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. 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 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 be 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 range from 18 inch (45 cm) to 30 ft (10 m). The air temperature may vary from ambient to 900 °F (490 °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 $6,000 per foot. Failure to properly integrate design forces may lead to catastrophic duct collapse. Overdesign of ductwork is expensive.
Large, round process ductwork is usually fabricated from 1/4 in (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 [Do (ft) * T plate (in) /wind pressure (psf)] Section = p * L (spacing, ft) * Do (ft) * Do (ft)/Fb (20,000 at ambient T)
See API 360 for design of wind ovaling stiffeners
Round duct support rings are spaced, as require at up to about 50 ft centers (14m). At this spacing the main support rings are designed for the sum of suction pressure stresses & support bending moments.
Duct work pressure drop: 60% to 80% of high temperature process duct work pressure drop occurs in the process equipment, baghouses, mills & 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)
There are several references for process duct work.
Cement, lime and lead industry accepted dust loads (for structural loading): Process ductwork is intended to convey large volumes of dust. some of this dust will settle to the bottom of the duct during power outages and normal operation. The percentage of duct cross section filled with dust is assumed to be as follows:
To minimize the build up of dust, each material has a minimum carrying velocity, lime = about 2800 fpm., cement about 3200 fpm, and lead dust about 4200 fpm.
Dust density depends on industry, cement dust density = 94 PCF, lime industry = 50 PCF, lead oxide dust = 200 pscf.