Choosing the Right Filter Media Is Critical to Proper Fume Management in Welding Applications
August 6, 2012
Choosing the right filter media will improve the overall function of the equipment, reduce maintenance, improve airflow, lower costs and improve air quality in the working environment.
Ask any safety manager about cleaning the air in a welding environment; the end game is filtering the fume as well as removing it from the work area. OSHA is taking a harder look at weld fume management, specifically hexavalent chromium (CrVI or Cr6+). Simply relocating welding fume is not an option. It must be captured effectively. And while you can see smoke disappear into a filtration system and clear air comes out, much of the fume itself is actually invisible to the naked eye. Choosing a system should be based on a combined examination of the system and understanding of filter technology. Matching the best filtration with a high efficiency system will lower costs over time.
Weld fumes of all types, whether from carbon steel, aluminum, stainless and others, carry the same basic fume particle signature. Weld fumes are thermally generated particles that require high levels of filtration. Investing in the best equipment but pairing it with the wrong filter media can reduce your expensive fume extraction system into nothing more than a dust collector.
The ultimate decision of what filter media is right for your application boils down to two equally important factors:
Protection of the worker. Filters must allow higher airflow and efficient capture over a long period of time. By providing a higher airflow, they’re protecting the workers as expected. The high-efficiency filter captures the thermally generated particulate.
Cost of the equipment, filters and overall operation. Choosing a filter with qualities that include high efficiency, low static pressure drop and surface loading will lower operating costs. Longer filter life will save costs.
Types of filter media
Different types of filter media are offered by numerous companies, each promising cleaner air and the best filtration package available. There are literally thousands of filter configurations and designs. You have to peel back the layers in order to understand what you’re really getting. The crowded market can be classed into five basic filter categories. Cellulose, cellulose blend, spunbound polyester, melt-blown composites and surface-enhanced configurations.
The majority of filters on the market are depth-loading, capturing particulates as the air flows through the media. Depth-loading filter media come in a variety of densities. The particles are introduced into the filter and caught in the media of fibers and trapped. As the depth-loading media filter captures particles, it remains functional up to a certain capacity until air can no longer effectively pass through it.
Airflow suffers when the equipment is required to draw air through depth-loaded or plugged filter media. As the media gathers fume and other particles, the filter becomes obstructed, resistance increases and the airflow measurement in cubic feet per minute (cfm) decreases. The fume extraction system is forced to draw more energy and work harder to draw through loaded or plugged filters to maintain the desired airflow. The resistance continues to grow until the filter is replaced. Generally the depth-loading filter is a disposable type that is thrown away at the end of its work cycle.
Type of depth-loading filters include:
- Cellulose. One-time-use-only cellulose filters are used for general filtration of larger size particulate and in general have lower capture efficiencies. Cellulose filters are not the best choice for weld fume extraction. As the filter is dirtied, energy costs go up and airflow becomes more restricted.
- Cellulose Blend. Just what it sounds like: filters blended with cellulose and other materials to increase filter efficiency. These disposable filters are not the best choice for weld fume extraction.
- Spunbond Polyester. Non-woven polyester. Particles are captured in the material. Spunbond polyester can be washed and reused. It falls short of desired fume filtration efficiencies as its intended use is not for the collection of weld fume.
- Melt-blown Composites. Melt-blown technology utilizes microfibers to make a tighter weave that captures more particles. This media type still is depth-loading, and like its cellulose cousins, changes airflow behavior as particles become trapped within the filter. This media type confines particles that are located between the base media and the microfiber weave, thus reducing filter life and its ability to be cleaned.
A surface-loading filter, on the other hand, is less restrictive to the airflow. Surface-enhanced media will capture particles as they pass by the surface, not through the media. These filter media are designed to surface-load with particles but retain longer functionality.
Surface-enhanced technology, like in the Miller FilTek™ XL filter series, are highly effective at capturing weld fume particles. The FilTek XL filter captures the particles on the surface of the media verses depth loading.
Maintenance is easier and the filter lasts longer. By design, surface loading filters allow the collected fume to release with ease when a reverse pulse cleaning cycle is performed. Surface-loading filters can last up-to three times longer than depth-loading filters.
Example of a Surface-Loading Filter:
- FilTek XL from Miller Electric Mfg. Co. Innovative, surface-enhanced filters like the FilTek XL offer improved airflow and efficiency because the fume is captured on the surface of the filter. With surface-loading filters, weld fume does not penetrate deep into the filter, enabling particles to be easily released during the cleaning process and reducing pressure drops. The surface-enhanced media, like the Filtek XL from Miller, is perfect for the capture of weld fumes.
Choosing the filter, know the MERV
MERV stands for "Minimum Efficiency Reporting Value." MERV ratings are used to gauge the ability of the filter media to remove particles from the air and is a reliable standard for measuring the efficiency of the filter.
The MERV rating of a filter describes its effectiveness. The MERV scale ranges from 1 to 16, and measures a filter's ability to remove particles from 0.3 to 10 microns in size. To give you an idea of the scale of a micron, one-thousandth of an inch is 25 microns; 100 microns is about the thickness of a piece of paper or a human hair. Filters with higher ratings not only remove more particles from the air, they also remove the smaller particles that are most concerning for worker protection.
You can see on the following chart that cellulose, cellulose blend and spunbond polyester filters are not adequate for weld fume filtration. Typically the minimum for weld fume extraction is MERV 13 or greater. Put the wrong filter with your fume extraction system, and your system will be ineffective. The quality of the return air suffers when the filters MERV value is not adequate for the particulate being collected.
The bottom line is that selecting a filter specific to welding fume extraction is important because most weld fume particles are less than 1 micron in size. Airborne dust is large in comparison – 1 micron and larger – and is easily captured. As discussed, the minimum recommended MERV-rated filter for weld fume is 13, but filter media designed specifically for weld fume with a MERV rating of 15 or 16 are more robust at dealing with weld fume, capturing 95 percent of weld fume particles and filtering out submicron particles.
Power efficiency with the right filter
FilTek surface-loading filters hold up to six times more dust given the same pressure drop across the filter media compared to common cellulose filters. This ability to hold more dust at lower pressure drops means the collection system blower load is lower overall for a longer period, resulting in higher energy efficiencies and lower energy costs over time.
With the right filter, this feature has two specific benefits.
First, by loading on the surface and not within the filter, particles can be released much easier during the cleaning process. The result is less compressed air usage.
Second, pressure drop across the filter is kept much lower, which results in lower operating costs and stronger suction power for a cleaner workspace. Airflow (cfm) should be measured at the point of extraction (fume generation point). Measuring at the hood gives the most accurate measurement of how effective fume extraction is. Your system is more efficient at the same level of protection, lowering your energy bill.
Keeping it clean
You have a number of options to choose from when selecting a fume extractor. Disposable filter models have lower initial expenditures, however, per-filter costs add up as you frequently change out the system’s filter. Automated/online and offline cleaning models offer less maintenance and longer filter life. The automated model options will cost more at install but have a greater long-term value, as the primary filter life is comparable to multiple disposable filters.
No matter what type of filtration system you install, the filter media matters. Choosing a surface-loading filter will not only clean the air, but lower operating costs and save on the consumption of energy. Airflow is minimally affected with the correct surface-loading filter, ensuring that your system is not straining through a clogged filter and instead is providing consistent extraction at the workstation.