Collecting and Testing Dust (continued)

Dust-Collecting Methods

For the majority of dust-testing situations, small-sample or bench testing will suffice. Common bench tests include:

1. Particle size analysis, which reveals the dust's particle size distribution down to the submicron range. This information determines the filtration efficiency required to meet emissions standards. The dual-laser particle analyzer shown in Figure 1 can pinpoint both the count (the number of particles of a given size) and the volume or mass spread of the dust. Knowing both is important because many dusts are mixed.

For example, the exhaust dust from a plasma cutter includes submicron carbon particles mixed with much larger steel particles. Scientific testing is the only way to identify the tiny particles of carbon dust, which helps you choose the appropriate equipment and filter media. Sieve analysis is a related test that measures particle size larger than 100 microns.

2. A video microscope, which provides visual analysis of the dust shape and characteristics. Together with particle size analysis, this tool is vital for proper equipment selection, often helping to determine what type of collector should be used. For example, a microscope may be needed to see oil in the dust-a common occurrence with processes involving oily steel. Oil can cause serious problems with dry-dust collectors, sometimes dictating the use of an alternate system.

3. Pychnometer testing, which determines the true specific gravity of the dust as opposed to the bulk density. Specific gravity is the weight of a given material as a solid block. For example, aluminum weighs 165 pounds per cubic foot. Bulk density is the weight of the same material in the form of dust. For example, flame-sprayed aluminum dust weighs only 1 to 2 pounds per cubic foot. Pychnometer testing can help to determine the efficiency of cyclonic-type dust collectors.

4. A moisture analyzer, which measures a dust's moisture percentage by weight. This information can help to prevent or troubleshoot moisture problems that could affect filter performance. A humidity chamber is used to see how quickly a dust will absorb moisture. This test helps to identify hygroscopic (moisture-absorbent) dust. Hygroscopic dusts require widely pleated filter cartridges or bag-type filters, as these sticky dusts cause tightly pleated filters to plug up.

5. Abrasion testing, which measures the relative abrasiveness of dust. This knowledge helps to determine the optimal design of dust-handling components, including valves, inlets, and ductwork. For example, when capturing a highly abrasive dust such as cast-iron grindings, the collector must be designed with low inlet velocity. If inlet velocity is too high, the dust will re-entrain on the filter elements, abrading the filters and causing premature wear.

6. Terminal velocity testing, which pinpoints the air velocity required to lift the dust. This information helps to determine correct filter housing size and bag length. Horizontal convey velocity testing reveals the optimal velocity needed to move the dust horizontally, aiding in proper ductwork system design. Sliding angle/angle of repose testing determines the angle at which dust forms freely, aiding in hopper and dust discharge design. This test further identifies whether the dust tends to stick or agglomerate.

In some cases, after bench testing is completed, a lab might need further information to troubleshoot an existing collector problem or to predict the behavior of an unusual or difficult dust. In these situations, full-scale testing using one or more dust collectors may be needed. Full-scale testing also can help fabricators to meet particularly strict emission requirements involving toxic dust and fumes such as those emitted when cutting or welding galvanized material.

Figure 2 - Four type of dust collectors are contained in this full-scale testing apparatus.

Figure 2 shows a full-scale testing apparatus equipped with four types of dust collectors. Tests can be run on any or all of the collectors to determine the best equipment choice for the application using either real-time or accelerated testing that simulates actual operating conditions. In addition to comparing the collectors, many performance variables also can be evaluated, including different media types, filter configurations, air-to-cloth ratios, temperatures, airflows, and dust loading conditions.

Customers may view the testing and make changes in a "what if" context to evaluate the impact of different variables.

After Testing Is Complete

The fabricator now can work knowledgeably with the equipment supplier to choose the appropriate collector and filter media type, the best air-to-cloth ratio, the proper can velocity (defined as the upward flow of air through the housing), and the best inlet and hopper design. The end result is a dust collection system that delivers reduced emissions, reliable operation, and optimal protection of workers and equipment.