EXPERIENCES WITH A HEAVY DUTY SHREDDER

Transcription

1 THIRTY-SEVENTH CONFERENCE EXPERIENCES WITH A HEAVY DUTY SHREDDER By C. D. CAMERON Pioneer Sugar Mills Limited, Ayr Introduction Examination of shredded preparation at Inkerman last season prompted an attempt to improve the quality by the simple expedient of decreasing the clearance between hammer and grid. This had the immediate effect of causing the motors to trip on overload, which seemed to indicate that the unit was under-powered. However, as it was not immediately possible to do anything about this it was decided to take a closer look at the operation of the unit in an attempt to find out what in fact was happening and whether improved work could be obtained within the installed power. History The shredder at Inkerman mill is a Greundler type heavy duty unit and was installed in 1964 at the same time as similar units were installed at Pioneer and Kalamia. The installation was generally as shown in Figure 1. The shredder is seven feet wide with a rotor weighing approximately eight and a half tons fitted with 99 hammers made from four by one and a quarter inch mild steel with hard facing each weighing approximately 20 pounds. Six rows of hammers were used and the unit was powered by two 350 hp motors direct coupled and running at 960 revlmm. The clearance from hammer to grid was one and a quarter inches and right from the outset it seemed that this installation was performing as well as any in the industry. Preparation was uniform and by observa- Fig. I-General arrangement lnkerman shredder.

2 198 THIRTY-SEVENTH CONFERENCE 1970 tion as fine as could be expected, and although the average motor load was high at 100 per cent with very large and frequent swings, there were no apparent problems with feeding and chokes above the unit were unheard of. The unit was operated in this manner for four years and rates up to 350 tons of cane per hour were not uncommon. Figure 2 is a reproduction of a typical chart from a recording ammeter taken from one of the motors. Full load current is approximately 420 amps. TABLE I-Preparation for week ended Fig. 2-Typical ammeter chart, original condition X 300 amp.

3 1970 THIRTY-SEVENTH CONFERENCE 199 However once the preparation was examined on the basis of p01 per cent broken cells as prescribed by the Sugar Research Institute (1968) it was realized that the performance, to say the least, was mediocre, and this led to the attempts to improve and the subsequent investigation which is the basis of this paper. Procedure In order that side effects be reduced to a minimum, as far as possible one modification only was made at any time, and although at times, in order to be sure of the effect, some changes were made in steps, only those which led to significant explainable results will be reported. As a comparison, pol per cent broken cells was the main criterion, although in the course of this work the.results of the whole year's operation were plotted using this measure of preparation against all extraction, and other factory control results. This is not reported in this paper but it can be said that within the range of results plotted no significant correlation could be obtained. Sight glasses were fitted to the shredder hopper so that the passage of cane could be observed, and it was immediately evident that a large percentage was thrown across the hopper by the top knives, to fall vertically on the top of the rotor. It could also be seen that the coarse cane was that which could pass cane outwards by centrifugal force and tended to force it up the angled feed chute. This in turn held back the remainder of the feed until SUEcient weight built up to force its way down. Each time this occurred the motor load would peak. First Ste : A curtain made of chain was hung behind the top knives to trap the W ying cane (Figure 1) and although this was not entirely effective the change in the motor load was quite distinct (Figure 3). Although there are still some power peaks these are due to cane continuing past the curtain, but the average motor load has been considerably reduced, which indicated that most of the load previously used had been absorbed in getting the cane into the shredder, tossing it about and dragging it through in lumps. The appearance of the preparation had changed, with more lumps and pieces of partly shredded sticks and the previous uniform effect was attributed to the tossing about the cane received before. Second Step: The curtain was modified to obtain a more effective screen and the clearance of the hammers was progressively reduced in an attempt to recover the preparation. Figure 4 and Table I11 indicate that although the preparation was improved to that of the original arrangement, the motor load has been considerably reduced. TABLE Il--Preparation for week ended

4 THIRTY-SEVENTH CONFERENCE 1970 TABLE Ill--Preparation for week ended Pol % broken cells However it was found that the closest clearance which could be tolerated was approximately one quarter inch. After that the shredder became very rough and although a change in hammer weight was considered we were not yet satisfied that the shredder was feeding as well as it should. Third Step: It appeared that the two grid bars located on the tangential entry to the shredder were acting as an obstruction and these were removed and plated over. In addition a further deflector plate was placed in the shredder hopper (Figure 1). Fig. 3-Typical ammeter chart, first step.

5 THIRM-SEVENTH CONFERENCE 20 1 TABLE IV--Preparation for week ended Monday Tuesday Wednesday Thursday Friday Average These moves ironed out more of the feeding problems and Figure 5 is a typical power chart towards the end of the season. It can be seen (Table IV) that even though the motor loads have been drastically reduced the preparation on a p01 per cent broken cells basis is slightly better. - 1icould also be seen that the coarse cane was that which could pass between the hammer and the grid but that the existing hammers would not operate at any less clearance even though feeding arrangements were now good. l Fig. &Typical ammeter chart, second step.

6 202 THIRTY-SEVENTH CONFERENCE 1970 Observations 1. The shredder would run smoothest with a greater clearance, indicating less deflection. 2. Poor feeding conditions can absorb as much as 70 per cent of the total load without increasing the effective work done. 3, For a given rotor speed the extent to which the clearance can be reduced is a function of the hammer weight and form. 4. Wear rate of hammer bushes indicated that rough running was connected with increased wear, therefore more deflection. Interpretation Neglecting factors such as bearing friction, windage etc. it is premised that the work done by the shredder is made up of two components : 1. Work done accelerating cane to the speed of the shredder; 2. Work done across the grid or shredding. This can be argued, as early high-speed still photographs indicate that the initial impact of a hammer on cane does little more than split it and the observations quoted show that more cane moves through untouched if the clearance is increased. Also, centrifugal force will tend to force the cane against the grid bars with it concentrating at the hammer tip so that the passage across each bar will extract further work. The work done in acceleration can be calculated, and for a constant rate and for an even feed will remain constant. If this is subtracted from the total recorded load, the remainder of the work must be done in shredding. As friction factors and such would remain fairly constant for any one unit, a general assumption such as this should allow reasonable prediction of the results of future performance. Analysis of Work Done Two papers (Shann and Cullen 1968, Crawford 1969) offer formulae representing the work done in a shredder, but the former appears to be best suited to analysing the results in a simplified form, sufficient to compare two sets of conditions. Consider the conditions which applied in the week ended 7th November 1969 : Weight of hammer M = 20 lb Radius of pivot R == 14 in Distance C of M to pivot (h) = 54 in Distance of tip to pivot (1) = 11 in Radius of centre of mass (R + h) = 194 in Numbers of hammers N = 99 Average motor load = 50 per cent = 350 hp at 960 rev/min Cane rate 300 tons per hour Energy expended per 320 X hammer blow 99 x960 = 121 ft lb Velocity at centre of mass of hammer = 164 ft/sec Velocity at tip = 210 ft/sec The cane falls approximately 10 feet down a chute angled at 40" to the horizontal Velocity at entry to shredder = 2 x 32.2 X 10 Sin 60 = 22 ft/sec

7 1970 THIRTY-SEVENTH CONFERENCE 203 Gain in energy by mass M accelerating from velocity V, to v2 Cane rate Energy gain = ft lb/sec 960 X 99 Hammer blows per second Work done per hammer in X = - 60 acceleration E, 960 X 99 = 79 ft lb :. Work done in shredding E, = = 42 ft Ib Thus on this assumption, 34.5 per cent of the energy is used in direct shredding. Consider now the deflection of the hammer due to these combined components. This can be calculated from the general formula. Energy at centre of mass (E) = -(l - Cos 8). Where 8 repre- gr sents an angle equivalent to the total work done on the hammer 8 = Cos-' (l X 14 = Cos-' (l - 20 X 164' X 5.5 = Cos-' (l' ) = Cos-' = 11". Prediction of Work for Modified Hammer It appears practical to design a hammer weighing 30 pounds with a centre of mass 7 inches from the pivot. Consider the conditions then using this hammer, but utilizing all the available power, i.e. 700 hp at 960 revlmin. Velocity at centre of mass = 177 ft/sec Work done per hammer = 242 ft lb Work done in acceleration is constant = 79 ft lbs. '. Work available for shredding = = 163 ft lb On the same approximation as before, the total deflection of the hammer is 10.5", a slight improvement on the previous condition. However, the percentage of total energy now applicable to shredding is 67.3 per cent of the new total, and is about four times that available from the previous hammer design. It is realized that in practice the arbitrary apportioning of energy will depend very greatly on the exit cane velocity, but a quite significant potential gain in performance is indicated.

8 204, THIRTY-SEVENTH CONFERENCE 1970 Effect of Width of Tip To obtain the proposed weight increase, the hammer width will be increased from 14 inches to 24 inches. The equivalent work calculated has all been taken at the centre of mass of the hammer, but in fact the shredding work is done at the tip. Net equivalent resistive force at tip = F. Net equivalent resistive force at C of M = F, F, x 5.5 For original hammer F - 11 For modified hammer F F, x7 =- 1 l Increase in available energy for shredding = = 121 ft lb :. Increase in effective force at tip X 7 X 100, 42 X 5.5 /D -

9 1970 THIRTY-SEVENTH CONFERENCE 205 On a basis of effective force per unit width of tip, this becomes 121 X 7 X 1.75 X X 5.5 X 2.5 % = 256% However the exposed cane area has been increased so that it is reasonable to assume that as the original setting considered was a half inch and preparation was 72 per cent p01 in broken cells, the modified arrangement should be expected to operate satisfactorily at a clearance of approximately one half of this with significant increase in rupture. Grid Settings One further point arises if one assumes a bulk density for shredded cane at 25 lb per cubic foot. The thickness of cane blanket at the calculated rate and tip speed is 0.06 inches and it thus seems reasonable to argue that the grid should be converging to a minimal clearance and should be continued as far as possible to obtain maximum work for available power. Conclusion Experimental work by other mills has resulted in trends towards higher speeds, which appears logical in an effort to gain in preparation, but where the installed unit will not tolerate higher speeds and the power is not available it appears that appreciable improvements can be.made by close attention to the internal performance of the individual shredder and the results could be predicted within the power available. Acknowledgments Thanks are extended to the Directors and Management of Pioneer Sugar Mills Limited for permission to publish this paper. REFERENCES Crawford, W. R. (1969). Mechanics of swing hammer shredders. Proc. Qd. Soc. Sug. Cane Technol., thirty-sixth Conf. Sham, D. S. and Cullen, R. N. (1968). Some aspects of shredder hammer design. Proc. Qd. Sug. Cane Technol., thirty-fifth Conf. Sugar Research Institute (1968). Technical Circular.