A DECANTER CENTRIFUGE

Abstract: The decanter centrifuge (1) comprises a bowl (2) and a conveyor screw (7) rotatably journalled in the bowl (2) and a reduction gear (22) providing a relative rotation of the screw relative to the bowl. The gear (22) is journalled separately relative to the bowl in separate bearings (25, 26). The housing and the driven shaft of the gear (22) are by means of flexural but torsionally stiff couplings (28, 29) connected with the bowl (2) and the conveyor (7), resp. Said flexural couplings result in that the gear (22) is dynamically insulated from the bowl (2) and does not influence the critical number of revolutions thereof. As a result of this the number of revolutions of the bowl - and thus the maximum allowable number of revolutions of the entire centrifuge - may be increased, thereby offering substantially improved separating properties of the centrifuge as a whole.

Description: A Decanter Centrifuge. This invention relates to a decanter centrifuge comprising a rotatably journalled bowl and a rotatable screw conveyor journalled in the bowl and of the type in which the conveyor is connected with the bowl through a reduction gear provided with a housing co- rotating with the bowl, a driven shaft connected with the conveyor, and a drive shaft whose number of revolu¬ tions determines the relative number of revolutions of the conveyor relative to the bowl. Such decanter centrifuges are used for separ¬ ating a liquid/solids mixture supplied to the interior of the bowl into a solids phase and one or more liquid phases. This is obtained by r.otating the entire bowl at a high, number of revolutions and driving at the same time the conveyor at a comparatively low number of revolutions relative to the bowl, that is effected by means of the reduction gear which may either be mech¬ anical or hydraulic. Due to the high number of revolu¬ tions of the bowl it is in all general applications only possible to transfer the necessary torque between conveyor and bowl by means of such a co-rotating gear. The separating effect of the centrifuge and its capacity depend on the number of revolutions and the inner diameter of the bowl and on the length of the separating space in such a manner that an increase of each of said parameters, other things being equal, will cause an increase of the separating effect and/or the capacity. The separating effect and the capacity of the centrifuge are, however, restricted by the critical number of revolutions of the rotating system and is further limited in practice in that an allowable maxi¬ mum number of revolutions which is somewhat lower than the critical number of revolutions is stipulated con- sidering the stresses occurring in the rotating parts and the working conditions of the centrifuge, such as expected wear, vibration of the decanter as a whole, and so on. The critical number of revolutions depends inter alia on the mass of the rotating parts, and by reducing said mass the critical number of revolutions may be increased. In view of the fact that the above mentioned co-rotating gear constitutes a substantial portion of the total rotating mass in a decanter centrifuge, it may be desirable to separate the gear from the bowl in order to increase the critical number of revolutions of the decanter centrifuge. The decanter centrifuge according to the inven- tion is characterized in that the reduction gear is rotatably journalled in separate bearings, in that its housing is connected with the bowl through a flexural but torsionally stiff coupling, and in that the driven shaft of the reduction gear and the conveyor are like- wise connected through a flexural but torsionally stiff coupling. By supporting the gear in separate bearings and eliminating the possibility of transferring bending moments from the gear to the bowl and the conveyor it is obtained that the gear from a dynamical point of view is largely completely insulated from the latter components and thus does not exert any influence on the critical number of revolutions of the decanter centri¬ fuge. This entails the advantage over and above the prior art centrifuges of the above mentioned type that the allowable maximum number of revolutions of the centrifuge according to the invention may be increased or, for example that the length of the separating space may be increased without increasing the critical number of revolutions. In both cases the result is an increase of the separating effect and/or the separating capa¬ city. In many cases, the indicated location of the reduction gear results in a series of additional ad¬ vantages. For instance when processing very hot pro¬ ducts there is some risk in traditional decanter cen- trifuges that the gear - certainly located in immediate heat-conducting contact with the bowl - becomes over¬ heated. In case of a mechanical gear such an overheat¬ ing may cause a reduction of the gear lubricating oil viscosity, at worst resulting in an expensive gear breakdown. It will be recognized that arranging the gear on separate bearings results in a substantial reduction of the possibility of heat transfer between the bowl and the gear. Such a decreased heat transfer is further ad- vantageous if the processed product is very sensitive to temperature and does not stand heating during the separation in the bowl. This applies for instance to various biotechnological processes. In this case too, it is an obvious advantage that the heat generated in the gear due to loss of effect is transferred to the bowl and thus to the product. In use, a decanter is frequently subjected to a substantial wear, particularly of the conveyor. Such wear may change the equilibrium of the decanter to such a degree that even in normal operation undesired vibra¬ tions occur which, moreover, often grow worse due to the presence of the relatively heavy gear. Decanter centrifuges with the gear positioned directly on the bowl are, moreover, sensitive to unbalances in the gear itself. As regards balancing it is therefore also a considerable advantage that the gear is a separate dynamic unit that does not influence the vibration con¬ ditions of the bowl. The invention will now be explained with refer- ence to the accompanying drawing illustrating a decanter centrifuge 1 substantially consisting of a horizontal, axially symmetrical bowl 2 including a cylindrical section 3 and a conical section 4 and which is rotatably supported in stationary bearings 5 and 6. The bowl 2 includes an elongated screw con- veyor 7 which by means of bearings 12 and 14 is rotatably journalled in relation to the bowl and con¬ sists of a central body portion 9 surrounded by a continuous screw flight 11. The suspension to be separated in centrifuge 1 is supplied through an inlet 15 of an inlet tube 16 extending coaxially with the axis of rotation of the centrifuge through a central passage 17 provided in conveyor body 9. The tube 16 ends in a transverse, radial passage 19 opening out into the separating space 20 of the centrifuge. After separation in this space the solids are discharged through radial apert¬ ures 21, while the liquid is discharged through one or more outlets, not shown, at the opposite end of the bowl. The relative rotation of the screw conveyor in relation to the bowl is provided by means of a gear 22 - e.g. an epicyclic gear - positioned on a separate shaft 24 rotatably journalled in stationary bearings 25 and 26. The housing and the driven shaft of the gear are via two co-axial flexural but torsionally stiff couplings 28 and 29 connected with the bowl 2 and the screw conveyor 7, respectively. The use of such a flexural coupling, that is incidentally known, results in a gear that dynamically speaking is almost completely separated from the bowl so that the mass of the gear does not influence the critical number of revolutions of the bowl. As stated by way of introduction, this permits the maximum allowable number of revolutions of the bowl and thus also of the centrifuge to be increased, resulting in a substantially improved separating capa¬ bility and capacity.

Claims: P A T E N T C L A I M S 1. A decanter centrifuge (1) comprising a rota¬ tably journalled bowl (2) and a rotatble screw conveyor (7) journalled in the bowl (2) and of the type in which the conveyor (7) is connected with the bowl (2) through a reduction gear (22) provided with a housing co- rotating with the bowl (2), a driven shaft connected with the screw conveyor (7), and a drive shaft whose number of revolutions determines the relative number of revolutions of the conveyor (7) relative to the bowl (2), characterized in that the reduction gear (22) is rotatably journalled in separate bearings (25, 26), that its housing is connected with the bowl (2) through a flexural but torsionally stiff coupling (28), and in that the driven shaft of the reduction gear and the conveyor (7) are likewise connected through a flexural but torsionally stiff coupling (29).

Inventors: CEDERKVIST JAN, GODDIK BJARNE, CEDERKVIST, JAN, GODDIK, BJARNE
Applicants: ALFA LAVAL SEPARATION AS, ALFA-LAVAL SEPARATION A/S
Application year: 1989
Main classification: B04B1
IPC codes: B04B1, B04B9