Energy-efficient and EPA-compliant Air Pollution Control

Most welding rod and wire contain more than 1 percent manganese and other hazardous contaminants, so almost all welding applications require fume extraction. Source capture is the most energy-efficient way to capture fume.

In 2011 the U.S. Environmental Protection Agency (EPA) began enforcing a subset of the National Emission Standards for Hazardous Air Pollutants, commonly referred to as MFHAP (metal fabrication hazardous air pollutants), which seeks pollution control to control emissions generated by metalworking processes such as welding and abrasive blasting. By November 2011 both new and existing facilities had to be in—and furnish proof of—compliance.

These standards apply to metal fabrication and machine shop manufacturers if the contaminants in their facilities are considered hazardous. The EPA defines hazardous as consisting of greater than 0.1 percent of cadmium, chromium, lead, or nickel or greater than 1 percent of manganese. Other manufacturing processes that are covered are dry abrasive blasting, machining, dry grinding and polishing, and spray painting (excluding thermal spray).

Regulations to control some of these hazardous contaminants had been in place for some time from worker safety organizations such as the Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health (NIOSH). The EPA regulation is different, though, in that its focus lies not on worker safety, but on controlling the indoor pollutants before they can become outdoor emissions.

The relatively new regulation requires:

• Controlling the pollutants so that no visible emissions can be seen discharging from the facility.
• Minimizing excess metal dust in surrounding areas.
• Eliminating hazardous materials where possible.
• Improving processes.

While the implications of this new regulation may have been extensive, it also presented metalworking companies with an opportunity to review their air pollution control strategies and adopt a more energy-efficient, productive, and healthier approach to air quality.

Venting Fumes Outdoors No Longer Allowed—nor Energy-efficient

Many manufacturers still control metal dusts and welding fumes by simply opening doors and windows or venting the fumes outdoors with roof or wall exhausters. Not only do these practices release the potentially harmful pollutants outdoors, they can be very expensive and energy-inefficient, especially when the indoor air is conditioned.

Because no visible emissions are allowed under the new EPA regulation, these methods are no longer viable options for facilities with processes that generate hazardous fumes and dust. As an alternative, facilities can choose from various means to control and capture indoor air pollutants, including source capture, extraction, and filtering.

Capture, Extract, Filter

Source Capture. The most effective and energy-efficient way to control fumes or other pollutants is to collect the pollutant at or near the point of generation, preventing migration throughout the facility (see lead image). Capturing at the source minimizes the amount of air required to control the pollutant and thus reduces the amount of energy required to do
so. Additionally, capturing the pollutant near the generation point better protects the worker’s breathing zone from the harmful pollutants.

The key to a successful source-capture system is a proper hood design that captures the fume effectively without reducing worker productivity. Source capture typically involves the use of extraction arms or fixed hoods ducted to workstation collection systems, or multiple hoods connected to one larger, central collection system.

One key consideration in selecting one of these approaches is the frequency of operation in each workstation. If all workstations are operating simultaneously, the best approach might be a central system. However, if some workstations operate intermittently, a dedicated system might be best so air is not drawn in continually, even when the stations are not in use, which wastes energy.

Ambient Extraction. Source capture is not always practical because of operational constraints such as component size and geometry or overhead obstructions such as cranes. In these cases, general ventilation or ambient collection systems can collect pollutants at fixed extraction points spread throughout a general area (see Figure 1).

Figure 1
In an energy-efficient ambient collection system, filtered, clean, warm air is diverted back into the facility in the winter and exhausted outdoors in the summer.

Ambient extraction systems require more airflow and energy than source-capture systems because they have to draw the pollutant across a larger area. Typically, the sizes specified for these systems are based on changing overall air volume at a prescribed frequency, usually eight to 12 air changes per hour.

Consider a 100- by 100-ft. welding area with 25-ft. ceilings and 10 welding stations. With source capture, 1,000 cubic feet per minute (CFM) per welding station usually is adequate, so the total requirement for 10 welding stations would be 10,000 CFM. With ambient extraction, using the minimum recommended air changes per hour (eight), the total requirement would be greater than 33,000 CFM.

There are several ways to minimize the amount of energy required for ambient extraction:

• Extract the fumes or pollutants at the roof level. Hot air rises, and so do welding fumes because of the thermal gradient created during the welding process. Collecting the fumes at the highest practical point takes advantage of this.
• Return filtered, clean air close to the floor level (ideally within 7 ft.) to create a system that pushes pollutants away from workers and toward the extraction points. This establishes a recirculation pattern that will both improve performance and return the conditioned air to the worker level.
• Equip the return air duct with a diverter valve so heated air returns back to the facility in the winter, returns cooled air in the summer, or exhausts air if needed.
• Use curtains or dividers to isolate the controlled area to help minimize the building volume that must be protected.
• Use variable-frequency drives to extract only the amount of air required to control the fume adequately. This will reduce the energy requirement on the blower significantly and also extend the filter life.

Filtration Technology. Metalworking dusts and welding fumes are composed of very small particulate; 70 percent can be smaller than 0.3 µm in diameter. Therefore, a filter technology is necessary that can provide the efficiency required to effectively capture the dust or fume—not just relocate it.

Nanofiber filter technology comprises a fine surface layer of fibers that traps the pollutants on the filter’s surface. This allows the collected dust to be released using less compressed air from the cleaning system. Because of the surface-loading properties of nanofiber filters, operating pressure is lowered also, so less energy is required to move the air, which extends filter life and helps reduce waste and lower operating costs.

Compliance Leads to a Safer Workplace

The new EPA regulations have forced many metal fabricators and machine shops to adjust their approach to indoor air pollution control. This is a good opportunity to implement an air pollution control strategy that not only achieves regulatory compliance and reduced emissions, but also creates a safer, more productive environment for workers.

Given the many variables and technical issues involved in designing a successful system, it is recommended that you consult an air pollution control specialist who understands these and other potential regulations, helping to ensure you get a system that best fits the needs of your operation.

Travis Haynam is director of business development for United Air Specialists, Inc., 4440 Creek Road, Cincinnati, OH 45242, 800-252-4647, [email protected], www.uasinc.com