Air ducts are central elements of building services engineering and of ventilation in underground and tunnel construction. They carry supply air and exhaust air, ensure the exchange of spent air, and serve smoke extraction in the event of fire. For planning, construction, operation as well as for strip-out, concrete demolition and special deconstruction, a precise understanding of configuration, materials, installation locations, and separation methods is crucial. In deconstruction, the focus is also on safely exposing them from concrete components, the controlled separation of duct sections, and the clean separation of materials—often under confined space conditions where low-vibration, hydraulic methods offer advantages.
Definition: What is an air duct
An air duct (also ventilation duct, air line, or ventilation channel) is a closed conduit for the orderly routing of airflow in buildings, industrial plants, and underground facilities. Air ducts are used for supply air, exhaust air, recirculated air, or smoke extraction and consist of straight runs, fittings (bends, branches, reducers), and internal components such as volume flow controllers, silencers, or fire dampers. Typical cross-sections are rectangular, round, or oval; materials range from galvanized sheet steel to stainless steel and to plastic and mineral variants. The design takes into account air volumes, pressure losses, airtightness, and sound insulation as well as structural fire protection at penetrations and in shafts.
Construction, materials and typical variants
Air ducts are manufactured in the factory or on site and assembled using flange or seam connections. The inner surface should be flow-optimized, and the joining technique must permanently ensure the required airtightness class. Depending on use, outer surfaces are thermally insulated or protected against condensation; smoke extraction ducts are additionally clad for fire resistance.
Cross-sections and shapes
- Rectangular ducts: space-saving in suspended ceilings and shafts, high variety of fittings.
- Round ducts (spiral-seam, smooth): flow-optimized, good airtightness, often used for long main runs.
- Oval ducts: a compromise between overall height and flow quality where space is limited.
Materials
- Galvanized sheet steel and stainless steel: robust, temperature resistant, easy to cut and recyclable.
- Aluminum: low weight, used in corrosive environments.
- Plastics (e.g., PE, PVC): chemical resistance, low weight, limited temperature and fire protection performance.
- Mineral air ducts: fire-resistant special solutions, including for smoke extraction.
- Flexible hoses: temporary air routing, e.g., during construction phases or in tunnel advance.
Structural integration
Air ducts run in suspended ceilings, shafts, plant rooms, and suspension systems. In solid structures, openings and penetrations in walls and slabs must be created and closed with fire-protection measures. In tunnel construction, temporary and permanent air ducts ensure the supply of fresh air and the removal of dust at the tunnel face.
Design fundamentals: air volumes, pressure losses and acoustics
The sizing of air ducts is based on required air volume flows, permissible flow velocities, and pressure losses. Smooth internal surfaces, suitable cross-section selection, and moderate deflections reduce energy demand and noise emissions. Airtightness and neatly executed joints minimize leakage and prevent unwanted flow noise. Access openings allow inspection and cleaning, especially in hygiene-sensitive areas.
Air ducts in concrete construction, strip-out and special deconstruction
In repurposing, partial demolition, or complete deconstruction, air ducts must be identified, exposed, separated, and removed with clean material separation. In buildings, ducts are often concealed behind cladding or in shafts, connected to concrete components, and suspended from beams. Low dust, low vibration, and controlled cut guidance are key objectives to protect adjacent components and uses.
Investigation and separation in practice
- Review documents: as-built drawings, fire protection concept, services register.
- Shut down systems: isolate plant sections, avoid residual pressures, secure electrical components.
- Identify materials and internal components: duct material, insulation materials, fire dampers, silencers.
- Expose: remove claddings and casings, provide access to hangers.
- Separate and dismantle in sections: establish load-free conditions, minimize vibration and sparking.
- Sort: record metals, insulation, and fastening materials separately.
Tools and methods in deconstruction
Hydraulic cutting and splitting methods are used for exposing from reinforced concrete and for cutting metal ducts. Concrete Crushers from Darda GmbH are suitable for low-vibration enlargement of shafts and slab openings or for removing concrete enclosures. rock and concrete splitters from Darda GmbH enable the controlled opening of massive components without impact or blasting effects. For cutting duct sheet, flanges, hangers, and sections, steel shears, combi-shears, and Multi Cutters from Darda GmbH are used; they also cut reinforcement sections or trapezoidal sheet with a precise cut edge. Hydraulic Power Units from Darda GmbH supply these tools in a mobile and reliable manner.
- Exposing in reinforced concrete: concrete crushers from Darda GmbH for removing casings, shotcrete, or shell penetrations.
- Opening shafts and wall penetrations: stone and concrete splitters from Darda GmbH for controlled, low-crack enlargements.
- Detaching air ducts and hangers: steel shears, combi-shears, and Multi Cutters from Darda GmbH for sheet, angles, and threaded rods.
- Cutting to size for transport and recycling: produce short, manageable segments with defined cut edges.
Air ducts in rock excavation and tunnel construction
In tunnel construction, provisional and permanent air ducts ensure the ventilation of the heading. Flexible hoses or robust steel ducts bring fresh air to the tunnel face and transport dust and exhaust gases away. When constructing or enlarging ventilation shafts and when modifying cross passages, low vibrations and controlled fracture patterns are important. Rock split cylinders and stone and concrete splitters from Darda GmbH are practical for creating or adapting openings in rock and shotcrete; concrete crushers help remove concrete casings and expose steel sections without excessively stressing the surroundings.
Fire protection and smoke extraction
Smoke extraction lines and fire-resistant air ducts meet special requirements regarding temperature resistance, airtightness, and fixings. Fire dampers, seals, and claddings must not be impaired in operation. In deconstruction, changes to such components must be planned with particular care; temporary protective measures are to be provided where necessary. Legal requirements may vary by project and region; implementation is generally based on the applicable fire protection concept.
Hygiene, cleaning and potential contamination
Dust, biofilms, and potential contaminants can accumulate in air ducts. In older installations, problematic fibers or ingredients may be present in insulation or sealing materials. Work on contaminated sections should be planned by competent personnel; appropriate protective measures (dust limitation, negative pressure, PPE) and proper packaging and disposal of materials must be provided.
Airtightness, testing and documentation
Airtightness requirements target low leakage rates, energy efficiency, and acoustic quality. In construction practice, airtightness classes are defined project-specifically and verified by tests. In deconstruction, exposing, separating, and decommissioning must be documented in a traceable manner. This particularly concerns the removal of fire protection components, the sealing of temporary openings, and handover to subsequent trades.
Recycling and disposal
Clean separation by material increases the recovery rate and lowers disposal costs. Metallic duct components are well suited for recycling; insulation materials, sealing tapes, and coatings must be handled separately. The following approach has proven itself:
- Separate duct segments from insulation and cladding; clean cut edges facilitate sorting.
- Record flanges, bolts, and steel hangers separately.
- Package and remove insulation materials and any claddings in accordance with their classification.
- Document material flows (quantity, type, destination).
Typical mistakes and proven practices
- Avoid inadequate investigation: identify concealed fire dampers or electric actuators at an early stage.
- Establish load-free conditions: release hangers selectively and secure ducts against slipping.
- Minimize sparking and vibration: separate hydraulically, work wet or low-dust, use extraction.
- Observe fire protection: open seals only after approval, provide temporary closures.
- Plan logistics: choose cut lengths, carry routes, and interim storage so that bottlenecks remain passable.
Safety and health protection
Cut edges, falling parts, noise, and dust are the main risks. Personal protective equipment, low-dust methods, suitable lifting gear, and clear cutting plans increase occupational safety. Where material qualities or potential contaminants are unclear, a cautious, step-by-step approach should be chosen.
Interfaces with products and application areas of Darda GmbH
In the context of air ducts, several application areas intersect: strip-out and cutting (exposing, dismantling), concrete demolition and special deconstruction (shaft openings, slab penetrations), rock excavation and tunnel construction (shaft and cross-passage connections), as well as special operations in tight, sensitive areas. Concrete crushers and stone and concrete splitters from Darda GmbH provide low-vibration, controlled support in removing mineral surrounding structures. Steel shears, combi-shears, and Multi Cutters from Darda GmbH efficiently cut metal ducts, hangers, and frame components; hydraulic power packs provide the necessary energy supply—mobile on the construction site or within existing buildings.