|1. Definition of a
dust aspiration system
|2. Single point
dust aspiration system
|4. Common problems with dust
All industries having to deal with solids have to deal with dust : powder processing industries of course, but also plastic industries, wood industries, mechanical workshops or even new technologies like addition manufacturing which is handling polymer or metal powders. It is crucial when handling dusts to be able to contain them for reasons of cleanliness, hygiene or safety (avoid that the operator inhales the dust and / or that ATEX risks are created). Systems to collect dust must then be designed, installed and operated properly. Such systems are typically referred to as Dust Aspiration Systems, Dust Collection Systems or Dust collectors.
Dust aspiration systems can have different design and different sizes. Typically, powderprocess.net distinguishes 2 categories :
Both types have in common to have a fan to suck air and carry dust away from the working area, and a filter to collect the dust.
Centralized systems have in addition some pipes / ducts to bring air and dust to a collecting hopper. The collecting hopper is typically using a filter (baghouse or cartridge) to capture the dust and make sure that clean air is rejected. To be noted that other processes can be used as dust collectors : cyclones, scrubbers, electrostic precipitators. These are however bigger systems beyond the scope of this page, more focusing on capturing small to medium localized emissions observed at a tipping station, for example, or when powder is poured out of a vessel.
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A solution that is particularly adapted to emissions of dust due to the manipulation of opened bags, typically on bags tipping stations, is to have a dust aspiration system directly mounted on the equipment from where the dust emission will happen.
Such a system is made of :
Figure 1 : examples of bags tipping stations with top mounted dust aspiration
If it is possible to close the equipment once the operation generating dust is completed, then the use of a pulse jet filter is a good choice as it will allow to unplug the filter and let the dust fall back in the process once the operator has completed the tipping, thus avoiding waste of materials.
As for all dust aspiration system, the sizing of the fan and of the filter is of great importance for ensuring the good performance of the system, i.e. to make sure dust is properly captured.
In the case of a tipping station, the fan must be strong enough to ensure that the air velocity through the opening is min ~0.5 m/s, with the filter sized so that the pressure drop is minimal (few mbar).
Table 1 : pros and cons of a
localized dust aspiration system
Fits well on bag tipping stations
Possible to recycle the captured dust directly
|Only 1 system per equipment, thus can be costly if many sources of emission|
A centralized dust aspiration system is located away from the source of dust emission and connected to it thanks to ductworks that will carry air and dust to the central collector. It is then possible to connect several equipment to a single centralized collection system. This is particularly attractive for example in the case of some mechanical workshop having several machines that can generate dust, or if a manufacturer has a high number of tipping stations installed close by one another. Typical applications of these kind of system are the following :
The system is made of a dust collector, which is a hopper equipped with a filter and connected to a fan that will suck the air from the different collection points to the receiving hopper. As for the localized system explained above, it is a good practice to have the filter equipped with an auto-cleaning system, typically pulse jet, in order to extend the life of the filter.
If the central dust extraction system is connected to several equipment, which is normally the case as it allows to make some investment savings vs having 1 system for 1 source of dust, the design of the ducting system is of prime importance. Indeed, the higher the number of ducts, the most difficult it is to ensure a good suction power is evenly spread for all equipment.
Figure 2 : example of central dust
Typical design requirements are the following :
In addition, the following design recommendations can be considered :
Table 2 : pros and cons of a central dust aspiration system
|Well adapted to multiple sources of dust emission||Maintenance of the dust is required
Sometimes balancing of the ducting not easy, or not well managed by operators
ATEX risk to be considered in the installation and in the receiver
May not be easy to recycle the dust collected if different materials are collected then mixed in the collector
This is a basic of dust aspiration, but actually not so often performed properly on the field.
The 1st thing is to have a proper hood design to have an efficient aspiration :
The pick-up velocity must then be adapted to the application, as requirement will vary according to the nature of the dust to capture, but also the dynamic conditions of the capture, i.e. if the dust is almost static, or if there are drafts that are pulling the dust away from the collection point. The following capture velocities can be found in the literature [SHAPA]
Table 3 : recommended dust capture
|Emission type||Pick-up velocity||Examples|
|No velocity||0.3-0.5 m/s||Vapors, welding fumes|
|Low velocity||0.5-1 m/s||Powder weighing, packaging, laser cutting|
|High velocity||1-2.5 m/s||Plasma cutting, high speed conveyor belt|
|High force||2.5-10 m/s||Grinding|
It is possible to estimate the flow required considering the advised pick-up velocity and the distance of the hood entry to the dust emission source thanks to the following formula [SHAPA] :
Q = V.(10X2 + A)
Q = required air flow (m3/min)
V = required capture velocity (m/min)
X = distance to dust emission (m)
A = open hood face area (m2)
The air velocity in ducts should neither be too high (energy waste, risk of abrasion), nor too low (dust build-up in the pipes). Depending on the dust type, the air velocities are recommended around 10 m/s for fumes and in between 15 (light dust) and 23 m/s (coarse particles) for solids ([SHAPA], [Maynard]).
Ducts conveying the air and powder must be studied in detail during the conception of the dust collection system. Indeed, it is very important to make sure that some design aspects have been correctly taken into account :
Reaching the design air pick-up velocities and conveying velocities in ducts require to size properly the fan and the filter according to the air flow requirement and the pressure drop that will be generated through the system.
Dust can lead to explosion if a dust cloud is submitted to an ignition source. Dusts, according to their nature, have different MIE making them more or less sensitive to explosions. However, one must consider that dusts are usually fine, which is decreasing the MIE and make dust collection a clear potential hazard for the factory.
As a consequence :
There are many examples of explosion in dust collectors, this topic must be taken with utmost seriousness by vendors and operators of the systems.
Table 4 : common problems with centralized dust collection systems
|Issue||Root cause and action|
|Dust not well aspirated||Pick-up velocity is too low : check the airflow, make sure the aspiration duct has a hood / enclosure and is close enough to the dust source, check if a damper has been changed|
|Build up in ducts||Air velocity in the ducts is too low : check the fan airflow, verify filter, check adequacy duct diameter / fan air flow|
|High energy consumption||Fan always at high speed even if no requirement : automatize the system with fan on VFD|
|Too much pressure drop through filter||Filter is clogged : check the filter, check the cleaning system pressure and frequency|
[Maynard] Six key considerations for proper dust collection system design, Maynard, PBE, 2018
[Constance] Getting it right the next time, Constance, PBE, 2019
[SHAPA] 10 Key steps for comparing dust extraction systems proposals, Whitehead, SHAPA, 2002