Monitoring by Control Technique - Cyclone
Centrifugal and inertial forces are used in a cyclone to separate particulate from the contaminated gas stream as it spirals through the cyclone. A cyclone's particulate collection efficiency is directly related to the particle size distribution of the contaminated gas stream. In general, cyclones are used to collect particles greater than 10 micrometers in diameter and are sometimes referred to as "pre-cleaners", because they are often used to reduce the inlet loading of particulate matter to downstream collection devices by removing larger, abrasive particles. However, a high efficiency cyclone can be employed to collect smaller particles, which requires a higher pressure drop and energy costs.
Cyclones have a relatively simple construction and generally no moving parts. Simple cyclones consist of an inlet cylindrical section, conical section, outlet gas duct, outlet dust tube, and collection hopper. Cyclones use an induced draft fan to impart centrifugal force on the gas stream within the conical section. A double vortex is created in the inside body and the incoming gas is forced into a spiraling motion down the cyclone. At the bottom of the cyclone body, the gas turns and spirals up through the center of the tube and out of the top of the cyclone. The centrifugal force of the spinning gas stream forces large particles toward the cyclone walls. Gravity causes the large particles to travel down the cyclone and into a hopper.
Cyclones are sized to provide the maximum inlet velocity possible for high separation without excessive turbulence. Multiclones or multicyclones consist of multiple small-diameter tubes in parallel, each of which acts like a small cyclone. This configuration combines the high efficiency of a small diameter with the ability to treat large gas volumes.
There are four types of cyclones, which are classified according to how the gas stream is introduced into the cyclone and how the collected dust is discharged. The four types include: (1) tangential inlet, axial dust outlet; (2) tangential inlet, peripheral dust outlet; (3) axial inlet, axial dust outlet; (4) and axial inlet, peripheral dust outlet.
Cyclones and multiclones are susceptible to many operational problems. The most common problems are erosion of cyclone components that come in contact with high velocity particles, plugging of the dust outlet or the gas inlet vanes, and air inleakage that affects the inlet velocity and collection efficiency of the cyclone.
Other specific information about cyclones can be found from EPA Fact Sheets. EPA plans to add cyclone costing to the EPA Air Pollution Control Cost Manual*, Section 6, Chapter 4 - Inertial Impaction (Planned Chapter).
For more information, see the box More About Cyclones.
The primary indicators of cyclone performance are the outlet opacity and inlet velocity. Other indicators of performance are the pressure differential across the cyclone and inlet gas temperature.
Costs of cyclones will be discussed in the upcoming EPA Air Pollution Control Cost Manual*, Section 6, Chapter 4 Inertial Impactors (Planned Chapter). Costs of monitoring systems, both Continuous Emission Monitors and parametric monitoring systems, are addressed in the EPA Air Pollution Control Cost Manual*, Section 2, Chapter 4-Monitors (Sixth Edition) (PDF)(41 pp, 125 K, About PDF)