Abstract:
The operation of dewatering and drying devices which consist of a dewatering centrifuge and a concentrically arranged spray drier may be disturbed by leaks between the drier housing and the centrifuge or by deposits and encrustation of solid particles inside the drier. In order to avoid these disturbances, the rotating outer surface of the centrifuge ( 1 ) is sealed with respect to the fixed front walls ( 13, 14 ) of the drier housing ( 11 ) by a sealing system in two or more stages which consists of rotary seals ( 160 ) and elastic or sliding sealing elements ( 180, 260, 300, 340 ). The rotating outer surface of the centrifuge ( 1 ) is provided, with turbulence-generating means ( 32, 33, 40, 42, 46 ), preferably torus-shaped turbulence-generating rollers, arranged inside the drier housing ( 11 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a device for dewatering and drying suspensions. A dewatering and drying device of this type is known from EP 0591299. 
     In the known dewatering and drying device, the 0.3-3-mm moist solid particles sprayed radially at high speed at the discharge of the centrifuge, preferably a full-jacketed helical-conveyor centrifuge, are diverted by suitable means, for example, diverting surfaces or a suitable gas flow, in the axial direction of the centrifuge and guided by the gas flow on a helical flight path in the drying chamber. Here the sprayed solid particles are flowed around at a high relative speed by the drying gas and dried. The drying chamber is a concentric annular chamber. It is embodied by the outer drier housing, the inside, rotating drum jacket of the centrifuge, or an inside housing surrounding the drum and the two housing end walls. The outside walls of the concentric drying chamber are stationary, and must be sealed, at least at one location, against the rotating parts of the centrifuge inside. 
     The rotary seal between the centrifuge rotor and the surrounding drier housing must overcome and tolerate a high relative speed, a gas-difference pressure between the inside and outside, and displacement movements due to thermal expansions and vibrations. The seal is intended to prevent or minimize the escape of gases from the drier interior to the outside, or the entrance of secondary air from the outside to the inside. 
     It has been seen that the seal gap between stationary housing parts and rotating centrifuge parts changes in an unacceptable manner particularly because of thermal expansion during heating processes in the startup phase, or with the occurrence of vibrations or changes in the temperature of the drier housing. This can lead to contact between the seal surfaces from time to time, and damage to or destruction of the seal. 
     To avoid this, the gap width must be selected to be large enough that thermal expansions and displacements of the drier housing do not lead to touching of the contactless seals. 
     A further disadvantage is that the gap also changes due to vibrations of the dewatering centrifuge inside the drier, because the rotating and non-rotating parts of the seal are respectively secured to different seal carriers. 
     An excessively-large seal gap is particularly disadvantageous in the operation of the centrifuge drier with an inert-gas atmosphere, because the entrance of the secondary air noticeably increases the oxygen content of the inert drying gas. 
     A further disadvantage of the dewatering and drying device known from EP 0 591 299 relates to the diverting surfaces for the solid particles that are spun out of the rotating centrifuge. Despite the use of wall scrapers that are secured to the rotating centrifuge drum, deposits and encrustations can occur on the diverting surfaces, as well as in the drier housing or the downstream devices (washer, cyclone) if the centrifuge effects poor mechanical pre-dewatering of the suspension, or if the solid particles are very sticky and moist. In continuous drying operation, this causes disturbances and breakdowns, which is economically disadvantageous. Up to now, attempts have been made to effect positive changes in the moisture behavior and stickiness of difficult-to-dewater suspensions by mixing them with additives prior to centrifuging. This measure is, however, quite expensive. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to implement constructive measures to avoid disturbances in operation, as caused by either seal leakages between the drier housing and the centrifuge or deposits and encrustations of solid particles, in a dewatering and drying device of the type mentioned at the outset. 
     The invention provides the generation of a free dispersion of the pre-dewatered solids through mechanically-induced rolling turbulences of the drying gas; good distribution of the dispersed solid particles in the drying gas; the most uniform possible distribution of the particle concentration in the drying gas; and the blowing away of encrustation layers that may build up. The concentration of the small, dispersed, moist particles in the drier chamber should be uniform and low, and the relative speed of the hot gas in relation to the particles should be as high as possible to assure rapid drying of the moist solid particles in flight. For example, elements that induce the gas flow and assure a powerful turbulence in the vicinity of the surfaces in the drier chamber, which are at risk for encrustation, or at the diverting surfaces, are secured to the outside of the rotating centrifuge drum so as to project into the drier chamber. The surfaces of the work chamber walls in the drier can be polished or coated with an anti-adhesive to promote the prevention of encrustation. The directing and guiding sheets built into the drier chamber purposefully influence the flow of the hot gas to effect a uniform gas distribution, avoid dead spaces and assure an intensive contact of the hot gas with the moist solid particles. Perforated walls through which gas flows are also suitable for preventing encrustations due to moist, sticky solid particles if the hot gas flowing in keeps the sticky particles away from the walls until the particle surfaces have dried sufficiently and, having a lower moisture content, lose their tendency to stick. Particularly in organic clarification sludges having a pronounced adhesive phase, the tendency to stick is especially strong in certain moisture ranges and must be overcome in fractions of seconds in flight. 
     The invention further provides a sealing of the radial end walls of the drier housing against the rotating jacket surface of the centrifuge with a rotary seal, which can keep the seal gap very narrow without the risk of mechanical contact between the rotating and non-rotating work surfaces of the rotary seal, and thus damage to or destruction of these surfaces. A further advantage of the rotary seal is that even uncontrollable, large displacement and expansion movements of the drier housing during the heating or cooling phase of the centrifuge drier, or stronger vibrations during the operation, do not affect the sealing function, despite the narrow gap of the rotary seal. The escape of inside gases or solids or the entrance of secondary air into the inert drying gas is virtually entirely prevented by the narrow seal gap. 
     A further advantage of the invention is the avoidance of encrustations and baked-on buildup, even in difficult-to-dewater sludges. This expands the use and application range of the device of the invention to products which, after the mechanical dewatering, yield a solid that is extremely sticky or possesses a very high moisture content. Breakdowns caused by baked-on buildup as a result of excessively-moist mechanical pre-dewatering in the centrifuge, and the associated costs, are also avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details, advantages and features of the invention are explained in detail by way of embodiments illustrated in the drawings. 
     Shown are in: 
     FIG. 1 a longitudinal section of a dewatering and drying device (referred to hereinafter as “centrifuge drier”) having perforated gas-guiding sheets; 
     FIG. 2 a longitudinal section of a centrifuge drier with directing sheets in the drier chamber; 
     FIG. 3 the dispersion zone of a centrifuge drier having rotating cleaning blades for the diverting surfaces of the dispersed particles; 
     FIG. 4 the dispersion zone of a centrifuge drier having rotating turbulence blades for keeping the drier walls clean; 
     FIG. 5 a combination of cleaning and turbulence blades for preventing encrustations in the interior of the drier and lines; 
     FIG. 6 a combination of turbulence and transport blades for keeping the interior of the drier clean; 
     FIG. 7 rotating turbulence disks in the drier chamber for generating rolling turbulences for re-dispersion; 
     FIG. 8 diverting surfaces for better dispersion and wider distribution of the pre-dewatered, moist solid particles; 
     FIG. 9 a longitudinal section of a centrifuge drier having a housing seal; 
     FIG. 10 a contactless labyrinth seal for a centrifuge drier; 
     FIG. 11 a contactless, threaded conveying seal for a centrifuge drier; 
     FIG. 12 a contactless, threaded conveying seal having a sharp-crested thread; and 
     FIG. 13 a contactless seal with shallow grooves. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the illustrated example, the dewatering and drying device (“centrifuge drier”) shown in FIG. 1 has a full-jacketed helical-conveyor centrifuge  1  of a design known per se. Instead of the illustrated full-jacketed helical-conveyor centrifuge, it is possible to use other centrifuges that are suitable for dewatering suspensions such as sludges, for example basket helical-conveyor centrifuges or three-phase separators, in which one phase is to be dried. 
     The full-jacketed helical-conveyor centrifuge  1 , referred to hereinafter as “dewatering centrifuge” or “centrifuge” for short, has a rotating drum  2 , which is rotatably seated at its axial ends on roller bearings  3 . The drum  2  tapers conically at one or both ends, and is provided at its tapered end with discharge openings  4 , which form the discharge zone  5  for the pre-dewatered solid  6 . The suspension, for example liquid sludge  8 , supplied through a pipe  7  into the interior of the centrifuge  1  is separated in the centrifuge  1  into a solid  6  and a clarified liquid  9  due to centrifugal forces, the liquid being sprayed out of the centrifuge  1  into a separate housing  10 , the central chute, at the other end of the drum jacket  2 . 
     The drier directly surrounding the centrifuge  1  is formed by an outside drier housing  11  and an inside housing  12  that surrounds the rotating drum  2 , or by the drum  2  itself and the two end walls  13  and  14 . The drying gas  15  is introduced, for example tangentially, into the drier chamber  17  through a hot-gas shaft  16 , then flows around the dispersed solid  6 , present in particle form, which is then diverted in the axial direction by the baffle cone  18 ; the gas then transports the dried solid particles in helical paths through the concentric annular chamber  19  to the discharge channel  20  of the drier housing  11 . From here, the drying gas  21  carrying the dried solid particles flows out through a pneumatic conveyor line, not shown, to a solids separator, and is separated again there into gas and a solids bed. 
     For uniformly distributing the hot drying gas  15  entering the concentric annular chamber  19 , and mixing it thoroughly with the solid particles diverted and slowed by the baffle cone  18 , a perforated sheet  22 , for example having a conical shape, is provided, through which the hot gas  15  flows. The perforated sheet  22  can comprise a conical surface or a series of sections having different conical angles, hole shapes, slots, free opening cross sections or partial solid-sheet sections for attaining the aforementioned effects. Full or partial annular gaps  23  can also be embodied between the perforated sheet  22 , the baffle cone  18  and/or the drier housing  11  for preventing an undesired accumulation of solids. The distributor sheet  22 , which can be flowed through, can also deviate from the cone shape and have a bowl shape, a cylindrical shape or a planar shape, or it can be a combination of different shapes. 
     FIG. 2 shows a combination centrifuge drier equipped with directing elements  25 ,  26  in the concentric annular drier chamber. The centrifuge drier is constructed from components similar to those in FIG. 1, and functions similarly to the drier of FIG.  1 . Instead of the perforated sheet  22 , however, helical directing sheets  25 ,  26  are built into the drier chamber  19 ; these sheets effect a restricted guidance of the flow of gas in the concentric drier chamber  19 , and prevent bypasses between the hot-gas entrance  16  and the gas exit  20 . The helical shape of the directing sheet  26  can preferably have a less-steep pitch than the directing sheets  25  disposed behind the directing sheet  26  in the axial direction. With a suitable embodiment of the directing sheet  26  (which is disposed in the entrance region of the hot gas  15 ), it is possible to reduce the number of directing sheets  25  extending over nearly the entire length of the drier housing  11  or on the directing,sheets  25 , or omit the sheets  25  altogether. The hot gas  15  (also called “drying gas”) entering, for example, tangentially is guided around nearly the entire circumference in the region of the discharge zone of the dispersed, moist solid  6  by a directing sheet  26 , and is penetrated there by solid particles. The solids-laden drying gas  15  is guided to the drier exit  20  through the helical directing sheets  25  in helical paths. The directing sheets  25  and  26  avoid dead zones, i.e., areas that are not flowed through, in the drier chamber  19 , and, overall, forcibly effect a predetermined minimum transport speed of the drying gas  15  and a uniform residence time of the dispersed solid particles. 
     FIG. 3 shows an enlargement of the discharge zone  5  of a combination centrifuge drier having two or more rotating cleaning blades  28 , which clean the diverting surface  29  of the baffle cone  18  with each rotor rotation. The pre-dewatered solid  6  is transported by the helical conveyor of the centrifuge  1  to the spraying edge  30 , and is ejected at high speed from the rotor  2 . The solid particles impact the surface  29  of the baffle cone  18 , and are broken into smaller particles and slowed there. The slowed particles fly at a greatly-reduced speed, and are diverted in the axial direction as a conical solid-spray mist into the drier chamber  19 , where they are flowed around intensively by hot gas and dried. The cleaning blades  28  are secured to the rotor behind the solids exit openings  31 , when seen in the direction of rotation, and are not showered by the exiting solid  6 . If, when very moist or sticky solid particles  6  impact the diverting surface  29 , a few particles are not reflected, and remain stuck on the diverting surface  29 , they are torn loose by the subsequent rotating cleaning blades  28  and spun into the drier chamber  19 . The blades  28 , which rotate at a high circumferential speed of about 60 m/s, also exert an aspirating and conveying effect on the surrounding hot gas  15   a ; consequently, the surrounding hot gas  15   a  partially conveys the solids dust located in the drier chamber  19  into the discharge zone  5 . The cleaning blades eject the dust-laden hot gas  15   a  aspirated by the blades  28  and the scraped solid particles into the drier chamber  19 , either radially or conically, depending on the shape of the guide surfaces. To intensify the gas conveyance, aspirating and directing sheets  32  can be mounted to the blades. 
     FIG. 4 shows the discharge zone  5  of a centrifuge drier, with a steeper angle of the baffle cone  18 , perforated gas-guiding sheets  22  and rotating blower blades. In contrast to the cleaning blades  28  in FIG. 3, the cleaning effect of the blower blades  33  is not based on a scraping effect, but on the blowing effect of the intensive gas flow  34  flowing out of the rotating nozzle  33  at a flat angle and onto the surface  29  of the baffle cone  18  to be cleaned. The gas conveyance through the blower blades  33  is particularly intensified by appropriate measures, such as large aspiration cross sections at the blade entrance  35 , directing elements in the blade and directed blowing at the blade exit. The aspirating effect of the dust-laden hot gas  15   a  at the blade entrance side  35 , and the hot gas  36  exiting the perforated gas-guiding surfaces  22 , keep the gas flow in the drier chamber  19 , with the dispersed solid particles  6 , away from the walls of the drier housing  11  and more toward the inside. Prior to impacting the surface  29  of the baffle cone  18 , the solid  6  flying from the spraying edge  30  of the centrifuge drum  2  enters the inflow region of the hot gas  15   a , which contains dust, and is conveyed by the blower blade  33 . The surfaces of the solid particles are thereby dried and coated with dry solids dust, so they lose their tendency to stick before contacting the surface  29 . To further reduce the sticking tendency, the diverting surface can also be coated with a suitable material, such as PTFE, enamel, ceramic or other anti-adhesive materials. The surface  29  can also comprise a perforated surface and be ventilated from the back. 
     FIG. 5 shows a combination of a rotating cleaning blade  28  and a blower blade  33 , which cooperates with a perforated gas-guiding sheet  22 . The surface  29  of the baffle cone  18  is cleaned by a rotating scraper  38  in connection with the blowing effect of the aspirated hot gas. The exiting jet  34  is not only directed at the surface of the baffle cone, but also blows tangentially onto the perforated gas-guiding sheet  22 . The side wall  39  that aspirates the hot gas can be slightly sloped with respect to the circumferential direction, or provided with openings to be able to aspirate more gas. The edges of the discharge openings  4  of the centrifuge  1  exert a conveying effect on the gas within the interior  37  of the centrifuge  1 . This conveying effect causes the moist-gas to be aspirated from the interior  37  of the centrifuge  1 , and hot, dry gas to be drawn in, so the moist solid  6  is already pre-dried in the helical pitch of the centrifuge  1 , with a long residence time, before being discharged. 
     FIG. 6 shows a combination of a turbulence blade  40  for keeping the drier chamber  19  clean, and a cleaning blade  28  for cleaning the surface  29  of the baffle cone  18 . The turbulence blade  40  possesses a high circumferential speed, and generates a strong vortex  41  of the drying gas in the drier chamber  19 . This avoids non-flowed-through dead zones, and the entering drying gas  15  is intensively mixed with the dispersed particles. As shown, the cleaning blade  28  can scrape or blow on a part of the surface  29  of the baffle cone, or the entire surface. The blades  28  and/or  40  can be rigidly secured to the rotor  2 , or secured thereto so as to oscillate. 
     In FIG. 7, rotating turbulence disks are built into the drier chamber  19  for generating rolling turbulences  43 . The drier housing  11  is embodied without a stationary inside housing  12 , which, in some embodiments of the centrifuge drier, surrounds the drum  2 . The concentric drier chamber  19  is therefore limited on the outside by a non-rotating cylinder wall, and on the inside by the rapidly-rotating centrifuge drum  2 . The rotating surface of the drum  2 , in connection with the rapidly-rotating disks  42 , induces a series of circulating, rolling turbulences  43  in the drier chamber  19 . These rolling turbulences  43  are driven by the rotating surfaces of the drum  2  and the disks  42 , create a high turbulence degree over the entire cross section, and even out the flow-through of the drier chamber  19  in the circumferential direction. The high turbulence degree of the rolling turbulences prevents deposits on the limiting walls of the drier housing  11 , compels a thorough mixing of drying gas and the dispersed solid particles, and generates a high drying speed for the moist solid particles in connection with an extremely-high water-evaporation rate with respect to the a drier volume. The axial movement of the entering hot gas  15  is evened out over the entire circumference by the passage gap  44  outside of the rotating disks  42 , and by the torus-shaped, rolling turbulences. Instead of the rotating disks  42 , other elements can also be used at the centrifuge drum  2  to generate rolling turbulences in the drier, such as a radial blade ring, axial or radial conveying wheels, beater arms or other known, suitable mounted parts. 
     In FIG. 8, one or a plurality of blade rings  46  is mounted to the outside of the rotating centrifuge drum  2  for creating a high turbulence degree in the drier chamber  19 , and for uniform axial conveyance and control of the residence time of the solids-laden drying gas. In addition to these functions, the blade rings  46  also effect a comminution of agglomerates in the drier chamber  19 . The surface  29  of the baffle cone  18  comprises a plurality of geometrically-assembled, smooth surfaces. At the impact zone  48  of the pre-dewatered, dispersed solid  6 , the surface comprises a flat cone adjoined further outward by a rounded surface contour  49 . The flat angle of impact of the dispersed, moist solid particles  6  against the smooth baffle cone  18  has a favorable effect on their reflection and further transport, despite the fact that they are broken into smaller particles  47 . The generally-desired, more severe diversion in the axial flight direction is effected further outward by the sliding of the particles on the rounded surface contour  49  of the baffle cone  18 . The additional sliding of the broken-down particles further reduces their entry speed into the drier chamber  19 , thus reducing the risk of baked-on buildup on the walls of the drier housing  11 . 
     The centrifuge drier shown in FIG. 9 again comprises a centrifuge, in the illustrated example a full-jacketed helical-conveyor centrifuge  1 , which is surrounded by an outside housing  11  of a spray drier. An inside housing  12  surrounds the centrifuge drum  2 . 
     The outside drier housing  11  and the inside housing  12  constitute the concentric drier chamber  19 , through which the drying gas  15  is conducted. The drying gas  15  is supplied through the tangential hot-gas shaft  16 , takes up the dewatered solid in the form of a dispersed-particle cloud in the region of the discharge zone  5 , transports the solid particles, with increased drying, through the drier chamber  19  in helical paths, and travels as a solids-laden gas  21  toward the exit channel  20 . The water separated in the centrifuge  1  is carried off in the central chute  10 . 
     The outside drier housing  11  is sealed at both end walls  13  and  14  against the rapidly-rotating centrifuge drum  2 . The gap  190  of the rotary seals  160  is formed by the centrifuge drum  2  and the sealing ring  170 , which, like the drum pedestals  210 , is rigidly connected to the base frame  220 . The seal gap  190  is guided exactly and in a stable manner by the mounting of the two work surfaces  2  and  170 , which form the seal gap  190 , to the same carrier  220 . Because of the eliminated suspension, the centrifuge drum  2  remains cold, even when hot gas  15  flows through the drier chamber  19 , and does not expand, whereas the drier housing  11 , through which hot gas  15  flows, expands significantly in the axial and radial directions. 
     The displacement movements of the two housing end walls  13  and  14  are compensated by a gas-tight, flexible compensator  180  or an elastic diaphragm, or a displaceable sliding ring  300 , with respect to the rigidly-mounted sealing ring  170 , so the seal gap  190  is not changed. 
     FIG. 10 shows in detail a contactless labyrinth seal for a centrifuge drier, which connects the sealing ring  170  that is rigidly mounted on the frame  220  to the axially- and radially-displaceable drier end wall  14  in a gas-tight manner by means of a compensator  180 . The flexible compensator  180  is connected in a gas-tight manner to both the sealing ring  170  and the end wall  14  by, for example, tightening straps  230  or other securing means. 
     The seal gap  190  between the crests  240  of the labyrinth seal and the rotating surface of the centrifuge drum  2  can be kept very narrow (0.3-0.5 mm), because the displacement movement of the end wall  14  is not transmitted onto the labyrinth seal. 
     All of the non-rotating parts are hatched from right to left; all of the rotating parts are hatched from left to right. 
     FIG. 11 shows a contactless rotary seal  160  in the form of a threaded seal for a centrifuge drier, with, for example, a vacuum existing in the drier chamber to the right of the end wall  14 . 
     The sliding and displacement movements of the end wall  13  or  14  of the drier during the heating or cooling phase of the drier housing  11  are compensated by a sheet-metal ring  260  that is sealed by heat-resistant O-rings  270 , and can slide on the housing end wall  13  or  14 , as well as on the rigidly-mounted sealing ring  170 . Because of the thread pitches  280  in the surface of the centrifuge drum  2 , the narrow seal gap  190  of the rotary seal  160  embodied as a threaded conveying sealing ring effects a conveying action that counteracts the vacuum in the drier, and a gas-counterpressure that prevents the entrance of secondary air into the drier chamber  19 . The thread pitches  280  can also be filled with a fluid sealing medium, for example water or sealing gas, which is conveyed through the thread pitches  280 . 
     FIG. 12 shows a contactless rotary seal  160  having a sharp-crested thread  310 , which rotates with a narrow gap  190  inside a soft cylinder surface  320 . The conveying action of the threaded seal compensates the vacuum prevailing in the drier. The displaceably-moving drier housing  11  is compensated by the sliding ring  300  in the gap. The sliding ring  300  itself is displaceably sealed by heat-resistant O-rings at both the drier end wall  14  and the rigidly-mounted sealing ring  170 . 
     FIG. 13 shows a contactless rotary seal  160  having shallow grooves, the seal rotating in a soft cylinder bushing  320  comprising sliding-bearing materials with a very narrow gap  190 . The displacement movement of the end wall  13  or  14  of the drier housing  11  is compensated by a sliding ring  340  that is resilient in the radial and axial directions.