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Ductwork by the Numbers
Sizing, pressure loss, fittings, fans, transitions, sheet and balancing for fabricated round and rectangular extraction ductwork, in SI units
Size and check ductwork with numbers: flow and velocity, round and rectangular equivalents, branches, friction, fittings, fan duty and fan laws, transitions, sheet and balancing.
Start readingAbout this book
Ductwork moves air: fresh air into a building, fume and dust out of a workshop, hot air away from a process. Every duct is sized, shaped and made with numbers. Its cross-section sets the air’s velocity for a given flow; its length, roughness and fittings set the pressure the fan must supply; its transitions and branches decide whether air goes where it is meant to; and its sheet and supports decide what it costs and whether it stays up. Most of the numbers are geometry, and the rest are a little fluid mechanics that a spreadsheet handles easily.
This book works through those numbers in the order a ducted system is designed and made. It covers flow, area and velocity; round and rectangular equivalents; branches and the main; friction in straight duct; fittings and velocity pressure; adding the losses to a fan duty; transitions treated for their effect on the air; fans, system curves and the fan laws; sheet, mass, supports and cost; and measuring and balancing the finished system. Flat-pattern development of duct fittings is a sheet metal topic in its own right and is not covered here.
It suits mechanical designers, drafters, fabricators and maintenance engineers. It assumes school algebra and a spreadsheet. The Library’s Tanks and Hoppers by the Numbers applies the same mensuration to vessels and is a natural companion.
The running case
Every chapter follows one fictional system. This is an illustration. A fabrication workshop installs a dust and fume extraction system, EX-1, serving three booths, B1, B2 and B3, each drawing 0.30 m³/s through its hood. B1, the furthest from the fan, feeds the start of the main; B2 and B3 join it at junctions N2 and N3. The main runs to a collector and fan, and the fan discharges through a short stack. The ventilation designer has specified a minimum duct velocity of 18 m/s to keep the dust moving.
The workshop, the system and its numbers are invented; none describes KEVOS® work or a real installation. Prices are illustrations in Australian dollars, excluding GST. Pressure-loss coefficients for fittings are labelled as illustrations: real values come from the fitting’s manufacturer or a recognised design reference.
Source limits and professional responsibility
The geometry in this book is exact and every calculation was checked by running it. The fluid mechanics uses standard equations with stated simplifications. The design of a real ventilation or extraction system, including its hood design, capture and transport velocities, fan selection, fire and smoke requirements, and the protection of workers from airborne contaminants, is governed by the National Construction Code, Australian Standards such as AS 1668.2 for mechanical ventilation and AS 4254 for ductwork, and work health and safety law, including the workplace exposure standards published by Safe Work Australia. This book is educational. Real systems must be designed and commissioned by competent people to the requirements that apply.
Who this book is for
Mechanical designers, drafters, sheet metal fabricators and maintenance engineers who size, draw, make or alter ducted extraction and ventilation systems in workshops and plants.
What you will learn
- Size round and rectangular ducts from flow and velocity, and choose standard sizes in the right direction.
- Tell equal-area from equal-friction rectangular equivalents and use each where it belongs.
- Estimate straight-duct and fitting pressure losses and add them up to a fan duty.
- Find a fan's operating point, apply the fan laws, and estimate a system's sheet, mass, support loads and cost.
- Measure flow with a pitot traverse and balance branches with numbers rather than guesses.
Inside this book
Chapters
- 01Flow, area and velocityRelate a duct's flow, area and velocity, convert flow units, size branches and mains for a minimum velocity, round in the right direction, and catch a branch upsized into trouble.
- 02Round and rectangular equivalentsConvert round and rectangular ducts by equal area, hydraulic diameter or the Huebscher diameter, know which a dust duct needs, and catch a comfort-duct habit that slowed the air.
- 03Branches and the mainSize a main as branches join it, by flow rather than by adding areas or diameters, shape junctions, see why paths must balance, and catch a main sized by adding diameters that ran at half speed.
- 04Friction in straight ductCalculate straight-duct loss with Darcy–Weisbach, the Reynolds number and an explicit friction factor, see how it scales, and catch two metres of flexible hose costing 94 Pa.
- 05Fittings and velocity pressureExpress hood, elbow, junction, transition and exit losses as multiples of velocity pressure, compare elbow types, total a path's fittings, and catch single-mitre elbows that added 410 Pa.
- 06Adding up the fan dutyTotal the index path's friction, fittings and equipment into a fan duty, size the branches' balancing, estimate air and shaft power and running cost, and catch a fan chosen for a clean filter.
- 07Transitions, expansions and contractionsTreat transitions as changes of velocity: sudden and gradual expansions, the Borda–Carnot loss, taper length, contractions and stack diffusers, and an abrupt step at a fan inlet.
- 08Fans, system curves and the fan lawsFind where a fan meets its system, see what clean and dirty filters do, apply the fan laws, compare a drive with a damper, and catch a speed increase that overloaded a motor.
- 09Sheet, mass, supports and costEstimate a duct system's sheet and mass from its lengths and sizes, load the hangers for a duct partly full of dust, build a labour estimate, and catch hangers sized for clean duct.
- 10Measuring flow and balancingMeasure duct flow with a pitot traverse, place the points, average velocities not pressures, check hood static pressure, balance in order, and catch a misleading centreline reading.
Apply This in Practice
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References and source notes
The manuscript and worked examples are original KEVOS® writing. References inform particular concepts; their authors do not endorse this book. Web references were consulted on 26 October 2026.
- William Neubecker, David Williams Company, New York. Mensuration for Sheet Metal Workers (1907). Supplied source, read as a subject map of the duct problems a sheet metal shop solves by mensuration: round, square and rectangular pipes of equal area, boots, branches and transitions. Its imperial examples and its equal-area rules are not reproduced; this book's examples, figures and exercises are its own, in SI units, and Chapter 2 explains where equal area falls short.
- The Engineering ToolBox. Equivalent Diameter (2026). Reference for the Huebscher equivalent diameter of a rectangular duct, 1.30 (ab)^0.625 ÷ (a + b)^0.25, used in Chapter 2 for round and rectangular ducts of equal friction.
- S. E. Haaland, Journal of Fluids Engineering. Simple and Explicit Formulas for the Friction Factor in Turbulent Pipe Flow (1983). Source of the explicit approximation to the Colebrook friction factor used with the Darcy–Weisbach equation in Chapter 4. The formula is applied, not reproduced from any table.
- Standards Australia. AS 4254.2-2012 Ductwork for air-handling systems in buildings, Part 2: Rigid duct (2012). Named by number and scope only, as the Australian Standard for the materials, construction and installation of rigid ductwork in buildings. No values are quoted from it.