Spiral Round Duct

Why Round?

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?

1. ASHRAE Advanced Energy Design Guides won’t be achieved – it’s a big deal to try and reduce energy use in commercial buildings by 30% and 50%. 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 fellow engineers, not a trade organization or contractor. 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. 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. With airflow, that gives you two unwanted problems that are directly related to turbulence – higher pressure drops and increased noise.

3. More duct will be needed – the only reasonable way to decrease pressure loss and noise in a duct (other than change its’ shape) 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). We’re not making this up! It’s pretty easy for you to confirm after a couple of moments on your Ductulator.

4. Duct will weigh more, therefore more hardware will be need to be screwed or welded – 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. 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:

5. Duct cost will increase – the cost of ductwork, both to buy/make and install, 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. If you pick an example from the chart above where rectangular duct weighs 50% more than round spiral duct, it only takes 44% of the labor to install the round spiral duct compared to the rectangular.

6. More hangers/supports will be needed – 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. More insulation will be added – it’s going to take more square feet of insulation to cover the increased perimeter/surface area of rectangular duct.

8. Duct will 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. ASHRAE Standard 90.1-2013 Energy Code might not be met – 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 6.4.4.2.2…” (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, the duct is designed to fail!

10. More duct sealant will be needed – 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. Within the rectangular joint, the biggest culprits are those pesky corners – the same ones that cause the turbulence that gives rectangular duct its 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. It will be harder to fit 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 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. 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 appealing. 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.

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 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 round spiral 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”. Round and flat oval spiral duct are neither hard to find or non- competitive.

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.

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.

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.

Round Spiral Duct

Round spiral lock-seam duct is the most efficient way to transport air in HVAC systems. The round profile has greater flow efficiencies than rectangular ducts. 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.

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.

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

Insulated Round Ducts

Insulated Round Checklist

Most HVAC ducts need to be insulated. At Spiral Pipe of Texas, our home area encompasses regions with high fluctuations in both temperature and humidity. 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 Galvanized Steels, A-40 and A-60 Galvannealed Steel, Black Iron, 3003 Aluminum, 304 and 316 Stainless Steels, PCD (Polyvinyl Coated Galvanized), AgION (Antimicrobial Steel).

Gauges – 26 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 Atmosphere™ 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.

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

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.

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.

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. 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. 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. 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. 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. It’s no surprise that most estimators predict installation rates of 50% more pounds per man-hour installed for round spiral duct than for rectangular duct. Don’t forget that we’re talking about a product that generally has 30% less pounds to start.

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. 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, fiberglass insulation is used. 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. 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.

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.

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:

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. With less inherent leakage, you won’t need to produce as much air.

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. Many of the “green” features of our products are not even things you can earn LEED credits. They actually are “prerequisites” –– preliminary requirements to get any level of LEED certification.

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:

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.