Essentials of industrial air quality
No other standards in the United States exist for air quality inside industrial plants aside from OSHA’s Permissible Exposure Limits (PELs).
OSHA laws on interior air pollution control concern air contaminants inside industrial buildings. The laws establish permissible exposure limits for over 500 regulated substances. These are routinely found in manufacturing operations that, through short or long-term exposure, can create unsafe working environments (see table).
Through OSHA rulings, employees have rights, which ensure them a safe, healthy, and hazard-free workplace environment. Employers are legally mandated to reduce workplace hazards, implement programs aimed at promoting job safety and good health, and meet OSHA standards.
Dilution ventilation is much less effective than local exhaust for reducing contamination from point source emissions. It should be used for control only when sources are distributed and not subject to capture by an exhaust hood.
Particulate and gaseous contaminants are most effectively controlled at the source so that they are not dispersed throughout the plant or allowed to increase to toxic concentration levels.
Because of varying sensitivities among people, one individual may react to a particular IAQ problem while others may have no ill effects. Symptoms that are limited to a single person can also occur when only one workstation receives the bulk of the pollutant dose. In other cases, complaints may be widespread.
A single indoor air pollutant or problem can trigger different reactions from different people. Some may not be affected at all. Information about the types of symptoms can sometimes lead directly to solutions. However, symptom information is more likely to be useful for identifying the timing and conditions under which problems occur.
Short work weeks can allow workers to have two full-time jobs, perhaps with similar exposures, and may result in overexposure even if neither job, by itself, entails overexposure.
Airborne particles can be in the form of smoke, mist, fumes, dust, and aerosols. The type of filtration used to remove airborne particles from airstreams depends on their size.
Visible particles, those greater than 50 microns, generally compromise less than 10% of the total airborne particles in an industrial environment. They tend to settle on horizontal surfaces, where dusting or vacuuming can remove them. Filtering equipment easily collects visible particles. Their greatest harm is the housekeeping problems they create.
Sub-micronic or invisible particles, less than 50 microns, can collect on vertical as well as horizontal surfaces. These singly invisible particles, some less than 10 microns, are the most harmful to human health. The human body is vulnerable to heavy concentrations of these small particles that are called a respirable or inhalable fraction.
This submicronic range is typical in oil mist, smoke, thermally generated hydrocarbons, welding smoke, and metal oxides. Because of their extremely small size, these particles bypass the body’s natural filtration mechanisms. Potential chemical reactions and immunological effects make these particles a very serious health threat.
The most practical way to control particulate in the respirable range, 0.1 to 10 microns, is two-stage electrostatic precipitators or reverse-pulse dust collectors. The preferred choice to filter and capture industrial dusts 10 microns and larger is with mechanical filtration devices (Fig. 1).
Electrostatic precipitators control air pollutants from processes in the range of 10 microns or smaller. They provide high collection efficiency at a low expenditure of energy. The precipitators operate by using high voltage to electrically charge or ionize particles in a process gas stream. The charged particles are then collected on electrically grounded plates.
This device can be combined with pre-cleaners, coolers, gas conditioners, and scrubbers, depending on the application and the pollutants involved.
Mist collectors are self-contained units that use centrifugal force from a rotating drum to collect and agglomerate mist particles, even submicron particles, against a throwaway filter liner (Fig. 2). For difficult combinations of mist and submicron smoke, prefilters and after filters can be added.
Mechanical filter devices are preferred for capturing large amounts and a wide variety of industrial dusts. They are designed primarily for dry dusts but some designs can tolerate a small amount of liquid contaminants. Excess fluid can cause filter failure due to plugging.
Cyclones are the simplest and oldest type of dust collection device. They operate by spinning the collected dust against the outside wall, using centrifugal force from intake air. Gravity and internal deflectors direct the dust-laden air in a downward spiral. The dust drops out and is discharged at the bottom. The clean airstream turns and spirals up the center and is exhausted.
Continuous-cleaning cartridge filter units use pleated paper or polyester as the filter media. The cartridges are cleaned online during dust collection. This maintains a relatively constant pressure drop across the filters, allowing a constant air volume for particulate capture.
Cartridge units offer high efficiencies and trap up to 99% of submicron particles and virtually 100% of larger particles. Selection is done by using an air-to-media ratio based on the type of contaminant (Fig. 3).
Bag house dust collectors are tubular bag devices that can be cleaned online during dust collection. An advantage of a jet bag collector is that it maintains a consistent pressure drop during the cleaning process, resulting in a relatively constant airflow and energy savings. These devices are very efficient in collecting fibrous and other large particles in high concentration levels (see cover).
Self-contained media fan/filter units are a simple form of filtration device. They remove dust particles 10-microns and larger. Contaminated air is passed through a prefilter where larger particles are collected, then through a primary filter where smaller particles are captured. These units offer the convenience of a total package that can be installed almost anywhere.
Downdraft welding tables are a specialized, self-contained workstation. They protect welders from toxic welding fumes and particles. A steel grating surface and shallow capture plenum form a dust-free work surface suitable for standing or sitting with comfortable knee and leg room. The particles are moved by a fan and collected in high-efficiency filters, then recirculated or exhausted from the area (Fig. 4).
The selection criterion for a filtration device depends on what and how much particulate must be collected. Because of the wide variety of choices available, it may be necessary to contact a specialist in this area for advice (Fig. 5).
PLANT ENGINEERING magazine extends its appreciation to AAF International for its assistance in the preparation of this article and in providing the cover illustration.
Typical exposure limits
|Substance||Threshold limit values||Short-term exposure limit|
|Time-weighted average 8-hr day, 40-hr week||15-min. with 60-min. break|
|Note: Threshold Limit Values refer to airborne concentrations of substances and represent conditions under which it is believed nearly all workers may be repeatedly exposed day after day without adverse health effects. Some workers may suffer effects at lower concentrations. From American Conference of Governmental Industrial Hygienists (ACGIH): 2002 TLVs|
|Acetic acid||10 ppm||15 ppm|
|Ammonia||25 ppm||35 ppm|
|Acetone||500 ppm||750 ppm|
|Welding fume||5 mg/m3|
|Benzene||0.5 ppm||2.5 ppm|
|Carbon dioxide||5000 ppm||30,000 ppm|
|Carbon monoxide||25 ppm|
|Carbon tetrachloride||5 ppm||10 ppm|
|Gasoline||300 ppm||500 ppm|
|Methyl acrylate||2 ppm|
|Methyl ethyl ketone||200 ppm||300 ppm|
|Trichloroethylene||50 ppm||100 ppm|
Common industrial, indoor air contaminants
Sources: Silicone caulking compounds
Effects: Eye, respiratory, and mucous membrane irritation
Sources: Improperly vented processes which produce combustion products
Effects: Difficulty concentrating, drowsiness, increased respiration rate
Sources: Fossil-fuel engine exhausts, tobacco smoke
Effects: Dizziness, headache, nausea, cyanosis, cardiovascular effects, and death
Sources: Combustion products from gas furnaces, welding, gas and diesel engine exhausts, tobacco smoke
Effects: Eye, respiratory, and mucous membrane irritation
Volatile Organic Compounds (VOC’s)
These include trichloroethylene, benzene, toluene, methyl ethyl ketone, alcohols, methacrylates, and polycyclic aromatic hydrocarbons
Sources: Paints, cleaning compounds, glues, silicone caulking materials, asphalt, gasoline vapors, tobacco smoke
Effects: Nausea, dizziness, headache, fatigue, and eye, respiratory tract, and mucous membrane irritation
Sources: Fibrous glass and mineral wool
Effects: Irritation to eyes, skin, and lungs, dermatitis
Sources: Air handling system condensate, cooling towers, high-humidity indoor areas, damp organic material, and porous wet surfaces
Effects: Allergic reactions and infections, chills, fever, muscle ache, cough, headache, nausea
Small particulate: 0.01 to 10 microns
Dry Welding Arc and flame cutting Metal spraying
Wet Flute grinding Cold heading Heat-treating
Large particulate: 10 to 100 microns
Dry Bulk powder manufacture Cement dust Foundry dust
Wet Water soluble machining Coolant mists Rolling mill over spray