Gas Cyclones and Swirl Tubes

Frontmatter

1. Introduction

Abstract

The subject of this book is centrifugal gas cleaning devices, namely cyclones used as gas-solids separators (for ‘dedusting’) and as gas-liquid separators (for ‘demisting’).

2. Basic Ideas

Abstract

In order to understand the practical working of cyclones, it is necessary to master a number of topics, which span a range of different disciplines. Fluid mechanics, particularly relating to swirling flows, particle motion in a fluid, and different aspects of particle properties, such as size and size distribution, shape, and density, are all topics relevant to the later chapters.

3. How Cyclones Work

Abstract

As mentioned in Chap. 1, cyclones work as a result of the centrifugal forces acting on the particles suspended in the swirling gas stream. This causes the particles, which are denser than the gas, to move outward to the cyclone wall, along which they are transported downward to the dust exit. The cleaned gas leaves near the centerline, in a reverse-flow cyclone through the roof. In a ‘once-through’ or ‘flow-through’ cyclone, the cleaned gas exits out the bottom¹.

4. Cyclone Flow Pattern and Pressure Drop

Abstract

Predicting the separation efficiency of cyclones involves predicting how particles behave in the separation space. In order to do this, we need to know the velocity distribution of the gas. Some researchers have made all-embracing models for both gas flow pattern and separation efficiency; others have concentrated on one or the other. We look at the two issues separately here.

5. Cyclone Separation Efficiency

Abstract

In the previous chapter we have examined various models for predicting the velocity distributions in cyclones. In this chapter we wish to first present a brief discussion of the issues, and to then present some of the literature models available for computing cyclone separation efficiency.

6. The Muschelknautz Method of Modeling

Abstract

Over a period of more than 30 years, Professor Edgar Muschelknautz, along with his students and co-workers, working mostly at the University of Stuttgart, have developed what may be, overall, the most practical method for modeling cyclone separators at the present time.

7. Computational Fluid Dynamics

Abstract

We have examined some of the most widely acclaimed and cited cyclone models. There is one more way of predicting the flow pattern, pressure drop and the separation efficiency in cyclones and swirl tubes, however: by Computational Fluid Dynamics, or ‘CFD’ for short.

8. Dimensional Analysis and Scaling Rules

Abstract

One might ask, here at the outset, “Why bother with’ scaling rules’ when one can simulate practically any size and type cyclone of interest with a good model?” While the latter is generally true, scaling—when based on the performance of a sufficiently large, geometrically similar laboratory model—can predict the performance of an industrial cyclone installation considerably more accurately than the models. It is also the writers’ experience that scaling rules are important for two additional reasons:

• Certain simplified scaling rules and dimensionless quantities allow the designer or practitioner to make quick, ‘back-of-the-envelope’ type calculations and decisions pertaining to cyclone design and performance.
• The scaling formulae allow one to better’ see’ the effects of changes in one variable upon another—both qualitatively and quantitatively.

9. Other Factors Influencing Performance

Abstract

In this chapter we will look at two special factors that strongly influence cyclone performance. These are solids loading and the ‘natural turning length’, both of which affect cyclone separation performance, wear and pressure drop, especially in cyclones with a tangential inlet. The natural turning length may also correlate with clogging.

10. Measurement Techniques

Abstract

A range of experimental or measurement techniques is available for determining the functioning of gas cyclones and swirl tubes. The choice of technique depends on the situation: the techniques giving the best results in the laboratory on relatively small-scale equipment under controlled conditions, may be quite different from those giving the best results in industrial equipment.

11. Underflow Configurations and Considerations

Abstract

The design, operation and mechanical condition of the underflow configuration can, and usually does, strongly impact the separation performance of a cyclone. In reality, the cyclone’s performance is just as good as its underflow design. It is the writers’ observation that about 90% of all cyclone related problems are due to problems associated with the inability of the ‘collected’ solids to properly discharge out the underflow. This can be due to a blockage or bridge of some sort or, more often, gas leakage into the cyclone via the underflow piping because of a poor underflow seal.

12. Some Special Topics

Abstract

This chapter presents information about three diverse topics relevant to cyclone technology; thus the title, “Some special topics”. Two of the topics related to the gas velocity in the separator: cyclone erosion and the critical deposition velocity. The last topic is the working of cyclones or swirl tubes under conditions of high vacuum.

13. Demisting Cyclones

Abstract

Until now we have been concerned with the separation of solid particles from gas streams. However, cyclones may be also utilized quite effectively to separate liquids contained in a carrier gas stream. The principles are the same but liquids pose some unique problems and some advantages relative to solids-collecting cyclones.

14. Foam-Breaking Cyclones

Abstract

Foams consist of cellular liquid structures, or lamellas, that are filled with gas. Some type of surfactant is required in order for foams to form—they cannot occur in pure liquids. If a gas, such as air, is sparged into a liquid containing a surfactant, the surfactant will form a double layer around the gas bubble, creating a collection of spherical foam bubbles. Such foams tend to be unstable and readily coalesce or break due to their high liquid content. More stable polyhedral foam, of most interest to us, is formed as a result of mechanical stresses, and is much more stable or difficult to break. Breaking of foam occurs in three stages: drainage of the cellular liquid comprising the walls, breakage of the foam walls, and diffusion of the gas out of the foam cells.

15. Design Aspects

Abstract

The modeling equations in the previous chapters are not enough to design a cyclone or swirl tube from scratch. The models do not perform well for uncommon designs, and they are not complete enough to be used to find an optimal design for a given duty directly. To assist those who may have the task of actually designing and constructing a cyclone or swirl tube we include some guidelines here. Although the majority of the discussion that follows directly pertains to “dedusting” cyclones and swirl tubes, most of it also applies equally well to demisting cyclones. Regarding the latter, see also Chap 13.

16. Multicyclone Arrangements

Abstract

In this chapter we wish to briefly discuss two types of multi-unit arrangements used in cyclone and swirl tube installations in industry. We also give some guidelines for the choice of arrangement in a given application including a worked example given in Appendix 16A. In addition, we briefly describe how to apply the modeling equations outlined in previous chapters to multicyclone arrangements.

Backmatter