Note: Descriptions are shown in the official language in which they were submitted.
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NETZSCH-Mohnopumpen GmbH V 825 19.06.2007
Feed screw for eccentric screw pump
The invention relates to an eccentric screw pump with a feed screw which
supplies the
suction region of the screw rotor mainly with medium to highly viscous media.
DE 101 60 335 Al shows an eccentric screw pump in this regard where a feed
screw is
arranged in the pump housing before the pump rotor. The screw is connected
with the
screw core via its entire inner contour.
DE 101 18 071 Al shows an eccentric screw pump about the coupling rod of which
a
hollow screw is arranged. This hollow screw is connected with a disc on the
drive side.
The other end in the suction-side region of the screw rotor has no connection
to a joint
or the coupling rod.
With a mixing device according to DE 1 277 819 dry substances are mixed with
liquid
and subsequently delivered by a pump. To this end, a mixing screw is seated in
the
region of a storage vessel, which mixing screw is connected on the one side
with a
motor and, on the other side with a screw pump. Liquid enters the screw region
between
the storage vessel and the pump. The feed screw consists of a helical band
which is only
fastened to the mixer shaft at one end by means of four braces.
A mixing and feed device is also shown in DE 43 18 177. The dry substances
enter the
region of a mixing screw via a hopper, while a liquid feed line also leads
into the region
of said mixing screw. Following the mixing operation, the mixture is
transported
onwards by a screw pump. The mixing screw itself consists of a region with a
solid
screw and a region with paddle and web-shaped mixing elements.
Each pump is designed for a determined delivery rate. To this end, adequate
medium
must always be available for the pump region on the suction side. The feed
screws,
which are arranged upstream of the actual screw or eccentric screw pump, can
therefore
deliver a multiple volume of the pump capacity. Because of this, a back-up
effect
develops in the so-called stuffing space in the suction region which is
associated with
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danger of bridge formation in the hopper above the screw. Because of this
stuffing
effect, substantially higher drive power than necessary must be made
available.
The object of the invention consists in adapting the stability of the feed
screw to the
required output while keeping the drive power constantly low even with
different
media.
This object is solved through the characteristics of claim 1. Further
developments of the
feed screw according to the invention are indicated from the characteristics
of the sub-
claims.
The design of a corresponding feed screw between the pump rotor and the drive
has
shown that the devices known from the prior art solve only part problems of
the object
according to the invention.
The design according to the invention is obviously dependent on which products
with
which viscosities, and, if applicable, present solid materials have to be
pumped.
According to the invention the normal embodiment concerns a feed screw having
at
least two perforations, wherein the webs formed between these perforations are
connected with their screw root with the coupling shaft. Depending on which
viscosity
the product has it may be practical to increase the number of perforations to
at least four
in order to facilitate the return flow of the medium and ensure more
homogenous
mixing-through, through which bridge formation of the medium is already
counteracted
on the suction-side end of the pump.
Through the perforations according to the invention, low-loss drive power
compared
with delivery devices from the prior art, designed to the condition of the
medium can be
installed.
In order to obtain the advantage according to the invention also in the
regions of the
couplings (joints) the screw also extends beyond this region for the purpose
of which
the pipe employed as coupling rod has strip-shaped pipe segments which are
connected
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with the screw. If the number of the pipe segments corresponds to the number
of the
perforations, a corresponding number of webs for their fastening is available.
Since for the return flow of the medium not only the number of perforations
but also
their area place a substantial role, it can be provided according to a version
according to
the invention to dimension the perforations between 30% and 70% of the height
of the
screw start. With low-viscosity substances the height of the perforations can
be selected
in the range from 20 % to 60 % of the height of a screw pitch.
As is shown in an exemplary embodiment the width of the perforations will
correspond
to the width of the webs because of the homogenous return feed and even
loading of the
screw. With high-viscosity media the return flow possibility must certainly be
improved
wherein the width of the perforations is greater than the width of the webs.
With low
viscosity substances the danger of bridge formation is relatively low so that
here the
width of the webs can be greater than that of the perforations.
To improve the return flow and thus reduce the stagnation effect the flow
along the
coupling shaft can be improved in that the webs have an inclination and thus
produce a
flow direction which is opposite to the course of the screw.
Depending on which design of the screw is required, the webs can be offset to
one
another by 30 to 120 per screw start. In order for the feed screw to be
stabilised about
its entire length through the coupling shaft the length of the pipe segments
is adapted to
the course of the feed screw.
Easier affixing of the feed screw to the coupling shaft is obtained in that
the feed screw
consists of several parts which enable better handling during the mostly
employed
welding operation.
The invention is exemplarily described in the following by means of exemplary
embodiments.
It shows:
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Fig. 1 Lateral view of the feed screw
Fig. 2 A three-dimensional representation of the feed screw
Fig. 3 A cross section of the feed screw
Fig. 4 Feed screw with joint part on both sides
Fig. 5 Lateral view of the feed screw according to Fig. 4
Fig. 6 Cross section of a feed screw
Fig. 7 Screw cross section with 6 perforations
Fig. 8 Screw cross section with 4 perforations
Fig. 9 Screw cross section with various perforation distances from the
coupling rod
Fig. 1 shows a design possibility of a feed screw 10 with a screw pipe 12. A
screw 14 is
welded on to the circumferential surface of the screw pipe 12. On both ends
16, 18 of
the screw pipe 12, pipe segments 20 for fastening the screw 14 are provided.
Each of the
pipe segments 20 ends at the point at which the screw 14 ends in axial
direction. In the
region of the pipe elements 20, couplings 22 for the joints not shown are
provided at
both ends. The screw 14 is manufactured from flat band-shaped material.
The design of the screw 14 can be more clearly seen in Fig. 2. From the
perspective
representation it becomes evident that the screw 14 is provided with
perforations 24 and
webs 26. Each screw root 34 of the webs 26 is connected with the pipe segment
20 or
the coupling shaft 32 for example through a welding operation. While the
medium in
axial direction flows from the rotor region back to the pump inlet through the
perforations of the feed screw closely along the screw pipe, the screw 14 with
its end
faces 28 transports the medium in the direction towards the pump rotor. The
pump rotor,
through a joint which is not completely shown, and which joint is fastened to
the
coupling 22, is in positive contact with the feed screw.
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Fig. 3 shows the embodiment and arrangement of the perforations 24 and webs 26
for a
feed screw for highly viscous media. Here, the large free areas of the
perforations 24
enable very good return flow possibilities for the medium in order to adapt
the
stagnation pressure in the stuffing space to the pump output. Here,
dehydration of the
medium and increased tendency toward bridge formation is prevented and
undesirably
high drive power avoided.
The perforations 24 according to this exemplary embodiment are wider than the
webs
26. The middle of the perforations in each case is located on the
midperpendicular and
is thus offset by 90 relative to one another which produces 4 perforations 24
and 4
webs 26 per screw pitch. The height of the perforations corresponds to
approximately
50% of the screw height.
A feed screw 10 is also shown in Fig. 4 and Fig. 5. Here, a coupling shaft 32
is provided
as drive component. A screw 14 is welded on to the coupling shaft 32 in the
region
between the joint components 30 which, as with all other exemplary
embodiments,
consists of individual screw segments. This embodiment of the screw 14 is
employed
for instance with low-viscosity media. By way of the large face areas 28
compared with
the areas of the perforations 24, more medium enters the rotor region while
the smaller
dimensioned perforations nevertheless prevent an increased need for drive
power.
The arrangement of the perforations 24 and their size of the feed screw 10
shown in Fig.
4 and 5 is shown in Fig. 6. Per screw pitch, three perforations each offset by
120 are
provided in the screw 14.
Additional exemplary embodiments for the screw design for one winding each are
shown in Fig. 7, 8, 9.
Fig. 7 represents an even distribution of 6 perforations 24 and webs 26. The
height HD
of the perforations is 50 % compared with the height HS of the screw 14. The
width BD
of the perforations corresponds to the width BS of the webs.
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In Fig. 8 the screw 14 has four perforations 24 and four webs 26 while the
width BD of
the perforations is greater than the width BS of the webs. The height HD of
the
perforations 24 is 50 % of the height HS of the screw 14.
The interrupted lines of Fig. 9 shows different size relationships with regard
to the
height HD of the perforations to the height HD of the screw 14, while three
perforations
24 are shown with a division by 120 .
