Industrial Fume Extraction Systems

Industrial fume extraction systems protect worker health by capturing airborne contaminants at the source before they enter the breathing zone. These localized exhaust systems provide superior control compared to general dilution ventilation for processes generating metal fumes, particulates, and toxic gases.

Welding Fume Characteristics

Welding operations generate complex fume compositions that vary by process and base material. The particle size distribution typically ranges from 0.1 to 1.0 micrometers, with most particles below 0.5 micrometers. This respirable fraction penetrates deep into the lungs, making effective capture critical.

Fume generation rates by welding process:

OSHA mandates a permissible exposure limit (PEL) of 5 mg/m³ for total welding fume as an 8-hour time-weighted average. ACGIH recommends a threshold limit value (TLV) of 0.02 mg/m³ for hexavalent chromium, commonly present in stainless steel welding fumes. These limits drive the requirement for effective source capture systems.

Source Capture Effectiveness

Capture efficiency depends on hood design, placement distance, and face velocity. The relationship between capture velocity and distance from the source follows the inverse square law for free-standing hoods. However, properly designed local exhaust arms can achieve 95-99% capture efficiency when positioned correctly.

Capture velocity requirements at distance from source:

For welding fumes with moderate thermal rise:

• Distance 12 inches: 200 fpm minimum capture velocity
• Distance 18 inches: 300 fpm minimum capture velocity
• Distance 24 inches: 400 fpm minimum capture velocity

The capture zone is the three-dimensional space where contaminants are drawn into the hood. For articulated fume arms, position the hood opening 12-18 inches from the weld point at a 30-45 degree angle to the work surface. This maximizes capture while minimizing interference with the welding operation.

Source capture effectiveness factors:

1. Hood design - Flanged openings increase capture by 25% compared to plain openings
2. Cross-drafts - Air currents above 50 fpm can reduce capture efficiency by 40-60%
3. Thermal plume utilization - Rising heat from welding aids capture when hood is positioned above work
4. Hood face velocity - Maintain 100-200 fpm at hood face for optimal particle entrainment

Local Exhaust Arm Systems

Articulated fume extraction arms provide flexible positioning for varied work configurations. These systems consist of a segmented arm with internal ductwork, hood assembly, and damper controls. The arm structure must support its own weight plus the negative pressure forces without sagging or requiring frequent repositioning.

Design parameters for exhaust arms:

• Hood diameter: 6-12 inches (larger for heavier fume loads)
• Arm reach: 6-10 feet typical (longer arms require stronger support)
• Duct velocity: 3500-4500 fpm to maintain particle transport
• Static pressure: 4-8 inches w.g. at hood inlet
• Volumetric flow: 400-1200 cfm per arm depending on hood size

The internal duct diameter typically ranges from 4 to 6 inches. Smooth interior surfaces minimize pressure loss and prevent particulate buildup. Incorporate a blast gate or butterfly damper at the arm base to balance multiple arms on a common system.

Position arm supports to allow 360-degree rotation and vertical adjustment through the worker’s reach envelope. Use counterbalancing mechanisms or gas springs to enable single-hand positioning. The hood should be positionable within 5 seconds by the operator.

System Design Considerations

Central vacuum systems serving multiple extraction points require careful sizing to maintain minimum transport velocity throughout the ductwork network. The main collector duct must convey particles at 4000-4500 fpm to prevent saltation and buildup.

Pressure loss components:

• Entry loss at hood: 0.25-0.5 velocity pressures
• Duct friction: 0.03 inches w.g. per foot for 4-inch duct
• Elbow losses: 0.5-1.5 velocity pressures per 90-degree turn
• Filter pressure drop: 2-6 inches w.g. depending on loading

Size the exhaust fan to overcome total system resistance plus a 25% safety factor. Use backward-inclined or airfoil centrifugal fans for energy efficiency and tolerance to particulate loading. The fan must deliver rated capacity at the maximum system resistance, which occurs when filters reach their cleaning setpoint.

Filtration requirements:

Primary filtration for welding fumes requires high-efficiency cartridge filters with MERV 14-16 rating or better. The filtration area must provide face velocity below 5 fpm to achieve acceptable filter life and minimize pressure drop. For 1000 cfm system flow, specify minimum 200 square feet of filter area.

Implement automatic filter cleaning using pulse-jet systems when pressure drop reaches 5-6 inches w.g. The compressed air pulse dislodges collected particulate into the hopper below. Size the hopper for weekly disposal intervals based on anticipated fume generation rates.

Health Hazard Control Verification

Verify system performance through direct-reading aerosol monitors positioned in the worker’s breathing zone during typical operations. Measurements should confirm exposure levels remain below 50% of the applicable PEL or TLV to provide an adequate margin of safety.

Performance testing protocol:

1. Measure capture velocity at hood face with vane anemometer
2. Verify duct velocity with pitot tube traverse at access ports
3. Record static pressure at hood, major duct sections, and filter inlet
4. Measure breathing zone exposure using personal sampling pumps
5. Document cross-draft velocities in work area

Conduct initial performance verification during system commissioning and annually thereafter. Maintain written records documenting compliance with OSHA respiratory protection standards (29 CFR 1910.134) and welding safety requirements (29 CFR 1910.252).

Implement a preventive maintenance program addressing filter replacement, ductwork inspection, and fan bearing lubrication. Quarterly inspection of flexible arms prevents deterioration of internal ductwork and maintains system effectiveness. Replace damaged hood assemblies immediately as compromised capture leads to increased worker exposure.

Metal Working Fume Control

Beyond welding, metal working operations including grinding, plasma cutting, and thermal spraying generate hazardous fumes requiring local exhaust control. Grinding operations produce larger particles (10-100 micrometers) that settle more readily but still require capture velocities of 150-200 fpm at 12 inches distance.

Thermal cutting processes generate the highest fume loads, often requiring dedicated high-volume extraction systems with 2000-4000 cfm capacity per cutting station. Down-draft tables provide effective control for cutting operations by drawing fumes away from the operator’s breathing zone through a perforated work surface.

The design approach for all metal working fume control prioritizes source capture over dilution ventilation. Well-designed local exhaust systems reduce facility heating and cooling costs by minimizing conditioned air exhausted from the building while providing superior worker protection.

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