Cyclone collectors can be grouped into two general types. The first is the conventional dry cyclone and the second is the multicyclone. The former is characterized by a relatively large housing with a tangential gas inlet and top central gas outlet, and the latter usually is configured with multiple rows of identical, individual, smaller diameter cyclones. The cyclones of the multicyclone collector are often made from castings, whereas the conventional dry cyclone is usually made from sheet or plate metals.
The conventional dry cyclone is a relatively simple device. Experience has shown that keeping them simple is the best formula for success. To accommodate various gas volumes, they are often grouped in pairs, quads, or even greater numbers.
The gas inlet velocity is usually at or above the conveying velocity of the particular dust being separated. Velocities of 40-65 ft/s are common. The inlet is often rectangular in shape so that the gas enters in wedge form at the tangent line of the cyclone. The width of the inlet is approximately one half the height of the inlet. If the dust is highly friable or abrasive, a velocity toward the lower velocity range is used. If the dust is both heavy and abrasive, a higher velocity must be maintained, so wear plates or even refractory linings are suggested at the gas inlet. The cylindrical body tube length in part dictates the number of turns, and the turning radius (tube diameter) controls the centrifugal force created at a given gas velocity. The higher the gas tangential velocity, the greater the number of turns, the higher the centrifugal force, and the greater the separation.
The cylindrical body length is usually two to three times the body diameter.
The gas outlet velocity is usually 55-65 ft/s and sometimes higher. This vortex finder or outlet tube usually extends down into the cylindrical body portion far enough to prevent dust from short-circuiting from the gas inlet to the outlet tube. An ascending vortex is formed in this tube that turns opposite in direction to the inlet spiral. On cyclones with high-tangential inlet velocities (greater than 100 ft/s), the outlet tube can also be equipped with turning vanes that control the gas swirl. The gas outlet diameter is often approximately one half the cylindrical vessel diameter. Care is taken to avoid having the outlet tube extend down too far into or near the conical section of the collector. If it does, dust near the wall will be drawn back up the outlet tube, lowering the efficiency. The outlet tube length is usually about 1.2—1.5 times the height of the gas inlet.
The tapered or conical portion of the cyclone should be smooth. It is usually made using multiple brake settings if made of metal. If the taper is dented or bumpy, re-entrainment of dust can occur. The taper usually has an angle of at least 60 degrees from the horizontal side. This angle exceeds the angle of repose of most dusts, therefore, bridging at the dust outlet can be reduced.
The gas outlet tube is sized for the expected dust flow rate and allows for a dust velocity of about 4-8 ft/s.
Multicyclone collectors are sized in a similar manner, however, a series of standard tubes are used. Each tube is designed for a given cubic feet per minute of gas flow, and then multiple rows are used to accommodate the design gas flow. Tube volumes of 500-1000 acfm each are common. This results in tubes of 9- to 12-inch inside diameter for many applications. Figure 4.6 shows
a multicyclone collector in cutaway. Notice that the tubes are mounted on a flat tube sheet and the outlet tubes are of varying length. The gas enters from the back of this particular view and exits out the top.
You can also see the vane section. Figure 4.7 shows this more clearly. The vanes look much like a turbine vane and are either cast as part of the tube or are separate pieces fitted into the tube. Quite often, a gas outlet vane is also used to enhance separation and to discharge the finer dust separated in the gas outlet tube.
The multiple cyclone collector works by causing the contaminant particle to move at high speed constrained by the limited radius of the individual tube.
The centrifugal force moves the particle to the tube surface, where it accumulates and drops by gravity down to the collecting hopper. To reduce short- circuiting of dust in the tube, a special outlet tube is used, often with vanes that impart a rotation to the ascending gas stream. Figure 4.8 shows the basic operating principles of the multiple cyclone.
The proper application of a cyclone collector starts with a knowledge of the type of dust being collected and its concentration.
Cyclone designers have accumulated data on a variety of dusts and their characteristics. Some dusts are spherical and others are fissured. Some particulate is oblong in shape. These characteristics affect their collectability using centrifugal force.
To make life easier, particulate is often characterized by its aerodynamic rather than physical diameter. The aerodynamic diameter can be considered to be its real-world effective diameter versus its actual physical characteristics as they would appear in, for example, a photograph. The aerodynamic diameter is obtained using a particle sizing device such as the cascade impac- tor, which separates particulate by size in accordance with its aerodynamic behavior.
Cyclone collector designers use the aerodynamic characteristics and loading to select the appropriate cyclone(s). If the dust loading is very high and the particulate is friable, for example, the designer may use a larger diameter cyclone with reduced turning radius. Often, cyclones are used in stages or groups where the gas flow is split into multiple streams and the separation conducted under more controlled conditions where the dust layer at the wall is thinner. Figure 4.9 shows a sketch of a multiple or dual cyclone. These units often share a single dust collection hopper and single rotary lock or discharge valve.
On multicyclone units, a condition called hopper recirculation can occur that reduces efficiency. When this condition exists, some dust-laden air goes into the inlet tube of one cyclone and short-circuits up the discharge tube of another cyclone. The telltale sign is usually an accumulation of dust immediately above the offending tube's gas discharge pipe. This often occurs when the defective cyclone's inlet vanes are broken or if the tube housing itself fails. When inspecting the interior of a multicyclone collector, look for these deposits. The offending tube can often be replaced without affecting its neighbors.
To allow the dust to exit the collecting hopper, a valve must be used to allow dust out but keep air from entering. Trickle valves and rotary locks are commonly used for this service (some cyclones in batch operation service use drum fittings that seal directly onto a receiving drum).
Trickle valves have counterbalanced plates inside of their housings that allow a measured weight of dust to discharge without allowing gas to enter or escape. One such trickle valve is shown in Figure 4.10. Counterweighted double-stage valves are also often used to reduce air infiltration. The external counterweight applies pressure to an internal plate that seals in the dust until the weight of the dust above the plate is sufficient to overcome the force of the
Dual cyclone on common hopper (Bionomic Industries, Inc.).
counterweight. The dust momentarily caught between the counterweighted plates acts as an additional sealing medium. These trickle valves may be single dump (one sealing chamber) or double dump (two sealing chambers) as shown in Figure 4.10. Sometimes, the discharge dust exits to a collecting drum or conveyor. Fabric filter or wet-type dust connection equipment is often used to collect and control fugitive dust in the particulate discharge area.
The rotary lock is another commonly used device to allow the uniform discharge of captured dusts from dry cyclones, dry precipitators, and other dry collection devices. Figure 4.11 shows such a rotary lock. These designs have evolved over the years to provide very effective discharge of collected dusts. The figure shows the lock with its drive mechanism (usually a gear reduction motor) removed.
Trickle valve (photo courtesy of the Wm. W. Meyer Co.).
Rotary lock (photo courtesy of Wm. W. Meyer Co.).
The inlet flange is shown at the top and the outlet flange is at the bottom. The dust flows vertically downward. The vanes are driven slowly by the rotation of the shaft shown to the right of the photo. Typically, a high-torque rotary motor or actuator is applied to the keyed shaft. At any given time, there is no direct passage of dust out or air in. These rotary locks operate much like the revolving doors used in hotels and other buildings that allow people and things to enter and leave with minimal air entry or loss. As with the trickle valve, the rotary lock may discharge into a collecting drum or conveyor.
A very common problem for any cyclone is excessive gaps in the rotary lock or trickle valve, allowing ambient air to be drawn into the cyclone hopper (if the cyclone operates under induced draft), which acts as an air lift to jettison the dust out of the collector. To mitigate this, rotary locks using adjustable end plates and rotor seals are used. With use of a motorized air lock, the end plate is constantly pressed against the rotating sealing vane in the device, thus reducing air leakage. These locks should be periodically inspected and adjusted as required but are often neglected given the dusty environment in which they must operate. It is literally a dirty job but someone has to do it.
Most cyclones must be installed vertically because their operation relies on stable, controlled vortices that Mother Nature (and the laws of physics) tells us operate best vertically. The ascending vortex that forms should be symmetric with the cyclone body otherwise, an imbalance in the rotational forces can occur. If this happens, dust from the cyclone body can be vacuumed out the outlet tube, causing a reduction in efficiency.
Another common problem is using an elbow immediately after the cyclone. The elbow can sometimes upset the spinning action of the ascending vortex, causing an imbalance. At least two diameters of straight ductwork at the cyclone discharge before the elbow usually solve the problem. Dry cyclones often use the involute-type outlet box to reduce these imbalances and produce a stable ascending vortex.
Excessive dust levels in the cyclone hopper can cause serious problems. Consider the spinning gases as a confined tornado inside the vessel. If you let the tornado (spinning vortex) touch down on the accumulated dust, the vacuuming action can lift the dust up and out of the collector. The vortex flow can often be reduced by the addition of a vortex breaker located above the trickle valve or air lock. Some facilities use bin detector devices to monitor the hopper dust level and actuate the rotary lock or trickle valve (above) to keep the level low enough to prevent touchdown.
For highly abrasive dusts, replaceable inlet scroll-wear plates are often used. Made of abrasion-resistant plate, they help reduce the erosive effects of such particulate.
In multicyclone collectors, a dust recirculation pattern can occur inside the cyclone modules. Gas (and dust) can migrate from tube to tube depending upon the differential pressure across the tubes. This can be mitigated by increasing the differential pressure between the hopper chamber and the clean air plenum. This is accomplished by pulling a draft on the hopper air space and directing the gas flow to a baghouse or other external dustcollecting devices.
Properly designed cyclone collectors are effective devices for the recovery of dry products and as primary collectors for subsequent additional air pollution control stages.