Suggestions for improved control in cane sugarfactories

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Suggestions for

IMPROVED CONTROL

IN CANE SUGARFACTORIES

PROEFSCHRIFT

Ter verkrijging van de graad van Doctor in de Technische Wetenschap aan de Technische Hogeschool te Delft, op gezag van de Rector Magnificus, Dr O. Bottema, Hoogleraar in de afdeling der Algemene Wetenschappen, voor een Commissie uit de Senaat te verdedigen op

Woensdag, 4 Juli 1951, des namiddags te 2 uur

door JAP KIE LING

Electrotechniscb Ingenieur Geboren te Djokjakarta

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Dit proefschrift is goedgekeurd door de promotor Prof. ir L, H, de Langen.

I

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E R R A T A . a

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2) page 107, first line, the formula must be read a s : ( 1 + ba n c ) - ^ 4 ^ = rba ( A Z - c n . A s ) .

c dt c

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/ <z/so gratefully remember the former and the present staffs of the

Research Institute of the Java Sugarindustry, especially Prof. Ir. E, C. von Pritzelwitz van der Horst and Dr. Ir. P. Honig for the perfect understanding that I always met with from them, and many instructive indications they have given me.

Finally I have to express my acknowledgement to Controller manu-facturing Companies and their representatives for all kinds of informa-tion they supplied me and to all those who assisted me in some way to write this booklet.

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FOREWORD

My interest for control originates from an intensive supervision of sugarfactories for about twenty years.

In cooperation with the Research Institute of the Java Sugarindustry at Pasuruan^ much supervision work has been done in the grinding-, steamgenerating-, power-, evaporator- panhouses^ etc., and many valuable riata have been obtained. But however important these data may be, the main, point was and is how to evaluate them and how to react to them

In many cases the difficulties were recognizedas inadequate reactions to fluctuating operating conditions, As an example I mention the vari-ous repercussions in a sugar factory to a shortage of steam, well-known to every sugar technologist,

As far as certain reactions could be expressed in definite rules, the need for an automatic intervention, a control, arose,

After a rest to recover from a long, tiring and delic ate u/ork during World War II and the Revolution at Java, I tried to concentrate on a purely technical scientific subject for the soothing of my mind.

I present the result as a modest contribution to the development of sugar technology.

I owe everything of my general and professional education to my Parents and my Teachers

I have to thank my Wife, my Brother and my Friends for their encouragement and assistance to write this thesis.

1 thank the Owners of the Oei Tiong Ham- Concern for being entrusted with the supervision of their sugarfactories, and I much regret that the late President Director Mr. Oci Tjong Hauw cannot witness the performance of this study.

I also gratefulh remember the former and the present staffs of the Researcf- f««'-' of the Java Sugarindustr^' <^rpecially Prof. Ir, P Honig fo> , erfect understanding that I a, " s met vjith from them, and many instructive indications they have given me.

Finally I have to express my acknowledgement to Controller manu-facluring Companies and their re'^res/'si'"'' -. for all kinds of informa-tion they supplied me and to . ..Ao assisted me in some way to irrite this booklet

May it also prove that West and East can peacefully cooperate on the hast terms, forgetting nation and race,

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Modern sugarmill-engines and steamturbines have their Hartung- Proell- shaft- and oil-p r e s s u r e governors, in most c a s e s oil-p r o v i d e d with a d e v i c e fora c o n s i d e r a b l e s oil-p e e d - v a r i a t i o n . But many old engines have kept their simple Gardner-governors and according to L.A, Tromp (Machinery and Equipment of the Cane Sugarfactory 1936, page 239) ' t h e Porter centrifugal governor, although o b s o l e t e for other engine-types, is still favoured by several d e s i g n e r s for s u g a r m i l l - e n g i n e s . "

Even automatic voltage-regulation in t h e e l e c t r i c powerhouse i s n o t y e t in general p r a c t i c e in Java sugarfactories; in small and medium p l a n t s hand voltage-regulation i s not seldom seen.

In the stearaplant there are p r e s s u r e controllers for the supply of live steam to the back-steam, which should be maintained at a minimum value. There are p r e s s u r e reducing valves in the filterpress-house, in the vacuumpan-house, in the centrifuges-house; some are very simple, some are of an improved design. A few factories have Ruth steam-accumulators, which accumulate the e x c e s s steamproduction of the steamboilers a t t i m e s o f small steam-demand of the prime e n g i n e s , and supply steam to the medium- and lowpressure plants at times of high consumption, when the p r e s s u r e in them drops below a p r e s c r i b e d limit. A specific p r e s s u r e control is that of a mudpress-pump i t s speed i s reduced, if the filter-p r e s s filter-p r e s s u r e r i s e s , the sfilter-peed is i n c r e a s e d , if the filter-p r e s s u r e drofilter-ps.

Another specific control, e s p e c i a l l y developed in the sugarindustry, i s the adaptation of the lime supply to the rate of flow of the raw j u i c e in the continuous carbonation plant at the sugarfactory Redjoagung, designed by the J a v a Sugar R e s e a r c h Institute at Pasuruan. Modern filtersystems (Revolving Autofilters, Rotary Drum Vacuum F i l t e r s ) and modem e l e c t r i c a l l y driven centrifuges have equipments for effective operation control, but in Java their application i s very limited, sometimes does not p a s s the experimental s t a g e . Finally we meet a full control of operation in automatic weighing equipments (for sugar-c a n e , raw juisugar-ce, masugar-ceration water, filterpress sugar-c a k e , final m o l a s s e s ) and in automatisugar-c liijricating s y s t e m s of bearings and e n g i n e s .

This study i s an attempt to i n v e s t i g a t e the need, the conditions, the p r i n c i p l e s and the performances of effective operation control in general and to prej-ent some detailed p r o p o s a l s for s p e c i a l a p p l i c a t i o n s .

NEED FOR CONTROL.

T h e n e e d for control a r i s e s from many r e a s o n s : 1. Safety and Reliability.

No h i g h - s p e e d engine (steamturbine, oilengine) runs without a speedcontrol and an emergency d e v i c e (speed limitation). In some c a s e s a wrong order of manipulations may c a u s e a c c i d e n t s or a failure of the desired effect. A well planned control (Program control) p r a c t i c a l l y e l i m i n a t e s t h e s e kinds of risk.

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2 Demand of high precision and regulabilify. 3. Demand of a high and uniform quality.

4 Demand of a high efficiency, which often includes a high rate of operation 5 Lack of labourers.

CONDITIONS OF CONTROL.

1. T h e p r o c e s s must be sufficiently studied that all factors, affecting the controlled variable(s) are qualitatively and quantitatively understood.

2. T h e controlled variable(s) and the factors influencing them can be adequately ob-served and measured.

3. T h e control apparatus and the control element should react truely and in a sufficient-ly rapid manner to the change of the controlled variable(s); conversesufficient-ly the controlled variable(s) should react quickly enough to the position variations of the final control element, and remain undistorted.

4. T h e r e i s an engineering staff, appreciating control and able to take care of the main-tenance of controllers.

5. The advantage of the controlled operation should be large enough to pay the cost of control within a r e a s o n a b l e short time.

In this connection a large rating a n d / o r a high price of the machinery unit is favourable. Therefore continuous operating machinery with high r a t i n g s i s e s p e c i a l l y suitable to be provided with effective control equipments

During and after World Vtar II great progress h a s been made in the theoretical knowledge and the practical a p p l i c a t i o n s of regulators and controllers. Automatic controllers and servomechanisms have been extensively applied for military p u r p o s e s ( a i r p l a n e s , guns) and for p e a c e i n d u s t r i e s , food-papei^ pharmaceutical- petroleumindustries,airconditioning, industrial and laboratory measuring d e v i c e s , e t c .

We have to realize the importance of t h e s e achievements, and study whether and how we could apply them in the sugar-industry.

It is not the main object of t h i s paper to develop the control theory s y s t e m a t i c a l l y , but to study the technological a p p l i c a t i o n s . We refer those who are not so familiar with the control theory, to recently published excellent books amd p u b l i c a t i o n s , of which a few arc mentioned in the reference l i s t .

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C H A P T E R I I

S T U D Y O F T H E P O S S I B I L I T I E S O F C O N T R O L IN T H E M A N U F A C T U R I N G P R O C E S S

Let us consider the manufacturing s t a g e s in a cane sugarfactory. GRINDING HOUSE.

The cane enters the grinding house in canecars, from which it i s transferred to a cane-carrier. This carries the cane to canecutters (revolving knives), shredders or crushers. The cut or crushed cane i s then ground s u c c e s s i v e l y in a set of three or more grinding mills to extract the cane juice. From the cane raw juice and bagasse are obtained. The raw juice l e a v e s the grinding house and g o e s to the jufce purification house. The bagasse i s transported to the boilerhouse, where it i s burnt in specially designed furnaces to produce steam for the prime movers in the power house, the large engines in the factory and for the sugar^manufacture.

The cutting, shredding, crushing and grinding i s a conrinuous process, which should go on without interruprion, day and night. Its rate should concur with the rate of the sugar-manufacturing departments. If the cane contains much juice with high sugai^content, the grinding rat» should be reduced. With dry cane with low sugar-content the grinding rate should be increased. Sometimes the grinding rate must be reduced on account of difficult-i e s of the cane-cuttdifficult-ing (radifficult-ins, unrdifficult-ipe cane) or difficult-interruptdifficult-ion of the cane transportardifficult-ion.

All these changes in the grinding rate are generally met by regulating the speed of the whole cutting, shredding» crushing and grinding train.

B e s i d e s this simultaneous reguladon of the whole train, there i s a need of individual speed regulation, because the ratio of the speed of the different units in the train is strongly dependent on the properties of the cane. T h e s e properties are not only specific for different kinds of cane (2967 POJ, 3016 POJ) but they a l s o vary with the soil and growing conditions.

This adaptation of speed of every unit in the grinding train i s done at sight of the thick-n e s s of the b a g a s s e layer, which p a s s e s the crusher or the mill, with electrical drive confirmed by the indication of the kW-meter, The adjustment of the ^ e e d i s done by moving a governor weight, turning a handwheel or a handle, or by pushing a push-button. The efficiency of the juice extraction depends on this ability and readiness to make the proper adjustments.

ï ' e therefore conclude, that there i s a want of grinding control of the driving engines or electromotors of shredders, crushers and grinding mills.

The conditions to install a control equipment are generally favourable because: 1. the cost of the control equipment will soon be paid by a better juice extraction; 2. the price of the cutring, shredding, crushing and grinding equipments i s highenough

to motivate an effecrive control;

3. the number of grinding s e t s in the grinding house i s limited, many factories have only one or two grinding s e t s , a few have three s e t s .

The old-fashioned steam-engines equipped with Meyer cut-off valves and Gardner or Porter governors, however, might prove to be too i n s e n s i d v e to react upon an automaric grinding control.

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THE STEAMBOILER HOUSE.

T h e steam generation i s a continuous p r o c e s s , the production varies according to the demand in the factory, where some large periodically operating a p p a r a t u s e s (vacuum p a n s )

strongly affect the steam demand, (references 1 and 2)

In principle the boiler p l a n t should be suitable to be fully automatically controlled, how-ever, there are some c o u n t e r a c t i n g factors.

There are two main problems in the boiler h o u s e , the combustion of the fuel and the transformation of h e a t into steam.

The second problem is common to all other steam generaring p l a n t s . Up till now there i s a preference of the older boiler types with large water contents and large heat accumula-tion to newer o n e s . For t h i s reason the generating rate of each unit i s rather limited in comparison with that of the large boilers of modern steam generating'plants.. Most sugar-factories in Java have about ten steamboilers. A large number of relatively small u n i t s would n e c e s s i t a t e the i n s t a l l a t i o n of a large numberof controller equipments, which would i n c r e a s e the e x p e n s e s . In Cuba and Hawaii, where larger u n i t s are i n s t a l l e d , the condit-ions for automatic control are more favourable.

The first problem however, that of fuel combustion, i s in our opinion more important. We have a s a fuel the b a g a s s e from the mills, contaning 45% to 52% water, which s h o u l d b e directly burnt in s p e c i a l l y designed furnaces. Predrying is not applied, but drying i s per-formed in the furnace itself, by a small auxiliary grate (offervuurtje) or by the h e a t radiat-ion of the flames. The result of t h i s combinatradiat-ion of two functradiat-ions in the b a g a s s e furnace i s , that the combustion i s strongly affected by the quality of the b a g a s s e , e s p e c i a l l y by i t s water content, though i t h a s been confirmed by p r a c t i c e that the fibre quality of different c a n e s o r t s sometimes more affects the combustion p r o p e r t i e s of the b a g a s s e . We know the s u c c e s s of full automatic control of the combustion in modem p l a n t s , in which the feed of coal or oil and the required combustion air are adapted to the steam demand, with a s p e c i a l correction dependent on v a r i a t i o n s of the fuel quality (See Guido Wtlnsch, Regier fur Druck und Menge, page 196/199).

But, for the r e a s o n s above mentioned, it s e e m s to u s , that the combustion of b a g a s s e i s not yet sufficiently mastered to apply full automatic control. Automatic control cannot be s u c c e s s f u l before the p l a n t can smoothly respond to a hand operated control. After the combustion will be better mastered - p e r h a p s by a s e p a r a t e predrying of the b a g a s s e , a powdering and combustion in cyclone burners - and larger boiler units are i n s t a l l e d , three or five for the whole factory, the automatic control of the boiler plant might be recon-sidered.

Of course some degree of control can be applied. Generally there are some p r e s s u r e re-ducing v a l v e s between the steamline s y s t e m s of different p r e s s u r e , a s the boilers for the powerplant and the mill e n g i n e s gjenerate steam of higher p r e s s u r e than those for the steamsupply to h e a t e r s , evaporators and vacuum p a n s . Some p l a n t s have automatic water-feed.

Finally for a truly r e l i a b l e balance of»the fuel u t i l i z a t i o n , the automatic weighing of the combusted b a g a s s e i s n e c e s s a r y .

THE JUICE PURIFICATION HOUSE.

T h e raw juice from the grinding m i l l s i s weighed in the i n t e r e s t of a r e l i a b l e sugar b a l a n c e of the manufacture. Automaric juice weighing i s now in common p r a c t i c e in J a v a . T h e raw juice i s then h e a t e d in a continuous p r o c e s s to a temperature which i s dependent on the method of j u i c e purification. T h i s heating can be e a s i l y automatically controlled with some heater controller of well known controller manufacturers (Taylor, Foxboro, Askania, Elliott, Sauter, e t c . ) .

In J a v a three main methods of purification are applied: defecation for the manufacture of raw or brown sugar (HS); s u l p h i t a u o n and carbonation for the manufacture of white su;;ar (SHS).

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treatment with lime. The mixing of j u i c e and lime can be done periodically in a batch p r o c e s s or continuously. The limed juice then e n t e r s the s e t t l i n g t a n k s , where the c l e a r j u i c e i s separated from the mud. In J a v a a l a r g e number of s e t t l i n g tanks, operating p e r i o d i c a l l y , i s in common p r a c t i c e . T h e r e i s , however, a tendency for l a r g e continuously working clarifiers (Etorr) which h a v e p t o v e d their merits in other sugar producing c o u n t r i e s . For a continuous mixing of juice and lime the automaric control i s not difficult. T h e flow of lime added may be r e g u l a t e d in ratio with the j u i c e flow. A correction can be made in the approximately s e t ratio according to the indication of a pH-meter.

T h e control of continuous clarifiers i s sdso not difficult with the modern means now available.

The control of a large number of defecators and s e t t l i n g t a n k s might prove to require too many equipments and too much c o s t .

The mud, after b e i n g - r e h e a t e d to about 100 d e g r e e s C, i s s e n t to f i l t e t p r e s s e s , where i t i s separated into a clear filter juice and into filtercake.

The filtercake, after being weighed and sampled for i t s sugar content, is removed a s w a s t e , while the clear juice i s generally added to the clear juice from the settling t a n k s or from the c l a r i f i e r s .

In J a v a the u s e of Kroog f i l t e r p r e s s e s i s still the regular p r a c t i c e . This type requires a periodical operarion. T h i s fact and the large number of u n i t s make the introduction of fully automatic control l e s s a t t r a c t i v e .

According to L . A . T r o n ^ (Machinery and Equipment of the Cane sugarfactory, 1936, p a g e 398) rotary drum vacuum filters have been applied in s e v e r a l canegrowing c o u n t r i e s for defecarion scum filtration. With this filter type the mud can be handled in a continuous p r o c e s s , thus s u i t a b l e to be automatically controlled.

T h e r e i s a trend to return the mud from the clarifiers to the b a g a s s e layer between the grinding mills. In t h i s c a s e the flow of mud returned should be proportioned to the flow of b a g a s s e p a s s i n g the mill. T h i s can be done by applying a dosing mudpump, mechanic-ally coupled with the mill, to which the mud i s r e t u m e d .

In the sulphitarion p r o c e s s the j u i c e i s h e a t e d to about 75 d e g r e e s C, after which it i s treated with lime and S O - - g a s in sulphitarion t a n k s . Generally the p r o c e s s is p e r i o d i c a l , though the J a v a Sugar Research Insritute at P a s u r u a n made e x p e r i m e n t s with continuous s u l p h i t a t i o n . The treated j u i c e i s then reheated to about 90 d e g r e e s C and s e n t to s e t t l -ing t a n k s , where the clear j u i c e s e p a r a t e d from the mud.

The automatic control of the heating and reheating i s no problem.

T h e sulphitarion p r o c e s s -is more ready for automatic control in the continuous than in the periodical performance. The flows of lime and S 0 2 - g a s should be regulated according to the flow of the raw j u i c e (parameter-control). The ratio for the lime added can be kept constant; the flow rate of the S 0 2 - g a s can be regulated according to the indication of a pH-meter.

About the automaric control of the settling p r o c e s s and the filtration of the mud, we refer to our d i s c u s s i o n of the defecation p r o c e s s .

In the carbonation p r o c e s s the raw j u i c e , after being h e a t e d to about 55 d e g r e e s C, i s mixed with lime and C 0 2 r g a s . T h e treated juice i s not s e t t l e d in t a n k s , but i s entirely filtered through f i l t e r p r e s s e s .

The filtercake, after being weighed and sampled, i s carried off a s w a s t e .

T h e clear j u i c e i s again treated with COT-gas to a pH of about 8. 6, heated to about 75 d e g r e e s C, and filtered for the second time. The filtercake i s generally retumed to t h e first carbonated j u i c e , the clear second carbonated j u i c e i s treated with SO^-gas to a pH of about 6. 5, h e a t e d to 95 - 100 d e g r e e s C and sent to the multiple effect evaporators, to be concentrated to 60 - 70% brix.

For the suitability of automatic control of the h e a t i n g , the carbonation, the filtrarion and t h e sulphitation, we refer to our c o n c l u s i o n s about the a n a l o g o u s p r o c e s s e s of the defecarion and the sulphitarion purification.

Auxiliary departments are: 1. t h e preparation of milk of lime

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2, the burning of sulphur to SOo-gas

3 the burning of limestone to CaO and C02-gas.

All t h e s e operations are generally done continuously. It would not be difficult to control the preparation of milk of lime and the burning of sulphur. A difficult problem seems the control of the lime kiln, because we do not only want enough well burnt lime and enough C 0 2 - g a s , with high C02-content but the burning process in die kiln should also be maintained in the proper state

THE MULTIPLE E F F E C T EVAPORATOR.

The object i s to concentrate the varying flow of thin clear juice with varying density to a thick juice of constant prescribed density in a conrinuous process, at the highest p o s s i b l e rate.

The conrinuous operation, the limited number of s e t s (generally not more than two) and the high price of the evaporaring apparatus, extremely favour the fully automatic control. Up till now a fully automatic control i s not yet considered in Java pracrice. There are only a few trial applications to maintain aut )marically a favourable constant level of the juice in the calandria. (Baltus Boulogne, Pasuruan, patent Nr 24S>91, 25-6-1927/15-4-1931, N.V. Ned.Ind. Industrie, patent Nr 14346, 7-6-1924/15-8-1925, Emmen, De Rietsuiker-fabrieken op Java, 3d edirion, page 423/427).

About other canegrowing countries we meet a descriprion of a Full Automaric Juice Level Control in Tromp's book, page 422/423. There i s a publication in 'Taylor Technology* vol. 2 Winter 1949 Nr 3 about a complete control of mulriple effect evaporators by J.G. Ziegler, and there i s a publicarion of A.C. Camp in Sugar, June 1950.

We shall d i s c u s s these systems in detail, after which we shall present a new proposal, for which a patent application i s made by Dr.Ir. P. Honig and author.

To the evaporation belongs the condensing plant, which i s generally of the jet system. Although this plant generally functions properly, the regularion of the rate'Zrr injected cooling water according to the temperature of the waste water seems desirable to save power.

THE VACUUM PANS.

The thick juice i s boiled in several s t a g e s in the vacuum pans to obtain a sriff mass, which contains sugar crystals and syrup (massecuite).

Although there-have been proposals to boil in a continuous process, the present practice i s still to boil periodically in the vacuum pans.

The price of vacuum pans i s high enough to morivate a full control, but the rather large number of pans and the complicated handling of so many kinds of liquors and syrups, which cannot yet be sufficiently standardized, interfere with the trend of a perfect control Indeed controller manufacturers have developed systems for vacuum pan control (e.g. Foxboro), some of them based on the temperature, or on the absolute pressure. We may also base this control on the conducuvity of the mass (Java Sugar Research Insritute) or on the heat transmission coefficient (Uetmar Jansen) or on the viscosity of the mass.

but these feamres are nodirect physical or chemical measure of thetendency to crystallize of the oversaturated sugar solurions.

For the rime being we shall have to content o u r s e l v e s with a parrial, gradually progress-ing control of the vacuum pan operation.

THE CRYSTALLIZERS.

The different kinds of massecuite (generally four) are cooled in crystallizers, before entering the centrifuges, which separate the crystals from the mother syrup.

It i s the aim thai In the crystallizers the sugar c r y s t a l l i z e s from the syrup to the exisring crystals, according to the progressing drop of temperature, in order to increase the re-covery of sugar from the massecuite.

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The ordinary p r a c t i c e i s the crystallization in a batch operation. The contents of one vacuum pan are entirely dropped into one c r y s t a l l i z e r .

The Werkspoor rapid cooling c r y s t a l l i z e r o p e n s the p o s s i b i l i t y of a continuous lization. It i s the purpose to regulate t h e cooling according to the velocity of crystal-lization, a s the cooling would be u s e l e s s if the only effect i s a more supersaturated mother syrup without a gain in the sugar c r y s t a l s ,

At p r e s e n t , however, we have no quickly indicating, fully reliable meter of the super-s a t u r a d o n , super-sothat the b a super-s i super-s of a control i super-s not yet a v a i l a b l e . P e r h a p super-s the electrical conductivity or the heat transmission coefficient can be u s e d for the time being.

THE SUGAR C E N T R I F U G E S .

In the sugar centrifuges the m a s s e c u i t e i s separated into sugar c r y s t a l s and syrup. For each kind of m a s s e c u i t e there i s usually a special groop of

centrifuges-The centrifugal p r o c e s s i s discontinuous and performed in a great number of u n i t s . centrifuges-There are t e n d e n c i e s to i n c r e a s e the capacity per unit and to approach a continuous p r o c e s s : self - dischargement, automatic electric command according to a time schedule (program control) e t c . , but the continuous operating centrifuge i s still remote.

The automatic command according to a time s c h e d u l e p r e s u m e s uniform proporties of the m a s s e c u i t e . T h e s e , however, cannot be realized in all c a s e s , on account of the variation of quantity and quality of the nonsugars in the j u i c e , even after the purification A more reliable b a s i s of control would be the syrup content of the sugar in the centrifugal basket, but at this moment we have no quick indicating equipment to measure this syrup content. Finally we come to

THE SUGAR DRYERS AND THE SUGAR SIEVES.

T h e s e equipments operate continuously, generally controlled by a thermometer for the hot air and by a speedometer for the revolutions of the dryer drum.

As far a s the a p p a r a t u s e s are well designed with the required overcapacity, they operate satisfactorily, h e n c e the need for an improved control seems to be l i t t l e .

In our general description of sugarmanufacture we have indicated the p l a c e s where an effective control i s d e s i r a b l e .

We shall now make some general remarks about automatic control itself, after which we shall p r e s e n t detailed p r o p o s a l s for a complete control of some important operations in a cane sugarfactory.

In h i s book: " P r i n c i p l e s of Industrial P r o c e s s Control", Donald P . Eckman stated: "An automatic control can be defined a s the maintenance of a balanced s t a t e in a p r o c e s s by measuring one of the conditions representing the balance and providing an automatic counteraction to any change in the condition- The balance in the p r o c e s s may be any form of balance of energy, very often heat or p r e s s u r e , "

His conception i s i l l u s t r a t e d by the following scheme:

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C. Dellaert (De Ingenieur Nr 35, 2-9-1949 W 63) wrote about the same subject in h i s summary about Automatic Control:

"The main feature of any automatic control i s a c l o s e d cycle of regulating with four e s s e n t i a l p a r t s : the i n s t a l l a t i o n , the observing organ, a transmission, and the correcting o r g a n . '

T h i s definition of automatic control seems to be generally accepted nowadays. L e t u s call this principle A, the c l o s e d loop system.

However, there was another principle applied in earlier d a y s : if the c a u s e of the variation. i s recognized, we can u s e it to compensate the change of the controlled s t a t e . This principle i s applied in the cumulatively compounded direct current shunt d y n a m o . In an ordinary shunt dynamo the voltage drops, when the dynamo i s loaded, b e c a u s e there i s a l o s s of voltage in the Ohm-resistance of the armature, and b e c a u s e t h e armature current h a s a demagnetizing effect on the field of the shunt wound p o l e s . In a cumulatively compounded dynamo the armature current i s u s e d to i n c r e a s e the shunt w i n d i n g s of the poles. We shall call this p r i n c i p l e B, the open loop system.

Let us now compare principle A with principle B.

Suppose, that there i s no appreciable lag between the change in the p r o c e s s and the indication of the measuring instrument, that the transmission of the impulse through the controller to the final control element, and the r e s p o n s e of the state in the p r o c e s s to the adjustment of the final control element are fast enough, then principle A can keep a smaller deviation from the desired s t a t e .

The quick Brown Boveri & Cie. voltage-regulation, and the Tirrill voltage>[egulation belong to control-principle A.

If, however, there i s a considerable lag in the system, control-principle B seems to be preferable, though this method h a s other weak p o i n t s . As generally there i s no linear relation between the changing c a u s e amd the controlled s t a t e , the compensation cannot be perfect over the whole regulating range. In t h i s system there i s no e l e m e n t that r e s -ponds to the difference between the desired and the actual s t a t e , the compensation t a k e s p l a c e according to a predetermined relation. It depends entirely on the correct calibration. In inany c a s e s , however, a combination of the two p r i n c i p l e s i s p o s s i b l e . Principle B i s applied to make an approximate adjustment, while principle A i s added a s a more accurate correction For i n s t a n c e : such a combined control i s described by Guido WBnsch in h i s book: "Kegler fur Druck und Menge", when writing on the automatic control of a coalpow-der boilerfurnace. WUnsch wrote:"Figure 187 shows a steamboiler with coalpowcoalpow-der com-bustion for which the proportion coalpowder: comcom-bustion air i s governed by a ratio-con-troller. Variation of the kind, the moisture-content, the fineness of the powder, e t c . chan-g e s the quantity of required combustion-air. The adjustment i s done by t h e indication of the C02-meter."

The statement somewhat differs from that of Prof. C.S. Brown and Prof. D . P . Campbell ( P r i n c i p l e s of Servotnechanism, page 3 and 4) who considered the c l o s e d loop (A) and the open loop (B) separately and concluded that the closed loop system is superior to the open loop control. Their book therefore only t r e a t s c l o s e d loop or error-sensitive control .systems and specially that c l a s s t h a t are linearly and continuously error-sensitive. In the following schemes we shall propose to u s e the combination of both p r i n c i p l e s a s far a s wc think it more effective for the object in view.

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C H A P T E R I I I

C O N T R O L O F T H E G R I N D I N G O P E R A T I O N

We start with the sugarcane on the canecarrier, which conveys the c a n e s , unloaded from the c a n e c a r s to a crusher or to c a n e c u t t e r s . It i s most important that this carrier i s regularly full loaded to maintain a c o n s t a n t feed of the crushing or c u t t i n g . T h e carrier i s

Vo <2 Ccin-e. carrier dr/v^

driven at the top end. If it i s driven by a chain from the crusher, the velocity is rigidly proportional to that of the crusher. A clutch-couplingpenables an "on and off" regulating of the feed, while the crusher i s rotating continuously. With this system it is difficult to e q u a l i z e an irregular feed of the canecarrier, and tiresome for the operator.

In newer performances the canecarrier is provided with an individual engine- or electro-motor-drive with adjustable speed. T h e object i s :

1. to adjust an average q5eed according to the average operating rate of grinding; 2. to e q u a l i z e i n e v i t a b l e irregularities of the canefeed.

For an efficient operating it i s d e s i r a b l e that the speed of the carrier i s adjusted accord-ing to the load of the fed crusher or the fed c a n e c u t t e r s . That m e a n s , if the crushaccord-ingor the canecutting requires a power different from a predetermined value, the s p e e d of the carrier i s adjusted, sothat the speed i s i n c r e a s e d at a low load and d e c r e a s e d at a high ioad.

I. H e n c e the first condition for an effective grinding i s the speed regulation of the c a n e carrier according to t h e load of the crusher or t h e c a n e c u t t e r s , which it f e e d s .

T h e mode of speed control will be d i s c u s s e d further on.

The cruslied or cut c a n e s must be ground in three-roller mills, of which there are 3 to 6 s e t s . ITie three-roller mill c o n s i s t s of a toproller a, a front- or feedroUer b and a

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or dischargeroller c (fig. 3) ITie crushed or cut c a n e p i e c e s are fed to the c l e a r a n c e be-tween the toproller and the feedroller, where they are p r e s s e d and yield part of their j u i c e . They are then conveyed over a trashplate d to the backopening between toproller a and backroller c, where they are p r e s s e d for the second time. The object i s that after t h e two p r e s s i n g s the c a n e p i e c e s l e a v e the discharge-opening, containing a s l i t t l e j u i c e a s possible'. The p r e s s e d c a n e p i e c e s are called b a g a s s e .

In the older types of mills the rollers are fixed in the housing, s o t h a t ( apart from the e l a s t i c i t y of the caps, the bolts, etc.) the feed- and the discharge openings are invariable. In newer types of mills the toproller i s provided with hydraulic rams, which enable the toproller to move up and down according to the quantity of b a g a s s e p a s s i n g . T h e two mill openings are variable in order to have a constant p r e s s u r e on the b a g a s s e at different t h i c k n e s s e s of the layer However, there are also minimum openings of the feed and the d i s c h a r g e ; hence, if the b a g a s s e - l a y e r is too thin, the p r e s s u r e on it becomes insufficient to effect a good j u i c e - e x t r a c t i o n .

T h e millopenings are c a l c u l a t e d for a predetermined average grinding rate according to some empirical method, in which the fibrecontent of the c a n e , the revolution of each canemill, the eventual average travel of the toproller, are taken into account. Of course the a s s u m p t i o n s are not fully realized in p r a c t i c e . So after grinding h a s starred the open-ings are readjusted experimentally; the speed of the mills may also require r e a d j u s t m e n t s . Once the openings are determined, the rate of the grinding can only vary by changing the speed of the mills. T h e s e s p e e d v a r i a t i o n s are required due to the irregular grinding quali-ty of the cane, to the daily rate of cutting c a n e s , to the transportation of c a n e s , to the r a t e of the sugarmanufacturing p r o c e s s , e t c .

The canemills are driven by steamengines or by electromotors with speedregulation. In Cuba the combined drive of two or more mills i s in com.mon p r a c t i c e , but in J a v a the in-dividual drive of e a c h mill i s preferred for i t s g r e a t e r p o s s i b i l i t y to adjust the roller-s p e e d roller-s to the b e roller-s t p r e roller-s roller-s i n g performance of each mill.

Mill-steamengines are provided with speedgovernors of different types centrifugal weight-and springloaded governors, iBertia-governors, oilpressure-governors, which have t h e character of keeping a constant speed at different l o a d s , but contain an adjustment to

14

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vary the e n g i n e s p e e d by moving a weight, straining a govemorspiing or by regulating an o i l v a l v e . Many e n g i n e s h a v e a remote control with cord- or c h a i n - t r a n s m i s s i o n or an electrical pushbutton,

Electromorors can be of the three p h a s e induction t y p e , three phase-commutator type or direct-current type. They have a speedregulation according to their own type. Three p h a s e induction-morors with r e s i s t a n c e regulation in t h e rotorcircuit, though simple, c h e a p and r e l i a b l e , are uneconomical and u n s t a b l e in the speedregulation, b e c a u s e at a small load the speed a l w a y s approaches the synchronous velocity. The command of the speedregu-lation of electromotors i s generally carried out by remote control.

Up till now t h e speedregulation i s done by a foreman supervising the grinding-house. If h e s e e s that t h e feed-space before a crusher or a mill i s conrinuously insufficiently fill-ed, or if he s e e s that a crusher or a mill i s continuously overffill-ed, he c h a n g e s t h e s p e e d of the engine or the electromotor to make the n e c e s s a r y correction.

If the speed i s not properly adjusted a poor pxessing or an overstraining of the rollers and the driving machinery will be t h e r e s u l t . L e t u s consider t h e speed-torque-diagram

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of a s t e a m e n g i n e .

At different but fixed adjustments of the governor loading weight or spring the speed-torque c h a r a c t e r i s t i c s a r e represented by t h e full l i n e s a , . . . aiQ. At no-load a minimum torque i s required to overcome t h e friction at no-load. T h i s fricaon-torque i n c r e a s e s with higher s p e e d s , so the line I which c o n n e c t s the no-load-points i s somewhat inclined. T h e line m i n d i c a t e s t h e maximum torque of the engine which i s given by i t s dimensions and the maximum s t e a m p r e s s u r e , it i s a l s o somewhat i n c l i n e d due to steam-pressure drop at higher s p e e d s .

T h e required theorerical torque for different grinding r a t e s i s r e p r e s e n t e d by t h e dotted l i n e s b . . . b , . . With empty m i l l s t h e l o a d only c o n s i s t s of t h e friction in d i e bearings and the transmission g e a r s . At a certain rate of grinding the required torque i s l e s s at higher s p e e d s b e c a u s e the b a g a s s e - l a y e r b e c o m e s thinner, so t h e b-lines tend to a n e g a t i v e s l o p e towards t h e s p e e d - a x i s .

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The full grinding rate at maximum speed i s represented by point P . T h e r e should still be a certain margin between the required torque and the maximum torque, b e c a u s e the grinding rate i s only an average conception, it i s liable to a c o n t i n u o u s variation. At somewhat i n c r e a s e d feed the engine should not stop but overcome the temporarily higher load. P^ is left to the maximum torque-line. If now at the maximum speed the maximum grinding rate drops, say to % of full rafe, then the operation i s indicated by point P , The speed is only slightly higher, but the torque i s considerably lower. T h i s means that the mill i s grinding a thinner layer than t h é ^ o s t efficient t h i c k n e s s of the b a g a s s e layer; the juice-extraction h a s become l e s s effective.

We do not say that juice-extraction i s best 'with the thickest layer of b a g a s s e , b e c a u s e the opinions of expert m i l l e n g i n e e r s differ. In Cuba and Hawaii millengineers prefer thin-ner layers than in Java. A thin layer d e c r e a s e s p r e s s u r e but f a c i l i t a t e s juice-discharge;

a thick layer i n c r e a s e s the p r e s s u r e but interferes w5th juice-discharge. So t h e r e must e x i s t a certain t h i c k n e s s for a most favourable juice-extraction, and a deviation from this most favourable value at both s i d e s means a l e s s effective extraction.

2, We have thus found a second condition of effective grinding. It should be done at the most favourable t h i c k n e s s of the b a g a s s c - I a y e r and the adjustment of t h i s condition should be carried out by an automatic control.

Tlie impulse for this control can be:

1. T h e t h i c k n e s s of the b a g a s s e - l a y e r on the b a g a s s e - c a r r i e r

T h e t h i c k n e s s of the b a g a s s e - l a y e r on the carrier i s , however, only a very rough measure of the quantity (by weight) ground, b e c a u s e on the carrier the b a g a s s e i s expanded. De-pendent on the properties of the c a n e , the t h i c k n e s s of a given weight of b a g a s s e v a r i e s considerably. B e s i d e s , the t h i c k n e s s i s not uniform over the whole width of the b a g a s s e carrier. Finally the instrument to measure the t h i c k n e s s may interfere with the free p a s s -age of the b a g a s s e ,

2. the s t r e s s of some loaded part of the cane-mill or of the driving engine, 3. the torque transmitted from the driving engine or electromotor to the mill,

4. the mean s t e a m p r e s s u r e in the enginecylinder (Ir. L . P . de Stoppelaar Archief voor de Suikerindustrie 1940, page 161),

5. provided a constant steampressure, the p e r c e n t a g e of the admission of live steam to the enginecylinder.

T h i s impulse should, after an eventual amplification, command the ^ e e t ^ a d j u s t m e n t of the governor or of the electromotor. For this c a s e we recommend a closed loop system, b e c a u s e the p r o c e s s - l a g i s not large, and b e c a u s e there i s no fixed relation between the quantity of cane or b a g a s s e ground and the required speed of the mills. B e s i d e s , the continuous measurement of the quantity of cane ground i s not a simple problem.

T h e different modes of control will be d i s c u s s e d l a t e r in d e t a i l .

The extraction of juice i s not merely done by repeated p r e s s i n g s , but waterraacerarion i s applied to dilute the remaining juice in the b a g a s s e , i n c r e a s e the volume and facilitate the p r e s s i n g . The macerationwater i s generally added to the b a g a s s e before the l a s t mill. T h e extracted diluted juice i s transported before the precedent mill to macerate the b a g a s s e to be ground, e t c . until the diluted j u i c e arrives on t h e b a g a s s e before the second mill, from which the e x t r a c t e d j u i c e i s not retumed before the first mill, but d i r e c d y mixed withthe juice from the first mill. The water i s t h u s utilized in a multiple maceration. T h e importance of a uniform distribution of macerationwater and maceration j u i c e s i s generally recognized and p r a c t i s e d .

T h e addition of macerationwater proportional to the quantity of c a n e or t h e quantity of fibre ground i s also p r a c t i s e d . But the performance up till now i s rather p o o r . It i s true that the maceration w a t e r i s weighed hourly, but the quantity of cane ground i s not determin-ed hourly. The quantity of raw juice i s measurdetermin-ed hourly, but it i s affectdetermin-ed by the quantity of macerationwater itself. Also on account of the irregular j u i c e content of t h e c a n e , it i s unserviceable as a measure of the c a n e or fibre ground.

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T h e macerationwater i s generally taken from an elevated supply-tank. Even if we put an indicating watermeter in the waterline we have no control indication to adjust the proper flow of the macerationwater.

For t h i s reason we s u g g e s t the insertion of a dosing pump in the watersupply line, this dosing pump to be connected with the l a s t mill, sothat the quantity of macerationwater i s proportional to the number of revolutions of the l a s t mill. By t h i s arrangement we ef-fect that:

1st. the water addition i s automatically stopped, if t h e l a s t mill s t o p s ;

2nd. s i n c e the speed of the l a s t mill is adjusted to grind a constant t h i c k n e s s of the b a g a s s e , the water addition i s approximately proportional to the quantity of b a g a s s e ground. As t h i s b a g a s s e h a s been p r e s s e d i n t e n s e l y , We approach a proportionality widi t h e fibre ground.

3. We t h u s have obtained a third condition of effective grinding: Addition of maceration-water by a dosing pump, coupled with the l a s t mill.

As a dosing pump we can u s e some type of rotary displacement-pump (gear pump, Imo-pump. Mono-pump). It should be provided with an adjustment of the waterquantity trans-ported, s o t h a ( a percentage of 15% to 30% of macerationwater, calculated on the quantity of cane ground, can be supplied.

T h e proposed control belongs to the open loopr system, b e c a u s e it s e e m s simple to con-n e c t the quacon-ntity of c a con-n e or fibre groucon-nd to the con-number of revolutiocon-ns of the dosicon-ng pump. S P E E D REGULATION OF THE CANESUGAR DRIVE.

Speedregulation of the canecarrier drive.

On account of the r e a s o n s we have explained above we want to regulate the carrier speed according to the load of the crusher, shredder or c a n e c u t t e r s , which is fed by the ceirrier. The carrier i s driven by a steamengine or by an electromotor.

a. The carrier i s driven by a s t e a m e n g i n e .

Usually the driving steamengine h a s no speedgovernor, but i s only provided with a hand operated valve which i s handled by a workman. For our purpose a speedgovernor with an ample s p e e d v a n a t i o n (about 65% of the maximum speed) is required. T h i s speed adjust-ment i s usually performed by varying the strain of a loading governorspring.

The impulse of the speedregulation should come from a measuring d e v i c e , indicating the load of the crusher, shredder or c a n e c u t t e r s .

As an example we represent an adjustment of a governorspring by an electromotor, but of course a hydraulic or a pneumatic motor can a l s o be used. In order to prevent over-shooting we can insert an interrupter in the e l e c t r i c line to the electromotor, or a thermo-r e l a y . ( F i g . 5 and F i g . 6)

b . The carrier i s driven by an electromotor.

We have s t a t e d that the speed variation should be about 65% of the maximum speed. T h u s the application of a DC shuntmotor or a three p h a s e commutator motor i s preferable. As we know, the .speedregulation of a common three p h a s e inductionmotor i s not s t a b l e , a s at a low load, the speed t e n d s to r i s e to the synchronous speed.

T h e speedregulation of a DC shuntmotor i s done by a r e s i s t o r adjustment in the field circuit of the motor, the speed i s t h u s regulated by turning a gear or a handle of the s h u n t r e s i s t o r . T h i s rotation can again be done by an electric, pneumatic or hydraulic motor. ( F i g . 7 and F i g . 8)

T h e speed regulation of three p h a s e commutator motors i s effected by a displacement of b r u s h e s , which is usually performed by turning a handwheel. Thus the s p e e d regu-lation can be carried out by the same means a s the rotation of a r e s i s t o r gear.

We have to consider the c a s e , that the crusher or shredder or canecutter s t o p s , and the load indication i s small or zero, Then the carrier will be commanded to run with the high-est s p e e d , overfeeding the crusher, shredder or cutter. In order to prevent t h i s , the line of the carrier steamengine should be shunted behind the steamvalve of the steam-engine of the crusher, shredder or canecutter. If t h i s steam-engine i s stopped, the carrier steam-engine i s stopped at the same time. L i k e w i s e if an electromotor i s used, the cable of the

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motor should be taken behind the main switch of the crusher- shredder- or canecuttermotor. If for t r i a l s or other p u r p o s e s the carrier should run alone, the steam or electricity should be supplied by an auxiliary valve or switch. While using this auxiliary equipment, the automatic speedregulation i s switched out, it i s automatically switched in, when the auxiliary supply is cut off,

GRINDING CONTROL BY SPEEDVARIATION OF THE MILL. (Shredder, crusher, canecutter

l i g 9 represents a diagram of the grinding c h a r a c t e r i s t i c s and of the e n g i n e - or motor-c h a r a motor-c t e r i s t i motor-c s .

f 1 is the no-load torque of the engine or motor alone,

f2 i s the no-load torque of the engine or motor together with the intermediate gears and mill (crusher, shredder or canecutter) a s a function of the revolutions n

We a s s u m e that the theoretical torque T - to work the c a n e or b a g a s s e i s proportional to the t h i c k n e s s of the layer. For a given rate of the canefeed, the t h i c k n e s s is inversely proportional to the grinding speed n, thus the theoretical torque T . as a function of n i s a hyperbola. By adding f, and T . we get the total required grinding torque T we get T j a s i function of the grinding speed n. We call t h e s e the grinding c h a r a c t e r i s t i c s

( b j , b2 ... h^l . Let P represent a certain grinding s t a t e . The engine- or motorcharacteristic (at a

con-stant speed adjustment) i s a'^, the grinding torque is T^, the t h e o r e t i c a l grinding; torque i s T P , which i s proportional to the t h i c k n e s s of the layer ground.

Now we suppose that the feed rate c h a n g e s to a new s t a t e . If the speed adjustment is the same, the grinding s t a t e i s represented by, Q (S), Tlie speed n i s not appreciably changed, but the grinding torque h a s become T S and T ^ ( T . and Ti"). T h i s m e a n s t h a t the layer ground h a s changed accordingly. If in the grinding s t a t e P the t h i c k n e s s was the most favourable one for an intensive extraction (crushing, cutting) the new condition d e v i a t e s from it. In order to maintain the fomier favourable t h i c k n e s s , w e must readjust the speed of the driving engine or driving motor, sothar the new s t a t e U (V) i s obtained,

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Ki^t^ott nut

The new engine- or motorcharacteristic will become a (a ). For this grinding control we need:

a. an adequate measuring device for the torque T • or T , b. a transmitting d e v i c e , the controller,

c. a final control element, in this c a s e the governor a. Measuring d e v i c e .

1. The most s u i t a b l e method would be the measurement of the s t r e s s in the roller- or canecutter shaft. But if the s t r e s s m e a s u r e -ment of the revolving shaft i s too complicat-ed or too i n a c c u r a t e , we can also measure the s t r e s s in some loaded stationary part of the mill (crusher, shredder, canecutter) for i n s t a n c e o f the kingbolts or of i t s nuts.t( F i g . 10) A modem method of s t r e s s measurement i s that with e l e c t r i c a l strain g a u g e s . Werefi-r to P h i l i p s Technical Information MA 3 for a brief description of t h i s method.

According to t h i s communication, not only elongations can be measured, but a l s o

con-t r a c con-t i o n s of con-the loaded macon-terial, con-the dimensions of con-the paper r e s i s con-t a n c e slips are 51 x 16 mm^ for 600 Ohm, and 30 x 8 mm^ for 120 Ohm r e s i s t a n c e . Undoubtedly t h i s method can • be developed as an impulse element for a controller.

2. If the l o s s e s are neglected, t h e torque transmitted by an electromotor can be approximate-k W

ly measured a s the quotient —r ,—r r r-. The kW-indication c a n be converted ' ^ n ( r e v o l u t i o n s / s e c )

into an e l e c t r i c current e.g. by a "Siemens Ringrohr', that i s a r i n g t u b e , containing a r e s i s t a n c e of a platinum alloy and mercury; dependent on the ringposidon the r e s i s t a n c e i s partly short circuited.

T h e tachometer can be equipped with a "Siemens Ringrohr', to co n v e r t i t s indication a l s o into an e l e c t r i c current. ( E u c k e n - J a c o b , Der Chemie Ingenieur Vol. II F i g . 81 ) The quotient of the two currents can be determined with some type of quotient meter, e.g. the Bruger crossedcoil meter for DC or an elec trodynamicaldoublecoil instrument ( E u c k e n - J a c o b Vol.11, p a g e 54-58) for AC.

If the motor is a DC shuntmotor, the torque i s the product of the armature current I " i d the magnetic induction B. If the field core i s not saturated, B is proportional to the fi id current i , and therefore also to the voltage a c r o s s the field winding = i r . Hence the torque can be measured

as a product of 1 x ( ' ^ r ^ ) with an electrodynamical instrument.

With an electromotor drive we can also u s e rtie kW-in-dication a s the primary im-p u l s e for the controller, but the setpoint i s automati-cally adjusted by the indi-cation of the speedometer, sothat the s e t kW-indication i s varied in ratio with the number of revolutions. We t h u s use a r e v o l u t i o n / k W ratio controller, which i s a standard performance of controller manufacturers

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( e . g . at 2 r e v o l u t i o n s / m i n . of t h e mill 150 k>..yfor a 34" x 7 8 " mill,). ' mill.)

If the speedvariation of the electromotor i s limited, (in p r a c t i c e 25% is sufficient) we can also omit the setpoint adjustment, t h u s admitting a certain deviation of t h e torque from the most favourable value

With a steamengine drive the problem i s more difficult, b e c a u s e up till now there is no instrument for a direct indication of the "power or the torque developed,

If p i s the i n s t a n t a n e o u s value of the p r e s s u r e in the steamcylinder, S = stroke, p, = mean b a c k p r e s s u r e , the mean working s t e a m p r e s s u r e p = -^ J ( p - P u ) dS or p S = / ( P - P b )

dS-The mean torque i s T = a.S.p = a j ( p - p . ) dS

The power P = n T , wnere n = numberof r e v o l u t i o n s / s e c . _ We could design a device to measure T = -f ƒ( p - p , ) dtt ( T f T ).

According to an indication of Prof Ir, L . H . de Langen, we can c a l c u l a t e for a sufficient number of theoretical diagrams the v a l u e s of T and T , and so determine their ratio Vie then obtain approximately the transformation of T into T , By applying a lever with variable fulcrum, we can carry out this transformation. The form of the supporting surface

can be drawn a s the envelope of different lever p o s i t i o n s , ( F i g . 12) .•

•a o <»

i2

tOySéeam cyZindcr

to Jia.ckjates.su.rt.

Fi^/Z Control o/

o. sieetmem^i-n^ iy

the Theetrt cylineéer

<Sieei

mpres.s*//t-We could a l s o consider:

= J /(p-p, ) dS = i /(p-p. )

dS dt

We could convert ( p - p , ) into an electrical current, e.g. by moving a variable r e s i s t a n c e . We could also convert -y-' into a voltage by applying the induction law in some form. By using an electrodynamical meter with a large moment of inertia, we could obtain;

T ^ ^ P - P b ) ^ ' l S d t = „ o r / ( p - p ^ ) . ^ § - d t = t 1 p . S

S o p = - ^ t . « o r ' « = -^

*^m S t

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As f = /3n, have = p . S. n. p

Hence " would indicate the power of the engine, which could be u s e d for the grinding control just a s in the c a s e of the e l e c t r i c a l drive.

But unfortunately such a device h a s not been r e a l i z e d .

For the p r a c t i c a l application it seems e a s i e r to u s e other impulse e x c i t e r s .

If the steamengine h a s a throttle-governing device with constant % admission, we can u s e the steampressure behind the throttling valve a s a measure of the t o r q u e developed by the engine, to regulate the engine speed.

If the engine h a s an automatic cut-off governor, we can u s e the % of the steamadmission as a measure of the torque of the engine. With a spring governor, the position of the go-vernor rod to the steam slide valve determines the % of admission, it can thus be used to operate e l e c t r i c a l contactors or the flapper of a pneumatic controller.

If we have a shaft governor, we must derive the impulse from the position of the a d m i s -sion e c c e n t r i c .

With longer admission the eccentricity i n c r e a s e s , which can be transformed into the stroke of a balancing lever.

In Fig. 13 a small stroke means a large average r e s i s t a n c e of the e l e c t r i c circuit, the bimetallic element i s l e s s heated. A long stroke gives a small a v e r a g e r e s i s t a n c e , the

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nozzle.

finei^nteciic

nozzle

cylindor (-on troL óy

ih« jieamadmission

bimetallic element is hotter. The movement of the bimetallic element can be used to o p e r a t e t h e flapper for a nozzle of a pneumatic controller

In F i g . 14 the lever i s connected with a weak spring and a strong damping cylinder in order to transform the balancing movement into a steadily floating one. The end of the lever can carry the flapper for t h e n o z z l e of a pneumatic controller.

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If we have an o i l p r e s s u r e governor, the admission is determined by t h e piston p of an operating cylinder, it can be used to move a pilot valve which r e g u l a t e s supply to an oilcylinder, adjusting the opening of the oilrelief valve a.

The insertion of a pilot valve

üov^rninq

osition the oil

fiiy-Ttff» and an oilcylinder is done to

have a slow adjustment of

the relief valve a.

b. The transmitting d e v i c e is the controller, which can be purchased from well known controller manufacturers. There are several methods of impulse t r a n s m i s s i o n , which all have the purpose to e s t a -blish a new steady position of the final control element in the smoothest and quick-e s t way. In thquick-e squick-eququick-encquick-e of their complexity we mention: 1. the on and off control (two position control)often sup-plemented by a neutral zo-ne (three p o s i t i o n control), 2. the single speed control, 3- the proportional speed

con-trol,

4. the proportional (position) control, a s far a s n e c e s s a -ry supplemented by a reset and rate control.

For a full understanding of the various modes we refer to textbooks on control. c. The final control element i s for our purpose;

1. A loading governor spring.

2. A regulating relief valve of an o i l p r e s s u r e governor.

When describing the measuring means we hare already explained how the final control elements are a c t u a t e d .

3. As far as there i s a need for the regulation of a weight loaded governor we suggest to r e p l a c e the movable weight by a pneumatic or hydraulic diaphragm. (Fig, 16 and

17)

It is i n t e r e s t i n g to draw the attention to the servo-operating governor, described by M. Tolle (Regelung der Kraftmaschinen 3d edition, page 739). This i s a servo governor with a retarded, but adjustable r e s e t . T h e piston of the main operating cylinder A is con-nected with the cylinder of the damping cylinder B, and this again with the fulcmm of the loading governor spring Its tension i s automatically changed proportional to the main piston position, thus a l s o proportional to the load. In this way the offset in the engine c h a r a c t e r i s t i c can be varied from p o s i t i v e to zero, even to n e g a t i v e . Such a governor can a l s o be used for our grinding control, b e c a u s e it effects a higher s p e e d with increasing load, if the offset is made n e g a t i v e .

From the diagram we s e e that with a negative offset the torque does not i n c r e a s e so much a s without the automatic spring adjustment. But the speed regulation is n o t so perfect, especisilly, if the grinding c h a r a c t e r i s t i c of the cane f l u c t u a t e s .

It seems useful to remark, that with the grinding control we proposed, we do not work with a governor with a negative offset, b e c a u s e such a governor i s u n s t a b l e . As a matter of

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lA/dn admission teau.la.tio7i

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of vs/e-igAi loae&d ^ovmrnors

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ntgetitve offset (Tolle)

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fegt/laîort iîÂ Ttegiaéiye offset

fact the engine c h a r a c t e r i s t i c remains p r a c t i c a l l y unchanged by the spring adjustment, if only this is performed gradually. Then the engine speed is automatically changed sothat the engine torque tends to the d e s i r e d v a l u e , and the measuring means i n d i c a t e a deviation tending to zero,

In order to prevent misunderstandings we want to s t r e s s our opinion that with a grinding control a s we have proposed, the highest efficacy of juice extraction is not at all guaranteed.

T h e s u c c e s s of the j u i c e extraction depends on many factors, we mention the proper con-struction of the canecutter, shredder, crusher or mill, the proper design and adjustment of the feeding s p a c e and the feeding equipment (feeding rollers), the quality, the proper adjustment of k n i v e s , shredder- cmsher- or mill-rollers, the correct form of trash turner p l a t e s , e t c .

But if t h e s e important things have been carefully attented to, the proposed grinding con-trol will effect optimum r e s u l t s from them.

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C H A P T E R I V

C O N T R O L O F T H E M U L T I P L E E F F E C T E V A P O R A T O R S a. Study of the existing s y s t e m s .

As stated before, the multiple effect evaporation i s a continuous p r o c e s s of a large capacity, which is extremely suitable to be automatically controlled.

The importance of a thick j u i c e of constant density or s o l i d s content, d e l i v e r e d by the evaporators, is evident, a s it i s the material from which is boiled a crystal containing m a s s e c u i t e in the vacuum p a n s . Now crystallization can t a k e p l a c e in two manners: the formation of new c r y s t a l s (primary crystallization) and the deposit of sugar on existing sugar crystals. In the boiling p r o c e s s in the vacuum p a n s sometimes primary c r y s t a l l i zation i s desired, but mostly the secondary crystallization i s wanted, to obtain a m a s s e -c u i t e with regular grains, free of sugar dust (false grain). The formation of primary c r y s t a l s i s more liable to happen in too much concentrated thick juice, which then con-t a i n s cryscon-tallizacon-tion nuclei of i con-t s own. So con-too con-thick a j u i c e may incon-terfere wicon-th con-the boiling of a good m a s s e c u i t e

On the other hand the e x c e s s of water in too thin a juice h a s to be evaporated in the p a n s in simple effect, which means a higher steam consumption and a d e c r e a s e in boiling rate of the vacuum p a n s .

In spite of i t s importance little care h a s been spent on e s t a b l i s h i n g an effective control of the multiple effect evaporators.

T h e few attempts made were based mostly on the maintenance of a c o n s t a n t , most efficient level in the calandria, according to the r e s u l t s of experiments, made by Dr. C l a a s -sen and by Kerr. According to Kerr the most favourable r e s u l t s are obtained at an immers-ion of the evaporating vertical tubes between 30% and 40%.

We have found a description of a Full Automatic J u i c e Level Control in Tromp's book: Machinery and Equipment of the Cane sugarfactory, first and second edition, page 4 2 2 / 423:

" J u i c e level regulators are shown in Fig. 421, fitted to all bodies of the multiple effect. In many i n s t a n c e s , only the juice level of the first body is regulated by an automatic d e v i c e , the other bodies being controlled by throttling gate valves in the j u i c e l i n e s between the b o d i e s . In Fig. 441 such a Full Automatic Juice L e v e l Control Apparatus i s shown, which h a s been applied on the first bodies of the multiple evaporators in beet

sugarfactories.

It will be obvious that a constant j u i c e level will achieve an improved evaporating per-formance, a s the hydrostatic head remains constant, and generally an i n c r e a s e d heat transmission, thus giving an i n c r e a s e d capacity to the evaporator.

The juice level a c t s on a float in the v e s s e l a and operates a double s e a t e d valve, which g i v e s a c c e s s to the system relay b, which in turn controls the a c c e s s of live steam to the regulating valve c in the j u i c e discharging line to the first body. The live steam relay e n t e r s into the circuit, a s the vapour p r e s s u r e of the first body generally will not be sufficient to operate the juice control valve and thus a medium of higher p r e s s u r e i s applied.

A low juice level in the evaporator will speed up evaporation whereas too high a level will retard it and it will therefore be obvious that, for optimum evaporating r e s u l t s , a controlled j u i c e level will show paramount advantages .