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 . 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 to . The air temperature may vary from ambient to . Process ductwork is subject to large loads due to dust buildup, fan suction pressure, wind, and earthquake forces. 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 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.
Duct support types Fixed supports typically are designed to resist lateral movement of the duct. Depending on the support geometry, fixed supports may also resist rotation of the duct art the support. Sliding supports are typically supported on Teflon, (or other material) pads, isolated from the duct so that temperature and dust does not damage the sliding surface. Link supports are often "bents", or braced frames down from the duct support ring (frame) to a foundation or support plane. if the bent is long enough, hinges are not required to allow for duct thermal growth. Rod or hanger supports are similar to link supports, but due to the flexibility of the rod supports, that are easier to design and detail. Guide supports: Often rings inside a structural frame, with angle guides, that allow the duct to grow vertically while restraining the duct laterally for wind loads. Unusual "support" conditions (details): Hinges at expansion joints Tension ties across dual fixed supports Designs that allowing duct elbows to flex under unusual support conditions Other unusual design models.
Duct design loads
For cement plant and Lime plant process ductwork, duct loads are a combination of: 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. Duct internal dust loads (bottom of duct): which vary considerably with duct slope. These loads need to be approved by the client, but are often used as follows: For duct sloping 0 degrees to 30 degrees, duct internal dust is 25% of duct cross section. For duct sloping 30 degrees to 45 degrees duct dust loads are reduced to 15% of cross section, plus internal duct coating loads. For ducts sloping 45 degrees to 85 degrees, duct internal dust is 5% of duct cross section, plus internal duct coating loads. For ducts sloping over 85 degrees. Because of the potential for high dust loading, most process ductwork is run at a 30 to 45 degree slope.
2a) Duct dust loading in non-process ducts (2-foot diameter and smaller), such as conveyor venting ducts are sometimes run horizontally and can be filled to 100% of cross section.
2b) Power plant internal duct dust loads are coordinated with the client, and are sometimes used at 1 to 2-foot of internal ash loadings.
3) Duct internal, coating dust loads, which sometimes are used as a coating of dust on the internal perimeter.
4) Duct suction pressure loads. Most process duct loads have design pressures of of water pressure. This suction pressure operates to cause suction pressure collapse on the duct side walls. Also this pressure operates perpendicular to the duct "expansion joints" to create an additional load on the duct supports that adds to dead, and live loads. Please note: duct pressure loads vary with temperature, as the gas density varies with temperature. A duct pressure of 25 inches of , at room temperature may become 12 inches to 6 inches at duct operating pressures.
5) Duct wind loads
6) Duct Seismic loads
7) Duct Snow loads, normally inconsequential, as snow will melt quickly unless the plant is in shutdown mode.
8) Top of duct dust loads, often used as zero, since plant dust generation is much less now, than in the past.
9) Duct suction pressure loads, act perpendicular to end of duct cross section, and can be significant. For a duct designed for 25" of water at a startup temperature of 70 degree F, on an 8-foot in diameter duct, this is equal to 8000 pounds at each end of the duct.
Round ductwork
The majority of cement plant process ductwork is round. This is because the round duct shape does not bend between circumferential stiffeners. Therefore, bending stiffeners are not required, and round ductwork requires fewer and lighter intermediate stiffeners than rectangular ductwork. Round cement plant duct stiffeners are sometimes about 5% duct plate weight. Rectangular cement plant duct stiffeners are 15 to 20% times duct plate weight. Power plant ductwork is often larger. Power plant ductwork is usually rectangular, with stiffener weights of 50% (or more) times duct plate weight. (this is based on personal experience, and my vary with loads, duct size, and industry standards)
Large, round process ductwork is usually fabricated from mild steel plate, with ovaling stiffening rings at on center, regardless of diameter. These lengths allow for resistance to wind ovaling and resistance to out of round when shipping by truck. This also works well with fabricator equipment.
The typical intermediate rings are designed for wind bending stresses, reduced as required by the yield stress reduction at working temperatures. The typical rings are fabricated from rolled steel plate, angles or tee's welded together to create the ring cross section required. Rings are fabricated from any combination of plate, tee or W shape that the shop can roll. Rings are usually mild carbon steel, ASTM A36 plate, or equivalent. The location of ring butt welds should preferably be offset 15 degrees(+/-)from point of maximum stress to minimize the effect of weld porosity on weld allowable stress.
See US Steel Plate, volume II for empirical ring spacing, and wind bending stress: Spacing = Ls = 60 sqrt [Do (ft) * t plate (in) /wind pressure (psf)] Section = p * L (spacing, ft) * Do (ft) * Do (ft)/Fb (20,000 at ambient T) This reference is older, but a good starting point for duct design.
SMACNA, (2ND Edition) chapter 4 has many useful formulas for round ducts, allowable stresses, ring spacing, effect of dust, ice, and live loads. The basic factor of safety for SMACNA, 3, is larger than typically used on typical structural engineering projects, of 1.6. Under SMACNA the critical ring spacing for rings is L = 1.25 * D (ft) sqrt (D(ft)/t(inches)), which is similar to tubular steel structures, L = 3.13 * R sqrt (R/t). In effect, using Spacing = 60 sqrt [Do (ft) * t plate (in) /wind pressure (psf)] is conservative.
Allowable bending and compression stress in ducts can come from several sources.
See API 560 for design of wind ovaling stiffeners