ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 19-2012 DUCT CONSTRUCTION.pdf

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1、19.1CHAPTER 19DUCT CONSTRUCTIONBuilding Code Requirements . 19.1Classifications 19.1Duct Cleaning 19.1HVAC System Leakage. 19.2Air-Handling Unit Leakage . 19.5Residential Duct Construction. 19.5Commercial Duct Construction . 19.6Industrial Duct Construction . 19.8Antimicrobial-Treated Ducts 19.8Duct

2、 Construction for Grease- and Moisture-Laden Vapors. 19.9Rigid Plastic Ducts. 19.9Air Dispersion Systems 19.9Underground Ducts 19.10Ducts Outside Buildings. 19.10Seismic Qualification . 19.10Sheet Metal Welding. 19.10Thermal Insulation. 19.10Specifications . 19.10HIS chapter covers construction of H

3、VAC and exhaust ductTsystems for residential, commercial, and industrial applica-tions. Technological advances in duct construction should be judgedrelative to the construction requirements described here and toappropriate codes and standards. Although the construction materi-als and details shown i

4、n this chapter may coincide, in part, withindustry standards, they are not in an ASHRAE standard.BUILDING CODE REQUIREMENTSIn the U.S. private sector, each new construction or renovationproject is normally governed by state laws or local ordinances thatrequire compliance with specific health, safety

5、, property protection,and energy conservation regulations. Figure 1 illustrates relation-ships between laws, ordinances, codes, and standards that can affectdesign and construction of HVAC duct systems (note that it may notlist all applicable regulations and standards for a specific locality).Specif

6、ications for U.S. federal government construction are pro-mulgated by agencies such as the Federal Construction Council, theGeneral Services Administration, the Department of the Navy, andthe Veterans Administration.Because safety codes, energy codes, and standards are developedindependently, the mo

7、st recent edition of a code or standard may nothave been adopted by a local jurisdiction. HVAC designers mustknow which code compliance obligations affect their designs. If aprovision conflicts with the design intent, the designer shouldresolve the issue with local building officials. New or differe

8、ntconstruction methods can be accommodated by the provisions forequivalency incorporated into codes. Staff engineers from themodel code agencies are available to assist in resolving conflicts,ambiguities, and equivalencies.Smoke management is covered in Chapter 53 of the 2011 ASH-RAE HandbookHVAC Ap

9、plications. The designer should con-sider flame spread, smoke development, combustibility, and toxicgas production from ducts and duct insulation materials. Code doc-uments for ducts in certain locations in buildings rely on a criterionof limited combustibility (see NFPA Standard 90A), which is inde

10、-pendent of the generally accepted criteria of 25 flame spread and 50smoke development; however, certain duct construction protectedby extinguishing systems may be accepted with higher levels ofcombustibility by code officials.Combustibility and toxicity ratings are normally based on testsof new mat

11、erials; little research is reported on ratings of aged ductmaterials or of dirty, poorly maintained systems.CLASSIFICATIONSDuct construction static pressure classifications typically usedon contract drawings and specifications are summarized by Table 1.The classifications are from SMACNA (2005) for

12、sheet metal duct-work, and NAIMA (2002a) for fibrous glass duct board. Negative-pressure flat oval duct systems can be designed by using 2500 Pasheet gages with the negative-pressure rectangular duct reinforce-ment welded to the duct. The most common flexible ducts arelisted with 2500 Pa maximum pos

13、itive-pressure ratings and any-where from 125 to 500 Pa negative-pressure ratings, but there arelisted flexible ducts with pressures as high as 4000 Pa and as lowas 3000 Pa.Air conveyed by a duct adds both static pressure and velocity pres-sure loads on the ducts structure. The load from static pres

14、sure dif-ferential across the duct wall normally dominates and the mean staticpressure is generally used for duct classification. Turbulent airflowadds relatively low but rapidly pulsating loading on the duct wall.Duct design is based on total pressure calculations as discussedin Chapter 21 of the 2

15、009 ASHRAE HandbookFundamentals.From these calculations, the designer should specify the static pres-sure classification of the various duct sections in the system. Allmodes of operation must be considered, especially in systems usedfor smoke management and those with fire dampers that must closewhe

16、n the system is running.DUCT CLEANINGDucts may collect dirt and moisture, which can harbor or trans-port microbial contaminants. Design, construct, and maintain ductsto minimize the opportunity for growth and dissemination ofmicroorganisms. Recommended control measures include accessfor cleaning, pr

17、oper filtration, and preventing moisture and dirtaccumulation. NADCA (2006) and NAIMA (2002b) have specificinformation and procedures for cleaning ducts. Air dispersionThe preparation of this chapter is assigned to TC 5.2, Duct Design.Fig. 1 Hierarchy of Building Codes and Standards19.2 2012 ASHRAE

18、HandbookHVAC Systems and Equipment (SI)systems should be cleaned by following the manufacturers instruc-tions. Owners should routinely conduct inspections for cleanliness.HVAC SYSTEM LEAKAGEFor the purposes of this chapter, and this section in particular,ductwork includes straight duct, flexible duc

19、t, sheet metal and rigidfiberglass plenums, and fittings (e.g., elbows, transitions, tees, wyes)for distribution and extraction of air. It does not, however, includeduct-mounted components (e.g., terminal units, access doors/panels,attenuators, coils, fire/smoke dampers, balancing and control damp-e

20、rs). A system consists of the supply air handler, return fan, exhaustfan, plenums, and all ductwork that connects the air handler to theconditioned space.HVAC system air leakage increases building energy consump-tion. It also reduces the systems ability to control and deliverintended flows and press

21、ures, and to manage spread of contami-nants. In addition, leakage can cause noise problems, drafts in theconditioned space, and dirt and dust deposits on the duct exterior.The leakage energy impacts depend upon building and system type.For small buildings with single-zone air distribution systems se

22、rvedby equipment such as packaged rooftop cooling units and furnaces(e.g., houses, commercial buildings with floor area less than 2300m2), 75 to 95% of the HVAC site energy is used for space heatingand cooling (Thornton et al. 2010; Walker and Sherman 2008;Zhang et al. 2010), and the impacts are mos

23、tly on the thermal side.For large buildings with central multizone air distribution systemsserved by equipment such as central chillers and boilers (e.g., mid-and high-rise offices, supermarkets and retail stores with a floor areaof 2300 m2or more), 20 to 80% of HVAC site energy is used by fans(Huan

24、g et al. 1991; Leach et al. 2009, 2010) and the impacts aremostly on fan power. All of these effects are strongly influenced bythe location of leaks relative to conditioned space.If supply air leaks to an unconditioned attic or crawlspace, or tothe outdoors, the lost heating or cooling must be repla

25、ced. In thiscase, heating or cooling fluid flows or temperature differences mustincrease or the system must run longer to meet the load. If insteadsupply air leaks to a ceiling return plenum adjacent to conditionedspaces, the largest impact is on fan power and the fan must run fasteror longer. The h

26、eat associated with this added fan power also createsan additional cooling load. Return leakage can also be important.For example, leaks from a hot attic into a return duct heat the returnair, which in turn reduces system cooling capacity.Because the relationship between fan power and airflow is som

27、e-where between a quadratic and cubic function depending on the sys-tem type, an increase in airflow to provide the desired service andcompensate for system leakage means that fan energy consumptionincreases significantly. Field measurements by Diamond et al. (2003)showed that a leaky VAV system (10

28、% leakage upstream and 10%downstream of terminal box inlet dampers at operating conditions)uses 25 to 35% more fan energy than a tight system (2.5% upstreamand 2.5% downstream at operating conditions). For an exhaust sys-tem with 20% leakage, the fan has to move 25% more air to meet thespecified flo

29、ws at the grilles, which causes fan power to increase95%.System SealingIt is recommended that all ductwork and plenum transversejoints, longitudinal seams, and duct penetrations, including dampershafts, be sealed. Openings for rotating shafts, wires, and pipes ortubes should be sealed with bushings

30、or other devices that mini-mize air leakage but that do not interfere with shaft rotation or pre-vent thermal expansion. Component leakage should not be used fortemperature control of associated motors and electronics. Sealingthat meets the above requirements is in compliance with ASHRAEStandards 90

31、.1-2010 and 189.1-2009, the International Mechani-cal Code(ICC 2012a), the International Energy ConservationCode(ICC 2012b), the International Residential Code(ICC2012c), and the Uniform Mechanical Code(IAPMO 2012). Spirallock seams need not be sealed. Duct-mounted equipment, such asterminal units,

32、reheat coils, and access doors, should be specified aslow leakage so that the combined HVAC system can meet air leak-age criteria set by the designer, the ASHRAE Handbook, standards,and codes.Sealing that would void product listings, such as for fire/smokedampers, is not required. It is, however, re

33、commended that thedesign engineer specify low-leakage duct-mounted components.For example, some manufacturers of UL-listed and -labeled fire/smoke dampers allow sealing and gasketing of breakaway duct/sleeve connections; all can provide sealed non-breakaway duct/sleeve connections.SealantsGeneral. A

34、ll tape, mastic, rolled sealants, aerosol/spray appliedsealants, gaskets, and nonmetallic mechanical fasteners shouldBe used in compliance with the manufacturers instructionsBe tested to UL Standard 723 (ASTM Standard E84) and have aflame spread index equal to or less than 25 and a smoke developedin

35、dex equal to or less than 50Maintain their airtightness over the service life of the componentto which they are appliedSheet Metal Ductwork. All joints, longitudinal and transverseseams, and connections in sheet metal ductwork should be securelyfastened and sealed with welds, gaskets, tapes, mastics

36、, mastic-plus-embedded-fabric systems, rolled sealants, or aerosol sealants.Pressure-sensitive tapes, rolled sealants, mastics, gaskets, andaerosol sealants used to seal sheet metal ductwork should be testedfor durability using ASTM Standard E2342 and have a minimum60-day time to failure. Heat sensi

37、tive and heat activated tapes shouldnot be used as a sealant on any metal ducts. In particular, cloth-backnatural latex-rubber adhesive duct tape should not be used regard-less of UL designation.For exterior applications, mastics should be tested using ASTMStandard C732 artificial weathering tests a

38、nd show no signs of vis-ible degradation (e.g., washout, slump, cracking, loss of adhesion)after being exposed to artificial weathering.Tapes should be used only on joints between parallel surfaces, oron right-angle flat joints.Table 1 Pressure Classification for DuctworkaType DuctStatic Pressure Cl

39、ass, Pa+125 125 +250 250 +500 500 +750 750 +1000 1000 1000 1000 1000 1000Rectangular Round Flat oval bFibrous glass duct boardcFlexible duct: fabric and wired Notes:aColumns with a dot indicate that construction standards are available for the pressure classes shown.bSame reinforcement as rectangula

40、r duct, except reinforcement mechanically attached to duct.cFibrous glass duct board must be UL Standard 181 listed.dFlexible duct must be UL Standard 181 listed and labeled.Duct Construction 19.3Rigid Fiberglass Ductwork. Rigid fiberglass ductboard shouldbe sealed following the NAIMA (2002b) standa

41、rd using materialslisted and labeled to the UL Standard 181A standard. There arethree closure and air sealing systems for fiberglass duct board. Thefirst two use tapes (marked “181A-P” or “181A-H”); the third sys-tem is a fiberglass mesh and mastic system (marked “181A-M”). Inthe latter case, a laye

42、r of mastic is applied to the joint, a strip offiberglass mesh is embedded into the mastic, and then a finish coatof mastic is applied over the mesh.Flexible Duct. Tapes, mastics, and rolled sealants used to closeflexible ducts and connectors should be listed and labeled to ULStandard 181B, Part 1 o

43、r Part 2; be marked “181B-FX” or “181B-M,” respectively; and be used in accordance with their listing.Mechanical fasteners for use with nonmetallic flexible ductsshould be either stainless steel worm-drive gear clamps or non-metallic straps listed and labeled to UL Standard 181B, Part 3, andbe marke

44、d “181B-C.” Nonmetallic fasteners should have a mini-mum tensile strength rating of 670 N and be suitable for continuoususe at the maximum temperature to which they will be exposed.When nonmetallic fasteners are used, beaded fittings are required,and the maximum duct positive operating pressure shou

45、ld be limitedto 1500 Pa.Leakage TestingRationale. System leakage testing, including ducts and duct-mounted components, is recommended, because leakage data col-lected by researchers over several years (1998 through 2004) for 10systems in nine U.S. large commercial buildings see Figure 2 inWray et al

46、. (2005) showed that seven had substantial leakage flows(10 to 26%). The other three systems had much smaller leakageflows (3 to 4%). These data include variable-air-volume (VAV),constant-air-volume (CAV), and dual-duct systems, and both high-pressure (main) and low-pressure (branch) system sections

47、. ForVAV systems, data include fan-powered and cooling-only boxes.Most systems supplied air through rectangular diffusers, but someused slot diffusers. Ductwork pressurization tests conducted byLawrence Berkeley National Laboratory (LBNL) on the same ninebuildings at a test pressure of 250 Pa also s

48、howed that, on average,the leakage area for branches was about three times more than formains see Figure 1 in Wray et al. (2005).Scope. It is recommended that supply air (both upstream anddownstream of the VAV box primary air inlet damper), return air,and exhaust air systems be tested for leakage du

49、ring construction toverify (1) good workmanship, and (2) the use of low-leakage com-ponents as required to achieve the design allowable system leakage.Systems may be tested at operating conditions and/or during con-struction before the installation of insulation and concealment ofductwork, but after the system section to be tested is fully assembled.As a minimum, 25% of the system, based on duct surface area,should be tested during construction and another 25% if any of theinitial sections fail. If