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Industrial Ventilation Fundamentals - A Primer on Indoor Air Quality

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Individuals may be exposed to a variety of contaminants in a manufacturing plant.  Common airborne contaminants include carbon monoxide from poorly-tuned forklifts, welding smoke, dust, process vapors, and volatile organic compounds.  Levels of these types of contaminants are typically low enough that they due not pose a hazard to employees, but there are some instances when these levels can rise, causing discomfort or even serious illness.

There are a number of methods for dealing with indoor contaminants.  Many Iowa manufacturers ventilate their plants with either natural ventilation (doors and windows), mechanical ventilation (wall or ceiling exhaust fans, paint booths, make-up air units, etc.), or a combination of the two methods.  Unfortunately, as outdoor temperatures fall, employees often shut doors and windows to keep the plant warm.  As doors and windows are closed, exhaust fans, paint booths, and dust collection systems can be starved for air if there is not a sufficient supply of makeup air.  The performance of ventilation systems can then degrade and the building pressure may decrease to a point where down drafting of combustion vents occur, creating an additional health risk.

Air quality does not refer exclusively to contaminants.  Excessive humidity levels in a plant can also lead to manufacturing problems.  High humidity levels can hasten mold growth and corrosion of metals.  Variations in humidity can cause paper to expand and contract, causing difficulties for printers.  High humidity levels can significantly slow the rate at which process drying occurs.

There are four main strategies for improving indoor air quality:

  • eliminating or reducing sources of contamination
  • capturing contaminants at the source followed by cleaning and re-circulation or exhaust
  • diluting by increased ventilation
  • general air cleaning

General air cleaning is usually not the primary choice for air quality control since it is typically inefficient to try to remove contaminants after they have been dispersed.  It is often used as a secondary tactic to capture residual matter not captured by other methods.  Here we focus on the first three approaches.

Eliminating or reducing contaminants is often the most overlooked, yet possibly most economical method of improving indoor air quality.  Approaches include:

  • Design of products - Efficient design of products can reduce contaminates generation.
  • Improved manufacturing processes - Some processes can be enclosed to contain contaminants or another process that generates fewer contaminants might be substituted.
  • Tune equipment - Adjusting machinery can significantly affect contaminant production.  For example, less than one turn of a jet adjusting screw on an LPG-powered forklift can change CO emissions from 50,000 ppm to 500 ppm without a noticeable change in engine operation, exhaust color, or exhaust odor.
  • Proper operation - The correct operation of machinery can have a large impact on indoor air contaminants.  By employing proper painting methods, the Iowa Waste Reduction Center’s STAR program (Spray Technique Analysis and Research) has reduced material by 29% and cut hazardous air emissions by 31%.

The second method of improving plant indoor air quality involves containment and removal of the contaminant before it escapes into the work environment (local source control).  If the contaminant is diluted by plant air, a much greater volume of plant air must be exhausted or cleaned to remove the contaminant.  Hood design is important and can have a large impact on the effectiveness of the extraction process.

Flexible-arm fume extractors can be used if the source moves within a region.  Once the contaminated air is extracted, it can be exhausted to the atmosphere, cleaned and then exhausted, or cleaned and returned to the plant.  The Iowa Department of Natural Resources (DNR) view of plant exhaust depends on whether the exhaust is fugitive in nature or is a regulated pollutant.  Air quality construction permits may be necessary if regulated pollutants are discharged through a vent, exhaust fan, or smokestack.

Methods to clean contaminated air depend on whether the contaminant is a gas or particulate.  Gases can be removed by ventilation fans, paint booths, etc., but can also be cleaned by numerous chemical and physical processes if it is necessary to return the air to the plant.  The cost of these methods must be weighed against the cost to replenish the exhaust air.

Methods to collect particulate contaminants are divided into two basic groups: filters and dust collectors.  Filtering is used to remove low concentration levels of contaminants.  The heavy loads associated with industrial settings are typically removed with a dust collection system. A variety of methods are available:

  • Electrostatic precipitators are commonly used to remove oil mists and welding smoke.
  • Fabric collectors filter the air that passes through them; captured particulates are removed from the fabric by mechanical agitation or air motion.
  • Wet collectors, or scrubbers, collect particulates by passing the air stream past a liquid; high temperature and moist streams can be handled.
  • Dry centrifugal collectors are typically used to remove large particles from an air stream; the cyclone separator is one of the more common devices in this category.

The method and equipment chosen depends on numerous factors including type of contaminant, particle size distribution, particulate volume, flow rate, moisture level, cost, and contaminant disposal.

Dilution by ventilation is the third method of decreasing exposure to indoor air contaminants.  This approach is conceptually simple - provide sufficient make-up air to the plant to dilute contaminants to safe levels.  This is a common approach in many Iowa manufacturing plants that do not use air conditioning.  Correctly sized roof- or wall-mounted vent fans, together with open windows and doors, can provide sufficient ventilation for three seasons.  Unfortunately, if doors are shut during winter months, indoor air quality can suffer.  Heated makeup air units, possibly with energy recovery technology, can be used to remedy this problem.

To keep your employees and your company healthy and productive, it is important to understand what your employees are exposed to, what is being vented to the atmosphere, and what the most efficient method of maintaining indoor air quality is in your plant.  OSHA regulates the quality of indoor air.  Employee exposure to contaminants can be ascertained by implementing personal employee sampling, where employees wear a monitor for up to eight hours at a time.  This data can be gathered by OSHA Consultation at no cost, by outside consultants for a fee, or possibly by your insurance company.  If OSHA Consultation finds that employees are exposed to levels above their permissible exposure limits, then you must correct what they find.  Questions concerning requirements should be directed to OSHA Consultation at 515-965-7162.

Information on DNR requirements concerning discharging contaminants can be obtained from DNR’s Des Moines offices at 515-281-8189.  The Iowa Waste Reduction Center at 800-422-3109 currently has a program to assist small manufacturers (less than 100 employees) with DNR permitting at no cost.

To ensure that your building meets applicable codes and standards, a qualified HVAC expert should conduct the design of your plant ventilation system.

Bibliography

1999 ASHRAE HANDBOOK - Heating, Ventilating, and Air-Conditioning APPLICATIONS, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1999.

Cox, R. A., Improving Indoor Air Quality, Ciras News, Vol. 34, No. 2, 2000.

Greiner, T., Manage Risk of LPG-Powered Forklifts, CIRAS News, Vol. 34, No. 1, 1999.

Industrial Ventilation – A Manual of Recommended Practice, 23rd Edition, American Conference of Governmental Industrial Hygienists, 1998.

Ventilation for Acceptable Indoor Air Quality, ASHRAE Standard 62-1989.