DUPLEX EVAPORATOR : Its Concept And Induction In Sugar Industry

SYNOPSIS:

Conservation of energy is the talk of the day in Sugar Industry. The minimum theoretical requirement of thermal energy for converting 1 ton of white sugar is a definite figure which amounts to 36.8% of cane when bagasse as captive fuel is used for generating steam. Process, steam consumption is controlling factor for generation of by-product power or captive electricaal and mechanical power required in the process. Ever since Robert type multiple effect evaporator was introduced in the sugar industry, the best combination was DEVC (Double Effect Vapour Cell) and Quad. Of late, deficient long tube pre evaporator (risingfilm and fallingfilm) has been in use but both have got their inbuilt deficiencies. Present effort is to introduce a Duplex Evaporator to eliminate deficiencies of both pre Evaporators The author has come to terms that Duplex Evaporator and Sweeping Calendria give best results with respect to utilization of heating surface and reduction of sugar loss in the Evaporator Station. Duplex Evaporator is very effective in the sense that conservation of total energy and production of by-product electrical energy would fit in the whole System Design. This paper deals with the theoretical aspect of design as well as performance of Duplex with respect to any conventional system design. Keywords: Duplex Evaporator, Dropwise Condensation, Sweeping Calendria, Thermal Energy Conservation, Heat Transfer Coefficient, Optimal Heating Surface, Long Tube Evaporator.

INTRODUCTION

DUPLEX EVAPORATOR AS COMPARED TO RISINGFILM & FALLING FILM EVAPORATOR

Risingfilm long tube evaporator

1. Semi Kestner Ever since Semi Kestner has been introduced in sugar inustry it posed two serious problems:- a) Semi Kestner requires high pressure exhaust steam. b) Due to countercurrent flow pattern i.e. juice film rising along with vapour bubble upwards and condensate trickling down to the bottom adversely affect retention time and exposure to high temperature for long duration which results in drop in purity between inlet clear juice and outgoing partially evaporated juice. c) Risingfilm Evaporator always has given higher evaporation rate than Robert Evaporator or even any other type of evaporator but with some sacrifice of technical efficiency. 2. Fallingfilm Evaporator a) Since it has been introduced in this country investment does not commensurately add advantage to rate of evaporation per sq. ft. H.S. and requires additional power consumption for recirculation of juice and due to much longer tubes 10.12 Mt. long which added additional burden in maintenance and operation of the equipment. b) Rate of evaporation has invariably been found to be less than that of Robert design Evaporator. 3. Duplex Evaporator In view of above mentioned inherited deficiencies it has become a must to develop some design which can eliminate deficiency of long tube Risingfilm Evaporator and long tube falling film evaporator. Merits of two systems design long tube risingfilm giving rise in higher evaporation rate and falling film having cocurrent flow of juice and condensate resulting in lesser drop in purity between inlet juice and partly evaporated outlet juice. Both have advantage & inherent deficiencies. Objective of development of Duplex Evaporator is to complete elimination of the deficiency of two systems as discussed above and high breed merits of both in the systems i.e. higher rate of evaporation as risingfilm and lower drop in purity in fallingfilm evaporator and also to avoid partial starvation of heating surface in rising film as well as fallingfilm. In Duplex evaporator design there are two Calendrias one above the other. The secondary Calendria reduces retention time and at the same time increases the rate of evaporation due to very high mass velocity in the evaporator. Whereas in primary calendria steam entry is at bottom and juice also enters at the bottom which is concurrent and takes the advantage of falling film evaporator and reduces drop in purity from inlet juice purity to partially evaporated outlet juice. The net effect and advantages of Duplex evaporator are as follows:- 1) It does not need very high exhaust steam pressure and temperature as the steam enters into the bottom and it sweeps the calendria to exit at the periphery which completely eliminates starvation of heating surface and produces complete saturation of steam going out of the calendria. 2) The exit exhaust steam is completely saturated as it remains in contact with the condensate in calendria before it exits. Thus the exit steam which is completely saturated enters in secondary calendria through central steam entry mechanism gets totally condensed which eliminates destruction of reducing sugars in Secondary Calendria because it never comes in contact with superheated high pressure steam and when juice is virgin clear juice as it is known, inversion rate is inversely proportional to concentration of reducing sugar in C.J. The combined effect of primary and secondary calendria gives rise to higher rate of evaporation and nearly eliminates destruction of sugar material due to dynamic characteristic of juice and aerodynamic characteristic of steam inside the calendria.

Salient technical features are as follows: - 1) Overall heat transfer coefficient is 1.4 times higher than Robert Evaporator 2) Retention time is less by 1.5 time of the Risingfilm Evaporator 3) Simultaneously heat transfer from outside the Calendria Shell and inside the tubes takes place which are rare as it avoids radiation loss of heat from Secondary Calendria. 4) Cleaning is easier for two Calendria than one long tube rising or Fallingfilm Evaporator.

DUPLEX EVAPORATOR : THE CONCEPT BEHIND (Dropwise Filmwise Condensation)

Heat transfer coefficients for drop wise condensation are four to eight times higher than those for film wise. Steam is the only pure vapour known to condense in drop wise manner under certain required conditions. Film of condensate on the tubethan those for film wise. Steam is the only pure vapour known to condense in drop wise manner under certain required conditions. Film of condensate on the tube wall contributes the controlling resistance (Pg - 252 -253, D.Q .Kern "Process Heat Transfer") With downward flow of condensing vapours at high velocity, the measured heat transfer coefficients are as much as ten times higher than those predicted for film type condensation neglecting the effect of vapour velocity (Pg -336, McAdams, Heat Transfer) An increase in the mass velocity of the fluid past the surface is accompanied by an increase in the individual coefficient. If corresponding - individual resistance 1/h is a substantial fraction of the total resistance 1/u, the overall coefficient will increase (Pg - 189, McAdams) Tube side *Nu = h,i D / K = 0.0115 (Re) 0.90 (Pr) 0.33 Re = Reynolds Number Pr = Prandtl Number Nu = Nusselt Number Calendria side (outer wall of tubes) ho De /K = 0.36 (DeGs 55 (Cp m / K ) 0.33 )( m/ms)0.14 De - Shell side equivalent diameter G - s Mass velocity jH = (hoDe/K ) (Cp m / K ) 0.33 (m/ms)0.14 (Pg 137 DQ Kern) 1/U = 1/ho + 1/hio + Rd [hio =hi(A 1 /Ao ] smaller coefficient is the controlling film coefficient. If the difference is large, enough for sugar solution is significantly lower than that for steam with sweeping calendria technology even if h steam is increased further, it will not affect you. All these equations are applicable for Newtonian fluids and for sensible heat transfer or in other words these equations are not applicable for two phase fluid flow i.e. (vapour +Liquid inside the tube) and simultaneous phase change from liquid phase to vapour phase with instant transfer of latent heat of vaporization. The only Newtonian equation which holds good and applicable is Q= =WCp DT = UA LMDT Measurement of Q in the evaporator is calculated as W x l By substituting the Q value and LMDT in last equation, overall U is calculated which is most practical. In this particular case, condensation is very rapid due to sweeping of calendria and increase in mass velocity inside the tube in the ratio as per calculation given below: 1. In case of 1700 m2 heating surface, one body having 45 mm OD and 42 mm ID and 2m thick. and when the heating surface is divided in the ratio 1100 m2 and 600 m2 for Primary to secondary calendria respectively. 2.1 The primary calendria having heating surface 1100 m2 the no of tubes of same dimension 45 mm OD, 42 mm ID and tube length 2 m, no of tubes would be 4121. 2.2 The secondary calendria having heating surface 600 m2 the tube sizes having 45 mm OD, 42 mm ID and 1.5 m length the no of tubes is 3000. 2.3 The cross sectional area of each tube, both in primary and secondary is same which is equivalent to .00138 m2. Say, this is called as A. 2.4 The total cross sectional area of primary calendria would be A x 4121. Similarly total cross sectional area of secondary calendria would be A x 3000. Total cross sectional area of single body evaporator would be A x 6390. Since we are concerned with mass velocity as the controlling factor for overall heat transfer coefficient, the ratio of mass velocity for primary and secondary will be in the ratio 1:1.55 and for secondary calendria 1:2.1. That is with respect to single body mass velocity would be 1.55 times more in primary calendria and 2.1 times more in secondary calendria. The weighted average as far as distribution of heating surface and number of tubes due to difference in length (2 m effective length in primary and 1.5 m in secondary) is calculated. The weighted average of increase in heat transfer coefficient with respect to one single body having equivalent heating surface will be 1.96 times more. However, we have taken a conservative figure of 1.4 times than the theoretical calculation of 1.96 times. The maximum heat transfer coefficient inside the tube having boiling liquid as reported in literature is 5678 W/M2 H.S./oC. and the condensing steam as 11356 W/M2 H.S./oC.and overall U 2981 W/M2 H.S./oC.. Whereas in the duplex evaporator resulting U has been calculated as 7966 W/M2 H.S./oC.. This figure is very much realistic and achievable.

Technical Description of Duplex Evaporator

The 'Duplex Type Evaporator' as shown in accompanying fig.No.1 consists of five major parts namely, 1. Primary Calendria (Bottom Calendria) 2. Duplex calendria i.e. second calendria above the first calendria inter connected through the Exhaust Steam Flow Line & Juice Flow Line. 3. Centrifix Catchall at the top for passing vapours from both the calendria at on optimal velocity to reduce the friction loss and pressure drop. 4. Exhaust Steam/vapour connection after sweeping the primary calendria connected to Duplex Calendria in series which acts as total condensing system. 5. Interconnecting juice line showing the thin juice entering first into the Duplex Calendria and after partial evaporation it flows to the primary calendria which takes care of the rise in viscosity in the juice which immerges from the Duplex Calendria and at the same time the Heat Transfer coefficient of the primary is enhanced due to sweeping of calendria. The 'Duplex Evaporator' as shown in accompanying drawing is actually one single body having Two calendrias of different Heating Surface incorporated in the shell one above the other. The Duplex Calendria is made up of S.S.304 to avoid the repair and maintenance cost and also to reduce the scale formation in the Tubes. Floating calendria of Duplex Evaporator will be made entirely of stainless steel including SS body/tube plate and tubes. The steam inlet will be of central steam entry type and down take will be at the periphery. The juice outlet from the duplex (floating) Calandria will be fed to the bottom of the primary calandria at the bottom . The concentrated juice from the primary Calandria will be taken to the succeeding body due to differential vacuum/pressure. The additional heat transfer area that can be accommodated in any existing evaporator body will be 60 percent of the total heat transfer area.

Technical Specification Of Duplex Evaporator

The body consists of two calendrias (1) the bottom calendria known as primary calendria having heating surface of 1100 M2 (2) the Secondary Calendria having heating surface 600 M2 made of S.S.-304.

Specification of Primary Calendria:-

Bottom Tube Plate Dia = 4100 mm, Top Tube Plate Dia = 4050 mm (Thk. for tube plate 28 mm)

Tubes Specification:-

ID 42 mm OD 45 mm Tube length 2070 mm made of S.S.304 and No. of tubes 4164 Ammonia Gas removal system is not needed as the calendria is swept through and the vapour/exhaust steam is introduced in second calendria on top with central steam entry and peripheral downtake. Clear Juice enters at the bottom of the Secondary Calendria through S.S.Feed Pipe 200 mm ID and is distributed evenly through a deflector.

Specification for Secondary Calendria (S.S.304):-

Bottom tube plate dia = 4050 mm, Top Tube Plate dia. = 3760 mm Thk.16 mm S.S. 304 for Tube Plate. Both the tube plates are made of S.S.304 quality. The body of the Calendria inner and outer are made of 8 mm thk. S.S.304. Bottom saucer is also made of 8 mm thk. S.S.304. Feed distribution is from the bottom nozzle fitted at the centre of the bottom saucer with a dish deflector to evenly distribute the juice. Steam entry is from the top with divergent nozzles inside this Calendria. Two ammonia coils are provided to vent out the non condensable gas to the atmosphere/vacuum system. Steam inlet pipe is provided with an S.S.Jacket with annular space to maintain vacuum in between the pipe and jacket. The Vapour generated from the primary Calendria passes through the annular space between the vapour space and the outer shell of the Secondary Calendria.

Tube Specification (For Secondary Calandria) :-

The entire calendria is made of S.S.304. Tube ID 42 mm, OD 45 mm, made of S.S.304 tube length 1500 mm and no. of tubes = 3004 and is covered under our supply. In the periphery the non condensable gas removal system has been provided through two nos. of S.S. circular coil of different dimensions for removal of light and heavy gases. On the top of the body, Centrifix Catchall has been provided to separate any entrained particles from vapour phase. The vapour from Primary Calendria is partly used for keeping the outer shell of the secondary calendria heated which is a unique feature in this design. Heat transfer takes place not only in calendria but also outside the calendria. The vapour from primary calendria passes through annular space between the vapour body and the outer shell of the secondary calendria. The vapour phase is under the same pressure and it passes through the Centrifix Catchall and to the succeeding body through the top vapour outlet.

CONCLUSION

1. Looking into heat transfer coefficient as calculated from the concept of mass velocity is about 1403 Btu/hr.sq.ft.oF. whereas maximum heat transfer coefficient of Robert type evaporator for rising film is only 800 Btu/hr.sq.ft. oF, and falling film is about 400 Btu/hr.sq.ft. oF. 2. Since two calendria are placed one above the other the space requirement is less than one single evaporator. 3. In falling film additional power consumption due to recirculation ratio is much higher than rising film or duplex. 4. Less formation of colouring matter due to less contact time between heating medium and juice inside the tube. 5. The total heating surface requirement for a particular duty is about 40% less than the Robert type evaporators in combination with rising or falling film.

ACKNOWLEDGMENT:

We are grateful to  Dr. Baishali B. Paul  for editing and compiling the manuscript.

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