1、2010 ASHRAE 387ABSTRACTThis paper presents results of an experimental program to determine the leakage class for three cross section flat oval ducts for positive and negative internal pressures. Sealed and unsealed duct leakage tests were performed. The leakage data were found to be a power law func
2、tion of static pressure differ-ence between the interior and the exterior of the duct. The power law exponent ranged from 0.675 to 1.065 for sealed conditions, and 0.437 to 0.729 for unsealed conditions. Gener-ally the exponent found for an individual duct tested in the negative pressure mode was le
3、ss than that for the correspond-ing positive pressure mode. Leakage class values ranged from 0.034 to 1.048 mL/s per m2(0.23 to 2.2 cfm per 100 ft2) for sealed duct tests, and between 297.4 to 1342.2 mL/s per m2 (197.2 to 366.5 cfm per 100 ft2) for unsealed tests where sheet metal screws were not us
4、ed to assemble the duct sections. The flat oval sealed leakage results were lower than those for spiral and longitudinal seam circular ducts reported in the ASHRAE Handbook. INTRODUCTIONUnsealed duct leakage depends on the machinery used for its fabrication, material thickness, assembly methods empl
5、oyed, and workmanship during installation. Duct leakage tests by ASHRAE/SMACNA/TIMA (1985) and Swim and Griggs (1995) have verified that a power law relation can represent the duct leakage from longitudinal seams and trans-verse joints of assembled duct sections. It was also confirmed that for the s
6、ame duct construction the behavior of duct leak-age was almost the same under negative and positive pressure. These tests have suggested that for unsealed or unwelded ducts the joint leakage dominates, as the longitudinal seam leakage is about 10% to 15% of the total duct leakage.There is a wide ran
7、ge of products and sealing methods available for ducts. A forecast of leakage class attainable by commonly used duct construction and sealing methods, based on the data obtained by Swim and Griggs (1995) and ASHRAE/SMACNA/TIMA (1985), is available in the Duct Design chap-ter of the ASHRAE Handbook (
8、2009). These data do not account for the presence of fittings, a realistic spacing of rect-angular and round duct transverse joints and duct-mounted components, such as access doors and balancing dampers. The leakage classes were calculated under the assumption of 0.82 joints per meter, or 25 joints
9、 per 100-ft of duct length. The stud-ies cited above suggest that duct system leakage rates are primarily a function of the geometry of the joint and seams, the sealing used (if any), and the pressure difference between the inside and outside of the duct. Future leakage classes should be adjusted fo
10、r typical rectangular and round transverse joint spac-ing, 1.2 m (4 ft) and 3.1 m (10 ft), respectively. Ducts possessing a lower proportion of joints would have less leakage in both the sealed and unsealed categories.Many researchers have previously attempted to perform in-situ measurements of the
11、effects of duct leakage on air distribution system performance and building envelope infil-tration; representative studies are by Modera (1989), Yuill and Musser (1997), Proctor (1998), and Walker (1999). Most of these studies have tended to compare different sealing tech-niques. Performance studies
12、 by Xu et al. (2000, 2004) of air distribution systems in light and large commercial buildings have reported air leakage ratios from one-quarter to one-third of the fan-supplied airflow in constant-air-volume systems. The air leakage from the ducts including supply and return, Flat Oval Duct Leakage
13、 Class MeasurementD.C. Gibbs S. Idem, PhDAssociate Member ASHRAE Member ASHRAED.C. Gibbs is a mechanical engineer with BWSC, Inc., in Nashville, TN. S. Idem is a professor in the Department of Mechanical Engineering at Tennessee Tech University, Cookeville, TN.AB-10-0032010, American Society of Heat
14、ing, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions (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
15、.388 ASHRAE Transactionswere reported in terms of the ASHRAE-defined leakage class with the reported values within a range much higher than the leakage classes predicted by the ASHRAE Handbook (2009) for unsealed ducts.Leakage measurement studies by Aydin and Ozerdem (2006) on round and rectangular
16、ducts for positive internal pressures were conducted along with a branched duct system having different duct diameters; the results were fitted to the power law model developed by J. Stratton from ETL data (ASHRAE/ SMACNA/TIMA 1985) and confirmed by Swim and Griggs (1995). Apparently leakage data ar
17、e unavailable in the literature for flat oval ducts. As a result the present project was initiated to study the leakage characteristics of spiral seam galvanized steel flat oval ducts.EXPERIMENTAL PROGRAMExperiments were performed to estimate the leakage class of three cross sections of flat oval du
18、cts. The tests conformed to requirements of SMACNAs HVAC Air Duct Leakage Test Manual (1985). The duct cross sections are listed in Table 1. In every instance the ducts were constructed from 24-ga galva-nized sheet metal with spiral seams. The test section consisted of a total of six duct sections.
19、Each duct section was 1.2 m (4-ft) in length, and individual sections were connected by means of standard beaded slip couplings. Then 19 mm (-in.) thick plywood caps secured with duct tape were used to seal the ends of the ducts (refer to Figure 1). Thereafter, the system was pres-surized with a wet
20、/dry shop vacuum connected to the ductwork by a combination of PVC and dryer hose tubing. The shop vacuum was capable of producing in excess of 2.5 kPa, equiv-alent to approximately 10-in water pressure or vacuum in the ductwork. A ball valve was used to regulate the pressure from the shop vacuum. T
21、he makeup air flow rate entering the enclosed system (which equaled the leakage rate) was deter-mined by measuring the pressure drop across a calibrated Meriam Instruments laminar flow element (LFE), model 50MC2-2. Care was taken to ensure that sufficient straight runs of tubes were mounted at the e
22、ntrance and exit of the LFE.Figure 1 Duct leakage test setup.Table 1. Flat Oval Duct Cross Section and SealingCross Sectionmm mm(in. in.)Seam Type Joint Type Joint SealingSurface Aream2(ft2)356 152 (14 6)Spiral(RL-1)Beaded Slip Sealed/Unsealed 6.48 (69.7)381 102(15 4)Spiral(RL-1)Beaded Slip Sealed/U
23、nsealed 6.42 (69.1)559 152(22 6)Spiral(RL-1)Beaded Slip Sealed/Unsealed 9.45 (101.7)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 use only. Additional reproduction, distr
24、ibution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.2010 ASHRAE 389Ambient pressure was measured with a Fortin-type barometer with an accuracy of 0.25-mm (0.01-in.) of mercury. Pressure taps constructed from 6.4 mm (1/4 in.) diameter cop
25、per tubing were soldered onto the outer duct surface at the center of the second and fifth ducts in the test section. Flexible tubing was used to construct piezometer rings at both measurement locations. The piezometer rings were connected to a single electronic manometer using flexi-ble tubing to m
26、easure the average static gage pressure in the test section. Similarly electronic manometers were used to measure the pressure drop across the LFE, and the gage pres-sure at the inlet to the LFE. Likewise the air temperature enter-ing the LFE was measured using a type-K thermocouple with a measureme
27、nt accuracy of 0.25C (0.5F). The latter two measurements were performed to calculate air density enter-ing the LFE using the ideal gas law. All gage pressure measurements were performed with a measurement accuracy of 0.01 kPa (0.05 in. H2O).Per Table 1, for a sealed leakage test all duct joints were
28、 first sealed carefully with duct tape. For an unsealed leakage test all but the center joint (between the third and fourth ducts comprising the test section) were sealed with duct tape. The spiral seams were not sealed in any manner, and there were no other duct wall penetrations, i.e., sheet metal
29、 screws were not used to assemble the duct sections. It was deemed that this would represent a worst-case leakage scenario. SMACNA (1985) notes that “helical (spiral) lock seams are exempt from sealing requirements”. More rigorous sealing means such as mastic sealants or gaskets were not used on the
30、 joints, as it was required that the duct sections were to be re-used in other pres-sure loss tests. In performing the leakage tests the static pres-sure was increased gradually in steps, and thereafter the pressure was reduced by adjusting the control valve. The ducts were tested under both positiv
31、e and negative pressures, which were not allowed to exceed 1 kPa (4 in. wg.) in order to strictly limit any observable duct deformation.DATA REDUCTIONBy convention, the ASHRAE Handbook (2009) defines the leakage class CL as the leakage rate in mL/s per m2of duct surface area, at a static gage pressu
32、re of 1 Pa. This implies the following(1 SI)Similarly, the leakage class can be interpreted as the leak-age rate in cfm per 100 ft2of duct surface area at 1in. wg. Hence(1 I-P)In Equation 1 the exponent N = 0.65 represents a mean value obtained by averaging extensive ETL data (ASHRAE/ SMACNA/TIMA 19
33、85). As defined by Equation 1, the leak-age classes are dimensional quantities, and have different values depending on the units that are employed. However using straightforward units conversions, it can readily be shown that if N = 0.65 the classes are related as follows(2)Equation 2 is the basis f
34、or the leakage rate charts presented in the ASHRAE Handbook (2009).In terms of the major and minor flat oval duct dimensions, A and a, respectively, the duct perimeter length is given by Equation 3.(3)Therefore the total duct surface area is (4 SI)(4 I-P)where n is the number of duct sections in the
35、 test section, and is the length of an individual duct section.A laminar flow element is constructed by breaking up the overall flow passage into numerous small-diameter channels that are in parallel, typically by inserting a honeycomb in the device. Although the total air flow through the LFE may n
36、omi-nally be turbulent based on its Reynolds number, the flow through individual small channels is laminar. The volumetric flow rate through a laminar duct is a linear function of pressure drop. The LFE was calibrated to measure air flow rate Qst (in units of scfm) at standard conditions, i.e., for
37、pst= 29.92 in. Hg (760 mm) and Tst= 70F (21.1C). Therefore the actual makeup air flow rate Qa was calculated using Equation 5, based on the ideal gas law.(5 SI)(5 I-P)Sealed and unsealed leakage tests (ASHRAE/SMACNA/TIMA 1985, Swim and Griggs 1995) have confirmed that duct leakage can be represented
38、 by Equation 6(6)where C is a constant reflecting the area characteristics of the leakage path, is the normalized static pressure differential from the duct interior to the exte-rior, and N is a constant exponent relating turbulent or lami-nar flow in the leakage path. It is straightforward to solve
39、 for the constant coefficients in Equation 6 by log-linearizing the data and fitting a least squares curve through the data, such that(7)CL1000 Qps0.65-=CLQps0.65-=CL,SI1.408 CL,IP=P 2 Aa()a+=AsPnL 1000=AsPnL12=LQapstpa-Ta273+Tst273+-Qst=Qapstpa-Ta460+Tst460+-Qst=QaCps*()N=ps* pspref()=log QaN log p
40、s*log C+=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 use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted w
41、ithout ASHRAEs prior written permission.390 ASHRAE TransactionsThe leakage rate at a normalized static gage pressure is obtained by substitution into Equation 7.(8)Therefore the leakage class can be calculated by Equation 9.(9 SI)The leakage rate per 100 ft2of duct surface is proportional to duct su
42、rface area. Hence the leakage class can also be expressed as follows.(9 I-P)RESULTSResults of leakage rate measurements are plotted in Figures 2 through 4 for different duct configurations as a func-tion of static pressure difference. The data are displayed on log-log plots. A least squares power la
43、w curve fit was employed per Equation 7, and the duct leakage coefficients, C and N, were found. The linear correlation coefficient R is shown with the least squares results. For the curve fit equations in Figures 2 through 4 the coefficients are in I-P units only; the corresponding SI values were c
44、alculated using suitable units conversions based on experimentally determined values of the power law exponent N. The dual unit coefficients are given in Tables 2 and 3 for sealed and unsealed ducts. In general, the power law exponent for a duct leakage test was greater in the positive pressure mode
45、 than the negative pressure mode. Duct leakage classifications based on Equation 9 for both positive and negative pressure systems were found and tabu-lated. In Tables 4 and 5, duct leakage class is given for config-urations with all transverse joints sealed and one transverse joint unsealed. For te
46、sts performed with all joints sealed the leakage class was found to be between 0.034 to 1.048 mL/s per m2(0.229 to 2.150 cfm per 100 ft2). Likewise, the respective values for the unsealed configuration ranged between 297.4 to 1342.2 mL/s per m2 (197.2 to 366.5 cfm per 100 ft2). From these results it
47、 should be noted that for a poorly sealed or unsealed system leakage can be significant. In-situ measurements performed by Xu et al. (2000, 2004) have shown that leakage classes for duct systems in large commercial buildings with poorly sealed duct-work range from 34 to 757 mL/s per m2.Figure 2 356
48、152 mm (14 6 in.) flat oval duct leakage data.Figure 3 381 102 mm (15 4in.) flat oval duct leakage data.ps* 1=Qa( ps* 1) C=CLCAs-=Figure 4 559 152 mm (22 6 in.) flat oval duct leakage data.CLC100As-=2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org
49、). Published in ASHRAE Transactions (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.2010 ASHRAE 391Table 2. Flat Oval Duct Leakage Coefficients with All Joints SealedNominal Duct SizeA aPositive Pressure Negative PressureCmL/s (ft3/min)NC
