source of information for design engineers. McGill AirFlow Corporation assumes no responsibility for the performance of duct system components installed in the. The ASHRAE Fundamentals Handbook chapter on duct design and the SMACNA HVAC Duct Systems Design Manual contain additional information on . The use of 26 gage ( millimeters) was Handbook chapter on duct design and the added for 4”, 6” and 10” wg and expanded SMACNA HVAC Duct Systems.
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HVAC Systems Duct Design. By SMACNA. Share This! Select format: Book; PDF; Combo (Book & PDF). $ QTY: Add to Cart. Rules of Duct Design (ACCA Manual D – Residential Duct Systems). looked up in an appendix of ACCA Manual D, ASHRAE or SMACNA guides. Information Required for Duct. Construction. 1. A comprehensive duct layout indicating sizes, design airflows, pressure class, and routing of the duct system. 2 .
Temporary internal supports can be appropriate at times. In some cases for example, pocket locks and standing seams , the metal in the duct counts in the joint quali- The T—1 drive slip connection provides sufficient ri- fication. Where dimensions, sizes, and arrangements of elements of duct assembly and support systems are not provided in these standards the contractor shall select configurations suitable for the service. See Table for rod and strap load limits. Thus the sheet displacement was measured relative to the duct itself. Further research and confirmation is needed in this area.
Authori- Precaution: Small differences occur in the diameter ties and contractors are invited to evaluate them by of ducts and fittings. Proper clearances are necessary. Verify suitability of fit, particularly when procure- ment is from outside sources. Pressure and Velocity Classification, 8. Tees and laterals, pages 3—10 and 3— Rectangular main to round branch, page 2—9. Sealing requirements, page 1—9.
Flexible connections, pages 3—15 to 3— Specifications for duct and fittings, page 3—1. Dampers, page 2— Longitudinal seams, page 3—7. Access doors, page 2— Transverse joints, page 3—8.
Hangers, page 4—8. Aluminum duct Schedule, page 3—6. Elbows, page 3—9. Polyvinyl coated steel or stainless steel: Use 7. Inline offsets and transitions, page 2—9.
The angles sizes are: If companion flange joints are used as reinforcements, those for to mm diameter shall be 38 x 38 x 6. See criteria in Sec- tion 7 S3. Spiral flat oval duct is machine-made from round spi- S3. It can also be made with longitudinal seam, joint, and connection arrangements seams. Flat oval duct has considerably less flat surface that S3. By UL Standard , a flexible connector is defined S3. They do not apply to diameter centerline radius.
Ducts should ex- service for conveying particulates, corrosive tend a few inches beyond the end of a sheet fumes and vapors, high temperature air, cor- metal connection before bending.
Ducts rosive or contaminated atmosphere, etc. Separate installation limitations for those of the manufacturer shall govern. Adhesives shall be chemi- cally compatible with materials they contact. If S3. These photographs depict typical accessories but do not represent all available accessories. Coincidence S3. Maximum per- recommended procedures are followed. A connection S3.
Avoid does not reduce the internal diameter of the contacting the flexible duct with sharp edges duct when the supported section rests on the of the hanger material. Damage to the vapor hanger or saddle material. In no case will the barrier may be repaired with approved tape. Narrower hanger material may flexible duct or treat the tear as a connection.
This S3. UL, NFPA, and most codes Diffusion Council covering thermal, make distinctions between these two products in their acoustical and friction ratings limits of application. Regulations governing these forms of duct Thermal Insulation Manufacturers Association should be checked especially for floor penetrations, ceiling air plenums, and fire rated floor-ceiling or The most common metallic duct is aluminum; how- roof-ceiling assemblies.
Nonmetal ducts are available in a wide va- These installation provisions were prepared for round riety of materials and nominal shape-retaining rein- ducts; however, they may also be usable for flexible forcements. Machines for producing the ducts are flat oval ducts.
Flexible ducts may come to the installer in com- Some types of flexible duct have received listings as pressed form in a variety of lengths. Their length can components of fan unit or air terminal unit systems, be determined by a measurement taken with a 25 lb. Repeated ditions of those listings. Sections S3. Round duct wall Materials commonly used for this application include thicknesses in these standards are generally accept- galvanized steel, vinyl chloride-coated steel, and able for below-grade installation.
Ribbed or corru- stainless steel. Glass fiber-reinforced resin, asbestos, gated styles have additional crushing strength. Tem- cement, tile, and other nonmetal ducts are also used.
Ducts are not generally deemed to be or required to Ducts should have continuous bedding. Ducts should always be above the wa- ter table. The designer should carefully evaluate the Ducts to be embedded in concrete are subject to float- exposure to moisture or ground water and require va- ing and they must be restrained. The first pour should por barriers, sumps, porous fill, and subsoil drainage be the base support for the duct and anchors should pipe as necessary. CSI Specification provides be included.
Twelve gage 2. The top of 2. Corrosion resistance is an important characteristic of Ducts buried in sand or pea gravel are not known to both in-slab and under-slab ducts.
The Portland Ce- float. Porous fill and earth fill should not be dumped ment Association has guidelines for protection of directly on ducts in trenches. Fill should be firmly but metals in contact with concrete. The Handbook addresses the corrosion of materials in soil first foot of fill should be shovelled on top of the duct.
The expansion nail is a lighter duty fastener. Some in Tables 4—1 to 4—3 and Figures 4- 1 to 4- 8. They should be used with a retaining clip. Powder-actuated shall be installed as required to maintain alignment. Welded studs Horizontal ducts shall have a support within two feet may be installed using special welding equipment. Upper attachments to structures flange will support either a rod or a band type hanger. The The duct hanging system is composed of three ele- wiring in the cells and the concrete above the deck ments, the upper attachment to the building, the hang- preclude the use of fasteners, such as sheet metal er itself, and the lower attachment to the duct.
The screws, that must pierce the deck. In all cases, the upper attachments to the decking should be in place before Concrete inserts must be installed before the concrete the application of fireproofing materials. The simplest insert is a piece of bent flat bar. Manufactured inserts are avail- Upper attachment methods should be selected with able individually or in long lengths; the latter are gen- care. A safety factor of 4 or 5 based on ultimate fail- erally used where many hangers will be installed in ure is practical unless it can be shown that few unpre- a small area, or where individual inserts cannot be dictable variables exist and that quality control is dis- precisely spotted at the time of placing the concrete.
For hangers made of round steel rod, use been poured and the forms have been removed.
Their uncoated hot-rolled steel except where the installa- application allows greater flexibility than concrete in- tion is in a corrosive atmosphere. These fasteners should not be used in certain light- The lower attachment is the connection between the weight aggregate concretes, or in slabs less than 4 hanger and the duct section. Fasteners that penetrate inches mm thick. Expanding concrete anchors should be made of steel. Holes for expanding fasteners are drilled either by a A straight duct section is actually a box section beam carbide bit or by teeth on the fastener itself.
The ex- of considerable strength. As in many structures, the 6! Duct joints, however, are normally strong enough quate hanging system can be disastrous. In any multi- to permit maximum hanger spacing at 8 2.
If one of these fails, mediate joints. Very wide ducts require closer hanger an even greater load is transferred to the next. The re- spacing in order to limit individual hanger loads to sult is a cascading failure in which an entire run of safe values.
They also require intermediate hangers duct might fall. Rectangular risers should be supported by angles or channels secured to the sides of the duct with welds, Figures in this manual show typical hanger construc- bolts, sheet metal screws, or blind rivets.
Here again, tions. When special conditions require high safety for ducts over 30 inches mm wide, caution must factors or the ability to withstand vibrations, individu- be used in fastening the support to the sheet because al concrete or steel attachments can be specified to the expansion of the sheet due to internal pressures be capable of supporting test loads equal to the mini- will tend to tear the fasteners out.
Riser support inter- mum rating listed when they are tested in accordance vals should be at one or two story intervals, i. Another method is to support the riser by its rein- latest edition. See pages 3—19 to 3—21 for support of forcing. The load can be transferred to the riser sup- flexible duct. Place fasteners in series, not side by side.
Dimensions other than gage are in inches. Tablesallowsfor duct weight, 1 lb. For upper attachments see Fig. For trapeze size see Table 4- 3 and Fig. Straps are galvanized steel; other materials 9. Dimensions other than hanger spacing are in 5. Table allow for duct weight, 4. For trapeze size see Table and Fig. Construction Manual. Spacing Dia. Straps are galvanized steel; rods are un- are to be installed, adjust hanger sizes to be coated or galvanized steel; wire is black an- within their load limits; see allowable loads nealed, bright basic, or galvanized.
All are with Table 4- 1. Hanger spacing may be ad- alternatives. See Fig. For industrial grade supports, in- cluding saddles, single load trapeze lads, longer pans and flange joint loads, see 3. See Figs. If heavier ducts ports. See Table 4- 1 for rod and strap load limits. Some perforated metals have the appearance of wire mesh screens. Ducts should be sup-. Ducts Each installation of a roof-mounted HVAC unit or that are interrupted at the curb should overhang the roof-supported duct involves customized design re- top of the curb or be flashed to divert water over the quirements.
The construction details and recommen- curb. Ducts that are continuous through the curb dations here are therefore advisory and depend on should have flashing that slopes over the curb and is contract documents for clarification.
Openings in sealed to the duct with caulking or a suitable tape. Ad- roofs require coordination of the architectural, struc- equate clearances between ducts and roof penetration tural, mechanical, and electrical contract drawings.
See Figure 5- 4. The height of equipment and ducts above the roof lev- el may be influenced by snow loading, snow drifting, Curbs may be supplied with rooftop units or provided and wind loading as well as esthetic considerations. The equipment manufacturer may Designers must specify constructions appropriate for outline flashing methods, structural opening require- the specific locality and circumstances.
With considerable All ducts that are not watertight through the use of pitch in the roof, a subbase may be required to adapt welded constructions or protective shields and are ex- to a pre-engineered curb.
Furthermore, curb mount- posed directly to weather and solar radiation should ings may incorporate vibration isolation features. Duct Section 1. Exterior duct sealant treatment should consist of ap- Attach supports with a minimum number of duct pen- plying products marketed specifically as forming a etrations. If exposed to direct sunlight it should also be ultraviolet ray- and ozone- If airtight, waterproof flexible insulation jackets are resistant or should, after curing, be painted with a applied on positive pressure ducts, the installation compatible coating that provides these plus weather should accommodate some duct leakage; ducts are resistance.
The term sealant is not limited to materials not completely airtight. Asphalt-based compounds should not be used for When moving rooftop units across the roof, handle sealing ducts.
Duct systems should not be pressurized until the seal- Supports for ducts should be as indicated in Figure ant has had time to cure.
Follow the sealant manufac- 5- 4. Unless otherwise prescribed by the HVAC equipment manufacturer, ducts should be flanged for attachment Pitch pockets require periodic maintenance and are to equipment with mechanical fastening plus exterior not permanently watertight.
They are not recom- duct sealant. Typical connections are shown in Figure mended. The attachment method should accommodate disconnection if this is required for routine mainte- Designers should carefully consider the proximity of nance of the equipment. The direction and elevation of dis- Where vibration isolation material is required at the charges may be controlled by codes or standards such connection of ducts to equipment, such material as NFPA—89M, 90A, 91, 96, or When equivalent construction is S6.
Use construction appropriate for the pressure classification. They shall not be used for Casing on fan discharge shall be of the desig- structural support of equipment. Wall and roof deflec- tions at the rated pressure shall not exceed S6. Use table to determine maximum panel 3. For casings with interior support angle, use width for load class, panel span, and panel larger of spans either side of angle to select gage.
Hinges Insulation Door Size No. Hinges No. In theory, the On one square foot 0. The following form air distribution. However, as a practical matter, table relates static pressure to pounds per square foot: Static Pressure in. Pa psf Several alternative constructions are illustrated. The concrete curbs are nor- Single wall casings may be constructed from continu- mally poured on top of the finished floor.
In order to ous standing seam reinforced panels or one of the al- prevent the forces on the casing walls from shearing ternative constructions. The same gage of metal is off the curb, the curb should be securely doweled to used on all sides.
Galvanized steel is standard sheet the floor. If the floor is waterproofed, the curb should material.
Black iron stiffeners are standard. Some offer acoustical control should be limited to approximately 20 inches through a perforated inner liner. This is an adequate size for personnel and most equipment. There are so many possible variations of the double- Larger doors should be avoided since they break the wall casing construction that it is impractical to detail continuity of the wall reinforcing.
This ar- this type of casing, it is suggested that strength of the rangement utilizes the air pressure rather than the panels be determined by structural calculations or door latches to force the door against the sealing gas- pressure tests on mock-ups. Particular attention ket. Piping penetrations must be carefully sealed, espe- All joints, seams, and connections should be sealed. Gasketing may suffice for some as- semblies. The casing up to the suction side of the fan is normally Attention to workmanship and inspection of the pres- constructed as a conventional low-pressure casing.
It is eliminator sections of the casing to handle condensa- not necessary to build these casings to withstand these tion on the cooling coil.
However, the fan and damper con- trols must be carefully coordinated so that it is impos- Conventional drains without deep seal traps will not sible for such a negative pressure condition to occur. Drainage may be di- Safety relief panels or dampers may be designed into rectly into the sewer system through a floor drain in the system to prevent damage. The static pressure levels ranging from 0. Tests were conducted in both ing members is 0. The study re- sure. Committees conclude that the functional criteria formation or failure.
Transverse joints must be able to withstand 1. The criteria used by SMACNA in its test program and Where a transverse joint acts as a reinforcing mem- in development of new duct tables is as follows.
The sheet must resist both deflection caused by inter- A duct section between adjacent hangers must be able nal pressure and vibration due to turbulent air flow.
The joints and sheets listed to maintain an approximately rectangular cross sec- in the current construction tables are not specifically tion, sheet deflections for ducts were limited as listed designed to support the weight of a person.
The sup- in Figure 7—1. The current test program did not include vibration analysis. Commentary also appears with Table 1—1 and in the notes for Tables 1—3 to 1— It was See Table 1—2 and the discussion of leakage in rela- concluded that the limited risk of an occasional prob- tion to pressure level.
Even high-pressure ducts are lem is preferable to postponing use of multiple sheet not absolutely airtight. Careful selection of closure gages until the boundaries of stability can be com- methods can ensure adequate performance.
However, pletely defined. Crossbreaking or beading of unbraced duct sides larg- er than certain dimensions is effective in dealing with Leakage is primarily a function of the static pressure commercial tolerances on out-of-flatness, natural sag differential. It is independent of joint orientation and from dead weight, and with the flexure reversals that of velocity levels of fpm If pressure does not produce a taut and snaplock seams is low compared to that in trans- sheet, vibration may result.
Beading is considered as verse joints. Also, crossbreaks establish an initial deflection which, when added to that generated 4. Noise generation and transmission. Exposure to damage from the rigors of han- dling and transportation, weather, tempera- Furthermore, the ability of a reinforcing member to ture extremes, flexure cycles, chemical cor- perform its function is critically affected by the loca- rosion, or other in-service conditions.
These variables change with the negative pressure and posi- 6. Support including supplemental loads, if tive pressure modes. A duct system is an assembly whose primary function is to convey air between specified points. In fulfilling In establishing limitations for these factors, consider- this function, the duct assembly must perform satis- ation must be given to the effects of the pressure dif- factorily with regard to certain fundamental perfor- ferential across the duct wall, airflow friction losses, mance characteristics.
Elements of the assembly are air velocities, infiltration or exfiltration, as well as the sheets duct envelope , reinforcement, seams, and inherent strength characteristics of the duct compo- joints.
With regard to the duct assembly and its ele- nents. Dimensional stability deformation and Selected specific performance requirements for rec- deflection.
Only applicable for the determination of a leakage a. Leakage and sag referenced to sup- port points are measured before and at the end of one 3.
I Burst and Collapse Tests hour. Ducts shall be as- 3. Duct Wall Limit 4. Blowers may be operated in- rimeter. A bleed valve may be installed in the fan discharge manifold to provide fine flow 5. For the purposes adjustment. The systematic error in an instru- ment can be minimized by suitable calibra- Step 1. Set up the dial indicator s over the test tion procedures. Random error in an instru- point s. Record the dial indicator reading D1 at ment can to some extent be reduced by zero gage pressure.
Determine the sag for top and bot- careful technique in reading and by choosing tom flat surfaces as required for section 4. Wet and dry- Step 2. Pressurize the duct to the target classifica- bulb thermometers, transducers, or sensors tion. Examine the duct. Calibration ther- mometers, transducers, and sensors shall be Step 3. Relieve pressure and record the dial indica- calibrated over the range of temperatures ex- tor reading D3. A dial indi- NIST. Duct pressure shall Step 5.
Relieve the pressure. Re- be measured with a manometer or other in- cord the dial indicator reading D4. This relates to the performance requirements in Sec- tion 3 of this test standard.
The DAVE calculation 5. Laminar Flow Meter. The Figure 7- 3B manifold shall be 1. Diagram of the test specimen. Pip- ing between the manifold and a test duct end 2. Complete description of the specimen: The sequence of observing and recording of model of machines on which seams, joint data, the increments of pressure, the zero members, and intermediate members are roll pressure level deflections, etc.
Any corrective adjustments made in the con- formed stiffener and joint members; type, dition of the specimen after the start of test- size, and spacing for fastener, e. The location and nature of any failure weld, or self-tapping screw ; type of seal- points or conditions.
Include any ob- served imperfection or irregularity which 6. Record the observations during, or after assembly of parts. Use precautions to avoid injury from parts dislocated at failure 3.
The test setup including apparatus, least pressure. If feasible, increase the pressure on the speci- men until buckling, permanent deformation, or sepa- ration of parts occurs. This will indicate the safety Show by written analysis and commentary that any factor of the construction and show the nature of the features that are different from the reinforcement and failure. Observe that the SMACNA pressure classifi- ducts to the same extent that the published cations are positive and negative at several standards do; pressure levels.
Tests for negative pressure qualification are similar to those described b. Tests in the negative pressure mode are more METHOD 2 critical for sheet deflection and joint and stiffener buckling than those in the positive pressure mode.
See the diagrams of models Present substantial evidence of historical acceptabili- for functional standards. A test in one mode ty for the use intended, and show that the previous will not substitute for one in the other mode. Construct, test, and rate specimens of the contem- The pressure capacity of ducts is usually plated design. Show that this ap- proach will not impair or reduce the performance of 4. The deflection limits are at the rated pressure the entire assembly.
The freedom-from-structural-failure limit is at a safety factor level. Make sure that end caps and their attachment are secure enough for the test pressure range. Test a full specimen. Construct a specimen using the End cap failure at lower than target pressure desired scheme of sheet thickness, joint type, inter- makes more testing necessary.
Conduct tests in the positive or negative mode of pressuriza- 6. Returning to zero pressure and checking tion, as desired. Use instrumentation and follow pro- joints or reinforcements between pressure cedures that will produce laboratory accuracy.
Re- level increments enables testers to identify cord the proceedings and observations. Write set, the residual deflection in excess of that conclusions showing equivalence to the construction originally present at atmospheric pressure. Include a diagram of Some set may occur from slippage or cold- the specimen tested. Considerable set may 6! Where riser or horizontal duct supports cess deflection may occur at the next pres- transmit loads to joints or reinforcements, these loads sure level.
Periodic release and restoration of can affect the integrity of the duct system. This Leakage Test Manual for evaluation of leak- is not true for some connection and rein- age control.
Allow sealants to cure before forcement systems. The investigation of a single specimen will 7. Typical failures under positive pressure are not provide enough data to confirm adequa- joint separation from the duct, opening of cy for all duct sizes, gages, and reinforce- joints, tearing of joint corners, screw or rivet ment arrangements. Also, marginal failure pullout from duct near corners, and longitu- of one specimen will not necessarily mean dinal seam separation.
Reinforcements that the construction is necessarily unsuit- break at corners or, if they are not corner- able. Typical failures under negative pres- Inserting flow meters in the air line between sure are buckling of duct wall at the corners, the blower or vacuum unit and the specime- buckling of joints and reinforcements near nand recording air leaks at stabilized pres- the center, openings anywhere in the duct sure levels, while evaluating the structure surface that change in size or orientation to can confirm joint separation or seal degrada- adversely affect seal effectiveness, fastener tion and provide other useful information.
Joints and intermediate reinforcements in- fluence the deflection of the duct wall at ! Checking the amount of wall midpoint deflection can lead to the development of new reinforce- Joint separation ment schedules. The corners of the duct may Seam separation deform more between joints than at joints, Permanent warp, buckling, or collapse and the duct ends at the joint may more lon- Excessive sag or misalignment at zero pressure gitudinally as well as laterally.
Both of these Excessive deflection in duct wall, reinforcements, conditions can contribute to span midpoint or joints under pressure deflection. This suggests that the wall Component or fastener breaking, pullout, or slip deflection measurement might better be ref- Changing alignment or fit-up of components causing erenced diagonally to the duct wall edges at loss of seal effectiveness joints and referenced transversely from cor- Stress cracking ner to corner at midspan.
Susceptibility to damage from routine handling and from impact at assembly and installation Sponsors of new or proprietary transverse joining sys- tems are encouraged to have ther tests witnessed and certified by a disinterested responsible party such as.! Evidence of equivalency Insufficient support causing misalignment of duct should include information on the E1 rigidity classifi- sections cation calculation.
SMACNA does not endorse or ap- Support detaching from duct assembly if attached prove proprietary constructions and does not validate Support deflecting under load, adversely affecting their analysis or tests as being equivalent to SMAC- duct integrity or shape retention NA classifications.
Authorities are invited to evaluate Support contact with duct being periodic and causing such constructions based on evidence presented by noise their sponsors. This arrangement prevented false readings caused by movement of the testing system relative to the building structure.
The static pressure, differential orifice ions is intended. Because the top, bottom, and sides of a nate rumble. The object of the test was to determine the behavior Duct widths tested were 10 , 12 , 18 , of a variety of rectangular duct constructions for vari- 24 , 36 , 48 , 54 , 60 , ous air velocities and static pressures.
Behavior was 72 , and 84 inches mm. Ducts wider measured in terms of deflection and vibration ampli- than 84 in. Air from ing pieces on each end. Longitudinal seams were but- the fan passed through a calibrated orifice and a ton punch snap lock for 24 0. Then it 1. Lock seams were used for 18 gage discharged into a plenum with an outlet damper. Air 1. Reinforcing angles were secured damper and the outlet damper at the end of the sys- with sheet metal screws and tie rods on each end.
A manually operated bypass damper was used Transverse joints were flanged and edge-welded. Four joints. The square-throat elbow with turning vanes was installed upstream of the test specimen in order to For each width of duct, the sheet gage, reinforcing create a turbulent airflow like that found in actual size, and reinforcing spacing were varied in order to installations.
The pressure in the test specimen was determine the lowest cost construction that would sensed by a static pressure probe installed in each of perform within the standards. The displacement of the sheet was measured by a calibrated linear potentiometer. The Constant-pressure test runs were made on each duct movable arm of the potentiometer was secured to the specimen, varying the velocity continuously from sheet by a permanent magnet, and the housing of the about 6, 33 down to about 2, fpm 10 mps.
Thus the sheet displacement was Pa static pressure as measured in the specimen. Increasing reinforcing spacing in- 4! Increasing duct width increases deflec- tion. Static Pressure Average d. Increasing internal static pressure in- in. WG Velocity creases deflection. Pa fpm mps e. Increasing sheet gage decreases deflec- 1. Amplitude of vibration 8.
Increasing velocity increases vibration. The oscillograph traces were examined to determine, c.
Increasing duct width increases vibra- for each pressure, the maximum sheet deflection and tion. Increasing internal static pressure de- value established as standard. With amplification creases vibration. Increasing sheet gage increases vibra- equal to a movement of the sheet of. Why would a light gage sheet resist vibration more than a It was not feasible to conduct the tests to establish the heavy gage sheet, and why would a sheet vibrate more exact relationship between the threshold velocity and at lower pressures?
Instead it was decided to arbitrarily Careful observation of all specimens showed that the establish minimum threshold velocities for various sheet acts more as a membrane than as a beam. A static pressures. The intent was that the resulting cri- sheet acting as a beam would require a much greater teria would cover all of the operating conditions in section modulus, calling for thicknesses beyond those normal- to high-velocity air conditioning systems. However, a relatively heavy sheet, say 16 gage 1.
Based on their responses, Table A lists the minimum threshold ve- The side of the duct is easy to move back and forth locities used in evaluating the test results.
A 16 gage 1. In all cases the performance of the duct specimen fol- lowed the same pattern. Deflection of the sheet a relatively heavy gage sheet is subjected to low- range operating pressures, some concave areas might a. If an area is near this equilibrium condition, weight of the sheet and the internal forces which then relatively small but rapid variations in static cause the waves.
Here again, the equilibrium static pressure caused by turbulence will make that area vi- pressure for heavy gage sheets, can occur well within brate violently. In addition to the random waves in commercial sheets, the sheet forming the top of the duct will sag and form a concave area between the reinforcing From the foregoing, it might appear that it would al- members.
This is due to the dead weight of the sheet ways be best to use a light gage sheet. However, as and occurs at zero or low internal static pressures. The the duct width increases, it is also necessary to in- top sheet will not be stretched taut until the internal crease sheet gage in order to meet the deflection crite- static pressure is adequate to overcome both the dead ria. It was agreed vex shape is to cross break it. Unfortunately, cross that the maximum deflection and amplitude values set breaking results in concave areas between the break as standard were reasonable enough to prevent such lines.
The technique of cross breaking is successful contact. Where ducts are covered with insulation, in reducing vibration when applied to low velocity vibration transmission by contact is further mini- ducts because internal static pressures are seldom mized.
OISE to the turbulence found in high velocity ducts. There- fore, the forces causing vibration are much smaller. As a practical matter, the foregoing two problems were not as great a concern as the possibility of exces- The test results showed that the effect of cross break- sive noise.
Due to the high ambient noise caused by ing high velocity ducts is dependent on the sheet gage. Our consultants evaluat- ducts. On the other hand, higher pressures were re- ed the test runs by visual observation, by touch, and quired to stretch taut the heavy gage sheets which had by ear.
On this basis, they unanimously agreed that been cross broken than those not cross broken. It was the maximum amplitude of vibration established as observed that in general, cross breaking was not bene- a standard was well within the limits of vibration con- ficial to high velocity duct performances. OISE ous specimens and system components. Magnetic tape recordings were made of the vibration of duct Vibration of the sheet is undesirable: A pre- amplifier was used that had uniform characteristics throughout the audio frequency range.
After the test 1. If it can cause fatigue failure in the sheet; runs, the consultants listened to the recordings in a quiet room and judged them. Certain qualitative judg- 2. If it causes other objects to vibrate by con- ments were made concerning these listening tests. If it makes excessive noise.
The levels in Table B indicate the relative vibration Complete and definitive testing in these three prob- under the various conditions of test on a logarithmic lem areas was not practical under the financial and scale as used for sounds.
The absolute values are of time limitations of this program. It was instead de- no importance, but the relative values are similar to cided to rely on the judgment of experienced parties those obtained if it had been possible to measure noise to determine the criteria for acceptability.
Thirty ven- directly on a decibel scale; that is, for an increase of tilation contractors and design engineers witnessed 3, the ratio of vibration is doubled. The basis of interpretation of the 0. In our tests, the differences in Fatigue failure in steel ducts is uncommon. The con- relative vibration however are significant enough to sultants had seen fatigue failure only in cases of much assume that an increase in radiated sound will result more severe vibration, usually where there were stress from an increase in vibration of the sheet.
The signifi- reversals such as that created by a poorly secured cance of the tests was the determination of trends turning vane flapping in the airstream. Fur- 6, fpm The differences that are noted therefore are minimum and it can be expected that Position Relative considerable more differences would have occurred Vibration under ideal test conditions.
Outlet plenum box 32 16 ga. The lowest vibration in the system was obtained in 2. The read- The thinner duct appears to sound significantly less Vibration readings on a 42 x 8 in. This is in part due to less duct specimen, constructed of a 10 feet 3. Our study of noise radiation was conducted under less than ideal conditions. However, its results appear to Under these test conditions, the vibration intensity of substantiate the assumption that light gages in high the thinner duct was consistently lower in level than velocity duct will not result in greater noise radiation the heavier duct and showed a maximum difference than heavy gages.
Gage Min. Tolerances are valid for 48" and 60" wide coil and cut length stock - other dimensions apply to other sheet widths and to strip. The steel producing industry recommends that steel be ordered by decimal thickness only. Thickness and zinc coating class can be stenciled on the sheet. The gage designation is retained for residual familiarity reference only.
Minimum weight in this table is based on the following computation: Minimum sheet thickness minus 0. G90 stock would be comparably calculated from: Actual weight may be near G90 coating is 0. First Second Manufacturers Standard Gage is based on a theoretical steel density of Thus, the weight basis associated with thickness specifications as Thus, U.
The table is based on 48" width coil and sheet stock. See ASTM Standards A cold rolled order form , A properties of hot rolled and cold rolled sheet of commercial quality , and A hot rolled order form. Metric conversions listed here are straight multiplications for comparison purposes. Individual manufacturers may quote other tolerances. Standards B- Bright Annealed Finish - A bright cold-rolled finish retained by annealing in a controlled atmosphere furnace.
The series weight is based on ASTM- A covers the structural grade of stainless steel not used for ducts. Width Min. Alloy is of slightly lower density. Specification references: Other useful references are published by the Aluminum Association: Based on galvanized steel.
Investigate tolerances for closer estimates. Construction conforming to the standard does not prevent some traditionally acceptable rumble noise under sudden 2.
Spacing in column 3 refers to joint- to- joint, joint- to- intermediate, or intermediate- to- intermediate. The same sheet thickness must be used on all sides of duct. Construction tables are prepared in "narrow scope" for 2" water gage and 1" water gage pressure classifications.
This applies to ducts of 20 ga. Contractors Association, Chantilly, Va. OR NEG. Angle 20 ga. Spacing in column 3 refers to joint- to- joint, joint- to- indermediate, or intermediate- to- intermediate. Standing reinforcement is not required on duct sizes above this heavy line. Flat Slips and Drives or other choice may be used. Flat Slips and Drives must not be less than one gage below duct gage and not less than 24 gage.
See Table 1- Tie rod options are not listed here. Standing reinforcement is not requird on duct sizes above this heavy line. None of the requirements of the standards are waived by this chart. Ratio of fittings to straight duct A. What is the Preference in Primary Vari- 9. Size changes, i. Metal Stock Size: Crossbreaking or beading obligations 2. Joint spacing: Standardized Pressure Classification Sup- 3. Joint type? Intermediate Reinforcement: Joints and intermediate reinforcements are labor intensive units and they may be more costly than the savings in a reduction in met- al in duct wall thickness.
Use of a thicker duct wall or a stronger joint over a wider range of duct sizes than those required can be cost effective. Examples in- 1. Sealing requirements clude using a 22 gage duct wall where 22 ga. Leak Test requirements, if any However, substituting a larger-than-mini- mum element in one primary variable does not justify reducing another primary vari- 3.
Some joint classifications have flanges or 4. Amount of Prefabrication off-site portions of the duct incorporated in their rat- ings; others do not. A thorough study is ad- vised. Transportation storage and hoisting damage risk and logistics control 4. Flat joint systems can qualify when backed up with reinforcements. Sealing expense and effectiveness may make this alternative 6. The larger a duct at a given pressure, the larger the reinforcement and the closer the A.
For each pressure level and a constant duct reinforcement spacing on a selected gage. For each combination of sheet thickness, sheet the closer the reinforcement spacing. For a given sheet thickness and constant duct sheet deflection is not controlled by rein- size, reinforcement size, and reinforcement forcement size nor reinforcement position. The complexity of these factors requires the services of a radiological physicist, who determines The purpose of these data sheets is to acquaint the air extent of shielding, materials for shielding usually conditioning engineer with means for shielding duct- lead or concrete and the thickness of the shielding- work and other openings that penetrate protective bar- material.
After the radiological physicist has done the riers around radiation facilities, particularly X-ray basic design for this shielding, the protective barrier rooms. Also, coordination be- time of exposure tween air conditioning contractor and shielding fabri- cator can best be achieved by understanding and fore- thought on the part of the air conditioning designer. The prime consideration in preventing penetration of rays is is density of the shielding material.
Lead is Figures 1 to 4 give some idea of the area of shielding the densest of any commonly available. Where space required around ductwork. They show various duct is at a premium, particularly in modern buildings, and installations which penetrate the protective barrier for where utmost radiation protection is demanded, lead walls or partitions of X-ray rooms.
Lead shielding is is invariably used. Lead is useful, especially where used to cover these openings, the approximate extent neutron and gamma rays are concerned, in that it does of which is indicated in terms of simple equations in not itself become contaminated and emit harmful involving the opening dimensions and wall thickness. These are conservative estimates, which will aid the air conditioning designer to understand what what to expect as to the area of shielding ductwork.
The ra- Lead, usually in sheet form, is used to line the walls, diological physicist actually determines for each case floor, and often the ceiling of rooms containing radi- the lead thickness and the exact amount of shielding ation facilities. Openings through the barrier for air required. Goodman, formerly with Meyer, Strong, and Jones now has secondary radiation is emitted by an irradiated mate- his own consulting practice in New York City.
He has a masters rial. Primary radiation requires more protection be- degree in mechanical engineering from the University of Wiscon- cause its energy level is higher. Hollands is Chief engineer, in charge of design of radiation Sheet lead is not structurally self-supporting, so must shielding materials and equipment, for Bar- Ray Products, Inc. The extent of the protective barrier for medical instal- lations is summarized below so that the air condition- When lead thickness is greater than 3.
These joints ducts can be located above this line, shielding around are lapped with sheet lead angles or lead strips, the them is obviously unnessary. Nails, bolts, screws, structural slab. The ceiling or slab above and the floor or other fasteners used to secure the lead sheet or pan- may also be lead lined, depending upon output of the el must be covered with lead of thickness equal to the machine and other conditions.
For industrial X-ray lead sheet. Lead headed nails may be used as shown work, wall shielding may extend to the ceiling. Both in Figure 5. For lead shielding of 1. The flexible leader vinyl sheets tions may apply. In any event, the radiological physi- can be applied in layers where heavier than 1. If the duct has a flexible tion. Where concrete is considered for the shielding vinyl sheets could be applied over it more readily than material, it is often more practical to use lead of other forms of shielding.
Where recesses occur in concrete barriers for equipment, lead backing, equivalent to the thickness of the con- Duct hangers are best installed on the outside of the crete removed, should be provided. For rectangular ducts, Of the many publication available on the subject of trapeze hangers would be the most practical. For de- radiation protection, these two are the most useful: It is very important to install shielding properly during the 2. Moreover, equipment such as dampers should never In addition, the New York City Health Department be put in the shielded section of the ductwork, as re- publishes the New York City Health Code require- pairs to this equipment would be very costly if the ments dealing with radiological hazards Article shielding must be dismantled.
A simple way to avoid penetration of the protective Notice: Moshart, Jr. Daniel, St. Petersburg, FL Donal J. Sandvik, Arlington, VA! Lauderdale, FL John H. Floyd W.
Related Papers. The mere presence of sealant at a connection, however, does not ensure low leakage. Applying sealant in a spiral lockseam can result in poor seam closure and less satisfactory control. No single sealant is the best for all applications. Selecting the most appropriate sealant depends primarily on the basic joint design and on application conditions such as joint position, clearances, direction of air pressure in service, etc.
The listing of certain duct products by recognized test laboratories may be based on the use of a particular joint sealing product. Such a component listing only reflects laboratory test performance and does not necessarily mean that the closure method can routinely be successful for the contractor or that it will withstand in-service operation of the system on a long-term basis.
Many manufacturers produce liquid sealants specifically for ducts. They have the consistency of heavy syrup and can be applied either by brush or with a cartridge gun or powered pump. Liquid sealants normally contain 30 to 60 percent volatile solvents; therefore, they shrink considerably when drying. They are recommended for slip-type joints where the sealant fills a small space between the overlapping pieces of metal.
These sealants are normally brushed on to round slip joints and pumped into rectangular slip joints. Heavy mastic sealants are more suitable as fillets, in grooves, or between flanges. Mastics must have excellent adhesion and elasticity. Although not marketed specifically for ductwork, high quality curtain wall sealants have been used for this application. Oil-base caulking and glazing compounds should not be used. Durable materials such as soft elastomer butyl or extruded forms of sealants should be used in flanged joints.
For ease of application, gaskets should have adhesive backing or otherwise be tacky enough to adhere to the metal during joint assembly. The choice of open cell or closed cell rubber gaskets depends on the amount and frequency of compression and on the elastic memory. Nothing in this standard is intended to unconditionally prohibit the use of pressure sensitive tapes. Several such closures are listed as components of systems complying with UL Standard tests. There are no industry recognized performance standards that set forth peel adhesion, shear adhesion, tensile strength, temperature limits, accelerated aging, etc.
The variety of advertised products is very broad. The shelf life of tapes may be difficult to identify. It may be only six months or one year. Although initial adhesion may appear satisfactory, the aging characteristics of these tapes in service is questionable. They tend to lose adhesion progressively at edges or from exposures to air pressure, flexure, the drying effects at the holes or cracks being sealed, etc.
Application over uncured sealant may have failures related to the release of volatile solvents. Sea air may have different effects on rubber, acrylic, silicone-based or other adhesives. Tapes of a gum-like consistency with one or two removable waxed liners have become popular for some applications. They are generally known as the peel and seal variety and have been used between flanges and on the exterior of ducts. Such tapes are typically of thicknesses several times that of tapes traditionally known as the pressure sensitive type.
Some may have mesh reinforcement. Others may have metal or nonmetal backing on one surface. Hot melt and thermally activated sealants are less widely known but are used for ductwork. The hot melt type is normally a shop application.
Thermally activated types use heat to either shrink-fit closures or to expand compounds within joint systems. There are several combinations of woven fabrics fibrous glass mesh, gauze, canvas, etc. Glass fabric and Mastic GFM used for fibrous glass duct appears to adhere well to galvanized steel. Surfaces to receive sealant should be clean, meaning free from oil, dust, dirt, rust, moisture, ice crystals, and other substances that inhibit or prevent bonding. Solvent cleaning is an additional expense.
Surface primers are now available, but their additional cost may not result in measurable long-term benefits. No sealant system is recognized as a substitute for mechanical attachments. Structural grade adhesive systems are being developed to replace spot welded and soldered connections of metals.
They have lap shear strengths of to psi KPa or more. SMACNA is not able to comprehensively define their characteristics at this time; however, authorities are encouraged to monitor their development progress and consider their use. The shelf life of all sealant products may be one year or less; often it is only six months.
The installer is cautioned to verify that the shelf life has not been exceeded. The contractor should carefully consider the effects of loss of seal and fire potential when welding on or near sealed connections. NFPA Standard 90A requires adhesives to have a flame spread rating not over 25 and a smoke developed rating not over The number in the cell is minimum duct gage; the letter code is type of joint or intermediate reinforcement, whichever you choose.
This applies for joint-to-joint, joint-to-intermediate, or intermediate-to-intermediate intervals. If the maximum short side reinforcement spacing thus found exceeds a joint spacing that you are committed to, go to the column with the joint spacing to find the joint size. Even though the duct gage listed at this width-spacing cell may be less, the joint rating cannot be less than at this cell.
In the table, an entry such as HG means that the H reinforcement size may be downsized to a G per section 1. This does not apply for joints that require tie rods on both sides of the joint.
In some schedules, only the tie rodded construction is given. Kt is an example. Use Chapter 7 to evaluate these. Very large ducts may require internal hangers as shown in Figure or may require other internal supports to provide shape retention. Such internal supports should be illustrated on the contract drawings. The rectangular duct construction standards provide the following options for constructing ducts: Not all options exist at all sizes and all static pressure classes.
The options are provided to correlate performance with economy and the preference of fabricators and specifiers. If the table requires a letter code, all joints on that side must qualify for the minimum code letter related to the minimum gage and the spacing. Spacing refers to letter code: Columns 3 to 10 are alternatives. Sometimes, if a project calls for small amounts of ductwork in many size ranges or pressure classes, it may be more economical to select heavier constructions than are required, so that fewer variations are needed.
The duct construction tables define relationships between static pressure, width, wall thickness, reinforcement spacing, and reinforcement strength so that ducts have adequate strength and acceptable deflection limits. The greater dimension of a duct determines the duct gage for all four sides.
This applies to reinforced and unreinforced ducts. The first step in determining construction requirements is to locate the table with the applicable static pressure. Duct sides having a gage listed in the second column of Tables to do not require reinforcement. These are summarized in Table Flat type joints may be used at any spacing. Flat slips and drives must not be less than two gages lower than the duct gage or below 24 gage 0.
The T-1 drive slip connection provides sufficient rigidity to be treated as Class A, B or C reinforcement within the limits of Table This gives the appearance of increasing the range of unreinforced duct sizes.
The arrow indicates that the right most value continues to the end of the row because the minimum duct gage and reinforcement grade remain the same for shorter spacings.
Any cell within a row is an acceptable selection for that duct width. Reinforcement spacings of 10 feet 3. See appendices 13 to 17 for discussion of variables that affect choices. First investigate the duct side with the greater dimension because this side dictates the duct gage.
Then find the smaller duct dimension in the first column, and on the same horizontal line locate the duct gage of the wide side.
If the duct gage is in the second column, no reinforcement is required on that side; otherwise, the minimum reinforcement code is the letter listed under the spacing used. The actual duct gage may occur in a column giving allowable spacing greater than will be used. In such a case the minimum reinforcement grade is that associated with the actual spacing. The reinforcement spacing in Tables to denotes distance between two joints or two intermediate reinforcements or from a joint to an intermediate member.
Any joint or reinforcement member having a corresponding letter code in Tables through may be used to comply. The letter code for reinforcement corresponds to a stiffness index number EI. This is the modulus of elasticity multiplied by a moment of inertia that is based on the contributing elements of the connector, the reinforcement, the duct wall, or combinations of these. Unless other evidence of adequate strength and rigidity is presented, equivalent construction must meet the EI index associated with the code letter.
In some cases for example, pocket locks and standing seams , the metal in the duct counts in the joint qualification. A minimum gage of duct that is heavier than the duct gage shown in Tables through may be indicated by the joint specifications in Tables and Flat slips or drives or any flat joint shown may be used at one of the spacing limits, provided that a backup member of the intermediate type is used with them; the joint is then rated by the backup member taken from Table Tie rod duct construction described on pages through is also an alternative.
An entry such as HG indicates that on 18 gage 1. The table entry Ht, for example, designates 18 gage duct with H class joints and intermediates having tie rods or straps at intervals not exceeding 60 inches mm. See Figure Other construction that meets the functional criteria in Section VII may be provided.
If the duct is of 24 gage 0. If the duct is of 26 gage 0. Substitution of C or D for B would still not permit a reinforcement spacing greater than 8 feet 2.
Grade D is required for 5 feet 1. If you did not have the 5 feet 1. This introduction does not review all aspects of construction. It is an overview. Certain other limitations will appear later in the text and tables. For example, certain joints have been assigned maximum pressure classes. However, other limits may be acceptable if they can be shown to result in equivalent construction. See the appendix for other useful tables and commentary. Unless otherwise specified or allowed, rectangular ductwork shall be constructed in accordance with Tables through l and with details associated with them.
Uncoated steel, prepainted steel, steel with metal coating such as aluminum or aluminum-zinc compounds, and stainless steel may be used if a minimum corresponding base metal thickness and material strength is provided. Lock forming grades of such material must be used. The use of alternative materials requires specification or approval by a designer. The surface conditions, hardness, ductility, corrosion resistance, and other characteristics of these materials must be judged acceptable by the designer for the planned service.
Specifications that refer to use of material that is two gages heavier mean two numbers separated in a series that uses both odd and even number progression; e. A reinforcement code classification letter and EI index higher than indicated must be substituted when the tables do not provide the specific construction details for a lower classification.
A higher rated construction member may also be substituted for convenience. Joint spacing on unreinforced ducts is unlimited. On ducts that require reinforcement, joint spacing is unrestricted except that the joint itself must qualify for the minimum reinforcement code associated with the reinforcement spacing.
Ducts that are of heavier gage, smaller dimensions, and smaller panel area and those that are lined or externally insulated are not required to have crossbreaking or beading.
Fittings shall be reinforced like sections of straight duct.
On size change fittings, the greater fitting dimension determines the duct gage. Where fitting curvature or internal member attachments provide equivalent rigidity, such features may be credited as reinforcement. Duct wall thickness, joints, seams, and reinforcements must be coordinated to provide proper assembly. Other construction that meets the functional criteria in Section VII or is as serviceable as that produced by the construction tables may be provided.
See page Circles in the Table denotes only column numbers. For column 2, see Fig. For columns 3 through 9, see Introduction to Schedules.
The number in the box is minimum duct gage; the alphabet letter is the minimum reinforcement grade for joints and intermediates occurring at a maximum spacing interval in the column heading. A letter to the right of the gage gives a tie rodded reinforcement alternative. For beading or crossbreaking, see Fig. The number in the box is minimum duct gage; the alphabet letter is the minimum reinforcement grade for joints and intermediates occurring at a maximum spacing in the column heading.
For beading or crossbreaking. Both modes are accepted when neither is given. C and H denote cold formed and hot rolled ratings; when neither is listed, either may be used. See tie rod options elsewhere. Joints T-2 and T through T are restricted to Pa to mm length at Pa and are not recommended for service above Pa.
For T, see tie rod downsize options in Tables to ; one rod for two angles. R means Tie Rodded. See Figures , , , and tie rod text. Internal ties shall be one of the methods shown in Figures and When more than one tie rod is used at a crossection of the duct, the design load may be proportionately reduced.
When one of these backs up a joint, the attachment options are the same. The attachment of tie rods or tie straps as in Figure by welding, bolting, riveting, or screwing within one inch 25 mm of each side of joints T, T, and the T and T series.
Each tie rod may be sized for one half of the load in Table Only one tie rod is required for joint T On 18 gage 1. For positive pressure service, several alternatives are available for compliance with Tables and M. One half inch For negative pressure rods, tubing, pipe, or angles are alternatives. The selection steps are as follows:. Holes made in the duct wall for tie rod passage shall be of minimum size and shall be sealed in accordance with the provisions of Sections 1.
Except as limited by joint specifications and certain mandatory uses, tie rod alternatives are indicated in Tables through for reinforcement sizes listed to the right of duct wall thickness.
G denotes the size with tie rod on 22 gage in HG nomenclature. Tie rods shall be galvanized steel. All internal ties, whether of rod, tube, pipe, or angle for shall be of material having the same nature and corrosion resistance as the duct wall material. Concealed components shall not be subject to nor cause galvanic.
When the internal ties are integrated with supports such as those in Figure , they shall be selected to be suitable for additional duty. Refer to Figure When ties occur in two directions in the same vicinity, they shall either be prevented from contacting or be permanently fastened together. Refer to Figures , , , and for basic tie rod application details on medium and large size ducts. See Figure for construction schedules. Smaller reinforcements than would otherwise be required can be used when a tie rod is placed in the duct so as to lock reinforcements on opposite sides of the duct together to restrain their bending.
Ties can be attached to intermediate reinforcements or to joint-backup-reinforcements or to certain joints able to withstand the forces involved. The duct dimension defines tie rod length. Duct size and weight and operating pressure determine tie rod size, shape and internal geometry. Pipes and angles were not sized for positive pressure because other options seemed more economical; they may be used. See tie rod text and figures. If more than one tie is used, the load is proportional.
Applicable for positive and negative pressures. Not all W by Rs load condition listed in Table occur in Tables through Also, loads for widths less than mm may be calculated for Table M. This assumes that threaded connections carry the load.
If rod s are welded to lugs on the duct wall, weld stress must be limited to 13, PSI. If rod s are welded to lugs on the duct wall, weld stress must be limited to kPa. Rod specification is diameter and threads per inch. The table gives maximum allowable load in pounds; see Table for assumed loads for various width, pressure, and reinforcement spacing combinations. X means the size is not acceptable at this greater length. The table gives maximum length and maximum load; see Table for assumed loads.
Blank spaces are not economical. The table gives maximum length and maximum load; see Table M for assumed loads. Length LEN is in mm. For the load from Table , select a pipe from Table that has that load capacity and is within the length limit.
Divide the load from Table by a trial size angle area in 2 ; divide the result by If this Ksi load is not in excess of one in Table for a listed length that is also not exceeded, the size is acceptable.
If unacceptable, select a size with a larger area and check again. Divide the load from Table by a trial size angle area mm 2 ; divide the result by If this Kpa load is not in excess of one in Table M for a listed length that is also not exceeded, the size is acceptable. You may interpolate for lengths between those listed. The modulus of elasticity of aluminum is one-third that of steel and the yield strength is approximately one-half that of steel. Table gives the metal thickness conversion comparison.
Tables and and notes explain how to adapt the steel duct reinforcement schedules to create comparable aluminum tables. Nevertheless, these provisions are more reliable than the tradition of simply increasing the duct gage by two size numbers. No failure at the rated pressure is anticipated, and none has been reported since this approach was introduced in However, if the joint is one that enfolds an angle or bar T, or T , equivalency is based on changing the thickness of the connector only and retaining a galvanized steel bar or angle.
Otherwise, if the bar or angle must be aluminum, change the thickness and dimensions as necessary to accommodate the aluminum extrusions. Use a flat connector, change its thickness per Table , and back it up with a galvanized steel member from Table attaching it with aluminum fasteners , or back it up with an aluminum angle from Table that is equivalent to the required steel angle code.
The alternative of using tie rods is also acceptable for aluminum. It is assumed that the reason for using aluminum duct would necessitate use of aluminum internal tie rods.
However, since aluminum tie rods are not in this standard, the user will have to qualify his own selections. Round aluminum duct construction is given in Table Any aluminum shape substituted must have a moment of inertia three times that of steel and have 30, psi minimum yield strength. For Tables , , and , a connector not using angles or bar stock must have its thickness increased per Table and its dimensions increased per Table For Tables , , and , a connector using angles or bar stock must have its aluminum thickness increased per Table and must use either aluminum stock or galvanized stock from Table For Table , use only galvanized steel members in Table or the equivalent aluminum members.
Use either galvanized steel members of dimensions given or aluminum members having both thickness and dimension conforming to Table Other suitable aluminum shapes having a moment of inertia three times that of steel may be used. Consider the need for dielectric isolation by zinc chromate paint, asphalt impregnated paper, bituminous paint, or other method. Follow construction details for steel construction standards unless they are superseded by data on this page or by other considerations pertinent to aluminum.
Use a lock-forming grade of sheet material. Transverse joints shall be selected and used that are consistent with the static pressure class, applicable sealing requirements, materials involved, duct support intervals, and other provisions for proper assembly of ductwork outlined in the construction standards. The precise type, size, location, and material of fastenings used in joint assemblies are sometimes left open to prudent judgment for the specific service.
Notching, bending, folding, and fit-up tolerances shall be appropriate for the composite assembly. When there is a choice of materials and methods, do not use such latitude as to create a deficiency in the integrity of the ductwork.
See paragraphs 1. Where the text and illustrations in sections 1 through 5 indicate certain sealing provisions independent of the provisions in paragraphs 1. Where bar or angle stock is incorporated in a joint, it shall be secured. Fasteners used on steel duct shall be steel. They may be zinc or cadmium coated. Standard or self-drilling sheet metal screws may be used as appropriate.
Blind rivets using pull-through mandrels are not permitted if they leave holes for air leakage. Where only bolts or welds are specified, other types of fastening are not allowed. Unless otherwise specified, joints shall comply with all of the provisions in Sections I and II except the commentaries. Use gage not less than two gages less than duct gage, with 24 gage 0.
See qualification as reinforcement in Table Use slip gages per joint T Gage shall be 24 0. Use slips conforming to T Drive a 16 gage 1. Use 24 gage 0. Fasten as on Joint T The standing portion of this cleat may be formed per illustration T to hold flat bar. Fasten the bar stock to the connector at ends and intermediate positions.
The angle must be fastened to the connector or the duct wall. Seal and fold corners. Stagger joints on adjacent duct sides if using standing seams on all four sides. Hammer longitudinal seam at ends of standing seam. Close seam per Joint T15 notes. Fasten angle to the vertical leg of the seam. On 24 0. The angle is otherwise fastened normally. For additional tightness, place sealant between the angle and duct or seal the weld.
If the faces of flanges are flush, thick-consistency sealant may be used in lieu of gasket. Otherwise, use a gasket suitable for the specific service and fit it up uniformly to avoid its protruding inside the duct. Mating flanges are formed on the ends of the duct in double flange style to create a tee shape when assembled.
A minimum of 16 gage 1. Flanges may also be assembled per Figure Joint Ta. This is a modified version of T that is assembled per Figure Joints Ta and Tb. Assembly specifications are given in Figure Ratings in Table may be adjusted when combined with EI rated flat bar stock or members from Table The supplemental reinforcements may be attached to the duct wall on both sides of the joint or use single members if such are fastened through both mating flanges.
There are several proprietary duct connection systems available and in use. However, SMACNA does encourage the development and use of new technology and it invites authorities to consider alternative constructions. Authorities may evaluate alternatives using analyses and tests such as those described in Section VII of the standards or using other means they deem appropriate.
Consult the manufacturers of alternative systems for ratings, assembly requirements and recommendations. Note that joints previously shown in earlier editions can still be used. T-4, 9, 17, 18, 19, 20, and 23, omitted due to infrequent use, may still be used per the first edition if acceptable to the specifying authority. Seams shall be selected for the material, pressure classification, and other construction detail appropriate for the service.
Seams shall be formed and assembled with proper dimension and proportion for tight and secure fit up. Notching shall be minimal and consistent with transverse joint make-up needs. Although they are not illustrated here, seams may be of butt weld, corner weld, plug weld or spot weld design. They may also be of the spiral lockseam type. Type L-1, Pittsburgh seam: Use on straight ducts and fittings. Type L-2, Button punch snaplock: It is not recommended for ducts of aluminum or other soft metals.
Type L-4, Standing Seam: Standing seams may be used on duct interiors with due consideration for velocity level. See Figure for inside standing seam construction. Type L-6, Double Corner Seam: Machines are available to automatically close this seam.
In some localities it is known as a slide lock seam. They assume that system designers understand friction and dynamic losses in HVAC systems and have accounted for these in the design of systems for particular projects.
The purpose of this section is to provide geometries, configurations, and construction detail alternatives that relate to and enhance the performance of fittings that the designer may incorporate in his systems.
More construction detail is provided than is given in design handbooks. The many alternatives included in this document would not have the same pressure loss factors.
Also, equipment manufacturer installation requirements may differ from the illustrations herein. To the extent that the designer is inconsistent with the presumption stated in S1. When not preempted Section S1 will control the construction of fittings to a significant extent S1. Similar specifications are in Section 3 and 4. S paragraphs for Section 2 begin on page 2. Compliance with Figures in Section 2 that are not specifically referenced in the S specification text is presumed when not preempted by specifications external to this document.
However, the inclusion of performance requirements in this section, as in other sections, is intended to acknowledge the provisions of Section S1. Designers must show all required air volume control devices on the contract drawings. Nothing in this document implies an obligation to provide volume control devices that are not on the contract drawings.
Illustrating dampers on contract drawings relieves contractors from interpreting damper requirements. The damper designs illustrated in Figures and are for reduced volume control, not for positive shut off.
Orifice plates or perforated metal with required pressure-drop characteristics may be used in lieu of dampers to set up permanent loss in duct runs. Multiblade damper styles are normally parallel blade for two position operation; opposed blade for modulating position. Motor operators for dampers should develop sufficient torque to operate properly.
The motor supplier should select operators carefully. In certain cases, a fire damper may be used for flow rate control. If it serves a dual function, its operation as a fire damper must not be impaired.
The installation must not develop noise or vibration. Volume control devices that are capable of throttling flow over wide pressure differentials without generating noise are normally special procurement items.
Low-pressure drop dampers should not be used for wide-pressure differentials. The designer must carefully evaluate pressure change in ducts and provide pressure relief measures where necessary. System status changes, as in smoke control mode or energy conservation use, impose different requirements for normally open, normally closed, and modulating dampers. Such detail now sets forth material, wall thickness, and joint construction, among other features. Furthermore, much data on the shape, size, and location of kitchen range and other hoods is published in the book Industrial Ventilation , by the American Conference of Governmental Industrial Hygienists.
Chapter 4 is devoted to hood design data and Chapter 5 to specific operations involving hoods and ducts. Moreover, new emphasis on energy conservation has prompted the increased use of localized exhaust and makeup air. These and similar industry changes have resulted in reliance on customized designs rather than standard designs such as those formerly published by SMACNA.
Designers should consult these references, illustrate the complete design on contract drawings, and make limited reference to the duct construction detail in this manual, if necessary.
Flexible duct liner of the specified material, thickness, and density shall be furnished and installed where shown on the contract drawings. Unless otherwise indicated, the net free area of the duct dimensions given on the contract drawings shall be maintained. The duct dimensions shall be increased as necessary to compensate for liner thickness.
All transversely oriented edges of liner not receiving metal nosing shall be coated with adhesive. Liner shall be neatly butted without gaps at transverse joints and shall be coated with adhesive at such joints before butting. Liner shall be folded and compressed in the corners of rectangular duct sections or shall be cut and fit to ensure butted edge overlapping.
Longitudinal joints in duct the liner shall not occur except at the corners of ducts, unless the size of the duct and standard liner product dimensions make them necessary. Except as noted in paragraph S2.
Longitudinal joints in liners shall be coated with adhesive when velocities over fpm Metal nosings that are either channel or zee profile or are integrally formed from the duct wall shall be securely installed over transversely oriented liner edges facing the airstream, at fan discharge and at any interval of lined duct preceded by unlined duct.
In addition, where velocities exceed fpm Where dampers, turning vane assemblies, or other devices are placed inside lined ducts or fittings, the installation must not damage the liner or cause erosion of the liner. The use of metal hat sections or other buildout means is optional; when used, buildouts shall be secured to the duct wall with bolts, screws, rivets, or welds. It is primarily used for its sound absorption characteristics, although it may have some thermal resistance value.
Molded round liner is available. Metal wall inner lining is available for all conventional duct shapes; typically it is of 22 gage 0. The double-wall style of lined duct is used where increased resistance to damage is desired or where erosion of the inner surface might occur. Standard flexible liner is normally shop-installed. Minor damage to the liner surface may occur in transportation and handling. Small cuts, tears, or abrasions may be repaired with fire retardant adhesive.
Material that has significant damage cannot be considered to be in new condition. Liner is normally prequalified for a certain resistance to moisture absorption, mold growth, and degradation from high humidity. In such cases, drying or other corrective measures recommended by the material manufacturer should be followed. Installing two layers of material to meet a minimum liner thickness is not recommended. In addition, pay special attention to the leading edge conditions.
Normally, duct linings must be interrupted at fire dampers to avoid adverse effects on damper operation and at heat sources to meet minimum clearances specified in an equipment listing. Some appliances are rated for a zero clearance to combustible material. Liner adhesives are usually water-based or solvent-based, and they may be flammable in wet or dry states.
Designers should select adhesives that meet construction and code requirements. So-called safety standards may involve tests that report various characteristics but do not meet up to a hazard classification under installed conditions. Contractors are invited to follow ventilation, storage, and other precautions published by the adhesive manufacturers. Three types of fasteners are commonly used with duct liners.
For each type of fastener, a specific pin length is appropriate for each type and thickness of liners. It is important that the proper pin length be used, otherwise a faulty installation will result. Fasteners designed to be secured with adhesives have a large base on which to apply the adhesive. After waiting enough time to achieve adequate bond strength which will vary, depending on the air temperature-impale the duct liner on the pin and add the spring clip or washer.
Mechanically secured fasteners form a positive attachment to the sheet metal. Typically, they are impact-applied, hardened steel fasteners which bite into the sheet metal. Weld-secured fasteners are attached by two techniques: Correct adjustment of the timing devices is necessary to obtain a solid weld without burn-through. The type of pin that is applied before duct liner installation takes a spring-clip or washer.
Pins with pre-attached. Depending on the type of fastener, discoloration or dimpling may be evident when fasteners are properly attached to the sheet metal. This does not affect the serviceability of the fastener or of the sheet metal. Fitting classes available for designer use in project specifications or contractor selection as being fit for the project specifications that adopt these standards are as follows. Category listings are not intented to preclude different selections for fittings that function as area change, direction change, converging flow, diverging flow, or special purpose.
Category listings also do not necessarily apply to their end connections to other fittings, straight duct sections or equipment. The preceding categories may have additional forming prescriptions such as rolled, stamped, gored, spun, pleated, semi-pleated, or other methods.
For purposes of distinction, openings in sections of straight ducts to receive taps of any connection method are not deemed to be fittings; but connection thereto may be specified by a prescribed method. Round ducts shall be constructed in accordance with Tables and Uncoated, polyvinyl coated, aluminum alloy coated or aluminum-zinc alloy coated steel or stainless steel may be used if a minimum corresponding base metal thickness and material strength is provided.
Lockforming quality is required. The use of an alternative material requires specification or approval by a designer. Fittings shall have a wall thickness not less than that specified for longitudinal-seam straight duct in Tables and The diameter of fittings shall be appropriate for mating with sections of the straight duct, equipment, and air terminals to which they connect. Sleeves, collars, and fittings to connect a round duct to a rectangular duct or to flexible ducts shall conform to S3.
See Figures and and pages to Nothing in this specification is meant to imply that the designer cannot by project specification designate acceptable construction methods. The use of a saddle or a direct connection of a branch into a larger duct is acceptable. Where they are used, the diameter of the branch shall not exceed two-thirds of the diameter of the main and protrusions into the interior of the main, are not allowed.
Direct connection of a branch into a main shall include mechanical attachment sufficient to maintain the integrity of the assembly. All saddle fittings shall be sealed at all pressures. Where other limitations are not stated, mitered elbows shall be based on the velocity of flow and shall be constructed to comply with Table Ducts shall be suspended in accordance with Section IV.
Additional supports shall be added if necessary to control deflection of ducts or to maintain. Round duct has a high strength to weight ratio, uses the least material to convey air at a given friction loss, and is comparatively easy to seal.
The wall thickness suitable for positive pressure application is generally less than that for negative pressure. For positive pressure and low negative pressure , girth ring reinforcement is not necessary. However, rings may be used to maintain the round shape to facilitate handling, shipment, or connection. The tables indicate that a 10" W. Some of the constructions in the tables will qualify at higher negative levels. For spiral ducts, higher negative pressure service information and bursting pressure in positive mode is available from industry sources.
This manual also does not indicate preference for any one type of longitudinal seam. The length of spiral seam duct is limited by considerations such as in-line fitting frequency, potential for damage in shipment, maneuverability of the sections on the job, the number of support points needed to place the duct in its final location, and other factors.
The most popular transverse joints are the slip or lap types. The flanged joint is used in ducts over 60" mm in diameter because of its advantage in retaining the circular shape. Access to joints for makeup and orientation in vertical or horizontal positions will influence the choice of connection. Friction loss data is provided in these design manuals. Where fittings of comparable or better performance are illustrated in duct design handbooks, designers are encouraged to consider allowing a substitution.
Omissions from this document are not intended as prohibitions against using other constructions. Double-wall rigid round duct is available from several industry sources.
It is used for its acoustical value, and the perforated typically metal inner wall provides resistance to erosion of the duct liner. Round spiral seam ducts with thinner than traditional wall thickness and with one or more corrugations ribs formed between the lock seams have been introduced in industry. Some of these forms have been tested for compliance with UL Standard and have qualified for Class O listing. As the industry develops more experience with these in installation and service, and as more functional performance criteria are identified, it is anticipated that such forms will be added to SMACNA construction standards.
Authorities and contractors are invited to evaluate them by information currently available. Small differences occur in the diameter of ducts and fittings.
Proper clearances are necessary. Verify suitability of fit, particularly when procurement is from outside sources. An alphabet letter in the table means that reinforcement angles or their equivalent must be used at the foot interval following the letter. The angle sizes are:. An alphabet letter in the table means that reinforcement angles or their equivalent must be used at the meter interval following the letter. Construction of aluminum duct and fittings shall otherwise correspond in the same relationship as for steel duct.
Sheet material shall be alloy H14 unless otherwise specified. Aluminum fasteners shall be used. Structural members if used shall be alloy T6 or galvanized steel as a related in table on page for rectangular duct.
Hangers in contact with the duct shall be galvanized steel or aluminum. Fittings shall conform to the thickness schedules in Table , shall conform to the seam, joint, and connection arrangements permitted for round duct, and shall be reinforced to conform to paragraph 3. See criteria in Section 7.
Supports shall conform to those permitted for rectangular duct, with the overall dimensions taken as references. Flat oval duct combines the advantages of round duct and rectangular duct because it may fit in spaces where there is not enough room for round duct, and it can be joined using the techniques of round duct assembly.
Spiral flat oval duct is machine-made from round spiral lockseam duct and is available in varying sizes and aspect ratios.
It can also be made with longitudinal seams. Flat oval duct has considerably less flat surface that is susceptible to vibration and requires less reinforcement than a corresponding size of rectangular duct. The deflection of the flat oval duct under pressure is related to the flat span rather than the overall width of the duct.
Any round duct fitting can have an equivalent fitting made in flat oval. As in rectangular duct, a hard bend elbow denotes the bend in the plane of the duct width, whereas an easy bend elbow denotes the bend in the plane of the duct height. Any branch fitting can be made with the branch tap either round or flat oval. The tap of the flat oval fitting can be located anywhere on the circumference of the fitting body.
If the diameter of a round tap is greater than the height of the flat oval body, a transition can be made from flat oval to round, providing an equivalent area at the base of the transition. These provisions apply to ducts used for indoor comfort heating, ventilating, and air conditioning service. They do not apply to service for conveying particulates, corrosive fumes and vapors, high temperature air, corrosive or contaminated atmosphere, etc.
It is presumed that project specifications define the specific materials, pressure limits, velocity limits, friction rate, thermal conductivity, acoustical ratings, and other attributes. By UL Standard , a flexible connector is defined as a flexible air duct not having certain flame penetration, puncture, and impact tests. Bends shall be made with not less than 1 duct diameter centerline radius.
Ducts should extend a few inches beyond the end of a sheet metal connection before bending. Ducts should not be compressed. Ducts shall be located away from hot equipment such as furnaces and steam pipes to avoid excess temperature exposure. Illustrations of accessories, sleeves, and collars are representative of classes of items.
The use of components not precisely identical to these is acceptable. If the application guidelines dictated by the flexible duct manufacturer are more stringent than the specifications in this manual, those of the manufacturer shall govern. The provisions for sealing ducts specified on page apply. Adhesives shall be chemically compatible with materials they contact. Collars to which flexible duct is attached shall be a minimum of 2" 51 mm in length. Sleeves used for joining two sections of flexible duct shall be a minimum of 4" mm in length.
Collars and sleeves shall be inserted into flexible duct a minimum of 1" 25 mm before fastening. Ducts larger than 12" in mm diameter shall have at least of five 8 sheet metal screws.
Non metallic flexible duct shall be secured to the sleeve or collar with a draw band. If the duct collar exceeds 12" mm in diameter the draw band must be positioned behind a bead on the metal collar. Insulation and vapor barriers on factory-fabricated ducts shall be fitted over the core connection and shall also be secured with a draw band.
These photographs depict typical accessories but do not represent all available accessories. Coincidence with proprietary features is unintentional. The standard is not intended to limit the selection or the development of accessories for use with flexible duct.
A connection to another duct or to equipment is considered a support point. Hanger or saddle material in contact with the flexible duct shall be wide enough so that it does not reduce the internal diameter of the duct when the supported section rests on the hanger or saddle material. In no case will the material contacting the flexible duct be less than 1" wide. Narrower hanger material may be used in conjunction with a sheet metal saddle that meets this specification.
To avoid tearing the vapor barrier, do not support the entire weight of the flexible duct on any one hanger during installation. Avoid contacting the flexible duct with sharp edges of the hanger material. Damage to the vapor barrier may be repaired with approved tape. If the internal core is penetrated, replace the flexible duct or treat the tear as a connection.
Terminal devices connected by flexible duct shall be supported independently of the flexible duct. UL, NFPA, and most codes make distinctions between these two products in their limits of application. Connectors are more restricted and are currently limited to 14" 4. Regulations governing these forms of duct should be checked especially for floor penetrations, ceiling air plenums, and fire rated floor-ceiling or roof-ceiling assemblies.
These installation provisions were prepared for round ducts; however, they may also be usable for flexible flat oval ducts. Some types of flexible duct have received listings as components of fan unit or air terminal unit systems, and they may be governed independently by the conditions of those listings.
The most common metallic duct is aluminum; however, galvanized steel and stainless steel varieties are available. Nonmetal ducts are available in a wide variety of materials and nominal shape-retaining reinforcements. Machines for producing the ducts are available from several suppliers. Flexible ducts may come to the installer in compressed form in a variety of lengths.
Their length can be determined by a measurement taken with a 25 lb. Repeated flexure of metallic ducts will probably result in fatigue stress cracking. Sections S3. Compressing duct increases first cost and friction loss.
The minimum length refers to the practical route between connection points but not to the degree that the material is overstressed or to the degree that all available stretch is removed. This installation standard is applicable to ducts placed in or beneath concrete floors or in areas free from vehicle traffic.
Materials commonly used for this application include galvanized steel, vinyl chloride-coated steel, and stainless steel. Glass fiber-reinforced resin, asbestos, cement, tile, and other nonmetal ducts are also used.
Ducts are not generally deemed to be or required to be waterproof. Ducts should always be above the water table. The designer should carefully evaluate the exposure to moisture or ground water and require vapor barriers, sumps, porous fill, and subsoil drainage pipe as necessary. CSI Specification provides useful references for subsoil drainage.
The top of drain tile should be below the bottom of the duct. Corrosion resistance is an important characteristic of both in-slab and under-slab ducts. The Portland Cement Association has guidelines for protection of metals in contact with concrete. The strength of round ducts makes them the preferred shape for underground application. Round duct wall thicknesses in these standards are generally acceptable for below-grade installation.
Ribbed or corrugated styles have additional crushing strength.