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The selection of a particular duct design is dependent on those characteristics that are important to the end user. The need for a specific characteristic or combination of characteristics such as pulling strength, flexibility, or the lowest coefficient of friction will dictate the type of duct required.
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.
A supersonic diffuser is a duct that decreases in area in the direction of flow which causes the fluid temperature, pressure, and density to increase, and velocity to decrease. These changes occur because the fluid is compressible. Shock waves may also play an important role in a supersonic diffuser.
Duct dead loads: are often simplified (in Cement plant usage) by using duct plate weight, multiplied by 1.15 as a stiffener allowance, as duct stiffeners usually weigh less than 15% times the duct plate weight. Duct stiffener allowance for rectangular power plants ductwork may be 50% to 100% of the duct plate weight.
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.
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.