SYMPOSIUM ON SUGAR ENGINEERING

SYMPOSIUM ON SUGAR ENGINEERING

Session-Wednesday, 1st February, 9-15 A.M.

RECENT DEVELOPMENTS IN SUGAR CANE MILLS

MR. K. S. ARNOLD, Chairman

INTRODUCTION

A number of advances have been made in the detailed design of mills over recent years, and some important developments have taken place, all of which are contributing to the higher crushing rates and improved efficiencies now being obtained. Some of the developments are radical in character, and give indication that certain items of plant-accepted as standard over a long period of years-are in process of being superseded by equipment of entirely different types. Apart from new developments, there are also some changing trends in the use of certain items of equipment associated with milling plants.

In this Symposium the more important of recent developments-apart from those already dealt with in the papers-are briefly described, and questions in relation to these are raised so as to stimulate discussion among delegates. It is hoped that both those responsible for the operation of mills and those engaged in their design and construction will contribute information gained from their various experiences and will also ventilate their problems.

The following aspects are dealt with :-

(a) Cane Shredders. (d) Intermediate Carriers. (b) Roller Grooving and Surface (e) Juice Strainers.

Speed. (f) Mill Roller Bearings. (c) Mill Feeding Devices. (g) Hydraulic Pressure systems.

(a) CANE SHREDDERS

(Introduced by Mr. W. M. Livie, Australia)

Shredders for the disintegration of cane in the milling process have been employed for over 30 years but have never been universally adopted. In certain territories they have been largely employed, in others only occasional applications have been made, and in some none at all. In certain countries, where at first they were extensively installed, their popularity has wankd, while in others there are indications of growing popularity.

At one time almost all the larger mills in Hawaii used the hammer-type shredder. Some factories subsequently removed the shredder permanently, while others-having removed them-have now re-installed them. Today 11 out of a total of 31 tandems, are equipped with shredders in that territory.

The hammer-type was also popular for a period on certain mills in the West Indies, but has now been discarded. They are still in favour on certain mills in South Africa and further installations are contemplated.

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Formerly the National shredder was used extensively in Australia, but only a few are retained in service today. Around the year 1920 there were 3 mills in that country using the hammer-type shredder, about 8 more: have since added shredders of this type, and there are iidications that furthei installations

1 may be made. The hammer-type shredder does not appear to have been installed as yet

in any factory in India. A number of Maxwell shredders were, however, formerly in operation but it is believed that these have all now been discarded.

As adjuncts to cane mills, shredders do not appear at any time to have been adopted od the majority of mills in any cane sugar country, and have experienced a varying degree of popularity. While they are undoubtedly well suited to deal with certain specific conditions-such as the preparation of extremely hard canes like Uba-their essential value on a milling plant still remains undecided. The almost universal adoption of cane knives-with in some cases two sets to a tandem-doubtless reduces considerably the advantages to be derived by the use of a shredder.

From the general experience in the use of shredders in many countries the f o ~ l o w i ~ ~ questions naturally arise :-

(a) Will the addition of a shredder to a milling plant always be justified by the improved results obtained-either by way of increased crushing rate, higher extraction, or a combination of both ?

(b) If not always justified, what are the factors which should decide a particular application ?

DISCUSSION

Mr. Livie stated that of the 12 Colonial Sugar Refining Company's Mills in Australia, 6 employed hammer-type shredders, and 4 National shredders. Good preparation of cane can be ensured either by knives or shredders. In one factory this shredder has been removed and two sets of high speed knives installed, resulting in an increase in crushing rate of 15 tons per hour.

Mr. J. H. vicklin stated that in about 25 other factories in Queensland 10 shredders were in use.

Mr. 0. M. Henzell said that he knew of only two hammer-type shreddeis operating in the West Indies, and these were placed after two-roller crusherg, He considered that with adequate preparation by knives and with a feeder caile carrier placed at right-angles to the main carrier, a shreddet was not necessaiy'.

Mr. 5. 0. Sason said that of 30 tandems operating in the Philippines 8 were provided with shredders. In some cases the direction of rotation of the shredders had been reversed.

Mr. D. K. Bralima said that in India 3 Maxwell shredders were discarded as the resulting fine bagasse had given trouble in the furnaces. He considered, however, that shredders allowed higher imbibition being applied, resulting in

,:? increased extraction:

I HARI CHANB 53,5 I

Mr. R, Lagesse considered that two sets of knives gave adequate preparation 1 for satisfactory crushing rates and high extraction and preferred these to shredders,

Chairman's Conclusion. The discussion showed a wide variation in opinion on the usefulness of shredders. Good preparatory work by cane knives appeared to render additional disintegration by shredders unnecessary f ~ r satisfactory mill performance, There were indications that when the cane preparation was carried out intensively by shredders, the finely divided bagasse gave trouble in the furnaces.

(b) ROLLER GROOVING AND SURFACE SPEED (Introduced by Mr. Hari Chand, India)

In order to achieve higher crushing rates there has recently been a marked increase in the surface speeds of mill rollers, these speeds being increased up to the highest amounts consistent with the rollers gripping the cane and maintaining the feed without slipping. An important factor in this trend is the type and dimensions of the surface grooving of the rollers, and practice in this connection varies considerably. The general practice is to employ coarse pitched deep V-grooving in the earlier units of a milling train, and gradually to decrease the pitch in succeeding mills. In the latter mills of a train, some operators prefer grooving of relatively coarse pitch, whereas others consider that fine pitch grooving is essential for maintaining good extraction.

While the advantage of Messchaert drainage grooves is full recognised, some operators consider that these are only beneficial on the feed rollers of mills and should not be employed on the discharge rollers. Chevron and other types of transverse grooving is also extensively employed on the earlier mills of a train to assist feeding in some cases on top rollers only, and in others also on the feed rollers.

The difference in grooving practice for achieving high crushing rates are appreciable, and delegates are asked to contribute views on the type and size of mill roller grooving for giving the best over all results under present-day conditions.

DISCUSSION

Mr. Hari Chand described a case in which by increasing the surface speed from 20122 ft. per minute to 35/38 ft. per min. on a 14-roller 24" x48" plant, and by adding an additional mill, the crushing rate was progressively increased) from 400 tons per day to 1200 tons per day. He considered that speeds above 40 ft. per min. would sacrifice extraction efficiency.

He considered that grooving of 3 14" pitch in the last mill, properly maintained, resulted in improved extraction, as more imbibition could be applied due to improved juice drainage compared with finer pitch grooving.

Mr. G. Mazery considered that a satisfactory mill grooving was 2" pitch for the first two mills, and either 314" or 112" for the subsequent mills. He considered that ample capacity was assured with surface speeds up to 35 ft. per min. and that troubles experienced at higher speeds, and lowering of extraction, did not justify operating faster.

Mr. 0. M. Henzell spoke in favour of having wider pitched grooving on feed rollers-generally double that on the top and back rollers.

Mr. B. M. Sawhney considered that the wider pitched grooving was preferred, as with fine pitch roller surfaces were liable to wear flat during a 1ong;crushing season.

Mr. J. P. Mukherji preferred differential angle grooving in the top and bottom rollers, with 60' angle for the top and 35/40" angle for the bottom rollers. He preferred using srr.all angle grooves in the bottom rollers to assist juice drainage and to avoid the use of Messchaert grooves. Wear on Messchaert grooves allowed considerable dropping of cush-cush, and the roller surface is liable to break away at the edges of these grooves.

Mr. W. M. Livie preferred the angle of grooving to be 55". Chairman's Conclusion. Whereas emphasis is often placed on the crushing

of fibre-and crushing rates are commonly compared on a tons fibre per hour basis-increased crushing rates have brought the juice drainage problem into prominence, thus stressing that it is not fibre which we are crushing, but cane. This drainage problem has led to coarse grooving being favoured, particularly on front rollers, with a tehdency also to make the angle of groove deeper. Coarse grooving also doubtless affords a better grip on the cane and helps to prevent

(c) MILL FEEDING DEVICES

(Introduced by Dr. W. R. Crawford, Australia)

Under the most propitious conditions there is naturally a limit beyond wh

underside of the cane blanket.

W, R. CRAWFORD 534

n bagasss and introduces it in a compacted form under pressure to the ing three-roller crushing unit. Reciprocating pusher feeders are also still in Australia-both the horizontal and vertical types, and often a combjnation h. Neither the two-roller feeder nor the pusher types have, however, so

een adopted in other countries.

The following questions naturally arise in regard to the use of feeding

to be gained by ihe installation of two-roller feeders he additional mechanical equipment employed ?

DISCUSSION

. Dr. Crawford explained that Australia had more experience than other countries in dealing with saturated bagasse, as a result of the maceration bath system employed. In these circumstances it is necessary to have forced-feeding of a mill in order to avoid slip. He considered that the self-feeding mill was restricted in surface speed, and therefore limited in capacity. The 2-roller feeder invented in Australia presented to the mill a compact thickness of feed of uniform density and exercised a steady pushing action. The 2-roller feeder was run 10% faster than the following mill. In particular cases this feeder had enabled 66" mills to crush up to 130 tons her hour.

Mr. F. L. Stewart asked for information on the power requirement and cost of the Zroller feeder as compared with a 3-roller mill.

Mr. W. M. Livie replied that for a 72" mill 50 B. H. P. was required, and the installation normally provided an increase in crushing rate of around 12%, and lowered the moisture content of the bagasse emerging from the mill.

Mr. J. P. Mukherji enquired about the increased centre distances necessary between mills for accommodating 2-roller feeders.

Dr. Crawford replied that the centre distance required was around 29 ft. Chairman's Conclusion. With the higher crushing rates now required the

discussion showed that some form of mill-feeding device was necessary, and new mills were now normally provided with such equipment. He described the application of the single-roller feeder placed below the apron delivery and of a slat-type intermediate carrier, which was now becoming popular, and could be readily installed on existing mills.

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(d) INTERMEDIATE CARRIERS

(Introduced by Mr. 0. M. Henzell, British wedt Indies)

Intermediate carriers of two types have been in general use-t,he slat aqd the scraper types. At different periods one or other type has gained favour, and at the present time the slat type is in more general use.

Each type possesses certain advantages over the other. The slat-type presents a continuous bagasse blanket to the mill, and the action ~f the slats at the delivery end of the carrier assists in the feeding of the mill. On the other hand, this type is liable to be dirty and to allow excess imbibition liquid to drain through below. I t is also not so readily cleaned and sterilised as the scraper-type, Replacement costs of slats and chains are high due to rapid wear and corrosion resulting from the action of juice and grit.

Efforts to overcome the maintenance problem of the slat-type are constantly being made, some of these being the provision of hardened bushes in the chains, stainless steel link pins, and galvanised slats. A recent development entirely discards the metallic slat and chain apron, and replaces this by an endless rubber belt. A large new milling plant recently installed in Cuba has been fitted with rubber belt intermediate carriers.

Having regard to the contribution which intermediate carriers make towards I I the satisfactory operation of a milling plant, the following questions arise

in connection with modern practice :-

(a) Does the slat-tfpe meet present-day requirements better than the scraper type ?

(b) On account of the facility now offered by the use of steam turbine drive to shorten the distance between mills, is it desirable to introduce the shortest possible intermediate carrier, or will this be detrimental to imbibition efficiency ?

(c) What are the results of experience in the use of rubber belt intermediate carriers and are there any other possible types of apron suitable as substitutes for the metallic slat and chain arrangement ?

DISCUSSION

Mr. Henzell said that for high' tonnage rates on mills the slat type of carrier was essential, and that the drag type only had an application on small mills with short centre distances. He preferred relatively short carriers, and considered that,, with cane properly prepared by knives, satisfactory imbibition efficiency 'could be obtained without employing long and complicated carrier arrangements:

He emphasised that all current forms af intermediate carriers were unsatisfactory, and that there was ample scope for improvement, The rubber . . belt apron gave of being a valuable dqvelopment.

C. Me NICOL 539

r. W. R. Crawford considered that for the best milling results the feed should be continuous. The scraper type carrier broke the feed into sections

ere unsatisfactory.

. McNicol stated that Meniche carriers were designed for a particular ing rate, but when operators appreciably exceeded these rates trouble

Mr. G. H. Jenkins said that in Australia the wide spacing of mills did not the use of Meniche carriers. He also considered that this type did not

allow of efficient imbibition.

, to the Australian view that lengths up to even 90 ft. are justified by the lting improvement in extraction.

(e) JUICE STRAINERS (Introduced by Mr. C. McNicol, India)

For a long number of years the conventional cush-cush strainer and elevator remained standard equipment on all milling plants. This consisted of stationary '

straining screen to which the expressed juices from the individual mills flowed ough gutters. Scrapers attached to endless chains driven from the mill removed cush-cush from the screens, and elevated this to a cross conveyor which

This form of strainer is now being largely superseded by other types for straining the extracted juice passing to process, and by the pumping of imbibition juice directly back on to the mills in its unstrained state by means of chokeless pumps. Strainers for the process juice are either of'the rotary screen type, or (the mechanical or magnetic vibrating types. They may be located in any convenient situation either over the mill or on a platform alongside.

It appears that the conventional combined cush-cush strainer and elevator is destined to be displaced entirely by these newer straining and pumping arrangements. In the transition period opinions are being formed on the merits of the various new types of equipment, and delegates are asked to give the results of their experiences, and to discuss trends of further development.

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DISCUSSION . Mr. McNicol stated that for high crushing rates no; employed the s

and chain type cush-cush strainer and elevator was an unsatisfactory arran He pointed out that rotating or vibrating types ?f strainer were more and occupied less space, apart from being much more efficient in straining a

Mr. J.P. Mukherji stated that 10 years ago he had discarded the convent cush-cush strainer and replaced this with a vibrating type with beneficial re With the old type of strainer there was a purity drop between primary and mill juice of 3.5, but this had been reduced to 1.5 with thenew arrangements.

I Chairman's Conclusion. It would appear from comments that the forme conventional type of strainer was now being largely discarded. The opinion been expressed that more attention should be paid by manufacturers to particular application of the new types of strainer to the straining of cane jui as a number of problems still existed.

(f) MILL ROLLER BEARINGS

(Introduced by Dr. Langen, West Germany) The use of journal type bearings for the rollers of sugarcane mills

the present been almost universal. There have been occasional applications roller bearings in the past-one noteworthy instance occuring in India 25 ye ago when two 14-roller 18" x 36" plants were designed and manufactured b Calcutta engineering firm for Indian factories. There has, however, been general adoption of roller bearings in any country. In the past few years, howev greater attention has been directed to the subject, and certain machinery fi are now manufacturing mills with the crushing rollers carried in roller bearings, some mills so fitted are now being erected in India.

The main advantage claimed for the use of roller bearings is the reducti in friction and corresponding lower horse-power demand for driving the mi On the other hand, the large-diameter roller bearings required are apprecia more expensive than the equivalent journal bearings. Furthermore, roller bearin require to be carefully lubricated and maintained, as otherwise deterioration w result and be accompanied by heavy replacement costs.

The main problem of applying roller bearings to mills is to exclude entrance of juice, so as to prevent the consequent corrosion and damage to finely finished surface of the rollers and races. Forms of sealing device have n been developed whereby the bearings can, with reasonable assurance, safeguarded against risk of juice entry. Here again, however, the seals m be maintained in a satisfactory state if they are properly to perform their functi and it is obvious that with roller bearings a much stricter standard of maintena must be enforced than with the journal type.

The following are some questions which arise in connection with application of roller bearings.

(a) What is the proportional saving in power effected by the use of ro

J. P. MUKHBRJI 541

(b) Are the sealing arrangements for preventing entry of juice into the roller bearihgs absolutely effective ?

DISCUSSION

Dr. Langen desc~ibed the application of roller bearings to heavy machinery and the recent dev'elopment in the case of sugar cane mills. He stated that, as the relative outside diameter was larger than in the case of journal bearings,it was not possible to fit roller bearings into existing headstocks. A new design of headstock had therefore been involved. He stated that there was a saving in power of 20125% by the use of roller bearings and that some of the advantages

' to be obtained were the absence of a cooling water system, no lubrication attention necessary during operation, improved cleanliness of the mill, and lower maintenance costs. If properly maintained he considered that the life of roller bearings on sugar mills should be at leasti 10 years.

Mr. W. M. Livie asked how many years had roller bearings been in service on any particular mill.

Dr. Langen replied that experience covered 4 years. Dr. W. R. Crawford asked what was the maximum size of mill on which

roller bearings had been fitted. Dr. Langen replied that the largest size so far was 32" x 6 0 . Mr. H. E. B. Scriven considered that during the off season the static condition

of the bearings might result in corrugated etching of the races by the rollers. Mr. Atma Singh considered that unless the sealing arrangements were

perfect, even a little juice entering into roller bearings would cause considerable damage. He also asked how long would it take to change a roller bearing should one break in service, particularly withlregard to the race having to be forced off the shaft.

Dr. Langen replied that about 2 hours was required for changing a bearing. Mr. J. R. Leclezio considered that the claim of 25% saving in power was *

unduly high as the major proportion of the power in a mill was demanded by the crushing of the cane, and was not due to bearing friction.

Chaivman's Conclusion. As a recent development the application of .. roller bearings to mills was obviously exciting considerable interest, and engineers

had many questions to ask regarding performance. It appeared a little early to judge the position and more experience would doubtless require to be gained. Opinion on the amount of power saved by the use'of roller bearings varied considerably and no authoritative information appeared to be available on this important aspect. Some experimental work was obviously necessary in order to establish comparative data on the difference in power effected by the use of roller bearings.

(g) HYDRAULIC PRESSURE SYSTEMS

(Introduced by Mr. J. P. Mukherji, India) I

a From the time hydraulic pressure regulation was first applied to the rollers of sugar cane mills seventy years ago, the weight loaded accumulator-with no

change in general design-has continued until recent years toebe the standard pressure arrangement employed.

Nearly 20 years ago the air-hydraulic accumulator was introduced in which air at a pressure of 200/300 p. s. i. g. is employed in an intensifier fo provide hydraulic pressures up to 4,500 p. s. i. g. for direct application to the top cap cylinders on mills. This type of accumulator represents a marked advance over the conventional pattern, as it dispensed with weights and can be placed conveniently in an &mated position close to the mills from which point pressure adjustment can be readily made.

More recently the gas-oil accumulator has been introduced for duty on sugar mills, this consisting of a steel pressure cylinder inside which gas at high-pressure is contained within a rubber bag. Due to its simplicity, small size, and absence of moving parts, this type of accumulator is now being extensively adopted on new mills, and is also replacing weight loaded accumulator on many existing plants.

While in the majority of cane-sugar countries hydraulic pressure regulation on mill top rollers is regarded as essential and is applied on all plants, an exception exists in the case of Australia where a large number of mills operate without pressure regulation of any type, the top rollers being fixed in position.

Questions which might be considered in regard to hydraulic pressure regulation are :-

(a) Is it necessary to,have the top roller of a mill " floating " on the bagasse blanket under the control of a uniform pressure, or is it preferable to operate with the top roller fixed in position thereby maintaining constant openings between rollers and trashplate ?

(b) Is the gas/oil rubber bag accumulator the most satisfactory form of pressure equipment so far evolved ? If so, is it likely that this may in turn be superseded by other improved types ?

DISCUSSION

Mr. J. P. Mukherji considered that the recent rapid development in the use of the gasloil accumulator on mills was evidence of its superiority over the "weight-loaded type. He stated, however, that some operators expected the accumulator to increase capacity and extraction, whereas these aspects depended equally on the maintenance of the hydraulic cylinders in g ~ o d condition and the proper setting of the mills to ensure even floating of the top rollers. He referred to the quick response of the gasloil accumulator relative to experienced with the weight-loaded type.

Mr. W. M. Livie spoke in favour of the Australian practice of fixed top rollers without hydraulic loading, He considered that hydraulic loading was only necessary in cases where more than one mill is driveri'by one engine. With

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J.P. MUKHERJI 543

erly adjusted settings and even feed he contended that the fixed top gave a r and would enable the moisture content in bagasse to be kept st 2% lower than when hydraulic pressure equipment was employed.

fitted on the top rollers bearings of mills, it has been found that the top s were not floating, but were in effect operating as fixed rollers. In such

airmarz's Conclusion. It was obvious that the newer forms of hydraulic

ler.