Quality Duct Systems

From Start To Finish

Spiral Round Duct

If you could do just one thing to your duct design that would make it more energy efficient and less expensive, would you do it?

Symbol Denotes Spiral Duct, Oval and Round

You might recognize the two symbols above. The one on the left is used to denote that a duct will be round. The one on the right denotes that a duct will be flat oval. Most of the time, for a typical HVAC system, they indicate you want spiral duct.

But for some reason, we often see engineers leaving these symbols off of their drawings. Perhaps they can’t find them in their CAD libraries. But when you leave them off, you are usually going to end up with poor-performing and costly rectangular duct.

Anti-Microbial Duct

The demand for antimicrobial products is increasing. More and more owners and engineers are requesting it for its effectiveness in inhibiting the growth of damaging bacteria, mold and fungi. AgION™ antimicrobial-coated steel ductwork is durable and should be incorporated into HVAC systems. Learn more about Anti-Microbial Duct products.

Why Round?

Without spiral round duct, here are a few of the things that could happen.

1. You won’t follow the ASHRAE Advanced Energy Design Guides – it’s a big deal to try and reduce energy use in commercial buildings by 30% and 50%. And ASHRAE has worked extensively to help its’ members achieve these goals by compiling research and assembling cost feedback that might be impractical for a single engineering firm to do. The Advanced Energy Design Guides were written by your fellow engineers, not a trade organization or some contractor that bought a coil line and needs to get the most use from it. They specifically state “Round duct is preferred over rectangular duct. However, space (height) restrictions may require flat oval ductwork to achieve the low-turbulence qualities of round ductwork.”

2. You’re going to get more turbulence, and turbulence is bad – you don’t even have to be an engineer to know those square corners of rectangular ductwork cause far more turbulence than no corners (round) or rounded corners (flat oval). We all kind of know without trying it that putting a rectangular drain line downstream of your toilet would pretty much assure regular visits from the plumber. Things just don’t flow as well when you have square corners. And with airflow, that gives you two unwanted problems that are directly related to turbulence – higher pressure drops and increased noise.

3. You’re going to need more duct – the only reasonable way to decrease pressure loss and noise in a duct (other than change its’ shape — which we are trying to get you to do) is to slow the air down. Yes, you need to make the duct bigger. For aspect ratios 2:1 to 4:1 — pretty typical for rectangular ducts — the perimeter relative to an equivalent round diameter is 30 to 55% (2013 ASHRAE Handbook – Fundamentals, Chapter 21 “Duct Design”, page 12, about midway down the right side of the page). We’re not making this up! And it’s pretty easy for you to confirm after a couple of moments on your Ductulator.

4. Your duct’s going to weigh a lot more, and need a whole lot more hardware screwed/welded to it – you’ve probably heard or used the term ”built in accordance with SMACNA”. That’s most often referring to the “2005 SMACNA HVAC Duct Construction Standards – Metal and Flexible”. It is an excellent structural standard that’s mostly showing you one thing — how to limit deflection. Whether dealing with how much an assembled length of duct deflects (hanger spacing and seismic bracing) or how much the walls of a duct deflect (limiting metal fatigue, low-frequency noise generation and how the duct may impede other items within the building), the goal is to keep a duct system as static and motionless as possible. Flat surfaces deflect. Round/curved surfaces have little-to-no deflection in positive and low pressures. The SMACNA manual will have you increase the gauges or add reinforcing to limit deflection. So not only do you have to increase the amount of duct (perimeter) to reduce pressure loss and noise, you also have to increase the weight of that perimeter to reduce deflection.

Here are a few examples:

Static Pressure and Duct

5. Your duct’s going to cost a lot more – the cost of ductwork, both to buy/make it and to install it, has a direct correlation to weight. Keep in mind the chart above. Even if someone says their cost per pound is less for rectangular duct, that’s quickly negated because it takes a lot more pounds of rectangular duct to carry the same amount of air. Installation labor skews even more in favor of round duct. Weight-based labor formulas generally allow 50% more pounds per man-hour for installing round and flat oval spiral duct versus rectangular. So if you pick an example from the chart above where rectangular duct weighs 50% more than spiral round duct, it only takes 44% of the labor to install the spiral round duct compared to the rectangular.

6. You’re going to need more hangers/supports – about 50% more. Most rectangular ducts need to be supported at least every 8’-0”. For round and flat oval spiral ducts, that support spacing is 12’-0”.

7. You’re going to need more insulation – it’s going to take more square feet of insulation to cover the increased perimeter/surface area of rectangular duct.

8. Your duct is going to leak twice as much – rectangular duct constructed to Seal Class A (transverse and longitudinal seams sealed, openings for rotating shafts sealed) are expected to leak 6 cfm/100 ft2 duct surface area at 1” WG. Round and flat oval spiral ducts constructed to Seal Class A are expected to leak 3 cfm/100 ft2 duct surface area at 1" WG.

9. You’re going to have a hard time meeting the ASHRAE Standard 90.1-2013 Energy Code – it states “Ductwork and all plenums with pressure class ratings shall be constructed to Seal Class A, as required to meet the requirements of Section…” (the Duct Leakage Class). It doesn’t say you have to test it, but you are expected to meet the same standards as the duct you are required to test. That Duct Leakage Class is 4 cfm/100 ft2 at 1” WC. Rectangular duct meeting the required Seal Class A is expected to leak 50% more than the allowance. Round and flat oval spiral duct is expected to leak only 75% of the allowance. Perhaps we should pause a moment to let these numbers sink in. If more than 33% of your total square footage of ductwork — high pressure, low pressure, medium pressure, return air and exhaust — is rectangular duct, even constructed to Seal Class A, instead of round and flat oval spiral duct, you are designed to fail!

10. You’re going to need a lot more duct sealant – even if you don’t test the duct to find that you’ve failed the energy code, you’re still required to construct the duct to Seal Class A. When you do that, you’ll get a pretty good picture of why rectangular duct leaks at least twice as much as spiral duct. First of all, you have to seal the longitudinal seams of the rectangular duct. You don’t have to seal the spiral lockseams of the spiral ducts. That is specifically stated in the ASHRAE standard, and testing has confirmed that spiral seam leakage is negligible — between a Duct Leakage Class of 0.02 and 0.3. Most leakage for all ducts occurs at the joints, and you are going to have about twice as many duct joints to make with rectangular duct (standard 56” lengths from a TDC coil line) versus spiral duct (commonly supplied in 10’-0” lengths). That by itself should give you an expectation of twice the leakage. And within the rectangular joint, the biggest culprits are those pesky corners — the same ones that cause the turbulence that gives rectangular duct it’s higher pressure drops and increased noise. Let’s pick an example from our chart above — 18”φ versus it’s equivalent rectangular size of 24 x 12.

Example #1
100 lineal feet of 18”φ spiral duct
Ten 10’-0” lengths of spiral duct = 9 joints
18” x π x 9 joints = 509 lineal inches of sealing

Example #2
100 lineal feet of 24 x 12 rectangular duct
100 ft x 12 in/ft x 2 longitudinal seams = 2,400 lin.in. of longitudinal joint sealing
100 feet / 56 in/section = 21.43 sections = 21 joints
21 joints x 72 perimeter inches = 1,512 lin.in. of joint sealing
Total = 3,912 lineal inches of longitudinal and joint sealing

11. You’re not going to have an easier time fitting the duct into the allotted space – there seems to be this mistaken idea that if ceiling space is tight or congested, your solution is to use rectangular duct. First of all, flat oval duct is going to work anywhere rectangular would, but without the pressure drops, noise, leakage and weight. Second, anyone that’s ever seen a bad contractor snake a 25’-0” length of flex duct through an attic — and we really recommend that you don’t allow that — should understand the concept that a round duct can be maneuvered more readily than rectangular duct without compressing the area. And with slip-joint construction you trim duct lengths of round and flat oval spiral ducts without compromising their integrity or performance. Have you ever looked at rectangular duct that’s been “field adjusted”? It’s usually not very pretty. In any case, you shouldn’t base the design of a commercial duct system on the premise that what you draw probably won’t fit, so you need to plan for the contractor to have to change everything in the field. If that’s the case, please make a big note on the plans — so we know to avoid that project.

12. The duct won’t be easier or faster to get – this one really doesn’t make sense for commercial construction, yet we hear it all the time. Yes, many sheet metal contractors have their own shops and could — in theory — go out there and make the duct for your project right now. In the real world, they try to keep their shops busy and your job will wait in line. If their shop is not busy, their cost to make rectangular duct will be much higher. If you really need the duct fast, there are dozens of places within a one-day shipping frame of your jobsite where you can find spiral round duct and fittings in stock for immediate shipment in the size range you should need on a fast-track project. Most commercial duct projects go through months of coordination and detailing, so getting any duct — regardless of what you think shop lead time is — is a matter of planning, not “availability”. And round and flat oval spiral duct are neither hard to find or noncompetitive. We really hate to plug our competition, but within a 600-mile radius of our plant in Ft. Worth, Texas you can find more than 100 spiral duct machines and at least 15 ovalizers. No, availability is not an issue

Do you see where we’re going with all this? The numbers stack up overwhelmingly in favor of round and flat oval spiral ducts. The ASHRAE Handbooks and Design Guides tell you to use round and flat oval ducts over rectangular ducts. ASHRAE research projects prove better performance from round and flat oval ducts. Other testing projects — from manufacturers, trade groups and government organizations like the Department of Energy — support the advantages of round and flat oval duct over rectangular. Countless real-world examples where a contractor or manufacturer converted a mostly rectangular duct design to round and flat oval spiral duct, have proven you can get better performance at a better installed cost.

But if you don’t use those symbols above and indicate you want round and flat oval duct for your project, you will almost certainly get rectangular duct. It will not perform as well, it will probably cost more, and you will have done the owner no favors. As obvious as it may be that you should have used round and flat oval duct, the contractor probably needs the rectangular work to pay for his shop and no one has the time to re-design the job and show how the duct system could have been much better.

So if you just haven’t been designing round and flat oval spiral duct because you couldn’t find the right symbols, give us a call. We’ll help you add them to your library.

Spiral Round Duct

Spiral round lock-seam duct is the most efficient way to transport air in HVAC systems. The round profile greater flow efficiencies than rectangular ducts. Because round ducts are inherently stronger than rectangular profiles, they can be lighter metal gauges and can be installed in longer spans. The 4-ply spiral lock-seams of “spiral duct” create a product that is stronger than traditional welded-seam round ducts of the same gauge. Spiral lock-seams have practically no leakage, and are the only mechanical seams exempted from sealing under energy standards such as ASHRAE Standard 90.1-2013.

At Spiral Pipe of Texas, we make products that meet or exceed the SMACNA HVAC Duct Construction Standards – Metal and Flexible (2005). We make diameters ranging from 3” to 92” and metal thicknesses from 26 gauge to 16 gauge. The following is our construction standard for positive pressures to +10” WG.

10" WG --- Standard Round Galvanized Ducts

Standard fittings to be spot welded and sealed. Continuously welded seam fittings are available if specified. All PCD and SPOT Agion fittings are riveted and sealed.

Available Materials

Galvanized Steel — G60 and G90
Mill Phosphatized Steel (Paint Grip)
Galvanealed Steel — A60
f Stainless Steel — 304L and 316L
3003 Aluminum
SPOT Agion (anti-microbial coating)
Polyvinyl Coated (PCD)
Black Iron

Available Transverse Connectors

Standard Slip-Fit / Couplings
SPOT Flange
SPOT Triple Rib Gasketed
Angle Rings
Wonder Flange
Long Seam Weld Flange

Special materials and connectors may be available. Please contact your nearest representative for details.

Standard (Flat) Spiral Seam PipeCorrugated Spiral Seam Pipe

Insulated Ducts

Most HVAC ducts need to be insulated. At Spiral Pipe of Texas, our home area of the Central and Southern US encompasses regions with high fluctuations in both temperature and humidity. So, it’s no surprise that we have been at the forefront of developing and implementing insulated duct products. Factory-insulated duct products reduce field labor, as well as giving consistency and product features you won’t get with thermal wrap. Duct insulation is also critical in reducing HVAC system noise. Here are some of the product options for round insulated duct that are available from Spiral Pipe of Texas.

Double-wall Insulated Round Ducts

Double-wall construction allows you to contain the insulation media between an inner and outer metal shell. The insulation is protected from external damage — a critical concern for ductwork on rooftops and out of the building envelope. Where ducts are exposed to view, insulation is concealed for a more pleasing appearance.

The internal metal shell gives protection from insulation erosion, maintains a smooth duct cross-section, and makes cleaning the ducts easier. There are quite a few options available for double-wall ducts, so it’s important that critical features are specified. Here are some of them.

Outer Metal Shell

For HVAC air ducts, the outer shell is the structural and pressure containment portion of the product, and is the basis of construction. Unless noted otherwise, double-wall ducts are fabricated in accordance with Chapter 8 of the SMACNA HVAC Duct Construction Standards Metal and Flexible (Third Edition – 2005) and the outer shells are constructed in accordance with Chapter 3 of the same standard.

It is important to note that the dynamic airflow performance of a duct system is based on the inner shell, and most duct sizing on mechanical drawings are I.D. Yet, it is the outer shell that determines gauge, reinforcing and construction, as well as defining how a duct fits within the physical constraints of a building. Fitting dimensions (elbow centerline radii, tee fitting length, tap clearance to end, etc.) are based upon the outer shell, not the inner.

Straight duct construction – available as spiral lock-seam duct (corrugated and non-corrugated) and welded longitudinal-seam duct.

Fitting construction – fully-welded or tack-welded and sealed.

Materials – G90 and G60 galvanized steels, “paint-grip” mill phosphatized steel, A-60 galvanealed steel, black iron, aluminized steel, 3003 aluminum, 304-L and 316-L stainless steels, PCD Polyvinyl Coated Duct, Agion anti-microbial steel.

Gauges – 28 to 16 for galvanized spiral lock-seam, 28 to 18 for stainless steel spiral lock-seams, 0.025” to 0.080” for aluminum spiral lock-seams, 22 gauge to 3/8” for welded longitudinal-seam duct.

Appearance and finish – standard factory finish (no special cleaning, welds spray-painted, mill markings, labels and piece-marks visible), paint-ready (welds not painted, removeable labels), and factory painted. SPOT has additional literature giving more specific details for our architectural duct products.

Inner Metal Shell

Solid – standard metal liner in gauges and construction to contain the insulation and maintain concentricity and structural integrity. Available in all materials offered above for outer shells.

Perforated metal – standard material is a 23% open area pattern with 3/32” holes on 3/16” centers, staggered.

Perforated metal with erosion barrier – standard perforated material with an additional protective layer of Mylar between the insulation and the perforations opening into the airstream.

Note — the Mylar film is an excellent erosion and vapor barrier, but it’s use may affect the acoustical performance of a double-wall duct, as well altering the flame spread and smoke development numbers.

Insulation Layer

Standard Material – our standard product is a 0.75# density Knauf AtmosphereTM Duct Wrap with ECOSE® Technology. The insulation is thicker than the intended annular space between metal shells (1.5” thick for 1” nominal spacing) so that net density is after fabrication is approximately 1.00#. This material is a highly resilient inorganic glass mineral wool with bio-based binders. It does not contain phenol, formaldehyde, acrylics or artificial colors. It carries a GREENGUARD Gold Certification.

Thickness — standard 1” and 2” annular spacing. Other thicknesses available.
Installed R-Value — R=4.2 for 1” spacing, R=8.4 for 2” spacing
Surface Burning Characteristics — Flame Spread 25 and Smoke Developed 50 (tested in accordance with UL 723, ASTM E 84 and NFPA 255.
Temperature Range — up to 350°F
Mold Growth — no growth (ASTM C 1338)
Note — metal spacers are not used to maintain concentricity between inner and outer shells. Though sometimes specified, they make duct assembly with slip-fit transverse joints more difficult, create thermal bridges between the interior and exterior of the duct, and have a negligible effect on actual R-value. In actual practice, insulation compression is more of a concern for larger diameters of duct where double-wall flanges are typically used for transverse connectors, eliminating any need for spacers.

Alternative Material

Alternative Material – Armacell AP/Coilflex® elastomeric foam duct liner. Fiber-free, non-particulating foam alternative to glass fiber insulations. Standard 1” thickness (larger annular thicknesses obtained by wrapping successive layers) with R-value R=4.2. Flame spread/smoke developed index of 25/50 (ASTM E 84) and temperature use limit up to 180°F. GREENGUARD Gold Certified. Made with EPA registered Microban® antimicrobial product protection.

Single-wall Insulated Round Ducts

Single-wall Insulated Round Ducts

Spiral Pipe of Texas has been a pioneer in the use of Johns Manville Spiracoustic Plus® round duct liner system, providing some of the largest systems ever produced with outer shell diameters up to 92”Φ. Like our double-wall products, the outer metal shell is the basis of construction. The Spiracoustic Plus® liner has factory-made, evenly spaced kerfs that, when inserted into the outer metal shell, allow it to evenly conform to the inside of the duct.

The product is a high-density fiber glass board with a factory-applied black acrylic coating applied to the surface and transverse edges, JM’s Permacoat®. After the liner is assembled into the outer metal shell, SPOT provides the finishing touches by using JM’s SuperSeal® coating product to dress up any exposed edges, as well as utilizing mechanical fasteners as needed to assure the liner stays permanently affixed.

Spiracoustic Plus Liner

Spiral Pipe of Texas single-wall insulated duct with Spiracoustic Plus® liner may have some advantages for your project over traditional double-wall insulated products. With no metal inner shell, the product is lighter in weight — as much as 30% lighter in many sizes. Labor needed for transverse joint assembly can also be reduced since a single slip joint is all that is necessary. The product is ideally suited for systems with long, straight runs of duct — gymnasiums, cafeterias, airports, sports arenas and convention centers.

Available Insulation Thicknesses — 1”, 1 ½” and 2”
R-Value (tested in accordance with ASTM C518) — 1” (R=4.3), 1 ½” (R=6.4), 2” (R=8.4)
Diameters Available (i.d.) — 1” (6” to 90”Φ), 1 ½” (11” to 89”Φ), 2” (14” to 88”Φ)
Density — 4.0 pcf
Operating Temperature — up to 250°F
Maximum Air Velocity (ASTM C1071) — 6000 fpm
Surface Burning Characteristics — Flame Spread 25 and Smoke Developed 50 (tested in accordance with UL 723, ASTM E 84 and NFPA 255)
Fungi Resistance — does not breed or promote (ASTM C1338), no growth (ASTM G21)
Bacteria Resistance — no growth (ASTM G22)

Are We Green?

The short answer is “YES”

The slightly-less-short answer is: “If you are not using spiral ducts from Spiral Pipe of Texas, then you aren’t really green at all.”

Of course, a provocative response like that deserves some elaboration. And we are happy to provide it. For the last 20 years it has become fashionable for companies to describe themselves as “green”, and we see plenty of advertising for construction components festooned with leaves and butterflies. Our term for this is “green-washing”. To be fair, if you have a product where environmental consciousness was not previously a concern, it is a good idea to find out what is “green” about your product, try and do better, and reassure consumers that you care. Spiral duct has always been green — long before the term came into being.

The spiral duct industry started in Northern Europe at the end of World War II. As economies recovered, new and better buildings were incorporating ventilation and conditioning systems that had been developed during the previous few decades. But they had three problems — energy was expensive, raw materials were also expensive and often in short supply, and there was a shortage of skilled labor with so many young men lost in the war. To solve all three problems, spiral ducts were developed. Round is the most efficient shape for transporting air. It has lower pressure drops, saving fan horsepower, and there is less heat gain/loss through the surface than with rectangular shapes.

Round is also the strongest shape for transporting air. You need to limit surface deflection to prevent metal fatigue and duct rumble, and metal thickness and additional reinforcement were the ways of accomplishing this with traditional rectangular ducts. Round ducts have virtually no wall deflection in normal operating conditions, and the helical spiral seams strengthen the duct even more.

As a result, spiral duct systems usually weigh 30% less than equivalent rectangular duct systems — less material! Spiral ducts are normally produced in longer lengths than rectangular ducts. Where rectangular duct is normally provided in 4 or 5 foot nominal lengths, the standard for spiral duct is 10 feet with lengths as much as 20 feet commonly provided. So, it’s no surprise that most estimators predict installation rates of 50% more pounds per man-hour installed for spiral round duct than for rectangular duct. And don’t forget that we’re talking about a product that generally has 30% less pounds to start with.

So, here we have a product — spiral duct — that uses less energy, less material and less labor than other methods of transporting air. By anyone’s definition, this is an inherently “green” product. But let’s take this a few steps further. Most spiral duct systems produced at Spiral Pipe of Texas are galvanized steel and, where insulation is required, use fiberglass insulation. Galvanized steel is one of the most recycled materials on the planet. Approximately 70% of all steel and 30% of all zinc consumed are made of recycled material. And at the end of service, virtually 100% of galvanized steel ductwork can be recycled. For our insulated products, our standard for double-wall products is Knauf Friendly-Feel Duct Wrap or an equivalent product.

The Knauf product contains three primary ingredients: 1) sand — one of the world’s most abundant and renewable resources; 2) a minimum 50% recycled post-consumer glass content; 3) a new binder technology that reduces binder embodied energy by up to 70% and features bio-based materials rather than non-renewable petroleum-based chemicals traditionally used. They contain no phenol, formaldehyde, acrylics or artificial colors.

So, spiral duct is a “green” product made from “green” materials. Let’s look further at some of the regulations and compliance standards we work under. Most states, including our home state of Texas, have adopted state-wide energy codes. In Texas, we use the ANSI/ASHRAE/IES Standard 90.1–2013 “Energy Standard for Buildings Except Low-Rise Residential Buildings”. It calls for ductwork to be constructed to Seal Class A. Part of the requirement involves the contractor assembling our ducts using sealant, but our duct components themselves meet Seal Class A.

The only mechanical seams that do not have to be sealed under this standard are the spiral lock seams of spiral duct. They have been proven to have almost no inherent leakage. Per this standard, all ducts should be constructed and installed to meet a Duct Leakage Class 4, regardless of whether testing is required or not. According to the ANSI/SMACNA HVAC Air Duct Leakage Test Manual-2011, the expected leakage performance of a round or flat oval spiral duct system, assembled to a Seal Class A, is a Duct Leakage Class 2. Our products greatly outperform the energy codes with only half the leakage that is allowed.

And we take duct leakage seriously, because it’s considered to be the single largest preventable energy waste in commercial and residential buildings — even more of a waste than leaving lights and A/C on when the building is unoccupied. To go further, ASHRAE is producing a series of Advanced Energy Design Guides for commercial construction. These are guides for achieving progressive amounts of energy savings toward a net zero energy building and are available for different building types.

Their recommendation for Ductwork Design and Construction is:

“Low-energy use ductwork design involves short, direct and low pressure drop runs. The number of fittings should be minimized and should be designed with the least amount of turbulence produced…. Round duct is preferred over rectangular duct. However, space (height) restrictions may require flat oval duct to achieve the low turbulence qualities of round ductwork.”

So, we know from ASHRAE and SMACNA that round, flat oval and spiral duct is the way to go for efficiency and low leakage. Additionally, ASHRAE publishes a Duct Fitting Database that includes dynamic efficiency data for all duct and fitting configurations. Using it, you can see the actual performance efficiencies of round and flat oval spiral ducts and fittings over equivalent rectangular ducts. These lower pressure drops optimize the duct system performance and allow the transport of air at lower fan horsepower. And with less inherent leakage, you won’t need to produce as much air to start with.

Which brings us to the question of “LEED”. Leadership in Energy and Environmental Design (LEED) is one of the most popular green building certification programs used worldwide. It is an important first step in promoting “green” construction in commercial buildings. In a perfect world (at least for us) the LEED guideline would simply state “Use round and flat oval spiral ducts and fittings from Spiral Pipe of Texas — and here’s your points!” Unfortunately, it’s not that simple. First, many of the “green” features of our products are not even things you can get LEED credits for. They actually are “prerequisites” — preliminary requirements to get any level of LEED certification.

  • “EA Prerequisite: Minimum Energy Performance Required — Option 1” states that you must demonstrate an improvement of 5% in a proposed building rating compared to ANSI/ASHRAE/IES Standard 90.1-2010. That standard calls for all ducts to meet Seal Class A and Leakage Class 6. Remember, we meet the 2013 version of Standard 90.1 with a Leakage Class 4 and an expected performance for round and flat oval spiral ducts of a Leakage Class 2. Option 2 is to comply with both the ANSI/ASHRAE/IES Standard 90.1-2010 and the ASHRAE 50% Advanced Energy Design Guide. Once again, that’s round and flat oval spiral ducts and fittings.
  • “MR Prerequisite: Construction and Demolition Waste Management Planning Required” — As stated above, our ducts are virtually 100% recyclable, so any excess, waste, or demolished product should not end up in a landfill.
  • “EQ Prerequisite: Minimum Acoustic Performance Required” applies to schools and a maximum background noise level of 40 dBA from HVAC systems must be achieved. Our round and flat oval products outperform metal rectangular and nonmetal ducts in true sound attenuation. With less or no flat surfaces, additional airflow-induced noise from “oil-canning” is eliminated. Turbulence is the main source of airflow generated noise, and our lack of square corners eliminates much of the airflow turbulence.

This does not mean you can’t get “LEED points” for using spiral duct and fittings. You just might have to do a bit more work. The following are potential sources for LEED credit:

  • “EA Credit: Optimize Energy Performance” — you can get as many as 18-20 points on a project for achieving higher levels of energy performance beyond the prerequisite standards. Lower leakage and higher efficiency than the Standard 90.1-2010 baseline will result in lower fan horsepower, as well as less demand on chillers and boilers. There are energy simulation programs available where such savings can be translated into percentage improvement in energy performance, and from there to LEED “points”.
  • “MR Credit: Building Product Disclosure and Optimization – Sourcing of Raw Materials: Option 2. Leadership Extraction Practices” — we can contribute, along with other materials used in the building, to the 1 point which can be awarded in terms of the recycled content of our products.
  • “MR Credit: Construction and Demolition Waste Management – Option 2. Reduction of Total Waste Material” — we can also be a contributor to the available 2 points. For this credit, you must not generate more than 2.5 pounds of construction waste per square foot of the building’s floor area. At Spiral Pipe of Texas, our standard practice is to coordinate the project with our customers and produce product and quantities matching our Coordinated Assembly Drawings. We can’t speak for the rest of the products on the jobsite, but there should be no construction waste from our products.
  • “EQ Credit: Low-Emitting Materials” can give 1-3 points for reducing concentrations of chemical contaminants. One of the categories is “interior adhesives and sealants applied on site”. You certainly need to seal the ducts to meet the above requirements. But using a duct system from Spiral Pipe of Texas can reduce or eliminate much of the on-site use of sealants. Our Triple-Rib gasketed duct system allows you to install sealed ducts without site-applied duct sealants. Even when sealants are used, round and flat oval spiral duct systems typically use far less sealants than rectangular systems. First, there is no longitudinal seam to be sealed since the spiral lockseams are exempted from sealing. Round and flat oval ducts have smaller perimeters than equivalent rectangular ducts, so there is less transverse sealing. Longer lengths of spiral duct results in less transverse joints to seal in the first place. Finally, the traditional slipfit joints in spiral pipe require less volume of sealant per length of seam than required for most rectangular joint types.
  • “EQ Credit: Construction Indoor Air Quality Management Plan” has an available point to which we can contribute. One requirement is to protect absorptive materials stored on-site and installed from moisture damage. We offer shipment of our products using the SPOT DuctShield plastic film protection installed on all open ends.
  • “EQ Credit: Acoustic Performance” has 1-2 points available for reducing HVAC background noise and sound transmission. Round and flat oval ducts have less airflow generated noise than rectangular ducts, and our double-wall and insulated products increase actual sound attenuation through a duct path. We also offer a full line of duct silencers.
  • “IN Credit: Innovation” has 1-5 points available “to encourage projects to achieve exceptional or innovative performance”. Even though round and flat oval ducts are known to be more efficient than rectangular ducts, as well as usually less expensive, quieter, and less total material weight, we still see about 70% of commercial duct systems designed primarily with rectangular duct. True innovation would be to reverse that trend and design around highly efficient round and flat oval ducts from Spiral Pipe of Texas.

So, we hope this answers the question “Are Your Products Green?” Spiral Pipe of Texas is proud to make products that are “green” throughout their history and origin, in their manufacture, and through their use. We just usually do it without letting our marketing people put leaves and butterflies on our literature and website.


Your Round Solution for Rooftop HVAC Duct Systems

Finally, there is a solution to your rooftop duct system needs that is the right shape, the right material and the right price. We have combined our almost 40 years of quality metal duct fabrication with a willingness to innovate and explore new technologies. The result is PhenoliDuct. We combine the proven technology of our double-wall duct manufacturing with the properties of Kingspan KoolDuct® to give you the perfect rooftop ducting solution.

It’s Round!

Rooftop duct should always be round when possible. The round shape sheds rainwater and prevents excessive snow and ice loads. But for most of the country the main reason is wind. A round duct experiences only about 40% of the wind force a comparable rectangular duct would see. Hanger and support spacings are greater for round ducts. And this is truly a round duct, not an 8 or 12-sided adaptation from rectangular duct. Our standard construction uses our spiral duct as the outer shell and a round metal longitudinal-seam duct for the inner shell. Your performance is the same as for our other round duct products.

It’s Closed-Cell!

Owners and designers want a product that will not wick or retain moisture. The Kingspan KoolDuct® insulation is closed cell.

Exceptional Thermal Performance!

Our standard product uses the KoolDuct 45mm board. This product has an R-value of 12.0. It is machined to fill a 2” annular space in our double-wall construction, and our process eliminates all potential thermal gaps. This is 50% more thermal performance in a 2” insulation product than you would get with fiberglass or elastomeric insulations.

Exceptional Surface Burning Characteristics!

The KoolDuct® board used as our insulation has flame spread and smoke developed indices of 10 when tested per ASTM E84. This greatly out-performs other insulations that struggle — and often fail — to reach the minimum 25/50 indices required by most codes. We can provide desirable characteristics such as fully-welded fittings without worrying about setting the insulation on fire. Our customers can also perform field-modifications to the duct without creating a fire hazard.

It’s Durable!

Sheet metal ducts, and particularly spiral ducts, are proven to be durable in outdoor applications. Other rooftop products have been introduced, including some made with phenolic board. Most are simply one or two layers of insulating board with an external lagging applied. These have neither the strength nor durability of double-wall metal construction.

It Is Easy to Install!

If you have installed double-wall spiral duct, you already know how to install this product. Actually, it’s even easier to install than our fiberglass insulated products. The rigidity of the phenolic board and the close tolerances of our annular construction eliminate the need for an inner coupling. Most installations will only need external bolted companion angle rings or our economical SPOT-Flanges.

Lots of material and fabrication options!

We can provide the inner and outer metal shells as galvanized steel (standard), paint-grip galvanized steel, epoxy powder-coated steel, PVC-coated steel, black iron, aluminized type 1 steel, 3003-H13 aluminum, 304 stainless steel or 316 stainless steel. We can provide the metal shells as spiral duct or longitudinal seam welded duct, and in thicknesses from 10 gauge through 26 gauge. We offer spiral lockseam construction, tack-welded and sealed construction, MIG and TIG welding. We can even offer the product as flat oval or rectangular construction. We can factory-assemble manifolds of duct and fittings to minimize field work. Essentially, we can offer just about all products from our SPOT catalog in PhenoliDuct construction.

Phenomenal Price!

This is the good part. We are a lot less expensive than the other closed-cell insulated duct products — elastomeric, foam-filled and even other phenolic products. We aren’t spending time and money trying to overcome properties that don’t lend themselves to the application. We simply developed a way to use the best rooftop metal duct fabrication — SPOT’s double-wall round spiral duct — with the best rooftop insulation material — Kingspan’s KoolDuct® phenolic system. The result is 50% more R-value at 20-50% less cost.

Insulation Characteristics SPOT PhenoliDuct Fiberglass Fiberglass Johns Manville Permacote® Linacoustic® AP/Armaflex® AP/Armaflex® AP/Armaflex FS® K-Flex® Duct Liner Gray K-Flex® Duct Liner Gray Ductmate PolyArmor™ Ductmate PolyArmor™ PremiPour™ 202M Baytherm® 9912 PTM Dual-Tech® ThermaDuct™ R-8.1 ThermaDuct™ R-12 Johns Manville Spiracoustic Plus® AP/Spiralflex®
Insulation Thickness 1-3/4" 1" 2" 2" 1" 2" 2" 1" 2" 1" 2" 1" 1" 1-3/16" X 2 1-3/16" 1-3/16" 1-3/4" 2" 1"
R-Value 12.0 4.2 8.0 8.0 4.2 8.0 8.0 4.2 8.0 4.2 8.0 6.7 6.6 16.0 8.1 12.0 8.4 4.2
Fiber-Free Yes No No No Yes Yes Yes Yes Yes No No Yes Yes Yes Yes Yes No Yes
Closed Cell (≥90%) Yes No No No Yes Yes Yes Yes Yes No No No Yes Yes Yes Yes No No
25/50 Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes No No Yes Yes Yes Yes No
Density (PCF) 3.5 1.0 1.0 ~3.8 ~5.0 ~5.0 ~5.0 3 – 4 3 – 4 1.6 1.4 2.0 2.2 3.5 3.5 3.5 ~3.8 ~5.0
Water Absorbtion, % by volume Round 1.15% <3% <3% <3% 0.20% 0.20% 0.20% <0.2% <0.2% 0.24% 0.24% 0.05 3.56% 1.15% 1.15% 1.15% <3% 0.20%
Round Yes Yes Yes Yes No No No No No Yes Yes Yes Yes No No No Yes Yes
Flat Oval Yes Yes Yes Yes No No No No No Yes Yes No No No No No No No
Rectangular Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No No Yes Yes Yes Yes No
Smoother Metal Inner Airway Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No No
Durable External Shell Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Can you weld metal on it without it smoldering, smoking or catching on fire? Yes Yes Yes Yes No No No No No No No No No No No No Yes No
Maximum Pressure WG (positive) 10" 10" 10" 10" 10" 10" 10" 10" 10" WG 10" 10" 6" WG 10"
Maximum Pressure WG (negative) -6" -6" -6" -6" -6" -6" -6" -6" -6" WG -6" -6" -6" WG -6"
Cost $$$ $ $$ $$ $$$$ $$$$$ $$$$$ $$$$ $$$$$ $$ $$$ $$$$ $$$$$ $$$$$$ $$$$ $$$$$ $$ $$$$$
Install Labor $ $ $ $ $ $ $ $ $ $ $ $ $ $$ $$ $$ $ $$
Double-Wall Sheet Metal Double-Wall Foam Filled Built-Up Insulation Board with External Lagging Lined Duct (no inner metal shell)

Vocational Woodworking Shop Dust Collection Systems

Most engineers and contractors that work with educational facilities are familiar with standard HVAC design practices and SMACNA duct construction standards. But there are many specialty duct systems that fall under lesser known standards and criteria. The ducts used in vocational woodshop dust collection systems may outwardly appear to be the same spiral lockseam duct and fittings used for HVAC applications. The static pressure ranges may fall within those found in the SMACNA HVAC Duct Construction Standards. However, the actual requirements for a duct system in this application are higher and involve other standards. This Duct Construction Guide combines those various requirements into a single reference. This guide is not intended to cover commercial and industrial woodworking facilities. It lists the references used here, and most have greatly expanded ranges of size and capacity that can be used for those applications. The scope of this guide is as follows:

  • Vocational woodworking shops in secondary and technical education facilities
  • Duct systems operating at -15” WG and less
  • Duct systems 24” diameter and smaller
  • Systems of less than 12,000 ACFM
  • Systems in operation less than 1,000 hours per year
  • Galvanized steel or black iron

Dust collection ductwork may be fabricated to a more robust standard than typical HVAC duct, but the cost does not have to be egregiously higher. The products and methods we recommend are chosen to meet the requirements in the most cost-effective way. Dust collection ducts are recommended to be round in shape. Rectangular and flat oval ducts would require heavier metal gauges and extensive reinforcement. They also have less-efficient airflows and corners where particulate can be trapped. Traditional round “blowpipe” systems could be used, utilizing longitudinal seam construction. But a system using spiral lockseam construction will typically use lighter-thickness materials, with diameter/thickness ratios of as much as 1800. Greater stiffness is inherent in their construction.

Most vocational woodworking shop dust collectors operate in the range of -6 to -15” WG. For the relatively small diameters, the gauges are sufficient to avoid collapse. The particulate transported — sawdust and small wood chips — does not cause much impact damage or abrasion. Frankly, one of our greatest concerns we try to prevent is unintentional damage caused by the users. More than once, we’ve been consulted on jobs where HVAC-grade duct was installed and subsequently damaged. It seems a popular method for breaking up debris clogs is to “whack the duct with a big stick” instead of utilizing the cleanouts or disassembling duct for cleaning. The fittings and flange attachments need to be fully-welded construction. Most HVAC-grade fittings and assemblies use tack-welds, screws or rivets that are then sealed with mastics. Mastics should not be used on the interior of sawdust collection systems. Wood chips and sawdust would become embedded in the mastic, causing a chronic clogging issue. For all of the above reasons, the minimum gauge used for a vocational woodworking shop dust collection system duct is 22 gauge. In addition to durability, that is also the lightest gauge that can be consistently welded in most production facilities.

Minimum Required Gauges for Galvanized Steel Spiral Lockseam Pipe and Fittings

Table 1

Diameter-2" WG-4" WG-6" WG-10" WG-15" WG
Spiral PipeFittingsSpiral PipeFittingsSpiral PipeFittingsSpiral PipeFittingsSpiral PipeFittings
4"22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.
6"22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.
8"22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.
10"22 ga.20 ga.22 ga.20 ga.22 ga.20 ga.20 ga.18 ga.20 ga.18 ga.
12"22 ga.20 ga.22 ga.20 ga.20 ga.18 ga.20 ga.18 ga.18 ga.16 ga.
14"22 ga.20 ga.20 ga.18 ga.20 ga.18 ga.18 ga.16 ga.18 ga.16 ga.
16"22 ga.20 ga.20 ga.18 ga.18 ga.16 ga.18 ga.16 ga.16 ga.14 ga.
18"20 ga.18 ga.18 ga.16 ga.18 ga.16 ga.16 ga.14 ga.16 ga.14 ga.
20"20 ga.18 ga.18 ga.16 ga.18 ga.16 ga.16 ga.14 ga.16 ga. + A114 ga.
22"18 ga.16 ga.18 ga.16 ga.16 ga.14 ga.18 ga. + A114 ga.16 ga. + A114 ga.
24"18 ga.16 ga.16 ga.14 ga.16 ga.14 ga.18 ga. + A114 ga.16 ga. + A214 ga.

A1 = min. 1″ x 1″ x 1/8″ angle ring reinforcing (12 foot maximum spacing) with 9 stitch welds 1.0″ long

A2 = min. 1″ x 1″ x 1/8″ angle ring reinforcing (12 foot maximum spacing) with 9 stitch welds 1.5″ long

Design and Layout

Dust collection systems are typically designed using the Constant Velocity method. Each item of shop equipment has a recommended flow rate of exhaust air — either provided by the manufacturer or taken from a reference such as the ACGIH “Industrial Ventilation” (see Table x). Woodworking equipment generally pro-duces two types of debris. Sanders and band saws produce fine sawdust. Table saws, lathes, joiners and planers produce larger chips. Both types of debris have a Minimum Transport Velocity. That is the velocity that must be maintained from the shop equipment to the collector to avoid debris settling in the duct and potentially producing clogs. As ducts from different shop equipment merge into the trunk, the highest of the Minimum Transport Velocities must be maintained from that point to the collector.

EquipmentTypical Woodshop Equipment

Table 2

SourceShop EquipmentExahust Flow Rate, acfmMin. Duct Velocity
Up to 2" blade width3503507003500
2-3" blade width3505509003500
VS-95-02Floor Table SawbaseguardTOTAL
Up to 16" saw blade dia.5451006454000
Over 16" saw blade dia.7851008854000
Saw with dado blade7851008854000
VS-95-03Radial Arm SawbottomtopTOTAL
VS-95-10Single Drum SanderTOTAL
Drum surface up to 200 in23503500
200 – 400 in25503500
400 – 700 in27903500
VS-95-12aDisc SanderTOTAL
Disc diameter up to 12"3503500
12 – 18"4403500
VS-95-13Horizontal Belt SandersheadtailTOTAL
Belt width up to 6"4403507903500
6 – 9"5503509003500
9 – 14"80044012403500
VS-95-15Woodworking LatheTOTAL
Knife length up to 6"3504000
6 – 12"4404000
12 – 20"5504000
Table 13-95-1Single Planers or SurfacersTOTAL
Knife length up to 20"7854000
20 – 26"10004000
26 – 32"14004000
Table 13-95-1Double Planers or SurfacersbottomtopTOTAL
Knife length up to 20"55078513354000
20 – 26"785110018854000
26 – 32"1100140025004000
Table 13-95-1Floor SweepTOTAL
6" dia.8004000
8" dia.14004000

Woodworking shop dust collection systems are designed for all equipment hookups to be drawing their recommended exhaust flow rates at any time the system is operating. Even though a saw or sander may not be in use, shutting off the exhaust flow from it would cause a decrease in trunk flow between that equipment branch and the collector. Those sections would drop below the Minimum Transport Velocity and clogs would potentially form. All of this makes sizing the ducts fairly easy. You know your cumulative exhaust flow rate (cubic feet per minute). You divide that by the prevailing Minimum Transport Velocity (feet per minute). The result is Square Feet of duct diameter. Simply round that down to the next available duct size. Table 3 below shows the available sizes of spiral lockseam duct from Spiral Pipe of Texas and their area.

Duct Sizes and Area

Table 3

Typical Flow (cfm)
Dia.Area (ft.2)at 3500 FPMat 4000 FPM

Energy efficiency is as important in dust collection as it is in any other type of ventilation. But the need for Minimum Transport Velocities prevents design methods such as static regain from being used to reduce system pressure drop. Duct system layout, exhaust flow rate and the cumulative duct and fitting pressure losses dictate the system pressure — and the required fan horsepower. Creating an energy efficient system really comes down to designing the most efficient duct system layout — have the shortest possible distance between the shop equipment and the collector, and get there on the straightest line. It also helps to use efficient fittings, and we will go over them below.


Preferred Elbows Vocational Woodworking Shop Dust Collection Systems - Spiral Pipe of Texas

Elbows and bends should be a minimum of two gauges heavier than the straight ducts of the same size used in a system. Elbow centerline radii should be a minimum of 2.0 with 2.5 recommended. All 90-degree elbows to 6” diameter should be 5-piece, larger sizes to be 7-piece construction. Angles other than 90-degrees should have a proportional number of segments. However, prefabricated elbows of “smooth construction” can be used. Where possible, we recommend the use of these die-formed elbows with a centerline radius of 1.5. The loss coefficients for these elbows is actually less than for segmented elbows with radii of 2.0 and 2.5, so you get better efficiency at a lesser cost. They are only available, however, in the smaller diameters. It should be noted that not all “die-stamped” or “pressed” elbows meet the minimum gauge requirements. Some are as light as 25 gauge. At Spiral Pipe of Texas, we only use 20 gauge die-formed elbows for this type of system, and only where the system design pressure allows 20 gauge construction in that size.

Branch Fittings

Branch Fittings Diagram Vocational Woodworking Shop Dust Collection Systems - Spiral Pipe of Texas

Woodshop dust collection systems use “tapered body” branch fittings. Most HVAC duct systems use “barrel” type fittings where the branch enters a constant-sized trunk. Any upstream reductions are made by putting a reducer on the upstream end. The problem with this type of fitting is that Minimum Transport Velocity is not maintained. Debris will settle out where both the tap and reducer meet the barrel, potentially causing clogs. Tapered body transitions are required, and a maximum 30-degree branch angle is preferred. The length of the tapered body should be a minimum of 5 times the difference of the upstream and downstream diameters. You should not try to effectively turn these branch fittings into 90-degree branches by combining the 30-degree lateral tap with a 60-degree elbow. That adds unnecessary field joints, additional pressure drop, plus an expensive elbow (60-degree elbows are not commonly available in die-stamped construction). The branch fittings in the trunk duct should be located on the collector side from the woodshop equipment, on a straight line from the elbow turning vertically down to the equipment.

Branch Fittings Diagram Vocational Woodworking Shop Dust Collection Systems - Spiral Pipe of Texas

Branch fittings entering the trunk directly opposite each other (“crosses”) should not be used. They create turbulence within the duct, leading to higher pressure drops and unpredictable transport velocity performance. If two items of shop equipment are located close to each other on opposite sides of the dust collector main, you should try to position them where their corresponding branch inlets, if extended across the profile of the main, do not overlap. The more separation, the better.

Branch Fittings Diagram Vocational Woodworking Shop Dust Collection Systems - Spiral Pipe of Texas

The last branch in the system should be made with a capped lateral, and the branch should be no more than 6” from the end. The removable end cap is used as a cleanout.

Blast Gates and Floor Sweeps

Blast Gates Vocational Woodworking Shop Dust Collection Systems - Spiral Pipe of Texas

Most woodshop dust collection systems have manual blast gates at each piece of shop equipment. Simple aluminum cast-body gates are not expensive and have a thumb-screw for locking them in place. They should be located in the vertical duct riser from the shop equipment, approximately 42” above the floor for ease of access, and five diameters away from elbows.

Floor Sweep Vocational Woodworking Shop Dust Collection Systems - Spiral Pipe of Texas

They are generally not used for balancing the system or shutting off unused equipment. They are most commonly used in conjunction with floor sweeps and shop cleanup. The exhaust flow rates of the floor sweeps are not usually included in the total system volume. This is a cost-saving measure. A smaller system design flow means smaller duct and a smaller fan. When sweeping up the shop floor, you close the blast gates on the shop equipment, open the blast gates to your floor sweeps, then turn your dust collector on. You should do a separate system calculation to determine that your fan, with only floor sweeps open, can maintain the Minimum Transport Velocity for the cumulative floor sweeps of 4,000 FPM to avoid potential clogs.

Flexible Hoses

Flexible ducts are commonly used for final connection to shop equipment. This allows some movement and positioning of normally fixed equipment. Such flexible duct should be as short as possible (2 foot maximum length recommended) and have a minimum amount of bending. The flexible duct should be a non-collapsible hose, rated for the pressure and for the intended use. Some equipment has a recommendation for a small-diameter flexible hose of longer length to be used where the attached pickup point is not fixed (blade guards on table saws and traveling heads on radial arm saws). Length should be limited to the expected movement.

Duct Joint Assembly

A lot of money — and limitations — get added to shop dust collectors through the overuse of proprietary duct assembly systems. The idea of a system you can put together with external V-clamps sounds great, and you certainly avoid putting obstructions into the airstream like you would with traditional screw-and-glue HVAC duct joints. But to get those nice rolled duct ends, you have to go to longitudinal seam duct, and that limits your maximum lengths (shorter duct equals more joints to put together) and you end up with a lot of custom lengths or expandable lengths. We recommend a more practical — and less expensive — system of duct connectors.

Vocational Woodworking Shop Dust Collection Systems - Spiral Pipe of Texas

The main trunks of dust collection systems are relatively straight runs in predictable and fixed positions. Most of the cutting and adjusting happens in the smaller runouts to the shop equipment. Our recommendation is to fabricate the main trunk lines of 14” diameter and larger as cut-to-length spiral lockseam pipe and fittings with flanged ends. Light gauge angle rings, such as the SPOT Flange, are sufficient for these applications and can be welded to spiral lockseam pipe ends and van-stoned to fitting ends. These flanges can be connected with self-tapping sheet metal screws be-cause such screws will not penetrate the airstream. If a joint is designed for disassembly, pre-punched angle rings can be used, and the flanges bolted together.

Duct Joint Assembly Vocational Woodworking Shop Dust Collection Systems - Spiral Pipe of Texas

For ducts 12” diameter and smaller, we recommend us-ing the MU Overcollars from METU-System. These are a wide gasketed clamp with a single tightening bolt. They are very economical and make joint assembly fast. Best of all, you don’t have to pre-order all of your small diameter ducts cut-to-length. This is very important when you are try-ing to make hard connections from your dust collector to shop equipment you may not even have yet.

Some material handling duct systems require seams and joints to “lap in direction of airflow”. This is intended to reduce particulate buildup when transporting long fibers that could snag on exposed metal ends. They are not required or recommended by any of the references used in this guide for vocational woodshop dust collection systems. Proper spiral duct systems follow tolerance standards that minimize the gap between male-sized fitting and coupling ends and the female-sized ends of spiral duct. The goal is a “friction fit”. You should have a little bit of metal scraping when you insert a slip fitting into a section of spiral duct.


Cleanouts Vocational Woodworking Shop Dust Collection Systems - Spiral Pipe of Texas

Cleanouts should be used in horizontal duct runs, near elbows, junctions and vertical runs. They should be spaced a maximum 12 feet apart for ducts 12” diameter and smaller, and maximum 20 feet apart for larger diameters. For small diameter branches to equipment, the removal of a duct section with the use of the MU Overcollars is an effective cleanout. For the larger primary trunk ducts, removing a section for cleanout is less desirable and practical. The most cost-effective method for producing a good cleanout is through the use of removable caps. We have already mentioned a removable cap for the last branch of the trunk ducts. Within straight sections of trunk ducts, lateral branches with removable caps can be added. For 90-degree directional changes in the trunks, removable caps can be utilized by having either heel-tapped elbows or using a lateral branch + elbow for the primary flow (for 12” and smaller). Recommended cap size is trunk diameter – 2” for branches from the main, up to a maximum 12” diameter cap size. Caps on the through-end of branch fittings should be the same diameter as the trunk duct. The removable caps are fixed in place with an MU Overcollar, so they can be easily removed with a single bolt.


The recommended maximum leakage for a dust collection system is 2%. As with any duct system, a substantial portion of the leakage occurs at the duct joints. In woodshop collection systems, the additional sources of leakage are the slide blast gates and cleanouts. The use of the gasketed MU Overcollars will eliminate most leakage at joints and cleanouts. Other types of cleanouts such as split sleeves and hinged access doors have been commonly used, but once opened, they are notoriously difficult to keep sealed. Rectangular connections and openings should be avoided because of their tendency for corner leakage.

Suspension and Support

For suspension and support, the weight of dust collection duct is calculated as the weight of the duct itself, plus the weight of the sawdust filling half the duct. Table 4 has nominal design weights for Vocational Woodworking Shop Dust Collection Systems using spiral lockseam duct. Ducts can be suspended using appropriatelysized cable hangers, teardrop strap hangers or saddles. Hangers should be located as close as possible to joints and elbows (2” recommended) and maxi-mum 12 feet apart. Longer hanger spacing can be used if the duct joints are flanged and guidelines found in the SMACNA Round Industrial Duct Construction Standards are followed (sections 4.10 and 5.8). Duct systems covered in this guide are not intended to be walked on, so the 250 pound maintenance load can be omitted.


  • Industrial Ventilation — A Manual of Recom-mended Practice for Design (30th Edition – 2019) — ACGIH
  • Round Industrial Duct Construction Standards (2nd Edition – 1999) — SMACNA
  • NFPA 91 – Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Particulate Solids – 2020 — NFPA
  • Chapter 19 “Duct Construction” — 2016 HVAC Systems and Equipment Handbook — ASHRAE
  • Chapter 21 “Duct Design” — 2017 Fundamen-tals Handbook — ASHRAE
  • Chapter 8 “Educational Facilities” — 2019 HVAC Applications Handbook — ASHRAE
  • Chapter 33 “Industrial Local Exhaust Systems” — 2019 HVAC Applications Handbook — ASHRAE

Spiral Duct Weights

Table 4

Duct Weights (per lineal foot
22 gauge20 gauge18 gauge16 gauge
Dia.Area (ft.2)Duct OnlyDuct + ParticulateDuct OnlyDuct + ParticulateDuct OnlyDuct + ParticulateDuct OnlyDuct + Particulate

Submittal Downloads

1995 SMACNA Standards – Spiral Pipe and Fittings

2005 SMACNA Standards – Spiral Pipe and Fittings

Single Wall

Double Wall



Vocational Woodworking Shop Dust Collection Systems

Spiral Pipe of Texas

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