Cyclone dust separators collection efficiency

Other Centrifugal Collectors. Cyclones and modified centrifugal collectors are often used to remove entrained Hquids from a gas stream. Cyclones for this purpose have been described (167—169). The rotary stream dust separator (170,171), a newer dry centrifugal collector with improved collection efficiency on particles down to 1—2 pm, is considered more expensive and hence has been found less attractive than cyclones unless improved collection in the 2—10-pm particle range is a necessity. A number of inertial centrifugal force devices as well as some others termed dynamic collectors have been described in the Hterature (170). 

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. 

Venturi scrubbers Water is sprayed into the gases as they pass through a venturi. The acceleration of the gas flow in the venturi throat causes an intensive mixing of both media. The dust particles are damped, making them heavier, so that they can be separated in a cyclone or other system placed downstream. If the gas flow drops, the venturi throat is adjusted to maintain the collection efficiency... 

Cyclones are low-cost particle collectors that have many potential applications in coal gasification systems however, they have low efficiency for collecting particles smaller than 10 pm but above this size collection efficiency can be at least 90%. Conventional applications for cyclones include use as precleaners, entrainment separators, and for controlling dust emissions from coal grinding and pulverizing (Chapter 6). 

The proportion of each particle size fraction which is precipitated in a cyclone of particular shape and rating is called the fractional dust collection efficiency and is plotted as a percentage against the particle size diameter (in microns). The curve thus obtained is called a separation curve. It is an important criterion for comparing different cyclones with one another. Table6 indicates, by way of example, the overall collecting performance of a cyclone for dust with known particle size distribution. 

Cyclone separators (Fig. 5) these rely on the action of centrifugal forces on the dust particles carried along in the swirling stream of gas. The particles are thus flung radially outwards to the wall of the cyclone, from where they fall into the dust hopper. The centrifugal force which determines the collection efficiently is directly proportional to the mass of the particles and to the square of the circumferential velocity, but inversely proportional to the radius of the cyclone ... 

In coal mines, dust samples are collected by both coal mine operators and the Mine Safety and Health Administration (MSHA) using a size-selective sampling device (cyclone) that separates out dust in a way that reflects the efficiency of deposition in the gas-exchange region of the lungs. This so-called respirable size fraction has a lung deposition efficiency of 100% at 1 rm or below, 50% at 5 [im, and zero efficiency for particles of 7 jrm and upward (NIOSH 1995). 

Consider the separation of dust particles from air by means of two cyclones connected in the following fashion. Feed air containing dust enters cyclone 1 the feed particle size distribution is Ff (Pp) = l-e"" p. The underflow from cyclone 1 is introduced with a small amount of air as feed to a small cyclone 2. The underflow from cyclone 2 is collected as dust from the system. The overflow air from cyclone 2 is mixed with the overflow air from cyclone 1 to obtain the cleaned air. The grade efficiency of the ith cyclone is given by... 

Centrifugal Separation Centrifugal force can be utilized to enhance particle collection to several hundredfold that of gravity. The design of cyclone separators for dust removal is treated in detail in Sec. 17 under Gas-Solids Separations, and typical cyclone designs are shown in Fig. 17-43. Dimension ratios for one family of cyclones are given in Fig. 17-36. Cyclones, if carefully designed, can be more efficient on hquids than on solids since liquids coalesce on capture and are easy to drain from the unit. However, some precautions not needed for solid cyclones are necessary to prevent reentrainment. 

As a simple and efficient particle separation device, cyclone collectors can be used for anything from dust removal in a fluid stream to material collection in the fluid conveying system. However, the cyclone is not suitable or economical for the separation of extremely small particles (say, less than 1 /xm), which frequently occur in industrial processes. It is recommended that the size of particles to be separated in an industrial ventilation cyclone be in the region of around 10 to 100 p.m. However, for the purpose of aerosol sampling, the size of particles to be separated may be much less than 10 jxm. 

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. 

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