Screw press for squeezing out fibrous material

A screw press for pressing fibrous material, in particular sugar beet pulp, having a pair of adjacent counter-rotating shafts parallel axes, the shafts each having a helical structure winding in the opposite direction to the helical structure of the other shaft, a perforated filtering cage enclosing said helical structures, a loading hopper for feeding the fibrous material to the press, a discharge aperture for the exit of the pressed material, the helical structures having a helix (22, 32) and a helical element (23, 33), which has at least one perforated surface and a helical interspace with the outer surface of the shaft, the perforated surface having a length (L) along the axis of the shaft which is less than the pitch (P) of the helix so as to receive the helix of the adjacent shaft.

TECHNICAL FIELD

The present invention relates to a screw press for squeezing out fibrous material, in particular sugar beet pulp.

Presses of this type are generally used for squeezing out the liquid contained in fibrous materials, such as sugar beet pulp.

In the present invention the term “fibrous material” means any material having a ligneous (fibrous) component and a liquid component, and able to be pressed to separate the liquid component from the ligneous component.

PRIOR ART

Presses are known comprising two screws which rotate parallel to each other side by side with their helixes offset within a perforated walled filtering cage.

Presses of the aforesaid type have been known in the sector for many years.

According to the said known art the material to be pressed is fed radially from above to one end of the filtering cage through a loading hopper. The material fed in this manner is pressed by the screw, rotated by suitable drive means, and urged during pressing towards that end distant from the feed end, from which it leaves, totally or partially dewatered, through a discharge opening.

During the advancement of the material through the press, the liquid component present in the fibrous starting material traverses the perforated wall of the filtering cage. This liquid component is collected in a sump positioned outside the cage and is conveyed towards an exit opening. Generally, each screw has a pitch which decreases in the direction of advancement of the material.

Dewatering of the fibrous material is achieved by the pressure generated by forces to which the material is subjected during the pressing. This pressure, which enables the liquid component of the fibrous material to emerge through the filtering cage, is a function of the particular geometry of the press, and of the small gap present between the crest of the screw and the filtering cage.

Presses are also known in which the screws are conical with their diameter increasing from that end at which the fibrous material to be pressed is fed, to the opposite exit end. In this case, the helixes present a variable height decreasing in the same direction to safeguard the parallelism between the screws.

In presses of this type the material is also pressed in the radial direction with consequent efficiency increase compared with presses with cylindrical screws.

However these presses are known not to accomplish their purpose perfectly.

In this respect, problems often arise such as the tendency of the material to be dragged into rotation by the screws, the tendency of the material to stagnate against the walls of the screws, and the tendency of the material to undergo damage in the region present between the screw crests and the outer filtering cage.

Consequently the pressing pressure has to be increased to facilitate the removal of the liquid portion from the material during pressing.

According to a solution known in the art, to achieve an increase in the degree of pressing, in conventional presses screws have been used in certain circumstances comprising a shaft surrounded by a helical hollow box structure having a perforated surface for passage of part of the liquid portion of the fibrous material. This structure forms with the outer surface of the shaft an interspace coaxial to the shaft.

In the present invention the term “helical structure” means a hollow box structure with a portion of perforated surface extending along the direction of the shaft axis.

Presses are known in which the shafts are surrounded by a hollow conical structure having its diameter increasing from that end at which the material to be pressed enters, to the opposite exit end; the perforated surface of this structure forms with the outer surface of the shaft an interspace for collecting part of the liquid portion of the pressed material. Consequently. the height of the helix of these presses is chosen such as to decrease in the same direction to safeguard the parallelism between the shafts; this helix is known as conical.

Hence in the state of the art, presses are used having a helical structure comprising a conical perforated surface and a conical helix.

Although these improvements have achieved an increase in the degree of squeezing obtainable for a starting fibrous material, the choice of the helical structure is restricted by the press geometry.

In this respect, a press with an interspace having a conical outer surface must be associated with a conical helix, as it is not possible to have a conical interspace associated with a cylindrical helix.

There is therefore a strongly felt requirement for a double screw press having a helical filtering structure which is independent of the press geometry and which presents an interspace having a conical outer surface also associated with a cylindrical helix.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide a double screw press for pressing fibrous material, in particular sugar beet pulp, having structural and functional characteristics such as to satisfy the aforesaid requirements while at the same time obviating the stated drawbacks of the known art.

MODES OF IMPLEMENTING THE INVENTION

With reference to said figures, structurally and functionally equivalent parts are indicated on the drawings by the same reference numerals. These will be described once only for descriptive brevity.

In the accompanying figures, the reference numeral1indicates overall a double screw press for pressing fibrous material, in particular sugar beet pulp.

In the illustrated examples, the press1comprises a pair of screws consisting of adjacent counter-rotating shafts20,30having their respective axes X-X and X′-X′ disposed parallel to each other.

About each shaft20,30there is wound a box-like helical structure21,31, described in greater detail hereinafter, with its cross-section increasing in the direction of advancement of the material.

The shafts20,30are rotatably supported at their respective ends by two robust supports2and3, one of which, indicated in the figures by2, comprises internally installed drive and transmission means, preferably electrical, not shown. Said means rotate the shafts20,30at the same speed but in the opposite direction.

Around the helical structures21,31there is disposed a perforated walled filtering cage4which follows the external profile of the structures. The cage4encloses as an exact fit said structures21,31which are hence enveloped by the cage4. Said cage4comprises at least two intersecting cylindrical portions4′ and4″ supported by a suitable central rod5and by two lateral rods6, as shown inFIGS. 4,8,12and16. “The filtering cage4comprises on the outer part of the perforated wall a plurality of hoops7to oppose the pressure exerted by the fibrous material on said cage during pressing.”

Said hoops7are provided with holes8traversed by suitable tie rods9to ensure their consistency.

At that end comprising the support3the filtering cage4is provided with a loading hopper10for feeding the material to be pressed.

At the opposite end of the cage4comprising the structure2containing the drive and transmission means, a discharge aperture11is provided for exit of the pressed fibrous material.

About the filtering cage4there is provided a collection sump12for collecting the liquid component of the pressed material. This sump12is supported by suitable support feet13which, together with the lower part of the support2containing the drive and transmission means, support the entire press1.

The collection sump12collects in its interior the liquid component of the fibrous material fed to the press1, this liquid component being conveyed towards an exit opening14by virtue of a slight slope.

According to the present invention, each box-like helical structure21,31comprises a helix22,32wound about the outer surface of the shaft20,30and a helical element23,33flanking the helix22,32.

Each helical element23,33comprises a perforated conical surface24,34and two radial side walls25,26and35,36to hence form with the outer surface of the shaft20,30a collection interspace27,37with its cross-section increasing in the direction of advancement of the material.

The perforated surface24,34of the helical element extends along the axis of the shaft whereas the two side walls25,26and27,37are disposed substantially perpendicular to axis of the shaft20,30.

According to a preferred embodiment and as shown in the accompanying figures, the function of one of the side walls25,35is performed by one of the sides of the helix22,32, whereas the other side wall26,36remains free.

Each helical element23,33flanking the respective helix22,32presents a helical extension with a pitch equal to that of the helix22,32.

In practice, the collection interspace27,37is helical and extends parallel to the helix.

During the pressing of the fibrous material, a part of the liquid component of the fibrous material passes through the filtering cage4, while the other part of the liquid component passes through the perforated surface24,34of the helical element23,33.

Consequently, that liquid component part of the pressed material which passes through the perforated surface24,34undergoes within each interspace27,37, during rotation of the shafts20,30, a roto-translation towards that end of the shaft at which the pressed material also emerges. Preferably, as shown in the figures, the two helixes22,32have a variable pitch which decreases progressively from the feed end10to the discharge aperture11. The same applies to each helical element23,33flanking the respective helix22,32.

The perforated surface24,34of each helical element23,33of the box-like structure21,31has a length L along the axis of the shaft20,30which is less at every point than the pitch P of the helix22,32.

The difference between said pitch P and the distance L between the two side walls25,35and the walls26,36of the helical element is equal at least to an amount sufficient to receive the helix32,22of the adjacent shaft30,20.

Essentially, the helix22wound about one of the two shafts20is interposed between a side of the helix32and the free side wall36of the helical element33of the adjacent shaft30and vice versa.

Each helical element23,33of the press1presents its perforated surface24,34as an enveloping conical surface.

In the described examples and as shown in the accompanying figures, the perforated surface24,34of the helical element23,33extends conically.

In a first embodiment the press1comprises a pair of cylindrical shafts20.30, and a pair of cylindrical helixes22,32(FIGS. 1-4).

In this respect, the term “cylindrical helix” means a helix the most external part of which can be enveloped by a cylindrical surface.

This configuration enables better pressing of the fibrous material compared with that attainable with presses of the known art.

In proximity to the material discharge aperture11, the compartment formed by the pitch of the helix22straddling the plane comprising the axes X-X, X′-X′ of the two shafts23,30can be substantially occupied by the helix32of the adjacent shaft30, so substantially reducing the gap present between the two helixes22,32.

This configuration prevents the fibrous material being able to rotate with the helix during pressing.

According to a possible variant, the helix22,32is formed by a hollow box-like structure communicating with said helical interspace27,37and having at least one perforated side.

According to a further embodiment of the present invention, the press1comprises a pair of cylindrical shafts20,30, and a pair of conical helixes22,32formed by a hollow box-like structure communicating with the helical interspace27,37and having at least one perforated side (FIGS. 5-8).

In this respect, the term “conical helix” means a helix the most external part of which can be enveloped by a conical surface.

In the embodiment shown inFIGS. 5-8, instead of reaching the surface of the shaft20,30along a single plane, the free side wall26,36of each helical element23,33can be formed with a stepped profile (when viewed in section), as shown inFIG. 5.

Another possible modification to the aforedescribed embodiments can be the use of a conical instead of cylindrical shaft, as shown in the embodiment ofFIGS. 9-12.

As will be apparent from the aforegoing description, the double screw press for pressing fibrous material, in particular sugar beet pulp, according to the present invention, enables the said requirements to be satisfied and the drawbacks stated in the introduction to the present description with reference to the known art to be overcome.

In this respect, by virtue of the combination of said characteristics a progressive pressing of the sugar beet pulp is obtained until a degree of controlled dewatering is achieved which is comparatively greater than the dewatering obtainable with known presses.

Moreover a virtually total absence of material damage is achieved together with a substantial reduction in the dragging rotation of the material by the press helix.

An advantage is also the provision of a conical interspace with cylindrical helixes, hence making the draining volume independent of the press geometry.

Numerous variations and modifications, all contained within the scope of protection of the invention as defined in the following claims, can be made to the aforedescribed press to satisfy contingent and specific requirements.