Guide To Collide Technology

Digital Re-print - May | June 2009 Feature: GTCTM Feature title: Guide to collide technology Grain & Feed Milling Technology is published six times a year by Perendale Publishers Ltd of the United Kingdom. All data is published in good faith, based on information received, and while every care is taken to prevent inaccuracies, the publishers accept no liability for any errors or omissions or for the consequences of action taken on the basis of information published. ©Copyright 2009 Perendale Publishers Ltd. All rights reserved. No part of this publication may be reproduced in any form or by any means without prior permission of the copyright owner. Printed by Perendale Publishers Ltd. ISSN: 1466-3872

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GTCTM

Feature

e d i l l o C o t e Guid

GTC

TM

technology

by Jonathan Woo, Wyes Machinery Shanghai Co Ltd, China

T

here are two key reasons for high-energy consumption, low throughput and a rise in temperature of ground materials coming from a hammer mill. They are: 1. Poor hammer milling capacity

Large amounts of colliding energy is wasted as heat due to the high rate of eccentric collision. This only makes materials spin and bounce around in grinding chamber, and does not lead to further size reduction.

2. Poor screening capacity There’s definitely a material circulation layer being swept along the inner surface of the screen, hindering correctly sized particles from getting through the screen holes in time. As these particles rub against the screen, and each other and crushed by hammers, their size is continually reduced by attrition and collision. Energy is wasted in the production of heat, while throughput is restricted and particles sizes become too small.

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Guiding the fluidized bed of material

GTC Technology is a size reduction method that manages to guide the fluidizedbed of material to collide with hammers. It greatly increases the probability of frontal collision between materials and hammers requiring the same energy consumption It makes full use of the kinetic energy within the fluidized-bed of material, and succeeds in enlarging impact capacities 31 times that of single rotor hammer mill, which could dash material into shatters once even eccentric collision It stops the formation of a material Figure 1: hammer mill for high-speed circulation layer effecphotography tively It highly improves 1: Hopper 2: Control pulls plate the efficiency of 3: Screen screening capacity 4: Hammer mill by enlarging by 20 Main technical parameter include: percent the effective screening area and Diameter of rotor: 400mm keeps agitating the fluidized-bed of mateWidth of grinding chamber: rial, thereby reducing 580mm the density of the Quantity of hammer: material layer against four rows and eight hammers the screen surface Dimension of hammer: It promises to 120 x 40 x 5mm produce more than Distance from hammer tip to screen: 40 percent more 5mm throughput under Shape of grinding chamber: same energy conwater-drop sumption; in other Feed material through hopper on top words, it saves more of hammermill than 30 percent on the energy requirement under same production capacity It promises temperature rises of less

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than 15°C when grinding aquatic and poultry feed

Figure 2

Y

Figure 3

225

Poor hammer milling capacity

5

184

10

143

13

102 30 Materials, such 11 61 as corn could be broke up by using 20 little energy when frontal collision. -21 -97 However, much power is -62 needed for eccen-103 -205 tric collision. This -134 -93 -52 -11 30 71 112 153 194 X is because when -237 -183 -129 -75 -21 33 87 141 195 X eccentric collision happens, a moment of gyration is generated between designed hammer mill (see Figure 1) in order into the hammer impacting area after collidthe impact point and the barycenter of corn. to study these effects. ing with the screen. In general, it only makes the corn spin One side of hammer mill was equipped Reason for this phenomenon include corn but does not necessarily break it up. That with plexiglass, whole the grinding chamber fed into a hammer mill with low speed cannot falls into the category of elastic collision; the was visible through a 10mm outside screen. pass through the material circulation layer. impact power turns into the form of heat The camera’s shutter rate was 5,000fps, expoThe first tracked corn particle was broke which is wasted. sure was 1/25,000s and the hammer mill’s up within 0.4ms at a relative speed 57.45m/s However, corn could be broke up along rotation speed was 3314rpm, with the ham- when frontal colliding with hammer. its mid-point and maximum length due to mer tip speed at 69m/s when photographed. The second tracked corn particle the bending movement if the impacting Two videos were captured separately rebounded into hammer impacting area speed is fast enough. During the process of when grinding corn at normal load and after collided with screen, as the first one milling with hammer mills, eccentric colli- below normal load. had done. sion happens mostly, with large amounts of Videos were analysed and processed in However, it spun and bounced in the energy being wasted. professional image processing system. chamber when there is eccentric conllision, Therefore, methods used manage to Some typical corn particles were tracked colliding with hammer. It needed further increase the probability of frontal collision and their movements were captured (see impacting in order to achieve size reduction. between the materials and hammers and Figure 2-3). It could be seen from the video Eccentric collision happens mostly in the two also enlarge the impact velocity that improve that there is definitely a material circulation videos. the efficiency of hammer mills. layer formed on the screen surface in chamber. Poor screening capacity To see what’s going on in a working hamThe corn particles first tracked did not fall mer mill, researchers designed experiment into hammer impacting area. They moved According to traditional size reduction by using high speed photography technology. on to the hammer impacting area along the theory in working hammer mill chambers, High-speed photography required a specially hammer rotating direction, and rebounded and due to centrifugal force, bigger particles

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Researchers checked the materials in each sampling probes and found that average particle sizes was similar along entire radial direction, all around ensemble average size of sample, 1.12±0.04mm. (Ensemble average size of sample was tested by mixing all samples from the probes.) Density (material mass per unit volume) distribution of the material circulation layer along the radial direction was low to high. That is, the bottom of material circulation layer, which was close to the inner screen surface, density was high; the closer to the center of chamber the lower it became.

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stay tight and close to the inner screen surface while the smaller particle are further from the screen. Big heavy particles are difficult to exit through the screen holes and block the holes, while small light particles are far from the screen hole. They also cannot easily exit through the screens, thus forming a material circulation layer with outer big particles and inner small ones. In view of the above, various shapes of grinding chambers were developed to disrupt the material circulation layer, such as hexagonal, elliptical, water-drop etc. However, some researchers carried out corn grinding experiments to compare every chamber shapes and they found that feeding was at a low speed, compared with round chamber, hexagonal, elliptical, water-drop shaped chambers’ capacity per kilowatt were higher; but when feeding at a higher speed, capacity per kilowatt of hexagonal, elliptical chambers were lower than round one, while water-drop one was similar to round one.

This is because when feeding at a higher speed, stagnated materials appear at every corner of the hexagon, at the two ends of ellipse and top frontal impacting screen area of the water-drop chamber (see Figure 4). Stagnated materials reduce real screening area and as big particles find it hard to bounce at these places, capacity drops. If you continue to increase the speed of feeding, the stagnated materials will grow in size to fill the chamber so that it becomes a round one, and the real screening area is reduced sharply. In truth, however, big and small particles are distributed evenly in the material circulation layer; density distribution is near the inner screen surface as density is increased, so that the material circulation layer is looser on the inner area and tighter on the outer area. Therefore, reducing the density of material on inner screen surface can largely improve the screening capacity of hammer mills. Researchers designed a special hammer mill (see Figure 5) to acquire material circu-

lation layer distribution data directly, which revealed the truth. There’s a fixed partition between grinding chamber and fan chamber. Eight sampling probes (four as a group) were installed in a radial direction evenly spaced on a fixed partition (see Figure 6). Sampling probes connect grinding chamber and fan chamber (negative pressure), high breathability sampling bags covered to end of sampling probes to the fan chamber side. Then distribution data of material circulation layer along radial direction could be acquired.

Main technical parameter of hammer mill Width of screen: 200mm Diameter of screen hole: �2mm Motor power: 5.5kW Hammer tip speed: 60m/s Feeding speed: 700kg/h Corn moisture content: 12.5 percent Single grinding quantity: 9.5kg

Figure 5: 1: Straw for material discharge | 2: Hammer | 3: Rotor 4: Grinding chamber | 5: Hopper | 6: Screen 7: Sampling probe | 8: Sampling bag | 9: Outlet 10: Fan chamber | 11: Fixed partition | 12: Belt pulley

GTC Technology GTC Guide (fluidized-bed of material diversion plate) ensures high probability of frontal collision between fluidizedbed of material and hammers.

GTC Guide is a plate similar to a cuboid, whose section is a parallelogram with two sides bowing inward. It is installed between two rotors and through the center of grinding chamber, which separates the whole chamber into chamber A and B It blocks the whole chamber’s center space; chamber A and B could communicate through the tunnel at the top and bottom of GTC guide. So the GTC Guide can guide as well as divert. Guide: GTC Guide guides material into neighbouring chamber’s hammer impacting area at a suitable angle, making moving direction of the fluidized-bed of material in the opposite direction to that of the hammer. Divert: GTC Guide avoids low efficiency collision between two fluidized-bed of material in two chambers, and ensures high probability of frontal collision between fluidizedbed of material and hammers.

GTC Technology enlarges impacting capacity Fluidized-bed of material collides with hammers of neighbouring chamber in opposite direction; at a speed around 70 percent

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Figure 4: Stagnated materials appear at places intend to disrupt material circulation layer

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R180

R158

R192

0

w

Screen

R204 Movable partition

V2

V1

Figure 8: If v is hammer tip speed, m1 is hammer mass, v2 is particle speed, m2 is particle mass, then hammer and particle’s momentum (M) and kinetic energy (E) before collision are: 1

M1 = m1 v , M2 = m2 v 1

2

E1 = -12 m1 v 2, E2 = -12 m2 v 2 1

Sampling probes Figure 6: Distribution of sampling probes

2

Hammer and particle could be viewed as a whole after collision. The whole mass is, (m1 + m2), hammer and particle have same speed v, momentum (M) and kinetic energy (E) are:

M = (m1 + m2)v of hammer tip speed. Enlarging GTC hammer mills’ impacting capacity 31 times those of single rotor hammer mills. It could dash materials into shatters once even eccentric collision. Hammer milling capacity and efficiency highly improved, temperature rise of ground material obviously lowered.

into neighbouring chamber and colliding it with the hammers. Continually disrupted, the trend of the fluidized-bed forms an inner loose layer and an outer tight layer in terms of density distribution. Thus, it reduces the density of material layer against the screen surface, greatly increases the screening capacity.

E1 = -12 (m1 + m2)v2 (1) Based on the law of momentum conservation:

M = M1 + M2 I.E. (m1 + m2)v = m1v1 + m2v2 So: v =

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m1 + m2

(2)

Take (2) into (1):

E=

(m1v1 + m2v2)2 2(m1 + m2)

Change in kinetic energy after collision is: E = E1 + E2 - E So:

Meanwhile, the unique double chamber frame structure enlarges by 20 percent the Figure 7: A 3D schematic effective screen drawing of GTC Guide area at the bottom, which also contributes much to reduction GTC Technology improves the in the density of material efficiency of screening capacity layer against the screen As mentioned before, the density dissurface, thereby improvtribution of fluidized-bed of material is that ing the efficiency of the the near the inner screen surface the greater screening capacity. the density, so the fluidized-bed of material is loose near the inside and tighter on the outside. More inforMation: Consequently, reducing the density of Jonathan Woo material on the inner screen surface can largely improve the screening capacity of Wyes Machinery hammer mills. Shanghai Co Ltd The GTC hammer mill has double grindTel: +86 21 67196821 ing chambers and a GTC Guide. Fax: +86 21 57107983 Its unique frame structure keeps agitatEmail: [email protected] ing the fluidized-bed of materials, guiding it Web: www.wyes.cn

m1v1 + m2v2

E =

m1m22 2(m1 + m2)

(v1 - v2)2

(3)

Change in kinetic energy E cause particle crushed. In single rotor hammer mill, fluidized-bed of material moves in the same direction with hammers at about 70% of hammer tip speed, I.E. v2 = 0.7v1, according to vector sum principle, from (3): E

sin gle

=

m1 + m2 2(m1 + m2)

(0.3v1)2

(4)

While in GTC hammer mill, fluidized-bed of material is guided to feed into neighboring grinding chamber in opposite direction with hammers, so: E

GTC

=

m1 + m2 2(m1 + m2)

(1.7v1)2

(5)

Compared (5) with (4):

=

EGTC Esin gle

≈ 31

(6)

(6) illustrating that GTC hammer mills’ impacting capacity is 31 times that of single rotor hammer mills.

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