Glacier Technology - Understanding Your Dust

Understanding your dust:

Six steps to better dust collection

Lee Morgan and Mike Walters, Farr Company
Almost every piece of process equipment is a powder and bulk solids handling plant depends on a dust collector to safely control dust or to reclaim valuable product. So why do most users treat dust collector selection as a mysterious art rather than scientific process, without considering exactly what kind of dust they need to collect and what performance they need? This article helps take the mystery out of the process by explaining how to analyze your application needs and dust characteristics a part of choosing a dust collector. Sections detail the six steps in this process.

Are you happy with your dust collector’s performance? If not, you have company: Most users are dissatisfied with how well their collectors operate.

This problem typically stems from neglecting or misunderstanding details about he unit’s performance when the collector is selected. After all, in most cases a dust collector doesn’t contribute to the plant’s bottom line, so the user just doesn’t devote the same attention to choosing a collector as to choosing a process machine.

But choosing your dust collector without doing enough research is dangerous. A poorly performing or failing dust collector can shut down your entire plant faster than any other piece of equipment. Without a dust collector, dust from process or handling machines can quickly create unsafe working conditions and the dust can’t be reclaimed as product.

The performance you should expect
You can expect the following performance from a properly selected dust collector:

The dust collector’s emissions meet your requirements.
The dust collector’s airflow is consistent to capture dust at its source.
The filter requires little maintenance at predictable intervals, and the filter media lasts reasonably long.
The dust collector operates trouble-free with minimal maintenance.
The captured dust is easy to handle and discard or reclaim.

Selection factors to Consider
One problem that often contributes to a poor collector choice is the dust collector manufacturer’s concentration on air (or gas) flow to the exclusion of other selection factors. But to ensure the dust collector performs as it should, you need to evaluate several critical factors, including:

What dust quantity must the unit collect–will it need to capture 2 lb/h or 2 t/h?
What type of dust do you have? For instance, it’s not enough to categorize the dust as silica. Is it sand or fume silica?
What is the dust’s particle size distribution?
What shape are the dust particles: long skinny fibers, uniform spheres, or jagged crystals?Is the dust flammable?
Is the dust hygroscopic?
Is the clean air returned to the plant or exhausted to the atmosphere?
What kind of requirements such as OSAH, EPA, or plant insurance must the dust collector meet?

By taking a close look at your dust, you’ll have the information you need to choose the right dust collector. Have your dust analyzed with quantitative, scientific tests. Following these six steps will provide the data you need and help you apply the information to choosing a collector.

Do a site survey to gather information about your application.
Obtain a dust sample.
Send the sample to a lab for analysis.
Have full-scale tests run on the sample, if required.
Select the appropriate filter media and dust collector.
After installation, verify that the dust collector performs as required.

Before exploring theses steps in detail, a word about the labs that can provide dusts analysis: A handful of independent labs have testing experience specific to dust collection. These labs charge from about $300 to $1,000 for lab tests, depending on the scope of the tests you require, and up to about $10,000 for full-scale tests. Some dust collector manufacturers also offer test services; most offer lab tests and several major manufacturers also provide full-scale tests. A manufacturer’s testing services often are free or involve only a nominal chare. An application engineer in the independent lab or at the manufacturer’s facility will test your dust and work with you to select the right collector for your needs.

Do a site survey to gather information about your application.To gather data about your application, do a site survey. This information should accompany the dust sample (as discussed in step 2) you send for testing because the data provides a context for the tests and ensures the results will be meaningful.

Typically, you’ll supply the site survey information on a data sheet, as shown in Figure 1. In addition to the information listed on the form, supply other facts to make the analysis more complete. This additional data should answer questions like these:

Does your plant currently have dust collectors that filter this dust? If so, how well do they perform?
Where do you intend to locate the new collector? Is headroom limited in this spot?How does the dust behave: Is it sticky some of the time? Does it cling to vertical surfaces or solidify after it sits in the open air?
What service life do you want from the filter media?
How do you want to handle the dust after it’s collected? For instance, will you collect it in drums or convey it to a large silo?

Obtain a dust sample.
Even if your dust seems to be a common type, such as grain or wood dust, something in your process can cause the dust to behave differently than the norm. This is why all dusts must be sampled for testing.

Before collecting your sample, review the dust’s Material Safety Data Sheet (MSDS) so you can protect yourself from any hazards associated with the dust.

Collect your sample properly. For the lab analysis, you’ll need about a 1-pint sample; if you require full-scale tests as well, you’ll need a much larger sample (as discussed in step 3). Be sure the sample represents the dust, with the same particle size and moisture content as the dust to be captured by the collector. For instance, a sample taken form dust swept from you plant floor isn’t representative of dust that will be captured from the air by the dust collector; the floor sweepings will contain impurities and larger particles with enough mass to have fallen to the floor. If the dust comes from an existing collector’s hopper, the sample won’t represent the dust’s true particle size distribution because it doesn’t contain the particles collected on the filters.

If you have an existing dust collector, the best sample to send to the lab is a dirty filter bad or cartridge. If you don’t have an existing collector, consult the lab for information about collecting a representative sample. Regardless of how you collect the sample, transport it to the lab in an airtight container to preserve the sample’s moisture content.

Send a copy of the MSDS with the sample; the MSDS is important to the application engineer because it provides information about your dust’s chemical reactants and OSHA-mandated personal exposure limits.

Send the sample to a lab for analysisTesting 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 I.

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 my 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 weigh particles of various diameters).