A centrifugal bambara groundnut sheller was designed,constructed and evaluated.The sheller consists of a feed hopper with a flow rate control device,shelling unit,parating unit and power system.Nine different impellers were ud for the investigation at three different pod moisture contents.Results of the performance tests were examined using a3x3x3factorial design with moisture content,impeller slot angulations and number of impeller slots as variables.Results showed that moisture content,impeller slot angulations,number of impeller slots and interaction between the variables statistically affected the per-formance indicators(shelling efficiency,percentage of breakage, percentage of partially shelled pods,percentage of unshelled pods and winnowing efficiency)at the significance level of5%.The forward facing impeller with eight slots gave the best performance of96%shelling efficiency,3.4%breakage,0.6%partially shelled pods.No unshelled pods were recorded,and winnowing efficiency of97.3%was recorded at an average feed rate of215.8kg h-1.
K e y w o r d s:bambara groundnut,centrifugal impeller,pods, winnowing
INTRODUCTION
Bambara groundnut(Vigna subterranean L.Verdc)is an indigenous African crop grown across the continent from Senegal to Kenya and from the Sahara to South Africa (Atiku et al.,2004).Bambara grou
ndnut is the third most important grain after groundnut and cowpea(Ezeaku,1994). In parate reports by Ezue(1977)and Atiku(2000)it was noted that in Nigeria bambara groundnut is widely produced in Borno,Anambra,Plateau,Taraba,Sokoto,Bauchi, Benue,Kano,Yobe,Adamawa and Gombe states.xianqi
Goli(1997)reported that bambara groundnut contains about63%carbohydrate,19%protein,and6.5%oil and is consumed in different forms.Other reports,by Akani et al. (2000),Atiku et al.(2004)and Linnemann(1988),stated that the ed of bambara groundnut can be ud for baby food,human consumption,industrial products and for animal feed.Linnemann(1990)reported that bambara groundnut flour has been ud in making bread in Zambia, and Brough et al.(1993)noted that the milk prepared from bambara groundnut gave a flavour preferred to that of milks from cowpea,pigeon pea and soybean.According to Atiku (2000)the fresh bambara groundnut ed is cooked before eating.It is ud as main food,snacks,relish and medicine, and has a high ceremonial value.
Kay(1979)recommended that after harvesting bambara groundnut is dried in the sun to about8to12%moisture content(w.b.).The dried ed could be taken as snacks after roasting or milled to flour for other food.
Despite the economic importances,no commercial production and no industrial u of the crop take place in Nigeria.According to Akani et al.(2000)rearch is con-centrated only on the agronomic aspect,while the proces-sing aspects have been neglected.The pod of bambara groundnut is very hard and the cracking methods are still traditional.The cracking methods vary from locality to locality depending on the quantity produced.Some com-munities u mortar and pestle to crush dry pods.Some beat them with sticks on flat ground,others u stones to crush the pods on flat ground.The methods have the disadvan-tages of damaging the eds,and are slow and tiresome.
Atiku et al.(2004)evaluated the performance of a bam-bara groundnut sheller working on the principle of rollers and pneumatically parating the shells from the eds. They got the maximum shelling and winnowing efficiencies of80and79.5%,respectively,at pod moisture content of5% (w.b.)and feed rate of93.6kg h-1.The percentage of dama-ged ed was about20%,the percentage of partially shelled (broken and unbroken eds)and percentage of unshelled pods were10and7%,respectively,while the machine loss accounted for3%.
Int.Agrophysics,2007,21,269-274
Development and performance evaluation of impact bambara groundnut sheller
©2007Institute of Agrophysics,Polish Academy of Sciences *Corresponding author’s e-mail:
Adigun and Oje (1993)reported that nuts who shells/pods cannot be easily broken by the roller cracker are commonly cracked using a centrifugal cracker.Makanjuola (1975)evaluated some centrifugal impaction devices for shelling melon (egusi)eds (Citrullus vulgaris )and found that a centrifugal impact method is a good technique for shelling melon eds.He evaluated three types of impellers (A,B and C)with four slots,eight slots,and two parallel plates,respectively.He concluded that impeller type A was the most effective of all the three types tested.Odigboh (1979)developed and tested a prototype impact melon (Citrullus vulgaris )shelling machine that gave about 96%shelling efficiency and 100%winnowing efficiency.He also evaluated three types of impellers at different vane angulations with four vanes each.Impeller type A had radially positioned vanes,type B vanes positioned at 45°to the radius,while type C vanes at 90°to the radius of the circle.He concluded that impeller type B gave the best combination of higher shelling and low percentage of damage.
Oluwole et al .(2004)developed and tested a sheanut cracker working on the principle of impaction and pneuma-tically parating the shells from the kernel.They got crack-ing efficiency of 100%and winnowing efficiency of 97%.They evaluated three types of impeller,similar to tho of Odigboh (197
9)but with slight differences in the vanes angulations.Impeller type A had radially positioned vanes;type B and type C had their vanes positioned at 45and 30°to the radius respectively.They concluded that the radially positioned vanes impeller gave 100%effective cracking efficiency.Akani et al .(2000)determined the optimum impact energy for shelling bambara groundnut at pod moisture content range of 5-8%(w.b.)and found that the impact energy ranged from 0.24to 0.59J.
DESCRIPTION OF THE IMPACT SHELLER
The machine,similar in principle to Odigboh’s impact melon (Citrullus vulgaris )sheller (Fig.1),consists of a feed hopper,pod shelling unit paration unit and power unit.Figure 2shows the asmbly details of the machine.The conical shaped hopper is mounted on the tool frame and is held in place by a hopper support frame.The ba is connected directly to the shelling unit.A nut flow control device is located between the hopper and the shelling chamber.The shelling unit consists of a 410mm diameter by 150mm height cylindrical shell made from 5mm steel sheet,who inner surface rves as the cracking surface,and an impeller of 370mm diameter made of 25by 50mm
rectangular steel pipe similar to that described by Dicken (1961)as shown in Fig.3.The impellers were divided into three groups:A –impeller slots positioned at 45°to the tangent at the slots outlet (for
ward facing)reprented with subscript F;B –radially positioned impeller slots (at 90°to tangent at the slots outlet)repre-nted with subscript R;and C –impeller slots positioned at 135°to the tangent at the slots outle (backward facing)reprented with subscript B.
270 F.A.OLUWOLE et al
.
Fig.1.Bambara groundnut
sheller.
Fig.2.Centrifugal impaction device:1–hopper,2–hopper sup-port frame,3–cracking chamber,4–impeller,5–transition chan-nel,6–shaft,7–pulley,8–bearing,9–belt,10–frame,11–ed collection chute,12–electric motor,13–chaff-outlet,14–inner funnel,15–paration compartment,16–axial fan.
Each group has impellers with eight,four and two slots.The cylindrical shell is covered with a transparent plastic cover 400mm diameter of 4mm thickness with a concentric open-ing (80mm diameter)at the center (pods inlet).The impeller is concentrically positioned within the cylindrical shell and horizontally mounted on a vertical shaft to give a clearance that is greater than the size of the ed with cracking surface.Figure 4shows the impeller mounted in operational position.This impeller is driven by a vertical shaft powered by a 0.5hPa lectric motor through a system of belt and pulley.The paration unit compris the transition channel (through which the mixture of ed and broken pods from the shelling chamber flows into the paration compartment),the parating chamber (where the ed and pods are parated by air current),the inner funnel shapped wire mesh for eds collection,and the axial fan mounted directly on the electric motor,which is powered by the elec-tric motor to supply the air stream required for winnowing.At the upper part of the parating unit is the shells outlet.In
between the parating compartment and the shells outlet is a transparent cylindrical plastic column to obrve the shells upward movement.The components are asmbled and mounted on a rectangular tool frame that gives the machine a compact design and a sturdy outlook.
PRINCIPLE OF OPERATION
To operate the device,the nut flow control device is clod completely.The hopper is then filled with a known number and mass of nuts.The main switch is then switched on to actuate the electric motor which runs the impeller in the cracking chamber.As the impeller reaches its operating speed,the nut flow control device is opened to allow nuts to flow into the impeller eye.The incoming nuts slide and roll on the inner surface of the rectangular pipe,the centrifugal force developed as a result of the rotation of impeller throws the nuts against the cracking surface and caus the nuts to crack.The eds and the shelled pods flow down through the clearance between the impeller and the cylindrical shell and fall on an inclined transition channel that leads to the parating compartment.Here the shells,which are lower in density than the eds,are lifted up by the air stream from the axial fan and blown out through the shells outlet.The denr eds fall through the air stream into the inner wire mesh funnel to the ed collection chute.Figure 4shows the impeller mounted in the operational position.
PERFORMANCE TESTS
To carry out the performance tests,nine types of impellers were evaluated.Each of the impellers was ud to shell bambara groundnut at three different moisture levels in the range of 5.3-12.2%(d.b.).Some quantity of bambara groundnut pods (N T )were randomly lected and weighed.The pods were poured into the hopper while the pods flow control was completely clod.The main control switch was switched on;as the impeller attained the operating speed,the pod flow control was opened to allow the pods to flow into
DEVELOPMENT AND PERFORMANCE EVALUATION OF IMPACT BAMBARA GROUNDNUT SHELLER 271
Fig.3.Groups of impellers:A,B,and
C.
A
C
B
8slots 4slots 2
slots
Fig.4.Impeller mounted in the operational position.
the eye of the impeller at an average feed rate of215.8kg h-1. The pods were carefully collected after going through the impeller,and the number of pods fully shelled without broken eds(N1),number of pods fully shelled with broken eds(N2),number of pods partially shelled(N3)and number of unshelled pods(N4)were determined at the end of each run.The quantities of shells winnowed out(M SW)and of tho collected with the eds(M UW)were determined and recorded.
Each of the tests was replicated five times at each of the moisture level and for each of the evaluated impellers. The performance was evaluated on the basis of the following indices(Oluwole et al,2007b):
shelling efficiency,
h S T
N N
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1100,(1) percentage of broken eds,
h b T
N N
=(/)
2100,(2) percentage of partially shelled pods,
h p T
N N
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3100,(3) percentage of unshelled pods,
h u T
江南春温泉N N
=(/)
4100,(4) winnowing efficiency,
h w SW TS
M M
=(/)100,(5) where:N T–number of pods fed into the impeller:
M TS=M SW+M UW.动态口令卡
The results were statistically analyd using factorial design.
RESULTS AND DISCUSSION
Tables1and2show the results of the performance tests analys.Table1shows that moisture content(M),number of impeller slots(f)and impeller slot angulations(S)signi-ficantly affect the shelling efficiency(h S),percentage of broken eds(h b),percentage of partially shelled pods(h p), percentag
e of unshelled pods(h u)and winnowing effi-ciency(h w)at the significance level of5%.The interaction between moisture content and number of impeller slots(M x f) has a significant effect on the shelling efficiency and the per-centage of partially shelled pods at the significance level of 5%,while the interaction between moisture content and im-peller slots angulations(M x S)significantly affects the shel-ling efficiency,the percentage of partially shelled pods and the winnowing efficiency at the significance level of5%.团小组会
Table2shows that the forward facing impeller con-sistently gave the best performance.Similar findings were reported by Oluwole et al.(2004)and Oluwole et al.(2007a) in performance evaluation of a sheanut cracker and effect of moisture content on crackability of bambara groundnut using a centrifugal cracker,respectively.Table2also reveals that the impellers with eight slots consistently gave the best performance.Similar findings were reported by Oluwole et al.(2007b)in an evaluation of some centrifugal impaction devices for shelling bambara groundnut.
At all the moisture contents employed and for each of the impeller groups the shelling efficiency,percentage of broken eds and the winnowing efficiency incread with increa in number of impeller slots,while the percentage of partially shelled pods and the percentage of unshelled pods decread with increa in the number of impeller slots.This is becau the higher th
e number of impeller slots the higher the number of pods that will emerge from the impeller and
272 F.A.OLUWOLE et al.
hit the cracking surface.
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