Cyclones in parallel

There are two common reasons for choosing to install more than one cyclone or swirl tube in parallel. One is that one device handling the whole gas stream 

If geometrically similar cyclones or swirl tubes of different sizes are operated at the same inlet velocity, Vrcs and vecs will also be similar. The equation therefore shows that the cut size is roughly proportional to the square root of the vortex finder diameter. Thus, in geometrically similar cyclones, the cut size will be proportional to the square root of the characteristic cyclone dimension, say D. Incidentally, since vecs and Vrcs are proportional to the inlet and outlet velocities, it can be also observed from inspection of Elquation (5.2.1) that the cut size for geometrically similar cyclones is inversely proportional to the square root of any characteristic velocity such as the gas superficial inlet or outlet velocity. 

Additionally, Eqs. (4.3.18) and (4.3.19) indicate that the Euler number and, therefore, the pressure drop, are independent of the cyclone size for geometrically similar cyclones or swirl tubes if the inlet velocity is kept constant. We can thus gain in efficiency without increasing pressme drop by splitting up the solid-laden process stream over two or more cyclones or swirl tubes and operating them in parallel. 

Parallel arrangements of cyclones come in a wide variety of configurations. Their inlet distributor geometry is normally comprised of either  

a segmented section of rectangular ducting, such as that shown in top portion of Fig. 16.2.1, or 

7 WORKED EXAMPLES WORKED EXAMPLE 9.1 - DESIGN OF A CYCLONE 

Determine the diameter and number of gas cyclones required to treat 2 m /s of ambient air (viscosity, 18.25 x 10 Pas density, 1.2 kg/m ) laden with solids of density 1000 kg/ m at a suitable pressure drop and with a cut size of 4 nm. Use a Stairmand HE (high efficiency) cyclone for which Eu — 320 and Stkso — 1.4 x 10 .  

This is too high and we must therefore opt for passing the gas through several smaller cyclones in parallel. 

Assuming that n cyclones in parallel are required and that the total flow is evenly split, then for each cyclone the flow rate will he q = 2/ . 

Therefore from Equations (9.1) and (9.2), new cyclone diameter, D = 1.014/n° . Substituting in Equation (9.21) for D, the required cut size and v (2.476 m/s, as originally calculated, since this is determined solely by the pressure drop requirement), we find that 

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Although performance curves are valuable in assessing classifier performance, frequently the cyclone overflow size analysis is used more than the d Q of the cyclone. In practice, clusters of cyclones (in parallel) are used to handle large capacities. Cyclones are manufactured in sizes ranging from 0.01 to 1.2 m in cyclone diameter, ie, the cylindrical section at the top (2,10). Capacities mn from 75 to 23,000 L/min. Materials of constmction vary widely. Rubber-lined or aH-polyurethane cyclones are used when abrasion is a problem. 

Air Flow Typical gas flow rates for a single cyclone unit are 0.5 to 12 standard cubic meters per second (smVsec) (1,060 to 25,400 standard cubic feet per minute (scfm)). Flows at the high end of this range and higher (up to approximately 50 smVsec or 106,000 scfm) use multiple cyclones in parallel (Cooper, 1994). There are single cyclone units employed for specialized applications which have flow rates of up to approximately 30 smVsec (63,500 scfm) and as low as 0.0005 smVsec (1.1 scfm) (Wark, 1981 Andriola, 1999). 

For conventional cyclones, it is recommended that the inlet fluid velocirv be around 10-20 tn/s and the conical angle of the cyclone be usually made smaller than 25°, If a single cyclone cannot meet the large fluid throughput required, then the use of multiple cyclones in parallel should be con.sidered. 

Example C. Suppose, in the previous Example B, performance must be achieved at a pressure loss less than 10" H20 requiring therefore multiple cyclones in parallel. 

This still exceeds the design specification of 10" H20 and could have been anticipated, since to meet this pressure drop criterion, Vi from Example A must not exceed about 75 ft/sec which would require 6 cyclones in parallel as opposed to the 2 in this Example C. 

Assume the designer does not desire to use 6 cyclones in parallel, but must still meet all the specifications in Example B (i.e., cannot accept 15.35" H20 pressure drop) but is willing to accept 4 cyclones in parallel. Could 4 cyclones suffice (by a reduction in inlet velocity compensated in performance by an increase in exit gas velocity) ... 

Early fluid-catalyst units employed a bank of many small-diameter cyclones in parallel, but this practice was superseded by the use of a smaller number of large-diameter cyclones (97). These are typically 3 to 5 ft. in diameter and 10 to 15 ft. high, although rough-cut separators with diameters up to 8 ft. have been reported (272). Two stages of cyclones in series are ordinarily used in the reactor. In units without Cottrells, two stages are usually employed also in the regenerator. Three... 

This is clearly too large compared with the standard design diameter of 0.203 m. Try four cyclones in parallel, = 0.42 m. 

Operation mechanism, which can be (a) two or more cyclones in parallel, (b) two or more cyclones in series, (c) recycling portion of the gas outlet stream back to the feed, (d) the use of secondary air flow,... 

Each roaster off gas system contains two cyclones in parallel. No established changes have occurred to cyclone availability since the introduction of oxygen emichment. 

Hydrocyclones In the range of 80-90% External Shear damage due to vortices interaction Good Possible Robust static device. No special maintenance required Well established designs from reputed manufacturers Multiple cyclones in parallel are a feasible altemalive. High harvest rates/ smaQ cyclones result in shear damage... 

The conventional hydrocyclone design procedures have been based on a rather simplistic view of the hydrocyclone function the cyclone size is selected from the capacity and available pressure drop requirements, with the cut size not being a free choice but fixed by the former two requirements (reduction in cut size can only be achieved by using a greater number of smaller cyclones in parallel). This approach ignores completely the effect of the underflow orifice size on the cut size, and also on the solids concentration in the imderflow. The procedure based on the model in section 6.6.6 centres... 

Countercurrent separator systems can be found in many diverse industries. One example is in the production of potato, com or wheat starch. Here, washing usually means removal of one sohd from another, i.e. gluten from starch. The gluten has to be sheared off the starch particles and hydrocyclones are ideally suited to this duty. The density of the sohds is low, however, (around 1500 kg/m ) and this combined with low particle size necessitates the use of small diameter (10 mm) cyclones in parallel arrangements. The number of stages used is often high (eight or nine, up to 24 in extreme cases). 

The reduction in the cut size of the whole arrangement, as opposed to that of the single-pass arrangement, is clearly a function of the recycle ratio Qlq if the cut size of the hydrocyclone remains the same (i.e. greater recycle ratios are matched by a greater number of identical cyclones in parallel), the flow ratio Rf is low (which is often the case in clarification duties) and the geometric standard deviation (i.e. the steepness) of the grade efficiency curve is 1.8 (a typical value for hydrocyclones), then the rate of decrease in cut size with the recycle ratio Qlq is as shown in Table 16.3. 

Therefore, two 0.717 m diameter Stairmand HE cyclones in parallel will give a cut size of 3.65 pm using a pressure drop of 1177Pa. 

If we have n cyclones in parallel then assuming even distribution of the gas between the cyclones, flow rate to each cyclone, q = Q/n and from Equation (9.2),... 

Placing a large number of cyclones in parallel in a common bin can result in distribution problems because it will be easier for the gas and solids to flow through the closest cyclone than one located some distance away from the inlet. Multiple inlets to the common vessel reduce this problem, but result in increased complexity and cost. 

It is possible, though rare, to install one or more cyclones in parallel with an existing cyclone, or cyclones, in order to reduce velocities and, hence, the rate of wear. However, unless the velocities in the existing design are higher than they need to be in order to achieve the desired separation, this is not normally an option. 

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