Common sense dust collector system maintenance and troubleshooting

Andy Winston, BWF Envirotec, USA, outlines how when problems arise in the dust collection system of a cement plant, the best solution is usually to make minor changes to several components.

A cement operation has many systems working together. When problems arise, detailed troubleshooting usually pinpoints several areas to address. So why is it whenever there is an airflow problem, a dusting problem, or an emissions problem, the dust collector alone is blamed? Is the system bottleneck just the dust collector or the dust collector filters? In most cases the answer is no.
In a cement plant, the dust collection system has five major components: collection hood(s), ductwork, dust collector, fan, and material handling or discharge equipment. If any of these areas are poorly designed or operating inefficiently, the entire system will not perform properly. Instead of making major changes to one component (often the dust collector), usually the solution is to make minor changes to several components. 

Collection hoods (the entrance to the dust collector system)
It is very easy to tell if you do not have adequate airflow at a collection hood, because there is dust everywhere. However, a collection hood can be designed and located to deliver too much airflow or suction, resulting in heavier than desired dust loading into the dust collector system. To design an efficient collection hood, capture velocity and duct velocity must be understood.
- Capture velocity is the speed of the air at any point in front of the hood or at the hood opening necessary to overcome opposing air currents and to capture the dust laden air at that point by causing it to flow into the hood.
- Duct velocity is the speed of the air once in the ductwork. In order to move air and particulate to the dust collector, the duct velocity must be equal to or slightly greater than the minimum air velocity required to move dust particles in the air stream.

The collection hood must generate a flow pattern and capture velocity sufficient to control dusting without collecting excessive dust. In cement application such as conveyor belt transfers, bucket elevators, air slides, vessel loading and crushing and grinding, the goal is to de-dust the area without moving material off the conveying equipment.
If the capture velocity is too high at the collection hood, excessive particulate will be introduced into the hood, ductwork and eventually to the dust collector. Inside the dust collector the filters will be subjected to heavier than designed dust loading. This leads to the dust collector operating at higher than desired differential pressure, requiring accelerated filter cleaning. Under these conditions, the filters wear out faster and require more cleaning energy.
The solution to this problem is to make sure all collection hoods are designed and located properly. The area around the collection hood should be enclosed as much as possible. This prevents excessive outside air being pulled into the system, requiring greater air flow to de-dust the area. The hood area needs to be large enough to maintain capture velocities of 200 ft per minute (fpm) to 250 fpm (1 m/sec to 1.3 m/sec).
By correcting the poorly designed and located hoods you will improve the dust collector system by reducing the grain loading.
Once the collection hoods are designed and located properly, blast gates need to be installed to achieve system balance. Due to changing airflows through the system, adjustments are needed to ensure proper capture velocity at each hood. 

Ductwork (dust highway)
The ductwork is the highway of the dust collection system. Proper duct velocity for most cement applications should be between 3500 and 4000 fpm (18 – 20 m/sec.) Speeds lower than 3500 fpm will allow dust in the airstream to fall out, causing dust buildup in the duct. Speeds higher than 4000 fpm could result in duct abrasion, especially in duct branches and elbows. If the ductwork is sized too small, the result is high velocity in the duct. This is evident in the large percentage of ductwork with holes and patches in almost all plants. The average plants spend a lot of time and money patching holes rather than correcting the design of the duct work. An increase in grain loading mixed with high velocities in ductwork only speeds up the wear. If the ductwork is sized too large, the velocities will be too low and result is dust fallout and build-up in the duct. Dust build-up in the duct creates a maintenance headache. In addition if enough dust build-up occurs it reduces the cross sectional area of the duct which affects overall air flow and air velocities at the collection hoods.
Adding additional vent hoods and ductwork to an existing system must be evaluated. Can the system handle the additional air requirement? Do I have to resize the ductwork in order to maintain the proper velocity based on the air flow increase? If not done properly the add-on collection hoods and ductwork can create an imbalance in the system.
Ductwork into the dust collector should have a straight run of at least eight diameters. This ensures the air and dust loading to the dust collector is evenly distributed over the whole dust collector. Uneven loading can cause abrasion of the filters and in extreme cases wear through the dust collector hopper walls. This is a very common mistake on small and large dust collector systems. In some instances the duct cannot be changed, resulting in the use
of turning vanes in order to evenly distribute the dust and air. The turning vanes can correct design flaws, but unfortunately they will wear and require maintenance and replacement eventually. It is much easier to design the ductwork correctly, avoiding adding to the work load of the maintenance department.

Dust collector
If we have corrected the collection hood design and location and we are transporting the dust and air to the dust collector at the right speed our dust collector has a better chance of working properly. With that said we can’t just throw any dust collector into the system and expect it to function properly.
Dust collector sizing is critical to a proper operating system. The air-to-cloth ratio must be right for the application. The air-to-cloth ratio is the relationship between the amount of air flowing through the dust collector and the total amount of filter media in the dust collector. The correct sizing (air-to-cloth ratio) of a dust collector is based on dust loading, dust particle size, temperature, moisture and dust weight.
Velocities are also important inside the dust collector. The velocity at the bottom of the filter bags, can velocity, has a bearing on how well filter bags clean. The recommended velocity is less than 300 fpm (1.5 m/sec). When a row of filters are cleaned, the dust on the filter surface is blown off and moves down the filter, some falling into the hopper. If the can velocity is too high the dust will not effectively fall from the filter bags, creating re-entrainment of dust on the filter were it was before cleaning. High can velocity results in higher than desired differential pressure and cleaning cycles. If the can velocities are excessively high it will also lead to filter bag abrasion. The high can velocity is caused by too much airflow through the dust collector housing. The solution to excessively high can velocity is to increase the housing size of the dust collector or reduce the airflow. Slightly high can velocity dust collectors can see an improvement of bag cleaning by replacing the filter bags and cages with shorter pleated filters. The shorter filter creates a lower velocity below the bags and aids the dust to fall in the hopper before it reaches the filters. This should be addressed on a case by case basis.
Dust collector cleaning controls play a critical role in dust collector operation. All dust collectors in a cement plant should be cleaned based on differential pressure. It is the only way to maintain consistent differential pressure in a dust collector. Consistent differential pressure yields consistent air flow and velocities through out the system. Cleaning on demand ensures the filter bags are only cleaned when necessary to maintain set differential pressure, thus prolonging filter life. The start and stop cleaning differential set points should never be more than ½ in. WC (13 mm) apart. Each dust collector will be different, but the average dust collector will operate very well between 5 in. and 6 in. WC (127 and 132 mm). Changing the cleaning controls to differential pressure is simple and inexpensive.
It is recommended to monitor all aspects of the system, including mechanical operations, differential pressure, emissions, hopper levels, number of cleaning cycles, and temperature. Example: Installing a sound monitoring system on a dust collector will alert personnel when a problem occurs, instead of waiting for a disaster.

The fan (the engine)
The fan is the engine that supplies energy to the dust collector system. An improperly sized fan can hinder the performance of the other components (hood, ductwork, dust collector). The fan must be sized to handle a calculated volume of air at a calculated system pressure. If the fan is undersized, it cannot move the desired air volume. If the fan is moving more air volume than needed, it puts unnecessary stress on the whole system, which leads to numerous maintenance problems.
If the fan is not sized for the correct system pressure, it will not move the desired air volume. This usually occurs when existing fans are used on a new application with only the air volume capacity taken into consideration. Just speeding up a fan will not always result in more air volume. System pressure must be calculated and matched to a fan curve.

Dust handling equipment
At this point in the dust collector system, the dust has been captured (hood), transported (ductwork), filtered and collected (dust collector). Now the collected dust in the hopper must be conveyed away from the dust collector. To do this properly conveyors and air locks must be sized correctly to evacuate the hopper(s). Once the dust has been collected in the hopper, it should be taken completely away from the dust collector. If hoppers are allowed to stay full of dust, the air steam coming into the hopper lifts the dust and it is collected back on the filters. This creates a circulating dust load in the dust collector and causes the filters to be cleaned at an increased pace, raising differential pressure and shortening filter life.
Air locks must be on the bottom of each hopper before the conveying equipment.
If there is no air lock there will be a flow of air up through the hopper against the flow of material you are trying to discharge from the hopper. This will create another re-entrainment of dust that has been already collected. Once again this requires the filter bags to clean more and most likely raise differential pressure. Airlocks must be installed and used as they are intended. In practical terms, do not take rotors out of rotary air locks and do not wire double flap valves open.

Summary
When experiencing airflow or dusting problems with your dust collector system, check all of the system’s components first. Do not settle for a quick fix in one area. Usually a quick fix just moves the problem to another area of the system. Understanding and using sound guidelines on all system components, and properly fixing the real problem areas, is the best way to create and maintain a reliable dust collection system and reduce maintenance cost.

About BWF Envirotec:
BWF Envirotec is the international market leader in the field of filter media for industrial filtration. The product portfolio ranges from needlona® brand filter media, including the PM-Tec® membrane product line to Pyrotex® KE ceramic filter elements. BWF Envirotec filter media find applications in the aluminium, cement and steel industry, in power plants and waste incineration plants, among others, or also in the food industry.

BWF Envirotec was the first manufacturer in the world to introduce needle felts as a filter medium in 1968. With its head office in Offingen/Bavaria and production plants in Germany, China, the USA, Italy, Turkey, Russia, India and Austria, in addition to a sales network in more than 50 other countries, BWF Envirotec guarantees economically viable solution concepts and technical services in line with the market.

BWF Envirotec is part of the BWF Group. With more than 1400 employees throughout the world, the company operates in the field of industrial dedusting, production of technical felt materials, wool felts and in plastics technology.