Ducting Raw Material

All ducting shall be fabricated of LFQ (Lock Forming Quality) grade prime G.I. raw material furnished with accompanying Mill Test Certificates.
Galvanizing shall be of 270 gms/sq m (Total coating on both sides).
All transverse duct connectors (Flanges/Cleats) and accessories/related hardware, such as support systems, are galvanized.

Fabrication Standards

All ductwork including Straight Sections, Tapers, Elbows, Branches, Show Pieces, Collars, Terminal Boxes and other Transformation Pieces are factory fabricated.
Coil Lines to ensure location of longitudinal seams at corners/folded edges, to obtain the required duct rigidity & low leakage characteristics. No longitudinal seams shall be permitted along any face of the duct. The starter unit shall produce 300mm centre to centre beading on the sheet with Pittsburgh and TDF notching from the die mounted on it.
All Ducts, Transformation Pieces & Fittings to be made on CNC profile cutters (Plasma Cutting Machine), for required accuracy of dimensions, location and dimensions of notches at the folding lines.
All edges to be machine treated using Lock Formers, Flangers and Roller for turning up edges.

Selection of G.I. Gauge and Transverse

Unless specified or allowed, rectangular ductwork shall be constructed in accordance with the Table for Rectangular Duct Construction, as per 2005*-SMACNA, and with details associated with them.

The duct gauge tables are based on G?90 coated galvanized steel of lock forming grade, conforming to ASTM Standards A653 and A924.

Duct Construction shall be in compliance with norms as per SMACNA standards, with reinforcement as listed below for rectangular ducts - for different pressure class with TDF flanges and without TDF flanges.
All transverse connectors shall be 4-Bolt TDF/Slip-On Flange System (supplied by Manufacturer).
All Ducts, Transformation Pieces & Fittings to be made on CNC profile cutters (Plasma Cutting Machine), for required accuracy of dimensions, location and dimensions of notches at the folding lines.
Non-toxic, AC–Applications grade P.E. or PVC Casketing is required between all mating flanged joints. Gasket sizes should conform to flange manufacturer’s specification.

Duct Construction

The fabricated duct dimensions should be as per approved drawings and connecting sections, to avoid any gaps. The following factors are taken care of to achieve this.

Dimensional Tolerances: All fabricated dimensions shall be within +/- 1.0 mm per metre. To obtain required perpendicularity, permissible diagonal tolerances shall be +/- 1.0 mm per meter.
Each & every duct piece shall be identified by a color coded sticker which shows specific Part Number, Job Name, Drawing Number, Duct Sizes, Gauge, Isometric Drawing and Blank Part details.
Ducts shall be straight and smooth on the inside. Longitudinal seams shall be air tight and at corners only - which shall be either Pittsburgh or Snap Button Punch as per SMACNA Practice, to ensure air tightness.
Duct sides that are 19 inches (483 mm) and over and are 20 GA (1.00 mm) or less, with more than 10 sq ft (0.93 sq m) of un-braced panel area, shall be cross broken or beaded, unless they are lined or externally insulated. Ducts that are of heavier gauge, smaller dimensions, and smaller panel area and those that are lined or externally insulated are not required to have cross breaking or beading.
Changes in dimensions and shape of ducts shall be gradual (between 1:4 and 1:7). Tuning Vanes or Air Splitters shall be installed in all bends and duct collars, designed to permit air to make the turn without appreciable turbulence.
Elbows, Bends, Offsets etc. should be fabricated with a width to radius ratio at intervals of not less than 1.0 to 1.5. Alternatively, turning vanes should be provided so that the aspect ratio of the various sections, so formed by the vanes, will be the least. All fittings shall be provided as per details in SMACNA 2005 Chapter 4.

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