Decanter centrifuge

Abstract

The invention relates to a decanter centrifuge comprising a rotatable drum (10) which tapers conically into a solids outlet (50) and forms a liquid outlet (56) at the opposite end, a stationary inlet tube (42) which leads axially into the drum for supplying a suspension into the interior of the drum, and a screw (20) which is rotatably arranged in the drum and which surrounds an outlet end (44) of the inlet tube and forms a helix (30, 32) that radially reaches the inner circumferential surface of the drum (10). The invention is characterized in that a portion of the inlet tube (42) that axially projects into the interior of the drum has a mechanical vibration damper (60).

Description

[0001] DECANTER CENTRIFUGE

[0002] The invention relates to a decanter centrifuge with a rotationally drivable drum which tapers conically towards a solids outlet and forms a liquid outlet at the opposite end, a stationary inlet pipe leading axially into the drum for feeding a suspension into the interior of the drum, and a screw which is arranged in the drum in a rotationally drivable manner and surrounds an outlet end of the inlet pipe and forms a helix which extends radially to the inner circumferential surface of the drum.

[0003] Decanter centrifuges of this type are used to separate mixtures that typically consist of a solid and a liquid phase. However, there are also applications involving two liquid phases and one solid phase. These mixtures are referred to here, without restriction of generality, as "suspensions."

[0004] An example of a decanter centrifuge of this type is described in WO 2019 / 081177 A1. The suspension is pumped into the centrifuge via the central, vertical inlet pipe, where centrifugal force creates a liquid ring, a so-called "pond," on the inner peripheral wall of the bowl. Due to the centrifugal force, the heavier particles are separated on the inner wall of the bowl. Due to a small speed difference between the screw and the bowl, the solids are transported axially by the screw flights to the solids outlet. Finally, the solids cake is pushed out of the liquid ring and discharged via outlets in the bowl shell. At the same time, the clarified liquid flows out of the opposite side of the bowl via the liquid outlet.

[0005] The inlet pipe is rigidly mounted outside the drum, with one end extending freely into the drum's interior. The axial position of the inlet pipe's opening is selected so that the suspension enters the drum's interior in a position that is optimal for the process. Since the drum rotates, a liquid-tight passage for the inlet pipe must be provided in one of the drum's end walls, allowing relative rotation of the two components. Any imbalance in the drum, caused for example by the mass distribution of the suspension currently present in the drum, can be transmitted to the inlet pipe via this passage, causing it to vibrate. The excitation frequency of these vibrations corresponds to the drum's rotational speed.To enable high speed and thus efficient material separation without causing damage due to resonant vibrations in the inlet pipe, the inlet pipe must have the highest possible natural frequency and correspondingly the highest possible flexural rigidity. This requires a correspondingly thick wall for the inlet pipe, so that the outer diameter of the inlet pipe must be relatively large in relation to the inner diameter. However, a large outer diameter requires a correspondingly large bearing for the screw and thus ultimately a large screw body diameter, which has process-related disadvantages in certain applications. If, on the other hand, the inner diameter of the inlet pipe is reduced, the suspension must be pumped into the inlet pipe at a higher pressure for the same throughput, resulting in greater energy losses.In some known decanter centrifuges, attempts have been made to further stabilize the inlet pipe by adding additional support bearings. However, these approaches require considerable design effort. The object of the invention is to create a decanter centrifuge in which natural vibrations of the inlet pipe can be avoided even at high speeds. This object is achieved according to the invention in that the section of the inlet pipe that extends axially into the interior of the bowl has a mechanical vibration damper.

[0006] The principle of operation of a vibration absorber, as it is known, is based on the fact that a balancing mass, the so-called absorber mass, is suspended from a vibrating system in such a way that it is capable of oscillating at the system's natural frequency. In resonance, the absorber mass then oscillates in counter-phase with the main mass of the system, thus reducing the oscillation amplitude of the main mass. By damping the oscillation of the absorber mass, vibration energy can be removed from the system, thus largely suppressing the occurrence of resonant vibrations of the main mass.

[0007] In the case of the inlet pipe of the decanter centrifuge according to the invention, the damper mass is suspended on or in the inlet pipe in such a way that it is capable of oscillating in any radial direction of the inlet pipe. The resonance frequency of the damper mass is selected to correspond to the fundamental frequency of the inlet pipe's bending natural vibrations. Since the vibration damper suppresses resonance vibrations of the inlet pipe at this frequency, unwanted resonance vibrations only occur at the next higher vibration mode of the inlet pipe. However, the first harmonic of the inlet pipe's natural frequency lies at a frequency approximately six times higher than the fundamental frequency, enabling significantly higher centrifuge speeds without causing destructive resonance vibrations.

[0008] Advantageous embodiments and further developments of the invention are specified in the subclaims. In one embodiment, the damping mass is accommodated inside the inlet pipe. This has the advantage that the inlet pipe has a uniform outer contour along its entire length, so that it can be easily inserted through the opening in the end wall of the drum during assembly. For example, the damping mass can be annular so that the suspension can flow through the interior of the ring. Pressure losses in the flow of the suspension can be minimized by integrating the annular damping mass, including the annular radial movement space for this damping mass, into a Venturi nozzle that locally constricts the internal cross-section of the inlet pipe.Since this cross-sectional constriction extends only over a very short longitudinal section of the inlet pipe and thus the flow velocity of the suspension is only increased over a short distance, flow losses are minimized.

[0009] The axial position of the vibration absorber can be selected to correspond to a node of the first harmonic. This avoids a reduction in the natural frequency of the first harmonic.

[0010] In the following, an embodiment example is explained in more detail using the drawing.

[0011] They show:

[0012] Fig. 1 is an axial section through a decanter centrifuge according to the invention;

[0013] Fig. 2 is an enlarged longitudinal section of a vibration damper in the decanter centrifuge according to Fig. 1; and

[0014] Fig. 3 is a section taken along line III-III in Fig. 2. The decanter centrifuge shown in Fig. 1 has a drum 10 which is axially divided into a cylindrical section 12 and a conical section 14 and is closed off at both ends by respective hubs 16, 18. Arranged inside the drum 10 is a screw 20 which, like the drum, is divided into a cylindrical section 22 and a conical section 24. In the cylindrical section 24, the screw has an inner casing 26 which is closed off from the cylindrical section by an end wall 28 and which has a helical helix 30 on its outer circumference which extends as far as the inner surface of the cylindrical section 14 of the drum.The cylindrical section 22 of the screw has a helical helix 32 which has a smaller height than the helix 30 and is supported with its inner circumference on the axially extending outer edges of supporting walls 34 which extend in the axial direction of the drum and are arranged in a star shape around the axis of the drum.

[0015] The cylindrical section 22 of the screw is mounted on the hub 12 by a bearing 36, and the inner shell 26 of the conical section 24 is mounted in the hub 18 by a bearing 38. The hub 12 of the drum, in turn, is rotatably mounted in a housing (not shown) by a bearing 40. Another bearing (not shown) serves to support the drum 10 at the opposite end. The drum 10 and the screw 20 can be driven at slightly different speeds by rotary drives (not shown).

[0016] An inlet pipe 42, which is rigidly mounted outside the drum 10 in a frame (not shown), runs coaxially through the hub 12 and terminates inside the cylindrical portion 12 of the drum. An outlet end 44 of the inlet pipe 42 is surrounded at a short distance by the radially inner longitudinal edges of the supporting walls 34.

[0017] The supporting walls 34 are welded to the inner shell 26 of the conical section 24 of the screw and to a housing of the bearing 36, forming a flexurally and torsionally rigid support structure for the screw 32. The operation of the decanter centrifuge is described below.

[0018] The drum 10 and the screw 20 are driven at slightly different speeds so that the screw 20 runs slightly faster than the drum 10. An emulsion to be decanted is pumped into the interior of the drum 10 via the inlet pipe 42 and deflected radially outward by a baffle 48. The radial support walls 34 accelerate the emulsion in the circumferential direction so that, due to its inertia, it spreads into a thin film on the leading surface of each support wall and flows radially outward due to centrifugal force. However, the radial component of the flow velocity of the suspension remains relatively low due to the friction on the support walls 34.

[0019] On the inner peripheral surface of the cylindrical part 12 of the drum, the suspension collects to form a liquid ring or "pond", the inner surface of which is shown in dashed lines in the upper half of Fig. 1. Due to centrifugal force, the heavier solid phase of the suspension settles on the inner surface of the drum and, due to the differential rotation of the screw and drum, is driven by the helix 32 to the left in Fig. 1, i.e. towards the conical end of the drum. The last turn of the helix 32 presses the solid cake into the conical section 24 of the screw and is then taken over by the helix 30 and transported to the tapered end of the conical section 14 of the drum, where it is discharged via a solids outlet 50 formed by radial discharge openings.The hub 12 at the right end of the drum 10 is interrupted by openings 52, which, together with an annular weir 54, form a liquid outlet 56. The inner diameter of the weir 54 determines the position of the inner surface of the liquid ring. With increasing suspension input, the depth of the "pond" increases, and as soon as the level of the inner diameter of the weir 54 is reached, the clarified liquid flows in a slow, calm axial flow in the spaces between the supporting walls 34 to the liquid outlet 56. The flowing liquid is only slightly swirled, so that the solid phase can be separated very effectively.

[0020] By replacing the weir 54, the depth of the pond can be varied depending on the operating conditions.

[0021] An end wall of the bearing 36 mounted on the hub 16 forms a fluid-tight passage 58 for the inlet pipe 42. If an imbalance occurs in the rapidly rotating drum 10, the inlet pipe 42 can be excited to flexural vibrations by interacting with the passage 58. To prevent the material of the inlet pipe 42 from fatigued and ultimately breaking, the flexural vibrations of the inlet pipe 42 should be prevented from resonating with the rotational speed of the drum 10. According to the invention, however, the inlet pipe 42 is dimensioned such that the fundamental frequency of the flexural vibrations, i.e., the frequency of the lowest vibration mode, is lower than the maximum rotational speed of the drum 10. This allows for a low wall thickness and, accordingly, a large inner diameter with a relatively small outer diameter of the inlet pipe 42.Accordingly, the bearing 36 can also be dimensioned small in the radial direction so that it does not hinder the outflow of the clarified liquid even at great pond depths.

[0022] When the centrifuge is started and the speed is increased, it eventually reaches the natural frequency of the lowest vibration mode of the inlet tube 42, causing it to resonate. However, these resonant vibrations are suppressed by a vibration damper 60 built into the inlet tube 42, which is shown enlarged in Figs. 2 and 3.

[0023] The inlet pipe 42 is interrupted at the location of the vibration damper 60, and the gap is filled by an outer pipe 62, which has the same outer diameter as the inlet pipe 42, and by an inner pipe 64 arranged coaxially in the outer pipe 62, which rigidly connects the two sections of the inlet pipe 42 separated by the gap. The inner pipe 64 therefore has a slightly greater axial length than the gap between the two sections of the inlet pipe and merges at both ends into conical rings 66, 68, which are welded to the inlet pipe 42 and to the outer pipe 62. Together with the inner pipe 64, the conical rings 66, 68 form a Venturi nozzle through which the emulsion can flow with minimal loss.

[0024] The inner ring 64 and the outer ring 62 together define an annular chamber 70, which accommodates an annular damper mass 72 with radial play. In the example shown, the damper mass 72 has a plurality of pockets 74 distributed around the circumference in its outer circumferential surface (Fig. 3), each of which holds a leaf spring 76 subject to bending stress, the apex of which rests against the inner circumferential surface of the outer ring 62. In this way, the damper mass 72 is resiliently suspended in the annular chamber 70, allowing it to vibrate in any radial direction. The annular chamber 70 can be filled with a high-viscosity oil via a closable connection 78, which dampens the vibrations of the damper mass 72.

[0025] The resonance frequency of the oscillating system formed by the damper masses 62 and the leaf springs 76 is selected such that it corresponds to the natural frequency of the lowest vibration mode of the inlet pipe 42, and the axial position of the vibration damper 60 is selected such that the damper mass 72 can be excited to vibrate by the lowest vibration mode of the inlet pipe 42. In the case of resonance, the bending vibrations of the inlet pipe 42 are therefore canceled out by the antiphase vibrations of the damper mass 72, thus achieving effective vibration suppression. The natural frequency of the next higher vibration mode of the inlet pipe 42, which would have a node at the location of the vibration damper 60, is so high that it is significantly above the maximum speed of the

[0026] Drum 10 and therefore cannot be excited to resonant vibrations.

[0027] In this way, a high speed of the centrifuge is possible without damage to the inlet pipe 42 due to resonance effects.

Claims

PATENT CLAIMS A decanter centrifuge with a rotatably driven drum (10) which tapers conically towards a solids outlet (50) and forms a liquid outlet (56) at the opposite end, a stationary inlet pipe (42) leading axially into the drum for supplying a suspension into the interior of the drum, and a rotatably driven screw (20) arranged in the drum, which surrounds an outlet end (44) of the inlet pipe and forms a helix (30, 32) which extends radially to the inner circumferential surface of the drum (10), characterized in that an axially into the The section of the inlet pipe (42) projecting inside the drum has a mechanical vibration damper (60).

2. Decanter centrifuge according to claim 1, in which the vibration damper (60) is arranged completely within the outer cross-section of the inlet tube (42).

3. Decanter centrifuge according to claim 1 or 2, wherein the vibration damper (60) has an annular damper mass (72) which forms a passage for the suspension supplied via the inlet pipe (42).

4. Decanter centrifuge according to claim 3, in which the damping mass (72) is received in an annular chamber (70) which is bounded by an outer tube (62), an inner tube (64) and by conical rings (66, 68) formed at both ends of the outer and inner tubes.

5. Decanter centrifuge according to claim 4, wherein the conical rings (66, 68) are profiled such that they, together with the inner tube (64), form a Venturi nozzle which narrows the inner cross-section of the inlet tube (42) over a limited length.

6. Decanter centrifuge according to claim 5, wherein the annular chamber (70) is filled with a viscous liquid.

7. Decanter centrifuge according to any one of claims 4 to 6, in which the damping mass (62) is held on the outer tube (62) and / or on the inner tube (64) by means of leaf springs (74).

8. Decanter centrifuge according to any one of the preceding claims, in which the The vibration damper (60) is arranged in the axial position of a vibration node of the first harmonic of the fundamental vibration of the inlet pipe (42).

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