Dust collection systems
by Diane Cave, P Eng, Element6, Canada
In agricultural industries, dust collection systems are a necessary evil. They are required to remove unwanted dust from the process but do not contribute to the bottom line. Often the need to upgrade and maintain dust collection systems are forgotten during annual requests for capital, which can eventually lead to an inefficient, outdated, and over-utilised system.
Whether purchasing a new dust collection system or retrofitting an existing system, this article will discuss the key parameters to consider to keep your dust collection system capturing 95 percent of dust and keeping personnel in a safe, clean work environment.
There are four main parts to a dust collection system (the big four): the fan, the dust collector, the ducting and the hoods. Each of these parts will be reviewed in turn, along with an explanation of common mistakes made which result in an inefficient design. There are other smaller parts to consider as well, but if the big four are functioning, the dust collection system will remain in the range of 95 percent dust capture.
The fan is the driver of the system. It supplies the momentum required to collect and move dust to the dust collector. There are two main factors when sizing a fan—air volume and static pressure. These two factors are dependent on each other and can be seen on any fan curve.
Air volume is typically a fixed amount and can be determined through an analysis of each collection point. Airflow requirements can be gathered from industry standards, from manufacturer's supplied information or from some good old-fashioned design work. One recommended resource would be any edition of the ACGIH Industrial Ventilation A Manual of Recommended Practice.
Over the life of a dust collection system, air flow requirements will remain fixed as long as collection points are not added or removed. The static pressure will increase over the life of the system mainly due to dirty filter media. In many cases, fans are sized for the lowest possible static pressure drop and only consider the duct work. To design a fan for a dust collection system properly, the following items also need to be taken into account:
- Dirty filters
- Inlet isolation
- The stacks or return air system
Not sizing the fan for the full system often results in an undersized fan that will not meet the required performance specifications.
The dust collector
The dust collector, also known as a filter, or a baghouse is the main part of a dust collection system. This is where the dust is collected. When installing a new dust collector or changing the capacity of an existing system there are three major design parameters to consider:
- Interstitial velocity
- Can velocity
- Air-to-cloth ratio (filter velocity)
Often air-to-cloth ratio is the only parameter that is considered when sizing a dust collector, but all three parameters are equally important.
The interstitial velocity is the upward velocity of air between the filters. If the interstitial velocity is too high, then the dust pulsing off the filters during a cleaning cycle will not fall off the filters. Instead, the dust will be re-entrained back onto the bag surface. This results in a high pressure drop and reduced efficiency of the filters.
The can velocity is the air velocity in the space below the filters. If the can velocity is too high, material will remain suspended in the air and not drop out. Eventually, the air in the hopper of the dust collector will become saturated with material and dust will start to collect in the hopper. At the same time, the static pressure drop in the dust collector will be very high, potentially exceeding the capacity of the fan.
The air-to-cloth ratio is the amount of air going through the area of filter media each minute. It is calculated by dividing the volume of airflow by the area of filter media in the dust collector. If this value is too high, the dust will be driven into the filter media and create a high pressure drop in the dust collector.
Each of these parameters is determined by the dust being collected. If any or all of these parameters are too high, they will increase the static pressure within the dust collector, likely to a point beyond the fan capabilities which will result in reduced efficiency of the entire system, and specifically the lifespan of the filter media.
The duct work
The duct work or ducting is a highway used to transport material from the collection point to the dust collector. All the material in the ducting must be transported at a certain speed (transport velocity) or the material will settle out in the ducting (saltation point).
The saltation point is based on the material. Is the system being designed for flour or for sugar? A rule of thumb is that a new system should be designed with a higher than calculated transportation velocity. This is because as the dust collection system ages (belts stretch, filters get dirty) the static pressure of the dust collection system will increase, and the dust collection system transport velocity will decrease. One must ensure that material will be transported efficiently throughout the life of the dust collection system.
When modifying an existing dust collection system care must be taken to ensure that the addition of new ducting branches does not affect the air flow in the remainder of the system. It is common to see retrofitted systems that look like an octopus.
Ducting branches have been placed on the system with little thought to design, resulting in an inefficient system and a very dusty environment. Prior to adding new ducting branches, or closing existing ducting branches, the fan and the transportation velocity must be assessed to determine the impact on the existing system. It all comes back to the fan. If the new ducting branches exceed the air volume and static pressure capacity of the fan, then the modifications will not capture the desired 95 percent of material.
If dust clouds are visible in your process, that dust can be collected with a hood, with containment, or a combination of both. Hoods must be placed in the direction of material flow, as close as possible to the equipment and have sufficient air flow to remove the dust and not to plug.
Too often hoods are placed in locations that are too far from the dust source with insufficient air volume. The quantity of air required for a hood is dependent on how much material is being collected, the distance from the collection point and the speed of the dust.
Whether the dust collection system being installed is new or a retrofit to an existing dust collection system, if you keep in mind the 'big four' the dust collection system will remain efficient over its life, and last longer. The key goal is to maintain the system to capture a minimum of 95 percent of all dust, and keep personnel in a safe, clean environment.