Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 409

HVAC (heating, ventilation, and air conditioning) system —Ventilating systems designed pri-
marily to control temperature, humidity, odors, and air quality.
in. wg (inches of water)—A unit of pressure. One inch of water is equal to 0.0735 in. of mer-
cury, or 0.036 psi. Atmospheric pressure at standard conditions is 407 in. wg.
Indoor air quality (IAQ), sick-building syndrome, tight-building syndrome —The study,
examination, and control of air quality related to temperature, humidity, and airborne
contaminants.
Industrial ventilation (IV) —The equipment or operation associated with the supply or exhaust of
air by natural or mechanical means to control occupational hazards in the industrial setting.
Laminar flow (also streamline flow )—Air flow in which air molecules travel parallel to all
other molecules; laminar flow is characterized by the absence of turbulence.
Local exhaust ventilation —An industrial ventilation system that captures and removes emit-
ted contaminants before dilution into the ambient air of the workplace.
Loss —Usually refers to the conversion of static pressure to heat in components of the ventila-
tion system (e.g., hood entry loss).
Manometer —A device that measures pressure difference; usually a U-shaped glass tube con-
taining water or mercury.
Minimum transport velocity ( MTV) —The minimum velocity that will transport particles in a
duct with little settling; MTV varies with air density, particulate loading, and other factors.
Outdoor air (OA) —Outdoor air is the “fresh” air mixed with return air (RA) to dilute con-
taminants in the supply air.
Pitot tube —A device used to measure total and static pressures in an airstream.
Plenum —A low-velocity chamber used to distribute static pressure throughout its interior.
Pressure —Air moves under the influence of differential pressures. A fan is commonly used
to create a difference of pressure in duct systems.
Pressure drop —The loss of static pressure across a point (e.g., “the pressure drop across an
orifice is 2.0 in. wg”).
Replacement air (compensating air, make-up air) —Air supplied to a space to replace
exhausted air.
Return air —Air that is returned from the primary space to the fan for recirculation.
scfm ( standard cubic feet per minute )—A measure of air flow at standard conditions: dry air
at 29.92 in. HG (760 mmHg) (gauge) and 68°F (20°C).
Slot velocity —The average velocity of air through a slot. Slot velocity is calculated by divid-
ing the total volume flow rate by the slot area (usually, V s = 2000 fpm).
Stack —A device on the end of a ventilation system that disperses exhaust contaminants for
dilution by the atmosphere.
Standard air, standard conditions —Dry air at 70°F (20°C), 29.92 in. Hg (760 mmHg), 14.7
psi, 407 in. wg.
Static pressure (SP) —The pressure developed in a duct by a fan; the force in inches of water
measured perpendicular to flow at the wall of the duct; the difference in pressure between
atmospheric pressure and the absolute pressure inside a duct, cleaner, or other equipment.
SP exerts influence in all directions.
Suction pressure —An archaic term that refers to static pressure on the upstream side of the
fan. See static pressure.
Total pressure ( TP )—The pressure exerted in a duct; the sum of the static pressure and the
velocity pressure. Also called impact pressure , dynamic pressure .
Turbulent flow —Air flow characterized by transverse velocity components as well as velocity
in the primary direction of flow in a duct; mixing velocities.
Velocity ( V )—The time rate of movement of air; usually expressed as feet per minute.
Velocity pressure ( VP )—The pressure attributed to the velocity of air.
Volume flow rate ( Q )—Quantity of air flow expressed in cfm, scfm, or acfm.