Ductwork Fabrication and Installation

What problem it solves

The ductwork is the part of a ventilation system you do not see, yet it determines performance more than anything else. However well the fan is selected, if the duct is sized wrongly the system cannot deliver its design airflow. While carrying air from one point to another, a duct has to do three things at once: pass the target airflow, stay within an acceptable pressure drop and not generate noise.

A smaller duct section makes the material cheaper, but the air velocity increases. As velocity rises, pressure drop grows with the square of the velocity; the fan has to overcome this resistance for its whole life. So an initial saving on the ducts comes back as operating cost, with interest. Choosing the section is a calculation that balances investment against operating cost.

Fabrication types

Rectangular galvanised duct

Preferred where height is limited, inside suspended ceilings and in narrow passages. Sheet thickness is chosen according to the largest side of the duct and the operating pressure. On large sections, cross-breaking or stiffening profiles are used to stop the sheet from drumming.

Joints are flanged; a gasket is placed between the flanges and they are bolted together. Airtightness is decided here: a leaking duct never delivers part of the calculated airflow to its destination, and this loss is invisible on site — it only shows up in measurement.

Spiral round duct

For the same cross-sectional area it gives lower pressure drop and better airtightness. A round section naturally resists pressure and needs no stiffening. It is also preferred for its appearance in exposed installations.

Joints are made with gasketed couplings. Bends, reducers and tees are supplied as standard fittings or made to measure.

Special fittings

Every site has a point that does not quite match the drawings: a transition around a column, a run that has to pass under an existing beam, a junction between two ducts of different sizes. These parts are made to measure. This is what fabricating in our own workshop means on site: the measurement is taken, the part is made the same week and installation does not wait.

Insulation

Duct insulation serves three different purposes, and which one applies changes the choice of material:

1. Heat loss/gain. Reduces energy loss on ducts carrying conditioned air.
2. Condensation. Prevents sweating on the outer surface of ducts carrying cooled air; the vapour barrier is critical here.
3. Sound. Internal acoustic lining reduces fan noise carried into the room. In hygienic areas, faced materials that do not shed fibres are used.

Installation

Installation starts with setting up the support system correctly. Hanger spacing is set according to duct section and weight; insufficient support leads to sagging and joints opening over time. Flexible connections at fan outlets and anti-vibration mounts on hangers keep vibration out of the structure.

Site conditions set the installation plan. In facilities where production does not stop, the system is divided into sections and each section is brought into service during planned shutdowns. In high-ceiling areas, platforms and lifting equipment are planned in advance; this directly affects installation time.

Handover and airtightness

Once fabrication and installation are complete, the system is checked for airtightness and airflows are measured and balanced. Results are recorded grille by grille. This record documents that the system meets its design values and is the reference for any future revision.

Section selection and velocity

When choosing a duct section, the air velocity inside it is decisive. Velocity affects three things at once: pressure drop, noise and the space the duct occupies.

Higher velocities are used in main ducts and lower velocities in branches approaching the room. If velocity is not kept low near the grille, supply noise is heard in the room. In spaces that need to be quiet, such as offices and laboratories, this limit is set even lower; in production areas, where background noise is already high, higher velocities can be allowed.

In rectangular ducts, the aspect ratio also matters. The same cross-sectional area can be achieved with a very flat rectangle, but the perimeter grows; the friction surface increases, so pressure drop rises and more sheet is used. A ratio close to square is preferred; if height is restricted, flatness is accepted — but with its cost known.

Revisions and extensions

Facilities change: a new machine arrives, an area grows, a line is removed. When extending existing ductwork, the most common mistake is not calculating the effect of the new branch on the rest of the system.

When a new branch is added to a main duct, the total resistance changes; the fan's operating point shifts and airflow in the existing branches drops. The result is weaker ventilation in the old areas while the new area works. That is why every extension starts with measuring the existing system: what the fan delivers, what each line draws, and where the balance will shift when the new branch is added.

In most cases the answer is not a new fan. Rebalancing the branches, enlarging the section of one branch or shifting the fan's operating point with a speed controller is enough.

Materials and durability

Galvanised sheet is used in standard applications. If the environment is different, so is the material:

• Humid and chemical environments: stainless steel or specially coated material
• High-temperature process exhaust: heavier gauge sheet and heat-resistant gaskets
• Lines carrying abrasive dust: heavy gauge sheet or wear plates at bends and impact points
• Hygienic areas: cleanable internal surfaces and sealed joints

If the material is not chosen correctly from the start, a duct line that corrodes through within a few years both stops production and brings the cost of fabricating it again.

What made-to-measure fabrication means on site

When working with standard-size ducts, the site always throws up fit problems: a beam is lower than expected, a column is not where the drawing shows it, an existing service crosses the route. This is where made-to-measure fabrication comes in; the part is produced to the dimension taken on site and installation is completed the same week.

How long ductwork lasts

A galvanised duct system made from the right material, installed on proper supports and suited to its environment can serve for the life of the building. Ducts are not worn out by time but by conditions: condensate, abrasive dust, chemical vapour and stress caused by insufficient support.

That is why periodic inspection of the ductwork is as important as the fan and filters. When hanger fixings, flange gaskets and insulation integrity are checked regularly, the ductwork does not stop production with an unexpected failure.