Balancing Terms

Air Entrainment:: The flow of room air (secondary air) that is pulled behind a current of primary air from an outlet, creating a mixed-air path.
Air Outlet:: A diffuser, grille, register, or other opening that releases supply air into the conditioned space.
Air Inlet:: A return-air or exhaust-air grill, register or other opening that receives air from the conditioned space or return-air plenum, channeling it into a return-air duct or mixed-air plenum.
Comfort Zone:: The desired temperature and humidity of a conditioned area, generally, between 68 and 79 degrees Fahrenheit and between 20 and 60 percent relative humidity.
Diffuser:: An air-distribution outlet, usually in a ceiling, with various deflectors arranged to promote mixing of primary air.
Draft:: A localized feeling of coolness or warmth caused by one or more of the following: high air velocity, the direction of air flow, or low ambient temperature.
Drop:: The vertical distance that the lower edge of a horizontally projected air stream drops between the outlet and the end of it throw.
Dumping:: The rabid falling of cold air that occurs when a variable air volume (VAV) terminal or other device reduces air flow, and the supply air leaves the diffuser at very low velocities.
Effective Area:: The net area of an air outlet or inlet through which the air can pass. Usually denoted as Ak.
Face Velocity:: The average velocity of the air leaving a supply grille or entering a return- or exhaust inlet.
Grille:: A wall-, ceiling-, or floor-mounted louvered or perforated covering for an air-passage opening.
Occupied Zone:: The conditioned space – from the floor to about six feet above the floor.
Outlet Velocity:: The average velocity of air emerging from an outlet or opening. (The term “face velocity” is sometimes used when referring to air flow from outlet grilles).
Register:: A grille with a built-in or attached damper assembly.
Residual- or Room Velocity:: The air velocity in the occupied zone.
Smudging:: Black marks on ceilings and outlets, generally caused by dirt particles suspended in the room air. This dirt is entrained in the mixed air stream and deposited on the ceiling and outlet.
Spread:: The divergence of the air stream after it leaves the outlet (Fig. 4.1).
Surface Effect:: The effect caused by entrainment of secondary air when an outlet discharges air directly against, or parallel to, a wall or ceiling.
Terminal:: A mixing box, variable-air-volume box, or other device located at the end of the supply duct, from which air is introduced into the conditioned space.
Terminal Velocity:: The maximum air velocity of the mixed air stream at the end of the throw. Throw: The distance an air stream travels after leaving the outlet before it is reduced to its specified terminal velocity,
Vena Contracta:: The smallest area of an air stream leaving an orifice.