A comprehensive online glossary of terms and definitions related to built environment, heating, ventilating, and air conditioning (HVAC), as well as refrigeration, building envelope, electrical, lighting, water and energy use, solar power, concentrating solar power (CSP), and measurement terms.
HVAC and Solar energy engineering Terminology
There are currently 253 terms in this directory beginning with the letter R.
(1) measure of the acoustical absorption properties of a room; the average absorption of all the surfaces in a room times the total surface divided by the average reflection. (2) (also known as thermal resistance), quantity determined by the temperature difference, at steady state, between two defined surfaces of a material or construction that induces a unit heat flow rate through unit area (R = ΔT/q). R-value is the reciprocal of thermal conductance.
radar doppler effect
shift of a radar echo due to relative motion of target and radar source. Note: Doppler radar can differentiate between fixed and moving targets and can measure target velocity.
radial-bladed impeller (paddle-bladed impeller)
one in which the blades are flat and disposed radially from the fan hub to the outer edge.
radial-energy distribution system
system of radial feeders extending outward from a centrally located district energy plant (heating or cooling). Each feeder normally is composed of one supply pipe and one return pipe.
total radiant flux emitted from a surface through unit projected area by unit solid angle. It includes the self-emitted radiation plus reflections from sources other than the object evaluated as interpreted from the direction of measurement. The term intensity of radiation is sometimes used as a synonym for radiance. See also [[radiosity]].
radiance at a point on a surface
the quotient of the flux incident on an element of a surface containing the point, by the area of that element, measured in watts per square meter, W/m2 (Btu/ft2).
radiating rays of light; emitting or reflecting beams of light; vividly shining; glowing; brilliant.
radiant asymmetry temperature
difference between the plane radiant temperature of the two opposite sides of a plane element. See [[plane radiant temperature]].
a surface of low emissivity (less than 0.1) placed inside an attic or roof space above (but not touching) the distribution system to reduce radiant heat transfer.
energy passing through space in the form of electromagnetic radiation (such as light or ultraviolet or infrared radiation) or as a stream of particles (e.g., electrons or protons).
radiant flux density
measure of radiant power per unit area flowing across or onto a surface. (Also called irradiance).
a heating or cooling surface that delivers 50% or more of its heat transfer by radiation, which may be either an integral part of the building (e.g., floor or ceiling heating) or detached from the building elements (e.g., suspended ceiling panel).
radiant reflectance (luminous reflectance)
ratio of the reflected radiant (or luminous) flux to the incident radiant (or luminous) flux.
radiant transmittance (luminous transmittance)
ratio of the transmitted radiant (or luminous) flux to the incident radiant flux.
a sensible cooling system that provides more than 50% of the total heat flux by thermal radiation.
a sensible heating system that provides more than 50% of the total heat flux by thermal radiation.
radiated sound power level
sound power that radiates from terminal casings and induction ports for induction terminal units.
act or process of radiating, specifically the process by which energy is emitted from molecules and atoms, owing primarily to internal temperature change.
radiation angle factor
(also known as shape factor), when the space above an element of surface is partially occupied by a body exchanging radiant energy with the surface, the angle factor represents the fraction of the angular field of view where energy exchange is taking place.
irradiation, using moderate doses, to enhance keeping quality by killing most of the spoilage microorganisms present; viruses are not affected.
(1) device designed to intercept radiated heat. (2) in a furnace, a sheet of metal or other material supported between the heat exchanger and the casing to protect the casing from heat; not to be considered as a heat transfer surface.
represents the net amount of infrared radiation absorbed by gases in the atmosphere. The radiative forcing of a gas depends on the efficiency with which it traps infrared radiation and its concentration in the atmosphere. Atmospheric concentration depends on emission rates and the atmospheric lifetime of the gas.
total radiant flux that leaves unit area of a surface. The sum of radiant flux emitted and reflected by the surface, plus any radiant flux transmitted through that surface. Compare to [[exitance]].
The ability of a generating unit to change its output over some unit of time, often measured in MW/min.
an error that causes readings to take random values on either side of some mean value. Measurements may be precise or imprecise depending on how well an instrument can reproduce subsequent readings of an unchanged input.
random-access memory (RAM)
memory providing access time that is independent of the address and is addressable for both reading from and writing into memory.
(1) difference between the highest and the lowest operational values, such as pressure, temperature, rate of flow, or computer values. (2) region between limits within which a quantity is measured, transmitted, or received, expressed by stating the lower and upper range values.
theoretical thermodynamic cycle used in steam engines, comprising four principal stages: (1) vaporization of water under high pressure, (2) expansion of steam, (3) condensation of steam, and (4) pumping of the water back to initial pressure.
absolute temperature scale conventionally defined by the temperature of the triple point of water equal to 491.68°R, with 180 divisions between the melting point of ice and the boiling point of water under standard atmospheric pressure (1°R = 1°F). See [[triple point]].
spot-cooling effect produced in a tube into which gas is introduced tangentially, producing vortex flow.
molar weights of nonvolatile nonelectrolytes, when dissolved in a definite mass of a given solvent under the same conditions, lower the solvent’s freezing point, elevate its boiling point, and reduce its vapor pressure equally for all such solutes.
rated battery capacity
The term used by battery manufacturers to indicate the maximum amount of energy that can be withdrawn from a battery under specified discharge rate and temperature. See also battery capacity.
rated final resistance
the operating pressure loss at the airflow rate at which a disposable device (e.g., filter) should be replaced or renewed, as recommended by the manufacturer, expressed in Pa (in. of water).
rated module current (A)
The current output of a photovoltaic module measured at standard test conditions of 1,000 w/m2 and 25°C cell temperature.
Rated power of the inverter. However, some units can not produce rated power continuously. See also duty rating.
the assigned values of those performance characteristics, under stated rating conditions, by which a unit may be chosen to fit its application. These values apply to all units of like nominal size and type (identification) produced by the same manufacturer.
portion of apparent power that does no work. It is measured commercially in kilovars. Reactive power must be supplied to most types of magnetic equipment, such as motors. It is supplied by generators or by electrostatic equipment such as capacitors.
(1) capable of being quickly and easily reached for operation, maintenance, and inspection. (2) see [[accessible]].
energy or work producing part of apparent power. The rate of supply of energy, measured commercially in kilowatts. The product of real power and length of time is energy, measured by watthour meters, and expressed in kilowatt hours (kWh).
a vessel in the refrigerating system designed to ensure the availability of adequate liquid refrigerant for proper functioning of the system and to store the liquid refrigerant when the system is pumped down.
a positive displacement compressor that changes internal volume of the compression chamber(s) by the reciprocating motion of one or more pistons.
air taken from a space and returned to that space, usually after being passed through a conditioning system. The part of the return air that is reused. Air removed from a space and reused as supply air.
a domestic or service hot-water distribution system that includes a closed-circulation circuit designed to maintain usage temperatures in hot-water pipes near terminal devices (e.g., lavatory faucets, shower heads) in order to reduce the time required to obtain hot water when the terminal device valve is opened. The motive force for circulation is either natural (due to water density variations with temperature) or mechanical (recirculation pump).
a remote unit with cooling that is provided at the dispensing valve and accomplished by circulating cold water or cold carbonated water through one of the lines and returning the same to a refrigeration unit.
recirculation of discharge air
(1) a condition pertaining to air-cooled condensers in which a portion of the discharge air enters along with the fresh air; the amount of recirculation is determined by equipment design, placement in regard to adjoining objects, and atmospheric conditions. The effect is generally evaluated on the basis of the decrease in unit capacity. (2) condition pertaining to cooling towers and evaporative condensers in which a portion of the discharge air enters along with the fresh air; the amount of recirculation is determined by equipment design, placement in regard to adjoining objects, and atmospheric conditions. The effect is generally evaluated on the basis of the increase in entering wet-bulb temperature compared to the ambient.
refrigerants reprocessed to the same specifications as new refrigerants by any means, including distillation. Such refrigerants have been chemically analyzed to verify that those specifications have been met.
The action of a free electron falling back into a hole. Recombination processes are either radiative, where the energy of recombination results in the emission of a photon, or nonradiative, where the energy of recombination is given to a second electron which then relaxes back to its original energy by emitting phonons. Recombination can take place in the bulk of the semiconductor, at the surfaces, in the junction region, at defects, or between interfaces.
an application of the commissioning process requirements to a project that has been delivered using the commissioning process. This may be a scheduled recommissioning developed as part of an ongoing commissioning process or it may be triggered by use change, operations problems, or other needs.
(1) cooling of air that has been previously heated. (2) lowering the temperature of air that has been previously heated by a mechanical heating system.
drawings that record the conditions of the project as constructed. These include any refinements of the construction or bid documents.
recording thermometer (temperature recorder; thermograph)
a thermometer that indicates temperature by traces on a moving chart; recording may be done on a cylinder, a circular chart, or a strip chart.
portion of thermal input to a prime mover and that is not converted to mechanical power and can be reclaimed.
recovered or reclaimed heat
comes from internal heat sources. It is used for space heating, domestic or service water heating, air reheat in air conditioning, process heating in industrial applications, or other similar purposes. Recovered heat may be stored for later use.
refrigerants removed from a system in any condition without necessarily testing or processing them.
(1) externally cooled heat exchanger in the high side of a refrigerating absorption system for condensing absorbent and separating it from the refrigerant before passing it to the condenser. (2) in electricity, a device for converting AC to DC current.
to reduce contaminants in used refrigerants by separating oil, removing noncondensables, and using devices such as filter driers to reduce moisture, acidity, and particulate matter.
refrigerants for which contaminants have been reduced by oil separation, removal of noncondensable gases, and single or multiple passes through filter driers or other devices that reduce moisture, acidity, and particulate matter.
reduced input rate with manually adjustable modulating control
the input rate resulting from burner operation at the manufacturer's recommended adjustment setting or the midpoint setting.
reduced pressure detector check (RPDC)
back-pressure and back-siphonage-type device designed to serve as a detector check on fire protection systems where contaminants are involved. Note: RPDC includes a line-size approved, reduced pressure zone backflow preventer with a metered bypass into which has been incorporated a three-quarter inch, approved, reduced pressure zone backflow preventer.
reduced pressure zone backflow preventer
back-pressure and back-siphonage-type device designed to operate under continuous pressure where contaminants are involved.
reduced-absorptivity exterior coating
an exterior finish applied to roof systems in order to reduce the absorption of solar radiation. The solar absorptivity must be 0.4 or less.
reduced-heat input rate
for furnaces or boilers equipped with automatic modulating control, the input rate resulting from the lowest factory-adjusted setting of the control.
(1) a type of check valve that restricts flow of fluids to a single direction and consists of thin strips of flexible metal, fiberglass, or other materials fixed on one end. They open and close with changing pressures across opposite sides of the valve. (2) compressor valve consisting of a plate having a number of slots and flexible steel strips fitted over the slots.
dry-media-type filters that are carefully measured for resistance and initial efficiency immediately after a test system is qualified. These filters serve as references to ensure that the test system continues to operate as it did when it was qualified.
the temperature at an appropriate, fixed location within the test zone, for example, at 42 in. (1.1 m) above the floor for an office space served by a mixing air system.
(1) portion of the incident radiation on a surface that is reflected from the surface. Note: for an opaque surface, the sum of the reflectance and the absorptance is unity at equilibrium. Absorptances and reflectances are of various types, as are emittances. (2) the ratio of the light reflected by a surface to the light incident upon it.
reflective thermal insulation
insulation that reduces radiant heat transfer across spaces by use of one or more surfaces of high reflectance and low emittance, for example, aluminum foil.
portion of the radiation-striking unit area of a surface that is not absorbed or transmitted by the surface. See [[reflectance]].
(1) refrigerating fluid used for heat transfer in a refrigerating system and that absorbs heat at a low temperature and low pressure of the fluid and transfers heat at a higher temperature and a higher pressure of the fluid, usually involving changes of state of the fluid. Compare to [[coolant]]. (2) in a refrigerating system, the medium of heat transfer that picks up heat by evaporating at a low temperature and pressure and gives up heat on condensing at a higher temperature and pressure. (3) substance that changes phase or condition (e.g., from liquid to vapor [evaporation] or from bonded to solid to vapor [desorption]) in the process of absorbing heat within the air-conditioning equipment.
a refrigerant composed of two or more different chemical compounds often used individually as refrigerants for other applications.
(1) actual amount of refrigerant in a closed system. (2) weight of refrigerant required for proper functioning of a closed system.
device used in conjunction with a thermostatic expansion valve to ensure equable refrigerant distribution from the valve to individual parallel sections of an evaporator.
refrigerant heat rejection
total useful capacity of a refrigerant condenser for removing heat from the refrigerant circulated through it.
splitting of refrigerant compounds by reaction with water (e.g., reaction of dichlorodifluoromethane or methyl chloride with water, forming acid products).
refrigerant mass flow rate
the mass flow rate of the volatile refrigerant, which is potentially mixed with lubricant.
refrigerant pressure-imposing device
device or portion of equipment used for the purpose of increasing refrigerant pressure.
recirculates liquid refrigerant from the refrigerant sump at the bottom of the evaporator to the evaporator tube bundle in order to effectively wet the outside surface and enhance heat transfer (when used).
refrigerant pump-out system
dedicated apparatus for transfer of refrigerant from a chiller or other refrigerating system to a separate and distinct storage vessel.
vessel in a refrigerating system designed to ensure the availability of adequate liquid refrigerant for proper functioning of the system and to store the liquid refrigerant when the system is pumped down.
refrigerant recovery unit
a machine that removes refrigerant from a refrigerating system, preventing refrigerant discharge to the environment.
process of cooling refrigerant below condensing temperature for a given pressure; also, cooling a liquid below its freezing point where it can exist only in a state of unstable equilibrium. See [[supercooling]].
refrigerated plate freezing
heat removal by direct contact of the packaged product with refrigerated plates.
refrigerated truck end bunker
a refrigerated vehicle where the space given to the ice or cooling element is in the end of the truck or rail car.
thermodynamic cycle of a system that transfers heat from a low-temperature reservoir to a high-temperature reservoir.
technique of design, manufacture, application, and operation of refrigerating machinery and its primary equipment.
refrigerating heating system
interconnected parts forming a closed circuit in which refrigerant is circulated and having the condenser located to transfer heat to the zone to be heated.
amount of heat per unit time imposed on a refrigerating system for the required rate of heat removal.
refrigerating service load
quantity of heat to be removed from a refrigerated space to compensate for opening doors, presence of personnel, heat from electric lights, etc.
a system that, in operation between a heat source (evaporator) and a heat sink (condenser) at two different temperatures, absorbs heat from the heat source at the lower temperature and rejects heat to the heat sink at the higher temperature. See also [[refrigerating unit]].
refrigerating system classification
refrigerating systems are classified according to the degree of probability, low or high, that leaked refrigerant from a failed connection, seal, or component could enter an occupied area. The distinction is based on the basic design or location of the components.
refrigerating system contaminants
contaminants include water (the most important and universal contaminant); dirt; particles; organic materials such as waxes, acids, and sludges; or other products of chemical reactions taking place while the system is operating.
unit assembly composed of a compressor, evaporator, condenser, and expansion device, used for refrigerating and for extracting heat.
refrigerating equipment forming a part of the refrigerating system, including any or all of the following: compressor, condenser, generator, absorber (adsorber), liquid receiver, connecting piping, and evaporator.
refrigerating-system performance factor
ratio of the useful refrigerating effect of the system to the power input.
device capable of providing the necessary difference in pressure between the high- and low-pressure sides of the refrigerating system.
(1) any use of mechanical- or absorption-refrigerating machinery for applications other than the comfort of human beings. Compare to [[cooling]]. (2) process of extracting heat from a substance or space by any means, usually at a low temperature.
a stable fluid that is compatible with system components, will form a friction-reducing film between rubbing surfaces and seal critical clearances, and has low-temperature transport properties suitable for the application in which it is used.
a stable fluid that is compatible with system components, will form a friction-reducing film between rubbing surfaces, will seal critical clearances, and has low-temperature properties suitable for the application.
plate, wall, or partition that is designed to perform one or more of the following functions: (1) prevent contact of food with refrigerated surfaces, (2) prevent dripping of condensate on food, (3) regulate and/or direct circulation of refrigerated air.
refrigerator cabinet breaker strip
separate insulating element or integral insulating extension of the cabinet interior surfaces around the periphery of the cabinet door or drawer opening(s), which functions as a thermal barrier to minimize heat flow to the cabinet interior (i.e., it breaks a thermal leakage path).
refuse-derived fuel (RDF)
refuse used as a combustion fuel source to provide heat in a district-heating plant.
the airstream used as a carrier for the desorbed moisture and/or a mechanism to transfer heat for the regeneration of the desiccant in a dry desiccant system.
device used to heat the solid desiccant regeneration air or the liquid desiccant.
process of using heat that must be rejected or absorbed in one part of the cycle to perform a useful function in another part of the cycle.
regenerative cycle gas turbine
simple cycle gas turbine to which has been added a heat exchanger to warm incoming combustion air with heat from exhaust gases in order to increase thermal efficiency.
heat rejected in one part of a system and used to perform a useful function in another part.
appropriately qualified and licensed professional engineer. See also [[design professional]].
valve capable of regulating the flow of water through a condenser according to changes in condensing pressure or water temperature.
agencies whose rules are mandated by law or by owner’s specification. Examples include the U.S. Coast Guard, American Bureau of Shipping, and U.S. Public Health Service.
application of sensible heat to supply air that has been previously cooled below the temperature desired for maintaining the temperature of the conditioned space.
raising the temperature of air that has been previously cooled either by mechanical refrigeration or an economizer system.
relative humidity (rh, RH)
(1) ratio of the mole fraction of water vapor to the mole fraction of water vapor saturated at the same temperature and barometric pressure. (2) ratio of the partial pressure or density of water vapor to the saturation pressure or density, respectively, at the same dry-bulb temperature and barometric pressure of the ambient air.
relative light transmission
the light transmission of a dust spot sampling target relative to that of a translucent standard.
(1) electrical mechanism that uses the current in a control circuit to open or close electric contacts. (2) fluid (liquid or pneumatic) device that uses variations in fluid pressure to actuate final control devices.
in freeze drying, that part of the water contained in the product and that may be extracted without spoiling the quality of the product.
any movement of refrigerant out of its containment, including, but not limited to, by a leak, by an action of filling or testing, or by failure.
the rate of release, in actual liters per minute (Lpm), of tracer gas during a hood test.
(1) mathematical probability that a device will perform its objective adequately for the period of time intended under the operating conditions specified. (2) probability that a device will function without failure over a specified time period or amount of usage. See [[accuracy]]; [[precision]]; [[repeatability]]. (3) probability that an instrument’s repeatability and accuracy will continue to fall within specified limits.
(1) all return air that is discharged directly to the outside or exhausted by separate exhaust fans. (2) building return air discharged by the air-handling unit (AHU) equipment to control building pressure when an HVAC system is operating in the economizer cycle.
thermostat in which the sensing bulb can be located at a distance from the instrument proper while remaining flexibly connected to it.
energy obtained from sunlight, wind, earth, geothermal sources, or bodies of water to provide heating, cooling, lighting, or water-heating services to buildings. See [[nondepletable energy]].
empirical unit of resistance to water vapor flow through a material or construction. One rep = 1 hour – square foot – in. Hg pressure difference between the two surfaces per grain (avoirdupois) water vapor (h·ft2·in. Hg/gr). (The resistance may be stated in other units consistent with this value.) The rep is the reciprocal of the perm (1 rep = 1/perm). It is not an SI unit.
(1) the reconstruction or renewal of any part of an existing building for the purpose of its maintenance. (2) to restore to good or sound condition within the following constraints: operation must be fully restored without embellishment and failure must have occurred.
(1) closeness of agreement among consecutive measurements of the output for the same value of input approaching from the same direction. Compare to [[accuracy]]; [[precision]]; [[reliability]]. (2) closeness of agreement among repeated measurements of the same variable under the same conditions. (3) the ability to obtain the same observed value in repeated experiments. Lack of repeatability is commonly associated with precision error, the random component of the total error.
outdoor air that is used to replace air removed from a building through an exhaust system. Replacement air may be derived from one or more of the following: makeup air, supply air, transfer air, and infiltration. However, the ultimate source of all replacement air is outdoor air. When replacement air exceeds exhaust, the result is exfiltration.
estimate of the current or future cost to replace existing facilities either as currently structured or as redesigned to embrace new technology with facilities that will perform the same functions.
repetitions of measurements at the same conditions that are taken to estimate the uncertainty in the results.
studious inquiry; usually critical and exhaustive investigation or experimentation with the aim of revision of accepted conclusions in light of newly discovered facts.
The amount of generating capacity a central power system must maintain to meet peak loads.
control method using a remote or external signal to modify the setpoint of a controller.
the occupant or occupants of an individual unit in a multiple-occupancy residential building who have entered into an agreement with the owner.
engineer employed by the owner to represent the owner’s interests at the project site during the construction phase.
spaces in buildings used primarily for living and sleeping. Residential spaces include, but are not limited to, dwelling units, hotel/motel guest rooms, dormitories, nursing homes, patient rooms in hospitals, lodging houses, fraternity/sorority houses, hostels, prisons, and fire stations.
residential health care facility
a facility, building, or portion of a building that provides housing and services for a resident or group of residents.
in a freeze-dried product, the ratio of the mass of residual water to the original mass of product.
(1) property of an electric circuit, or of any object used as part of an electric circuit, that determines for a given current the rate at which electric energy is converted into heat or radiant energy and that has a value such that the product of the resistance and the square of the current gives the rate of conversion of energy. (2) property opposing movement of material or flow of energy and involving loss of potential (voltage, temperature, pressure level). (3) thermal resistance. See [[electrical resistance (Ω)]]; [[thermal resistance]].
The property of a conductor, which opposes the flow of an electric current resulting in the generation of heat in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit current flow. The unit of resistance is ohms.
(1) dimensionless group equal to four times the Fanning friction factor. Also known as Darcy-Weisbach coefficient and Darcy number 1. (2) dimensionless number used in the study of flow resistance; equal to the resistance force in flow divided by one-half the product of fluid density, the square of fluid velocity, and the square of a characteristic length.
resistance heating element
the electrical conducting medium that is heated by an electric current and that also dissipates this heat into the air or a fluid.
resistance temperature device (RTD)
temperature sensors that utilize the predictable change in electrical resistance of some materials with changing temperature. They are commonly made of platinum. There are two broad categories, film and wire-wound types. Film temperature sensors have a layer of some resistive material, such as platinum, on a substrate: the layer may be extremely thin, perhaps 1 micrometer. Wire-wound temperature sensors can have greater accuracy, especially for wide temperature ranges. The coil diameter provides a compromise between mechanical stability and allowing expansion of the wire to minimize strain and consequential drift.
welding with resistance heating and pressure, the work being part of the electrical circuit.
(1) electric load with all energy input converted to heat and light. (2) electric load without capacitance or inductance or one in which inductive portions cancel capacitive portions at the operating frequency.
resistive voltage drop
The voltage developed across a cell by the current flow through the resistance of the cell.
(1) for digital techniques, the number of discrete values that can be indicated by the digital word from zero to full scale (e.g., a ten bit binary word has a resolution of one point in 1024). (2) in analog to digital conversion, resolution refers to the smallest input increment that can be measured and indicated by an output change. This can also be called sensitivity.
condition of high-vibration response. The exact resonance of a system in forced vibration exists when any change, however small, in the frequency of excitation causes a decrease in the response amplitude (i.e., when the driving frequency equals the natural frequency).
the frequency at which operation of the equipment leads to a peak in the response spectrum. Note: for lightly damped structures, the resonance frequency can be taken to be the natural frequency.
resorption-type refrigerating system
system in which the refrigerant vapor is not condensed to a pure liquid but is absorbed in a weak solution from which it is subsequently evaporated at a lower temperature to produce refrigeration.
resource energy impact
product of an energy resource [includes application of resource utilization factor (RUF)] anticipated to be used in providing fuel or energy to a building site, multiplied by an RIF for that particular form of energy resource. Total resource energy impact is the sum of all resource energy impacts for a building project.
resource impact factor (RIF)
multipliers applied to fuel and energy resources required by a building project to permit a quantitative evaluation on the economy of those resources resulting from the selection of on-site fuel and energy forms. Availability and social, economic, environmental, and national interest issues are considered.
resource utilization factor (RUF)
multiplier, applied to the quantity of fuel or energy delivered to a building site, that provides a quantitative estimate of the energy resources consumed in providing that fuel or energy. Variant multipliers account for the burden of processing, transporting, converting, and delivering fuel or energy from the point of extraction to the building site.
particles small enough to be inhaled into a nonciliated portion of the lung. Peak deposition of respirable particles occurs within the size range of 0.2 to 5.0 micrometers. Particles greater than 10 micrometers in aerodynamic diameter are not respirable.
(1) breathing process of animals. (2) production of carbon dioxide and heat by ripening of perishables in storage.
in plants, heat created during the respiratory process (absorption of oxygen and evolution of CO2).
(1) output, expressed as a function of time, resulting from the application of specified input. (2) time (preferably in seconds, may also be in cycles of supply frequency) required for the output quantity to change by some agreed-on percentage of the differential output quantity in response to a step change input. Note: in measurement, the initial and final output quantities shall correspond to the test-output quantities. The response time shall be the maximum obtained including differences arising from increasing or decreasing output quantity or time phase of signal application. (3) time for a measuring sensor to reach 90% of the final value after a step change. For a measuring system that includes only one exponential time constant function, the 90% response time equals 2.3 times the time constant.
of the adsorbent, the amount remaining after a saturated bed reaches equilibrium in clean air.
the commissioning process applied to an existing facility that was not previously commissioned. The same process for retrocommissioning needs to be followed from predesign through occupancy and operations to optimize the benefits of implementing the commissioning process philosophy and practice.
modification of existing equipment, systems, or buildings to incorporate improved performance, updated operation, improved energy performance, or all three. Derived from retroactive refit.
the savings measurement approach that determines energy or demand savings through the use of meters to isolate the energy flows for the system(s).
air removed from a space to be recirculated or exhausted. Air extracted from a space and totally or partially returned to an air conditioner, furnace, or other heating, cooling, or ventilating system.
return flow nozzle
nozzle for a mechanical atomizing oil burner in which part of the oil supplied to the atomizer is withdrawn and returned to storage or to the oil line to the atomizer.
compressor in which the suction valve is located in the cylinder head of the compressor.
persistence of sound to bounce around in an enclosed space after the sound source has stopped.
time in seconds for sound energy to decay 60 decibels. A common reference is often the time for the sound in the 500 Hz octave band to decay 60 dB.
device or control where the action of the device or control decreases (or increases) as the variable increases (or decreases). Compare to [[direct acting]].
reverse current protection
Any method of preventing unwanted current flow from the battery to the photovoltaic array (usually at night). See also blocking diode.
cycle obtained when a fluid is made to follow the different thermodynamic stages of a cycle in the reverse order.
reverse cycle defrosting
defrosting an evaporator by reversing its function with that of the condenser.
airflow within the hood when smoke released in the hood moves forward, toward the front of the hood. This term does not apply to the forward motion of the roll inside the hood that occurs in the upper cavity of the hood above the hood opening or to the cyclonic motion that occurs behind a closed horizontal sash.
(1) output pressure of a reversing relay that changes in opposition to the input signal (i.e., as the input pressure increases, the output pressure decreases). (2) pressure differential existing in a heat pump reversing valve to affect a reversal of flow through the valve.
reverse return piping system
two-pipe system in which the heat transfer medium supplied to the first load is the last returned to the heat transfer equipment. A system in which the water return piping from terminal units is sized to provide equal lengths for balanced flow rates. Compare to [[direct-return piping system]].
maximum useful work obtained for a given change of state. It includes heat supplied from other systems but excludes work done on the surroundings.
relay designed to reverse the direction of current flow or of function, on actuation.
a dimensionless number, designated Re, that indicates whether the fluid flow is laminar or turbulent. For flow in a pipe, transition generally occurs between Reynolds’ numbers of 2300 and 4000.
ribbon (photovoltaic) cells
A type of photovoltaic device made in a continuous process of pulling material from a molten bath of photovoltaic material, such as silicon, to form a thin sheet of material.
dimensionless number used in studying the stratified flow of multilayer systems; equal to the acceleration of gravity g times the density gradient of a fluid divided by the product of the fluid’s density and the square of its velocity gradient at a wall (symbol NR).
rigid duct flow area
calculated by using the average inside duct dimensions determined by measurement of a minimum of three representative sections of the duct envelope.
granular deposit of opaque ice on a surface, formed by quick freezing of supercooled water droplets.
alternating-current component from a direct-current power supply arising within a power supply.
rise of an air jet in mixing air diffusion
vertical distance (hv) between the highest horizontal plane tangent to a specified isovel and the center of the core of an air jet.
(1) measure of the time required for a circuit to change its output from a low voltage level to a high voltage level, normally from 10 to 90 percentage points. (2) time required for the output of a system (other than first order) to make the change from a small specified percentage (often 5% to 10%) of the steady-state increment to a large, specified percentage (often 90% to 95%) either before overshoot or in the absence of overshoot.
rock bed regenerative cooling system
system of air conditioning in which packed beds of crushed stone or gravel are used for both evaporative cooling and heat energy storage.
the rotation of air in the upper cavity of the hood. The roll is induced by the momentum of the air entering the hood through the hood opening.
roll bond evaporator
evaporator consisting of two metal plates that are weld bonded together (with the exception of the printed circuit forming the refrigerant passage, which is obtained by inflation under pressure).
filter in which the filter medium is a continuous belt on movable rolls to bring clean filter media into the airstream, either automatically or manually.
bearing consisting of a number of rollers in a cage between the inner and outer races.
rolling piston compressor
special type of small rotary compressor having a rotor aligned eccentrically within the stator, used in domestic refrigerators.
the upper portion of the building envelope, including opaque areas and fenestration, that is horizontal or tilted at an angle of less than 60° from horizontal.
all components of the roof/ceiling envelope through which heat flows, thus creating a building transmission heat loss or gain, where such assembly is exposed to outdoor air and encloses a heated and/or mechanically cooled space.
roof spray cooling
system that reduces heat gain through a roof by cooling the outside surface with a water spray; suited for only temporary treatment because high humidity may be introduced by air intakes on the roof.
rooftop air conditioner
packaged air conditioner mounted on a roof, the conditioned air being discharged directly into the rooms below or through a duct system.
room air conditioner
an encased assembly designed as a unit primarily for mounting in a window, through a wall, or as a console. It is designed primarily to provide free delivery of conditioned air to an enclosed space, room, or zone. The room air conditioner includes a primary source of refrigeration and dehumidification, means for air circulation, air cleaning, and heating and may include means for ventilation and humidification.
a test facility consisting of a room-side compartment and an outdoor-side compartment, each of which is equipped with instrumented reconditioning equipment. The output of this equipment is measured and controlled to counterbalance the room-side net total cooling effect of the air conditioner under test.
room cavity ratio (RCD)
a factor that characterizes room configuration as a ratio between the walls and ceiling and is based upon room dimensions.
room criterion curve (RC curve)
a series of curves of octave band sound spectra in a system for rating the noisiness of an occupied indoor space; an actual octave band spectrum is compared with this set of curves to determine the RC level of the space. Rates sound from 16 to 16,000 Hz octave bands.
root mean square (RMS)
(1) for a sinusoidal motion, the RMS value is 0.707 times peak. (2) for a velocity measurement, root mean square is often used when vibrations are random or consist of a number of sinusoidal vibrations of different frequencies. The RMS value is a measure of the effective energy used to produce the vibration of the machine. (3) root mean square is the square root of the time average of the sound (vibration) wave(s).
rotary atomizing burner
burner in which atomization is accomplished by feeding oil to the inside of a rapidly rotating cup.
a positive displacement compressor in which the change in internal volume of the compression chamber is accomplished by the rotary motion of a positive displacement.
process of deliberately interrupting preselected loads from an electric power system, as a nonroutine remedy of energy management implemented through transmission supervision and substation automation, on a substation bus or distribution feeder basis in a sequentially timed pattern for the purpose of matching demand to temporarily limited supply.
rotating vane anemometer
device consisting of rotating-propeller-type vanes; the air velocity is indicated from the rotational speed of the vanes.
ratio of size of projections from the surface of a pipe or duct to the diameter of the pipe or duct.
runaround heat exchanger
finned-tube coils (closed system) or spray chambers (open systems) in which a liquid is circulated by gravity or pump action through a heat source exchanger and then through a heat sink exchanger. Antifreeze may be used in the coil loop and a desiccant in the spray system.
regenerative-type closed secondary system in which a continuously circulated fluid absorbs heat from the primary system fluid at one place and sends it to the primary system fluid at another place.
(1) the period of time between the start and the stop of a cycle. (2) the period of time between the start of refrigeration after a defrost termination and the beginning of the next successive defrost.
(also known as pressure relief device or pressure limiting device), valve or rupture member designed to relieve pressure at a predetermined setting by mechanical failure of the disc. A rupture disc is a single-use device and must be replaced after the incident (as opposed to a relief valve, which may be manually or automatically reset).