Abstract:
An expansion and degassing container is in an expansion and degassing device for connecting to a circuit system for a circulating liquid. Because the amount of circulating liquid in the circuit system constantly changes during operation, liquid is withdrawn from the expansion and degassing container by way of a pressure holding pump, or supplied via a supply line, if needed. The supply line for supplying the refilling liquid is provided as a closed line, a system separator being arranged at the beginning region of the line and the end region thereof penetrating the container cover of the expansion and degassing container in a pressure-tight manner. The valve of the supply line is controlled according to the liquid level in the container. A pressure load cell, which additionally controls the valve according to the pressure, is present as a unit for decreasing an undesirable level of negative pressure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to an expansion and degassing device for connecting to a circuit system, in which a circulating liquid is pumped in a loop and undergoes pressure and volume changes. Such a device is known from the Swiss patent specification CH 694 895 A5. 
         [0002]    The circuit systems that may be used for expansion and degassing devices of the type referred to herein are described as a cooling circuit system and a heating circuit in CH 694 895 A5. In these cases, the circulating liquid also undergoes temperature changes. As a result of the heating and cooling of the circulating liquid during operation, or optionally by the activation and deactivation of individual loads that are charged with the circulating liquid, the pressure and volume of the circulating liquid change constantly during operation. Consequently, a volumetric content of the circulating liquid must be constantly withdrawn or replenished, and the required optimal operating pressure must be maintained. For this purpose, the known expansion and degassing device has a basically closed and rigid expansion and degassing container, which serves for intermediate storage and receives or releases fluctuating amounts of the circulating liquid. The expansion and degassing container is additionally used for functions such as maintaining the pressure in the system, degassing the circulating liquid, and feeding refilling liquid, as well as a settling tank for salts and minerals. To the extent possible, contact with the atmospheric oxygen is to be avoided during all these functions, because this causes the entire installation to corrode. 
         [0003]    The connection between the circuit system and the expansion and degassing device is established by a bypass or parallel line in a partial flow, which in CH 694 895 A5 is referred to as a “circulation line.” This line connects the flow line of the circuit system to the return line thereof, so that the circulating liquid likewise constantly flows in a loop in the bypass or circulation line. 
         [0004]    A container supply line, which leads into the interior of the expansion and degassing container, is connected to a first connecting point of the bypass or circulation line via an overflow valve. Downstream of this first connecting point, the bypass or circulation line has a second connecting point into which the discharge port of a pressure holding pump leads. The intake line of the pressure holding pump likewise extends into the interior of the expansion and degassing container. The interior of the expansion and degassing container is thus connected by way of the pressure holding pump to a first point, and by way of the overflow valve to a second point, of the circulating liquid of the circuit system that is pumped in a loop. Excess amounts of the circulating liquid flow into the expansion and degassing container via the overflow valve and via the container supply line; conversely, if additional amounts of circulating liquid are required in the circuit system so as to maintain the pressure, these amounts are withdrawn from the expansion and degassing container by the pressure holding pump. 
         [0005]    This constant change results in pressure fluctuations in the substantially closed expansion and degassing container, wherein predetermined positive and negative pressures are to be reached. The pressure in the interior of the expansion and degassing container is typically slightly reduced as compared to the outside atmospheric pressure. The container interior can only be connected to the outside atmosphere by way of pressure control valves. For this purpose, an air bleed and safety valve (gas drain valve) is provided in CH 694 895 A5. When gases have collected in the interior of the expansion and degassing container and apply a particular pressure, the air bleed and safety valve opens and allows these gases to escape through a water seal into the atmosphere. As a result, neither the circulating liquid of the circuit system nor the fresh refilling liquid comes in undesirable contact with the gases to be discharged. The outflow of the gases also causes a pressure equalization and results in the closing of the air bleed and safety valve. At this point, no outside air can thus flow into the expansion and degassing container. The air bleed and safety valve can have a simple mechanical construction and control; optionally, liquid components may also leave the container interior via this valve. The expansion and degassing container can be filled with liquid to the level of the air bleed and safety valve. 
         [0006]    If the level of liquid in the interior of the expansion and degassing container has dropped to a particular lower level, fresh refilling liquid must be replenished. According to CH 694 895 A5, the solenoid valve of a supply line is opened for this purpose by the control electronics when a lower level switch is activated. This line ends with free outflow via a float chamber, which is arranged on the container cover of the expansion and degassing container. The refilling liquid thus flows as a free stream into the float chamber, can degas into the ambient air, and collects in the float chamber to a particular height, at which the float control is activated and allows the refilling liquid to flow downward into the expansion and degassing container. The interruption of the supply line by a section with a free stream also serves the purpose of preventing container liquid from flowing back into the supply line of the refilling liquid. However, the outflow of the refilling liquid in a free stream has the disadvantage that oxygen from the atmosphere can be taken up. Another disadvantage of the known design relates to the float control, because air can likewise enter the refilling liquid via the valve that is controlled by the float. This is in particular the case when the movable valve body, which generally is a valve disk, lifts off or is lifted off the seat thereof. 
         [0007]    According to CH 694 895 A5, provisions have already been made to the effect that impermissibly high negative pressure occurs in the expansion and degassing container. In extreme cases, the container could then implode. According to CH 694 895 A5, a so-called vacuum breaker is thus arranged on the container cover of the expansion and degassing container, which is to say a valve that establishes a connection to the outside atmosphere when a limit value for an impermissibly high negative pressure is reached. While the inflow opening of the vacuum breaker is again located inside a plumbing unit, this does not change the fact that air can find its way into the interior of the expansion and degassing container when the vacuum breaker is activated. According to CH 694 895 A5, this must be tolerated because the vacuum breaker is only provided for disturbances that occur as exceptions. 
         [0008]    In addition, expansion and degassing devices are known, which operate with pressure holding pumps and in which the closed expansion and degassing container is provided with a diaphragm. The diaphragm divides the interior of the container into a gas compartment and a liquid compartment. For example, the diaphragm can be designed in the manner of a sack, which contains the liquid and is located inside the expansion and degassing container. An intermediate space, which receives a gas, this being air for example, is formed between the outer side of the sack and the inside wall of the expansion and degassing container. If the circulating liquid expands when the temperature increases, the sack forming the diaphragm absorbs the change in volume with the air. The drawbacks that arise due to the pressure control valves in the device comprising the rigid expansion and degassing container according to CH 694 895 A5 are thus avoided. However, the material of which the diaphragms are made likewise causes problems. The diaphragm material can be rubber or butyl rubber, for example. If these materials are in constant contact with certain circulating liquids, such as heating water for example, and subject to constant strong deformation, material fatigue may result. Gases, including oxygen, can diffuse through the diaphragm into the circulating medium of the pipe network, and thus there is a further risk of corrosion and silting. The pipe networks must be bled more frequently as a preventive measure, which incurs additional costs. Because the deformability of the diaphragms is limited, the pressure range between the positive pressure and negative pressure that an operating program can cover is also limited. Finally, the diaphragms must also be replaced more frequently, in accordance with pressure container regulations, so as to assure flawless functioning. This is because the temperatures cause the plasticizer to evaporate, and the rubber material becomes porous and decomposes. 
       SUMMARY OF THE INVENTION 
       [0009]    Thus, it is the object of the invention to improve the expansion and degassing device of the type mentioned above, with the aim that the same can be operated over a relatively large pressure range between positive pressure and negative pressure in the expansion and degassing container, yet has a simple and reliably functioning construction, that the absorption of oxygen from the atmosphere into the container is completely excluded, and that the backflow of container liquid into the refilling liquid is also safely prevented (system separation). 
         [0010]    The overflow valve is designed so that the medium to be degassed flows from the heating circuit into the degassing container, and more particularly in accordance with the pressure in the container that is desired during normal operation and can be set at the overflow valve. This desired pressure (by way of example) is 0.8 to 0.9 bar and is thus slightly below the outside atmospheric pressure of 1 bar, the pressure differential at which the overflow valve opens being set in keeping with the static positive pressure of 2 bar (by way of example) that is present at this point of the heating system. This proven characteristic has been derived from the generic prior art according to CH 694 895 A5. 
         [0011]    So as to completely exclude the absorption of oxygen, two modifications are made to the container (characterizing features a and b) over the disclosure of CH 694 895 A5. First, the vacuum breaker, the purpose of which was to prevent excessive negative pressure in the degassing container by the supply of air (or supply of gas in general), is eliminated; the function thereof is performed in another manner, and more particularly by the supply of fresh water (feature b). 
         [0012]    Previously, according to the disclosure of CH 694 895 A5, the supply of fresh water was carried out so that the fresh water valve, which is located in the “open” line of the fresh water replenishment system, was only controlled according to the level. The novel supply of fresh water is now carried out so that the solenoid valve is located in a “closed” supply line (feature a) and opens not only according to the level, but also according to the pressure (feature b). As a result, a vacuum breaker, which allows either atmospheric air or inert gas (in any case a gaseous medium) to flow in, can be dispensed with. An essential part of the invention is thus that, instead of a gas, a fluid or liquid medium is used with the fresh water supply so as to limit the negative pressure. Because the vacuum breaker is eliminated, and also because the fresh water supply line is no longer connected to the atmosphere, the degassing container is completely protected from all oxygen. 
         [0013]    As a result, the negative pressure in the expansion and degassing container is limited (i) by opening the overflow valve at a relatively low negative pressure relative to the atmosphere and (ii) by way of a novel negative pressure load cell, which can open the fresh water solenoid valve that is already present elsewhere. In an older application according to DE 10 2010 047 514, the fresh water was replenished by way of the solenoid valve in a closed supply line, wherein the solenoid valve was only controlled according to the level. Instead of the “open” vacuum breaker according to CH 694 895 A5, a safety valve comprising a connected inert gas container was used in DE 10 2010 047 514. This also represented an already successful solution, which in the particular operating states of replenishment and negative pressure limitation replaced the connections that were previously open to the atmosphere with the respective closed connections. However, this “closed” solution was more complex than the “open” solution in the present example. It was found in the invention that a supply of nitrogen can also be dispensed with. 
         [0014]    Due to the design of the expansion and degassing container, which is to say, with such oxygen degassing, an oxygen content of less than 0.1 mg/l, typically 0.05 mg/l, is achieved in the circulating medium of the system. The introduction of oxygen due to practically inevitable leaks in the heating circuit and the oxygen that is contained in the likewise inevitable supply of fresh water are reliably removed from the system by the degassing in the container before these can contribute to corrosion. 
         [0015]    Because the refilling liquid in the now closed supply line flows through the container cover into the interior of the closed and pressure-tight expansion and degassing container, any contact between the circulating liquid and the free atmosphere at this point is excluded, and the degassing and deliming take place only within the container. Thus, undesirable corrosion by atmospheric oxygen is limited at this point. 
         [0016]    The system separator at the inlet of the supply line is constructed in accordance with DVGW (German Technical and Scientific Association for Gas and Water) guidelines and thus is a reliable component. This component is certain to prevent backflowing container liquid from mixing with fresh refilling liquid. This is because, when the pressure in the supply line downstream of the system separator rises in comparison with the pressure at the inlet port, and exceeds a particular limit value, the inlet-side backflow preventer of the system separator closes and conducts container liquid that has penetrated into the supply line, for example, through a secondary line to a collection site, for example a spillover siphon. 
         [0017]    However, the entire advantageous effect of the expansion and degassing device according to the invention is also achieved by precluding any penetration of atmospheric air into the interior of the expansion and degassing container during the necessary function of vacuum breaking (this being the function of limiting the negative pressure in the container). The negative pressure must be limited so as to prevent the container liquid from boiling and evaporating and so that a massive design is not required for the container. If the negative pressure in the expansion and degassing container becomes impermissibly high, a liquid medium flows into the upper region of the container. 
         [0018]    Because a negative pressure exists, generally only a very small amount of gas is taken up by the container liquid. In addition, pressure equalization takes place very rapidly. Any excess gas leaves the expansion and degassing container via the air bleed and safety valve (gas drain valve) already mentioned in connection with the prior art, as soon as the pressure is once again high enough in the expansion and degassing container. 
         [0019]    A major advantage of the expansion and degassing device according to the invention is that it operates without diaphragms. This prevents the difficulties that result from the materials of the membranes being prone to wear. The expansion and degassing device according to the invention preserves the proven pressure-tight and rigid expansion and degassing container and operates with proven valves that generally have a simple mechanical function. In addition, this allows the device according to the invention to be operated over a wide pressure range between positive pressure and negative pressure in the expansion and degassing container. The modular design of the device, which is proposed in CH 694 895 A5, can be preserved. 
         [0020]    It is possible with the expansion and degassing device according to the invention to expel all gases, such as hydrogen, nitrogen, oxygen, sulfur hydrocarbons and carbon dioxide, from the circulating liquid. An inert fluid develops in the heating circuit, wherein the pH value rises compared to the replenishment liquid and the electric conductance value decreases. The pH value should be between 8.2 and 10.0 and the electrical conductivity should be less than 100 microsiemens per cm for a low-salt medium. As a result, corrosion by oxygen, which is at a concentration of less than 0.1 mg/l, is negligible. 
         [0021]    Maintaining the pressure in the heating circuit likewise means that the circulating liquid must be constantly undersaturated, so as to allow for the uptake of substances and so that the pipes and walls of the installation are cleaned of substances. 
         [0022]    Degassing also provides improvement in the heat transfer in the circuit system, so that primary energy is saved, and thus less carbon dioxide is released. 
         [0023]    Moreover, the substantial degassing of the water or medium has the advantage that the expansion of the water under the influence of the temperature is lower (comparable to mercury), whereby the pressure fluctuation that must be compensated for is also lower. 
         [0024]    Advantageous refinements are described in the dependent claims, and the characteristics claimed therein are described in more detail in the exemplary embodiment. 
         [0025]    According to claim  7 , the invention also relates to the circuit system of a building heating installation, in which circulating water is pumped in a loop and undergoes pressure and volume changes, wherein the invention in this field of application is that of providing the building heating installation with an expansion and degassing device according to at least one of claims  1  to  6  so as to treat the circulating water. Claim  8  contains the corresponding design for a cooling circuit system. 
         [0026]    In the case of the building heating installation, the refilling liquid is fresh water, and the circulating water is treated in a particular manner for heating purposes. The device according to the invention is particularly well suited for the purposes of building heating installations in the low temperature range, and also in the range of the cooling systems, because these operate in temperature ranges in which the density of the water increases and the absorbing power rises. However, a number of other fields of application, such as refrigeration circuits, are also possible. 
         [0027]    The invention will be described in more detail hereafter based on one exemplary embodiment in three figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a longitudinal sectional view of a schematic illustration of the expansion and degassing device according to the invention; 
           [0029]      FIG. 2  shows an enlarged detail of  FIG. 1 ; and 
           [0030]      FIG. 3  shows a diagram similar to that of  FIG. 1 , comprising an associated electronic control region. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    In  FIG. 1 , reference numeral  1  denotes the collectivity of an expansion and degassing container. The container  1  comprises the container wall  2 , which forms the cylindrical lateral wall and can be made of stainless steel. The container cover  3  and the container bottom  4  are likewise made of stainless steel and are detachably fastened to the container wall  2 . The container wall  2  can additionally be surrounded by a casing  5  made of a reflective and insulating material. Details of this casing  5  have already been described in the aforementioned CH 694 895 A5 or in DE 103 29 740 A1. In addition, a drain valve  6  is provided in the container bottom  4 . So as to monitor the level of the liquid in the expansion and degassing container  1 , a level indicator is present thereon, which can be a glass gauge  7 . A floating body can trigger particular switch and control functions by reaching an upper level switch  8  or a lower level switch  9 . Moreover, a closable inspection opening  10  is present, which is used for cleaning and repair work. 
         [0032]    An intake line  11  penetrates the container cover  3 . The line ends in a check valve  12  in the region of the bottom third of the expansion and degassing container  1 . The check valve  12  ensures that, during operation, only liquid from the lower region is taken in; liquid should, however, be prevented from flowing back downward into the container  1 . The intake line  11  leads to a pressure holding pump  13 . The outlet of the pressure holding pump  13  on the pressure side leads into a circulation line ( 16 ,  17 ), which is connected to the circuit system of the entire installation as a bypass or parallel line; for details, again refer to CH 694 895 A5 (refer to FIG. 4 in particular). A supply line  16  of the expansion and degassing device is connected to the pressure line of the circuit system, while a return line  17  of the expansion and degassing device is connected to the return line of the circuit system. The supply line  16  and the return line  17  thus together form the circulation line. The flow directions in the lines  16  and  17  are indicated by the directional arrows  16   a  and  17   a . A connecting line  18  leads from the supply line  16  in a first branch  18   a  to an overflow valve  15 , which is provided at the upper end of a container supply line  14 . A connection for a pressure gage  19  and/or a pressure control device  19  or the like is also provided in the first branch  18   a  of the connecting line  18 . A second branch  18   b  conducts the pressure and is secured by a tamper-proof valve. This valve is used to allow the pressure and function of the container  1  to be set, without impacting the circulation line. The second branch  18   b  of the connecting line  18  forms the classic expansion line, and is integrated in the dynamic zero point of the system. The static pressure of the system is made available by the second branch  18   b , via which the excess amounts of circulating liquid are conducted into the expansion and degassing container  1  using the pressure-controlled, adjustable overflow valve  15 . Static and dynamic pressures are present in the lines  16  and  17 . The pressures in the lines  16 ,  17  and  18  are equalized by way of the connecting line  18 . 
         [0033]    The line section comprising the connecting line  18  and the first branch  18   a  is also used for pressure equalization and is therefore also referred to as the expansion line. If, during operation of the circuit system, the circulating liquid circulating therein expands, the resulting pressure increase in the connecting or expansion line  18  causes the overflow valve  15  to open and the additional volume to be discharged via the container supply line  14  to the expansion and degassing container  1 . The overflow valve  15  is designed as a differential pressure valve and is protected from pollution by a screen. The screen retains coarse particles, which can be washed up from the liquid circuit, while small particles are flushed through without difficulty because of the pressure gradient between the operating pressure in the return line  17  and the significantly lower internal pressure in the expansion and degassing container  1 . The container supply line  14  having the air outlet  14   a  already allows gases to be released, wherein the air outlet  14   a  is located in the upper region of the container supply line  14  in a regionally widened area (partial housing). 
         [0034]    Advantageously, a safety valve, with or without a nitrogen bottle, is no longer required at the container cover  3 . 
         [0035]    Another assembly relates to the supply and removal of supplemental or refilling liquid into and from the expansion and degassing container  1 . For improved clarity, the enlarged illustration according to  FIG. 2  was selected for this purpose. This shows the case where the circuit system is a heating system, in which the circulating liquid is specially treated water. The refilling liquid is fresh water in this case. The supply takes place via a closed fresh water supply line  23 , which is collectively denoted by  23  and the end region  23   a  of which is a rigid pipeline that penetrates the container cover  3  in a pressure-tight manner. This fresh water supply line  23  is connected to a system separator (mechanical disconnector)  24  by way of a solenoid valve  27 . The connecting line between the system separator  24  and the solenoid valve  27  is composed of a flexible line section  25  and an intermediate piece  26 . The fresh water flows into the system separator  24  via the inlet port  24   a  thereof. Such system separators are known in terms of the mode of operation thereof and are available on the market as ready-to-install units for additional plant engineering components. Such system separators are composed of two backflow preventers that are connected in series and equipped with a ventable center zone. The function thereof in the expansion and degassing device according to the invention will be described hereafter. 
         [0036]    The latter assembly also includes an air bleed valve  30 , which is provided on the container cover  3  and can establish a connection between the container interior and the outside atmosphere. The air bleed valve is based on the function of a flap plate  31 , which lifts under the action of slight positive pressure in the interior of the expansion and degassing container  1  and thus allows the positive pressure to be released. Excess air can thus escape from the container interior to the outside, as can excess amounts of liquid, if the liquid inadvertently reaches the inside of the container cover  3 . The guided flap  31  is then likewise lifted and allows this excess liquid to flow into an intermediate chamber  32 . 
         [0037]    A first spillover line  28  leads from the system separator  24  to a spillover siphon  29 . Moreover, a second spillover line  33  leads from the intermediate chamber  32  likewise to the spillover siphon  29 . 
         [0038]    The described expansion and degassing device operates as follows: 
         [0039]    It shall be assumed again that the circuit system is a building heating installation and that the water in the expansion and degassing container  1  is approximately at the level of the region limit  34 . A settling zone A is formed there, in which salts and suspended oxide particles or solids, notably lime, are deposited. Here, the intent is to supply additional fresh water to the expansion and degassing container  1 . This will automatically take place no later than when the lower level switch  9  is activated. The solenoid valve  27  will then open. 
         [0040]    The fresh water flows via the system separator  24 , the flexible line section  25  and the intermediate piece  26  through the solenoid valve  27 , and finally via the end region  23   a , directly into the expansion and degassing container  1 , without coming in contact with atmospheric oxygen. During the inflow, an expansion region E, and beneath the same a smoothing region B, form in the expansion and degassing container  1 , see also the region limit  35 , as is described in detail in CH 694 895 A5. The inflowing fresh water, as well as the water that is already present in the container, degas in the vacuum or in the negative pressure of the expansion region E, so that substantially all gases escape. 
         [0041]    Meanwhile the smoothing region B exerts a piston action, as a result of which the pressure in the expansion region E rises when the level of liquid in the expansion and degassing container  1  increases, whereby the slight negative pressure initially present in the expansion and degassing container is ultimately eliminated. A slight positive pressure lifts the guided flap  31  of the air bleed valve  30 , so that the air and gas components present in the container  1  escape into the intermediate chamber  32  and thus transition into the outside atmosphere, without coming in contact again with the fresh water or the circulating water of the circuit. 
         [0042]    At the same time, the escaping of air and gas components from the expansion and degassing container  1  results in pressure equalization, so that the flap  31  closes tightly again under the influence of gravity. Thus, no oxygen-containing outside air reaches the interior of the expansion and degassing container  1 . 
         [0043]    During the operation of the circuit system, the circulating water will gradually warm and consequently expand. This leads to a pressure increase in the secondary circuit connected in parallel comprising the supply line  16  and the return line  17 , with this increase acting on the overflow valve  15  via the connecting or expansion line  18  and the first branch  18   a . This overflow valve opens at a defined limit value and allows the excess circulating water to flow through the container supply line  14  downward into the expansion and degassing container  1 . There, the gases contained in the fluid escape, as was already described for the fresh water. The level of liquid in the expansion and degassing container  1  thus continues to rise. 
         [0044]    It is possible for the amount of water that is present in the expansion and degassing container  1  to rise unexpectedly from the level of the lower level switch  9  to the container cover  3 . In the fixed fresh water supply line  23 , this may lead to a gradual back pressure, which acts on the system separator  24  and causes the same to be activated. In this case, the system separator  24  will block the supply of fresh water in keeping with the function thereof and open to the first spillover line  28 . The water coming from the expansion and degassing container  1  thus drains via the first spillover line  28  into the spillover siphon  29 . The system separator  24  thus protects the fresh water present at the inlet port thereof from the liquid coming from the expansion and degassing container  1 , and this liquid can no longer flow into the fresh water network. 
         [0045]    In contrast, if the circulating water present in the circuit system cools, this takes up less volume, so that the pressure control device  19  is activated and the electronic system switches on the pressure holding pump  13 . The amount lacking in the system is thus replenished and the operating pressure is maintained in the circuit system. The check valve  12  consequently opens, and the water present in the expansion and degassing container  1  is supplied via the intake line  11  to the return line  17 . This consumption can be very high, so that the water level in the expansion and degassing container  1  drops very quickly to the lower level switch  9 . This creates a significant negative pressure in the expansion and degassing region E. There is the risk that the container may implode. In addition, the fluid in the settling region A will partially evaporate under highly negative pressure. 
         [0046]    So as to prevent the risk of implosion and boiling, the solenoid valve  27  in the fresh water connection opens when the negative pressure is too high. As a result, refilling liquid flows into the expansion and degassing container  1 . This also prevents the interior of the expansion and degassing container  1  from coming in contact with the atmospheric oxygen. The inflow of liquid into the expansion region E causes pressure equalization and thus limitation of excessively high negative pressure. This prevents the possible implosion of the container  1 , without the water present in the container  1  making contact with the oxygen. In contrast to the widely common prior art, a diaphragm is not required. Under negative pressure, the gases escape even more readily from the heating water in the settling region A and the smoothing region B. Swirling of the water results in even better degassing. 
         [0047]    In the exemplary embodiment according to  FIGS. 1 and 2 , the pressure measurement signal of the negative pressure load cell  36  acts directly on the solenoid valve  27 , so as to open the same if necessary. This negative pressure function takes priority over the conventional level function that is initiated by the lower level switch  9 , which is to say whenever the negative pressure signal (of  36 ) requires opening and, at the same time, the level signal (of  9 ) prefers closing, the solenoid valve  27  is opened. Direct actuation can take place by way of a 220 volt signal, to which the switch elements are adjusted. If too much medium enters the entire system during a priority process, an excess amount flows off via the siphon  29 . 
         [0048]    The novel negative pressure function also takes priority over maintaining the pressure in the system and optionally causes the pressure holding pump  13  to stop. 
         [0049]    As an alternative,  FIG. 3  shows an electronic control unit  37 , which is located in an electronic control region A 2 , including the network cable connection  38 . The alternative function of the same is to convert the measurement signal of the negative pressure load cell  36  into an opening signal for the solenoid valve  27  in the fresh water replenishment ( 23  to  27 ), again with priority over the level measurement signal. In this alternative it is also possible to use 220 volt signals, either entirely or in part, as control signals. The priority switching in this alternative can take place inside the control unit  37 , wherein the two control signals advantageously can also be weighted against each other.