Abstract:
In a cooling circuit which comprises a refrigerant compressor incorporating a suction port and a pressure chamber incorporating a pressure port, a condenser which is arranged in the cooling circuit downstream of the pressure port and comprises a fluid collecting chamber in which a reservoir of refrigerant is formed, an evaporator which is located in the cooling circuit between the condenser and the suction port, a feed unit which is connected at one side to the refrigerant reservoir and to the pressure chamber at the other side and which serves for supplying refrigerant from the refrigerant reservoir to the pressure chamber which incorporates a pumping unit for the refrigerant, it is proposed that in order improve this cooling circuit in such a manner that it is realizable in an economically meaningful manner that the pumping unit comprise a pressure-tight closed housing which is provided with only one inlet and one outlet as access points and a pumping element which is movable for pumping the refrigerant be arranged in the pumping chamber thereof.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application is a continuation of international application number PCT/EP2014/073575 filed on Nov. 3, 2014. 
         [0002]    This patent application claims the benefit of International application No. PCT/EP2014/073575 of Nov. 3, 2014 and German application No. 10 2013 112 670.7 of Nov. 18, 2013, the teachings and disclosure of which are hereby incorporated in their entirety by reference thereto. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The invention relates to a cooling circuit comprising a refrigerant compressor incorporating a suction port and a pressure chamber incorporating a pressure port, a condenser which is arranged in the cooling circuit downstream of the pressure port and comprises a fluid collecting chamber in which a reservoir of refrigerant is formed, an evaporator which is located in the cooling circuit between the condenser and the suction port, a feed unit which is connected at one side to the refrigerant reservoir and to the pressure chamber at the other side and which serves for supplying refrigerant from the refrigerant reservoir to the pressure chamber and which incorporates a pumping unit for the refrigerant. 
         [0004]    Cooling circuits of this type are known from the state of the art such as DE 43 38 939 C1 for example. 
         [0005]    In these known cooling circuits however, complex pumping units were provided for the feed unit and this has led to economically non-realizable solutions. 
         [0006]    Consequently, the object of the invention is to improve a cooling circuit of the type indicated in the preamble of the main Claim in such a way that it is realizable in an economically more meaningful manner. 
         [0007]    In accordance with the invention, this object is achieved in the case of a cooling circuit of the type described hereinabove in that the pumping unit comprises a pressure-tight closed housing which is provided with only one inlet and one outlet as access points, and a pumping element which is movable for pumping the refrigerant is arranged in the pumping chamber thereof. 
       SUMMARY OF THE INVENTION 
       [0008]    The advantage of the solution in accordance with the invention is to be seen in that it is then possible to utilise simply built pumps having a very low handling capacity which will suffice for the application in accordance with the invention, such pumps having a very low leakage rate for the pressurised refrigerant in keeping with their permanently closed construction and being producible economically and operable economically. 
         [0009]    A low leakage rate in keeping with the permanently closed devices of this type amounts to 3 g/year or less per connection port under a pressure of at least 0.25 times the maximally permissible pressure. 
         [0010]    In consequence, a cooling circuit of the type described in the preamble of the main Claim can be operated efficiently when using the solution in accordance with the invention. 
         [0011]    In the context of the present invention, liquefaction of the refrigerant is effected in a condenser in the event of a subcritical mode of operation as is the case in the usual commercially used refrigerants, but in the event of a supercritical mode of operation however, only cooling of the refrigerant without liquefaction thereof occurs. 
         [0012]    Consequently, liquid refrigerant collects in the fluid collecting chamber in a subcritical mode of operation, but a cooled gaseous refrigerant collects in the event of a supercritical mode of operation. 
         [0013]    It is particularly advantageous hereby for the pumping chamber to be arranged in a pressure-tight closed pumping chamber housing. 
         [0014]    In the case of a solution of this type, the pumping chamber is thus arranged directly in a pressure-tight housing. 
         [0015]    Furthermore, it is advantageously envisaged in this solution that the pumping element be driven by an electromagnetic or magnetic force that is effective through the pumping chamber housing. 
         [0016]    A solution of this type is particularly advantageous since the pumping chamber housing can then be of very small volume and consequently this very small volume can be closed in pressure-tight manner in a simple manner and with simple means so that the constructional realization of the solution in accordance with the invention is thereby particularly simple in regard to the cost thereof. 
         [0017]    A pressure-tight termination in the sense of the solution in accordance with the invention is to be understood in particular as a termination which is free of a mechanical feed-through for a drive of the pumping element, i.e. all the complex sealing measures that are necessary in the region of a mechanical feed-through for a drive but which nevertheless lead to leakages at the requisite pressures of more than 15 bar for example, preferably of more than 20 bar and still better of more than 25 bar can be avoided. 
         [0018]    The most varied of possibilities are conceivable in regard to the construction of the pumping element. 
         [0019]    Hereby, one particularly simple and economically realizable solution envisages that the pumping element be in the form of a piston. 
         [0020]    A piston of this type could, for example, be a conventional piston of a piston pump. However, a particularly simple and expedient solution envisages that the piston be constructed in the form of a spring-loaded oscillating piston so that it can then move in oscillatory manner due to the spring loading thereof. 
         [0021]    In particular, a oscillating piston of this type can easily be driven in oscillatory manner by means of a solenoid coil. 
         [0022]    To this end, an alternative solution envisages that the pumping element be in the form of a pumping element which rotates about an axis and is thus, in particular, also driven in rotary manner. 
         [0023]    A rotary pumping element of this type permits of a multiplicity of simply realizable forms of pump having a rotary pumping element. 
         [0024]    For example hereby, provision is made for the rotary pumping element to be a gear wheel in a set of gear wheels in a gear pump. 
         [0025]    A particularly expedient solution envisages that the pumping unit be controlled by a refrigerant supply control unit. 
         [0026]    With the aid of the refrigerant supply control unit, there is then the possibility of not only controlling the processes of switching on and switching off the pumping unit but, by controlling the delivery rate of the pumping unit, it is also possible to control the cooling of the pressure chamber and thus of regulating the temperature of the pressure chamber in order to hold the temperature within a range about a given threshold value. 
         [0027]    As an alternative or in addition to the initially described solution in accordance with the invention, a further solution of the object specified hereinabove envisages that a gas discharge unit be associated with the pumping unit, wherein said unit comprises a gas discharge line which conducts away gaseous refrigerant from the feed unit. 
         [0028]    The advantage of this solution is to be seen in that it is then possible to remove a gas cushion of refrigerant which is formed during the switching off periods in the region of the pumping unit and in particular at the inlet side of the pumping unit and which, in the case of pumping units having a low flow rate, leads to them only beginning to pump liquid refrigerant after at least a long start-up time or only occasionally pumping liquid refrigerant or not pumping any liquid refrigerant at all during the envisaged run time since, once a build-up of a gas of the refrigerant at the inlet side of the pumping unit has occurred, the low delivery rate of the pumping unit is insufficient to pump away quickly enough the gas that has been formed by the input of heat so that eventually, the pumping unit is unable to reliably pump liquid refrigerant, this being something that is essential for the solution in accordance with the invention because the envisaged low delivery rates of the pumping unit in accordance with the invention then only lead to a meaningful cooling process in the pressure chamber if they are delivering liquid refrigerant which can then evaporate in the pressure chamber and thereby absorb heat. 
         [0029]    For example hereby, the gas discharge line could lead to a refrigerant path at an intermediate pressure level, to an intermediate pressure port of the compressor for example. 
         [0030]    A particularly expedient solution envisages that the gas discharge line lead into a refrigerant path of the cooling circuit which is at suction-side pressure so that there is a large pressure difference available for discharging the gaseous refrigerant from the feed unit and it can thus be rapidly removed. 
         [0031]    Herein, the refrigerant path of the cooling circuit which is at suction-side pressure is to be understood as the entire refrigerant path running from the evaporator to the suction chamber of the compressor. 
         [0032]    It is particularly expedient if the gas discharge line leads into the refrigerant path which is at suction-side pressure prior to the suction port of the compressor so that, in the event of liquid refrigerant being supplied by way of the gas discharge line to this refrigerant path, there is an opportunity for it to evaporate before reaching the suction chamber. 
         [0033]    In particular, it is expedient if the refrigerant path which is at suction-side pressure runs from the suction port of the compressor in the compressor housing through a motor compartment of the compressor in order to cool it so that evaporation of any liquid refrigerant that is being supplied can be assisted by joining the gas discharge line prior to the motor compartment or into the motor compartment. 
         [0034]    Furthermore, one expedient solution envisages that the gas discharge unit be connected to a supply line section of the feed unit leading to the inlet of the pumping unit. 
         [0035]    However, the supply of refrigerant over the gas discharge line to the refrigerant path which is at suction-side pressure can also be used to good effect for cooling the compressor. 
         [0036]    To this end, a throttle or an expansion unit is preferably provided in the gas discharge line, for example, prior to the junction thereof into the refrigerant path which is at suction-side pressure, said throttle or expansion unit expanding and thus cooling the refrigerant before it enters the refrigerant path which is at suction-side pressure so that this cooled refrigerant can be supplied to the compressor at the suction-side. 
         [0037]    Another advantageous solution envisages that the gas discharge unit be connected to a discharge line section of the feed unit leading to the pressure chamber. 
         [0038]    A particularly expedient solution envisages that the gas discharge unit be connected to an inlet of the pumping unit and/or to an outlet of the pumping unit in order to enable the pumping unit to be supplied with liquid refrigerant insofar as possible before it starts or immediately after it has started so that it will pump liquid refrigerant to the pressure chamber. 
         [0039]    In order to be able to activate and deactivate the gas discharge unit, provision is preferably made for an on-off valve to be associated with the gas discharge unit and in particular, for it to be provided in the gas discharge line. 
         [0040]    Preferably hereby, the on-off valve is controllable by a refrigerant supply control unit in such a way that the gas discharge unit is activated thereby either before or when switching-on the pumping unit and firstly, during a definable time period for example, gaseous refrigerant and possibly some of the liquid refrigerant flowing after it is conducted away until such time as only liquid refrigerant is present at the pumping unit for the purposes of pumping it to the pressure chamber. 
         [0041]    Hereby for example, a length of time is selected in such a way that at the end of this period it is ensured that liquid refrigerant is present at the inlet of the pumping unit in every operational state of the cooling circuit. 
         [0042]    A further advantageous solution, which reduces the supply of liquid refrigerant to the suction-pressure-side of the refrigerant path, envisages that the length of time be adjustable so as to be variable in correspondence with the actual operational state of the cooling circuit wherein the operational state of the cooling circuit is detected by sensors such as temperature and/or pressure sensors for example. 
         [0043]    However, as an alternative thereto, there is also the possibility of detecting whether liquid refrigerant is already present in the supply line section and/or in the discharge line section and/or in the gas discharge unit by means of a liquid sensor for example, and then, if this is the case, to deactivate the gas discharge unit. 
         [0044]    As an alternative or in addition to the previously described mode of operation, the conveyance of liquid refrigerant in the gas discharge line can also be detected by the provision of a throttle in the gas discharge line so that when refrigerant flows therethrough the temperature of the refrigerant is reduced due to the expansion process and this reduction of temperature is detectable by a sensor located downstream of the throttle and/or a sensor located upstream of the throttle so that, upon the occurrence of a reduction of temperature corresponding to the expansion of liquid refrigerant, the gas discharge unit is deactivated. 
         [0045]    Furthermore, the invention relates to a method of operating a cooling circuit comprising a refrigerant compressor incorporating a suction port and a pressure chamber incorporating a pressure port, a condenser which is arranged in the cooling circuit downstream of the pressure port and comprises a fluid collecting chamber in which a refrigerant reservoir of refrigerant is formed, an evaporator located in the cooling circuit between the condenser and the suction port, a feed unit which is connected at one side to the refrigerant reservoir and to the pressure chamber at the other side and serves for supplying refrigerant from the refrigerant reservoir to the pressure chamber which incorporates a pumping unit for the refrigerant and with the aid of which refrigerant is supplied by means of the feed unit to the pressure chamber for the purposes of cooling it, wherein gaseous refrigerant is conducted away from the feed unit by means of a gas discharge unit. 
         [0046]    Further features of the invention form the subject matter of the following description and the graphical illustration of some exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]      FIG. 1 , shows a schematic illustration of a first exemplary embodiment of a cooling circuit in accordance with the invention; 
           [0048]      FIG. 2 , a schematic illustration similar to  FIG. 1  of a second exemplary embodiment of a cooling circuit in accordance with the invention; 
           [0049]      FIG. 3 , a schematic illustration of a first exemplary embodiment of a pumping unit provided in accordance with the invention; 
           [0050]      FIG. 4 , a schematic illustration of a second exemplary embodiment of a pumping unit in accordance with the invention and 
           [0051]      FIG. 5 , a schematic illustration of a variant of the second exemplary embodiment of the pumping unit in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0052]    A first exemplary embodiment of a cooling circuit  10  employing a circulating refrigerant in accordance with the invention which is illustrated in  FIG. 1  comprises a compressor for the refrigerant which may be in the form of a piston compressor for example and bears the general reference  12 . 
         [0053]    However, the compressor  12  could also be implemented in the form of a scroll compressor, a rotary piston compressor, a vane compressor or a rotary screw compressor. 
         [0054]    In the case of a piston compressor  12 , a drive motor  16  is arranged in a compressor housing  14  and it drives pistons  18  in one or more cylinders  22  which are arranged in a cylinder block  24  of the compressor housing  14  and are closed off by at least one cylinder head  26 , wherein the at least one cylinder head  26  comprises a suction chamber  32  and a pressure chamber  34 , wherein refrigerant that is to be sucked-in is supplied to the suction chamber  32 , supplied therefrom to the one or the plurality of cylinders  22  and then, after being compressed by the cylinders  22 , is delivered to the pressure chamber  34 . 
         [0055]    The pressure chamber  34  is provided with a pressure port  38  for compressed refrigerant and a pressure line  42  leads from the pressure port  38  to a condenser  44  which liquefies the pressurised refrigerant in the case of a subcritical mode of operation, or cools it in the case of a supercritical mode of operation, and then supplies it to a fluid collecting chamber  46  in which a refrigerant reservoir  48  consisting of refrigerant, liquid refrigerant in a subcritical mode of operation, is formed. 
         [0056]    Hereby, the fluid collecting chamber  46  can be integrated into the condenser  44 . However, as illustrated in  FIG. 1 , the fluid collecting chamber  46  could also be arranged in a collector vessel  52  which forms the fluid collecting chamber and is arranged in the refrigerant circulation path  10  between the condenser  44  and an evaporator  54 . 
         [0057]    However, the fluid collecting chamber could also be arranged in a supply line which leads to the evaporator  54  and has been widened-out in correspondence with the volume required. 
         [0058]    For example, a supply line  56  leads from the condenser  44  to the collector vessel  52  in order to supply the liquefied refrigerant thereto and an evaporator supply line  58  leads from the collector vessel  52  to the evaporator  54 , wherein the evaporator supply line  58  is arranged relative to the fluid collecting chamber  46  in such a way as to take liquid refrigerant from the refrigerant reservoir  48  but not vaporous refrigerant. 
         [0059]    For its part, the evaporator  54  is additionally provided with a control valve  62  which controls the inflow of refrigerant to the evaporator  54 , wherein the refrigerant in the form of a liquid refrigerant then evaporates in the evaporator  54  in a subcritical mode of operation under given pressure conditions and, in a supercritical mode of operation, cools down due to the expansion process and thereby absorbs heat. 
         [0060]    The refrigerant that has been evaporated in the evaporator  54  is then fed back to the compressor  12  through a suction line  64  of the refrigerant circulation path  10  which is at suction pressure, wherein for example, a suction port  66  of the compressor  12  is arranged in such a way that the evaporated refrigerant coming from the suction line  64  firstly flows round a drive motor  16  which is arranged at the suction end, cools it and then enters the suction chamber  32  of the cylinder head  26 . 
         [0061]    Since, in particular, refrigerants having a high compression index such as the refrigerants R407A, R407F, CO 2 , NH 3  for example reach high compression end temperatures, there is substantial heating of the cylinders  22  and the cylinder head  26  in the case of high pressure conditions, especially in the region of the pressure chamber  34  and overall, this then leads to heating of the compressor housing  14  so that the efficiency of the compressor  12  is impaired due to heat transfer losses. 
         [0062]    The heating in the region of the pressure chamber can also lead to chemical decomposition of a lubricant being conveyed by the refrigerant mass flow and thus, as a consequence thereof, to the breakdown of the compressor and to contamination of the system. 
         [0063]    For this reason, there is provided a feed unit bearing the general reference  70  for the supply of liquid refrigerant from the refrigerant reservoir  48  into the pressure chamber  34  of the at least one cylinder head  26  so that cooling of the pressure chamber  34  is achieved by evaporation of the supplied liquid refrigerant in the pressure chamber  34 . 
         [0064]    Herein, the feed unit  70  comprises a supply line  72  which either opens out directly into the refrigerant reservoir  48  or branches off from the evaporator supply line  58  and opens out into the pressure chamber  34  of the at least one cylinder head  26 . 
         [0065]    However, as the pressure in the fluid collecting chamber  46  is lower than the pressure in the pressure chamber  34  due to pressure losses in the pressure line  42  and in the condenser  44 , it is in a range of between 0.5 and 5 bar for example in the case of the refrigerants R407A and R407F or refrigerants having similar working pressures and is still lower in the case of high pressure refrigerants such as CO 2  for example, there is provided in the supply line  72  a pumping unit  74  for liquid refrigerant which raises the liquid refrigerant from the pressure level in the fluid collecting chamber  46  to at least slightly above the pressure level in the pressure chamber  34  or at most to 5 bar above the pressure level in the pressure chamber and moreover, there is provided in the supply line  72  between the pumping unit  74  and the point of entry thereof into the pressure chamber  34  a check valve  86  which permits the pumping unit  74  to be switched off at any desired time point. 
         [0066]    Hereby, the pumping unit  74  comprises an inlet  76  which is connected to a supply line section  78  of the supply line  72  and also an outlet  82  which is connected to a discharge line section  84  of the supply line  72 , wherein the discharge line section  84  leads from the outlet  82  to the pressure chamber  34 . 
         [0067]    In particular, a check valve  86  is arranged in the discharge line section  84 , said valve permitting the pumping unit  74  to be switched off if a high temperature above a desired value does not occur in the pressure chamber  34  and only allowing it to switch on when a temperature above a desired value occurs in the pressure chamber  34  of the cylinder head  26 . 
         [0068]    In order to operate the pumping unit  74 , there is provided a refrigerant supply control unit  90  which detects the temperature in the pressure chamber  34  or in a region of the at least one cylinder head  26  bordering the pressure chamber  34  by means of a temperature sensor  92  and then always switches on the pumping unit  74  when the temperature in the pressure chamber  34  or in a region of the cylinder head  26  bordering the pressure chamber  34  exceeds a predetermined threshold value so that the supply of refrigerant to the pressure chamber  34  only occurs in a sub-critical mode of operation when the threshold value is exceeded, whereby the refrigerant then evaporates in the pressure chamber  34  and thereby absorbs heat and thus cools the gaseous refrigerant present in the pressure chamber  34  as well as the cylinders  22  and the compressor housing  14 . 
         [0069]    In the case of a drive motor of the compressor which is arranged at the pressure end, cooling of the drive motor by the supply of liquid refrigerant is also possible if the refrigerant emerging from the pressure chamber  34  flows around this motor. 
         [0070]    The threshold value lies within a range of 80° C. to 150° C. for example, preferably within a range of 110° C. to 130° C., and in particular, in a range of between 115° C. and 125° C. 
         [0071]    Furthermore, there is preferably provided a safety cut-out which switches off the compressor  12  in the event that a maximum temperature in the pressure chamber  34  is exceeded, wherein the maximum temperature lies within a range of 130° C. to 150° C., preferably within a range of 135° C. to 145° C. 
         [0072]    In order to additionally ensure that the threshold value is not significantly exceeded, the refrigerant supply control unit  90  controls the pumping unit  74  in such a way that the quantity of material being delivered by the pumping unit  74  is regulated, wherein the pumping unit  74  is designed such that a maximum delivery rate thereof is sufficient to meet the envisaged working conditions in order to prevent the threshold value from being permanently exceeded. 
         [0073]    The additional refrigerant being supplied to the pressure chamber  34  by way of the supply line  72  then flows to the condenser  44  through the pressure line  42  in addition to the refrigerant that was compressed by the at least one cylinder  22 , and it is liquefied in the condenser  44 . 
         [0074]    In particular, the supply of refrigerant to the pressure chamber  34  is effected until such time as the temperature measured by the temperature sensor  92  drops back below the threshold value. 
         [0075]    In order to ensure that the pumping unit  74  conveys substantially only liquid refrigerant, there is associated with the supply line  72  a gas discharge unit  100  which comprises a gas discharge line  102  that branches off from the supply line section  78  in the first exemplary embodiment illustrated in  FIG. 1 , wherein said gas discharge line may join the cooling circuit  10  between the evaporator  54  and the at least one cylinder  22  which represents a flow path of the refrigerant that is at suction pressure. 
         [0076]    This means that the gas discharge line  102  can, for example, open out into the suction line  64  or into the suction port  66 , or, it can open out in the compressor housing  14  into the flow path running therethrough for the refrigerant that is being sucked in and is flowing to the suction chamber  32  or, it could also open out directly into the suction chamber  32  as is illustrated in  FIG. 1 . 
         [0077]    For the purposes of activating the gas discharge line  102 , there is provided therein a gas discharge valve  104  which is controllable by the refrigerant supply control unit  90 . 
         [0078]    For example, activation of the gas discharge line  102  is effected by opening the gas discharge valve  104  when switching on the pumping unit  74  or before switching it on so that, due to the large pressure gradient between the pressure in the supply line section  78  of the supply line  72  and the suction pressure of the compressor  12 , the gas that collects in the supply line section  78  as a result of the heating process occurring when the pumping unit  74  is switched off is supplied to the compressor  12  at the suction end via the gas discharge line  102  and in consequence liquid refrigerant flows thereafter from the refrigerant reservoir  48  into the supply line  72 . 
         [0079]    If liquid refrigerant is present at the inlet  76  of the pumping unit  74 , the refrigerant supply control unit  90  can close the gas discharge valve  104  and thus deactivate the gas discharge line  102  since the pumping unit  74  can then convey the liquid refrigerant present at the inlet  76  thereof and bring the pressure up to a level such that this refrigerant will flow into the pressure chamber  34  of the at least one cylinder head  26  in order to be evaporated in the pressure chamber  34  and thus—as described—to cool down in the pressure chamber  34 . 
         [0080]    The presence of liquid refrigerant at the inlet  76  of the pumping unit  74  can be ensured in the most varied of manners. 
         [0081]    A first possibility envisages the opening of the gas discharge valve  204  for a time period that is definable in regard to the length of time thereof, wherein the length of time is measured in such a way as to ensure that liquid refrigerant will definitely be available at the inlet  76  of the pumping unit  74  at the end of the period under all the usual operating conditions. 
         [0082]    Hereby, the length of time can be fixed in accordance with the maximum length of time that is necessary under all possible operating conditions. 
         [0083]    However, it is also possible to detect the operating conditions occurring at different positions of the cooling circuit by means of sensors such as sensors for the ambient temperature and/or sensors for the temperatures in the evaporator and/or pressure sensors for example and to set the length of time in a variable manner in accord with the particular operating conditions that have been detected. 
         [0084]    A second possibility envisages that the presence of liquid refrigerant be detected by means of at least one liquid sensor  106 . Hereby, this liquid sensor  106  can be arranged in the supply line section  78  such as directly before the inlet  76  of the pumping unit  74  for example and/or in the gas discharge line  102  such as at the point of branching from the supply line section  78  and/or at the point of entry into the refrigerant path which is at suction-side pressure for example. 
         [0085]    As soon as the temperature in the pressure chamber  34  or in the part of the cylinder head  26  bordering the pressure chamber  34  has dropped again to such an extent that it is under the threshold value, the refrigerant supply control unit  90  switches off the pumping unit  74  so that the feed unit  70  will be inactive until such time as the threshold value for the temperature in the pressure chamber  34  or in the part of the cylinder head  26  bordering the pressure chamber  34  is exceeded once again. 
         [0086]    In the first exemplary embodiment, the gas discharge line  102  branches from the supply line section  78  of the supply line  72  as directly as possible before the inlet  76 , preferably at the inlet  76 , in order to conduct away all the gas from the supply line section  78  before the inlet  76  of the pumping unit  74  so that the pumping unit  74  sucks in as little as possible or, if possible, no gaseous refrigerant at all, but rather, when it is switched on, it can directly pump out liquid refrigerant. 
         [0087]    As an alternative thereto, provision is made in a second exemplary embodiment of a cooling circuit  10 ′ in accordance with the invention that is illustrated in  FIG. 2  for the gas discharge line  102 ′ to branch out from the pressure port  38  or from the supply line  72 , for example, between the outlet  82  of the pumping unit  74  and the check valve  86  in the discharge line section  84  so that, upon activating the gas discharge line  102 , there will be a pressure gradient through the pumping unit  74  which permits certain flooding of the pumping unit  74  with liquid refrigerant so that, after deactivating the gas discharge line  102  by closing the gas discharge valve  104 , the pumping unit  74  as a whole is flooded with liquid refrigerant and thus immediately begins to pump liquid refrigerant. 
         [0088]    Hereby, the check valve  86  prevents the process of conducting away the gaseous refrigerant from negatively affecting the pressure in the pressure chamber  34 . 
         [0089]    This solution is attractive in particular if the pumping unit  74  is constructed in such a way that a pressure gradient occurring therein in the pumping direction can assist the conveyance of gaseous refrigerant. 
         [0090]    In all other respects, the second exemplary embodiment of the cooling circuit in accordance with the invention in accord with  FIG. 2  is constructed in the same way as the first exemplary embodiment so that the same parts are provided with the same reference symbols and reference can be made to the full extent of the remarks relating to the first exemplary embodiment in regard to the description thereof. 
         [0091]    In regard to the construction of the pumping unit  74 , no detailed indications have as yet been given. 
         [0092]    A first exemplary embodiment of a pumping unit  74  in accordance with the invention which is illustrated in  FIG. 3  is constructed in the form of a piston pump  110  which comprises a piston  112  that moves in a reciprocating linear manner in an oscillation direction  114 . 
         [0093]    To this end, an outer piston surface  116  of the piston  112  is guided in a pumping chamber  118  such as to be moveable in the direction of oscillation  114  and, for example, it is arranged between two springs  122  and  124  so that the piston  112  can move in the pumping chamber  118  in the direction of oscillation  114  due to the compression of one of the springs  122 ,  124  and the relaxation of the other one of the springs  122 ,  124 . 
         [0094]    For example, the piston  112  is provided with a passage  126  so that the medium that is to be conveyed can flow therethrough. 
         [0095]    Furthermore, a first variable volume  134  is formed between a first end face  132  of the piston  112  and the inlet  76  and a second variable volume  138  is formed between a second end face  136  of the piston  112  and the outlet  82 . 
         [0096]    Moreover, an inlet valve  142  is arranged between the inlet  76  and the first volume  134 , an outlet valve  144  is arranged between the second volume  138  and the outlet  82  and yet another piston valve  146  which is arranged in the passage  126  for example is associated with the piston  112 . 
         [0097]    If the piston  112  moves in such a way that the second volume  138  becomes smaller whilst the first volume  134  grows larger, then the outlet valve  144  opens and the liquid refrigerant flows out of the piston chamber due to the reduction of the second volume  138 . At the same time, the inlet valve  142  opens since the first volume  134  has become larger so that liquid refrigerant can enter the pumping chamber  118  via the inlet  76 . 
         [0098]    The piston valve  146  remains closed hereby. 
         [0099]    If then the piston  112  moves in such a way that the first volume  134  becomes smaller and the second volume  138  grows larger, then both the inlet valve  142  and the outlet valve  144  close whilst the piston valve  146  opens and liquid refrigerant can thus be transferred from the first volume  134  through the passage  126  into the second volume  138 , whereupon the piston valve  146  opens. 
         [0100]    The movements of the piston  112  in the direction of oscillation  114  are enabled by an electromagnet  152  which is arranged outside the pumping chamber  118 , the magnetic field thereof being effective on the piston  112  in such a manner that the latter is moved in reciprocating manner in the direction of oscillation  114 . 
         [0101]    For example, this is made possible by virtue of the magnetic field of the electromagnet moving the piston  112  in a direction leading to a reduction of the second volume  138 , and removal of the magnetic field of the electromagnet  152  leads to the piston  112  moving in a direction producing a reduction of the first volume  134  due to the effect of the springs  122  and  124 , and a renewed activation of the electromagnet  152  again leads to a movement of the piston  112  in a direction producing a reduction of the second volume  138 . 
         [0102]    Since the piston pump  110  works at pressures in a range of over 15 bar, preferably in a range of over 20 bar and still better in a range of over 25 bar, the piston pump  110  is constructed in such a way that a pumping chamber housing  154  forming the pumping chamber  118  is hermetically sealed and in particular is also connected to the inlet  76  and to the outlet  82  in hermetically sealed manner, wherein in this case, the electromagnet  152  does not have to be arranged in a hermetically sealed housing  156 , but rather, the field effect thereof is effective on the piston  112  through the pumping chamber housing  154 . 
         [0103]    Another solution envisages that the piston pump  110  comprise a hermetically sealed outer housing  156  which encompasses the pumping chamber housing  154  and which is connected to the inlet  76  and to the outlet  82  in hermetically sealed manner, wherein the electromagnet  152  is also located within the outer housing  156  at a pressure level lying above 15 bar, preferably above 20 bar and still better above 25 bar. 
         [0104]    In both cases it is thus possible to hold the liquid refrigerant that is to be conveyed in a housing which encloses it hermetically and in particular one which is free of mechanical feed-throughs for the drive for the piston  112 . 
         [0105]    A second exemplary embodiment of a pumping unit  74 ′ in accordance with the invention is illustrated in  FIG. 4 . This pumping unit is in the form of a rotary pump  170  which comprises a pumping element  172  that is driven in rotary manner about an axis  174 . 
         [0106]    For example, the pumping element  172  is a gear wheel of a gear pump which cooperates with a further gear wheel that is not visible in  FIG. 4 . 
         [0107]    Here, liquid refrigerant is advanced from the inlet  76  to the outlet  82 , wherein the pumping element  172  is arranged in a pumping chamber  176  which is connected to both the inlet  76  and the outlet  82 . 
         [0108]    The pumping chamber  176  here is arranged in a pumping chamber housing  178  which does not comprise any sort of mechanical feed-throughs for the drive means for the pumping element  172 . 
         [0109]    Rather, the drive for the pumping element  172  is effected by means of magnetic coupling between a rotor  184  of a drive motor  182 , wherein the rotor  184  is arranged in an interior space  186  of a motor housing  188  adjoining the pumping chamber housing  178  such that it is coaxial with the pumping element  172  so that the reciprocally acting magnetic interaction between the rotor  184  and the pumping element  172  is effected through the motor housing  188  and the pumping chamber housing  178 . 
         [0110]    Furthermore, a stator  192  which encloses the rotor  184  and is effective for the rotary motion of the rotor  184  is arranged in the interior of the motor housing  188 . 
         [0111]    In this exemplary embodiment, in which the pumping element  172  in the pumping chamber  176  does not comprise any sort of mechanical feed-throughs for the operation of the pumping element  172 , the pumping chamber  176  is connected exclusively to the inlet  76  and the outlet  82  whilst the drive means for the pumping element  172  is provided by means of a reciprocally acting magnetic interaction between the pumping element and the rotor  184 . 
         [0112]    In a variant of the second exemplary embodiment of the pumping unit  74 ′ which is constructed in the form of a rotary pump  170 ′ and is illustrated in  FIG. 5 , the pumping chamber housing  178 ′ and the motor housing  188  form a hermetically sealed unit so that the interior  186  of the motor housing  188  accommodating the rotor  184  and the stator  192  can adopt the same pressure level as the pumping chamber  176 . 
         [0113]    In this case, it is thus also possible for the pumping element  172  to be coupled to the rotor  184  by means of a mechanical shaft  194  so that the rotor  184  together with the shaft  194  and the pumping element  172  represent a mutually non-rotationally connected unit which rotates about the axis  174 . 
         [0114]    Consequently, in this exemplary embodiment too, there also arises the possibility of working with the pumping element  172  at a very high pressure level of over 15 bar for example, still better, a level of over 20 bar and even better of over 25 bar without leakage losses due to mechanical feed-throughs for the drive means. 
         [0115]    With regard to the maximum delivery rate of the compressor  12  in all of the preceding exemplary embodiments, the delivery rate envisaged for the feed unit  70  amounts to less than 100%, still better less than 50% and preferably less than 30% of this maximum delivery rate of the compressor  12 . 
         [0116]    In the case of the two embodiments of the pumping units  74  and  74 ′ in accordance with the invention, the dimensioning thereof is effected such that the maximum handling capacity of a pumping unit of this type amounts to 100 litres or less per hour so that one or possibly more parallel-working, very small pumping units having a very low power consumption can be used. 
         [0117]    Preferably hereby, the handling capacity of one of the pumping units  74 ,  74 ′ of this type that are to be used is at least 0.3 litres per hour, still better at least 0.3 litres per hour [sic], or more. 
         [0118]    Furthermore, in both embodiments of the pumping units  74 ,  74 ′, additional regulation of the handling capacity of the pumping units  74 ,  74 ′ can be effected by means of the refrigerant supply control unit  90  so that the handling capacity of the pumping units  74 ,  74 ′ can be adapted to the requisite cooling performance in the pressure chamber  34  by the supplied refrigerant.