Collecting and Testing Dust (continued)

How Testing Succeeds Where Guesswork Fails

Company X, which used a wheel-blasting system to finish steel beams, was not attaining adequate dust pulse-down filter cleaning with its cartridge-type collector, resulting in high delta P. (Pressure drop or "delta P" in inches of water gauge [w.g.] is the measurement of how dirty the cartridges are.) The company sent a used cellulose media filter cartridge to a laboratory for dust testing.

An initial particle size analysis revealed a very fine dust mixed with larger metal dust, which was confirmed when the number and volume distributions of the dust were measured (see Figures 3A and 3B).

Dust analyzed under a video microscope supported this finding. The larger steel particles were surrounded by blotches of agglomerated mill-scale dust (see Figure 4). This finer dust is the carbon that is pulverized off the steel by the blasting wheel.

Figure 3 - These graphs show particle number and volume distribution of the same dust.

Figure 4 - Agglomerated mill-scale dust.

While these procedures provided useful information on the dust's size and characteristics, the bench test results did not offer an adequate explanation for the current dust collector problems. A full-scale test was carried out to provide more in-depth analysis.

The lab engineers hypothesized that special silicone-treated filter media might perform better than the standard cellulose media. Silicone had been shown to prevent finer dusts from impregnating the media. The focus of the full-scale test was to compare the differential pressure drop of the two media under similar conditions. A media change can reduce pressure drop, thereby increasing filtering efficiency.

In the lab, the cleaning system in the test collector was set to pulse automatically every 15 seconds. A 55-gallon drum of blasting dust was fed gradually into a cartridge dust collector on the full-scale test apparatus. A computer, which controlled the feed rate as well as air-to-cloth, air volume, and airflow ratios, produced real-time graphs of the collector's delta P and emissions.

To test the two media, lab personnel set the inlet dust feed at a very high rate to simulate dust loading in an accelerated manner. The feed rate also determined how fast the delta P would climb across the filter media. As the test continued, the engineers varied the feed and pulse rates and observed results to determine whether the filters were performing satisfactorily.

Satisfactory performance is defined as a recovery in differential pressure after half of the filter elements are pulsed. If performance is not satisfactory, settings are adjusted to maintain a constant load of dust. This process is repeated over several hours.

Using this test approach, the standard media stabilized at a pressure drop of 2.7 inches w.g. The silicone-treated media, by contrast, stabilized at 1.9 inch w.g.-a 0.8-inch w.g. reduction. Based on these results, the engineers determined that the silicone-treated media performed better and could be expected to experience lower pressure losses under actual operating conditions. In real-life experience, this pressure drop reduction also can be expected to result in improved airflow, lower energy usage, and longer filter life.

Without testing, Company X would have had to resort to trial and error to resolve its dust collection problem. Testing solved the problem and led to performance enhancements that could not otherwise have been anticipated.

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