Cyclones scale

In the vector equation of motion, the form assumed by the vertical component when all Coriolis, earth-curvature, frictional, and vertical-acceleration terms are con- sidered negligible compared with those involving the vertical pressure force and the force of gravity. The error in applying the hydrostatic equation to the atmosphere for cyclonic-scale motions is less than 0.01%. In extreme situations, the strong vertical accelerations in thunderstorms and mountain waves can be 1% of gravity, hypsithermal period... 

The use of the two dimensionless groups Eu and Sffcso in cyclone scale-up and design is demonstrated in the worked examples at the end of this chapter. 

This Stokes number is different from that used in cyclone scale-up in Chapter 9. The cyclone scale-up Stokes number Stkgo incorporates the dimensionless ratio of particle size to cyclone diameter, i.e. ... 

Since separation performance, as measured by Stk or Stkinso, is virtually independent of Re it is not necessary to maintain a constant Re between the model and the protot3rpe when attempting cyclone scale-up. In most such laboratory studies, it is far preferable to use air rather than water. This also makes it easier to find a test dust, since many dusts give problems with solubilization or swelling or incomplete particle wetting, among other things, when dispersed in water. 

The turbulent fluidized bed has a similar or slightly lower soHds volume fraction than the vigorously bubbling bed. There is considerable transport of soHds out of the turbulent bed and the bed level is not very distinct. Large-scale cyclones are needed to return soHds to the bed. On average, the bed inventory passes through the cyclones several times per hour. 

Dust Separation It is usuaUy necessaiy to recover the solids carried by the gas leaving the disengaging space or freeboard of the fluidized becl GeneraUy, cyclones are used to remove the major portion of these sohds (see Gas-Sohds Separation ). However, in a few cases, usuaUy on small-scale units, filters are employed without the use of cyclones to reduce the loading of solids in the gas. For high-temperature usage, either porous ceramic or sintered metal has been employed. Multiple units must be provided so that one unit can be blown back with clean gas while one or more are filtering. 

Regardless of the source, the resultant oil slicks are essentially surface phenomena that are affected by several transportation and transformation processes. With respect to transportation, the principal agent for the movement of slicks is the wind, but length scales are important. Whereas small (i.e. relative to the slick size) weather systems, such as thunderstorms, tend to disperse the slick, cyclonic systems can move the slick essentially intact. Advection of a slick is also affected by waves and currents. To a more limited extent, diffusion can also act to transport the oil. 

References 15 through 23 provide additional information on cyclone separators and the design and scale-up principles. 

An alternative method of using the scaling factor, that does not require redrawing the performance curve, is used in Example 10.4. The cyclone should be designed to give an inlet velocity of between 9 and 27 m/s (30 to 90 ft/s) the optimum inlet velocity has been found to be 15 m/s (50 ft/s). 

Calculate the cyclone diameter for an inlet velocity of 15 m/s (50 ft/s). Scale the other cyclone dimensions from Figures 10.44a or 10.44/). 

These equations can serve as a guide for estimating performance but cannot be expected to provide precise predicted behavior. However, they can be used effectively to scale experimental results for similar designs of different sizes operating under various conditions. For example, two cyclones of a given design should have the same efficiency when the value of Nst is the same for both. That is, if a given cyclone has a known efficiency for particles of diameter d, a similar cyclone will have the same efficiency for particles of diameter d2, where... 

Cyclone type and size of hydrocyclone Geometrical proportions Scale-up constants Running cost criterion... 

Matsen (1985) pointed out a number of additional problem areas in scale-up such as consideration of particle size balances which change over time due to reaction, attrition and agglomeration. Erosion of cyclones, slide valves and other components due to abrasive particles are important design considerations for commercial units which may not be resolved in pilot plants. 

Most of the simulation effort has been applied to fluidized bed combustors which use relatively large size particles. Simulation can also be used for other fluidization processes in the petrochemical industry. Research should be undertaken to identify the proper scaling parameters for beds fluidized with smaller particles. Similar simulations may also apply to components such as cyclones. 

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