1、Screening Vibrating screens Principles -Vibrating screens save space and weight and operate on little power because the screening surface may be actuated by vibrating, gyrating or pulsating movement of small amplitude, but at frequencies that normally exceed 3,000/min. Selection of Proper Vibration
2、Sereen -Be sure the screen supplier knows all details of the application. The centrifugal force factor, or combination of frequency of vibration (speed) and amplitude (throw), may affect performance of any vibrating screen. Also, a correct combination of slope and direction of mechanism rotation is
3、vital for inclined screens. Usually, the larger the opening, the greater the amplitude needed for a screen. If the throw is too small, the material may clog or wedge in the openings. Increasing the throw beyond what is required to prevent blinding or plugging does not necessarily increase the life o
4、f the bearings and reduce screening efficiency. Increased rate of travel permits more tonnage to be passed over the screen per unit of time. For a given tonnage, a faster rate of travel results in a thinner bed of material and high screening efficiency. Maximum slope is reached when the material tra
5、vels too fast for the fines to penetrate the ribbon of material and reach the apertures in the screen cloth. At this point an excessive amount of fine material passes over the screen with the oversize, resulting in poor efficiency. When an existing screen is to be used for an application other than
6、that for which it was originally intended, check with the Supplier to see if any of the operating characteristics need modification and if the the screen is structurally suitable for the new application. The operator can get the correct vibrating screen by providing the supplier with the following i
7、nformation: Maximum tons per hour to be screened, including any circulating load or any surges in the feed rate. A complete size consist or sieve analysis of the material or, if available, an estimated analysis. Type of material and weight per cubic foot in broken state. Separations desired on each
8、deck. Surface moisture carried by the material if screening is to be dry or amount of water with feed if wet. Special operating requirements or conditions such as temperature, abrasiveness, corrosiveness or other physical characteristics of the feed, 辽宁科技大学本科生毕业设计 第 2 页 efficiency or product require
9、ments which determine selection of screening surface, or installation problems which affect screen size selection or capacity. General Types -Vibrating screens may be divided into two main classes: mechanically-vibrated and electrially-vibrated. The former can be subdivided into classes based on how
10、 the vibration is produced-by eccentrics； by unbalanced weight； by cams or bumpers. They can also be subdivided as inclined and horizontal. Sizes -Vibrating screens are made in standard sizes of from 12 in. to 10 ft wide and from 2 1/2 ft to 28 ft long. Common practice dictates that the length of th
11、e screen should be 2.5 times the width for dry screening. For wet screening, wider and shorter screen is best. Screens for scalping ahead of primary crushers, operate at a slope of from 12 degrees to 18 degrees and have openings as large as 11-in. square. The eccentric throw for openings from 5 to 1
12、1 in. is usually 1/2 in.； for openings from 3 to 5 in. about 3/8 in.； and for smaller openings 1/4 in. The screening surface consists of a heavy cast desk, perforated steel with or without skid bars welded between the holes, rod deck, etc. The mechanical shaking screen comprises a rectangular frame,
13、 with perforated steel or wire cloth screening medium. It is usually inclined and suspended on loose rods or cables. These screens now are used mainly for special tasks of coarse screening, having given way to vibrating screens. Electrically-Heated Screens -Electrically heated screen cloth decks hav
14、e afforded better screening and less dust. Modern or updated screening operations, now handling clays, limestone, potash, salts, phosphates and various hydroscopic materials, report minimum dust loss when equipped with heated screens. The controlling factor in this improvement is the electrically-he
15、ated screen doth deck. Any vibratory screen with fine opening (less than 1/2 in.) can be equipped with low volt-age-high amperage resistance heating. The principle of electric heating is based on the fact that small diameter wire of screen cloth (especially stainless steel) serves as a conductor, bu
16、t offers resistance to a high-amperage current. This resistance causes heating of the wire when powerful transformer and specially designed bus bars connected to screen doth decks push up to 6,000 amps current into a circuit. The current is safe and shock-proof because voltage is low, ranging from a
17、pproximately 1 1/2 to 16 volts. Workers can do their jobs around electrically-heated screens without special precautions. Heated screens are effective in preventing moisture content of material from causing buildup and blinding. The screen wire is kept at 100 degrees to 130 degrees F depending on ch
18、aracter and tonnage of the material processed. This temperature is not high enough to weaken the wire cloth or screen structure, nor does it suffice to drive off moisture held in material. The warm wire stays dry, breaking the surface tension that otherwise would bind damp material to cold, damp met
19、al. This differential or 辽宁科技大学本科生毕业设计 第 3 页 warm, dry screen wire versus cold, damp material can be maintained economically through transformer control settings. With every opening in the heated screen mesh protected against blinding, there is no guesswork about what size particle will be delivered
20、. A uniform, unvarying quality to meet tough specifications comes through day after day, no matter what the weather. (High humidity makes trouble with unheated screens.) Plants operating heated screens will have no trouble with excessive amounts of fine, dusty material that formerly sifted through r
21、educed meshes on clogged screens. Service life of screen cloth is greatly increased when electric heating ends the punishment of old-fashioned cleaning methods, such as rough pounding or brushing. Blowtorch flames put too much heat in one spot and bouncing chains added to wear and tear. But, screens
22、 kept clean and open with electric heating reportedly are seldom mistreated and last up to eight times as long. Heated decks end the threat of pile ups and strains that can break the mesh. Economical operation of electric heating for screens requires one transformer for installations of up to three
23、panels of screen cloth (maximum area 4- x 12-ft). Two transformers are used on longer decks. Any deck (top, center or bottom) can be heated. Electric heating is most desirable of all where a screen surface is hard to get at. Changing heated screens does not involve more unbolting than needed for ord
24、inary screens. In specifying screens to deliver a uniform particle through heated mesh, the first thing to consider is weight per square foot of the wire cloth needed to set up appropriate resistance. Most calculations are based on square openings. Slotted openings must be identified as to width of
25、clear opening, diameter of wire and number of wires per inch before weight per square foot can be found in any screen cloth manufacturers catalog. Having determined the usable weight per square foot of screen cloth, the next step is to select the size of clear opening needed, making no allowance for
26、 reduction in this size as formerly was the case when material stuck to the wires. Heated wires will maintain the clear opening at all times, making it possible to screen finer without blinding. From a wire catalog, select a mesh weighing no more per square foot (can weigh less) than determined usin
27、g the method above with the clear opening characteristics desired. This often turns out to be a more efficient screen because the wire diameter will be smaller and the percentage of the open area will be greater. For example, where a 1/8-in. clear opening was desired (but often blinded) on unheated screens, the wire diameter was 0.63 and the open area was 44 percent. The cloth weighed 1.43 lb/ft. 2. For a heated 3- x 12-ft screen, the weight must not exceed
