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 Governmenlal Industrial Hygienists (ACGlH). 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).
| Dust Type | TW, mg/m3 |
|---|---|
| Graphite, except fibers | 2 |
| Gypsum | 0.5 |
| Mica | 3 |
| Particles not otherwise classified | |
| Inhalable | 10 |
| Respirable | 3 |
| Silica - amorphous | |
| Diatomaceous earth (uncalcined) | |
| Inhalable | 10 |
| Respirable | 3 |
| Precipated silica | 10 |
| Silica, fume | 2 |
| Silica, fused | 0.1 |
| Silica, crystalline cristobalite | 0.05 |
| Quartz | 0.1 |
| Tridymite | 0.05 |
| Tripoli | 0.1 |
| Silicon | 10 |
| Talc, with no asbestos fibers | 2 |
| Zinc oxide | |
| Fume | 5 |
| Dust | 10 |
Table 1 - ACGIH recommended indoor air quality limits
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 than 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 detetmining 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.
Figure 1 - A full-scale dust testing apparatus can be brought onsite to test actual conditions.
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.
