Rein: Circulation in Vacuum Pans
CIRCULATION IN VACUUM PANS
Peter W. Rein1, Luis F. Echeverri1 and Sumanta Acharya2
Audubon Sugar Institute, Louisiana State University AgCenter
2
Mechanical Engineering Dept., Louisiana State University
Baton Rouge, LA 70803
1
ABSTRACT
The crystallization of sucrose involves complex processes that require the correct design
of vacuum pans and precise operation. Numerous parameters, such as tube dimensions,
downtake size and pan geometry, determine the quality of the sugar produced and the
throughput, which results in certain vacuum pans giving better performance than others. A
particularly important factor is the circulation, which is strongly interrelated with the convectiveboiling heat transfer in the calandria and determines, to a large extent, the velocity of
crystallization and the uniformity of the conditions inside the vessel. In this paper several factors
that affect boiling and circulation in vacuum pans are identified. The use of Computational Fluid
Dynamics in the analysis of circulation and in the evaluation of alternatives for hydrodynamic
optimization is discussed. Options for making changes to batch pans to improve the
characteristics of the pan are identified, either to change the geometry or assist circulation. Some
results with steam-assisted circulation (steam jiggers) are given, which show how the
performance of a pan can be improved.
Keywords: Circulation, vacuum pans, crystallization.
INTRODUCTION
It is widely accepted that the circulation of the massecuite within vacuum pans must be as
high as practically possible in order to achieve the maximum throughput of the equipment and
produce a good quality sugar. The benefits associated with good circulation can be listed as:
• Quality: Improved crystal size distribution and lower sugar color due to uniformity of
fluid conditions within the vacuum pan. Good circulation prevents stagnant regions, hot
and cold zones that can induce differences in the super saturation level and crystal growth
rate.
• Recovery: A higher massecuite dry substance concentration is possible, leading to a
higher crystal yield and a lower purity of molasses.
• Capacity: Improved heat transfer leads to a higher crystallization rate and increased
throughput.
• Energy efficiency: Use of vapor from later evaporator effects becomes possible (e.g.
vapor 2 or vapor 3).
The design of batch vacuum pans has evolved to a practically uniform basic geometry
(central downtake, straight side walls, fixed calandria, tubes of diameter 100 mm and length 800
- 1300 mm, bottom angle 18-25°), which may be assisted mechanically with an impeller (stirrer)
or by injecting a gas to improve circulation. This design has evolved essentially by trial and
error.
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Journal American Society of Sugar Cane Technologists, Vol. 24, 2004
Although valuable experimental work on the sugar boiling process has been done and
technological development has led to proven designs of batch and continuous vacuum pans, there
is still a lack of understanding of the important issues affecting vacuum pans design. This paper
discusses some factors that affect the circulation in vacuum pans, alternatives for the analysis of
flow characteristics and options for enhancing circulation.
HEAT TRANSFER AND CIRCULATION
Convective-boiling heat transfer in the calandria tubes is the driving force for the
circulation of massecuite. The buoyancy that results from the density difference between the
vapor-phase and the surrounding massecuite is the main driving force, following a strong
interaction between heat transfer and circulation. Any mechanism that reduces the evaporation
rate will lower the circulation, and conversely, any reduction in the massecuite velocity will be
detrimental to heat transfer (Bosworth, 1959). The balance between the buoyancy forces and the
resistance to flow determines the circulation rate. The hydraulic resistance or friction depends
essentially on the massecuite viscosity and the geometry of the passages within the pan, which
should be smooth and offer minimum obstruction to flow (Webre, 1959).
The circulation mechanism in vacuum pans has not been completely understood due to
the complexity of performing measurements inside the vessels (Wright, 1966) and the numerous
operational regimes that are possible. Ideally, it is expected that a flow of massecuite and vapor
rises after passing through the calandria tubes and reaches the surface. The vapor formed crosses
the free massecuite surface before going to the condenser, while the liquid moves in a radial
direction towards the center and goes back to the bottom through the downtake, to enter into the
calandria tubes again. However, evidence exists which indicates that serious variations from this
ideal circulation pattern could occur (Wright, 1966).
Using experimental measurements, Rouillard (1985) identified that evaporation rates, and
therefore circulation, are increased as the heating steam pressure, the vacuum and the purity are
raised. Conversely, longer tubes, higher strike levels and high concentrations reduce the
evaporation rate. The evaporation rate changes dramatically during each strike, decreasing
progressively and finding its minimum value at the last stage of each cycle (Table 1).
Table 1. Average evaporation rates measured in South Africa.
Type of Boiling
Grain / Seed
A
B
C
Maximum
kg /(m2 h)
61
38
25
18
DESIGN ISSUES AFFECTING CIRCULATION
2
Minimum kg
/(m2 h)
8.2
22
6.2
3.5
Rein: Circulation in Vacuum Pans
Calandria
The calandria is essentially a tube and shell heat exchanger. The exchange area is
normally around 6 m2/m3, but for high strike grades, where evaporation rates are higher, it can be
increased to 9 m2/m3 (van der Poel et al., 1998). The different variations are shown in Figure 1.
Horizontal flat plate calandria: This is the most common design. A single central
downtake is normally used, with vertical tubes arranged around it, constituting an efficient
design of simple construction and maintenance. The diameter of the downtake is 30-50 % of the
calandria diameter, usually around 40 % for natural circulation pans, while smaller downtakes
can be used when a stirrer is provided.
Inclined plate calandria: The plates of the calandria are angled between 10º and 25º to
the horizontal. The top plate is made horizontal in some cases.
Floating calandria: The downtake is located in the outer annular region. Although it was
developed in an attempt to improve the circulation characteristics, measurements involving the
use of a radioisotope tracer showed that the conventional central downtake performs better
(Wright, 1966). The combination of the annular downtake with a small central downtake has
also been tried, resulting in an inefficient design, prone to the formation of false grain (Webre,
1959)
Disengagement
vapor 1.5-3.0 m
Strike
Height 1.25 m
0.4*Dcalandria
58°
Tubes
D 0.1m
10-25°
L 0.7-1.2 m
18°
Inclined plate calandria
Conical enlargement
Conical bottom
Floating calandria
Straight side
Saucer bottom
Horizontal flat calandria
Straight sides
"W" bottom
Figure 1. Vacuum pan designs.
The downtake is sized to give a circulation ratio, defined as the ratio of the cross
sectional area of the tubes to the area of the downtake, less than 2.5 to get a pan with reasonable
circulation.
Tube diameter and length
The high viscosities of the massecuites demand the use of large diameters to overcome
the friction with the available buoyancy. The most common diameter is 100 mm (4 in). Tubes
above this size exhibit less resistance and promote circulation, but the lower area/volume ratio is
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Journal American Society of Sugar Cane Technologists, Vol. 24, 2004
unfavorable (Webre, 1959) and the graining volume increases (Rouillard, 1985). Tubes with a
length between 900 and 1500 mm (36-60 in) have been used, but currently they are designed
normally between 700 and 1200 mm. Short tubes are selected for low-grade pans, while longer
tubes are chosen for high-grade pans or when a stirrer is provided. Short tubes give the best heat
transfer coefficients, and probably there is no justification for tubes longer than 900 mm, even if
it results in a decrease in the heat exchange area (Webre, 1959).
Pan shape
Bottom: The bottom shape should promote an even distribution of the massecuite across
the calandria, without restricting circulation or providing stagnant areas, and allow the discharge
of massecuite within an acceptable time.
Conical bottom: When coil pans dominated, a conical bottom was the preferred option,
ensuring a smooth discharge of the massecuite. In the case of calandria pans, this shape is
believed to lead to a significant stagnant zone. The bottom angle does not need to be greater
than 20º (Bosworth, 1959).
“W” bottom: While smaller angles of the bottom reduce the footing volume, they reduce
the discharge velocity and lengthen the strike time. Two discharge valves are recommended.
Usually the angle of the bottom is between 17 and 25º (van der Poel et al., 1998).
Conical enlargement (flared pans): Some designers adopted a conical enlargement of the
body above the calandria to increase the capacity of the vessel without increasing the strike
height and give a lower graining volume to final strike volume. However, this was largely
discredited after a negative effect on circulation was recognized (van der Poel et al., 1998).
Numerous comparisons have shown that straight-sided pans perform better than “low-head
conically-enlarged” pans (Chen and Chou, 1993).
Disengagement height above the massecuite: Enough space above the massecuite must
be provided for disengagement of the liquid from the vapor, minimizing entrainment and
allowing for froth explosion, which occurs often on starting a boiling or on cutting over. The
disengagement volume should represent between 65-100 % of the strike volume, being normally
between 1.5–3.0 m but preferably as high as possible.
Strike height
As the strike height increases, the hydrostatic pressure in the calandria increases, raising
the boiling temperature. As a result, the available temperature difference becomes smaller,
leading to a reduction in evaporation rate and consequently reduced circulation. The situation is
particularly critical at the end of the strike, when the highest level coincides with the maximum
density and viscosity of the massecuite, all parameters that impair circulation.
Tests performed by Austmeyer (1986) suggested that a maximum heat transfer
coefficient is reached when the height of the strike is 800 mm above the top tube plate, after
which it starts reducing progressively as the massecuite level increases. A height around 1.2 –
4
Rein: Circulation in Vacuum Pans
1.6 m could give the best balance between quality, performance and capacity. The height may be
higher with high grade or refined boilings.
Unsteady nature of boiling
Boiling heat transfer or evaporation is an intrinsically unsteady process, characterized by
intermittent nucleation and a fluctuating generation of vapor. The bubbles in general display
random behavior, which is influenced by many factors like bubble size, coalescence, the slip
velocity, densities, viscosity, surface tension, and hydrostatic head, making the analysis complex.
Evaporation is usually considered a steady state phenomenon, but in reality, it is chaotic and
difficult to characterize in detail.
Wright (1966) proposed an “eruptive” mechanism after performing measurements of the
circulation patterns in high-grade pans. The results showed an intermittent flow in the tubes and
downward currents over the top calandria plate. It was proposed that the massecuite remains
stagnant within the tubes until enough superheat is reached, and then a large burst of vapor
erupts and expels the fluid upwards inside the vessel. The vapor escapes, while massecuite
penetrates the tube from above and below. In a similar way, other theories involving nucleation,
coalescence of bubbles, formation of slugs or columns of vapor, temperature differences,
flashing, eruptions, etc. have been proposed to describe the boiling in vacuum pans, but the lack
of information has not enabled reasonable conclusions to be drawn.
Forced or assisted circulation
Stirrers: Pan stirrers if properly designed can significantly improve the performance of a
pan. The assisted circulation improves heat transfer and so shortens the duration of the batch
boiling, thus improving capacity. It has also been shown that stirrers improve the quality of
high-grade sugar produced (Rein, 1988). This is a consequence of the better circulation leading
to more homogeneous crystallization conditions within the pan.
The first stage of a strike is characterized by a high evaporation rate. An intense
evaporation occurs, and stirrers do not have much effect in this period. This suggests that the
effect of buoyancy forces due to temperature differences and mechanical circulation are small in
comparison to the circulation induced by vapor bubbles. As the strike height is increased, the
hydrostatic pressure of the massecuite in the calandria increases the massecuite temperature,
leading to a reduction in temperature gradient between massecuite and the heating vapor. During
the last stages of the strike, the evaporation is minimal, resulting in a low bubble flow. The
effect of the forced circulation becomes important at this stage, and is reflected in higher heat
transfer coefficients with respect to natural circulation vacuum pans (Austmeyer, 1986). This is
shown in Table 2.
There is no general agreement about the cost effectiveness of a stirrer. Its installation
gives all the advantages associated with good circulation mentioned previously, and promotes
circulation at the end of the strike. In contrast, high capital costs, air leakage and high power
consumption, particularly at the end of the strike, work against the use of stirrers.
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Journal American Society of Sugar Cane Technologists, Vol. 24, 2004
Table 2. Typical heat transfer coefficients in cane sugar vacuum pan boiling (Bubnik et al.,
1995).
Start
End
Natural circulation
(W / m2K)
570
32
With stirrers
(W / m2K)
640
224
Mechanical agitators provide the option of achieving an acceptable heat transfer with a
temperature difference as low as 20° C, compared to at least 45-50º C in the absence of a stirrer.
The use of lower pressure vapors becomes possible (e.g. coming from the 2nd or 3rd evaporator
effect), allowing reductions in the factory steam requirements.
A stirrer can be used with a smaller diameter downtake, thus enabling a larger heating
area to be installed for a given pan diameter.
Jigger steam: The installation of a sparge pipe under the calandria to blow low pressure
steam is another option to assist the circulation, reducing the massecuite specific gravity in the
calandria and reducing the effect of hydrostatic head. The simplicity, low cost and absence of
moving parts of this alternative make it particularly straightforward to put into practice. Venting
of incondensables into the jigger arrangement can be done to reduce the consumption of vapor, if
steam economy is important.
It should be noted that the vapor admitted through the ring does not condense, but passes
straight through the massecuite without causing any superheating or dissolution of crystal. The
advantage of jigger steam is that it can be shut off at any time, unlike a stirrer, and can be
adjusted to give the required degree of circulation.
Air injection: The injection of air instead of vapor has been adopted in some Spanish
factories, allowing an increase in circulation with low energy consumption (15 % of that required
by stirrers), low costs, and without breakage of crystals (van der Poel et al., 1998). Injection of
compressed air at 750 kPa preheated to 65-70º C beneath the calandria of A pans was reported to
be effective in reducing the boiling times by 11 % and to increase recovery without an
appreciable effect on the pan vacuum (Stobie, 1999). However this option requires a much larger
vacuum pump or ejector to remove the additional air and is not recommended here.
APPLICATION OF CFD IN THE SUGAR INDUSTRY
The development of numerical techniques and codes for the solution of fluid mechanics
and heat transfer problems has evolved particularly rapidly in the last few years, making it
possible to resolve complex problems through the discretization of the geometry and numerical
solution of the governing equations. This approach is known as Computational Fluid Dynamics
(CFD) and has proved to be a valuable tool for research and analysis of engineering problems in
many different industries. Currently there is a strong trend towards CFD assisted design,
although experimentation still remains important, particularly for the most complicated cases.
6
Rein: Circulation in Vacuum Pans
The application of CFD in the sugar industry has much potential and has been already
used to some extent in the analysis of juice clarifiers (Steindl, 2001; Chetty, 2002) and bagasse
boilers (Dixon, 2001), where important improvements have been reported. A promising
application of CFD is the analysis of flow in vacuum pans, in view of the importance of
massecuite circulation and the complexity of the problem. A correct numerical approximation of
the flow patterns inside the vessel could give valuable information for the optimization of the
design of vacuum pans.
Analysis of the fluid flow inside vacuum pans
A first attempt to analyze numerically the flow pattern inside vacuum pans was
developed by Bunton (1982), who wrote a program to solve the flow field, requiring some
considerable assumptions in the process. Brown (1986) obtained results with a commercial CFD
code for a stirred crystallizer, representing the calandria tubes as rings and neglecting the vapor
phase. A more refined model was developed by Stephens (2002), who using the rings approach
developed a numerical solution for boiling in the calandria, which was coupled with a
commercial CFD code (CFX) for the solution of the flow in the rest of the domain.
In general, vacuum pans have been modeled using two-dimensional axisymmetric
isothermal two-phase models. The massecuite has been assumed to be Newtonian, and the
computational domain has been restricted to the zone below the massecuite free surface. The
numerical results have suggested undesirable recirculation zones, as well as an important
reduction in circulation when the strike height and the viscosity are increased. Nonetheless,
alternatives to reduce the resistance to the flow or improve the fluid circulation have not been
explored or reported.
The Audubon Sugar Institute in conjunction with the LSU mechanical engineering
department is working currently on the study of the flow in vacuum pans. The main
characteristics of the CFD model and some initial results are presented here.
CFD Model
In a vacuum pan there are three phases present (liquid: mother liquor - gas: water vapor solid: sugar crystals). However, for convenience the case can be analyzed as a two-phase flow
(liquid and vapor), which is an acceptable approach considering the high viscosity of the
massecuite (Bunton, 1982).
For the grid generation and CFD solution, a commercial code has been used (FLUENT).
A two-dimensional axisymmetric problem has been considered to simplify the geometry. A grid
independence study was performed, finding that between 90,000 and 180,000 quadrilateral
elements the results were the same. It is important to use a grid fine enough so that the accuracy
of the results is independent of the grid points used.
The Eulerian-Eulerian multiphase model was selected, which is the most appropriate
considering the complex interaction of the phases and the high volumetric fraction of the
secondary phase (vapor) in the tubes and above the calandria. With this model, a Eulerian
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Journal American Society of Sugar Cane Technologists, Vol. 24, 2004
treatment is used for each phase, resulting in coupled transport equations for the liquid and the
vapor phase. Solving the transport equations for both phases requires substantial computational
effort, and can require several days of processing before a solution is reached.
The temperature field is assumed isothermal. Thus the calculations are limited to small
temperature differences. Since the vapor bubbles are the main driving force for the circulation of
massecuite, the calculations are expected to be representative of the real situation. The heat
transfer phenomena associated with the process have been studied extensively by Rouillard
(1985) and Austmeyer (1986).
As driving force for the circulation of massecuite, steam is assumed to be injected in the
zone corresponding to the calandria tubes. The calandria is represented as a set of concentric
rings (Brown, 1992; Stephens, 2001). It is assumed that the same evaporation takes place across
the calandria. However, knowing that tubes with less circulation evaporate less, a velocity profile
across the calandria smoother than in real vacuum pans is expected. The steam is injected
without any momentum, and rises due to its density difference with the surrounding massecuite.
As a case study, a 90 m3 pan was selected, which is considered a modern well-designed
pan giving a good performance. In addition three other cases, a pan with a conical enlargement,
a pan with a steam jigger installed, and a pan with a conical bottom, have been undertaken in this
study.
The properties of the massecuite and the amount of steam to be injected in the calandria
were determined using figures considered normal for conditions between the middle and the end
of a B-strike (Table 3).
Table 3. Massecuite properties and evaporation used to represent a B-strike.
Strike height (m)
Absolute pressure (kPa)
Density massecuite (kg/m3)
Dynamic viscosity massecuite (Pa.s)
Evaporation Coefficient (kg/m2.h)
Density vapor (kg/m3)
Dynamic viscosity vapor (Pa.s)
1.0
14.0
1490
52.1
11
0.183
1.12E-05
RESULTS AND DISCUSSION
CASE 1. Reference case: W-bottom, straight sides
The flow simulations have illustrated how the acceleration of vapor rising through the
calandria tubes reduces the local static pressure, which falls below the pressure in the downtake.
The created pressure difference promotes the up flow of massecuite in the calandria and down
flow in the downtake.
8
Rein: Circulation in Vacuum Pans
a. Contours volume fraction of vapor (%)
b. Contours of static pressure (Pa)
c. Vapor velocity vectors
d. Massecuite velocity vectors
Figure 2. CFD results for straight-sided, W-bottom, vacuum pan.
The resultant distribution of the flow across the calandria is even, with exception of the
ring located closest to the downtake, which has a smaller flow and suggests that the tubes located
in this region have less massecuite flowing through them.
The velocities below the calandria are low enough to let the hydrostatic head determine
the pressure field, resulting in practically the same static pressure in the inlet of all the tubes
(Figure 2.b). Consequently, unless a blatantly wrong geometry is chosen for the bottom, it will
not have any important implication on the even distribution of the massecuite across the
calandria, which is desirable from the heat transfer point of view and to have uniform conditions
within the vessel.
An undesirable large vorticity over the top tube plate was found for all the runs. This
vorticity seems to originate in the region where the flow coming up from the tubes meets
tangentially the down flow at the wall of the downtake (Figure 2.d). As a consequence of this
vorticity, there is an important reduction in the useful area of the downtake. There is also a
narrower area for the liquid that travels in radial direction towards the center, and some
9
Journal American Society of Sugar Cane Technologists, Vol. 24, 2004
recirculation of part of the liquid in the zone over the top calandria plate. This vorticity does not
contribute at all to the desired circulation, and could lead to different crystallization rates for
crystals that are trapped there.
The results indicate a small vorticity over the calandria plate close to the sides of the pan
(Figure 2.d). However, this does not show any effect on the circulation through the outer rings,
suggesting that it does not influence significantly the flow of the massecuite through the
calandria tubes.
CASE 2. Conical enlargement.
The results indicated that the conical enlargement leads to another large vorticity over the
top tube plate, between the sides and roughly the middle of the calandria width (Figure 3). This
vorticity interacts with the vorticity located toward the downtake, and its size exceeds
expectation. The additional vorticity involves a large downflow near the sides, which has a
negative momentum in the vertical direction, opposing the rise of massecuite, and that results in
a reduction of 19 % in the circulation in the outer rings (Figure 4). The outer rings represent
precisely the zone of the calandria where more tubes are present.
a. Contours volume fraction of vapor (%)
b. Massecuite velocity vectors
Figure 3. CFD results for vacuum pan with conical enlargement.
10
Rein: Circulation in Vacuum Pans
0.20
0.18
Liquid velocity (m/s
0.16
W-bottom
W-flared
W-jigger
V-bottom
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Radius (m)
Figure 4. Liquid velocity results across the calandria for different pan geometries.
CASE 3. Circulation assisted with steam jiggers.
A simplified steam jigger system is considered, consisting of a single sparge ring under
the calandria. The steam injected below the calandria increased substantially the circulation in
the pan, accelerating the flow particularly in the rings located over the steam jigger (Figures 4
and 5). This indicates that the tubes located closer to the points where the gas is injected will
have a very high circulation. Vorticity over the top tube plate increase, but no significant
reduction in the circulation in the other tubes was observed. The average circulation increased
24% respect the same geometry without steam injection. It also appears to reduce the
recirculation in the downtake and that it may be better to introduce the vapor uniformly below
the calandria to induce a higher flow in all the tubes.
a. Contours volume fraction of vapor (%)
b. Massecuite velocity vectors
Figure 5. CFD results for vacuum pan assisted with steam jiggers.
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Journal American Society of Sugar Cane Technologists, Vol. 24, 2004
CASE 4. Conical or “V” bottom.
The flow field over the top tube plate and the flow distribution across the calandria were
similar to that found for the W-bottom reference case, but with a massecuite flow 5 % higher.
The higher circulation is attributed to the smaller resistance of the bottom to the downflow and
smoother change of direction. The expected stagnation zone at the lowest part of the pan is small.
a. Contours volume fraction of vapor (%)
b. Massecuite velocity vectors
Figure 6. CFD results for straight-sided with conical or V-bottom.
In spite of the assumptions made in modeling the problem, the results obtained agree with
experience, indicating that the W-bottom is efficient in distributing the massecuite, and that the
flared sides or conical enlargement impairs the circulation, while the use of steam jiggers
improves circulation.
This work will continue using CFD to evaluate the factors affecting circulation in
vacuum pans (e.g. pan geometry, strike height, viscosity, type of calandria, downtake area, and
steam jigger design). A scale model designed and constructed specifically to study the
circulation in vacuum pans with laser anemometry techniques (PDPA) will be used to verify the
computational results. Finally, the CFD will be applied to explore alternatives for improving the
circulation in batch vacuum pans, and extended to the analysis of continuous vacuum pans.
PRACTICAL OPTIONS FOR IMPROVING CIRCULATION
Stirrers
The stirrer itself is located in the downtake if it is an axial flow impeller, or else just
below the downtake in the case of a radial flow impeller. Installed power should be in the range
1 to 1.7 kW/m3. Tip speed has to be kept below 7 m/s (23 ft/s), or else false grain will form.
12
Rein: Circulation in Vacuum Pans
Each stirrer should be designed for the particular pan and its duty individually. There is
no universal stirrer for all occasions. If good results are to be obtained, it is seldom possible to
transfer a stirrer from one pan to another without redesigning the arrangement.
Conversion of flared to straight-sided pans
As shown earlier, the conical enlargement above the calandria has a negative effect on
pan circulation. This flaring of pans was done to increase the strike/seed ratio of pans, but this
design is now largely obsolete. However, there are a large number of these pans still installed in
sugar mills. Old pans at Lula and Alma factories have been re-tubed and at the same time have
been changed from flared to straight-sided pans by increasing the calandria diameters. The Lula
conversion is reported in detail at this conference (Bergeron and Carline, 2003). As a result of
the conversion, pan volume increased by 25 % and the heating surface by 52 %. The circulation
ratio is affected adversely, but the circulator and the absence of the flared body lead to a
significant net improvement in pan performance. C pan boiling times dropped by 2 hours.
Condensate and incondensables removal
Adequate arrangements for the removal of condensate and incondensable gases must be
made. It is important that these details are given proper attention, as they can be the cause of
under-performance if not properly designed. Condensate is generally removed from the lowest
point of the shell, from more than one outlet. It is good to be generous in sizing these outlets.
The size of the condensate drains should be based on liquid outlet velocities of less than 0.45 m/s
(1.5 ft/s) at maximum evaporation rate.
Incondensables need to be purged by the steam flow and are generally located at a point
furthest from the steam inlet. This ensures a positive purging of incondensables. The best
arrangement is a radial flow of steam from a vapor belt around the calandria. As the steam flow
is radially inwards, the incondensables offtake arrangements should consist of two rings around
the downtake, one at the top and one at the bottom of the calandria. Care needs to be given to
the size and number of holes in the incondensables offtake rings to ensure uniform offtake
around the downtake, and that sufficient quantity is vented. The two incondensable offtakes
should be separately vented. Incondensable quantity is estimated as about 100 mg/kg of steam.
In order to ensure total venting of incondensables, the amount vented should be 100 times the
quantity of incondensables. Thus the total quantity to be vented should be about 1 % of the
steam flow to the calandria.
Thermostatic valves can be used to bleed incondensables if steam economy is important.
Removing Resistance to Circulation
Any unnecessary steel inside the pan represents a resistance to circulation. Attempts
should be made to get rid of any unnecessary ironware in the massecuite. Louvers or any other
steel devices installed in the pan in the mistaken idea that they will improve circulation should be
removed.
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Journal American Society of Sugar Cane Technologists, Vol. 24, 2004
Condensate and incondensable venting lines should not be run through the massecuite.
Incondensable gases should be vented through the outside of the calandria, and not up through
the massecuite. Likewise, condensate outlets should be positioned at the periphery of the pan,
and should not run from the bottom tube plate down through the massecuite.
The syrup or molasses feed system should also represent minimum obstruction to the
circulating massecuite. The feed should be introduced through the periphery of the pan below
the calandria or through minimum sized pipes or channels on the floor of the pan. If the feed is
conditioned and at a higher temperature than the boiling massecuite, the feed must be directed
under the calandria, so that the flash will aid circulation.
Steam-assisted circulation
Sparging of steam underneath the calandria has often been used as a means of improving
circulation. It has the effect of increasing the velocity of the massecuite through the tubes, thus
increasing the rate of heat transfer to the massecuite and further improving circulation.
If the pan is badly designed and/or the massecuite level gets too high, there comes a time
with a high viscosity massecuite when the pan stops boiling. Heat transfer in the tube is
insufficient to cause significant boiling and movement of massecuite slows. This is a vicious
cycle. As the heat transfer rate reduces, the rate of movement of massecuite drops, and heat
transfer drops even further. Steam assisted circulation can get such a pan to start boiling again.
Thus the effect of steam assistance, often called “steam jigger” is to enable a higher strike height
to be achieved as well as a reduced boiling time.
Before the 2002 season, steam jiggers were fitted to three C massecuite pans at Raceland.
These pans are fitted with stirrers, but it was still considered that the circulation in these pans
could be improved, as it was not possible to run them continuously on vapor 1, and high Brix C
massecuites could not easily be achieved. The steam jigger consisted of a 51 mm (2”) ring, fitted
under the calandria, one-third of the width of the calandria from the pan periphery. It had 32 x
9.5 mm (3/8”) holes drilled in the pipe, facing downwards. These pans were all nominally 39.6
m3 (1400 ft3) pans, with the diameter at the calandria 0.4 m (13 ft) and above the calandria 0.47
m (15.5 ft). These pans already had circulators fitted, but still the steam jigger had a marked
effect on boiling times and strike heights.
The sizing of the system was based on vapor injection at a maximum rate of 25 kg/m3 hr,
giving a maximum vapor 1 rate of 1.15 ton/hr. This compares with estimated steam flows to the
pan of 17 ton/hr dropping to about 3.7 ton/hr on average. The number of holes and their size is
determined by the required flow rate of vapor. The sizing of the ring has to be calculated so that
the pressure drop through the ring is less than 10 % of the pressure drop through each hole. This
ensures that roughly equal flows will be achieved through every hole (Knaebel, 1981). This
simple arrangement may not be satisfactory in a larger pan, where a different sparging
arrangement should be used to give a more uniform distribution of vapor under the calandria.
14
Rein: Circulation in Vacuum Pans
Comparison of the performance before and after the installation of the steam jiggers is
complicated by the fact that processing conditions in 2002 were the worst experienced in many
years as a result of the severe weather conditions. Nonetheless the following was achieved:
•
The pans could be boiled throughout the full cycle using vapor 1. In 2001, the pans were
run on exhaust during the final stages of the boiling.
Average C massecuite Brix increased from 95.6 to 96.8. After the most severe processing
problems in October and November were over, this enabled the mill to achieve their
lowest recorded molasses target purity differences.
Boiling times were roughly the same in both years in spite of conditions which in some
mills led to pans stopping boiling altogether.
•
•
It should be borne in mind too that the effect of installing a steam jigger system can be
expected to be most significant in a pan without a circulator.
During November, the stirrer in pan 1 stopped working. Instead of stopping to repair the
stirrer, the mill was able to continue operating the pan with the stirrer in the downtake
representing a restriction to circulation. Figure 7 shows that boiling times increased, but the pan
operation continued
Pan #1 Boiling Time For November 2002
10
9
8
Hours For Boiling
7
6
5
4
3
Pan with Circulator
and Jigger Steam
2
Pan without
Circulator, Only Jigger
Nov. 18,2002
1
0
0
10
20
30
40
50
60
70
80
Strike #
Figure 7. Boiling times in Raceland pan 1 during November 2002.
15
90
100
Journal American Society of Sugar Cane Technologists, Vol. 24, 2004
CONCLUSIONS
Circulation in a vacuum pan is one of the most important parameters affecting
performance. A number of factors which influence the operation of vacuum pans have been
identified. Some relatively minor changes can be made to improve capacity and performance.
However it is clear that most designs are far from being optimal. CFD appears to offer a tool for
progressing efforts to improve the design and operation of both batch and continuous pans.
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Rein: Circulation in Vacuum Pans
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17