Thursday, February 9, 2012

RP-1335 Test Effects of Diffuser Performance

ASHRAE Study 1335 on the effects of typical inlet conditions on ceiling diffusers and their performance began in 2009 at the University of Nevada, Las Vegas (UNLV) laboratories.

The goal of this ASHRAE study was to test and determine the impact on manufactures catalog performance data for ceiling diffusers created by typical field-installed inlet conditions.

ASHRAE Standard 70-2006, is the current “Method of Testing the Performance of Air Outlets and Air Inlets” used by manufactures to obtain catalog performance data. Standard 70 requires diffusers to be tested with a minimum of 3 diameter equivalents of straight duct ahead of the inlet with even flow throughout the duct. 

So the question is, “What happens when diffusers are mounted in buildings and how much variation in performance will we see with typical field installed conditions?”

When building air distribution systems, designers and system installers require accurate quantitative information on how the installed system will perform to achieve optimum efficiency and comfort, and they rely on performance data from manufactures catalogs. But catalog performance reflects perfect inlet conditions. If field installations adjustments are required to the manufactures data due to typical field installation procedures, the extent of these adjustments to the performance data of throw, pressure loss, and sound will be shown in the results of this study.

This study incorporated the performance data of multiple installations using six different types of typical ceiling diffusers. The data in this report compares the performance results of these various field installations to the performance data collected per ASHRAE 70-2006. All diffusers were first tested per ASHRAE 70-2006 to determine the base performance data and then various modifications to the installation were conducted.

Full scale testing was done in the UNLV laboratory with diffusers mounted in various inlet conditions over a large range of flow rates with various duct approach angles, as well as:
  • hard duct vs. flex duct
  • various straight duct heights above the diffuser
  • using an elbow attached to flexible duct
  • close coupling duct installations
In addition, the effect of various dampers connected directly to the diffusers with multiple inlet conditions was also studied.



Installations with flexible duct connections and tight bends affect diffuser performance significantly due to uneven air flow through the diffusers.


The throw from the diffuser is affected with elbows directly attached to the diffuser inlets. The throw from the a diffuser with an 90-degree elbow directly attached to diffusers of all types tested showed an average throw increase in the (forward) direction and a decrease in the (backward) direction. Dampers, depending upon the design, can reduce this asymmetry problem a significant amount. These dampers, however, also add a significant sound increase.


The data shows that a flexible duct elbow to a diffuser as compared to an all metal elbow has a greater pressure loss. This data also shows that the higher the pressure loss of the diffuser, the less the effect of inlet conditions and damper conditions. The average pressure loss increase due to dampers on the face of diffusers is about 50%. This data shows that dampers directly mounted on the diffuser inlet should be avoided.


A very interesting group of information deals with the amount of straight duct required after an elbow to obtain the same performance data as shown in manufactures catalogs.  This data shows that three diameters of straight duct down-stream from the elbow typically resulted in the same sound levels as cataloged and tested by ASHRAE 70-2006. However, elbows directly connected to diffusers typically increase the NC. Dampers also contribute to sound increases. Dampers in the full open condition can add to NC levels. Flex duct elbows averaged higher than rigid duct. The sound data from plaque diffusers was interesting. This data showed that with a fixed diffuser face free area as we have with plaque diffusers, that the increase in sound is not as critical as compared to diffusers of this type with inlets having a greater free area than the diffuser face.       

Close coupling is when diffusers are connected directly below a ducting system. The length of duct between the supply duct and the diffuser was studied. The results were very similar to those seen with elbows to diffusers. Another variable was also seen which deals with the main duct velocity and the pressure loss of the diffuser as related to the velocity pressure in the main supply duct. In general, as the main duct velocity increased greater than the diffuser inlet velocity, the sound from the diffuser increased. Once again, open dampers also increased the sound.


In general, the data show that dampers should be mounted as far up stream form the diffusers as possible. Dampers directly mounted on diffusers cause high velocity air streams on the diffuser cones and can cause significant sound increases.


As part of the report, a set of tables were developed to easily predict how various installation configurations will affect diffuser performance. A set of three reports were given at the Winter ASHRAE 2012 meetings in Chicago, and copies of this data are now available through ASHRAE.

Leon Kloostra - Titus Senior Chief Engineer

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