Round Ductwork
Large, round process ductwork is usually fabricated from 1/4-inch (6 mm) steel plate, with stiffening rings at 15 ft (5 m) to 25 ft (8 m) on center, regardless of diameter. These lengths allow for shipping by truck, and fabrication with most 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 tees's welded together to create the ring cross section required.
See US Steel Plate, volume II for empirical ring spacing, and wind bending stress: Spacing = Ls = 60 sqrt 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.
See API 560 for design of wind ovaling stiffeners
See Tubular Steel structures, chapter 2, 9 & 12 for the allowable stresses for thin, round ducts, their allowable stresses, elbows, elbow softening coefficients, and some procedures for the design of duct support rings. These allowable stresses can be verified with select review of chapters of US Steel Plate, Blodgett Design of plate structures, Roark & Young, or API 650.
Round duct support rings are spaced, often at three diameters, or as require at up to about 50 ft centers (14 m). At this spacing the main support rings are designed for the sum of suction pressure stresses & support bending moments.
Round ductwork allowable compressive stress is = 662 /(d/t) +339 * Fy (tubular steel structures, chapter 2)
Duct work pressure drop: 60% to 80% of high temperature process duct work pressure drop occurs in the process equipment, baghouses, mills and cyclones. However, since motor 1 (one) horsepower cost roughly $1000/year (US$), duct efficiency is important. Minimizing duct pressure drop is important. most ductwork, non-equipment pressure drop occurs at transitions and changes of directions (elbows). The bests way to minimize duct pressure drop ($) is to use elbows with an elbow radius to duct radius exceeding 1.5. (For a 15-foot duct, the elbow radius would therefore equal, or exceed 22.5 ft.)
(Duct pressure drop and energy loss, occur due to turbulence)
Process duct pressure drops (US practice) are usually measured in inches of water. A typical duct operates at about 25 inches total suction pressure, with 18" of pressure loss in the bag house, and 3" of pressure lost in duct friction, and 4" lost in elbow turbulence.
Round duct work suction pressure collapse, in ducts over 6 feet in diameter, is prevented with rings at supports, and roughly 3 diameter centers.
Round duct support rings are traditionally designed from the formula's found in Roark & Young. However this reference is based on point loads on rings, while actual duct ring loads are based on almost uniform bottom dust. Therefore these formulars have a factor of safety of 2 above actual stresses. These forces are also combined with suction pressure stresses. Suction pressure forces concentrate on the rings, as they are the stiffest element present.
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