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Air Diffuser Selection
Surface Effect
An airstream moving adjacent to, or in contact with, a wall or
ceiling surface creates a low-pressure area immediately adjacent
to
that surface, causing the air to remain in contact with the surface
substantially throughout the length of throw. This surface effect,
commonly referred to as the Coanda effect, counteracts the drop
of
a horizontally projected cool airstream.
Ceiling diffusers exhibit a surface effect to a high degree because
a circular air pattern blankets the entire ceiling area surrounding
each outlet. This effect diminishes with a directional discharge
that
does not blanket the full ceiling surface surrounding the outlet.
Linear diffusers, which discharge the airstream in a single direction
across the ceiling, exhibit less surface effect than radial pattern
discharge; however, the effect is greater with longer diffusers
and with
diffusers that have spread accessories at the outlet face. Sidewall
grilles exhibit varying degrees of surface effect, depending on
the
spread of the particular air pattern and the proximity and angle
of
airstream approach to the ceiling.
In many installations, the outlets must be mounted on an exposed
duct and discharge the airstream into free space. In this case,
the airstream entrains air on both its upper and lower surfaces.
As a result,
a higher rate of entrainment is obtained and the throw is shortened
by 30% of the equivalent throw along a surface. Because there
is no
surface effect with diffusers installed on exposed ducts, with
a cooling differential the supply air drops more rapidly toward
the floor
unless the outlet surface provides an upward deflection to the
discharge stream. Conical and louver face diffusers with radial
patterns
normally exhibit this upward deflection. Linear and flush perforated
face diffusers normally do not possess this characteristic.
Temperature Differential
Heated, horizontally projected air rises and then falls as it
cools. Downward projection of cooling air or upward projection
of heated air increases with an increase in the temperature difference.
Similarly, downward projection of heated air and upward
projection of cool air decreases with an increase in the temperature
difference.
Low-temperature supply air (i.e., in the range of 38 to 45°F)
requires special attention to the environment in which the diffusing
and distribution equipment is operating. Cold starts in a saturated
environment will cause condensation in all but specially treated
equipment. Ramp starts, with a gradual decrease of the relative
humidity of the indoor environment, avoid condensation except
with an unusually high internal load or high infiltration.
Sound Level
The sound level from an outlet is a function of its discharge
velocity and the transmission of system noise. For a given air
capacity, a larger outlet has a lower discharge velocity and corresponding
lower generated sound. A larger outlet also allows a higher level
of
sound to pass through the outlet, which may appear as outlet-generated
sound. High-frequency sound can be the result of excessive
outlet velocity but may also be generated in the duct by the moving
airstream. Low-pitched sounds are generally mechanical equipment
sound and/or terminal box or balancing damper sound transmitted
through the duct and outlet to the room.
The cause of the sound can usually be pinpointed as outlet or
system sounds by removing the outlet core during operation. If
the
sound remains essentially unchanged, the system is at fault. If
the
sound is significantly reduced, it may be caused by a highly irregular
velocity profile at the entrance to the diffuser. The velocity
profile should be measured. If the velocity varies less than 10%
in the
air outlet entrance neck, the outlet is causing the noise. If
the velocity profile at the entrance indicates peak velocities
significantly
higher than average, check the manufacturer’s data for the
sound at
the peak velocity. If this rating approximates the observed sound,
the velocity profile in the duct must be corrected to achieve
design
performance. Note that a high-velocity free stream jet does not
cause a high sound level until the jet impinges against an interfering
surface or edge.
Smudging
Smudging is the deposition of dirt particles on the air outlet
or
surface that is contiguous with the outlet. Dirt particles may
be
either in the room air that is entrained in the discharge or in
the air
supply to the outlet. Smudging is more prevalent with ceiling
diffusers and linear diffusers that discharge the air parallel
to the
mounting surface than with grilles that discharge air perpendicular
to the surface.
Dirt from room air is deposited most frequently at the edge of
the
stream, where the entrained air comes in contact with the surface,
rather than at the center of the stream, which tends to wipe the
surface with clean supply air. Edges of the stream occur at interruptions
in the discharge stream, such as at a blank section of a linear
diffuser
or at the corner of a directional rectangular diffuser.
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