CYCLONES

Description

Cyclones provide a simple and relatively inexpensive method of removing particles from gas streams. Particles greater than 10 µm in size can be removed with a high collection efficiency.

Cyclones come in two basic designs, the reverse-flow cyclone and the straight-through cyclone. Both types utilise the higher density of the dust particles compared to the gas. The gas stream is spun and enters a vortex and the dust particles are thrown outwards to the cyclone wall where they can be guided into a collection hopper.

Reverse-flow cyclones are funnel-shaped. The feed gas enters at the top of the unit, either tangentially or axially. The gas is encouraged to spiral either by the tangential entry or by a series of vanes. The gas continues to spiral down into the tip of the cyclone in an increasing vortex motion. Particulate matter is slowed by contact with the walls of the cyclone and drops into a hopper below. The clean air continues to spiral but moves upwards through the centre of the device and out through the exit. The cyclone may be irrigated with water to prevent dust re-entrainment.

In straight-through cyclones the clean air does not reverse direction in the unit. The dirty fraction exits through an annular slot.

Individual cyclones can either specialise in high throughput of gas or high collection efficiencies. To attempt to achieve both, cyclones can be arranged in parallel. To collect fragile agglomerates efficiently, cyclones can be arranged in series and the gas velocity lowered.

Application

Cyclones are used to remove coarse iron oxide dust in the steel industry, dust in sawmills, and fly ash in boilers and power-generation plants. They can also be used in powder paint spraying applications to recycle paint that otherwise would be lost.

Cyclones are frequently used as a pretreatment to remove coarse dust or to lower the dust burden on more expensive dust removal equipment.

Sizing

Cyclones have the advantage of low capital and running costs and low maintenance requirements. However, the units must be sized correctly for the dust size and gas volume and velocity under consideration otherwise efficiencies are much reduced. Cyclones are not an appropriate means of collecting dust particles of less than 5 µm in size.

In general, cyclones that operate on flue gases do not require an additional power source. However, other applications may have a fan to drive the gas through the cyclone, and in a typical application a 10 kW fan would be required to treat 15,000 m3/hour of gas.