Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE ~NVENTION
The invention relates to apparatus for removing
pulverized particles or other type of particles carried along
in a stream of gas, comprising a substantially cylindrical
housing having a first portion provided with a tangentially
directed inlet for the stream of gas to be purified, means
for flushing the inner wall of the housing located in an
annular zone which extends transversely of the centerline
of the housing and means for permitting the gas stream to
exit. The housing is provided with an eduction pipe which
exits outside the housing in such a manner that the stream
of gas to be purified flows through an axial track inside the
housing. A similar scrubber in a particular embodiment is
- shown from U.S. Patent No. 2,696,275 according to which the
stream of gas is injected into a number of cyclone paths via
a stationary blade rim and then passes along a moistened inner
wall, after which -the scrubbed stream of gas is removed
upward through the eduction pipe.
One problem which arises in scrubbers is that sorne wall
portions inside the housing are moistened occasionally as
a result of local turbulence, so that there is a precipitation
or deposit of the particles present or o~ the dissolved
material. These deposits, also known as scruffs, may be
the cause of pollution or of b~cterial growth as a result
of which the sanitary character of the scrubber is lost.
The occurrence of such problems is particularly promoted by
; the turbulence of the stream of gas inside the housing.
SUMMARY OF THE INVENTION
It is the object of the invention to provide a scrubber ~ ~`
which does not exhibit the disadvantages mentioned and which
may come up to high demands from a sanitary point of view.
According to the invention, this object is obtained in tha-t
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means for flushing are made up of an annular gutter which
via a large number of outlets exits into a second por-tion of
the housing which widens gradually within which portion the
central eduction or out]et pipe extends.
For obtaining the lowest possible turbulent flow of the
stream of gas, it is favorable if the transition between the
inner wall of the first to the second portion of the housing
runs so fluently that the stream of gas does not show an
inclination toward disengaging or losing contact with the
inner wall. As a result thereof, no air-turbulence and
spatters of fluid will occur at the point of inlet of fluid
owing to the gradual curve of the inner wall.
As a result of the above mentioned measures, the inner
wall of the housing is efficiently flushed without the risk
of leaving drops of fluid spraying away. The particles in
the circulating stream of gas are urged outward in the
direction of the flushed inner wall, and a sharp separation
occurs between the dry inner wall in the Eirst portion of
the housing and the inner wall of the second portion of the
housing which is kept wet by complete flushing.
~; In an embodiment of the scrubber according to the
invention the flushing gutter forms part of an annular
intermediate element merging with the two portions of the
cylindrical housing. This embodiment simplifies the
construction of the apparatus and in addition allows a certain
thermal isolation of the gutter with respect to the inner
wall of the housing to be brought about as a result of which
unwanted condensation of fluid against the dry wall portion
is prevented. In the flusing gutter fresh fluid is supplied
continually as a result of which this fluid will circulate
in the gutter.
The length of the central eduction or outlet pipe
inside the second (flushed) portion of the housing is
important in that it influences the percentage of catch of
the particles from the stream of gas. In the scrubber
according to the invention the central eduction pipe extends
a distance past the annular flushing zone which is many
times larger than the distance between the pipe and the
inner wall of the second portion of the housing.
It is furthermore important that the tangentially
directed supply for injecting the stream of gas takes place
via a slit-shaped aperture with a height which is practically
equal to the height of the first portion of the housing and
with a width which is smaller than the distance between the
central eduction pipe and the inner wall of the first housing
portion. This contributes to the suppression of a turbulence
and to the promotion of the transport of the particles -to the
inner wall of the housing. An e~tension of time that -the
stream of gas remains in the housing may be obtained by
lengthening the central eduction pipe.
Another important measure for preventing bacterial growth
may be found in a reduction of the temperature of the fluid
`- collecting on the bottom. The method applied for this
; purpose relates to the operation of the scrubber described
~ above which is connected with, for instance, an apparatus for
. ~ . spray-drying milk, in which a first portion of a flushing
fluid is utilized for flushing a zone along the inner wall of
the scrubber. According to the invention, a second portion
~` of this fluid is supplied in a controlled way to the lower
portion of the flushing zone for cooling purposes. This
means that the bacterial growth is fought not only on the dry
3~ surface of the eduction pipe (wlth high temperature) but also
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~; on the wet surface by means of a low temperature. The
foregoing and other objects and features of the invention
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will become apparent from the following description and
accompanyiny drawings.
BRIEF DESCRIPTION OF TlIE DRAWINGS
Fig. 1 is a side-view with partial cross-section of
the scrubber;
Fig~ 2 is a cross-section on an enlarged scale of a
variant of a detail of the apparatus according to Fig. l;
Fig. 3 is a top-view according to the arrow III in
Fig. 2;
Fig. 4 shows schema~tically a complete apparatus.
DESCRIPTION OF A PREFERRED EMBODIMENT
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As may be seen from Fig. 1, the apparatus comprises a
substantiallv cylindrical housing 1 which includes a first
(upper) portion 2 and a second (lower) portion 3 which is
somewhat widened with respect ot the portion 2. The first
portion 2 of the housing 1 is provided with a tangentially
directed inlet 4 for the stream of gas to be scrubbed.
Inside the housing there is an educti.on or outlet pipe 5
for the scrubbed gas; pipe 5 extends into the lower part
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of portion 3 of the housing and exits a~t the portion 2
of this housing. Between the inlet 4 and the bottom of
the eduction or outlet pipe 5, there is an annular gas track
A which is traversed by the stream o gas to be scrubbed.
Next to and partly past this track A there is provided a
flushing zone B. At the end located near the gas inlet 4,
the flushing zone B is limited by means for flushing the
inner wall of the housing. These means are located in an
annular zone which stands transversely to the centerline 6
of the housing 1, and they comprise an annular gutter 7 which,
via a large number of passages 8 communicates with the widened
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second portion 3 of the housing 1, i.e. the beginning of
~- the flushing zone B. At its o-ther end the zone B is limited
by the bottom 9 of the housing 1. The fluid in the bottom 9
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is collected in an annular gutter 9' as a result of the
inward curve 10. A discharge line 11 is connected with
this gutter 9', while moreover there is provided a connection
14 hereafter explained.
As may be seen from Fig. 1, the transition between the
inner wall of the first portion 2 of the housing 1 to the
second portion 3 of the housing 1 runs smoothly in such a
way that the turbulence of the stream of gas is suppressed.
In the embodiment illustrated, the passages 8 between the
gutter 7 and the inner wall of the lower portion 3 of the
housing are in the form of short flow sections that efficiently
flushes the inner wall with moistening fluid without
splashing. Such inner wall flushing means makes it possible
that the outside of the eduction or outlet pipe 5 remains dry
and that the inner wall of the lower portion 3 is continually
moistened and will not show dry spots. The moistening Eluid
required for this purpose may be supplied to the gutter 7 in
such a way that a circulating stream will occur which promotes
a uniform flushing of the inner wall of the housing. The
gutter 7 may be double-walled or be isolated in any other
manner with respect to the upper portion 2 of the housing, so
that local cooling of the inner wall is prevented and, in
this manner, no condensation drops will precipitate from
the stream of gas. On the top of the gutter 7 is open and
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~e~a~d by a lid 12. This allows the passages 8 to be reached
from the outside Eor cleaning, if necessary.
The lid 12 allows the Elushing gutter 7 to form a
closed space so that the fluid may be supplied under some
overpressure (the pump pressure). This may promote the
stream through the passages 8.
The length of that portion of gas eduction or outlet
pipe 5 which is located inside the housing 1 and the distance
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C between this pipe and the lnner wall of the second portion
3 of the housing 1 are important inasmuch as a little distance
C increases the possibility that the particles entrained in `!
the stream of gas will touch the inner wall, and a great
length A ensures a sufficient stay. Preferably the distance
A, in which the central eduction pipe 5 is located in the
flushing zone B, will be many times greater than the said
distance C (for instance ten times). It is also important
that the tangentially directed supply of the stream of gas
is effected via the slit-shaped aperture with a height D
illustrated in Fig. 1, which height is practically equal to
that of the first portion 2 of the housing. The width E of
the supply 4 is preferably smaller than the distance F between
the central eduction pipe 5 and the inner wall of the first
portion 2 of the housing.
Figs. 2 and 3 show a variant of the flushing means.
The gutter 7 is therein part of an annular intermediate
element 13 with which the two portions 2 and 3 of the
cylindrical housing 1 are connected~ Fig. 3 gives a picture
of the number of ?assages 8 (in this case bores) between
the gutter 7 and the inner wall o~ the portion 3. At a
housing diameter of about 1700 mm, the radius of curvature
R has a value of about 25 mm.
The fluid discharged at outlet 11 is not contaminated
with bacteria and may be conveyed to evaporation and drying
apparatus where valuable particles carried thereby may be
recovered. It is observed that the dimensions of the passages
with respect to the total amount of f luid supplied are
such that the inner wall of the portion 3 of the housing
remains wet completely down to the bottom 9 in order to
prevent any beginning of precipitation of bacterial growth.
The most important advantage of the scrubber according to the
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invention is that -the particles emanating from a stream
of gas from a drying apparatus may be caught without
contamination because the inner wall of the housing 1 is
flushed uninterruptedly in the flushing zone B.
The eduction or outlet pipe 5 remains dry as a result
of the centrifugal action of the rotary stream of air by
which a sharp separation between the wet and the dry portions
of the scrubber is maintained. It is of vital importance
that the eduction pipe 5 remains dry, because:
(a) no drops are carried in the air stream;
(b) no bacterial growth may occur in dry surfaces in
; any event if the temperature on the surface is sufficiently
high (above 70C).
Growth of bacteria in the wet portion B is prevented
as a result of high flushing rate which does provide
sufflcient time for bacterial growth. In this manner the
interior of the housing 1 remains clean and a return of the
fluid to an evaporating or drying plant is possible and the
loss of valuable particles is limited. It may be observed
that the invention is particularly intended to the processing
of dairy products such as milk or whey.
In an embodiment, the following dimensions, amounts,
temperatures and densities may occur:
Diameter of-the upper portion of the
housing 1 1700 ~m
Diameter of the portion 3 of the
housing 1 1750 mm
Diameter of the eduction pipe 5 1100 mm ;
Leng-th of the gas track A
outside the pipe S 3200 mm
Length of ~he flushing zone B 4000rr~m
Flushing fluid milk
Velocity of the stream of gas
via inlet 4 25 m/sec.
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Amount of powdered milk by weight in 3
this stream of gas 210 mg/m
Temperature of the stream of gas 95C
Temperature of the fluid 5-15C
Weight ratio gas/fluid about 10
Stay of the moistening fluid in the
flushing zone B about 10 sec.
Percentage of the Eluid carried off
via the bottom 9 with respect
to the total amo~mt supplied as
a result of partial evaporation
of the fluid film 96%
Fig. 4 is a schematic presentation illustrating the
use of the scrubbers according to the present invention
in combination with other apparatus for the preparation of
dried condensed milk. The scrubber is shown generally by
numeral 15 which is connected on one side to a spray dryer
16 for condensed milk and a separating cyclone 17, and on the
other side to a tank 18 of cool milk. The milk is introduced
into the scrubber 15 through annular zone 13. Line 11 conducts
the milk discharged from scrubber 15 to tank 19 from which
; the fluid is conveyed to evaporator 21 by pump 20 and
intermediate cooler 20'. In the evaporator 21 the r.lilk is
; condensed to a solids content of 50%. This condensed milk
product is then conveyed to spray dryer 16 which is provided
with an inlet 22 for hot air and a gas outlet 23 which is
connected by line 23 through cyclone 17 to inlet 4 of scrubber
15. Powdered milk product is discharged from dryer 16 and ,
cyclone 17 through lines 24 and 25 respectively which connect
into discharge 26.
The cool flushing milk supplied from tank 18 to the
annular zone 13 of the present scrubber will undergo a
temperature increase during its passage through the flushing
portion of the scrubber (see B in Fig. 1) and may approach
a temperature of, for example, 40~C, and which may increase
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bacterial growth. In order to avoid any such bacterial
contamination a line 27 may be provided between the fluid
milk transfer line and an intake 14 in the bottom of scrubber
15. In this manner a mixing of warmer flushing milk in
zone B of the scrubber with cooler milk from tank 18 occurs
resulting in a ternperature in the order of about 15C in the
milk in the bottom of the scrubber.
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