Fisher Klosterman: How to Buy a Cyclonic Dust Separator

The prospector pans for gold in the gravel deposits along the outside bends of the lower reaches of a mountain stream. The farmer separates the heavier cream from cow’s milk using a centrifuge. The laundry spins the wet clothes at high speed to extract water. All of these events are demonstrations of forces related to the cyclone. Cyclone dust collectors have been used and misused throughout world industry for more than 100 years. Many of out most useful inventions were discovered by accident or by observations of natural phenomena. This is probably what led to the cyclone. Our forebearers knew if they built a vessel that caused a liquid or gas to flow in the circular motion of a whirlpool, tornado or cyclone, the heaviest/densest portion of the carrier fluid or conveyed foreign particlees would be cast to the outside of the curved path.

Are all cyclones created equal?

Our forebearers though all cyclones where “created equal” and as long as they resembled a cylinder with attached cone, they would do the job! In a most general and crude way they were right. However, the maximum utilization  of these helpful natural forces becomes a more sophisticated study in terms of application. To maximize the efficiency of separation requires a precise design engineered to exactly fit the the many variables possible. The parameters of each specific application must be carefully defined. The resulting cyclone will achieve its highest level of efficiency (99.9+% is possible) only when operated within the envelope of its specifications.

Long history playing as supporting actor!

History has given cyclone separators somewhat of a “bad rap” and wrongfully relgating them to the lowly stage of a first-stage filter for crude separation assignments. Most engineers thought the final filtration job had to be performed by the more costly baghouses and scrubbers. In good measures this “bad rap and minor role” stems from an ignorance of application knowledge combined with the aforementioned “cyclone is a cyclone” misconception. The difference in comparative efficiency can be as much as 100% between grabbing any available cyclone and applying a unit designed by specialists in high efficiency cyclonic separators. Two additional important considerations are the safety factor wherein it is a matter of fact cyclones are always safer from fire or explosion than devices using fabric filters and the lower maintenance costs since there are no filters to replace.

Many of the “misusers” have been known to install used cyclones left over or inherited from a totally different discontinued application. THis practice goes on without any regard to proper design criteria. It would be comparable to running a Clydesdale in the Kentucky Derby simply because you already owned him – and isn’t a horse a horse?

Five points of cyclone design
Cyclones work on the principal of maximizing the effect of a combination of major and minor forces to remove –
A) Solids from gases.
B) Liquids from gases.
C) Solids from liquids.

The forces involved in separating different matter in a cyclone –
1) Mass rotational forces – can occur as centrifugal, inertial or gravitational.
2) Drag forces – opposes the mass forces to effect separation of particulate from a gas flow.
3) Other forces – including fluid resistance, dynamic pressure, centripetal, and reactance.

Complications to the proper interaction of these forces exist due to the following:
A) Incorrect definition of the entering gases.
B) Incorrect definition of the entering particulate.
C) Incorrect shape of the cyclone.
D) No consideration of proper ducting design.
E) No consideration of material discharge.
F) No consideration to changes in the nature of the carrier gas and conveyed material during collection (i.e. lack of grounding, stickiness, etc.).

It is a known fact that many cyclones installed in the last century do not do not approach their full collection potential because of a lack of the knowledge and experience required to develop a proper design, based on the conditions at hand. (Cyclones are the most suitable dust collection device when dealing with high process temperature and pressures (≥2000°F ≥500 Atm). Many texts have been published that offer formulas for generic cylcone designs. These work well for bulk materials with a large particle size of consistent aerodynamic shape. Ultra high efficiency cyclone models have been perfected in recent years that achieve extremely high efficiencies (exceeding 99%) on particulate, down to approximately 5 microns Stokes equivalent diameters (even at specific gravities of ≤1.0). Since every statement made to this point deserves a defining publication volume all by itself, it is important that we direct this discussion on the factos that maximize efficiency. A cyclone, if design precisely, can do a much better job than has commonly been assumed in the past. In order to improve efficiency we will take a closer look at the complicating factors (listed previously) that affect performance –

A) Incorrect definition of the entering gas that acts as the conveying medium for the particulate. Proper definition is based on an accurate description of the gas as it enters the cyclone inlet. The gas conditions are defined as an actual cubic measure per unit of time, at a defined gas density and viscosity/ All of these can deviate from standard (ex SCFM) depending on the gas pressure/density caused by geographical location or process pressure along with humidity conditions and temperature.

B) Incorrect definition of the entering particulate is a major problem because it too must be defined as it enters the cyclone inlet. In many cases, a customer who wants to replace an existing cyclone of marginal efficiency, with an ultra high efficiency design, defines the material, based on a previous outlet emissions test or material being discharged from the existing cyclone . This is totally unacceptable. So,what is the recourse, if one is unable to obtain an accurate inlet sample? One approach is to consult a high efficiency cyclone expert who has the means to computer model the existing cyclone , based on an accurate measurement of the inlet, body, cone, discharge, vortex finder and and vortex breaking receiver hopper. By modeling the existing cyclone accurately at defined inlet gas flow and particulate conditions, an expert can design a predictable cyclone replacement that will achieve a higher, improved, efficiency. Other factors that must be taken into consideration are space requirements and the allowable pressure drop across the cyclone. In some cases, space and efficiency requirements may dictate the use of a dual, quad, or more, parallel arrangement and/or an increase in pressure drop, to create higher centrifugal force for collection. In most cases, it is also recommended that a representative sample of particulate, as it would be entering the inlet, be sent to a qualified laboratory to be tested to be tested for an aerodynamic particle size distribution. The test will provide a list of percentages of Stokes equivalent micron sizes finer than various sizes. This allows all the particulate to be described in spherical ball equivalents of a homogeneous density and is the easiest way to predict efficiency. This can be best understood by a simple demonstration-

1) Take any two “identical” pieces of paper.
2) Assume that each is an enlarged “particle” of material that has been magnified many times and we drop them from the same height at the same time, we can note that
3) They twist and turn as they fall to the floor.
4) They do not land in the same place on the floor.
5) One will likely take a longer time to land on the floor than the other. But how can this be, if the two “particles” are identical? The reason lies in the aerodynamic effects and forces which are not obvious from the physical properties of the particles. Also, as “Chaos” theory would predict, there are numerous small variations in the particles, and how are they dropped, which affect the outcome. To add further definition to the test, pick up the two “particles” (pieces of paper) and re-drop the first one and watch it flip flop around and land somewhere other than straight below the drop point. Now take the second “particle” (piece of paper) which is the same shape, size and weight of the original, and crumple it into a small ball (to change its shape into a more aerodynamic shape). Drop it and you will see that it falls much faster to the floor. You will also note that it falls in a more predictable path (i.e. straight down to the floor). From this demonstration, it becomes apparent why it is necessary to define the particulate distribution in an aerodynamic manner for the design of inertial collectors. It is also noted that the above exercise assumes that the two “particles” have identical densities, but in many applications we may be dealing with mixed materials with individual specific gravities.

C) Incorrect shape is a major factor in any contest of bad cyclone designs. Two cyclones with identical diameters but different geometric configurations of inlet, code, vortex finder, and discharge can create a difference of maximum proportion.

Cyclone A has the potential for a number of problems-
1) The inlet may not provide an adequate inlet velocity and profile.
2) The tangential inlet may encourage-

A) Vortex finder abrasion.
B) Improper inlet flow due to vortex finder interference.
C) The potential exists for a direct short circuit path allowing particulate to go up the vortex finder prior to reaching the maximum centrifugal force potential it will take to get the material to the cyclone wall where it will fall to the receiver.

3) Bad internal design that can maximize turbulence to redirect particulate that may be normally caught, to the inner reverse flow vortex.
4) Potential for high wear at the body/cyclone joint due to a too abrupt transition preventing material to smoothly transfer from the body to cone.
5) Low profile cone (too short) promotes wear and reentrainment.
6) NO vortex breaking receiver hopper between cyclone and airlock.

D) Improper duct design is the most common reason for inadequate air volume to the cyclone. How many times is a system designed for a given air volume, and a fan purchased that cannot deliver that volume because the static pressure capacity of the fan ordered. In addition, it seems that there is a natural design tendency to put an elbow connection at the inlet of most cyclones. For optimum efficiency conditions it is necessary to allow 6-8 diameters of straight duct to the cyclone inlet to prevent “Weighing” the air to the outside of the elbow and into the cyclone.

E) Poor design of material discharge point can open up the potential for major reentrainment. It is believed by many that a cyclone, with the fan upstream, does not require a vortex breaker hopper or an airlocK. This is not true. The truth is that the internal vortex, whether it is induced positively or negatively, has a suction that can reentrain particulate. A vortex breaker and an airlock should always be a system consideration whether the fan is located upstream or downstream.

F) Overlooking any possible changes in gas and material during collection has caused manuy serious performance problems. One of the primary concerns relative to high temperature gas applications is any case where an uninsulated cyclone can transfer enough heat to cause condensation to form internally. In Such cases material collected dry may become very wet. Another potential problem area is related to electrostatic charge buildup leading to material bridging and in some cases ignition of the dust, a fire or explosion. Proper grounding is a requisite of all dust collector applications. Sticky materials may require special interior surface treatment, easy release or low friction coatings to prevent buildup.

G) Although it is a foregone conclusion that any type of collector will require maintenance from time to time, it is an established fact that a properly designed cyclone will require less maintenance than most baghouses or scrubbers.

Trouble shooting when efficiency is unsatisfactory

When a problem becomes apparent it is important that a proper plan of action be followed. Some basic clues may include-
1) If a cyclone is discharging wetted material that should not be wet, check to see if the inlet temperature versus the outlet temperature causes a dew point problem, and consider insulating the walls.
2) If the cyclone has plugged at the discharge, check the following-

A) Does the material appear to be different than expected, i.e. wet, hot or sticky?
B) Check for air entrainment that can fluidize material above the airlock.
C) Is the airlock side adequate or does it need servicing?

3) If a cyclone worked well initially and has subsequently deteriorated in operation, look for the following-

A) Air leaks at doors, airlocks or abraded holes.
B) Dents in the walls.
C) Fan deterioration.
D) A system change which can include system additions to the original design or system operating condition changes.

Cyclones-Often the best choice.
The fact that a cyclone, being one of the oldest designs, has earned an unjust image as a low efficiency collector. A well designed cyclone dust collector can be your best equipment friend, especially when its operation has been based on optimizing the compatability between carefully defined inlet conditions and the studied application of those forces that make it work. In most products recovery or pollution control situations, cyclones can be effectively incorporated as a process collector or final filter. In those cases where the final filter requires a permit and must qualify as Best Available Control Technology (BACT), cyclones have not fared well historically against baghouses and scrubbers. Some leniency is given on a case-to-case basis, taking into account energy, environmental, and economic impacts. In such cases a cyclone will come up the winner.

Amrein, David, “How to Buy and Cyclonic Dust Separator.” Pollution Equipment News, April 1995.

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