Dan Navicky | Aerodyne Environmental
Many powder and bulk solids manufacturers know that incorporating a dust collection system into their process is important due to the benefits the system provides. But installing and operating a dust collection system is just the tip of the iceberg. Manufacturers will also want to ensure that their dust collection system is operating as efficiently as possible. This article describes the necessary questions to ask about the different parts that make up a dust collection system in order to determine whether the system is properly and efficiently operating.
We all know that if we don’t regularly maintain our vehicles, we’ll end up having issues with them in the future. Whether we forget to change the oil, replace the brakes, or get a tune-up, these and other issues will eventually catch up to us. Why do we think the proper operation of our dust collection system is any different? Dust collectors are made of various pieces, all of which have to be operating correctly for the system to work as desired. Usually, a dust collection system consists of a dust collector, exhaust fan, ductwork, and hoods and pickup points. Additional accessories such as airlocks, controls, instrumentation, and dampers can also affect the system’s operation.
How do you know if every part of your dust collection system is working as it should? The following questions and information will help you evaluate your dust collection system to determine if the system is operating up to par. And after reading through this guide, if you think your dust collection system needs a tune-up, contact your supplier or a local dust collection system expert and have him or her come out to help get your system working properly.
Hoods and pickup points
Dust collection systems use hoods and pickup points to collect dust from the facility, whether the dust is from the ambient air or directly off the equipment producing the dust. To ensure the system is operating efficiently, take note of whether the areas near these pickup points are dusty. Is there any dust in the air, is the air hazy, or is breathing in the area a bit difficult? Perhaps you notice dust piles around the area or a layer of dust on the equipment? The dust you see can be the result of the hoods’ and pickup points’ airflow being too low to pick up the dust, the hoods and pickup points not being in the correct locations, or a lack of hoods and pickup points in the area. Dust piles and layers of dust can become airborne, and if the dust is explosive, the airborne dust can lead to an explosion, potentially causing significant damage and injury.
Along with checking the atmosphere and surrounding area for dust, you want to determine if you can feel airflow being pulled into the hood or pickup point. Can you see any dust being captured? You should be able to feel the airflow and see the dust being captured by the hood or pickup point. Often, dust collection systems are modified to fit into the surrounding equipment, but any modification could cause a change in airflow at other areas in the system if those areas aren’t taken into consideration when the modifications are made. Also, opening or closing dampers can cause diminished airflow at hoods, resulting in less dust collection.
Another question to ask is if the dust collection system ever worked as expected? Sometimes a dust collection system is implemented into a manufacturer’s process, but the system has never fulfilled its expectations. Being honest with yourself in answering this question is important. If you realize your dust collection system has never been up to par, know that changes can be made to get the system working better.
The dust collection system’s ductwork carries the dust away from the pickup points. The ductwork, as shown in Figure 1, is an integral part of the system. An important question you’ll want to ask regarding your system’s ductwork is does the ductwork have holes in it? Holes in the ductwork allow air to leak into the system, resulting in less air being picked up at your hoods and pickup points, which ultimately means less dust being picked up. Or perhaps there are no holes in your ductwork, but maybe your ductwork is plugged up? Plugged-up ductwork is caused by low air velocities moving through the ductwork. Low air velocities are unable to keep the dust entrained, so the dust falls out of the airflow and builds up within the ductwork, eventually leading to plugging. Low air velocities also cause low airflow at your hoods, which can lead to a risk of explosion when the dust isn’t sucked up into the system but lingers in the air and on the equipment.
Also, ask yourself if you have added or decommissioned pickup points on your dust collection system. Adding or removing pickup points can cause the system to be out of balance. If you have made pickup point changes without reevaluating the whole dust collection system, your system may not be operating correctly. The entire dust collection system works in coordination, so when a change is made to one part, the rest of the system needs to be taken into account to ensure the system is working harmoniously.
One last question that may not be so obvious to ask is how often you measure the airflow moving through the ductwork. Just like making sure you have enough gas in your car, you should be measuring the ductwork airflow on a regular basis to ensure the system is operating at the appropriate level for your application. Keeping records of this information will help you see patterns of airflow strength and detect issues should any arise.
There are a variety of different dust collectors available from baghouses and cartridge collectors to cyclones and wet scrubbers. Each dust collector type has different features to consider when ensuring that the dust collection system is efficiently running. We’ll only address a few of the dust collector aspects here, so contacting a dust collection system expert is wise as they can help you diagnose any issues you may be having.
Baghouse and cartridge collectors. Baghouse and cartridge dust collectors, as shown in Figure 2 and Figure 3, respectively, are relatively the same in that they both use fabric filters to separate the dust from the airflow. After a while, the dust builds up on the filter and the filter needs to be changed. If you have a baghouse or a cartridge dust collector, you’ll want to focus on the filters to ensure efficient operation of your dust collector. Ask yourself when the last time was that you changed filters. Do you feel like you’re changing them too often? Some plants have to change filters twice a year while others haven’t changed their filters since they were installed. However, both of these filter maintenance scenarios could have issues. If you’re changing your dust collector’s filters too often, you might have the wrong filter type or too high of a dust loading on the filters, which will get expensive either way. If you haven’t ever changed your filters, the dust buildup on the filters may be restricting airflow, so it’s time to inspect them.
Another important filter question is what is the pressure drop across your filters? Pressure drop is the pressure difference between two points in an airflow system. In this case, it’s the pressure before the filters and the pressure after the filters. Pressure drops over 6 inches water gauge are usually a sign that you need new filters soon. A low pressure drop can mean that you have holes in your filters or your filters haven’t properly developed a dust cake on them so there’s not as much material catching the dust as it flows through. Both high and low pressure drops can cause operational and environmental compliance issues, as the more dust there is in the facility and surrounding area, the greater the explosion risk.
Aside from checking your filters, when was the last time you did a maintenance check on your dust collector? If you haven’t done a maintenance check on your dust collector in the past year or two, now is the time. Filters, tube sheets, diffusers, air valves, manifolds, and a variety of other items can all develop issues that will keep your dust collector from operating as efficiently as possible. Just as with your car, a checkup every now and then will help prevent a small problem from becoming a major problem.
Cyclones. This dust collector type, as shown in Figure 4, uses centrifugal force to separate dust particles from the airflow. Cyclones are very low maintenance compared to other dust collectors, but they don’t have the same dust-removal efficiency as other dust collectors. If you have a cyclone, monitor the pressure drop through the cyclone. If the pressure drop suddenly decreases or increases, this usually means that something in the system has changed and is affecting the airflow, in which case you should investigate.
Along with consistently checking your cyclone’s pressure drop, ask yourself if you have checked your cyclone for holes. Cyclones often wear faster than other dust collectors due to the friction that’s imparted on the cyclone’s internal walls by the dust, so a yearly check for pinholes or wear is always a good idea. Pinholes and wear decrease the cyclone’s ability to separate dust from the airflow, allowing entrained dust to remain in the system or escape out of the pinholes.
Cyclones have lower removal efficiencies than most other dust collectors, so your best bet for having an efficient dust collection system is to have a filter right after your cyclone. If you don’t, you’ll want to regularly monitor the emissions to make sure you’re not emitting too much dust.
Cyclones are often used as prefilters for other dust collectors. The cyclone removes the larger dust particles, thereby decreasing the dust loading on the main dust collector. Often, installing a cyclone prefilter can solve some of the issues mentioned in this article. Cyclone prefilters allow you to collect dust locally, so the dust can be reused or recycled. They also reduce the material loading for baghouses, cartridge collectors, and wet scrubbers. This reduced loading helps extend filter life and decreases compressed-air usage, which can help save money. For wet scrubbers, cyclone prefilters help to decrease the blowdown rate (the amount of water that leaves a wet scrubber sump), thereby saving on water and water treatment costs, and possibly allowing you to reuse the collected material.
Wet scrubbers. Wet scrubber dust collectors have the same concept as the previous dust collectors — to filter dust out of an airstream — but go about filtering dust in a different manner. Wet scrubbers use water droplets (that are usually much larger than the dust particles themselves) to filter out the dust. When a dust particle makes contact with a water droplet, the dust joins the water droplet, making the dust easier to separate from the airflow. Wet scrubbers are available in a variety of styles, such as venturi, packed-bed, sieve-tray, cloud chamber, and even wet fan. All of them have different characteristics to look out for, but the following are some basic points that can indicate the system isn’t operating as it should.
Be aware of what the pressure drop is through the wet scrubber. A higher- or lower-than-normal pressure drop always tells you that something bad is happening within your wet scrubber. A higher pressure drop usually means that dust is building up in the system, whereas a lower pressure drop usually means that less airflow is going through the system. Either way, the appropriate airflow volume isn’t being achieved and that needs to be corrected.
Another question to consider is do you monitor your water usage and the overflow’s particulate loading. The wet scrubber’s overflow stream removes the particulates and dissolved solids from the system. Particulate loading that’s too high will cause issues in the recycle pumps and water treatment facility, possibly leading to premature pump failure. Particulate loading that’s too low will cause the scrubber to use more water than is necessary.
Lastly, are you overworking your water treatment plant? Water treatment is expensive so if you can lower the amount of water going to the plant, you could save money. The amount of water going to the plant is dependent on the amount of the dust being captured by the collector. One way to lower the dust loading is with a prefilter cyclone.
The exhaust fan usually provides all the motive force — the energy needed to move the air through the system — in a dust collection system. Therefore, if the fan isn’t working properly, the whole system will have issues. You’ll want to determine if the fan is vibrating excessively or making more noise than usual. Excessive fan vibrations and unusual fan noises can both mean that something is wrong with the fan. Damage to the impeller or dust buildup on the impeller can cause unnecessary vibrations and noise and should be looked into. Some fans do get unbalanced over time, which can cause the fan to stop working.
You’ll also want to know if any of the fan bearings are running hot. A bearing running hot can be caused by a fan or bearing issue, such as excessive shaft wear, an unbalanced shaft load, or a lack of lubricant for the bearing. While unlikely, hot bearings have been known to cause fires or explosions if all the conditions are right.
Other miscellaneous items to check on in your dust collection system include dampers, airlocks, and controllers and instrumentation.
Dampers can cause airflow issues when they aren’t operating correctly or are accidentally opened or closed when they shouldn’t be. Worn-out airlocks under dust collectors can cause air leakage into the system or prevent dust from leaving the system. Broken, plugged-up, or uncalibrated controllers and instrumentation can provide false readings, which affect the system’s operation, or they can cause the system to shut down, which hurts productivity levels.
Dust collection systems are complex, so diagnosing system issues isn’t always simple. However, treating your dust collection system as you would your car will allow you to identify minor issues before they develop into major problems. Regular maintenance inspections are a good idea. If you’re having a plant shutdown, don’t forget about the dust collector. A planned plant shutdown is a great time to change your dust collector’s filters and bearings or add a prefilter. A maintenance inspection a few weeks before the plant shutdown can also allow you to be prepared to do any major repairs during the shutdown.
If after evaluating your dust collection system you think your system isn’t operating as it should be, don’t hesitate to call an expert. Not all dust collection systems are the same and neither are all applications.
For further reading
Dan Navicky (440-543-7400) is the industrial cyclones and airlocks applications engineer at Aerodyne Environmental. He graduated from the University of Dayton where he majored in chemical engineering. He has 20 years of experience in selling and designing air pollution systems, including cyclones, baghouses, cartridge collectors, wet scrubbers, airlocks, and industrial fans.
Aerodyne Environmental • Chagrin Falls, OH
440-543-7400 • www.dustcollectorhq.com
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