With energy costs going through the roof and no relief in sight, HVAC system designers and specifiers are taking a much harder look at the many choices in ductwork available to them today. One question that is being asked more and more is whether to choose rectangular or spiral duct for an HVAC system. If the healthy state of the spiral duct manufacturing industry is any indication, the trend in the United States is moving towards spiral duct, as specifiers become more enlightened to the advantages of circular duct.
The inherent energy- and labor-savings features of the duct’s design are what are causing this shift. This changeover has already happened in Western Europe, where the cost of energy has always been higher than that in the U.S. In the region of Scandinavia, for example, the percentage of the spiral duct market share has gone from 5 percent 40 years ago to 85 percent currently.
Some of this changeover resulted from specialist manufacturers intensifying their level of automation, standardization, and R&D, but the bottom line has been that growth was spurred along by the competitive edge created by spiral duct.
The design, installation, and performance benefits of spiral duct are many, with some appealing to HVAC contractors and others to HVAC system designers. Since energy efficiency is the biggest advantage.
AIRTIGHTNESS = ENERGY EFFICIENCY
Quality spiral duct, which includes round, oval, and flat oval configurations, exhibit a very high level of air tightness. Many leading manufacturers of sheet metal spiral duct can guarantee a line of spiral duct that meets or exceeds the highest air leakage standard, Class 3, recognized by the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA). Since some duct is manufactured with self-sealing double-gaskets, the connection is guaranteed to prevent duct leakage.
Duct air leaks can waste energy and cause IAQ problems, such as condensation. Loose-fitting joints or blow outs of improperly maintained seams can cause problems in supply ductwork, while air leakage can allow soil gases and molds in crawl spaces and below slabs to enter return ductwork. When leakage is virtually eliminated, the cost for filtering, heating, cooling, and distributing the air is kept to a minimum. In addition, studies have shown that round duct has a lower airflow resistance, which significantly contributes to the energy efficiency of the system.
One industry study demonstrated that a shift in the U.S. to more airtight systems would mean an annual energy savings of approximately 10 TWh (terawatt hours, each equivalent to one billion kilowatt hours), which is comparable to the annual energy product of three nuclear power plants.
So that the correct flow of heated or cooled air can reach the areas to be conditioned, fans must transport the total airflow. A poorly designed or leaking duct system needs larger, and often more expensive fans to achieve the desired effect, as well as more space in the building’s design, where extra space is often at a premium.
With the many flow measurement units specially designed for circular duct that are now available, HVAC engineers also find that it is easier to measure the airflow passing through circular ductwork. With these relatively inexpensive devices, the ventilations systems can include fixed measuring units as a low-cost method of enabling regular check ups or continuous monitoring.
Another benefit of spiral duct that contributes to the overall IAQ of buildings with spiral ductwork is the ease of duct cleaning, using industry-recognized duct cleaning equipment outlined in the North American Insulation Manufacturers Association (NAIMA) Duct Cleaning Guide.
Aesthetics plays a very important role today as mechanical engineers, architects, and other decision makers are specifying ductwork. Innovative design elements often include exposed ductwork, to give a property a contemporary look. Spiral duct with gasketed joints requires no unsightly sealant at the connections, which enhances the clean look of the ductwork. Contributing to the attractive look of the duct is the fact that this type of duct requires fewer joints and flange connections, and requires less supports and hangers. In addition, the tight, gasketed seal allows the system to remain free of dirt streaking, common with manually sealed systems, ensuring that it will look good for many years. And those systems with RGS-3 registers do not require register taps, since they are mounted directly to the ductwork, again giving it a progressive, streamlined appearance.
Finally – of concern to the HVAC system designer – is the fact that round duct that includes a double wall system with a non-woven poly fabric liner can deliver a level of acoustical performance competitive with other types of ducts.
How do the HVAC contractors who install ductwork view the choice of spiral ductwork? The overall cost of spiral ductwork can be relatively low because installation moves more quickly than with other forms of duct. Correct fit at each joint can be counted on with premium spiral duct because standardized components are manufactured to tolerances of -0.25 percent. And elimination of sealants speeds installation along, as well.
Likewise, the lighter weight of spiral ductwork enables one worker to install some sections of round duct into place, rather than two, which also adds to labor savings. Manufacturers say that the complete weight for a typical system comprising a normal combination of straight ducts, bends, and diffusers, can typically be 30 to 40 percent lower for a circular system.
Other advantages to the contractor are the ability of circular duct to be nested, with various sizes resting together, which helps with storage and transportation, and the duct’s inherent strength, created by the strong seams made during the fabrication process, which gives each section an increased rigidity. This also reduces the need for additional stiffeners and hangers during installation.
Specifying the best duct system for each project requires a careful study of the many choices of duct products and their inherent benefits. With energy costs on the mind of everyone much more in recent years, it always pays to study and compare ductwork design before selecting the best type to be used for a specific application.