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Dust control : Dust Collection Systems

Design and operations tips for efficient dust collection

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Page summary
1. Definition of a dust aspiration system
2. Single point dust collection
3. Centralized dust aspiration system
4. Common problems with dust collection systems


1. Definition of a dust collection system

What is a dust aspiration system ?

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 :

  • Local dust aspiration system : small system mounted locally, directly on the equipment where the dust emission can happen
  • Centralized dust collection system : larger system allowing to collect powder at different locations

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.

Looking for other dust aspiration systems, or specific components ? check these other pages out : FILTERS, GAS / SOLIDS SEPARATION. By PowderProcess.net

2. Single point dust aspiration

2.1 How does it work ?

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 :

  • A filter that will collect the dust
  • A fan that will provide the driving force to suck air located at the point of emission of dust and carry the dust with it
Bag Tipping station with dust aspiration system

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).

2.2 Pros and cons of a localized dust aspiration system ?

Table 1 : pros and cons of a localized dust aspiration system

Pros Cons
Compact system
No duct
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

3. Centralized dust collection system

3.1 What is a central dust aspiration system ?

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 :

  • Woodworking
  • Metal cutting, grinding
  • Material handling (tipping stations)

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.

Central Dust Collection System

Figure 2 : example of central dust collection system

3.2 How to design an efficient dust collection system ?

Typical design requirements are the following :

  • The air velocity at the extraction point (pick-up velocity) must be high enough to capture the dust
  • The air velocity in the ducts must be high enough to allow to carry the dust to the reception hopper
  • As a consequence, the fan must be designed to ensure those velocities, and the filter must be sized according to the max capacity of the fan
  • The whole ducting system must be well balanced in order to avoid to have some emission points without enough air for a proper aspiration. Dampers or other balancing systems are then used.

In addition, the following design recommendations can be considered :

  • Ducting must be as short and straight as possible in order to reduce the pressure drop (thus the size of the fan and the energy consumption)
  • The velocity must neither too high (energy waste, some risks of attrition) nor too low (deposits of dust leading to duct blockages)
  • The system can be automatized, with a fan on frequency drive, so that the suction power is adapted to the need (energy savings), if only few collection points are actually in use at a given time
  • It must be thought in advance how the dust will be handled : is it possible to collect it in bags for example, or, better, is it possible to recycle it directly back to the process

3.3 Pros and cons of a central dust collector ?

Table 2 : pros and cons of a central dust aspiration system

Pros Cons
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

3.4 Pick-up hood and pick-up velocity

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 :

  • It is 1st better to have a profiled hood, adapted to the application, than just a tube. It can be required to have an enclosure in order to have efficient capture [SHAPA]
  • The hood must be positioned close to the dust emission source : the highest velocity is of course at the duct entry when the diameter is narrower, but it must be kept in mind that the velocity will be 70% lower at ~1/2 diameter of the pick-up point while it is 90% lower at 1 diameter [Maynard]
  • The hood must be positioned in a way that it does not pull the dust to the operator face [Constance]

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 velocities

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)

With

Q = required air flow (m3/min)
V = required capture velocity (m/min)
X = distance to dust emission (m)
A = open hood face area (m2)

3.5 Air velocity in ducts

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]).

3.6 Ductwork

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 :

  • The diameter of the piping must be such that the required air velocity is reached
  • The pressure drop must be minimized (will allow to reduce the size of the fan and make savings both at investment and in electricity spent for running the system)
  • The construction of the duct must reduce the risks of leakage
  • The system must be well balanced which means that all pick-up point should have an optimum air flow. [Maynard] is proposing to balance the system by design, which means that the diameter of the different branches is adjusted so that the need of each pick-up point is met, not leaving the place to the a manual adjustment by an operator for example. It is possible although it may not be very flexible all along the life of the installation, that is why many systems still resort in using dampers / blast gates at each pick-up points to adjust the draft. It is indeed more flexible but it should be managed with care to make sure no uncontrolled system modification is carried out that may unbalance the system. The blast gates must also be regularly cleaned
  • Branches should be connected to the main duct through inclined inlet, not 90 degrees ones [Maynard]

3.7 Fan and filter

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.

  • A fan is a centrifugal air mover. As such, one must be careful to that the airflow generated is not constant with the pressure, which is normally the case with a positive displacement blower like a lobe blower. It means that if the pressure drop in the ducts is not properly evaluated, or if operators close too much the dampers, the total air flow through the fan will decrease and thus prevent to have a good capture and conveying of the dust.
  • In order to generate energy savings, the fan can be equipped with a frequency drive that will vary the speed according to the actual demand. It however requires some degree of automation but the payback is usually worth it.
  • A section of straight pipe is recommended at the inlet of the fan [Constance], if the inlet of the fan is made of an elbow, the flow of air may not be constant and the fan can suffer from pulsation and lower airflow than expected.
Fan typical performance curve
Graph 1 : fan performance curve
  • The filter must be sized large enough in order to reduce the pressure drop and the frequency of maintenance (changing of filters) required.
  • As explained in powderprocess.net page on filtration, the baghouse or cartridges must be sized with a proper air to cloth ratio, which will ensure a lower pressure drop and then a better performance of the whole system.
  • The filter must be equipped with an automatic cleaning system which very often takes the shape of a pulse jet system : compressed air is injected backwards to the filter. The air flow and the deformation of the filter allow to make dust fall from the filter and clean it. The cleaning can be triggered according to a timer, or according to the pressure drop (may be better from an energy point of view). Pressure drop triggering cleaning are typically 8-10 mbar g. If it is not possible to bring back the pressure drop below this value, then the filter must be changed.

3.8 Dust explosion protection

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.

4. Common problems with dust extraction 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

Source

[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