Cyclone Design.

Equipment can be broadly described as a cyclone if it derives centrifugal separational ability through the use of a pump-induced tangential velocity in a stationary vessel. Furthermore the tangential velocity can be partly or wholly induced by some kind of impeller mechanism. Cyclones have long been used in industry and technology as simple de-dusting devices, for the removal of particulates from contaminated gas streams (Lapple, 1950; Stairmand, 1952). They have also been used to collect size- selected samples of airborne dusts (Davies, 1952) and airborne micro-organisms (Errington and Powell, 1969). Due to the widespread use of cyclones it is important to try and understand their particle collection characteristics.

Cyclones generally consist of two sections, an upper cylindrical section and a lower conical section. The gas flow is introduced tangentially into the upper section and is constrained to take a downward spiral motion. A fraction of this flow, referred to as the underflow, leaves the cyclone through a duct at the apex of the conical section, while the remained reverses direction and swirls upwards in a central air core to exit from the vortex finder (see Figure 1.2).

Separation of the particles is due to the centrifugal force caused by the spinning gas stream. This force throws the particles outwards and onto the cyclone wall where they are collected.

A second important flow feature is short circuit flow. A fraction of the flow forms a thin stream that follows the top of the cyclone and the outer wall o f the vortex finder, before

Chapter 1. Introduction

turning rapidly through 180 ° at the tip of the vortex finder and leaving the cyclone through the overflow. The practical implication is that particles following this stream are more likely to escape than be captured.

overflow vortex finder inflow a

J

Figure 1.2 Schematic representation of a cyclone

The flow field inside a cyclone is very complex. In addition to the interaction of the particles with the fluid (also called the carrier phase), the fluid swirls and re-circulates along the length of the cyclone. The cause of this swirl and its effect on particle separation can be described by examining the individual velocity components of the flow.

The tangential velocity component of the flow, illustrated schematically in Figure 1.3, has been shown by Kelsall (1952) to increase as the diameter decreases.

Chapter 1. Introduction air core locus of vortex constant finder tangential wall velocity cyclone wall g 0

1

I

Figure 1.3 Tangential velocity distribution (from Kelsall, 1952)

The radial velocity component is generated because not all the fluid can enter the underflow but some must be discharged through the vortex finder. The radial velocity varies from a maximum at the cyclone wall to zero at the air core (see Figure 1.4). A force balance is generated between the centrifugal force and the force generated by the inward radial flow. Fine particles have a small centrifugal force and are carried inwards whereas large particles penetrate against the flow to the wall. At the cyclone wall the radial velocity component is at its greatest while the tangential component is fairly moderate. Thus only the larger diameter particles will tend to stay at the wall. At smaller radia from the core, the radial velocity component is reduced and the centrifugal forces increase. Thus smaller particles will attain a balance of forces and lose their radial velocity component.

Chapter 1. Introduction vortex air finder core wall cyclone wall (D C o §' c 0) a

I

Figure 1.4 The radial velocity distribution (from Kelsall, 1952)

The axial velocity component is responsible for the particle discharge from a cyclone, it does not take part in the force balance. The axial velocity is greatest at the wall diminishing towards the air core. There exists a locus of zero vertical velocity (see Figure 1.5). Inside this locus the direction of the vertical velocity is upwards and all particles in this zone will go to the vortex finder. Those particles that are balanced by acting forces to stay at this point form the size that has an equal opportunity of passing to either the underflow or overflow (see Section 1.3.3.2)

Chapter 1. Introduction vortex air finder core wall locus of zero cyclone vertical velocity wall

<u 0 1 C I 3

I

Figure 1.5 The axial velocity distribution (from Kelsall, 1952)

The operational characteristics of a cyclone are mainly determined by the density differences of the fluid and particulate phases and the high rotational velocities and centrifugal forces that are imparted due to the injection of both phases into the upper part of the cyclone.

1.3.2 Optimisation of Cyclone Design.

Cyclone collection efficiency is defined as the fraction of the particles of a given size that are retained by the cyclone. There are eight common dimensions used to characterise cyclone collection efficiency, often expressed as a ratio to the cyclone body diameter. However from the literature it is clear that there is still a great deal of uncertainty over the best way to optimise the design of the cyclone. This problem is compounded further due to variations in the feed rate and feed composition from process to process.

Chapter 1. Introduction

The effect of cyclone diameter on efficiency has been reported by Kim and Lee (1990). The authors recognised that the smaller the diameter of the cyclone the greater the collection efficiency. This is paid for by an increased pressure drop at the same flow rate or by the need to accept a smaller flow rate at the same pressure requirement. An increase in the overall length of the cyclone has been shown to give an increase in both capacity and efficiency for a given pressure drop (Rietema, 1961).

To allow an opportunity for re-entrainment of the particles in the short circuit flow it is usual to remove the overflow stream by means of a vortex finder (Svarovsky, 1979). Re- entrainment then occurs as particles flow down the outside wall. Increase in the length o f the vortex finder therefore allows more time for this re-entrainment and increases the efficiency of separation of the course particles. However, the majority of the fine particles reach the overflow in the return stream from the apex of the cone. An increase in the vortex finder length therefore allows less time for their re-entrainment (Bradley, 1965) and consequently causes a decrease in efficiency. An optimum length therefore exists, dependent on feed size and distribution, and cut point in relation to this size distribution.

The principal design variables that control cyclone performance are the three aperture sizes; feed, overflow and underflow. The feed and overflow sizes control the size of separation and the pressure drop, the underflow size controls the flow ratio. However it has now been recognised that there is probably no such thing as an optimum diameter for each aperture applicable to all duties and to all sizes o f cyclones (Bradley, 1965)