Rectangular Duct Fittings by the Numbers

About this book

Rectangular duct is made of flat panels folded at the corners, and its fittings look simpler than round ones: no curves to roll, no girths to calculate, just rectangles and a few angles. That simplicity is misleading. A rectangular elbow has two different shapes of piece that must match along curved edges. An offset that moves the duct sideways and down at once has faces whose true shapes appear in no view on the drawing. A transition from one rectangle to another has faces that look like rectangles and are trapezoids of different heights. And every fitting must allow for seams and joints that a roll former or a folder makes in its own way.

This book develops those fittings with numbers. Each chapter shows the traditional drawing method and calculates the same lines, so that a pattern can be marked on the bench, generated in a spreadsheet or checked against a CAD unfold. It starts with straight duct and its seams, works through elbows, offsets, transitions and branches, then covers joints and stiffening and ends with the take-off that turns a drawing into a cutting list.

It is for sheet metal workers, ductwork fabricators, apprentices and drafters, and for engineers who check fabrication drawings. It assumes school algebra and trigonometry. Readers who want round fittings should start with the Library’s Sheet Metal Patterns by the Numbers; readers who want airflow, pressure loss and fan duty should read Ductwork by the Numbers. This book is about making the rectangular pieces fit.

The running case

Every chapter follows one fictional job. This is an illustration. A small manufacturer is adding a make-up air supply to its assembly hall, and a sheet metal shop is making the rectangular ductwork. The duct is 600 × 400 mm inside, in 0.8 mm galvanised steel, with Pittsburgh-lock longitudinal seams and slip-on transverse joints. The run needs:

• a transition from the air handler’s 800 × 500 mm outlet to the 600 × 400 mm duct, 600 mm long, flat on top;
• straight duct in 1,200 mm lengths, with cross-broken panels;
• a radius elbow turning the duct through 90° in the plane of its 400 mm dimension, with a 200 mm throat radius, and a mitred elbow where space is tight;
• a single-plane offset moving the duct 300 mm sideways over a run of 900 mm;
• a diagonal offset moving it 300 mm sideways and 250 mm down at once over 1,000 mm, to clear a beam and a sprinkler main;
• a 45° shoe branch, 300 × 200 mm, off the side of the main to a second zone.

The manufacturer, the shop, the dimensions, the seam allowances and the mistakes are invented. None describes KEVOS® work or a real business.

How to use the book

Chapters 1 and 2 set the conventions, including which dimension of a rectangular duct is in the plane of a bend and how panel blanks are measured. The chapters on fittings can then be read in any order, although Chapter 6 on diagonal offsets builds on Chapter 5. Chapter 9 on joints applies to every fitting, and Chapter 10 brings the run together. Values are given to 0.1 mm so that the arithmetic can be followed; on the bench, rectangular fittings are marked to the nearest half millimetre.

Source limits and professional responsibility

The geometry is exact, and every calculation in this book was checked by running it. Seam, cleat, flange and stiffening dimensions depend on the shop’s machines and on the specification for the job, and the ones in this book are illustrations, to be calibrated with trial pieces as Chapter 9 shows. Duct gauges, reinforcement spacing and joint types for a real system come from its specification and the standards it calls up, which this book names but does not quote.

Air-handling systems affect health, comfort and, through fire dampers and smoke control, safety. Their design, sizing and installation are matters for a competent designer and the specification for the job, under the Australian standards and building requirements that apply. This book teaches the geometry of fittings. It is not a design for any duct system.