There are special cases where instead of sheet metal you can use fabric ductwork such as made by DuctSox. Watch the video below of the John Mansville Spiracoustic Plus liner material being added very easily to spiral duct and fittings. In order to provide acoustical properties, which is the ability to attenuate or reduce sound transmission, lined round spiral duct can be used by adding liner to the interior of the duct. They also have a double-wall version, in addition to a single and double-wall flanged system, and one for oval.Įngineers will also specify that the duct be made by a particular manufacture so that a certain level of quality can be assured to the owner. Watch the enclosed video to see how easy the ductwork goes together. The rubber gasket provides a tight seal, making additional sealing unnecessary. There are various manufactures that make proprietary systems that are meant to save labor such as the Eastern Sheet Metal’s gasket fitting system. Unreinforced Round Duct Special Round Ducts and Fittings You can also see in the table that the larger the round size the greater or thicker the metal gage required. This indicates that the spiral seam is stronger and more rigid then a longitudinal seam. But, spiral seam can use 26 gage from size 16” to 24” round, while longitudinal seam is limited to a maximum of 18” round when using 26 ga. You can see that from 4” to 14” round (Column #1), both Longitudinal Seam (Column #2) and Spiral Seam (Column #3) require the same 28 gage minimum. Spiral allows for lighter gages then longitudinal seams when using Unreinforced Round duct as shown in the SMACNA table below. Sheet Metal Rollers Longitudinal vs Spiral Seam The video quality isn’t the best but it shows how you can weld galvanized and stainless steel round ductwork, and how to adjust the machine for various sizes. In addition to the spiral seam there are various other types of seams that run straight from one end to the other (longitudinally), such as a longitudinal welded seam as shown in the video below. The use of rings helps maintain the roundness of the duct as the sizes get larger. Reinforcement rings are generally not required for positive pressure and low negative pressure round ducts. Spiral duct is not limited in length, except as required to fit on a truck or in an elevator, or as needed for the insertion of a fitting. Manufactures usually stock standard lengths, but if you have your own spiral machine you can fabricate any length. Round duct is used to connect between fittings, accessories and flex duct. Round duct is manufactured with various seam types, the most common being spiral or snap lock. Manufactures make various types and connection methods, so you need to become familiar of what is commonly used in your area. If the round doesn’t start from a piece of equipment or from a square to round, then it will need some form of starting fitting. Round fitting types and methods of construction can vary by region and between companies.Įngineers use software programs that have standardized duct and fitting symbols, in addition to the ability to create their own representations of what duct and fittings look like on the drawings, but once you know what to look for you can identify the slight differences between designs.ĭepending on the situation ducts either start at some cooling or heating source air conditioner, air handler or an HVAC accessory like a VAV terminal, or transition from a rectangular duct as in a square to round fitting. In order to do a takeoff, you will need to be able to understand what rectangular duct and fittings look like and how they are represented on the drawings.
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