Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
- 1319096
PRECRYSTALLIZER WITH HEATED SCRAPERS
Back~round of th_ Invention
The invention concerns a precrystallizer,
comprising a tube surrounded by a cooling jacket and
equipped with inlet and outlet openings for a melt to be
precrystallized. A motor-driven rotor body with radially
projecting scrapers revolves in the area of the tube
wall.
Precrystallizers are known, e.g., from German
Patent Document No. 32 09 747. They are used to
accelerate the process of solidification of materials
provided in the form of melts, wherehy nucleation for the
crystallizing process is initiated. This is effected by
cooling the melt at the wall of a tube containing the
melt. Desirably, the growth of crystals on the tube wall
should be prevented, because such crystals are not
adapted for further processing operations and would
render the precrystallizers useless. Accordingly,
scrapers connected with the rotor bodies are provided,
which revolve to remove cry tals tending to grow on ~he
tube wall. A disadvantage of such precrystallizers
consists of the fact that the effectiveness of the
mechanical scraping process is not always adequate, and
moreover, crystals may become baked onto the scraper
dges whereby the scrapers are no longer able to perform
their proper scraping function and may undesirably stress
and deform the tube wall leading to a reduction of the
cooling capacity and an imbalance of the rotor. To
eliminate the problem, attempts were made heretofore to
connect several precrystallizers in parallel preceding
1319096
the production apparatus, ~e.g., a droplet former), so
that a usable precrystallizer is always available, while
non-usable precrystallizer~ are being readied for
operation by being dismantled or having the solidified
material melted away. Such a facility i~, however,
expensive.
Precrystallizers of the afore-mentioned general
type are further known, wherein the scrapers are in the
form of hollow bodies through which a heating medium is
flowing, e.g., see German Patent Document 19 17 089.
Steam is also flowing as a heating medium through the
tube. The scrapers are in the form of radially
projecting hollow shovels which rotate within the tube.
These layouts have an undou~ted advantage o~er unheated
scrapers relative to the adherence of crystals. ~hey do,
however, have the disadvantage that the heating medium is
connected only through radially extending stub lines with
the cavities of the scrapers, so that heat transfer is
very poor because the heating medium is largely
stationary (non-flowing) in the scrapers. The result is
an undesirable condensation of steam inside the scrapers
and a resultant inadequate heating, leading to the baking
of crystals, especially along the radially outer edges of
the scrapers.
It is an object of the present invention to
develop a precrystallizer of the afore-mentioned type so
that the ormation of crystals on the ~crapexs is safely
prevented.
1319~96
Summar~ of Preferred Embodiments
of the Inventlon
,
To attain this object, it is proposed to mount
~he hollow scrapers by means of at least two hollow arms
onto the rotor body and to series~connect the arms by
flow passages in the rotor body to carry the heating
medium. It has been found that with such a con~iyura-
tion, a flow of heating fluid is induced through the
scrapers to insure that an adequately thorough heating
~0 of the scrapers takes place to prevent the adherence of
crystals. Significantly longer operating times of the
precrystallizers (between cleanings) may be achieved in
this manner. The crys~als scraped from the tube wall are
not retained on the scrapers for a long period, in view
of the revolving motion of the scrapers, so the crystals
scraped off are not returned by the heating process to a
melt condition, which in itself would be undesirable
since the formation of crystalæ is to be initiated in
the precrystallizer.
However, the heating is adequate to protect
the scrapers themselves against the settling of the
crystallizing melt. ~his may be further enhanced by
providing good mixing of the melt in the area of the
~crapers, as set forth below.
~f stea~ is used as the heating medium, it is
conducted through one of the armæ into the associated
scraper and returned to the out~ide through the rotor
body as a condensate only after flowing through the
scrapers. It would also be possible to use a strongly
heated melt as the heating medium, which would be cooled
1319096
during its passage through the scrapers and the
associated flow channels to the extent that it could be
transferred subsequently to the inner space of the tube
of the precrystallizer. This configuration would have
the advantage that no return conduits would be needed in
the rotor body itself. However, such a design involves a
certain effort for the control of the temperature of the
melt used as the heating medium, which would have to be
returned to the melt to be recrystallized at an
appropriate temperature.
The arms can be shaped to induce a motion of
the melt. Thus, even in the radially inner zone of the
scrapers, i.e., in the area between the fastening arms
for the scrapers, there is effected a movement of the
melt to mix it with zones enriched in crystals.
Simultaneously, it also occurs that the crystals removed
by the scrapers from the walls are removed from the
scrapers themselves by the movement of the melt generated
in these areas, so that the retention time of the
crystals, as indicated above, is relatively short. By a
suitable shaping of the arms, an axially directed mixing
motion of the melt may be effected in the precrystal-
lizer, which motion takes place either in one direction
or alternately, in both axial directions. It has been
found that in this manner, due to better mixing, the
efficiency of precrystallizers of this type is improved
relative to the enrichment of the melt in crystals.
~319~96
Also, alternating accelerations of the melt may be
obtained, which firstly contributes to the prevention of
the adhesion of crystals to the rotor body and secondly
effects a uniform mixing and thus good preparation of the
melt for the subsequent solidification process~
Brief_Description of the Drawin~s
The objects and advantages of the invention will
become apparent from the following detailed description
of preferred embodiments thereof in connection with the
accompanying drawings, in which like numerals designate
like elements, and in which:
FIGURE 1 is a schematic longitudinal sectional
view through a precrystallizer and melt conduit system
according to the invention, with portions of an outer
thermal insuiation broken away;
FIGURE 2 is a schematic longitudinal sectional
view of a rotox hody whioh can be used in FIG. 1;
FIGURE 3 is a side elevational view of another
embodiment of the rotor body which could be used in
FIG. 1, with portions thereof broken away to expose the
interior;
FIGURE 4 is an enlarged detailed view of a
scraper used on the rotor body of FIG. 3;
FIGURE 5 is a sectional view taken along the
line V-V in FIG. 4;
FIGURE 6 is a sectional view taken along the
line VI-VI in FIG. 4 depicting a astening arm for a
scraper;
~319~
FIGURE 7 is a view similar to FI~. 6 of a
modified form of fastening arm; and
FIGURE 8 is a view similar to FIG. 6 of yet
another form of arm.
Detailed Description of Preferred
Embodiments of the Invention
_
Figure 1 shows a precrystallizer 1 comprising
a tube 3 surrounded by a cooling jacket 2. The tube is
supplied with a melt of the material to be precrystal-
lized, by means of a pump (not shown) and a valve 31.The melt passes through a line 32 and a heat exchanger 33.
The tube 3 i8 provided with an inlet opening 34 and an
outlet opening 35, whereby the melt may be conveyed into
the tube and removed from it. The melt may then be
transported in the precrystallized state in a conven-
~ional manner by means of a pump 36 to a processing
apparatus, for example, a droplet former. The tube 3
is further equipped with a parallel bypass line 37,
whereby a pump 38 returns the material not conveyed by
the pump 36 in a loop to the inlet opening 34. All the
piping of this circulation and of the feeder line 32 is
in the form of heated double walled tubes ~now shown in
detail) and is equipped with additional thermal
insulation 39, depicted schematically.
The melt passes axially through the tube 3 and
crystallizes in the region of the tube wall due to the
lower temperature of the wall. The melt is preven~ed
from s~ttling and baking on the wall by a rotor body 5
equipped with radially projecting scrapers 6. The
scrapers 6 are mounted ~y means of radially projecting
~3~90~
arms 8 on the rotor body 5, which is rotated by a drive
motor 4. The scrapers 6 revolve in the area of the tube
wall and prevent the crystals formed in the cooler ~all
zone from settling on said tube wall. The scrapings are
returned to the melt as nuclei for continued
crystallization. The temperature in the cooling jacket 2
is maintained constant, in a known manner, by means of an
appropriately controlled he~t exchanger 40 and a
circulating pump 41 for the schematically indicated
cooling medium circulation 42.
Depicted in FIG. 2 is a first embodiment of the
rotor body 5' which comprises a closed hollow body,
equipped at a first end 5b wi~h a bearing journal 43 and
at a second end 5a with a conventional bearing journal 12
for the connection of a rotating packing box~ Two
channels 15 and 16 extend concentrically, wherein tha
channel 15 annularly surrounds the inner channel 16.
Steam conduits 44 and 45 emanate from the outer
channel 15 to the scrapers 6 locatled nearest to the rotor
second end 5a and extend through the arms 3~O A first
row 6b of scrapers ~ is mounted on one side of the rotor
body along a longitudinal center plane of the rotor
body 5', with a second row of scrapers 6a being
coordinated with the row Sb on the diametrically opposed
side of the rotor body S'. The individual scrapers 6 are
distributed in an axially uniform manner over the length
of the rotor body 5', in a manner such that the scrapers
of each row are longitudinally offset relative to the
adjacent scrapers of the opposing row by about one
scraper length.
~he ~nlet 44 to the first scraper 6 of the
13~19~9~
row 6b may therefore extend approximately radially, while
the inlet 45 for the row 6a should extend radially and
axially in the approximate shape of an S, to reach the
respective arm ~'. All of the scrapers 6 are in the
form of hollow bodies, the internal cavities of which
communicate with one another by means of the hollow
arms 8' and connecting lines in the form of separate
conduits 9 installed within the rotor body 5'. Thus,
steam from the bearing journal 12 enters the rows of
scrapers 6a, 6b through ~he inlets 45, 44, respectively,
and travels successively in series through the scrapers
of the respective rows, and then exits a last scraper in
each row ancl passes into a return pipe 60 disposed in the
rotor body 5'. The steam has by now condensed, and the
condensate travels out of the rotor body through the
channel 16.
The scrapers at the front end 5a are therefore
hotter than the downstream scrapers, since the steam has
been cooling during its flow. This cooling, howeverl
taXes place in counterflow relationship to the movement
of the melt in the tube 3, which is introduced in the
area of the irst end 5b of the rotor body 5' and removed
in the area of the second end 5a.
Figure 3 shows a second embodiment of the rotor
body 5" which is not continuously hollow, but is equipped
with a center return bore 22 for the condensed steam.
The steam, which here again is supplied through a packing
box bearing journal 12' at the second side 5a of the
rotor body 5~ (which in FIG. 3 is revers~d relative to
FIG. 2), flows through the scrapers of rows 6a and 6b.
However, the steam does not flow separately through the
1319~96
rows 6a, 6b. Rather, the steam is conducted from the
first scraper 6 of a irst row 6b through a bore 11
passing through the rotor body 5" to the first scraper 6
of ~he other row 6a. Thus, the steam travels zigzag-like
through the rotor body 5 n . The steam exits the last
scraper of the row 6a and enters the bore 22. The
bores 11 are formed in solid portions of the rotor body.
Figures 4 to 6 show in detail the configuration
of the scrapers used for ~he rotor body 5~. The scrapers 6
comprise gutter-shaped or U-shaped bodies 7, closed-off
on both ends by plates 46 which are tightly welded to the
gutter body 7. A blade 20 is welded tightly to the
gutter body 7 to form therewith an internal steam passage
in the gutter body 7, which passage communicates with an
inner side of the blade 20. An edge 20a of the blade 20
forms a radially outermost point o~ the scraper 6. The
radially outer surface of the gutter body 7 extends from
the blade 20 in a direction extending circumferentially
rearwardly (with reference to the direction of blade
travel) and radially inwardly from an imaginary circle 47
concentric with the outer circumference of the rotor body 5n.
The rear side of the gutter body 7 is rounded in this
embodiment to facilitate the flow of me~t around the
scraper 6 and retard the adhesion of crystals to the
scraper 6.
It is further seen in FIGS. 4 to 6, that the
arms 8n each include a $eeder bore 48 disposed within a
profiled body in the form of an isosceles triangle. Each
of the bores 48 could be connected by means of connecting
1319~9~
bores 11 with the adjacent scraper of the other row or~
as indicated in FIG. 5, by means of connecting bends 49,
tightly inserted in the corresponding openings 50 of the
rotor body 5 n .
The position of the triangular arms 8~ is chosen
so that the equal legs 25 of the triangle 25 form guide
surfaces 13 located obliquely relative to the direction
of rotation 21~ The longest side 51 of the triangle 26
is located in a radial plane. By means of this
configuration, in the course of the re~olution o~ the
rotor body 5" the melt locatPd between the blade 20 and
the arms 8" is impacted by the guide surfaces 13 and
~hereby kept in motion. The area between the blade 20
and the rotor body 5~ is therefore being moved axially
relative to the rotor hody 5~ during the rotation of the
rotor body 5n. The melt is thus prevented from settling
on the arms 8`' or the blade 20 by such motion, as well as
by the heating ac~ion. As seen in FIG. 5, the forward
and rear portions of ~he arms 3" are heated additionally
hy blind bores 52 issuing from the bore 48 to further
xesist the adhesion of me`lt.
The effect of mechanical action on the melt in
the area of the arms ~ay obviously be attained by other
configurations of the arms. For example, FIG. 7 shows a
configuration of arms 8''' in droplet form, with the
location of the droplet pro-file relative to the direction
of rotation 21 being such that one side of the droplet
shape forms baffle surfaces for the melt.
The motion of the melt may further be affected
by the fact that, as shown in FIG. 3, the sides 25 of the
triangle 26 of the arms 8~ are always aligned in a
~3~9096
11
definite manner~ Thus, for example in FIG. 3, in the
upper row 6b of the scrapers 6, the sides 25 of each arm
8" face toward the second end 5a, while the sides 25 of
the arms 8" of the lower row 6a of scrapers face toward
the first end 5b. This configuration has the advantage
that the melt within the tube 3 is induced to flow in a
kind of circulating motion in the axial direction. It
has been found that mixing of the melt and the enrichment
in crystals is e pecially enhanced in this manner~ It
would also be possible to orient the guide surfaces 13 o
all of the arms in one direction or turn the associated
guide surfaces of the arms of adjacent scrapers against
each other. All of these measures enhance the mixing
motion of the melt. This can also be achieved by
operating the drive motor 4 of the rotor bodies 5', 5"
and 5 discon~inuously or intermittently. Thus, the motor
may be, or example, stopped briefly so that it would
cause an acceleration of partial areas of the melt
following short pauses. It is further possible to merely
vary the driving velocity of the motor, which again would
lead to accelerating effects on thle melt.
For such an intermittent operation of the
precrystallizer, arm configuration~ such as those shown
for the heated scrapers in FIG. 8, are especially
suitable. ~ere, a section 14 of each arm is made freely
pivotable around the axis of the bore 48' over a certain
angle defined by stops (not shown). If, therefore, the
forces 21a and 21b acting on the arm section 14 are
periodically reversed as couId be achieved by the
intermittent starting and stopping of the drive motor,
the arm sections 14 would pivot in one or the other
13190~6
direction, thereby affecting the melt always in the same
direction, even though the movement is reversed at the
scrapers themselves. The effect o heat.ing the scrapers
anA arms to prevent the adhesion of crystals to these
parts of the xotor body is thereby again enhanced.
It will be appreciated that in accordance with
the present invention, the scrapers are connected in
series to assure that heating fluid is induced to flow
through the scrapers. This eliminates the presence of
stagnant areas in the scraper which may become too cool.
Furthermore, the rows of scrapers can be connected
independently in series or the scrapers of one row can be
connected in series with the o~her row, whereby a heating
pattern of any desired type can be obtained. The arms
which moun~ the scrapers can be shaped to induce motion
of the melt in any desired direction(s).
Although the present invention has been
described in connection wi~h prefe:rred embodiments
thereof, it will be appreciated by those skilled in the
art that modifications, additions, deletions, and
su~stitutions may be made without departing from the
spirit and scope of the invention as defined in the
appended claims.
