Abstract:
The present invention provides a method and a device for dewatering a hydraulic fluid of a hydraulic system, in particular in the aerospace sector, comprising a container which has a sorbent, a feed which supplies the hydraulic fluid from the hydraulic system to the container for the hydraulic fluid to be passed through the sorbent such that it can be dewatered in a dewatering mode of the device, and a return which returns the dewatered hydraulic fluid from the container to the hydraulic system in the dewatering mode of the device. The hydraulic fluid can be dewatered continuously and very efficiently by the method and the device according to the invention.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 12/827,300 filed Jun. 30, 2010, which is a continuation of Application No. PCT/EP2008/068193 filed on Dec. 22, 2008 and which claims the benefit of U.S. Provisional Application No. 61/009,967, filed Jan. 4, 2008 and German Patent Application No. 10 2008 003 179.8, filed Jan. 4, 2008, the entire disclosures of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for dewatering a hydraulic fluid, in particular in the aerospace sector, and to a device for implementing a method of this type. Furthermore, the invention relates to a unit for dewatering a hydraulic fluid of a hydraulic system, to a method for controlling a unit of this type, to an aircraft or spacecraft with a device or unit of this type and to a floor maintenance machine with a device or unit of this type. 
     Although the present invention and the problem on which it is based can be applied to any vehicles, they will be described in detail in respect of an aircraft. 
     The hydraulic fluid used in aircraft hydraulic systems is typically very hygroscopic. The increasing water content, due to the uptake of water, in the hydraulic fluid results in the formation of acids as well as in other undesirable chemical changes. When there is a specific water content, valves and pumps can suffer from corrosive damage which cannot be tolerated in view of the particular safety requirements imposed in air transport. 
     One possibility of avoiding the problems associated with an increasing water content is to completely replace the hydraulic fluid. However, this is expensive, entails long aircraft immobilisation times and necessitates a separate disposal of the replaced hydraulic fluid. 
     DE 10252148 B3 discloses a method and a device for dewatering a hydraulic fluid according to the preamble of claims  1  and  8  of the present invention. In the known method, water is separated from the hydraulic fluid by pervaporation on a membrane which is permeable to gas and water and impermeable to the hydraulic fluid, the membrane being charged on the permeate side with a rinsing gas stream of a water vapour partial pressure which is lower than in the hydraulic fluid. 
     A disadvantage of the known method is that the dewatering procedure can only be carried out relatively slowly. 
     SUMMARY OF THE INVENTION 
     Therefore, the object of the present invention is to provide an improved possibility of dewatering a hydraulic fluid, with the provision in particular of a rapid dewatering of large quantities of hydraulic fluid. 
     This object is achieved by a method which has the features of claim  1  and/or by a device with the features of claim  8 . 
     According thereto, a method is provided for dewatering a hydraulic fluid, in particular in the aerospace sector, the hydraulic fluid being passed through a sorbent which removes water from the hydraulic fluid. 
     A sorbent which is brought into direct contact with the hydraulic fluid removes water from the hydraulic fluid significantly faster than is possible by the membrane separating method known from the prior art. 
     Furthermore, a device is provided for dewatering a hydraulic fluid of a hydraulic system, in particular in the aerospace sector, comprising a container, a feed and a return. The container has a sorbent. The feed supplies the hydraulic fluid from the hydraulic system to the container such that the hydraulic fluid can flow through the sorbent for the hydraulic fluid to be dewatered in a dewatering mode of the device. The return guides the dewatered hydraulic fluid from the container back to the hydraulic system in the dewatering mode of the device. 
     This simply constructed solution makes it possible to bring a hydraulic fluid into contact with a sorbent, the hydraulic fluid flowing through the sorbent and thus the hydraulic fluid being continuously dewatered. The advantages which have already been mentioned apply accordingly. 
     Furthermore, a unit for dewatering a hydraulic fluid of a hydraulic system, in particular in the aerospace sector, with at least two of the devices according to the invention is provided. According to a method for controlling the unit of the invention, the devices are only switched alternately into the regenerating mode by a common control means. 
     Here, by “only” it is meant that the devices are never in the regenerating mode at the same time. The advantage of this feature is that the hydraulic fluid can be dewatered in an uninterrupted and rapid manner. 
     In addition, an aircraft or spacecraft with the device according to the invention or with the unit according to the invention is provided. 
     With an aircraft or spacecraft of this type, there is no appreciable increase in the water content of the hydraulic fluid due to the continuous dewatering by the device or the unit. Consequently, the immobilisation times of the aircraft or spacecraft are considerably reduced. 
     Furthermore, a floor maintenance machine with the device according to the invention or with the unit according to the invention is provided, it being possible for the floor maintenance machine to be connected to a hydraulic system of an aircraft or spacecraft for dewatering the hydraulic fluid. 
     A floor maintenance machine of this type can avoid the entrainment of additional components in the aircraft or spacecraft, which advantageously entails a reduction in the flying weight. 
     Advantageous embodiments and developments of the invention are provided in the subclaims. 
     According to a preferred development, the water content of the hydraulic fluid is determined before and/or after the hydraulic fluid has passed through the sorbent. 
     By means of the water content in the hydraulic fluid, it is possible to detect whether the sorption capacity of the sorbent is exhausted, i.e. the sorbent is no longer capable of absorbing water or absorbing water in an adequate amount per unit of time. 
     According to another preferred embodiment, if the measured water content, in particular the water content after the hydraulic fluid has passed though the sorbent, is above a first limiting value, a regenerating mode is initiated to regenerate the sorbent. 
     In particular, the water content is to be measured after the hydraulic fluid has passed through the sorbent, since then it can be clearly and immediately established whether the sorbent still has an adequate sorption capacity. 
     For example, the first limiting value can correspond to 0.5% water content in the hydraulic fluid which is the maximum value permitted in aviation. A method of this type can be implemented very easily in terms of control. The first limiting value is preferably set slightly below the 0.5% limit, for example at 0.3% or 0.4%, so that the water content in the hydraulic fluid never rises above the predetermined 0.5% limit, even during the regeneration of the sorbent. 
     The term “regenerating mode” as used herein combines the types of operation required for a resumption of the dewatering mode after it has been established that the sorption capacity of the sorbent is exhausted. For a resumption of the dewatering mode, it is necessary to restore the sorption capacity of the sorbent. 
     As will become clear from the following explanations, there are a total of five possible different types of operation of the device: the device can be in dewatering mode or regenerating mode. In turn, the regenerating mode is divided into an emptying operation, re-drying operation, filling operation and/or cleaning operation. 
     According to a further development, if the value of the difference of the measured water content before and after passage of the hydraulic fluid through the sorbent is below a second limiting value, the second limiting value being determined before or after the passage as a function of the measured water content, a regenerating mode is initiated to regenerate the sorbent. 
     The value of the difference indicates to what extent the sorption capacity of the sorbent is exhausted. For example, if the sorption capacity is already substantially exhausted, the value of the difference will be correspondingly small, but only if a certain water content is present in the hydraulic fluid; when there is a very low water content in the hydraulic fluid, the value of the difference must inevitably be small. This improvement of the method takes these facts into consideration, the second limiting value being determined as a function of the measured water content. 
     According to the development described above, it is possible to detect early on whether the sorbent will have to be replaced in the near future. 
     In a further preferred development, in the regenerating mode the sorbent is separated from the hydraulic fluid and is re-dried. The term “re-drying” as used herein is understood as meaning the removal of the water which is absorbed in the sorbent. 
     According to a further preferred embodiment, the sorbent is re-dried by heat and/or by reduced pressure. These are very simple measures for re-drying the sorbent. 
     In a further preferred embodiment, the re-drying procedure is carried out at least by reduced pressure and the end of the re-drying procedure is determined by the pressure falling below the limiting value for the change in pressure. 
     When the pressure falls below the limiting value, it is then established that a sufficient amount of water has been removed from the sorbent in order to restore the sorption capacity thereof. 
     In a further preferred development, the degree of contamination of the hydraulic fluid is measured and, when the degree of contamination exceeds a contamination limiting value, the sorbent is rinsed with a cleansing agent, after being re-dried, to remove particles of dirt from the sorbent. 
     Not only the absorption of water, but also the incorporation of particles of dirt in the sorbent can impair the sorption capacity of the sorbent. For this reason, when the sorbent is correspondingly contaminated with particles of dirt, it has to be cleaned. In the decision whether the sorbent is to be cleaned, the period of time over which the contamination limiting value has been exceeded can also be considered. This provides an indication of the amount of dirt incorporated in the sorbent. 
     In a further preferred embodiment, the hydraulic fluid is passed through the sorbent again after the regenerating mode. 
     Thus, the original state is restored, and the device can answer its purpose, namely the dewatering of the hydraulic fluid. 
     In a further preferred development, the sorbent is selected from the group consisting of silica gel, sepiolite and molecular sieve, and/or the hydraulic fluid is based on phosphate ester. 
     Phosphate ester is a hydraulic fluid widespread in aviation. The sorbents can advantageously have geometries with large surfaces to thus achieve a high sorption capacity. 
     According to a preferred development, provided in the feed and/or in the return are moisture sensors for measuring the water content in the hydraulic fluid and a control means is also provided which is connected in terms of signalling to the moisture sensors. 
     Of course, the moisture sensors could also be provided in the hydraulic system itself, however for specific applications of the device, particularly in connection with a floor maintenance machine, this is unfavourable, because it would require the provision of such sensors in every aircraft, instead of a one-off provision of such sensors in the floor maintenance machine. 
     Moisture sensors of this type are preferably based on a capacitive measurement, particularly also bearing in mind the temperature of the hydraulic fluid. 
     In a further preferred embodiment, the control means switches the device out of the dewatering mode into a regenerating mode to regenerate the sorbent when the measured water content, in particular the water content in the return, is above a first limiting value. 
     According to a further preferred development, the control means switches the device out of the dewatering mode into a regenerating mode to regenerate the sorbent when the value of the difference of the water content in the feed and in the return is below a second limiting value, the control means determining the second limiting value as a function of the measured water content in the feed or in the return. 
     According to a further preferred embodiment, the container is coupled with the feed by a feed valve and is coupled with the return by a return valve. The hydraulic fluid in the container can thus be controlled in a flexible manner. 
     The feed valve is preferably provided at an upper end of the container and the return valve is preferably provided at a lower end of the container, “upper” and “lower” relating to the ground. 
     In a further preferred embodiment, the container is coupled with a compressed air line by a compressed air valve, the control means closing the feed valve and opening the return valve in an emptying operation of the regenerating mode, the compressed air discharging the hydraulic fluid out of the container into the return through the open return valve. 
     An emptying procedure of this type can be realised in a simple manner and it takes place very rapidly. 
     As used herein, the term “closed” valve is understood as meaning a state in which the valve prevents the fluid from flowing through it and the term “open” valve is understood as meaning a state in which the valve allows the fluid to flow through it. 
     In a further preferred embodiment, the control means again closes the compressed air valve in the emptying operation when a filling level sensor which is connected in terms of signalling to the control means indicates that the hydraulic fluid has been emptied out of the container. 
     This measure prevents compressed air from being pressed into the return, as a result of which it could pass into the hydraulic system and cause damage therein. 
     In a further preferred embodiment, the container is coupled with a vacuum line by a vacuum valve, the control means closing the return valve in a re-drying operation, downstream of the emptying operation, of the regenerating mode and opening the vacuum valve, the vacuum which then prevails in the container re-drying the sorbent. 
     The “vacuum” is nothing more than the reduced pressure which has already been described in connection with the method. The vacuum prevailing on the sorbent leads to the evaporation of the water absorbed in the sorbent, the water vapour resulting therefrom being removed via the vacuum valve. 
     In a further preferred development, the container is coupled with a vent line by a ventilating valve, with a heating means being provided, the control means closing the return valve in a re-drying operation, downstream of the emptying operation, of the regenerating mode, opening the ventilating valve and connecting the heating means for supplying heat to the sorbent to re-dry the sorbent. 
     The supply of heat to the sorbent for evaporating the water absorbed in said sorbent is an additional or alternative possibility for re-drying the sorbent to the embodiment which has already been described, in which the sorbent is re-dried by applying a vacuum. Both embodiments are advantageously used at the same time, in which case at least the amount of heat is supplied by the heating means which is removed from the sorbent during the evaporation process. In this respect, the ventilating valve can serve simultaneously as a vacuum valve and correspondingly the vent line can serves as a vacuum line. Thus, the re-drying procedure can take place very efficiently and it is possible to economise on components. 
     According to a further preferred embodiment, the container is coupled with a vent line by means of a ventilating valve, the control means opening the ventilating valve and the feed valve so that the container can be filled with hydraulic fluid in a filling operation, downstream of the re-drying operation, of the regenerating mode. 
     In order to switch the device back into the dewatering mode, it is necessary for the feed valve to be opened, thereby enabling hydraulic fluid to again flow into the container. However, for this, the air in the container must be able to escape. This can take place through the open ventilating valve. The feed valve must finally be re-opened to allow the hydraulic fluid to re-flow out of the hydraulic system into the container with the sorbent and out of the container again through the return into the hydraulic system. 
     According to a further preferred development, in the filling operation, the control means re-closes the ventilating valve and opens the return valve when a filling level sensor which is coupled in terms of signalling with the control means indicates a desired filling level. Then the control means again switches the device from the regenerating mode into the dewatering mode. 
     This embodiment prevents hydraulic fluid from flowing into the ventilating line. Instead, it can be shut off immediately when the container has been adequately filled with hydraulic fluid. Thereafter, it is possible to resume the dewatering mode. 
     In a further preferred embodiment, a contamination sensor is provided which measures a degree of contamination of the hydraulic fluid and makes this measurement available to the control means, the container being coupled with a cleansing agent feed by a cleansing agent feed valve and being coupled with a cleansing agent return by a cleansing agent return valve, the control means switching the device into a cleaning operation to remove contamination from the sorbent after the re-drying operation and before the filling operation when the control means establishes that the degree of contamination exceeds a contamination limiting value, in which case the control means closes the vacuum valve and/or the ventilating valve and opens the cleansing agent feed valve and cleansing agent return valve, the cleansing agent then flowing through the sorbent and, in so doing, removing contamination therefrom. 
     According to a further preferred development, the cleansing agent feed and the cleansing agent return are coupled with a cleaning container, and a cleansing agent pump and a filter are provided for cleaning the cleansing agent, the cleansing agent pump circulating the cleansing agent through the container, the cleansing agent return, the cleansing agent container, the filter and the cleansing agent feed in the cleaning operation, with the filter filtering contamination out of the cleansing agent. 
     Thus, contamination can be easily removed from the sorbent, the contamination itself being collected in a filter. 
     The filter is preferably provided with a contamination indication and is preferably provided to be replaceable. This makes it possible to replace the filter as soon as it is contaminated. 
     According to a further preferred development, the cleansing agent container has a ventilation, the control means closing the cleansing agent return valve and opening the compressed air valve in the cleaning operation after circulation of the cleansing agent for the discharge thereof from the container, the compressed air discharging the cleansing agent into the cleansing agent feed and compressed air escaping out of the cleansing agent container via the ventilation. 
     The cleansing agent is removed very quickly from the container by compressed air. The excess pressure resulting thereby in the cleansing agent circulation, since the cleansing agent circulation is interrupted by the closed cleansing agent return valve, can advantageously escape via the ventilation. 
     In a further preferred embodiment, the cleansing agent pump and the filter are arranged in the cleansing agent feed or in the cleansing agent return, a cleansing agent discharge line being provided with a cleansing agent discharge valve which bypasses the cleansing agent pump and/or the filter, and to empty the container, the control means opens the cleansing agent discharge valve and shuts off the cleansing agent feed valve or the cleansing agent return valve. 
     This embodiment allows the cleansing agent to be discharged very rapidly from the container, because it does not have to flow through the cleansing agent pump or the filter which constitute a high flow resistance. Furthermore, a flow through the filter in the opposite direction could result in the contaminant particles, trapped in the filter, being distributed in the cleansing agent circulation. 
     In a further preferred embodiment, a cleansing agent contamination sensor is provided which measures a degree of contamination of the cleansing agent and makes this measurement available to the control means, the control means supplying a warning signal to an indicator when the degree of contamination of the cleansing agent exceeds a cleansing agent contamination limiting value. Thus, it can be ensured that the cleansing agent is replaced when it is itself contaminated. For specific types of contamination, it is quite possible that the filter is not capable of adequately cleaning the cleansing agent, particularly in the case of fluidic contamination in the cleansing agent. 
     According to a preferred development of the unit according to the invention, four of the devices, for example devices A, B, C and D are provided, the common control means of which only switch them alternately into the dewatering mode, emptying operation, re-drying operation and filling operation. 
     In other words, when device A is in the dewatering mode, device B is in emptying operation, device C is in re-drying operation and device D is in filling operation. Thus, the required amount of sorbent per container can be minimised, since the amount of sorbent provided in each container must last just as long as the longest operation lasts (emptying operation, re-drying operation or filling operation). It is thus possible to minimise the size of the containers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention will be described in detail on the basis of embodiments with reference to the accompanying figures of the drawings. 
       In the figures: 
         FIG. 1  shows a unit with four devices according to an embodiment of the present invention; 
         FIG. 2  shows a moisture sensor according to the embodiment; 
         FIG. 3  schematically shows a circuit diagram according to the embodiment; 
         FIG. 4  shows one of the devices of  FIG. 1  with an associated cleaning means according to the embodiment, the cleansing agent flowing through the container; and 
         FIG. 5  shows the arrangement of  FIG. 4 , the cleansing agent having been emptied out of the container. 
     
    
    
     In the figures, the same reference numerals denote the same or functionally identical components, unless indicated otherwise. 
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  schematically shows a unit  1  for dewatering a hydraulic fluid of a hydraulic system  2 , for example the hydraulic system of an aircraft. In the case of the present embodiment, the hydraulic fluid is a phosphate ester. 
     The unit  1  is preferably a component of a floor maintenance machine, as typically found in airports. 
     The unit  1  has a first device  3 , a second device  4 , a third device  5  and a fourth device  6 . Each of the devices  3  to  6  has a container  10 , all the containers  10  being fluidically coupled with the hydraulic system  2  by a common feed  11  and a common return  12 . 
     The unit  1  is coupled with the hydraulic system  2 , for example during maintenance of the aircraft with the hydraulic system  2  and is of a temporary nature, i.e. the connection  13  of the feed  11  and the connection  14  of the return  12  to the hydraulic system  2  are configured to be detachable. 
     Arranged in the feed  11  and in the return  12 , downstream of the connections  13  and  14  are in each case stop valves  15 ,  16  which are each opened after the unit  1  has been coupled with the hydraulic system  2  and are closed before the unit  1  is uncoupled from the hydraulic system  2 . This prevents residual hydraulic fluid from issuing out of the unit  1  after the uncoupling of the hydraulic system  2 . 
     A hydraulic pump  17  which pumps the hydraulic fluid through the unit  1  is preferably arranged downstream of the stop valve  15  in the feed  11 . 
     A filter  18  with a contamination indication is preferably arranged in the feed  12  downstream of the hydraulic pump  17 . A corresponding filter  22  with a contamination indication is also preferably arranged in the return  12  downstream of the stop valve  16 . The filters  18 ,  22  filter contamination particles out of the hydraulic fluid. If the contamination indications of the filters  18 ,  22  indicate that said filters  18 ,  22  are contaminated, they can be replaced. 
     A flow sensor  23  is preferably provided in the feed  11  downstream of the filter  18 . The flow sensor  23  can establish whether and how much hydraulic fluid is flowing through the unit  1 . 
     Connected to the flow sensor  23  in the feed  11  is preferably an adjustable pressure reducing valve  24  which can adjust the pressure in the hydraulic fluid which is supplied to the containers  10 . 
     A nonreturn valve  25  connected to the pressure reducing valve  24  in the feed  11  prevents the hydraulic fluid from flowing against the direction of flow provided with reference numeral  26  in the feed  11 . 
     The feed  11  downstream of the nonreturn valve  25  preferably has a safety line  27 , connecting this to the return  12 , with a safety valve  28 . In the normal state, the safety valve is in the position shown in  FIG. 1 , in which it prevents hydraulic fluid from flowing from the feed  11  into the return  12  through the safety line  27 . However, if an error then occurs which prevents the hydraulic fluid from flowing from the feed  11  through the container  10  into the return  12 , but the pump  17  is still subsequently supplying hydraulic fluid, the safety valve  28  is opened if a specific limiting value for the permissible hydraulic fluid pressure is exceeded and the hydraulic fluid can then flow away from the feed  11  into the return  12 . Thus, damage to lines and valves, for example can be prevented. 
     Downstream of the filters  18  and  22 , the feed  11  and the return  12  preferably have a respective moisture sensor  32  and  33  which measure the water content in the hydraulic fluid. 
       FIG. 2  shows by way of example one of the moisture sensors  32 ,  33  which projects with its moisture probe  32   a  into the feed  11  and there capacitively measures the moisture of the hydraulic fluid. The moisture sensor is also equipped with a temperature probe  32   b  which provides a temperature of the hydraulic fluid. The measured temperature is incorporated in the determination of moisture of the hydraulic fluid. 
     According to the present embodiment, only two moisture sensors  32 ,  33  are arranged in the feed  11  respectively in the return  12 . In the same way, it is possible for each of the devices  3  to  6  to have two moisture sensors, one of which is provided upstream and the other is provided downstream of the container  10 , so that the water content can be individually determined upstream of and downstream of each container  10  for each of the devices  3  to  6 . However, the variant shown in  FIG. 1  is relatively economical in terms of parts, since it manages with only two moisture sensors  32 ,  33 . 
     The devices  3  to  6  are configured identically. For this reason, in the following the construction thereof will be described by way of example with reference to device  3 . 
     The container  10  is configured as a cartouche, i.e. as a cylindrical container which extends substantially vertically to the ground  40  (not shown further). In the following, “upper” and “lower” always relate to the ground  40 . 
     At its upper end  29 , the container  10  is fluidically coupled with the feed  12  by a feed valve  34  configured as an electromagnetically actuatable 2/2 directional control valve and at its lower end  30 , it is fluidically coupled with the return  12  by a return valve  35  configured as an electromagnetically actuatable 2/2 directional control valve. 
     In the open position of the feed valve  34  and of the return valve  35 , shown in  FIG. 1  for device  3 , hydraulic fluid can flow from the feed  12  into the container  10  and out of said container  10  again into the return  12 . 
     In the closed position of the feed valve  34  and of the return valve  35 , shown in  FIG. 1  for device  5 , hydraulic fluid cannot flow either from the feed  11  into the container  10 , or from the container  10  into the return  12 . 
     Arranged between the return valve  35  and the return  12  is preferably a nonreturn valve  36  which prevents hydraulic fluid from flowing out of the return  12  into the container  10  at any time. This prevents a mutual influencing of the containers  10  of the devices  3  to  6 . In particular, the nonreturn valve  36  seals off a container  10  which is in emptying operation, described in detail later on, from the pressurised hydraulic fluid in the return  12 . 
     Provided on the container  10  are an upper filling level sensor  37  and a lower filling level sensor  38  which generate a signal when the filling level in the container  10  falls below a first limiting value or when a filling level in the container  10  exceeds a second limiting value. The filling level sensors  37  and  38  are preferably arranged on a measuring column  39 , the lower end of which is fluidically connected to a line  43  connecting the return valve  35  to the return  12  and the upper end of which is connected to the upper end of the container  10 . 
     A level  44  of the hydraulic fluid in the measuring column  39  corresponds to the level  45  of the hydraulic fluid in the container. According to the present embodiment, the lower filling level sensor  38  only generates a signal when the line  43  is at least partly empty so that the level  44  in the measuring column falls below the position of the filling level sensor  38 . This ensures that the filling level sensor  38  only generates a signal when the container  10  is completely empty. 
     In its interior, the container  10  has a sorbent  46 , for example a silica gel. The sorbent  46  is capable of removing water out of the hydraulic fluid. 
     Furthermore, the container  10  has a heating means  47  which is configured, for example as heating elements, through which current flows when an electromagnetic switch  48  is closed and the heating elements generate heat which heats the sorbent  46 . 
     At its upper end  29 , the container  10  can be fluidically coupled with a compressed air line  53  by a compressed air valve  52  configured as an electromagnetically actuatable 3/3 directional control valve. The compressed air line  53  can be charged with filtered compressed air by a compressor  54  and a filter  55  connected downstream. 
     Furthermore, the container  10  can be fluidically coupled with a vent line  56  by the compressed air valve  52 , the vent line  56  having a filter  57  and a ventilation  58  at which atmospheric pressure prevails. 
     The compressed air valve  52  has a first position in which the container  10  is not coupled with the compressed air line  53  or with the vent line  56 . In a second position, the container  10  is coupled with the compressed air line  53 . In a third position of the compressed air valve  52 , the container  10  is coupled with the vent line  56 . 
     Furthermore, the upper end  29  of the container  10  can be fluidically coupled with a vacuum line  63  by a vacuum valve  62  configured as a 2/2 directional control valve, the vacuum line  63  preferably having in the following sequence: a settling container  64 , a vacuum pump  65  and preferably a water separator  66 . The settling container  64  protects the pump from solid and liquid constituents. The vacuum pump  65  can charge the vacuum line  63  with a vacuum (based on atmospheric pressure). 
     The vacuum valve  62  has two positions: in a first position, as shown in  FIG. 1  for device  3 , the vacuum line  63  is uncoupled from the container  10 , i.e. there is no vacuum in the container  10 . In a second position of the vacuum valve  62 , the container  10  is fluidically coupled with the vacuum line  63  and there is a vacuum in the interior of the container  10 . 
     Particles of dirt in the air which has been sucked up can be filtered out in the settling container  64  to protect the vacuum pump  65 . The water separator  66 , for example an electrostatic separator removes the water from the air, sucked up out of the container  10 , which water is possibly contaminated with hydraulic fluid (or with additives thereof). 
     Furthermore, a control means  67  is provided which is connected in terms of signalling with all the switchable elements  15 ,  16 ,  17 ,  24 ,  34 ,  62 ,  48 ,  35 ,  54  and  65  to control them and is connected in terms of signalling with all the signal-emitting elements  18 ,  22 ,  33 ,  23 ,  32 ,  37 ,  38 ,  68  and  69  to evaluate signals therefrom (the electrical lines have not been shown for reasons of clarity). The control means  67  is preferably configured as a flexibly programmable SPC (stored-program control). 
     The control means  67  is preferably connected to an indicator  73  (see also  FIG. 3 ), on which, for example measured values, the different operating states of the individual devices  3  to  6  or also warning signals, for example that a filter should be replaced, can be displayed. 
     The circuitry of the control means  67  is shown schematically in  FIG. 3 . By way of example, the control means  67  is connected to the moisture sensor  32 . Furthermore, the control means is connected to the indicator  73  which has already been mentioned. The control means  67  is also connected to a warning light  64  to warn an operator of the unit  1 . The control means  67  powered by a plug power pack  75  can be programmed flexibly by a PC (personal computer)  76  which, for example, allows the input of various limiting values for the permissible water content of the hydraulic fluid, which values can differ for different types of aircraft, for example. 
     Of course, each of the devices  3  to  6  could have a respective compressed air line  53 , vent line  56 , vacuum line  63  and control means  67  (with respectively associated components), however, according to the present embodiment, in order to economise on parts, devices  3  to  6  are provided with a common compressed air line  53 , vent line  56 , vacuum line  63  and control means  67 . 
     In  FIGS. 4 and 5 , the device  3  is shown supplemented by a cleaning means  80 . Of course, each device  3  to  6  can have a cleaning means  80  of this type. 
     A cleansing agent feed  81  is fluidically coupled with the line portion  82  connecting the return valve  35  to the container  10  and a cleansing agent return  83  is fluidically coupled with the line portion  84  connecting the feed valve  34  to the container  10 . 
     Provided in the cleansing agent feed  80  or in the cleansing agent return  83  are firstly respective stop valves  85 ,  86  which, in the closed state, ensure that no cleansing agent  87  penetrates unintentionally into the lines  82 ,  84 . 
     A discharge line  92  preferably branches off from the cleansing agent feed  81  downstream of the stop valve  85 , it being possible for said discharge line  92  to be fluidically coupled with a cleansing agent container  94  by a discharge valve  93  configured as an electromagnetically actuatable 2/2 directional control valve. 
     Downstream of the discharge line  92 , the cleansing agent feed  81  has a cleansing agent feed valve  95  configured as an electromagnetically actuatable 2/2 directional control valve, a cleansing agent pump  96  and preferably a cleansing agent filter  97  with a contamination indication, downstream of which the cleansing agent feed  81  opens into the cleansing agent container  94 . 
     Provided in the cleansing agent return  83 , downstream of the stop valve  86  is a cleansing agent return valve  98  which is configured as an electromagnetically actuatable 2/2 directional control valve, downstream of which the cleansing agent return  83  opens into the container  94 . 
     The cleansing agent container  94  is also oriented substantially vertically to the ground  40  and has at its upper end  102  a ventilation  103  above a filter  104 . 
     Each device  3  to  6  can now be operated in the types of operation as listed in the following: in a dewatering mode, see  FIG. 1 , device  3 ; in an emptying operation associated with a re-drying mode, see  FIG. 1 , device  4 ; in a re-drying operation associated with the regenerating mode, see  FIG. 1 , device  5 ; and in a filling operation associated with the regenerating mode, see  FIG. 1 , device  6 . 
     In the dewatering mode shown for device  3  in  FIG. 1 , the hydraulic fluid flows from the hydraulic system  2  by the effect of the pump  16  through the feed  11  into the container  10  and there flows through the sorbent  46  which removes water from the hydraulic fluid. Thereupon, the hydraulic fluid flows out of the container  10  into the return  12  and then returns into the hydraulic system  2 . 
     During this procedure, the moisture sensors  32 ,  33  are constantly measuring the water content in the hydraulic fluid. The moisture sensor  32  provides the control means  67  with the measured water content in the feed as a measured value MZ and the moisture sensor  33  provides said control means with the water content measured in the return as a measured value MR. 
     The control means  67  compares the measured value MR with a limiting value G 1  which is, for example 0.45% water content and is thus just below the maximally permissible water content in the hydraulic fluid of 0.5%. 
     If the control means  67  then establishes that the measured value MR is above the limiting value G 1 , it decides that the sorbent  46  no longer has an adequate sorption capacity for permanently keeping the water content of the hydraulic fluid below 0.5%, i.e. the maximally permissible value. The control means  67  then switches device  1  into the regenerating mode and, in this mode, initiates the emptying operation, as shown for device  4  in  FIG. 1 . 
     Additionally or alternatively, it can be provided that the control means  67  constantly determines the value of the difference BD between the measured value MR and the measured value MZ and compares this value BD with a limiting value G 2 . The limiting value G 2  is preferably calculated as a function of the measured value MZ. In this respect, the limiting value G 2  is a value, to be expected, of the difference with a sorbent  46  of a “normal” sorption capacity. These values can be recorded in a table, for example. 
     In addition, the flow rate DR, indicated by the flow sensor  23 , can also be used in determining the limiting value G 2 , because the flow rate influences the value, to be expected, of the difference between the measured values MZ and MR; for example with a high flow rate, the active time of the sorbent  46  on the hydraulic fluid is reduced. Therefore, a lower difference value will be expected. 
     If the control means then establishes that the value BD is above the value G 2 , it likewise switches the device into the regenerating mode and, in so doing, initially switches into the emptying operation, as shown in  FIG. 1  for device  4 . The second calculation method allows an earlier prediction that the sorption capacity of the sorbent  46  is exhausted. 
     For the emptying operation, the control means  67  closes the feed  11  by means of the feed valve  34  and connects the compressed air valve  52  such that compressed air flows from the compressed air line  53  into the container  10 . In so doing, the hydraulic fluid in the container  10  is discharged by the compressed air  105  into the return  12  through the open return valve  35 . The lower filling level sensor  38  indicates to the control means  67  when the container  10  is completely empty and even when a portion of line  43  is empty. This ensures that the container  10  is completely empty. 
     The control means  67  then again switches the compressed air valve  52  such that no further compressed air flows from the compressed air line  53  into the container  10 . The control means  67  then closes the return valve  35  so that the container  10  is no longer fluidically coupled with the return  12 . 
     Thereafter, the control means  67  switches into re-drying operation, switching the vacuum valve  62  such that the container  10  is connected to the vacuum line  63  and there is a vacuum in the container. The vacuum evaporates the water absorbed by the sorbent  46  and the water escapes through the vacuum valve  62  and line  63 . 
     The control means  67  also switches the switch  48  such that current flows through the heating elements of the heating means  47  and the sorbent is heated. This measure further stimulates the evaporation of the water absorbed in the sorbent  46 . By means of the pressure MD measured by the pressure sensor  68  in the vacuum line, the control means  67  constantly calculates the temporal change in pressure MDZ and compares this with a limiting value for the change in pressure GD. When the value MDZ falls below the value GD, it is then established that the amount of water absorbed in the sorbent  46  has fallen to a desired (low) content. Thereupon, the heating means  47  is disconnected again by switching the switch  48  and the vacuum valve  62  is reclosed. 
     There is then the possibility of again cleaning the sorbent  46 , i.e. to free the sorbent from particles of dirt incorporated therein from the hydraulic fluid. Whether the device is switched into a cleaning operation of this type can take place, for example on the basis of a measured value which is indicated to the control means  67  by the filter  22  and which reflects the extent to which the hydraulic fluid is contaminated with particles of dirt. If the degree of contamination exceeds a predetermined limiting value, the control means  67  can decide to switch into the cleaning operation. 
     In the cleaning operation, the stop valves  85 ,  86  (see  FIGS. 4 and 5 ) and the cleansing agent feed valve  95  and the cleansing agent return valve  98  are opened. The discharge valve  93  is closed. 
     The control means  67  then starts up the pump  96  and the cleansing agent  87  is circulated through the sorbent  46 , as a result of which particles of dirt are flushed out of the sorbent  46 . The flushed out particles of dirt are in turn filtered out of the cleansing agent  87  by the filter  97 . After a certain amount of time, when it can be assumed that the sorbent  46  is clean, the control means  67  switches off the pump  96  again, closes the cleansing agent feed valve  85  and the cleansing agent return valve  98  and opens the discharge valve  93 , as shown in  FIG. 5 . 
     The control means  67  then switches the compressed air valve  52  such that compressed air  105  flows from the compressed air line  53  into the container  10  and, in so doing, discharges the cleansing agent  87  out of the container  10  into the cleansing agent feed  81  (see  FIG. 5 ), the cleansing agent  87  then being discharged through the discharge line  92  and through the open discharge valve  93  into the cleansing agent container  94  and it displaces the air  106  present in the cleansing agent container  94  out of the cleansing agent container  94  through the filter  104  and the ventilation  103 . The compressed air valve  52  is re-closed so that no more compressed air flows into the container  10  when it is established that all the cleansing agent  87  has been displaced out of the container  10 . A suitable sensor (not shown) can be provided for this purpose. 
     If the measured signal which is made available by the cloudiness sensor  99  to the control means  67  and indicates a cloudiness of the cleansing agent  87  exceeds a limiting value for the permitted cloudiness of the cleansing agent, the cleansing agent  87  can be replaced at this time. 
     Hereafter or, if it is established that a cleaning operation is unnecessary, directly after the re-drying operation, the control means  67  switches into the filling operation and opens the feed valve  34  and switches the compressed air valve  52  such that the container  10  is connected to the vent line  56 , whereupon the hydraulic fluid flows out of the feed  11  into the container  10  and displaces the compressed air  105  in the container  10  out of said container into the vent line  56  through the filter  57  and ventilation  58  (shown in  FIG. 1  for device  6 ). 
     If the level  45  of the hydraulic fluid in the container  10  rises to a specific level, it activates the filling level sensor  37  and the filling level sensor  37  indicates to the control means  67  that the container is full again. 
     Thereupon, the control means  67  switches device  3  back into dewatering mode, in which the hydraulic fluid is again dewatered by means of the sorbent  46 . 
     The control means  67  only switches devices  3  to  6  alternately into the dewatering mode, emptying operation, re-drying operation and filling operation. This means that when device  3  is in dewatering mode, device  4  is in emptying operation, device  5  is in re-drying operation and device  6  is in filling operation (see  FIG. 1 ). 
     It is conceivable to provide a further device according to the invention, in which case the control means  67  only switches devices  3  to  6  and the other device alternately into dewatering mode, emptying operation, re-drying operation, cleaning operation and filling operation. 
     Although the present invention has been described on the basis of a preferred embodiment, it is not restricted thereto, but can be modified in many different ways. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  unit 
           2  hydraulic system 
           3  device 
           4  device 
           5  device 
           6  device 
           10  container 
           11  feed 
           12  return 
           13  connection 
           14  connection 
           15  stop valve 
           16  stop valve 
           17  hydraulic pump 
           18  filter 
           22  filter 
           23  flow sensor 
           24  pressure reducing valve 
           25  nonreturn valve 
           26  flow direction 
           27  safety line 
           28  safety line 
           29  upper end 
           30  lower end 
           32  moisture sensor 
           32   a  capacitive probe 
           32   b  temperature probe 
           33  moisture sensor 
           34  feed valve 
           35  return valve 
           36  nonreturn valve 
           37  filling level sensor 
           38  filling level sensor 
           39  measuring column 
           40  ground 
           43  line 
           44  level 
           45  level 
           46  sorbent 
           47  heating means 
           48  switch 
           52  compressed air valve 
           53  compressed air line 
           54  compressor 
           55  filter 
           56  vent line 
           57  filter 
           58  ventilation 
           62  vacuum valve 
           63  vacuum line 
           64  settling container 
           65  vacuum pump 
           66  separator 
           68  pressure sensor 
           69  filling level sensor 
           73  indication 
           74  warning light 
           75  plug power pack 
           76  line 
           80  cleaning means 
           81  cleansing agent feed 
           82  line 
           83  cleansing agent return 
           84  line 
           85  stop valve 
           86  stop valve 
           87  cleansing agent 
           92  discharge line 
           93  discharge valve 
           94  cleansing agent container 
           95  cleansing agent feed valve 
           96  cleansing agent filter 
           98  cleansing agent return valve 
           99  cloudiness sensor 
           103  ventilation 
           104  filter 
           105  compressed air 
           106  compressed air 
         BD difference value 
         DR measured flow rate 
         G 1  limiting value 
         G 2  limiting value 
         GD pressure limiting value 
         MDZ change in pressure 
         MD measured pressure 
         MZ measured water content in feed 
         MR measured water content in return