Welded duct is HVAC ductwork fabricated with seams permanently joined through welding processes such as MIG (GMAW), TIG (GTAW), or spot welding, creating an airtight and structurally robust connection. Typically constructed from carbon steel, galvanized steel, or stainless steel, welded duct is the preferred choice for high-pressure, high-temperature, or hazardous exhaust applications where mechanical seaming or tape-sealed joints would be insufficient. It is sometimes referred to as “black iron duct” when fabricated from uncoated carbon steel.
Technical Details and Specifications
Welded duct is manufactured from sheet metal typically ranging from 16 gauge (1.52 mm) to 10 gauge (3.42 mm), with heavier gauges selected for larger diameters or higher-pressure systems. The two most common welding methods used in fabrication are:
- MIG (GMAW): Gas metal arc welding, favored for its speed and suitability for carbon steel and stainless steel in production environments.
- TIG (GTAW): Gas tungsten arc welding, preferred for thinner materials and applications requiring precise, clean welds with minimal spatter, such as stainless steel exhaust systems.
Welded seams are designed to achieve very low air leakage rates, often less than 1% at operating pressure. Duct sections may be fabricated in round (spiral welded) or rectangular configurations, with round duct generally offering superior structural performance at higher pressures. Weld quality is verified through visual inspection and, when specified, non-destructive testing (NDT) methods such as dye penetrant testing or radiographic examination.
Common Applications
- Industrial ventilation: Systems handling corrosive fumes, abrasive particulate, or elevated temperatures where standard lock-seam duct would degrade or fail.
- High-pressure HVAC systems: Commercial and laboratory environments operating at elevated static pressures, often above 6 inches water gauge (w.g.).
- Exhaust systems: Kitchen exhaust, fume hood exhaust, and hazardous material exhaust where leakage poses safety or code compliance risks.
- Cleanroom and pharmaceutical facilities: Environments requiring leak-tight, smooth-interior ductwork for contamination control.
Related Standards and Codes
Welded duct fabrication and installation are governed by several industry standards:
- AWS D1.1: Structural Welding Code for Steel, covering welding procedures and welder qualifications for carbon steel duct.
- AWS D1.6: Structural Welding Code for Stainless Steel, applicable to stainless steel ductwork.
- SMACNA HVAC Air Duct Leakage Test Manual: Establishes testing procedures and acceptable leakage rates by duct pressure class.
- NFPA 96: Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations, which requires welded grease duct construction.
Practical Significance
Welded duct costs more to fabricate and install than standard snap-lock or slip-and-drive ductwork due to the skilled labor and inspection requirements involved. However, its long-term value is evident in applications where air leakage, structural failure, or contamination would create safety hazards, energy waste, or regulatory violations. In high-pressure systems, the energy savings from reduced leakage alone can justify the higher upfront investment. Specifying welded duct where conditions demand it is a critical decision that affects system performance, occupant safety, and code compliance throughout the life of the installation.