Proper mist collection necessary for shop floor safety

The metalworking fluids (MWFs) commonly applied during machining processes generate airborne mists that must be carefully controlled. Otherwise, the mists pose a variety of health risks to workers and create a dirty and unsafe work environment. It is important to understand the hazards associated with oil mists, as well as exposure limits set by the Occupational Safety and Health Administration (OSHA) and other entities. This article will review these issues and describe the role of proper mist collection in keeping machine tools and the surrounding shop area clean.

Regulations and Guidelines

The emissions generated by machining processes are formed because of a combination of mechanical and thermal effects and fall into three general categories:

• Coolant-mist liquid aerosols, which are formed via condensation as a result of cooling or mechanical processes.
• Coolant vapors, i.e., gaseous-phase substances that result from heating liquid coolant.
• Coolant fumes, the finest solid particles in the air, which are generally formed during combustion processes.

Three different entities have established and published exposure limits that apply to MWFs: OSHA, National Institute for Occupational Safety and Health, and American Conference of Governmental Hygienists (Table 1 below).

​Table 1. Exposure Limits for Metalworking Fluids

OSHA also published the Metalworking Fluids: Safety and Health Best Practices Manual in 2001, which incorporates recommendations from multiple sources. It does not carry the force of law or regulation, but it is meant to be advisory and informational. Its goal is to help employers develop prevention programs that will create safer workplaces for employees exposed to MWFs.

The list of adverse health effects associated with MWFs is long. It includes respiratory ailments such as asthma, chronic bronchitis and hypersensitivity pneumonitis; pancreatic, colon, bladder and liver cancers and tumors of the brain and respiratory organs; contact dermatitis, eczema and other skin disorders; irritation to the respiratory tract and mucous membranes; and changes in organs, as well as potential nerve damage, damage to genetic material and immune
disorders.

The exposure limits defined in Table 1 have been developed to protect workers against many of these adverse health effects. If your facility is meeting OSHA requirements but workers are still experiencing symptoms, it may be necessary to set lower limits.

There are two general categories of MWFs used in machining processes:

• Emulsion coolants: Water-soluble and water-mixed coolants are cooling-lubricant concentrates that are diluted with water, to their usage concentration, prior to use. The oil or lubricant proportion is typically about 5 to 11 percent. They offer effective heat dissipation and are used primarily for cooling.
• Pure, or “straight oil,” coolants: Non-water-soluble coolants are not mixed with water and are used according to the composition provided by the manufacturer. Straight oils are generally used for their excellent lubricating properties–unlike water-mixed emulsion compounds, which provide some lubrication but primarily cool.

Figure 1. A fiberglass V-bag-style mist collector.

Mist Collectors

A variety of equipment captures mists generated when these coolants and lubricants are applied. Most commonly used are multiple-stage, fiberglass V-bag mist collectors (Figure 1). These units feature a first-stage Chevron metal filter, a second-stage aluminum mesh filter and a third-stage fiberglass V-bag with a 95 percent ASHRAE (formerly American Society of Heating, Refrigerating and Air-Conditioning Engineers) efficiency rating. The efficiency rating is a bit misleading in that it has nothing to do with oil removal. The ASHRAE rating system is used to measure efficiency in removing dry particulates. Most units also offer an optional fourth-stage HEPA final filter for added protection and cleanliness. A HEPA filter is usually designed to remove 99.97 percent of airborne particles measuring 0.3µm or greater in diameter.

Also prevalent are centrifugal-type mist collectors that use a rotating drum to spin the oil (Figure 2). Typically, a pad inside the unit functions as a final filter, but most contaminants are removed by the rotating action of the drum. If chips collect in too high a volume, the unit can become unbalanced and malfunction, creating a health hazard and a maintenance headache.

Figure 2. A centrifugal-type mist collector.

Both styles of collectors are usually designed for double duty, meaning they are used on straight oil and emulsion coolants. As a rule of thumb, they work well in light-duty applications but are not as effective for heavy-duty use and long production runs. V-bag and centrifugal collectors have a relatively low initial cost but are limited in terms of run time, filter service life and filtration efficiencies.

Because of these limitations, many parts manufacturers recognize that one collector size and type does not fit all applications. Straight oils and emulsion coolants have different properties and characteristics. As a result, consider a
collection system that is specifically engineered for use with one type or the other.