Abstract:
To replace bromotrifluoromethane (R-13B1) in very low temperature refrigeration, the invention proposes to employ a mixture containing, on a mass basis, 25 to 50% of difluoromethane and 50 to 75% of pentafluoroethane. 
     A third component may be incorporated in this mixture.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of refrigeration and its subject-matter is more particularly the use of mixtures of cryogenic fluids of low boiling point which have no or little effect on the environment, in order to replace bromofluorocarbons (BCF) in very low temperature refrigeration systems (evaporation temperature lower than -45° C.). 
     BACKGROUND OF THE INVENTION 
     Chlorofluorocarbons (CFC) or hydrochlorofluorocarbons (HCFC) were widely employed in the field of low and very low temperature. Thus, R-22 (chlorodifluoromethane) and R-502 (azeotrope of chlorodifluoromethane and chloropentafluoroethane) with single- or two-stage systems made it possible to reach temperatures ranging down to -35° C., while R-13 (chlorotrifluoromethane) and R-503 (azeotrope of trifluoromethane and chlorotrifluoromethane) with cascade systems (with two compressors) made it possible to go down to -80° C. 
     In these cases, by virtue of thermodynamic properties close to those of the CFCs, hydrofluorocarbons (HFC) such as HFC-23 (trifluoromethane), HFC-116 (hexafluoroethane), HFC-32 (difluoromethane), HFC-125 (pentafluoroethane), HFC-143a (1,1,1-trifluoroethane) and HFC-134a (1,1,1,2-tetrafluoroethane), pure or mixed, have enabled substitutes to be developed. 
     However, a fluid had been developed in the past for making the transition between the above two configurations. This is a bromofluorocarbon (BFC), namely bromotrifluoromethane (R-13B1) which made it possible, with a single compressor, to obtain temperatures of -60° C. with a single- or two-stage system. 
     Like the CFCs, R-13B1 has been subjected to rules by the Montreal Protocol because of its large effect on ozone, and its production has been prohibited since Jan. 1, 1996. 
     Among the HFCs available at present none exhibits properties close to those of R-13B1, as the table below shows. In particular, none has a boiling point close to that of R-13B1, which would make it possible to go down to temperatures close to -60° C.: 
     
                       TABLE 1______________________________________                      Vapour                            LiquidBoiling    Critical              ΔHv at                      d     d      PressureT          T       1 atm   at 1 atm                            at 25° C.                                   at 25° C.(° C.)      (° C.)              (kJ/kg) (kg/m.sup.3)                            (kg/dm.sup.3)                                   (bar)______________________________________R-13B1 -57.7   67      119   8.72  1.538  16.2HFC-23 -82.1   25.9    238.6 4.68  --     --HFC-116  -78.3   19.7    117.7 9.00  --     --HFC-32 -51.7   78.4    381.5 2.98  0.961  16.9HFC-125  -48.1   66.3    169.7 6.73  1.189  13.8HFC-143a  -47.4   72.9    220.2 4.88  0.927  12.6HFC-134a  -26.4   101.1   215.9 5.28  1.206  6.7HFC-218  -36.7   71.9    104.4 10.25 1.323  8.8HFC-290  -41.6   96.7    427.8 2.40  0.494  9.3______________________________________ 
    
     HFC-116 is the fluid nearest to R-13B1, but its critical point, which is too low (&lt;20° C.), does not allow it to be employed in mono-stage systems with a condensation temperature higher than 30° C. 
     In the case of the other HFCs, which have higher critical temperatures, their thermodynamic properties and, in some cases, their flammability mean that no pure substance can replace R-13B1. 
     DESCRIPTION OF THE INVENTION 
     The objective of the present invention is therefore to provide a solution to the replacement of R-13B1 by means of mixtures of low boiling point which are nonflammable, containing at least two hydrofluorocarbons. 
     It has now been found that mixtures containing, on a mass basis, 50 to 75% of HFC-125 and 25 to 50% of HFC-32 are substitutes capable of replacing R-13B1. 
     The preferred mixtures are those which contain, on a mass basis, from 65 to 75% of HFC-125 and 25 to 35% of HFC-32. Among the mixtures according to the invention a very particularly preferred mixture contains approximately 68% of HFC-125 and 32% of HFC-32. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the replacement of R-13B1 the boiling point is a very important criterion. Inspection of FIG. 1, appended, which gives the boiling temperature of 32/125 mixtures at 1 atm as a function of the content of HFC 32 shows that the 32/125 mixtures according to the invention make it possible to obtain a boiling point lower than -51° C., and therefore fairly close to that of R-13B1. 
     It is seen that, in the case of HFC-32 contents higher than 20%, the boiling temperature remains stable (difference smaller than 1 K). 
     Furthermore, the measurements of flammability of 32/125 mixtures at various temperatures (25 and 100° C.) at atmospheric pressure according to ASTM Standard E 681-85 show that the mixtures are nonflammable in the case of HFC-32 contents lower than 50%. 
     The HFC-32/125 mixtures according to the invention are quasi-azeotropic mixtures, that is to say that they exhibit a small difference in pressure between the bubble and dew points (&lt;1%) and that they have a vapour pressure very close to that of R-13B1. 
     The data for a mixture containing, on a mass basis, approximately 68% of HFC-125 and 32% of HFC-32 are collated in Table II. 
     
                       TABLE II______________________________________    R-13B1Temperature    pressure            Pressure (bar) of the 32/125 mixture(° C.)    (bar)   Bubble     Dew    Difference______________________________________-60° C.    0.908   0.656      0.651  0.005-40° C.    2.198   1.750      1.734  0.016-20° C.    4.566   3.935      3.900  0.035 0° C.    8.452   7.794      7.725  0.069 20° C.    14.344  14.017     13.899 0.118 40° C.    22.826  23.392     23.221 0.171______________________________________ 
    
     The data at 40° C. for 32/125 mixtures of various compositions are collated in Table III, and FIG. 2 shows the pressure differences with R-13B1 at various temperatures. 
     
                       TABLE III______________________________________32/125 Mixture         Bubble pressureweight %      (bar)______________________________________50/50         24.1735/65         23.5532/68         23.3930/70         23.2725/75         22.95R-13B1        22.83______________________________________ 
    
     The vapour pressure at high temperature is an important factor for a cryogenic plant. In fact, in air condensation systems the condensation temperatures can reach 40 to 50° C. However, the existing hardware has been designed for pressures corresponding to those of R-13B1. In these conditions an excessive increase in pressure is not acceptable by the system. The mixtures according to the invention, containing 25 to 50% by weight of HFC-32, make it possible to obtain a low boiling point without giving rise to excessively large delivery pressures. 
     It has furthermore been found that it is possible to add one or two compounds cited in Table I to the 32/125 binary mixtures. In particular, HFC-23 and HFC-143a can be cited, thus forming ternary mixtures for the same application. Table IV below shows that the addition of a third component (X) such as HFC-23 or HFC-143a to the 32/125 mixtures makes it possible to obtain boiling points which are closer still to that of R-13B1 (E=-57.7° C.). 
     
                       TABLE IV______________________________________EXAMPLESHFC-32/HFC-125/X mixture               BoilingX            Weight %   temperature (° C.)______________________________________HFC-23       25/63/12   -59.5&#34;            30/65/5    -55.1&#34;            40/52/8    -56.5&#34;            31.5/58.5/10                   -57.9HFC-143a     22.5/67.5/10                   -51.1&#34;            45/45/10   -52.1&#34;            31.5/58.5/10                   -51.6&#34;            20/60/20   -50.9&#34;            40/40/20   -52.1______________________________________ 
    
    
    
     The following examples illustrate the invention without limiting it. 
     EXAMPLE 1 
     The results presented below were obtained on a cryothermostat designed to run on R-13B1. 
     After a base line was obtained with the latter, 32/125 and 32/125/23 mixtures according to the invention were tested on this equipment. 
     The only changes introduced when the fluid was changed were an adjustment to the expansion valve and an optimization of the charge of cryogenic fluid. 
     If the liquid and vapour densities of the mixtures according to the invention are compared in relation to R-13B1, a decrease of 55 to 65% in the density of the vapour drawn in by the compressor and a decrease of approximately 30% in the liquid density are observed in all cases. It is therefore appropriate to load the plant with a charge approximately 50% smaller than that of R-13B1 and to close the expansion valve until the evaporator is fed correctly. 
     To give an example, optimization results produced with a 32/125/23 mixture containing 26/63/11% by weight are given in Table V. The trial consists in lowering the temperature of the refrigerant bath. The bath temperature and the high and low pressures were measured throughout the trial period. 
     As shown in FIG. 3, appended, which illustrates the change in the temperature of the bath and in the condensation pressure as a function of time, two stages of operation are observed. The first stage (down to approximately -30° C.) is performed at constant rate of cooling and pressure. 
     
                                           TABLE V__________________________________________________________________________               Stage B    Stage A    Final point of the trial         Pressure               Bath  Low Temperature of theCharge    Expansion    Rate High            Low               temperature                     pressure                         vapour drawn(g) valve*    (° C./min)         (bar)            (bar)               (° C.)                     (bar)                         in (° C.)__________________________________________________________________________615 Open 2.6  22 1.9               -51   1.05                         -39.5615 Closed    2    21.4            1.5               -54.2 0.67                         2.7922 Closed    2.1  22.1            1.6               -53.41230    Closed    2.1  24 1.6               -52.8__________________________________________________________________________ *Open expansion valve means that the setting initially made for R13B1 has not been altered. Closed expansion valve means that the valve of the expansion device was closed again in relation to the initial setting. 
    
     In the case of the expansion valve set for R-13B1 a low pressure which is too high and a temperature of the vapour at the compressor suction which is too low are observed, thus indicating an excessive feed of the evaporator. 
     Conversely, if the plant is loaded with the same charge as that of R-13B1 (namely 1.2 kg), an increase in the high pressure is observed, indicating an excessively fed condenser. 
     EXAMPLE 2 
     Thermodynamic performance of various mixtures according to the invention, compared with R-13B1, was tested on a cryothermostat without any major alteration to the system apart from the optimization of the charge of cryogenic fluid and the closure of the expansion valve. 
     Table VI summarizes the thermodynamic performance seen during an attempt to lower the temperature of the refrigerant bath with R-13B1 and with various mixtures of HFC-32, HFC-125 and HFC-23 at an ambient temperature of 23±1° C. 
     
                       TABLE VI______________________________________    FLUIDS          32/125  32/125  32/125/23                                 32/125/23    R-13B1          (50/50) (32/68) (26/63/11)                                 (30/65/5)______________________________________Stage ARate (° C./min)      1.38    2.5     2.06  2.07   1.94High pressure      18      20.3    18.6  22.1   20.5(bar)Compressor exit      47      62      53    55     56temperature (° C.)Power absorbed (W)      800     986     860   933    892Stage BFinal temperature      -59     -54.5   -54   -54    -54of the bath (° C.)OverallparametersConsumption (kWh)      1.22    0.69    0.74  0.78   0.8Temperature      122     64      74    73     76descent period (min)______________________________________ 
    
     Stage A corresponds to the stationary stage where the rate of cooling and high pressure are constant: this corresponds to a lowering of the bath temperature from 10 to -30° C. 
     Stage B corresponds to the lowering of temperature from -30° C. to the final temperature attained. 
     The mixtures according to the invention make it possible to obtain refrigerant bath temperatures of the order of -54° C. with a single-stage system initially envisaged to operate on R-13B1. 
     Although having a power absorbed by the compressor which is higher than that of the R-13B1, the mixtures according to the invention permit an overall gain in energy of approximately 40% because of their higher rate of cooling. 
     The mixtures according to the invention which contain 25 to 50 mass % of HFC-32 and 50 to 75% of HFC-125 and those containing optionally a content lower than 12% of HFC-23 or lower than 20% of HFC-143a allow R-13B1 to be replaced in very low temperature refrigeration systems. 
     The mixture containing approximately 32 mass % of HFC-32 and 68 mass % of HFC-125 is the best compromise for the replacement of R-13B1. 
     Although the invention has been described in conjunction with specific embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all of the alternatives and variations that fall within the spirit and scope of the appended claims.