Table of Contents:
Dry Cyclone Collectors
Dry, cyclonic separators disengage entrained dust from a carrying gas stream. They are often called cyclone collectors, multicyclones, cyclones, cyclonic separators, or cyclonic dust collectors.
Typical Applications and Uses
Cyclone collectors are used for product recovery of dry dusts and powders and as primary collectors on high-dust loading (more than 2-5 grs/dscf, or grains per dry standard cubic foot) air pollution control applications.
A common application is the rotary dryer. Used to dehydrate various products from grain to manure, direct- or indirect-fired rotary dryers often use cyclone collectors to capture the entrained dust prior to a secondary collector (such as a Venturi scrubber). The rotating action of the dryer entrains a portion of the product as the product tumbles through the hot, drying air. This product is often valuable in dry form, so the cyclone is used to disengage the dust from the gas stream and be recovered. The residual dust is air conveyed to the downstream device.
Figure 4.1 shows a large-diameter cyclone collector attached to a gas-fired rotary dryer for agricultural product recovery. The cyclone is the large white vessel in the center of the photograph. The large cyclone diameter (in effect increasing the turning radius of the particle path) is used to provide a more- gentle separation of material that would otherwise reduce in size if excessive gas velocities were used. For particulate that resists breakage and size reduction, tighter radius cyclones can be used, resulting in a more compact design. Dryer/cyclone designers select the optimum tangential velocity and turning radius based upon the characteristics of the material to be separated.
Another application is on wood waste or bagasse (sugar cane) boilers where light-entrained ash can be collected in suitably designed cyclones. On wood waste applications, smaller diameter cyclones are often used in “banks"
Rotary dryer and cyclone (Duske Drying Systems).
where each cyclone handles less than 1000 actual cubic feet per minute (acfm) of flue gas. The cyclones are arranged in parallel flow. The smaller diameter cyclones have a shorter turning radius and are more efficient in particulate separation. These are called multiple cyclone collectors.
One of the most common uses of cyclones is to protect fans from abrasive dusts. Many dust-producing process applications operate under induced draft. Placing a well-designed cyclone collector ahead of the fan helps protect the latter from abrasive wear and improves the operating life of the fan. If the cyclone alone cannot meet emissions guidelines, another type of device may be used after the fan.
Cyclones are also used to collect trim from paper machines. The edges of the formed paper are trimmed to size using knives and the edge is air conveyed in a continuous ribbon of paper back to a cyclone, then repulped in other equipment, and returned to the paper-making process.
Other uses are sawdust collection, separation of air entrained product from pneumatic conveying systems, primary separation in vacuum cleaning systems, fiber separation, and similar applications where the particulate is heavy enough to be influenced by centrifugal forces.
Dry cyclones are not generally used on particulate that is under 5-(tm aerodynamic diameter because these size particles (about one tenth the diameter of a human hair) resist inertial separation. Particles under about 5-pm aerodynamic diameter are so small that they lack the required inertial characteristics relative to the carrying gas (usually air) for separation such as a tendency to settle or follow a trajectory and instead are influenced by the movement of the carrying gas itself.
In the past, dry cyclones were often installed with the fan located ahead of the cyclone. In this arrangement, gases and particulate passed through the fan, which was typically used to provide the motive force for ventilating the gases through the system. The fan was therefore located “hot," with the gas stream typically above the saturation temperature, and therefore corrosion issues were often reduced. Since the cyclone was mounted after the fan, the cyclone was considered to be positive, i.e. operating under positive pressure. Of course, the ductwork from the fan to the cyclone and the cyclone itself was under positive pressure, therefore, any leaks were out instead of in. Another downside to moving the particulate through the fan was that the fan wheel and abrasion could in many applications reduce the size of the particulate, thus making the particulate harder to collect.
With the advent of stricter air emissions codes, the trend is to place the fan after the cyclone. This places the cyclone under negative pressure, and the possibility that the particulate will reduce in size is less than in the positive mode. Since the cyclone would be running under suction (lower than atmospheric pressure), the cyclone is in a negative mode. Given that the dust in the cyclone is situated at below atmospheric pressure, the dust is typically discharged using rotary locks.
When drying high-density products such as manure, blood meal, bone meal, etc., a primary drop-out device such as a shallow-cone cyclonic collector (the shallow cone is used to enhance the removal of the material from the cyclone) can be used. This device reduces the particulate loading to the secondary cyclone. Being a more-lazy design, i.e. lower gas velocities, the larger material tends to separate from the gas stream with less size reduction.
The higher efficiency secondary cyclones are then located after the primary drop-out device on the high-density product applications. On low- density applications, the dryer exhaust could go directly to the higher efficiency cyclone, thereby eliminating the primary separation stage. The higher efficiency cyclones may be configured in single or multiple arrangements depending upon the gas flow parameters, particulate characteristics, and desired efficiency.
Figure 4.2 shows a cyclone installation depicting all of the components mentioned above. The primary cyclone (drop-out device) is in the center of the picture. Note the shallow cone at the base of the device. To the left of the primary collector and elevated is the higher efficiency secondary cyclone. The chamber at the top of the secondary cyclone is where the cleaned
Cyclone installation on a Duske dryer (Uzelac Industries, Inc.).
gases exit. (Notice the length of the cone at the base of the secondary cyclone.) The gases then move to the induced draft fan, which is located behind the (angled trough) product screw conveyor.
One step up of the "complexity ladder" from settling chambers is the family of dust separation devices known as cyclone collectors. These devices primarily use centrifugal force (inertial separation) to spin the entrained particulate from the carrying gas stream. To a lesser extent, they can be considered to be settling chambers wrapped in a cylindrical shape to save space.
The gas stream is typically directed into a cylindrical portion of the device so that a spinning motion is created and sustained for a required number of turns or revolutions to achieve the desired separation. Some designs use a single tangential gas inlet; others use fixed vanes that impart rotational forces to the gas stream. As the gas spins (Figure 4.3), the higher specific gravity dust
is thrown outward toward the containing vessel wall, where it accumulates and slides down the wall surface into a receiving chamber, usually a hopper or other essentially quiescent zone, where the dust accumulates out of the moving gas stream. The dust is usually discharged through a trickle valve or motorized air lock/feeder that prevents air leakage or re-entrainment while allowing the dust to exit.
The typical cyclone includes the following components as seen in Figure 4.4: A tangential gas inlet is used (sometimes incorporating a curved “involute" section) to gradually direct the gas stream for smooth tangential release into the cyclone body. The cyclone body itself is typically a vertical-walled cylinder. The tapered hopper and disengaging section are used to accumulate and separate the dust. The vortex finder (or gas outlet tube) is used to control the ascending vortex. The outlet involute is used to increase the radius of rotation and slowly slow down the spinning gas stream so that the ascending vortex stability is maintained and the rotary airflow is converted to an essentially linear flow with minimal pressure loss.
In general, the more spin cycles or turns imparted to the gas stream, the greater the separation efficiency. Cyclone collector housings are therefore designed to provide a varying number of spins or turns, depending on the
Basic cyclone collector components.
application. Many dry cyclone collectors use a disengaging hopper to separate the collected material from the gas stream. The hopper is shown at the lower portion of the drawing in Figure 4.5.
A limiting factor, however, is the friability of the particles (dust) themselves. A highly friable dust is one that easily breaks down into smaller, more difficult to collect, dust particles as they rub together. Because a cyclone collector inherently throws the dust close together near the vessel wall, the interaction between the particles becomes critical in the design. A limit can be reached wherein the spinning of the dust stream and the friable nature of the dust achieves equilibrium, and no more dust can be separated.
Because an inertial force is used (centrifugal force and its reaction force), the particles most influenced by cyclonic action are quite large. Generally, low- friability particles over 5-gm aerodynamic diameter may be best separated
Dry cyclone (Bionomic Industries, Inc.).
using centrifugal force. Sand particles, for example, are relatively easy to separate with a dry cyclone. Starch, fly ash, and other powders that tend to reduce in size are more difficult to remove with cyclones alone, therefore, cyclones are often followed by additional pollution control devices on those applications.
4.4 Primary Mechanisms Used
Centrifugal force and, to a lesser extent, settling are the forces used in cyclone collectors.
Contradicting forces and effects are same-charge electrostatic forces that could inhibit separation as well as the friability of the particles themselves. Some particulate acquires a charge as it passes through ductwork or a cyclone (piezoelectric effect), thereby making separation more difficult. If the particulate or dust becomes reduced in size, it is more difficult to collect because the effective centrifugal force applied to the particle is a function of its mass.