The method of test is described in this standard, and the allowable measured deflection of the duct wall of a round duct under negative pressure is D/200 (diameter divided by 200). DPTS-2005." Rather than causing a duct to “fail” (collapse) then working backwards, it gives a maximum amount the duct wall can deflect. The SMACNA HVAC Duct Construction Standard has a Chapter 11 titled “Functional Criteria” which has the “SMACNA Duct Performance Test Standard No. But that’s not how we are supposed to judge a commercial HVAC duct. Collapse is arguably the ultimate failure, and many in the industry chose minimum gauges based on the predicted collapse of the duct (based on those old collapse tests) with a safety factor of 2 or 3 applied.
Much of what we know about spiral duct in negative pressure use comes from collapse tests. The goal was to determine up to what size stocked positive pressure duct could also be used for commercial HVAC negative pressure applications. In addition, many SPIDA members produce spiral duct with corrugation, a practice that has been shown to increase strength, rigidity and durability of the product. SPIDA and SMACNA jointly sponsored and funded a study, performed at Tennessee Tech University, titled “Determine the Maximum Negative Operating and Collapse Pressure of Stocked Spiral Duct With and Without Corrugations.” This study focused on the limited range of “stock” spiral duct being produced by SPIDA members with diameters up to 42”. As a further question, has much of the toilet exhaust, ducted return and general exhaust in commercial HVAC construction been designed and installed as rectangular duct because a heavier (and more expensive) spiral duct has been required? SPIDA finds this highly questionable since positive pressure product has undoubtedly been used for negative pressure applications with good results. Yet, if you look at the SMACNA negative pressure tables, there is only a very small range at which you can use this stock spiral duct.Īs this table shows, stock spiral duct should only be used in sizes up to 16” diameter at a maximum -2” WG and only to 10” diameter (12” diameter if you have minimum 26 gauge stock) at a maximum -4” WG. No doubt, plenty of stock positive pressure spiral duct has been used for toilet exhaust systems with no adverse consequences. The nature of “stock” is that material is taken for use without control by the manufacturer on where and how it is used. For those, the SMACNA HVAC Duct Construction Standard has separate tables, Tables 3-10 and 3-11, for minimum gauges. These systems are seldom designed for negative pressures greater than -2” WG or tested at greater than -4” WG. Commercial HVAC systems often have toilet exhaust, ducted return air and general exhaust systems.
14 spiral duct manual#
Those are generally higher negative pressures and their construction is covered by the Industrial Ventilation Manual and the SMACNA Round Industrial Duct Construction Standards. This round stock can be used for virtually all commercial HVAC supply, outside and makeup air systems.īut what about negative pressure applications? We’re not talking about fume hoods, dust collectors and industrial exhaust. The use of 28 gauge material is allowed through 14” diameter per this standard, though most SPIDA members use 26 gauge in those sizes because of material availability, durability of product and some local codes mandating a minimum 26 gauge for commercial construction. This gives not only the advantage of lower cost from duct produced more efficiently, it also greatly reduces lead time by using products that can literally be acquired “same-day.”Ī survey of SPIDA members indicates that stock spiral duct is usually made in accordance with gauges for up to 10” WG positive pressure in accordance with the Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) HVAC Duct Construction Standards. A high percentage of an HVAC duct system could be installed using “stock” rather than cut-to-length or custom duct. Fabricating “stock” is highly efficient and results in minimal waste. These are fabricated not only for their own use, but may also be inventory for reps, secondary distributors or contractors. Spiral Duct Manufacturers Association (SPIDA) members often fabricate an inventory of standard lengths of round spiral ducts.
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