A manufacturer of grinding wheels in Niagara Falls, N.Y., needed to replace an existing dust collector used to collect dust coming off a rotary kiln. Initially, a bag house collector was considered to be the obvious solution. However, because the problem involved high temperatures – 700° F and above – specific efficiency requirements, moisture content problems, and future maintenance issues, the purchase of an Aerodyne SplitStream grinding dust collector was justified.

Aerodyne SplitStream^™ Cyclone Dust Collector

The volume of gases being handled was estimated at 4,000 CFM at 700°F, according to the specifications of their previous cyclone. Using this information, a S4500 SplitStream grinding dust collector was selected and installed.

When the collector was started and the flows were checked, the company discovered that only 2,500 CFM at 500°F was needed to ventilate the kiln. Even at this lower-than-expected inlet flow rate, the results were excellent. No visible carry-over was detected, and the SplitStream dust collector captured materials much finer than previously collected.

The plant was satisfied it had made the correct decision and avoided the headaches and maintenance issues of a bag house. After several   months in operation, abrasion   wear   was   not   evident  on   the   collector. Consequently, the company ordered two more Model 4500 SplitStream grinding dust collectors.

The lack of a filter media and Aerodyne’s unique design make the SplitStream collector perfect for applications involving high temperatures and abrasive materials.

The Aerodyne SplitStream dust collector achieves high efficiency by forcing dirty gases into a powerful centrifugal motion. The centrifugal action throws dust particulate out of the gas stream. A secondary air stream carries the dust particulate to the hopper, keeping dust away from the collector walls and reducing sticking and abrasion. As a result, the SplitStream dust collector virtually eliminates maintenance problems common to other types of cyclones. The prevention of particulate contact with external walls is a major factor in the unit’s ability to achieve high efficiency ratings.

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We all know that if we don’t regularly maintain our cars, we will end up having issues in the future. If we forget to change the oil, change the brakes or get a tune up, it will eventually catch up to us. Why do we think 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 operate as desired. Usually, a dust collection system consists of the dust collector, exhaust fan, ductwork, and hoods/pickup points. Additional accessories such as airlocks, controls, instrumentation, and dampers all can affect the operation of the system.

The following questions with quick explanations will help you evaluate your dust collector. If after going through this guide you think your dust collector needs a tune-up, contact a local expert or Aerodyne Environmental and have them come in to help you get your system working properly.

Pickup Points

1. Are the areas near the pickup points dusty? Is there any dust in the air, is it hazy, or is breathing in the area a bit difficult?

2. Are there dust piles around the area? Dust piles and layers of dust can become airborne and if the dust is explosive, it can lead to an explosion causing significant damage and injury.

3. Can you feel airflow being pulled into the hood or pickup point?Often times, dust collection systems will be modified and this could cause a loss of airflow at other areas in the system.

4. Did the dust collection system ever work as expected? Sometimes a dust collection system has never fulfilled expectations. This doesn’t mean that changes can’t be made to get it working better.

Ductwork

1. Does the ductwork have holes? Holes allow air to leak into the system, this will cause less air to be picked up at your hoods and pickup points.

2. Are you plugging up your ductwork? Plugged up ductwork is caused by low air velocities through the ductwork. This will cause low airflow at your hoods and is an explosion danger.

3. Have you added or de-commissioned pickup points on your system? Adding or removing pickup points can cause the system to be out of balance. If you have done this without re-evaluating the whole system, your system may not be operating correctly.

Dust Collector

There are a variety of different dust collectors available—baghouse and cartridge collectors, cyclones, and wet scrubbers. Each one of them has different things to look at. We will only address a few of them here. Contacting a dust collection expert such as Aerodyne Environmental can help you diagnose issues.

Baghouse and Cartridge Collectors

1. When was the last time you changed filters? Some customers have to change filters twice a year, others haven’t changed them since they were installed. Both examples could have issues.

2. What is the pressure drop across your filters? Pressure drops over 6” W.C. usually tells you that you need new filters soon. A low pressure drop can mean that you have holes in your filters or haven’t properly developed a dust cake on the filter.

3. When was the last time you did a maintenance check on your dust collector? If you haven’t done it in the past year or two, it might be time. Filters, tube sheets, diffusers, air valves, manifolds and a variety of other items can all develop issues.

Cyclone

1. Have you checked your cyclone for holes? Cyclones often wear faster than other dust collectors, so a yearly check for pin holes or wearing is always a good idea.

2. Do you have a filter after your cyclone? Since cyclones have lower removal efficiencies than most other dust collectors, if your cyclone doesn’t have a filter afterward, you might want to monitor the emissions regularly to make sure you’re not emitting too much dust.

Wet Scrubbers

1. What is the pressure drop through the wet scrubber? Higher than normal or lower than normal pressure drops always tell you something is happening. Higher pressure drops usually means that dust is building up in the system. Lower pressure drop usually means that less airflow is going through the system.

2. Do you monitor your water usage and the particulate loading of the overflow? The overflow stream of the wet scrubber removes the particulate and the dissolved solids from the system.

3. Are you taxing your water treatment plant? Water treatment is expensive so if you can lower the amount of water going to it, you could save money.

Exhaust Fan

The exhaust fan usually provides all the motive force in a dust collection system; therefore, if the fan isn’t working properly the whole system will be having issues.

1. Is the fan vibrating excessively?
2. Is the fan making more noise than usual?
3. Are the bearings in the fan running hot?

Excessive vibrating and unusual noises can mean that something is wrong with the fan. Damage to the impeller or dust build up on the impeller could cause this. Some fans do get unbalanced over time and could cause the fan to stop working. A bearing running hot could be caused by an issue with the fan or an issue with the bearing. Hot bearings, while unlikely, have been known to cause fires or explosions if all the conditions are right.

Miscellaneous

Other items to check on your dust collection system include dampers, airlocks, controllers and instrumentation. Dampers not operating correctly, or accidently opened/closed can cause operating issues. Airlocks under dust collectors can cause air leakage into the system or prevent dust from leaving the system. Controllers and instrumentation can cause the system to shut down, or provide wrong reading, thereby affecting the operation of the system.

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Dust collection is often one step in a series of steps used to treat an airstream or process stream. Often times the airstream will require heating or cooling, gas scrubbing, and/or dilution of gas stream, etc. All of these processes usually require additional equipment to work in tandem with the basic dust collector. Adding additional equipment is not only expensive, but it consumes floor space and often purchasing new equipment will get overlooked simply due to lack of space to accommodate it.

Standard dust collectors often have operational issues with abrasive, sticky, and fibrous dusts. Standard pre-filters for these applications can have high maintenance costs, however without a pre-filter the particulate will often foul or erode the process equipment. Sticky dust will plug filters and foul heat exchangers. Abrasive dust will prematurely wear heat exchangers and dust collectors. Fibrous dust will foul in tight areas. Wet scrubber applications can experience higher water usage when the particulate amount increases.

There are 5 applications that utilize standard dust collectors:

1. Temperature Adjustment of an Airstream

The SplitStream cyclone uses a secondary airstream to convey the dust from the walls of the cyclone down into the hopper for dust collection. The secondary airstream can be used to heat or cool the primary airstream before it is vented outside or fed to other process equipment. The two airstreams will combine in the cyclone and they will leave together at a uniform temperature.

2. Abrasive Dust

Abrasive dust will shorten process equipment’s life span by eroding away the walls of the equipment. Due to the design of a standard cyclone, the lifespan is shortened quicker. A standard cyclone is designed so that the dust is conveyed to the outside of the cyclone, where it hits the walls and fall into the hopper. Abrasive dust will also wear out filter bags, the inlet distributor, tube sheets, and the baghouse walls.

3. Sticky and Hygroscopic Dust

Sticky and hygroscopic dust can also cause issues with standard dust collectors and cyclones. Sticky dust will accumulate in any dead zones in standard cyclone. This causes a gradual reduction in the internal volume of the cyclone, and changes the geometry of the cyclone, which could cause a change in the removal efficiency of the unit.

4. Fibrous Dust

Fibrous dust causes many issues for standard dust collectors. The fibrous dust floats in the airstream and often will be re-entrained back in to the air exiting the cyclone. In baghouses and cartridge collectors, fibrous dust when cleaned off the filters, will float in the dirty air plenum. This increases the concentration of the dust in the unit. The dust will not settle into the hopper to be removed from the system. Overtime this will cause the dirty air plenum to plug up, thereby plugging up the dust collector.

5. Oxygen Reduction

Many dusts are explosive and with growing scrutiny of new and existing operations by inspectors and insurance companies, existing processes are being found that require explosion protection. One way of protecting from explosions is to prevent an explosion from ever occurring by lowering the oxygen level of the system below the Lower Explosive Level (LEL). This is done by adding an inert gas such as nitrogen to decrease the concentration of oxygen.

Aerodyne’s SplitStream cyclone offers a strong advantage over standard dust collectors. Coupled with its unique design and its ability to be configured multiple ways, it is the ideal dust collection solution. Whether it is using it as a pre-filter for an existing system or replacing a faulty dust collector in a plant, the SplitStream will reduce maintenance, increase efficiencies, and improve the overall process.

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Most industrial dust collection systems use a baghouse or a cartridge collector. Whether your baghouse or cartridge collector is 50 CFM or 100,000 CFM, there are advantages to putting a cyclone before the baghouse or cartridge collector. Here are 5 advantages to utilizing a cyclone in a dust collection system.

1. Increased Removal Efficiencies

Cyclones will increase your total dust collection efficiency when added before an existing baghouse. Cyclones remove the larger, coarser dust before the particulate reaches the baghouse. This helps to lighten the load on filters, which results in less dust in the airstream and overall higher removal efficiency for the whole system.

2. Longer Filter Life

Baghouses and cartridge collectors require compressed air to knock off the dust. The use of the compressed air forces the bag to expand around its cages during high pressure bursts. The constant expansion of the bag causes it to wear out at a faster rate. Putting a cyclone ahead of the baghouse will remove most of the larger dust from the baghouse, thereby minimizing the amount of dust coming into the baghouse or cartridge collector.

3. Recovery of Product

If the dust being collected is valuable (such as gold or silver) or needs to be/can be reused, simply using a baghouse or cartridge collector will cause the material to get lost on the filters. A cyclone doesn’t require any filters and all the recovered material will eventually be removed through the dust discharge valve.

4. Easy Maintenance

Baghouses and cartridge collectors are extremely high maintenance. They require confined space entry to remove and replace the bags. Furthermore, bag replacement can be a time consuming process. Cyclones, however, require very little maintenance. At most, plant engineers have to observe the pressure drop every so often, and inspect the walls of the cyclone to insure that is has not worn down from the application.

5. Allows Baghouses to Operate in Difficult Applications

Bags and cartridges are not well suited for fibrous, sticky, or hydroscopic dust. Cyclones perform in these applications and prevent the material from reaching the filters. Placing a cyclone in front of a baghouse or cartridge will help prevent time and money spent on repairing or replacing the filter, and ultimately will lengthen the life of the filter.

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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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