Duct (flow)
Ducts are conduits or passages used in heating, ventilation, and air conditioning to deliver and remove air. The needed airflows include, for example, supply air, return air, and exhaust air. 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.
A duct system is also called ductwork. Planning, sizing, optimizing, detailing, and finding the pressure losses through a duct system is called duct design.
Materials
Ducts can be made out of the following materials: They areGalvanized 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 or wrapped externally with fiberglass blankets. When necessary, a double walled duct is used. This will usually have an inner perforated liner, then a 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 sections. 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 suit, but the most common stock sizes range evenly from with being most commonly used. Stock pipe is usually sold in joints. There are also joints of the non-spiral type pipe available, which is commonly used in residential applications.
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 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 micrometers for external use or for higher mechanical characteristics.There are various types of rigid polyurethane foam panels available, including 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.
A rigid phenolic insulation ductwork system is listed as a class 1 air duct to UL 181 Standard for Safety.
Fiberglass duct board (preinsulated non-metallic ductwork)
Fiberglass duct board panels provide built-in thermal insulation and the interior surface absorbs , 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 groove with 45° sides which does not quite penetrate the entire depth of the duct board, thus providing a thin section acting as a hinge. The duct board can then be folded along the groove 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.
Flexible ducting
Flexible ducts are typically made of flexible plastic over a metal wire coil to shape a tube. They have a variety of configurations. In the United States, the insulation is usually glass wool, but other markets such as Australia, use both polyester fiber and glass wool for thermal insulation. A protective layer surrounds the insulation, and is usually composed of polyethylene or metalized PET. It is commonly sold as boxes containing of duct compressed into a length. It is available in diameters ranging from as small as to as big as, but the most commonly used are even sizes ranging from.Flexible duct is very convenient for attaching supply air outlets to the rigid ductwork. It is commonly attached with long zip ties or metal band claps. However, the pressure loss is higher than for most other types of ducts. As such, designers and installers attempt to keep their installed lengths short, e.g. less than or so, and try to minimize turns. Kinks in flexible ducting must be avoided. Some flexible duct markets prefer to avoid using flexible duct on the return air portions of HVAC systems, however flexible duct can tolerate moderate negative pressures. The UL181 test requires a negative pressure of 200 Pa.
To use flexible ducting in a system, make sure to pull the duct tight so you get the full internal diameter. This reduces resistance and improves airflow, as well as ventilation efficiency. Minimize bends and kinks as much as possible, since they can affect how well the airstream flows through the ductwork.
There are a few types of flexible ducting – Polyurethane, Aluminium & Aluminium insulated, Acoustic and Rectangular flexible ducting, as well as semi- and combi-flex.
Fabric ducting
This is actually an air distribution device and is not intended as a conduit for conditioned air. The term fabric duct is therefore somewhat misleading; fabric air dispersion system would be the more definitive name. However, as it often replaces hard ductwork, it is easy to perceive it simply as a duct. Usually made of polyester material, fabric ducts can provide a more even distribution and blending of the conditioned air in a given space than a conventional duct system. They may also be manufactured with vents or orifices.Fabric ducts are available in various colors, with options for silk screening or other forms of decoration, or in porous and non-porous fabric. The determination which fabric is appropriate can be made by considering if the application would require an insulated metal duct. If so, an air-permeable fabric is recommended because it will not commonly create condensation on its surface and can therefore be used where air is supplied below the dew point. Material that eliminates moisture may be healthier for the occupants. It can also be treated with an anti-microbial agent to inhibit bacterial growth. Porous material also tends to require less maintenance as it repels dust and other airborne contaminants.
Fabric made of more than 50% recycled material is also available, allowing it to be certified as green product. The material can also be fire retardant, which means that the fabric can still burn, but will extinguish when the heat source is removed.
Fabric ducts are not rated for use in ceilings or concealed attic spaces. However, products for use in raised floor applications are available. Fabric ducting usually weighs less than other conventional ducting and will therefore put less stress on the building's structure. The lower weight allows for easier installation.
Fabric ducts require a minimum of certain range of airflow and static pressure in order for it to work.