Adding a radial opposed blade damper to a diffuser neck, pictured in Figure 13, to the neck of the
diffuser will not only increase pressure drop, but affect the air pattern as well.
Figure 13: Radial Opposed Blade Damper
The light blue isovel represents the changes to the air pattern after installing a radial opposed blade
damper. The damper has significantly changed the pattern of the perforated diffuser as seen in Figure
14.
Figure 14: Perforated Diffuser
The pattern has morphed into an almost radial pattern, with no distinct air jets. Some directional
diffusers are positioned such that air jets from one diffuser will intersect with air jets from a different
diffuser. This will cause the air to deflect downwards, towards the occupied zone, helping to properly
mix the space. In this case, the change in air pattern could mean that this junction would not occur
creating discomfort to the occupants as the conditioned air would remain above the occupied zone.
The directional diffuser in Figure 15 retained its four-way pattern, however, with a shortened throw and a probable increase in drop.
Figure 15: Directional Diffuser
Once again, the square plaque diffuser remains relatively unchanged, with the pattern only slightly affected by the addition of the damper (Figure 16). The radial opposed blade damper, due to its design, retains a fairly even air distribution across the entire area of the inlet.
Figure 16: Square Plaque Diffuser
Butterfly Damper
The green isovels in the following three figures represent the air patterns produced by the diffusers
with a butterfly damper (Figure 17) installed.
Figure 17: Butterfly Damper
Figure 18 indicates adding a butterfly damper does not affect the pattern of the perforated diffuser, but will nonetheless shift the pattern in a counter clockwise direction.
Figure 18: Perforated Diffuser
The pattern of the directional diffuser is also left unmarred, but as seen in Figure 19, the throw is reduced in all four directions. As in the previous cases, a reduction in throw will, in all likelihood cause an increase in drop which can decrease occupant comfort levels.
Figure 19: Directional Diffuser
The square plaque diffuser experiences a decreased throw as well, potentially resulting in an increased drop, and therefore decreased occupant comfort, depicted in Figure 20.
Figure 20: Square Plaque Diffuser
Radial Damper
Radial dampers,
as shown in Figure 21, operate by limiting the flow through the duct with
bowtie shaped plates. A major disadvantage to using this style of damper is a drastic increase in
pressure drop due to the uneven reduction in cross section of the inlet area.
Figure 21: Radial Damper
The radial damper isovels are displayed in a purple color. Of the 5 inlet conditions presented,
Figure 22 depicts the most profound modification seen to a diffuser throw pattern. The radial damper
has altered the pattern from four distinct jets to a, six-way star like pattern.
Figure 22: Perforated Diffuser
The diffuser retains its four-way pattern, but the addition of the radial damper shortens the throw and consequently increases the drop. Compared to the four previously discussed inlet conditions, the radial damper causes the most dramatic effect on air pattern of the square plaque diffuser, although still much less than with the other diffusers. Similar to earlier examples, the directional diffuser shown in Figure 23 sees a less drastic change in air pattern.
Figure 23: Directional Diffuser
The normally radial pattern of the plaque diffuser (Figure 24) is transformed into a more oblong shape, increasing throw in some directions, and reducing throw in others.
Figure 24: Square Plaque Diffuser
Conclusion
This study has revealed the different degrees to which ceiling diffusers are affected by an assortment of common field inlet conditions.
Depending on the type of diffuser selected, different inlet conditions can have a varying impact on the performance of a diffuser ranging from minor to significantly detrimental. This demonstrates that proper diffuser selection and consideration of field inlet conditions is important. Certain diffusers have more yielding design qualifications, and should be used in instances where less than ideal inlet conditions exist.
Primarily, the square plaque diffuser experienced minor changes to its overall pattern and throw when tested with the five inlet conditions. Conversely, it was shown that the perforated diffuser was the most affected by varying inlet conditions.
By choosing the square plaque diffuser in areas where these inlet conditions are expected and
unavoidable, the risk of potential discomfort for the occupants can be reduced.
For more information on this and other topics, please see the Price Engineer’s HVAC Handbook or email grd@priceindustries.com with specific GRD questions.