Pneumatic pressure conveyor for fine material

There are two superimposed pressure vessels, the upper of which has a closable material inlet connected to a source of fine material and has a lower end in the form of a cone with a bottom outlet. The lower pressure vessel has a material inlet which is connected through a valve to the bottom outlet of said cone, and has a pneumatic feed conduit connected to its lower portion as a material outlet. A conduit is connected to both vessels for equalizing the pressure therein after the inlet of the upper vessel has been closed and before said valve is opened. A plurality of nozzles are uniformly distributed throughout said cone and are connected to a source of gas under pressure for aerating material in said cone, each of said nozzles having an inside diameter between 0.2 and 0.6 mm.

BACKGROUND OF THE INVENTION 
This invention relates to a pneumatic pressure conveyor for fine material, 
with two superimposed pressure vessels, wherein the closable material 
inlet aperture of the upper pressure vessel is connected to a feeder for 
the material and the lower section of the lower pressure vessel is 
connected to a pneumatic feed conduit, and wherein at least the upper 
pressure vessel has a material aerating cone pressurisable by gas, the 
outlet aperture of said cone being connected by a valve member to the 
inlet aperture of the other pressure vessel in such a manner that when the 
upper vessel is pressurized and the inlet aperture to the vessel is 
closed, the valve member is opened in order to fill the lower vessel, and 
wherein both pressure vessels are connected to pressure conduits or 
pressure equalising conduits. 
Pneumatic pressure conveyors are very widely known which comprise only one 
pressure vessel and hence can only expel fine material from the pressure 
vessel discontinously, since the vessel must first be filled, then closed 
and finally pressurised so that the material can be pneumatically expelled 
from the pressure vessel. 
In order to be able to convey with pneumatic pressure conveyors as 
continuously as possible, various constructions have therefore been 
provided (see for instance German DAS No. 1212871 and German OLS No. 
1903539) wherein two pressure vessels are disposed one above the other in 
the manner initially described and joined together. With these 
constructions, while the fine material in the lower pressure vessel is 
being pneumatically extracted, with the outlet aperture closed and the 
inlet aperture opened in the upper pressure vessel, it is possible for 
further fine material to reach the pressure conveyor and be received in 
the upper pressure vessel. When the upper pressure vessel is sufficiently 
filled, its inlet aperture is then closed, the vessel is pressurised (so 
that approximately the same pressure prevails in both pressure vessels) 
and then its outlet aperture is opened, so that fresh material is supplied 
to the lower pressure vessel from which material can be continuously 
extracted. 
The difficulty with this known device is that, at least in the upper 
pressure vessel, the fine material often tends to form bridges, which 
hinder or prevent the pneumatic conveyance. To alleviate this difficulty, 
the known pressure conveyors have been altered by providing at least the 
upper pressure vessel with a material-aerating cone pressurisable by gas, 
whose casing has portions that are porous, such as for instance ceramic 
plates, sintered metal plates and the like, through which the aerating gas 
(e.g. air) is fed into the container. This measure has ind ed proved 
satisfactory in ordinary mixing and storage silos, but cannot achieve 
satisfactory loosening of the material present in the pressure vessel, 
this being mainly due to the fine material undergoing marked compaction 
during the pressure equalisation. 
SUMMARY OF THE INVENTION 
The invention is therefore directed to the problem of providing a pneumatic 
pressure conveyor, of the type initially defined, which will permit 
reliable movement of the compressed fine material out of the upper 
pressure vessel and thus ensure continuous pneumatic conveyance. 
According to the invention this problem is solved in that the 
material-aerating cone in the upper pressure vessel is provided with a 
number of nozzles, uniformly distributed over the length and periphery of 
the casing, and having an inside diameter of between about 0.2 and 0.6 mm. 
In the pneumatic pressure conveyor embodying the invention, when the fine 
material which has been heavily compacted due to the pressure equalisation 
between the two vessels is to be moved from the upper pressure vessel into 
the lower vessel, the nozzles in the aerating cone are pressurised with 
gas which can pass through said nozzles deeply into the fine material and 
hence cause effective loosening of the material, so that bridge building 
by the material can be prevented with great reliability. It has been found 
that penetration of the aerating gas into the fine material and hence the 
loosening of this material is influenced to an extremely favourable extend 
if the internal diameter of the nozzles is between about 0.2 and 0.6 mm. 
In accordance with one form of the invention it is also desirable that the 
loosening air be blown into the upper pressure vessel in a pulsating 
manner by means of a control unit by which the duration of the inpulses 
and their frequency can be varied according to the feed properties of the 
fine material being conveyed. This embodiment is especially desirable for 
fine material which tends to form bridges very readily and has 
unfavourable feed properties. 
The duration of the impulses of loosening air may be at least 0.2 second. 
The interval between impulses may be from 0.25 second to 30 seconds. 
When the nozzles are pressurized to cause gas to pass through the nozzles 
deeply into the fine material, it is necessary that the absolute pressure 
at which the gas is supplied to the nozzles be at least 4 percent but not 
more than 89.3 percent greater than the absolute pressure in the pressure 
vessels. For example, if the pressure in the vessels is 6 bar, gas may be 
supplied to the nozzles at a pressure from 6.25 bar to 11.4 bar. If the 
pressure in the vessels is 30 bar, gas may be supplied to the nozzles at a 
pressure from 31.2 bar to 56.8 bar.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The pneumatic pressure conveyor 1 in accordance with the invention as shown 
in FIG. 1 includes an upper pressure vessel 2 with a lower pressure vessel 
3 disposed below it, the inlet aperture 4 of the lower vessel being 
connected to the outlet aperture 5 of the upper pressure vessel, through a 
valve member 6. In usual fashion, both pressure vessels 2 and 3 have a 
generally cylindrical main portion, having at the top a domed cover, and 
connected at the bottom to a conical portion. The two pressure vessels 2 
and 3 are coaxially disposed one above the other. 
In the centre of the domed cover 7 of the upper pressure vessel 2 is an 
inlet aperture 8 at which is disposed a further valve member 9. Above this 
valve member 9 there can be connected either material feeding conduit or, 
as in the example shown, a storage container 10. 
Connected to the cover 7 of the upper pressure vessel 2 there is also an 
air-discharge conduit with an air release valve 11, and also the one end 
of an equalisation conduit 12 whose other end is connected to the domed 
cover 13 of the lower pressure vessel 3. 
Terminating in the cone 14 of the lower pressure vessel 3 is a pneumatic 
feed conduit 15 passing through said vessel from top to bottom, by which 
the fine material is extracted from the pressure conveyor 1. In the 
embodiment shown in FIG. 1 the cone 14 of the lower pressure vessel is 
closed at its bottom by a base 16 to which loosening air can be applied, 
said base being coated on its inner side for instance with porous 
material. 
The bottom section of the lower pressure vessel 3 may naturally also be 
formed in such manner that the material to be conveyed runs downwards out 
of the cone 14, where it may be received by a horizontal pneumatic 
conveyor conduit. 
In order to ensure that the fine material runs properly from the upper 
pressure vessel 2 into the lower pressure vessel 3, the lower exit cone of 
the upper pressure vessel is formed as an aerating cone 17. It is provided 
with a number of nozzles 18 which are uniformly distributed over the 
length and periphery of the cone casing (see FIG. 2). In the embodiment 
shown in FIG. 2 the casing 19 of the aerating cone 17 is made with a 
double-wall, i.e. it comprises two funnel walls 19a and 19b spaced from 
each other, which define between them a space 20 for the supply of 
loosening gas (e.g. air). This intermediate space 20 can be a single 
undivided space surrounding the inner funnel wall 19a. As may also be 
clearly seen from FIG. 2, the nozzles 18 are formed directly in the inner 
funnel wall 19a. The outer funnel wall 19b of the casing 19 is in this 
case provided with two connector stub pipes 21, 21a which are connected to 
a joint aerating gas feed conduit 22 (see FIG. 1). 
For many applications it may also be advantageous to divide the space 
between the two funnel walls horizontally and/or vertically, thus 
producing a number of sub-divisions for feeding the aerating gas, in which 
case a separate stub pipe for connecting to the aerating gas feed conduit 
is provided for each sub-division. Each sub-division of the space can then 
be supplied with aerating gas separately, for example following a 
particular sequence. 
While in the embodiments of the aerating cone described above the nozzles 
are formed directly in the inner funnel wall (e.g. 19a) of the 
double-walled casing, the casing of the aerating cone may instead comprise 
a single cone wherein the nozzles are individually set and then connected 
to an aerating gas feed conduit. In this case it is obviously also 
possible to dispose the nozzles in groups, so that when necessary aerating 
gas is blown into the pressure vessel only over specific sections of the 
casing. The same effect can obviously be achieved with an aerating cone 
having individually inserted nozzles as with the cone described with 
reference to FIG. 2; however, in general the form of construction in FIG. 
2 represents the simpler design. 
In forming the nozzles care should be taken in all cases that the internal 
diameter lies between 0.2 and 0.6mm. In the tests on which the invention 
is based, nozzles with an internal diameter between 0.3 and 0.45mm have 
proved particularly effective. The number and the diameter of the nozzles 
is preferably determined by the properties of the fine material to be 
conveyed. It has also been found that if the nozzles are effectively 
distributed over the cone casing, only a relatively small number of them 
are needed to achieve the desired loosening and the prevention of bridge 
formation. 
As shown in FIG. 1, in the aerating gas feed conduit 22 there is disposed a 
control device 23 which controls the supply of loosening air to the 
aerating cone 17 and to its nozzles 18 in such a manner that aerating gas 
is blown pulsatingly through the nozzles 18 into the upper pressure vessel 
2. The control device 23 is in addition so designed that the duration of 
the impulses and their frequency can be varied according to the feed 
properties of the fine material to be conveyed. With such a control device 
any sub-sections present in the double-walled cone casing, or the 
individual nozzles set in the cone casing (singly or in groups) can 
naturally be controlled correspondingly. Depending on the design and 
assembly of the pressure conveyor, the aerating gas can be derived from a 
different gas source. A very simple possibility, however, is to branch the 
aerating gas feed conduit from the conduit which is used to supply the 
conveying air. It is also possible to provide a separate blower for 
feeding aerating gas to the nozzles. 
Finally it may also be mentioned that the bottom cone 14 of the lower 
pressure vessel can obviously also be formed in the manner described above 
as an aerating cone (with uniformly distributed nozzles.)