Design and Development of Rasp Bar Mill for Size Reduction of Maize

: Particle size affects many characteristics in the manufacturing process. Controlling the particle size helps to assure that the milled material will be consistent and repeatable with respect to firstly color , that is, uniform particles assure batch-to-batch color consistency , Secondly, flowability that is critical to packaging, tableting, weighing. Thirdly, uniformity , that is , consistent bulk density. Fourthly , d ensity - helps control shipping costs and minimize dust. Fifthly, reconstitution , that assures the desired dissolution rate , Sixthly, c hemical reaction that is vital for uniform, controlled chemical change. Finally, taste that allows precise portion control for consistent taste . The aim of this project is to design and develop a rasp bar mill for size reduction of maize seeds for different purposes. The fabricated rasp bar mill is capable of feeding maize around 60-180 kg/h, delivering of grits 60-90%, with sieve effectiveness 90-100% at a power requirement of 800-1400W,for different sieve perforation sizes of 5, 6, 7, 8 mm respectively.


Introduction
Maize  1.1 Literature review: In general "size reduction" is taken to mean the disintegration of solid substances by mechanical forces without altering their solid state. This also includes the division of liquids into drops or gases into bubbles. However, the physical and chemical condition of the disintegrated material may alter, particularly when inhomogeneous substances are present. The preparation for separation according to material components, e.g. grinding grain, is therefore, one of the classical tasks of size reduction techniques. Size reduction or "comminution" is the unit operation in which the average size of solid pieces of food is reduced by application of grinding, compression or reduction of particle size is an important operation in many chemical and other industries. The important reasons for size reduction are:  Easy handling  Increase in surface area per unit volume  Separation of entrapped components impact forces

Introduction
In all types of size reduction, there are three types of forces used to reduce the size of foods: 1. compression forces, 2. impact forces, and 3. shearing (or attrition) forces.
Cereal grains generally are the primary source of energy in feedlot diets. Availability of energy from the grain depends largely on the type of grain used as well processing of that grain. [13].
A variety of grain processing techniques are used including grinding, steam flaking, and compiling high moisture corn to ferment. Each processing method differs in its nutritional efficacy [13] and each has a unique associated cost. [14]. Criteria for size reduction An ideal crusher would (1) have a large capacity, (2) require a small power input per unit of product, and 3) yield a product of the single size.

Design details
The Comminutor operates by feeding material uniformly into a chamber in which a rotating blade assembly reduces the particles of the material by cutting or impacting them. The material discharges through a screen which regulates final particle size at the outlet of the milling chamber. The blade and screen act in conjunction to determine final product-sizing. [9] A gravity feed throat introduces material tangentially to the rotation of the blades. Other throats are available for production machines, such as a metered feed throat, liquid inlet throat, etc. Here, in this project feed throat was located 90 o to the face plate of the mill machine.

Blade Profile:
[9]The type, quantity and shape of blade helps to determine the degree of reduction achieved based on the material being processed. Some blade styles offer flexibility of knife on one side and impact on the other. Knife-edged configuration is for gentle granulation and impact-edged for more aggressive reduction. In this project, stainless steel flat bars were used as it is food grain processing. Since it is low energy size reduction, reduction of the rasp bar speed should be done to decrease the percentage of cracked seeds by maintaining 600-1400rpm speed range. [3] Therefore 3 hp motor is used to supply power to have rotating speed 2880 rpm for machine operation and the rasp bar rotor speed is kept at 1440 rpm.
………..(4) Where, T1 & T2-fan belt tight side and slack side tensions T3&T4-motor belt tight side and slack side tensions RD-Diameter of pulley, mm Coefficient of friction between the belt and pulley is 0

Energy and power requirements in size reduction[2]
The cost of power is a major expense in crushing and grinding, so the factors that control this cost are important.

Open area[11]
Open area or perforation area for a sieve having triangular pitch is determined by T R Where R-hole diameter, mm T-Triangular pitch, mm According to the equation for 5mm sieve, perforation area is22.7mm 2 for 6mm sieve, perforation area is32.6mm 2 for 7mm sieve, perforation area is19.8mm 2 for 8mm sieve, perforation area is25.8mm 2

Crushing efficiency
Empirical relationships determined by Rittinger"s and Kick"s law: The work required in crushing is proportional to the new surface created. This is equivalent to the statement that the crushing efficiency is constant and, for a given machine and material, is independent of the sizes of feed and product. If the sphericities F a (before size reduction) and F b (after size reduction) are equal and the machine efficiency is constant, the Rittinger's law can be written as where P -the power required, W -the feed rate to crusher, g/s -the average particle diameter before crushing,mm , -the average particle diameter after crushing, mm Kr -Rittinger"s coefficient. [12] Rittinger"s law is applicable for feed size less than 0.05mm.

Bond crushing law [12]
Bond crushing law is applicable for feed size in between 0.05mm to 50mm.The work required to form particles of size Dp from very large feed is proportional to the square root of the surfaceto-volume ratio of the product, sp/vp. Since F s = 6/Dp, it follows that (P/ ṁ = Kb/(Dp)^0.5

3.Materials and method
Maize rasp bar milling or the corn gritting is a kind of dry milling, which can be simply defined as process of impacting corn grains, that leads to grain size reduction and passing through a screen. Electrically operated small scale Rasp Bar Mill was developed and tested for maize. The mill consists with 5 major components; feeding funnel, grinding chamber, power supplying unit, blower unit and collecting outlet. Grinding chamber consists of four (04) ……….. (9) sets of rectanguler blades attached to a rotor shaft that leads to size reduction of grains and a sieve, that has round perforations, enclosed the blades to prevent leaving grits from the grinding chamber until they are at least as small as the sieve openings. There are four sieves having 5mm, 6mm, 7mm and 8mm diameter perforations that can use separately according to the preference. Three horse power (03hp)with rotating speed 2880 rpm motor supplies power for machine operation.

Testing methodology
Performance of the developed gritting unit was evaluated in terms of output capacity, gritting efficiency, power requirement against different moisture levels.(i.e.8%,9%,10%,11%) Different moisture levels were obtained by oven drying. The Moisture content was measured by Grain Moisture meter GMK 303A G-WON manufacturer. Maize grains flowed under gravity to the gritting chamber where impact of revolving gritting occures. Rotor with 4 sets of rasp bars gritted the grain. Pneumatic cleaning is used to remove light, chaffy and dusty materials out of the grain while heavier materials move downward. Air is generated by a mechanical fan. Light material get collected into the cyclone of which inlet is fixed into the grain falling path. Figure 5 shows the change of gritting efficiency with the sieve size. When the perforation size is larger efficiency is higher.

ENGINEER 43
sets of rectanguler blades attached to a rotor shaft that leads to size reduction of grains and a sieve, that has round perforations, enclosed the blades to prevent leaving grits from the grinding chamber until they are at least as small as the sieve openings. There are four sieves having 5mm, 6mm, 7mm and 8mm diameter perforations that can use separately according to the preference. Three horse power (03hp)with rotating speed 2880 rpm motor supplies power for machine operation.
Following parameters were determined  Physical properties of the maize  Out put efficiency(%)  gritting capacity (kg/h)  feeding capacity (kg/h)  percentage of grits (%)  screen effectiveness (%) Machine and operating parameters  Screen dimensions  Sieve size  Screen pitch  Hopper capacity  Rpm of the unit The motor was started and spined the rasp bars. Dried maize was fed through the hopper. The hopper capacity was around 1000g. Small bits of plant material exit through the punched holes at the bottom.

Testing methodology
Performance of the developed gritting unit was evaluated in terms of output capacity, gritting efficiency, power requirement against different moisture levels.(i.e.8%,9%,10%,11%) Different moisture levels were obtained by oven drying. The Moisture content was measured by Grain Moisture meter GMK 303A G-WON manufacturer. Maize grains flowed under gravity to the gritting chamber where impact of revolving gritting occures. Rotor with 4 sets of rasp bars gritted the grain. Pneumatic cleaning is used to remove light, chaffy and dusty materials out of the grain while heavier materials move downward. Air is generated by a mechanical fan. Light material get collected into the cyclone of which inlet is fixed into the grain falling path. Figure 5 shows the change of gritting efficiency with the sieve size. When the perforation size is larger efficiency is higher.  Figure 6 shows the change in power requirement for size reduction of maize, with respect to the perforation size at moisture level 11%. Maximum power requirement was recorded in the perforation size of 6mm. This may be due to higher perforation area of this sieve compared to the other sieves.

Figure 7 -Gritting capacity vs perforation size
Drastic change of gritting capacity at 11% (wet basis) moisture content was observed in the perforation size 8mm. This may be due to higher opening area in the seive.  Drastic change of feeding rate was observed in the perforation size 8mm at moisture level 11%(wet Basis. This may be due to the higher opening area in the seive. In all cases, seive effectivness are greater than 86%. The speed of the rasp bar rotor and speed of the fan were kept at 1440 and2880 rpm respectivly.  The same experiments were carried out to different moisture levels.i.e.8%, 9%, 10%, 11%(wet basis). An increase of gritting efficiency was noticed at perforation size 6mm. This may be due to highest perforation area as the pitch of this sieve was 10mm compared to other sieves.

Figure 10-Feeding capacity vs perforation size
Higher feeding capacity was observed at perforation size 6mm. This may be due to higher perforation area of this sieve. Human error may be the reason for different capacities of the sieve with perforation size 5mm.

Figure 11-Seive effectiveness vs perfortion size
A low seive effectiveness was observed in perforation size 5mm at all moisture levels. This may be due to lower perforation area of this seive.

Figure 12 -Power requirment vs perforation size
High power requiement was observed at perforation size 5mm. This may be due to the lower perforation area of this seive. Power was supplied by 3hp motor.The speed of the rasp bar rotor and the speed of the fan were kept at 1440 and 2880 rpm respectivly in all experiments

Conclusions and recommendations
The study revealed that the fabricated maize gritting machine is capable of gritting maize at different moisture levels of maize and different seive perforation sizes. i.e. 5 , 6 , 7 , 8 mm in diameter.The product comes out is grits without cover and germ.The optimum performance was obtained at the perforation size 6 and the moisture content of 8%(wet basis). The overall efficiency of unit is 47% as calculated. Size reduction creates heat while in operation but it can be neglected. The brokens do not contain much heat. Therefore no color change appears. The manufacturing cost is calculated as Rs. 150,000/= according to the present market prizes.(2013).This can be a self employment for a person. The brokens can be boiled as it is for consumption with coconut scrapes and can be mixed with milk rice or can be used for making corn flakes.