Abstract:
An apparatus for level control in a fluid tank is provided, wherein the fluid tank is designed to accommodate a supply of a fluid. The apparatus includes at least one conveying line through which the fluid can flow and which has at least one control opening which is hydraulically connected to the fluid tank. The control opening is designed to at least largely prevent fluid flowing out of the conveying line and into the fluid tank when at least one minimum level in the fluid tank is exceeded, and the control opening is further designed to allow fluid to flow out of the conveying line and into the fluid tank when the at least one minimum level in the fluid tank is undershot.

Description:
This nonprovisional application claims priority under 35 U.S.C. § 119(a) to German Patent Application No. 102008022960.1, which was filed in Germany on May 9, 2008, and which is herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an apparatus and to a method for level control in a fluid tank. Apparatuses and methods of this kind can be used, in particular, in cleaning appliances for cleaning washware. For example, the apparatus and the method can be used in dishwashers for dishes, containers or similar washware, in particular in dishwashers for commercial use. In addition to single-chamber dishwashers, the invention can advantageously be used in multi-chamber dishwashers too, for example in conveyor-type dishwashers. The invention can be used with particular preference in conjunction with treatment apparatuses for removing impurities from a cleaning liquid. For example, a treatment apparatus of this kind can comprise a device for separating dirt, preferably a centrifugal separator for separating impurities using centrifugal force. 
     2. Description of the Background Art 
     Cleaning apparatuses for cleaning washware are known from the extremely wide-ranging fields of natural sciences, engineering and medical technology. In the text which follows, reference is made to dishwashers in particular, but other types of cleaning appliances, for example cleaning appliances for cleaning medical equipment, articles for patient care, cleaning appliances for cleaning containers and trays or other types of cleaning apparatuses, can benefit from level control according to the invention. In general, the invention can also be used in other types of fluid systems in which level control of a fluid in a fluid tank is important. 
     The treatment of cleaning liquids, for example water with or without added assistants (for example detergents, rinse aids or the like) plays an important role in many cleaning apparatuses. Treatment apparatuses which are designed to remove impurities, for example liquid or solid impurities (for example suspended particles), from the cleaning liquid are often provided for this purpose. As an alternative or in addition to, for example, filters which can carry out the treatment operation, the technique of centrifugal separation is often used to treat cleaning liquids. Such dirt separators which are based on centrifugal separators are often also called “cyclones”. Centrifugal separators of this kind can be used particularly advantageously in conjunction with dishwashers, as is known from DE 1 965 828 A1, DE 1 703 612 A1, DE 660 23 00 U, DE 198 36 739 A1 or DE 1 250 979 A1. In the text which follows, reference is made to all of these documents for possible refinements of a centrifugal separator and for functional principles of such centrifugal separators. Centrifugal separators are designed to make the cleaning liquid to be purified eddy, for example in the form of one or more vortices. By virtue of this eddying, impurities are separated off due to their inertia. 
     If a tank of a dishwasher, for example a washing tank, is equipped with (that is to say is connected to) a treatment apparatus of this kind, in particular a centrifugal separator, in order to clean the cleaning liquid continuously or in specific cycles, it is often necessary for a partial quantity of the cleaning liquid which is routed through the treatment apparatus to flow out of this treatment apparatus together with the separated dirt, for example as part of a circuit. However, this reduces the level of cleaning liquid in the associated supply tank. This loss should be permanently compensated for, so that the ability of the cleaning apparatus to function is maintained. 
     This level compensation can be performed in various ways. For example, a level sensor can be used to detect whether the level in the supply tank has fallen below a permissible quantity. If this is the case, a refilling system can be put into operation. However, this means increased outlay on equipment. 
     In another procedure, cleaning liquid flows, for example out of a downstream tank of the cleaning apparatus, into the supply tank, from which the treatment apparatus is fed, and supplements the filling level. However, if the quantity of water flowing in is not sufficient for operation- or method-related reasons, a circulation pump for cleaning and/or the circuit through the treatment apparatus, for example, has to be disconnected. This results in the disadvantage that there is no longer any cleaning function in the corresponding section of the cleaning apparatus in this case, or that the treatment of cleaning liquid is put out of operation. 
     A further disadvantage of the described procedures is that an increased quantity of cleaning liquid is consumed. For example, continuous refilling of water to maintain the function of the cleaning apparatus can be associated with increased water consumption. Although substantially no water is lost during the washing process in the apparatus described in DE 1 250 979 A1, this advantage has to come at the expense of additional complexity of a dirt-collection container at the waste-water outflow of the centrifugal separator and an additional stop valve at the outlet of this dirt-collection container. 
     Overall, the apparatuses known from the prior art therefore have the disadvantage that a considerable amount of additional outlay on a sensor system and additional apparatuses is required in order to control the liquid level in the supply tank, if any such control is provided at all, and this considerably increases the costs and the technical outlay on the cleaning apparatus overall. This also results in an increased susceptibility of the cleaning apparatuses to faults. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an apparatus for level control in a fluid tank, which apparatus can be used, in particular, in a cleaning apparatus with a treatment apparatus, and which apparatus at least largely avoids the disadvantages of the above-described prior art. In particular, a high degree of functional reliability as regards level control and, at the same time, minimal outlay on a sensor system or other apparatuses should be ensured. 
     An embodiment of the invention provides an apparatus for level control in a fluid tank. The fluid tank is designed to accommodate a supply of a fluid, in particular a liquid. This liquid may be, for example, a cleaning liquid, in particular water with or without added assistants, for example detergents, rinse aids or similar assistants. The fluid tank may be configured, for example, as a pressureless fluid tank in which the fluid is accommodated up to one level. 
     The apparatus further comprises at least one conveying line through which the fluid can flow. The conveying line may be configured, for example, as a pipe with a rectangular, square or round cross section or a cross section of some other configuration. The dimensions of this conveying line can be matched to the requirements of the respective overall apparatus in which the apparatus for level control is used. 
     The conveying line can have at least one control opening which is hydraulically connected to the fluid tank. In this case, a hydraulic connection is understood to be a connection in which the fluid of the fluid tank can act on the control opening in a hydraulic manner in order to open and to close said control opening. In the process, the fluid preferably comes into direct contact with this control opening in order to open or to close said control opening. The hydraulic connection may be established, for example, by the control opening being arranged directly in the fluid tank or, as an alternative or in addition, being arranged in a control tank which is hydraulically connected to the fluid tank, for example a secondary tank. In this way, the same fluid level can be set, for example, in the control tank as in the actual fluid tank. 
     The control opening can be designed to at least largely prevent fluid flowing out of the conveying line and into the fluid tank when at least one minimum level in the fluid tank is exceeded. The control opening can further be designed to allow fluid to flow out of the conveying line and into the fluid tank when the at least one minimum level in the fluid tank is undershot. The control opening is preferably configured without any moving components in this case and can be opened and closed solely by being opened or unblocked by the fluid by means of the hydraulic connection. Level control in the supply tank is accordingly preferably performed automatically, that is to say without a sensor system and/or without moving components, so that a level in the supply tank can be kept at a predefined value, or so that the fluid can be prevented from falling below the at least one predefined minimum level. However, as an alternative or in addition, valves, floats or similar moving components or sensors which, for example, allow the at least one control opening to be opened and closed depending on the level in the fluid tank may nevertheless be provided. 
     In this way, the proposed apparatus can therefore be used to realize simple and cost-effective level control which is not sensitive to faults. The minimum level being undershot results in the control opening being unblocked, and fluid flowing out of the conveying line and into the fluid tank again raises or at least does not further reduce the level in the fluid tank. 
     The minimum level can be predefined, for example, by a position of the control opening, for example vertical positioning of the control opening. “Undershooting” and “exceeding” can cover a “&lt;” or “≦” and, respectively, a “&gt;” or “≧” relationship. Since the control opening has a physical extent in practice, the minimum level can also comprise a level range, within which the control opening is gradually unblocked or closed, instead of an infinitely small level limit. 
     Instead of a single control opening, it is also possible to predefine a plurality of control openings or a plurality of minimum levels. In this way, gradual level regulation can be performed by, for example, the individual control openings being unblocked or closed one after the other. 
     In an embodiment, the conveying line can have at least one control section which is arranged within the fluid tank or within a control tank which is hydraulically connected to the fluid tank. The control opening can be arranged in this control section. For example, the conveying line can run at least partially horizontally in the control section, wherein, for example, the control opening can be arranged in the horizontal region. However, another arrangement of the control opening is also possible. The control opening can be arranged, for example, on a lower face of the conveying line in the control section, for example in an at least horizontally running region of the control section. 
     As described above, the control opening can be configured as a purely mechanical rigid opening without moving components. For example, the control opening can be configured without any moving components and can have a specific shape and size which are particularly well suited to the respective use. By way of example, a rectangular, square, round or oval opening or an opening configured in some other way can be provided, for example an opening with a flow cross section which takes into account the respective flow conditions in the overall apparatus. 
     The apparatus can further comprise at least one conveying device, for example at least one pump. This conveying device should be designed to convey fluid out of the fluid tank and into the conveying line. 
     The present invention can be used, in particular and as described above, in conjunction with a cleaning apparatus for cleaning at least one item of washware, for example as a constituent part of one of the above-described cleaning apparatuses, for example a dishwasher, a cleaning machine for containers or similar washware. Use in commercial dishwashers, that is to say in dishwashers which have at least two tanks for accommodating cleaning liquid, is particularly preferred. Said invention can be used, for example, in single-chamber dishwashers and/or in conveyor-type dishwashers. 
     The cleaning apparatus can include at least one supply tank for accommodating a cleaning liquid. As described above, this supply tank can be configured as a pressureless or pressurized supply tank. Use in open supply tanks, that is to say supply tanks which have at least one opening, for example, in the direction of a cleaning chamber of the cleaning apparatus, that is to say can be subjected to the action of cleaning fluid from this cleaning chamber, is particularly preferred. 
     The cleaning apparatus can further include at least one apparatus for level control according to one or more of the above-described embodiments. According to the invention, this apparatus can be used to perform level control in the supply tank of the cleaning apparatus. The apparatus can have one or more associated supply tanks of this kind and, in particular, may be able to keep the level in the at least one supply tank at a predefined value without a sensor system or moving components or to prevent or slow down a minimum level of the cleaning liquid in the supply tank being undershot at least within predefined limits. In this sense, the term “level control” is to be understood within the scope of the present invention overall as meaning that, in addition to control in the actual sense, that is to say active adjustment to a setpoint value by virtue of corresponding compensation measures, said term can also cover undershooting of the at least one minimum level being prevented or slowed down. 
     The apparatus for level control can be designed, in particular, in the cleaning apparatus in such a way that the control opening is hydraulically connected to the supply tank, while one end of the conveying line is connected to a further tank of the cleaning apparatus, preferably a tank with a lower degree of purity, or an outflow system of the cleaning apparatus. The cleaning apparatus can comprise, in particular, at least two cleaning zones, wherein a transportation apparatus of the cleaning apparatus can ensure that the washware is transported through the at least two cleaning zones in a transportation direction. The supply tank can then have at least one first associated cleaning zone. The cleaning apparatus can be designed, in particular, to convey the cleaning liquid out of the supply tank and into the conveying line. A pump or a similar conveying device can, for example, be provided for this purpose, as has been described above. In this case, pumps can also be used synergistically, said pumps being present in the cleaning apparatus in any case, for example conveying pumps for supplying wash systems, spray nozzles or the like, or waste-water pumps. The further tank, in which the end of the conveying line of the apparatus for level control issues, should then preferably be a tank which has at least one associated second cleaning zone which is arranged upstream in the transportation direction. In this way, for example, overflowing of used cleaning liquid from a relatively clean cleaning zone into a cleaning zone with a lower degree of purity can be controlled via the conveying line in such a way that, at least within certain limits, overshooting of a minimum level in the relatively clean supply tank due to excessive amounts of fluid being conducted away into the upstream further tank can be prevented or at least slowed down. 
     In a further embodiment, which has already been described in the introduction, the cleaning apparatus can include at least one treatment apparatus for removing impurities from the cleaning liquid. This treatment apparatus can comprise, for example, at least one of the above-described devices, for example a filter or similar treatment apparatuses. However, for the reasons cited above, it is particularly preferred to use at least one centrifugal separator since the comparatively large liquid streams are generally required in such centrifugal separators in order to transport impurities away. The treatment device can be connected to the apparatus for level control, in particular to transport waste water away through the conveying line. 
     The treatment apparatus can therefore have at least one dirt outlet for conducting away a soiled fraction of the cleaning liquid, and at least one clean-liquid outlet for conducting away a cleaned fraction of the cleaning liquid. In particular, the dirt outlet can be connected to the conveying line. In contrast, the clean-liquid outlet can be arranged to allow the cleaned fraction to flow into the supply tank. By way of example, the treatment apparatus can further have a feed for supplying purified cleaning liquid, which feed is connected to the supply tank. Cleaning liquid can be conveyed out of the supply tank and to the feed by means of a pump, after which the purified fraction is returned to the supply tank, whereas the soiled fraction is transported away through the conveying line. 
     The cleaning apparatus can, for example, have at least one wash system for subjecting the washware to the action of the cleaning fluid, in particular at least one nozzle system. This wash system can be supplied with wash liquid from the supply tank via a wash line, wherein the feed of the treatment apparatus can be connected to the wash line. In this way, the purified fraction which is supplied to the feed of the treatment apparatus can be diverted away out of the wash line. 
     The apparatus for level control and the corresponding method and the cleaning apparatus have numerous advantages over known apparatuses and methods and permit novel, advantageous embodiments of level control arrangements. For example, the conveying line can be configured as a tubular waste-water line of the treatment apparatus, in particular of the centrifugal separator, and can be routed horizontally through the supply tank, for example via a specific section, said conveying line then forming the control section or a part of the control section. This control section can then be arranged, for example, at a predefined minimum height of the filling level of the supply tank, for example a dishwasher tank. Further on, the conveying line can then be guided out of the tank region of the supply tank. In this way, it is possible, for example, for the waste water, that is to say cleaning fluid containing a high proportion of dirt for example, to be conducted, in particular, into the dishwasher tank which is upstream in the transportation direction. However, as an alternative or in addition, the waste water can also be conducted directly into a waste-water connection of the machine, as part of an outflow system. 
     The horizontal line section can have the control opening of a specific size on its lower face. As long as the filling level in the supply tank is equal to or above the level of the control opening, the pressure of the cleaning fluid in the supply tank closes the opening, and waste water from the centrifugal separator can continue to flow in the direction of the waste-water pipe without obstruction. However, if the filling level in the supply tank falls below the predefined minimum level, the control opening on the lower face of the horizontal pipe section is unblocked. Starting from this point in time, waste water from the centrifugal separator can no longer flow in the direction of the outlet, but can fall back into the tank. This can prevent the liquid level in the supply tank dropping any further. Any such drop could result, for example, in a circulation pump of the corresponding supply tank, which feeds a wash system for example, being supplied with an insufficient quantity of cleaning liquid for long or short periods of time, as a result of which an entire section of the cleaning apparatus, for example the dishwasher, would be inoperative, for example. 
     During normal operation of the cleaning apparatus, a sufficient quantity of cleaning liquid, in particular water with or without assistants, can flow out of a downstream zone, for example out of a fresh-water rinse arrangement, to the described supply tank, so that the quantity of waste water which is required by the centrifugal separator can be compensated for. In specific operating states of the cleaning apparatus, for example in the case of special washware, water may be, as it is called, entrained, so that the quantity of water which flows in in a regular manner is not sufficient and therefore the desired minimum level in the supply tank would be undershot. This can be remedied by the inventive refinement of the apparatus and of the cleaning apparatus with minimum outlay. 
     The treatment apparatus can be specifically associated with the supply tank. In this way, it is possible, for example, to establish level control for the supply tank which is automatic and is able to keep the level in the supply tank at a predefined value without a sensor system or moving components. 
     The conveying line, which can be configured as a waste-water line for example, can be configured, for example, as a tubular conveying line and can have, for example, a circular, square or approximately square, rectangular or approximately rectangular cross section or another cross section. The control opening, which can be arranged on the lower face of the conveying line for example, can be matched to the size of the conveying line and/or to the volumetric flow rate prevailing during operation and can have, for example in the case of a pipe with an inside diameter of approximately 22 mm and a volumetric flow rate of approximately 80 l per minute, a length of approximately 20 mm and a width of approximately 20 mm too. 
     The described apparatus for level control in one or more of the above-described embodiments can be used, in principle, for any processes in which level control is required. Simple level control, preferably without a complex control system and/or sensor system, can be ensured in this way. This is particularly advantageously noticeable in said cleaning apparatuses, in particular in dishwashers. In the case of such dishwashers, the apparatus for level control can be used, for example, to also control other processes which make level control necessary. Therefore, entirely generally, wash liquid can be conveyed away out of a tank by a pump for example, and undershooting of a specific minimum level in the corresponding tank can be avoided. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIG. 1  shows a sectional illustration of a cleaning apparatus according to the invention with an apparatus for level control; and 
         FIG. 2  shows a detailed illustration of the apparatus for level control. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates one possible exemplary embodiment of a cleaning apparatus  110  according to the invention. In this exemplary embodiment, the cleaning apparatus  110  is configured as a conveyor-type dishwasher and has a transportation apparatus  112  for transporting the washware. The washware is placed on the transportation apparatus  112  at a charge point  114 , for example directly or using racks, and is transported through the cleaning apparatus  110  in a transportation direction  116 , in order to then be removed again, in the cleaned state, at a discharge point  118  at the outflow end of the cleaning apparatus  110 . 
     In the illustrated exemplary embodiment, the cleaning apparatus  110  has a plurality of cleaning zones through which the washware is passed in succession. Therefore, a pre-wash zone  120  is provided and has a pre-wash tank  122  and a pre-wash system  124  which is fed from the pre-wash tank  122  by means of a pre-wash pump  126 . 
     A main-wash zone  130  with a main-wash tank  132  and a main-wash system  134  which is fed from the main-wash tank  132  by means of a main-wash pump  136  is arranged downstream of the pre-wash zone  120  in the transportation direction  116  and is separated from said pre-wash zone optionally by a separating curtain  128 . 
     A rinse zone  138  which is divided into a pump rinse zone  140  and a fresh-water rinse zone  142  is in turn connected downstream of the main-wash zone  130 , in a manner separated by an optional separating curtain  128 . The rinse zone  138  comprises a common rinse tank  144  and a pump rinse system  146  of the pump rinse zone  140  and also a fresh-water rinse system  148  of the fresh-water rinse zone  142 . Whereas the pump rinse system  146  is fed with rinse liquid from the rinse tank  144  by means of a rinse pump  150 , the fresh-water rinse system  148  is fed from a water feed  154  via a fresh-water line  152 . 
     A drying zone  156  which can have, for example, a blower  158  and an exhaust-air apparatus  160  is connected downstream of the rinse zone  138  and is separated from said rinse zone again by an optional separating curtain  128 . The fresh-water line  152  can be passed, for example, through the exhaust-air apparatus  160 , for example a heat exchanger of this exhaust-air apparatus  160 , and can further have a boiler  162  in order to bring the inflowing water to a temperature which is required for the rinsing process. 
     It should be noted that the cleaning apparatus  110  shown in  FIG. 1  is to be understood merely by way of example and that other types of cleaning apparatuses can also be configured in a manner according to the invention. In particular, the cleaning apparatus  110  can be varied in terms of the type and configuration of the wash zones and the type and configuration of the wash systems. Completely different cleaning apparatus superstructures are also feasible, for example single-chamber systems or similar superstructures. 
     The cleaning apparatus  110  further comprises an apparatus  164  for level control which, in the illustrated exemplary embodiment, serves to control a level  166  in the rinse tank  144  and, at the same time, is used to treat the cleaning liquid in the rinse tank  144 . However, it should be noted that this apparatus  164  for level control, with and without the function for treating cleaning liquid, can also be used, as an alternative or in addition, in other regions of the cleaning apparatus  110 , for example in one or more of the other wash zones  120 ,  130 ,  138 ,  140 ,  142 . 
       FIG. 2  shows a detailed illustration of this apparatus  164  according to  FIG. 1 , and the structure and the manner of operation of this apparatus  164  will be explained with reference to said detailed illustration. However, this apparatus  164  can also be used in other types of apparatuses which require level control. 
     In the illustrated exemplary embodiment, the apparatus  164  comprises a treatment apparatus  168  for treating cleaning liquid from the rinse tank  144 . In the illustrated exemplary embodiment, this treatment apparatus  168  is configured as a centrifugal separator  170  and comprises a feed  172  for supplying cleaning liquid which is to be purified, a clean-liquid outlet  174  for conducting away a cleaned fraction of the cleaning liquid, and a dirt outlet  176  for conducting away a soiled fraction of the cleaning liquid, that is to say a fraction which contains a concentration of impurities. For possible refinements of the structure of the centrifugal separator  170 , reference may be made, for example, to the prior art documents cited in the introduction, and so these details are not discussed in the following description. However, as an alternative or in addition to the centrifugal separator  170 , the treatment apparatus  168  can also comprise other types of treatment apparatuses, for example filters or similar apparatuses. 
     In the illustrated exemplary embodiment, the feed  172  of the treatment apparatus  168  is connected to the rinse tank  144  via a wash line  178  of the pump rinse system  146  and is therefore supplied with cleaning liquid as a secondary stream from the pump rinse system  146  by means of the rinse pump  150 . The centrifugal separator  170  separates off suspended matter which accumulates in the cleaning liquid, and thus improves the washing result. The cleaned wash liquid, which emerges from the clean-liquid outlet  174 , returns to the rinse tank  144  via an outflow  180 , for example in free fall. In contrast, the second fraction of the centrifugal separator  170 , which fraction has a concentration of dirt and emerges at the dirt outlet  176 , is conducted, in the illustrated exemplary embodiment, into one of the wash zones which is connected upstream in the transportation direction  116 , in this example into the main-wash tank  132  for example, via a conveying line  182  which can also serve as a waste-water line  184 . However, as an alternative or in addition, it is also possible to introduce the waste water from the conveying line  182  at another point of the cleaning apparatus  110 , for example in another of the illustrated tanks. It is also possible, again as an alternative or in addition, to introduce the waste water directly into a waste-water system of the cleaning apparatus  110  or into a waste-water system of the building. In the illustrated exemplary embodiment, only one zone of the cleaning apparatus  110  is further equipped with a centrifugal separator  170 . This does not restrict the possible embodiments. It is also possible to equip a plurality of or all of the zones with such centrifugal separators  170 , and similarly also only individual, other zones instead of the rinse zone  138  cited in the example. Furthermore, instead of an individual apparatus  164  or an individual centrifugal separator  170 , it is also possible to use a plurality of such apparatuses  164  or a plurality of such centrifugal separators  170  or treatment apparatuses  168  for each wash zone. 
     Furthermore, level control by the apparatus  164  in the rinse tank  144  will be explained with reference to  FIG. 2 . Therefore, the conveying line  182  has a substantially horizontally running control section  186  which is hydraulically connected to the rinse tank  144  or some other supply tank, in which the level of the cleaning liquid is to be controlled, via a control opening  188 . In the illustrated exemplary embodiment, the control opening  188  is arranged on the lower face of the conveying line  182  in the control section  186 , and therefore is directed into the rinse tank  144 . Reference numerals  166  indicate possible levels of the cleaning liquid in the rinse tank  144 , wherein the upper of the illustrated levels  166  is level with the control opening  188  and therefore at the same time characterizes a minimum level  190  beneath which the level of the cleaning liquid in the rinse tank  144  should not fall if possible. This minimum level  190  can be adjusted, for example, by moving the conveying line  182  in the vertical direction. It goes without saying that levels other than the illustrated levels  166  in FIG.  2  are possible. In the illustrated exemplary embodiment, one end  192  of the conveying line  182  projects into the main-wash tank  132 , which is connected upstream of the rinse tank  144 , via an aperture  194  in a wall  196  between the tanks  132  and  144 , and forms an outlet opening via which liquid can flow out of the waste-water line  184  and into this main-wash tank  182 . 
     During operation of the apparatus  164 , the centrifugal separator  170  is, as described above, supplied with a secondary stream from the pump rinse system  146 . The cleaned fluid of the first fraction falls back into the rinse tank  144  via the outflow  180 . The waste water from the centrifugal separator  170 , which waste water has a concentration of dirt, is conducted, in the example, into the main-wash zone  130  via the waste-water line  184 . The control section  186  of the tubular waste-water line  184  of the apparatus  164  runs at least partially horizontally through the rinse tank  144  at the predefined height of the minimum level  190 . 
     Further on, this waste-water line  184  then leads out of the rinse tank  144  and conducts the waste water (cleaning fluid with a high proportion of dirt) into the preceding dishwasher tank for example, in this case the main-wash tank  132 . The horizontal control section  186  with the control opening  188  of a predefined size defines the minimum level  190 . As long as the filling level in the rinse tank  144  is at or above the level of the control opening  188 , the waste water from the centrifugal separator  170  continues to flow in the direction of the end of the conveying line  192  without obstruction and emerges at this end  192 . If, in contrast, the filling level in the rinse tank  144  falls below the predefined minimum level  190 , for example in a manner caused by particular operating conditions such as water being entrained, the control opening  188  on the lower face of the horizontal control section  186  is unblocked. Starting from this point in time, the waste water no longer flows out of the centrifugal separator  170  in the direction of the end  192 , but falls back into the rinse tank  144  through the control opening  188 . This prevents the liquid level in the rinse tank  144  dropping further. If the liquid level in the rinse tank  144  is further supplemented by the water which flows in in a regular manner, for example from a downstream zone of the cleaning apparatus  110 , the control opening  188  is closed again, and the waste-water stream from the centrifugal separator  170  again emerges at the outlet opening at the end  192  of the conveying line  182 . The control opening  188  on the lower face of the control section  186 , which is routed level with the minimum level  190 , is preferably matched to the size of the pipe of the tubular control section  186  and/or to the volumetric flow rate prevailing during operation and, for example in the case of a pipe with an inside diameter of approximately 20 mm and a volumetric flow rate of approximately 80 l per minute, is approximately 20 mm long and approximately 20 mm wide. 
     By virtue of the example illustrated in  FIG. 1 , an apparatus  164  according to the invention for level control, in particular with a treatment apparatus  168  (with or without a centrifugal separator  170 ) can also be used on conveyor-type dishwashers which have fewer or more tanks than illustrated, or else on a single-tank dishwasher. The invention further describes, in conjunction with  FIG. 2 , that the control section  186  is arranged directly in the rinse tank  144  and is routed level with the minimum level  190 . However, a design is also feasible in which the hydraulic connection between the rinse tank  144  and the control opening  188  or the control section  186  is established in a different way. For example, the hydraulic connection can be established in such a way that the control section  186  is not routed directly in the rinse tank  144  but in a separate control tank, for example in a secondary container which is hydraulically connected to the rinse tank  144 . 
     In addition to the integration of a treatment apparatus  168  into the apparatus  164  for level control, the apparatus  164  can also be used for other processes in the cleaning apparatuses  110  in which wash fluid is conveyed away out of a tank, for example by a pump, and undershooting of a specific minimum level in the corresponding tank is to be prevented. Accordingly, the apparatus  164  can be advantageously used in conjunction with the treatment of cleaning liquid, but it can also be used in a profitable, cost-effective and simple manner for other types of level control too. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.