February 19, 2016
It's no secret that companies woefully need qualified welders. Many lump the issue into manufacturing's broader skilled-labor crisis, but companies lacking welders face another problem: It can be dirty, very unpleasant and sometimes dangerous work.
Note a key phrase: can be. It doesn't have to be, and installing proper fume extraction may help matters greatly. That's according to Patrick Gilmour, business development manager for Clawson, Mich.-based RoboVent Product Group, Inc.
Weld fumes present unique challenges. They're hot, especially when close to the welding arc. They waft, rising relatively slowly from the weld work zone. If a fume-collection device is too far from the work zone, the weld smoke won't be collected, but extracting too close to the welding arc can present a serious fire hazard. “A fire in the fume extraction system can be a major disaster,” Gilmour says. “It can put all the workers in the parking lot— and that's a lot of downtime.”
As a foundation, weld-fume collection must accomplish several things: collect as much smoke as possible, ensure the fumes enter the system at a safe temperature, and filter them with enough media to handle the area where fume is being extracted.
As Gilmour explains, this is often easier said than done.
The Basics of Welding Smoke
About 85 percent of welding smoke consists of particulate, most of which comes from the welding electrode, with sizes ranging from 0.1 to 5.0 micron. The width of human hair is approximately 60-120 microns. The remaining 15 percent consists of gas, including CO2 and argon. During welding, about one to two percent of the weldjoint metal in carbon steel converts to collectable particulate; for aluminum, it rises to between six and eight percent.
Welding produces, not surprisingly, a lot of hot air, between 750 and 1,000 F near the arc. Acting under the same principles as a hot air balloon, the smoke wafts up, cools, then descends down into the breathing area between 6 and 8 feet above the shop floor.
“Note the blue haze you see at the roof line of many fab shops,” Gilmour says. The haze represents heated weld fume that, without proper fume extraction, will descend back down to the floor.
Filtration Problems and Solutions
To use an industry buzzword, capture efficiency remains imperative. “It doesn't matter how fine a filter you have if you can't get the welding smoke into it in the first place,” Gilmour says.
If excess weld smoke hangs in the air even after a fume-extraction system is installed, two things may have occurred. One, the system may not have an adequate collection system for welding, with the mouth of the system too far away from the welding source. Two, the filter may be overwhelmed with the amount of smoke it needs to collect, leading to frequent changeouts of the filter media.
“Many companies change out their filters every one to three months,” Gilmour says. “That's a lot of money spent on filters,” adding that the baseline target for filter changeouts should be between six and eighteen months, depending on the application. This can be accomplished by first matching the filter medium to the application and, second, ensuring the filter area is adequate to handle the amount of CFM (cubic feet per minute) needed for the operation.
According to the textbook way of selecting filter media, a two-to-one air-to-cloth ratio represents the typical way of doing things. Extracting fume out of 2,500 square feet requires 1,250 square feet of filter cloth (typical filter cartridges have about 200 square feet of cloth inside).
However, consumables can take a bite out of the wallet, air filter cartridges included. To ensure longer filter life, Gilmour says, a shop can “over-rate” the ratio with 25 percent more cloth. This, he says, ensures filters will last longer—twice as long or more in some cases.
A properly designed cartridge filter designed for welding today should be 99.99% efficient at capturing particulate down to 0.5 micron, Gilmour says. Better filters are available but are much more expensive. He does not recommend the older-style “box” filters, which are only about 60 percent efficient. Electrostatic and ionizing filters also have about 60-percent efficiency.
If a generic fume and dust extraction system collects weld smoke along with other mist material that may include oil, filter life may be shorter. “A filter is made up of paper and polyester,” says Gilmour, “and oil is difficult to remove from the paper.”
Welding produces flammable particulate, so fume extraction should be designed to control and contain those sparks. Designing a system that stops the sparks at the source involves a spark arrestance device that has been thoroughly tested in heavy welding applications, and will not allow the spark into the duct.
Today, some collection systems force fume into specific air patterns that flow at a consistent rate with limited acceleration. Significant acceleration of a spark literally adds fuel to the fire, giving it a healthy dose of oxygen before entering the system's duct work. “If the spark enters the duct work,” Gilmour explains, “it's too late.”
The application and specific welder position must be considered and analyzed. If smoke forms a plume behind the welder, the system will extract it by pulling it across the welder's breathing space, which, of course, defeats the system's purpose.
“With revelations about hexavalent chromium in recent years, we want to know exactly where that fume is going at all times between the welding arc and the hood”, says Gilmour.
Standing fans, open shop doors and similar elements can all affect where weld fume goes, and all should be taken into account when considering a fume-extraction system.
Gilmour recommends localized fume extraction for welding, over against systems that may be 20 or 30 feet over a welder's head. “If a hood is placed at a certain distance from the welding arc, the air is isolated to a small box. This way, the problem can be addressed much more directly than would be possible with general plant air.”
Many fume-collection systems, says Gilmour, seem to fail in the summertime, when the doors may be open to let breeze onto the shop floor. A moderate breeze can, in fact, move air rather quickly—at about 700 feet per minute. That is no match for a fume-collection system designed to collect slow-moving welding smoke at 80-150 feet per minute. Solutions may involve closing the shop door or altering floor layout.
Duct work placement is critical for proper fume extraction. Overhead cranes can create major headaches. “When a plant becomes reconfigured”, says Gilmour, “be sure to consider the duct work during the planning stages.”
Also note that fume extraction uses negative-displacement technology, not the positivedisplacement systems (i.e., bringing air into the facility) of common HVAC duct work. Unlike HVAC, a single small hole in a fume-extraction system can decimate its effectiveness, so systems must be built for durability and handled with care.
Though filtering welding fumes, the system does not exhaust the air to the outside. Costs will increase when exhausting air, since that air must be replaced with fresh air from the outside.
This can be an effective system, particularly in warmer climates. But during a winter in most climates, the replacement air must be heated. For this reason, Gilmour explains, such exhaust systems can make heating bills skyrocket.
Safety First, Budget a Close Second
Gilmour emphasizes that although fume extraction shouldn't needlessly drain the company purse, it first and foremost should help make a safe environment for the welder.
For some applications, like pressure vessels and confined-space work, fume extraction systems would be difficult to design to provide adequate protection. For these, personal protective equipment (PPE), including respirators, may be a consideration.
“When fume extraction can do the job”, says Gilmour, ”and when it is done properly, it can make a happier, more productive welder.
Technical Brief: Fume Extraction and Shielding Gas
Some may think fume extraction could suck welding gases away from the arc, and hence, hurt welding quality. While breezes from open docking bays and the like may well affect critical welding operations, fume extraction systems usually don't present a problem, says Gilmour. True, systems that extract fume directly from the tip of a welding gun may present issues. But in most circumstances the concern is unfounded.
“To suck shielding gas away from the heat-affected zone would require in excess of 400 cubic feet per minute,” Gilmour explains. “But under a back draft hood or enclosure with negative pressure, you're not going to get to this. We're operating in the range of between 100 and 200 feet per minute.”
Technical Brief: The Challenge of Automation
Comparing arc-on time between a manual welder and a robot always raises eyebrows. The average person can keep the arc on up to 30 percent of the time during his shift; for a robot it's up to 90 percent. And with robotic setups using tandem wires, they now produce more fumes in less time than ever.
A shop should consider these characteristics when designing a fume-extraction system in an automated cell, which sometimes requires up to three times the square footage of filter media that would be required in a manual setup. Otherwise a shop will find itself frequently replacing expensive filters overwhelmed by the amount of fume flowing into it.