Ventilation Duct

Last crawled date: 1 year, 12 months ago

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

Ducts can be made out of the following materials:

Galvanized steel
Galvanized mild steel is the standard and most common material used in fabricating ductwork because the zinc coating of this metal prevents rusting and avoids cost of painting. For insulation purposes, metal ducts are typically lined with faced fiberglass blankets (duct liner) or wrapped externally with fiberglass blankets (duct wrap). When called for, a double walled duct is used. This will usually have an inner perforated liner, then a 1–2" layer of fiberglass insulation contained inside an outer solid pipe.

Rectangular ductwork commonly is fabricated to suit by specialized metal shops. For ease of handling, it most often comes in 4' sections (or joints). Round duct is made using a continuous spiral forming machine which can make round duct in nearly any diameter when using the right forming die and to any length to suite, but the most common stock sizes range evenly from 4" to 24" with 6"-12" being most commonly used. Stock pipe is usually sold in 10' joints. There are also 5' joints of non-spiral type pipe available, which is commonly used in residential applications.

Aluminium
Aluminium ductwork is lightweight and quick to install. 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. The parts are then typically cut at 45°, 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.

Aluminum is also used to make round spiral duct, but it is much less common than galvanized steel.

Polyurethane and phenolic insulation panels (pre-insulated air ducts)
Traditionally, air ductwork is made of sheet metal which was installed first and then lagged with insulation. Today, a sheet metal fabrication shop would commonly fabricate the galvanized steel duct and insulate with duct wrap prior to installation. However, ductwork manufactured from rigid insulation panels does not need any further insulation and can be installed in a single step. Both polyurethane 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. There are various types of rigid polyurethane foam panels available, including a water formulated panel for which the foaming process is obtained through the use of water and CO2 instead of CFC, HCFC, HFC and HC gasses. Most manufacturers of rigid polyurethane or phenolic foam panels use pentane as foaming agent instead of the aforementioned gasses.

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