Recirculating Air From Dust Collectors (continued)

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. Spunbond 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.

Monitoring

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 filler 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.