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Filtration and Vacuum Specialists since 1976

A Scientific Review of Dust Collection
Reference material by: Scientific Dust Collectors

Recycling Sciences Solvent Recovery Still
Because our newsletters are a service to our valued customers we have decided to share some important and educational information on Scientific Dust Collection. Over the next few months we will be focusing on the use of Dust Collectors. We felt that the extensive information and it's importance in the industry would be very useful in helping our customers make an informed decision on their needs for dust collectors in their businesses. Because the information is extensive we will be spreading it over several months.

We will begin with the different types of dust collectors.

  • Cyclones and Inertial Separators
  • Airwashers (Scrubbers)
  • Electrostatic Collectors
  • Filter Media
  • Mechanical Cleaning Collectors (Shaker Collectors)
  • High Pressure Reverse Fan Cleaning Collectors
  • Pulse Jet Baghouse Collectors

This newsletter will focus on the Different Types of Dust Collectors

The simplest type of collector is an inertial separator. This design depends on slowing the flow through the system so that the air velocity is not sufficient to hold the particles in suspension in the air stream. Figure 1-1 illustrates this design which utilizes both inertial and gravity forces upon the dust particles.

As dirty air enters the inlet of the collector, the air immediately reacts to an internal baffle that cause the dirty air to take a downward direction which is followed by a 180 degree upward turn. The inertial and gravity forces drive the particles toward the open hopper.

The hopper is shaped such that it intercepts the particles. The particles will often agglomerate and slide toward the hopper outlet. This agglomeration will allow the collection of smaller particles than those particles that might be captured by only the action of gravity and inertia forces.

A common application of this type of collector is a pre-filter to separate the large particles that might harm some collector models. On process venting hot applications, it will remove large sized hot particles that are not cooled by the process gas. This design also has limited application as a Spark Trap since sparks often have buoyancy and are little affected by gravity or inertial forces.

Centrifugal Collectors

are more commonly known as "cyclones" and depend on centrifugal force to move the dust particles toward the wall of the collection chamber.

The dust laden air enters the collector tangentially at the top and the flow forms a vortex pattern as it travels down the inside vertical wall or barred of the cyclone (see figure 1-s). The tangential forces propel the particles toward the wall. In the whirling air stream, these particles are held against the wall by the centrifugal forces, agglomerate, and slide downward toward the cone of the hopper. The acceleration exerted on the particle is according to the centrifugal equation:

A = Rw2

where w is the rotation radians per second, R the radius of rotation, and A is the acceleration on the dust particles. If we assume that the inlet velocity to the cyclone is a fixed velocity V, then:

w = V/R

and since the force F is from the familiar equation:

F = MA

where M is the mass of the particle.

We can deduce the following:

The forces on the larger particles are greater than the smaller particles since the larger particles have more mass.

A smaller diameter cyclone has higher forces than a large diameter cyclone. But, as we can see in Figure 1-2, the air can take multiple revolutions as it travels down the barrel of the cyclone. The efficiency of the collector depends on the size of the particle, the exerted force, and the time that the force is exerted on the dust particles. When the force brings the dust to the cyclone barrel and it is agglomerated, the dust will slide down the wall. The designer has a choice of designing a cyclone with a small diameter and a shorter barrel or a larger diameter with a longer barrel to get the same performance.

High narrow inlets reduce the distance that the dust must travel to reach the wall. In designing ducts for carrying these air streams, the transitions must be smooth to get the maximum performance from the cyclone.

As far as the dust carrying capacities, there are two opposite characteristics. In general, small diameter cyclones will cost dust efficiently even at relatively low loads (0.1 to 6 grains per actual cubic foot), but the pressure drop ill range from 6 to 10 inches w.c. (water column). However, at high dust loads, some of the dust outlets may have a tendency to plug. Large diameter cyclones can handle dust loads in the 50-100 grains per cubic foot range with low pressure drops (1 1/2" to 3" w.c.), but the collector efficiency will be lower at the low dust loads because the dust particles may be swept from the walls of the collector before the dust particles may be swept from the walls of the collector before the dust particles can agglomerate.

The first generation cyclones (Figure 1-3) had low pressure drops (1 1/2" to 2" w.c.) and relatively large diameters. These collectors were usually arranged so that a fan would blow the dust laden air stream into the inlet. The bottom of these collectors were at atmospheric pressure and the collected dust would drop into a bin or truck.

Dust Discharge Considerations. In high performance high pressure drop cyclones (Figure 1-4), a very intense vortex is formed inside the main swirling stream at the discharge point. If this dust is allowed to collect at this junction, it will reentrain and be swept upward into the outlet tube. Expansion hoppers are necessary to allow the dust to be discharge through an airtight feeder. Also, in some heavy moisture applications, they can be effective in "wringing" out moisture before moving onto the baghouse.

Multiclone Collectors with vane spinners are a very effective compromise. These are illustrated in Figure 1-5. The sloped dirty air plenum allows for effective air and dust distribution on the dirty side and even distribution on the clean air side. The most prevalent design uses 6 inch diameter barrels. These multiple cyclones were often applied in boilers as the only acceptable dust collectors. Most recently, they are used as the preliminary cyclones and followed by more efficient fabric collectors to meet discharge codes.
There are other unique methods of designing inertial separators. Figure 1-6 is a rotary dry centrifugal unit which has specially designed blades that serve the dual function of a fan and the acceleration of the dust particles which are thrown against the scroll of the inertial se3parator. The housing is fabricated of cast iron for maximum abrasion resistance. These were commonly applied in venting grinding applications and were limited to relatively small volume flows.

Louver type collectors are a rather specialized form of centrifugal or inertial collectors. The louvers have very narrow spacing which causes the dust laden air to make a very abrupt change in directions. The dust particles are thrown against the flat surfaces, agglomerate, and fall into the lower part of the collector. These are effective in collecting very light loads of fine dust. Heavier loads would quickly plug the collector. There is a portion of the air stream that is separated in order to remove the dust from the dirty side of the collector. This side stream is usually vented into a small diameter cyclonic centrifugal collector. One of the common applications of a louver collector is to reduce the load entering the replaceable panel filters. Figure 1-7 outlines the construction and design of this louver design. These louver designs are limited to inlet loads of less than 0.5 grains per cubic foot load.

Mechanical collectors are mostly used as a preliminary filter in front of other filters or dust collection devices. They can increase the overall efficiency of a solids separation process, especially when the final collector is a water scrubber or an electrostatic precipitator. Also, they are sometimes used for capturing the larger particulates from an air stream where this separation fits into process requirements.

The collection efficiency of these mechanical "cyclone" or inertial separators have some limitations and will not perform as well as cartridge or baghouse collectors. The fact that these mechanisms have few internal parts is a definite advantage, however, ongoing and future requirements for higher filtration efficiency are causing these devices to take a "back seat" to other more sophisticated methods.

We would like to aknowledge the contribution of Scientific Dust Collectors for their contributions to this newsletter. Exerpts taken from " A Scientific Review of Dust Collection" by Scientific Dust Collectors.

Look for "Airwashers (Scrubbers) next month.

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