单螺杆挤出机构设计【SJ-150】
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编号无锡太湖学院毕业设计(论文)相关资料题目: 同向旋转型双螺杆挤压机及挤压部件设计 信机 系 机械工程及自动化专业学 号: 0923081 学生姓名: 陈 玉 指导教师: 戴宁(职称:副教授 ) (职称: ) 2013年5月25日目 录一、毕业设计(论文)开题报告二、毕业设计(论文)外文资料翻译及原文三、学生“毕业论文(论文)计划、进度、检查及落实表”四、实习鉴定表无锡太湖学院毕业设计(论文)开题报告题目: 同向旋转型双螺杆挤压机及挤压部件设计 信机 系 机械工程及自动化 专业学 号: 0923081 学生姓名: 陈 玉 指导教师: 戴宁 (职称:副教授) (职称: )课题来源自拟课题科学依据(包括课题的科学意义;国内外研究概况、水平和发展趋势;应用前景等) (1)课题科学意义挤压加工技术作为一种经济实用的新型加工方法广泛应用于食品生产中,并得到迅速发展。采用挤压技术来加工食品,只要简单地更换挤压模具,便可以很方便地改变产品造型。挤压研究内容包括原料经挤压后微观结构及物理化学性质的变化,挤压性能及原料本身特性对产品质量的影响等,为挤压技术在新领域的开发应用奠定了基础。挤压机有多种机型,本文主要研究螺杆挤压机,它主要由一个机筒和可在机筒内旋转的螺杆等部件组成。 螺杆挤压机按螺杆数量分为单螺杆挤压机和双螺杆挤压机两大类。单螺杆与双螺杆挤压机的主要差别是其中物料的允许水分范围以及加工能力的差别应是最值得注意的方面。本文主要研究双螺杆挤压机。双螺杆挤压机是多螺杆挤压机中的一种,是在单螺杆挤压机的基础上发展起来的。在双螺杆挤压机的机筒中,并排安放两根螺杆,故称双螺杆挤压机。双螺杆挤压机按螺杆方向不同,可分为同向旋转和反向旋转两大类,本文主要研究同向双螺杆挤压机。同向旋转相较反向旋转,其优越性是很明显的,但这并非说同向旋转不存在问题。首先是推送效率问题,反向旋转式螺杆的挤压建立在类似于齿轮泵的原理上,物料在双螺杆内的流动不是由于摩擦牵引作用,而是因为机械的强制推送,物料能均匀分配给两螺杆,所以推送效率高。但双螺杆同向旋转式,按螺杆旋转方向,物料只送往螺杆转向端,物料分布偏向一方,其推送性只及反向旋转的一半左右。 (2)挤压机的研究状况及其发展前景与传统生产工艺相比,挤压加工极大地改善了谷物食品的加工工艺,缩短了工艺过程,丰富了谷物食品的花色品种,降低了产品的生产费用,减少了占地面积,大大降低了劳动强度,同时也改善了产品的组织状态和口感,提高了产品质量。本世纪30年代末期,首次把挤压机应用于方便食品谷物的生产中,1936年第一台应用于谷物加工的单螺杆挤压蒸煮机问世,并在行业中取得成功。40年代末期,随着挤压技术的发展,挤压机的应用在食品领域中进一步扩大,深受欢迎。50年代初,迅速发展的挤压蒸煮由于省时省力,很大程度上取代了当时的饼干焙烤。利用挤压技术处理淀粉等,取得了较好的糊化效果。60年代中期,挤压机进一步发展完善,到了70年代,许多国家纷纷展开挤压机理的探讨,进一步研究各种谷物及蛋白类食物在挤压过程中发生的一系列变化。目前美国生产的大型挤压机生产能力已达每小时几顿至十几吨,挤压产品遍及全国各地及食品业和饲料业,有关挤压技术和设备的专利已达百余份。日本长期以来对挤压技术及理论,尤其是谷物膨化淀粉的性质方面做了大量的研究。1979年生产的挤压食品就有300多种,大规模地把挤压技术应用于快餐食品及饲料生产工业中。西方许多国家如英国,法国,德国,意大利,瑞士等对挤压技术也做了大量的研究。近年来,国外挤压食品已成为一大类方便食品。量的研究。近年来,国外挤压食品已成为一大类方便食品。随着人民生活水平的提高和饮食结构的变化,随着对挤压机理研究的不断深入和新型挤压设备的研制开发,挤压食品的品种和产量将会日益增多,并朝着高效节能,产品风味多样化和美味化方向发展。(3)同向旋转型双螺杆挤压机的研究状况及发展前景双螺杆挤压技术近几年发展迅速。研究表明,与单螺杆挤出技术相比,双螺杆挤出技术具有无法比拟的优越性能,如物料能充分、彻底混合揉捏,并且在双螺杆挤出机运转时,由于双螺杆互相啮合而具有自行擦净的功能,避免了单螺杆挤出机经常出现的螺杆堵塞的物料在套筒表面产生结焦的现象。同时双螺杆挤压机还具有广泛的原料适应性这一显著优点,解决了单螺杆挤压机无法处理高水分和高脂肪物料这一瓶颈。双螺杆挤出机因其具有突出的高效工作性能,受到了食品行业的广泛重视。作者根据收集的相关文献,对双螺杆挤压机在食品工业中的应用、双螺杆挤压加工对食品中营养成分的影响、双螺杆挤压技术的发展前景等进行综合的分析和论述,期望有益于我国双螺杆食品挤压蒸煮的研究与发展。食品成分十分复杂,通常是若干种原料混合在一起加工,进入挤出机的物料更是由多种复杂多变的生物高分子混合构成,而且,食品挤压过程往往或高或低地伴随着一定量的水分进行,构成所谓的低湿挤压加工和高湿挤压加工。两种水分含量不同的挤压蒸煮加工,对挤压系统运行的影响也有极大差别,因此双螺杆挤压机在物理模型建立和数学模型求证方面存在困难,这也是挤压技术面临的最大问题。这一问题的解决,将会大大提高双螺杆挤压技术的研究水平。从世界食品发展潮流看,挤压食品占有重要地位。由于它能为消费者提供色、香、味、营养俱全的食品,是其它食品加工手段不可比拟的。发达国家已把蒸煮挤压食品单列为一大类食品,并在保健食品挤压技术、功能性食品挤压技术、超临界流体挤压技术、米粉挤压技术、点心与早餐等即食谷类食品加工、挤压太空食品等方面开展了广泛深入的研究。我国在这一新兴领域也开展了一些研究工作,但尚缺乏深度及广度。因此从事该领域的研究将大有作为。目前有关双螺杆挤压膨化机在水产饲料加工中的报道很多。但对有关鱼肉双轴挤出组织化的研究尚缺乏系统性。水产品不但将成为人们摄取动物蛋白质的主要来源之一,而且也可以缓解人增地减、食品不足、优质蛋白质缺乏的问题。利用双螺杆挤压技术研究开发低值水产资源,将具有显著的综合效益。拟采取的研究方法、技术路线、实验方案及可行性分析拟研究方法、技术路线: 根据课题所确定的挤压机种类,用途及生产能力确定和面机的主要构件(例如螺杆,机筒)机构形式和尺寸参数,运动参数及动力参数(电机功率)。根据挤压机主要构件的形式,性质及运动参数,拟定整机的机械传动链和传动系统图。计算并确定各级传动的传动比,皮带转动,齿轮转动等传动构件的结构参数及尺寸,拟定机器的结构方案图。 根据结构方案图,在正式图纸上拟定传动构件及执行构件的位置,然后依次进行执行构件及传动系统设计机体,操纵机构设计,密封及润滑的结构设计。研究计划及预期成果研究计划:2012年10月12日-2012年12月31日:按照任务书要求查阅论文相关参考资料,完成毕业设计开题报告书。2013年1月1日-2013年1月27日:学习并翻译一篇与毕业设计相关的英文材料。2013年1月28日-2013年3月3日:毕业实习。2013年3月4日-2013年3月17日:同向旋转型双螺杆挤压机的主要参数计算与确定。2013年3月18日-2013年4月14日:同向旋转型双螺杆挤压机总体结构设计。2013年4月15日-2013年4月28日:部件图和零件图设计。2013年4月29日-2013年5月20日:毕业论文撰写和修改工作。 预期成果:根据提供的主要构件参数而计算出的传动构件的参数,尺寸及机体等是合理的,可以进行正常的生产组装,最终达到同向旋转型双螺杆挤压机的工作要求。特色或创新之处机器操作噪音小。故障率低,使用寿命长。已具备的条件和尚需解决的问题1、设计方案思路已经非常明确,已经具备使用CAD制图的能力和了解同向旋转型双螺杆挤压机原理结构等知识。2、使用CAD制图能力尚需加强,结构设计能力尚需加强。指导教师意见 指导教师签名:年 月 日教研室(学科组、研究所)意见 教研室主任签名: 年 月 日系意见 主管领导签名: 年 月 日英文原文Mixing effects of constituting elements of mixing screws in single andtwin screw extrudersD.J. van Zuilichem, E. Kuiper, W. Stolp, T. JagerDepartment of Food Technology and Nutritional Sciences, Wageningen Agricultural Uniersity, Food and Bioprocess Engineering Group, P.O. Box 8129,Bomenweg 2, 6700 EV Wageningen, NetherlandsReceived 21 April 1998; accepted 29 March 1999 Abstract In extrusion, mixing of solids and melts has always been problematic, leading to diverse models describing the melting process. It isfound that for foods based on cereals, only a few are valid, due to the simultaneous presence of water and high viscous non-Newtonian material. Mixing trials are summarised for single and twin screw extruders, with particulate solids of different particle sizes like, maizegrits, wheat flour, sucrose crystals and glucose syrup. Special mixing-heads for single screw extruders like, pineapple-heads, and slottedflight sections for counter-rotating twin screw extruders are investigated. The effects of screw geometry on mixing has been measured using co-rotating twin screw extruder modular elements like: single lead elements, mixing paddles and reversed pitch elements in a translucent model extruder by means of a study of flow phenomena and residence time distribution RTD measurements. Mixing effects are reported and their influences on viscous dissipation, residence time, curve spread and stagnancy are explained. 1999 Elsevier Science S.A. All rights reserved.Keywords: Single screw extruders; Twin screw extruders; Mixing1. IntroductionPart of the definition of food extrusion cooking is thatthe food material on its way through the open channel of a single screw extruder s.s.e. , is sheared, mixed and com-pressed. In the case of a twin screw extruder t.s.e. With closely intermeshing screws, the food is transported in so-called C-shaped chambers, which are enclosed by the neighbouring intermeshing screw and the barrel wall see Fig. 1 .The food powder is taken up at the feed port in the C-shaped chamber of one screw, is conveyed, internally mixed, forced through the gaps and clearances of this screw or eventually transferred to the neighbouring screw and finally this C-shaped chamber delivers its content at wx the die 13,23 . The differences in the way of operation between s.s.e.s and t.s.e.s are obvious and each of them will ask for its own design of modular screw parts that will enhance and improve the mixing effects of the screws wx 14,17 .Mixing operations in cooking extruders can be distin-guished in three different types: the first one is coarse mixing or longitudinalraxial mixing. The second one is dispersive mixing or comminution of the disperse phase.The third one is distributive mixing or radial mixing. The operative mechanism to realise coarse mixing is the rota-tional movement of the screws, causing residence time distribution RTD . In case of plug flow however, there will be no coarse mixing. These different mechanisms are responsible for the mixing effects. For dispersive mixing the function of the comminution is to break agglomeratesof the disperse phase into smaller ones. The operative mechanism is the existence of elongational forces andror shear stresses. Distributive mixing is caused by the action of a screw element, distributing the concentration of the disperse phase over the total volume of the chamber or channel of the extruder cross-section in such a way that the concentration of the disperse phase in each volumetric element equals the averaged concentration. This require- ment is stricter when the elementary volume to be consid-ered is smaller. Although totally different, the three mech-anisms described are not independent of each other. The anisms described are not independent of each other. The three types of mixing should therefore be carefully distinguished. It is possible to design screws with mixing elements on different locations, provoking a certain combination of the three mixing types at a certain axial location.The possibilities of these combinations are strongly dependent of extruder geometry, properties of food materials and properties under extrusion conditions. Although a lot of knowledge is gathered concerning the use of well-known mixing elements from the plasticating industry, incorporation of this knowledge into designs of s.s.e.s and t.s.e.s are not always successful for food products and other biopolymers. There are several reasons for this phenomenon. The first disturbing factor when comparing plastic polymers with biopolymers is the difference in feed to the extruder, e.g., the presence of multiple solids, water and eventually vegetable oil in the case of biopolymers.Secondly, biopolymers do not have a well-defined melt-index like many chemical polymers and in general theirviscosity behaviour differs from that of chemical polymers wx 1 . Thirdly, there are a number of degradation reactions of irreversible nature occurring simultaneously when a biopolymer is subjected to a temperature and stress field in an extruder, changing the physical and chemical properties wx drastically from location to location 15 . In this field numerous papers have been published by well-known au-thors from research institutions like CAFTrRutgers university, IFR Norwich, INRA, CEMEF France. The aim of this article is summarising the possibilities and solutions for mixing problems for single screw and counter-rotating twin screw extruders and giving better insight in the functions of screw elements in co-rotating twin screw extruders.2. Mixing mechanisms2.1. Longitudinal or axial mixingWhen food particles during their lifetime in an extruder are subjected to axial mixing, this can be seen and measured by a changing RTD. Extremes in this case are plug flow, where all particles have the same residence time, or a wide RTD which is caused by axial mixing. Some particles will stay in the extruder cooker for a process time shorter than the average, whilst others will dwell for much longer than the average when different flow systems are compiled see Fig. 2 . Fig. 3 shows the theoretical RTD for a material with a coefficient ns1 in the power-law equation for the shear dependency of the viscosity in an s.s.e.This figure also shows the RTD data, measured whenprocessing corn grits in a Battenfeld s.s.e. at two moisture y1 . contents v s0.142, 0.20 kg kg and screw revolu- w y1 wx tions ranging from Ns1.33, 1.66, 2.0 rev s 22 . The RTD data are represented as an cumulative exit age or F-function, which shows the fraction F of the inflow,wx which has left the extruder after t seconds 3,4,16 . The E-function, the derivative of the F-function, shows the so-called exit age distribution of food particles, fed into the extruder at an infinite small time and is related to the F-function with: t Ft s Et dt 1 H 0 The average residence time t of the food material in the cooking extruder is calculated with: ts tE t dt .2 H When the F-function in Fig. 3 is observed, it follows that most of the corn grits have an exit age between t and t , 0 whereas t (3r4t . These measurements show that for a 0 single screw extruder there is a strong influence of the moisture content and the number of revolutions of the screw on the shape of the F-function and the breakthrough wx point 3 .The operating principle of twin screw extruders can be described with the existence of two parallel series of Continuously Stirred Tank Reactors CSTR , where each wx C-shaped chamber represents such a CSTR 13 . In Fig. 4,a schematic is given of this principle and the leakage-gaps and -flows between the chambers and between the series CSTRs are indicated. The leakage flows through the calendar-, side-, flight- and tetrahedron-gaps Q , Q , Q , csf. Q , res. and forced conveying of the chambers results in a t wx nett positive mass flow from feed port to the die 13,17 .This approach was the basis of a series of publications of the Wageningen research group on the RTD of extrusion wx cooking, which is described below 812,2426,28 . The RTDs of t.s.e.s are more asymmetric compared to acascade of CSTRs. The different parallel series of C-shaped chambers in a t.s.e., can be summed up to an RTD model of CSTRs in series. These CSTRs travel from feed port to die, during which the chamber volume changes and these chambers loose material in the opposite direction wx see Fig. 4 . Jager et al. 11 presented a way by which the RTD could be calculated. The average residence time and the curve widths are calculated from the uncorrected E-curve using Todds method of recording the passing-by of 16%, 84% and 92% of tracer material. They are corrected with a systematical error as calculated by a Monte Carlo wx procedure 18 . The uncorrected average residence time is calculated using equations for the zero, first and second. time moment M , M , M of the measured extinction 012. Ct as: M s t iCt dt .3 H i tsM rM 4 10And M2 1 U N s 5 2 MM yM 02 1 NU gives the number of mixers of a series of CSTRs. Ahigher number of NU stands for a flow that resembles a more plug-flow nature. RTD models can be divided in curve-fit models, which describe the curve shape and in models that describe the axial mixing in the extruder reactor in a predictive andror descriptive way. An example of a descriptive model is the plug-flow model with wx axial dispersion 16 . The axial dispersion is described with a constant effective longitudinal dispersion coefficient IDe 2 y1. wx . m s 21 . The Peclet number Pe , the ratio of the average axial convective transport and the transport by dispersion is equal to: : L Pes 6 IDe : in which L is the extruder length and is the average axial velocity. The axial dispersion model and a model of a wx cascade of CSTRs do give comparable RTDs 16 . Jager wx wx has shown in 1991 11 and 1992 12 that both models forextruders can be combined and that Pe and N are related as: Pe2 U N s 7 U yPe 2Pey1q10 It is possible to use this RTD-model for different twin screw extruder designs, to evaluate their working behaviour. This can be realised by introducing a so-called chamber mixing-coefficient, which describes the ratio of mixed and total leakage flows in each CSTR of the model. The value of this coefficient is expected to be dependent on the gap-dimensions between the chambers. Especially narrowly intermeshing counter-rotating t.s.e.s may have such narrow RTDs, that they contribute, without mixing elements, very little to axial mixing. The gaps in co-rotat-ing t.s.e.s act differently than those in counter-rotatingt.s.e.s, since they allow considerable larger leakage flowswx and increased axial mixing 13 . The influence of a certainconstituting element on axial mixing can be measured withwx the RTD-response 8 .2.2. Dispersie mixingFor successful dispersive mixing a shearrelongational stress field is needed to break particulates and fluids into smaller pieces or to disperse them in the viscous liquid. The stresses are related to velocity gradients, e.g., for shear:tUshg 8 . U y2 . in which t sshear stress N m , hsdynamic viscos- y1 . ity Pa s , gsshear rate s . The separation of particles occurs normally perpendicular to the velocity gradient see Fig. 5 . In the case of solid and liquid particulates, the magnitude of stresses determine the dispersion. Shear stresses are as effective for dispersion as elongational stresses. In the case of slightly elastic liquids, which is the case for many foods, the stresses in anelongational flow may therefore be much larger than in shear flow. Machine parts in extruders, subjecting the material to elongational flow, like calendar gaps, tetrahedron gaps and internal mixing elements are good dispergators for this reason. The shear rate calculated over the channel depth is usually too low to cause dispersion. A possible solution is to increase the leakage gaps in s.s.e.s and t.s.e.s and indeed single screw extruder cookers with fairly deep channels, large flight gaps and high rpm have become popular. For t.s.e.s the total leakage flow is obviously larger. For counter-rotating cooker-extruders the calendar gap is most suitable for dispersion, whilst in co-rotating t.s.e.s a comparable dispersion effect only can be realised by installing many kneading discs. It is known that s.s.e.s and t.s.e.s of the self-wiping type are only suitable for dispersive mixing actions with special arrangements.U . From t shg Eq. 8 follows that the viscosity should be high to provide attractive shear stresses. This means for food particles, that dispersive mixing should take place early in the melting process, as close as possible to the feed section.Elements that provide the total mass flow to pass through a high shear zone, are called shear elements. For single screw food extruders some designs have become popular, of which must be mentioned the use of shear rings on the screw which is imitated from the so-called blisterdesign in plasticating extrusion and the shear effect of mixing pins, in, e.g., expander cookers. In twin screw extruder cookers the most remarkable design is the one wx used in APV-Baker t.s.e.s, an adjustable barrel valve 19 .Here an adjustable valve of which the position can be controlled from the topside of the barrel, is mounted between two blister discs, of which both are providing shear and control the degree of fill at the same time. The behaviour of food material in co-rotating t.s.e.s, provided wx with such a barrel valve is investigated 26 . Anotherdesign, well-known in counter-rotating t.s.e.s, is the so-called drossel element, a pressure barrier with relatively short length, made as a two-start short pitch screw ele-wx ment. This design was investigated by Jager et al. 10 for food products in a conical counter-rotating t.s.e.2.3. Distributie mixing This mixing effect can be based on particle distribution always unavoidably combined with shear effects at the same time. For distributive mixing the effects of mixing are more or less proportional to the total shear g, written as Weighted Average Total Shear WATS .d t WATSsgs dt 9 H d x 0y1 . in which drd xsaverage shear rate s , tstotal resiwx dence time s , gsshear 3 . For single screw extrusion some distributive mixing element designs have become popular see Fig. 6 , like the pineapple mixing section, the slotted screw flights, the mixing pins in expander designs and the special design of wx static and reciprocating pins of the Buss co-kneader 5 . The cavity transfer mixing section from Fig. 6 has not yet proven its use in food extrusion, due to cleaning problems.However, for twin screw extruders there are design limitations and only the combination of pairs of slotted screw flights, kneading blocks and reversed screw elements are applied. When using these elements, there willbe an intensive exchange of solids and melt between the screws and within the screw channels providing optimalmixing and by means of this a constant product quality will follow.中文译文单螺杆和双螺杆挤压机中混合元件的混合性能摘要: 在挤压加工中固体和熔体混合一直存在问题,导致描述熔化过程的模型不同。研究发现,基于谷物的食物,只有少数是有效的,由于同时存在水和高粘性的非牛顿材料。混合试验总结了单双螺杆挤压机用于不同粒径的固体颗粒,比如玉米糁、面粉、蔗糖晶体和葡萄糖浆的固体颗粒时的效果。研究了特殊单螺杆挤压机的混合头,比如销钉螺杆以及异向旋转双螺杆挤压机中的沟槽式螺杆段。混合螺杆的几何形状的影响已使用同向旋转双螺杆挤压机模块化元素来衡量,如:如单头元件、混合浆板和一个透明挤出机模型中的反向倾斜元件(RTD测量)测量。报告显示混合效果,并解释他们的粘性耗散,停留时间,曲线传播和停滞带来的影响。关键词:单螺杆挤压机; 双螺杆挤压机; 混合1、介绍 食物挤压成型定义的一部分是食物原材料通过开放渠道的单螺杆挤出机时被剪切、混合和压缩。然后在双螺杆挤压机中下有紧密啮合的螺杆,食品在所谓的C形室中被传输,由相邻啮合的螺杆和螺筒密闭。 食品粉末由一根螺杆从材料仓内获得,在内部对其进行混合并传输,用压力通过缝隙或螺杆的间隙最终转移到邻近的螺杆,最后这个C形室输送其到末端。单螺杆挤压机和双螺杆挤压机的运作方式区别很明显但是都要求自身的模块化螺杆零件的设计可以加强和改善螺杆混合效果。 图一:C形腔室 食用混合操作挤出机可以分为三种不同类型:第一个是粗混合或纵向混合。第二个是分散的混合或粉碎分散相混合。第三种是分布混合或径向混合。螺杆实现了粗糙的运行机制的
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