A ventilation air duct is the basic element of the ventilation system. Ventilation ducts are passages used to transport air – supply fresh outside air and remove stale indoor air. Ventilation ductwork can be divided according to its shape, namely, circular, rectangular and oval (a compromise between rectangular and round ventilation ducts).
It can be also distinguished according to the material from which it’s made, e.g. rigid spiral pipe is usually made of galvanized steel sheet; flexible ducts often made of aluminium, polyester foil, etc. By far the most popular ductwork types used for both residential and industrial ventilation systems are rigid spiral ducting and rectangular ducting.
What is a spiral duct?
Spiral duct is a rigid spirally wound tube manufactured of steel metal, most often galvanised steel, but also: stainless steel, aluminium or copper. Spiral ducting has female ends, therefore in order to connect two duct sections a male coupler needs to be used as a connection piece.
Of course, spiral ductwork is more than just pipe and fittings, a complete ventilation system, depending on the project, may consist of the following components:
Spiral ducts are made of 137 mm and 140 mm wide spirally rolled steel sheet metal, using so-called tubeformers, i.e. machines for spiral air ducts production. It is important for the duct manufacturer to buy these metal strips or cut them yourself so that they have an exact dimension tolerance. Otherwise, even a small margin of error can cause the spiral duct machine to close the lockseam incorrectly. The machine that cuts large steel coils, e.g. with a width of 1250-1500 mm into different narrower steel strips is called a slitting machine.
The most popular spiral duct machines known in the HVAC industry are SPIRO International machines - tube formers. Therefore, since SPIRO tubeformers are incredibly acknowledged brand among spiral duct manufacturers, many HVAC professionals refer to spiral ducts as simply spiro ducts.
Watch a short video of spiral pipe production LINK
Spiral pipe making machines that are available on the market allow the production of ducts up to 2500 mm. However, such large diameters of circular ventilation ducts are almost never used. When the larger diameters are needed, we typically choose rectangular ducts. Therefore, most spiral duct manufacturers, including us, have forming heads up to 1600mm and offer spiral ducting from 80mm to 1600mm.
Tubeformers are machines used for spiral duct production
As we've already mentioned spiral ducts as a standard are made of galvanised steel sheet, galvanised ducting is a default not only for us but most of the duct manufacturers. You could ask, but what type? In general, spiral ductwork manufacturers use several types of steel grades for their production. The most common steel grades are DX51D (primarily used for spiral ducts), DX53D and DX54D. In short, the difference between these steel grades is flexibility or formability during production. What's more, it’s a hot-dip galvanised steel with a zinc coating that protects against corrosion.
The zinc coating (Z) is shown in grams per square meter (g/m2) on both sides. In order to visualize the coating thickness, You can see below an example for the amount of zinc layer in g/m2 and approximately, the minimum thickness of these zinc coatings in brackets:
As a fellow manufacturer, wholesaler or duct fitter You can probably recognize such coating designations as Z100, Z140 and Z275. Galvanised steel Z275 is said to be a good market standard when it comes to spiral ducts, although Z140 and Z100 are also used.
Another duct parameter that You should consider when selecting the ducting type for the specific project is its corrosivity class. You need to determine which corrosivity category applies to a given facility [environment] and check, according to the standard BS EN ISO 12944-2 (see table below), what thickness loss will occur in time. The standard defines six classes: C1, C2, C3, C4 and C5. 
This loss can be very different. Depending on the corrosion rate, how aggressive the environment is, it may be slower or faster. What's why choosing the correct coating thickness is essential for ventilation ductwork life expectancy. You can calculate in years when the chosen coating in a specific corrosive environment can simply end.
The ducts are sold is 3 m lengths as a standard, though it’s possible to manufacture circular ducts of any length from 0.5 to 12 m with a tolerance of up to 2 mm.
Spiral ductwork is available in diameters from 80 mm to 1600 mm – and most of the standard diameters is available from stock. The duct wall thickness depends on the diameter and the required pressure in the ductwork. Spiral ducts are manufactured from 0.4 mm to 1.8 mm thick metal sheet - where the material used is determined by the pressure and mechanical requirements of the system - and the price. 
The spiral ducts in diameters of 160 mm or greater are also available with corrugation for improved stiffness and vacuum strength. The corrugations are available as a standard feature for diameters of 250 mm to 1600 mm. To “relieve” duct suspension components which both fasten the ductwork as well as maintaining its rigidity, it is recommended to use ducts with additional stiffening corrugations while reducing the wall thickness. 
Example of additional, stiffening corrugations on the spiral duct
The growing trend of energy efficiency, influences also the increase in demand for airtight ventilation systems, in order to limit to the minimum any leakages. Therefore if our project requires the highest airtightness class, we need to look for the ventilation systems with as little duct connections, joints as possible as well as being equipped with a highly efficient sealing method. In that case of ductwork airtightness, circular ducts are far superior to rectangular ones.
When compared to rectangular ductwork, spiral ducting has significantly fewer "duct-to-duct connections" which reduces the risk of leakage, the standard length of a spiral duct is 3 m, although we can easily manufacture ducts 6 meter long and even up to 12 m, while the typical length of a rectangular duct is from 1 to 1.5 m.
As for the sealing method, lockseam which is the overlap of metal sheet that the spiral duct machine makes should be airtight, as long as the production processes are correct. However, this lock seam can be additionally sealed, if necessary. The spiral pipe’s lock seam can be sealed with a sealing compound, e.g. an EPDM seal or high-temperature silicone. Yet, such additional sealing could be required for a very specific installation, e.g. one of our clients used spiral ductwork system with NBR seals for oil mist applications.
SPIRAL® system components are tested for their airtightness using the above diagram
Alnor's ventilation spiral duct system is manufactured in two air tightness classes: B or D according to EN 12237: 2005.
In airtightness class B, spiral ducts and duct fittings are connected with each other by means of slide-in connections, protected against disconnection by means of sheet metal screws or rivets and sealed with a self-adhesive sealing tape made of polyethylene (PE) covered with fabric, 50 + 100 mm wide.
In airtightness class D, spiral ducts and duct fittings are connected with each other by means of slide-in connections, protected against disconnection with self-tapping screws or tight rivets. Duct fittings are factory-equipped with EPDM rubber seals, protecting the duct connections from leakages.
The question remains: Is good quality spiral ductwork enough to meet high airtightness standards? In short, unfortunately, it doesn't. In order to reach airtightness class D, we need to work us a team, meaning a duct manufacturer or a wholesaler must deliver high-quality ducts and fittings with seals that ensure airtight joints. Next, these components must be safely delivered and stored to avoid the risk of damages e.g. breaking of the seals. Finally, when the time for the ductwork assembly comes, the on-site installation must be conducted according to the best standards.
As a reference, You can check Alnor's Good Practices for Assembly, Transport and Storage of SPIRAL ductwork.