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Installation effects on air outlet performance, part I: ideal performance-testing optimization and results.

This paper is based on findings resulting from ASHRAE Research Project RP-1335.


It is widely known that under typical field conditions, ceiling diffusers and other air outlets are typically installed with inlet conditions significantly different from those specified in AHRAE Standard 70-2006 (ASHRAE 2006), resulting in performance differences from manufacturer published performance data. The air duct running to the diffuser may be rigid or flexible, have a hard radius near the diffuser, or have a 90 degree or angled entrance into the device. Also, many have an air-balancing device at or near the diffuser inlet. These inlet conditions can dramatically change the performance of outlets compared to data obtained following Standard 70.

Dynamics of air movement can have an effect on the air distribution system performance. Supply air traveling within ductwork develops considerable momentum. When the supply-air duct empties its air into the air-conditioned space, this momentum is utilized to help mix the supply air with the room air. How air is delivered and removed from a space is known as room air distribution (Rock and Zhu 2002). The spread of air indicates the divergence of the air-stream after leaving the outlet. This knowledge leads into the description of the flow as it propagates away from the diffuser into the four zones of expansion (Spengler et al. 2001). These zones of expansion play a major role in the analysis of the supplemental data recorded during this project.

Installation variations that can result in significant performance variation from ASHRAE Standard 70 (ASHRAE 2006) installation predominantly concern the length and type of the duct branch, how duct turns are accomplished, how the duct approaches the diffuser and the use of an inlet air damper. Without sufficient length to develop a uniform flow profile, the flow in duct branches too close to the VAV (variable-air-volume) terminal or in a previous branch being nonuniform, an increase in pressure loss is often the result. If elbows and junctions, such as those made to avoid obstructions in the path of the ductwork, are not constructed with minimal friction effects, pressure loss and aerodynamically generated noise will increase (ASHRAE 2007).

Each diffuser manufacturer has published sound levels, for a given diffuser and flow rate, under ideal conditions, per ASHRAE Standard 70 (ASHRAE 2006). However, duct connections encountered in the field result in significantly different and usually higher pressure loss and sound levels for the same airflow rate. The magnitude of the performance effects associated with installation variations will also depend on variations out of the control of the installer such as variable flow rates. Diffusers are designed to optimally distribute the air at some particular load condition and air volume, but in a typical VAV type installation, air volume rate will vary. Consequently, diffuser throw, room airflow velocity, and sound levels can be significantly different from that specified design point (ASHRAE 2007).

This research, funded by ASHRAE, Inc. (American Society of Heating, Refrigerating and Air-Conditioning Engineers) under project 1335-RP, identified quantitative data on the performance differences between ASHRAE Standard 70 baseline data (ASHRAE 2006) and typical field installations for ceiling diffusers. The results are organized into three papers covering the step-by-step process taken to determine the quantitative differences between Standard 70 data and typical field installations that take into account different inlet conditions. This paper covers two main aspects of the research. First, the design of a diffuser supply air inlet plenum that exceeds Standard 70 requirements and gives a method for efficient supply plenum design for any throw room. Second, the Standard 70 testing using the optimized plenum, where the baseline data for the output measures (throw, sound, and pressure data) were recorded and analyzed for each type of ceiling diffuser.


ASHRAE Standard 70-2006 is the accepted method of testing the performance of air outlets (ASHRAE 2006). The standard defines laboratory methods of testing air outlets used to terminate ducted and non-ducted systems for distribution and return of building air. For air outlet testing, the standard describes two methods of testing: ducted and plenum. The ducted method can be considered an ideal installation with a long, vertical rigid duct leading to the diffuser. The plenum method is a laboratory simulation of an ideal installation, with the diffuser inlet connected to a large plenum with ideally stagnant air. The plenum method is often the method of choice because it allows for quick change-out of diffusers with different inlet shapes and sizes, and a smaller requirement for vertical space above the ceiling in the test facility.

The standard specifies that construction of the plenum should provide uniform and unidirectional air velocities such that the velocity measured at any location in the entrance plane of the test device must not vary more than 10%. The standard also notes that since practical considerations will limit the shape and volume of the plenum, equalization devices may be required to accomplish the flow uniformity, but does not specify the equalization device construction or placement within the plenum. Also, the designer is left to decide the volume, shape and supply air inlet configuration of the plenum. Thus, the design of the plenum and accompanying flow equalization device has become an art based on knowledge of the physics of airflow and experience in test device design.

The objective was to design and build a plenum that could be used to characterize diffuser performance, as well as develop guidance for other designs with similar flow objectives. The goal for that objective was that the performance of the plenum should not only meet Standard 70 (ASHRAE 2006) specifications, but come as close as possible to ideal. The main quality measure was the level of uniformity of airflow at the entrance of the test diffusers. A secondary objective was to develop and perfect the test methods for measuring diffuser throw and sound generation for the laboratory and the design of experimental methods used to test for parameter and noise variation effects (Fowlkes and Creveling 1995). These testing and experimental design methods were to be used for all the following tests performed in this project.


The one output measure was the airflow velocity across the plane of the diffuser inlet. The quality characteristic derived from this measure was the variation in airflow velocity across the plane. To meet the experimental objectives economically, a Taguchi designed fractional factorial experiment (a reduced experimental subset of a full factorial experiment) was developed using the four test parameters most likely to be used for similar-use plenums and most likely to affect the output (Fowlkes 1995). Other parameters were held constant during the experiment. The fractional factorial experiment does not test every combination of the levels of each parameter together, requiring fewer test runs, while obtaining enough data to accurately analyze the effects of each parameter on the system. To cover the standard range of test airflow rates and diffuser inlet diameters, testing was performed near the two extremes of airflows reported in published performance data for two different inlet diameters. The ideal energy transformation for the plenum is for air to flow into the plenum and exit at the inlet to the diffuser with a uniform vertical velocity and zero horizontal velocity. Thus, any variations from ideal would be variations in either direction or airspeed across the plane of the diffuser inlet. Variations of interest in the experiment would be large enough and across enough of the inlet plane to cause some variation from ideal conditions in diffuser output. Considering a circular diffuser inlet, a variation in flow velocity that would likely result in a significant variation in diffuser output velocity would exist over a 90 degree section of the inlet. This assumes that less extensive variations would tend to diffuse into the flow and not be detectable in the output flow. Therefore, sampling every 90 degrees or four samples around the inlet was used. These measurements were taken equally spaced, approximately 1.5 in. (38 mm) inside of the circumference. To cover velocity variations in the radial direction a center test point was added. Thus, in total, five measurements were made in the inlet plane for each test run. From these measurements a standard deviation for each run condition was determined.

In product optimization, noise conditions are conditions that are not controlled by the designer but are set by the product user or result from external factors not controlled by either the designer or user. In this case, noise is not associated with sound but with nonplenum design factors that cause variation in the output, similar to the meaning of noise in the signal-to-noise ratio. The noise conditions considered that could affect the output were the diffuser inlet size and the volume airflow rate. Tests were conducted with the volume airflow rate at a low and high level that corresponds to the typical low and high levels reported in the diffuser performance specifications. Thus, for an 8 in. (0.2 m) diameter inlet, low flow was 200 cfm (5.7 cmm), high was 800 cfm (23 cmm), and for a 12 in. (305 mm) diameter inlet, low was 400 cfm (11.3 cmm) and high was 1200 cfm (34 cmm).


Test conditions are parameters that the designer can adjust to obtain optimum performance. For the plenum, three parameters determined the design of the flow equalization device in the plenum between the top inlet to the plenum and the inlet to the diffuser. The flow equalization device tested was a circular perforated plate. The flow equalization device parameters included the distance from the plenum top inlet, the size of the flow equalization disk, and the percentage of open area (due to perforation dimension and quantity) of the disk. A fourth plenum design parameter was the ratio of flow from the top to the flow from two inlets on the sides of the plenum. A list of the test parameters and noise conditions is shown in Table 1. A picture of the plenum exterior showing the inlet configuration is shown in Figure 1, where the air flows into the rectangular plenum through all three ducted inlets and out the bottom through a 24 (0.61 m) by 24 in. (0.61 m) square hole. The plenum has dimensions of 72 in. (1.8 m) long by 41 in. (1.04 m) high by 45 in. (1.14 m) wide. The top inlet duct has a 12 in. (0.31 m) diameter and the two side inlet ducts are 10 in. (0.25 m) in diameter. An 18 in. (0.46 m) outlet transition piece is placed between all ducts and the plenum wall. The different types of flow equalization disks and their sizes are shown in Figure 2.
Table 1. Test Parameter and Noise Condition List for Test Array

Parameter           Low State        Mid State       High State

1. Inlet air     All from center  Center and side  Center inlet
configuration    inlet            inlets open      partially

2. Distance of   4 in. (0.1 m)    8 in. (0.2 m)    12 in. (0.31 m)
equalization to
center inlet

3. Size of flow  8 in. (0.2 m)    12 in. (0.31 m)  18 in. (0.46 m)
equalization     diameter         diameter         diameter

4. Open area of  50%              40%              13%

Noise Condition  Low State                         High State

1. Diffuser      8 in. (0.2 m)                     12 in. (0.31 m)
inlet diameter

2. Volume flow   Low                               High


Obviously, there are many more parameters in the plenum design that could affect the inlet velocity variation. Among them are plenum size (three dimensions), diameter of plenum flow inlet ducts, number of plenum flow inlet ducts, design of the inlet cone attached to the diffuser inlet, and length of straight duct between the diffuser inlet and the inlet cone. Based on logical argument, a designer can generally assume that increases in any of these parameters would not result in significant degradation of the results presented here, and could result in some improvement. In this experiment a small plenum with relatively small diameter plenum inlet ducts, one to three plenum inlet ducts, and minimum straight duct between the diffuser inlet and the inlet cone were used, thus providing an economical value for these parameters while still designing to exceeding Standard 70 requirements (ASHRAE 2006).

The diffuser inlet condition used for the experiment was a round inlet formed by the back of a standard perforated diffuser with a short inlet cone attached to the diffuser, as shown in Figure 3. The inlet cone is expected to create a smooth transition of the airflow from zero velocity to the velocity of the diffuser inlet. The core outer diameters were 14 in. (0.36 m) for the 8 in. (0.2 m) inlet, 16 in. (0.41 m) for the 10 in. (0.25 m) inlet, and 18 in. (0.46 m) for the 12 in. (0.31 m) inlet.

Laboratory Instrumentation. All tests for this project were run under steady-state isothermal conditions. Plenum volume airflow was set using a variable-frequency drive fan motor and measured with an Ebtron precision airflow/temperature meter in the supply duct with an installed airflow accuracy of [+ or -]3% of reading and a repeatability of [+ or -]0.25% of reading. Since the output of concern is airspeed variation, the absolute value of the volume airflow (a noise condition) is not critical so long as it is close to the noise condition. In these tests, volume airflow within 5% of the target level was considered acceptable. Each of the five measurements for a test run were an average of measurements taken every two seconds over at least one minute. Airspeed measurements were taken using a TSI VelociCalc Plus Multi-parameter Ventilation Meter with a published accuracy of [+ or -]3% of reading. Measurements taken for this experiment are used for comparison and not for absolute values. The replication variation (from one test to another) of the instrument is anticipated to be less than 1.3%.

Statistical Noise Experiment. Prior to performing the optimization experiment, a noise experiment was performed, where the plenum test parameters are set at a nominal level and the noise factors are varied from high to low to determine the effect of the noise factor levels on the output variation. The noise factors for this test were diffuser inlet size and volume flow rate. The nominal design configuration had all four test parameters set at a mid-state. For the noise test, a full factorial test was performed where all four combinations of diffuser inlet size and volume flow rate were tested. Airspeed data was taken at the five measurement points previously described. The five individual airspeed measurements were normalized by the average of the five measurements. A signal-to-noise ratio (S/N) was calculated for each test run using the formula:

S/N = -10[Log.sub.10][s.sup.2] (1)

where s is the sample standard deviation of the five normalized airspeed measurements.


Signal-to-noise is a standard parameter used for determining the level of output variation due to parameters in the a matrix; in this case the parameters were the two noise factors. The higher the signal-to-noise ratio, the smaller the output variation due to noise. From the test results, it was determined that variation increases significantly with size of inlet and slightly with increased airflow. The goal of the parameter experiment to follow is to determine the parameter levels that achieve the lowest airflow velocity variation under all the noise conditions. As a result, to create a robust design where output is less sensitive to noise effects, parameter optimization testing should include levels of the noise conditions that result in the highest output variation. Therefore, the noise factors and levels chosen for the optimization testing resulted in two noise conditions: High and low airflow with all test runs at the largest diffuser inlet diameter.

Optimization Experiment. A Taguchi Orthogonal Array was used to test the system for all the parameter variations and the noise condition of the two airflow rates (Fowlkes 1995). The L9 array, shown in Table 2, was chosen for this experiment. The array required nine tests and has the ability to evaluate the main effects of four parameters at three levels. The array is balanced by choosing parameter levels such that any condition of any parameter is tested with an equal number of high, mid and low conditions of the other parameters. This testing method reduces interaction effects in the average output and exposes all parameters to the different levels of the other parameters.
Table 2. Test Array with One 4-Level Parameter, Four 3-Level
Parameters, and One Noise Parameter at 2 Levels

                     Parameters                          Noise
Test  Inlet          Flow        Flow         Flow       Low
No.   Configuration  Equalizer   Restriction  Equalizer  Flow,
400   Flow,
                     Distance,                Diameter,  cfm (0.189
                     in. (m)                  in. (m)    cms)

1                 1     4 (0.1)            1    8 (0.2)
2                 1     8 (0.2)            2   12 (0.3)
3                 1    12 (0.3)            3  18 (0.46)
4                 2     4 (0.1)            2  18 (0.46)
5                 2     8 (0.2)            3    8 (0.2)
6                 2    12 (0.3)            1   12 (0.3)
7                 3     4 (0.1)            3   12 (0.3)
8                 3     8 (0.2)            1  18 (0.46)
9                 3    12 (0.3)            2    8 (0.2)

Test  High



Similar to the noise experiment, the five airspeed measurements were normalized by the average of the five measurements for each noise condition and a signal-to-noise ratio (S/N) was calculated for each run from the ten normalized airspeed measurements (five at each noise condition).


The information used to analyze the optimization experiment included the main effects on airspeed variation from each of the four parameters, the interaction between parameters in airspeed variation and the statistical significance of the main effects of each parameter.

Plots of the main effects for the optimization experiment are shown in Figure 4. The plots show the signal-to-noise ratio at the three different levels of each parameter. These results show that inlet flow condition 3 (flow primarily from the side inlets), flow equalizer position 3 (18 in. [0.46 m]) from the top plenum inlet), flow equalizer disk size 3 (18 in. [0.46 m]) diameter disk), and flow equalizer open area 3 (most restrictive) produced the highest signal-to-noise ratio, which should equate to the configuration with the lowest flow velocity variation. In other words, this configuration is predicted to have the least variation in output across the inlet area of the diffuser and at the two test airflow rates.

The significance of the main effects was calculated using an analysis of variance. The results are shown in Table 3 (Note that only two to three digits are significant). The adjusted sum of the squares (Adj SS) shows how much of the total variation was due to the corresponding factor. For example, inlet configuration had an adjusted sum of the squares of 0.000740, the total sequential sum of the squares was 0.00282, thus the fraction of the variation due to inlet configuration was 0.00074/0.00282 or roughly 0.26 or 26%. The F-statistic, the higher the more significant, shows the ratio of variation due to that factor and the variation due to noise when taking into account degrees of freedom. The P-value shows the significance of the factor variation in terms of the probability that this variation could be due to random sampling. Normally, a confidence level of 95%, or P-value of less than 0.05 or 5% is needed to consider results significant. For inlet configuration, the P-value shows a 0.7% chance that the variation was due to random sampling (a 99% confidence level). The summary of this analysis shows that the variation measured for inlet configuration, flow equalization distance, and flow equalization diameter were significant and the variation measured for flow restriction was marginally significant. This gives valuable information on the levels of confidence that should be given to the results.
Table 3. Analysis of Variation of Standard Deviations for the
Optimization Test

Source          Degrees    Adj SS      Adj MS       F      P

Inlet                 2   0.0007396  0.0003698   9.24  0.007

Flow Equalizer        2   0.0006198  0.0003099   7.74  0.011

Flow                  2   0.0003064  0.0001532   3.83  0.063

Flow Equalizer        2   0.0008012  0.0004006  10.01  0.005

Error                 9   0.0003602    0.00004

Total                17  0.00028237


Before going with the levels with the highest signal-to-noise ratios from the main effects, the interaction effects plots were checked to determine if there were any strong interactions that would indicate any inaccuracy in a choice based solely on the main effects. The interaction plots are shown in Figure 5. These plots show a strong interaction between some parameters. Generally speaking, in an interaction plot, parallel lines show no interaction, non-parallel but matched increasing or decreasing lines show moderate interaction, and non-parallel and non-matched increasing or decreasing lines show strong interaction. For example, for interaction between inlet configuration and flow equalization distance, the top left plot, we see non-parallel and non-similar increasing or decreasing lines showing a strong interaction between inlet configuration and flow equalizer distance. At a flow equalization distance of 4 in. (0.1 m), inlet configuration 1 has the highest standard deviation, whereas at a flow equalization distance of 12 in. (0.31 m), inlet configuration 1 has the lowest. This and many of the other interactions could be anticipated on physical grounds since the flow equalization device was primarily in line with the top plenum inlet and thus would be expected to have its greatest effect on inlet configuration 1 and only a small effect on inlet configuration 3.



For the optimum configuration predicted as inlet configuration 3, flow equalization distance 12 in. (0.31 m), flow restriction level 3, and flow equalization disk size 18 in. (0.46 m), the model predicted a standard deviation of 0.31%. Note, however, that the interaction plots showed that although the lowest average standard deviation for the levels of inlet configuration was level 3, the lowest single value of standard deviation occurred with inlet configuration 1, when the other parameters were at the level predicted to be the optimum. Because of the high interaction levels, verification testing was done for inlet configuration at all three levels, while the other three parameters were held at the predicted optimum levels. The predicted and measured results are shown in Table 4. These verification tests show that although all actual measured standard deviations were well within Standard 70 guidelines, they were higher than predicted by the model. Also, instead of plenum inlet configuration 3, configuration 2 had the lowest standard deviation. These inaccuracies in the model can be attributed to the interaction between parameters. The experimentally verified optimum configuration, Test Number 2 in Table 4, had a standard deviation of 0.8158%. This means that any value must be off by slightly over six standard deviations to reach the limit of [+ or -]5 (6 * 0.8158 = 4.8948) percent variation. This happens less than one time per million.
Table 4. Verification Testing Results

                             Test Parameter
Test Number       Inlet           Flow           Flow
              configuration   equalization   restriction
                             distance, in.

1                         1        12 (0.3)            3

2                         2        12 (0.3)            3

3 (predicted              3        12 (0.3)            3


Test Number       Flow       Standard Deviation (%)
              diameter, in.

1                 18 (0.46)                  1.3763

2                 18 (0.46)                  0.8158

3 (predicted             18                  2.6261

Through plenum optimization testing, it was determined that by having the inlet cone directly on the diffuser inlet gave the least amount of variance in flow at the diffuser inlet. However, it was discovered that in the case of a perforated diffuser with an 8 in. (0.2 m) inlet that had 0 in. of added height from cone to diffuser inlet, the throw was significantly decreased when compared to testing using the vertical ducted method of Standard 70. Further investigation showed that a section of 7-10 in. (0.18-0.25 m) of straight duct added to the 8 in. (0.2 m) diffuser inlet resulted in diffuser output identical to using the ducted method. Experimentally, the addition of the straight duct onto the 12 in. (0.31 m) inlet resulted in slightly higher standard deviation of flow velocity across the inlet of the diffuser but improved output performance. Clearly, uniform inlet flow conditions alone do not guarantee ideal performance and the addition of a short vertical duct to the diffuser inlet probably adds a certain stability to the flow, perhaps by reducing any horizontal flow component, improving the plenum method of testing.

One additional, but untested, improvement to flow equalization might be to have a cone with a smooth rate of increase in cross-sectional area such as an exponential horn. This report used a linear cone, with a constant rate of cross-section area increase. That cone was very inexpensive to obtain, yet may have caused flow variation at the diffuser inlet.


Ideal diffuser performance data was collected experimentally for six typical ceiling diffuser types using primarily an airflow supply plenum in accordance with ASHRAE Standard 70. Testing covered two diffuser parameters (diffuser type and inlet diameter) and one system parameter (diffuser inlet neck velocity). From the experimental data, diffuser throw, sound power, and pressure differential across the diffuser were determined for each test condition. These results were used as a baseline for comparison against results from typical field installation conditions.


Output measures and derived output measures included:

1. Room 1/3 octave band sound power level and resulting calculated room NC (noise criteria) level

2. Pressure difference between the inside of the plenum and the test room

3. Diffuser throw distance from center of diffuser

Sound power level in the test room was measured at 1/3 octave bands from 25 to 10,000 Hz. From a subset of those levels, the octave band sound power levels from 125 to 4000 Hz is calculated, which are then used to determine the room noise criteria (NC) level based on ANSI/ASA 12.2 as recommended by ASHRAE Standard 70 (ASHRAE 2006). The total pressure in the plenum is measured according to ASHRAE Standard 70-2006 as the difference between the pressure in an air line with four pressure taps spaced around the perimeter of the inside of the plenum and the pressure in the throw room. The static pressure in the plenum is assumed to be the same as the total pressure. In the few cases where a vertical duct was used for the diffuser inlet instead of the plenum, the static pressure was measured at three duct diameters from the diffuser inlet. Total pressure in the duct was determined by summing the static pressure and the velocity pressure. The plenum was positioned above the ceiling so that the diffuser was installed flush with the suspended ceiling, and ten feet from the nearest wall. There were no obstructions breaking the plane of the ceiling. Air velocity magnitude was measured with a horizontal scan from the diffuser using a vertical array of air velocity sensors shown in Figure 6. The measurement thus covered a vertical plane. That plane was normally perpendicular to the edge of the diffuser, but in a few cases, the maximum throw direction was found to be nonperpendicular to the diffuser edge. The maximum throw direction was identified from results of a scan in the vertical plane parallel to the diffuser edge and 6 feet (1.83 m) downstream. From diffuser throw scans in the maximum throw direction, the maximum velocity at each distance from the diffuser was the maximum velocity measured in the vertical plane from which the 150 (0.76), 100 (0.51), and 50 fpm (0.254 mps) throw distances were determined.

Replicate tests for several cases were conducted using the vertical ducted method described in the ASHRAE Standard 70-2006 (ASHRAE 2006) as a crosscheck of the plenum method used for Standard 70 testing. This method called for a minimum vertical inlet duct length of six diameters and a pressure ring measurement at three diameters from the diffuser inlet. In this setup, the vertical duct should be sufficiently long enough for the flow to stabilize with a uniform velocity cross-section before entering the diffuser. The results showed that the results from the plenum testing were nearly identical to results from the vertical ducted method. Replicate testing was also completed for a few cases to verify the repeatability of the diffuser performance results using the plenum method. All replicate testing had nearly identical results to the original testing.


All tests for this project were run under steady-state conditions. Volume airflow corresponding to the required inlet velocity was set and allowed to stabilize. Testing was performed under isothermal conditions.

The sound signal was captured by the analysis computer and converted to 1/3 octave band sound levels. In post-processing, the 1/3 octave band levels were transformed to 1/1 octave band levels and corrected for a room background sound level. Then, for each octave band a room reverberation correction and a standard room correction of minus 10 dB, to convert from sound pressure to sound power, were applied to the noise criterion curves to obtain the noise criteria level (NC) for the sound generated by the discharge (ASHRAE 2009). The room reverberation correction was calculated two ways and the results averaged. The first way was derived from the time it takes for the sound pressure level in a room to decrease 60 dB and the second was derived from the measured sound level in the room with a calibrated source. It was recommended by ASHRAE, in Standard 70, that the 1/1 octave band levels of most significance were: 125, 250, 500, 1000, 2000, and 4000 Hz center frequencies (ASHRAE 2006).

The noise criterion curves produced by Beranek specify maximum sound levels permitted in each octave band for a specific NC curve (Reynolds 1997). Algorithms based off the NC curve levels for each center frequency determined the overall NC level for each test.

The height of each sensor in the vertical array of air velocity meters is listed in Table 5, where TV17 is the top sensor. The sensor vertical array was scanned in the direction of maximum flow velocity on the diffuser side that had the longest run distance from the center of the diffuser.
Table 5. Velocity Meter Sensor Locations

Sensor Number  Height from Floor  Height from Ceiling
                   (in. [m])           (in. [m])

TV17               107.25 (2.72)          0.75 (0.02)
TV16               104.75 (2.66)          3.25 (0.08)
TV15                 102.5 (2.6)           5.5 (0.14)
TV14                  100 (2.54)              8 (0.2)
TV13                97.75 (2.48)         10.25 (0.26)
TV12                 95.5 (2.43)          12.5 (0.32)
TV11                   93 (2.36)            15 (0.38)
TV10                   90 (2.29)            18 (0.46)
TV9                    85 (2.16)            23 (0.58)
TV8                    78.75 (2)         29.25 (0.74)
TV7                 73.75 (1.87)         34.25 (0.87)
TV6                 67.75 (1.72)         40.25 (1.02)
TV5                    56 (1.42)            52 (1.32)
TV4                    44 (1.12)            64 (1.63)
TV3                    31 (0.79)            77 (1.96)
TV2                    25 (0.64)            83 (2.11)
TV1                    16 (0.41)            92 (2.34)

Inlet duct velocity was the noise factor for the Standard 70 testing. Inlet duct airflow velocities used were 1200 (6.1), 800 (4.1), and 500 fpm (2.54 mps). These velocities cover a large span of typical diffuser flow conditions, including what is typically seen in actual installations. For the test array, this parameter was considered a noise condition because it is not controlled by the installation. Under modern systems, where a VAV unit of some type is used, a typical diffuser will be required to perform under large variations in airflow.

Two design parameters were used in the performance characterization experiment. They were diffuser type and diffuser inlet diameter. The design parameters and noise condition with corresponding levels are shown in Table 6. Pictures of the six different types of diffusers are shown in Figure 7.

A full factorial array was used to set up the experiment. The experimental outputs were used to extract the main effects of the test parameters and the variation of those main effects due to the noise conditions. The test array has 18 runs, one parameter at six levels and one parameter at three levels--specifically, six different diffusers, each at three inlet diameters. There were three sets of output measures (throw, sound level, and pressure loss) at each noise condition. Note that for diffusers that have a square inlet (modular core and louvered), the inlet size is the diameter of the round to square adaptor used.
Table 6. Test Parameters and Noise Conditions with Corresponding States

Parameter   State    State     State 3       State 4     State  State 6
              1        2                               5

1.         Square   Plaque   Perforated  Modular     Round  Louvered
Diffuser                     round neck  core
type                                     perforated

2. Inlet   8 in.    10 in.   12 in.
diameter   (0.2     (0.25    (0.31 m)
           m)       m)

Noise      Low      Medium   High State
Condition  State    State

1.         500 fpm  800 fpm  1200 fpm
Diffuser   (2.54    (4.1     (6.1 mps)
inlet      mps)     mps)

From the diffuser velocity profile data, the data was normalized so that diffusers of the same type with different inlet sizes, at differing velocities, could be compared side-by-side. To do this, the zone plot method of the diffuser velocity data was used. This method is described in Appendix C of ASHRAE Standard 70-2006 (ASHRAE 2006) and is characterized by the zones of expansion from an air outlet. Using this method, the nondimensional diffuser discharge velocity is plotted corresponding to the nondimensional measurement distance. The nondimensional discharge velocity is the ratio of discharge velocity to inlet duct air velocity, [V.sub.x]/[V.sub.k]. The nondimensional measurement distance is the measurement distance divided by the square root of the diffuser neck area, X/[([A.sub.k]).sup.1/2]. The results are plotted on a log-log plot and a linear regression curve is drawn to pass through the data points. For any diffusers characterized by this regression curve, all data points should fall within [+ or -]20% of the line as shown in Figure 8 (ASHRAE 2006).


The log-log plot in Figure 8 shows a zone linear regression line and the corresponding [+ or -]20 percent error lines. This line has been split into three zones labeled 2, 3, and 4. These are standard zones that have a slope based on typical discharge flow velocity phenomena. Zone 1 (not shown in Figure 8) is known as the short zone and was rarely seen during analysis of the experimental data. Zone 2 is called the transition zone and can extend eight to ten diameters from the air outlet. Zone 3 is the zone of fully established turbulent flow and is usually the region that reaches the occupied zone making its importance the greatest. Zone 4 is the terminal zone meaning the residual velocity decays into large-scale turbulence at a rapid rate (Goodfellow and Tahti 2001). The regression equations for each zone are as follows:

Zone 1: y = a * [x.sup.(0)] +b (2)

Zone 2: y = a * [x.sup.(-1/2)] + b (3)

Zone 3: y = a * [x.sup.(-1)] + b (4)

Zone 4: y = a * [x.sup.(-2)] + b (5)

After taking the log of both sides of each equation, notice that the exponent of x is actually the slope of the regression line in each zone, the value a is the y value at x = 1 and b is ideally zero.



The ideal energy transformation is for air to flow into the diffuser inlet and exit at the diffuser discharge with no pressure loss other than velocity pressure, no sound generation and the intended discharge velocity profile. Actual ideal installation measurements show variations from the theoretical ideal output due to flow restrictions in the diffuser, turbulence induced in the flow due to diffuser structure and anomalies in discharge throw due to the geometry of the diffuser design.

Data from the square diffusers are presented as an example of the testing and analysis performed on all the diffusers. For all test runs, airflow velocity meter measurements were used to plot the diffuser discharge velocity versus distance from the diffuser at various heights below the ceiling. Figures 9 and 10 show the results for a 12 in. (0.31 m) square diffuser and an 8 in. (0.2 m) square diffuser respectively. Both are at 1200 fpm (6.1 mps) inlet flow velocity displayed in a linear graph of velocity at increasing distance from the diffuser measured at various distances from the ceiling and also in a velocity contour plot. The contour plot gives a good visual representation of the velocity field for a vertical plane extending from the edge of the diffuser out to the farthest distance air velocity was measured. In the top plot of each figure, the plot line labeled TV17-v is the top velocity sensor closest to the ceiling at 0.75 in. (0.02 m) from the ceiling, while TV16-v is 3.25 in. (0.08 m) from the ceiling. Both plots show that the flow leaving the diffuser outlet is primarily confined to the space near the ceiling. As distance from the diffuser increases, the flow slowly spreads out and mixes with air in the room, resulting in the velocity dropping off and slowly spreading downward. This is a typical ceiling diffuser velocity profile. From this data comes the throw data distance from the diffuser center for the velocity points of 150 (0.76), 100 (0.51), and 50 fpm (0.254 mps).

Also measured was the room noise criteria (NC) level using a rotating boom microphone. In all cases, there were minimum twenty 5-second samples taken over a two-minute time period for each diffuser at each of the three flow rates. Samples were excluded from the average if there was evidence of a temporary background noise above the background noise level. Noise criterion data was not attainable at levels below 21.5 dB due to the background noise level in the throw room. The plenum total pressure was recorded every four seconds during the velocity meter flow testing and averaged for each test run. Note that the results from the testing are for use as a baseline for comparison with field installation testing results and may not match with Standard 70 data published by all manufacturers.




Following are examples of the analysis of two types of diffusers examined for Standard 70 data.

Square Diffuser. The zone plot for all square diffusers tested is shown in Figure 11. The data includes runs with different neck sizes and manufacturers at several different duct air velocities. It is apparent that the throw data fits within the [+ or -]20% margins specified by Standard 70. In this case, the nondimensional data for the different diffusers nearly overlap one another. When comparing the same size of square diffuser from different manufactures, sound data and total pressure data are not very different. Because of the similarity of baseline performance, it is proposed that the differences in performance found in field installations in these manufacturers design of the square diffuser can be used to predict the performance change in other manufacturers diffusers.

Perforated Round Diffuser. Figure 12 shows the data for perforated round neck diffusers of two different manufacturers at various inlet sizes and duct air flow velocities. It is clear that the throw performance in the two manufacturer's designs are different and do not fit in the [+ or -]20% range of the same linear regression. However, all diffusers in the plot have flow in zone 3 over approximately the same horizontal range. Much like the square diffuser, the sound and pressure data for the perforated round diffuser were almost identical for corresponding inlet sizes and manufacturers. Even for this case it is proposed that the differences in performance found in field installations in these manufacturers design of the square diffuser can be used to predict the performance change in other manufacturers' diffusers.


Replicability and verification of the Standard 70 data obtained using the optimized plenum show that the data can be used as a baseline data set for comparison to real field installations. The data conforms to the linear regression described by the zone plot formulation. This standardized, baseline data can be compared to field installation recorded data as an ideal case that will determine how much the field installation varies to that of the Standard 70 data. From this comparison, a set of calculated algorithms can be developed to determine the effect each physical parameter has on the field installations.

Differing manufacturers' diffuser performance data may or may not fit the same zone plot because not all diffuser types perform the same given identical conditions. However, the similarity of the performance of diffusers of the same type should allow using the performance differences of field installations to ideal for one diffuser to be used as a guide to predict the performance differences of other manufacturers' diffusers of the same type. Refer to the latter two articles in the series, part II: "Field Air Outlet Throw and Pressure Loss Performance Difference From Ideal" and part III: "Field Air Outlet Sound Generation Performance Differences From Ideal" for an in-depth analysis of how baseline characterization data captured in part I was used to compare real world field conditions to ideal conditions and produce algorithms for predicting diffuser performance.


Results from this research project show that the Standard 70 plenum method is an accurate and economical method to test for ideal diffuser performance. A method for designing an optimum performance plenum was developed and implemented. The method can be a useful example for future plenum design work and gives experimental data on how to build an optimum plenum. Several findings in the research could be applied to enhancing the guidance given in Standard 70. It is recommended that a statistical guide be given in regards to the allowed variation of airflow velocity at the diffuser inlet. For example, rather than within 10 percent, a standard deviation of 3 percent would result in nearly all measurements within 10% and also 95.4% within 6% and 68% within 3%. This would achieve better flow uniformity. Finally, adding to Standard 70 a provision requiring a rigid vertical neck of one duct diameter or a 10 in. minimum would help prevent cases as discovered in this research where despite diffuser inlet velocity within specifications, diffuser throw performance differed from the vertical ducted method.



ASHRAE. 2006. ASHRAE Standard 70-2006. Method of Testing the Performance of Air Outlets and Air Inlets.

Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

ASHRAE. 2007. ASHRAE Handbook--HVAC Applications, Chapters 47 and 56. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

ASHRAE. 2009. ASHRAE Handbook--Fundamentals. Chapters 32 and 34.1. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Fowlkes, W. and C. Creveling. 1995. Engineering methods for robust product design. Using Taguchi Method in Technology and Product Development. 1st Edition. Reading, MA: Prentice Hall.

Goodfellow, H. D. and Tahti, E. 2001. Industrial Ventilation Guide Book. San Diego, CA: Academic Press.

Reynolds, Douglas. 1997. Engineering Principles of Acoustics and Noise Control. Las Vegas, NV: DDR.

Rock, B.A. and D. Zhu. 2002. Designer's Guide to Ceiling-Based Air Diffusion. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Spengler, J. D., Samet, J. M., and McCarthy, J. F. 2001. Indoor Air Quality Handbook. New York: McGraw-Hill Companies, Inc.

Brian Landsberger, PhD

Associate Member ASHRAE

Douglas Reynolds, PhD

Life Member ASHRAE

Zaccary Poots

Associate Member ASHRAE

Brian Landsberger is manager of Las Vegas Innovative Product Design, LLC, Las Vegas, NV. Douglas Reynolds is a professor at the University of Nevada, Las Vegas, NV, and Zaccary Poots is air distribution engineer at Nailor Industries, Houston, TX.
COPYRIGHT 2012 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
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Author:Landsberger, Brian; Reynolds, Douglas; Poots, Zaccary
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2012
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