Abstract:
The invention relates to a method for adjusting a cryogenic refrigeration apparatus including a plurality of liquefiers/refrigerators arranged in parallel in order to cool a single device. The method includes a step of calculating in real time the dynamic mean value of at least one operating parameter for all the liquefiers/refrigerators. The apparatus controlling in real time the at least one valve for controlling the stream of working gas of at least one liquefier/refrigerator in accordance with the difference between the instantaneous values of the parameter relative to said dynamic converge toward said dynamic mean value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a 371 of International PCT Application PCT/FR2015/051492 filed Jun. 5, 2015, which claims priority to French Patent Application No. 1457100 filed Jul. 23, 2014, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to a method for adjusting a cryogenic refrigeration apparatus and to a corresponding apparatus. 
         [0003]    The invention relates more particularly to a method for adjusting a cryogenic refrigeration apparatus comprising several refrigerators/liquefiers arranged in parallel to cool one and the same application, each refrigerator/liquefier comprising a working circuit for a working gas equipped with at least one valve for controlling the flow of working gas, the refrigerator/liquefiers in parallel using a working gas of the same kind such as pure gaseous helium, each refrigerators/liquefier comprising a working gas compression station, a cold box intended to cool a flow of working gas leaving the compression station to a cryogenic temperature at least close to its liquefaction temperature, said flows of working gas cooled by each of the respective cold boxes of the refrigerators/liquefiers being mixed and then placed in a heat exchange relationship with the application in order to give up frigories thereto, the cold working gas having exchanged heat with the application then being divided into several return flows distributed respectively through the respective compression stations. 
         [0004]    The invention relates to what is referred to as “large-scale” refrigeration apparatuses employing several refrigerators/liquefiers in parallel in order to cool one and the same user application 
         [0005]    A “refrigerator/liquefier” denotes a device which subjects a working gas (for example helium) to a thermodynamic cycle of work (compression/expansion) that brings the working fluid to a cryogenic temperature (for example a few degrees K in the case of helium) and where appropriate liquefies this working gas. 
         [0006]    One nonlimiting example of such an apparatus is described in application no. FR2980564A1. 
         [0007]    The refrigeration cycles (which generate cold) are said to be “closed” at the level of each refrigerator. What that means to say is that the flow of working gas that enters the cold box of a refrigerator/liquefier reemerges for the most part from this same cold box. By contrast, the flow of working gas is said to be “open” at the level of the application that is to be cooled, which means to say that the gas from the various refrigerators/liquefiers is mixed therein. The flow of working gas supplied by the refrigerators/liquefiers is therefore pooled for cooling the application then returned separately to each refrigerator by a distribution system. 
         [0008]    Adjustment of the refrigerators of such an apparatus generally involves manually positioning the control valves of the working circuit (from and to the application that is to be cooled). 
         [0009]    Suitable adjustment becomes more difficult when the apparatus comprises a great many interfaces and when the thermal loads that need to be cooled vary over time. This is because static adjustment of the valves may be unsuitable if the flow rate and/or pressure of the system vary. 
         [0010]    The fluctuating thermal loads of the application indeed generate fluctuations in the flow rate through the compressors. 
         [0011]    If this is not corrected, certain refrigerators/liquefiers will recuperate more working gas and cold than others. Thus, certain refrigerators/liquefiers may diverge from their nominal operating point. Certain components of these refrigerators/liquefiers may therefore be used at their limit (compressors, turbines, etc.) whereas the other refrigerators/liquefiers will be underutilized. The overall cold power of the apparatus and the efficiency thereof will therefore be reduced. 
         [0012]    Providing systems for control and adjusting the independent flows for each refrigerator/liquefier may lead to a system which overall is unstable in which the loads and flow rates will be distributed inconsistently between the refrigerators/liquefiers. In addition, the specific features of helium (a density that varies greatly as a function of temperature) lead to a phenomenon in which the imbalances between the refrigerators are amplified. 
         [0013]    The distribution of helium flow rates between the refrigerators is performed generally via a common helium feed pressure and the resistance (pressure drop) of the circuit returning to the source of pressure (compressors). 
         [0014]    When one refrigerator/liquefier receives in relative terms more cold gas coming from the application, the mean temperature of the return circuit drops and the pressure drop of the circuit is therefore reduced. Specifically, the density of the gas may change more rapidly than the speed of the gas through the circuit. This drop in pressure drop in a circuit leads to a relative increase in the flow rate of cold gas accepted into the circuit concerned and therefore leads to divergence within the apparatus. 
         [0015]    It is an object of the present invention to alleviate all or some of the disadvantages mentioned hereinabove of the prior art. 
       SUMMARY 
       [0016]    To this end, the method according to the invention, in other respects in accordance with the generic definition given thereof in the above preamble, is essentially characterized in that it comprises a step of simultaneous measurement, for each of the refrigerators/liquefiers, of the instantaneous value of at least one and the same operating parameter from: a flow rate of what is referred to as a “return” flow of working gas returning to the compression station, a flow rate of what is referred to as an “outbound” flow of working gas circulating through the cold box having left the compression station, a differential in temperature of the working gas between, on the one hand, the outbound flow of working gas and, on the other hand, the return flow of working gas, both flows being situated in the cold box in one and the same temperature range, the method comprising a step of real-time calculation of the dynamic mean value of the at least one operating parameter for all the refrigerators/liquefiers, the apparatus performing real-time control of the at least one working gas flow control valve of at least one refrigerator/liquefier as a function of the difference between the instantaneous values of the parameter with respect to said dynamic mean value, so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers to converge toward this dynamic mean value. 
         [0017]    This particular feature allows the apparatus to be adjusted dynamically in order to react automatically to the variations in refrigerator parameters (temperature, pressure, flow rate, level, etc.). 
         [0018]    This adjustment makes it possible to get as close as possible to the predetermined optimum operation (calculated beforehand) in which the various refrigerators/liquefiers operate identically (same flow rates/pressure/temperature of the working gas in the circuit). 
         [0019]    In order to meet this requirement, the method compares one of the dynamic parameters indicative of the operation of a refrigerator and compares it against the mean of this same parameter across all the other refrigerators. The control action of the method uses this difference in value of the parameter to modify the set point of the regulators existing on each refrigerator having an impact on the parameter. That then also modifies the mean of the parameters and therefore the set point is also updated. This is a control system which may be qualified as being “in cascade” with a set point that is “dynamic” that causes each parameter to converge toward the mean of this parameter across the various refrigerators. 
         [0020]    Moreover, embodiments of the invention may comprise one or several of the following features:
       the refrigerators/liquefiers are identical, the apparatus performing real-time control of the at least one working gas flow control valve of at least one refrigerator/liquefier as a function of the difference between the instantaneous values of the parameter with respect to said dynamic mean value, so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers to converge toward a determined identical value,   the refrigerators/liquefiers are identical, the apparatus performing real-time control of the at least one working gas flow control valve of at least one refrigerator/liquefier as a function of the difference between the instantaneous values of the parameter with respect to said dynamic mean value in order at once to cause said instantaneous values of the flow rates of the return flow of working gas returning toward the compression stations to converge toward a determined identical flow value, to cause the differential in temperature of the working gas between the outbound flow of working gas in the cold box and the return flow of working gas returning toward the compression station to converge toward a determined identical temperature differential value and to cause the flow rate of the flow of cooled working gas at the outlet of each cold box to converge toward a determined identical flow rate value,   the compression station of each refrigerator/liquefier comprises two compressors arranged in series on the working circuit and respectively designated “low-pressure compressor” and “medium-pressure compressor”, a bypass circuit for selectively bypassing the low-pressure compressor comprising at least one variable-opening controlled bypass valve, the method comprising simultaneous measurement, for each of the refrigerators/liquefiers, of the operating parameter consisting of the instantaneous value of the flow rate of the return flow of working gas returning toward the compression station, the method comprising a step of real-time calculation of the dynamic mean value of the operating parameter for all the refrigerators/liquefiers, the apparatus performing real-time control of the opening/closing of each bypass valve as a function of the difference between the instantaneous values of the operating parameter of the refrigerator/liquefier concerned in order to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers to converge toward this dynamic mean value,   the method comprises simultaneous measurement, for each of the refrigerators/liquefiers, of the differential in temperature of the working gas between, on the one hand, the return flow and, on the other hand, the outbound flow at the same temperature level in the cold box, control of each bypass valve being corrected as a function of the discrepancy between said differential in temperature for the refrigerator/liquefier concerned and the mean of said temperature differential calculated for all of the refrigerators/liquefiers, the opening/closing of each bypass valve being reduced when the temperature differential for the refrigerator/liquefier concerned increases in terms of absolute value with respect to the mean of said temperature differential,   at the outlet of the compression station, each refrigerator/liquefier comprises a variable-opening controlled outlet valve, the method comprising simultaneous measurement, for each of the refrigerators/liquefiers, of the operating parameter consisting of the instantaneous value of the flow rate of the outlet flow of working gas, the method comprising a step of real-time calculation of the dynamic mean value of the operating parameter for all the refrigerators/liquefiers, the apparatus performing real-time control of the opening/closing of each outlet valve as a function of the difference between the instantaneous values of the operating parameter of the refrigerator/liquefier concerned so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers to converge toward this dynamic mean value,   each outlet valve is controlled according to a pressure set point measured at the outlet of said valve, the apparatus performing real-time control of the opening/closing of each outlet valve so as to reduce the pressure set point when the instantaneous value of the flow rate of the flow of gas at the outlet of the compression station of the refrigerator/liquefier concerned is greater than said dynamic mean value, and vice versa,   the working circuit comprises, in the cold box of each refrigerator/liquefier, a main pipe comprising a working gas cooling exchanger immersed in a cryogenic tank of liquefied working gas and a secondary pipe forming a bypass of the main pipe upstream of the cryogenic tank and opening into the latter so as to be able to deliver thereto liquefied working gas produced by the cold box, the main pipe comprising a variable-opening controlled downstream valve situated downstream of the cooling exchanger, the method comprising simultaneous measurement, for each of the refrigerators/liquefiers, of the operating parameter consisting of the instantaneous value of the flow rate of the outlet flow of working gas in said main pipe downstream of the cooling exchanger, the method comprising a step of real-time calculation of the dynamic mean value of this operating parameter for all of the refrigerators/liquefiers, the apparatus performing real-time control over the opening/closing of each downstream valve as a function of the difference between the instantaneous values of this operating parameter of the refrigerator/liquefier concerned in order to make said instantaneous values of said operating parameter of the various refrigerators/liquefiers converge toward this dynamic mean value,   the secondary pipe is provided with a variable-opening distribution valve the opening of which is increased in the event of an increased production of liquefied working gas in the cold box, in that control of each downstream valve is corrected as a function of the state of opening of the distribution valve so as to reduce the opening of the downstream valve when the opening of the distribution valve increases, and vice versa,   the cold box of each refrigerator/liquefier comprises a plurality of heat exchangers for cooling the working fluid and a bypass pipe for bypassing at least some of said exchangers supplying working gas at the outlet of the cold box, said bypass pipe being connected to the rest of the working circuit in a heat exchange relationship with the exchangers via variable-opening respective controlled bypass valves, the method comprising simultaneous measurement, for each of the refrigerators/liquefiers, of the operating parameter consisting of the instantaneous value of the flow rate of the flow of gas in said bypass pipe, the method comprising a step of real-time calculation of the dynamic mean value of this operating parameter for all of the refrigerators/liquefiers, the apparatus performing real-time control of the opening/closing of at least one of the bypass valves as a function of the difference between the instantaneous values and the dynamic mean value of this operating parameter of the refrigerator/liquefier concerned, so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers to converge toward this dynamic mean value,   the working circuit comprises, inside the cold box of each refrigerator/liquefier, a plurality of exchangers for warming up the cold working fluid that has exchanged heat with the application, the working circuit comprising a pipe for returning the return flow of working gas returning to the compression station, the return pipe comprising a portion that is subdivided into two parallel branches referred to respectively as the “hot” leg and as the “cold” leg, the hot leg bypassing at least some of the warming up exchangers, the cold leg being thermally coupled to the warming up exchangers, the working fluid having exchanged heat with the application returning to the compression station being distributed through the hot leg when its temperature is above a determined threshold or the cold leg when its temperature is below the determined threshold, each hot leg comprising a variable-opening controlled regulating valve, the method comprising a simultaneous measurement, for each of the refrigerators/liquefiers, of the operating parameter that consists of the instantaneous value of the flow rate of the flow of gas in said hot leg, the method comprising a step of real-time calculation of the dynamic mean value of this operating parameter for all the refrigerators/liquefiers, the apparatus performing real-time control of the opening/closing of the valve of the hot leg as a function of the difference between the instantaneous values and the dynamic mean value of this operating parameter of the refrigerator/liquefier concerned, so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers to converge toward this dynamic mean value,   each cold leg comprises a variable-opening controlled regulating valve, the method comprising simultaneous measurement, for each of the refrigerators/liquefiers, of the operating parameter consisting of the instantaneous value of the flow rate of the flow of gas in said cold leg, the method comprising a step of real-time calculation of the dynamic mean value of this operating parameter for all the refrigerators/liquefiers, the apparatus performing real-time control of the opening/closing of the valve of the cold leg as a function of the difference between the instantaneous values and the dynamic mean value of this operating parameter for the refrigerator/liquefier concerned, so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers to converge toward this dynamic mean value.       
 
         [0032]    The invention may also relate to any alternative device or method comprising any combination of the features above or below. 
         [0033]    The invention may also relate to a cryogenic refrigeration apparatus comprising several refrigerators/liquefiers arranged in parallel to cool one and the same application, each refrigerators/liquefier comprising a working circuit for a working gas equipped with at least one valve for controlling the flow of working gas, the refrigerators/liquefiers in parallel using a working gas of the same kind such as pure gaseous helium, each refrigerator/liquefier comprising a working gas compression station, a cold box intended to cool a flow of working gas leaving the compression station to a cryogenic temperature at least close to its liquefaction temperature, said flows of working gas cooled by each of the respective cold boxes of the refrigerators/liquefiers being mixed and then placed in a heat exchange relationship with the application in order to give up frigories thereto, the cold working gas having exchanged heat with the application then being divided into several return flows distributed respectively through the respective compression stations, the apparatus comprising electronic control logic connected to simultaneous measurement means, for measuring, for each of the refrigerators/liquefiers, the instantaneous value of at least one and the same operating parameter from: a flow rate of what is referred to as a “return” flow of working gas returning to the compression station, a flow rate of what is referred to as an “outbound” flow of working gas circulating through the cold box after having left the cold box, a differential in temperature of the working gas between, on the one hand, an outbound flow of working gas within the cold box and, on the other hand, the return flow of working gas in the cold box, the electronic logic being configured for real-time calculation of the dynamic mean value of the at least one operating parameter for all the refrigerators/liquefiers, and to perform real-time control of the at least one control valve controlling the flow of working gas from at least one refrigerator/liquefier according to the difference between the instantaneous values of the parameter compared with said dynamic mean value in order to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers to converge toward this dynamic mean value. 
         [0034]    The invention also relates to any alternative device or method comprising any combination of the features above or below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    Further specifics and advantages will become apparent from reading the following description, given with reference to the figures in which: 
           [0036]      FIG. 1  depicts a schematic and partial view illustrating one example of the structure and operation of an apparatus able to implement the invention, 
           [0037]      FIG. 2  depicts a schematic and partial view of a detail of the apparatus of  FIG. 1 , illustrating an example of the structure and operation of part of the compression stations and of the cold boxes of the refrigerators/liquefiers of the apparatus, 
           [0038]      FIG. 3  depicts a schematic and partial view of a detail of the apparatus of  FIG. 1 , illustrating one example of the structure and operation of part of the working circuit at the outlet of the compression stations, 
           [0039]      FIG. 4  depicts a schematic and partial view of a detail of the apparatus of  FIG. 1 , illustrating one example of the structure and operation of part of the working circuit at the level of the liquefied working gas storage reservoirs, 
           [0040]      FIG. 5  depicts a schematic and partial view of a detail of the apparatus of  FIG. 1 , illustrating one example of the structure and operation of part of the working circuit at a bypass pipe bypassing cooling exchangers of the cold box, 
           [0041]      FIG. 6  depicts a partial and schematic view of a detail of the apparatus of  FIG. 1 , illustrating one example of the structure and operation of part of the working circuit at a return pipe returning working gas to the compression station. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0042]      FIG. 1  schematically illustrates a cryogenic refrigeration apparatus comprising three refrigerators/liquefiers (L/R) arranged in parallel to cool one and the same application  1 . Conventionally, each refrigerator/liquefier L/R comprises a working circuit for a working gas which is equipped with at least one working gas flow control valve. 
         [0043]    Each refrigerator/liquefier comprises its own station  2  for compressing the working gas and its own cold box  3  intended to cool the flow  30  of working gas leaving the compression station  2  to a cryogenic temperature at least close to its liquefaction temperature. 
         [0044]    The flows  30  of working gas cooled by each of the respective cold boxes  3  of the refrigerators/liquefiers L, R are mixed and then placed in a heat exchange relationship with the application  1  in order to give up frigories thereto. The cold working gas having exchanged heat with the application  1  is then split into several return flows  31  distributed respectively across the compression stations  2 . 
         [0045]    The parallel refrigerators/liquefiers L/R use a working gas of the same nature such as pure gaseous helium. 
         [0046]    The apparatus  100  preferably comprises electronic control logic  50  comprising for example a microprocessor (a computer and/or controller). The electronic logic  50  is connected to measurement members for simultaneous measurement, for each of the refrigerators/liquefiers L/R, of the instantaneous value of at least one and the same operating parameter regarding the working gas in the working cycle of each of the refrigerators/liquefiers L/R. For the sake of simplicity,  FIG. 1  does not depict these measurement members (examples thereof will be illustrated in  FIGS. 2 to 6 ). 
         [0047]    The at least one operating parameter measured for each refrigerator/liquefier L/R preferably comprises at least one out of: a flow rate of the return flow of working gas returning to the compression station (after exchanging heat with the application or a return flow of working gas returning directly to the compression station without passing via the application  1  or certain parts of the cold box  3 ), a flow rate of the flow of cooled working gas at the outlet of the cold box (after having left the compression station), a differential in temperature of the working gas between, on the one hand, the flow of working gas in the cold box (heading toward the application) and, on the other hand, the return flow of working gas returning to the compression station (from the application). 
         [0048]    The electronic logic  50  is configured (for example programmed) to perform real-time calculation of the dynamic mean value of the at least one operating parameter for all the refrigerators/liquefiers L/R and for performing real-time control of the at least one working-gas flow control valve of at least one refrigerator/liquefier L/R as a function of the difference between the instantaneous values of the parameter with respect to said dynamic mean value. More specifically, the electronic logic is configured to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers R/L to converge toward this dynamic mean value. 
         [0049]    What that means to say is that each refrigerator/liquefier L/R is controlled in its working cycle as a function of an operating mean of the whole set of refrigerators/liquefiers L/R, so as to cause all the refrigerators/liquefiers L/R to converge toward this mean. 
         [0050]    This adjustment may be implemented via controllers of the “proportional integral” (PI) type for controlling the working-gas circuits. 
         [0051]    For preference, the apparatus performs real-time control of the at least one working-gas flow control valve of at least one refrigerator/liquefier (L/R) as a function of the difference between the instantaneous values of the parameter with respect to said dynamic mean value, so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers R/L to converge toward this dynamic mean value. 
         [0052]    Various examples of the control of the apparatus will be described with reference to  FIGS. 2 to 6  respectively. All or some of these various examples may be implemented cumulatively or alternatively in order to adjust the operation of such an apparatus  100 . 
         [0053]    As partially illustrated in  FIG. 2 , the compression station  2  of each refrigerator/liquefier may comprise two compressors  12 ,  22  arranged in series on the working circuit and referred to respectively as the “low-pressure compressor”  12  and the “medium-pressure compressor”  12 . The low-pressure compressor  12  receives the relatively hot working gas returning at low pressure (return flow  31 ) having passed or not passed through the cold box  3 . 
         [0054]    Each compression station  2  comprises a bypass circuit  14  for selectively bypassing the low-pressure compressor  12  and which is equipped with a variable-opening controlled bypass valve  4 . 
         [0055]    The apparatus comprises, for each of the refrigerators/liquefiers L/R, a sensor  13  for measuring the operating parameter consisting of the instantaneous value of the flow rate Q of the return flow  31  of working gas returning to the compression station  2 . This measurement sensor  3  is, for example, situated within the cold box  3 , upstream of one or more exchangers  26  which both cool toward the working gas toward the application and heat the working gas returning toward the compression station  2 . 
         [0056]    The electronic logic  50  may perform real-time calculation of the dynamic mean value of this operating parameter for all the refrigerators/liquefiers L/R. The electronic logic  50  performs real-time control of the opening/closing of each bypass valve  14  as a function of the difference between the instantaneous values of the operating parameter of the refrigerator/liquefier concerned so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers R/L to converge toward this dynamic mean value. 
         [0057]    For example, the opening/closing of each bypass valve  14  is controlled according to a pressure set point CP according to a formula of the type CP=A-B.ΔQ, where A is a predetermined pressure value, B is a predetermined coefficient (dimensions=pressure/flow rate) and ΔQ is the differential (dimensions=flow rate) between, on the one hand, the dynamic mean value of the flow rate of the three coolers and, on the other hand, the instantaneous flow rate of the refrigerator/liquefier concerned. 
         [0058]    In addition, each refrigerator/liquefier L/R may comprise a sensor  15  for measuring the temperature differential DT=T 31 -T 32  of the working gas between the return flow  31  (returning to the compression station) and the “outbound” flow  32  (toward the application  1 ) which are situated in the cold box ( 3 ) in a part of the circuit that has one and the same determined temperature range. 
         [0059]    The expression “one and the same temperature range in the cold box” means points on the working circuit at which the outbound flow  32  (toward the application that is to be cooled  1 ) and return flow  31  (toward the compression station  2 ) are situated at the same level with respect to the cooling exchangers of the cold box  3  (for example, the two measurement points are situated in legs of the circuit which are situated between two same cooling exchangers). What that means to say is that the two points on the circuit have relatively similar temperatures, for example differing by just a few degrees Kelvin (typically between 0.1 and 4° K. of difference). 
         [0060]    The outbound flow  32  is, for example, the flow of working gas leaving a cooling exchanger of the cold box (for example at the outlet of the first heat exchanger which cools the working gas after it has passed through the compression station  2 ). The return flow  31  in the same temperature range is the part of the working circuit in which the working gas returns toward the compression station  2  before entering this same heat exchanger. According to one advantageous feature, the control of each bypass valve  14  may be corrected as a function of the discrepancy between said temperature differential DT=T 31 -T 32  for the refrigerator/liquefier L/R concerned with respect to the mean of said temperature differential DT=T 31 -T 32  calculated for all of the refrigerators/liquefiers L/R. This temperature differential DT=T 31 -T 32  is indicative of the imbalance in the flow rates of working gas between the return flow  31  (toward the compression station) and the outbound flow  32  (toward the application  1 ). 
         [0061]    For example, the opening of each bypass valve  14  may be increased when the temperature differential DT=T 31 -T 32  for the refrigerator/liquefier L/R concerned increases (in terms of absolute value) with respect to the mean of said temperature differential. This control will have the effect of reducing the imbalance in the flow rates of the working gas between the return flow  31  (toward the compression station) and the outbound flow  32  (toward the application  1 ). 
         [0062]    As illustrated schematically in  FIG. 3 , at the outlet of the compression station  2 , each refrigerator/liquefier L/R may, on the outlet pipe  30 , comprise a variable-opening controlled outlet valve  11 . 
         [0063]    In addition, each refrigerator/liquefier L/R may comprise a measurement sensor  16  for measuring the operating parameter consisting of the instantaneous value of the flow rate of the flow  30  of gas at the outlet of the compression station  2 . 
         [0064]    As previously, the electronic logic  50  may be configured to perform real-time calculation of the dynamic mean of this operating parameter for all the refrigerators/liquefiers L/R. The electronic logic  50  may perform real-time control of the opening/closing of each outlet valve  11  according to the difference between the instantaneous values of the operating parameter of the refrigerator/liquefier concerned so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers R/L to converge toward this dynamic mean value. 
         [0065]    For example, the opening/closing of each outlet valve  11  is controlled according to a pressure set point CP according to a formula of the type CP=C+D.ΔQ, where B is a predetermined pressure value, C is a predetermined coefficient (dimensions=pressure/flow rate) and ΔQ is the differential (dimensions=flow rate) between, on the one hand, the dynamic mean value of this flow rate for the three coolers and, on the other hand, this instantaneous flow rate for the refrigerator/liquefier concerned. 
         [0066]    As illustrated in  FIG. 4 , the working circuit of each refrigerator/liquefier may, in the cold box  3 , comprise a main pipe  19  comprising an exchanger  20  for cooling the working gas which is immersed in a cryogenic tank  21  of liquefied working gas and a secondary pipe  23  forming a bypass of the main pipe upstream of the cryogenic tank  21 . The secondary pipe  23  opens into this tank  21  into which it delivers the liquefied working gas produced by the cold box  3 . 
         [0067]    Each main pipe  19  comprises a variable-opening controlled downstream valve  5  situated downstream of the cooling exchanger  20 . Each apparatus comprises a sensor  24  of the operating parameter consisting of the instantaneous value of the flow rate of the flow of working gas in said main pipe  23  downstream of the flow cooling exchanger  20 . 
         [0068]    The electronic logic  50  may be configured to perform real-time calculation of the dynamic mean value of this operating parameter for all the refrigerators/liquefiers L/R and to perform real-time control of the opening/closing of each downstream valve  5  as a function of the difference between the instantaneous values of this operating parameter of the refrigerator/liquefier concerned so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers R/L to converge toward this dynamic mean value. 
         [0069]    For example, the secondary pipe  23  is equipped with a variable-opening distribution valve  25 , the opening of which is increased in the event of increased production of liquefied working gas in the cold box  3 . In addition, control of each downstream valve  5  may be corrected according to the degree of opening of the distribution valve  25  so as to reduce the opening of the downstream valve  5  when the opening of the distribution valve  25  increases, and vice versa. 
         [0070]    As illustrated in  FIG. 5 , the cold box  3  of each refrigerator/liquefier L/R may comprise a plurality of heat exchangers  26  for cooling the working fluid and a bypass pipe  27  bypassing at least some of said exchangers  26 . This bypass pipe  27  bypassing the exchangers  26  provides downstream working gas leaving the cold box  3 . 
         [0071]    As depicted, the bypass pipe  27  is connected to several portions of the working circuit in a heat exchange relationship with the exchangers  26  via respective controlled bypass valves  6 ,  7 ,  8  (valves with variable opening). 
         [0072]    Each refrigerator/liquefier may comprise a measurement sensor  28  for measuring the operating parameter consisting of the instantaneous value of the flow rate of the flow of gas in said bypass pipe  27 . The electronic logic  50  may comprise a step of real-time calculation of the dynamic mean value of this operating parameter for all the refrigerators/liquefiers L/R and for the real-time control of the opening/closing of at least one of the bypass valves  6 ,  7 ,  8  as a function of the difference between the instantaneous values and the dynamic mean value of this operating parameter of the refrigerator/liquefier concerned, so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers R/L to converge toward this dynamic mean value. 
         [0073]    For example, the opening/closing of the bypass valve  7  is controlled according to a pressure set point CP according to a formula of the type CP=G+H.ΔQ, where G is a predetermined pressure value, G is a predetermined coefficient (dimensions=pressure/flow rate) and ΔQ is the differential (dimensions=flow rate) between, on the one hand, the dynamic mean value of this flow rate for the three coolers and, on the other hand, this instantaneous flow rate for the refrigerator/liquefier concerned. The other bypass valves  6 ,  8  allow adjustment of the temperature of the circuit for the refrigerator/liquefier concerned. As illustrated in  FIG. 6 , the working circuit may, in the cold box  3  of each refrigerator/liquefier L/R, comprise a plurality of exchangers  26  for warming up the cold working fluid that has exchanged heat with the application  1 . The working circuit additionally comprises a return pipe  29  for the flow  30  of working gas returning to the compression station  2 , the return pipe  29  comprising a portion that is subdivided into two parallel legs  129 ,  229  respectively referred to as the “hot” and “cold” leg. The hot leg  129  does not exchange heat with at least part of the heating heat exchangers  26 . The cold leg  229  itself exchanges heat with several warming up exchangers. The working fluid that has exchanged heat with the application returns to the compression station  2  and is distributed into the hot leg  129  when its temperature is above a determined threshold or into the cold leg  229  when its temperature is below the determined threshold. Each hot leg  129  comprises a variable-opening controlled regulating valve  9 . 
         [0074]    Each cold box  3  comprises a measurement sensor  130  for measuring the operating parameter consisting of the instantaneous value of the flow rate of the flow of gas in said hot leg  129 . 
         [0075]    The electronic logic  50  may be configured to perform real-time calculation of the dynamic mean value of this operating parameter for all the refrigerators/liquefiers and to perform real-time control of the opening/closing of the valve  9  of the hot leg  129  as a function of the difference between the instantaneous values and the dynamic mean value of this operating parameter of the refrigerator/liquefier concerned, so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers to converge toward this dynamic mean value. 
         [0076]    For example, the opening/closing of each valve  9  of the hot leg is controlled according to a pressure set point CP according to a formula of the type CP=I+J.ΔQ, where I is a predetermined pressure value, J is a predetermined coefficient (dimensions=pressure/flow rate) and ΔQ is the differential (dimensions=flow rate) between, on the one hand, the dynamic mean value of this flow rate for the three coolers and, on the other hand, this instantaneous flow rate for the refrigerator/liquefier concerned. 
         [0077]    Similarly, each cold leg  229  comprises a variable-opening controlled regulating valve  10  and a measurement sensor  131  for measuring the operating parameter consisting of the instantaneous value of the flow rate of the flow of gas in said leg  229 . The electronic logic  50  may be configured to perform real-time calculation of the dynamic mean value of this operating parameter for all the refrigerators/liquefiers and to perform real-time control of the opening/closing of the valve  10  of the cold leg  229  as a function of the difference between the instantaneous values and the dynamic mean value of this operating parameter of the refrigerator/liquefier concerned, so as to cause said instantaneous values of said operating parameter of the various refrigerators/liquefiers R/L to converge toward this dynamic mean value. 
         [0078]    As before, the opening/closing of each valve  10  of the cold leg may be controlled according to a pressure set point CP according to a formula of the type CP=K+L.ΔQ, where K is a predetermined pressure value, L is a predetermined coefficient (dimensions=pressure/flow rate) and ΔQ is the differential (dimensions=flow rate) between, on the one hand, the dynamic mean value of this flow rate for the three coolers and, on the other hand, this instantaneous flow rate for the refrigerator/liquefier concerned. 
         [0079]    It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.