A screw press which effects a continuous ex¬ pelling of fluid from various moist material masses, is described in Norwegian Patent Specification No.

56,471 (from 1935) and in the main features the screw presses are built continuously as is described in said patent specification.

Gradually as screw presses have found application in new areas it has been found that when materials are concerned which give good squeezing out of fluid - and with this also a compact and hard press cake with great friction between the material and the screw surface - there is obtained with the construction of screw element which is descrived in the afore-mentioned Norwegian patent -specification, and insufficient thread engagement between the screw elements over a large portion of the length of the screw element, so that the press cake rotates to too great a degree together with the screw elements and with this the advancement of material is reduced and the effect of squeezing out of fluid is reduced. A consequence of the material

rotating to a large degree with the screw element, is that the pressure which is built up internally in the press basket already begins to decrease strongly at a distance from the discharge end of approximately 30% of the total length of the screw element. This situ¬ ation is determined by means of special pressure measuring cells which are mounted at suitable mutual distances along the press basket.

With the present invention the aim is to elimi- nate the afore-mentioned disadvantages with these types of presses and particularly the objective is a screw press arrangement .where the forward feeding of the material, the squeezing out of fluid and the build up of pressure in the screw press can be improved. This is achieved according to the invention due to the screw element at the discharge end in a length of between 10 and 30%, preferably approximately 20%, of the total length of -the- screw element having a dis¬ charge portion with a core diameter increasing uniform- ly towards the discharge end and a uniformly decreasing press basket inner diameter, the sum of the internal press basket radius and core radius in any radial plane in said discharge portion being constant and sub¬ stantially corresponding to the centre distance between the screw elements".

When it is stated above that the sum of internal press basket radius and core radius substantially corresponds to the centre distance between the screw elements, it is meant that the screw threads - in the instances the discharge portion is provided with screw threads - substantially form a sliding abutment against the core surface of the adjacent screw element.

In the two-screwed presses hitherto used one has been heavily committed to maintaining a particular press cake thickness when for the screw presses of a particular press size and press capacity there is first determined the core diameter of the screw element and

the thread diameter and with this also the centre distance between the screw elements. According to Norwegian Patent Specification 106,253 it has been made possible to reduce the press cake thickness by cutting in grooves in the core of the screw element for the reception of the threads of the adjacent screw element. An increase of the press cake thickness has been able to be effected relatively easily, but then with the consequence that the material slip becomes greater, by virtue of too small a narrowing down of the material passage in the accumulation zone between the screw threads. For a ready built press there has thus been hitherto an opportunity to alter the press cake thickness to a minimum degree only. According to the invention said discharge portion can in a manner known per se be constructed with or without screw threads and the press basket over a corresponding length has- a-separately replaceable portion. As a result one can freely choose all according to which material which is to be treated internal press basket diameter and external core diameter as required, so that optimum squeezing out conditions can be ob¬ tained for the relevant material. Said in another way, the greater the conisity which is employed -for the dis- charge portion, the smaller the discharge cross-section of the press cake which is obtained and with this also a correspondingly high material pressure in the dis¬ charge end together with improved squeezing out con¬ ditions. In an instance where the discharge portion is without screw threads this part of the press will act as an effective throttle arrangement, and the throttle effect can be regulated as required by employing different displaceable basket sections. It must be mentioned that the press can act just as effectively with screw elements with opposite thread directions and hence opposite directions of rotation,

as with screw elements with the same thread direction and hence the same direction of turn.

When the press cake leaves the press, it has the shape of a hollow cylinder in accordance with the shape of the passage of the press at the discharge end of the screw element. In the construction of the screw element the aim is optimum efficiency with a high degree of squeezing out of fluid and a particular press cake thickness at the discharge of the press, the height of the gap between the screw elements and the threads respectively at this portion giving a corre¬ sponding height. Apart from the-desire to have such a completely mutual engagement as possible between the threads in the two screw elements it is preferred according to the invention that the screw element in a length of up to 50% of the total length of the screw element, including said discharge portion, has a thread radius corresponding to--the distance from the axis of the screw element to the inner surface of the press basket and to the outer core surface of the adjacent screw element respectively.

Further features of the invention will be evident from the following description having regard to the accompanying drawings, in which: Fig. 1 to 3- show a vertical longitudinal section, a horizontal section and a cross7section of a screw press of known construction.

Fig. 4 shows a horizontal section of a press according to the invention, in which there is schema- tically shown the relationship between the inner walls of the press basket and the screw elements.

Fig. 5 shows a horizontal section according to the invention where the discharge portion is without threads. Fig. 6 shows for comparison the path of the pressure curve for the material in the press according to the known construction and according to the inven-

OMPI Λ. WIFO

tion.

Fig. 7 shows for comparison the volume curve path for the material in the press according to the known construction and according to the invention. In Fig. 1 to 3 there is shown generally a press of known construction. On a stand 10 there is fastened a braced, sectionally divided drainage means in the form of a press basket 11 which surrounds in a rela¬ tively tightly fitting manner two rotating screw ele- ments 12, 13 with associated threads 14, 15 and with centre axes extending mutually parallel. The screw elements 12, 13 rotate in opposite directions, but alternatively can be designed in a known manner so that they rotate in the same direction. The supplied material^ that is to say the wet mass which is to be pressed, is fed at the one end of the press via a vertical downwardly exte ding hopper 16 to a feeding-in portion—17-of the screw elements 12, 13 and is transported by means of the screw elements end- ways through the press via its central portion 18 and discharge portion 19 to a vertical downwardly extending exhaust duct 20 for the squeezed out mass, that is to say the press cake itself. The screw elements 12, 13 are mounted at the feeding-in portion in bearings 21, 22 and at the discharge portion in bearings in connection with a toothed wheel gearing 23 and an associated driving motor(not shown further), a reduction in revo¬ lutions per minute being provided for from motor to screw elements. Below the whole of the press basket 11 there is arranged a collecting tray 24 with outlet pipe 25 for the luid ^" which is squeezed out of the material or the mass of material.

In the known construction which is illustrated in Fig. 1 to 3, there is total thread engagement, that is to say the thread on the one screw element reaches totally inwards to the core of the adjacent screw ele¬ ment, only at a relative ^" ! short portion at the inlet

end and the discharge end of the screw elements, while a relatively extended central portion, which has a uniformly increasing screw core diameter, has a varying degree of thread engagement. The press basket according to the invention as illustrated in Fig.. 4, is equipped at the feeding-in end with a first, cylindrical press basket portion 30 which is followed by a second, relatively short, coni- cally tapered press basket portion 31 and a third, rela- tively extended, cylindrical press basket portion 32 together with a fourth, conically tapered press basket portion 33 at the discharge end. Screw threads 36, 37 of the screw elements 34, 35 have an external diameter which corresponds to the inner diameter of the press basket in any radial plane in the longitudinal direction of the press, so that there is obtained a stripping slide abutment between the screw thread and the inner surface of the press basket in the longitudinal directio of the whole press. In the first press basket portion 30 cores 38, 39 of the screw elements have a minimum dia¬ meter and a substantially cylindrical outer surface or an outer surface with a slightly increasing diameter. In the second press basket portion 31 and in the first part of the third press basket portion 32 there is a re- latively heavily increasing core diameter, while in the largest part of the third press basket portion 32 there is a substantially cylindrical outer surface or an outer surface with a slightly increasing diameter and in the fourth press basket portion 33 there is a relatively heavily and uniformly increasing core diameter. In the illustrated embodiment complete thread engagement is achieved between the screw elements mutually (that is to say the threads of the one screw element contact the core of the adjacent screw element) , over approximately 50% of the total length of the screw element. In other words there is included in said section of 50% of the total length of the screw element the bulk of the length

of the screw elements in the fourth press basket portion 33 (with the exception of the outermost portion of the screw elements which is without threads) and an adjoining substantial portion of the screw elements in the third press basket portion 32. One achieves by this only a central portion of approximately 20-25% of the length of the screw elements having incomplete thread engagement (that is to say forms substance slip between threads of the screw elements) . In the illustrated embodiment the fourth portion 33 of the press basket constitutes approximately 20% of the total length of the screw elements, but if . . desired can constitute up to 30% of the total length of the screw elements . In the illustrated embodiment threads are arranged on approximately the whole of the fourth portion 33. If necessary threads can be left out of the screw- ^' elements in the whole of the fourth portion 33 as shown in-Pig-, 5.

In Fig. 6 there is shown a curve A (chain line) which illustrates the pressure for a particular sub¬ stance which passes a press according to a known con¬ struction and a curve B (fully drawn line) ^' which illustrates the pressure for the same substance which passes a press according to the invention. It is evi- dent that with the known construction a significant pressure drop is obtained towards the discharge end from a point which lies at a distance of 30-40% of the length of the press, reckoned from the discharge. According to the invention there is ensured a higher pressure level over a greater part of the press length with the same outlet pressure.

In Fig. 7 there is shown a curve a (chain line) which illustrates the substance volume-course measured at different length distances from the inlet of the press in the known construction and a curve b (fully drawn line) which illustrates the substance volume course in the solution for a corresponding press

according to the invention. In the curve portion b there is illustrated the substance volume-course at the conical discharge portion according to the invention.