.

compound-refrigerating system

multistaged refrigerating system where a single charge of refrigerant circulates through all stages of compression.

compressed liquid

a liquid for which the existing pressure is greater than the saturation pressure for the given temperature.


ease with which a fluid may be reduced in volume by the application of pressure. Compressibility depends on the state of the fluid as well as the type of the fluid itself.

compressibility coefficient

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.

compressibility factor

relative variation of the departure from the perfect gas laws.

compressing cycle

refrigerating cycle composed of four principal stages: vaporization of the refrigerant, compression of the vapor, liquefaction of the vapor, and expansion of the liquid.


process by which the pressure of a gas is increased by reducing its volume.

compression economizer

device that reduces compressor energy use by introducing intermediate pressure gas into the compressor during the compression stroke.

compression efficiency

ratio of work required to compress, adiabatically and reversibly, all the vapor delivered by a compressor (per stage) to the actual work delivered to the vapor by the piston or blades of the compressor.

compression joint

multipiece joint with cup-shaped threaded nuts which, when tightened, compress tapered sleeves so that they form a tight joint on the periphery of the tubing that they connect.

compression ratio

ratio of the absolute pressure after compression to the absolute pressure before compression.

compression stage

each part at compression whereby the total compression of a gas is accomplished by several compressors in series to reduce the compression ratio for each stage.

compression stroke

that movement of a piston in a compressor cylinder during which the gas is compressed and discharged.

compression tank

pneumatic cushioning device, operating at system pressure, that absorbs fluid expansion as a result of temperature change and prevents unnecessary periodic operation of the relief valve. Compare to expansion tank.

compression volume ratio

ratio of the volume of compression chamber at intake of gas to volume at discharge in positive displacement compressors.

compression-type refrigerating system

System in which refrigeration is affected by the vaporization at low pressure in a heat exchanger (evaporator) of a liquid refrigerant, the vapor thus formed being restored to the liquid state by mechanical compression to a higher pressure and subsequent cooling in another heat exchanger (condenser).


(1) device for mechanically increasing the pressure of a gas. (2) often described as being either open, hermetic, or semihermetic to describe how the compressor and motor drive is situated in relation to the gas or vapor being compressed. Types include centrifugal, axial flow, reciprocating, rotary screw, rotary vane, scroll, or diaphragm. 1. device for mechanically increasing the pressure of a gas. 2. specific machine, with or without accessories, for compressing refrigerant vapor.

compressor calorimeter

apparatus for determining the refrigerant flow rate and, subsequently, the capacity of a refrigerant compressor by measuring the heat input required to balance the refrigerating effect produced in the evaporator by the compressor.

compressor capacity reducer

(1) design maximum rate of heat removal by the refrigerant assigned to the compressor in a refrigerating system. This is equal to the product of the mass rate of refrigerant flow produced by the compressor and the difference in specific enthalpies of the refrigerant vapor at its thermodynamic state entering the compressor and the refrigerant liquid at saturation temperature corresponding to the pressure of the vapor leaving the compressor. (2) device, such as a clearance pocket, movable cylinder head, or suction bypass, by which compressor capacity can be adjusted without otherwise changing the operating conditions.

compressor clearance pocket

space of controlled volume to give the effect of greater or less cylinder clearance, thereby changing compressor capacity.

compressor discharge

that part of the compressor at the high-pressure side.

compressor discharge stroke

that part of the piston stroke between the opening of the discharge valve and the top dead center.

compressor displacement

actual volume of gas or vapor at compressor inlet conditions moved by a compressor per revolution or per unit of time.

compressor economizing

process whereby a side port in the compressor (usually a screw compressor or multiwheel centrifugal compressor) is used to provide refrigerant subcooling, resulting in an improvement in overall system efficiency.

compressor oil return

transport of oil from the evaporator to the compressor.

compressor or condensing unit efficiency

the ratio of the work absorbed for compressing a unit mass of refrigerant in a compressor or condensing unit to the work absorbed for compressing the same unit mass of refrigerant in an isentropic compressor or condensing unit. (Also known as isentropic efficiency.)

compressor or condensing unit efficiency

(also known as isentropic efficiency), the ratio of the work absorbed for compressing a unit mass of refrigerant in a compressor or condensing unit to the work absorbed for compressing the same unit mass of refrigerant in an isentropic compressor or condensing unit.

compressor piston displacement

volume swept by a piston during one stroke or one revolution of the crankshaft.

compressor refrigerating effect

rate of heat removal by the refrigerant assigned to the compressor in a refrigerating system. This is equal to the product of the mass rate of refrigerant flow produced by the compressor and the difference in specific enthalpies of the refrigerant vapor at its thermodynamic state entering the compressor and refrigerant liquid at saturation temperature corresponding to the pressure of the vapor leaving the compressor.

compressor saturated discharge temperature

the saturation temperature corresponding to the refrigerant pressure at the compressor discharge, usually taken at or immediately downstream of the compressor discharge service valve (in either case on the downstream side of the valve seat), where discharge valves are used.

compressor starting - no load start

(also known as unloaded start), practice of starting a compressor after equalizing pressures in high- and low-side pressures.

compressor surge

condition achieved in a centrifugal compressor when the momentum of the refrigerant gas through the compressor is insufficient to overcome the thermal lift requirement. Direction of flow temporarily reverses through the compressor until the lift requirement decreases. The condition repeats until the operating condition is corrected. Accelerated wear and damage can eventually result.

compressor theoretical displacement

total volume swept by the working strokes of all the pistons of a compressor per revolution of the crankshaft or per unit of time.

compressor unit

a refrigerating component, designed to compress a specific refrigerant vapor, consisting of compressor, prime mover, and regularly furnished accessories.

compressor unloader

(1) device for controlling compressor capacity by rendering one or more cylinders ineffective. (2) device on or in a compressor for equalizing the high- and low-side pressures for a brief period during starting in order to decrease the starting load on the motor.

compressor volume ratio

ratio of volume of compression chamber at intake of refrigerant gases to the volume at discharge in positive displacement compressors.

compressor work

(1) (theoretical), enthalpy difference along an isentrope. (2) mechanical energy required by, or load imparted to, the piston of a compressor or shaft of a centrifugal compressor.

computer and data processing room (CDPR) unitary air conditioner

a unitary air conditioner, for a computer and data processing room, consisting of one or more assemblies that include a DX evaporator or chilled-water cooling coil, an air-moving device, and air-filtering devices. The air conditioner may include a compressor, condenser, humidifier, or reheating device.

computer memory

general term for the computer equipment that holds information in any (usually binary) language in electrical, optical, or magnetic form. This equipment also receives information for storage and gives out the stored information for storage and later use. The word “memory” usually means storage inside the computer, while “storage” refers to optical and electrical media storage outside of the computer.

computer simulation

(1) (general) representation of an actual system by analogous characteristics of some device easier to construct, modify, or understand. (2) (physical) the use of a model of a physical system in which computing elements are used to represent some but not all of the subsystems. (3) computer-aided decision process in which proposals are tested in a computer before one or more of the proposals are considered for use (e.g., DOE-2).

computer software

programs and instructions put into a computer.

computer software library

(1) computer subprograms called into an automation system program to perform a special assignment. (2) general collection of software packages available for a particular data-processing system.

computer storage

device that stores information temporarily during data transfers. Clarified by buffer. See computer memory.

computer-aided drafting (CAD)

equivalent to conventional drafting, only performed on a computer. Points, lines, and symbols are used to convey design intent or detail construction means and methods. Most often plotted onto paper media and published in that form for drawings and specifications and delivered to the owner, contractor, and reviewing authorities and agencies for approval and actual construction.

computer-aided manufacturing (CAM)

system in which a computer directs the manufacture and assembly of a product.

computer-based system

energy management system in which a computer is the central controlling device. See energy management system (EMS).

concealed fixing

installation of an air--terminal device where the attachment to the duct, wall, or ceiling is hidden from view to room occupants.

concentrating collector

a solar collector that uses reflectors, lenses, or other optical elements to concentrate the radiant energy passing through an aperture onto an absorber with a surface area smaller than the aperture area.


the quantity of one constituent dispersed in a defined amount of another.


liquid formed by condensation of a vapor. In steam heating, water condensed from steam, in air conditioning, water extracted from air, as by condensation on the cooling coil.

condensate return pump

pump used to transfer condensate from one point in a system to another receiver, usually installed with a receiver tank and a float valve, the pump being controlled by tank level.

condensate subcooling heat exchangers

a variation of solution heat exchangers, used on steam-fired, double-effect machines and on some single-effect, steam-fired machines. Uses the condensed steam to add heat to the solution entering the generator.


change of state of a vapor into a liquid by extracting heat from the vapor.

condensation point

temperature at which a vapor liquefies if the latent heat is removed at standard or stated pressure. See also dew point, saturation temperature, boiling point.


a heat exchanger in which the primary heat transfer vapor changes its state to a liquid phase.

condenser coil

a condenser constructed of pipe or tubing exposed to air

condenser cooling liquid

the fluid used as the condensing media in a liquid cooled, self-contained refrigerator.

condenser liquid flow rate

the mass flow rate of liquid through the condensing unit under the conditions specified.

condenser load

for water-cooled condensers, the amount of heat added to the water removed from the load, the integrated product of the flow rate through the condenser and the temperature difference across the condenser.

condenser receiver

a water-cooled condenser with the tubes or pipes grouped in the upper portion of the shell, leaving the lower section of the shell for use as a receiver.

condenser subcooling

number of degrees that a pressurized liquid is cooled lower than its saturated temperature at that pressure.

condenser tube

heat exchanger tube manufactured to special requirements such as tolerances, finish, and temper.

condensing furnace

one that circulates the products of combustion and extracts available heat to a point that causes condensation to occur. Some of this latent heat of condensation is recovered as usable energy, resulting in higher operating efficiency.

condensing heat rejection

(1) portion of the total refrigerant heat-rejecting effect of a condenser, which is used for condensing the entering refrigerant vapor to a saturated liquid at the entering refrigerant pressure. (2) the portion of the total heat rejection of a condenser that is used for desuperheating and condensing the entering refrigerant vapor to a saturated liquid. This is the product of the mass rate of refrigerant flow and the difference between the enthalpy of the entering refrigerant vapor and that of the saturated refrigerant liquid at the leaving pressure.

condensing pressure

pressure of a gas at which it condenses.

condensing pressure valve

automatic valve responsive to inlet pressure to prevent compressor discharge pressure from falling below a specific value. Normally used as a form of head pressure control (backup valve) but also used on refrigerating systems for maintaining hot-gas defrost systems.

condensing temperature

The temperature of a fluid at which condensation occurs or constitutes the dew point for zeotropic mixtures.

condensing unit

(1) an apparatus for processing low-pressure refrigerant vapor back into high-pressure liquid refrigerant to be used for cooling a refrigerator. (2) machine designed to condense refrigerant vapor to a liquid by compressing the vapor in a positive displacement compressor and rejecting heat to a cooling medium. A condensing unit usually consists of one or more positive displacement compressors and motors, condensing coils, liquid receivers, and other devices mounted on a common base.

condensing unit calorimeter

apparatus for determining refrigerant flow rate and, subsequently, the capacity of a condensing unit by measuring the heat input required to balance the refrigerating effect produced in the evaporator by the condensing unit.

condensing unit capacity

rate of heat removal by the refrigerant assigned to the condensing unit in a refrigerating system. This is equal to the product of the mass rate of refrigerant flow produced by the condensing unit and the difference in the specific enthalpies of the refrigerant.

condensing unit refrigerating effect

rate of heat removal by the refrigerant assigned to the condensing unit in a refrigerating system. This is equal to the product of the mass rate of refrigerant flow produced by the condensing unit and the difference in the specific enthalpies of the refrigerant vapor entering the unit at a specified superheat and the refrigerant liquid leaving the unit at a specified subcooling.

condition line

on a psychrometric chart, the infinite number of wet- and dry-bulb temperatures which will satisfy the requirements of an air supply for a given room temperature.

conditioned air

(also treated air) air treated to control its temperature, relative humidity, purity, pressure, and movement.

conditioned space

that part of a building that is heated and/or cooled and/or humidity controlled for the comfort of occupants. Compare unconditioned space.

confidence level

the probability that a stated interval will include the true value. In analyzing experimental data, a level of 95% is usually used.

confidence limits

used for multisample data and uncertainty interval (used for single-sample data), the range of values that can be expected, given a stated probability, to include the true value. For example, a statement that the 95% confidence limit is 5 to 8 means that there is a 95% probability (19 chances out of 20) that the interval between 5 and 8 will contain the true value.

connection in parallel

system in which flow is divided among two or more channels from a common starting point or header.

connection in series

system in which flow through two or more channels is in a single path entering each succeeding channel only after leaving the first or previous channel.


substantial agreement reached by concerned interests according to the judgment of a duly appointed authority after a concerted attempt at resolving objections. It implies much more than a simple majority, but not necessarily unanimity.

consensus process

procedures adopted by a standards-developing organization to reach consensus.

consensus standard

in ASHRAE, a standard developed by the consensus process reflecting a consensus of professional opinions of the members of a committee of balanced interests with the support of other experts who review and comment on drafts during open public review.

conservation of momentum law

when a system of masses is subject only to internal forces that masses of the system exert on one another, the total vector momentum of the system is constant.

console air conditioner
constant cut-in

used in refrigerating devices constructed to permit the cut-in point to remain constant while providing a variable cut-out (variable differential) range when the setting is changed.

constant pressure expansion valve

valve that maintains a constant output pressure regardless of the input pressure.

constant-level valve

device for maintaining within a reservoir a constant level of fluid (e.g., oil fuel for delivery to an oil burner).

construction checklist

a form used by the contractor to verify that appropriate components are onsite, ready for installation, correctly installed, and functional.

consumer’s water system

all potable water piping, valves, fittings, and appurtenances on the premises side of the service connection. It is the secondary component of a public water system.

contact cooling

cooling by direct contact with a cold surface.

contact freezing

(1) a contact freezer is a freezer in which the product is frozen by contact with a refrigerated surface. (2) freezing of produce by direct contact with a refrigerated surface. Crust freezing (shell freezing) is very quick freezing of the outer part of a product (mainly poultry), and final freezing is completed by conventional methods.

contact icing

process of chilling in which finely crushed ice is placed in direct contact with the product in its unpacked or packed state.

continuous air barrier

the combination of interconnected materials, assemblies, and sealed joints, and components of the building envelope that minimize air leakage into or out of the building envelope.

continuous dimming

a lighting control strategy that varies the light output of a lighting system over a continuous range from full light output to a minimum light output in imperceptible steps without flickering.

continuous insulation (ci)

insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings. It is installed on the interior or exterior or is integral to any opaque surface of the building envelope.

contra-rotating fan

see fan types.

contract documents

include a wide range of documents that will vary from project to project, with the Owner’s needs, and with regulations, laws, and countries. Contract documents frequently include price agreements, construction management processes, subcontractor agreements or requirements, requirements and procedures for submittals, changes, and other construction requirements, timeline for completion, and the construction documents.


in construction terminology, the person or entity responsible for performing the work and identified as such in an owner/contractor agreement.


to regulate the operation of equipment.

control action

(of a controller or a controlling system), nature of the change of the output produced by the input (e.g., direct-acting or reverse-acting devices).

control detecting element

in a control measuring unit, the element that responds directly to, or senses, the variable to be measured. See sensor.

control device

a specialized device used to regulate the operation of equipment.

control element

mechanism that directly acts to change the value of the controlled variable (such as actuators or relays).

control function

process of maintaining building conditions such as HVAC systems, lighting systems, and irrigations systems, etc.

control logic

control logic is the diagrammatical flow chart of operations of programming for software that controls the operations of the program. The control logic responds to commands from an input and generates an output to perform operation-related tasks. Control logic can be modeled using a state diagram, which is a form of hierarchical state machine. These state diagrams can also be combined with flow charts to provide a set of computational semantics for describing complex control logic.

control panel

assembly of the indicating devices and remote control units required for the operation of a system.

control point

the process output value to maintain setpoint. Setpoint plus offset is equal to control point.

control power element

actuator in an automatic control.

control sequence
control system

(1) the methods and means of governing the performance of any apparatus, machine, or system. (2) system governing the starting, stopping, direction of motion, acceleration, speed, and retardation of the moving member. (3) designation of how the equipment is governed (i.e., by an attendant, by automatic means, or partially by automatic means and partially by an attendant). (4) one or more of the components in a mechanism responsible for interpreting and carrying out manually initiated directions.

control temperature

the measured temperature at the location of the controlling device for a specific purpose (e.g., a room thermostat).

control-measuring element

element used to measure the status of a controlled variable.

controlled device

device that receives a signal from a controller and acts on the process plant to vary its operating condition in accordance with the information received. In HVAC & R systems, controlled devices typically are valves, dampers, and motors.

controlled medium

substance that is to be maintained at a specific value of temperature, concentration, or flow rate.

controlled variable

in either a closed-loop or open-loop control system, the measured physical phenomenon (e.g., temperature, humidity, pressure, lighting, CO2) that causes a controller to respond in an effort to reduce or minimize the deviation from a desired value.


device for regulation of a system or component in normal operation, manual or automatic. If automatic, the implication is that it is responsive to changes of pressure, temperature, or other variables whose magnitude are to be regulated.

controller error signal

difference between the control point (actual value of the controlled variable) and the setpoint. This quantity may have a positive or a negative value.

controller gain

ratio of change in controller output to the change in the value of the sensed value.

controller proportional band

range of controller output as it goes from one extreme to the other.

convective film coefficient

constant of proportionality relating the convective rate of heat transfer at a surface to the temperature difference across the air film on that surface.


surface designed to transfer its heat to a surrounding fluid largely or wholly by forced and/or natural convection.

convector radiator

terminal unit used in hot-water or steam systems to deliver heat to a space (but primarily by convection and not radiation).

conversion burner

burner intended for field installation that changes the fuel type of an existing furnace or boiler from oil or coal to a gas-burning system.

cooking effluent

matter (such as moisture, vapor, products of combustion, smoke, and particulate matter) rising from cooking equipment during equipment operation,.


to remove heat in an environment generally at a temperature not below 33°F (–1°C). Compare to refrigerant.

cool down

reduction of space temperature down to occupied setpoint after a period of shutdown or setup.

cool storage

technology or systems used to store cooling capacity. Normally applies to comfort or air-conditioning applications. Compare to cold storage and ice storage.


a single-phase fluid (usually a liquid) used for transferring heat from one place to another. Sometimes referred to as heat transfer fluid, brine, and/or secondary refrigerant (see refrigerant).


reduction of space temperature down to occupied set point after a period of shutdown or setup.


thermally insulated enclosure kept at a reduced temperature by a refrigeration system.

cooler refrigerating effect

rate of heat absorption by a refrigerating medium (air, water, brine, etc.) flowing through a cooler at stated conditions. It is measured as the product of the mass flow rate of the refrigerating medium and the difference in specific enthalpies of the refrigerating medium entering and leaving the cooler.


(1) Removal of heat, usually resulting in a lower temperature and/or phase change (2) Lowering temperature

cooling air

(1) ambient air used to remove heat from a device, space, or system. (2) cooled air used to lower the temperature of a space or products stored in a space.

cooling and heating heat pump
cooling capacity

(also known as total cooling capacity), design maximum rate at which equipment removes heat from a fluid under specified conditions of operation.

cooling coil

an arrangement of pipes or tubes, not enclosed in a pressure vessel, that can be used either with refrigerant or secondary coolant to provide cooling or cooling with dehumidification.

cooling degree days (CDD)

see degree day.

cooling design temperature

the outdoor dry-bulb temperature equal to the temperature that is exceeded by 1% of the number of hours during a typical weather year.

cooling design wet-bulb temperature

the outdoor wet-bulb temperature equal to the temperature that exceeds a stated number of hours during a typical weather year. The value is normally stated as a percent. This value is applicable to cooling systems where the main purpose is dehumidification and the prevention of mold and mildew.

cooling effectiveness

the primary air dry-bulb temperature reduction divided by the primary air entering dry-bulb temperature less the entering secondary wet-bulb temperature.

cooling efficiency ratio (CER)

a ratio calculated by using the formula: CER = (C+ FE)/E where: C = cooling capacity, Btu/h (W), FE = fan electrical input, W × 3.413 Btu/W (W), E = total electrical input (W).

cooling energy

the sum of all site energy in kilowatt-hours required to provide cooling via vapor compression, ventilation, dehumidification, humidification, evaporation, absorption, adsorption, or other means.

cooling energy consumption

the site electric energy consumption of the mechanical cooling equipment including the compressor, air-distribution fan (regardless of whether the compressor is on or off), condenser fan, and related auxiliaries.

cooling liquid flow rate

the flow rate of liquid refrigerant required for all cooling purposes in a compressor or condensing unit.

cooling load

(1) amount of cooling per unit time required by the conditioned space or product. (2) heat that a cooling system must remove from a controlled system over time.

cooling load factor (CLF)

ratio of the cooling building load to the steady-state cooling capacity.

cooling medium

substance used, with or without a change of state, to lower the temperature of other bodies or substances. See coolant. See refrigerant.

cooling season

that portion of the year that the outdoor air temperature is above 18.3°C (65°F).

cooling system

apparatus for lowering the temperature of a space or product to a specified temperature.

cooling system energy coefficient of performance

a ratio calculated by dividing the net total cooling capacity in watts by the total power input in watts (excluding reheaters and humidifiers) at any given set of rating conditions. The net total cooling capacity is the total gross capacity minus the energy dissipated into the cooled space by the blower system.

cooling tower

heat transfer device, often tower like, in which atmospheric air cools warm water, generally by direct contact (evaporation).

cooling unit

unit that includes means for cooling and which may also include means for other air-handling-unit functions.

cooling-tower cell

smallest tower subdivision that can function as an independent heat exchange unit. It is bounded by exterior walls or partitions. Each cell may have one or more fans or stacks and one or more distribution systems.

cooling-tower fill
cooling-tower fogging

fog condition created when the exhaust air or plume from a cooling tower, which is essentially a saturated air/water vapor mixture warmer than ambient air, becomes supersaturated so that part of the water vapor condenses into visible liquid droplets.

cooling-tower packing

(also known as tower fill), that part of a crossflow, counterflow, or natural draft tower consisting of splash bars, vertical sheets of various configurations, or honeycomb assemblies, tile, or other materials which cause the water to break up into droplets to effect heat and mass transfer between the circulating water and the air flowing through the tower.

cooling-tower plume

visible exhaust from a cooling tower. Compare to cooling-tower fogging.

cooling-tower ton

total heat rejection capacity of a cooling tower, traditionally, 15,000 Btu/h. Note: this value is based on 25% compressor heat added to a ton of refrigeration. Current energy-efficient equipment may have lower values than traditional values.

coordination drawings

drawings showing the work of all trades to illustrate that equipment can be installed in the allocated space without compromising equipment function or access for maintenance and replacement.

COP degradation factor (CDF)

a multiplier (=1) applied to the full-load system COP or COP2. CDF is a function of part-load ratio. (Also see part-load ratio.)

core area

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

core area of a sand trap louver

product of minimum height (h) and minimum width (b) of the front opening of a sand trap louver assembly with the louver blades removed. Also see core area of an air terminal device.

core area of an air terminal device

area of an air terminal device located within a convex closed surface of minimum area, inside of which are all openings of the air terminal device through which the air can pass.

Coriolis acceleration

(a) acceleration which, when added to the acceleration of an object relative to a rotating coordinate system and to its centripetal acceleration, gives the acceleration of the object relative to a fixed coordinate system. (b) vector that is equal in magnitude and opposite in direction to that of definition (a).

Coriolis effect

(a) deflection relative to the earth’s surface of any object moving above the earth, caused by the Coriolis force. Note: an object moving horizontally is deflected to the right in the northern hemisphere and to the left in the southern. (b) the effect of the Coriolis force in any rotating system. Also called Coriolis deflection.

Coriolis force

velocity-dependent pseudoforce in a reference frame that is rotating with respect to an inertial reference frame, it is equal and opposite to the product of the mass of the particle on which the force acts and its Coriolis acceleration.

corrected effective temperature

effective temperature corrected by accounting for the effect of radiation. See also operative temperature.

correcting variable

output from controlling device that inputs to the controlled element.

corrective maintenance

classification of expended or reserved resources used to predict and correct impending failure. Corrective action is strictly remedial and always performed before failure occurs. The identical procedure performed in response to failure is classified as a repair. Corrective action may be taken during a shutdown caused by failure, provided the action is optional and unrelated. Corrective maintenance is carried out on all items where the consequences of failure or wearing out are not significant (less important items), and the cost of this maintenance is not greater than preventive maintenance.

corresponding state

states of fluids when the ratios of their state variables (pressure, temperature) to the critical values of these variables have equal values.


rusting or deterioration of a substance (usually a metal) because of a reaction to its environment.

corrosion inhibitor

(1) typically, a chemical agent that protects internal machine parts from the corrosive effects of the absorbent solution in the presence of an air chemical agent that slows corrosion of metal parts of a system. (2) substance added to a brine or other cooling medium.


capacity of an environment or environmental factor to bring about destruction of a specific metal by the process of corrosion.

Coulomb’s law

the attraction or repulsion between two electric charges acts along the line between them, is proportional to the product of their magnitudes, and is inversely proportional to the square of the distance between them.

counterflow heat exchanger

heat exchanger in which fluids flow in opposite directions approximately parallel to each other, inlets for the two fluids are at opposite ends of the exchanger.

counterflow heat transfer

directional pattern of the heat transfer fluids used in energy-exchange equipment where the warmest fluid “A” indirectly contacts the warmest fluid “B” at the entering side of the equipment and the coldest fluid “A” indirectly contacts the coldest fluid “B” at the leaving side of the equipment. Most energy exchange equipment is designed to use this method of heat transfer as it creates the highest log mean temperature difference (LMTD). When an energy exchange equipment is designed for counterflow heat transfer, and it is correctly connected in the field, the results would be loss in heat transfer capacity.

counterflow tower

one in which air, drawn in through air inlets at the tower perimeter (induced draft) or forced in (forced draft) at the base by the fan flows up through the fill material in a direction opposite to the falling hot water.


air terminal device intended to be installed above a natural ventilation exhaust duct with the aim (by creating negative pressure and depending on wind speed) of avoiding reverse flow and increasing flow rate. It may or may not have moving parts.

creep action

slow make/break action of a switch mechanism in a controller, as differentiated from snap action or toggle action.

critical circuit

the hydronic circuit that determines the minimum differential pressure that the pump must produce to satisfy the zone loads (e.g., the circuit with the most-open valve). The critical circuit is the one with the highest pressure drop required to satisfy its load. At part-load conditions, the critical circuit can change based on zone loads.

critical discharge point

the point in the load profile at which the combination of the required discharge rate and the current storage inventory causes the discharge temperature from the thermal storage device to rise to its highest value.

critical loads

Individual loads deemed to be critical to the operation of the facility or process.

critical nucleate boiling heat flux

(peak nucleate boiling heat flux), heat flux for which the surface coefficient of heat transfer between a heating wall and a liquid under nucleate boiling is a maximum. Also called maximum nucleate boiling heat flux.

critical peak period

A time period during which a special high price for electricity is applied as a way to reduce demand.

critical point

the location on a plot of thermodynamic properties at which the liquid and vapor states of a substance meet and become indistinguishable. The temperature, density, and composition of the substance are the same for the liquid and vapor phases at this point. The density, pressure, specific volume, and temperature at the critical point are referred to as the critical density, critical pressure, critical volume, and critical temperature, respectively.

critical process

processes with environmental control needs that are more constrained than occupancy comfort parameters. Examples of typical processes or areas that have unique needs and are thus critical are as follows: printing, papermaking, textiles, computer rooms, broadcasting studios, food processing, medical and hospital areas, cleanrooms, controlled laboratories, unusual safety and health needs, potentially explosive areas, cold storage, milling and machining, casting, glassmaking, and other specialized manufacturing or process spaces.

critical refrigerant charge

compromise refrigerant quantity required by a system to maximize performance when a capillary or fixed restriction expansion device is used.

critical speed

operating speed at which the vibration of a unit reaches an unacceptable limit.

critical state
critical velocity

(1) in fluid mechanics, the velocity above which flow in a pipe is no longer laminar. (b) velocity at which given phenomena occur.

cross connection

(1) connection between supply and return line in a hydronic system. May be used to balance system pressure, maintain a minimum circulation flow rate or temperature, etc. (2) in a piping system, a connection in which a pipe carrying potable water is connected to a closed vessel (or system) that is above atmospheric pressure and that contains nonpotable fluid. This is typically the point where a backflow preventer is required.

cross pipe

fitting with four branches in the same plane with right angles between them.

cross transmission

spread of infectious disease from an infected individual to a susceptible person.

cross ventilation

(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.

cross-connection control backflow prevention

(first line of defense) installation of a backflow preventer or a vacuum breaker at each cross connection on a premise to protect both premise system and the main system.

crossflow heat exchanger

heat exchanger in which fluids flow perpendicular to each other. Compare to counterflow heat exchanger.

crossflow tower

one in which air, drawn or forced in through the air intakes by a fan, flows horizontally across the fill section perpendicular to the falling hot water.


migration between airstreams.


undesirable transfer of energy from active signal line(s) to one or more independent signal lines, creating signals that may reach proportions to cause system errors.

cryocooling (cryogenic cooling)

cooling below –244°F (–153°C, 120 K).

cryogenic liquid

liquefied gas below –244°F (–153°C, 120°K).


science that deals with the production of very low temperatures and their effect on the properties of matter.

cryogrinding process

(freeze grinding) , grinding at a low temperature of a substance that otherwise could not be ground or would be spoiled by the temperature rise resulting from the operation.


eutectic mixture of which one component is water. See also eutectic solution.


device designed for producing an ultrahigh vacuum by condensation or adsorption of a gas at a very low temperature, usually below –320°F (–196°C, 77 K).


batch operating apparatus in which a cryogenic liquid or solid evaporates to maintain a cryotemperature, which need not be constant but may vary in a predetermined fashion.

cryotemperature (cryogenic temperature)

temperature within a few degrees of absolute zero (2.2 K).


surface cooled below –244°F (–153°C, 120 K) in order to condense vapors. Can be used to reduce pressure.

cup anemometer

device with several (often hemispherical) cups attached to the ends of symmetrical radial arms that rotate by air motion (wind) at a speed proportional to the wind velocity.

Curie point

temperature above which a given ferromagnetic substance becomes paramagnetic.

Curie-Weiss law

susceptibility of a paramagnetic substance above the Curie temperature varies inversely as the excess of the temperature above that point.

Curie’s law

magnetic susceptibilities of most paramagnetic substances are inversely proportional to their absolute temperatures.

Current Facility Requirements (CFR)

a written document in which the Owner details the current functional requirements of a facility and the expectations of how it should be used and operated. This may include goals, measurable performance criteria, cost considerations, benchmarks, success criteria, and supporting information to meet the requirements of occupants, users, and Owner(s) of the facility.


initiation of an event for which control is applied.

cut-in point

predetermined value (pressure, temperature, etc.), at which operation commences.


cessation of an event for which control is applied.


(1) complete course of operation of working fluid back to a starting point, measured in thermodynamic terms (functions). (2) complete series of values of a periodic quantity that occur during a period. (3) in alternating current, the time for a change of state from a zero through a positive and a negative maximum and back to zero. (4) interval of space, time, or a sound wave in which one set of repetitive events or phenomena is completed. (5) process or series of processes wherein the initial and final states of the system are identical. Therefore, at the conclusion of a cycle, all the properties have the same value as at the beginning. (6) set of operations that is repeated regularly in the same sequence. The operations may be subject to variations on each repetition. (7) the period of equipment operation from cut-in to cut-in.

cycle defrosting system

a system in which the refrigerated surfaces of the general refrigerated compartments are defrosted while maintaining nominal refrigerated food temperatures. Defrost water is disposed of automatically or collected in a container for subsequent manual removal.

cycle of concentration

represent the accumulation of dissolved minerals in the recirculating cooling water. Draw-off (or blowdown) is used principally to control the buildup of these minerals. Increasing the amount of minerals present in the water by cycling can make water less aggressive to piping; however, excessive levels of minerals can cause scaling problems. As the cycles of concentration increase, the water may not be able to hold the minerals in solution. When the solubility of these minerals has been exceeded, they can precipitate out as mineral solids and cause fouling and heat exchange problems in the cooling tower or the heat exchangers. The temperatures of the recirculating water, piping, and heat exchange surfaces determine if and where minerals will precipitate from the recirculating water. The use of water treatment chemicals, pretreatment such as water softening and pH adjustment and other techniques can affect the acceptable range of cycles of concentration. Concentration cycles in the majority of cooling towers usually range from 3 to 7. The water may also be filtered and dosed with biocides and algaecides to prevent growths that could interfere with the continuous flow of the water. In closed-loop systems, corrosion inhibitors may be used. 1. in boilers, the ratio of chlorides in the boiler water to the chlorides in the feedwater. 2. in cooling tower operation, the ratio of chlorides in the recirculating cooling tower water to the chlorides in the makeup water.

cycle thermal efficiency

ratio of the integrated energy output to the integrated energy input of a process or machine for a single cycle of operation.


(1) continuous oscillation occurring without periodic stimuli. A situation in a closed-loop system where the controller output to an input change causes instability. (2) periodic change in the controlled variable from one value to another. Also called hunting.

cycling life

expected total duration of lifetime cycling expressed in time or number of events.

cyclone filter

funnel-shaped device for removing particles from air or other fluids by centrifugal action or force.

cylinder head

in a reciprocating engine, pump, or compressor, the end of a cylinder opposite to that from which the piston rod or connecting rod projects.