Cyclones reverse-flow cyclone

FIGURE 13.1 A schematic diagram of a classical reverse-flow cyclone. 

Table 12-1 Standard Designs for Reverse Flow Cyclones...

Circulating Beds These fluidized beds operate at higher velocities, and virtually all the solids are elutriated from the furnace. The majority of the elutriated sohds, still at combustion temperature, are captured by reverse-flow cyclone(s) and recirculated to the foot of the combustor. The foot of the combustor is a potentially very erosive region, as it contains large particles not elutriated from the bed, and they are being fluidized at high velocity. Consequently the lower reaches of the combustor do not contain heat-transfer tubes and the water walls are protected with refractory. Some combustors have... 

There are a number of different forms of cyclone but the reverse flow cyclone represented in Fig. 1 is the most common design used in the industry. The cyclone consists of four main parts the inlet, the separation chamber, the dust chamber and the vortex finder. Tangential inlets are preferred for the separation of solid particles from gases [1]. In this study, the numerical simulation deals with the standard case of reverse flow cyclone with a tangential rectangular inlet. Cyclone dimension used in this simulation are as shown in Table 1. 

The most commonly used design is the reverse-flow cyclone, as shown in Figure 10.43 other configurations are used for special purposes. In a reverse-flow cyclone, the gas enters the top chamber tangentially and spirals down to the apex of the conical section it then moves upward in a second, smaller-diameter spiral, and... 

FIGURE 20 Schematic of reverse flow cyclone flow configuration. 

FIGURE 53.1 Shape and dimensions of reverse-flow cyclone. 

Figure 53.1 shows a typical reverse-flow cyclone in which the necessary elements consist of a gas inlet that produces the vortex an axial outlet for cleaned gas, and a dust-discharge opening. There are a number of different arrangements and modiflcations that offer variations in performance and overcome some of the limitations of the conventional reverse-flow cyclone. These modiflcations can be summarized according to the following classifications [18] ... 

Finally, reversible-flow cyclones can be used for gas cleansing. These have the highest turndown ratio and good efficiency versus drop size (commercial claim 3 + pm, depending on design). They do, however, have a pressure drop higher than that of vane packs and axial flow cyclones. 

Gas-liquid (gas dominated) separation done by axial-and reverse-flow cyclonic devices operating at 20-200 g. These are also used for polishing at gravity separator gas outlets. 

Sand-liquid separation done by reverse-flow cyclonic devices operating at 10-500 g. These have been commercially available in the mining industry for a long time, and are extensively used for sand cleaning in the offshore industry... 

Figure 9.2 Schematic diagram of a reverse flow cyclone separator...

Figure 9.7 Total separation efficiency and pressure drop versus gas flow rate through a reverse flow cyclone...

There are two primary types of cyclones the reverse-flow cyclone and the uniflow cyclone. The reverse-flow (Fig. 1) is by far the most common. It is called a reverse-flow cyclone because the gas-solids mixture enters the cyclone tangentially at its periphery, spirals around the barrel, and then the gas reverses flow and exits through a gas outlet tube (also called the vortex finder or the vortex tube) at the top of the cyclone. The solids spiral down around the barrel of the cyclone at an angle of approximately 15 degrees and enter the cyclone cone attached to the bottom of the barrel. The solids exit the reverse-flow cyclone at the bottom of the cyclone cone. 

Over the decades that cyclones have been used, many different reverse-flow cyclone geometries have been tried to improve efficiency, prevent particle attrition, prevent erosion of the cyclone wall, or prevent particle buildup on the cyclone surfaces. However, there are a few basic types that have emerged as the most popular over the years. Some of these cyclone types are shown in Fig. 3. The cyclones shown in this figure are the tangential inlet cyclone, the volute Met cyclone, and the axial inlet cyclone. This last type of cyclone uses axial swirl vanes to impel the gas solids mixture into rotary centrifugal motion. 

In a tangential-inlet reverse-flow cyclone, the cyclone inlet translates the linear inlet gas flow into a rotating vortex flow. As shown in Fig. 1, the gas solids mixture enters an annulus region between the outer wall of the cyclone and the outer wall of the gas outlet tube. As the gas solids mixture spirals downwards, it sets up a vortex with an axial direction downward toward the solids outlet. 

Grade and Overall Efficiency Collection efficiency is a complex exponential function of inlet velocity air density and viscosity particle mass, size, shape, and roughness and cyclone design configuration. A number of attempts have been made to predict the grade efii-dency of conventional reverse-flow cyclones. Several modeling approaches have been reviewed by Licht [18] and the results of these models are summarized in Table 45.5. 

To separate particles from gaseous streams, cyclones are frequently used in large-scale practices. In fact, cyclone based dust collectors are one of the most widely used devices for removing larger-sized particles. Virtually all cyclones used industrially are reverse-flow cyclones. There are two other types of cyclones rotary-flow (CiUberti and Lancaster, 1976a, b) and uniflow (Ter Linden, 1949). Only the reverse-flow cyclone will be considered here. 

The flow pattern of the vortex motion of the gas in reverse-flow cyclone is quite complex. First, it is three-dimensional second, the flow is turbulent An exact analysis is therefore difficult Soo (1989) has summarized a fundamental analysis of velocity profiles and pressure drops in such a cyclone. He has also analyzed the governing particle diffusion equation in the presence of electrostatic, gravitational and centrifugal forces. He has then provided an analytical expression for partide collection efficiency under a number of limiting conditions. We wiU, however, opt here for a much simpler model of particle separation in a cyclone developed by Clift et id. (1991). This approach is based on a modification of the original model by Leith and Licht (1972). The model will be... 

Figure 7.3.15. (a) Geometry of a conventional reverse-flow cyclone (b) particle trajectory in a cyclone having an idealized flow pattern. (After Flagan and Seinfield (1988).) (c) Modified cyclone geometry for analysis. (After Dietz (1981).)... 

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