Dust collection cyclone efficiency

Cyclone Efficiency. Most cyclone manufacturers provide grade-efficiency curves to predict overall collection efficiency of a dust stream in a particular cyclone. Many investigators have attempted to develop a generalized grade-efficiency curve for cyclones, eg, see (159). One problem is that a cyclone s efficiency is affected by its geometric design. Equation 15 was proposed to calculate the smallest particle size collectable in a cyclone with 100% efficiency (157). 

The collection efficiency of cyclones varies as a function of particle size and cyclone design. Cyclone efficiency generally increases with (1) particle size and/or density, (2) inlet duct velocity, (3) cyclone body length, (4) number of gas revolutions in the cyclone, (5) ratio of cyclone body diameter to gas exit diameter, (6) dust loading, and (7) smoothness of the cyclone inner wall. Cyclone efficiency will decrease with increases in (1) gas viscosity, (2) body diameter, (3) gas exit diameter, (4) gas inlet duct area, and (5) gas density. A common factor contributing to decreased control efficiencies in cyclones is leakage of air into the dust outlet (EPA, 1998). 

Within the range of their performance capabilities, cyclones are one of the least expensive dust-collection systems. Their major limitation is that, unless very small units are used, efficiency is low for particles smaller than five microns. Although cyclones may be used to collect particles larger than 200 microns, gravity-settling chambers or simple inertial separators are usually satisfactory and less subject to abrasion. 

Separation based on rotating flow principles is one of the most common operations involved in gas-solid flows. This section describes the fundamental rotating flow principles and their applications to cyclone operation. The efficiency of dust collection in cyclones is also described. 

Current industry trends are toward the installation of enclosed conveyors and the construction of enclosed areas with negative pressure aspiration for oilseed unloading and meal load-out. Highly efficient dust collection systems and control equipment (cyclones and bag houses) are used throughout the processes to control both fugitive and point source emissions. 

Vatavuk (1990) pointed out that a key dimension in the sizing of a cyclone is the inlet area. Properly designed cyclones can remove nearly every particle in the 20-30 micron range. Typically, cyclone separators have efficiencies in the range of 70-90%. Because of the low efficiency of these units, they are often used as a first stage of dust collection, or are referred to as primary collectors. 

Cyclones offer the least expensive means of dust collection. They give low efficiency for collection of particles smaller than 5 im. A high efficiency of 98 percent can be achieved on dusts with particle sizes of 0.1 to 0.2 im that are highly flocculated. 

The removal of dust particles, typically 1 to 1,000 microns in diameter, from gas streams (also called gas cleaning) is accomplished on an industrial scale by four main types of equipment (1) bag filters, (2) cyclones using centrifugal force, (3) electrostatic precipitators, and (4) venturi scrubbers using washing with a liquid. Reasons for dust collection include air-pollution control, elimination of safety and health hazards, recovery of a valuable product, improvement in the quality of other products, and reduction of equipment maintenance. Typical ranges of particle size that can be efficiently removed by each of the four methods are as follows ... 

As Leith and Licht and Koch and Licht indicate, this design method does not include dust loading as a parameter. Higher dust loading improves efficiency however, the quantitative effect cannot be predicted. Also, a purge flow of 10% of the gas out the bottom of the cyclone can increase overall collection efficiency by as much as 20-28%, according to Crocker et al. ... 

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