How rectangular duct is made

Round duct is rolled; rectangular duct is folded. Its four sides are flat panels, joined at the corners either by folds in one piece of sheet or by seams between pieces. That changes how patterns are made. There is no girth to calculate on a mean line, because the corners are tight folds rather than gentle curves. There are no cut ordinates around a circumference, because every face is flat. Instead, each face of a fitting is developed separately as a flat shape, and the faces are matched along their edges.

This chapter sets out the vocabulary and conventions used through the book: how rectangular duct is described, which faces of a fitting are cheeks and which are wrappers, how a straight length is folded from one, two or four pieces, and how much a tight fold changes a blank. It introduces the running case, a make-up air duct of 600 × 400 mm, and ends with an elbow that was made correctly and turned the wrong way.

Describing a rectangular duct

A rectangular duct has two dimensions, and the order in which they are written matters. The convention used here, and in much of the trade, is that the first dimension is the width seen on the drawing and the second the depth, with both inside dimensions. The running case’s duct is 600 × 400 mm: on the plan it is 600 mm wide, and it is 400 mm deep.

For a fitting that changes direction, a second convention matters more: which dimension lies in the plane of the bend. An elbow that turns a 600 × 400 duct in plan, so that the duct stays level, has its 600 mm dimension in the plane of the turn. One that turns the same duct from horizontal to vertical has its 400 mm dimension in the plane of the turn. They are different fittings with different patterns, and a drawing must say which.

Trade shorthand varies between shops and regions, and words such as “easy way” and “hard way” are used in opposite senses in different places. This book avoids them. It states the dimension in the plane of the bend in millimetres, every time.

Cheeks and wrappers

Every rectangular fitting that turns or shifts the duct has two kinds of face.

The cheeks are the two faces that lie in the plane of the bend. On a radius elbow, they are flat pieces shaped like a quarter of an annulus, and they carry the elbow’s shape. On a mitred elbow, they are flat L-shapes with a diagonal mitre line.

The wrappers are the two faces that run around the bend, the throat on the inside and the heel on the outside. On a radius elbow, each wrapper is a flat strip, curved to the cheek’s arc when assembled. On a mitred elbow, the wrappers are flat panels that meet at the mitre.

The cheeks are cut to shape; the wrappers are cut to length. That division of labour is what makes rectangular fittings quick to make once their patterns are right.

Figure 1Cheeks and wrappers in the running case's fittings

Fitting	Cheeks are	Wrappers are	Chapter
Radius elbow, 400 in the plane of the bend	Quarter annuli, 200 to 600 mm radius	Strips, 600 wide, curved to the cheeks	4
Mitred elbow	L-shaped panels with a mitre line	Flat panels meeting at the mitre	3
Single-plane offset, 600 in the plane	Two parallelogram-ended panels	Flat panels folded at each joint	5
Diagonal offset	No cheeks: all four faces are parallelograms	| 6
Transition, 800 × 500 to 600 × 400	No cheeks: four flat trapezoids	| 7

Folding a straight length

A straight length of the running case’s duct can be folded in three common ways.

1. One piece, folded at three corners and seamed at the fourth. The blank is the whole perimeter plus the seam’s allowance. It uses the fewest seams but the largest blank, and a long blank is awkward to handle and fold.
2. Two L-shaped pieces, each folded at one corner and seamed to the other at two corners. Each blank is one width plus one depth. The two pieces are identical, which halves the number of patterns.
3. Four flat panels, seamed at all four corners. Used for large duct or when panels are cut from narrow coil, it needs no folding of whole panels but has the most seams.

The shop’s choice depends on its folder’s length and capacity, the coil or sheet widths it buys and the size of the duct. For the running case, the shop makes straight duct as two L-shaped pieces, which fit its folder and its 1,200 mm wide coil.

Figure 2Ways to make a straight length of rectangular duct

• Straight duct, 600 × 400 inside
  • One piecethree folds, one corner seam, blank about 2,000 mm plus seam
  • Two L-shaped piecesone fold each, two corner seams, blanks about 1,000 mm plus seams
  • Four panelsno folds of whole panels, four corner seams, blanks 600 and 400 mm plus seams

How much a fold changes the blank

A folded corner is a bend of 90° to a small inside radius, typically about one sheet thickness. As the Library’s Sheet Metal Parts by the Numbers explains, the length a bend consumes is its bend allowance, calculated with a K-factor that places the neutral line inside the mid-thickness for tight bends.

This is an illustration. For 0.8 mm sheet folded to an inside radius of 0.8 mm with a K-factor of 0.44, the bend allowance of a 90° fold is π ÷ 2 × (0.8 + 0.44 × 0.8) = 1.81 mm. Dimensioning to the inside, each flat face is its inside dimension less 0.8 mm at each folded end, so each fold adds 1.81 − 1.6 = 0.21 mm to the blank compared with simply adding the inside dimensions.

So an L-shaped piece of the running case’s duct, 600 mm plus 400 mm inside with one fold, has a net blank of 1,000.21 mm before seam allowances, and a one-piece wrap with four folds would be 2,000.84 mm. For 0.8 mm sheet the correction is a fraction of a millimetre per fold, small enough that many shops ignore it on duct, but it is not zero, and on heavier gauges it grows.

The mean-line girth used for rolled round duct does not apply here. A sheet metal worker who calculated the one-piece wrap as the perimeter of the mean line, 2 × (600.8 + 400.8) = 2,003.2 mm, would cut the blank 2.4 mm too long, because a tight fold does not behave like a gentle roll.

Where the seams go

On rectangular duct, longitudinal seams go at the corners, never in the middle of a face. A corner seam is stiff, because the seam’s layers form a small angle section along the edge, and it sits where the folded corner would otherwise be. A seam in the middle of a face would add a ridge where the panel most needs to be flat and would complicate cross-breaking.

With two L-shaped pieces, the seams fall at two diagonally opposite corners. Which two is a choice the job sheet should record, because it matters at fittings: a seam that runs into the throat of an elbow, or along the edge where a branch is cut in, makes that fitting harder to make and seal. The running case puts the straight duct’s seams at the top-left and bottom-right corners, looking downstream, and keeps the same positions through every fitting where it can.

Seams also decide which way pieces go together. A Pittsburgh lock has a pocket on one piece and a flange on the other, so an L-piece is not simply reversible: the pocket edge and the flange edge must be at the right ends. The shop marks every blank with “P” and “F” on its edges before folding, so that the roll former is fed the right edge.

Reading the run from the drawing

The designer’s drawing shows the duct as a centreline with sizes, usually on a plan and one or more sections. Turning that into fittings means reading three things at every change of direction or level.

1. The centreline’s path: where it turns, by how much, and in which plane. A turn in plan is a horizontal bend; a rise or fall is a vertical bend; a change in both at once is a diagonal offset or a compound bend.
2. The duct’s orientation along that path: which dimension is horizontal at each point. Rectangular duct cannot rotate about its own axis without a special fitting, so its orientation is fixed from the air handler to the last outlet, and every fitting must preserve it.
3. The clearances: beams, sprinkler mains, cable trays and lights that the duct must pass. The running case’s diagonal offset exists because a beam and a sprinkler main cross the path within a metre of each other.

The fabricator sketches the run in three dimensions, even roughly, before developing any fitting. A sketch shows at once which faces of each fitting are cheeks, which dimension is in the plane of each bend, and where seams would run into trouble.

Panel stiffness

A flat panel of thin sheet is flexible. When air flows through a duct at pressure, or when the fan starts and stops, a large flat panel bows in and out, and can drum or “oil-can” with a bang. Rectangular duct therefore needs its panels stiffened: by cross-breaking (a shallow diagonal fold that turns the panel into a very flat pyramid), by beading (rolled ribs across the panel), by the transverse joints themselves, and on larger sizes by external angles or rods. Chapter 2 covers cross-breaking and Chapter 9 the joints. The stiffening a particular duct needs depends on its size, gauge and pressure, and comes from its specification.

The running case

This is an illustration. A manufacturer’s assembly hall needs fresh air supplied through a rectangular duct along its roof trusses. The shop is making the run in 0.8 mm galvanised steel, 600 × 400 mm inside, with these fittings, each developed in a later chapter:

• a transition from the air handler’s 800 × 500 mm outlet, flat on top, 600 mm long;
• straight lengths of 1,200 mm, cross-broken;
• a radius elbow with the 400 mm dimension in the plane of the bend and a 200 mm throat radius, and a mitred elbow in a tight corner;
• a single-plane offset of 300 mm sideways over a 900 mm run;
• a diagonal offset of 300 mm sideways and 250 mm down over a 1,000 mm run;
• a 45° shoe branch, 300 × 200 mm, off the side of the main.

The shop’s longitudinal seams are Pittsburgh locks made on a roll former, and its transverse joints are slip-on joints with drive cleats, as Chapter 9 sets out. Its coil is 1,200 mm wide, which is why the straight lengths are 1,200 mm.

Figure 3The running case's duct in numbers

600 × 400 mmInside size0.8 mm galvanised steel

2,000 mmInside perimeterabout 2.0 m² of sheet per metre of duct

12.6 kg/mMass of plain ductbefore seams, joints and stiffening

1,200 mmStraight lengththe coil width

The error: an elbow turned the wrong way

This is an illustration. The first radius elbow for the run was drawn on the job sheet as “R ELBOW 600 × 400, 90°, THROAT R200”. The fabricator made it with the 600 mm dimension in the plane of the bend: cheeks of 600 mm across, from a 200 mm throat radius to an 800 mm heel radius, and wrappers 400 mm wide.

It was a well-made elbow, and it was the wrong one. The run needed to turn from horizontal to vertical, with the 400 mm dimension in the plane of the bend, so that the duct’s 600 mm width stayed across the truss. Offered up, the elbow turned the duct correctly but rotated its section a quarter turn, so the duct leaving it was 400 wide and 600 deep, and it would not meet the next length.

The fault was found at the trial assembly on the shop floor, not on site, because the shop assembles each run’s fittings dry before dispatch. The job sheet’s description had been ambiguous: the same words describe both elbows. The shop’s job sheets now state every bend as “400 IN PLANE OF BEND” or “600 IN PLANE OF BEND”, and the cheek’s size across the bend is written beside it. The second elbow, with cheeks from 200 to 600 mm radius and wrappers 600 mm wide, is the one Chapter 4 develops.

Figure 4Describing a rectangular fitting on a job sheet

• Do: Inside dimensions, width first as drawn600 × 400
• Do: Dimension in the plane of each bend400 in plane of bend
• Do: Throat radius, or mitredthroat R200
• Do: Angle of the bend or offset, and its direction90°, horizontal to vertical, turning up
• Do: Seam type and corner positionsPittsburgh, two diagonally opposite corners
• Do: Transverse joint at each endslip-on with drive cleats
• Avoid: Bend described only as "600 × 400 elbow"the elbow made turning the wrong way

Exercise: blanks for a smaller duct

A branch duct is 300 × 200 mm inside, in 0.8 mm sheet, made from two L-shaped pieces. Using the same folding allowance of 0.21 mm per fold, find each L-piece’s net blank length, and the blank of a one-piece wrap. What would the mean-line perimeter have given, and by how much would it be wrong?

Worked answer.

• Each L-piece has one fold: 300 + 200 + 0.21 = 500.21 mm.
• A one-piece wrap has four folds around a perimeter of 1,000 mm: 1,000 + 4 × 0.21 = 1,000.84 mm.
• The mean-line perimeter is 2 × (300.8 + 200.8) = 1,003.2 mm, 2.36 mm longer than the folded wrap needs.

The difference is the same for the small duct as for the large one, because it comes from the four folds, not the size. For folded duct, the correction is a number per fold; for rolled duct, it is a number per circumference.