AIR HANDLING
UNIT FANS
AFAN is an air pump that creates a pressure difference and
causes airflow. The impeller does work on the air, imparting
to it both static and kinetic energy, which vary in proportion,
depending on the fan type.
Fan efficiency ratings are based on ideal conditions; some
fans are rated at more then 90% total efficiency. However,
actual connections
often make it impossible to achieve ideal efficiencies in
the field.RINCIPLES OF OPERATION
All fans produce pressure by altering the velocity vector
of the flow. A fan produces pressure and/or flow because
the rotating blades of the impeller impart kinetic energy
to the air by changing its velocity. Velocity change is
in the tangential and radial velocity components for centrifugal
fans, and in the axial and tangential velocity components
for axial flow fans
The centrifugal return fan pulls air from occupied building
space through the return air ducts. The return fan has airfoil
type blades which are the most efficient among all centrifugal
fan blades. Some of this air is exhausted outside through
the exhaust air damper, while a small amount of it (variable)
continues through the air handling unit to mix with air drawn
in from outdoors. This mixture of outside air and return air,
before additional heating and cooling, is called mixed air
The mixed air is filtered before entering the supply fan.
The airfoil type centrifugal supply fan pushes the air through
the heating and cooling stages of the AHU. The air is then
distributed through a system of ductwork to all areas of
the
Centrifugal fan impellers produce pressure from (1) the
centrifugal force created by rotating the air column contained
between the blades and (2) the kinetic energy imparted to
the air by virtue of its velocity leaving the impeller.
This velocity is a combination of rotative velocity of the
impeller and airspeed relative to the impeller. When the
blades are inclined forward, these two velocities are cumulative;
when backward, oppositional. Backward-curved blade fans
are generally more efficient than forward-curved blade fans.
Axial flow fans produce pressure from the change in velocity
passing through the impeller, with none being produced by
centrifugal force. These fans are divided into three types:
propeller, tube axial, and vaneaxial. Propeller fans, customarily
used at or near free air delivery, usually have a small
hub-to-tip ratio impeller mounted in an orifice plate or
inlet ring. Tubeaxial fans usually have reduced tip clearance
and operate at higher tip speeds, giving them a higher total
pressure capability than the propeller fan. Vaneaxial fans
are essentially tubeaxial fans with guide vanes and reduced
running
Fan selection
Fan Noise
Fan Control
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