DESIGN AND PRELIMINARY TESTING OF A HAMMER MILL WITH END

Ebunilo.P.O.et al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 1581-1593

DESIGN AND PRELIMINARY TESTING OF A HAMMER MILL WITH END-

SUCTION LIFT CAPABILITY SUITABLE FOR COMMERCIAL PROCESSING OF

GRAINS AND SOLID MINERALS IN NIGERIA

BY

EBUNILO P. O., OBANOR A. I. AND ARIAVIE G. O.*

DEPARTMENT OF MECHANICAL ENGINEERING UNIVERSITY OF BENIN

P.M.B 1154

BENIN CITY

NIGERIA

*ARIAVIE G.O

RM 126, MECHANICAL ENGINEERING DEPARTMENT

UNIVERSITY OF BENIN

BENIN CITY, NIGERIA

ABSTRACT

A hammer mill with an end-suction lift capability has been designed, fabricated and tested. The tests were carried out by comparing the product of a conventional hammer mill with that of the new hammer mill with end suction lift capability. The preliminary test results obtained using grains show that the efficiency of a conventional hammer

mill and its ability to produce an output of specific size of 400m-600m for both maize and cowpeas is low when compared with the output of the new hammer mill. The new hammer mill, which is a partially closed system while in operation and utilizes suction power, virtually eliminates environmental pollution usually associated with the operation of conventional hammer mills and can be used to mill commonly occurring Nigerian grains like millet, sorghum, maize, cowpeas, guinea-corn and soya beans into flour and also for pulverizing locally occurring solid minerals like clays into powder. This would diversify storage options for the grains, deepen and widen the available food choices for all Nigerians and enhance food security and rural development.

Keywords

Hammer mill, design, testing, solid mineral processing, Nigerian grains, food security, rural development.

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INTRODUCTION

A conventional hammer mill is a device consisting of a rotating head with free- swinging hammers, which reduce rock, grains or similarly hard objects to a predetermined size through a perforated screen [Sci-Tech Dictionary, 2003]. Hammer mills are widely utilized in the agricultural, wood, mining and chemical industries.

The vast majority of Nigerians live in the rural areas [Okpala,1990] and are predominantly engaged in agriculture[Anifowoshe,1990]. Nigeria is blessed with equatorial [Morgan and Moss, 1965], tropical[Clayton, 1958], guinea [Jones, 1963], sudan [Olajire, 1991] and sahel [Pande, et al, 1993]climatic zones; thus making her suitable for the profitable cultivation and production of a wide variety of grains and tubers[FAOSTAT,2004]. The farmers rely on ancient and antiquated methods that are inefficient for storing the grains and tubers[Biewar, 1990], [Igbeka and Olumeko, 1996], [Adejumo and Raji, 2007], and thus lead to large storage losses due to rodents, damp, fungi and natural decay[Agboola, 1992], [Agridem, 1995]. Furthermore, the grains and tubers in their unprocessed states are bulky, difficult to transport and fetch very low prices in the market [Dixon, et al, 2001], [Taylor, et al, 2006]. This is a major cause of poverty amongst rural farmers [Killick, 1990], [Umoh, 2003] that encourages rural-urban migration [UNEP, 2006]. Thus, to guarantee access to food [Simon, et al, 2003], [Sanchez, et al, 2005], reduce rural-urban migration and encourage sustainable development [Barber, 2003], [Altieri, 2004], the processing of agricultural products like grains and tubers or solid minerals like clays and feldspars into more valuable products by the use of hammer mills must be encouraged and fostered.

Conventional hammer mills that are extensively employed in the processing of solid minerals and grains suffer from a number of weaknesses that greatly hamper their productivity, efficiency and effectiveness. These weaknesses include the following:

(a) The conventional hammer mill cannot produce material whose particle size is less than 400µm. For the most commonly processed solid minerals like kaolin, dolomite and feldspar, the particle sizes produced are relatively large and they cannot be directly used in the pharmaceutical, paint, textile, tyre, chemical, paper and glass industries or for grains as flour to make bread, biscuit and foo-foo for local consumption[Beintema and Stads, 2004], [Eyo, 2008].

(b) The fineness of the particles produced depend on the hole size of the screen sieve employed. Large particles can block the holes of the sieve screen thereby, reducing the output of the hammer mill.

(c) Milling rates fall rapidly as the moisture content of the raw material increases. (d) To maintain the output, the screen sieves are continuously changed. Hence, it requires the acquisition of a

lot of expensive accessories which cannot be produced locally. (e) Excessive dust particles are usually released into the atmosphere where hammer mills are operating. This

constitutes a health hazard for the human operators and environmental pollution for the surrounding plants, animals and human communities.

Ebunilo et al. (2001) has shown that Nigeria is abundantly blessed with kaolin, dolomite, feldspar, limestone, granitic and terrazzoic rocks. Other solid minerals reportedly found in Nigeria include gold, uranium, silver, gypsum, bauxite and lead. These are all initially found in rock formation and are blasted out as boulders. The boulders are usually very large and must be broken into smaller boulders that can be carried by hand, wheel barrows and shovels and loaded into trucks. The trucks are then sent to the milling site.

The defects and shortcoming of currently used hammer mills have meant that most hammer mill operators and owners in Nigeria are running their business at marginal profit levels. This is because virtually all the hammer mills being utilized are old designs. These machines were originally designed and manufactured in Britain and the United States of America in the early 1930’s (Lynch and Rowland, 2005). They were brought into Nigeria by the tin mining companies in Jos and were copied by local artisans. Since then, there have been no significant improvements in their design or method of operation. The lack of innovation in the areas of design and operating principles of hammer

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mills has constituted the greatest hindrance facing the growth of solid minerals and grains processing industries in Nigeria.

Although it is known that highly efficient, economical and fast speed models of hammer mills are being utilized in the chemical, powder, nuclear and food or grain processing industries, a recent search on the internet revealed that there is no single drawing of any type, model or prototype available on the web. This implies that manufacturers of this equipment regard their designs as proprietary.

Moreover, there are no published articles describing the working principles of these new designs. Thus, a successful growth and development of the solid minerals and grains processing sector of the Nigerian economy would depend largely on the design and fabrication of indigenous machines and equipment [Spangenberg, 2002]. These would be machines and equipment whose technology, maintenance, replacement, upgrading, efficiency and reliability are well understood and undertaken locally without the need for minimal contribution from any foreign expert or technology. This paper reports on the design and preliminary testing of a hammer mill with end suction lift capability and hopes that the commercialization and widespread application of the device will contribute significantly to the growth of the solid mineral and grains processing industries in Nigeria.

AIMS AND OBJECTIVES

The aims and objectives of this paper is to show that a hammer mill with an end suction lift capability (sieveless hammer mill), capable of continuous operation without clogging would increase throughput, percentage and quality

of particle sizes lower than 400m suitable for producing basic and composite flours used in making bread, noodles, infant foods and foo-foo. Also, as the hammer mill is fabricated locally, its functionality is not dependent on tight tolerances, allowances or clearances.

MATERIALS AND METHOD

DESCRIPTION OF CONVENTIONAL HAMMER MILLS

Conventional hammer mills operate on the principles of impact and pulverization. Essentially, a hammer mill consists of a number of steel hammers radially and axially spaced on a steel shaft or rotor which rotates at a high speed in a strong housing (usually made of thick steel sheets). As the material is fed into the mill from a feed hopper, the hammers strike it with great force and rapidly pulverize it. At a surface on the bottom of the housing and close to the tip of the hammers is a screen or sieve. The pulverized materials in the form of fine particles pass through the sieve and are collected. The fineness of the particles is regulated by the use of sieves of different mesh sizes. A conventional hammer mill is shown in Fig.1. This is typical of the model developed in the 1930s (Agricola, 1950). In the 1970s and 1980s, pneumatic systems were incorporated in the fabrication of

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Hopper

Hammer carriers

Sieve screen

Discharge

Rotating shaft

Swinging hammer

Rigid frame

Feed

Fig.1. A Conventional Hammer Mill

hammer mills to enhance the transfer, storage, separation and processing of the fine particles produced by the mills (Lynch and Rowland, 2005). Automatic control of the feed was introduced to prevent overloading of the crushing chamber. This was achieved by using a cut-out switch to control the periodic opening and closing of the feed hopper gates in the chamber. The hammers are allowed to swing freely instead of being rigidly attached so as to absorb shock loads encountered when they come into contact with very hard substances or material. These innovations which improved the capability and operational life of hammer mills are not yet incorporated in the design and fabrication of local hammer mills (Onokpe et al 2000).

PROBLEM DEFINITION AND CONCEPTUAL DESIGN

The conceptual design was based on the principle of design by analysis [Norton, 2006]. The methodology adopted was to examine the most critical defects of conventional hammer mills and provide solutions. Thus the following defects or problems were identified and corresponding solutions were proffered.

Problem 1

As a result of wear and corrosion the sieve screen holes enlarge or burst thereby allowing larger than desired particles to pass through.

Solution 1

Eliminate sieve screens. Introduce an endless sieve that is a dimensionally controlled “open gate”.

Problem 2

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After several hours of hammer mill operation, the sieve screen holes are clogged thereby reducing its efficiency and capacity.

Solution 2

The solution to problem 1 eliminates problem 2

Problem 3

Wet materials become elastic and therefore absorb most of the impact energy of the hammer without breaking. This reduces the efficiency of conventional hammer mills.

Solution 3

Introduce a fan to induce forced convection and rapid drying of material

Problem 4

Adequately broken particles can be collected when they fall through the sieve hole by gravity. Due to the relatively large gap between the hammers and the screen, this will be inadequate and therefore clearly inefficient.

Solution 4

Solution 3 eliminates problem 4 as pressurized air can lift particles of sufficient sizes through great distances. This is observed in tornadoes and cyclones.

Problem 5

Materials being crushed by conventional hammer mills cannot be recycled until they are reduced to the required size before trying to force them through the sieve holes. This is probably the greatest cause of burst holes.

Solution 5

A mechanical separator, which rotates at the same speed as the shaft ensures that all solid particles above certain sizes are blown back into the hammer mill chamber until they are ground or broken by impact into fine particles.

Problem 6

Some of the particles produced by hammer mills are in the form of dust. They usually constitute 5- 10% of the raw materials and are lost as dust into the atmosphere. They constitute serious health hazard to the human operators of the hammer mills as they enter the lungs (which can lead to cancer) and ears (which can lead to hearing loss), eyes (which can lead to blindness), et cetera

The dust particles escaping into the atmosphere would eventually settle on roofs of buildings, leaves of trees, and on animals thus causing pollution and damaging the ecology of the immediate environment.

Solution 6

Install a large sedimentation chamber with long tubes so as to virtually remove all the dust at the point of generation. This is greatly aided by solution 3 as the fan generates the required suction pressure. The design factors considered to be of utmost importance in this design were function, maintenance, reliability, safety and cost (Norton, 1999). The introduction of a sieveless screen for separating the right size of particles from the unwanted ones ensures that most of the maintenance and reliability problems of constant sieve de-clogging, checking and servicing, repair and

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changing are virtually eliminated as the machine functions as designed. Elimination of the sieves usually associated with conventional hammer mills also eliminates their greatest running costs which constitute of stockpiling of very expensive sieves that cannot be manufactured in Nigeria or repaired locally, machine down time and idle time due to clogged or burst sieve holes that reduce its availability, reliability and effectiveness.

MACHINE SPECIFICATION

Since this hammer mill was designed to be utilized in the rural areas of Nigeria where technical skills are least available, the following machine specifications were considered during design namely:

a) Increased throughput

b) Continuous operation

c) Efficiency of machine independent of very tight tolerances and allowances during fabrication or assembly d) Local availability of fabrication materials and spare parts

DESCRIPTION OF HAMMER MILL WITH END SUCTION LIFT CAPABILITY

The final conceptual design of the new hammer mill with end suction lift capability arose from the consideration of many concepts involving the use of feed hoppers, cams, springs, hammers, winnowers, feed hopper, a milling chamber, and a shaft coupled to a high-speed electronic motor through two vee-belts. The following machine elements are attached as shown in Fig.2. to the shaft, namely: free-swinging hammers, collars, a mechanical separator, a centrifugal fan, two bearings (one at each end)and a pulley. Two square cross-sectional pipes are attached to the centrifugal fan end of the shaft. The pipes are taken to the settlement chamber of the cyclone. From here, one of the pipes is connected to the collection point while the other is re-routed back to the feed hopper.

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Fig.2: New Hammer Mill

OPERATING PRINCIPLES OF HAMMER MILL WITH END SUCTION LIFT CAPABILITY

The raw material is fed into the hammer mill through the feed hopper. The feed hopper is chamfered to facilitate unidirectional flow of the raw material by gravity to the milling chamber. Here, the hammers strike the material which breaks into small pieces each time there is a successful hit.

The pulverized material is prevented from leaving the milling chamber until it has been reduced to fine particles on subsequent impact by the hammers by the unique action of the mechanical separator. The mechanical separator is a rotor having two equal arms chamfered at the ends. It is housed in a frustum chamber that is mounted in such a way that its inlet is larger than the outlet. The separator rotates at the same speed as the shaft which is directly coupled to it. As the shaft rotates at high speed, the blades of the separator form a partially closed system at low pressure with the frustum end and only very fine particles of the pulverized material can pass through. The larger particles or uncrushed materials are recycled to the crushing chamber.

The centrifugal fan generates a suction pressure which depends on the speed of rotation of the blades. The induced air/particle mixtures are vented through the two overhead pipes. The two pipes are of such length and cross-sectional area to enhance the movement of the dust laden, air and prevent the sedimentation of the particles in them. At the sedimentation chamber or cyclone, the air from the lower vent containing less number of particles is re-routed back into the feed hopper. This causes a pressure drop in the feed chamber and prevents dust particles from escaping through the feed hopper. The air from the higher vent passes through the cyclone and as it expands, the velocity of the particles is sharply reduced to a negligible value as they come into contact with the walls of the cyclone. The

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particles assemble on the walls of the container which are directed downward. The tiny particles now fall by gravity and are collected and bagged.

DETAIL DESIGN

This section attempts to show the basic equations used in the design of the hammer mill and the principles adopted. The major components of the machine include the hammers, shaft, bearing, centrifugal fan, mechanical separator, cyclone casing and electric motor.

The Hammers

The centrifugal force on the hammers, Fh, is given by

Fh = Nh mh rh ωh2 . . . (1) [Hannah and Stephens, 2004]

Where,

Fh = centrifugal force

Nh = number of hammers

mh = mass of each hammer

rh = radius of hammer

ωh= angular velocity of hammer

Assuming inelastic impact between the hammers and material, the velocity of material, Vm, given by

Vm = . . . (2a) [Khurrmi and Gupta, 2007]

Or

Vm = . . . (2b)

Where

Vm = velocity of material being milled

mm = mass of material being milled

Nm = number of material impacted

The minimum width of hammer, wh, to withstand the centrifugal force at impact is given by

wh = dh + . . . (3) [Morrison and Crossland, 2000]

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Where

wh = width of hammer

dh = diameter of hammer

th = thickness of hammer

σh = working stress on hammer

Mechanical Separator

Let us assume that the loading is such that failure can only occur by bending or shearing of the blades.

(a) Failure by bending of the blades

Fb = σytb(wb-db) . . . (4 ) [Hall, et al, 1998]

Where,

Fb = bending force

σy = yield stress

tb = thickness of blade

wb = width of blade

db = diameter of blade

(b) Failure by shearing of blade

Fs = τ tb (wb-db) . . . (5) [Hall, et al, 1998]

Where

Fs = shear force

τ = shear stress on blade

Power Required by Machine

The power required by the hammer mill, Phm, is given by

Phm = Tω . . . (6) [Khurmi and Gupta, 2008]

Where

Phm = power

T = torque

ω = angular velocity

The Shaft

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It is assumed that the shaft is subjected to both axial and tangential forces. The diameter of the shaft, dsh, is determined using the Soderberg criterion and maximum shear stress theory. Thus

= . . . (7) [Hall, et al, 1998]

Where

FS = factor of safety

Kf = actual stress concentration factor

σu = ultimate tensile stress

M = bending moment

σe = endurance limit of shaft material

Tm = mean torque

dsh = shaft diameter

V-Belt and Pulley

The centrifugal tension, Fbo, is given by

Fbo = m’be Vbe2 . . . (8) [Morrison and Crossland, 2000]

where

Fbo = centrifugal tension

m´be = mass of belt per unit length

Vbe = linear velocity of belt.

The power transmitted by the belt, Pbe, is given by

Pbe = (Ft1 – Ft2) Vbe . . . (9) [Khurmi and Gupta, 2007]

Where

Pbe = power transmitted

Ft1 = tension on tight side of belt

Ft2 = tension on slack side of belt

The turning moment on the pulley, Mp, is given by

Mp = (Ft1- Ft2)rp . .. (10) [Hannah and Stephens, 2004]

Where

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Mp = turning moment

rp = radius of pulley

Centrifugal fan

The centrifugal fan provides the pressure that enables the new hammer mill to have end suction lift capability. The total pressure, Pt, of the air is related to the static pressure, pst and dynamic pressure, pv, by the formula

Pt = Pst +Pv . . . (11) [Welty, et al, 1999]

Where

Pt = total pressure

Pst = static pressure

Pv = dynamic pressure

TESTING AND RESULTS

A prototype of a hammer mill with end-suction lift capability has been produced. Most components of the machine were produced using locally available raw materials. The machine was tested using maize and cowpea grains.

To compare the performance of the conventional hammer mill with that of the new hammer mill, equal quantities (7kg) each of maize and cowpea grains taken from the same population were fed into the respective machines. The outputs of the machines at the end of 1min. of operation are shown in Table 1 for the conventional hammer mill and Table 2 for the new hammer mill. The results shown in Tables 1 and 2 are the average of 3 test results conducted for each machine and crop under similar conditions.

Table 1: Output of Conventional Hammer Mill

Crop

Output (kg/min) Coarse (1mm – 2mm)

Medium (601µm – 1mm)

Fine (400µm - 600µm)

Maize Cowpea

3.0 3.2

1.8 1.9

1.2 1.3

Table 2: Output of New Hammer Mill

Crop

Output (kg/min) Coarse (1mm – 2mm)

Medium (601µm – 1mm)

Fine (400µm - 600µm)

Maize Cowpea

- -

- -

1.5 1.6

The power source of the hammer mill is a three phase electric motor with a power capacity of 3.73kW and a rotational speed of 3000r.p.m.

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DISCUSSION

The particle size distribution of the outputs of the machines indicates that while the conventional hammer mill produces particles in the coarse, medium and fine categories, the new hammer mill produces particles in the fine category only for the two grains that were tested.

This output affirms the performance of the mechanical separator which ensures that only particles of a size desired by the user or client are obtained from the new hammer mill. The incorporation of a feedback mechanism that returns uncrushed and coarse materials back to the crushing chamber for more fracturing assist in ensuring that only very fine particles are obtained from the machine. These features of the new hammer are very important because they ensure that products of high quality and desired specification are obtained. Thus the crushing and pulverization of solid minerals and agricultural products can be accomplished efficiently.

CONCLUSION

This paper has focused on the design and preliminary testing (after fabrication) of a hammer mill with end suction lift capability. The major components of the new hammer mill are the hammers, shaft, bearing, centrifugal fan, mechanical separator, cyclone, casing and electric motor. Most components of the hammer mills were produced using locally available raw materials.

The preliminary tests carried out on the new hammer mill confirm that it produces very fine particles as required . This contrasts sharply with the coarse, medium and fine particles produced by the conventional hammer mill. Thus the quality control and productivity of the new hammer mill are considerably greater than those of the conventional hammer mill.

The environmental pollution associated with the use of conventional hammer mills is virtually eliminated in the new hammer mill. Thus there is no health hazard experienced by the operator of the new machine. It is hoped that the commercialization and widespread application of the new hammer mill will contribute significantly to the growth of the agricultural and solid mineral processing industry in Nigeria. Furthermore, the new hammer mill with end suction lift capability when used in processing grains that are abundantly produced in Nigeria would reduce processing losses, produces flour with longer shelf life (dry flour), enhances greater consumer choice, ensures new markets for domestic cereals and legume crops, reflects a more effective response to changing market requirements and increases food security for Nigeria.

ACKNOWLEDGEMENTS

The authors wish to appreciate the contribution of Mr. O.O. Onokpe, Mr. I.B. Onosode and Mr. E.O. Ekamah in the design, fabrication and preliminary testing of the hammer mill with end-suction lift capability.

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DESIGN AND PRELIMINARY TESTING OF A HAMMER MILL WITH END