Insulated Flat Oval Ducts
If you compare internally insulated rectangular ducts versus internally insulated flat oval ducts, you will usually see a big price advantage for the rectangular duct. That’s because you insulate rectangular ducts – at least 25 years ago – with glued and pinned duct liner. Round and flat oval ducts had to be double-wall construction. We’ve had a huge change in the industry since 1990 because we now see the industry move away from duct liner and towards external insulation wrap.
Externally Insulated Ducts
Even where ducts are internally insulated for acoustics, we are now seeing rectangular ducts required to be double-wall construction. Rectangular double-wall duct is more expensive than round or flat oval double-wall duct. For external wrapping, that old advantage for insulating rectangular versus flat oval not only goes away, it actually favors flat oval. Because of those rounded corners, an “equivalent” flat oval duct has less surface area – and less required insulation – than its equivalent rectangular duct.
See above. More efficient, higher velocity ducts can be smaller.
Insulated Flat Oval Ducts
There are a lot of misconceptions about ductwork, but the one that is probably most egregiously wrong is the one about “double-wall flat oval duct is just too expensive to use.” While based loosely on a few facts, and maybe kind of true for the way ducts were fabricated and installed 25 years ago, the notion is outdated and needs to be reevaluated for modern duct systems. You should be using more flat oval duct and less rectangular to remain competitive, as well as meet modern performance standards. Let’s take a look at a few of the facts about these modern duct systems.
‡ The $1.75/CFM/year is a consensus figure used by several industry committees to determine payback for system improvements and includes both energy and cost savings from reduced equipment and ductwork for lower net CFM’s.
‡‡ Installed duct costs are based on the 1992 National Mechanical Estimator (Ottaviano) for installed duct systems with mark-up factors for labor and material.
‡‡‡ From 1992 NME, installed cost of insulated medium pressure rectangular duct at $6.88/ft2 versus $6.52 comparative cost for installed double-wall round spiral duct ($3.35/ft2 installed single-wall spiral X 2.3 double-wall mark-up X 0.92 ft2 adjustment X 0.92 duct sealing adjustment).
‡‡‡‡Example from 30% Advanced Energy Design Guide for K-12 School Buildings.
Insulated flat oval ducts from Spiral Pipe of Texas are provided as “double-wall” construction. The outer metal shell is the basis of construction, designed to contain the air at the specified pressure. The inner metal shell holds the insulation material in place, and assures a smooth airstream. The use of metal spacers to maintain liner concentricity is not recommended. We recommend that for flat oval ducts with sufficient flat span to exhibit “sagging”, that flanged transverse connectors be used. They greatly increase field installation productivity while reducing deflection in both the inner and outer shells.
Outer Metal Shell
The outer metal shell of straight duct may be spiral flat oval lock-seam duct or longitudinal seam welded duct. SPOT makes 516 spiral flat oval duct sizes that can be used for double-wall construction, excluding only the 3” and 4” minor axis duct sizes. We make double-wall flat oval duct with outer shell dimensions ranging from 13 x 5 to 124 x 36. Standard construction and gauges are in accordance with SMACNA standards for +10” WG. Fittings can be made with either fully-welded construction or “tack-welded-and-sealed” construction.
Spiral lock-seam outer shells are normally provided in corrugated construction for increased rigidity. They can be ordered without corrugations by request. Insulated flat oval duct is increasingly used as an architectural feature in buildings, we offer the same appearance and finish options as we do for our round ducts. Double-wall insulated flat oval ducts can be made in lengths up to 20’ – 0”, but we recommend that ducts in exposed applications be limited to 8’ – 0” in length. This allows for duct to be shipped “standing up” to minimize damage to the appearance. SPOT recommends that even non-exposed double-wall flat oval ducts be limited to 10’ – 0” lengths because they have twice the weight of single-wall ducts and most construction sites lack the equipment to safely offload large sections.
Outer metal shells are available in all materials used for our round products – G90 Galvanized Steel, A40 and A60 Galvannealed Steel, 3003 Aluminum, 304 and 316 Stainless Steel, AgION (Antimicrobial Steel), and PCD (Polyvinyl Coated Galvanized).
Inner Metal Shell
The inner metal shell may be spiral flat oval lock-seam duct or longitudinal seam welded duct. They are available as solid metal, perforated metal (23% open area), and perforated metal with Mylar erosion barrier. Inner metal shells are available in the same materials as the outer shells. The function of the inner shell is to contain the insulation liner material and maintain the cross-sectional profile of the airstream. It’s not the “pressure shell”. Unless specifically ordered, standard construction is to tack-weld all seams except the spiral lock-seams and omit sealing. Spiral flat oval lock-seam duct is provided corrugated to increase rigidity.
Our standard insulation material is 0.75# density Knauf Atmosphere™ Duct Wrap with ECSOE® Technology. This glass mineral wool product is purchased in thicknesses 50% greater than the intended annular space, then compressed to approximately 1.0 PCF density. It is available in 1” and 2” thicknesses. As an alternative “fiber-free” insulation, we also offer Armacell AP/Coilflex® elastomeric foam insulation in 1” and 2” thicknesses.