Basic Calculations for Occupational Safety and Environmental Health Professionals - Mathematics and Statistics for the Environment - page 408

15.6.1 d eFinitions
Because ventilation is one of the most important environmental and engineering control techniques
used by environmental practitioners, they must be familiar with and understand the following ven-
tilation terms and definitions.
acfm ( actual cubic feet per minute )—Measure of gas flowing at existing temperature and
pressure.
ACH, AC/H (air changes per hour) —The number of times air is replaced in an hour.
Air density —The weight of air in pounds per cubic foot. Dry standard air at a temperature of
68°F (20°C) and barometric pressure 2992 inHg (760 mmHg) has a density of 0.075 lb/ft 3 .
Anemometer —A device that measures the velocity of air. Common types include the swing-
ing vane and the hot-wire anemometer.
Area ( A )—The cross-sectional area through which air moves. Area may refer to the cross-
sectional area of a duct, a window, a door, or any space through which air moves.
Atmospheric pressure —The pressure exerted in all directions by the atmosphere. At sea level,
mean atmospheric pressure is 29.92 inHg, 14.7 psi, 407 in. wg (water gauge), or 760 mmHg.
Branch —In a junction of two ducts, the branch is the duct with the lowest volume flow rate.
The branch usually enters the main at an angle of less than 90.
Canopy hood (receiving hood) —A one- or two-sided overhead hood that receives rising hot
air or gas.
Capture velocity —The velocity of air induced by a hood to capture emitted contaminants
external to the hood.
Coefficient of entry (C e ) —A measure of the efficiency of a hood's ability to convert static
pressure to velocity pressure; the ratio of actual flow to ideal flow.
Density correction factor —A factor applied to correct or convert dry air density of any tem-
perature to velocity pressure; the ratio of actual flow to ideal flow.
Dilution ventilation (general exhaust ventilation) —A form of exposure control that involves
providing enough air in the workplace to dilute the concentration of airborne contaminants
to acceptable levels.
Evase —A cone-shaped exhaust stack that recaptures static pressure from velocity pressure.
Fan —A mechanical device that moves air and creates static pressure.
Fan curve —A curve relating pressure and volume flow rate of a given fan at a fixed fan speed
(rpm).
Fan laws —Relationships that describe theoretical, mutual performance changes in pressure,
flow rate, rpm of the fan, horsepower, density of air, fan size, and power.
Flow rate —Volume flow rates are described by the conservation of mass formula: Q = V × A ,
where Q is volume, V is velocity, and A is the cross-sectional area of air flow.
Friction loss —The static pressure loss in a system caused by friction between moving air and
the duct wall, expressed in wg/100 ft, or fractions of velocity pressure per 100 ft of duct
(mm wg/m; kPa/m).
Gauge pressure —The difference between two absolute pressures, one of which is usually
atmospheric pressure.
Head —Pressure (e.g., “the head is 1 in. wg”),
Hood —A device that encloses, captures, or receives emitted contaminants.
Hood entry loss ( H e )—The static pressure lost (in inches of water) when air enters a duct
through a hood. The majority of the loss usually is associated with a vena contracta
formed in the duct.
Hood static pressure ( SP h )—The sum of the duct velocity pressure and the hood entry loss;
hood static pressure is the static pressure required to accelerate air at rest outside the hood
into the duct at velocity.