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A duct with less than 0.75 mg/100m 2 is considered to be clean, per the NADCA standard. [7] A Hong Kong standard lists surface deposit limits of 1g/m 2 for supply and return ducts and 6g/m 2 for exhaust ducts, or a maximum deposit thickness of 60 μm in supply and return ducts, and 180 μm for exhaust ducts. [8]
Air-blown cable installation requires the use of a device that injects a high volume of air into the duct, at pressures as high as 20-25 psi. The viscous drag forces generated by the rushing air along the length of the cable act to reduce or overcome the friction between the cable and the duct.
There are two major systems to classify ductwork airtightness, one based on European standards, the other based on ASHRAE standard 90.1-2010.Both are based on the leakage airflow rate at a given ductwork pressure divided by the product of the ductwork surface area and the same ductwork pressure raised to the power 0.65.
Ducts move with changes in internal temperature. Ducts are assumed to have the same temperature as their internal gasses, which may be up to 900 °F. If the internal duct temperature exceeds 1000 °F, refractory lining is used to minimize the duct surface temperature. At 1000 °F, ducts may grow approximately 5/8 inch per 10 feet of length.
Liquidtight flexible metal conduit (LFMC) is a metallic flexible conduit covered by a waterproof plastic coating. The interior is similar to FMC. The interior is similar to FMC. Flexible metallic tubing ( FMT ; North America ) is not the same as flexible metallic conduit (FMC) which is described in US National Electrical Code (NEC) Article 348.
The ducts can be suspended on any type of ceiling, as the ducts only weigh approximately 1–5 kg depending on the duct size. Air socks are mounted on extruded aluminium rails or cables, these are hung from the ceiling and continually support the ducts along their length.