AIR DIFFUSION COUNCIL 1901 N. Roselle Road Suite 800 Schaumburg, IL 60195 847-706-6750 Fax 847-706-6751 www.flexibleduct.org October 16, 2017 Gabriella Davis Secretary, IAPMO Standards Council The IAPMO Group – West Building 4755 E. Philadelphia Street Ontario, CA 91761 Subject: Appeal to Standards Council Reference: Item #72 Dear Gabriella, Following please find the Air Duct Council’s appeal to the Standards Council for the above referenced Item. Please inform me if there is anything else needed to file this appeal to the Council. Respectfully, Ralph Koerber Chairman, ADC Engineering Committee Cc: John Falk (ADC President), Jack Lagershausen (ADC)

October 16, 2017 Ralph Koerber Chairman, Engineering Committee Air Duct Council 1901 N. Roselle Road, Suite 800 Schaumburg, IL 60195 Executive Summary: My name is Ralph Koerber and I am the Chairman of the Air Duct Council (ADC) Engineering Committee. This is an appeal to the Council on the Item #72 for technical merit and substantiation. Item #72 seeks to remove the exception for residential occupancies in 603.4.1 which was established during the revision cycle for the 2015 UMC. Significant technical issues were raised in opposition to removing the residential exception during public comment and at the UMC TC meeting in May. The TC voted to reject Item #72 at this meeting. Specifically, there is no technical reason to prevent proper sizing and installation of flexible air ducts in residential occupancies when the length exceeds 5 feet. Therefore I am requesting that the Council uphold the TC decision during its meeting in May to retain the residential exception by rejecting Item #72. Page 1 of 3 ADC Appeal, IAPMO Standards Council (16 Oct 2017)

October 16, 2017 Ralph Koerber Chairman, Engineering Committee Air Duct Council 1901 N. Roselle Road, Suite 800 Schaumburg, IL 60195 Appeal on Item #72: My name is Ralph Koerber and I am the Chairman of the Air Duct Council (ADC) Engineering Committee. ADC is the trade association representing the North American manufacturers of flexible air ducts and air connectors used in HVAC applications. I am employed as Vice President, Technical Services for ATCO Rubber Products, Inc., 7101 ATCO Drive, Fort Worth, Texas. ATCO manufactures UL Listed Class 0 and Class 1 Flexible Air Ducts and Air Connectors for commercial and residential HVAC systems. I am a current voting member of the UMC Technical Committee. I am appealing acceptance of Item #72. Summary of Actions Related to Item #72: A proposal was submitted by Mr. Randy Young which strikes the residential exception in 603.4.1 and prevents flexible air ducts from being more than 5 feet length in residential occupancies. At the first TC meeting in May 2016, this proposal was accepted. Substantial comments were received during the public comment period in opposition to acceptance of this proposal (Item #72). After review of these comments and with considerable deliberation at the second TC meeting in May 2017, the TC voted to accept Public Comment 02 thus rejecting Item #72 and retaining the residential exception. A motion to accept Item #72 as submitted was made at the Association Meeting in September 2017 and the lack of technical substantiation for the proposal was again debated. The IAPMO membership approved the motion however this motion did not receive the necessary 2/3rds acceptance by the TC (9-15-1). In addition the TC did not receive the necessary 2/3rds acceptance regarding the suitability of current language in the UMC (15-9-1) thus sending Item #72 to the Council as an appeal. Page 2 of 3 ADC Appeal, IAPMO Standards Council (16 Oct 2017)

Reason for Appeal: I am appealing Item #72 on the grounds that this 5 feet limit proposed for flexible air ducts in residential occupancies was not based on any technical substantiation that justifies this limit. UL Listed air ducts are tested and approved per the referenced standard in this code and they are intended for use without an arbitrary limit of length when installed in accordance with the listing and per the manufacturer’s installation instructions. Properly sized and installed flexible air ducts in lengths longer than 5 feet will perform similarly and efficiently in residential duct systems. An arbitrary limit of 5 feet length is not substantiated by any sound technical study that looks at properly sized and installed flexible duct runs. There was no technical justification provided in Item #72 to exclude flexible duct lengths longer than 5 feet or to substantiate the removal of the exception for residential occupancies. In fact, the studies referred to in the original code proposal themselves concluded that a properly installed flexible air duct performs as well as alternative duct materials. The multitude of statements and testimony made in opposition to Item #72 during the comment phase and 2017 TC meeting point out this lack of technical substantiation for removing the residential exception. I also wish to point out to the Council that UMC TC action on Item #75 places 11 new installation requirements for flexible air ducts into the body of the code to help improve and foster proper installation. This action by the TC is the appropriate response and these new requirements for flexible duct should be given the opportunity to affect future installations rather than a restriction of use. I respectfully request the Council uphold the earlier TC decision and maintain the residential exception by rejecting Item #72. Respectfully, Ralph Koerber Chairman, ADC Engineering Committee Page 3 of 3 ADC Appeal, IAPMO Standards Council (16 Oct 2017)

AIR DUCT COUNCIL 1901 N. Roselle Road Suite 800 Schaumburg, IL 60195 847-706-6750 Fax 847-706-6751 www.flexibleduct.org 28 October 2017 Summary: Experimental Comparison of Pressure Loss in Typical Flexible and Sheet Metal Residential Duct Systems To provide realistic information regarding pressure drop characteristics for flexible air ducts and typical snap-lock sheet metal ducts and fittings in residential applications, the Air Duct Council contracted with Tennessee Technical University to conduct this testing program. The goal of the project was to demonstrate the actual pressure drop associated with the use of both duct types when each are properly installed per applicable model codes and standards and the specific installation instructions. Data from this program would be useful in understanding the comparative differences between the two duct types and helpful in formulating opinions in future development of codes and standards. Specifically, this evaluation would demonstrate the air flow performance of flexible duct versus that of sheet metal duct in residential applications when flexible duct length exceeds the proposed five (5) feet limitation. The evaluation consisted of two installation scenarios - A. Multi-line duct system sections comparing metal and flexible duct where the metal and

flexible ducts were the same diameter (i.e. flex ducts not sized per ACCA Manual D).

B. Duct system sections comparing metal and flexible duct where all ducts were sized in accordance with the procedures in ACCA Manual D using the pressure drop data from ASHRAE Duct Fitting Data Base (metal) and the flex duct manufacturers published friction loss data.

The results from this test program demonstrate two (2) main points - 1. When flexible air ducts are properly installed but not correctly sized (installed at the same

diameter as the sheet metal ducts), the pressure loss penalty was approximately 17%.

2. When flexible air ducts are properly sized and installed, both the sheet metal and the flexible duct system sections performed basically the same in terms of pressure drop.

In conclusion - Properly sized and installed residential HVAC systems using flexible air ducts will perform similar to sheet metal ducts. Proposed five (5) feet limitations for flexible duct in residential construction is not substantiated nor warranted.

Experimental Comparison of Pressure Loss in Typical Flexible and Sheet Metal Residential Duct Systems

By

A. Paruchuri

Department of Mechanical Engineering Tennessee Tech University

Cookeville, Tennessee 38505 USA

S. Idem, Ph.D. Department of Mechanical Engineering

Tennessee Tech University Cookeville, Tennessee 38505 USA

A. Paruchuri is a Graduate Student in the Department of Mechanical Engineering, Tennessee Tech University, Cookeville, TN. S. Idem is a Professor in the Department of Mechanical Engineering, Tennessee Tech University, Cookeville, TN. Corresponding Author: Stephen Idem Department of Mechanical Engineering Tennessee Tech University 115 W. 10th St. Cookeville, TN 38505-0001 Tel. (931) 372-3607 Fax (931) 372-6340 E-mail: sidem@tntech.edu

1

ABSTRACT

Tests were performed to measure air flow characteristics of multi-branch duct

systems comprised of wire-wound flexible ducts and rigid sheet metal ducts. When

flexible duct systems were not sized and installed per applicable standards required to

design residential duct systems, the total system pressure loss penalty was as much as

44%, when expressed in terms of the measured system loss coefficients. However, the

difference in pressure loss for typical residential duct systems constructed from properly

sized and installed wire-wound flexible ducts and similar systems constructed from rigid

steel ducts was negligible at design conditions.

INTRODUCTION

This paper describes a test program to measure air flow characteristics of wire-

wound flexible ducts and rigid sheet metal ducts. The work involved testing of one (1)

multi-line section and one (1) system section in three configurations each. The goal was

to measure and contrast the pressure loss in typical residential duct systems constructed

either from wire-wound flexible ducts installed per the Fifth Edition of the ADC Flexible

Duct Performance & Installation Standards, and analogous systems constructed from

rigid steel ducts. The tests setups in this study were intended to duplicate those previously

evaluated by ADC in their laboratory. Therefore the present testing provided

corroboration of the ADC test methods and data reduction. The test results are valuable

for characterizing the pressure loss when either wire-wound flexible ducts or rigid sheet

metal ducts are correctly installed in typical residential duct systems. The tests and data

reduction were performed in compliance with ANSI/ASHRAE Standard 120-2017,

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“Method of Testing to Determine Flow Resistance of HVAC Ducts and Fittings”, except

as noted below.

Numerous accounts of pressure loss measurements for straight ducts are available

in the literature. For example, Hutchinson (1953) evaluated the duct roughness of round

aluminum ducts, and Griggs et al. (1987) studied the flow resistance of round sheet metal

ducts. In the latter instance both longitudinal welded seam construction and spiral ducts

were considered, and the impact of the joint design on duct roughness was studied. Flow

characteristics of rectangular ducts have been reported by Huebscher (1948), and Griggs and

Khodabakhsh-Sharifabad (1992). Heyt and Diaz (1975) investigated spiral flat oval duct

pressure drop characteristics. Abushakra et al. (2004) measured pressure loss in 152 mm (6

in.), 203 mm (8 in.) and 254 mm (10 in.) wire-wound flexible ducts. The pressure loss was

correlated in terms of a Pressure Drop Correction Factor (PDCF) that accounted for the

effects of duct compression. Weaver and Culp (2007) and Culp and Cantrill (2009) studied

fully-stretched flexible ducts having diameters of 305 mm (12 in.), 356 mm (14 in.), and 406

mm (16 in.). Culp (2011) presented PDCF correlations for 152 mm (6 in.), 203 mm (8 in.),

254 mm (10 in.), 305 mm (12 in.), 356 mm (14 in.), and 406 mm (16 in.) flexible ducts that

possessed compression ratios ranging from 0% to 45%, in the absence of sag. Flexible ducts

that were joist supported were also considered in that study, in order to characterize the

impact of natural sag and long-term sag.

Loss coefficient data for a wide range of HVAC duct system fittings have been

presented in the literature. Brooks (1993) measured loss coefficients for a variety of

round mitered elbows with and without vanes, as well as rounded and mitered offset

fittings. Main and branch loss coefficients for diverging flow in tees with a rectangular

3

main to rectangular or round taps were also reported. Townsend et al. (1996a), Kulkarni

et al. (2009a), and Kulkarni et al. (2009b) studied loss coefficients for easy bend, hard

bend, and mitered flat oval elbows. Sun et al. (2015) reported pressure loss coefficients

for flat oval transition fittings. Townsend et al. (1996b), Idem and Khodabakhsh (1999),

Idem (2003), and Gibbs and Idem (2012) reported diverging flow branch loss coefficients

for flat oval tee and lateral fittings in terms of power law functions of branch-to-common

flow rate ratio. Kulkarni et al. (2011) employed a logarithmic model to correlate branch

and main loss coefficients for converging flow flat oval tees and laterals as functions of

branch-to-common and main-to-common flow rate ratio, respectively, and the geometry

of the fitting. Nalla and Idem (2012) measured branch and main loss coefficients for

round saddle-tap tees, operated in both the diverging and converging flow modes.

Kulkarni and Idem (2015) characterized zero-length loss coefficients for controlled bends

of 45° and 90° in nonmetallic flexible ducts under fully stretched conditions for

dimensionless bend radii of r/D = 1 and r/D = 1.5.

SCOPE OF TESTS

A multi-line duct system in three separate configurations were constructed per

Figures 1 through 3; these are referred to as System Sections #1A, #2A, and #3A. The

static gage pressure was measured at a location one duct diameter upstream of the 90∞

elbow that preceded the test section by means of pressure taps configured as a

piezometric ring and connected to a micromanometer using pressure tubing. The

measured flow rate was varied in even increments over a range of values and converted

to standard air conditions. The resulting static gage pressure loss [Pa (in. WC)] was

plotted as a function of standard air volume flow rate [L/s (SCFM)], where standard air

4

density was defined as rstd = 1.204 kg/m3 (0.075 lbm/ft3). In addition a dimensionless total

pressure loss coefficient was determined by fitting a straight line curve through the total

pressure loss versus velocity pressure data using the least squares method, and evaluating

the resulting slope with the intercept forced to zero. The resulting pressure loss

coefficients were tabulated to illustrate the difference between using the same diameter

(i.e., not properly sized per ACCA Manual D) metal and flexible duct for each section.

Likewise duct systems in three separate configurations were assembled per

Figures 4 through 6; these are referred to as System Sections #1, #2, and #3. ADC

indicated that for these cases all ducts were appropriately sized per the procedures in the

latest edition of ACCA Manual D. Once again the static gage pressure was measured at a

location one duct diameter upstream of the 90∞ elbow that preceded the test section using

pressure taps configured as a piezometric ring and connected to a micromanometer using

pressure tubing. The flow rate was adjusted to approximately 255 L/s (540 cfm), and

converted to standard air conditions. The flow rate at each system sub-section was

controlled by adjusting dampers mounted at prescribed locations within each apparatus.

Therein the flow rates at diffusers #1 and #2 were set to 76 L/s (160 SCFM), whereas the

flow rate at diffusers #3 and #4 were set to 47 L/s (100 SCFM) and 57 L/s (120 SCFM),

respectively. In each instance the flow rates were confirmed by means of a low flow

balometer installed at each diffuser. The resulting static gage pressure loss [Pa (in. WC)]

was reported at these balanced flow conditions.

EXPERIMENTAL SETUPS

The test setups designated as System Sections #1 and #1A were constructed using

30 gauge galvanized steel snaplock ducts having a longitudinal seam. The test setups

5

referred to as System Sections #2, #2A, #3, and #3A employed wire-wound flexible

ducts. All sheet metal connections were secured using three #8 ¥ 1/2” self-tapping screws

spaced approximately 120° apart. Likewise circumferential joints and longitudinal seams

were sealed used UL181B-M mastic. Sheet metal ducts were supported at a maximum

1.52 m (5 ft) spacing using wooden stands. All flexible duct connections were made in

accordance with “Flexible Duct Performance & Installation Standards Fifth Edition”.

Flexible ducts were strap supported at a maximum 1.23 m (4 ft) spacing. The dampers

were installed per the drawing locations, and plumbers putty was used to seal any air

leakage. The test setups were in compliance with Figures 8 (airflow measuring chamber)

and 17 (test setup) of ASHRAE Standard 120-2017.

The lengths and diameters of System Sections #1A and #1 are noted on Figures 1

and 4, respectively. Similarly the nominal diameters of the flexible ducts comprising

System Sections #2A, #2, #3A, and #3 are indicated on Figures 2, 3, 5, and 6,

respectively. The flexible duct lengths utilized in System Sections #2 and #3 conformed

closely to the corresponding steel duct lengths used in System Section #1. Likewise the

flexible duct lengths employed in System Sections #2A and #3A were consistent with the

analogous steel duct lengths incorporated in System Section #1A. In each instance the

flexible ducts were first fully stretched using a sufficient axial force that was maintained

for a prescribed time period, as mandated by ASHRAE Standard 120-2017. Therein the

flexible ducts were allowed to relax while resting on the laboratory floor, and then cut to

the desired length. Every effort was made to ensure that all bends present in the flexible

duct setups were as smooth as possible.

System Sections #1A, #2A, and #3A

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In this portion of the test program, both registers for each test apparatus were set

to their wide-open positions with the throw directed outward. PH4 boots were installed in

each test apparatus. The static gage pressure Dps,1-2 was measured adjacent to the

upstream 45∞ elbow using a piezometric ring as a function of the standard air volume

flow rate. A steel wye was installed in System Section #2A, whereas a ductboard splitter

box was utilized in System Section #3A. Similar fastening techniques were employed as

discussed subsequently under system sections #1, #2, and #3.

System Sections #1, #2, and #3

For System Section #1, 90∞ steel elbows were installed upstream of the PH7

boots/registers. However, for tests conducted on System Sections #2 and #3, the elbows

upstream of the boots/registers were constructed using wire-wound flexible ducts. In

every instance a dimensionless bend radius r/D = 1 was present. This was accomplished

by mounting the inner liner of each flexible duct to the collar of the boot, and fastening

per the ADC Flexible Duct Performance & Installation Standards, Fifth Edition (2010).

Draw bands were used on the inner liner of flex ducts in order to fasten the joints and

boots to ducts, and another two layers of approved tape was applied to avoid leakage.

Therein fabric straps were utilized to restrain each elbow, in order to better achieve the

desired bend radius. Considerable care was taken not to constrict the inner liner with the

support restraint when constructing the elbows.

TEST APPARATUS

A 30 hp centrifugal fan was employed to provide air flow through each test

apparatus. The forced draft mode was employed in the experiments. A VFD was used to

control the flow rate through the test section. A multiple-nozzle chamber that was in

7

compliance with the requirements of ASHRAE Standard 120-2017 was used to

accurately measure the volume flow rate through the test setup. The data reduction

equations were taken directly from ASHRAE Standard 120-2017. Screens located in the

nozzle chamber upstream of the nozzle board ensured that the air flow was almost

uniform before entering the flow nozzles. Unused nozzles were blocked by vinyl balls.

The pressure drop across the nozzles was measured using two piezometric rings located

38 mm (1.5 in.) on either side of the nozzle board. Both sides were connected to a

micromanometer which read the pressure drop across the nozzle in terms of inches of

water with a scale readability of 0.025 mm (0.001 in.). The upstream piezometric ring

was also connected through a ‘T’ to a digital manometer, which measured the nozzle

chamber static pressure with a scale readability of 0.25 mm (0.01 in.). The ambient wet-

bulb and dry-bulb temperatures were measured using a compact lab pyschrometer with a

scale readability of 0.6∞C (1∞F). Likewise the atmospheric pressure was measured using a

mercury barometer with a scale readability of 0.25 mm (0.01 in.) of mercury.

A bell mouth was employed to ensure the air flow exiting the nozzle chamber

entered the entrance length duct appropriately. The static gage pressure loss

measurements were performed using pressure taps mounted on the rigid steel ducts

upstream of the test section. The pressure taps were configured as piezometric rings, in

order to average the measured pressure at each measuring plane. A micromanometer was

employed to measure the pressure drop over the test section in terms of inches of water

with a scale readability of 0.025 mm (0.001 in.). For each pressure drop test, the blower

motor was allowed to run for several minutes in order to obtain stable conditions. Air

8

flow rates were obtained for each duct configuration. Readings recorded at each test point

were as follows:

∑ Ambient dry-bulb temperature ∑ Ambient wet-bulb temperature ∑ Barometric pressure ∑ Approach dry-bulb temperature ∑ Approach static pressure ∑ Nozzle pressure differential ∑ Test section static gage pressure loss

CALCULATIONS

In this study the system pressure loss coefficient for System Sections #1A, #2A,

and #3A was defined as the ratio of the total pressure loss through the duct assembly to

that of velocity pressure measured immediately upstream of the test section, and was

given by:

v

2t,1

pΔp

C -= (1)

It is apparent from Equation 1 that the system loss coefficient includes pressure losses

associated with both the fittings and the straight duct sections, i.e., it is not a zero-length

loss coefficient as defined by ASHRAE Standard 120-2017. The velocity pressure was

based on the measured average velocity in the duct upstream of the test section, corrected

back to standard conditions of density:

2stdv Vρ21p = (2)

It is straightforward to demonstrate that Equation 2 can be expressed in terms of the

measured standard volume flow rate, wherein:

42stdstd

2v DQ8p r

p= (3)

9

The term ‘D’ represents the nominal duct diameter immediately upstream of the test

section for System Sections #1A, #2A, and #3A.; in this instance that dimension was 203

mm (8 in.). Recognizing that each system blew down to atmospheric pressure, the

measured total pressure loss was evaluated as follows:

v21,s21,t ppp +D=D -- (4)

The total pressure losses were measured experimentally at each flow rate, and the least

squares method was employed in order to obtain an overall loss coefficient, such that:

v21,t pCp ◊=D - . (5)

The slope of the curve 21t,Δp - plotted against vp with the intercept forced through zero

was interpreted as the loss coefficient for system. For System Sections #1A, #2A, and

#3A, the system loss coefficient factor was determined as the loss coefficient measured

for each system, divided by the loss coefficient of System Section #1A:

Loss Coefficient Factor = CSystem Section #nACSystem Section #1A

(6)

This was evaluated at design conditions, i.e., with all registers in their wide open

positions.

The uncertainty of the system loss coefficient calculation was estimated as

follows:

xx

yx2N,2/ S

stC -a±=D (7)

The quantity ta/2,N-2 is the student’s t-statistic with N-2 degrees of freedom. In this case N

represents the number of points in the data set, and a = (1-c) is the assumed confidence

10

level (95%). In this instance let xi = pv,i and yi = Δpt,i,. The value of yxs was determined

using

( )[ ]21

N

1i

2iiyx xyy2N

1s ˜̃¯

ˆÁË

Ê-

-= Â

=

(8)

where yi is the actual value at point i, and y(xi) was the value obtained by the least

squares fit at point i. Likewise the Sxx in Equation 7 was calculated from the expression

Â=

-=N

1i

2i

2xx )xx(S (9)

where x is the mean x value.

At design conditions, i.e., with all dampers and registers wide open, the measured

static pressure for each of System Sections #1, #2, and #3 was normalized by the static

pressure measured for System Section #1. Hence the system pressure ratio was calculated

as follows:

System Pressure Ratio = ∆ps,1−2(#n)∆ps,1−2(#1)

(10)

In this instance the quantity ‘n’ denotes the system section number. Likewise for System

Sections #1, #2, and #3 it was deemed important to compare the sum of flow rates

measured at standard conditions through each of the registers (using the low flow

balometer) to the flow rate determined by means of the calibrated nozzle chamber. This

assessment was performed for both for design and balanced conditions. In the latter

instance the flow was controlled using dampers located in each branch. The difference

between those conditions was characterized in the following manner:

% difference = Total Register Flow Rate− Nozzle Flow RateNozzle Flow Rate

× 100 (11)

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Likewise in the present study it was proposed to quantify the initial unbalance in System

Sections #1, #2, and #3 as follows:

% unbalance = �∑ (Qi,des−Qi,bal)2ni=1

∑ Qi,balni=1× 100 (12)

In this case the variable ‘n’ represents the number of branches in the system. Moreover

Qi,design indicates the flow rate measured using the low flow balometer in each branch at

design conditions, and Qi,balanced represents the measured flow rate in each branch at

balanced conditions.

RESULTS System Sections #1A, #2A, and #3A

A summary of tests results from System Sections #1A, #2A, and #3A is provided

in Table 1. The static pressure loss Dps,1-2 was measured over a prescribed ranges of flow

rates, which were corrected to standard conditions. In every instance the data were

generated by setting all registers to their wide-open position. The static pressure loss is

plotted as a function of the standard air flow rate standard air flow rate in Figure 7 for

System Sections #1A, #2A, and #3A. A quadratic curve for static pressure loss [Pa (in.

WC)] through the system as a function of standard air flow rate [L/s (SCFM)] was fit to

the data using the least squares method. This yielded the following system equations and

their associated coefficients of determination.

System Section #1A:

00.1R;1002.5Q1013.59Q1060.1p 2122321,s =¥-¥+¥=D---

- (13 SI)

00.1R;1002.2Q1012.1Q1040.1p 2132621,s =¥-¥+¥=D---

- (13 I-P)

System Section #2A:

12

00.1R;1042.2Q1031.30Q1080.2p 2122321,s =¥-¥+¥=D---

- (14 SI)

00.1R;1072.9Q1075.5Q1048.2p 2242621,s =¥-¥+¥=D---

- (14 I-P)

System Section #3A:

00.1R;1088.68Q1042.17Q1090.3p 2122321,s =¥-¥+¥=D---

- (15 SI)

00.1R;1077.2Q1030.3Q1052.3p 2242621,s =¥-¥+¥=D---

- (15 I-P)

Figure 8 illustrates the measured total pressure loss for System Section #1A as a

function of the velocity pressure upstream of the test section (based on standard

conditions of air density); the resulting system loss coefficient C = 6.048. Figure 9 shows

the corresponding system loss coefficient for System Section #2A; for that case C =

7.055. For System Section #3A the steel wye mounted in System Section #2A was

replaced with a ductboard splitter box. As shown in Figure 10, the resulting system loss

coefficient was C = 8.701. In Figures 8 through 10 the solid lines represent the actual

trend followed by the measured data, whereas the dashed lines denote the uncertainty of

loss coefficients with 95% confidence.

System Sections #1, #2, and #3

A summary of all test data obtained for System Sections #1, #2, and #3 is

provided in Table 2. The static pressure loss Dps,1-2 was measured over a prescribed

ranges of flow rates using a VFD to control the fan speed. In every instance the measured

actual volume flow rate was corrected back to standard conditions. The resulting data are

plotted in Figure 11. Least-squares curve fitting was therein employed to calculate the

following system pressure loss curves and their resulting coefficients of determination:

System Section #1:

13

00.1R;1070.51Q1000.2Q1000.7p 2332521,s =¥+¥-¥=D---

- (16 SI)

00.1R;1008.2Q1028.1Q1026.7p 2452721,s =¥+¥-¥=D---

- (16 I-P)

System Section #2:

00.1R;1090.3Q1000.2Q1000.7p 2352721,s =¥+¥-¥=D---

- (17 SI)

00.1R;1093.3Q1038.2Q1029.7p 2352721,s =¥+¥-¥=D---

- (17 I-P)

System Section #3:

00.1R;1087.27Q1080.10Q1000.8p 2132521,s =¥+¥-¥=D---

- (18 SI)

00.1R;1012.1Q1000.7Q1000.8p 2252721,s =¥+¥-¥=D---

- (18 I-P)

The static pressure loss was likewise evaluated for System Sections #1, #2, and #3

at the desired design conditions (approximately [255 L/s (540 cfm)]), i.e., with all control

dampers set to their wide open positions. Under those circumstances it is apparent that for

System Section #1 the measured design static pressure Dps,1-2 = 51.0 Pa (0.205 in. WC).

For System Section #2 the measured design static pressure Dps,1-2 = 52.5 Pa (0.211 in.

WC). Similarly for System Section #3 the static pressure at design conditions Dps,1-2 =

51.3 Pa (0.206 in. WC). A calibrated air flow capture hood with a 0.41 m ¥ 0.41 m (16 in.

× 16 in.) fabric hood was used to measure the flow rates at each of the four registers. Per

the manufacturer’s recommendation, the one-point measurement mode was employed.

Hence the closed vent mode was used for register air flow measurements between 4.72

L/s (10 cfm) and 70.8 L/s (150 cfm), and the open vent mode was utilized for

measurements between 70.8 to 236 L/s (150 to 500 cfm). Table 1 also indicates the

measured air flow rate at the registers for design conditions, i.e., with all control dampers

set to their wide open positions.

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Testing was also conducted on System Sections #1, #2, and #3 to measure the

performance under balanced conditions. In those instances the control dampers were

adjusted to achieve the desired standard air flow rates at each of the four registers, as

measured at each using the balometer capture hood. Under those conditions the flow rate

sum indicated by the balometer for System Section #1 was 261 L/s (554 SCFM), whereas

the calibrated flow nozzle yielded a flow rate of 255 L/s (540 SCFM). Likewise at

balanced conditions for System Section #1, the static pressure Dps,1-2 = 93.4 Pa (0.375 in.

WC). For System Section #2, the flow rate sum indicated by the balometer was 254

SCFM (538 SCFM), whereas the corresponding flow rate measured using the calibrated

flow nozzle was 243 L/s (515 SCFM). Moreover, the static gage pressure measured at

balanced conditions was Dps,1-2 = 120.8 Pa (0.485 in. WC). For balanced conditions in

System Section #3, the flow rate sum indicated by the balometer was 260 L/s (551

SCFM), and the corresponding flow rate measured using the calibrated flow nozzle was

251 L/s (532 SCFM). Under those circumstances the static gage pressure Dps,1-2 = 63.5 Pa

(0.255 in. WC).

Referring to Figure 11, it is apparent that System Section #1 (comprised of all

steel ducts) exhibited a static gage pressure at all air volume flow rates that conformed

closely to System Section #2 or System Section #3, which were constructed using wire-

wound flexible ducts. Each apparatus was assembled such that the total length of each of

the duct sections were equal. The layout of each apparatus was likewise similar.

However, the duct diameters employed in System Section #2 downstream of the plenum

boxes were upsized by 1 in., relative to the duct diameters employed in System Section

#1, with the exception of the SE branch which consisted of the same diameter ducts for

15

both the System Section #1 and #2. The volume flow rates measured using the balometer

capture hood generally exhibited a positive bias relative to the values obtained from the

flow nozzle. The bias was not constant, and tended to increase directly in proportion to

the volume flow rate.

CONCLUSIONS

In the present test program, when the multi-line duct systems constructed from

wire-wound flexible ducts possessed the same diameters as those fabricated using rigid

steel ducts, e.g. System Sections #1A, #2A, and #3A, the total system pressure loss

penalty was 17% as expressed in terms of the measured system loss coefficients. When

using a duct board triangle box in place of the metal wye the pressure loss penalty was as

much as 44%. It is evident that data acquired from those experiments fell within the

stated 95% confidence level. Since each apparatus possessed equal cross section ducts

(based on their nominal diameters), and in every instance exhibited similar layouts, the

system loss coefficients provide a convenient method of measuring and comparing the

total pressure loss for typical residential duct systems constructed either from steel or

wire-wound flexible ducts when the flexible ducts are not properly sized per ACCA

Manual D.

Multi-branch duct systems are often designed using the equal friction method,

where a static pressure loss per unit length is selected and used to size all ducts in the

system. That design approach should be applied to low velocity systems with

symmetrical duct layouts, or in those instances where the duct lengths from the fan to the

terminal units are approximately equal. However the flow through a multi-branch duct

system does not precisely equal the design flow rate in each branch, due to uncertainties

16

associated with the fitting loss coefficients and duct roughness, and because ducts can be

manufactured only in discrete diameter intervals. Hence the system must be balanced by

adjusting dampers located in each branch. This increases the static pressure in the main

trunk downstream of the fan. Testing performed on System Sections #1, #2, and #3

indicated that the difference in pressure loss for typical residential duct systems

constructed from properly sized and installed wire-wound flexible ducts and similar

systems constructed from rigid steel ducts was negligible at design (unbalanced)

conditions. Balancing these duct systems by adjusting dampers located in each branch

increased the static pressure in the main trunk leading from the fan. The smallest pressure

difference between design and balanced conditions was achieved with System Section #3,

whereas the greatest difference was observed with System Section #2. Both of those duct

systems were constructed using wire-wound flexible ducts. Likewise System Section #3

exhibited the least initial unbalance, and System Section #2 displayed the largest initial

unbalance. In contrast, tests performed on System Section #1, which was constructed

using rigid steel ducts, yielded a static pressure difference between design and balanced

conditions that was intermediate to that observed in the analogous wire-wound flexible

duct systems. This applied as well to the initial flow unbalance.

NOMENCLATURE

C = loss coefficient, dimensionless D = diameter, mm (in.) N = number of data points, dimensionless n = number of branches in system, dimensionless

vp = velocity pressure, Pa (in. WC) Q = flow rate at standard air density, L/s (SCFM) R2 = coefficient of determination, dimensionless r/D = radius-to-diameter ratio, dimensionless

2xxS = sum of squared deviations of C about the curve, dimensionless

syx = standard error of C about the curve, dimensionless

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ta/2,N-2 = student’s t-statistic with N-2 degrees of freedom V = average velocity, m/s (ft/min) x = independent variable x = mean value y = dependent variable ΔC = loss coefficient uncertainty

tpΔ = total pressure loss, Pa (in. WC)

spΔ = static pressure loss, Pa (in. WC) r = density, kg/m3 (lbm/ft3) Subscripts 1 plane 1 2 plane 2 bal balanced condition des design condition i data point “i” std standard REFERENCES

ACCA. 2014. Manual D Residential Duct Systems, Third Edition. Arlington, VA: Air Conditioning Contractors of America.

ADC. 2010. Flexible Duct Performance & Installation Standards, Fifth Edition. Schaumburg, IL: Air Duct Council.

ASHRAE. 2017. ASHRAE Standard 120-2017: Method of Testing to Determine Flow Resistance of HVAC Ducts and Fittings. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Abushakra, B., I. S. Walker, M. H. Sherman. 2004. Compression Effects on Pressure Loss in Flexible HVAC Ducts. International Journal of Heating, Ventilating, Air-Conditioning and Refrigeration Research, 10(3):275-289.

Brooks, P.J. 1993. New ASHRAE Local Loss Coefficients for HVAC Fittings. ASHRAE Transactions, 99(2):169-193.

Culp, C., D. Cantrill. 2009. Static Pressure Losses in 12”, 14” and 16” Non-Metallic Flexible Ducts with Compression and Sag. ASHRAE Transactions, 115(1):622-628.

Culp, C.H. 2011. HVAC Flexible Duct Pressure Loss Measurements. ASHRAE Research Project RP-1333, Final Report.

Gibbs, D.C., S. Idem. 2012. Measurements of Flat Oval Diverging-Flow Fitting Loss Coefficients. ASHRAE Transactions, 118(1):1146-1153.

Griggs, E. I., Khodabakhsh-Sharifabad, F. 1992. Flow Characteristics in Rectangular Ducts. ASHRAE Transactions, 98(1):116-127.

Griggs, E.I.; Swim, W.B.; Henderson, G. H. 1987. Resistance to Flow of Round Galvanized Ducts. ASHRAE Transactions, 93(1):3-16.

Heyt, J. W., Diaz, M. J. 1975. Pressure Drop in Flat-Oval Spiral Air Duct. ASHRAE Transactions, 81(2):221-232.

Huebscher, R.G. 1948. Friction Equivalents for Round, Square and Rectangular Ducts.

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ASHVE Transactions, 54:101-118. Hutchinson, F.W. 1953. Friction Losses in Round Aluminum Ducts. ASHVE

Transactions, 59:127-137. Idem, S. 2003. Main Loss Coefficient Measurements for Flat Oval Tees and Laterals.

ASHRAE Transactions, 109(1):456-461. Idem, S., F. Khodabakhsh. 1999. Influence of Area Ratio on Flat Oval Divided Flow

Fitting Loss Coefficients. HVAC&R Research 5(1):19-33. Kulkarni, D., S. Khaire, S. Idem. 2009a. Measurements of Flat Oval Elbow Loss

Coefficients. ASHRAE Transactions, 115(1):35-47. Kulkarni, D., S. Khaire, S. Idem. 2009b. Influence of Aspect Ratio and Hydraulic

Diameter on Flat Oval Elbow Loss Coefficients. ASHRAE Transactions, 115(1):48-57.

Kulkarni, D., J. Cui, S. Idem. 2011. Laboratory Testing of Converging Flow Flat Oval Tees and Laterals to Determine Loss Coefficients. HVAC&R Research, 17(5):710-725.

Kulkarni, D., S. Idem, 2015, “Loss Coefficients of Bends in Fully Stretched Nonmetallic Flexible Ducts,” Science and Technology for the Built Environment, 21:413-419.

Nalla, A.N., S. Idem. 2012. Laboratory Testing of Saddle-Tap Tees to Determine Loss Coefficients. ASHRAE Transactions, 118(1)1131-1145.

Sun, Y., S.E. Ford, Y. Zhang. 2015. Laboratory testing of flat oval transitions to determine loss coefficients. Science and Technology for the Built Environment, 21(4):386-395.

Townsend, B, F. Khodabakhsh, S. Idem. 1996a. Loss Coefficient Measurements in Divided-Flow Flat Oval Fittings. ASHRAE Transactions, 102(2):151-158.

Townsend, B., F. Khodabakhsh, S. Idem.1996b. Loss Coefficient Measurements for Flat Oval Elbows and Transitions. ASHRAE Transactions, 102(2):159-169.

Weaver, K., C. Culp. 2007. Static Pressure Losses in Nonmetallic Flexible Duct (6”, 8” & 10”). ASHRAE Transactions, 113(2):400-405.

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Table 1. Summary of Test Data for System Sections #1A, #2A, and #3A

System Loss Coefficient Loss Coefficient Factor (Eq. 6) #1A 6.05 1.00

#2A 7.06 1.17

#3A 8.70 1.44

Table 2. Summary of Test Data for System Sections #1, #2, and #3

System Condition 21,sp -D ,

Pa (in. WC)

System Pressure

Ratio (Eq. 10)

Nozzle Flow Rate, L/s

(SCFM)

Register Flow Rate, L/s (SCFM) %

Difference (Eq. 11)

% Unbalance

(Eq. 12) NE NW SE SW Total

#1 Design 51.0 (0.205) 1.00

255 (540)

69 (146)

75 (158)

72 (152)

46 (98)

261 (554) 2.6 7.0

Balanced 93.4 (0.375) 247

(524) 78

(165) 76

(161) 56

(119) 49

(103) 259

(548) 4.6

#2 Design 52.5 (0.211) 1.03

257 (544)

59 (125)

73 (155)

72 (153)

61 (130)

266 (563) 3.5 10.6

Balanced 120.8 (0.485) 243

(515) 75

(159) 76

(160) 57

(120) 47

(99) 254

(538) 4.5

#3 Design 51.3 (0.206) 1.00

253 (536)

73 (155)

75 (159)

70 (148)

47 (99)

265 (561) 4.7 5.3

Balanced 63.5 (0.255) 251

(532) 77

(163) 76

(162) 57

(121) 50

(105) 260

(551) 3.6

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Figure 1. Configuration of System Section #1A (Dimensions in I-P Units)

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Figure 2. Configuration of System Section #2A (Dimensions in I-P Units)

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Figure 3. Configuration of System Section #3A (Dimensions in I-P Units)

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Figure 4. Configuration of System Section #1 (Dimensions in I-P Units)

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Figure 5. Configuration of System Section #2 (Dimensions in I-P Units)

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Figure 6. Configuration of System Section #3 (Dimensions in I-P Units)

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Figure 7. System Curve with All Registers Wide Open – System Sections #1A, #2A, and #3A

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Figure 8. System Section #1A Loss Coefficient

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Figure 9. System Section #2A Loss Coefficient

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Figure 10. System Section #3A Loss Coefficient

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Figure 11. System Curve with All Control Dampers and Registers Wide Open – System Sections #1, #2, and #3

Test Setup of System Section #1A

Test Setup of System Section #2A

Test Setup of System Section #3A

PH4 Boot Installed in System Section #1A, #2A, and 3A

Wye Installed in System Section #2A

Splitter Box Installed in System Section #3A

Test Setup of System Section #1

Test Setup of System Section #2

Test Setup of System Section #3

R/D = 1 Flexible Duct Elbow with Fabric Strap Restraint