Kieron P. O’Connell 

It is useful to remind ourselves why we clean the air. I encourage people to remember the 4Ps.

  • product protection: product quality can be severely impacted by contaminated air with resultant wastage and cost;
  • plant and equipment protection: removal of airborne contaminants can provide significant reductions in maintenance and replacement costs, from protection of surface coatings to mechanical equipment and electrical components;
  • personnel protection: it is critical to protect employees and contractors on ethical and moral grounds alone, nevertheless, the savings in terms of reduced absenteeism, healthcare costs, and replacement staff training can be significant; and
  • public and environmental protection: once again, we have an ethical and moral obligation not to damage the environment in which we operate and live. In addition, there is the necessity to comply with laws and regulations, the avoidance of fines or sanctions, and the maintenance of public goodwill.  



To properly capture, transport, and collect airborne contaminants, the nature of the contamination and the air in which it is entrained must be understood. Beyond the simple measureable parameters such as air temperature and humidity there are many other questions that need to be addressed. Parameters can include, but are not limited to:

The material, or combination of materials, that comprise the dust; the specific gravity of these materials; the shape of the dust particles, e.g. compact, smooth, angular, fibrous; the electrostatic characteristics of the dust; the agglomerating characteristics of the dust; the average and maximum mass of dust in the air at any time; time-dependent variations in the dust load; a particle size distribution analysis; entrained sensible moisture in the air; oils, fats or grease entrained in the air; dust having adhesive properties (is it ‘sticky’?); dust presenting an explosive hazard; and seasonal factors that may result in system changes such as internal condensation.

A typical client will normally not have answers to all of these questions and the cost of testing to obtain them can be time  consuming and expensive. Fortunately, the experienced air filtration specialist will have designed for the same or similar applications and based on that knowledge and on the answers to at least some of these questions, will be able to guide the client to the best dust collection solution.

Factors to be considered before selecting a dust collection system:

The buyer can expect to be operating the dust collector chosen for many years to come so it is important to ask all of the questions beforehand that will ultimately result in a satisfactory selection. The following list covers many of these:

  • size, based on space available;
  • indoor or outdoor location;
  • continuous or intermittent operation;
  • local structural requirements, wind and seismic;
  • suitability to the application;
  • availability and costs of utilities such as power, compressed air, water;
  • handling and disposal of collected material;
  • legislative and code requirements;
  • support and maintenance capabilities;
  • particulate removal efficiency required;
  • is the material to be collected explosive?
  • materials of construction and finishes, is the material to be collected corrosive?
  • installed cost, including peripherals and accessories;
  • ease of operation and operating cost;
  • ease of maintenance and expected maintenance costs; v availability of replacement parts;
  • expected operating life; and
  • warranty and guarantees.



Dust collectors can be separated into a number of general categories as follows:

  • continuous duty self-cleaning dry media dust collectors; v intermittent duty self-cleaning dry media dust collectors;
  • wet hydrostatic precipitators;
  • wet centrifugal dust collectors;
  • high pressure wet venturi dust collectors; v electrostatic precipitators; and
  • cyclones and drop-out boxes.


Continuous duty self-cleaning dry media dust collectors are required to operate continuously and clean on-line, with minimal down-time required for maintenance, usually only during scheduled outages. The self-cleaning style dry dust collector uses pulses of compressed air, or high pressure airflows, to reverse the airflow through the filter material displacing the collected dust to a collection hopper for removal. The cleaning mechanism can operate at normal airflow conditions.

These collectors are available in two general types: baghouses and cartridge collectors. Baghouses are preferred for high inlet dust load applications up to 20 grains per ft3 (45.767 grams per m3) whereas most cartridge collectors are suitable for moderate dust loads up to 3 grains per ft3 (6.865 grams per m3).

Intermittent duty self-cleaning dry media dust collectors must shut down regularly for cleaning off-line and lend themselves to intermittent manufacturing processes where dust is not produced continuously. The intermittent style dry dust collector cleans by means of a shaking mechanism that vibrates the filter material displacing the collected dust to a collection hopper for removal. The cleaning mechanism is only effective without airflow. This type of product is ideal for venting work stations in wood, metal, plastics and ceramic dust applications.

Intermittent duty collectors are generally lower in cost than continuous duty collectors.

Wet hydrostatic precipitators use a combination of induced aerodynamic and hydrostatic forces to transfer particulate in the airstream into a water bath from where it settles out into a hopper from where it can be removed automatically or manually. Such collectors can also be operated continuously. The hydrostatic precipitator is less efficient than a dry media collector for the collection of very small (submicron) particles, however it does offer a significant advantage: 

It is an ideal collector for the collection of highly explosive dusts since it removes the dust from the oxygen rich airborne environment, the water eliminates any sources of combustion, and the encapsulation of the dust in the water prevents the formation of a dispersed explosive dust cloud.

These are the collectors of choice for collecting the explosive dust formed when working light metals such as aluminium, titanium and magnesium. While there are ways to handle some explosive dusts in dry fabric dust collectors (but not light metals), the solutions required often cost more than the original dust collector.

Wet hydrostatic precipitators can handle heavy inlet air dust loads (up to 10 grains per cubic foot).

Wet centrifugal dust collectors inject controlled sprays of water into the air entering the device and through a combination of mixing, impaction and centrifugal force, cause the particulate and water to mix and then be separated from the airstream. Such collectors use specially designed blades to enhance this process and increase efficiency.

An important feature of the wet centrifugal dust collector is that it is both an air moving device AND a dust collector. This negates the need for a separate fan with all the attendant cost savings. It also means that the product has a small footprint and will fit into spaces where other types of dust collector will not.

Wet centrifugal dust collectors are designed to handle inlet air dust loads up to 2 grains per cubic foot.

High pressure wet venturi dust collectors are the preferred wet collector for difficult wet, sticky, oily or self-combusting dusts when very high efficiencies on small and sub-micron particulate are desired. The contaminated air is forced through a narrow venturi throat at very high velocity where it is mixed with a water spray. The resultant water-particulate mixture is then removed using a cyclonic separator.

The disadvantages of the wet venturi dust collector are the quantities of water required and the high energy needed to force the air through the venturi. However, it is often the only dust collector that can be effectively used for some applications. .

Electrostatic precipitators use electrostatic forces to collect the particulate contained in an airstream. There are two main types: high-voltage single state precipitators (Cottrell type) which use high voltage (40,000–70,000 volts) DC discharge electrodes; and low-voltage dual state precipitators (Penny type) which use a much lower voltage (a 13,000-15,000 volt DC supply with intermediate supply of 7,500).

The large Cottrell-style collectors are typically used in large power plant applications but have been losing ground in recent times to media collectors due to high initial cost. The smaller Penny style collectors are excellent for removing smoke and oily particulate from the airstream in industrial applications but have also lost ground to media filters, again because of higher initial cost.

The advantages of electrostatic precipitators are their high removal efficiency and low energy usage because the open plate design presents almost no barrier to airflow.

Cyclones and drop-out boxes are often used as pre-cleaners to eliminate the larger particulate and much of the mass from an airstream ahead of a high efficiency dust collector. Cyclones induce a rotational centrifugal force into the airstream which causes the heavier particulate to separate from the lighter air. Drop out boxes use change of direction and slowing of the air velocity to cause the heavier particulate to drop out of the airstream.

Cyclones may remove between 70% and 80% of the mass of particulate in the airstream. They are ineffective in removing small size particulate.


Regardless of your needs, there is an emissions control system that can meet them. Be prepared to reflect on why you are cleaning the air and know and understand the technologies available that are specifically suitable to your particular application.  

Keiron O’Connell, BSc (1st Class Hons), MBA, is a 35 year professional in the air cleaning industry and has spent his entire air pollution control career with AAF International. He has developed, designed, applied, marketed, promoted and sold air cleaning products to all sectors of the air cleaning market including commercial, industrial, power, pharmaceutical and nuclear industries.

AAF International offers a comprehensive range of air filtration solutions. From its world headquarters in Louisville, Kentucky,AAF maintains operations in 22 countries with more than 3,000 employees worldwide. AAF is supported in its international ventures through the resources of its parent company Daikin Industries, Ltd., based in Osaka, Japan, a diversified international manufacturing company and a global leader in air conditioning.

Today, selling under the AAF® andAmericanAirFilter® brand names,AAF clean air products and systems offer comprehensive clean air solutions worldwide. Its products are high quality and high performance, from simple roughing filters, to air pollution control, to gas containment removal, to the highest efficiency filters.

AAF International is committed to applying environmentally friendly practices in all aspects of business operations.