Ducts are used to distribute conditioned air throughout building and return air from conditioned space to AC. Duct material maybe metal or non metal. The most common metal ducts are galvanized iron or aluminium. It may be round, rectangular or square and Flat oval. Metal ducts are typically used in headers and branches, in some places flexible ducts are used for the final connection from branches to diffusers because they are easier to align than rigid metal ducts.
Air velocities in duct can create noise if the velocities are high and these need to be maintained at optimum level. Ducts may have acoustic fibre liners to reduce noise especially just downstream of the fan.
Types of duct materials
Ducts come in a variety of materials and design types as discussed below
Metal
Metal ducts are the mostly used and can be cleaned aggressively. The most common metal used in commercial building is galvanized iron. But metal ducts must be insulated properly to prevent heat transfer loss or sweating. It can be fabricated to many shapes such as circular, square, rectangular or flat oval. Excessive air leakage from metal supply ducts can occur as a result of loose fitting joints and connections. It is available in different gauge thickness and is selected based on smacna standards.
Flexible Ducts
Flexible ducts are often used to connect supplier branches to registers or decreases because they are easier to work with and achieve a good connection. Usage of long flexible ducts is restricted due to increase in frictional losses.
Fabric Ducts
Fabric ducts are used as open ducts. Fabric is used in spaces like Auditorium go down and factories. The main advantage of using fabric is labour time can be reduced and there is no need of duct insulation. Permeable and non permeable fabrics are available in market. Normal grills or diffusers are not used with Fabric ducts. The air flow pattern is maintained through nozzles or orifices that come within the fabric ducts.
Pre- Insulated Ducts
Pre insulated ducts, it is clear from the name itself that it is made of some insulated material, polyurethane foam or high density thermocol. So there is no need of any further layer of thermal insulation to prevent heat transfer losses. This can be used in residential or commercial HVACs applications.
Concrete
Concrete ducts are sometimes used in commercial buildings to house pipes and cables. It may be used as a return duct in some buildings. The concrete may be painted or unpainted. Cracks or holes in concrete return ducts located in crawl spaces or below Slabs could allow soil gases, moisture and mould spores to enter the circulating airstream.
