A few years ago I supplied a rectangular system for a plating plant: 1,800 × 700 mm main, 1.0 mm galvanised, 850 Pa static at the fan. The shop formed TDF flanges on the main because that is what the line does, and the first section looked beautiful on the trestles. Three days after commissioning, the maintenance fitter sent me a photograph of a transverse joint with the gasket squeezed out along one side and a 4 mm gap at the corner. The corner insert had not failed. The flange leg had bent.
That is the honest limit of an integral flange. Not the sealant, not the corner, but the 20 mm leg of 1.0 mm sheet that has to carry the bolt load across a 1.8 metre span. Once I understood it that way, deciding between TDF, TDC and a welded angle frame stopped being a matter of habit and became arithmetic.
Three things, in this order.
The flange leg bends. A TDF leg is a folded strip of the duct material, typically 20 or 30 mm high. Its resistance to the bolt clamping force comes from that short fold, and the force is not small: an M8 bolt torqued by hand to a sensible 12 to 15 N·m pulls roughly 3 to 4 kN. Across a 1,800 mm side with bolts at 150 mm you have twelve bolts, but the sheet between them is what has to stay flat. On 0.8 mm material the leg starts to yield visibly at around 700 to 900 Pa when the duct is also being pulled by the system pressure.
The corner insert rocks. A corner piece is only as good as the pocket it sits in. When the leg deflects, the pocket opens and the insert loses its interference fit, which is exactly when the joint starts to leak rather than merely look tired.
The gasket compresses unevenly. A compressed EPDM gasket wants about 30 percent deflection to seal. If the flange is not flat, you get 50 percent in the middle and almost nothing at the corner, and the leak appears at the exact place where two flange legs meet, which is also the hardest place to reach with a sealant gun after the fact.
For a shop with a standard TDF flange forming machine and TDC capability, my working rule is that integral flanges are comfortable up to about 1,200 mm on the long side at 500 Pa, and up to 1,000 mm at 750 Pa. Above 1,500 mm on any side, or above 1,000 Pa, I move the main runs to welded angle frames and keep integral flanges for the branches. This is not a code statement, it is where my failure records sit, and it lines up with the reinforcement spacing tables in most national codes once you include the flange as part of the stiffening.
Angle iron flanges are sold by section: 25 × 3, 30 × 3, 40 × 4, 50 × 5, 63 × 6. The weight per metre is easy to calculate and worth knowing because it drives both cost and handling.
| Section | Weight, kg/m | Bolt size | Duct side, mm | Typical pressure | Comment |
|---|---|---|---|---|---|
| L25 × 3 | 1.11 | M6 | up to 800 | up to 500 Pa | Rarely worth it over TDF |
| L30 × 3 | 1.34 | M8 | 800 to 1,200 | 500 to 750 Pa | The workhorse for retrofit flanges |
| L40 × 4 | 2.39 | M8 / M10 | 1,200 to 1,600 | 750 to 1,000 Pa | Needs a punched hole line to be economic |
| L50 × 5 | 3.73 | M10 | 1,600 to 2,000 | 1,000 to 1,500 Pa | Weld distortion becomes the main problem |
| L63 × 6 | 5.72 | M10 / M12 | 2,000 and above | 1,500 Pa and above | Usually with intermediate stiffeners |
Bolt spacing is 150 mm as a default and 100 mm on the first and last 300 mm of each side, because the corners see the highest load. For 1,000 Pa and above, drop the whole side to 120 mm. Anyone who tells you 200 mm is fine has not had to reseal a joint at height.
There are two routes and they suit different shops.
Cut four lengths at 45 degrees, tack, check the diagonals, weld, redrill or punch the holes, and touch up the zinc. On a 2,000 × 1,000 frame the diagonal check is the step people skip: a 3 mm difference between diagonals is a frame that will fight every joint downstream, and it takes twenty seconds with a tape to catch.
Welding is where quality drops in a hurry. A 3 mm fillet with MIG at 120 to 140 A is enough for a 4 mm angle; running a fat 6 mm bead to look strong puts so much heat into the leg that the frame bows 2 to 4 mm over 2 metres, and then the gasket does the work of hiding it. Weld two opposite corners, let the frame cool, then the other two.
Galvanised angle loses its coating 30 to 50 mm either side of every weld. Cold zinc-rich paint at 60 to 80 µm dry film is acceptable in most indoor work and will not survive a coastal plant for long, which is worth telling the client before they discover it in year three.
For round duct and spiral connections, the flange is a rolled angle ring. A section bender does this in one pass and produces a far better ring than any welded polygon. The machines I know best take L30 × 3 up to L40 × 4 on a 3 kW drive with a minimum ring diameter of 350 mm, and the bigger hydraulic units go to L100 × 10 with a 600 to 650 mm minimum diameter, which is where you need them for large spiral mains and for tank work.
Two settings decide whether the ring fits: the roller gap, which should be a light drag on the angle rather than a squeeze, and the curvature, which on a hydraulic machine with a grating scale can be set and repeated. A ring that comes out 4 mm under size will not go over the duct end without a hammer, and a ring 6 mm over size leaks at the overlap. I aim for 1.5 mm over nominal bore and let the clamp do the rest. If you are setting up this kind of work from scratch, the specifications on a hydraulic angle iron bending machine are a reasonable reference for what each frame size can actually roll.
Take a 2,000 × 1,000 duct, perimeter 6.0 m, 1.0 mm galvanised, 1,000 Pa. Sheet weight is about 7.85 kg per square metre, so a 1.5 m section carries roughly 70 kg of steel in the duct itself: about 40 to 50 US dollars of material at current coil prices.
An L40 × 4 frame for that perimeter needs 6.0 m of angle plus 0.2 m waste, which is 14.8 kg at roughly 1.00 to 1.20 USD/kg in most export markets: 15 to 18 dollars. Add four mitre cuts and welds at 3 minutes each, punching 44 holes, and a gasket: another 6 to 8 dollars of labour and consumables at a 15 USD/h burdened rate. Call it 22 to 26 dollars per joint, or roughly 4 dollars per metre of perimeter.
The same joint in TDF costs four corner inserts at 8 to 12 cents, a bead of mastic, and about 3 minutes of labour: under 2 dollars. So the angle frame is ten times the joint cost, but only about 35 percent of the value of the steel in that section, and it is the difference between a system that holds 1,000 Pa and one that does not. That is the number to put in front of a client who asks why the main is not TDF like the branches.
| TDF (integral, 30 mm) | TDC (integral, grooved) | Welded angle frame | |
|---|---|---|---|
| Material added per joint | None, formed from the blank | None, formed from the blank | 6 to 15 kg of angle |
| Labour per joint | 3 to 4 min | 3 to 4 min plus fastener | 12 to 20 min including welding |
| Practical duct size | to 1,200 mm at 500 Pa | to 1,500 mm at 750 Pa | unlimited with stiffeners |
| Gauge range | 0.5 to 1.2 mm | 0.6 to 1.2 mm | 0.6 to 2.0 mm, any |
| Leakage behaviour | good when new, degrades if leg deflects | good, groove holds gasket better | best, stiff face for the gasket |
| Site assembly | insert corners, clamp | insert corners, power fastener | bolted, needs access all round |
| Where it wins | branches, small plant rooms, fast jobs | medium mains, exposed ductwork | large mains, high pressure, plant rooms |
Notice that TDC sits between the two and that its advantage is not stiffness but gasket retention: the groove gives the gasket a home, so it does not extrude when the joint is pulled. On 1,200 to 1,500 mm duct at 750 Pa, TDC is often the cheapest thing that works, and it can be formed on the same line as TDF if the tooling is there. If you are comparing that option, the setup differences between a plain and a grooved TDC flange forming machine are worth understanding before you order tooling for a market you have not built for yet.
Four things separate a good bolted joint from a bad one.
Gasket. 3 mm EPDM with adhesive backing, continuous around the frame, mitred at the corners with no gap. 5 mm is for rough or warped frames and it costs you bolt preload. Neoprene hardens in hot plant rooms; EPDM does not.
Bolt sequence. Start at the centre of each side and work outwards, then go round again at full torque. Doing it corner-first traps the gasket at the middle and the joint leaks there for the rest of its life. Torque for M8 is 12 to 15 N·m, for M10 20 to 25 N·m. A ratchet with a mark on the handle is enough; a torque wrench is better if the crew will actually use it.
Alignment. The two frames must be parallel within 1 mm over the joint face. On a 2,000 mm side that usually means a hanger within 300 mm of each flange, and it means the duct is not being pulled into line by the bolts. If you need a clamp to close a 5 mm gap, the run is out of line and the gasket will never seal.
Support. A 6 m perimeter angle frame with duct attached weighs 60 to 90 kg. Hanger spacing of 2.5 m is normal for the duct, but put one within 600 mm of every flange, and use a trapeze rather than a single drop rod on anything over 1,200 mm wide.
The most practical arrangement on a real job is a hybrid: angle frames on the plant room mains and risers, TDC on the medium runs, TDF on branches and terminal connections. Two rules keep that clean. First, never change flange system at a joint that will be assembled at height, because the transition piece is where mistakes happen. Second, order the transition spools from the shop with both flange types formed on the same blank, so the joint count does not go up.
The other thing worth doing is a trial fit in the shop. Assemble one length of each type, bolt it up, and look at the gasket line with a torch. Twenty minutes in the shop saves a day of resealing on site, and it is the cheapest quality control available to a fabricator.
Can I put an angle frame on duct formed on an auto line? Yes, and it is common. The line cuts the blank and forms the lock seam and bead; the shop then welds or rivets the angle frame to the duct end instead of forming an integral flange. You lose the speed advantage of the line at that end, which is why the decision is usually made per diameter rather than per job.
Rivets or bolts on the frame? Rivets are faster and fine for indoor low-pressure work if you use 4.8 mm steel rivets at 75 mm spacing. For 750 Pa and above, or anywhere the joint will be opened for maintenance, use bolts: a riveted joint cannot be re-tightened, and gaskets do take a set over five years.
How much does a rolled angle ring spring back? On L30 × 3 at a 400 mm diameter, expect 2 to 4 mm of diameter growth when the rollers release. Roll 1 to 2 percent under the target and check the first ring before running a batch. On the larger sections the springback is proportionally smaller but the machine load is much higher, so check the drive capacity against the section before promising a size.
Is a welded angle frame worth it below 800 mm? Usually not. At that size a TDF or TDC joint is stiff enough, faster, and cheaper, and the extra 20 minutes of welding per joint is time you will not get back. Reserve angle frames for the sizes and pressures where the integral flange genuinely runs out of stiffness, and the rest of the shop will be more productive for it.