ASHRAE AB-10-002-2010 Measured and Predicted Pressure Loss in Corrugated Spiral Duct.pdf

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1、380 ASHRAE TransactionsABSTRACTAn experimental program was initiated to determine the absolute roughness of three corrugated circular spiral ducts. The average absolute roughness value for these ducts was 0.85 mm (0.0028 ft). Pressure loss tests were likewise performed on a round standard spiral sea

2、m duct using Pitot-static tubes mounted at the duct centerline, and also using wall static pres-sure taps mounted on the duct surface at identical axial loca-tions. The absolute roughness values obtained by these two approaches were indistinguishable. The resulting average abso-lute roughness for th

3、e standard spiral duct was 0.12 mm (0.0004 ft). Pressure loss predictions for corrugated and standard spiral ducts are compared. INTRODUCTIONCorrugated spiral ducts are increasingly being used in aboveground HVAC systems. The presence of corrugations increases the rigidity and structural strength of

4、 the duct, so that lighter gauges can be employed in many applications. Under those circumstances corrugated ducts may be less expensive and easier to install than conventional spiral seam ducts. Furthermore their enhanced resistance to permanent deforma-tion allows them to be stacked higher for tra

5、nsportation and storage.In the present research the roughness characteristics of corrugated galvanized spiral ducts having a somewhat different corrugation profile from that examined by Kulkarni et al. (2009). The objective of this research was to verify whether minor corrugation profile and seam pi

6、tch variations have a significant impact on the resulting abso-lute roughness and roughness category. The test procedure was validated by measuring pressure loss characteristics of a standard conventional spiral seam duct per ASHRAE Standard 120. Those results were then compared to data presented pr

7、eviously in the literature. Pressure loss characteristics of conventional galvanized steel ducts with continuously rolled spiral seams have been described in Griggs et al. (1987). Similarly, roughness factors for galvanized steel spiral seam ducts with varying numbers of ribs have been reported in G

8、riggs et al. (1987). In these instances the data were not obtained by tests conducted in accordance with ASHRAE Standard 120. Until recently there has been a lack of corrugated galvanized spiral round duct information available to the designers of duct systems. However Kulkarni et al. (2009) used AS

9、HRAE Standard 120 to obtain pressure loss data for one particular corrugation configuration. EXPERIMENTAL PROGRAMThree round, spiral, 24 gauge, galvanized steel ducts with four corrugations between helical seams were tested in this project. The geometric details of the corrugations and seams are sho

10、wn in Figure 1; in the present study the depth and configuration of the corrugations differed from those tested in Kulkarni et al. (2009). The duct diameters were 203 mm (8 in.), 356 mm (14 in.), and 508 mm (20 in.). In each case the duct sections were 3.05 m (10 ft) in length, and were connected by

11、 beaded slip couplings possessing integral sealing gaskets. Each joint was further wrapped by commercial duct tape. The 356 mm (14 in.) diameter non-corrugated galvanized steel ducts tested in this project possessed a standard spiral seam (RL-1 seam per SMACNA (2005) having a pitch of 121 mm (4.75 i

12、n.). The 3.05 m (10 ft) duct sections were connected by beaded slip couplings and sealed using duct tape.Measured and Predicted Pressure Loss in Corrugated Spiral DuctD.C. Gibbs S. Idem, PhDAssociate Member ASHRAE Member ASHRAED.C. Gibbs is a mechanical engineer with BWSC, Inc., in Nashville, TN. S.

13、 Idem is a professor in the Department of Mechanical Engineering at Tennessee Tech University, Cookeville, TN.AB-10-0022010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal use o

14、nly. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.2010 ASHRAE 381The test apparatus shown in Figure 2 (excerpted from ASHRAE Standard 120) was used to measure the pressure loss characteristics of the

15、corrugated and standard spiral duct. In every instance the duct test apparatus consisted of an entrance duct section to achieve fully developed flow (upstream length), the test section, and a tail portion (down-stream length). All tests were conducted with a plenum cham-ber and bellmouth combination

16、 situated between the upstream nozzle chamber and the downstream test section, per ASHRAE Standard 120. The plenum chamber had one settling screen with a 46.8% open area. The dimensions for each test setup are listed in Table 1. The duct diameters were measured in three planes and averaged. The appa

17、ratus, dimen-sions, and test procedures were in compliance with ASHRAE Standard 120.For tests conducted on corrugated ducts and conventional spiral ducts the pressure loss was measured using the static pressure ports of Pitot-static tubes mounted at the duct center-line at axial locations as prescri

18、bed by ASHRAE Standard 120. The standard spiral duct tests were also repeated using static wall pressure taps soldered onto the duct surface (at precisely the same locations as the Pitot-static tubes) in order to measure the pressure loss; the Pitot-static tubes were with-drawn from the test section

19、 for these tests. The pressure taps Figure 1 Geometric details of round corrugated duct.Figure 2 Straight duct test setup (ASHRAE 2008).2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). F

20、or personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.382 ASHRAE Transactionswere fashioned into a piezometric ring using flexible plastic tubing. The piezometer rings were connected to a

21、 single micromanometer by means of flexible tubing so as to measure the pressure drop across the test section. For all pressure loss tests static gage pressure was measured at each location by inserting tees into the pressure tubing. This procedure allowed for the determination of whether the use of

22、 Pitot-static tubes to measure pressure loss would yield similar results to measure-ments performed using wall static pressure taps.The system was blow-through. Airflow was generated by a 30-hp centrifugal fan. A cylindrical nozzle chamber was used for flow measurement, and a variable frequency driv

23、e was used to control air flow through the system. Screens mounted upstream and downstream of the nozzle board inside the chamber were used to settle the flow. The nozzle board contained four long-radius spun aluminum flow nozzles having throat diameters of 51-mm (2-in.), 102-mm (4-in.), 152-mm (6-i

24、n.) and 203-mm (8-in.). The nozzles were mounted on a 25-mm (1-in.) thick plywood board. Various combinations of flow nozzles were employed, depending on the desired flow rate. Nozzles that were not used were blocked using smooth vinyl balls. The pressure drop was measured by two piezometer rings lo

25、cated 38-mm (1.5-in.) on each side of the nozzle board, with both sides connected to a manometer.In every case pressure drop measurements over the test section and across the nozzle board were performed using liquid-filled micromanometers having a measurement accuracy of 0.025-mm (0.001-in.). Likewi

26、se, the pressure upstream and downstream of the test section was measured by means of inclined liquid-filled manometers having a readability of 0.25-mm (0.01-in.). Static pressure in the nozzle chamber was measured using an electronic manometer having the scale read-ability of 0.25-mm (0.01-in.). Ho

27、wever, because of observed pressure fluctuations associated with static pressure measure-ments in the nozzle chamber and test section, these measure-ments were presumed to exhibit an accuracy of 0.63 mm (0.025-in.). The air temperature in the nozzle chamber was measured using a mercury thermometer h

28、aving a scale readabil-ity of 0.5C (1.0F). The dry-bulb and wet-bulb temperatures of the ambient air were measured using an aspirated psychrom-eter, with an accuracy of 0.5C (1.0F). The test section temperature was not measured directly, but was assumed to be the same as the temperature of the air i

29、nside the nozzle chamber. Ambient pressure was measured with a Fortin-type barometer, with an accuracy of 0.25-mm (0.01-in.) of mercury. All measurements of temperature and pressure in this project were in compliance with ASHRAE Standard 120. All dimensional measurements in these experiments were as

30、sumed to have an accuracy of 1%.DATA REDUCTIONIn this study all data reduction complied strictly with ASHRAE Standard 120. The Darcy friction factor was calcu-lated by Equation 1; the plane locations are depicted in Figure 2.(1 SI)(1 I-P)The flow rate for each test point was calculated by Equa-tion

31、2, where 5 denotes the section upstream of the nozzle and 6 indicates the nozzle throat.(2 SI)(2 I-P)Additional equations necessary to support the flow calcu-lation per Equation 2 can be found in ASHRAE Standard 120. The Reynolds number in the test section was determined by Equation 3.(3 SI)(3 I-P)T

32、he average air velocity in the duct V was defined by the continuity equation using Equation 4.(4 SI)(4 I-P)The measured pressure loss data were plotted on a Moody diagram in terms of friction factor f as a function of relative Table 1. Test Setup DimensionsRound Corrugated and Standard Spiral Duct T

33、estsNominal Duct Diameter mm (in.)Measured Duct Diametermm (in.)LZ-1m (ft)L1-2m (ft)Tail Duct Lengthm (ft)203 (8) 203.2 (8.0) 2.8 (9.2) 5.3 (17.3) 1.1 (3.5)356 (14) 358.1 (14.0) 4.6 (15.0) 9.1 (30.0) 1.5 (5.0)508 (20) 508.0 (20.0) 6.1 (21.0) 12.8 (42.0) 2.1 (7.0)fpf,1 2L1212-1V12D 1000()-=fpf,1 2L12

34、1V11097()2D 12()-=Q 1000 Yn2 ps,5 65- CnAn()=Q 1098 Ynps,5 65- CnAn()=Re11V1D11000()1-=Re11V160()D112()1-=V1Q11000A-=V1Q1A-=2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (2010, Vol. 116, Part 2). For personal

35、use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.2010 ASHRAE 383roughness /D and Reynolds number. These quantities are related by the Colebrook equation.(5 SI)(5 I-P)The relative roughness was d

36、etermined iteratively by fitting the experimentally determined friction factors to the Colebrook equation using the least squares method; this approach is described in more detail in Kulkarni et al. (2009).The measurements were subjected to an uncertainty analysis based on the method of Kline and Mc

37、Clintock (1953), as prescribed by ASHRAE Standard 120 for random variations of the measured quantities. In every instance the measurement uncertainty estimates were performed with a 95% confidence level. TEST RESULTSThe friction factor data for tests performed on the corru-gated ducts are plotted as

38、 Moody diagrams in Figures 3 through 5. The horizontal bars through the data points represent the range of expected uncertainty in the measured Reynolds numbers, with a 95% confidence limit. Similarly the vertical bars through each point depict the range of expected uncertainty in the measured frict

39、ion factor, with a confidence limit of 95%. The absolute roughness data for the corrugated galvanized spiral ducts tested in this study are summarized in Table 2. The average absolute roughness value for corrugated ducts was 0.85 mm (0.0028 ft). These results compare closely to an absolute roughness

40、 value of 0.74 mm (0.0024 ft) reported in Kulkarni et al. (2009) for a simi-lar corrugation profile. Likewise standard spiral duct pressure loss data obtained using either Pitot-static tubes or wall static pressure taps with piezometer rings are depicted in Figure 6. Absolute roughness values for st

41、andard galvanized steel spiral ducts obtained are also provided in Table 2. The average abso-lute roughness for standard spiral ducts was 0.12 mm (0.0004 ft). Griggs et al. (1987) reported an absolute roughness for 254 mm (10 in) diameter unribbed standard spiral ducts having a nominal joint spacing

42、 of 3.66 m (12 ft) equal to 0.06 mm (0.0002 ft).PREDICTED DUCT PRESSURE LOSSEquation 1 can be rearranged such so as to obtain the Darcy equation.(6 SI)(6 I-P)Figure 3 Moody diagram for 203 mm (8 in.) round spiral corrugated ducts.Figure 4 Moody diagram for 356 mm (14 in,) round spiral corrugated duc

43、ts.1f- 2 l o g D13.7-2.51Re1f-+=1f- 2 l o g12 D13.7-2.51Re1f-+=Figure 5 Moody diagram for 508 mm (20 in.) round spiral corrugated ducts.pf1000fLD-V22-=pf12fLD- V1097-2=2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transact

44、ions (2010, Vol. 116, Part 2). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.384 ASHRAE TransactionsIn order to assess the influence of relative roughness on duct pressure loss,

45、a parametric study was performed over a range of typical duct diameters, lengths, and volumetric flow rates. For corrugated ducts an absolute roughness of 0.85 mm (0.0028 ft) was employed. Based on the measurements performed in the present study, an absolute roughness of 0.12 mm (0.0004 ft) was assu

46、med for standard spiral ducts; it is noted that the Friction Chart in the Duct Design chapter of the ASHRAE Handbook (2009) assumes a “medium smooth” roughness of 0.09 mm (0.0003 ft) based on data reported in Griggs et al. (1987). The Colebrook equation was used to solve iteratively for the friction

47、 factor by means of a standard root-solving procedure. In every instance standard conditions of temperature and pressure were assumed when calculating air thermal properties. The resulting predicted pressure loses are presented in Tables 3 through 5, where the quantity pc represents the friction pre

48、ssure loss for corrugated ducts, and p indicates the friction pressure loss for standard spiral ducts. The label - denotes a pressure loss that is smaller than 0.25 Pa (0.001 in. wg). Referring to the Darcy equation, the pressure loss in the duct is proportional to length. Hence at a given flow rate and duct diameter, the pressure results presented in Tables 3 through 5 can readily be extended to other duct lengths by means of straight