what is duct

Ductwork is used to convey air between the air systems and the terminal equipment and spaces which these air systems serve.


Common cross-sectional shapes for ductwork are rectangular, round, and flat oval. Rectangular ductwork is normally used for main ducts and runouts to air devices (air inlets and outlets) that have a rectangular neck. Round ductwork is normally used for runouts to variable air volume (VAV) terminal units or supply air devices, which commonly require round duct connections. Flat oval ductwork is not commonly used. However, when it is used, it is usually for portions of the duct systems that require a pressure classification exceeding 2 in. w.c.


Galvanized steel is the most common construction material for duct systems. The galvanized sheet steel should have a minimum galvanized coating of G60; however, G90 galvanized steel should be specified due to its greater corrosion resistance. More costly materials such as stainless steel, aluminum, and carbon-steel have specific applications for duct systems. Stainless steel ductwork is often used for laboratory fume hood exhaust systems because of its corrosion resistance and the potential for corrosive chemical vapors to be conveyed through the duct system. Aluminum ductwork is not as resistant to corrosion from chemical vapors as stainless steel; however, it is suitable for use in damp environments and to convey moist air. For this reason, and because it
is less costly than stainless steel, it is commonly used for HVAC systems serving indoor swimming pool areas (natatoriums) and exhaust systems serving commercial dishwashers and shower rooms. NFPA Standard 96—Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations requires carbon-steel ductwork for exhaust systems serving kitchen exhaust hoods.

Duct systems are usually externally insulated. However, double-wall duct systems that consist of both outer and inner ducts separated by an insulating material are available. Sometimes the inner duct will be perforated for improved sound attenuation qualities. It is also common for portions of duct systems, especially those serving noise-sensitive areas, to be lined on the interior with 1 in. of sound attenuating/ insulating material called duct liner.15 Normally the length of ductwork that is lined on the interior is limited to the first 10 ft of supply and return air ductwork connected to air handling units and the first 5 ft of supply air ductwork downstream of VAV terminal units.

The final connections to supply air diffusers are often made with flexible round ductwork. Flexible ductwork consists of a wire-reinforced plastic inner duct surrounded by a plastic-faced layer of flexible fiberglass insulation. The use of flexible ductwork for the final connection to supply air devices simplifies the installation process and also provides a measure of sound attenuation for the supply air duct system. However, the length of flexible ductwork should be limited 8 ft because of
the potential for excessive bending in the ductwork that could restrict the airflow through the duct.

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