Sheet, mass, supports and cost

Ductwork is bought by the kilogram or the square metre, made by the hour and hung from the building by the hanger. All three follow from the same few numbers: each section’s size, length and sheet thickness. A take-off of those numbers gives the system’s sheet area, its mass, the load on each support and most of its cost, and a take-off done carefully at the design stage is the difference between a quotation that holds and one that does not.

This chapter calculates EX-1’s sheet area and mass, section by section, with its elbows and an allowance for joints and fittings; loads its hangers, including the case of a duct partly filled with dust; and prices the sheet. The running case finds that the hangers on the 250 mm main were first sized for the clean duct’s mass, less than half of what they must carry if the duct partly fills.

Area and mass per metre

A round duct’s sheet area per metre of length is its circumference, π times its mean diameter. Its mass per metre is that area times the sheet’s thickness and density. For a rectangular duct, the circumference becomes the perimeter.

The sheet thickness of a duct depends on its size, its pressure, what it carries and the standard it is built to; for rigid ductwork in buildings, AS 4254.2 sets requirements for construction, which this book does not reproduce. The thicknesses below are illustrations, plausible for a light extraction system: 0.8 mm for the 140 mm ducts and 1.0 mm for the larger ones, in galvanised steel taken at 7,850 kg/m³, with the zinc coating’s small extra mass ignored.

Figure 1Sheet and mass per metre for EX-1's sections

	Thickness	Area per metre	Mass per metre
140 mm round	0.8 mm	0.442 m²	2.78 kg
200 mm round	1.0 mm	0.631 m²	4.96 kg
250 mm round	1.0 mm	0.789 m²	6.19 kg
160 × 310 mm rectangle	1.0 mm	0.940 m²	7.38 kg

The take-off

This is an illustration. EX-1’s straight duct, by size:

• 140 mm: 8 m from B1 to N2 and 3 m in each of B2 and B3, 14 m, 6.19 m², 38.9 kg;
• 200 mm: N2 to N3, 6 m, 3.79 m², 29.7 kg;
• 250 mm: N3 to the collector less the 4 m crossing, and the 6 m stack, 12 m, 9.46 m², 74.3 kg;
• 160 × 310 mm: the mezzanine crossing, 4 m, 3.76 m², 29.5 kg.

The elbows add their own sheet. A 90° elbow with a centreline radius of 1.5 diameters has a centreline length of π ÷ 2 × 1.5 × D, so each 140 mm elbow is about 330 mm long on its centreline and each 250 mm elbow about 589 mm. EX-1’s four 140 mm elbows and two 250 mm elbows add about 1.51 m² and 11.0 kg.

That is about 24.7 m² and 183 kg before joints, flanges, the junctions’ fittings, the transitions and the access doors. An allowance of 15% for those, in this illustration, brings the total to about 28.4 m² and 211 kg. A detailed take-off would list each of them instead of using an allowance, and a quotation for a fixed price should.

Figure 2EX-1's ductwork mass by section

1. 140 mm duct38.9 kg · 18%
2. 200 mm duct29.7 kg · 14%
3. 250 mm duct74.3 kg · 35%
4. Crossing rectangle29.5 kg · 14%
5. Elbows11.0 kg · 5%
6. Joints, fittings and doors, 15%27.5 kg · 13%

Joints and seams

Each length of duct joins the next, and the joint adds material: a slip joint overlaps the next length; a flanged joint adds two flanges and a gasket; a spiral duct’s seam is a lock with several layers of sheet. Each also has a longitudinal or spiral seam of its own, which uses a little extra sheet beyond the circumference. Those allowances depend on how the duct is made, and a fabricator’s own figures for its methods are better than a general percentage. The point for the take-off is that they exist, and that a take-off at the bare circumference understates the sheet.

Thickness and material

Mass is proportional to thickness, so a change of gauge changes the take-off directly. This is an illustration. If the 250 mm main and stack were made in 1.2 mm sheet instead of 1.0 mm, for wear resistance, their mass would rise from 6.19 to about 7.43 kg/m, and the 12 m of 250 mm duct from 74.3 kg to about 89 kg, some 15 kg more on the hangers and in the quotation.

Material changes mass more. The same 250 mm duct in 1.0 mm aluminium, at about 2,700 kg/m³, would weigh about 2.13 kg/m; in 1.0 mm stainless steel, at about 7,900 kg/m³, about 6.23 kg/m, almost the same as galvanised steel. Stainless is chosen for corrosive or hygienic duties, not for weight; aluminium for light, non-abrasive duties. A dust system carrying abrasive grinding dust, like EX-1, is usually galvanised or mild steel, with heavier gauges at the wear points.

Access doors and cleanouts

A dust duct needs access doors where dust is most likely to collect and where a blockage is most likely to form: near elbows, downstream of expansions, at the low points of horizontal runs and at intervals along long straight runs. Each door is a fitting that adds sheet, a frame, a seal and a few minutes of fitting, and it adds a small leak path if its seal is poor. This is an illustration. EX-1 has six access doors, one near each of its six elbows, and one more at each end of the mezzanine crossing, eight in all, which are part of the 15% allowance in the take-off but should appear as their own line in a detailed estimate.

Supports

Horizontal ducts hang from the structure on hangers, at a spacing set by the duct’s stiffness, its joints and the standard or specification it is built to. The load on each hanger is the mass of the duct between hangers, plus anything it may carry.

This is an illustration. At a hanger spacing of 3 m, each hanger on the 250 mm main carries 3 × 6.19 = about 18.6 kg of clean duct. The hanger, its fixing and the structure it hangs from must carry that load with the safety factors their own design requires, which is a structural matter beyond this book.

The error: hangers for clean duct

This is an illustration. EX-1’s hangers on the 250 mm main were first selected for the clean duct’s 18.6 kg per hanger. The design review asked what the duct would weigh if it partly filled with dust, as it might if the fan stopped with the duct loaded, or if a blockage formed. The designer’s allowance was a duct filled to a quarter of its depth with dust of a bulk density of 800 kg/m³, in this illustration.

A 250 mm duct filled to 62.5 mm, a quarter of its depth, holds a circular segment of about 0.0096 m² per metre, by the segment formula of Tanks and Hoppers by the Numbers, and at 800 kg/m³ that is about 7.7 kg/m of dust. The duct and its dust then weigh about 6.19 + 7.68 = 13.9 kg/m, and each hanger carries about 41.6 kg, 2.24 times the clean figure. The first selection would have been overloaded at the very moment the duct was hardest to reach and clear.

Figure 3Load per hanger on the 250 mm main at 3 m spacing

Clean duct18.6 kg

Duct a quarter full of dust41.6 kg

The fix was to reselect the hangers for the loaded case and to state the dust allowance on the drawing, so that whoever checks or changes the supports later knows what they were designed to carry.

Vertical ducts and the stack

Vertical ducts are supported at intervals by brackets that carry the whole weight above them, and the stack, standing outside, also carries wind. This is an illustration. EX-1’s 6 m stack of 250 mm duct weighs about 6 × 6.19 = 37 kg, carried by a bracket at the roof and guyed or braced above it. Its wind loading, like that of any structure on a roof, is assessed to AS/NZS 1170.2 by whoever designs its supports; the geometry’s contribution is the stack’s mass, height and diameter.

Cost

This is an illustration, in Australian dollars excluding GST. At A$4.20 per kilogram for galvanised sheet, EX-1’s 211 kg of ductwork costs about A$886 in material. Fabrication and installation usually cost several times the sheet: cutting, rolling, seaming, forming elbows and junctions, fitting, sealing and hanging. A fabricator’s labour estimate is best built from its own records for each kind of piece, so many minutes per metre of straight duct of each size, so many per elbow, per junction, per transition, per hanger, rather than as a multiple of the sheet. The take-off of this chapter gives the quantities that such an estimate needs.

This is an illustration of how such an estimate is built, with times invented for the example rather than taken from any fabricator’s records:

• 32 m of round straight duct at 0.4 h per metre, made and installed: 12.8 h;
• 4 m of rectangular crossing at 0.8 h per metre: 3.2 h;
• six elbows at 1.5 h each: 9 h;
• two junctions at 1.5 h each: 3 h;
• two transitions at 1.0 h each: 2 h;
• eight access doors at 0.5 h each: 4 h;
• twelve hangers at 0.5 h each: 6 h.

That totals 40 h, about A$3,800 at A$95 an hour excluding GST, more than four times the sheet. The elbows, junctions and transitions, ten pieces in all, take 14 h, almost as much as the 16 h for all 36 m of straight duct, which is the usual pattern and the reason a take-off that counts fittings carefully prices a job far better than one that counts metres.

Hangers and fittings

Hangers belong near joints, where a duct is weakest, and on both sides of heavy or awkward fittings. Fittings that carry concentrated loads or forces, such as blast gates, orifice plate flanges, access door frames and the connection to the collector, are best supported on their own hanger so that the duct’s sheet does not carry them. Flexible connections, like the short hose at B1, carry no load at all: the duct and the hood on either side must each be supported independently, or the hose will stretch and leak.

Spiral and rolled duct

Round duct is made in two main ways. Spiral duct is wound from a strip with a continuous helical lock seam, in long lengths, on a machine; it is stiff, consistent and economical for straight runs. Rolled duct is rolled from a flat blank with a single longitudinal seam, in lengths limited by the sheet, and suits short runs, odd sizes and heavier gauges. Both have the same circumference for the same diameter, so the take-off’s area is the same, but spiral duct’s lock seam uses a little more strip than its bare circumference, and the spiral manufacturer’s mass per metre, which includes it, should be used in the take-off whenever it is available.

From take-off to quotation

A take-off becomes a quotation when every quantity has a price, and the commonest errors are omissions rather than wrong prices.

Figure 4Items a ductwork take-off should list

• Do: Straight duct by size and gaugeMetres and kilograms
• Do: Elbows, junctions and transitionsCount by type and size
• Do: Joints, flanges and gasketsCount by size
• Do: Access doorsCount and size
• Do: Hangers and bracketsCount, with the load each is selected for
• Do: Orifice plates and blast gatesCount and size
• Do: Sealant and fixingsBy length of joint
• Avoid: Sheet priced on bare circumferenceUnderstates every line
• Ask: Fan inlet and outlet piecesOften on the fan supplier's scope; confirm

Weight for handling

The mass of each piece also matters on site. The heaviest single pieces in EX-1 are the 250 mm lengths and the crossing rectangle, each a few tens of kilograms, which two people can handle at height on a scissor lift but not one person on a ladder. A take-off that lists each piece’s mass lets the installer plan the lifts, and work health and safety law requires that manual handling risks be assessed and controlled, which starts with knowing what each piece weighs.

Exercise: the 200 mm section’s hangers

At the same 3 m spacing and the same dust allowance, a quarter of the depth at 800 kg/m³, what load does each hanger on the 200 mm section from N2 to N3 carry, clean and loaded?

Worked answer. Clean, the 200 mm duct weighs 4.96 kg/m, so each hanger carries 3 × 4.96 = 14.9 kg. Filled to a quarter of its 200 mm depth, 50 mm, the duct holds a segment of about 0.0061 m² per metre, so about 4.9 kg/m of dust at 800 kg/m³. The loaded duct weighs about 4.96 + 4.91 = 9.9 kg/m, and each hanger carries about 29.6 kg, roughly twice the clean figure.

The ratio of loaded to clean load is smaller than for the 250 mm main, 2.0 against 2.24, because a duct’s dust capacity grows with the square of its diameter while its sheet grows only with the diameter. Larger ducts are proportionally more affected by a dust allowance, which is why the largest ducts’ supports are the ones most often found undersized.