Recirculating air from dust collectors

Whether dust collectors are used in a plant to control indoor air quality (IAQ), keep equipment clean, and/or recover high-value process dusts, many plants are considering recirculating the air back into the plant downstream of collectors instead of exhausting it outdoors. When using recirculating dust collection systems, special safety and performance concerns must be addressed.


Key concepts
OSHA regulations apply when recirculating contaminated indoor air.
Recirculated indoor air can save energy.
Select dust removal equipment based on performance, not efficiency.
Safety issues
Meeting OSHA IAQ standards
Filter media
Fire safety and explosions

Whether dust collectors are used in a plant to control indoor air quality (IAQ), keep equipment clean, and/or recover high-value process dusts, many plants are considering recirculating the air back into the plant downstream of collectors instead of exhausting it outdoors. When using recirculating dust collection systems, special safety and performance concerns must be addressed.


There are at least three positive reasons to recirculate indoor air.

Less regulatory paperwork

When contaminated air is exhausted outdoors, the EPA must be satisfied that the exhausted air is in compliance with current standards, a process that involves time-consuming permit applications, testing, and regulatory paperwork. By containing the air totally inside a building, the plant engineer deals with OSHA instead of the EPA — a less daunting prospect, even though OSHA indoor air quality standards are becoming increasingly stringent.

OSHA does not require permits or collector testing, but they do require a plant to meet certain indoor air quality standards, no matter how compliance is achieved.

OSHA may require the use of a personal monitor on an individual in the workplace and perform an 8-hr, time-weighted average (TWA) test to make sure contaminants are below allowable levels. Unlike the EPA, they will not test the dust collector or require that a permit be filed for the system.

In some states, the local EPA may still want to permit an indoor collector. Plant engineers should check with local agencies to find out their position on recirculating dust collectors.

Substantial energy savings

When a dust collector is designed to recirculate heated or cooled air back through the plant, the cost to replace that conditioned air is eliminated — an expense that can be substantial.

Consider the example of a 10,000-cfm dust collector where the outside temperature is 10 F. Use of a recirculating dust collection system could save an estimated $1600 per month during the winter — the approximate cost to heat an equivalent amount of replacement air to 70 F, based on an energy cost of $0.60 per ccf (100 cu ft of natural gas), an 8-hr day, and a 5-day work week.

Dust collecting systems for welding shops with high ceilings can often improve the efficiency of a heating system by taking hot air off the ceiling and delivering it at ground level.

Fig. 1. A full scale dust testing apparatus can be brought on site to test actual conditions

Being a good neighbor

An indoor dust collection system is not subject to unneeded scrutiny by commercial or residential neighbors. Outdoor systems and exhaust stacks can be a frequent source of community concern and potential complaints. These issues are eliminated with a recirculating system.

Safety issues

Some types of contaminants must be exhausted outdoors. This includes combustion gases and unusual gas stream constituents that cannot be adequately handled by a particulate removal system. When in doubt about a given application, ask a supplier or consultant with dust collection engineering expertise three key questions:

  • How can OSHA emission requirements be achieved?

  • How can fire safety/explosion concerns be met?

  • How should the air be recirculated for energy efficiency and worker comfort?

    • Meeting OSHA IAQ standards

      A crucial concern with any recirculating dust collector is to ensure the system has adequately removed the dust to protect workers' health. To do so, several factors must be evaluated.

      The first step is to ascertain the allowable indoor limit for the dust being captured. OSHA has set an indoor limit of 5 mg of nonspecific or nuisance dust (&10 microns in size) per cubic meter of air. Toxic dusts, such as silica, carry an indoor limit of only 0.05 mg per cubic meter — 100 times stricter than the allowable threshold for nuisance dust.

      While OSHA guidelines must be met, plants should follow the guidelines published by the American Conference of Governmental Industrial Hygienists (ACGIH). The guidelines in this manual are often just a little tighter than those OSHA has adopted, and by meeting these guidelines, the plant engineer can ensure regulatory compliance (see Table 1).

      Next, select a particulate removal system that meets IAQ requirements. Whatever brand or type of equipment is used, obtain a guarantee from the manufacturer for the maximum emissions rate (milligrams of dust per cubic meter of air) for the equipment over an 8-hr TWA.

      Do not accept efficiency stated as a percentage, even if the manufacturer states 99.99% efficiency. OSHA only cares that the quantified amount of dust in the air is below established limits. If the established limit is an average of 5 mg/cu m, the manufacturer must provide a guarantee of something less that that, preferably at least half of the limit.

      In most cases, a high efficiency cartridge dust collector will be the system of choice. These systems can be designed to produce emissions well below OSHA limits, except with the most hazardous dusts.

      Today's cartridge collectors can be used for a wider range of processes than in the past, thanks to the advent of new media, improved pleat spacing, and increased application knowledge.

      Pulsejet baghouses often have barely acceptable emission performance. Recirculation directly off a baghouse is generally not recommended unless a very high efficiency media is used.

      To determine the best collector design for a recirculating application, dust testing is strongly recommended, especially wherever toxic dusts are involved. A qualified test laboratory can perform a series of bench tests on a representative dust sample to determine its characteristics — which can influence collector design.

      For example, a particle size analyzer reveals the particle size distribution of the dust, down to the submicron range. This information can be very helpful in determining the filtration efficiency needed to meet indoor emission standards.

      Additional tests can provide a visual analysis of the dust, determine its specific gravity, identify moisture content, absorbency, abrasiveness, and other characteristics. These properties aid the engineer in selecting filter media, hardware, and other components based on scientific analysis rather than guesswork.

      In some cases, after bench testing is completed, full-scale testing, using one or more actual dust collectors, may be needed (Fig. 1). Full-scale testing is sometimes used to predict the behavior of an unusual or difficult dust. It can help ensure compliance with strict emission standards for processes that generate toxic dust and fumes — for example, the cutting or welding of galvanized material.

      Tests can be run using either real-time or accelerated testing that simulates actual operating conditions. Many performance variables can be evaluated in this manner — including different media types, filter configurations, air-to-cloth ratios, temperatures, airflows and dust loading conditions. Plant engineers may view the testing and make changes in a "what if" context to evaluate the impact of different variables.

      Filter media

      Selecting the right filter media is critical to meeting emission requirements. The most commonly used media is a nonwoven cellulose/synthetic blend that provides an economical choice for dry dusts and operating temperatures up to 240 F.

      Polyester/silicon blended media with a melt-blown synthetic applied to the surface deliver superior efficiencies — achieving emissions as low as 1 mg/cu m or less, far below the OSHA threshold for nuisance dusts. They also feature a smooth surface that offers better dust release characteristics, for more efficient cartridge cleaning.

      Spun-bond media are excellent performers where there are hot, moist gas streams, sticky dusts, or the need for frequent wash-down of cartridges. Spun-bond filters do have a place in recirculating systems. But due to their higher cost, they are usually limited to use with difficult dusts.

      Filter suppliers can provide efficiency curves to help compare performance of different filtration media on various size dust particles. When evaluating media, the plant engineer should avoid placing too much emphasis on efficiency claims stated in percentages. These claims say little about actual emission performance.

      No matter what goes into a dust collector, the air that comes out must meet OSHA requirements. A reputable dust collector manufacturer should be able to guarantee the emission performance of a system handling a specific dust.


      What if a cartridge should rupture, releasing dust back into the plant? In this event, workers may be exposed to unacceptable contaminant levels. To avoid this possibility, the use of a safety monitoring system is recommended with recirculating dust collectors.

      Such systems typically include a side-access housing, prefilter, and high efficiency ASHRAE filter, which together form a backup system to keep emissions at acceptable levels in the event of a dust collector failure.

      Where toxic dusts are present, a safety monitoring system is mandatory and should always use a HEPA filter as the final filter. HEPA filters, commonly found in critical applications, achieve near-zero emissions and allow plant engineers to meet even the most stringent indoor air requirements.

      A remote monitoring and control system is an option worth considering, especially for large pollution control systems with multiple collectors. These control systems can electronically monitor an entire network of dust collectors, providing automatic alarming of fault conditions and troubleshooting problems as soon as they occur. They can also help to lower emissions and extend filter life through electronic control of cleaning cycles.

      Fire safety and explosions

      If the dust being captured is explosive or flammable, special safety concerns must be addressed. While highly explosive dusts such as aluminum powder should be exhausted outside, many other flammable dusts can be handled with a recirculating dust collector, as long as adequate safety precautions are taken.

      Explosions are a big concern. The explosive power of a dust is denoted as Kst, the rate of pressure rise (see Table 2). Both NFPA and Factory Mutual use this value in formulas to calculate the amount of explosion vent area required for a dust collector. Collectors requiring explosion vents should be located outside and vent away from buildings and populated locations.

      If the collector must be located inside, the plant engineer can duct an explosion outside through a very short duct (9 ft or less). This design adds back-pressure and usually requires reinforcing the collector to handle the increase in pressure. If it is not feasible to duct to the outside, the collector should be outfitted with an explosion suppression system. An indoor suppression system may cost more than the dust collector itself.

      Fire is another safety concern with a dust collector. Welding, metal grinding, and similar operations can cause sparks to enter the collector inlet and start a fire. To prevent this, a spark suppression and detection system should be installed in the inlet ducting to sense and extinguish sparks or flames. Several different suppression systems are available utilizing water, chemicals, or inert gas.

      A diversion system that incorporates spark detection and diversion valves called "abort gates" is also available. When a spark is detected, the abort gate diverts the recirculated air stream to the outside, arresting the flame front before it reaches the dust collector. Companies that specialize in this equipment can help design a spark detection and extinguishing system.


      After the air has been adequately cleaned and safety issues addressed, how should the air be recirculated downstream of the dust collector? Ideally, to maintain balance and optimize energy conservation, the return air duct should allow distribution to the same areas from where the air was originally exhausted. A common mistake is to draw the air out of one room and recirculate it elsewhere, creating areas of negative and positive pressure.

      A well-designed recirculating system not only saves energy; it can actually enhance worker comfort. For example, a system that serves multiple welding stations might consist of one long duct with adjustable diffusers at each station. This design allows personnel to use the diffusers like individual fans, directing the air toward or away from their workstations as desired. The Industrial Ventilation Manual (see "More info" box) expands on this area in great detail, and its use is recommended.

      There are two possible return air configurations. A general ventilation system with zone return, used in cold climates, will recover heat from the ceiling and return it to the work area (Fig. 2). This configuration is also useful when the process does not allow the use of source capture hoods. A major disadvantage is that a general ventilation dust collection system requires larger airflows, fans, and filters, resulting in higher equipment and operational costs.

      Fig. 2. General ventilation arrangements with zone returns captures ambient air for filtration

      Fig. 3. Source capture ventilation with zone returns uses hoods for capturing dust where it is generated

      In a source capture system with zone return, hoods are added over each workstation (Fig. 3). This is a more efficient air distribution system with lower airflow, fan, and filter requirements. However, it can only be used with stationary processes.

      — Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135,

      ACGIH recommended indoor air quality limits

      Dust typeTWA, mg/m3
      Graphite, except fibers2
      Particles not otherwise classified
      %%POINT%% Inhalable10
      %%POINT%% Respirable3
      Silica - amorphous
      %%POINT%% Diatomaceous earth (uncalcined)
      %%POINT%% Precipitated silica10
      %%POINT%% Silica, fume2
      %%POINT%% Silica, fused0.1
      %%POINT%% Silica, crystalline cristobalite0.05
      %%POINT%% Quartz0.1
      %%POINT%% Tridymite0.05
      %%POINT%% Tripoli0.1
      Talc, with no asbestos fibers2
      Zinc oxide

      Dust cloud ignitability and explosibility

      Weight %&Size (um)Median size umKs Bar*m/s
      Particle size distribution
      Dust type50025012571633220
      Fat powder (48% fat)100752479220
      Fish meal68231232035
      Fructose from filter993917150102
      Barley grain dust79512583240
      Oats grain dust6424829514
      Wheat grain dust483080112
      Coffee from filter1009989&1090
      Coffee refined10011
      Cocoa bean shell dust10068
      Cocoa/sugar mixture532050043
      Potato granulate10021
      Potato flour865326176569
      Rice flour10057
      Rye flour947658152979
      Soy bean flour85635020110

      More info

      Guidelines for meeting IAQ standards can be found in the back of the Industrial Ventilation Manual, A Guide of Recommended Practice, 23 rd edition, published by the ACGIH and available by calling 513-742-2020.

      Author Information
      The author is available to answer questions about recirculating indoor air. Mr. Morgan can be reached at 800-479-6801, , or visit the website at .

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Doubling down on digital manufacturing; Data driving predictive maintenance; Electric motors and generators; Rewarding operational improvement
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Research team developing Tesla coil designs; Implementing wireless process sensing
Commissioning electrical systems; Designing emergency and standby generator systems; Paralleling switchgear generator systems
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me