The difference between spiral and rectangular ductwork

If you are specifying ductwork for commercial heating and ventilation applications, it’s important to understand the difference between spiral and rectangular types.

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Ducting is used to deliver and remove air and is a vital part of maintaining a safe and pleasant workplace environment. Choosing the right kind of ductwork is important to the success of the installation.

Rectangular ductwork

Rectangular ducts are usually made of sheet metal, most commonly galvanized steel, although other materials are sometimes used. Their shape means they are easy to stack and ship and they can be adapted to a variety of installation requirements. Where unusual widths and heights are needed rectangular ducts are a more viable option.

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Spiral ductwork

A spiral duct is usually manufactured from metal, commonly galvanised steel, but aluminium is often used too, and it can also be made of fibreglass or a composite of wire and plastic. Whatever it’s made of, it uses less material than a rectangular equivalent. It can be manufactured in different lengths, but generally comes in 20-foot long sections to make for easy transportation. Spiral ducts can be connected using flanges or with slip joints making them easier to seal effectively.

Pros and cons

A spiral duct is likely to be less subject to leaks than a rectangular one. This is due to the fact that fewer joints are involved in its construction and also because creating joints is easier; however, it’s better to rely on a good mechanical joint than to apply sealants. Because a round duct has lower pressure drop properties, it means fan speeds can be reduced. This helps to cut fan noise and also eliminates the drumming effect that can sometimes be a problem with rectangular ducts.

Insulation is another factor. Spiral ducting is more rigid and therefore easier to insulate, but it must be insulated on the outside. Rectangular ducts give you the option of insulating on the inside, which can help to keep your building warmer.

The big advantage of spiral is that it tends to be cheaper for any given installation because less material is used in its construction. Its flexibility also means that most installations can be completed using off the shelf parts thus minimising the need for custom fabrications or for modifications to be carried out on site.

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