Note: Descriptions are shown in the official language in which they were submitted.
WO 96/09872 2 2V/1 9/..Q PCT/FI95/00524
~
Li-quid distributor for an evaporator
The present invention relates to a liquid distributor in an
evaporator, which distributor is intended in particular for
liquid suspensions containing solid ingredients and comprises
at the upper end of the heat exchange surface a transverse
conduit frame which includes liquid conduits for distributing
over the entire width of the surface a liquid introduced from
the side of the evaporator.
In an evaporator in which the liquid flows as a thin film on
the heat exchange surface, the surface should be covered with
liquid throughout in order to achieve an efficient transfer of
heat. When a liquid suspension which contains solid ingredients
is being evaporated, increasing concentrations of solids at the
boundaries of the dry and liquid-covered areas of the heat
exchange surface would additionally cause very rapid soiling of
the heat exchange surface. An even distribution over the heat
exchange surface of the liquid to be evaporated is thus an
absolute prerequisite for efficient operation of the evap-
orator.
The keeping of the heat exchange surface covered with liquid
can be promoted by increasing the liquid amount fed onto the
surface. However, this increases the consumption of energy for
pumping and increases the thickness of the flowing liquid film,
thus lowering the heat transfer coefficient. For this reason,
very narrow liquid conduits have generally been resorted to in
liquid distributors in evaporators, but such conduits tend to
become clogged when solids-containing liquid suspensions are
being evaporated.
From Finnish lay-open print 86961 there is known a heat ex-
changer intended for the evaporation of liquid, the heat ex-
changer having adjacently positioned bags formed from plastic
WO 96/09872 22 09 4'8 PCT/FI95/00524
2
membrane, the membrane surfaces of the bags serving as heat
exchange surfaces. The liquid to be evaporated is introduced
onto the outer surfaces of the bags from conduits adjacently
positioned in honeycombs at the upper end of the evaporator and
distributed over the entire width of the bags. The heating
medium used is vapor directed via the same honeycombs to the
inside of the bags, the vapor being vapor produced in the evap-
oration and compressed in a compressor.
The apparatus described in Fl publication 86961 is intended for
the distillation of seawater. In addition, the publication
mentions as an intended use of the apparatus the concentration
of solutions and suspensions, such as bleaching effluents.
However, in this case the apparatus has the deficiency that the
fiber material and other solids present in the suspension tend
to clog the obliquely oriented narrow liquid conduits in the
honeycombs.
An object of the present invention is to provide a liquid dis-
tributor for an evaporator, a liquid distributor in which the
problem of prior-art systems, the clogging of conduits by
solids present in the suspension, is avoided and which is
therefore especially suitable for the treatment of wood-
processing industry waste waters which contain fiber materials,
such as the waste liquor from pulping or bleaching effluents,
or for the treatment of food-industry waste waters which form
carbonate, oxalate and other precipitates. The liquid distrib-
utor according to the invention is characterized in that it has
an obliquely downwards slanted feed conduit and a plurality of
distribution conduits branching out therefrom and leading to
the heat exchange surface, and that at each branching point of
the feed conduit and a distribution conduit the common wall
surface of the conduits, downstream relative to the feed flow, is rounded so
that the dividing of the flow takes place on the
curved dividing surface formed by the wall.
WO 96/09872 2 2 948 PCT/F195/00524
3
In a manner deviating from the oblique liquid conduits starting
from the side of the evaporator in accordance with Fl publica-
tion 86961, the liquid distributor according to the present
invention comprises a common oblique feed conduit which divides
the liquid into distribution conduits departing therefrom,
which distribution conduits may be substantially vertical. The
liquid to be evaporated flows in the feed corlduit in a cascade-
like manner from one dividing surface at a branching point of'
the conduits to another, the curved dividing surface dividing
the stream at the given point into a principal component which
will continue forwards in the feed conduit and a component
passing via the distribution conduit to the heat exchange sur-
face. The dividing is regulated, for example, by the radius of
curvature of the dividing surface and by the angle at which the
feed conduit flow impinges upon the dividing surface, and on
the basis of preliminary experiments it is possible, by regu-
lating these according to the type of the suspension to be
evaporated, to accomplish a substantially equal division of the
liquid among the distribution conduits. In addition, it has
been observed that, in accordance with the objects of the in-
vention, the curved dividing surfaces effectively control the
tendency of the conduits to become clogged by solid ingredients
of the suspension, for example by eliminating from the liquid
flow path any sharp edges to which agglomerations of solids
could adhere.
At a branching point, on the upstream side of the feed flow,
i.e. on the side opposite to the curved dividing surface, the
feed and distribution conduits are preferably delimited by wall
surfaces terminating in a common sharp tip. The sharp tip con-
stitutes a step from which the inflow of the feed conduit will
jump over the mouth of the distribution conduit onto the divid-
ing surface, where the dividing of the flow takes place. The
distribution conduit wall surface which terminates in the tip
may be rounded in a manner corresponding to the dividing sur-
face.
WO 96/09872 2 2V v9'F 8 PCT/FI95/00524
4
In an evaporator intended for the treatment of a fiber suspen-
sion, the radius of curvature of the dividing surfaces may be,
for example, within a range of approx. 10-30 mm,-preferably 20-
30 mm, and the width of the distribution conduits may vary
within a range of approx. 10-30 mm, preferably approx. 15-
25 mm. The width of distribution conduits branching out from
the feed conduit may be the same for all c-onduits, or it may
increase progressively from the side of the evaporator towards
its middle. '
It is preferable to construct the liquid distributor according
to the invention so that it comprises two mutually symmetrical
feed conduits which start on the opposite sides of the evap-
orator and extend to the middle of the evaporator in such a
manner that each feed conduit will feed liquid to its own half
of the evaporator.
The liquid distributor according to the invention can be used
in a heat exchanger of the type according to Fl lay-open print
86961 to direct a liquid suspension to be evaporated onto the
outer surfaces of bags of plastic membrane. The liquid dis-
tributor extending in the transverse direction from one side of
the bag to the other in this case distributes the liquid over
the entire width of the heat exchange surfaces of the bags. The
vapor produced from the liquid may be directed to a compressor
and from there, having been compressed to higher pressure and
temperature, to the inside of the bags as heating vapor, which
will recondense to liquid in the heat exchange process. How-
ever, the use of heating vapor produced in some other manner is
equally possible.
The liquid distributor according to the invention can be con-
structed from two vertical wall elements which have been fabri-
cated from, for example, plastic by injection molding, and be-
tween which the liquid conduits are formed. For example, in the
WO 96/09872 220 948 PCT/F'I95/00524
above-mentioned heat exchanger comprising bags of plastic mem-
brane, a wall element of the liquid distributor may at the same
time serve to delimit vapor conduits leading to the inside of
the bags on its opposite side. The feeding of liquid and vapor
into the heat exchanger can thus be arranged by attaching to
each other said elements, which will delimit, between them,
alternately liquid conduits leading to the space between the
bags and vapor conduits leading to the inside of the bags.
Owing to the compact structure, the liquid conduits can be made
maximally wide, which will also prevent their tendency to
become clogged.
The invention is described below in greater detail with the
help of examples, with reference to the accompanying drawings,
in which
Figure 1 depicts a vertical section of a liquid dis-
tributor according to the invention,
Figures 2-4 depict a wall element of the liquid distributor
according to Figure 1, in horizontal sections
11-11, III-III and IV-IV of Figure 1,
Figure 5 depicts a partial horizontal section of three
wall elements attached one to another, the wall
elements forming a distributor for both the
liquid to be evaporated and for the heating
vapor, and
Figure 6 depicts on a larger scale the branching points
of the conduits of the liquid distributor ac-
cording to Figure 1.
The liquid distributor according to the invention for an evapo-
rator comprises a conduit frame which is made up of two verti-
cal wall elements 1, 2, for example, injection molded from a
plastic material, these wall elements delimiting between them-
selves the liquid conduits 3, 4 of the distributor. The liquid
conduits 3, 4 have been produced by forming, in connection with
injection molding, recesses in the wall element 1 according to
WO 96/09872 2 G.+~ ('V~(j9jaS PCT/FI95/00524
V i ~
6
Figures 1-4 and by closing these on their open side by a second
wall element 2 in accordance with Figure 5. Figure 5 addition-
ally shows vapor conduits 5 closed by means of a wall element 1
according to Figures 1-4, which vapor conduits will be returned
to in greater detail hereinafter.
The liquid conduits, which can best be seen'in Figures 1 and 6,
comprise a feed conduit 3 starting from the distributor end 6
located on the side of the evaporator, the feed conduit extend-
ing obliquely downwards to the middle of the distributor, and a
plurality of substantially vertical distribution conduits 4
which branch out from the feed conduit at branching points 7
and lead to the heat exchange surface (not shown) of the evapo-
rator. Starting from one end of the distributor, the width D
(cf. Figure 6) of the distribution conduits 4 branching out
from the feed conduit 3 may be constant or grow progressively,
conduit by conduit, towards the middle of the distributor. In
its other half the distributor comprises a symmetrically cor-
responding system of liquid conduits made up of feed and dis-
tribution conduits 3, 4. The feed flow of the liquid to be
evaporated, such as a suspension which contains solid fiber
material, is directed in accordance with arrows 8 in Figure 1
to the feed conduits 3, from where it is divided into partial
flows according to arrows 9, into the distribution conduits 4
leading to the heat exchange surface, substantially equally
among the distribution conduits. Thus the liquid can be dis-
tributed from the distribution conduits 4 evenly over the en-
tire width of the heat exchange surface.
Via its end in the middle of the distributor, the feed conduit
3 communicates with the vapor spaces of the evaporator so that
during operation the feed conduit is in part filled with vapor
14 formed in the evaporation. It is necessary to regulate the feeding in of
the liquid to be evaporated in such a manner that
the feed conduit 3 is not in its entirety filled with liquid,
because otherwise the produced siphon effect would hamper even
WO 96/09872 2200948 PCT/FI95/00524
7
distribution of the liquid.
The branching points 7 of the conduits and the division of the
liquid flow at them into partial flows passing to the heat
exchange surface is seen on a larger scale in Figure 6. The
feed flow traveling in the feed conduit 3 is indicated in the
figure by numeral 8 and the partial flows separating into the
distribution conduits 4 by numeral 9. At each branching point 7
of the conduits the wall surface common to the conduits, down-
stream relative to the feed flow 8, is formed into a curved
dividing surface 10, the radius of which is indicated by R. At
each branching point 7, on the upstream side the wall surface
of the distribution conduit 4 is rounded correspondingly, and
the wall surface of the conduit 4 and the wall surface of the
feed conduit 3 together form a sharp feeding tip 11 from which
the arriving feed flow 8 jumps over the end of the distribution
conduit onto the dividing surface 10, on which the division of
the flow takes place at each given point. The successive divid-
ing surfaces 10, terminating in feeding tips 11, are dimen-
sioned so that the feeding tip of a dividing surface lower in
the direction of the flow is at each given point disposed a few
millimeters below the theoretical continuation of the feeding
tip of the next dividing surface upstream. The impinging angle
of the flow 8 to the tangent of the dividing surface 10 is
indicated in the figure by a. The division of the feed flow 8
into partial flows 9 depends on the composition of the liquid,
on the radius R of curvature of the dividing surface, and on
the impinging angle a, which in turn is dependent on, for
example, the velocity of the flow and the width D of the dis-
tribution conduits 4, and the distribution may be regulated by
varying the said values. According to preliminary experiments,
during the operation of the distributor solid material adheres
momentarily to the dividing surfaces 10 at the impinging points
of the feed flow 8, but as the accumulation of material in-
creases it either continues forward in the feed conduit 8 or
falls into the distribution conduit, i.e. in operation the
WO 96/09872 2 2 " 4v PCT/FI95/00524
$
device is self-cleaning. The impinging angle a of the feed flow
8 is advantageously set close to a right angle, for example,
within a range of approx. 90-110 .
As already pointed out above, the liquid conduits 3, 4 in the
distributor are located in accordance with Figure 5 between two
wall elements 1, 2 fastened to each other. The locking of the
elements 1, 2 to each other is effected by-.snap fastenings of'
vertical projections 12 in element 1 according to Figures 1-4
to corresponding recesses 13 in the other element 2. The heat
exchange surface being of plastic membrane, the upper end of
the membrane may be locked (not shown) between the elements 1,
2 fastened to each other.
The said second wall element 2 in Figure 5 is designed so that,
together with a third element, element 1 according to Figures
1-4, it delimits vertical vapor conduits 5 from which heating
vapor is directed to that side of the heat exchange surface
which is opposite to the liquid to be evaporated. When bags
made of plastic membrane are used, the heating vapor is direct-
ed via conduits 5 to the inside of the bags and the liquid to
be evaporated is directed via conduits 3, 4 onto the outer
surfaces of the bags. By fastening elements 1, 2 according to
Figure 5, equipped with projections 12 and recesses 13, alter-
nately one to another it is possible to construct a liquid and
vapor distributor which has in alternation liquid conduits 4
leading to the spaces between the bags of plastic membrane and
vapor conduits 5 which lead to the inside of the bags, the con-
duits distributing the liquid and the vapor evenly over the
entire width of the heat exchange surfaces formed by the bags.
For an expert in the art it is clear that the various embodi-
ments of the invention are not limited to that described above =
by way of example but may vary within the accompanying claims.
