GLOSSARY

‘T’ to ‘Z’

TEMPERATURE, DEWPOINT: The temperature at which the condensation of water vapor in a space begins for a given state of humidity and pressure as the temperature of the vapor is reduced. The temperature corresponding to saturation (100 percent relative humidity) for a given absolute humidity at constant pressure.

TEMPERATURE, DRY-BULB: The temperature of a gas or mixture of gases indicated by an accurate thermometer after correction for radiation.

TEMPERATURE, MEAN RADIANT (MRT): The temperature of a uniform black enclosure in which a solid body or occupant would exchange the same amount of radiant heat as in the existing non-uniform environment.

TEMPERATURE, SATURATION: The temperature at which no further moisture can be added to the air-water vapor mixture. Equals dew point temperature.

TEMPERATURE, WET-BULB: Thermodynamic wet bulb temperature is the temperature at which liquid or solid water, by evaporating into air, can bring the air to saturation adiabatically at the same temperature. Wet bulb temperature (without qualification) is the temperature indicated by a wet bulb psychrometer constructed and used according to specifications.

TERATOGEN: Any substance, agent, or process that blocks normal growth of the fetus, causing one or more developmental abnormalities in the fetus. Teratogens act directly on the developing organism or indirectly, affecting such supplemental structures as the placenta or some maternal system. The type and extent of the defect are determined by the specific kind of teratogen and its mode of action. It also depends on the embryonic process affected, genetic predisposition, and the stage of development at the time the exposure occurred. The period of highest vulnerability in the growing embryo is from about the third through the twelfth week of gestation. The reason is that at this period differentiation of the major organs and systems occurs. Susceptibility to teratogenic influence decreases quickly in the later periods of growth. Among the known teratogens are chemical agents, including drugs, as thalidomide, alkylating agents, and alcohol. Infectious agents, especially the rubella virus and cytomegalovirus have the same effect. Other teratogens include ionizing radiation, particularly x-rays, and environmental factors, as the general health of the mother or any trauma in the uterus that may affect the fetus, especially during the later stages of pregnancy.

TERMINAL VELOCITY: The point at which the discharged air from an outlet decreases to a given speed, generally accepted as 50 feet per minute.

THERM: Measurement used by gas utilities for billing purposes. 1 Therm = 100 cubic feet of gas = 100,000 Btu.

THERMAL COMFORT: That condition of mind which expresses satisfaction with the thermal environment.

THERMAL ENVIRONMENT: Those characteristics of the environment which affect a person's heat loss.

THERMAL SENSATION: A conscious feeling commonly graded into the categories of cold, cool, slightly cool, neutral, slightly warm, warm and hot.

THERMODYNAMIC WET BULB TEMPERATURE: Also called the Adiabatic Saturation Temperature - is that temperature at which water, by evaporating into the air, can bring the air to saturation adiabatically at the same temperature. The wet bulb temperature measured with an appropriate psychrometer can approach the thermodynamic wet bulb temperature.

THERMODYNAMICS, LAWS OF: Two laws upon which rest the classical theory of thermodynamics. These laws have been stated in many different, but equivalent ways.

The First Law:; (1) When work is expanded in generating heat, the quantity of heat produced is proportional to the work expended; and, conversely when heat is employed in the performance of work, the quantity of heat which disappears is proportional to the work done; (2) If a system is caused to change from an initial state to a final state by adiabatic means only, the work done is the same for all adiabatic paths connecting the two states); (3) In any power cycle or refrigeration cycle, the net heat absorbed by the working substance is exactly equal to the net work done. 

The Second Law: (1) It is impossible for a self-acting machine, unaided by an external agency, to convey heat from a body of lower temperature to one of higher temperature); (2) It is impossible to derive mechanical work from heat taken from a body unless there is available a body of lower temperature into which the residue not so used may be discharged); (3) It is impossible to construct an engine that, operating in a cycle, will produce no effect other than the extraction of heat from a reservoir and the performance of an equivalent amount of work.

THERMOPHILE: An organism that grows optimally within the temperature range of 113oF to 140oF (45o to 60oC).

THROTTLING RANGE: Generally applied to pneumatic controls, where it is defined as the change in the controlled temperature which causes the branch line pressure to change from maximum to minimum or vice versa.

THROW: The distance measured in feet that the air stream travels from the outlet to the point of terminal velocity. The throw is measured horizontally from the registers and ceiling diffusers, and vertically from perimeter diffusers.

TIME-WEIGHTED AVERAGE CONCENTRATION (TWA): Refers to concentrations of airborne toxic materials which have been weighted for a certain time duration, usually 8 hours.

TLV. THRESHOLD LIMIT VALUE: A time-weighted average concentration under which most people can work consistently for 8 hours a day, day after day, with no harmful effects. A table of these values and accompanying precautions is published annually by the American Conference of Governmental Industrial Hygienists.

TOLUENE: Hydrocarbon derived mainly from petroleum but also from coal. Sources of TNT, lacquers, saccharin, and many other chemicals.

TOTAL HEAT CONTENT: The sum of sensible heat and latent heat.

TOTAL HEAT (ENTHALPY): Total heat is the sum of the sensible heat and latent heat in an exchange process. In many cases, the addition or subtraction of latent and sensible heat at terminal coils appears simultaneously. Total heat also is called enthalpy, both of which can be defined as the quantity of heat energy contained in that substance.

TOTAL PRESSURE: The sum of the velocity and static pressure measured in inches of water.

TOTAL SUSPENDED PARTICULATE: The weight of particulates suspended in a unit of volume of air when collected by a high volume air sampler.

TOXICITY: 1. The degree to which something is poisonous. 2. A condition that results from exposure to a poison or to poisonous amounts of a substance that does not cause side effects in small amounts.

TOXIGENIC: Producing a toxin.

TOXIN: A substance produced by a living organism that injures tissues or alters the functions of another organism.

TRANSPORT VELOCITY: See Minimum Transport Velocity.

TWA: Time-weighted average.

TWO-POSITION: Essentially, on/off operation, or open/closed.

UNITARY SYSTEM: A room unit which performs part or all of the air conditioning functions. It may or may not be used with a central fan system.

UPPER EXPLOSIVE LIMIT (UEL): The highest concentration (expressed in percent vapor or gas in the air by volume) of a substance that will burn or explode when an ignition source is present.

VANEAXIAL: A type of fan completely contained in a section of round duct.

VAPOR: A substance in gas form, particularly one near equilibrium with its condensed phase and which does not follow the ideal gas laws; in general, any gas below its critical temperature.

VAPOR BARRIER: A moisture-impervious layer applied to the surfaces enclosing a humid space to prevent moisture travel to a point where it may condense due to lower temperature.

VAPOR PRESSURE: Vapor pressure denotes the lowest absolute pressure that a given liquid at a given temperature will remain liquid before evaporating into its gaseous form or state.

VARIABLE AIR VOLUME (VAV): A supply air system that holds the supply air temperature constant but varies supply air flow to compensate for the thermal loads in the space.

VEHICLE: Any contaminated inanimate material, such as water or food, through which microorganisms can be transported or dispersed to a susceptible host.

VELOCITY: A vector quantity which denotes, at once, the time rate and the direction of a linear motion.

VELOCITY, OUTLET: The average discharge velocity of primary air being discharged from the outlet, normally measured in the plane of the opening.

VELOCITY PRESSURE: The pressure produced by the movement of air, such as the wind, or the air coming from a fan.

VELOCITY, TERMINAL: The highest sustained airstream velocity existing in the mixed air path at the end of the throw.

VENTILATION: The process of supplying and removing air by natural or mechanical means to and from any space. Such air may or may not be conditioned.

VIRUS: An infectious agent that contains either RNA or DNA in its core surrounded by a protein shell, is able to alternate between intracellular and extracellular states, and replicates only when present in living cells.

VOC: "Volatile Organic Compund" Any organic chemical reacting with atmospheric ozone. This designation is not a reflection or indication of human health effects.

VOLATILE FUMES: The fumes from substances such as gasoline or alcohol; usually very flammable.

VOLUME: Cubic feet per pound of dry air in the air-water vapor mixture as used in pschrometrics.

VOLUME, SPECIFIC: The volume of a substance per unit mass; the reciprocal of density.

WALL OR CEILING EFFECT: The tendency of an air stream moving along a wall or ceiling surface to remain in contact with that surface. This effect extends the throw and reduces the drop of the air stream.

XEROPHILE: An organism with enzyme systems that enable it to grow optimally in dry places (deserts).

ZONING: The practice of dividing a building into small sections for heating and cooling control. Each section is selected so that one thermostat can be used to determine its requirements. 

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