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1、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, Fo
2、od 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 a
3、s 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
4、 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
5、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 12
6、0C, 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; De
7、sign; 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
8、 (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-ma
9、il: 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
10、 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 extrude
11、rs (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 eit
12、her 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
13、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
14、 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.Fi
15、g. 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 compres
16、s 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 whic
17、h 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
18、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
19、, 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 dr
20、ive 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
21、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
22、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 neces
23、sary. 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 direct
24、ly 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 t
25、he 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 p
26、oint 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
27、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 su
28、pport 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 pro
29、duce 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 satisfac
30、tory 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 s
31、ets 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 fix
32、ed 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
33、 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
34、 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 bot
35、h 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 sea
36、son 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 s
37、ized 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 significa
38、nt 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 furth
39、er 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:B
40、arrel 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 t
41、he 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
42、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
43、 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
44、 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 difficultie
45、s 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 over
46、come 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 fe
47、ed 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