Industrial Sheet Metal and Process Ductwork

Industrial Sheet Metal and Process Ductwork

Introduction to the Industrial Field

Industrial sheet metal represents a distinct sector from building sheet metal. While residential and commercial work primarily concerns thermal comfort and ventilation, industrial sheet metal deals with manufacturing processes, pneumatic material handling, chemical corrosion, and high temperatures. For the Red Seal exam, you must understand that design criteria, tolerances, and assembly methods differ radically from those in the commercial sector.

The term process duct refers to any duct transporting an industrial effluent: abrasive dusts, corrosive vapors, hot fumes, wood chips, or suspended chemicals. Unlike standard HVAC ducts, these systems are not governed by ASHRAE standards or the National Building Code for comfort, but rather by specific process requirements and the applicable safety codes for the materials being transported.

Classification of Industrial Systems

Categories by Content

Material Selection Criteria

The choice of base metal depends on three factors: the chemical nature of the product being transported, the service temperature, and the abrasiveness. For corrosive applications, stainless steel type 304 or 316 is common. Type 316 contains molybdenum, which improves resistance to chlorides and acids. For temperatures exceeding 400 °C, alloy steels or refractory stainless steel are used. Aluminum is generally prohibited for metallic dusts due to the risk of exothermic reaction.

Rule of thumb: the minimum thickness of an industrial carbon steel duct is 16 gauge (1.5 mm) for diameters up to 300 mm, and 14 gauge (1.9 mm) for larger diameters. For abrasive materials, add 2 additional gauges of thickness or install reinforcement plates at impact points.

Design and Calculations of Industrial Ducts

Transport Velocity and Flow Rate

The air velocity in a process duct must keep particles suspended. If the velocity is too low, particles settle and obstruct the duct. If it is too high, wear and noise increase unnecessarily.

Flow rate formula: Q = V × A, where Q is the flow rate in m³/s, V is the velocity in m/s, and A is the cross-sectional area in m². For a circular duct, A = π × D² / 4, where D is the inside diameter in meters.

Recommended minimum velocities (according to common industrial practice and dust collector manufacturer data):

Pressure Loss and Balancing

The total pressure loss of a system is the sum of friction losses in straight sections and fitting losses (elbows, connections, entries). For the exam, you must know that the pressure loss in a 90° short-radius elbow (R = 1.5 × D) is expressed as an equivalent length of straight duct. A long-radius elbow (R = 2.5 × D) reduces the pressure loss by approximately 30% compared to a short-radius elbow.

Balancing: in a system with multiple branches, balancing is done with dampers or by calculating diameters. The golden rule: the pressure loss of each branch must equal the pressure loss of the most unfavorable path. If a branch has less pressure loss than the main path, it will receive too much flow. You then install a balancing damper or reduce the branch diameter.

Thermal Expansion

For ducts transporting hot gases, expansion is a critical factor. The linear expansion coefficient of carbon steel is approximately 12 × 10⁻⁶ m/(m·°C). For a 10-meter length with a temperature rise of 200 °C, the total expansion is:

ΔL = 12 × 10⁻⁶ × 10 × 200 = 0.024 m (24 mm)

This expansion must be absorbed by expansion joints or compensators. Supports must allow axial movement: use sliding supports or roller hangers. Never rigidly fix a hot duct at both ends — this would cause buckling or weld failure.

Fabrication and Assembly

Joining Methods

For industrial ducts, continuous welding is required when the duct transports hazardous or flammable materials. CSA B149.1 (Natural Gas and Propane Code) requires welded vent ducts for certain gas appliances. Welds must be visually inspected and, for critical applications, tested by dye penetrant or radiography.

Fabrication Tolerances

Tolerances for industrial sheet metal are tighter than for commercial work. For a circular duct, the maximum deviation on diameter is ±1% of the nominal diameter. Ovalization must not exceed 2% of the diameter. For rectangular ducts, the tolerance on dimensions is ±3 mm for sides up to 600 mm, and ±5 mm beyond that.

Frequent exam trap: industrial ducts must be fabricated with joints oriented in the direction of flow to prevent particles from catching on surface irregularities. A transverse joint facing against the flow creates material accumulation and progressive obstruction.

Reinforcements and Stiffeners

Large rectangular ducts require reinforcements to withstand internal pressure (positive or negative). Negative pressure (vacuum) is particularly dangerous: a 600 mm × 600 mm duct with an internal pressure of −5 kPa experiences a force of 1,800 N on each face. Reinforcements can be angle iron, U-channels, or perimeter frames. The maximum spacing of reinforcements depends on sheet thickness, pressure, and panel dimension.

Industrial Ventilation Systems

Dilution Ventilation vs. Capture Ventilation

Dilution ventilation involves introducing clean air to dilute contaminants to an acceptable level. It is used when contaminants are mildly toxic and uniformly distributed. Capture ventilation (or local exhaust ventilation) involves capturing the contaminant at the source before it disperses. This is the preferred method for toxic contaminants or combustible dusts.

Capture Hoods

The design of a capture hood depends on the distance between the source and the hood, and the required capture velocity. Capture velocity is the air velocity at the point of release needed to draw the contaminant toward the hood.

Typical capture velocities:

The flow rate calculation formula for a slotted hood is: Q = V × (10 × X² + A), where X is the distance from the source to the hood in meters, A is the hood face area in m², and V is the required capture velocity in m/s. This formula is a practical approximation — it accounts for the fact that velocity decreases with the square of the distance.

Dust Collectors and Separators

Before the fan, the particle-laden air stream must pass through a separation device. Common types include:

Fan position: the fan can be installed upstream (pushing) or downstream (pulling) of the dust collector. For abrasive dusts, place the fan downstream of the dust collector to protect the fan wheel from erosion. For hot gases, verify the fan's maximum allowable temperature.

Dust Explosion Prevention

Conditions for a Dust Explosion

Five conditions must be present simultaneously: fuel (dust), oxidizer (oxygen), ignition source, particle dispersion, and confinement. Eliminating any one of these conditions prevents an explosion. In a duct, suspended dust forms a cloud; a spark or hot surface can ignite it.

Protection Equipment

Ducts transporting combustible dusts must be equipped with:

Grounding requirement: all metal ducts transporting dusts must be grounded to prevent static electricity accumulation. Grounding resistance must be less than 1 MΩ according to common industrial practices. Non-conductive connections (rubber sleeves, flexible joints) must be bridged with copper braids.

Applicable Standards

The National Fire Code of Canada and adopted provincial codes reference NFPA (National Fire Protection Association) standards for dust explosion prevention. Although NFPA is an American standard, it is widely adopted in Canada by reference. Relevant sections include NFPA 68 (explosion venting) and NFPA 69 (explosion prevention systems).

Vent and Flue Ducts

CSA B149.1 Requirements

CSA B149.1 (Natural Gas and Propane Code) governs the installation of vent ducts for gas appliances. Key rules for the sheet metal worker:

Double-Wall Ducts

For high-temperature gas vents, double-wall ducts with insulation are used. The inner wall is stainless steel (often 316L), the outer wall is galvanized or stainless steel, and the insulation is typically mineral wool. The spacing between walls varies from 25 to 75 mm depending on service temperature. These ducts are classified according to their maximum service temperature (for example, Class A for 540 °C, Class B for 400 °C).

Corrosion Control

Corrosion Mechanisms

Corrosion in industrial ducts results from the chemical action between the metal and the transported fluid. The main mechanisms are:

Corrosion Protection

Protection strategies include:

Exam trap: galvanized steel must never be used for ducts transporting acid vapors. Zinc dissolves rapidly in the presence of acids, even weak ones, and the duct will perforate in a short time. Similarly, aluminum is not suitable for alkaline vapors or for environments containing copper or brass (galvanic corrosion).

Supports and Anchors

Support Calculations

Industrial duct supports must withstand the weight of the duct, the weight of the contents (accumulated particles), and the dynamic forces of the fluid. The total load is the sum of these three components. For a 600 mm diameter duct in 3 mm steel, the linear weight is approximately 45 kg/m. With accumulated dust content (assuming 50% of the section filled), add approximately 15 kg/m. The support must therefore carry approximately 60 kg/m.

Maximum support spacing by diameter:

These values are practical guidelines; precise calculations must account for allowable deflection (generally L/600 for industrial ducts).

Special Supports

For hot ducts, roller or sliding supports are used to allow axial expansion. Spring supports are used when the duct undergoes significant vertical movement (vertical expansion). Fixed anchors are placed at points where you want to control the direction of expansion — for example, near expansion joints.

Testing and Commissioning

Leakage Testing

Before commissioning, industrial ducts must undergo a leakage test. For low-pressure ducts (less than 500 Pa), a visual inspection and joint verification often suffice. For ducts with higher positive or negative pressure, a smoke test or pressure test is performed.

Pressure test: close all openings, introduce air at a test pressure (generally 1.5 times the service pressure, with a minimum of 1 kPa), and check the pressure drop over a given period. The allowable drop is typically 10% of the test pressure in 10 minutes for new ducts.

Flow and Velocity Verification

After installation, measure the actual flow at each outlet or branch using an anemometer or Pitot tube. Measured flow rates must be within ±10% of design flow rates. If a discrepancy is found, adjust the balancing dampers.

Pitot tube method: measure the dynamic pressure (Pd) at several points across the duct section. Velocity is calculated by V = √(2 × Pd / ρ), where ρ is the air density (approximately 1.2 kg/m³ at 20 °C and 101.3 kPa). For a circular duct, take measurements along two perpendicular diameters at predetermined logarithmic positions.

Summary

Traps to Avoid