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 weigh 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 in 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 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 (46 cm) to 30 feet (9.1 m). The air temperature may vary from ambient to 1,000 °F (538 °C). Process ductwork is subject to large loads due to dust buildup, fan suction pressure, wind, and earthquake forces. As of 2009 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.
Round and rectangular duct structural design The structural design of ductwork plate is based on buckling of the plate element. Round ductwork plate design is based on diameter to duct plate thickness ratios, and the allowable stresses are contained in multiple references such as US Steel Plate, ASME/ANSI STS-1,SMNACA, Tubular Steel Structures, and other references. In actuality round ductwork bent in bending is approximately 30% stronger than a similar shape in compression, however one uses the same allowable stresses in bending as we do for compression. Round ducts require typical stiffeners at roughly 3 diameter spacing, or roughly 20 ft. O.C. for wind ovaling and fabrication and truck shipping requirements. Round ducts, larger than 6 feet 6 inches (1.98 m) in diameter (1/4" plate) require support ring stiffeners. Smaller-diameter ducts may not require support ring stiffeners, but may be designed with saddle supports. When stiffener rings are required they are traditionally designed based on "Roark", although this reference is quite conservative. Round duct elbow allowable stresses are lower than the allowable stresses for straight duct by a K factor = 1.65/(h 2/3power) where h = t (duct) * R (elbow) /(r(duct)*r (duct). This equation, or similar equations is found in Tubular Steel Structures section 9.9. Rectangular ductwork design properties is based on width-to-thickness ratios. This is simplified, normally to width=t/16, from corner elements or corner angle stiffeners, although in reality, the entire duct top & side plate does participate, somewhat in duct section properties.
Duct logic Duct logic is the process of planning for duct thermal movement, combined with planning to minimize duct dust dropout. 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. This movement must be carefully planned for, with cloth (or metal) expansion joints at each equipment flange, and one joint per each straight section of ductwork. Sloping ductwork at or above the duct dust angle of repose will minimize dust buildup. Therefore, many ducts carrying high dust loads slope at 30 degrees, or steeper.
Duct elbow geometry To minimize pressure loss in duct elbows, the typical elbow radius is 1 1/2 times the duct diameter. In cases where this elbow radius is not feasible, turning vanes are added to the duct.
Duct transition and elbow layout Process ductwork is often large (6-foot diameter to 18-foot diameter), carrying large volumes of hot dirty gasses, at velocities of 3000 to 4500 feet per minute. The fans used to move these gasses are also large, 250 to 4000 horsepower. Therefore, minimizing duct pressure drop by minimizing turbulence at elbows and transitions is of importance. Duct elbow radius is usually 1 1/2 to 2 times the duct size. The side slopes of transitions are typically 10 to 30 degrees. Note: the duct gas velocity is chosen to minimize duct dust dropout. Cement and lime plant duct velocity at normal operations is 3000 to 3200-foot per minute, lead plant velocities are 4000 to 4500-foot per minute, as the dust is heavier. Other industries, such as grain have lower gas velocities. Higher duct gas velocity may require more powerful fans than lower duct velocities.
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