How Well Do You Know Your Dust? (continued)

Bench Testing

Whether you are planning a new dust collection system or updating existing equipment, dust collection is a complex process affected by dozens of variables. Dust sample bench testing is an excellent tool for knowing your dust better, forming the basis for sound and accurate equipment selection.

Bench testing is beneficial in many ways. By identifying the dust characteristics properly, you can determine the right type of collector and filtration media for your needs and determine the right equipment sizing and air-to-cloth ratio needed for optimal energy savings and operational efficiency. This can help to minimize maintenance problems and meet more stringent emission requirements while extending filter life.

A few independent test laboratories have dust collection experience, and can perform bench testing at costs ranging from $300 to $1,000. Some equipment manufacturers have in-house test labs and offer free testing as a value-added service to customers. Find out whether you are obligated to buy a dust collector if the manufacturer conducts tests for you. The lab will ask you for a sample (see Note on Sampling Procedures) and should also ask for detailed application data. This data may include information on the process generating the dust, operating requirements, airflow and pressure-drop conditions, temperature and humidity, space constraints, and more. Without application data, no context exists for your test program, and test results will be less meaningful.

Common bench tests include:

1. Particle size analysis reveals the dust's particle size distribution down to the submicron range. This information determines the filtration efficiency required to meet emissions standards. A dual-laser particle analyzer 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. Sieve analysis is a related test that measures large particle sizes (>100 microns).

Particle-size analysis is used to reveal dust's particle-size distribution down to the submicron range.

A video microscope provides visual analysis of dust shape and characteristics.

2. A video microscope 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. Oil can cause serious problems with dry-dust collectors and may require the use of a filter media with an oleophobic or oil-resistant coating. In more drastic cases, an alternate collection system might be needed.

3. Pychnometer testing 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. This test can help to determine the efficiency of cyclonic-type dust collectors.

4. A moisture analyzer 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 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, the collector must be designed with low inlet velocity. Otherwise, the dust will re-entrain on the filter elements, abrading the filters and causing premature wear.

6. Terminal velocity testing pinpoints the air velocity required to lift the dust. This information helps to determine correct dust collector size and bag or cartridge filter size. Horizontal 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.