Cyclones

Cyclones are the most common gas cleaning devices widely used for trapping solid particles from gases.

Cyclones find application in the most varied branches of industry: in ferrous and non-ferrous metallurgy, chemical and oil industry, building materials industry, energy, etc.

With low capital costs and operating costs, the cyclones, depending on the characteristics of the dust trapped, the type and operation mode of the cyclone, provide an efficiency of cleaning gases of 80-95%.

Cyclones can be used both for preliminary cleaning of gases and installed before fabric filters or electrostatic precipitators, and independently.

The main elements of the cyclones are the inlet branch, the body consisting of a conical and cylindrical parts, an exhaust branch pipe and a hopper. The gas in the main types of cyclones enters the upper part of the body through the inlet pipe welded to the body. The dust is trapped by the centrifugal force that occurs when the gas moves between the body and the exhaust port. The trapped dust enters the hopper, and the cleaned gas is ejected through the exhaust pipe into the atmosphere or fed to an after-treatment.

Depending on the flow rate of the gas being purified, cyclones can be installed one by one (single cyclones) or combined into groups of two, four, six or eight cyclones (group cyclones). Cyclones can be used to purify gases from a few hundred to hundreds of thousands of cubic meters per hour.

Cyclones can be produced with a "left" and "right" rotation of the gas stream. Usually, it is customary to call the "right" rotation of the flow in the cyclone clockwise (if viewed from the side of the exhaust pipe), "left" - rotation of the flow counterclockwise. The direction of rotation is selected based on the conditions of the arrangement of the cyclone in the circuit, as well as the location of the cyclones in the group.

The efficiency of gas cleaning η in the cyclone is mainly determined by its type, size, dispersed composition and density of particles of the trapped dust, and also by the viscosity of the gas. As the diameter of the cyclone decreases and the gas velocity in the cyclone increases to a certain limit, the purification efficiency η increases.

The peculiarity of the operation of cyclones is that the efficiency of gas purification is sharply reduced when atmospheric air is sucked into the cyclone, especially through the hopper. Experimental studies have shown that 1% of the air sucking reduces the cleaning efficiency by a value of 1 to 4%, so the suction should be kept to a minimum.

For dust of a given disperse composition, it can be calculated based on the fractional efficiency curves given in the relevant regulatory materials, such as: "Guidelines for designing, manufacturing, installation, and operation of NIIOGAZ cyclones" (All-Union Association for Gas Purification and Dust Collection, Yaroslavl , 1971), and a number of others.

An important value characterizing the energy costs for gas cleaning by a cyclone is its coefficient of hydraulic resistance ξ. The coefficients of hydraulic resistance of the cyclones ξ, listed in the catalog are referred to the average gas velocity in the cross-section of the cylindrical part of the cyclone body.

Another, very important characteristic of the cyclone, is its resistance to abrasive wear, which determines the longevity of the apparatus. Abrasive wear is due to the concentration of particles at the walls of the cyclone and the dynamic action of the particles with the walls.

Studies of the nature of wear of various cyclone elements show that the greatest wear is observed in the upper part of the cyclone body at the entrance of the dusty gas into the cyclone and at the bottom of the conical part of the cyclone. The intensity of abrasion of cyclones depending on the specific operating conditions can reach 12-20 mm per year.

For more than a century of history of cyclone development, several dozen types of their designs have been developed. Only in the territory of the former USSR more than 20 different types of cyclones have been used. Conducted comparative tests of cyclones of various types have shown that the list of recommended cyclones can be limited to cyclones NIIOGAZ.

Analysis of requests for the supply of NIIOGAZ cyclones, such as CN-11, CN-15, CN-15U, CN-24, SDK-CN-33, SCN-40, SK-CN-34, showed that of all the cyclones NIIOGAZ demanded Only cyclones of average efficiency CN-15 and highly effective cyclones SCN-40 are used.

The practice of operating cyclones CN-15 showed that the resistance to abrasion in the cyclones of CN-15 is comparatively small. Another disadvantage of this cyclone is the complexity of manufacturing the screw housing cover. With the aim of eliminating the above-mentioned shortcomings, a cyclone with a flat cap SCN-50 was developed in JSC "NF NIIOGAZ" more resistant to abrasive wear. The service life of the SCN-50 exceeds the lifetime of the cyclone CN-15 several times, and the cleaning efficiency of gases is 2-4% higher. Cyclones SCN-50 are more simple in design, easy to manufacture. The metal capacity of the cyclone SCN-50, compared to the cyclone CN-15, is reduced by 15%.

A very promising direction in the field of inertial dust collection is the development of direct-flow cyclones, the gas velocity through the cross section in which reaches a value Wp=12-15 m/s with very little resistance. However, direct cyclones widely used in transport are very difficult to manufacture and require for their work an additional fan installed in the suction line. All this limits the use of a direct-flow cyclone for general industrial purposes.

At the present time, a cyclone design has been developed that combines the positive properties of countercurrent and straight-flow cyclones, which the authors have called a vertical direct-flow cyclone (VPC).

The exhaust outlet of the cyclone VPC passes through the hopper, which eliminates the condensation of moisture in the hopper when cleaning hot gases. The trapped dust, in this case, will not hang when unloading. Cyclones of the VPC type are less sensitive to foreign air sucks in the hopper because The rarefaction in the hopper is several times smaller than conventional (countercurrent) cyclones. These cyclones are well assembled in aspiration systems when installed in front of the fan.

Analysis of the dependence of the degree of air purification η on the average air velocity Wvx in the cyclone inlet allows us to state that the minimum flow velocity at the cyclone inlet, at which the apparatus starts to operate stably and efficiently, is Wvx=6 m/s. This is significantly less than conventional (countercurrent cyclones) and leads to an increase in the reliability of the cyclone.

The coefficient of hydraulic resistance of a single cyclone of a VPC is ξp=80. The coefficient of hydraulic resistance of the group cyclone of the VPC, consisting of four cyclones-ξp=104.

The metal capacity of cyclones of VPC is less by 35-45% than for conventional cyclones designed to purify the same volumes of gases..