遼寧科技大學本科生畢業(yè)設計 第 1 頁 Screening Vibrating screensPrinciples --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 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 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 of 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 travels 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 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 information: ? 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 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, 遼寧科技大學本科生畢業(yè)設計 第 2 頁 efficiency or product requirements 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 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 the 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 11 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, 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 have 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-heated 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, but 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 approximately 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 character 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 metal. This differential or 遼寧科技大學本科生畢業(yè)設計 第 3 頁 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. 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 reduced 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 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 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 ordinary 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 clear opening, diameter of wire and number of wires per inch before weight per square foot can be found in any screen cloth manufacturer's 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 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 using 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 遼寧科技大學本科生畢業(yè)設計 第 4 頁 1.1 lb/ft2. Two options were found in cloth with 1/8-in. clear opening: wire diameter .054 weighing 1.09 lb/ft2 providing 48.8 percent open area; and wire diameter .047 weighing 0.85 lb/ft2 with a 52.8 percent open area. Load conditions may make fine diameters of carbon steel wire inadequate. In such cases, stainless steel wire of larger diameter with greater load-carrying capacity will have the necessary resistance for good heating. Lighter wire with more open area yields higher tonnage and heated mesh stays open 100 percent. Two screen cloths connected in a series may be of different mesh sizes or clear openings, as long as they both weigh the same per ft2 and do not exceed the weight allowed for that particular size of screen. With heated screens, major savings in pollution control are possible. Also moisture content of material is reportedly increased in a range of 5 to 8 percent. This moisture is added during or after crushing-grinding operations and holds down dust during transfer of material and passage over heated screens. Warm wire handles 5 to 8 percent moisture in stride. The cost of heating equipment and mist spraying reportedly is less than the cost of having bag towers and precipitators. 振動篩的選擇振動篩的選擇原則：選擇振動篩的原則是所選擇的振動篩要節(jié)省空間、重量并且驅動的功率要小，因為篩選表面可以驅動并且發(fā)生振動。同時還要保證旋轉或脈動運動振幅要小,但振動篩的振動頻率在通常情況下是要超過三千次每分鐘的。選擇合適的振動顯示屏：要確定顯示屏供應商知道所有有關方面的使用細節(jié)。離心力或組合頻率振動（速度）以及振幅（投擲）等因素都可能影響到任何振動篩的正常工作。此外,需要有一個正確的組合坡度和方向的輪換機制，這對于顯示屏來說是至關重要的。通常情況下，振動篩需要大開放、大振幅的顯示屏。如果投入太少的話，則這些材料可能出現木屐或楔形開口等情況。增加投入以后，需要知道如何防止堵塞因為堵塞是不必要的，而且會增加日常軸承的磨損以及和降低篩選的效率。單位時間內材料上升速率曲線都要通過整個屏幕,因此該處材料以較快的速率通過較薄的車床的同時需要用較高的效率來進行篩選。當材料速度過快導致材料絲帶被擊穿并當材料到達屏幕布料開口處時出現最大傾斜角。此時大量的優(yōu)質材料通過屏幕的速度過快，從而導致效益不佳。 當一個現有的屏幕畫面出現故障時，如果該屏幕的結構還適合于新的應用，這時應及時進行檢查并詢問供應商，看看是否有需要修改的地方。操作者還可以通過供應商提供的下列資料得到良好的振動篩: ?通過最高每小時噸數進行篩選，包括任何循環(huán)負荷或任何突然增加的進給速度。?通過一個完整的粒度分析或對材料進行篩分分析，如果條件允許的話，最好使用估計分析法。?材料的種類和每立方米重量失效的概率。?各個篩板的預期斷裂的情況。?通過表面材料所攜帶的水分進行篩選，如果是潮濕的需要將水份曬干或添加一定數量的材料。?特殊的操作要求或條件，例如溫度、磨損情況、腐蝕情況或具有其他物理特性的材料、效率或產品的要求，確定篩選表面或安裝問題，因為這將直接影響到屏幕尺寸的選擇以及屏幕實際使用的能力。一般的類型：振動篩可分為兩大類:一類為機械振動型振動篩，另一類為電力振動型振動篩。 前者是根據振動是如何產生的可被細分入類—由偏心輪產生；由失衡的重量產生；由凸輪或防撞器產生；它們也可以按照傾斜和橫向進行細分。 尺寸：振動的屏幕通常被制造成寬為從 12 英尺到 10 英尺，長為從 2 1 / 2 英尺到 28 英尺。在通常情況下，干式篩選顯示屏的長度應該是寬度的 2.5 倍。對于濕式篩選來說，更寬和更短的屏幕是最好的選擇。為防止屏幕提前而導致被壓碎，操作斜率應保持在 12 度至 18 度左右，并有多達 11 種開口情況。異常投擲為開頭從 5 英尺到 11 英尺通常是 1/2 英尺；開頭為 3 英尺到 5 英尺，大約 3 / 8 英尺和較小的開口 1/4 英尺。篩選表面是由一輛重型鑄造服務臺、鋼穿孔或無打滑焊接鋼筋之間的空隙、篩網等部分組成的。機械振動篩的屏幕由一個長方形框架、已打孔的鋼或絲織物組成。它通常傾斜和暫停在懸浮松散桿或電纜上。這些屏幕現在主要用于有特殊任務的時侯，此時應讓位給振動篩的屏幕。電熱屏幕--電熱布屏幕面板提供了較佳的保護功能并且具有粉塵少等優(yōu)點?，F代或更新篩選作業(yè)，現在處理粘土、石灰石、鉀、鹽、磷和各種的吸濕材料，當配備暖簾時，報告的粉塵損失最低。 控制因素在這方面的改善是使用電熱幕小靈通平臺。任何振動篩的屏幕以良好的開幕(小于 1 / 2 英寸)，可配備低電壓使用年齡高的焙電阻來抵抗熱化。 電熱化的原則是基于這樣的事實:小直徑線材絲網布( 尤其是不銹鋼) 作為導體，但是能夠抵抗高焙電流。當強有力的變壓器和特別地被設計的母線接上屏 幕面板被推至 6000 安培電流進入電路時，這種抵抗導致導線的熱化目前是安全防震，因為電壓較低，從大約從 1 / 2 伏特至 16 伏特。因此工人們可以在沒有殊防范措施的情況下在電熱屏幕附近安全的工作。熱屏幕能有效地防止材料的水分累積和堵塞。根據材料本身的特征和被處理的噸位，屏幕導線能夠被被保留在 100 度到 130 華氏度。這個溫度并不足夠高導致減弱導線布料或屏幕的結構，也不足夠除去材料中的水分。 加熱導線使其保持干燥，否則會打破束縛在潮濕的材料中對冷, 潮濕的金屬的表面張力。這一差動或溫暖、干燥篩線對寒冷、潮濕的材料可以通過控制設定，而變得更加經濟合理。每個熱屏幕篩網的前端被保護，以避免堵塞。無法猜測多少尺寸的顆粒將被提供。無論是什么天氣，一個統一的、品質不變的、滿足苛刻的規(guī)格每天都有通過。(高濕度造成未加熱的屏幕的問題) 。操作熱的顯示屏不會有太多的麻煩 ,多灰塵的材料以前是在被堵塞的屏幕上通過減少篩網來過濾的。當電熱化結束后可以使用傳統的清潔方法清理屏幕，如果使用粗洗或電刷洗等方法，屏幕布料產品的使用壽命將會大大的提高。小型發(fā)焰裝置在一定的區(qū)域內產生大量的熱量，他所產生的連鎖反映就是增加了磨損以及斷裂。但是，據說屏幕電熱化保持清潔和開放很少被采用而且最多能使用 8 次。暖底甲板的物料堆積和應變，可以導致濾網被打破。對于電熱屏幕來說最經濟的操作是需要有一個變壓器裝置來為屏幕布料(最大面積 4×12 英尺) 。兩臺變壓器是用在更長的甲板。任何甲板( 頂部中心或底部)，都可以用來加熱。處理屏幕表面是很難獲取的地方，使用電熱化是最可取的方法。對于普通的屏幕來說，更換熱的顯示屏時并不需要過多的開插銷。在指定的顯示屏里通過加熱網提供一個一致的微粒，首先要考慮的事情是量出每平方英尺導線布料所要求的重量，以便設來定適當的阻力。大多數計算都是基于方形洞口的基礎上進行的。開槽開孔被識別是在量出每平方英尺的重量和找到任何屏幕布料制造商的產品目錄之前，通過前端精確的寬度，導線的直徑和每平方英尺一定數量導線的重量來實現的。被確定能用的每平方英尺屏幕布料的重量，下一步是清楚的選出所需要的開頭的大小，制造無加工余量尺寸大小減少到以前的情況時，材料貼到電線上。加熱導線將一直保持清晰的開放，使得畫面清晰無黑屏。 通過導線編目，僅僅通過重量選擇一個篩網(重量可以少稱量) 使用的方法是通過前端特征來決定的。這通常是一個更有效率的方法，因為這樣屏幕上的鋼絲直徑將變得更小，而且開放范圍的百分比將會變得更大。例如，在一個 1/8 英尺的未加熱的顯示屏上，它的前端能夠被清楚的顯示出來，(但是屏幕經常黑屏 )，其導線直徑為 0.63 及開放范圍的百分比為為 44%。布料重量為 1.43 磅/ 英尺 。一個被加熱的 3×12 英尺的顯示屏，重量不得超過21.1 磅/英尺 。兩個方案通過比較發(fā)現，布料以 1/8 英尺前端被清楚的顯示出來2的顯示屏上: 導線直徑為 0.054，重量 1.09 磅/ 英尺 ，開放范圍的百分比為248.8%；一個加熱 3×12 英尺的顯示屏上：線直徑為 0.047，重 0.85 磅/英尺 ，2開放范圍的百分比為 52.8%。 在負荷的狀態(tài)下可以解決碳鋼導線直徑不足的問題。在這種情況下，對于好的熱化來說，不銹鋼導線直徑以更大的承載能力來抵抗外部的載荷。更輕的導線并有更多開闊地帶能夠產生更高的噸位，而且熱的篩網能夠保持百分之百處于打開的狀態(tài)。兩塊屏幕布料被連系成一個系列，也許是網格尺寸大小不同或者是前端的清晰度不同。只要它們滿足每平方英尺的重量相同而且不超過該規(guī)定的重量，尤其是屏幕的尺寸。 對于振動篩的熱屏幕來說，通過控制從而大大節(jié)省污染是可能的。同時材料中的水分根據數據顯示增加了 5 至 8 個百分點。這些水分是在破碎粉磨作業(yè)的期間或之后產生的，通過灰塵在材料中的持續(xù)運動以及通過以上所述的熱屏幕來降低的。溫暖的導線的處理量在 5 %到百 8 %之間。供熱設備和噴霧設備的成本據說是低于袋子塔和沉淀器的成本。Screening Vibrating screensPrinciples --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 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 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 of 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 travels 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 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 information: ? 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 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, efficiency or product requirements 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 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 the 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 11 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, 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 have 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-heated 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, but 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 approximately 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 character 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 metal. This differential or 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. 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 reduced 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 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 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 ordinary 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 clear opening, diameter of wire and number of wires per inch before weight per square foot can be found in any screen cloth manufacturer's 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 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 using 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 1.1 lb/ft2. Two options were found in cloth with 1/8-in. clear opening: wire diameter .054 weighing 1.09 lb/ft2 providing 48.8 percent open area; and wire diameter .047 weighing 0.85 lb/ft2 with a 52.8 percent open area. Load conditions may make fine diameters of carbon steel wire inadequate. In such cases, stainless steel wire of larger diameter with greater load-carrying capacity will have the necessary resistance for good heating. Lighter wire with more open area yields higher tonnage and heated mesh stays open 100 percent. Two screen cloths connected in a series may be of different mesh sizes or clear openings, as long as they both weigh the same per ft2 and do not exceed the weight allowed for that particular size of screen. With heated screens, major savings in pollution control are possible. Also moisture content of material is reportedly increased in a range of 5 to 8 percent. This moisture is added during or after crushing-grinding operations and holds down dust during transfer of material and passage over heated screens. Warm wire handles 5 to 8 percent moisture in stride. The cost of heating equipment and mist spraying reportedly is less than the cost of having bag towers and precipitators.