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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 4" to 24" (10-60 cm) with 6"-12" (15-30 cm) being most commonly used. Stock pipe is usually sold in 10' (300 cm) joints.
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.
The following table gives flow rate Q such that friction loss per unit length Δp / L (SI kg / m 2 / s 2) is 0.082, 0.245, and 0.816, respectively, for a variety of nominal duct sizes. The three values chosen for friction loss correspond to, in US units inch water column per 100 feet, 0.01, .03, and 0.1.
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.
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.
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.
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