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

(1) dimensionless unit of acoustical absorption, equal to the equivalent sound absorption of one square foot of a surface of unit absorptivity (i.e., of one square foot of surface that absorbs all incident sound energy). (2) dimensionless unit of acoustical absorption, equal to the equivalent sound absorption of unit of area of a surface of unit absorptivity.

safe pressure

maximum pressure a system can be subjected to without component failure.

safety device

(1) ancillary device fitted to an apparatus or machine in order to prevent accident or damage in the event of abnormal functioning of the system. (2) control that causes equipment to cease operation when adverse or excessive conditions occur.

safety head

in a compressor, a cylinder head held in place by a spring of such strength that it will not be compressed during normal operation but will be compressed by solid matter or liquid coming between it and the piston, thereby protecting the compressor.

safety interlock

hydrometer calibrated in salt concentration.

salvaged material

material, component, or assembly removed in a whole form from a structure or site in which it was permanently installed and subsequently reused in the building project.

sampling tube

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

sand-trap louver

device intended to allow the passage of outdoor air or exhaust air while minimizing the ingress of airborne sand. Also see louver.

sash crack

sum of all perimeters of all ventilators, sashes, or doors based on overall dimensions of such parts expressed in feet (centimeters) (counting two adjacent lengths of perimeter as one).

sash movement effect

the maximum of the positional sash movement effects for all the positions tested on a particular hood.

sash movement performance rating

a series of letters and numbers consisting of the letters SME-AM, SME-AI, or SME-AU and a two- or three-digit number, (i.e., SME-AU yyy, SME-AI yyy, SME-AM yyy), where SME means “sash movement effect,” AM means “as manufactured,” AI means “as installed,” AU means “as used,” and yyy equals the sash movement effect, ppm. A test rating of SME-AM 10, for example, would indicate that the peak concentration of tracer gas measured during a sash movement test under the “as manufactured” test [with a tracer gas release rate of 4.0 Lpm] was 10 ppm.


an organic (carbon-containing) compound in which each carbon atom is joined to four other atoms, all of the chemical bonds in a saturated compound are single.

saturated air

(1) air that holds the maximum water vapor possible at a specified temperature and pressure. (2) moist air in which the partial pressure of the water vapor is equal to the vapor pressure of water at the existing temperature.

saturated condensing temperature

the saturation temperature corresponding to the measured refrigerant pressure at the condenser inlet for single component and azeotropic refrigerants, and the arithmetic average of the refrigerant dew-point temperature and the bubblepoint temperature corresponding to the refrigerant pressure at the condenser entrance for zeotropic refrigerants.

saturated liquid

liquid existing at the saturation temperature and pressure.

saturated water-vapor pressure

the water-vapor pressure at the saturation temperature corresponding to the reference pressure and without any liquid phase.


(1) condition for coexistence in stable equilibrium of a vapor and liquid or a vapor and solid phase of the same substance. As an example, steam over the water from which it is being generated. See vapor pressure. (2) physical adsorbent that contains all the contaminant it can hold at the challenge concentration, temperature, and humidity. This point is its activity, expressed as a percentage of the carbon mass or fraction (i.e., g contaminant/g adsorbent). (3) condition for coexistence in stable equilibrium of a vapor and liquid, or a vapor and solid phase of the same substance. As an example, steam over the water from which it is being generated. See vapor pressure.

saturation deficiency

dimensionless difference between the humidity ratio of an air sample and the saturation humidity ratio of air at the same temperature and pressure.

saturation effectiveness

the dry-bulb temperature reduction achieved by the ECU divided by the entering wet-bulb depression.

saturation pressure

the pressure at which vapor and liquid exist in equilibrium at a given temperature.

saturation temperature

the temperature where a substance changes between its liquid and its vapor phase. If the pressure in a system remains constant, a vapor at saturation temperature will begin to condense into its liquid phase as thermal energy is removed, and, conversely, a liquid at saturation temperature will begin to evaporate as thermal energy is applied.

savings determination

the process of separating a retrofit’s (energy conservation measure’s) effectiveness from a facility’s energy use pattern. It involves measurements of physical conditions and analysis of resultant data.

savings-measurement approach

the estimation of energy and demand savings associated with an energy conservation measure for a piece of equipment, a subsystem, or a system. The estimated savings are based on some kind of measured data from before and after the retrofit and may be calculated using a variety of engineering techniques. There are several different savings measurement approaches.


(1) the formation of thick corrosion product layers on a metal surface at high temperature. (2) the precipitation of water-insoluble constituents on a surface.

scale inhibitor

substance added to water used in condensers, boilers, piping, and cooling towers to prevent or minimize the formation of insoluble deposits.

scale setting

(1) control setpoint at which temperature is to be maintained. (2) indicated temperature to which a thermostat is set.

scale trap

in a refrigeration system, an arrangement for gravitational separation of the larger impurities from a refrigerant.

scan interval

an indication of how often scans are initiated. Throughput rate is the maximum rate the entire data acquisition system can accept, process, and transfer data.

scan rate

an indication of how fast a recording instrument proceeds from measurement to measurement within a given scan.


standard cubic feet per minute.

schematic design drawing

drawing illustrating the relationship of project components and/or the fundamental outline of a functional system.

Schmidt number

dimensionless number consisting of the mass transfer coefficient times a characteristic length divided by the mass diffusivity. Symbol Sc, Sc, or NSc. See Prandtl number.

Scotch boiler

fire tube boiler consisting of a cylindrical shell with one or more cylindrical internal furnaces in the lower portion and a bank of tubes attached to both end closures.

scraped heat exchanger

exchanger having a surface on which a fluid can be solidified and the solid layer scraped off.

scraped-surface freezer

apparatus in which a liquid freezes on the surface of a cooled drum to form a thin layer which is removed by a scraper.

scraper ring

piston ring of special section, designed to prevent oil from the crankcase entering the cylinder head.

screw compressor

a positive displacement rotary compressor that produces compression with two intermeshing helical rotors.

scroll compressor

positive displacement compressor in which the reduction in internal volume of the compression chamber is accomplished by an orbiting scroll (involute spiral) within a mating stationary scroll.

scrubber (air washer)

(1) device for reducing the CO2 content of a controlled-atmosphere storage room. (2) system to reduce noxious substances from a flowing stream, as in chimneys or process discharges.


device to prevent the passage of a gas or liquid into or out of a pipe, container, or along a shaft.

seal cap

a gasketed secondary containment cover for valve stems.

sealed compressor motor

hermetic unit with compressor and motor enclosed in the same housing without external shaft or shaft seals with the motor operating in the refrigerant.

sealed-unit compressor
seamless drawn tube

tube produced by drawing and forming operations from a pierced bar or hollow billet.


simplified energy analysis procedure

seasonal coefficient of performance cooling (SCOPC)

the total cooling output of an air conditioner during its normal annual usage period for cooling divided by the total electric energy input during the same period in consistent units (analogous to the SEER but in I-P or other consistent units).

seasonal coefficient of performance—heating (SCOPH)

the total heating output of a heat pump during its normal annual usage period for heating divided by the total electric energy input during the same period in consistent units (analogous to HSPF but in I-P or other consistent units).

seasonal energy efficiency ratio – air-source combined appliance (SEERca )

for the cooling season, the ratio of the total heat removed from the conditioned space to the total electrical energy input required to remove that heat, evaluated over all appliance operating modes. The quantity is expressed in units of Btu/Wh.

seasonal energy efficiency ratio – cooling only (SEER)

for the cooling season, the ratio of the total heat removed from the conditioned space to the total electrical energy input if the combined appliance operated exclusively in a space-cooling-only (COOL) mode. The quantity is expressed in units of Btu/Wh.

secondary air

(1) air for combustion supplied to the furnace to supplement the primary air (after ignition). (2) air prevailing in a treated space. (3) air used to reject heat to an ambient environment or elsewhere. (4) primary air recirculated through a terminal unit.

secondary condenser

condenser cooled by the evaporator of a secondary system.

secondary coolant (secondary fluid)

any liquid used for the transmission of heat without vaporization.

secondary fluid

a fluid of known properties (e.g., water, steam, or brine) that is used as a heating medium.

secondary refrigerant

(1) a volatile refrigerant (usually a single refrigerant or an azeotropic mixture) of known properties that is used as a heating medium. (2) volatile or nonvolatile substance in an indirect refrigerating system that absorbs heat from a substance in space to be refrigerated and transfers this heat to the evaporator of the refrigerating system.

secondary surface

(also known as indirect surface), in extended surface exchangers, the additional surface that is not in direct contact with both the heat-absorbing and heat-emitting media.

secondary transfer fluid

(1) the fluid that flows through the condenser as a coolant. (2) fluid that flows through a heat exchanger that transfers thermal energy from the primary transfer fluid to the application that uses solar energy.

seebeck effect

generation of an emf induced by a difference in temperature at the junctions of two dissimilar materials selected for their emf characteristics and temperature limits.

self-contained system

a complete, factory-assembled and factory-tested system that is shipped in one or more sections and has no refrigerant-containing parts that are joined in the field by other than companion or block valves.

self-contained, mechanically refrigerated drinking-water cooler

a factory-made assembly in one structure that includes a complete mechanical refrigerating system and that has the primary function of cooling potable water and also provides for dispensing such water, by either integral or remote means or both.

self-powered control (self-operated control, system-powered control)

method of control which requires no external or auxiliary power supply.

self-renewable air cleaner

devices that incorporate means to maintain performance at a nearly constant level or within preestablished limits. Examples are roll filters, filters equipped with vacuum-cleaning systems, and self-washing electronic air cleaners. The self-renewal action is accomplished by means incorporated within the device itself, actuation may be manual or automatic.

semi-synthetic lubricant

a mixture of synthetic and mineral-based lubricants.

semiautomatic changeover

selection of heating or cooling operation by means of a combination of manual and automatic switches, which usually are located at the room thermostat.

semiautomatic defrosting

defrost system in which the defrost cycle is started manually and ends automatically with automatic resumption of normal refrigeration at the end of defrost operation.

semiheated space

an enclosed space within a building that is heated by a heating system whose output capacity is greater than or equal to 3.4 Btu/h·ft2 (10 W/m2) of floor area but is not a conditioned space.

semihermetic compressor

a motor compressor assembly contained within a gas tight housing that is sealed by gasketed joints to provide access for servicing internal parts.

semipackaged primary indirect evaporative cooler (semipackaged primary IEC)

a component an indirect evaporative cooler provided with a primary air-moving device, that delivers primary air. This device also includes the entire water distribution, collection, and recirculation system with pump and piping.

semipackaged secondary indirect evaporative cooler (semipackaged secondary IEC)

a component an indirect evaporative cooler with integrated or nonintegrated primary and secondary air passages and provided with a secondary air moving device, which delivers secondary air. This device also includes the entire water distribution, collection, and recirculation system with pump and piping.

sensible capacity

the rate, expressed in Btu/h (W), at which the fan coil under test reduces or increases the dry-bulb temperature of the air passing through it.

sensible cooling capacity

the rate, expressed in W (Btu/h), at which the equipment lowers the dry-bulb temperature (removes sensible heat) of the air passing through it under specified conditions of operation.

sensible cooling effect

(1) amount of sensible heat removed from air during cooling. (2) difference between total cooling effect and dehumidifying effect.

sensible cooling panel

a panel designed for sensible cooling of an indoor space through heat transfer to the thermally effective panel surfaces from the occupants and/or indoor space by thermal radiation and natural convection.

sensible heat

the energy exchanged by a thermodynamic system that relates to a change of temperature.

sensible heat ratio

ratio of the sensible heat to the sensible plus latent heat to be removed from a conditioned space.

sensible heat ratio (SHR)

[also known as sensible heat factor (SHF)], the ratio of sensible heat transfer to total (sensible + latent) heat transfer for a process. Also see sensible heat and latent heat.

sensible heating capacity

the rate, expressed in Btu/h (W), at which the equipment raises the dry-bulb temperature (adds sensible heat) of the air passing through it under specified conditions of operation.

sensible heating panel

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


a measure of the smallest incremental change to which an instrument can respond.

sensitivity ratio

in instrumentation, ratio of a change in output magnitude to the change of input that causes it after the steady state has been reached.


device or instrument designed to detect and measure a variable.

separator filter

vessel that removes solids and entrained liquid from a liquid or gas stream using a combination of a baffle or a coalescer with a screening (filtering) element.

sequence of operation

(1) an organized narration specifying how the integrated functions of a device, system, or facility will perform. It should incorporate energy efficiency and environmental concerns with detailed, comprehensive control strategies, i.e., how each individual piece of equipment will be controlled and what information and adjustment will be available to the user. These may be provided in a combination of narratives, diagrams, and point lists for every unique type of equipment and for each system. (2) one in which successive operations are carried out in a sequence, either predetermined or resulting from the correct execution of each successive operation.


instrument that provides sequence control.

series fan-powered terminal

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

series perimeter loop-heating system

hot-water heating system in which each radiator is connected in series with the next and all flow returns to the boiler in the loop. Also known as a single pipe or one pipe system.

service agency

an agency capable of providing calibration, testing, or manufacture of equipment, instrumentation, metering, or control apparatus, such as a contractor, laboratory, or manufacturer.

service water heating

pertaining to the ease with which equipment in buildings can be serviced for maintenance purposes.

servo control

mechanism that automatically actuates a system according to the variation of input. The device is designed so that a small input power controls a much larger output power.

servo system

combination of devices for controlling a source of power in which the output (or some function thereof) is fed back and compared to some reference at the input, the difference of this comparison being used to effect the desired control.

set pressure

the pressure at which a pressure relief device or pressure control is set to operate.


reduction of heating (by reducing the setpoint) or cooling (by increasing the setpoint) during hours when a building is unoccupied or during periods when lesser demand is acceptable.


point at which the desired temperature (°F [°C]) of the heated or cooled space is set.

shading coefficient

ratio of absorbed and transmitted solar heat relative to fenestration fitted with shading devices to that occurring with unshaded single-strength glass.

shaft pressurization system

a type of smoke control system that is intended to positively pressurize stair and/or elevator shafts with outdoor air by using fans to keep smoke from contaminating the shafts during an alarm condition.

shaft ventilation

natural ventilation by means of a duct mounted vertically or near vertically.


where used in standards, codes, and regulations, the term is meant to indicate a required or mandatory compliance with codes or regulations.

shape factor

radiation angle factor.

sharp freezer

cold storage freezer room, generally kept at –30°F to –10°F (–34°C to –23°C), to receive unfrozen goods and freeze them.

shell-and-coil condenser

condenser in which the cooling medium circulates in a coil located in a shell containing the condensing refrigerant.

shell-and-coil evaporator

closed, cylindrical shell containing an evaporator coil which is in contact with the liquid to be cooled.

shell-and-tube condenser

condenser consisting of an outer shell containing multiple tubes.

shell-and-tube evaporator

evaporator in which the fluid to be cooled is passed through the tubes, which are immersed in the refrigerant.

shell-and-tube heat exchanger

nest of tubes or pipes, or a coil of tube or pipe, contained in a shell or container. The pipe(s) carries a fluid through it, while the shell is also provided with an inlet and outlet for flow of another fluid.

Sherwood number

dimensionless number consisting of the mass transfer coefficient times a characteristic length divided by the mass diffusivity. Symbols are Sh, Sh, or NSh.


place, building, or room where manufacturing or repairing is done.

shop drawing

drawing, diagram, schedule, and other data specially prepared for the work by the contractor or any sub contractor, manufacturer, supplier, or distributor to illustrate some portion of the work.

short cycling

excessive frequency of starting and stopping in an operating system.

short term exposure limit (STEL)

typically a 15 min. time weighted average (TWA) exposure that should not be exceeded at any time during a work day.

short-circuiting flow

entrainment flow with very poor mixing within the room because much of the supply air leaves the room without mixing with the room air.


where used in standards, codes, and regulations, the term is meant to indicate a provision that is not mandatory but that is recommended as good practice.


the point of operation where the fluid flow rate is zero.

SI (Système Internationale)

international system of units using the metric system, augmented by scientific units used in technology.

SI units (Le Système International d’Unités)

the international agreement on the metric system of units using meters, grams, and other designations, as opposed to I-P units in the Inch-Pound system.

sick building

building in which the indoor air quality is unacceptable to a substantial majority of people exposed. Volatile organic compounds (VOC) of various types and other contaminants are present in concentrations sufficient to act synergistically on many occupants, resulting in a sick building syndrome, usually when concentrations are too high. This syndrome does not conform to a particular illness and is difficult to trace to a specific source. See indoor air quality (IAQ), volatile organic compounds (VOC).

sick building syndrome

the term may apply to a combination of ailments (a syndrome) associated with a place of work, residence, or assembly. Sick building syndrome can be related to a lack of adequate outdoor air ventilation, improper exhaust, ventilation of odors, chemicals or fumes, or poor indoor-air quality. Other sources of sick buildings may be linked to contaminants produced by outgassing of some types of building materials, VOC, bacteria molds, etc. This syndrome does not conform to a particular illness and is difficult to trace to a specific source. See indoor air quality, volatile organic compounds (VOC).

sight glass

(1) a device for direct visual indication of the presence, level, flow, or condition of a confined fluid. (2) transparent tube or window (bull’s eye) used to indicate the liquid level in a boiler, tank, bearing, and similar fluid-containing equipment.

signal converter

device that changes one set of protocols, codes, modes, sequences, or frequencies to a different set. See also transducer.


device or unit installed in air duct systems to reduce air noise in the duct. See sound attenuator.

silica gel desiccant (aerogel)

form of silicon dioxide which absorbs moisture and may be used as a drying agent.

simulation model

a computer model that provides information on the energy-using systems in a building (e.g., HVAC, lighting, occupancy, plug loads, building envelope). The model serves as the input data for a specific computer building energy simulation program, along with weather data. When run, the computer simulation program will predict the energy use and demand in the described building for a time interval specified in the simulation model. Depending on the kind of simulation program and how it is set up to run, various kinds of output may be produced.

single phasing

interruption of any one conductor in a three-phase system.

single-acting compressor

compressor having one compression stroke per revolution of the crank for each cylinder.

single-duct air-handling system

system in which the air, having been conditioned, is distributed to various zones through a single duct.

single-duct terminal unit

a terminal unit supplied with one source of supply/exhaust air. This type of terminal excludes fan-powered terminal units.

single-duct unit

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

single-family house

building containing one or two living units or a townhouse, including a modular house.

single-island canopy hood

a single-island canopy hood is placed over a single appliance or appliance line. It is open on all sides and overhangs the front, rear, and sides of the appliance(s).

single-leaf damper

the flap centrally mounted or at one end (sometimes one or a combination of this damper is used as diverting element).

single-line diagram

a simplified schematic drawing that shows the connection between two or more items. Common multiple connections are shown as one line.

single-stage compression

compression from evaporator to condenser pressure by passing through one compressor stage only.

single-stage control

a control that cycles between two states, typically between on and off (maximum and minimum).

single-vane rotary compressor

sliding-vane compressor with one vane, sliding in a slot in the fixed casing, maintaining continuous contact with the rotor, and separating the suction and the delivery ports.

single-zone system

an HVAC system serving a single HVAC zone.

site solar energy

thermal, chemical, or electrical energy derived from direct conversion of incident solar radiation at the building site and used to offset consumption of purchased fuel or electrical energy supplies.

site-recovered energy

waste energy recovered at the building site that is used to offset consumption of purchased fuel or electrical energy supplies.

skylight well

the shaft from the skylight to the ceiling.

slab insulation

thermal insulation applied under slab on ground construction.

slab thermal insulation

semirigid insulation preformed into rectangular units having a degree of suppleness particularly related to their geometrical dimensions.

sliding vane compressor (rotating vane compressor)

rotary compressor having a rotor aligned eccentrically within a cylindrical housing, with one or several vanes located in slots in the rotor or in the housing.

sling psychrometer

hygrometer of two matched thermometers, one with its bulb wetted and the other dry, capable of being whirled rapidly on a sling to indicate the temperature differences related to relative humidity.


(1) difference between the speed of rotation of a motor and the rotational speed of the alternating current. See also two-phase flow. (2) phenomenon of a fluid slipping past a fixed surface with a finite velocity. A no-slip condition pertains to a boundary condition in a fluid flow problem where the slip velocity is assumed to be zero.

slip joint

method of constructing transverse seams in rectangular sheet metal ducts.

slot diffuser

elongated type of air diffuser consisting of a single or a number of slots.

slow freezing

freezing of produce without attempting to reduce the temperature rapidly below that of the zone of maximum crystallization.


product of decomposition of oil resulting from impurities, moisture, or chemical reactions and favored by excessive temperature. Sludge may be mushy, gummy, or hard.


effect produced by droplets of liquid refrigerant or oil, or a mixture of both, that reach the cylinder of a compressor.


suspension of a solid in a liquid. See ice slurry.

slush ice

small pieces of wet ice, usually wet snow ice or wet flake ice.

small hermetically sealed system

a factory-charged refrigerating system using less than 5 lb (2.3 kg) of refrigerant that is welded, brazed, soldered, or otherwise joined together in such a manner as to create a sealed system typically capable of operating for 10 years without maintenance or repair.


(1) small solid and/or liquid particles produced by incomplete combustion of organic substances, varying in size often in the range of 0.1 to 0.3 µm. (2) the airborne solid and liquid particulates and gases evolved when a material undergoes pyrolysis or combustion, together with the quantity of air that is entrained or otherwise mixed into the mass.

smoke barrier

an approved opening protection device designed to resist the passage of air or smoke that meets the requirements of UL Standard 555S-1983, Leakage Rated Dampers for Use in Smoke Control Systems. A combination fire and smoke damper should also meet the requirements of UL Standard 555-1990, Fire Dampers.

smoke control mode

a predefined operational configuration of a system or device for the purpose of smoke control.

smoke control system

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

smoke control zone

a space within a building enclosed by smoke barriers, including the top and bottom, that is a part of a zoned smoke control system.

smoke exhaust system

a mechanical or gravity system intended to move smoke from the smoke zone to the exterior of the building, including smoke removal, purging, and venting systems, as well as the function of exhaust fans used to reduce the pressure in a smoke zone.

smoke management system

an engineered system that includes all methods that can be used singly or in combination to modify smoke movement.

smoke zone

the smoke control zone in which the fire is located.


black marks on ceilings and air outlets, generally caused by dirt particles suspended in the room air. This dirt is entrained in the mixed airstream and is deposited on the ceiling and outlet.

snap action

response of a switch to very small movements of its actuating button or lever, changing rapidly and positively from one contact position to the other.

snap-in fastener

a connecting means for a component, such as an air-terminal device, where the installation is a hand press fit and retaining means are by counter turned rolls of metal or counterfacing dimple punches.

snaplock joint

longitudinal folded joint used for rectangular sheet metal duct construction in which the seam is snapped together.

Snell's laws of refraction

when light travels from one medium into another, the incident and refracted rays lie in one plane with the normal to the surface, are on opposite sides of the normal, and make angles with the normal whose sines have a constant ratio to one another. Also known as Descartes laws of refraction.

snubber (pulsation suppressor)

a restrictor placed in a liquid or vapor line to suppress pulsating or fluctuating pressures.

soft copper tube

deoxidized and dehydrated, seamless, soft copper tube, thoroughly annealed to ensure good bending and flaring characteristics.

soft soldering

hot joining operation in which the melting temperature of the filler metal is lower than 800°F (450°C). See soldered joint, brazed joint, welded joint.

soft water (softened water)

water free of calcium and magnesium salts.

softening point

temperature at which a solid loses rigidity or consistency in a standardized test procedure.

solar absorptance

ratio of the solar spectrum radiant flux absorbed by a body to that incident on it.

solar collector

a device designed to absorb incident solar radiation and to transfer the energy to a fluid passing through it.

solar collector absorber

part of the solar collector that receives the solar radiation energy and transforms it into thermal energy. It may possess a surface through which energy is transmitted to the transfer fluid, however, the transfer fluid itself can be the absorber.

solar collector aperture area

maximum projected area of a solar collector through which the unconcentrated solar radiant energy is admitted.

solar collector cover

material covering the aperture to provide thermal and environmental protection.

solar collector drain back

system in which the collector fluid is allowed to drain back to storage whenever solar energy is not being collected, i.e., when the fluid circulating pump is not operating.

solar collector drain down

system in which the collector fluid is drained from the system under prescribed circumstances.

solar collector gross area

maximum projected area of the complete collector module including integral mounting means.

solar collector instantaneous efficiency

ratio of the energy removed by the transfer fluid per unit of collector area to the total solar radiation incident on the collector per unit area (aperture or gross) during a test period for which the condition of the test corresponds to the steady state or quasi-steady state.

solar collector loop heater

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

solar collector tilt angle

angle between the horizontal plane and the plane of the collector aperture.

solar collector transfer fluid

fluid that flows through a solar collector.

solar compensator

device that resets the control signal to account for the effect of the sun’s radiation on a temperature-sensing element.

solar distribution fraction

the fraction of total solar radiation transmitted through the window(s) that is absorbed by a given surface or retransmitted (lost) back out the window(s).

solar energy

energy derived directly from the sun’s rays, both as diffuse and beam radiation (excludes indirect methods such as photosynthesis, wind, etc.).

solar energy source

source of thermal, chemical, or electrical energy derived from direct conversion of incident solar radiation at the building site.

solar fraction
solar heat gain

solar energy flowing into a space or structure. Units are expressed in kilowatts (Btu/h).

solar heat gain coefficient (SHGC)

the ratio of the solar heat gain entering the space through the fenestration area to the incident solar radiation. Solar heat gain includes directly transmitted solar heat and absorbed solar radiation, which is then reradiated, conducted, or convected into the space.

solar irradiance

rate at which solar energy is received, per unit area. See also irradiance.

solar loss through window

fraction of total solar radiation transmitted through the window(s) that is reflected by opaque surfaces and retransmitted back out the window(s).

solar noon

instant at which the sun reaches its maximum altitude on the horizon at any given location.

solar radiation

the transmission of radiant energy from the sun.

solar simulator

a source of radiant energy simulating the solar radiation.

solar time

the time of the day as indicated by the apparent position of the sun.

solar-optical properties

spectral, radiant, or luminous transmittance, reflectance, and absorptance within the range of wavelengths characterizing solar radiation, i.e., 300 to 3,000 nm.

soldered joint

a gastight joint formed by joining metal parts with alloys that melt at temperatures not exceeding 800°F (426.5°C) and above 400°F (204.5°C).


a gastight joint formed by a process where a filler metal or alloy is heated to melting temperatures above 400°F (205°C) and below 842°F (450°C), or, by the traditional definition that has been used in the United States, below 800°F (427°C) and distributed between two or more close fitting parts by capillary action. Soldering is used as a general term for the process of creating a soldered joint.

soldering flux

chemical substance that aids the flow of solder and serves to remove and prevent the formation of oxides on the pieces to be joined.

solenoid valve

a valve that is closed (or opened) by gravity, pressure, or spring action and opened (or closed) by the movement of a plunger due to the magnetic action of an electrically energized coil.

solid expansion thermometer

one based on the expansion of a solid substance (usually bimetallic).

solid state

one of the three states or phases of matter characterized by stability of dimensions, relative incompressibility, and molecular motion held to limited oscillation.

solid-state motor protection

protective device using electronic solid-state materials as opposed to conventional electromechanical devices.

solidification point

temperature at which a liquid substance will solidify, but not necessarily crystallize, on removal of heat (usually at standard atmospheric pressure). Compare to freezing point.


quantity of dissolved substance (solute) which is contained in a unit quantity of saturated solution at a given temperature and pressure.


substance which is dissolved in another.


substance capable of dissolving another substance.

sonic thermometer

a device used to measure and display air temperature using phase difference between transmitted and received signals across an acoustic path in the medium.

sonic velocity

speed of a pressure wave in a fluid.


an absorbent or adsorbent medium that remains completely or substantially in a single phase during the process of absorbing or releasing heat by means of a refrigerant. Such media include liquids and solids.


substance absorbed by or adsorbed on a sorbent.


material which extracts one or more substances present in an atmosphere or mixture of gases or liquids with which it is in contact, due to an affinity for such substances.


general term covering both absorption and adsorption.


a physical disturbance, vibration, or frequency transmitted by a solid, liquid, or gas that is capable of being detected by the human ear.

sound attenuation (noise reduction, sound damping, sound deadening)

process in which sound energy is absorbed or otherwise diminished in intensity.

sound attenuator

element inserted into the air-distribution system and intended to reduce the airborne noise in the system. See silencer.

sound decay rate

rate at which the sound pressure level in an enclosed space decreases after the sound source has stopped. It is measured in decibels per second.

sound insulation

acoustical treatment of constructions and equipment such as fan housings, supply ducts, and space enclosures to reduce the amplitude of the sound present by absorbing energy from the wave, or reducing the vibration amplitude by using damping or mass or both to reduce transmitted noise.

sound level

magnitude of a sound or a noise, usually determined by measuring the amplitude of variations in the sound pressure expressed in decibels.

sound power

see sound pressure level or sound power level. Sound power can not be measured directly.

sound power level (Lw)

a level of sound power that is ten times the logarithm to the base 10 of the ratio of the sound power generated by the source to a reference sound power. The reference sound power is 10–12 W or 1 picowatt (pW)

space-cooling-only (COOL) mode

an operating mode that occurs during space cooling when either the refrigerant-to-water heat exchanger water pump has automatically cycled off or the rate of heat transfer to the domestic water has become negligible. With respect to seasonal performance calculations, this mode occurs when the water-heating load is satisfied but a space cooling load remains.

space-heating-only (HEAT) mode

an operating mode that occurs during space heating when either the refrigerant-to-water heat exchanger water pump has automatically cycled off or the rate of heat transfer to the domestic water has become negligible. With respect to seasonal performance calculations, this mode occurs when the water-heating load is satisfied but a space heating load remains.

sparge pipe

pipe or tube containing a series of holes designed to equalize the distribution of one fluid stream into another.

spatial resolution

planar angle defined by the instrument instantaneous field of view, i.e., the resolution element.

specific capacity

a quantity that which a water well can produce per unit of drawdown.

specific enthalpy

enthalpy per unit mass of substance.

specific entropy

entropy per unit mass of a substance.

specific gravity

the ratio of the mass of a volume of liquid at 15.6°C (60°F) to the mass of an equal volume of water at the same temperature.

specific heat (CP)

ratio of the quantity of heat required to raise the temperature of a given mass of any substance one degree to the quantity required to raise the temperature of an equal mass of a standard substance one degree (usually water at 59°F [15°C]). The units are expressed in Btu/lb·°F [J/(kg·K)].

specific humidity

(1) ratio of the mass of water to the total mass of a moist air sample. (2) ratio of the mass of water vapor to total mass of a moist air sample (including water vapor and dry air) in a particular volume. Specific humidity is expressed as a ratio in units of lb of moisture/lb of dry air or grains of moisture/lb of dry air (kilograms of water vapor per kilogram of mixture). See also absolute humidity.

specific infiltration ratio

ratio of infiltration to leakage area, a normalized quantity that indicates the intensity of the weather relative to infiltration.

specific refrigerant flow rate

the refrigerant flow rate in lb/min (g/s) required to produce one ton (one kW) of refrigeration.

specific superheat

(1) difference in specific enthalpies of a pure condensable fluid between vapor at a given temperature above saturation and vapor at the dry saturated state at the same pressure. (2) superheat in a unit quantity of fluid.

specific volume (SV)

volume of a unit mass of a material. Usually expressed in ft3/lb (m3/kg). The reciprocal of density.


statement of a set of requirements to be satisfied by a material, product, system, or service that indicates the procedures for determining whether each of the requirements is satisfied. Note: it is desirable to express the requirements numerically in terms of appropriate units, together with their limits.

spectral thermal emittance

thermal emittance based on the radiant energy emitted per unit wavelength interval (monochromatic radiant energy).

speech interference level (SIL)

average of the sound pressure level measured in the three octave bands centered at 500, 1000, and 2000 hertz.

spinning-disc humidifier

apparatus using centrifugal action to atomize and distribute moisture into an airstream.

spiral fin (helical fin, strip fin)

continuous strip of metal helically wound and attached to a tube.

splash lubrication

lubricating system for a compressor with the oil in the crankcase fed to all moving parts and to cylinder walls by the splashing action of a dip or paddle integral or attached to the crankshaft ends of the connecting rod.

split air-conditioning system

air-conditioning system consisting of equipment provided in more than one assembly or enclosure, usually with supply air-distribution equipment housed separately from refrigerant-condensing equipment.

split condenser

condenser comprising several heat-exchanging components operating on one or several refrigeration systems.

split system
spontaneous ignition temperature

temperature of spontaneous ignition of a product in the absence of a flame, determined under standardized conditions.

spool piece

piece of pipe with flanges on both ends that is inserted into a pipeline in place of a valve or instrument.

spot cooling

cooling the air of a limited portion of an enclosed space without the use of walls or partitions.

spot welding

welding of lapped parts in which fusion is confined to a relatively small circular area. It is generally resistance welding, but may also be gas tungsten arc, gas metal arc, or submerged-arc welding.

spray cooling

cooling by spraying a low-temperature liquid on the object to be cooled.

spray freeze drying

freeze-drying process during which a liquid product is sprayed frozen prior to sublimation of the ice.

spray nozzle

orifice through which liquid is sprayed as in air washers and humidifiers.

spray pond

system for lowering the temperature of water by conductive and evaporative cooling of the water in contact with outside air. Water to be cooled is sprayed by nozzles into the air above a pond of water and allowed to fall into the pond.

spray wall tank

liquid cooling tank whose walls are sprayed with chilled water.

spray-type air cooler

an air cooler, with or without cooling surface, fitted to spray liquid continuously into the airstream.

spray-type air washer

enclosure in which water is sprayed by nozzles into the airstream to wash it.

spray-type evaporator

(1) shell-and-tube evaporator in which the tubes are sprayed by liquid refrigerant. (2) starved evaporator condition when an evaporator does not receive sufficient refrigerant.


(of an air jet in mixing air diffusion), maximum distance (dv) between two vertical planes tangent to a specified isovel and perpendicular to a plane through the center of the core of an air jet. There may be two different spreads, not always equal: one for the left side, the other for the right side (considered when looking at the treated space from the supply air terminal device).

spring range

range through which the signal applied must change to produce total movement of the controlled device from one position to another.


(general) property of a system or element by virtue of which its output will ultimately attain a steady state.

stabilized temperature

a condition in which a temperature is considered to be stabilized when three successive readings taken at intervals of 10% of the previously elapsed duration of the test, but not less than five minute intervals, vary no more than 5°F (3°C).


the portion of the exhaust system downstream of the draft diverter, draft hood, or barometric draft regulator.

stack effect

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

stagnation temperature

the temperature that exists at a stagnation point in a fluid flow. At a stagnation point, the speed of the fluid is zero, and all of the kinetic energy has been converted to internal energy. In incompressible fluid flow and in isentropic compressible flow, the stagnation temperature is equal to the total temperature at all points on the streamline leading to the stagnation point.

stand-alone system

management system that performs all control functions without direction from any other unit.

standard air

(1) air weighing 0.075 lb/ft3 (1.2 kg/m3), which approximates dry air at a temperature of 70°F (21.1°C) and a barometric pressure of 29.92 in. Hg (101.3 kPa). (2) dry air at 20.0°C and 101.324 kPa having a mass density of 1.204 kg/m3.

standard ambient temperature

reference condition for the rating of equipment, usually 68°F (20°C).

standard atmospheric pressure

The pressure of 101.325 kPa (defined by the International Civil Aeronautics Organization), equal to 1.01325 bar, equivalent to the pressure exerted by a column of 760 mm of mercury under standard gravity: g = 9.80665 m/s2. In I-P, units the value is approximately 14.696 psi, or 29.921 inches of mercury at 32°F.

standard barometric pressure

barometric pressure of 14.696 psi, 29.92 in Hg (101.325 kPa).

standard conditions

(1) set of conditions for experimental measurements. To allow comparisons to be made between different sets of data. Standard conditions for temperature and pressure are often necessary to define the standard reference conditions to express the volumes of fluids and other related physical quantities. Standard conditions are most usually defined as a temperature of 68°F (20°C) and an absolute pressure at sea level of 14.696 psi or 29.92 in. Hg (101.325 kPa). (2) set of physical, chemical, or other variables of a substance or system that define an accepted reference state or forms a basis for comparison.

standard cooling efficiency ratio (SCER)

ratio calculated from the capacity and power input values obtained at standard rating conditions.

standard cubic feet per minute (SCFM)

the volumetric rate of flow of air that is at standard conditions.

standard deviation

square root of the average of the squares of the deviations from the mean (root mean square deviation), a measure of the dispersion of any statistical data set of numbers or measurements.

standard rating

capacity in energy units per unit time based on tests performed under standard conditions.

standard rating conditions

rating conditions used as the basis of comparison of performance characteristics.

standard rating cycle

refrigerating cycle as defined by standard conditions.

standard temperature

the temperature that exists by virtue of the internal energy of the air only. If a portion of the internal energy is converted into kinetic energy, the static temperature will be decreased accordingly.

standard units

the common units of measurement, such as therms, cubic feet, gallons, kilowatt hours, tons, etc, may be I-P or SI units.

standby condition

the condition of operation in which reduced or no energy is being utilized.

standby power mode consumption (PW,SB)

the power used by a product or appliance when enabled but in the standby operating mode (Refer to 10 CFR 430).

standing-seam joint

folded joint used for transverse joints in rectangular sheet metal duct construction and for apparatus-casing seams.

Stanton number

dimensionless number used in calculation of forced convection, equal to the heat transfer coefficient of a fluid divided by the product of the specific heat at constant pressure, the fluid density, and the fluid velocity. Symbol St, St, or NSt. Also known as the Margoulis number M.


a set of procedures to be followed in the systematic initial sequencing or energizing of components, devices, equipment, and systems.

starting overload

(1) a device equipped with overload trip mechanisms for its protection. (2) starting a device with a load connected that will require the device to operate outside its designed operating range.


condition or stage in the physical being of matter or material (e.g., solid, liquid, or vapor phases of matter).

state diagram

diagram representing, in a system of suitable coordinates, the thermodynamic equilibrium states of a substance.

static discharge head

the static pressure of a fluid at the outlet of the pumping device, expressed in terms of the height of a column of the fluid or of the height of some manometric fluid that it would support.

static gage pressure

static pressure relative to the atmosphere.

static head

the pressure due to the weight of the fluid above the point of measurement. In a closed system, static head is equal on both sides of the pump.

static pressure

the actual pressure of the fluid, which is associated not with its motion but with its state. The pressure is exerted uniformly throughout the entire fluid. The portion of the fluid pressure which exists by virtue of the degree of compression only. If expressed as gage pressure, it may be negative or positive. In a dynamic system, static pressure is the difference between total and velocity pressures. in. H2O (kPa).

static pressure loss

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

static regain method duct sizing

method in which ducts are sized so that the regain in static pressure due to decreased velocity between two points totally or partially compensates for the frictional resistance between the points.

static suction head

the static pressure of a fluid at the inlet of the pumping device, expressed in terms of the height of a column of the fluid or of the height of some manometric fluid that it would support. It is a positive value. See static suction lift.

static suction lift

the static pressure of a fluid at the inlet of the pumping device, expressed in terms of the height of a column of the fluid or of the height of some manometric fluid that it would support. It is a negative value. (See static suction head).

static temperature

the temperature which exists by virtue of the internal energy of the air only. If a portion of the internal energy is converted into kinetic energy, the static temperature is decreased accordingly.


(1) indication of a device’s operating mode (ON or OFF). (2) state, position, or condition of an item.

status device

normally open, digital, contact device in which contact closure indicates a change of status, e.g., on-off, heating-cooling, day-night, etc.

steady state

state of a system in which movement of matter or energy phenomena are taking place when the various physical phenomena are independent of time.

steady-state condition

the condition existing when a uniform renewal cycle of a self-renewable air cleaner maintains essentially constant performance. Steady state is reached when the system is operating in a uniformly repetitive manner, for example, cycling between two resistance levels.


water in the vapor phase.

steam boiler

enclosed vessel in which water is converted into steam.

steam exhaust

steam discharged from a machine or system.

steam header

(1) horizontal piping connected to a boiler steam outlet. (2) in a location out of a boiler room, where a steam main extends into several branches located close together.

steam jet air ejector

device for air removal from closed vessels in which the operating pressure is less than atmospheric pressure.

steam jet refrigerating system

system in which high-pressure steam, supplied through a nozzle and acting to eject water vapor from the evaporator, maintains the requisite low pressure on one side and produces a high pressure on the other by virtue of compression in a following diffusion passage. Sometimes called an ejector cycle refrigerating system.

steam quality

fraction of vapor in a mixture of liquid and vapor, expressed as a percentage by weight.

steam trap

device for allowing the passage of condensate and preventing the passage of steam, or for allowing the passage of air as well as condensate. Common steam trap types include float and thermostatic (F&T) traps, inverted-bucket traps, and thermodynamic traps.

steam-atomizing burner

burner in which the firing oil is atomized by steam.

steel joist

any structural steel member of a building or structure made of hot-rolled or cold-rolled solid or open-web sections.

steel-framed wall

a wall with a cavity (insulated or otherwise) whose exterior surfaces are separated by steel framing members (i.e., typical steel stud walls and curtain wall systems).

Stefan-Boltzmann law

total energy radiated from a blackbody is proportional to the fourth power of the temperature of the body. Also known as the fourth power law.

step change

an incremental change in a variable, either by design or as the result of an interval between measurement, typically, an incremental change in a control setpoint.

step controller

multiple switch assembly in which a moving element trips multiple output steps successively.

step modulating control

a modulating control that cycles a burner between the reduced input rate and OFF if the heating load is light. If a higher heating load is encountered that cannot be met with the reduced input rate, the control goes into a modulating mode where it either gradually or incrementally increases the input rate to meet the higher heating load. If, at that point, a lower heating load is encountered, the control either gradually or incrementally decreases to the reduced input rate.

stepped piston compound compressor

(also known as differential piston compressor), single acting compressor for compound operation, the lower stage of which is compressed in a chamber in which the larger diameter of the stepped piston moves and the higher stage is compressed in a chamber into the smaller diameter piston chamber.


heating foods or other materials to kill microorganisms, usually to a temperature of 212°F (100°C) or higher.

still air freezing

freezing of produce in a room without mechanical movement of air.

Stirling cycle

theoretical thermodynamic cycle comprising two isothermal processes and two isochoric processes.

stoichiometric reaction

having the precise mass relationship of the elements in a chemical compound, or (quantities of reacting elements or compounds) in the same mass relationship as the theoretical combining mass of the elements involved. See combustion.


device that automatically feeds solid fuel to a boiler or furnace (used mainly with coal).

Stokes’ laws

wavelength of luminescence excited by radiation is always greater than that of the exciting radiation.

stonewall flow

(also known as choking flow), state at which gas flow through a pipe or a centrifugal compressor reaches conditions that significantly impede flow.

stop valve

a device used to shut off the flow.

storage capacity

the amount of thermal energy required to complete one charging cycle of a thermal storage device. The storage capacity will always be a greater value than the usable discharge capacity of the thermal storage device. Compare to discharge capacity.

storage disease

injury to produce occurring during storage. Compare to cold injury.

storage efficiency

discharge capacity divided by charge capacity.

storage factor

(1) ratio of the volume occupied by the quantity of a particular product stored to the maximum that can be stored, taking into account the requirements of that particular product. (2) reciprocal of capacity factor.

storage gas

in controlled-atmosphere storage, a gas or atmosphere with carbon dioxide added.

storage inventory

the amount of usable cooling energy remaining in a thermal storage device at any given time.

storage medium

the material in the storage device, independent of the containing structure, in which the major portion of the energy is stored.

storage water heater

a closed vessel in which water is heated by the combustion of fuels, electricity, or any other source and is stored and withdrawn for use external to the system at pressures not exceeding 160 psig (1102.4 kPa), including the apparatus by which heat is generated and all controls and devices necessary to prevent water temperatures from exceeding 210°F (98.9°C).

stored heat

heat from external or recovered heat sources that is held in reserve for later use.

straddle refrigerating unit (saddle unit, plug unit)

factory assembled refrigerating system mounted at high level in the insulated wall of a cold store with the evaporator inside the store and the rest of the unit outside.

straightening element

a device for withholding foreign matter from a flowing liquid or gas.


division into a series of layers, as with thermal gradients across a fluid in motion or at rest.

stratification index

a parameter that indicates the degree of thermal stratification in a storage device.

stratified airflow

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

stratified fluid

a region of fluid in which the density decreases monotonically in the upward direction and is stably stratified.

stratified storage

thermal storage vessel in which a thermocline exists.

stratified system

an air-distribution system that, during the cooling operation, limits the amount of mixing in the space and instead relies on thermal plumes to produce a stratified environment with cooler and fresher air near the floor and warmer and less fresh air near the ceiling. Examples are underfloor air-distribution system (UFAD) and DV systems.


force applied to a structural member, component, or assembly per unit of its area. See also mechanical properties, pressure vessel.

strong liquor (rich liquor, strong solution)

solvent with a relatively high concentration of dissolved refrigerant.

structural barrier

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

structural thermal insulation

insulation used as a part of the load carrying frame of a structure, such as the walls of a cold room or the body of a refrigerated vehicle.

structure borne noise

sound that arrives at a point of interest by propagation through a solid structure.

stuffing box

(1) device for use where a cable passes into a junction box or other piece of apparatus designed to render the joint watertight. (2) packing gland surrounding a shaft, stem, or rod to prevent leakage.

subacute care

a category of care requiring less intensity of care/resources than acute care. It falls within a continuum of care determined by patient acuity, clinical stability, and resource needs.


part containing terminals and switches, to which a thermostat is attached. It may also contain other auxiliary components.

subcooled liquid

liquid whose temperature is lower than the condensation temperature at its given pressure.


a heat exchanger for cooling liquid refrigerant below its condensing temperature at a given pressure.


at a defined pressure, the difference between a given liquid temperature and the bubble point temperature.

subcooling heat rejection effect

total refrigerant heat rejection effect less the condensing heat rejection effect.

subcooling refrigerating effect

additional refrigeration effect made available by subcooling the refrigerant liquid in the condenser.


change of state directly from solid to gas without passing through a liquid phase.

sublimation front (sublimation interface)

during the sublimation process, the dividing interface within the substance between a region which is fully hydrated and frozen and a region which is nearly completely dry.

submaster controller

(1) controller whose setpoint is automatically readjusted from a remote location over a predetermined range by variations in an applied signal from a master. (2) controller whose setpoint may be automatically reset at the command of either a conventional or master controller.

submerged arc welding

arc welding process wherein coalescence is produced by heating an arc or arcs between a bare metal electrode or electrodes and the work. Note: the arc is shielded by a blanket of granular fusible material on the work. Pressure is not used, and filler metal is obtained from the electrode and sometimes from a supplementary welding rod.

submerged coil condenser

(submerged condenser), condenser in which the piping is submerged in a vessel containing cooling water.


direct measurement of a commodity sold by a utility down line from the utility’s master meter. Submetering includes only that which is measured on the billing, i.e., it includes no distributed costs.

suction head

positive head on a pump inlet when the source of liquid supply is above the pump centerline. See also head.

suction inlet

(1) inlet of a pump. (2) port through which gas enters.

suction lift

combination of static suction lift and friction head in suction piping when the source of liquid is below the pump centerline.

suction line

(1) tube or pipe that carries the refrigerant vapor from the evaporator to the compressor inlet. (2) tube or pipe that connects any line from a store to the suction side of a pump.

suction pressure

(1) fan/pump: the pressure generated on the inlet or intake side of a fan or pump. (2) refrigeration: the intake pressure generated by the system compressor while operating. The suction pressure, along with the suction temperature, the pressure, and the wet-bulb temperature of the discharge air, are used to determine the correct refrigerant charge in the system. It is also called low-side pressure.

suction trap (suction line accumulator, liquid separator)

accumulator installed in the suction line between evaporator and compressor to trap liquid carryover from the evaporator and to prevent it from reaching the compressor.

suction vapor temperature

the temperature of the refrigerant vapor returning to the compressor or condensing unit.

suction-line filter

a device installed in the suction line of a refrigerating system for the purpose of removing solid contaminants. The device generally consists of a shell with fittings and an internal filtering element.

suction-line-filter drier

a suction-line-filter drier consists of a suction-line filter with the addition of desiccant for the purpose of removing moisture and other contaminants.

sun effect (solar heat gain)

solar energy flowing into a building, through both windows and structural materials.


chilling a product to a temperature very near or sometimes below its freezing point.

superconducting state (superconductive state)

low-temperature thermodynamic state in which a superconductor exhibits the property of superconduction.

superconduction (superconductivity)

property of certain substances characterized by their zero DC electrical resistivity for low currents when their temperature is sufficiently low.

superconduction critical temperature

temperature below which, in the absence of all external influences, a substance becomes superconducting.


(1) substance exhibiting or capable of exhibiting superconduction. (2) Type I: superconductor for which the Meissner state and the normal state can coexist in thermodynamic equilibrium. (3) Type II: superconductor capable of existing in the mixed state.

supercooled liquid

one in which the existing temperature is lower than the saturation temperature for the given pressure.


cooling a substance below the normal freezing point without solidification.


state of fluids at pressures and temperatures above their critical values. Also used loosely, and especially for helium, for the state of the liquid at pressures exceeding the vapor pressure.


(1) at a defined pressure, the difference between a given vapor temperature and the dew-point temperature. (2) the difference in temperature between the saturation temperature corresponding to the measured outlet pressure and the measured outlet temperature of the refrigerant vapor leaving the heat exchanger.

superheated steam

steam at a temperature higher than the atmospheric pressure boiling temperature.

superheated vapor

vapor at a temperature greater than the saturation temperature. The pressure and temperature of superheated vapor are independent properties, since the temperature can increase while the pressure remains constant. See superheat.


(1) group of tubes in a boiler that absorb heat from the products of combustion to raise the temperature of the vapor passing through the tubes above the temperature corresponding to its pressure or saturation temperature. (2) heat exchanger used on flooded evaporators, where hot liquid on its way to enter the evaporator is cooled by giving up heat to both dry and superheat the wet vapor leaving the evaporator.

supersaturated air

humid air that contains in a unit mass of dry air a mass of water vapor greater than would be contained in saturated air at the same temperature, the excess water may remain in the form of fog.

supersaturated vapor

vapor in a metastable equilibrium at a pressure higher than the saturation pressure corresponding to its temperature. See supersaturation.


(1) condition of metastable equilibrium in a solution where the solute remains dissolved at a temperature lower than the initial solidification temperature. (2) condition of metastable equilibrium in which a vapor is at a pressure higher than the saturation pressure corresponding to its temperature.

supply air

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

supply mains

(1) (pneumatic) the air supply piping to all controllers or other devices requiring a main air supply. (2) pipes through which the heating or cooling medium of a system flows from the source of heat or refrigeration to the runouts and risers leading to the heating or cooling units. (3) source of electric power to a system.

supply pressure

(1) energy source to a controller or auxiliary device. (2) pressure of supply mains of a pneumatic control system.

surface cooling

method of cooling air or other gas by passing it over cold surfaces.

surface dehumidifier

(1) air-conditioning unit designed primarily for cooling and dehumidifying air by passing the air over cooling coils that are below the dew point of the air. (2) dehumidifier with a surface at a temperature below the dew point.

surface effect

effect caused by entrainment of secondary air when an outlet discharges air directly against, or parallel to, a wall or ceiling.

surface film resistance

the reciprocal of the heat transfer film coefficient, Ri or Ro, in h·ft2·°F/Btu ([m2·K]/W). (Subscripts i and o often denote inside and outside surface film resistances and conductances, respectively.) For convection to occur, the surrounding space must be filled with air or another fluid. If the space is evacuated, heat flow occurs by radiation only.

surface film thermal conductance

time rate of heat flow from a unit area of a surface to its surroundings, induced by a unit temperature difference between the surface and the environment. Note: the environment is a fluid (liquids or gases).

surface reflectance

the fraction of the radiant flux falling on a surface that is reflected.

surface tension

forces in a liquid surface that cause it to remain continuous by the attraction between molecules.

surface thermostat

(1) temperature-controlling device located on a surface to respond to surface temperatures and initiate control actions. (2) automatic control device having a sensing element responsive to the temperature of the surface being controlled.


immediate outside of a thermodynamic system.

swaging tool

tool to expand or “bell out” the end of a tube to fit over another tube of the same diameter, or over a special fitting to accept such a shape.

swamp cooler

slang for evaporative cooler.

sweat joint

type of fitting formed by a melted filler metal alloy flowing by capillary action between the parts to be joined. See soldered joint and brazed joint.

sweep rate

the time rate of change of the frequency df/dt, where f is frequency and t is time.

swept volume

piston displacement.

swing check valve

valve where the action is a disc hinged within the body of the valve, either from one side or from a two-piece center point, to prevent reverse flow. Swing check valves are primarily used in horizontal piping, but some manufacturers allow both vertical (upward flow only) and horizontal piping installations. See check valve.

swing joint

arrangement of fittings and pipe provided for expansion, contraction, or movement in pipelines that allows one or more degrees of movement.


(1) rotating movement produced in a flowing mass of fluid (e.g., when the fluid enters an impeller without any tangential velocity component). (2) vortex caused by suction at the bottom of a shallow fluid tank.


device for making, breaking, or changing the connections in an electrical or pneumatic circuit.

synthetic lubricant

a lubricant compound manufactured from distinct chemical compounds.

synthetic oil

a synthesized material, often derived from petroleum compounds but containing no petroleum oil fractions, that can act as a lubricant.


a combination of equipment and auxiliary devices (e.g., controls, accessories, interconnecting means, and termi­nal elements) by which energy is transformed so it performs a specific function such as HVAC, service water heating, or lighting.

system effect

a phenomenon that occurs in fluid systems (fans, pumps, and distribution systems) that can create undesired conditions that cause reduced capacities in all or part of a system. System effect cannot be measured directly, but estimated calculations can be made.

system effects

conditions in a distribution system that affect fan and pump performance and related testing, adjusting, and balancing work.

system load management

subfunction of equipment (hardware) and programming (software) that implements direct and/or indirect load management, coordinated with other functions of energy management system.

system operation

parameters under which the building or systems operator can adjust components of the system to satisfy occupant comfort or process requirements and the strategy for optimum energy use and minimum maintenance.

system-refrigerating effect

rate of heat removal by the refrigerant in the evaporator of a refrigerating system.

systematic error

an error that persists and cannot be considered as due entirely to chance. Systematic error can be corrected through calibration.

systematic uncertainty

an uncertainty that persists and cannot be considered as due entirely to chance. Systematic errors can be corrected by calibration.

systems manual

a system-focused composite document that includes the operation manual, maintenance manual, and additional information of use to the owner during the occupancy and operations phase.