1. Will the dust collector ensure compliance?

Engineers today must deal with an increasingly complex alphabet soup of regulations as the EPA, OSHA and other organizations continue to tighten air quality and safety requirements. Meeting these requirements should be first and foremost in any dust collection game plan. Failure to comply may result in fines, production shutdowns or costly litigation. In one recent case, a federal jury awarded $20.5 million to the plaintiffs in a lawsuit involving inhalation of welding fumes.

OSHA has established permissible exposure limits for hundreds of dusts, ranging from nonspecific or “nuisance” dust to highly toxic substances. These limits are based on eight-hour time-weighted average (TWA) exposure.

One area of concern involves newly imposed limits on exposure to hexavalent chromium, a known carcinogen, generated by processes such as welding or cutting stainless steel, thermal spraying, and application of anticorrosion paints for aircraft and military use. OSHA has set thresholds as low as 5 micrograms per cubic meter TWA. This is 10 times stricter than the limits for some toxic dusts. Dust collectors will need to be equipped with very high-efficiency filtration media to meet such requirements.

How can engineers know if dust collectors will comply with emission thresholds? The equipment supplier should provide a written guarantee stating the maximum emissions rate for the equipment over an eight-hour TWA. Filter efficiency stated as a percentage is not an acceptable substitute, even if the supplier promises 99.9-percent efficiency. OSHA only cares that the quantified amount of dust in the air is below established limits.

While OSHA guidelines must be met, it is also good practice to follow the guidelines published by the American Conference of Governmental Industrial Hygienists (ACGIH). The guidelines in this manual are sometimes a little tighter than those OSHA has adopted.

Engineers also should become familiar with the National Fire Protection Association’s new NFPA 68 Standard on Explosion Protection by Deflagration Venting, which provides stringent and mandatory requirements for dust collection applications involving explosive dusts. The change from a guideline to a standard is enforced by OSHA, which in October 2007 launched a National Emphasis Program focusing on the safe handling of combustible dusts.

Explosive dusts can be organic or metallic in nature and are present in a long list of manufacturing industries, including agricultural, chemical, food, paper, pharmaceutical, textile and woodworking. The new directives are significant because many plants will now have to install updated dust collection/explosion venting equipment that is manufactured in accordance with NFPA standards to ensure compliance.

2. Will it fix the problem?

New filter designs with open, breathable pleats allow better media utilization for more efficient performance.

Though compliance is a major issue, it is not the only issue. A dust collector could be expected to reclaim valuable product, maintain a higher level of cleanliness in manufacturing areas, accommodate changes or expansions in the plant, or solve a performance problem experienced with an older dust collection system.

A good way to pinpoint objectives is by using a site survey form available from most equipment suppliers. This form typically calls for information on the process and the material to be collected, operating hours and conditions, electrical requirements, airflow and pressure ratings, and other specifics of the application.

The survey also will call for detailed information on the physical properties of the dust.

Even if the dust is a common type, such as wood dust, something in the process may cause it to behave differently. Therefore, dust should always be tested, preferably using a sample collected from used filters.

What are the median size and particle distribution of the dust? Is it in the shape of long fibers, uniform spheres or jagged crystals? Is it combustible? Is it sticky or hygroscopic? These are just some of the characteristics that can be determined through a series of bench tests available from independent laboratories and many equipment suppliers.

A site survey coupled with lab testing is a strong approach for determining the dust collector’s required filtration efficiency and pressure drop across the filter media and, from this, what type of collector design and media will be most effective for the application.

3. Will it perform reliably?

Collectors with horizontally mounted filters may be subject to problems from uneven dust loading.

Dust collection equipment often can be a maintenance headache, but this need not be the case. Reliability problems typically stem from neglecting or misunderstanding details about the unit’s performance during the initial selection process or when changes are made in the plant. By following the steps above, engineers can help to ensure more reliable performance from their dust collectors.

Although the site survey and lab analysis typically provide enough data, in some cases the engineer may opt to commission full-scale dust collection testing. Full-scale testing typically requires a large (55-gallon) dust sample that is run through dust collectors on a test rig in a simulation of real-life operating conditions. Pressure drop, dust load, filter media and other parameters can be varied to determine the optimal collector design. Full-scale testing is usually limited to analysis of difficult or hard-to-handle dusts, or applications where there is a history of chronic dust collector upsets.

When selecting equipment, it also helps to be aware of design and technological improvements that can enhance reliability and performance. Examples include the cartridge mounting orientation and pleat spacing.   Horizontal vs. vertical cartridge mounting: Some pleated filter cartridges are mounted on their sides. The biggest problem with horizontal mounting is that the dust does not get cleaned off the top of the filter, causing the dust to blind at least one third of the filter. Also, because incoming dust is dumped on the top of the filters, there is no pre-separation of heavy or abrasive particles from the air stream. This situation can shorten filter life or, in spark-generating applications, pose a fire hazard because any spark entering the collector will come into direct contact with filter cartridges.

An antidote to this problem is a system using vertically mounted cartridges. For example, the system may incorporate a high, side entry inlet with a series of staggered baffles that distribute the air and also separate out larger particles, dropping them straight into the hopper. This reduces the load on the filters and helps eliminate problems encountered with horizontal mounting.

Advances in pleat spacing: Most dust collection cartridges use tightly packed media configurations. Though they offer high efficiency, much of the media surface area is unavailable for filtering, allowing dust to remain trapped within the filter even after pulse cleaning.

A recently introduced pleating technology makes use of hot melt separators that open up the full length of the pleat, allowing the entire depth of the pleat to be utilized. Thus significantly higher air flows per square foot of media than what has been achieved in the past are achievable.

Because virtually all the media surface is exposed to the air stream, the filter holds more dust between cleaning pulses. The open, breathable design also results in significantly lower pressure drop as well as improved dust release characteristics during pulse cleaning, using fewer pulses, and therefore less energy.

4. Will it provide the best possible investment?

Before and after photos of a plasma cutting operation show the improved air quality that can be achieved with a good dust collection system. The cartridge collector is located outside.

Where feasible, the best way to maximize return on investment (ROI) is to recirculate air downstream of a dust collector. By recirculating heated or cooled air back through the plant, the cost to replace that conditioned air is eliminated. Plants using such systems have reported five- to six-figure annual energy savings. Also, dust collectors in welding shops with high ceilings often can improve the efficiency of a heating system by taking hot air off the ceiling and delivering it at ground level.

Another advantage of recirculating systems is the reduction in regulatory paperwork. By containing the air indoors, the engineer can deal with OSHA and avoid the EPA permitting involved when contaminated air is exhausted outside. Recirculating systems have special safety and performance concerns that must be addressed, but the payback can still be substantial.

Finally, it is not initial cost but total cost of ownership that counts. What will it cost to operate and maintain the unit and replace the filters? How much compressed air will it use? Can it save on maintenance of electrical components such as motors and control panels that are exposed to the dust? A reputable equipment supplier can accurately project these costs, help analyze the best ways to improve ROI, and maximize dust collector performance. PE