Process duct work

Process duct work conveys large volumes of hot, dusty air from processing equipment to mills, baghouses to other process equipment. Process duct work may be round or rectangular. Although round duct work costs more to fabricate than rectangular duct work, it requires fewer stiffeners and is favored in many applications over rectangular ductwork.

The air in process duct work may be at ambient conditions or may operate at up to 900 °F (482 °C). Process ductwork varies in size from 2 ft diameter to 20 ft diameter or to perhaps 20 ft by 40 ft rectangular.

Large process ductwork may fill with dust, depending on slope, to up to 30% of cross section, which can weight 2 to 4 tons per linear foot.

Round ductwork is subject to duct suction collapse, and requires stiffeners to minimize this. but is more efficient on material than rectangular duct work.

There are no comprehensive, design references for process duct work design. The ASCE reference for the design of power plant duct design gives some general guidance on duct design, but does not specifically give designers sufficient information to design process duct work.

Structural process ductwork carries large volumes of high temperature, dusty air, between pieces of process equipment. The design of this ductwork requires an understanding of the interaction of heat softening of metals, potential effects of dust buildup in large ductwork, and structural design principles. There are two basic shapes for structural process ductwork: rectangular and round. Rectangular ductwork is covered by the ASCE "The Structural Design of Air & Gas Ducts for Process Power Stations and Industrial Applications".

In the practical design of primarily round structural process ductwork in the cement, lime and lead industries, the duct size involved ranges from 18 inches (45 cm) to 30 feet (10 m). The air temperature may vary from ambient to 1000 °F (515 °C). Process ductwork is subject to large loads due to dust buildup, fan suction pressure, wind, and earthquake forces. As of 2009^[update] 30 ft diameter process ductwork may cost $7,000 per ton. Failure to properly integrate design forces may lead to catastrophic duct collapse. Overdesign of ductwork is expensive.

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