Duct (HVAC) – Wikipedia, the free encyclopedia

Ducts are used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. These needed airflows include, for example, supply air, return air, and exhaust air.[1] Ducts also deliver, most commonly as part of the supply air, ventilation air. As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort.

A duct system is often called ductwork. Planning ('laying out'), sizing, optimizing, detailing, and finding the pressure losses through a duct system is called duct design.[2]

Ducts can be made out of the following materials:

Galvanized mild steel is the standard and most common material used in fabricating ductwork. For insulation purposes, metal ducts are typically lined with faced fiber glass blanket (duct liner) or wrapped externally with fiber glass blankets (duct wrap).

Traditionally, air ductwork is made of sheet metal which is installed first and then lagged with insulation as a secondary operation. Ductwork manufactured from rigid insulation panels does not need any further insulation and is installed in a single fix. Light weight and installation speed are among the features of preinsulated aluminium ductwork, also custom or special shapes of ducts can be easily fabricated in the shop or on site.

The ductwork construction starts with the tracing of the duct outline onto the aluminium preinsulated panel, then the parts are typically cut at 45 degree, bent if required to obtain the different fittings (i.e. elbows, tapers) and finally assembled with glue. Aluminium tape is applied to all seams where the external surface of the aluminium foil has been cut. A variety of flanges are available to suit various installation requirements. All internal joints are sealed with sealant.

Among the various types of rigid polyurethane foam panels available, the foaming process of a new water formulated panel is obtained through the use of water and CO2 instead of CFC, HCFC, HFC and HC gasses. Most manufacturers of rigid polyurethane foam panels use pentane as foaming agent instead of the CFC, HCFC, HFC and HC gasses, so do manufacturers of rigid phenolic foam panels.

A rigid phenolic insulation ductwork system is listed as a class 1 air duct to UL 181 Standard for Safety.

Both polyurethane foam panels and phenolic foam panels are manufactured with factory applied aluminium facings on both sides. The thickness of the aluminium foil can vary from 25 micrometres for indoor use to 200 micrometres for external use or for higher mechanical characteristics. The finish for external ductwork exposed to the weather can be an aluminum or aluminium / zinc alloy coated sheet steel, a multilayer laminate, a fibre reinforced polymer or other waterproof coating.

Fiberglass duct board panels provide built-in thermal insulation and the interior surface absorbs sound, helping to provide quiet operation of the HVAC system. The duct board is formed by sliding a specially-designed knife along the board using a straightedge as a guide; the knife automatically trims out a "valley" with 45 sides; the valley does not quite penetrate the entire depth of the duct board, providing a thin section that acts as a hinge. The duct board can then be folded along the valleys to produce 90 folds, making the rectangular duct shape in the fabricator's desired size. The duct is then closed with outward-clinching staples and special aluminum or similar 'metal-backed' tape. Commonly available duct tape should not be used on air ducts, metal, fiberglass, or otherwise, that are intended for long-term use; the adhesive on so called 'duct tape' dries and releases with time, further the 'duct tapes' do not meet the required UL standards for fire resistance.

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Duct (HVAC) - Wikipedia, the free encyclopedia