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A simplified twin screw co-rotating food extruder: design, fabrication and testingS.A.M.A.N.S. Senanayake a, B. Clarke b,* Division of Agricultural and Plantation Engineering, The Open University of Sri Lanka, Nawala, Nugegoda, Sri LankaDepartment of Postharvest Technology, School of Agriculture, Food and Environment, SilsoeCollage, Cranfield University, Silsoe, Bedfordshire MK45 4DT,UKReceived 6 July 1998; accepted 10 February 1999AbstractA simplified co-rotating twin screw food extruder was designed, fabricated and tested in England, followed by extensive testing in Sri Lanka. It was built as a model to meet the specific product and financial constraints of less developed countries and was expected to be used in those countries to widen the production capabilities of extruded foods. The machine had an estimated delivery of 10 kg/h and was made mainly with mild steel. Two types of screw were made, one with a constant pitch of 14 mm and the other with varying pitch in segments of 14, 12 and 10 mm. The machine was powered by a 2.2 kW electric motor with electronic speed control .The machine also had electrical heating with a temperature controller and a pressure sensing device. The cost of fabrication of themachine was estimated at 2000 with most of the parts built in a fairly simple workshop. A mixture of rice and dried banana was successfully extruded as a potential snack food and on the basis of maximum expansion the best results was obtained from a barrel temperature of 120C, screw speed 125 rpm, feed moisture 15% and with a die orifice size of 3 mm. When the alternative compress ion screw was tested very similar results were achieved with no significant improvement in product expansion. 1999 Elsevier Science Ltd. All rights reserved.Keywords: Twin screw extruder; Design; Low cost; Snack food; Continuous cooker; Local construction; Cereal mixturesNomenclaturea Die diameter (mm)B Channel width (mm)C Screw circumference (mm)d Screw core diameterD Outer diameter of screws (mm)H Flight depth (mm)M Moisture content (% wet basis)n Number of fight turnsN Speed angular (rev/min)p Pitch (mm)Q Delivery rate (mm3/min)S Total helical length of screws (mm)t Temperature ()T Residence time (min)a Overlap angle of screw fights (degrees)d Calender gap (mm)e Side clearance (mm)q Product density (g/mm3 )/ Helix angle (degrees)* Corresponding author. Fax: +01525-863277; e-mail: b.clarkecran-eld.ac.uk0260-8774/99/$ see front matter1999 Elsevier Science Ltd. All rights reserved.PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 0 4 9 71. IntroductionExtrusion cooking is finding ever increasing applications in the food process industry. Apart from providing a means of manufacturing new products, it has successfully revolution is many conventional manufacturing processes (Harlow, 1985, Frame, 1994). Today, extruders come in a wide variety of sizes, shapes and method of operation. There are three types of food extruder found in industry: hydraulic ram, roller and screw type extruders (Frame, 1994). The screw extruders are very different to the other two having special features such as continuous processing and mixing ability. Single and twin screw types are both widely used in the food process industry. Unfortunately, most of the food extruders available in the market are either so costly that less developed countries cannot afford to buy them except by some form of assistance or outside investment or else are not appropriate for the wide variety of materials that need to be processed. As a result the growth of extrusion technology of food into these countries has been hindered despite its many advantages. Fig. 2. Plan drawing of the twin screw extruder with drive system. 1-V belt pulley, 2-gear box, 3-food seal, 4-ange clamp bolt, 5-die plate, 6-die, 7-two segments of the extruder chamber, 8-extruder screw.were made so that they could be externally screwed to the die plate.2.5. Drive system The machine was driven by an electric motor of 2.2kW using a twin belt drive between the motor and a gearbox shown in Fig. 2. The speed reduction in the box was2.08 while an electronic speed controller was used to control the speed continuously over the range required.Fig. 3. Front portion of barrel showing provision for heaters, temperature and pressure sensors. 1-slots for heaters, 2-end flanges, 3-side flanges to barrel, 4-hole for pressure sensor, 5-twin holes to form the barrel.2. Motor powerIn twin screw extruders the motor power is utilized mainly to compress and shear the food dough that squeezes through various gaps in the intermeshing screws and the gap between the screws and the barrel. When dealing with a wide range of foods under different process conditions the shear resistance can vary widely because of changes in the rheological behaviour which would prevent accurate estimate of the motor power. Owing to the unknown character therefore of the novel materials a motor power was selected based on that used for similar materials in similar sized extruders with a safety margin and from exploratory trials in the Brabender extruder. Rossen and Miller (1973) give a range of specific energy consumption figures for different extruders which ranged from 0.02 to 0.10 kWh/kg. At 10kg/h throughput this gave a maximum power requirement of 1 kW while the Brabender trials tended to indicatea power requirement of about half of this value. The 2.2 kW, 3 phase AC motor used was amply capable of supplying this power plus all other drive friction losses.3. Gear boxIn the co-rotating extruder the two screw shafts are driven at the same speed in the same direction. The main problem is that they are very close together. The gearbox was designed to drive two pinions, coupled to the shafts by shear pins, by using a gear wheel of more than double the width of the pinions. In this way the two pinions could t side by side driven simultaneously and maximise their diameter space as shown in Fig. 2. Lubricated phosphor bronze thrust bearings were used to resist the axial load generated by the material along the shaft.2.6. Heating and temperature controlHeating of the barrel to give necessary thermal input for cooking the food was done by two sets of cartridge heaters having capacities of 800 and 1200 W. The heaters were positioned in the grooves made on the top and bottom of the barrel towards the die end as shown in Fig. 3. A single temperature controller was set up together with a thermocouple to sense the temperature inside the barrel very close to die plate. Owing to the shortness of the barrel only one thermocouple was considered necessary. In an early design heaters were also used near to the feed hopper but were not used as they tended to cause premature gelatinization of the starch and blockage of the feed.4. Pressure sensorPressure measurements are not so important in the commercial production processes as it cannot be directly controlled to monitor the product characteristics. Neither was such a device needed as a safety measure as this was covered by an overload cut out on the electrical supply. However, in experimental work the measurement of pressure is useful to ascertain the relationship between the pressure and the other controllable parameters such as die size, temperature, moisture content and speed. In this study, a device was built using strain gauges mounted on a small cantilever beam in order to measure the pressure inside the extruder barrel (Fig. 4). A four arm strain gauge bridge was fixed at the point of maximum bending moment. The pressure was tapped from a small hole made in the die end of the barrel in which a plunger, sealed by an O-ring, actuated the cantilever beam to transmit the pressure force. The strain in the beam was detected as a voltage difference. This feature could have been used as an automatic safety cut-out but reliance was placed instead on belt slip in the initial drive stage and the motor itself had an overheating cut-out.Fig. 4. Position of pressure and temperature sensors on the extruder barrel. 1-location of strain gauges on the pressure sensor, 2-cantilever support to plunger, 3-temperature sensor.5. Testing and evaluationA range of rice and banana mixtures were selected as being both novel yet having high potential as processed foods in Sri Lanka. These materials are cheap and common crops in most developing countries and represent an opportunity to produce an attractive, nutritious and tasty snack food. This would provide labour, utilisation of excess perishable fruits in season and a means of storing them for at least one year in appropriate packages. The main product qualities were assessed as part of the same programme and shown to be satisfactory by Gamlath (1995). The rice was prepared in the form of grits (800 lm) and the banana was dried and milled to a similar sized powder which was mixed and flood fed from the feed hopper. Extrusion trials were carried out as given below. Sixteen combinations ofvariable levels were studied in two sets of experiments. In both sets the throughput was measured when the flow became stable.Initial trials indicated no significant difference in performance due to the variable pitch screws as a means of compressing the feed so all subsequent trials and the results quoted in this paper are for the fixed pitch screws. The extrudate diameter was measured using a vernier calliper immediately after extrusion and before any further drying took place which could cause some further reduction in ratio but not to affect the general result. All tests were replicated three times making 48 individual trials carried out in a fully randomised formatExperiment 1Fixed settings:Speed (N) 125 rev/minDie size (a) 5 mm diameterVariables:Barrel temperature (t) two levels (100C and 120C)Feed moisture content (M) four levels (15%, 20%,25%, 30%)Experiment 2. This experiment was carried out using fixed settings of barrel temperature and the feed moisture determined in experiment 1 on the basis that maximum product expansion represented the best quality.Fixed settings:Barrel temperature (t).120CFeed moisture content (M).15%6. Testing and evaluationA range of rice and banana mixtures were selected as being both novel yet having high potential as processed foods in Sri Lanka. These materials are cheap and common crops in most developing countries and represent an opportunity to produce an attractive, nutritious and tasty snack food. This would provide labour, utilisation of excess perishable fruits in season and a means of storing them for at least one year in appropriate packages. The main product qualities were assessed as part of the same programme and shown to be satisfactory by Gamlath (1995). The rice was prepared in the form of grits (800 lm) and the banana was dried and milled to a similar sized powder which was mixed and flood fed from the feed hopper. Extrusion trials were carried out as given below. Sixteen combinations ofvariable levels were studied in two sets of experiments. In both sets the throughput was measured when the flow became stable.Initial trials indicated no significant difference in performance due to the variable pitch screws as a means of compressing the feed so all subsequent trials and the results quoted in this paper are for the fixed pitch screws. The extrudate diameter was measured using a vernier calliper immediately after extrusion and before any further drying took place which could cause some further reduction in ratio but not to affect the general result. All tests were replicated three times making 48 individual trials carried out in a fully randomised formatExperiment 1Fixed settings:Speed (N) 125 rev/minDie size (a) 5 mm diameterVariables:Barrel temperature (t) two levels (100C and 120C)Feed moisture content (M) four levels (15%, 20%,25%, 30%)Experiment 2. This experiment was carried out using fixed settings of barrel temperature and the feed moisture determined in experiment 1 on the basis that maximum product expansion represented the best quality.Fixed settings:Barrel temperature (t).120CFeed moisture content (M).15%Table 1Results of Experiment 1 (Die orifice diameter=5 mm, screw speed=125 rpm)Temperature (C) Feed moisture (%) Throughput (g/s) Expansion ratio Pressure (MN/m2)100 15 3.76 1.01 2.97 100 20 2.56 1.00 2.38 100 25 2.04 1.00 1.83 100 30 1.25 1.00 1.38 120 15 2.16 1.06 2.91 120 20 2.00 1.05 2.07 120 25 1.18 1.01 1.59 120 30 1.02 1.00 1.38Variables:Die orifice diameter (a) two levels (3, 4 mm)Speed (N) four levels (100, 125, 150, 175 rev/min)4. Results and discussion4.1. Machine performanceGenerally the extruder performed very satisfactorily.The extrudates produced by the machine were fairly well expanded. During extrusion operations it did not become necessary to dismantle the barrel lengthways by splitting into two halves as it never seized up. In order to clean the screw and barrel the latter barrel was very easily pulled o. from the screws within a few minutes after extrusion. This was in part due to a shorter than usual barrel length. This suggests that the horizontal splitting of the barrel was not essential which would make the machining process of the barrel far easier. No serious difficulties were encountered as far as the operation of the machine is concerned, except initial feedingproblems due to a temperature rise close to the feed hopper. This happened because some heaters were installed a little too close to the feed point so these were later removed and the difficulties were overcome as mentioned earlier. Many extruders have cooling facilities in this region but these were not found to be necessary. Those heaters further from the feed point and close to the die end proved to be sufficient to gelatinize the rice grits. The extrudate was observed to change from a powder at feed to a continuous, expanded extrudate at exit although quantitative assessments of the degree of gelatinization were not carried out.7. Extruder settings and product characteristicsIt can be seen from Table 1 and Fig. 5 that the throughput dropped with each increase of feed moisture content at both the barrel temperatures used. When the feed moisture was increased from 15% to 30%, the throughput was reduced by 66.8% and 52.7% at 100 and 120barrel temperatures, respectively. This effect was probably caused by an increase in backflow allowed by the reduced viscosity which the increase in moisture produced. Another important observation made was the variation of product expansion with the pressure and feed moisture content. The expansion was found to be highest at the lowest moisture content with associated highest pressures (Fig. 6) and a steady reduction in both expansion ratio and pressure as moisture content increased. The product was well gelatinised but with low expansion ratio. The second series was designed to test a wider range of parameters and if possible increase the expansion ratio which was thought to depend on the die diameter.The results of Experiments 2 are tabulated in Table 2 below.Fig. 5. Throughput as a function of feed moisture content with die diameter 5 mm and screw speed 125 rev/min.Fig. 6. Pressure and expansion ratio as a function of feed moisture content at feed moisture 15%, die diameter 5 mm and screw speed 125 rev/minFig. 7 and Table 2 show that the throughput increased with the speed due to increased rate of material conveyance. The pressure changes with screw speed was not found to be significant. The product expansion, however, showed a downward trend with the increase of speed as evident from Fig. 8. This reduction can be attributed to the reduction of pressure and lower degree of gelatinization due to reduced residence time. At settings of 125 rpm, feed moisture 15%, temperatures 120C, die size 3 or 4 mm diameter a very acceptable product was achieved.The overall performance of the machine was found to be quite satisfactory in achieving all the parameter settings and measurements required. Each trial only lasted a few minutes in running time which was mainly spent in reaching equilibrium conditions indicated by the temperature reading but after 48 trials no significant wear was observed even though the prototype was in mild steel.Cleaning and maintenance was quick and simple and in the event of a complete seizure of the screws the barrel could be split on this machine.The gearbox was of a bolted construction to permit modifications but future designs should be welded together. The 2.2 kW motor was found to be amply capable and most of the time it only consumed about 0.5kW. No mechanical breakdowns were experienced.The prospects for use of this design in developing countries seem to be good from these experiments. Scale up to a higher capacity would bring some difficulties as discussed by Levine (1989); Singh, Smith and Frame (1998) and Yacu (1992) and although these issues were not addressed they are not considered to be insurmountable.Fig. 7. Throughput as a function of speed with feed moisture 15% and barrel temperature 120C.Fig. 8. Pressure and expansion ratio as a function of speed with 3 mm die size, feed moisture 15% and barrel temperature 120C.8. ConclusionsThe following conclusions were made from this study. Simplified extruders for specialised applications can successfully be made and operated in less developed countries to process local food materials. All components can be made in an unsophisticated workshop except gears, seals, motor, temperaturesensor and heaters. Simple machining processes such as drilling and boring can be used to produce twin holed barrels to accommodate the intermeshing screws. Horizontal splitting of the barrel is not essential in this type ofmachine so that fabrication of the barrel for these machines can be simple enough for developing country manufacture. A simple construction of gear box, using straight spur gears driven by a single large gear wheel is quite adequate to run the twin screws in the same direction. An attractive and acceptable snack food was produced from the prototype machine from mixture of cereals and fruits.ReferencesFrame, N. D. (1994). The technology of extrusion cooking. Blackie Academic and Professional, London.Gamlath, G. G. S. G. (1995). Nutritional, Physico-chemical and sensory evaluation of extruded cereals with perishables. Ph.D. thesis, Cranfield University, Bedford, England.Harlow, N. (1985). Revolutionising a cereal need. Food Processing, pp. 29-30.Harper, J. M., & Jansen, G. R. (1985). Production of nutritious precooked foods in developing countries by low cost extrusionTechnology. Food Review International, 1, 27 97.Harper, J. M. (1979). Food extrusion: critical reviews in food science and nutrition. Florida: CRC press.Harper, J. M. (1992). A comparative analysis of single and twin screw extruders. In J. L. Kokini, C.-T. Ho & M. V. Karwe, Foodextrusion science and technology. New York: Marcel Dekker.Hauck, B. W. (1985). Comparison of single and twin screw extruders- 2. Food Trade Review (Suppl. 5-9).Hauck, B. W., & Ben Gera (1987). Single and twin screw extruders. Milling, pp. 1820.Jansen, G. R., & Harper, J. M. (1980). Applications of Low cost extrusion cooking to weaning foods in feeding programs. FAOFood and Nutrition, 6(1), 2-9; (1) 15-23.Jansen, L. P. B. (1978). Twin screw extrusion. Amsterdam: Elsevier.Levine, L. (1989). Scale-up, experimentation and data evaluation. In C. Mercier, P. Linko & J. M. Harper, Extrusion cooking (pp. 57-90). USA, DC: American Association of Cereal Chemists.Martelli, F. G. (1983). Twin screw extruders- A basic understanding. New York: Van Nostrand Reinhold.Rossen, J. L., & Miller, R. C. (1973). Food extrusion. Food Technology (Chicago), 27(8), 46-53.Ryder, G. H. (1953). Strength of materials (pp. 259-261). LondonW8: Cleaver-Hume Press.Seiler, K. (1984). Extrusion cooking and Food processing. Food Trade Review, 124/127.Sahagun, J. (1977). Parameters a.ecting the performance of a low cost extrusion cooker. M.S. Thesis, Department of Agricultural and Chemical Engineering, Colorado State University, Fort Collins,USA.Singh, N., Smith, A. C., & Frame, N. D. (1998). E.ect of process variables and monoglycerides on extrusion of maize grits using two sizes of extruder. J. Food Engrg., 35(1), 91-109.Smith, A. (1986). Cooker extruder die design-some fundamentals.Food Trade Review, 56, August 1986, Impulse Food Suppl. (6-12).Yacu, W. A. (1992). Scale-up of food extruders. In J. L. Kokini, C. Ho & M. V. Karwe, Food Extrusion Science and Technology (pp. 465 -472). New York: Marcel Dekker.Van Zuilichem, D. J., Stolp, W., & Janssen, L. P. B. M. (1984). Engineering aspects of single and twin screw extrusion cooking of bio-polymers. In R. Jowett, Extrusion Cooking Technology. Amsterdam: Elsevier.一个简化双螺杆旋转食品挤压机：设计，制造和检验S.A.M.A.N.S.Senanayake,B.Clarke农业的和耕地工程学的分割。斯里兰卡的开放型大学， 纳瓦拉 ，斯里兰卡技术部，农业、食物和环境的学校，悉尔索农业大学，克兰菲尔德大学，悉索尔，贝德福德郡，MK45 4DT,UK收到1998年7月6日;接受1999年2月10日摘要一个简化双螺杆旋转食品挤压机的设计，制造和测试在英国，之后在斯里兰卡广泛的测试应用。它是作为一个模式来满足特定的产品和金融限制，较不发达国家，预计将在这些国家扩大生产能力，膨化食品。机器有一个 10 公斤小时的估计输出而且主要是碳钢所做。螺旋的两类型被做了，一个有固定摊位的14毫米和其他不同的音高段14, 12和10 mm。这个机器是采用变频电机与电子速度控制。该机也有电加热温度控制器和一个压力感应装置。制造成本的估计2000欧元，大多数部分建立在一个相当简单的工作坊。一个米的混合物和干燥的香蕉成功地当做潜在的点心食物和根据最大展延被挤出，最好的结果从一个 120 的滚筒式打印机温度被获得，螺旋加速 125个转/每分，输送水分 15%而且与一钢模囗 3 毫米的尺寸相当。当其它可能的压缩螺旋被测试非常相似的结果与没有产品展延中的重要的改良一起达成。1999全文电子期刊公司。版权所有。关键词：双螺杆挤出机，设计低成本，休闲食品，连续蒸煮器，地方建设，谷类混合物1.驱动系统这台机器是由电机和齿轮箱之间使用双皮带传动的电动机（220KW）驱动的（图2）。箱子减速器是用电子调控器来达到持续变化范围内的速度的要求的。图2用传动系统的对螺旋挤压机的平面图制图。单一三角皮带滑车 2-齿轮箱、 3 食物的封囗，4-凸缘夹子螺栓， 5 一钢模的屏，6-一钢模，挤压机室的 7- 两个片段，8-挤压机转。（1） 电机功率 在双螺杆挤出机电机功率的利用主要压缩和剪切食物生面团挤压，通过各种差距在啮合螺丝和螺丝之间的差距和桶。当处理一个范围广泛的食品不同工艺条件下的剪切阻力可能差异很大，因为变化的流变行为，防止准确的电机功率。由于未知的变化和新奇材料等，一个马达的能力被选择了基于一个安全界限和从布拉班德挤压机的探索试验在相似的按规定尺寸制作挤压机中为相似的材料。罗森 和铣床为从 0.02 到 0.10KWh/kg 的不同的挤压机提供多种比能量消费数据。在10kw吞吐量时，这给一个1kw的最大动力要求而布拉班德审讯倾向于表明一个相关的电源要求这个的一半值。2.2 KW，被用的 3 相位交流电动机加上所有其他磁盘摩擦损失等供给充足有能力。（2）齿轮箱 在同向旋转挤压机中，螺旋桥以相同的方向和相同的速度被驱使。主要的问题是他们在一起非常接近。齿轮箱被设计成驾驶两个小齿轮，藉着剪梢对桥加倍了，藉由使用齿轮比倍更多小齿轮的宽度。这样，这两个小齿轮会并排安装并且同时地驱使，如图 2 所显示的他们的直径空间增加至最大限度.被加油的磷青铜推力方位被用来抵抗沿着事物产生的桥方向的负载。2.加热与温度控制滚筒式打印机的加热给煮熟食物的必需的热输入分两组卷筒加热器有800W 和1200W的容量。加热器在槽向如 Fig.3 所显示的一钢模结束在滚筒式打印机的顶和底上被做中放置。一个温度控制器连同一个热偶一起被建立在滚筒式打印机内温度接近的死屏。由于滚筒式打印机的脆性所以只有一个热偶被视为必需的了。在一个早期设计暖气并且使用靠近进料斗现在不使用是由于他们倾向于原因为时过早糊化的淀粉和阻止费。图3: 加热器、温度和压力感应器的滚筒成绩准备的前面部分。1-为加热器起槽，2-结束凸缘，3-边凸缘快速前进，4-为压力感应器挖洞，5-双排电洞造形滚筒式打印机。3.压力传感器压力尺寸不是那样重要。在广告生产过程由于这不可能直接控制对显示器的产品特点。当这在电的供给上的外被超载了复盖且剪下的时候 , 一个不可或缺的装置即一个安全测量也是。然而，在实验的工作中，压力的测量有用确定压力和其他可控制的叁数，像钢模尺寸、温度之间的关系，水分足而且满足加速。在这研究中，一个装置被建造了并且使用在一个小展开的应变计的悬臂梁上为了是要在挤压机滚筒式打印机内有压力（图4）。四个臂应变计桥接器被濒于最大弯曲（力）矩定盘了。压力从一个小电洞，在一个被一个 O 环封闭的活柱，促使了悬臂梁传送压力的滚筒式打印机的钢模被挖被轻打。波束的应变当做一个电压差额被发现。这特征可能有作为一个自动安全断路器并且被信赖放置代替在皮带打滑在初步驱动器阶段且马达本身有一个过热开孔。图4：压力和在挤压机滚筒式打印机上的温度感应器的职务。在压力感应器上的应变计的 1-位置，2-悬臂对活柱，3 温度的感应器支援。4.测试与评估 多种米和香蕉混合物被选择了当做仍然有高电位为斯里兰卡的加
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