WO2001052999A1

WO2001052999A1 - Scraper for centrifuges

Info

Publication number: WO2001052999A1
Application number: PCT/EP2000/000454
Authority: WO
Inventors: Ernst-Ulrich Himmen; Dietmar GÜNTHER; Jürgen Giesselmann; Detlef Freyer
Original assignee: Bayer Biologicals S.R.L.
Priority date: 2000-01-21
Filing date: 2000-01-21
Publication date: 2001-07-26
Also published as: AU2000229051A1

Abstract

Scraper for centrifuges having a high cutting capacity and a high degree of stability, consisting of an L-shaped stem in which the active head presents, against the wall of the centrifuge, a cutting profile basically defined by a cutting angle T comprised between the active surface Sa of sliding of the shaving and the working plane PI tangent to the wall of the centrifuge along the cutting generatrix and by a rake S comprised between the facing surface Sf of the scraper towards the wall of the centrifuge and the working plane, the angles T and S being respectively comprised within the following ranges: 20° ≤ T < 35° and 4° ≤ S < 15°.

Description

SCRAPER FOR CENTRIFUGES Scope of invention

The present invention regards the sector of separation of blood plasma by centrifuging and subsequent collection of the precipitate in the form of a paste deposited on the walls of the centrifuge.

More in particular, the invention regards a scraper, or knife, which is to be used for removing the paste from the walls of the centrifuge and which has the purpose of carrying out this operation with utmost precision, without interfering with the walls of the centrifuge, and with the lowest possible absorption of energy. Prior art

In the present state of the art, the paste deposited on the walls is recovered by means of a scraping system consisting of a knife kept close to the wall of the centrifuge by an articulated mechanical system. An example of centrifuge comprising a similar scraping system is described in the US patent No. US 5733238, to which the reader is referred for the overall constructional diagram of the centrifuge.

In the above-mentioned patent, the scraper consists of a basically L-shaped stem, which is connected to the articulated system. The cross section of the stem has the shape of a truncated triangle, and the geometry of the stem presents cutting edges in the part designed to operate on the paste.

In use, the cutting edges of the scraper are made to approach the wall so that they are parallel to the generatrices of the cylindrical wall and are defined by the encountering of one first active surface, on which the scales of paste cut away from the walls (hereinafter referred to as "shavings") run, and by one second surface that faces the wall.

The solution described enables a substantial proportion of the paste to be detached from the wall and enables the subsequent collection of the said shaved paste by means of special systems (not described in further detail herein because they do not form part of the invention). However, the scraper proposed above presents certain limits as regards its capacity to remove the maximum possible amount of paste, so that much of the paste is lost during the centrifuge washing phase, and as regards the excessive cutting force required during removal of the paste.

In particular, the above limits are due to the tendency of the scraper to compress the paste against the walls, so causing some of the paste to escape between the cutting edge and the wall of the centrifuge. This effect leads to a loss of energy (greater cutting force required) and lower precision, which in particular means the loss of significant layers of paste, which are left on the wall of the centrifuge. Purpose of invention

The present invention sets itself the purpose of overcoming these drawbacks and proposing a scraper for centrifuges of high efficiency both as regards the precision that can be achieved and as regards the lower energy required during the scraping operation.

Summary of invention

For the above purposes, it has been possible, according to the invention, to arrive at a scraper for centrifuges that has a high cutting capacity and a high degree of stability. The scraper consists of an L-shaped stem in which the active head presents, against the wall of the centrifuge, a cutting profile basically defined by a cutting angle T comprised between the active surface Sa of sliding of the shaving and the working plane PI tangent to the wall of the centrifuge along the cutting generatrix and a rake S comprised between the facing surface Sf of the scraper towards the wall of the centrifuge and the working plane.

Other characteristic angles are the knife angle L between the active surface and the facing surface Sf of the scraper towards the wall of the centrifuge and the top rake D between the active surface and the plane Pn normal to the working surface at the cutting line. According to the invention, the angles T and S are respectively comprised within the following ranges: 20° < T < 35° and 4° < S < 15°.

Preferable values of the characteristic angles of preferred embodiments are given in the dependent claims. The advantages obtained basically lie in the fact that, with such a geometry, the scraper manages to provide a better efficiency and precision in cutting and manages to limit the compression of the paste against the walls of the centrifuge. List of figures

- Figure 1 is a schematic view of a cutting profile with the main angles indicated;

- Figure 2 is a schematic top view of a scraper in the position of use in a centrifuge for the separation of plasma; - Figures 3a, 3b and 3c show one first scraper according to the invention illustrated, respectively, in side view, front view, and working position;

- Figures 4a, 4b and 4c show one first scraper according to the invention illustrated, respectively, in side view, front view, and working position;

- Figures 5a, 5b and 5c show one first scraper according to the invention illustrated, respectively, in side view, front view and working position.

Detailed description of invention

With reference to Figure 2, a scraper 1 is made up of a stem, the initial portion 3 of which is supported by a mechanical system 4, whilst the second portion 5 of the stem is positioned parallel to the cylindrical wall 6 of the centrifuge and faces downwards looking into the sheet according to the representation of Figure 1.

During operation, the mechanical system 4, which is of a type in itself known and hence is not described in greater detail hereinafter, performs the function of bringing the cutting profile up to the wall, whilst the cutting feed motion is provided by the rotational motion of the wall 6 itself. With reference to Figure 1 , the angles of major importance that define the cutting profile of a scraper with reference to the wall 6 of a centrifuge and to the cutting edge 7 are illustrated schematically. The arrow in the figure indicates the direction of the cutting motion. According to the invention, it has been found that by limiting the value of the cutting angle T and the rake S within the ranges 20° < T < 35° and 4° < S < 15°, a greater cutting precision is obtained and a lower cutting force is required, with the advantage of it being possible to remove a larger quantity of paste from the wall 6 without the risk of damaging the wall and with the advantage of using less energy to carry out removal. In the above ranges it has been possible to identify certain preferred embodiments of scrapers, indicated as R1-R3 and completely defined by the following angles: TABLE 1

Cut Rake Knife Dev. i I

R1 31.7° 15° 16.7° 58.3°

R2 20.9° 10° 10.9° 69.1 °

R3 20.9° 4° 16.9° 69.1 °

The scrapers R1-R3 are completely illustrated in the respective figures (3a-3e) to (5a-5e), which present the values of the angles given in Table 1 and the sections of the scraper according to the cross-sectional planes A-A of the active portion and B-B of the portion of connection to the mechanism for supporting the scraper. Experimental data

In order to verify the results obtained, a centrifuge manufactured by CARR was used in which two suspensions were separated. The first suspension was obtained by mixing sawdust (Lignocel HB 120 produced by Rettenmaier & Sδhne GmbH & Co. KG) and alumina (Tonsil Standard produced by Sϋd Chemie) in water in the proportion of 3:1 ; the second was obtained by mixing lime powder (Ulmer Weiss XM produced by Eduard Merckle GmbH & Co.) and Tonsil Standard in water in the proportion of 2:1. The suspensions showed a behaviour that is very similar to that of blood plasma.

Once the suspensions had been centrifuged, the tests on the scrapers were conducted with a position angle X = 90° and a feed speed of the scraper at 21 mm/min.

The cutting force absorbed was evaluated by measuring the hydraulic pressure in the centrifuge circuit.

With both suspensions, the scrapers R1-R3 revealed a high cutting precision and reduced cutting forces, which enabled limitation of the compression of the paste between the facing surface Sf and the wall 6. The usefulness of a sharp shaving- breaker corner on the back of the cutting edge was also confirmed. The present invention has been described with reference to a preferred embodiment, but it is understood that equivalent modifications may be made by any person skilled in the branch without, however, thereby departing from the sphere of protection of the present industrial patent right.

Claims

CLAIMS 1. Scraper for centrifuges made up of an attachment stem (3) and of an active portion (4) for removal of the precipitate from the wall (6) of a centrifuge, the cutting profile of the scraper being defined along a cutting line (7) by an active surface Sa of sliding of the shaving and a facing surface Sf of the scraper towards the wall (6), characterized in that the cutting angle T comprised between Sa and a working plane PI tangent to the wall (6) along the cutting line (7) is comprised between 20° and 35°, and the rake S, comprised between Sf and PI, is between 4° and 15°. 2. Scraper according to Claim 1 , characterized in that the cutting angle T is 31.7° and the rake S is 15°. 3. Scraper according to Claim 1 , characterized in that the cutting angle T is 20.9° and the rake S is 10°. 4. Scraper according to Claim 1 , characterized in that the cutting angle T is 20.9° and the rake S is 4°. 5. Scraper according to Claim 2, characterized in that it presents a knife angle L comprised between Sa and Sf of 16.7° and a top rake D, delimited by Sa and the plane Pn normal to PI at the cutting line (7), of 58.3°. 6. Scraper according to Claim 3, characterized in that it presents a knife angle L comprised between Sa and Sf of 10.9° and a deviation angle of the shaving or top rake D, delimited by Sa and the plane Pn normal to PI at the cutting line (7), of 69.1 °. 7. Scraper according to Claim 4, characterized in that it presents a knife angle L comprised between Sa and Sf of 16.9° and a top rake D, delimited by Sa and the plane Pn normal to PI at the cutting line (7), of 69.1 °. 8. Scraper according to any one of the foregoing Claims, characterized in that it presents a shaving-breaker edge.