THE COMPLETE GUIDE TO WELDING
FUME EXTRACTION SYSTEMS:

HOW (AND WHY) TO KEEP YOUR FACILITY AIR CLEAN AND SAFE

CHAPTER FOUR: SOURCE CAPTURE SYSTEMS FOR WELD FUMES

Source capture is considered to be a best practice for weld fume collection and is preferred wherever possible. Source capture systems collect contaminants close to the source where they are produced, keeping them out of the breathing zone for manual welders and preventing the propagation of weld fumes throughout the facility. Source capture fume collection systems may be contrasted with ambient capture systems, which clean air for the whole facility. Most source capture systems use air filtration to filter fume and particulate out of the air. Cleaned air may be exhausted outside or returned to the facility (if certain criteria are met).

What is a source capture system for fume collection?
A source capture system collects contaminants from close to the source where they are produced. This may be accomplished via the use of hoods or enclosures that contain fume-producing processes or localized capture methods (such as fume arms) that pull weld fumes in as they are produced to prevent propagation through the facility.

A source capture system consists of the following basic elements: a capture hood, duct system, fan (blower) and exhaust ductwork. In a filtration source capture system, the hood is ducted to a dust collector that captures and filters the air to remove contaminants.

What are the advantages of source capture for weld fume collection?
Source capture systems are usually preferred for weld fume collection, especially in high-production environments. They offer significant advantages in energy efficiency and efficacy for weld fume removal.

Health and safety: Source capture removes weld fume before it enters the welder’s breathing zone or propagates through the facility, where others can breathe it in. It is usually the best way to minimize exposure to weld fumes and ensure compliance with OSHA PELs.
Energy efficiency: Because fumes are collected close to the source where they are generated, source capture minimizes the airflow (CFM) requirements for the dust collection system. Lower CFM means a smaller system and less energy use.
Maintenance: A smaller, more efficient dust collection system also minimizes maintenance costs and time.
Control: Source capture systems provide more control over fume collection, as they can be designed to target emissions from a specific source. Hoods, ductwork and dust collection systems can all be tailored for the application.

Is source capture required for weld fume extraction?
Source capture is not mandated for weld fume control, but it is recommended in most circumstances, especially for high-production welding environments and manual welding. Source capture prevents weld fumes from entering the breathing zone or moving throughout the facility, resulting in safer, healthier air.

What source capture methods are used for robotic welding?
For robotic welding, the entire process can be contained under a large hood or enclosure. Weld fumes are contained within the hood and exhausted or collected by a dust collector or wet filtration system. The hood should be designed to contain the particulate within the cell and not allow it to escape into the facility. No employees should be working inside the hood.

In some cases, the robotic welder may be equipped with a tip extraction hood that collects a majority of weld fume as it is created at the weld seam. This can be a useful alternative if the welder cannot be easily enclosed under a hood or if overhead cranes or other equipment are required. The amount of fume collected by this method may vary depending on the welding process and the way the robot moves. In some cases, it may be desirable to combine a tip extraction system with an ambient system to collect fugitive fume.

What source capture methods are used for manual welding?
Source capture for manual welding must pull weld fume away from the breathing zone of the welder. For this reason, the large hoods and enclosures used for robotic welders are not appropriate for manual welding. There are several source capture options for manual welding.

Backdraft Tables/Booths

A backdraft (or sidedraft) table or booth uses an intake plenum that is positioned to pull weld fume back and away from the welder’s face. The plenum may be combined with a work surface or booth for an all-in-one workstation and weld fume collection system. Backdraft tables are a good choice for smaller weldments that fit on the work surface. Unlike fume arms, they do not require any repositioning. Note: Downdraft tables, which pull air down rather than up, are not generally recommended for weld fume; because weld fume rises, downdraft tables may miss a portion of fume that rises beyond the reach of the extraction system before it can be collected. When using backdraft systems, make sure that the plenum is close enough to the weld fume source for efficient capture.

Fume Extraction Guns/Torches:

A fume gun (or extraction torch) combines fume collection with the welding torch itself. A fume gun has a small hood integrated into the tip of the weld torch, which is attached via a hose to a dust collection system. The best fume guns may capture between 90-95% of weld fume at the source; however, actual capture rates will depend in part on the technique used by the welder and the position of the torch. Hood position and airflow velocity for the capture system must be carefully calibrated to ensure efficient capture without disrupting the shielding gases. A fume gun can be a good option for large weldments or enclosed areas where other methods of source capture are impractical. Used correctly, it ensures efficient fume capture with little or no effort on the part of the welder and little or no impact on weld seam quality or welder comfort. Fume guns are only used with MIG welding.

Fume Arms

Fume arms are mobile arms with a small hood on the end that is positioned over the weld seam. It is important to ensure that the fume arm is always correctly positioned to pull weld fume as it is created; this may require the welder to reposition the arm several times if working on a larger piece. Fume arms are a practical and flexible source capture option that can be used with either smaller tabletop pieces or larger weldments.

WHAT ARE THE CONSIDERATIONS WHEN DESIGNING A SOURCE CAPTURE SYSTEM FOR WELDING?

When designing a source capture system, there are several considerations to keep in mind, including capture method, hood design, air velocity (CFM), and filtration.

Capture method: Make sure the capture method is appropriate for the type of welding (e.g., manual vs. robotic) and will keep weld fumes away from the welder’s face and areas where people are working.
Hood design: Using a smaller enclosure or collecting fume closer to the generation source/weld seam will minimize the volume of air that must be moved for efficient capture, which will reduce energy requirements and costs for fume collection. When using source capture methods such as fume arms, the distance between the fume source and the capture hood should be no more than one times the duct diameter.
Air velocity: Air velocity must be calibrated to ensure efficient capture of fume without disrupting the shielding gases. Air velocity will depend on the rate of capture you are aiming to obtain. Typically, the velocity at the weld source needs to be around 150 FPM; this is the velocity needed to influence the direction of the weld fume and prevent fume from escaping. Higher velocities waste air and risk disrupting the shielding gases and causing porosity in the weld. There is a balance between capture rate and weld quality which is important to maintain when using source capture. For hoods and enclosures, the velocity at all of the openings should be equivalent to the capture velocity of the particulate.
Filtration system: In most cases, a source capture solution is ducted to a filtration system (e.g., a cartridge dust collector or wet collector) to filter the air and collect particulates for proper disposal. This may be a single dust collector for each individual node (e.g., a robotic weld cell, fume arm, backdraft table, etc.) or a centralized system that collects fume from multiple nodes. Small portable dust collectors may be used with individual fume guns or fume arms, and some systems combine the hood and dust collection system in one unit (e.g., a backdraft table or fume arm with integrated dust collection). Filter media should be chosen to capture the small particulate in weld fume; typically, this will require filters rated at MERV 15 or above (see Chapter 6: Filtration Selection for Weld Fumes).

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How can fugitive fume from welding be captured?
Even with an efficient source capture system, there may still be a problem with fugitive weld fume in the facility. Fugitive weld fume commonly results from improper positioning of a fume arm, the use of smoking/cooling bins for cooling parts, or fumes rising from hot weld seams after a fume arm or fume gun has been repositioned. Make sure fume arms are properly positioned over the weld seam for maximum fume capture. Overhead hoods can also be designed to better capture rising fume from smoking parts. If the hood or enclosure does not fully capture all weld fume, a secondary ambient air filtration or general ventilation system may be needed to remove the remaining fume. Not sure where to start? Learn More About RoboVent Facility Testing and Engineering Services.