Cyclones

The simplest type of centrifugal device is the cyclone separator (Fig. 3.4), which consists of a vertical cylinder with a conical bottom. The centrifugal force is generated by the fluid motion. The mixture enters in a tangential inlet near the top, and the rotating motion so created develops centrifugal force which throws the particles radially toward the wall. 

Figure 3.4 A cyclone generates centrifugal force by the fluid motion.

Cyclones. Cyclones are also primarily used as prefilters. These also were discussed in Chap. 3 and illustrated in Fig. 3.4. The particle-laden gas enters tangentially and spins downward and inward, ultimately leaving the top of the unit. Particles are thrown radially outward to the wall by the centrifugal force and leave at the bottom. 

Cyclones can be used under conditions of high particle loading. They are cheap, simple devices with low maintenance requirements. Problems occur when separating materials that have a tendency to stick to the cyclone walls. 

Centrifugal demister or cyclone) devices rely on high velocities to remove liquid particles and substantial pressure drops are required in cyclone design to generate these velocities. Cyclones have a limited range over which they operate efficiently this is a disadvantage if the input stream flowrate is very variable. 

Hydrocylones have become common on offshore facilities and rely on centrifugal force to separate light oil particles from the heavier water phase. As the inlet stream is centrifuged oil particles move to the centre of the cyclone, coalesce and are drawn off upwards, while the heavier water is taken out at the bottom. 

The efforts of the experts from Pivdenny have made it possible for Ukraine to become firmly established in the first three of space powers (after USA and Russia). More than 400 earth satellites developed in Pivdenny have been in space In recent years experts have developed the unique camer-rockets Zenith and Cyclone, capable of taking 4 and 14 tons into orbit, respectively. No other carrier-rockets of this type exist anywhere in the world, so they were selected for the international project Sea Start and Globalstar The NDT experts from Pivdenny have made a great contribution to these development, as practically all the parts and components of the carrier-rockets are subjected to thorough control. 

Almost everyone has a concept of pressure from weather reports of tlie pressure of the atmosphere around us. In this context, high pressure is a sign of good weather while very low pressures occur at the eyes of cyclones and hurricanes. In elementary discussions of mechanics, hydrostatics of fluids and the gas laws, most scientists leam to compute pressures in static systems as force per unit area, often treated as a scalar quantity. They also leam that unbalanced pressures cause fluids to flow. Winds are the flow of the atmosphere from regions of high to low... 

From the tube dryer, fiber drops into a cyclone which separates the fiber from the moist, warm air. The fiber falls from the bottom of the cyclone into a large bin and in then moved to the mat forming line. If machine-blending of fiber with additives is used, it will occur at this point, immediately before the forming line. 

Includes cyclonic, dynamic, filtration, inertial impaction (wetted targets, packed towers, turbulent targets), spray chambers, and venturi. 

Fig. 2. Types of spiay towers (a) horizontal spray chamber (b) simple vertical spray tower (c) cyclonic spray tower, Pease-Anthony type (d) cyclonic spray...

Fig. 7. Cyclone generalized grade-efficiency curves. The soHd line is for the Lapple cyclone dimension ratios given in Figure 9. The dotted line is theoretical...

Fig. 8. Cyclone types commonly used (161) (a) conventional, large diameter, tangential inlet, axial discharge (b) smaller tube, tangential inlet, peripheral concentrated aerosol discharge (c) small tube axial inlet and discharge (d) smaller tube axial inlet, peripheral concentrated aerosol discharge.

Cyclone Efficiency. Most cyclone manufacturers provide grade-efficiency curves to predict overall collection efficiency of a dust stream in a particular cyclone. Many investigators have attempted to develop a generalized grade-efficiency curve for cyclones, eg, see (159). One problem is that a cyclone s efficiency is affected by its geometric design. Equation 15 was proposed to calculate the smallest particle size collectable in a cyclone with 100% efficiency (157). 

For smaller particles, the theory indicates that efficiency decreases according to the dotted line of Figure 7. Experimental data (134) (sofld line of Eig. 7) for a cyclone of Eig. 9 dimensions show that equation 15 tends to overstate collection efficiency for moderately coarse particles and understate efficiency for the finer fraction. The concept of particle cut-size, defined as the size of particle collected with 50% mass efficiency, determined by equation 16 has been proposed (134). 

This equation is for Eigure 9 cyclone dimension ratios. The term the effective number of spirals the gas makes in the cyclone, was found to be approximately 5 for Lapple s system (134). The soHd line grade-efficiency curve of Eigure 7 is also used with Lapple s cyclone, which is a somewhat taller, less compact cyclone than many commercial designs. 

Fig. 9. Dimension ratios ofLapple (134) cyclone design. From Ref. 160,...

Table 6. Technique for Calculating Cyclone Overall Efficiency for Dust Particles ...

Traditionally, cyclone dimensions are multiples of outiet pipe diameter D. Typical barrel diameters are 2D but efficiency increases at constant up to a 3Z9 barrel diameter. Efficiency also improves as barrel and cone length are increased at constant up to the natural length of the vortex. At constant inlet velocity, efficiency increases as outiet diameter (and all ratioed dimensions in a family of cyclones) is decreased. Improved efficiency is attained at the... 

Practically all cyclone performance data have been related to a present cyclone set of geometric ratios. One model for cyclone grade-efficiency curves has been tested against reported commercial cyclone efficiencies (159). A good fit was obtained. 

Cyclone Pressure Drop. Typical cyclone pressure drops range from 250 to 2000 Pa. Most data are reported for clean air flowing through the cyclone and these data are conservative for design purposes. Many investigators have unsuccessfully attempted to relate pressure drops to inlet and oudet dimension ratios. Manufacturers caUbration curves or experimental measurements on cyclones of similar dimension should be used where possible. If a rehable experimental measurement is available, however, the pressure drop at other conditions can be estimated by first evaluating the constant i in equation 17. 

Some empirical equations to predict cyclone pressure drop have been proposed (165,166). One (166) rehably predicts pressure drop under clean air flow for a cyclone having the API model dimensions. Somewhat surprisingly, pressure drop decreases with increasing dust loading. One reasonable explanation for this phenomenon is that dust particles approaching the cyclone wall break up the boundary layer film (much like spoiler knobs on an airplane wing) and reduce drag forces. 

Table 7. Dust Loadings and Cyclone Inlet Velocities above Which Erosion Is Excessive ...

Other problems affecting cyclone efficiency are usually caused by abuse or poor maintenance. Problems may arise from temperature warpage, rough interior surfaces, overlapping plates and rough welds, or misalignment of parts, such as an uncentered (or cocked) vortex oudet in the barrel. 

Other Centrifugal Collectors. Cyclones and modified centrifugal collectors are often used to remove entrained Hquids from a gas stream. Cyclones for this purpose have been described (167—169). The rotary stream dust separator (170,171), a newer dry centrifugal collector with improved collection efficiency on particles down to 1—2 pm, is considered more expensive and hence has been found less attractive than cyclones unless improved collection in the 2—10-pm particle range is a necessity. A number of inertial centrifugal force devices as well as some others termed dynamic collectors have been described in the Hterature (170). 

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