Abstract:
The invention relates to a thermostatic expansion valve having a valve element ( 33 ) which, for the throughflow of the refrigerant, closes and moves in the opening direction a valve seat ( 32 ) of a passage opening ( 29 ) arranged between the supply opening ( 27 ) and the discharge opening ( 31 ), and which is assigned to a first actuating element ( 36 ), the first actuating element ( 36 ) comprising a chamber ( 38 ) which is delimited with a first active face ( 37 ) and which contains a control charge ( 41 ), wherein an actuating element ( 46 ) is provided, which is thermally activated independently of the high pressure, the actuating movement of which actuating element ( 46 ) is coupled in terms of movement to the first active face ( 37 ) of the first actuating element ( 36 ) when a temperature-dependent actuating movement of the thermally activatable actuating element ( 46 ) acts counter to the actuating movement of the first active face ( 37 ) of the first actuating element ( 36 ), with a temperature threshold value of the thermally activatable actuating element ( 46 ) for an actuating movement being set to an identical value as the MOT (maximum operation temperature) of the control charge ( 41 ) of the first actuating element ( 36 ), which control charge ( 41 ) has a fluid density which lies below its critical density.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a thermostatic expansion valve for a refrigeration or heat-pump circuit as per the preamble of claim  1 . 
         [0002]    In transcritical refrigeration or heat-pump circuits, the high-pressure side dissipation of heat takes place usually above the critical pressure of the refrigerant which is used. On account of the resulting temperature gradient in the gas cooler, the pressure at the gas cooler outlet is a degree of freedom in the circuit process. Specifically in circuit processes which use CO 2  as refrigerant, it is highly important to adjust the high pressure into an optimum-efficiency range as a function of the ambient or gas-cooler-outlet temperature. In CO 2  air conditioning systems, usually only fixed throttles or externally-controlled expansion elements are used in the regulation of the refrigerant circuit. The former do not permit any adaptation of the high pressure to the process boundary conditions during operation. Externally-controlled expansion elements must for this purpose be regulated by electronic control elements whose responsiveness is insufficient in particular for automotive applications. Accordingly, said externally-controlled expansion elements cannot offer a sufficient level of operating reliability. Further disadvantages result from a high susceptibility to failure and high development and purchase costs. 
         [0003]    DE 102 49 950 B4 discloses an expansion valve for high-pressure refrigeration systems having a valve seat and a valve element which interacts with the valve seat, and a spring device which acts on the valve element, and an adjusting device for the spring arrangement, with the spring arrangement having at least one first spring and one second spring which act on the valve element. The first spring defines a working range and the second spring has a spring force which can be varied by the adjusting device. 
         [0004]    U.S. Pat. No. 6,012,300 discloses an expansion valve which has a chamber in which refrigerant is enclosed. The chamber is delimited by a diaphragm which acts indirectly on a valve element. The diaphragm is however also exposed to the high-pressure-side refrigerant. In particular, the active faces which are acted on by the refrigerant which is enclosed in the chamber and the further active faces which are acted on by the high-pressure-side refrigerant which passes from the gas cooler are identical. With the described expansion valve, no safeguard against high pressure above a maximum permissible value (for example 120 bar) is possible. In addition, a reliable start-up behaviour is not possible at inlet temperatures at the expansion valve above the critical temperature of the refrigerant. An operationally reliable application therefore cannot be realized with said expansion valve. 
         [0005]    DE 10 2005 034 709.6 discloses a thermal expansion valve which has a first and a second active face which are coupled in terms of movement to a valve element. The first active face is part of an expandable separating device which comprises a chamber with a control charge in the thermal head. The temperature of the high-pressure-side refrigerant can be sensed in this way. By means of said expandable separating device of the thermal head, the temperature-dependent pressure of the control charge in the chamber is transmitted to a temperature-independent spring element which is connected to the second active face which is also subjected to the high pressure. By means of said embodiment, it is intended to obtain a high-pressure limiting function in the supercritical regulating range. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore an object of the invention to further develop an expansion valve which can adjust the high pressure of a refrigeration or heat-pump circuit which can be operated transcritically and also subcritically within an optimum range and can autonomously prevent an exceedance of a maximum permissible value. 
         [0007]    Said object is achieved by means of an expansion valve as per the features of claim  1 . By using a thermally controllable actuating element whose actuating movement is coupled in terms of movement to a first active face of a first actuating element only when a temperature-dependent actuating movement of the second, that is to say of the thermally controllable actuating element acts counter to the actuating movement of the first active face of the first actuating element, it is made possible to provide a pressure limiting function or a safety function for preventing excessively high operating pressures, which requires no external activation. 
         [0008]    Here, a temperature threshold value of the thermally activatable actuating element for an actuating movement is selected which corresponds to a temperature value of the MOT of the control charge. The temperature threshold value is the temperature at which the thermally controllable actuating element generates an actuating or stroke movement. The working characteristic curve of the thermally controlled actuating element has the same gradient as the working characteristic curves of the control charge in the superheated vapour state, but in the opposite direction. The safety function is obtained in this way. In addition, an absolute pressure limitation, that is to say the realization of the MOP function (maximum operation pressure), is permitted at all temperature levels. While the first actuating element is acted on with pressure by a high-pressure-side refrigerant passing from the inner heat exchanger and absorbs the temperature of said refrigerant, the working behaviour of the thermally activatable actuating element is independent of the refrigerant pressure. 
         [0009]    According to a further advantageous embodiment of the invention, it is provided that a detachable mechanical coupling is provided between the first actuating element and the thermally activatable actuating element, and the thermally activatable actuating element engages on a first active face of the first actuating element or on a valve element which is connected to the first actuating element. Said mechanical coupling, which occurs above a predetermined temperature value, makes it possible, in normal operation at a conventional temperature threshold range, for the first actuating element to work independently of the thermally activatable actuating element, and the first control element is coupled in terms of movement to the valve element only when a further temperature rise takes place which demands the use of the safety function. 
         [0010]    The control charge of the first control element is preferably provided in a chamber which is embodied in the manner of a diaphragm or bellows and absorbs the temperature of the high-pressure-side refrigerant. The active face of the first actuating element is acted on by the temperature-dependent pressure of the control charge in the chamber of the actuating element and also by the high pressure. The resulting pressure difference generates an adjusting force which sets the valve element in motion and, as a function of the throttle properties of the associated valve seat, opens a certain flow cross section. 
         [0011]    It is preferably also possible for an additional, in particular preloaded spring element to be provided which intensifies the action counter to the high pressure. This has the result that an opening movement of the valve element takes place when the temperature-independent excess force, which is generated at the active face by the high pressure of the refrigerant system, is sufficient to overcome the preload of the in particular preloaded spring element and the force action of the chamber, as a result of which a passage between the valve seat and the valve element is opened or the cross section of the passage opening is enlarged. 
         [0012]    The control charge of the chamber of the first actuating element preferably has a charge density which lies below its critical density. It is preferably additionally provided that a substance mixture is selected for the control charge which has a critical temperature which lies above the critical temperature of the refrigerant to be regulated. In this way, the control charge has, in most temperature threshold ranges, a two-phase state with a high vapour proportion. Only when the energy absorbed by the control charge is sufficient to completely evaporate the liquid phase, which is present as a function of the prevailing filling density, does the control charge pass into the superheated vapour state. Under said circumstances, in the event of a further temperature rise, a control pressure is generated with only a smaller gradient than in the previous, two-phase state of the control charge, which gradient is not equal to zero. The temperature value above which said physical effect occurs is referred to as MOT (maximum operating temperature). The associated pressure value for the control charge is referred to as MOP (maximum operation pressure). It is additionally preferably provided that the temperature-independent force of the thermally activatable actuating element corresponds to the increase of the control charge of the first actuating element in the superheated state. In the event of a further temperature rise in the superheated vapour state, the pressure rises with only a considerably smaller gradient than in the previous, two-phase state. On account of the adaptation of the thermally activatable actuating element to said gradient, the safety function is realized in that the thermally activatable and high-pressure-independent actuating element acts in the opposite direction with the same gradient, so that a maximum operating pressure can be set which, in a desired manner, corresponds to a horizontal pressure profile at a MOP level. 
         [0013]    Said temperature value or temperature threshold value is preferably determined by the structural design of the thermally activatable actuating element. According to a first advantageous embodiment of a thermally activatable actuating element, it is provided that bimetal elements, in particular bimetal plates, which are stacked one on top of the other are provided. Said bimetal plates are for example arranged in the shape of a bellows. Said bimetal elements perform an actuating movement only above a certain temperature, as a function of their pre-setting. 
         [0014]    A second alternative embodiment for the design of a thermally activatable actuating element provides that a diaphragm, a bellows or a spring element, in particular a spiral spring or a spring bellows, is produced from a shape-memory alloy. A temperature-dependent activation can in turn be made possible in this way. 
         [0015]    A further alternative embodiment of the actuating element is provided by a filled, bellows-like spring element which is preferably filled with a medium which exists in the liquid state of aggregation above its vaporization pressure or below its saturation temperature. 
         [0016]    Suitable charge media are for example oil or generally hydrocarbons with a high boiling point. Said temperature displacement transducer elements are preferably hermetically sealingly joined diaphragm, corrugated-tube, bellows elements or else cylinder-piston units which exert high actuating forces by means of thermal expansion of their liquid filling. Said elements can be designed such that their stroke-temperature characteristic curve begins only above a certain temperature. 
         [0017]    It is preferably provided that the thermally activatable actuating elements have a pressure-independent device in order to preload them. It is made possible in this way for the temperature value at which the thermal safety function of the valve comes into action to be adjustable. A device of said type is preferably externally adjustable. An electronic or motor-driven activation can alternatively also be provided. 
         [0018]    It is additionally preferably provided that the chamber of the first actuating element, in particular an inner contour of the chamber, is guided by a sleeve or webs. This makes it possible for deformations as a result of the action of the control charge to be prevented. 
         [0019]    In a rest position of the valve element of the thermostatic expansion valve, it is preferably provided that a minimum passage opening is opened. This means that, when the temperature- and pressure-dependent excess force on the underside of the thermally activatable actuating element is not sufficient to overcome the preload of the latter, only an expediently predefined throttle cross section is opened, and the thermostatic expansion valve functions as a fixed throttle, as a result of which the high pressure in the circuit itself is set. 
         [0020]    The scope of the present invention therefore encompasses a transcritical or subcritical refrigeration or heat-pump circuit with an inner heat exchanger which makes possible a thermostatic expansion valve with an autonomously settable overflow function or safety function without for example an additional relocation of lines at the evaporation inlet. At the same time, the thermostatic regulating capability of the COP-optimum high pressure can be maintained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention and further advantageous embodiments and refinements thereof are described and explained in more detail below on the basis of the examples illustrated in the drawings. The features which can be gathered from the description and from the drawings can be applied according to the invention individually or together in any desired combination. In the drawings: 
           [0022]      FIG. 1  is a schematic illustration of a refrigerant circuit, 
           [0023]      FIG. 2  shows a state diagram for explaining the function of a refrigerant circuit having the thermostatic expansion valve as specified in the introduction, 
           [0024]      FIG. 3  shows a first embodiment of a thermostatic expansion valve, 
           [0025]      FIGS. 4   a,b  are a schematic illustration of a control charge characteristic curve and the action of the thermally activatable actuating element on the valve opening characteristic curve, 
           [0026]      FIG. 5  shows a state diagram of valve stroke characteristic curves at different operating pressures, 
           [0027]      FIG. 6  shows a second embodiment of a thermostatic expansion valve and 
           [0028]      FIG. 7  shows a third embodiment of a thermostatic expansion valve. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]      FIG. 1  shows a refrigerant and/or heat-pump circuit  11  of an air-conditioning system. In a refrigerant compressor  12 , a gaseous refrigerant, in particular CO 2 , is compressed. The compressed refrigerant is supplied to a gas cooler  13  where a heat exchange takes place between the compressed refrigerant and the environment in order to cool the refrigerant. The refrigerant which leaves the gas cooler  13  passes to an inner heat exchanger  14  which is connected to an expansion valve  15 . The expansion valve  15  has the effect firstly of limiting the pressure of the refrigerant and secondly of regulating the pressure of the refrigerant at the outlet of the inner heat exchanger  14 . From the expansion valve  15 , the refrigerant passes to an evaporator  16 . In the evaporator  16 , the refrigerant absorbs heat from the environment. Arranged downstream of the evaporator  16  is an accumulator  17  in order to separate refrigerant of the gaseous phase and of the liquid phase and at the same time to collect liquid CO 2 . The accumulator  17  is in turn connected to the inner heat exchanger  14 . 
         [0030]    The mode of operation of the air-conditioning system is now to be explained on the basis of the state diagram of  FIG. 2  in which the pressure p is plotted against the specific enthalpy H. A refrigerant, for example CO 2 , in the gaseous phase is compressed in the refrigerant compressor  12  (A-B). The hot, highly-pressurized, transcritical refrigerant is then cooled in the gas cooler  13  and in the inner heat exchanger  14  (B-C and C-D). The pressure is reduced in the expansion valve  15  (D-E) in order to evaporate the now two-phase (gaseous and liquid phase) refrigerant in the evaporator  16  (E-F), and to thereby extract heat from the environment. The COP is determined by means of the ratio of the enthalpy change Δi in the step E-F and the enthalpy change ΔL in the step A-B, that is to say COP=Δi/ΔL. 
         [0031]    The critical temperature of CO 2  lies at approximately 31° C., which is lower than the critical temperature (often &gt;100° C.) of fluorohydrocarbons which have hitherto been used in air-conditioning systems. This has the result that the temperature of CO 2  at the outlet of the inner heat exchanger  14  can be higher than the critical temperature of CO 2 . In said state, the CO 2  itself does not condense at the outlet of the inner heat exchanger  14 . The pressure at the outlet of the inner heat exchanger  14  must therefore be regulated. If, therefore, the external temperature is high, for example in summer, it is necessary to set a high pressure at the outlet of the inner heat exchanger  14  in order to obtain a sufficient cooling power. The outlet temperature at the inner heat exchanger  14  is dependent inter alia on the refrigerant-side temperature at the gas cooler outlet, which is in turn dependent on the ambient temperature. This means that the temperature of the CO 2  at the outlet of the inner heat exchanger  14  can also be used for the regulation of the COP-optimized high pressure, which is otherwise dependent on the refrigerant-side gas cooler outlet temperature. 
         [0032]    In the diagram as per  FIG. 2 , the characteristic curves  21 ′ and  21 ″ illustrate the COP-optimized regulating region. The double arrow in between denotes a valve stroke range of 0 to approximately 75% of the valve stroke. Illustrated between the characteristic curve  21 ″ and the characteristic curve  21 ′″ is the overpressure regulating region. By means of a further opening of the valve stroke beyond approximately 75%, an excess pressure can be dissipated. The characteristic curve  21 ″″ represents a settable high-pressure limit for the refrigerant circuit  11  which is to be regulated. Said high-pressure limit can be designed to be variable. 
         [0033]      FIG. 3  illustrates a first embodiment according to the invention of a thermostatic expansion valve  15  which permits operation of a refrigerant system as per a state diagram in  FIG. 2 . The expansion valve  15  comprises a valve housing  26  which has a high-pressure side supply opening  27  which leads into a high-pressure space  28 . The high-pressure space  28  is connected by means of a passage opening  29  to a low-pressure side discharge opening  31 . The passage opening  29  has a valve seat  32  in which a valve element  33  is provided in a closed position and separates the supply opening  27  with respect to the discharge opening  31 . 
         [0034]    Provided in the high-pressure space  28  is a first actuating element  36  which comprises a first active face  37  on which the valve element  33  is provided. A chamber  38  engages on said first active face  37  in the closing direction of the valve element  33 , which chamber  38  is embodied in the manner of a diaphragm or bellows. 
         [0035]    Additionally provided is a spring element  39  which for example surrounds the chamber  38  and preferably engages on the active face  37  in a preloaded manner and in the same force direction as the chamber  38 . In coordination with the size of the valve element  33  or the length of its shank or a stop element which is provided in the high-pressure space  28 , a preload of the spring element  39  and/or of the chamber  38  is made possible. 
         [0036]    The chamber  38  is preferably formed from a highly thermally conductive material. Provided in the chamber  38  is a control charge  41  whose pressure in the chamber  38  is temperature-dependent. When a high pressure acts on the high-pressure side, said high pressure acts against the active face  37  and opens the passage opening  29  if the acting high pressure has an excess force with respect to the preloaded spring element  39  and the pressure of the control charge  41  in the chamber  38 . The opening and closing movement is, in the COP-optimized regulating range, independent of a thermally activatable actuating element  46  which is likewise provided in the high-pressure space  28 . 
         [0037]    In the exemplary embodiment as per  FIG. 3 , the thermally activatable actuating element  46  engages on the first active face  37  opposite the chamber  38  and the spring element  39 , if provided. Alternatively, the actuating element  46  can also engage on the valve element  33  or additionally on the valve element  33 . The thermally activatable actuating element  46  is formed from bimetal plates which are stacked one on top of the other in the shape of a bellows. The bimetal plates can be preloaded by means of a pressure-independent device (not illustrated in any more detail), so that said bimetal places perform an actuating movement or a stroke movement only once the safety function is required. This is the case if the temperature of the refrigerant rises above the MOT. Accordingly, the preload of the bimetal plates or their material configuration is adapted to a temperature threshold value of said type. 
         [0038]    In the event of a sufficient excess force of the high pressure with respect to the pressure force of the chamber  38  and of the spring element  39 , if provided, by means of a predefined stroke characteristic curve, the optimum cross section is opened and therefore the optimum high pressure (COP-optimized range) is set as a function of the high-pressure-side outlet temperature of the refrigerant at the inner heat exchanger. 
         [0039]    The expansion valve  15  according to the invention makes possible an autonomously settable overpressure and safety function, so that the refrigerant circuit can operate with COP-optimized high pressure.  FIG. 4   a  is a schematic illustration of a characteristic curve  19  of a control charge in a chamber  38  of the first actuating element  36 , in which the pressure is plotted against the temperature up to the critical point. Since the control charge, which is present in two-phase form up to said point, passes into the single-phase, superheated gaseous state above the MOT value  20  for the circuit  11 , the pressure of the control charge continues to rise with only a considerably shallower gradient. The safety function can however only be obtained by means of a horizontal pressure profile from the MOT value  20 . Said further disadvantageous rise is compensated in one expedient embodiment of the present invention by means of the use of the thermally activatable actuating element  46 , whose characteristic curve is illustrated with  46 ′ in  FIG. 4   a . In this way, a valve opening characteristic curve  22  is obtained which is illustrated in  FIG. 4   b . Said valve opening characteristic curve  22  with the horizontal pressure profile at the MOP level leads to a maximum mass flow generation when the high pressure of the circuit  11  is situated thereabove, so as to result in a self-inhibiting generation of high pressure, because the temperature-induced pressure force of the chamber  38 , which acts in the closing direction of the valve element  33 , is compensated. The thermally activatable actuating element  46  can also act early on the opening cross section of the passage opening  29 , so that a rise of the high pressure above the MOP value is prevented. 
         [0040]    It is additionally to be mentioned that, although the refrigerant-side gas cooler outlet temperature is the preferred regulating temperature in the circuit with regard to COP optimization, the high-pressure-side outlet temperature at the inner heat exchanger  14  can likewise be used for the purpose of regulating the high pressure in a COP-optimum range. For this purpose, the outlet states at the inner heat exchanger  14  which correspond to each COP-optimum gas cooler outlet state are determined either by means of simulation or testing for the circuit in which the thermostatic expansion valve  15  described by this invention is used. A COP-optimized pressure profile therefore results by means of the high-pressure-side outlet temperature at the inner heat exchanger  14 , and said COP-optimized pressure profile is the aim of the optimum valve stroke characteristic curve  22  as per the state diagram in  FIG. 5 , in which the mass flow rate is plotted against the temperature. Said COP-optimum valve stroke characteristic curve  22  is restricted to one part, which is to be defined within the context of the application, of the entire valve stroke range, for example between 0 and 75%. This is illustrated in  FIG. 2  by the characteristic curves  21 ′ and  21 ″. The double arrow  22  shows the COP-optimized regulating range. Beyond the upper limit of the latter, the overflow function comes into action. If a mass flow rate characteristic curve  23  of the throttle point is designed, above said upper limit, that is to say until 100% of the total valve stroke range is reached, so as to be sufficiently steep that such a mass flow rate can flow out from the high-pressure into the low-pressure side, and therefore a further rise in the high pressure of the system can be prevented, one obtains the safety function, as claimed by the present invention, for preventing excessively high system pressures. 
         [0041]    By means of the arrangement of a thermostatic expansion valve  15  of said type at the evaporator inlet, one avoids complex line set relocation, as is necessary for example in the use of a thermostatic expansion valve as per the patent U.S. Pat. No. 6,012,300, since the valve described therein must absorb the refrigerant-side outlet temperature at the gas cooler—either by means of a local arrangement at the gas cooler outlet or by means of the relocation of a capillary line between the valve and gas cooler outlet. 
         [0042]      FIG. 6  illustrates an alternative embodiment to  FIG. 3 . In contrast to the latter, the thermally activatable actuating element  46  is produced as a spring element from a shape-memory alloy. Said actuating element  46  can be set in such a way that the stroke movement takes place only above a predetermined temperature threshold value. Here, the acting force can additionally also be determined by means of the cross section of the spring element. In addition, an electric activation of said thermally activatable actuating element  46  composed of the shape-memory alloy could also be possible. The further functions and variants described with regard to  FIG. 3  likewise apply to this embodiment. 
         [0043]      FIG. 7  illustrates a further alternative embodiment of a thermally activatable actuating element  46  to  FIG. 3 . In said embodiment, a hydraulically filled, bellows-like spring element is provided which permits the overflow function or safety function. The charges of the thermally activatable actuating element  36  comprise for example different oils and hydrocarbons. 
         [0044]    All of said features are in each case essential to the invention and can be combined with one another in any desired manner.