Workplace air quality has become an important issue to employers and employees. Both simple and complex manufacturing processes generate pollutants that produce harmful airborne particles throughout the workplace, affecting both the manufacturing process and the employees in the plant.
The pollutants — dust, smoke, and mist — can interfere with plant operation, contaminate process work, and necessitate the need for added labor to clean the facility continuously. These pollutants also affect employee health and safety; continued exposure to process emissions can cause health problems for employees and result in lost time. In addition, as the marketplace becomes more competitive, demands for quality employees are increasing, and available applicants can be choosy about working only for an environmentally concerned company.
Plant managers must give these issues due attention and decide where the least economic risk lies: in installing an air cleaning system or in doing nothing. For many, the least-risky choice is to install a system.
Finding the source
As an employer, it is difficult to look at a manufacturing area and realize how an air quality problem can be solved. Many times, it is easy to see the resulting particulate problem, but hard to identify the generating source. To improve the air quality and provide clean air, companies need to first find the source.
If the particulate is lightweight, it can be suspended in air for some time and can easily migrate to and contaminate other area so the plant. Often, dust will come from one particular small source and then escape, traveling over a large area and creating a housekeeping headache. Welding smoke and machining mist are examples of these types of contaminates. Heavy particulate is unable to travel to other areas, so it tends to remain in the location of generation.
A simple way to determine the source and type of particulate is to look around the area of generation and see if the material is widespread or in the immediate area. Look to see if a cloud of dust is coming from one particular location.
Once the source and particulate type are determined, two options are available for controlling the fugitive dust emissions: collection (source capture) or containment.
Source capture
The source capture technique collects contaminants directly at the point of generation, drawing off particulates before they have a chance to become airborne and spread throughout the plant.
Hooding is best used for operations in which the hood will not interfere with the operation or the operator, when the hood can be placed close to the source, or when the generated particles are moving in one direction. A hood is usually required to capture particles that are generated by processes such as metal deburring and grinding that produce active particles at high speed. As long as the grinding wheel or saw blade remains in a fixed direction, the hood can be used to catch the particles.
Because the particles come off the wheels or blades faster than the air speed going into the hood, the hood area acts as a target or receiver.
As a hood is placed further from the source, the airflow requirements increase, and the collection efficiency decreases. A simple rule of thumb for controlling active dust generated at high and moderate velocity is to keep the hood within 12 to 16 in. of the source. When a low-velocity process creates particles (such as welding), the hood should be placed 12 to 18 in. from the source.
As the airflow requirements increase, so does the size of the dust collector and associated system costs. For this reason, it is extremely important to determine beforehand if hooding can even be used.
Close hooding for large, and especially manual, operations is nearly impossible. The hood would interfere with the operator’s performance on processes such as manual grinding and deburring. Trying to place hoods completely around manufacturing operations is usually futile. Airflow considerations rise dramatically, and the results are marginal at best.
Another source capture solution is the portable dust collection system. These systems take source capture directly to the source of the pollutants using swing arm assemblies that draw fumes, dust and other airborne pollutants directly from the workspace into the collector. Collection units are available for ceiling mounting or in mobile, wheeled units. Models are also available with halogen lights on the swing arm to provide additional lighting right at the work.
Portable dust collection systems have long been available, but have earned a bad reputation because workers don’t use them correctly and find them inconvenient and difficult to move around. However, portables have been found to be ideal for applications with very tight, hard-to-reach work areas and varying sizes, shapes, and locations of materials where flexibility or collection equipment is most important.
When neither close hooding or portable source capture make sense, the next-best option is containment.
Containment
Containment is often an overlooked option. Containment systems are often thought of as additional, large enclosures inside a building. In reality, a containment system is just a larger hood than what would be used in source capture.
The enclosure is placed around a particular operation, such as deburring, grinding or sanding. It becomes the collection hood for all the materials being generated inside the boot. The objective is to prevent airborne materials from escaping the enclosure, thus ensuring material will not contaminate another portion of the facility.
Usually, the necessary air speed to contain airborne dust is 150 feet per minute (FPM) or greater. Any active generation of dust must be aimed away from the opening of the enclosure and preferably toward the rear of the enclosure, where it is collected. The incoming airflow is at the operators back, so the operator is immersed with clean air and kept from breathing in dirty air.
Enclosures can be drop curtains, dry-wall construction, or booth enclosures. The existing ceiling and walls can also be adapted to serve as a collection enclosure. Lighting is usually needed and must be installed according to the containment method selected.
If noise is an added problem with the operation, then attention should be given to reduce the transmission of the noise, both within the enclosure and outside it. “Sandwich walls” with perforated inner panels are a good choice for noise reduction.
The enclosure can be ducted to a remote dust collector or ducted with integral dust collection modules attached to the rear of the booth. In either case, filter access is a requirement. The enclosure can be divided into several booths, and multiple operations can be performed and evacuated into separate dust collectors. Different sections of the enclosure can be used at different times.
In the case of remote versus integral module, often the dust that is being collected will determine the type of dust collector that should be used. For example, aluminum deburring or part grinding is usually ducted outside because of the potential for explosion.
The enclosure containment method does not collect as much material as source capture does, but it provides the control needed to keep particles from spreading throughout the workplace, creating housekeeping and health problems.
Summary
Both source capture and containment are common approaches to controlling airborne particles from their sources. Although source capture tends to be the preferred method, when it is not practical and will not fit the particular operation, containment is an option. The main objective of both methods of pollutant collection is to provide a safe and healthy work environment.
Bob Hild is western region sales manager with United Air Specialists Inc., a Clarcor Co. Contact him at Tel. 800-992-4422, fax 513-891-4882.