Atomizer wheel for the atomization of suspensions of hard-wearing materials

An atomizer wheel comprising an annular chamber provided with a number of conical outlet holes lined with bushings extending a distance into the chamber.

This invention relates to an atomizer wheel for the atomization of 
suspensions of hard-wearing materials and of the type which concentrically 
with the hub of the wheel has an annular bowl-shaped space in the 
substantially cylindrical outer wall of which there are provided a 
plurality of discharge openings having linings of a wear-resistant 
sintered material. 
In this case "sintered material" means a material where the hard grains are 
stably interconnected no matter whether such interconnection has been 
effected by fusing together the surfaces of the grains or by embedding the 
grains in some basic substance such as, by way of example, is being used 
in the manufacture of wolfram carbide bodies. 
In the atomization of suspensions which contain solid particles of a hard 
material, especially sharp edged particles, for example a suspension of 
ceramic clay, a very hard .[..[.water.]..]. .Badd.wear .Baddend.is taking 
place on account of the very high velocities of discharge from the 
atomizer wheel caused by the centrifugal force. Thus, a conventional 
atomizer wheel made of the commonly used wear-resistant types of steel, 
will have an extremely short lifetime, sometimes only a few hours or even 
less before they have become so heavily worn as to be entirely 
unserviceable. 
With a view to reducing the wear it is known to employ wear-resistant 
sintered materials in such places where the wear occurs. 
On account of their hardness such sintered materials are practically 
speaking not machinable so that the wear-resistant parts to be used must 
be produced in their final shape by sintering which in conjunction with 
the circumstance that they do in all essentials only withstand compressive 
forces and must not, on account of their brittleness, be exposed to 
tensile or bending stresses of significance, has the effect that they only 
lend themselves to be produced in very simple geometric shapes. 
There is known an atomizer wheel of the type concerned having slot 
discharge openings where plates of such sintered material are inserted 
into the slot openings at the walls thereof across which the discharge is 
taking place. The plates are loosely disposed for the very reason of 
avoiding bending stresses and are retained by their end edges engaging a 
projecting edge provided at the external opening of the slot, against 
which latter edge they are firmly pressed by the centrifugal force during 
rotation of the wheel. This measure prolongs the lifetime of the wheel to 
some extent but it proves in practice that this is nevertheless only a 
rather limited improvement in that the liquid is by the centrifugal force 
pressed in between the loosely deposited plates and the adjoining surface 
of the wheel proper whereby the material of the wheel is quickly worn to 
such extent as to make the wheel discardable. 
Instead of the slot openings one has tried to use cylindrical holes in 
which there are disposed bushings of a wear-resistant sintered material 
but in this case it proves that the outer wall of the wheel is worn in the 
area surrounding the bushings so that the wheel is discardable after a few 
hours of operation so that practically no improvement is obtained by using 
the wear-resistant bushings. 
The invention has for its object to provide an atomizer wheel of the type 
concerned in connection with which a very considerable increase of the 
lifetime is obtained. One object of the invention is to provide the holes 
with linings of tapered bushings of wear-resistant sintered material, 
projecting into the bowl-shaped space. 
As a consequence of the tapered shape of the bushings they are during 
operation of the wheel pressed firmly against the correspondingly shaped 
wall surfaces of the openings by the centrifugal force so that any 
discharge of liquid between the bushings and the material of the wheel 
proper is avoided. As the bushings extend a distance into the annular 
space in the wheel, a layer of liquid will deposit on the outer wall 
during the rotation of the wheel. Owing to the centrifugal forces the 
heavier parts of the layer i.e. the suspended solid material is separated 
from the liquid in the layer and deposited directly on the wall so as to 
fill the area surrounding the inwardly extending portions of the bushings. 
The coating thus obtained is, to be sure, not wear-resistant in the proper 
sense of the word in that it consists of loosely interconnected particles 
which are incessantly torn way from the surface of the layer by the 
suspension flowing towards the openings of the bushings but the 
centrifugation of particles out into said layer will continuously have the 
effect that the torn-away particles are replaced by fresh particles so 
that the parts of the coating adjoining the wall are preserved and 
preclude any wear of said wall. In practice it proves that in this way 
lifetime can increase several hundred times. 
Although the bushings consist of the said wear-resistant sintered material, 
they will nevertheless be subject to wear but they are readily replaceable 
because they are only kept in place by the centrifugal force which, as a 
matter of fact, is inoperative when the wheel is at rest. The wear to 
which the bushings are exposed is unilateral and their lifetime may, 
therefore, be increased by turning them gradually as they are worn. 
The openings of the bushings may by way of example have a circular cross 
section but preferably they are of a square cross section. The square 
cross section determines four different well-defined positions of the 
bushings. Further the wearing surfaces are plane which has proved to 
involve a longer lifetime than curved wearing surfaces. 
A wearing groove may be produced in the annular space at the place where 
the surface of said coating adjoins the bottom of the wheel. This wear may 
be essentially reduced when the bottom of the bowl-shaped space has a 
conical shape sloping outwardly towards the outer wall. 
In a particular embodiment of the atomizer wheel according to the invention 
a layer of a wear-resistant material is provided on the bottom surface of 
the bowl-shaped space made for either in the shape of an embedded annular 
plate or a sintered surface coating at any rate in the outermost part of 
the bottom surface. In this place the wear may be less hard than in the 
bushings proper and through a surface coating of wear-resistant material 
it is possible to obtain the advantage that the lifetime of the wheel 
proper becomes a length sufficient for permitting the bushings being 
replaced a number of times before the wheel is discardable. If using an 
embedded ring this ring can also be replaced and, consequently, the 
longest imaginable lifetime of the wheel is attained. 
Sometimes it may be difficult or at any rate expensive to produce the said 
annular plate of sintered material, but excellent results are also 
obtained by using a ring of a wear-resistant metal alloy because the wear 
at this place is, as mentioned above, less hard than in the discharge 
openings.

The wheel illustrated in FIG. 1 is substantially of bowl-shape with a 
centrally disposed hub 1 whereby an annular space 2 appears between the 
hub 1 and the outer wall 3 of the wheel which space is further defined by 
the wheel portion 4 connecting the hub 1 with the outer wall 3 and forming 
the bottom of the annular space 2. Further, an annular cover plate 5 is 
secured to the upper side of the wheel, said cover plate surrounding the 
hub 1 in such a manner as to form between said hub and the annular plate 5 
an annular opening 6 leading into the space 2. 
The wheel is intended for being mounted into a known spray-drying apparatus 
on a shaft which during operation rotates at a very high velocity, a 
suspension being supplied to the space through the opening 6 and 
precipitated in atomized form through openings in the side wall 3 to a 
surrounding drying chamber wherein the fine particles formed by the 
atomization are dried so that their contents of solid matter fall to the 
bottom of the drying chamber in the form of a fine powder. 
FIGS. 1 and 2 show a single one of said openings, and according to the 
invention a bushing 7 of a wear-resistant sintered material, for example 
wolfram carbide, silicon carbide, aluminum oxide or tetraboron carbide, is 
disposed in each of said openings. 
These bushings 7 have conical outer surfaces with outwardly decreasing 
diameter and, consequently, they can be loosely disposed in said openings 
as they are pressed outwardly by the centrifugal force when the wheel is 
rotated and, consequently, they are immovably and tightfitting secured in 
the openings in the outer wall. 
At their outer ends the bushings 7 are preferably ground flush with the 
cylindrical outer surface of the wheel whilst their inwardly facing ends 
extend a distance into the space 2 beyond the outer wall 3 as seen in FIG. 
1. 
When the wheel commences to rotate the surroundings of the bushings 
adjoining the outer wall will very quickly be filled with a layer of the 
suspension as a consequence of the effect of the centrifugal force, as 
indicated by a dotted line 8. By the centrifuging effect the material of 
this layer is quickly separated so that the heavier parts, i.e. the 
contents of solid wearing particles will adopt .[.the.]. .Iadd.an 
.Iaddend.outer position .[.closed.]. .Iadd.close .Iaddend.to the outer 
wall 3, .[.whilst.]. .Iadd.while .Iaddend.the lighter parts, i.e. the 
liquid will lie innermost. In this manner the surroundings of the bushings 
7 are quickly filled with a layer of these solid particles which will thus 
protect the material of the wheel proper from wear. 
A certain wearing effect may occur at the bottom of the space 2 and 
primarily at the vicinity of the bordering line 8 for the material 
deposited on the outer wall 3. For the purpose of preventing the wheel 
from being quickly ruined by this wearing effect, an annular plate 9 of a 
wear-resistant material is embedded at the bottom of the space. If deemed 
appropriate, this material may likewise be a hard sintered material but 
since this area is subject to somewhat less wear it will often suffice to 
use a wear-resistant metal alloy, for example a wear-resistant type of 
steel. 
The embodiment shown in FIG. 3 differs from that shown in FIG. 1 in that 
instead of the annular plate 9 a layer 10 of a wear-resistant material has 
been applied to the bottom of the space 2, said layer extending a distance 
upwardly along the hub 1. Since the said layer is not replaceable in the 
same manner as that used in FIG. 1, it may be expedient that said layer 
consists of a wear-resistant sintered material. 
FIG. 4 shows a modified embodiment of the bushings having square 
cross-section instead of circular cross-section as shown in FIGS. 1 and 3. 
The bushings are tapered at least over part of its length 11 corresponding 
to the conical part of the bushings 7 in FIGS. 1 and 3. 
The bottom of the annular space may be conically sloping outwards from the 
hub 1 .[..[.as indicated in FIG. 1 by means of a dotted line 12.]..]..