Factory Engineering A NEW DESIGN OF RECTANGULAR RESISTANCE HEATER FOR REHEATING LOW GRADE MASSECUITES. N.A. Ramaiah and H.N. Gupta

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1 Factory Engineering I I A NEW DESIGN OF RECTANGULAR RESISTANCE HEATER FOR REHEATING LOW GRADE MASSECUITES N.A. Ramaiah and H.N. Gupta National Sugar Institute, Kanpur, India ABSTRACT A new design of rectangular electrical resistance heater for reheating low grade massecuites was developed. The heater was tested in direct consumption plantation white sugar factories operating with batch type and continuous centrifugal machines. To obtain a rise in temperature of about 100C (from 410 to 51%) the power requirement for the heater was 5.4 kw per tonne of massecuite. The resistance heater reduced the molasses purity by 1.5 to 2.0 units and contributed to increased recovery by 0.06 per cent. The head loss in the rectangular heater was lower than that in the earlier designs, which indicated the lower space requirements for installation of the rectangular heater between the crystalliser/pug-mill and the centrifugal machines. I INTRODUCTION To achieve better exhaustibility of molasses in sugar factories, the low grade massecuites are normally tight-boiled to Bx in vacuum pans, and cooled to 40-42OC in crystallisers for a period of hours. At these low teillperatures the massecuites become highly viscous, their viscosity ranging between and poises in direct consumption plantation white sugar factories employing a carbonation/sulphitation process for clarification of the cane juice. The high viscosity of massecuites offers operational difficulties in separating sugar crystals from molasses in centrifugal machines. The available designs of centrifugal machines can easily handle massecuites of viscosities ranging from to poises. The massecuites are therefore conditioned to reduce the viscosities to make possible their efficient curing in the centrifugal machines. This is achieved by reheating the massecuites by passing hot water in the heat exchangers of the crystallisers or by diluting the massecuites. These two methods, which have been in vogue in sugar factories for a long time, lead to re-dissolution to some extent of sugar crystals, which results in rise in the purity of molasses. Gundu Rao and Sastri2 reported 2-3 units rise in purity by these methods and a consequent loss in sugar recovery. To reduce this loss of sugar, a process of heating low grade massecuites by an electrical resistance method was developed at this Institute. This has been found to be very helpful in reducing viscosity of massecuites without any loss in recovery.

2 2376 FACTORY ENGINEERING EARLIER WORK ON ELECTRICAL RESISTANCE HEATING OF LOW GRADE MASSECUITES For the first time, in 1954, Doss and Vishnul working at the National Sugar Institute, Kanpur, conducted laboratory experim~nts to raise the temperature of massecuites by employing it as resistance between two electrodes. In these experiments massecuites could be heated from 40 to 52OC without effecting any appreciable redissolution of crystals. Following these experiments, detailed studies were made at the Sugar Research Institute, Mackay3 (Queensland) which showed that the re-dissolution of crystals took place to some extent in the resistance heating method and that this contributed to the rise in purity of molasses. The increase in purity depended upon the "residence time" in the heater; the purity rise being of the order of 0.1 to 0.45 units when the "residence time" was in the range of 1-2 minutes. The rate of re-dissolution of crystals increased when the heated massecuites were stirred. As a result of these observations it was considered that the resistance heating of massecuites would be ideally suited to the system in which low grade massecuites are purged in continuous centrifugals since, in that system, the interval between reheating and purging can be kept very low. In the year 1964 the first commercial design was developed by Wright4. This consisted of two concentric pipes which served as electrodes between which a potential difference of 250/440 volts was applied. The potential required depends upon the desired temperature rise and the design of the heater. The massecuites passed through the annular space between the two concentric tube-electrodes and was heated by the resultant current flow. The temperature was controlled by varying the voltage, by employing the silicon controlled rectifier system and the direct line step switching system. The length of the heater was about 1,7 metres. Generally, the outer electrodes had diameters ranging from 38 cm to 60 cm while the inner electrodes had diameters from 23 cm to 45 cm. The capacity of these heaters varied from 25 kg to 50 kg per minute and the head loss varied from 1 to 2 metres. LIMITATIONS OF THE EARLIER DESIGNS 4, The earlier designs, especially those of Wright4, required considerable head space above the centrifugal machines to receive a direct feed of massecuite from the crystallisers/pug-mill. The head required was of the order of 3-4 metres. Usually this amount of space is not available in most of the existing sugar factories. In such cases cold massecuites from the crystallisers must be pumped, as pointed out by Wright4, to a common receiver installed at a sufficient height to have a direct feed to the heater and then to the centrifugal machines in order to provide the required head. Pumping of the cold massecuites is difficult and poses a serious problem.

3 1. I- N.A. RAMAIAH AND H.N. GUPTA BASIC FEATURES OF THE RECTANGULAR ELECTRICAL RESISTANCE HEATER The development of a resistance heater which requires less head space appeared to be of considerable importance for the application of the system of electrical resistance heating of low grade massecuites. The basic features of the new design, developed by the present authors, ara shown in Fig. 1. It consists of a rectangular box made of 20 mm thick hylum sheet, in which 7 electrodes in the form of plates have been fixed vertically. The gap between the electrodes is 50 mm. Lengthwise, the electrodes have been divided into 3 parts which are referred to as the A, B and C parts. The electrodes of the heater (Fig. I-) are connected to a 3-phase power supply in two different manners: (a) Star system and (b) Delta system. The arrangements of the electrodes in 3 different parts (A, B, C) made it possible to operate or energise all the 3 parts simultaneously or A, B, or/and C at will, to control the temperature of the massecuites at the outlet of the heater. The electrical operating system as either the star connections or the delta connections made it possible to vary the rating ol the heater. These two parameters are found to be very significant for the successful operation of the new electrical heater. DESIGN PARAMETERS The length and the width, in other words the surface area, of the electrodes and the distance between them depends upon a number of parameters including the characteristics and quantity of the massecuites to be heated. The fundamentals involved are briefly discussed here. The power requirements of the heater can be calculated by the formula: where P is the power requirement in kilowatts AT is the required increase in temperature in OC C, is the specific heat of inassecuite in kj/kg OC W is the flow rate of massecuite in,kgpmi/n. i

4 2378 FACTORY ENGINEERING When the power connections are mpde in accordance with the star system, the surface area of the electrode can be computed by the following formula : where A is the surface area of each electrode in m2 d is half of the distance between two consecutive electrodes in m - K is the log mean specific conductance of the massecuite in micromho jm"' Pf is the power factor of the supply. V is the applied voltage in volts When electrodes are energised in accordance with the delta connections, the surface area of the electrodes is given by the formula: The pressure drop inside the heater is given by the following equation, applicable for streamline flow through parallel plates, where F is the head loss across the heater in m of massecuite L is the length ol the heater in m q is the geometric mean viscosity of the massecuite in poises B is the width of the heater in m. For working out the actual dimensions of the resistance heater, experirqental data on the following characteristics of massecuites are required (i) specific conductivity, (ii) viscosity and (iii) specific heat. Detailed experiments have been carried out on the computation of these physicochemical characteristics of massecuites in plantation white sugar factories in India employing the sulphitation process. These.experiments will be

5 N.A. RAMAIAH AND H.N. GUPTA 2379 described elsewhere. The lollowing are the data of the above physical characteristics of massecuites which were employed in the design of the Rectangular Electrical Resistance Heater: Property Magnitude Log mean specific conductivity (micro mho/m) Viscosity (poises) Specific heat (kj/kg C) FIGURE kw Rectangular Resistance Heater. NEUTRAL GRID 1 SECTION P-Q

6 2380 FACTORY ENGINEERING DESCRIPTION OF ELECTRICAL RESISTANCE HEATER FOR BATCH TYPE CENTRIFUGALS A resistance heater for heating the massecuites to be cured in a high speed batch type semi-automatic machine was designed and fabricated. The following are the dimensions of the heater: Overall length of heater Number of electrodes Distance between electrodes Width of each electrode 1.12 metre 7 45 mm 0.6 metre The design parameters of the heater were as shown in Fig. 1. PERFORMANCE OF T ~ RECTANGULAR E RESISTANCE HEATER WITH BATCH TYPE CENTRIFUGAL. MACHINES The first trial of the above heater was conducted during the crushing season , at Sanjivani Sahakari Sakhar Karkhana (Sanjivani Cooperative Sugar Factory), Kopergaon, Maharashtra (India). This factory had a continuous crystalliser. The massecuite at 102OBx was fed into the crystalliser at its starting point at 63-65OC. The cooling coils were filled with cold water to reduce the temperature of the massecuite in,the continuous crystalliser slowly over a period of hours. The temperature at the end of the continuous crystalliser was 40 C. The cooled massecuite was fed into the resistance heater directly through a chute and the heated massecuite was received in a pug mill. The stirrer of the pug-mill was not operated. The layout of the heater is shown in Fig. 2. K - CRYSTALLISER R - RESISTANCE HEATER P - PUG MILL M - CENTKIFUGAL MACHINE FIGURE 2. Layout of Resistance heater with batch type centrifugal machine. K - Clystalliser; R - Resistance heater; M - Centrifugal machine (batch); P - Pugmill.

7 N.A. RAMAIAH AND H.N. GUPTA

8 FACTORY ENGINEERING The electrodes were energised by adopting the star system. The feed rate from the heater was adjusted to 85 kg per minute. The massecuite was heated in the rectangular resistance heater from 40 to 51 C. The power consumed was 30 kw per 'hour for the fldw rate quoted. Experiments were conducted to find out whether there was any purity rise in the aolasses due to dissolution of crystals during the reheating process in the rectangular electrical resistance heater. As a control, the purity of the molasses taken from the cooled massecuite through the Nutsch apparatus before the massecuite entered the resistance heater, was also determined. Nutsch purity of molasses taken from the massecuite coming immediately after the heater was also determined. Similar tests were carried out with massecuites reheated in the conventional manner by passing hot water through the heat exchanger of the last set of crystalliser. The results obtained are summarised in Table I. ' The results obtained at Sanjivani factory showed that the extent of re-dissolution of the sugar crystal during the heating of the massecuite by the electrical resistance method was slight by comparison with that of conventional heating by passing hot water through the heat exchangers. In experiments with electrical resistance heating a purity rise of 0.13 to 0.20 unit in qolasses was recorded, whereas the rise was 2.18 to 2.20 units with the other method. In other words, use of the resistance heater gave a reduction in molasses purity of the order of 2 units. The successful performance of the rectangular heater at Sanjivani factory was examined by a number of technologists. DESCRIPTION OF THE RECTANGULAR RESISTANCE HEATER FOR CONTINUOUS CENTRIFUGAL MACHINES Another heater for reheating of low grade massecuites to be cured in a continuous centrifugal machine was designed. The design parameters were as follows: Overall length of heater metres Number of electrodes 7 Distance between electrodes 50 mm Width of each electrode 0.6 metre PERFORMANCE OF THE RECTANGULAR HEATER WITH CONTINUOUS CENTRIFUGAL MACHINES A rectangular resistance heater of the dimensions described above was fabricated and tested during the crushing season in another factory viz., the K.C.P. Ltd., Vuyyuru (Andhra Pradesh) which used a continuous centrifugal machine in fore-curing of C-massecuite. The details of the heater were the same as those shown in Fig. 1.

9 N.A. RAMAIAH AND H.N. GUPTA 2383 The layout of the heater was as show11 in Fig. 3. The photograph shows the rectangular resistance heater in operation (Fig. 4). I il R - RESISTANCE HEATER M - CENTRIFUGAL MACHINE (CON1 INUOUS) I I tinuous). FIGURE 3. Layout of Resistance heater with continuous centrifugal machine. K - Crystalliser; R - Resistance heater; M - Centrifugal machine (con- I FIGURE 4. Rectangular Resistance heater in operation with continuous centrifugal machine. R - Resistance heater; M - Continuous centrifugal machine. The cooled massecuite from the crystalliser was taken through a chute to the resistance heater. A perforated sheet was fixed at the end of the chute to eliminate entrance of lumps and foreign material into the resistance heater. The massecuite after being heated in the resistance heater was directly and continuously fed into the continuous centrifugal machine.

10 TABLE 11. Test data on the effect of resistance heating on purity rise in molasses, reduction in visconsity, colour and reducing sugars. Set Temp. Massecuite Nutsch's Purity rise Transmittance* Absorbance Reducing No. Particulars OC Viscosiw Bx. w. molasses due to sugar % poise heating in massecuite Bx. Pty. heater. m p mp mp mp Before heating After heating Before heating After heating : Measured at wavelengths 420 and 560 millimicrons.

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12 2386 FACTORY ENGINEERING In this particular factory, as head space between the massecuite leeding chute and the continuous machine was not adequate to accommodate a heater of 1.12 metre length, the resistance heater was only metre in length. The required power could not be obtained by the star system connection. Higher voltage at 440 V had therefore to be applied by using the delta system connection. The feed rate of the massecuite was adjusted to 100 kg per minute. The power consumption was of the order of 35 kw per hour for this Ilow rate. The temperature rise was found to be ll C. Tests were carried out to determine the rise in purity of molasses, the reduction in viscosity, the increase in capacity of the centrifugal machine etc. resulting from the use of the electrical resistance heater. The possible burning or caramelisation of reducing sugar in the resistance heater (primarly due to the application of 440 V) was also investigated. The results are given in Table 11. Some rise in purity of nlolasses due to heating of the massecuite in the resistance heater was recorded. The reduction in viscosity was appreciable. There was practically no colour formation or destruction of reducing sugar even at the higher voltage, such as 440 volts, applied. One significant achievement was the marked increase in the capacity of the centrifugal machine resulting from the reduction in viscosity due to the resistance heater. The capacity of the machine increased by 30 to 45%. No increase in the load of the machine was however recorded in spite of the increase in its capacity. The results are given in Table 111. DISCUSSION AND CONCLUSIONS The foregoing results showed that the rectangular electrical resistance heater developed by the authors was suitable to raise the temperature of cooled low grade massecuite to 51 C for easy purging in batch or continuous centrifugals. To obtain a temperature rise by about 10 C, the power requirement was of the order of 5.4 kw per tonne of massecuite. This figure is based on the actual data collected in sugar factories during trials on the heater, and is in accord with the theoretical calculations made by earlier workers1. In factories where electrification has been effected by installation of high pressure boilers and turbo-alternators, provision of the above quantity of power may not pose a serious problem. In other factories where power is already in short supply, arrangements for supply of additional power for the resistance heater may be made from an outside power supply grid. The rectangular resistance heater used with the continuous centrifugal machine at K.C.P. Ltd., Vuyyuru gave a rating of 35 kw when connected

13 on the delta system. It handled 100 kg of massecuite per minute and gave a temperature rise of ll C. The loss of head at this flow rate was computed to be 0.9 metre. The same heater could however increase the temperature by 15OC if the flow rate was maintained at 75 kg/min. The loss of head at this flow rate would be 0.7 metre. The corresponding figure for a tubular resistance heater (two concentric pipe design) would be 1.7 metres with a flow rate 50 kg/min. and the temperature would rise 15OC. The loss for a flow rate of 75 kg/min. and 100 kg/min. through a tubular heater would undoubtedly be more than 1.7 metres. The rectangular resistance heater on the star system, (250 volts) gave a rating of 12 kw. At this rating the temperature rise was 15OC at a flow rate ol 25 kg/min. The head loss under these conditions would be 0.25 metre in the rectangular resistance heater. For a tubular heater for similar duty and when operated on 250 volts, the head loss would be about 0.85 metre, which is considerably more than that for the rectangular heater. The rectangular resistance heater can be accommodated within a head space of 1.5 to 2 metres between the crystalliser/pug-mill and the centrifugal machines as compared with a head space requirement of 3 to 4 metres with the tubular type heaters. 'The electrical resistance heating resulted in lowering of molasses purity by about 1.5 units. This is likely to increase recovery of sugar by 0.06 per cent. The calculations are based on the assumption that the molasses formation is 4 per cent on cane, which is the average figure for nlost of the Indian plantation white sugar factories. The additional recovery more than offsets the expenditure on power and on the interest and depreciation of the cost of the equipment. No quantitative data on the economic are presented as these are subjective and vary with change in sugar prices from place to place. I REFERENCES 1. Doss K.S.G. and Vishnu. (1956). Laboratory experiments of resistance heating of massecuite. Proc. ISSCT, 9, Gundu Rao S.N. and Sastry P.V.S. (1951). The Fletcher Blanchard heat exchange units in the treatment of low-grade massecuites. Proc. D.S.T.A. India, 8, Technical Report No. 60 of March 1960 from the Sugar Research Institute, Mackay, Queensland. 4. Wright P.G. (1965). The resistance heater for the low-grade massecuite Proc. ISSCT, 12,

14 2388 FACTORY ENGINEERING UN NUEVO DISERO DE CALENTADORES RECTANGULARES DE RESISTENCIAS PARA RECALENTAR LAS MASAS COCIDAS DE BAJA PUREZA N.A. Ralnaiah y H.N. Gupta RESUMEN Un nuevo diseao de calentador de resistencias electricas rectangulares para recalentar las masas cocidas de baja pureza fue desarrollado. El calentador fue probado en consumo direct0 a la planta de fabricas de azljcar blanca operando con centrifugas tipo discontinua y con continuas. Para obtener un incremento en la temperatura de aproximadamente 10 C (de 41 to 5I0C), 10s requerimientos de potencia para el calentador fueron 5.4 KW por tonelada de masa cocida. El calentador de resistencias, redujo la pureza de las mieles de 1.5 a 2.0 unidades y contribuyo a un incremento en la recuperaci6n del 0.06%. La perdida de presi6n en el calentador rectangular fue menor que aquella de 10s disefios anteriores, lo cual indica 10s menores requerimientos de espacio para la instalacion del calentador rectangular entre el cristalizador y las maquinas centrifugas. I