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68 TRG-TRC013-EN
Glossary
fan-speed control A method of controlling fan capacity by varying its speed of
rotation—commonly accomplished using a variable-speed drive on the fan
motor.
forward curved (FC) A type of centrifugal fan with blades curved in the
direction of wheel rotation.
inlet vanes A device used to control the capacity of a fan by “preswirling” the
air in the direction of fan wheel rotation before it enters the wheel, lessening its
ability to “bite” the air and reducing its airflow capacity.
manometer A device used to measure small pressures within a duct system.
optimized static-pressure control An optimized method of VAV system static-
pressure control that uses the benefits of DDC control to continuously reset the
static-pressure set point of the system, so that the VAV terminal requiring the
highest inlet pressure is nearly wide open.
plenum fan
See
plug fan.
plug fan A type of centrifugal fan; it consists of an unhoused fan wheel with
airfoil fan blades and an inlet cone that pressurizes the plenum surrounding the
fan, allowing the air to discharge in multiple directions.
“riding the fan curve” A method of fan capacity modulation that involves no
direct form of control, but simply allows the fan to react to the change in system
static pressure and “ride” up and down its performance curve.
propeller fan A type of axial fan that is well suited for high volumes of air, but
which has little or no static-pressure generating capability.
SMACNA Sheet Metal and Air Conditioning Contractors’ National Association
sound trap A device installed in an air duct system to control discharge air
noise.
static efficiency The percentage of input power that is realized as useful work
in terms of static energy (pressure).
static pressure Pressure due to the perpendicular outward “push” of the air
against the duct walls.
surge A condition of unstable fan operation where the air alternately flows
backward and forward through the fan wheel, generating noise and vibration.
system effect Turbulence due to fan inlet and outlet restrictions, where the air
is not allowed to establish a uniform velocity profile, influencing fan
performance.
system resistance curve A curve representing the pressure that the system
(including the supply ductwork, duct fittings, terminal units, supply diffusers,
return grilles, coils, filters, dampers, etc.) creates over a range of airflows.