Understanding Your Dust (continued)

3 - Send the sample to a lab for analysis

Testing your sample in a lab is the best way to discover important information about the dust's physical characteristics. The lab analysis includes a series of bench tests of the 1-pint sample. The tests commonly included are listed in Table 1.

Particle size analysis is probably the most important procedure of all those in the lab analysis. The results list the particle size distribution of your sample in terms of both the quantity of particles of a given size and the particle diameters. Two common sizing techniques are listed in the table: sieve analysis, for particles over 100 microns, and dual-laser (time-of-flight) particle size analysis, for particles down to submicron sizes.

The application engineer will use this information to determine your dust collector's required filtration efficiency and pressure drop across the filter media and from this, what type of collector and media will be most effective at a reasonable cost. In a simplified example, if the particle size analysis shows that your dust has a large quantity of extremely fine particles, a cartridge dust collector with an on demand cleaning system may be best; if the results show that the dust contains many large particles, a baghouse dust collector with polyester felt bag filters may be best.

A thorough particle size analysis actually examines two particle size distributions: count and volume. The count distribution shows the number of particles in the sample and their diameters. This distribution can help the application engineer determine, for instance, if several submicron particles are masked by larger particles in the volume distribution. The volume distribution (also called mass distribution) shows the sample's mass spread (the weight of particles of various diameters).

Let's take a closer look at the difference between the count and volume distributions. It takes 1 million 0.5-micron particles to equal the mass of one 50-micron particle. We can demonstrate the difference between the two distributions by considering a hypothetical dust containing 1 million 0.5-micron particles and 99 50-micron particles. The dust is called a mixed dust because its particle sizes vary widely. Based on count, 99.99 percent of the dust is submicron size, but based on volume, only 1 percent is submicron. Knowing both distributions, we would select a dust collector that captures the submicron particles and cleans them off the filters. This collector would also have a specially designed inlet to allow the much larger particles to drop out of the airstream.

Such mixed dusts are common and often result from pneumatically conveying large, fragile bulk materials such as grains and plastic pellets. An example is thermographic powder used in printing. The fragile powder has a particle size ranging from about 30 to 80 microns, but after the powder is handled and conveyed, the particles break down and generate a dust with a particle size from 0.8 to 5 microns. Another mixed dust is emitted in the exhaust from a laser cutter. This device cuts steel and generates submicron-sized carbon smoke mixed with 30- to 70-micron steel particles.

Count and volume distributions for the laser cutter dust are shown in Figures 2a and b. A magnified view of the dust is shown in Figure 3.

If you relied solely on a visual analysis - merely looking at the laser cutter dust sample in your hand - rather than a lab analysis, you would never see the submicron particles.

In this case, you would probably specify a dust collector with 12-ounce polyester felt bags to handle the large steel particles. But this collector would emit carbon smoke haze, and the filters would probably blind. On the other hand, if you performed the lab analysis with video microscope analysis you'd clearly see the submicron particles and would choose an appropriate high-efficiency media, such as a pleated cartridge filter with cellulose or polytetrafluoroethylene (PTFE) membrane, to capture the particles.

The pychnometer test measures the particles' true specific gravity (rather than bulk density). This can help the application engineer calculate the efficiency of a cyclone and determine what gravity data to input for the dual-laser particle size analysis test.

The video microscope analysis provides a close-up view of the dust so the application engineer can determine the dust's particle shape and other characteristics important to dust collector selection. For instance, submicron, spherical dry particles such as polystyrene latex beads are hard to capture and require surface filler media such as P-84, PTFE membrane, pleated cellulose, or spun-bond polyester. Long fibrous particles such as fiberglass, grain dust, or wood dust can require wide bag filter spacing or wide pleat spacing on cartridge filters to avoid media bridging and packing. An oily dust such as chili powder can require filter media with an oleophobic (oil- and water-resistant) coating.

The video microscope analysis also reveals particle agglomeration. The dual-Iaser particle size analysis can indicate that the sample contains large particles, but the microscope can show thousands of particles stuck together in a ball. Once you see agglomerated particles, you need to know whether the particles agglomerate before or after they reach the dust collector. The site survey information you gathered in step I can help you answer this question. If the dust collector is very dry and subsequent moisture causes the particles to stick together, you know you can treat the dust as tiny submicron particles that won't agglomerate in the operating environment. Conversely, if the site survey reveals that the collector will be exposed to moisture, you need to select a collector with an effective cleaning system to prevent caking on the filters.

Other tests provide additional information about your dust. The moisture analysis measures the particles' moisture content by weight and helps you choose filter media, such as polypropylene or a Teflon-treated type, that can release wet particles during the cleaning cycle. The humidity chamber test indicates whether the dust is hygroscopic (absorbs moisture). If the dust is hygroscopic, bag filters or widely pleated cartridge filters rather than tightly pleated cartridge filters would be best.

The abrasion test reveals the dust's abrasiveness and - if required - helps you select an abrasion-resistant design for the dust collector inlet and other components, such as valves and ductwork. The terminal velocity test (also called the lift velocity test) helps you determine the right dust collector size and bag or cartridge filter size by pinpointing the air velocity needed to lift the dust. (Together, the dust collector size and filter size create the collector's can velocity, the upward airflow through the dust collector housing.) The horizontal velocity test reveals the optimal velocity for moving your dust horizontally and helps you design the collector's ductwork.

The sliding angle test and the angle of repose test determine the angle at which dust freely forms and help you determine the dust collector's hopper angle and any need for flow-aid devices, such as air pads that inject air into the hopper to assist discharge. Along with video microscope analysis, the sliding angle test helps to identify the dust's adhesive and cohesive properties - for instance, whether the dust is likely to stick to dissimilar surfaces or agglomerate.