HVAC ComparingVAVDuctDesigns

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FINAL REPORTNCEMBT070315

COMPARING VAV DUCT DESIGNS

Brian J. Landsberger, Ph.D.

Liangcai (Tom) Tan, Ph.D.

University Of Nevada, Las Vegas

Davor Novosel

National Center for Energy Management and Building Technologies


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FINALREPORTNCEMBT-070315

NATIONAL CENTER FOR ENERGY MANAGEMENT

AND BUILDING TECHNOLOGIES TASK 3:

COMPARING VAV DUCT DESIGNS

MARCH 2007

Prepared B :y

Brian J. Landsberger, Ph.D.

Liangcai (Tom) Tan, Ph.D.

University Of Nevada, Las Vegas

Davor Novosel


Prepared For:

U.S. Department of Energy

William Haslebacher

Project Officer / Manager

This report was prepared for the U.S. Department of Energy

Under Cooperative Agreement DE-FC26-03GO13072


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NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. Neither the

United States government nor any agency thereof, nor any of their employees, makes any warranty, express or

implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any

information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned

rights. Reference herein to any specific commercial product, process, or service by trade name, trademark,

manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by

the United States government or any agency thereof. The views and opinions of authors expressed herein do not

necessarily state or reflect those of the United States government or any agency thereof.

NATIONAL CENTER FOR ENERGYMANAGEMENT AND BUILDINGTECHNOLOGIES CONTACT

Davor Novosel

Chief Technology Officer


601 North Fairfax Street, Suite 250

Alexandria VA 22314

703-299-5633

[email protected]

www.ncembt.org


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TABLE OF CONTENTS

ACKNOWLEDGEMENTS.......................................................................................................................................... ix

EXECUTIVE SUMMARY.............................................................................................................................................1

1 PROJECT OBJECTIVE.............................................................................................................................................2

2 BACKGROUND.....................................................................................................................................................32.1 Duct Design Issues .......................................................................................................................................3

2.2 Performance Issues ......................................................................................................................................4

2.3 Installation Problems....................................................................................................................................5

3 METHODOLGY......................................................................................................................................................6

3.1 General Approach.........................................................................................................................................6

3.1.1 Identifying the Knowledge Gap ..............................................................................................................6

3.1.2 Design of the Experiment (DoE)..............................................................................................................6

3.1.3 Development of Laboratory and Test Procedures ....................................................................................7

3.1.4 Testing and Industry Guidance...............................................................................................................7

3.2 Aligning project scope with the knowledge gap..............................................................................................8

3.2.1 Installation Variations Identified From Literature Review.........................................................................8

3.2.2 Expert Committee Recommendations ....................................................................................................8

3.2.3 DOE Peer Review of the VAV Duct Design Variations Test Plan ...............................................................11

3.3 System performance characterization and measurement .............................................................................12

3.4 Design of the Experiment ............................................................................................................................14

3.4.1 Parameter Selection............................................................................................................................14

3.4.2 Test Matrix Selection and Modification ................................................................................................16

3.5 Laboratory Design and Instrumentation......................................................................................................20

3.5.1 Laboratory System Modifications And Capabilities ...............................................................................20

3.5.2 Laboratory Instrument Modifications and Capabilities..........................................................................23

3.5.3 Test Conditions, Measurement Setup and Procedures ..........................................................................27

3.5.4 Experimental Error...............................................................................................................................28

3.6 Analysis Procedures...................................................................................................................................29

3.6.1 Parameter Effects On Performance ......................................................................................................30

3.6.2 Airflow Distribution Performance .........................................................................................................30

4 RESULTS ...........................................................................................................................................................31

4.1 Energy Efficiency For Square and Slot Diffusers ...........................................................................................31

4.2 Noise Generation For Square Diffusers ........................................................................................................32

4.3 Air Distribution Variations Resulting From Installation Variations For Square and Slot Diffusers ....................33


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5 DISCUSSION .....................................................................................................................................................34

5.1 Energy Efficiency.........................................................................................................................................34

5.1.1 Square Diffusers .................................................................................................................................34

5.1.2 Slot Diffusers ......................................................................................................................................35

5.2 Sound Levels..............................................................................................................................................36

5.3 Air Distribution ...........................................................................................................................................366 CONCLUSIONS ..................................................................................................................................................38

7 REFERENCES.....................................................................................................................................................39

APPENDIX A - ENERGY EFFICIENCY TEST RESULTS .................................................................................................40

A1. Test Conditions and Results ........................................................................................................................41

A2. Parameter Main Effects Analysis .................................................................................................................45

APPENDIX B - NOISE GENERATION TEST RESULTS.................................................................................................47

B1. Test conditions and Results ........................................................................................................................48

B2. Parameter Main Effects Analysis .................................................................................................................50APPENDIX C - AIR DISTRIBUTION VARIATION TEST RESULTS ..................................................................................52

C1. Airflow Around Diffusers:Square Diffusers...................................................................................................53

C2. Airflow Around Diffusers:Slot Diffusers ........................................................................................................62

C3. Room Airflow Between Diffusers: Square Diffusers.......................................................................................71

C4. Room Airflow Between Diffusers: Slot Diffusers ...........................................................................................89


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LIST OF FIGURES

Figure 1. Schematic of typical branch installation variations ....................................................................................9

Figure 2. Different types of diffusers that are used in terminal duct installations. Top: two 4-foot slot diffusers, one

with a 8 inch round adaptor and one with a 10-inch oval adaptor. Bottom right: typical 2 by 2-foot louvered square

diffuser. Bottom right: Plaque type 2 by 2-foot square diffuser. ..............................................................................10

Figure 3. Schematic of non-ideal energy transformation in the target system ..........................................................12

Figure 4. Schematic of air distribution modifications for VAV system.......................................................................21

Figure 5. Picture of duct showing a hard turn in the duct and a 3-foot vertical entry into the diffuser (low state of

Parameter 4) ........................................................................................................................................................22

Figure 6. Picture of duct showing a 3-foot vertical duct section attached to the diffuser (high state of Parameter 3) .22

Figure 7. Picture of duct showing the duct running horizontal right before attachment to the diffuser (low state of

Parameter 3) ........................................................................................................................................................23

Figure 8. Test room diffuser locations and measurement scan pattern for square diffusers (left) and slot diffusers

(right). ..................................................................................................................................................................24

Figure 9. Measurement scan patterns for slot (upper) and square (lower) diffusers..................................................25

Figure 10. Directional sensitivity for the HT-412 velocity probe. For= 0o, flow is at a right angle to the direction ofthe probe shaft. ....................................................................................................................................................26

Figure 11.Traversing mechanism sensor ................................................................................................................27

Figure 12. Main effects plots for square (left) and slot (right) diffusers for the five test parameters and where the

performance measure is airflow rate ratio..............................................................................................................31

Figure 13. Noise criteria means for square diffusers when noise criteria levels have been adjusted to that estimated

for a standard airflow rate .....................................................................................................................................32

Figure 14. Airflow distribution from the test diffuser for condition 1 and 4 for square diffusers at 100% design airflow

............................................................................................................................................................................33

Figure 15. Airflow rate ratio means for square diffusers..........................................................................................45

Figure 16. Signal to noise ratio for airflow rate ratio for square diffusers. ................................................................45

Figure 17. Airflow rate ratio means for slot diffusers...............................................................................................46

Figure 18. Signal to noise ratio for airflow rate ratio for slot diffusers .....................................................................46

Figure 19. Noise criteria means for square diffusers without adjustment to a standard airflow rate .........................50

Figure 20. Noise criteria means for square diffusers when noise criteria levels have been adjusted to that estimated

for a standard airflow rate .....................................................................................................................................51

Figure 21. Signal to noise ratio for noise criteria for square diffusers when noise criteria levels have been adjusted to

that estimated for a standard airflow rate ..............................................................................................................51

Figure 22. Test condition 0 airflow velocity from diffuser measured in a 2 by 2 foot square pattern around the diffuser.

............................................................................................................................................................................53


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Figure 28. Test condition 6 airflow velocity from diffuser measured in a 2 by 2 foot square pattern around the diffuser.............................................................................................................................................................................59


............................................................................................................................................................................60


............................................................................................................................................................................61

Figure 31. Test condition 0 airflow velocity from diffuser measured across the face from 6 inches away from the

diffuser.................................................................................................................................................................62


diffuser.................................................................................................................................................................63

Figure 33. Test condition 2 airflow velocity from diffuser measured across the face from 6 inches away from thediffuser.................................................................................................................................................................64


diffuser.................................................................................................................................................................65


diffuser.................................................................................................................................................................66


diffuser.................................................................................................................................................................67


diffuser.................................................................................................................................................................68


diffuser.................................................................................................................................................................69


diffuser.................................................................................................................................................................70

Figure 40. Airflow distribution between diffusers for test condition 0, Square diffusers, at 100% design airflow......71

Figure 41. Airflow distribution between diffusers for test condition 0, Square diffusers, at 50% design airflow. ......72


Figure 43. Airflow distribution between diffusers for test condition 1, Square diffusers, at 50% design airflow. .......74





Figure 48. Airflow distribution between diffusers for test condition 4, Square diffusers, at 100% design airflow. ....79



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Figure 55. Airflow distribution between diffusers for test condition 7, Square diffusers, at 50% design airflow. .......86Figure 56. Airflow distribution between diffusers for test condition 8, Square diffusers, at 50% design airflow. .......87


Figure 58. Airflow velocity between diffusers for test condition 0, Slot diffusers, at 100% design airflow..................89

Figure 59. Airflow velocity between diffusers for test condition 0, Slot diffusers, at 50% design airflow....................90

Figure 60. Airflow velocity between diffusers for test condition 1, Slot diffusers, at 100% design airflow. ................ 91



Figure 63. Airflow velocity between diffusers for test condition 2, Slot diffusers, at 50% design airflow....................94Figure 64. Airflow velocity between diffusers for test condition 3, Slot diffusers, at 100% design airflow..................95






Figure 70. Airflow velocity between diffusers for test condition 6, Slot diffusers, at 100% design airflow................101







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LIST OF TABLES

Table 1. Possible Standardized Input Conditions ...................................................................................................11

Table 2. Parameter list for the experimental design following Department of Energy review .....................................15

Table 3. Taguchi Orthogonal L8 Array ....................................................................................................................16Table 4. Parameter List for Test Array ....................................................................................................................16

Table 5. Test Array with One 4-Level Parameter, Four 2-Level Parameters And One Noise Parameter........................17

Table 6. Expected effect of parameter variation on output ......................................................................................19

Table 7. Vertical locations of the traversing mechanism sensors .............................................................................25

Table 8. Airflow Rate Ratio predictions with parameter levels set at levels for high and low performance for square

diffusers ............................................................................................................................................................... 35

Table 9. Airflow Rate Ratio predictions with parameter levels set at levels for high and low performance for slot

diffusers ............................................................................................................................................................... 35

Table 10. Test conditions and results for square diffuser tests ...............................................................................41Table 11. Test conditions and results for square diffuser tests (continued) .............................................................42

Table 12. Test conditions and results for slot diffuser tests.....................................................................................43

Table 13. Test conditions and results for slot diffuser tests (continued)..................................................................44

Table 14. Test conditions and sound results for square diffuser tests......................................................................48

Table 15. Test conditions and sound results for slot diffuser tests...........................................................................49


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ACKNOWLEDGEMENTS

The authors obtained help and guidance from many outside sources during the course of this project. The

collaboration of those people steered this project in the productive direction toward filling essential

industry knowledge gaps and keeping the project focused on those goals. They also were very helpful in

applying practical knowledge on experimental techniques that are particular to ductwork and room

airflow testing. The expert committee members: Ted Carnes, Steve Kimmel, Richard John, Robert

Browning, and Michael Mamayek, contributed their expertise and guided the project team. Also, Dan

Int-Hout contributed multiple times to the success of the experiment testing.


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EXECUTIVE SUMMARY

This task compared the performance of conventional air distribution (CAD) systems, built according to

current design specifications and workmanship standards, with CAD systems that have been built with

common variations in construction and workmanship seen in typical field installations. Variations

examined in this task are those found in the ducted air distribution system from the variable air volume

(VAV) unit to the diffuser.

The results of this project testing revealed the quantitative differences in energy use, sound generation and

room air distribution due to variations in the installation of ductwork and diffusers after the VAV unit.

Specifically, for a constant supply air pressure:

Increasing flex-duct length from 6 to 35 feet decreased airflow by 11 percent

Decreasing diameter of the flex duct from 10 to 8 inches decreased airflow by 25 percent

One hard turn or a kink in the flex-duct near the diffuser decreased airflow by 11 percent andnoticeably increased noise level (5 dB)

The two standard types of diffuser tested had a 9 percent difference in airflow rate

For square diffusers, the absence of a vertical section in the flex-duct right above the diffuser (no

substantial length of vertical duct) actually resulted in a slight increase in airflow compared to aninstallation with a 40 inch vertical section but was accompanied by a significant (6 dB) increase

in flow generated noise and significant asymmetric diffuser discharge airflow.

A branch duct installed immediately after the VAV unit, compared to at least four duct diametersafter the VAV unit, in the square diffuser tests caused a slight decrease in airflow. The same

effect could not be determined from the slot diffuser tests.

Room air diffusion symmetry was affected primarily by the type of diffuser and the approach ofthe flex duct immediately before the diffuser. For square diffusers, the two levels of the flex duct

elbow near the diffuser condition, (1) three feet of vertical duct before the diffuser and (2) three

feet of horizontal duct with only 5 inches of vertical duct before the diffuser, gave significantly

different room air distribution patterns. Apparently the horizontal momentum of the airflow

carried through the diffuser creating an asymmetric airflow discharge pattern. For slot diffusers,which have an internal air plenum, the two levels of the flex duct elbow near the diffuser

condition did not result in significant differences in airflow discharge symmetry. The net effect

on room draft and air distribution performance index (ADPI) was not determined.

These effects were consistent for both the 100 percent design and 50 percent design airflowconditions tested

The tests were conducted in the new environmental systems room at UNLV under repeatable conditions

that replicated expected field installation variations. The test parameters and their test conditions were

determined with the help of the literature review, the advise of an expert panel and the advise of a

Department of Energy review panel. Quantitative effects were determined through the use of an

orthogonal test array. The orthogonal test array gives the main effect of parameter level variation under

both flow conditions in the fewest possible number of tests.

The results confirm many standard held beliefs and results of some previous testing by others, but the

results also show some standard held beliefs are not accurate. The main contribution of this research is

that it provides quantitative results that can be used to make energy efficiency and noise generation design

and installation decisions, and to predict the efficiency and noise levels of an installation.


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1 PROJECT OBJECTIVE

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1 PROJECT OBJECTIVE

This task compared the performance of conventional air distribution (CAD) systems, built according to

current design specifications and workmanship standards, with CAD systems that have been built with

common variations in construction and workmanship seen in typical field installations. Variations

examined in this task are those found in the ducted air distribution system from the variable air volume

(VAV) unit to the diffuser. A test protocol was developed and implemented in the UNLV Building

Technologies Laboratory (BTLab) to measure performance of CAD VAV systems with respect to energy

use, air distribution and acoustics. Test data were collected and analyzed to identify the sensitivity of

CAD systems to the typical variations.

The specific task objectives were:

Identify typical (with potential faults) field installations of ducted CAD VAV systems.

Conduct laboratory airflow, energy, and sound tests on selected typical installations of ductedVAV systems to develop a body of valid engineering design data for these systems.

Upgrade and modify the laboratory facilities at UNLV/CMEST to conduct airflow, energy, andsound tests on ducted CAD VAV systems.

Information gathered by the project team including guidance from the Department of Energy motivated

this research and guided formation of the specific project objectives. Those objectives cover gathering

information on past research that could be used to refine the project objectives and methodology,

identifying industry needs that will be used to define the project scope, design the experiment to fit the

project scope, develop a measurement protocol to meet the experimental design, develop analysis

methods used to determine the effects of installation variation on performance and disseminate this

information to the industry.


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2 BACKGROUND

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2 BACKGROUND

ASHRAE has sponsored several research projects for the purpose of updating friction loss coefficients for

duct fittings in rectangular, round, and flat oval ducts and for improving design methodologies for HVAC

air distribution systems in buildings. That research has helped to significantly improve the design of that

part of HVAC air distribution systems up to the point where branch ducts supply air to variable volume

terminal units and room air terminal devices (grills, registers, and diffusers). Design information on duct

design from variable volume terminal units to room air terminal devices is primarily anecdotal in nature,

and it is predicated on ideal system installations, which seldom if ever occur in field installations. There

are very little measured data concerning the effects on building air distribution, energy, and sound due to

variations in duct installations between air terminal units and room diffusers.

The installation of supply duct and VAV systems raises a number of issues that impact overall air

distribution system efficiency and performance. Although research information on specific details is

limited, there are established guidelines addressing common installation issues. The primary information

sources for VAV duct design are SMACNAs HVAC Systems Duct Design Manual (SMACNA 1990),

California Energy Commissions (CEC) Advanced Variable Air Volume Design Guide (Hydeman 2003),and the ASHRAE Handbook of Fundamentals (ASHRAE 2001). An additional important source is Mr.

Dan Int-Hout, Chief Engineer for Krueger. The purpose of this literature review was to extract published

information on the effect on the performance of the HVAC system of VAV terminal unit-to-diffuser duct

design and installation variations. Specific information sought included duct design issues, performance

issues, and installation problems.

2.1DUCTDESIGN ISSUES

The predominant duct design issues deal with duct leakage, noise and duct contamination.

Duct leakage is a frequently raised issue. Leakage in all unsealed ducts varies considerably with thefabricating machinery used, the methods of assembly, and installation workmanship (ASHRAE 2001). In

the buildings investigated by Xu et al. (Xu 2002), the duct systems leaked more than what is specified by

ASHRAE for unsealed ducts. Air leakage not only increases fan energy consumption and run-time, but

also increases the induced cooling load by the extra heat generated by the fan (Xu 2002). For large

commercial buildings the complete elimination of air leakage from the ducts has an electricity energy

savings potential on the order of 10 kWh/m2 per year. (Modera 2001). However, SMACNA (SMACNA

1990) cautions on the impracticality of obtaining zero leakage.

Noise is also an important consideration in HVAC duct design. Ducts serve as transmitters of break-in

noise while flexible ducts are effective attenuators of upstream noise sources (Int-Hout 1996). Issues with

flexible duct connections at the inlet of the diffuser include increased pressure drop, increased sound

levels, and non-uniform air distribution from the diffuser (Int-Hout 1996). Concerning VAV terminal

design, discharge noise is rarely an issue if the unit has hard duct on the inlet, a lined outlet plenum andflex duct between the plenum and diffusers VAV units above acoustical ceilings should have radiated

Noise Criteria levels no more than ~5 NC above the desired room NC rating. (Hydeman 2003).

Contamination of the duct walls is another important consideration particularly with respect to indoor

environmental quality. According to Hydeman (Hydeman 2003), it is yet to be determined how

significant a health issue duct liner retention of dirt and moisture truly is. However, Foarde et al. (Foarde

1996) acknowledged the contamination of fiberglass duct materials due to soil and moisture. Their study


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results suggest that dust accumulation and/or high humidity should be properly controlled in any HVAC

duct to prevent the growth ofP. chrysogenum (a species of fungus) (Foarde 1996).

2.2PERFORMANCE ISSUES

Variable airflow, proper unit sizing, and minimum airflow settings of VAV systems are the main

performance issues.

The variability of the flow rates common to VAV systems appears in the literature as a common issue.

Diffusers are designed to optimally distribute the air at some particular load condition and air volume.

Thus, the performance of outlets with regard to throw, room velocity and noise levels will vary greatly

with the discharge volume (SMACNA 1990). An adequate diffuser for VAV systems should be able to

perform across the whole operational range of a VAV system. As one option, Linder generally

recommends only linear slot diffusers for VAV systems (Linder 1997). Unfortunately, diffusers are often

being selected without regard to the effect of VAV turn-down (Int-Hout 2001), which results in the

degraded performance of diffusers at very low flow rates (Hydeman 2003). Diffusers with perforated

faces tend to have short throws at high airflow rates and thus may be unacceptable for VAV applications

(Int-Hout 2004).

The California Energy Commission suggests designing the HVAC system to efficiently handle auxiliaryloads that operate during off hours. HVAC systems may operate at only one-half of the design airflow for

the bulk of the time. Recognizing that VAV systems seldom operate at their design capacity, a reasonable

balance between first costs and energy costs often results in designs using smaller duct sizes. This design

approach results in a friction rate (airflow friction loss per 100 ft) at maximum design capacity that is very

high compared to a system designed around maximum expected capacity (Hydeman 2003). On the otherhand, optimizing the system at a dominant part load operating condition results in cost savings (Kim

2002). In a separate article, Kim reported a 7% reduction in energy consumption using the optimized part

load design approach compared to the duct area over the T-method, and a higher reduction compared to

the equal friction and static regain method (Kim 2002b). When considering proper room air distribution,

SMACNA suggests designing with overthrow at maximum design volumes to achieve acceptable throwat part load volumes (SMACNA 1990).

Over-sizing of VAV terminals can lead to significant operational issues and generally results from using a

safety factor when sizing the equipment. The outcome is quite often a reduced turndown ratio and a more

expensive system (Linder 1997). Additionally, over-sizing a VAV unit significantly reduces the velocity

of the air passing the velocity sensor at the same air flow as a properly sized unit, resulting in a velocity

pressure that is below the sensing range of the VAV unit manufacturers velocity sensor (Simon 2002).

The California Energy Commission design guidelines suggest setting the minimum airflow set point to

the larger of the lowest controllable airflow set point allowed by the unit and the minimum ventilation

requirement. In California, the minimum ventilation rate for an office is 0.15 CFM/sq.ft. In contrast, it is

common practice to have the minimum airflow set point between 30% and 50% of the cooling maximum

airflow set point, well above the guidelines. The recommended approach is a dual maximum scheme that

allows reduction of flow when heating or cooling load is low. There are many buildings operating

comfortably with lower than 30% airflow minimums (Hydeman 2003). Some issues that drive highminimum set points are stratification, short-circuiting and dumping of cold air. Room temperature non-

uniformities can result from insufficient flow at low loads due to low unit flow (Linder 1997). Also, if the

airflow set point is below the working range of the velocity controller, the unit may cycle between closed

and partially open, causing some varying sound levels (Int-Hout 2003). Taylor has developed a

simulation that considers many of the above mentioned design factors and determined optimum design

total pressure drop for the VAV unit that is on the path with the highest pressure drop (Taylor 2004).


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2.3INSTALLATION PROBLEMS

Installation problems predominantly concern the length and type of the duct branch, how duct turns are

accomplished and how the duct approaches the diffuser.

The length of the duct branch, the type of duct, and its radius affect the air delivery characteristics from

the VAV unit to the diffuser. Without sufficient length to develop a uniform profile, the flow in ductbranches too close to the VAV terminal or a previous branch is non-uniform, and hence causes an

increase in pressure loss. SMACNA suggests a sufficiently long duct section before elbows. In addition,

it highly emphasizes the use of turning vanes in elbows. However, the installation of elbows with only

some vanes and with no vanes is common practice. Eliminating every other turning vane from the vane

runners is believed to decrease the pressure drop. Research results, however, have revealed that thispractice more than doubles elbow pressure losses, and is definitely not recommended. More astounding

is the total elimination of vanes in the installation of fittings. This may be due to the fact that some sheet

metal contractors have found that they do not get paid for furnishing expensive fittings, such as ones with

turning vanes that were not shown on the project mechanical drawing. Without turning vanes, especially

in fittings used for avoiding obstructions, good airflow in the duct system can be totally destroyed(SMACNA 1990).

The advantages of installing vanes in elbows are significant but their purpose will not be met without theproper alignment of the vane rails. Improper alignment is a common problem found in the field. Vane

rails need to be aligned tangent to the flow to maintain uniformity in the flow. If the flow, however,

approaching the elbow is non-uniform, the turning vanes will instead assist in maintaining non-uniformity

in the flow downstream, and hence cause a pressure increase in the system upstream. The position of

fittings with respect to each other is also important for the air distribution. Hydeman et al. suggest

avoiding consecutive fittings to reduce the pressure drop, noting that in fittings less than six hydraulic

diameters apart the flow pattern entering subsequent fittings differs from the flow pattern used to

determine loss coefficients (Hydeman 2003). Thus, accurate loss calculation may be difficult.

Roughness of the ducts also affects the airflow distribution, and hence the energy used. The higher the

friction factor, the more energy it takes to push the air through the ducts. Straighter and smoother ducts

result in lower energy consumption and first cost (Hydeman 2003). The friction factor substantially

increases for not fully extended flexible ducts. For a straight flex duct extended only 70% of full length a

400% increase of the friction factor is expected (ASHRAE 2001).

The duct approach to the diffuser is also very important, since detrimental effects of improper duct

approach cannot be corrected by the diffuser itself. Both SMACNA and ASHRAE agree that for proper

diffusion, the velocity of the air stream must be as uniform as possible over the entire connection to the

duct and must be perpendicular to the outlet face. However, few outlets are installed in this manner. In

some cases, special devices can assist redirecting and stabilizing the airflow. Most ceiling outlets are

attached either directly to the bottom of horizontal ducts or to vertical take-off ducts that connect the

outlet with the horizontal duct (ASHRAE 2001; SMACNA 1990).

With respect to noise, SMACNA suggest placing the diffusers as far as possible from duct elbows and

branch take-offs to minimize noise transmission. For flexible ducts, SMACNA recommends to keep

bends as gentle as possible to diminish noise transmission, however, they also indicate the possibleneed for bends to assist in sound attenuation (SMACNA 1990). Moreover, direct duct connections to the

diffuser can result in noise levels by as much as 12 NC higher than catalog levels (Int-Hout 1996). An

offset of the flexible duct connection between the diffuser and the supply duct equal to the diffuser

diameter over a connection length equal to two diameters can increase the sound power level as much as

12 dB. Improperly used diffusers can also be a source of noise. Diffuser dampers cannot be used for

reducing high air volumes without inducing objectionable noise (SMACNA 1990).


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3 METHODOLGY

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3 METHODOLGY

To meet the project objectives, a detailed investigation methodology was developed and followed. To

determine scope and design of the experiment, and develop test procedures, the investigation used

information gathered from published literature, advise from an industry expert committee, and review

comments from a Department of Energy peer review panel. The test laboratory was designed anddeveloped and test protocols were developed and followed to conduct experiments called for in the

experimental design.

3.1GENERALAPPROACH

3.1.1 Identifying the Knowledge Gap

The goal of this task was to develop useful information for the HVAC industry on duct installation

between a VAV box and the diffuser. Therefore, collecting customer needs is essential to determine the

best direction and scope of the investigation. Some of the customer needs were extracted from the

literature review. The major source of customer needs came from a group of design and installation

experts assembled specifically for this project. Specific guidance sought included:

1. Consensus on a best or industry standard installation

2. Typical variations from that standard seen in the field

3. Predominance of the different variations and the magnitude of the variation encountered inthe field

4. Performance expectations from an industry standard installation

5. Expert opinion on the effect and importance of various installation variations

The information collected was used to determine the different installation parameters and to prioritize the

list based on the anticipated performance effect of variation in each parameter.

3.1.2 Design of the Experiment (DoE)

The design of the experiment is the crucial process used to devise a method to economically and

accurately determine the effects of parameter variation on the performance of the system. The system

field-operating environment is realistically simulated in the laboratory by testing under various

environmental conditions expected in the field. Important inputs to the design are the environmental

conditions, the physical installation parameters, the expected system performance and the parameters

expected effect on performance. Specific objectives of the experimental design are to determine:

1. What are the significant environmental conditions in the field2. Which parameters have a significant effect on the system performance

3. Which significant parameters are likely to be controlled by design or installation practices

4. Which significant parameters are likely to be not controlled by design or installation practices

5. What is the significant performance parameters to measure


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6. What is the most economical test matrix that can be used to capture the main effects ofparameter variation and achieve the objective of the experiment.

3.1.3 Development of Laboratory and Test Procedures

Laboratory and test procedures are needed to effectively and reliably perform the testing called for in the

experimental design. Specific objectives of the laboratory and test procedures development include:

1. Create a stable and repeatable laboratory test environment

2. Controls for modifying the test environment as required by the testing

3. Procedures for performing the various duct installations called for in the experiment design.

4. Sufficient instrumentation to monitor the test environment and accurately capture the targetperformance.

5. Data recording equipment and data reduction procedures for analysis of the results.

6. Understand the source and magnitude or experimental error

3.1.4 Testing and Industry Guidance

Experiments were conducted as called for in the experimental design using the laboratory and test

procedures. Data analysis, conclusions and recommendations focus on producing useful information and

guidance for the industry. Specific objectives of the testing and analysis of results include:

1. Conduct airflow, energy, and sound tests on a HVAC system with common installationvariations from a typical industry standard system

2. Determine the main effects of those variations on significant performance measures

3. Determine the importance of each type of variation and the target value for best performance4. Determine the quantitative advantage of holding a parameter on target and the quality loss for

off target performance.

5. Disseminate this information in a practical and usable format for industry use


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3.2ALIGNING PROJECT SCOPE WITH THE KNOWLEDGE GAP

Valuable assistance from outside sources was used to obtain the voice of the customer concerning the

knowledge gap in duct design and installation. The literature review, the project expert panel and a

Department of Energy review panel made particularly significant contributions toward identifying theknowledge gap.

3.2.1 Installation Variations Identified From Literature Review

The literature identified rules for proper installation of ductwork and gave reasons for following the rules.

In general, installations that result in unsteady flow at a branch connection or diffuser should be avoided

due to increased airflow pressure drop, increased noise and unpredictable flow balancing. Common

installation variations are branches to close to the terminal unit or to each other, flex duct too short or with

hard bends, and duct approach to the diffuser offset from the diffuser center.

3.2.2 Expert Committee Recommendations

To ensure relevance with up to date installation practices and problems, an expert committee for advising

the project team was established. This committee met at UNLV during the initial project methodology

development to identify the state of the art of VAV-to-diffuser duct design and common constructionvariations found in the field. The panel members evaluated a range of construction variations with regard

to good workmanship, commonly accepted construction practices and potential detrimental impact on the

air distribution performance of the system. The panel also identified several possible test setups and the

variations that should be tested in this Task.

The identified variations were generally expected to disadvantageously affect the performance of the air

distribution system due to either added flow restrictions, creation of uneven or disturbed flow in the ducts

or added imbalance of the system. The effect of two variations, perforated face on the diffuser for squarediffusers and round or oval side or top plenum inlet for slot diffusers, was not clearly known and could

negatively or positively impact the performance of air distribution system.

For square diffuser systems, the panel identified the following common variations:

1. First branch too close to VAV terminal discharge

2. Short length to diffuser

3. Flex duct offset

4. Bad radius in flex duct

5. Short vertical length to diffuser

6. Flex duct too long

7. Factory supplied damper in the face of diffuser grill

8. Perforated face on the diffuser


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For slot diffuser systems:

1. First branch too close to VAV terminal discharge

2. Short duct length to diffuser

3. Flex duct offset

4. Small radius turn in flex duct

5. Flex duct too long

6. Round or oval, side or end plenum inlet.

A sketch of a duct layout showing a normal branch and six variations is shown in Figure 1. Two typical

slot diffusers and two square diffusers are shown in Figure 2.

VAV Unit

Branch too close to

VAV unit dischar geProper branch

installationBranch too

short

Proper installation

Radius in branch

turn too small

Side View

Vertical duct offset Vertical duct

too short

Branch too long

Figure 1. Schematic of typical branch installation variations


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Figure 2. Different types of diffusers that are used in terminal duct installations. Top: two 4-foot slot diffusers, one with a 8 inch

round adaptor and one with a 10-inch oval adaptor. Bottom right: typical 2 by 2-foot louvered square diffuser. Bottom right:

Plaque type 2 by 2-foot square diffuser.

The expert panel suggested a separate series of tests for square diffusers and slot diffusers due to the

different fundamental air distribution characteristics of the two types of diffusers. Two-foot and four-

foot slot diffusers were discussed for the testing. Originally, when the test plan called for four square

diffusers in the test room, using four two-foot end slot diffusers in the test room would give the best

comparison to the square diffusers and are a common size of slot diffusers used in that size of room.

Later, after a series of preliminary tests, a room configuration with only two diffusers, either square or

slot, was determined to be the best for room air distribution and for testing. The panel initially worked

out a series of tests that were based on having one condition at the extreme or bad setting while holding

the other conditions at nominal or the good setting. This testing sequence was modified in favor of a

more efficient orthogonal array test sequence, which is discussed in detail in section 3.4 Design of the

Experiment.


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A summary of a preliminary test plan was submitted back to the expert committee for their review and

comment. Changes suggested by several members of the committee were subsequently incorporated into

the test plan.

A focus of discussion between the panel members and the team was determination of the VAV unit input

parameters. Debate centered around maintaining constant airflow, pressure or energy into the VAV unit.

The advantages and disadvantages of each approach are shown in Table 1. Normally, as many units donot have airflow sensing, airflow through the VAV unit is controlled by predetermined positioning of the

inline damper. Therefore, the team decided to use two fixed damper positions for the two noise

conditions. Only two of the three parameters: pressure, airflow and damper position, are independent.

Thus, for each damper position, only pressure or airflow, but not both, can be held constant for different

variations in the ductwork. Alternately, energy can be held constant by adjusting airflow until the airflow

and pressure condition results in the same energy. The team decided to hold pressure constant and allow

airflow to vary. This is more representative of field conditions where several VAV units are attached to amain air supply. In such cases, supply pressure is held constant and flow requirements are met by use of

in-line dampers, such as in a VAV unit. By holding the supply pressure constant while varying

installation conditions, the airflow will vary off target. This airflow variation is expected to be small

compared to the airflow difference caused by the two damper positions.

Table 1. Possible Standardized Input Conditions

Advantage Disadvantage

Constant Airflow Easy to control May be inconsistent with actualbuilding system operating

conditions

Constant VAV Inlet Pressure Easy to control Similar to actual building system

operating conditions

Will require airflow scaling todetermine noise effects of

parameter variation

Constant Input Energy Similar to actual operating

conditions in some small systems

May be difficult to determine root

cause of output variation

High and low value of the test parameters were provided by the industry expert panel.

3.2.3 DOE Peer Review of the VAV Duct Design Variations Test Plan

A Department of Energy sponsored peer review panel was presented with the details of the test plan. The

panel made a few recommendations in the test parameters.

Following the DOE peer review panel recommendations, a test parameter of closed and open center face

design diffuser was added, while perforated face diffuser and end slot diffuser testing was dropped.These changes are based on the types of diffusers most commonly used in the commercial building

HVAC industry. From the original seven parameters, short length to diffuserandflex duct too long werecombined into one four level parameter, flex duct length to result in six parameters for both square and

slot diffusers.

The DOE panel also concurred with the test plan to measure isothermal throw from the diffusers.

Estimating ADPI from isothermal throw measurements is an industry accepted method. The different


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installations used in this project are expected to result in changes in total airflow and flow pattern from

the effected diffuser.

3.3SYSTEM PERFORMANCE CHARACTERIZATION AND MEASUREMENT

Good performance measures posses the important characteristics of a strong relation to the customer

needs and being identifiable as the result of an energy transformation from input to output. It is alsoadvantageous that the performance measure be a leading indicator of customer satisfaction. System

designers require sufficient information to design a system that will provide the desired thermal and

acoustic comfort as economically as possible. The common indicator of thermal comfort is ADPI, and of

acoustic comfort is the noise criteria (NC) level. The information needed to create a good system design

includes diffuser airflow, diffuser airflow pattern, the expected pressure drop and noise generated from a

given installation.

The energy transformation takes place in the target system from VAV unit to the test room. The input is

airflow at a given rate, temperature and pressure. The desired output is airflow into the room at a desired

rate, temperature and distribution. The intended energy transformation consists of redirection of the flow.

The unintended consequences of the transformation are airflow decrease, pressure loss, noise generation,

heat loss or gain, and undesired airflow distribution. A depiction of the energy flow is shown in Figure 3.

This project deals with isothermal flow so heat transfer is not considered. A strong effort was made to

avoid airflow loss due to leakage between the VAV unit and the diffuser. Airflow decrease due to

leakage was not used as a parameter and any loss was considered experimental error in this experiment.

As previously mentioned, at a given inline damper setting airflow and system pressure drop are inter-

dependent.

Energy Outputnergy InputfAirflow

Figure 3. Schematic of non-ideal energy transformation in the target system

For this experiment, the supply air pressure was held constant. For a given VAV damper setting, airflow

decreased due to increased airflow resistance in the ductwork including diffuser. Thus, changes in

airflow were a good measure of the efficiency of the energy transformation. Before reaching the VAV

unit, the supply air passed through several silencer like plenums, nearly 15 feet of lined metal duct with

perforated inner surface and about 30 feet of large diameter flex duct. At the VAV unit the flow had no

significant noise content. All noise created in the room was noise that was generated from the VAV unit

to the diffuser. Thus, noise measured in the room was a good indicator of unintended energy

transformation in the target system. Airflow entered the VAV unit through a 4-foot straight metal duct

and was presumed to be uniform. The distribution of the air into the room, uniform or otherwise was a

fAirflow Energy Transformationf Flow Distributionf Pressuref NoisefTemperaturef

Temperature

Unintended Energy Inputf Noise

f Heat Transfer


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combination of the intended and unintended consequence of the installation from the VAV unit to the

diffuser. Airflow distribution was a good measure of part of the energy transformation in the target

system. For this experiment, airflow rate, airflow pattern and room noise levels were measured as

performance parameters.

Airflow rates were measured at each diffuser before and after each test run. The performance parameter

used was the ratio of test diffuser airflow to the airflow measured during the nominal case run. This gavea direct measure of the relative efficiency of the installation.

Room noise level was measured at two elevations for all test points during all test runs. An average of the

readings from the microphone at mid level during a scan around the diffuser was used for the

performance measurement. The measurement was converted to the Noise Criteria (NC). Noise Criteria is

designed for measuring HVAC related noise and is commonly used by the industry. Because of

intermittent extraneous noise in the test room at the two lowest octave bands used for calculating NC, the

measurements at those bands were omitted. Therefore, the reported levels were modified NC levels.

Those omitted bands covered the low frequencies typically associated with rumble. Rumble is a

characteristic of large fan noise or unstable flow in large ducts and the generation of such noise was not

expected from the ductwork in this experiment. The modified NC was expected to be a good

representation of the actual NC due to the ductwork installation. For nearly all measurements, they were

in fact the same.

Room diffuser airflow pattern and throw characteristics were measured to determine air distribution

changes due to variations in ductwork installation. If a room air distribution system is designed with the

assumption of directionally symmetric diffuser performance, then any significant deviation from that

symmetry could be detrimental to the room ADPI. However, most room designs, including the test room

for this task, have inherent compromises in the diffuser design. Most notably, the characteristic length

used to select diffuser location may not be symmetrical in every direction. In the test room used in this

experiment, the square diffuser characteristic length varied from eight to eleven feet. It is possible that

matching asymmetry in diffuser throw with asymmetry in characteristic length could be beneficial. On

the other hand, random anti-symmetrical throw is likely to be especially detrimental to ADPI. Clearly,

APDI has dual dependency on both diffuser throw and room air distribution design. The Task goals were

to produce data that could be used for different room designs, and in a sense, be room independent.

Therefore, diffuser throw variation were deemed to be a better indicator of the effect of variations in the

test parameters than ADPI measurements. Some parameter variations may not affect throw symmetry,

but may affect the system balance and, as a result, the overall efficiency with or without system

rebalancing.


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3.4DESIGN OF THE EXPERIMENT

The design of the experiment determines how economically and accurately the effects of parameter

variation on the performance of the system are captured. Parameters that have a direct effect on the

energy transformation were chosen and grouped as either control, test or noise parameters. Followingparameter selection, the most efficient test matrix was chosen and parameter levels were adjusted to best

capture the main effects of parameter variation.

3.4.1 Parameter Selection

The expert panel identified seven different conditions and two different types of diffusers that most likely

would be installed in a typical VAV system. A full factorial design of experiment (DoE) for those seven

conditions and two diffuser types at just two levels would involve hundreds of tests. Alternately, a basic

vary-from-nominal one parameter at a time test for four configurations would require 26 tests at each

airflow level (48 for two airflow levels) and would not give any information in cases where more than one

parameter is different from nominal. Along with total number of tests, the required labor and thedifficulty of achieving test parameters must also be considered. Each test involves configuring the duct

system and diffuser according to the selected levels of the parameters, running the HVAC system until

conditions such as air velocity and temperature have stabilized, then measure air velocity, temperature

and sound at all the required positions in the test room.

To achieve the task objectives in the budget and time allotted, an efficient experimental design was

required.

A DoE based on the Taguchi Orthogonal Array is a very efficient tool used to derive relationships

between parameters and the results. The Taguchi method of experimental design has gained wide

acceptance in industry and applied research for quality improvement in product and process design. The

method is used to determine the relationships between different parameters such as ingredients, strength,

shape, length and smoothness, and the quality level achieved in the output. The orthogonal property ofthe test array allows for easy and accurate statistical analysis. For example, from the experimental results,

the designer can determine the best level for a parameter, the sensitivity of the output to that parameter

and the sensitivity of the output to noise (changes in uncontrolled conditions) at different parameter

levels. In this experiment, the relations obtained were for the main effects. A main effect was defined

as the impact a particular parameter had on the output. In contrast, an interaction was defined as any

change in the effect of a parameter caused by a change in the level of a different parameter. In other

words, an interaction was the change in the output that was not caused by the main effect. When the

Taguchi Orthogonal Array is used, any interaction is spread out evenly through all the output of the

experiment. Thus, the interaction is not measurable, but fortunately, due to the spreading out of any

interaction effects on the experimental output, the interaction has a minimum impact on the main effect.

The Taguchi Orthogonal Array was used to test the system for all the parameter variations. Along with

the conditions obtained from the expert panel, analysis of the literature suggested an additional condition

of VAV terminal variable flow rate. One of the conditions suggested by the expert panel, flex duct

vertical section above the diffuser is offset, was incorporated into short vertical section above diffuser.

Knowledge of the physics of the system suggested that both parameters have a similar effect and have

strong interaction. In fact, short vertical section above diffuser, is just the extreme case of duct offset.

The conditions and resulting parameters are listed in Table 2. The table also lists the form of energy

conversion associated with that parameter and the type of parameter as either test, control or noise.


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The type definitions are:

Test Parameter. A parameter that is a cause of variation and is tested a several levels.

Control Parameter. A parameter that can be controlled and eliminated as a source of variation.

May be tested to evaluate different design option.Noise Parameter. A parameter that cannot be controlled, or is preferred not to be controlled, in

actual system use. Therefore a good system should be robust to changes in the noiseparameter. This parameter may be varied in the experiment to test for system robustness.

Table 2. Parameter list for the experimental design following Department of Energy review

Condition Parameter Energy Form Type of Parameter

Duct branch close to VAV terminal or

previous branch

Distance from branch to

VAV unit or previous

branch

Flow disturbance Test Parameter (x)

Duct very short or very long Distance from branch todiffuser


Short vertical section above diffuser Distance from elbow to

diffuser


Small radius turn in flex duct Size of radius Flow resistance / noise Test Parameter (x)

Closed or open face diffuser (square

diffuser only)

Closed / open Flow resistance,

disturbance

Control Parameter (C)

Round / oval diffuser plenum inlet

(slot diffuser only)

Round / oval Flow disturbance Control Parameter (C)

VAV airflow volume Flow rate Flow resistance / noise

/disturbance

Noise Parameter (N)

Note that the type of parameter classification (test, control or noise) is dependent on where in the whole

process of HVAC system creation the evaluation is performed. For example, a system designer may not

specify the exact diffuser or duct but instead just the source of air for each diffuser and the expected flow

for the anticipated room load. To that designer, some of the control parameters listed in Table 2 become

noise parameters such as diffuser choices, and variations in duct installation and room conditions.

Alternately, to the installer that chooses the diffuser and duct products, and directs the installation, many

of the test parameters in Table 2 become control parameters. Possible noise parameters for the installer

would be parameters such as flow rate, room load, room furniture and partition arrangement, variations

within the products, and variations from the designated installation.

For this experiment, it was assumed that diffuser choices are controlled but that exact installationconditions are subject to variation. The seven parameters in Table 2 were determined to be acceptable

parameters for the DoE. One practical consideration was that, based on the available time for testing and

how the design sequence affected ease of changing between tests, the total number of tests would not

exceed 35. To the maximum extent possible, test parameters that were expected to have a strong

interaction or were essentially the same parameter at different levels were combined and varied together


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as one parameter. It was decided to include the control parameter, diffuser inlet or face type, as a test

parameter to capture the effects of changing the diffuser type.

3.4.2 Test Matrix Selection and Modification

For the experiment itself, there were a discrete number of arrays to choose from. Considering the number

of parameters and the desire to economize on the number of tests, the L8 array, requiring 8 tests andhaving the ability to evaluate the main effects of up to 7 parameters at two levels, or 5 parameters, 1 at

four levels and 4 at two levels was chosen. The array shown in Table 3 is the 5-parameter configuration.

The first test shown has all parameters at level one while all other tests have the parameters at mixed

levels. When the test is actually conducted the order of tests is randomized to help reduce uncontrolled

noise interference in the results. The array is balanced by choosing parameter levels such that any

condition of any parameter is tested with an equal number of high and low conditions of the other

parameters. For example, parameter 2, level 1 is tested with parameter 4, level 1 twice and level 2 twice.

This testing method reduces interaction effects in the output and exposes all parameters to the different

levels of the other parameters. This is an excellent array to determine main effects as long as interactions

are low and effects for the two level parameters are close to linear.

Table 3. Taguchi Orthogonal L8 Array

TESTCONDITION PARAMETERS

Test No.Branch to elbow

distance

Branch to VAV

unit distance

Elbow to diffuser

distance

Bend in duct

radius

Diffuser/duct

type

1 1 1 1 1 12 1 2 2 2 23 2 1 1 2 24 2 2 2 1 15 3 1 2 1 26 3 2 1 2 17 4 1 2 2 18 4 2 1 1 2

The list of the five parameters for the two diffuser cases is shown in Table 4.

Table 4. Parameter List for Test Array

Parameter Low State Mid State High State

1. Branch to diffuser distance 6 (square), 9 (slot) feet 15 and 25 feet 35 feet

2. Branch to VAV unit distance 6 inches 54 inches

3. Elbow to diffuser distance 5 inches 40 inches

4. Bend in duct radius 5 inch radius, 120 degree

turn

30 inch radius, less than

90 degree turn

5. Diffuser design (square) Louvered Plaque

5. Duct size and connection (slot) 8-inch, round 10-inch, oval


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Table 3 is referred to as the inner array. The one noise test condition, airflow rate, still needed to be

added to the experiment. Noise conditions are added at the right of the matrix in what is called the outer

array. With only one noise condition, the outer array is just a one-row, two-column array. The complete

test array, inner and outer, for both diffuser types is shown in Table 5. One nominal test condition, listed

as test condition 0, was added to the array. Test parameters for this test condition were set at what are

considered standard installation practice levels and the more common diffuser was chosen. Test

condition 0 was not used in the calculations for main effects and signal to noise levels. It was included togive comparative information on the performance of a standard installation. The measured output for

each test is entered under each Noise Test Condition column. Thus, for each type of diffuser (square or

slot) there are eight test numbers, each done once for each noise condition. Because we have two diffuser

types, a total of 32 tests were required. The results of the experiment were used to determine the main

effects of the test parameters. The main effects were the average effects for that test parameter with the

other parameters at equal instances of all possible test levels and while exposed to the different noise

condition levels.

Using the main effects, it was possible to predict the results for all possible cases of parameter levels,

most of which were not tested. Also, for any given performance measure or combination of measures,

optimum settings of the test parameters can be determined. Parameters that can have continuous levels

(not just either/or) can be set at values between the extremes. Results for the 4-level parameter can shownon-linearity in the output and can show an optimum value between extremes. Because a noise parameter

is evaluated, robustness of the design to that noise condition can be evaluated. These results also can be

used to help with tolerance design.

Table 5. Test Array with One 4-Level Parameter, Four 2-Level Parameters And One Noise Parameter

TESTCONDITION PARAMETERS NOISETESTCONDITION

Test No. 1 2 3 4 5 1 21 1 1 1 1 12 1 2 2 2 23 2 1 1 2 24 2 2 2 1 15 3 1 2 1 26 3 2 1 2 17 4 1 2 2 18 4 2 1 1 20 Nominal 2 2 2 1


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Because the arrays are balanced and orthogonal, analysis of the results of the test is greatly simplified.

Analysis of the results produces values for the main effects (change in output when a parameter is

changed from one level to the other), and the signal-to-noise ratios (S/N), (the effect of noise and other

parameter variation on the output with the parameter at a given level). Those values are plotted for easy

parameter at all test levels. The main effects are calculated using the equation:

Al,i =j=1

j= n

Yl ,i,j

n(1)

where Al,i is the mean performance when parameter i is at level l, and Yl,i,j is the result for the j test

when parameteri is at level l. For example, for the mean when parameter 2 is at level 1, the results from

the four tests (numbered 1, 3, 5, and 7 in Table 5) are averaged.

The S/N calculation depends on the type of performance characteristic. The performance characteristic ofairflow is a larger the better characteristic and noise level is a smaller the better characteristic. The

signal-to-noise calculation uses the equation:

S/N= 10log10 MSD (2)

where MSD is the mean square deviation. For smaller the better:

MSDl ,i =j=1

j= n

Yl,i,j

2

n(3)

while for greater the better,

MSDl ,i =j=1

j= n

1/Yl,i,j2

n(4)

for level l of parameteri.

The mean gives the average performance when a test condition is at a specific level. Because the other

test parameters are equally balanced at all of their levels, this mean is the best indicator of expected

performance at that level. S/N, on the other hand, also measures the variation of performance when a test

condition is at a specific level. Because of the way S/N is calculated, the highest value of S/N is always

desirable in both larger and smaller the better performance measures. For the larger and smaller the better

performance measures, the mean and the S/N level normally indicate the same level as the best for any

given parameter. However, if performance has high variation at a specific parameter level, the S/N value


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may be lower than a level with a less desirable mean but lower variability. A designer then needs to

decide if the low variation is more desirable than the better mean. In many cases, low variation is chosen

and a different parameter is used to obtain a desirable mean. In the case of energy efficiency in ductwork,

the optimum mean may be chosen to maximize potential energy savings, while the lower variation may

be chosen so that performance can be better predicted.

Based on principals of physics and engineering experience, certain changes in performance were expectedfrom changes in the different parameters. Table 6 lists the expected change in different performance

characteristics with changes in the test parameter. This list was compiled before the experiment was run

to help build the design and measurement scheme so as to not miss those performance changes.

Table 6. Expected effect of parameter variation on output

ParameterExpected effect at

variation

Effect on

diffuser throw

symmetry

Effect on system

performance

Energy efficiency related

effect

Duct branch to diffuser distance Unstable flow Small to none Airflow balance

change

Resistance added by

balancing

Duct branch to VAV unitdistance Unstable flow Possibleasymmetry Airflow balancechange Resistance added bybalancing

Duct hard elbow to diffuser

distance

Flow velocity

direction

Significant

asymmetry

Flow noise

generation

Unknown

Bend in duct radius Flow restriction none Flow resistance Resistance added to

balance

Diffuser design (square

diffuser)

Throw pattern

variation, flow

resistance

Different throw

pattern

Possible resistance

change

Possible resistance change,

different balancing

Duct connection (slot diffuser) Flow resistance and

direction

Different throw

pattern

Possible resistance

change

Possible resistance change,

different balancing


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3.5 LABORATORYDESIGN AND INSTRUMENTATION

The UNLV test room was extensively modified to accommodate the requirements of this and other

NCEMBT projects. The modifications included new heat pumps for the room air system, rework and

extensive additions to the room ductwork, addition of a temperature control system for all test room

surfaces, the addition of extensive system monitoring instruments and new instruments for measuring theperformance characteristics.

3.5.1 Laboratory System Modifications And Capabilities

The laboratory system modifications that made this project possible were the addition of temperature

control of the room HVAC system and room surfaces, and the rework and extensive additions to the room

ductwork.

The room HVAC control was designed to supply air at a stable volume and temperature. The volume for

each test is maintained by a manually controlled fan run at a set speed. The supply air temperature was

conditioned by two multi-stage compressor heat pumps. Each heat pump was controlled by a variable

demand thermostat with a temperature sensor in the supply duct located just before the VAV unit. Tostabilize the supply air temperature, the air passed a heat exchanger with 15 gallons of circulating water

and glycol. The buffering effect of the heat exchanger reduced temperature changes in supply air to a rate

of less than 2F per minute under all operating conditions.

Temperature control of the room surfaces was accomplished by ducting conditioned air through a channelbetween layers in the walls, sub-floor and ceiling. Surface control was divided into thirteen

independently controlled zones. The zones could be heated or cooled to maintain the desired temperature

or heat transfer, on their respective surfaces. The four vertical surfaces (room walls) were covered with

1/4-inch wallboard backed by 1/8-inch of dense foam rubber like material. The wall covering added

sufficient insulation between the temperature control air ducting and the room interior surface to achieve

thermal buffering in the wall and create slower thermal convection through the wall. At the same time,

the control scheme for the surface temperature control was optimized to hold target and reduce

temperature variations. Tests indicate a temperature gradient along the walls of 4 F or less. Average

surface temperature variation was below 2 F. For this experiment, all the walls and ceiling were

maintained at 75 F to achieve isothermal conditions. Total heat load in the room was estimated at 150

watts from overhead florescent lighting, 80 watts from the instrument motion system, and 50 watts from

the instrumentation.

The room ductwork was modified to accommodate the CAD and under floor air distribution (UFAD)

testing. The UFAD tests were conducted under a separate NCEMBT task. A depiction of the supply

ductwork in the test room is shown in Figure 4. Starting with the main supply duct, a T-section with a

class III smoke damper on each

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