Cyclone Collectors are often employed as an Initial Stage Collector to lighten heavy dusts loads before being sent to a Primary Stage Fabric Collector.
By removing coarse particles from the gas stream, and allowing later stage Fabric Collectors to solely remove finer particles, these collectors increase the efficiency and service life of Fabric Collectors (Baghouses) and their Filters. Thus protecting your larger and most costly investment from unnecessary wear and tear.
It often named after the cyclone weather phenomenon, are large funnel shaped sheet metal tubes connected to ducts often used in woodshops, machine shops, manufacturing plants, and powder processing plants. Dust and debris are sucked in at the top. Air containing fine dust blows out of the other side of the top, while chips and large dust particles fall out of the bottom into a drum or bin. Dusty exhaust air is either blown outside or filtered again using media filtration.
As the dirty air enters the cyclone dust collector, it is forced into a swirling movement. This results in a centrifugal force acting on the dust particles suspended in the air stream. The particles, denser than the air, are forced to move outwards, towards the cyclone dust collector wall. They then fall downwards, towards the dust exit. The clean air is eventually directed towards the centre of the cyclone and leaves through the gas exit.
Cyclone dust collector efficiency is a function of the physical parameters of the particles and the design parameters of the unit. Cyclone efficiency increases with:
- Coarse particle size distribution
- Higher products specific gravity
- Lower gas density
- Smaller outlet diameter. An increase in pressure drop also results.
Inertial separators use inertia and gravity to separate dust particles from the dust filled air stream i.e. by slowing the flow of dusty air stream. When the speed of the dust filled air stream is slowed down, heavier dirt particles settle out from the air stream by gravity and fall into a hopper where they are collected.
There are two types of inertial separators. They are (1) Settling collectors (2) Baffle collectors.(3) Centrifugal collectors
Neither settling chambers nor baffle chambers are commonly used in the minerals processing industry. However, their principles of operation are often incorporated into the design of more efficient dust collectors.
Settling Dust Collectors:
Settling collectors separate dust from the dusty air stream by using a settling chamber (a large box) in ductwork carrying dusty air. When the dust filled air stream enters the large settling chamber its speed come down considerably owing to the sudden increase in size of its passage. Heavier dust particles settle out due to gravity from the slow air stream and are collected.
Baffle Dust Collectors:
Baffle collectors have a baffle plate (a flat plate) in the path of the dust filled air stream to slow it down. The air stream strikes the baffle plate and undergoes a sudden change in direction. The air stream flow and the baffle are designed in a way that the stream is first forced in a downward direction, followed by an upward 180 degree turn. With such abrupt changes in direction, the air flow slows down sharply. The heavier dust particles either strike the baffle plates due to their inertia or settle out by gravity when the flow slows down and slide into the hopper where they are collected. As they collect, the larger dust particles mop up the finer particles that escape the action of gravity and inertia, and improve the overall efficiency.
Uses of Inertial Dust Collectors:
Inertial separators are the simplest type of dust collector. They are normally used as a pre cleaner or a pre filter for collectors with greater efficiencies. As pre cleaners their main function is to separate large particles that could damage some dust collectors. In metal working operations they are also used as a spark trap and protect filters in collectors in a limited way.
Cyclones or Centrifugal Collectors
Cyclones (or centrifugal collectors) create a ‘cyclonic’ or centrifugal force, similar to water going down a drain, to separate dust from the polluted air stream. The centrifugal force is created when dust filled air enters the top of the cylindrical collector at an angle and is spun rapidly downward in a vortex (similar to a whirlpool action). As the air flow moves in a circular fashion downward, heavier dust particles are thrown against the walls of the collector, collect, and slide down into the hopper.
Cyclone Collector Design Considerations:
Cyclone dust collector efficiencies depend on,
Particle size (particles with larger mass being subjected to greater force)
force exerted on the dust particles
time that the force is exerted on the particles
can be designed with either large or narrow diameters depending on the application. Small diameter cyclones have high dust collection efficiencies at low dust loads (0.1 to 6 grains per cubic foot) and high pressure drop of 6 to 10 inches w.c. (water column). Owing to the small diameter they have the tendency to plug at high dust loads. Large diameter cyclones can handle high dust loads (50-100 grains per cu.ft) with low pressure drops (1.5 to 3 inch w.c.) efficiently. They are not very efficient at low dust loads.
To improve efficiencies, design considerations are,
- high narrow inlets reduce distances traveled by dust to the wall and thereby improve collection efficiencies
- small diameters have higher forces than larger diameter cyclones
- smooth transition ensures maximum efficiency.
Use of expansion hoppers in dust discharge:
In high pressure drop cyclones, dust collecting at the discharge point could be swept upward to the outlet tube. This phenomenon occurs due to the powerful inner vortex that is formed inside the main swirling stream at the discharge point. Use of expansion hoppers allows dust to be discharged through an airtight feeder. Expansion hoppers effectively squeeze out moisture in some heavy moisture applications
Multiple Cyclone Separators:
Multiple cyclone separators consist of a number of small diameter cyclones placed parallel to one another with vane spinners. The multiclones have a common inlet and outlet for air. The smaller diameter of the barrels and longer length makes them more efficient than regular cyclones. By being longer dust is retained inside for greater amount of time and smaller diameter of barrel increases centrifugal force, causing efficient separation of dust. The inclined dirty air plenum facilitates effective air and dust distribution in the dusty area and even distribution of clean air in the clean area. This type of collectors are commonly used in boilers or as preliminary cyclones.
Secondary Air Flow Separators
This type of cyclone uses a secondary air flow, injected into the cyclone to accomplish several things. The secondary air flow increases the speed of the cyclonic action making the separator more efficient; it intercepts the particulate before it reaches the interior walls of the unit; and it forces the separated particulate toward the collection area. The secondary air flow protects the separator from particulate abrasion and allows the separator to be installed horizontally because gravity is not depended upon to move the separated particulate downward.
Rotary Dry Centrifugal Unit:
They are centrifugal collectors with centrally designed blades that effectively disperse dust particles from the air stream against the walls of the collector. The dust particles slide down and are collected in a hopper while clean air is let out from the outlet. These units are commonly used in grinding applications. Limited to small volume flows, the housing of these centrifugal collectors is normally made of cast iron due to high abrasion.
Louver Type Collectors
In this type of a dust collector, louvers with narrow spacings are used in the collector to cause abrupt change in direction of incoming dust-filled air stream. Dust particles in the air stream collide against the flat surfaces and collect in the lower part of the collector. Louver type collectors are highly effective at light loads of fine dust (their use is limited to less than 0.5 grains per cu.ft.) and are used to reduce the load entering replaceable panel filters. These collectors plug at heavier loads. A part of the air stream is diverted into a small centrifugal collector. They are used for some specialized applications of collecting fine dust.
Being basic designs, the efficiency of these collectors is limited despite the advantage of having few internal parts. Inertial separators are normally used as a) preliminary filters b) to trap large particles from an air stream and c) to increase efficiency of a solid separation process with water scrubbers or electrostatic precipitators.
Cyclone dust collectors