Round Duct Joints

Round duct joints utilize the inherent structural advantages of circular geometry to create efficient air distribution systems with superior strength-to-weight ratios and reduced leakage potential compared to rectangular configurations. Joint selection impacts installation speed, system pressure capacity, and acoustic performance.

Longitudinal Seam Construction

The longitudinal seam forms the primary structural element of round ductwork, joining opposing edges of formed sheet metal into continuous circular sections. Seam integrity directly determines duct pressure capacity and air leakage characteristics.

Spiral Lockseam

Spiral lockseam construction represents the predominant manufacturing method for commercial round ductwork. The spiral forming process creates a continuous helical lockseam that provides superior strength and stiffness compared to longitudinal straight seams. The five-layer lock configuration distributes stress while creating an inherently sealed joint.

Spiral duct typically uses 26 to 16 gauge galvanized steel, with gauge selection based on diameter and pressure classification. The spiral angle typically ranges from 75 to 85 degrees relative to the duct axis, optimizing both structural performance and manufacturing efficiency.

Spiral lockseam duct accommodates static pressures up to 10 inches water column (w.c.) without supplemental reinforcement, depending on diameter and gauge. The continuous helical seam provides distributed stiffness that resists both radial and longitudinal deformation.

Longitudinal Seam Snaplock

Longitudinal snaplock seams create a four-thickness lock along a single axial line, suitable for low to medium-pressure applications. This construction method allows field fabrication of round duct from flat sheet metal, facilitating custom fittings and specialized configurations.

Snaplock seams typically require supplemental sealing with mastic or tape to achieve specified leakage classes. The single-line seam concentrates stress, necessitating appropriate gauge selection and potential reinforcement for higher-pressure systems.

Transverse Joint Connections

Transverse joints connect sequential duct sections to create extended runs. Joint selection balances installation labor, sealing performance, and structural requirements.

Slip Joint Couplings

Standard slip joints utilize a bell-and-spigot configuration where one duct end expands to receive the following section. Engagement depth typically equals 2 to 4 inches, varying with duct diameter and pressure classification.

Proper slip joint installation requires concentric alignment to ensure uniform gap spacing for sealant application. Eccentric joints create preferential leakage paths and stress concentrations that compromise long-term performance.

Beaded Slip Couplings

Beaded ends create circumferential reinforcement that increases joint stiffness while providing a positive stop for consistent engagement depth. The bead profile typically projects 0.125 to 0.25 inches radially, forming a mechanical interlock that resists separation under axial stress.

Beaded joints facilitate uniform sealant application by establishing controlled gap geometry. The bead serves as a sealant dam, preventing migration during application and curing.

Standing Seam Couplings

Standing seam connections mechanically lock adjacent duct sections through deformed sheet metal edges that create a four to five-thickness joint. This connection method eliminates separate coupling components while providing adequate strength for low-pressure systems.

Standing seam joints require specialized hand tools or pneumatic equipment to form the final lock configuration. Proper seaming technique ensures uniform compression around the entire circumference, preventing localized weak points.

Flanged Connections

Round duct flanges provide reinforced connections for high-pressure systems, equipment interfaces, and applications requiring frequent disassembly.

Companion Angle Flanges

Companion angle flanges for round ductwork utilize formed continuous angles attached to duct ends with sheet metal screws at regular intervals. The circular angle configuration requires careful forming to maintain radius consistency and prevent flat spots that compromise gasket sealing.

Bolt hole spacing typically ranges from 4 to 8 inches around the circumference, with closer spacing required for larger diameters and higher pressures. Bolt circle diameter should provide adequate clearance for wrench access while minimizing flange overhang.

Pressure Capacity and Reinforcement

Round duct geometry inherently resists internal and external pressure through hoop stress distribution. This structural advantage allows thinner gauge material compared to equivalent rectangular ducts, reducing material costs and system weight.

Spiral duct reinforcement typically becomes necessary at pressures exceeding 6 to 10 inches w.c., depending on diameter and gauge. Reinforcement methods include circumferential banding, embedded wire, or increased material gauge.

Installation Best Practices

Round duct joints require consistent alignment to prevent eccentric loading that induces ovaling deformation. Support spacing should limit vertical deflection to less than 0.5 percent of duct diameter to maintain circular cross-section.

Joint accessibility affects installation productivity, particularly in congested mechanical spaces. Slip joint installation requires sufficient clearance for axial alignment, while flanged connections demand access for bolt tightening.

Acoustic Considerations

Round duct joints generate less turbulence-induced noise compared to rectangular joints due to smooth circular profiles and absence of corner discontinuities. Properly sealed joints eliminate air-induced whistling caused by high-velocity leakage through gaps.

Spiral lockseam construction creates minimal acoustic disruption compared to longitudinal seams with mechanical fasteners. The continuous helical seam presents a smooth internal surface that reduces turbulent boundary layer formation.

Sealing Requirements

Seal class requirements dictate joint sealing methods. Seal Class A typically requires mastic or tape on all transverse joints, while lower classifications may permit sealed fittings only or selective sealing based on pressure zone.

Mastic application to round joints requires uniform coverage around the circumference with thickness adequate to fill gaps resulting from manufacturing tolerances. Typical mastic thickness ranges from 1/32 to 1/16 inch.