Process Duct Work
Encyclopedia
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.
s, 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. 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.
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)
Process engineering
Process engineering focuses on the design, operation, control, and optimization of chemical, physical, and biological processes through the aid of systematic computer-based methods...
to mills, baghouses to other process equipment. Process duct work
Duct (industrial exhaust)
Industrial exhaust ducts are pipe systems that connect hoods to industrial chimneys through other components of exhaust systems like fan, collectors etc...
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
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 metalMetal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s, 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
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
, lime and lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
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
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
forces. 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.
Round ductwork
Large, round process ductwork is usually fabricatedFabrication (metal)
Fabrication as an industrial term refers to building metal structures by cutting, bending, and assembling. The cutting part of fabrication is via sawing, shearing, or chiseling ; torching with handheld torches ; and via CNC cutters...
from 1/4 in (6 mm) steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
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
Yield (engineering)
The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed...
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
U.S. Steel
The United States Steel Corporation , more commonly known as U.S. Steel, is an integrated steel producer with major production operations in the United States, Canada, and Central Europe. The company is the world's tenth largest steel producer ranked by sales...
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
Suction pressure
The term diffusion pressure deficit was coined by B.S Meyer in 1938. Originally DPD was described as suction pressure by Renner. It is reduction in the diffusion pressure of water in solution or cell over its pure state due to presence of solutes in it and forces opposing diffusion...
stresses & support bending moments.
Duct work pressure drop: 60% to 80% of high temperature process duct work pressure drop
Pressure drop
Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another point downstream. "Pressure drop" is the result of frictional forces on the fluid as it flows through the tube. The frictional forces are caused by a resistance to flow. The main...
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)