Brochure

Glossary of Terms
Ceiling or Wall Effect
The tendency of an air stream moving along a wall or ceiling surface
to remain in contact with that surface.
Core Area
The total plane area of that portion of a grille, face, or register
bounded by a line tangent to the outer opening through which air can
pass. The core area is less than the register size. Example, a 14-in. x
8-in. register may have a core that is 1 in. less than the listed size; so,
the core area is 13in. x 7in. – 91 sq. in.
Diffuser
An outlet discharging supply air in a spreading pattern.
Diffusion
Distribution of air within a space by an outlet discharging supply air in
a spreading pattern.
Drop
The vertical distance between the base of the outlet and the bottom of
the air stream at the end of the horizontal throw.
Effective Area, Ak (Sq. Ft.)
The calculated area of an outlet based on the average measured
velocity between the fins.
Envelope
The outer boundary of an air stream moving at a specific velocity (for
example, a 50 fpm envelope).
Free Area
The total minimum area of the openings in the air outlet or inlet
through which air can pass.
Grille
A louvered covering for an opening through which air passes.
Induction
The process of drawing room air into the projected air stream due to
the velocity of the projected air stream (sometimes called aspiration).
Jet Velocity, Fpm (Face Velocity)
The average measured velocity of air passing between the fins.
Natural Convection Currents
Air currents created by a buoyancy effect caused by the difference in
temperature between the room air and the air in contact with a warm
or cold surface.
Outlet
Any opening through which air is delivered to condition a space.
Outlet Velocity, Fpm
The average velocity of the supply air, measured as it passes through
the plane of the opening in the supply outlet.
Pressure Loss, WG
Indicates how much total pressure is required to move air through a
register.
Primary Air
The mixture of supply air from the outlet and room air within the 1 50
fpm envelope.
Radius of Diffusion, Ft.
The horizontal distance (throw) from a ceiling diffuser to the point of
terminal velocity.
Register
A grille which is equipped with a damper or control valve, and which
directs air in a nonspreading jet.
Return
Any opening through which air is removed from a conditioned space.
Spread, Ft.
The maximum width of the total air stream at the point of terminal
velocity.
Static Pressure, PS
The outward force of air within a duct measured in inches of water.
Stratification Boundary
The boundary between room air currents moving faster than 1 5 fpm
and the stratification zone.
Stratified Zone
A region in which room air velocity is less than 1 5 fpm.
Temperature Difterential
The temperature difference between the primary and the room air.
Temperature Variation (∆T)
The temperature difference between points within the same space.
Terminal Velocity, Fpm
When the velocity of total air drops to 50 or 75 fpm, depending on the
particular application, it reaches terminal velocity. Terminal velocity is
not sharply defined for all applications.
Throw (Blow), Ft.
The horizontal distance an air stream travels after leaving a horizontal
sidewall outlet before maximum velocity is reduced to terminal
velocity. For a perimeter outlet, throw is the vertical distance the air
stream travels before maximum velocity is reduced to terminal
velocity.
Total Air
The mixture of projected air and room air set in motion by the supply
air.
Total Pressure, Pt
The sum of the velocity and static pressures measured in inches of
water.
Vane Ratio
The ratio showing depth of vane to minimum width between two
adjacent vanes.
Velocity Pressure, Pv
The forward-moving force of air within a duct measured in inches of
water.
NC Noise Criteria
A single number noise rating system that indicates what Broad Band,
continuous sounds are reasonably acceptable.
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Glossary of Terms