The main reason to use wet scrubbing for particulate removal is that you need wet scrubbing for chemical or fume removal. Wet scrubbers such as a packed tower are great for removing chemicals (contaminant) in the air, such as acids, bases, etc.

The Functions of Wet Scrubber

The wet scrubber sprays water droplets from nozzles which make contact with the water-soluble contaminant. Then based on the vapor pressure, the contaminant enters the water solution and leaves the air. Often times, the water has chemicals which react with the contaminant and forms a salt. This allows more of the contaminant to be collected.   In packed towers, the packing is used to break up water droplets into smaller and smaller particles, thereby increasing the water’s surface area and increasing the rate of removal of the contaminant.

The falling water droplets also come in contact with particulates. That water encloses the particulate as it makes contact, increasing the size and weight of the particulate. This makes it more likely that the combined droplet will fall to the sump and leave the airstream.

What is Blowdown?

The particulate then needs to be removed from the sump or else it can settle and eventually decrease the water in the system, which could cause operational issues. Abrasive material can wear out the instrumentation, piping, nozzles, and pumps. The wastewater created from the wet scrubber is called the blowdown. This blowdown liquid is removed from the sump either by diverting some from the recycle line or an overflow line to keep salts and particulate at a safe concentration. The particulate in the blowdown stream needs to be treated by a plant or municipal wastewater treatment plant so that it can be reused or disposed of. High amounts of solids in the blowdown stream can increase the price of treatment and cause maintenance issues on the equipment used for treatment. Many municipal plants will limit the water it allows to be treated.

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When we think about dust particles being picked up by dust collectors we think of the particles as small spheres. And often for theoretical removal efficiency of a dust collector the dust is assumed to be a sphere. This is done to simplify the calculations as spheres act predictably compared to other shapes. However in real life dust isn’t normally spherical in shape. Dust is often created by material rubbing against each other and breaking off. This material often breaks along weak points in the material structure, which often isn’t spherical in shape. The material can have a wide range of shapes.

The heavier the material the less the material shape affects how the particle acts in the airstreams. While the lighter the particle the more subjective the particle is to the forces being applied by the airstream. For example, two pieces of dust that weigh the same will act differently if the volumes of the two particles are different. The lighter unit (larger volume) will be harder to capture in a dust collector than the heavier one.

Spherical Particles

Spherical particles are easier to predict when flowing through an airstream. A spherical dust particle looks exactly the same no matter what angle you are looking at it. So basically the particle will react the same no matter which way a force acts on it. However as the particle becomes less spherical, the geometry of the particle offers more area for forces to affect it from certain angles and less from others. For example, a cylindrical particle will have smaller surface area if the force hits the round ends (let’s call these top and bottom) of the cylinder rather than hitting the longer straight walls (let’s call these the sides) of the cylinder. And the more surface area available the more force will be applied to the particle, which can cause the particle to move in the airstream, ex. spin, wobble, etc.

Particle vs. Filter

To understand how the shape of a particle can affect its removal in a dust collector let’s look at the cylindrical dust above heading to a fabric filter. If the particle reaches the filter with its side facing the filter, the chances of it getting through the small openings in the filter are very small. While if the particle reaches the filter with the top/bottom facing the filter, it could possibly slip through the opening and get past the filter. So as you see, the orientation of the particle could affect whether it is being collected or not. And since there are thousands of particles moving through the dust collector, there will be a small percentage that will hit the filter just right and pass through, thereby lowering the removal efficiency of the dust collector.

So if your dust collector isn’t getting the removal efficiency that you expected from theoretical calculations, the particle shapes could be the cause of the lower removal efficiency. Special particle size tests can be done to show the different shapes so that the particle shapes can be taken into account.

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One common mistake operators make in dust collectors is to use the dust collector hopper to store material. The hopper in a dust collector isn’t designed for this. Instead it is a temporary home for the dust collected while the material is being removed. This period should be as short as possible for the following reasons:

• The dust collector isn’t designed to support a hopper fully filled with material. The added weight could cause structural issues with the vessel and the supports.
• As the hopper gets fuller, there is a greater chance that material will be re-entrained into the airstream, thereby causing a lower removal efficiency, increased wearing on the housing, filters, etc.
• Large amounts of dust in the hopper could become airborne during an incident, which could fuel an explosion in the dust collector. Removing the material from the hopper isolates the material storage from the dust collector.
• Storage in the hopper could cause bridging or rat holing of the material. This could cause the material to backup into the separation zone (example: begin covering filters). This will cause major operational issues in the dust collector and decrease removal efficiency and airflow through the system. 

So when you are operating your dust collector, be sure to remove the dust collected in the hopper as soon as you can. Airlocks such as rotary valves, double dump valves and trickle valves (ex. Aerodyne’s Vacu-Valve) are ideal for keeping the process isolated from the outside while also allowing the collected material to leave the collection hopper.

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The airflow in dust collection systems is crucial to the proper operation of the system. The dust collection system consists of the hood/pickup points, ductwork, dust collector(s) and exhaust fan. The hood/pickup points are designed to capture the dust. This design requires a range of airflows to properly work. If too little airflow is going through the hood, dust will escape from the hood. If too much air is going through the hood, the system can capture material it isn’t supposed to (example picking up product from the conveyor belt, not just dust lingering over the belt in the air).

The Important of Ductwork

Ductwork is like a highway for the dust in the dust collection system. It allows the airflow to be directed to the dust collector from the pickup points/hoods. The ductwork should be sized so that the airflow velocity is fast enough to keep the dust in suspension, but taking into account that the faster the airflow the higher the pressure drop is through the ductwork. The minimum velocity required to keep the dust in suspension is dependent on the dust. It can vary, but usually 4500 FPM is a safe velocity. Elbows can also increase the pressure drop of ductwork so try to minimize the elbows when designing the ductwork. Make sure you know the pressure drop in your ductwork so you can have enough static pressure in your fan to keep the design airflow.

Pressure Drop in Dust Collectors

Dust collectors have a pressure drop associated with them. Usually the higher the pressure drop the greater your removal efficiency will be, however different technologies will have different pressure drop and removal efficiencies. For example 10”WC pressure drop on a cyclone will have a lower removal efficiency than 10”WC on a filter. Be sure you have enough static pressure to operate your dust collector throughout the normal life of it. A fabric filter will build up dust on it, thereby increasing its pressure drop over time. Be sure to have enough static pressure to handle the required airflow at the point the filters are dirtiest, or else your airflow will suffer.

Exhaust Fan vs. Airflow

The exhaust fan should be designed to provide the required airflow with enough static pressure throughout the operational life of the system. This means having enough static available at the higher airflow (pressure drop increased in hood, ductwork, and dust collector) and to handle dirty filters until they are replaced (or cleaned). It is often good to slightly oversize the fan and to use a variable frequency drive (VFD) to adjust the fan as required.

Another good device is a digital airflow meter (such as Aerodyne’s GPC airflow meter) which lets you monitor the airflow through the system to be sure it is operating as it was designed.

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Material of construction is extremely important to the durability of a piece of equipment. The suitability of the materials of construction is based on the process, which includes the different components, phases, temperature, and pressure. For example: a cyclone’s material of construction would be dependent on the following:

Material (dust) Being Collected

Properties of the dust being might dictate the materials of construction. Food or pharmaceuticals will usually require stainless steel to prevent / minimize contamination. Carbon steel is often acceptable for wood applications. Other times, the material properties require a special material. For example, abrasive material might require AR steel or a coating to help prevent erosion of the cyclone.

Chemical Composition

The chemical composition of the process can dictate the materials of construction. If a component will react with the materials of construction, it could cause premature failure. For example, water or high humidity can cause rusting of carbon steel so stainless steel might be better suited. Or if an acid is a component of the gas stream, then a high alloy metal or fiberglass construction might be better suited.

Temperature and Pressure

High or low temperature can cause materials to change their properties. Material can become brittle or they may react more with components. For example fiberglass can’t handle higher temperatures whereas metals usually can. And some material might have good compatibility at lower temperatures but very poor compatibility at higher ones. Sometimes pressure can affect the material properties, but usually it will affect the thickness of the walls.

Aesthetics

Some customers have plant requirements for their equipment. For example, a food or pharmaceutical facility may require stainless steel construction even though the equipment is on the waste process and everything will be disposed of.   In a similar vein, the finishing of the equipment might be determined because all the other equipment has a high polished finish and the customer wants it to fit in.

So, when determining the materials of construction, there are many factors that contribute. It is best if the end user, with help from the vendor(s), determine the materials of construction. The end user usually knows the process better than the equipment manufacturer and therefore is in the best position to determine the materials of construction.

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Rotary valves will occasionally jam. This can happen when material gets between the rotor and the housing or when oversized material cannot fit into the rotary valve pockets.   Depending on the type of jamming that is happening you handle the situation differently.

Space Between Rotor and Housing

Rotary valves have a space between the rotor and the housing. This space allows the rotors to rotate freely but it can also allow air to leak across the valve. When materials falls on the rotor edge it can buildup and jam the rotary valve. So for existing rotary valves, you might have to replace or modify the existing rotor. Beveling or chamfering the rotor edges help the material to fall off the edge and into the valve pockets.

When Jams Caused by ...

For jams caused by oversized material, a valve with larger pockets is required. This could mean a larger rotary valve or replacing the rotor with a rotor containing one less pocket. Please note if the rotary valve is on an explosive application, NFPA requires a minimum of six vanes on the rotor. If these options aren’t possible, looking into a double dump valve might provide larger clearance for oversized material to pass through.

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One of the issues when figuring out if your dust is explosive is the particle size of the dust and the location where you are assessing the dust. Material that consists of very large particles sizes (larger than 500 micron) tend not to be explosive. While larger particles can catch fire, they tend to prevent explosions. However, this might not be as straight forward as you think.

Is the Process Explosive?

Let’s assume that you have a conveyor belt that feeds a grinder which then empties onto another conveyor belt which feeds a bagging station. Let’s assume that the material starts out with a particle size distribution of 10% less than 500 micron and 90% greater than 500 micron of a combustible material. After it is fed into the grinder, its target size is around 200 microns. The question is, is this process explosive? Your 1^st thought would be that before the grinder, the dust is too large to be explosive, and you might want to take the material leaving the grinder and have it tested. So you test the material leaving the grinder on the conveyor and find out it isn’t explosive. Therefore you assume the whole process isn’t explosive. However, this might give you a false reading.

Is the Dust Cloud Explosive?

For example, the material falling on the conveyor belt that feeds the grinder creates a dust cloud. That dust cloud is made up of the finer particles, so it is probably made up mostly of the 10% that’s less than 500 micron. And this material could be explosive. Not only that, but if that 10% of the material particle size distribution has a significant amount of dust less than 100 micron, this could very well be the dust cloud which is much more likely to be explosive. And since the smaller the particle size the better the chance of the material being explosive, your dust cloud could be explosive dust. This would then float in the building and layer the surrounding equipment or rafters with explosive dust.

Now looking at the material leaving the grinder, even though it tested as not explosive this is the material on the conveyer or that was bagged. If the grinder feed creates a dust cloud then this will again be the finer dust. So while the majority of your material is around 200 micron, there is a small portion (now much greater than the initial amount that was less than 100 micron) that is very fine. And again, this portion of the dust is the dust that is creating the dust clouds at the exit of the grinder and in the bagging section.

So when you are collecting dust to see if it is explosive, be sure to collect the material that is pertinent. You should try to collect dust that is actually forming a cloud to make sure it is explosive and not lump it in with material that won’t form the cloud. If you have an existing baghouse or cartridge collector, use the material that is collected in the dust collector for testing.

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Now that it is spring, it’s time to start thinking about summer shut down! If you are planning on doing maintenance on your dust collection system now is the time to order your parts.

If you are changing bags also look into doing the following:

1. Pull your airlock off and perform any needed maintenance on it (replace bearings, change out rotors or plate, replace Vacu-Valve sleeves, etc.)
2. Have someone check your exhaust fan a few weeks before shutdown so that if the fan needs rebalancing or bearings replaced, you can schedule it during the shutdown.
3. Inspection of your baghouse a few months before shutdown so any damage in the housing, bad air valves, tube sheet, etc. can be replaced during the process.
4. Inspect the ductwork and look for holes and dust buildup. Make sure dampers are working.
5. Look into installing a cyclone pre-filter to
    
   1. Increase filter life
   2. Decrease compressed air usage
   3. Collect dust before contamination in the filters
6. Inspect explosion protection equipment and have any chemical isolators/suppression equipment maintained. Be sure to have the clearance of the rotary valve measured to be sure it is still in compliance with NFPA requirements.
7. Schedule a Dust Efficiency Clinic to evaluate the system performance so any modifications can be installed during plant shutdown.

Doing the above will help you revitalize your dust collection system and make sure you get the best possible performance out of it.

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Dust collectors have multiple accessories that can be used to help optimize the system. Some are used to protect the system, some are used to monitor the system, and some are used to make the system operate smoothly. The following blog is a quick description of a few of them.

Meters

• Airflow meters – allow you to measure the airflow through the system. Effective to know if your system is operating correctly.
• Differential pressure gage – used to monitor the pressure drop through the dust collector. Often can warn you if your dust collector is having an issue (sudden change in pressure drop) or when maintenance is required (high or low pressure drops).
• Level meter – allows you to monitor the amount of material in your hopper. This will tell you if you’ve bridged the hopper, your airlock stopped, etc.
• Temperature gage – allows you to monitor the temperature of your system and prevent damage to components such as filters, seals, instrumentation, etc.

Valves and Tools

• Vibrators – allow you to shake material in the hopper, preventing bridging.
• Air nozzles – blow compressed air into the hopper preventing dust from settling and bridging
• Airlocks – allows material to leave the dust collector hopper and prevent air from passing through (entering or leaving the system)
• Zero speed switch – used to monitor the airlock to make sure it is operating and inform the operators when it stops.
• VFD – perhaps the most useful of all this allows you to increase or decrease the airflow in the system by adjusting the operating speed of the exhaust fan.

Explosive Applications

• Explosion vent – mounted on the dust collector, it allows an explosive overpressure to relieve in a controlled fashion preventing destruction of the dust collector and surrounding areas.
• Flameless explosion vent – similar to the above but designed to cool down the release so no flame is vented out. Can be used indoors.
• Chemical suppression – used to inject an inert material into the dust collector which will prevent the dust in the vessel from exploding.
• Thermal sensor – monitors the heat in the dust collector and is used to activate chemical suppression.
• Pressure sensor – monitors the pressure in the dust collector and is used to activate chemical suppression when over pressure is measured.
• Isolation gate – installed before the dust collector, this prevents an explosion in the dust collector from moving back into the building and the rest of the process.

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A dust collection system consists of four major components. The pickup hood, ductwork, dust collector, and exhaust fan. For the system to operate efficiently all four components much be sized and used correctly. Unfortunately, one of the main components (the hood) is often overlooked. The hood design is crucial for the operation of the dust collector system. And without the proper hood design, your dust collector will not provide the dust pickup required for the safety of the equipment and workers.

The hood should be designed so that it will capture the dust without letting dust escape. This can often be difficult as the hood will need to be designed to pull air from all the areas generating dust while also provide the required access of the operator / machine.

Using the wrong hood can cause:

• Dust to escape the hood, thereby causing health, safety, and maintenance issues.
• Too much dust is captured, removing product or components from the process.
• Too much air is used, increasing the system size and cost.

When you are looking to install a dust collector system, pay attention the hood design of the system. Spend time selecting the proper hood for the application and don’t just go with the least expensive hood, as this could cause your entire dust collection system to fail at its job.

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