Mechanical separators: the explanation of its structure and operational principles Essay

6.12.4. Mechanical separators

Also known as cyclone separators or simply cyclones. Mechanical collectors use the inertia of the particles for collection. A cyclone separator is a device that utilizes the centrifugal force generated by a spinning gas to separate the suspended particles from the flue gas streams. The centrifugal action throws the heavy coarser particles to the cyclone wall where they can slide down into a collector. The separator operates with two vortexes as shown in Figure 6.7.

Figure 6.7

The particulateladen gas stream is forced (tangentially) to spin in a cyclonic or rotating motion to form the vortex. The vortex so formed develops a centrifugal force, which acts to throw the particles radially towards the wall. The gas spirals downward to the bottom of the cone, (forming an outer vortex) and at the bottom, the gas flow reverses to form an inner vortex which leaves through the outlet pipe at the top. It is in this outer vortex that removal of particulates occurs. This vortex acts as the settling chamber or zone.

The centrifugal force on the particles, in a spinning gas stream, is much greater than gravity therefore cyclones are effective in the removal of much smaller particles than gravitational settling chambers and require much less space to handle the same gas volumes. In fact, the centrifugal force is locally opposed by aerodynamic drag in the radial direction hence, the particles get carried towards the bottom of the cyclone. However, the finer particles exit from the top along with the flue gas.

6.12.4.1. Types of Mechanical Collectors

There are two main types of mechanical collectors

(a) Large diameter cyclones (as shown in Figure 6.8 (a))

known as Reverse flow Cyclone Separator

(b) Small diameter MultiCyclones.

The large diameter cyclones are usually 0.3 m to 1.8 m in diameter to remove particles ranging in size from 1.6 mm to more than 150 mm.

The Multi Cyclone Consists of small diameter tubes which from 75 mm to 300 mm in diameter. The small diameter MultiCyclone tubes range from 75 mm to 300 mm in diameter. The arrangement of small tubes or cyclones is shown in Figure 6.8 (b). The vanes located on the inlet of each of the tubes create the spinning movement of the gas stream. Most of the commercial tubes are 150 mm, 225 mm or 300 mm in diameter. Due to the limited gas handling capacity of each tube, large number of tubes is mounted in parallel to a single collector.

The small diameter of the cyclone tube creates more rapid spinning of the gas stream than is possible in large diameter cyclones. Furthermore, the particles moving outward in the spinning gas stream have a relatively shorter distance to travel in a small diameter multi cyclone tube before they reach the cyclone body wall. These features allow small diameter multicyclones to collect considerably smaller particles than large diameter cyclones can. The multicyclones are capable of removing particles having diameters down to 5 micrometers.