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.
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