A breath of fresh air — an overview of in-plant filtration systems

Facility evaluation and system selection

By Wayne Bonham

November 15, 2001

The article explains how to carry out a facility and process evaluation and discusses the basics of in-plant air filtration system selection.

Plant managers no longer question whether to use an in-plant air filtration system. Rather, they ask what type of system to use. Multiple collection and filtration system options are available from a variety of manufacturers, and different systems may function equally as well when cleaning in-plant air.

So, where should a metal fabrication facility begin when choosing a system, and how does it know when the correct choice has been made? This article explains how to carry out a facility and process evaluation and discusses the basics of in-plant air filtration system selection.

Why Install a System?

Companies with metal fabrication operations add air filtration systems to:

1. Create a healthier work environment. Airborne contaminants can be, at a minimum, an annoyance to employees. More important, contaminants often pose serious health risks.
2. Protect equipment. Airborne contaminants damage manufacturing equipment such as sensitive computer-driven controls, motors, and optics.
3. Reduce maintenance costs. Airborne contaminants often migrate throughout an entire facility, including manufacturing and office areas. As they settle on fixtures such as lighting and walkways, maintenance costs increase and productivity decreases.
4. Comply with regulations. The Occupational Safety and Health Administration (OSHA) has detailed ventilation requirements for welding and cutting metals. The work area must be ventilated mechanically. Shops that work with stainless steel and other metals containing carcinogens need to be particularly conscious of both the airborne contaminants levels that are allowed by regulation and the levels that are deemed acceptable by the company.
5. Reduce health insurance premiums and workers' compensation costs. The work team's health comes first, and a healthy work force is good for everyone.

A Look and Learn Tour

The first step when selecting an in-plant air filtration system is to make a facility and process evaluation. This analysis includes observing manufacturing and production operations such as the work process and layout, materials used, and existing exhaust and ventilation systems.

Airborne pollutants should be identified and measured professionally to determine a benchmark for collection and filtration methods for each process.

For welding processes, parameters include the type and amount of wire and metal being used and the weld methods being performed.For cutting operations, considerations include the type of metal being cut, the amount of metal removed from the sheet, the length of processing time, and air requirements.

With a proper and thorough evaluation, a correct air filtration system can be identified with relative accuracy, and calculations can be made to determine the likely end result of the installation, including contaminant collection efficiency levels and system/filter maintenance cycles.

Figure 1: Mechanical arm systems can be positioned close to breathing areas to remove smoke without obstructing lighting or sight lines.

Contaminant Collection

By incorporating the facility and process evaluation results with proper application engineering, an efficient and effective air filtration system can be identified. Welding and cutting applications most often are handled with any of three basic collection methods:

1. Source capture systems. Also known as mechanical arm systems (see Figure 1), these systems can be adjusted to bring the collection hood close to the worker's breathing zone without interfering with overhead lighting or the worker's sight line. Source capture maximizes pollutant capture, making this system more than 99 percent efficient.
   

   Source capture methods also can be integrated with portable filtration systems, combining the versatility of portable systems with the source capture system's collection efficiency.

2. Free-hanging systems. Facility layout often cannot accommodate source capture systems, in which case free-hanging, or ambient, collection methods may be best (see Figure 2). In this configuration, filtration units are ceiling-mounted, typically in a circular or oval pattern, depending on the layout of the work area. Airborne pollutants then are drawn into the filtration units by the air pattern created by the system.

   Pollutants are not removed from the worker's breathing zone immediately with free-hanging systems, but most free-hanging systems can reduce airborne particulate by 75 to 80 percent.

3. Directional draft systems. With directional draft systems, a slotted surface draws pollutants from the worker's breathing zone with high collection efficiency, often more than 99 percent. Horizontal workstations, commonly known as downdraft benches, provide a grated work surface that draws particulate down and away from the breathing zone.

   Vertical orientations, or power booth systems, can be positioned near permanent, or fixed, workstations where large items are worked on (see Figure 3). Power booth systems pull the particulate away from the breathing zone without interfering with the workplace or shop layout.

Filtration

The collection process and filtration method must be considered together. Typical filtration options for welding and cutting applications include:

1. Cartridge systems. Cartridge filters collect smoke, dust, and fumes from welding and cutting applications. The filters come in a variety of sizes and shapes and are designed to trap particulate mechanically in the filter pleats. They are available in renewable or disposable configurations.

   Disposable filters are discarded when they are filled with particulate and collection efficiency has dropped noticeably. Renewable cartridge filters are made of materials designed to release particulate during cleaning.

   Filter-cleaning systems often have pulsed-air or filter-wrapping configurations (see Figure 4). A filter-cleaning system that combines both pulsed air and filter wrapping may clean the filter to within 10 percent of the system's original pressure differential. Better cleaning extends filter life and overall system operating efficiency. In typical usage and maintenance cycles, filters commonly can stay in place for six to 12 months.

2. Bag filters. Dust collection from a variety of metal fabricating applications can be accomplished inexpensively with conventional cloth bag filters. Premium bag filters can remove up to 99 percent of contaminants.

   Vee-bag filters, constructed of polyester-type materials, are used for capturing sticky or oily dust that can be generated by coated metals. They usually work at a 95 percent efficiency level.

3. Electrostatic precipitators (ESPs). ESPs use electrostatic charging to ionize the contaminants as they flow through the air path. Ionized contaminants then are collected with oppositely charged collection plates. This collection method is best-suited for contaminants such as submicron smoke or oily particles that may clog other filters prematurely. Although efficient in collecting these types of contaminants, ESPs also can be maintenance-intensive compared to cartridge filters.

System Selection

System selection criteria often are balanced among one-time installation costs, operating costs, and filter costs and labor associated with system maintenance. Maintenance and wear are reduced on a system that exceeds an installation's basic operating requirements. It will operate more efficiently, and the larger system blower will lengthen maintenance cycles; however, initial equipment and system installation costs may be greater.

A system sized to the exact needs of the installation may require more filter maintenance during its useful life but may cost less to install.

Facilities management personnel, in cooperation with their systems provider, must determine their own balance among installation costs, operating efficiency, and maintenance costs when selecting an in-plant air filtration system.