Understanding Your Dust (continued)

4 - Have Full-Scale Tests Run On The Sample, If Required

Although the site survey and the lab analysis typically provide enough data, in some cases you'll need to have full scale dust collector tests run on your dust. You may need to do these tests if:

• Your dust collector has never operated properly and you want to diagnose and correct the problem.
• You want to predict how your sticky or hard-to-handle dust will behave in the collector.
• You want to know how your moist or hot process airstream will affect dust collection.
• Your plant must meet stringent emission requirements.
• Your dust has a history of causing collector "upsets." These can be caused by plugging (either from plugged filters that increase the pressure drop until airflow through the filters stops or from a plugged hopper discharge that causes dust to build up until it touches the filters) or filters that leak or break due to excessive airflow.

The full-scale tests typically require a dust sample large enough to fill a 55-gallon drum. The test apparatus includes various dust collectors, so the application engineer can run the dust through any of them to determine which unit is best for a given application. A portable version of such a test apparatus is shown in Figure 4.

The application engineer can perform each test using different air-to-cloth ratios (ACRs), dust loads, can velocities, and inlet velocities and compare the performances of different filter media, inlets, and housing designs. Emissions, pressure drop, and many other variables can be controlled and monitored during testing. When the tests are done, the engineer can determine the optimal collector type, filter media, ACR, cleaning frequency, and collection efficiency for your application.

Important test parameters include the ACR, dust load, can velocity, inlet velocity, and filter media. The ACR, which determines the airflow velocity through the filter media, is one of the most important parameters in selecting filter media. If the air velocity through the filter is too high, the dust will be forced deep into the media and won't be removed during the cleaning cycle. This will blind the filter and cause it to fail. A high ACR also re-entrains more dust on the filter. Conversely, a lower ACR requires a larger, more expensive filter. By optimizing your dust collector's ACR, you can get the best performance and economy out of your filter.

By varying the dust load, the application engineer can observe how a filter will perform at expected and extreme dust loads. In many applications, the ACR must be reduced as the dust load increases. For this reason, it's important to know the maximum dust load a given filter can handle while still meeting emission requirements and maintaining a steady pressure drop.

The can velocity is as important as ACR, although it's sometimes overlooked. Another related parameter is the interstitial velocity - the upward airflow through the interstices (spaces) in the dust collector's filter compartment. Both can and interstitial velocity are proportional to the ACR and the dust collector's total airflow capacity.

To better understand the influence of can velocity, consider this example: A dust collector with 12-foot-long bag filters can have the same filter surface area and ACR as a collector with 8-foot-long bag filters. However, the can velocity in the first collector is 30 percent higher, which can cause bag failure. A high can velocity tends to prevent dust from falling into the collector's hopper and promotes dust recirculation in the filters, which has the same effect as increasing the dust load at the inlet. The second dust collector will operate at a lower pressure drop with reduced emissions (because the dust load is effectively lower) and with longer filter service life.

The application engineer can also test the inlet velocity to predict how it can affect the dust collector's performance. With an abrasive dust, it's important to maintain a low inlet velocity. A high inlet velocity tends to increase dust re-entrainment on the filters and abrade the filters or cause them to swing inside the filter compartment, any of which can reduce filter life.

Filter media are available in dozens of types, ranging from standard polyester felt bags to more exotic ceramic bags and polyamide cartridges. The latter specialty media can cost as much as ten times more than polyester felt. So it's important to compare the performance of different media. Full-scale testing that compares filter media can help you select the best-performing, most cost-effective media for your application.

Different types of filter media also differ in their ability to control emissions. As you know, EPA and OSHA continue to tighten air quality control regulations. To obtain an EPA permit, you must know what your stack emissions are. Many plants find it easier to deal with OSHA, which is concerned with regulating indoor air quality, than with EPA. So they return filtered air to the plant and accept responsibility for ensuring air quality inside the workplace, thus avoiding EPA's time-consuming form-completion and permitting process. [Author's note: Air quality regulations differ from state to state, so check with your state authorities before deciding whether or not to recirculate dust collector exhaust to the plant.] But whether you try to meet EPA or OSHA requirements, use full-scale tests to determine a filter media's emission control performance with your dust.

5 - Select the appropriate filter media and dust collector

The site survey you did in step 1, the lab analysis completed in step 3, and, if required, the full-scale tests done in step 4 all combine to eliminate guesswork in selecting a dust collector and help you choose the best filter media with confidence.

Keep in mind your site survey and test results when considering these factors in selecting a dust collector and filter media:

• Choose the appropriate filter element (bag or pleated cartridge) and filter media.
• Determine the optimal ACR for your filter media.
• Calculate the proper can velocity, which determines the dust collector size.
• Select the appropriate inlet design.
• Design the collector hopper to quickly discharge the dust.

6 - After installation, verify that the dust collector performs as required

Once your dust collector is installed and running, make sure its operating parameters, such as pressure drop and filter service life, meet your requirements. Because processing technology continues to evolve, finer, hotter, and wetter dusts are created everyday and pose greater challenges to filter media. The dust control industry is responding with its own technology developments. To continue to achieve optimal dust collector performance, maintain a close relationship with your dust collector manufacturer and keep up with the latest developments in collectors and filter media.

 

***