Abstract:
A method for cooling a fluid or a body by means of at least a first vapour compression circuit containing a first heat transfer fluid and at least a second vapour compression circuit containing a second heat transfer fluid, the method including: measuring the temperature of the external surroundings; and setting the temperature of the second heat-transfer fluid to evaporation, according to the temperature of the external surroundings. Also, an installation suited to implementing this method.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a U.S. national stage application of International Application No. PCT/FR2013/050034, filed on Jan. 8, 2013, which claims the benefit of French Application No. 12.50746, filed on Jan. 26, 2012. The entire contents of each of International Application No. PCT/FR2013/050034 and French Application No. 12.50746 are hereby incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosure relates to a cascade refrigeration system designed to operate optimally and to a refrigeration process carried out in this system. 
       TECHNICAL BACKGROUND 
       [0003]    Refrigeration systems are generally based on a thermodynamic cycle comprising the vaporization of a fluid at low pressure (in which the fluid absorbs heat); the compression of the vaporized fluid up to a high pressure; the condensation of the vaporized fluid to give a liquid at high pressure (in which the fluid discharges heat); and the reduction in pressure of the fluid in order to complete the cycle. The choice of a heat-transfer fluid (which can be a pure compound or a mixture of compounds) is dictated, on the one hand, by the thermodynamic properties of the fluid and, on the other hand, by additional constraints. Thus, an important criterion is that of the impact of the fluid under consideration on the environment. In particular, chlorinated compounds (chlorofluorocarbons and hydrochlorofluorocarbons) exhibit the disadvantage of damaging the ozone layer. Thus, nonchlorinated compounds, such as hydrofluorocarbons, fluoroethers and fluoroolefins, are from now on generally preferred to them. 
         [0004]    Another environmental constraint is that of the global warming potential (GWP). It is thus essential to develop heat-transfer compositions exhibiting a GWP which is as low as possible and good energy performances. 
         [0005]    Some specific refrigeration systems are based on the use of several refrigeration circuits and in particular on two circuits coupled together, namely a high-temperature circuit and a low-temperature circuit: these systems are said to be “cascade” systems. The two circuits generally comprise different heat-transfer fluids. 
         [0006]    A cascade system exhibits a number of advantages in terms of safety. In particular, it is possible to use, for reasons of cost or performance, a certain heat-transfer fluid in the high-temperature circuit and to use another heat-transfer fluid, which is less flammable or less toxic, in the low-temperature circuit. Thus, the total charge of the most flammable or most toxic heat-transfer fluid is minimized and this most flammable or most toxic heat-transfer fluid is restricted to an unconfined region and/or to a region without risk of contact with the public or personnel in the event of escape. 
         [0007]    For example, carbon dioxide is a highly advantageous heat-transfer fluid due to its nonflammability, as well as from the environmental viewpoint. However, due to its low critical point, it is generally less effective than a conventional heat-transfer fluid (hydrocarbon, hydrofluorocarbon, and the like). An optimal solution may consist in using a cascade system comprising carbon dioxide in the low-temperature circuit and a conventional heat-transfer fluid in the high-temperature circuit. 
         [0008]    The documents WO 2008/150289 and WO 2011/056824 provide examples of cascade refrigeration systems. 
         [0009]    The paper  Theoretical analysis of a CO   2 - NH   3    cascade refrigeration system for cooling applications at low temperatures , by Dopazo et al. in  Applied Thermal Engineering,  29, 1577-1583 (2009), and also the paper  Experimental investigation on the performances of NH   3   /CO   2    cascade refrigeration system with twin - screw compressor , by Bingming et al. in  International Journal of Refrigeration,  32, 1358-1365 (2009), describe the performance of a cascade system using carbon dioxide in the low-temperature circuit and ammonia in the high-temperature circuit. 
         [0010]    However, a need still exists to improve the efficiency and the performance of cascade refrigeration systems and in particular a need still exists to minimize the overall energy consumption of these systems and also the associated environmental impact. 
       SUMMARY OF THE INVENTION 
       [0011]    The disclosure relates first to a process for cooling a fluid or a body by means of at least one first vapor compression circuit comprising a first heat-transfer fluid and of at least one second vapor compression circuit comprising a second heat-transfer fluid, the process comprising:
       in the first vapor compression circuit:
           the at least partial evaporation of the first heat-transfer fluid by exchange of heat with said fluid or body;   the compression of the first heat-transfer fluid;   the at least partial condensation of the first heat-transfer fluid by exchange of heat with the second heat-transfer fluid;   the reduction in pressure of the first heat-transfer fluid;   
           in the second vapor compression circuit:
           the at least partial evaporation of the second heat-transfer fluid by exchange of heat with the first heat-transfer fluid;   the compression of the second heat-transfer fluid;   the at least partial condensation of the second heat-transfer fluid by exchange of heat with an external medium;   the reduction in pressure of the second heat-transfer fluid;   
               
 
         [0022]    the process additionally comprising:
       the measurement of the temperature of the external medium; and   the adjustment of the temperature of the second heat-transfer fluid at the evaporation, as a function of the temperature of the external medium.       
 
         [0025]    According to an embodiment, the adjustment of the temperature of the second heat-transfer fluid at the evaporation is carried out continuously or is carried out at least once per hour. 
         [0026]    According to one embodiment, the process comprises the detection of variations in the temperature of the external medium and the adjustment of the temperature of the second heat-transfer fluid at the evaporation comprises an increase in the temperature of the second heat-transfer fluid at the evaporation if an increase in the temperature of the external medium is detected and a decrease in the temperature of the second heat-transfer fluid at the evaporation if a decrease in the temperature of the external medium is detected. 
         [0027]    According to one embodiment, the process comprises the calculation of an optimum evaporation temperature as a function of the measurement of the temperature of the external medium. 
         [0028]    According to one embodiment, the temperature of the second heat-transfer fluid at the evaporation is adjusted to the optimum evaporation temperature. 
         [0029]    According to one embodiment, the optimum evaporation temperature corresponds to the evaporation temperature for which the overall coefficient of performance of the first vapor compression circuit and of the second vapor compression circuit is at a maximum. 
         [0030]    According to one embodiment, the optimum evaporation temperature is defined by the formula T opt =A×T ext +B, in which T ext  is the temperature of the external medium in degrees Celsius, A is a dimensionless constant and B is a constant in degrees Celsius. 
         [0031]    According to one embodiment, the constant A has a value from 0.3 to 0.6, preferably from 0.4 to 0.45, and the constant B has a value from −50° C. to 0° C., preferably from −30° C. to −20° C. 
         [0032]    According to one embodiment, the fluid or body is cooled to a temperature of −50 to −15° C., preferably of −40 to −25° C. 
         [0033]    According to one embodiment:
       the first heat-transfer fluid is chosen from carbon dioxide, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and the mixtures of these, and is preferably carbon dioxide; and/or   the second heat-transfer fluid is chosen from ammonia, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and the mixtures of these, is preferably tetrafluoropropene and more particularly preferably is 2,3,3,3-tetrafluoropropene or 1,3,3,3-tetrafluoropropene.       
 
         [0036]    According to one embodiment, the compression of the second heat-transfer fluid is carried out by one or more compressors and the adjusting of the temperature of the second heat-transfer fluid at the evaporation is carried out by regulating said compressors. 
         [0037]    According to one embodiment, the regulating of said compressors comprises an adjustment of the speed of rotation of the compressors or is carried out by successively starting up and shutting down the compressors. 
         [0038]    According to one embodiment, the process is a process for cooling a compartment comprising products, preferably foods, which are deep-frozen or frozen. 
         [0039]    The disclosure furthermore relates to an installation for cooling a fluid or a body, comprising at least:
       a first vapor compression circuit comprising a first heat-transfer fluid;   a second vapor compression circuit comprising a second heat-transfer fluid;   a cascade heat exchanger, appropriate for exchanging heat between the first heat-transfer fluid and the second heat-transfer fluid;       
 
         [0043]    the first vapor compression circuit comprising:
       a first evaporator appropriate for exchanging heat between the first heat-transfer fluid and said fluid or body;   one or more first compressors;   a first expansion device;       
 
         [0047]    the second vapor compression circuit comprising:
       one or more second compressors;   a second condenser appropriate for exchanging heat between the second heat-transfer fluid and an external medium;   a second expansion device;       
 
         [0051]    the installation also comprising:
       a device for measuring the temperature of the external medium; and   means for adjusting the evaporation temperature in the cascade heat exchanger, as a function of the measurement of the temperature of the external medium.       
 
         [0054]    According to one embodiment, the installation additionally comprises a module for calculating an optimum evaporation temperature as a function of the measurement of the temperature of the external medium. 
         [0055]    According to one embodiment, the means for adjusting the evaporation temperature in the cascade heat exchanger are appropriate for adjusting the evaporation temperature in the cascade heat exchanger to the optimum evaporation temperature. 
         [0056]    According to one embodiment, the optimum evaporation temperature corresponds to the evaporation temperature for which the overall coefficient of performance of the first vapor compression circuit and of the second vapor compression circuit is at a maximum. 
         [0057]    According to one embodiment, the optimum evaporation temperature is defined by the formula T opt =A×T ext +B, in which T ext  is the temperature of the external medium in degrees Celsius, A is a dimensionless constant and B is a constant in degrees Celsius. 
         [0058]    According to one embodiment, the constant A has a value from 0.3 to 0.6, preferably from 0.4 to 0.45, and the constant B has a value from −50° C. to 0° C., preferably from −30° C. to −20° C. 
         [0059]    According to one embodiment, the installation is appropriate for cooling the body or the fluid to a temperature of −50 to −15° C., preferably of −40 to −25° C. 
         [0060]    According to one embodiment:
       the first heat-transfer fluid is chosen from carbon dioxide, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and the mixtures of these, and is preferably carbon dioxide; and/or   the second heat-transfer fluid is chosen from ammonia, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and the mixtures of these, is preferably tetrafluoropropene and more particularly preferably is 2,3,3,3-tetrafluoropropene or 1,3,3,3-tetrafluoropropene.       
 
         [0063]    According to one embodiment, the means for adjusting the evaporation temperature in the cascade heat exchanger comprise means for regulating the second compressors. 
         [0064]    According to one embodiment, the means for regulating the second compressors are appropriate for adjusting the speed of rotation of the second compressors or are appropriate for successively starting up and shutting down the second compressors. 
         [0065]    According to one embodiment, the installation comprises a compartment appropriate for receiving products, preferably foods, which are deep-frozen or frozen. 
         [0066]    The disclosure makes it possible to meet the needs felt in the state of the art. More particularly, it provides refrigeration processes and corresponding installations in which the overall energy consumption and the environmental impact are minimized. 
         [0067]    This is accomplished by adjusting the evaporation temperature of the heat-transfer fluid of the high-temperature circuit as a function of the external temperature (ambient temperature). It has been discovered that such an adjustment makes it possible to optimize the overall performance of the system. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0068]      FIG. 1  is a diagram of an installation according to an embodiment of the disclosure. 
           [0069]      FIG. 2  is a graph representing: (1) the change in ambient temperature during a typical day taken as an example (white circles, left-hand axis of the ordinates, values in degrees Celsius); and (2) an example of conventional change in the refrigerating capacity necessary for the preservation of deep-frozen foods during this typical day (black squares, right-hand axis of the ordinates, values in kW); this being as a function of the hours of the day (axis of the abscissae). 
           [0070]      FIG. 3  is a graph illustrating the optimum evaporation temperature (in degrees Celsius, axis of the ordinates) as a function of ambient temperature (in degrees Celsius, axis of the abscissae) for a cascade refrigeration system in which the heat-transfer fluid of the high-temperature circuit is: (1) HFO-1234yf (white squares); or (2) HFO-1234ze (black circles). 
           [0071]      FIG. 4  is a graph illustrating the total energy consumption of a refrigeration system during a typical day, in kWh, according to whether the refrigeration system is according to an embodiment of the disclosure (gray bars, evaporation temperature of the high-temperature heat-transfer fluid adjusted as a function of ambient temperature) or is a conventional system (black bars, evaporation temperature of the high-temperature heat-transfer fluid set at −10° C.). The two series of data correspond to the case where (1) the heat-transfer fluid of the high-temperature circuit is HFO-1234yf, and (2) the heat-transfer fluid of the high-temperature circuit is HFO-1234ze. 
           [0072]      FIG. 5  is a graph illustrating the TEWI index of a cascade refrigeration system over a typical day in various scenarios: conventional refrigeration system and HFO-1234yf in the high-temperature circuit (R1234yf bar); refrigeration system according to an embodiment of the disclosure and HFO-1234yf in the high-temperature circuit (opti R1234yf bar); conventional refrigeration system and HFO-1234ze in the high-temperature circuit (R1234ze bar); refrigeration system according to an embodiment of the disclosure and HFO-1234ze in the high-temperature circuit (opti R1234ze bar). The values correspond to the percentage of TEWI index with respect to the reference situation (conventional refrigeration system and HFO-1234yf in the high-temperature circuit). The conventional system is a system in which the evaporation temperature of the high-temperature heat-transfer fluid is set at −10° C., and the system according to an embodiment of the disclosure is a system in which the evaporation temperature of the high-temperature heat-transfer fluid is adjusted as a function of ambient temperature. 
           [0073]      FIG. 6  is a graph equivalent to that of  FIG. 4 , but with a conventional system where the evaporation temperature of the high-temperature heat-transfer fluid is set at −18° C. 
           [0074]      FIG. 7  is a graph equivalent to that of  FIG. 5 , but with a conventional system where the evaporation temperature of the high-temperature heat-transfer fluid is set at −18° C. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0075]    The disclosure is now described in more detail and without implied limitation in the description which follows. 
         [0076]    The term “heat-transfer compound” or “heat-transfer fluid” (or refrigerant) respectively is understood to mean a compound or a fluid respectively capable of absorbing heat by evaporating at low temperature and low pressure and of discharging heat by condensing at high temperature and high pressure, in a vapor compression circuit. Generally, a heat-transfer fluid can comprise just one, two, three or more than three heat-transfer compounds. 
         [0077]    The term “heat-transfer composition” is understood to mean a composition comprising a heat-transfer fluid and optionally one or more additives which are not heat-transfer compounds for the application envisaged. 
         [0078]    The disclosure is targeted at installations for cooling of fluid or a body, and at associated cooling processes. These installations can be stationary or mobile air conditioning installations or, preferably, stationary or mobile refrigeration and/or freezing and/or cryogenic installations. 
         [0079]    With reference to  FIG. 1 , according to one embodiment, the installation according to an embodiment of the disclosure comprises a first vapor compression circuit  10  (or low-temperature circuit), which comprises a first heat-transfer fluid, and a second vapor compression circuit  20  (or high-temperature circuit), which comprises a second heat-transfer fluid. A cascade heat exchanger  30  (or evaporator-condenser or refrigerant-to-refrigerant heat exchanger) provides the thermal coupling between the two vapor compression circuits. 
         [0080]    The first vapor compression circuit  10  comprises at least one first evaporator  11 , at least one first compressor  12  and at least one first expansion device  14 . Between the first compressor  12  and the first expansion device  14 , the circuit passes through the cascade heat exchanger  30 , which acts as condenser for this first circuit (first condenser). 
         [0081]    Fluid transportation lines are provided between all the components of the circuit. 
         [0082]    The vapor compression circuit  10  operates according to a conventional vapor compression cycle. The cycle comprises a change in state of the first heat-transfer fluid from a liquid phase (or liquid/vapor two-phase system) to a vapor phase at a relatively low pressure (in the first evaporator  11 ), then the compression of the fluid in the vapor phase up to a relatively high pressure (in the first compressor  12 ), the change in state (condensation) of the heat-transfer fluid from the vapor phase to the liquid phase at a relatively high pressure (in the cascade heat exchanger  30 ), and the reduction in the pressure in order to recommence the cycle (in the first expansion device  14 ). 
         [0083]    The second vapor compression circuit  20  comprises at least one second compressor  22   a ,  22   b ,  22   c , at least one second condenser  23  and at least one second expansion device  24 . 
         [0084]    Between the second expansion device  24  and the second compressor  22   a ,  22   b ,  22   c , the circuit passes through the cascade heat exchanger  30 , which acts as evaporator for this second circuit (second evaporator). 
         [0085]    Fluid transportation lines are provided between all the components of the circuit. 
         [0086]    The second vapor compression system  20  operates analogously to the first. 
         [0087]    It is possible to provide an accumulator  27  in the circuit in order to form a reserve of fluid in the liquid state. The level of the liquid in the accumulator varies according to the requirement of the installation as a function of the conditions of use. 
         [0088]    The first heat-transfer fluid receives heat from the part of the fluid or body to be cooled in the first evaporator  11 . For example, when the body to be cooled consists of one or more frozen or deep-frozen products (in particular foodstuffs), this body can be placed in a compartment, at least a portion of the walls of which is in direct contact with the first evaporator  11  (or at least a part of the walls of which belongs to the first evaporator  11 ). 
         [0089]    Alternatively, the exchange of heat between the fluid or body to be cooled and the first heat-transfer fluid can be carried out via an auxiliary circuit comprising a heat-exchange fluid, such as air or else a glycol compound, for example (with or without change in state). 
         [0090]    The first heat-transfer fluid gives up, in turn, heat to the second heat-transfer fluid, in the cascade heat exchanger  30  which provides the coupling between the two circuits. The transfer of heat from the first heat-transfer fluid to the second heat-transfer fluid brings about, on the one hand, the condensation of the first heat-transfer fluid and, on the other hand, the evaporation of the second heat-transfer fluid. 
         [0091]    Finally, the second condenser  23  allows the second heat-transfer fluid to give up heat to the external medium. The external medium is preferably the surrounding air. 
         [0092]    The exchange of heat between the second heat-transfer fluid and the external medium can be carried out either directly or via an auxiliary circuit of heat-exchange fluid (with or without change in state). 
         [0093]    Use may in particular be made, as compressors, in the abovementioned circuits, of single-stage or multistage centrifugal compressors or of centrifugal minicompressors. Rotary, piston or screw compressors can also be used. The compressors can be driven by an electric motor or by a gas turbine (for example fed by the exhaust gases from a vehicle, for mobile applications) or by gears. 
         [0094]    Use may be made, as heat exchangers for the implementation of the disclosure, of cocurrentwise heat exchangers or, preferably, countercurrentwise heat exchangers. It is also possible to use microchannel exchangers. 
         [0095]    Each item of equipment (condenser, expansion device, evaporator, compressor) can consist of one unit or of several units arranged in series and/or in parallel. When several units in parallel are used, as is the case for the second compressors  22   a ,  22   b ,  22   c  in  FIG. 1 , a distributor  25  and a collector  26  are provided, if necessary, in order to distribute the fluid into the various units and to collect the fluid resulting from the various units. 
         [0096]    It is also possible to provide several first vapor-compression (low temperature) circuits coupled to a single second vapor compression (high temperature) circuit or also several second vapor compression (high temperature) circuits coupled to a single first vapor compression (low temperature) circuit. 
         [0097]    The first heat-transfer fluid is preferably chosen from carbon dioxide, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and the mixtures of these. It can in particular be carbon dioxide. 
         [0098]    The second heat-transfer fluid is preferably chosen from ammonia, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and the mixtures of these. It can in particular be tetrafluoropropene and more particularly preferably 2,3,3,3-tetrafluoropropene (HFO-1234yf) or 1,3,3,3-tetrafluoropropene (HFO-1234ze), in the cis or trans form or in the form of a mixture of cis and trans forms. 
         [0099]    According to one embodiment, the first heat-transfer fluid is carbon dioxide and the second heat-transfer fluid is HFO-1234yf. 
         [0100]    According to another embodiment, the first heat-transfer fluid is carbon dioxide and the second heat-transfer fluid is HFO-1234ze. 
         [0101]    Other possible examples for the second heat-transfer fluid are:
       A mixture of HFO-1234yf and HFC-134a (1,1,1,2-tetrafluoroethane), which is preferably a binary mixture and which preferably comprises from 50% to 65% of HFO-1234yf and ideally approximately 56% of HFO-1234yf.   A mixture of HFO-1234ze and HFC-134a, which is preferably a binary mixture and which preferably comprises from 50% to 65% of HFO-1234ze and ideally approximately 58% of HFO-1234ze.   A mixture of HFO-1234yf and HFO-1234ze, which is preferably a binary mixture and which preferably comprises from 35% to 65% of HFO-1234yf and ideally approximately 50% of HFO-1234yf.   A mixture of HFO-1234yf, HFO-1234ze and HFC-134a, which is preferably a ternary mixture and which preferably comprises from 40% to 45% of HFC-134a, from 35% to 50% of HFO-1234ze and from 5% to 25% of HFO-1234yf.   A mixture of HFO-1234yf and ammonia, which is preferably a binary mixture and which preferably comprises from 15% to 30% of ammonia.   A mixture of HFO-1234yf, HFC-152a (1,1-difluoroethane) and HFC-134a, which is preferably a ternary mixture and which preferably comprises from 2% to 15% of HFC-134a, from 2% to 20% of HFC-152a and from 65% to 96% of HFO-1234yf.   A mixture of HFO-1234ze, HFC-134a and HFO-1336mzz (1,1,1,4,4,4-hexa-fluorobut-2-ene), which is preferably a ternary mixture.       
 
         [0109]    Within the above ranges, the proportions of the different compounds are proportions by weight. 
         [0110]    Various additives can be added to the heat-transfer fluids in the context of the disclosure in the vapor compression circuits. They can in particular be lubricants, stabilizing agents, surfactants, tracers, fluorescent agents, odorous agents and solubilizing agents. 
         [0111]    The stabilizing agent or agents, when they are present, preferably represent at most 5% by weight in the heat-transfer composition. Mention may in particular be made, among the stabilizing agents, of nitromethane, ascorbic acid, terephthalic acid, azoles, such as tolutriazole or benzotriazole, phenolic compounds, such as tocopherol, hydroquinone, t-butylhydroquinone or 2,6-di-(tert-butyl)-4-methylphenol, epoxides (alkyl, optionally fluorinated or perfluorinated, or alkenyl or aromatic), such as n-butyl glycidyl ether, hexanediol diglycidyl ether, allyl glycidyl ether or butylphenyl glycidyl ether, phosphites, phosphonates, thiols and lactones. 
         [0112]    Mention may be made, as tracers (agents capable of being detected), of deuterated or nondeuterated hydrofluorocarbons, deuterated hydrocarbons, perfluorocarbons, fluoroethers, brominated compounds, iodinated compounds, alcohols, aldehydes, ketones, nitrous oxide and the combinations of these. The tracer is different from the heat-transfer compound or compounds making up the heat-transfer fluid. 
         [0113]    Mention may be made, as solubilizing agents, of hydrocarbons, dimethyl ether, polyoxyalkylene ethers, amides, ketones, nitriles, chlorocarbons, esters, lactones, aryl ethers, fluoroethers and 1,1,1-trifluoroalkanes. The solubilizing agent is different from the heat-transfer compound or compounds making up the heat-transfer fluid. 
         [0114]    Mention may be made, as fluorescent agents, of naphthalimides, perylenes, coumarins, anthracenes, phenanthracenes, xanthenes, thioxanthenes, naphthoxanthenes, fluoresceins and the derivatives and combinations of these. 
         [0115]    Mention may be made, as odorous agents, of alkyl acrylates, allyl acrylates, acrylic acids, acryl esters, alkyl ethers, alkyl esters, alkynes, aldehydes, thiols, thioethers, disulfides, allyl isothiocyanates, alkanoic acids, amines, norbornenes, norbornene derivatives, cyclohexene, aromatic heterocyclic compounds, ascaridole, o-methoxy(methyl)phenol and the combinations of these. 
         [0116]    The choice may in particular be made, as lubricants or lubricating oils, of compounds chosen from oils of mineral origin, silicone oils, paraffins of natural origin, naphthenes, synthetic paraffins, alkylbenzenes, poly(α-olefin)s, polyol esters, polyalkylene glycols and/or polyvinyl ethers. Polyol esters and polyvinyl ethers are preferred. Polyalkylene glycols are very particularly preferred. 
         [0117]    Embodiment of the disclosure are very particularly appropriate for fluids or bodies to be cooled to a temperature of −50 to −15° C., preferably of −40 to −25° C. The temperature of the external medium typically varies from −10 to 50° C., in particular from 0 to 40° C. and very particularly from 10 to 35° C. 
         [0118]    The temperature of the evaporation of the first heat-transfer fluid (temperature in the first evaporator  11 ) is preferably from −60 to −20° C., more particularly from −50 to −25° C. 
         [0119]    The temperature at the condensation of the second heat-transfer fluid (temperature in the second condenser  23 ) depends on the external temperature and it is typically from 20 to 60° C., more particularly from 20 to 45° C. It can, for example, be +10° C. with respect to the external temperature. 
         [0120]    The condensation temperature of the first heat-transfer fluid in the cascade heat exchanger  30  depends on the evaporation temperature of the second heat-transfer fluid in this same exchanger. It can, for example, be +5° C. with respect to said evaporation temperature. 
         [0121]    In addition, an embodiment of the disclosure provides a device for measuring the temperature of the external medium  41  and also means for adjusting the evaporation temperature  42  in the cascade heat exchanger  30 , as a function of the temperature of the external medium which is measured. 
         [0122]    It has been found by the inventors that the overall performance of the installation is at an optimum (that is to say, that the energy consumption is at a minimum, for a given cooling temperature of the fluid or body to be cooled) when the temperature of the second heat-transfer fluid in the cascade heat exchanger  30  is adjusted as a function of the external temperature. The higher the external temperature, the higher the temperature of the second heat-transfer fluid in the cascade heat exchanger  30  has to be, for better effectiveness, and vice versa. 
         [0123]    According to a preferred embodiment, the evaporation temperature in the cascade heat exchanger  30  is adjusted to an optimal evaporation temperature, which is determined by a calculation module, as a function of the temperature of the external medium which is measured. 
         [0124]    The optimum evaporation temperature is preferred defined as being the evaporation temperature in the cascade heat exchanger  30  for which the overall coefficient of performance of the installation is at a maximum and for which the overall energy consumption of the installation is at a minimum (for a given refrigerating capacity and/or for a given cooling temperature of the cooled fluid or body). 
         [0125]    For a given installation, the optimum evaporation temperature can be determined either by directly using the data supplied in example 1 below in connection with  FIG. 3 ; or by carrying out a calculation analogous to that presented in example 1 below, for the installation in question; or also experimentally or empirically, by measuring the energy consumption of the installation for different evaporation temperatures of the high temperature circuit, and by establishing the correlation with respect to the external temperature. 
         [0126]    Means for determining the optimum evaporation temperature can be included in the installation. Alternatively and preferably, the function connecting the optimum evaporation temperature to the external temperature is determined beforehand and then only this function is incorporated in the abovementioned calculation module. 
         [0127]    The evaporation temperature in the cascade heat exchanger  30  can also be adjusted to a different temperature from the optimum evaporation temperature, in order to take into account other constraints. For example, it may be appropriate to limit the possible variations in the evaporation temperature in the cascade heat exchanger  30  to a certain temperature range T 1 -T 2 . In this case, the evaporation temperature in the cascade heat exchanger  30  is adjusted to the optimum evaporation temperature, if the latter belongs to the range T 1 -T 2 , or else it is adjusted to the temperature T 1 , if the optimum evaporation temperature is less than T 1 , and, finally, it is adjusted to the temperature T 2 , if the optimum evaporation temperature is greater than T 2 . 
         [0128]    Many other variations are possible. It is possible in particular to provide a delayed adjustment or a hysteretic adjustment of the evaporation temperature in the cascade heat exchanger  30  as a function of the temperature of the external medium, in order to prevent excessively frequent or excessively sudden adjustments. 
         [0129]    Generally, the optimum evaporation temperature is an increasing function of the temperature of the external medium. Consequently, it is desirable, when an increase in the temperature of the external medium is detected, for the evaporation temperature in the cascade heat exchanger  30  to be increased and, when a decrease in the temperature of the external medium is detected, for the evaporation temperature in the cascade heat exchanger  30  to be reduced. Or, according to another embodiment, the adjustment is such that, for all given temperatures T 1  and T 2  of the external medium with T 2 &gt;T 1 , the evaporation temperature in the cascade heat exchanger  30  is respectively adjusted to temperatures T 1 ′ and T 2 ′ with T 2 ′ greater than or equal to T 1 ′. 
         [0130]    The adjusting of the evaporation temperature in the cascade heat exchanger  30  can be obtained by regulating the second compressors  22   a ,  22   b ,  22   c . For example, the means for adjusting the evaporation temperature  42  in the cascade heat exchanger  30  can comprise means for adjusting the speed of rotation of the second compressors  22   a ,  22   b ,  22   c , or also means for successively starting up and shutting down the second compressors  22   a ,  22   b ,  22   c.    
         [0131]    The adjusting of the evaporation temperature in the cascade heat exchanger  30  can be carried out either continuously or at separate moments and, for example, at regular time intervals (every minute, every 15, 30, 45 or 60 minutes, and the like). The adjusting of the temperature can also be carried out by taking, for reference, a mean of the temperature of the external medium measured over a certain period, for example over 10 minutes, 30 minutes or 1 hour. 
       EXAMPLES 
       [0132]    The following examples illustrate embodiments of the disclosure without limiting it. 
       Example 1 
     Demonstration of an Optimum Evaporation Temperature 
       [0133]      FIG. 2  provides a typical example of the variation in the temperature of the external medium (ambient temperature) over a day, and also a typical example of the refrigerating capacity requirements over this day, in order to refrigerate compartments comprising frozen or deep-frozen products in a store of the supermarket type. 
         [0134]    The refrigeration installation is of the type represented diagrammatically in  FIG. 1 . The low-temperature circuit comprises carbon dioxide and the high-temperature circuit comprises HFO-1234yf or HFO-1234ze. 
         [0135]    For the low-temperature circuit, the evaporation temperature is −40° C., the overheating is 25° C. and the undercooling is 5° C. The compressor is a screw compressor with an isentropic efficiency according to the following equation: η iso =0.00476 τ 2 −0.09238 τ+0.8981, τ being the ratio of pressures (see Thermodynamic analysis of optimal condensing temperature of cascade-condenser in CO 2 /NH 3  cascade refrigeration systems by Tzong-Shring et al. in International Journal of Refrigeration, vol. 29, No. 7, 2006, pp. 1100-1108). 
         [0136]    The condensation temperature is 5° C. greater than the evaporation temperature in the high-temperature circuit. 
         [0137]    As regards the high-temperature circuit, the evaporation temperature is either fixed at a constant value (−10° C. or −18° C.) or is variable as a function of the external temperature. The overheating is 25° C. and the undercooling is 5° C. The compressor is a screw compressor with an isentropic efficiency according to the following equation: η iso =0.00060079 τ 2 −0.03002352 τ+0.90880781 (reference:  ASHRAE  2008  Handbook, HVAC system and equipments , Chapter 37, p. 22, Twin screw compressor,  FIG. 34 ). The condensation temperature is 10° C. greater than the external temperature. 
         [0138]    With the evaporation temperature as parameter (evaporation temperature in the high-temperature stage), the coefficient of performance (COP) is optimized as a function of the ambient temperature. The value of the COP for the entire installation corresponds to the following formula: 
         [0000]    
       
         
           
             
               COP 
               Cascade 
             
             = 
             
               
                 
                   COP 
                   1 
                 
                 · 
                 
                   COP 
                   2 
                 
               
               
                 1 
                 + 
                 
                   COP 
                   1 
                 
                 + 
                 
                   COP 
                   2 
                 
               
             
           
         
       
     
         [0000]    (in which COP 1  and COP 2  are the coefficients of performance of the low-temperature and high-temperature circuits). 
         [0139]    The correlation between the ambient temperature (T ext ) and the optimum evaporation temperature in the high-temperature circuit (T opt ) is visible in  FIG. 3 . 
         [0140]    The tendency equations are very similar for the two refrigerants tested: 
         [0000]      For HFO-1234yf,  T   opt =0.4411 ×T   ext −26.549 (in degrees Celsius).
 
         [0000]      For HFO-1234ze,  T   opt =0.4208 ×T   ext −26.107 (in degrees Celsius).
 
       Example 2 
     Gains Offered by an Embodiment of the Disclosure 
       [0141]    In this example, the optimum evaporation temperature demonstrated in example 1 is used to achieve energy savings. 
         [0142]    Thus, the graph of  FIG. 4  illustrates the comparison between: (1) the overall energy consumption of the installation operating in accordance with an embodiment of the disclosure (in gray), that is to say with an adjustment to the evaporation temperature of the high-temperature circuit to its optimum value as a function of the ambient temperature (as determined in  FIG. 3 ), hour by hour, it being assumed that the temperature changes over the day according to the curve of  FIG. 2 ; and (2) the overall energy consumption of the same installation operating conventionally (in black), with a constant evaporation temperature in the high-temperature circuit equal to −10° C. (this is the value most normally selected). 
         [0143]    The graph of  FIG. 5  illustrates a comparison between the same two situations but with respect to the TEWI (Total Equivalent Warming Impact) index as defined in Annex B to the standard EN 378-1:2008+A1:2010. In the graph, the indices are with respect to a reference of 100 for the installation operating conventionally with HFO-1234yf in the high-temperature circuit. 
         [0144]    The graphs of  FIGS. 6 and 7  are analogous to those of  FIGS. 4 and 5 , except that the installation operating conventionally operates with a constant evaporation temperature in the high-temperature circuit equal to −18° C. instead of −10° C. 
         [0145]    It has also been confirmed that an embodiment of the disclosure makes it possible to correctly anticipate (in particular on the basis of the graph of  FIG. 3 ) the daily energy consumption in the case of ambient temperatures which are either warmer or colder than those of the typical day of  FIG. 2 .