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
DWV: Drainage, waste and vent.
DAMPER: A device used to control the volume of
air passing through an air outlet, air inlet or
DEATH: The absence of life. Apparent death is
the end of life as indicated by the absence of
heartbeat or breathing. Legal death is the total
absence of activity of the brain, heart, and lungs,
as observed and declared by a physician.
DECIBEL (db): A unit of measure of the intensity
of sound. A decibel is one tenth of a bel; an
increase of 1 bel is approximately double the
loudness of a sound.
DECIPOL: a unit of perceived air quality. Air
on mountains or the sea has a decipol = 0.01;
city air with moderate air pollution has a decipol
= 0.05- 0.03; acceptable indoor air quality has
decipol = 1.4 (for 80% satisfaction)
DEGREE DAY: A unit, based upon temperature difference
and time, used in estimating fuel consumption
and specifying nominal heating load of a building
in winter. For any one-day, when the mean temperature
is less than 65oF, there exist as many degree-days
as there are Fahrenheit degrees difference in
temperature between the mean temperature for the
day and 65oF.
DEHUMIDIFICATION: The condensation of water vapor
from air by cooling below the dew point or removal
of water vapor from air by chemical or physical
DEHUMIDIFIER: (1) An Air cooler or washer used
for lowering the moisture content of the air passing
through it: (2) An absorption or adsorption device
for removing moisture from air.
DEHYDRATION: (1) removal of water vapor from
air by the use of absorbing or adsorbing materials;
(2) removal of water from stored goods.
DEMAND CONTROL: A device that controls the kW
demand level by shedding loads when the kW demand
exceeds a predetermined set point.
return to top
DEMAND CONTROLLED VENTILATION: a system having
sensor-driven control units that turn on ventilation
systems when gas concentrations (e.g., carbon
dioxide) exceed a preset level.
DENSITY: The ratio of the mass of a specimen
of a substance to the volume of the specimen.
DESICCANT: Any absorbent or adsorbent, liquid
or solid, that will remove water or water vapor
from a material. In a refrigeration circuit, the
desiccant should be insoluble in the refrigerant.
DESORPTION: the phenomenon in which an adsorbed
molecule leaves the adsorbent surface.
DEW POINT: The surface temperature at which moisture
will condense out of the air onto a surface; varies
with the relative humidity.
DEWPOINT, APPARATUS: That temperature which would
result if the psychometric process occurring in
a dehumidifier, humidifier or surface-cooler were
carried to the saturation condition of the leaving
air while maintaining the same ratio of sensible
to total heat load in the process.
DEW POINT DEPRESSION: The difference between
dry bulb and dew point temperatures (oF DB- oF
DEW POINT TEMPERATURE: The temperature at which
moist air becomes saturated (100% relative humidity)
with water vapor when cooled at constant pressure.
DIFFERENTIAL: The difference between the points
where a controller turns "on" and "off".
If a thermostat turns a furnace on a 68o and the
differential is 3o, the burner will be turned
off at 71o.
DIFFERENTIAL PRESSURE: The difference in static
pressure between two locations.
return to top
DIFFUSER: A circular, square, or rectangular
air distribution outlet, generally located in
the ceiling and comprised of deflecting members
discharging supply air in various directions and
planes, and arranged to promote mixing of primary
air with secondary room air.
DIFFUSION RATE: A measure of the tendency of one
gas or vapor to disperse into or mix with another
gas or vapor. This rate depends on the density
of the vapor or gas as compared with that of air,
which is given a value of 1.
DILUTION: a mitigation strategy that lowers the
concentration of airborne contaminants by increasing
the fraction of outdoor air in the supply air
DIMORPHIC FUNGUS: A fungus with the ability to
exhibit alternate growth forms, exhibiting yeast-like
cells at 98.6oF (37oC), and exhibiting filamentous
(mold) growth at room temperature.
DISINFECTANT: An agent that frees from infection
by killing the vegetative cells of microorganisms.
DISPERSION: The general term describing systems
consisting of particulate matter suspended in
air or other fluid; also, the mixing and dilution
of contaminant in the ambient environment.
DISPLACEMENT VENTILATION: a type of ventilation
system that uses a plug flow of air to purge occupied
spaces of contaminants. Typically such systems
require large volumes of air moving at very low
velocities. Because air moves directly through
the space rather than mixing with the room air,
supply air temperature at the air handler are
closer to those of the desired conditioned temperature.
DOMESTIC HOT WATER: Potable hot water as distinguished
from hot water used for house heating.
DOSIMETER (DOSE METER): An instrument used to
determine the full-shift exposure a person has
received to a physical hazard.
return to top
DRAFT: A current of air, when referring to the
pressure difference which causes a current of
air or gases to flow through a flue, chimney,
heater, or space; or to a localized effect caused
by one or more factors of high air velocity, low
ambient temperature, or direction of air flow,
whereby more heat is withdrawn from a person's
skin than is normally dissipated.
DROP: The vertical distance between the base
of the outlet and the bottom of the air stream
at the end of the horizontal throw.
DRY BULB, ROOM: The dry bulb (dew point, etc.)
temperature of the conditioned room or space.
DRY BULB TEMPERATURE: The temperature registered
by an ordinary thermometer. The dry bulb temperature
represents the measure of sensible heat, or the
intensity of heat.
DRY BULB TEMPERATURE, ADJUSTED: The average of
the air temperature and the mean radiant temperature
at a given location. The adjusted dry bulb temperature
is approximately equivalent to operative temperature
at air motionless than 80 fpm when the mean radiant
temperature is less than 120oF.
DRY-SCRUBBING: the process of removing heavy
concentrations of gaseous contaminants from an
air stream using adsorbers of chemisorbers.
DUCT: A passageway made of sheet metal or other
suitable material, not necessarily leak tight,
used for conveying air or other gas at low pressures.
DUCT VELOCITY: Air velocity through the duct
cross-section. When solid particulate material
is present in the air stream, the duct velocity
must exceed the minimum transport velocity.
DUST: An air suspension (aerosol) of particles
of any solid material, usually with particle size
less than 100 micrometers(um).
return to top
DX COIL: A direct expansion coil. The refrigerant
liquid turns to vapor in the coil, cooling the
air (or water) flowing across the coil.
DYSPNEA: difficult or labored breathing.
return to top