EP1902263A2

EP1902263A2 - Method for refrigerating a thermal load

Info

Publication number: EP1902263A2
Application number: EP06794443A
Authority: EP
Inventors: Didier Alo; Jean-Pierre Germain; Jean-Yves Thonnelier
Original assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 2005-06-02
Filing date: 2006-05-18
Publication date: 2008-03-26
Also published as: FR2886719B1; US20090193817A1; WO2006129034A3; WO2006129034A2; FR2886719A1

Abstract

The invention concerns a method for refrigerating one or more thermal loads which consists in providing a first CO2 source derived from a mechanical cold system, and in feeding one or more evaporators from said first source so as to cause the CO2 to evaporate thereby cooling said one or more thermal loads. The method is characterized in that it consists in providing a second CO2 source, consisting of a cryogenic storage of CO2, and in inputting CO2 from said second source, so that the flow rate of fluid feeding the evaporator(s) is a mixture of liquid CO2 derived from said first source and liquid CO2 derived from said second source.

Description

Process for refrigerating a heat load

The present invention relates to the field of the production of cold, including mechanical type and cryogenic type, and is particularly interested in applications in the field of food processing, such as freezing and freezing. Mechanical refrigerants have long used ammonia (NH ₃ ) or HCFCs (hydrochlorofluorocarbons) or HFCs (hydro-fluoro-carbons) as the refrigerant.

But we know that environmental legislation, evolving, penalized the use of these fluids, either for safety issues, especially for ammonia; for environmental issues as is the case for HCFCs and CFCs.

Manufacturers of refrigeration systems were therefore led to look for other refrigerants more in line with legislation and then focused their attention on so-called "natural fluids, such as carbon dioxide, air, water .. Their choice was focused in particular on a combined NH ₃ + CO2 combination in a cascade cycle which makes it possible to obtain cooling capacity at evaporation temperatures preferably comprised between -20 and -55 ^° C.

CO ₂ can also be used as a two-phase refrigerant fluid in combination with a compression cycle using a refrigerant, for example ammonia.

The new cascade system generations consist of two distinct compression assemblies: o the high-pressure circuit using as refrigerant ammonia or any other refrigerant suitable for cooling by the ambient environment or a cold source (temperature between 10 and 30 ^° C) and the low pressure circuit using carbon dioxide to supply the evaporator producing the useful cold.

As a reminder, it is known that the term "mechanical cold" system generally means a cold production system using a condensable steam compression cycle, and a "cryogenic cold" system.

EEMPLICITY KEY (RULE 26) a system producing cold from the phase change of a cryogenic fluid in an open circuit.

For the cooling, crusting or freezing of food products, two types of technologies are commonly used: - The cold called "cryogenic", uses the frigories released by the phase change of a cryogenic fluid, for example the sublimation of the carbon dioxide or the vaporization of liquid nitrogen. This technology works with open circuit ie lost fluids. The amount of investment is thus minimized. - The cold called "mechanical" uses the frigories released by the expansion of a refrigerant, for example carbon dioxide. This technology works in a closed circuit with a constant fluid load. The amount of investment is higher because it is necessary to provide the compression groups. It is conceivable that moving from a mechanical cold production system to a cryogenic system and vice versa, or mixing the two systems would provide the user with greater flexibility.

For example, industrialists who have chosen a mechanical cooling system could, for a given period of time, use the deep-freezer with a source of cryogenic refrigerants, which would have the advantage of improving the staggering of investments. during the launch period of the frozen product on the market.

Indeed, the amount of investment for a freezing unit with a production of mechanical cold is broken down into two substantially equivalent parts between the deep freeze and the refrigeration plant. It could be interesting for manufacturers to minimize their investments by replacing refrigeration production with cryogenic storage, the investment of refrigeration production being done only when the refrigeration requirements necessary to ensure production no longer allow the cryogenic system to be competitive, which may also have the advantage of minimizing financial risks during the product launch period. Moreover, any mechanical assembly requires periods of downtime to ensure their maintenance. It is generally accepted to stop the refrigeration production unit (ie the compressors) and the refrigeration system (the freezer) approximately 4 weeks per year to maintain the compressors and the various freezer components. During this maintenance period, the installation no longer produces. The possibility of working momentarily thanks to a cryogenic fluid would have the advantage of:

- Allow continuous production without stops due to maintenance periods. - To intervene in emergency of the refrigerated productions during breakdowns.

In conclusion, the simultaneous operation of mechanical cold and cryogenic cold, would allow to increase the refrigeration production during peak periods.

Solutions have already been proposed in the literature to overcome a stop of the compression circuit or to increase the production of cold or to optimize performance, it is a question according to these solutions to add:

- An exchanger between an external fluid cooling the internal fluid of the mechanical cold circuit, this exchanger is disposed between the evaporator and the compressor, or can be introduced into the low pressure bottle (see for illustrative document US 5331824A).

- A cryogenic fluid spray boom on the product, or at the cold battery (see for example the document US-5410886) or in the cooling space (see for example the document EP-0652409), or the setting up a cryogenic bath (see for example US-5220803, EP-0360224, EP-0361700).

an additional exchanger for the evaporation of the cryogenic fluid to take place in a circuit independent of the circuit of the mechanical system (see for example US-5996355).

An independent device for cooling the products, either at the inlet or at the outlet of the main cooling equipment using mechanical cold (see for example the document US-5220803). It has also been envisaged to couple cryogenic cold with mechanical cold to improve the performances of one or the other (see for example the documents US-6425264B1, US2002 / 0124587, US-5694776, EP-0208526, US-5343715). Thus, mixed cold using cryogenic and mechanical cold systems has been designed:

- to benefit from the advantage of both technologies: fast cooling by cryogenics and low cost of use of mechanical refrigeration;

- as a booster: cryogenic cold increases the cold production capacity of the mechanical system;

to recover cold vapors from the cryogenic fluid.

Mixed cold uses both technologies. The final "wanted" cooling effect may be due to mechanical cold and cryogenic cold or only due to one of the two systems. The system that does not contribute directly to the "wanted" refrigeration effect is used to allow the other system to operate under extended conditions (power, temperature).

It should be noted that in all known cases of coupling of mechanical cold and cryogenic cold, the fluids of the two systems do not mix. The two systems remain completely autonomous and parallel, at best a slight "coupling" is achieved by recovering the cold vapors of the cryogenic fluid to send them into the exchanger of the mechanical cold system and thus recover some frigories (otherwise, these cold vapors are simply sent to the surrounding atmosphere).

As will be seen in more detail below, the present invention seeks to provide a device in which, on the contrary, the "consumable" cryogenic fluid is CO ₂ and is mixed with the fluid of the mechanical system which supplies the evaporator of the installation using cold to cool a thermal load. The latter fluid is also CO ₂ whether it is the refrigerant "recycled" or the refrigerant fluid. This additional injection of CO ₂ operates before producing cold in the evaporator.

Since the fluid of the mechanical cold circuit must remain at constant mass, it is necessary that the quantity of CO ₂ of cryogenic origin injected removed from the network after being sprayed. As will be seen later this CO2 vapor extraction can be carried out at various points of the circuit.

According to the present invention, the proposed solution consists in mixing the fluids coming from an open circuit using CO2 as a cryogenic fluid and a closed circuit using CO2 as a refrigerant or as a refrigerant.

More specifically, the present invention relates to a method of refrigeration of one or more heat loads according to which there is a first source of CO ₂ from a mechanical cooling system, and where one or more evaporators are fed to from this first source in order to carry out the evaporation of the CO ₂ and thus cool said one or more heat loads, and characterized in that there is a second source of CO ₂ , constituted by a cryogenic storage of CO2, and in that a supply of CO _{2 is made} from said second source, so that the flow of fluid supplying the evaporator or evaporators is a liquid CO ₂ mixture from said first source and liquid CO ₂ from said second source.

A method of refrigerating one or more heat loads in which there is a first source of CO ₂ from a mechanical cooling system, and where one or more evaporators are supplied from this first source to evaporation of the CO ₂ therefrom and thereby cooling said one or more heat loads, characterized in that a second source of CO ₂ , constituted by a cryogenic storage of CO ₂ , is available, and in that the a supply of CO _{2 is made} from said second source, so that the flow of fluid supplying the evaporator or evaporators is a mixture of liquid CO ₂ from said first source and liquid CO ₂ from said second source; source.

According to one of the embodiments of the invention, the mechanical cooling system proceeds with the steps of:

- CO ₂ compression in the vapor state; condensation in a CO ₂ exchanger thus compressed;

- relaxation of CO ₂ and sub-cooled;

storage of CO ₂ thus expanded in a storage tank; and the following measures are implemented:

extracting CO2 from the storage tank to supply said one or more evaporators;

an amount of CO 2 substantially equivalent to that corresponding to said CO ₂ supply is fed to the outside according to one or each combination in the following ways: i) the fluid or a part of the fluid obtained at the outlet of the evaporator (s) is directed towards said storage tank and from this tank is made the extraction and evacuation of said CO ₂ flow to be discharged to the outside; j) the part of the fluid obtained at the outlet of the evaporator (s) is separated and discharged to the outside, the remainder being directed towards either said storage tank or towards said compression step; said supply of CO ₂ from said second source is carried out in said storage tank where it is mixed with CO ₂ originating from said evaporator or said expansion step.

According to another of the embodiments of the invention, an amount of CO ₂ substantially equivalent to that corresponding to said CO ₂ supply is exhausted to the outside, one or each combined in the following ways: i) the fluid or a portion of the fluid obtained at the outlet of the evaporator (s) is directed towards a storage tank and is made from this reservoir the extraction and evacuation of said CO ₂ flow to be discharged to the outside; j) the part of the fluid obtained at the outlet of the evaporator (s) is separated and evacuated to the outside, the remainder being directed towards either said storage tank or to a condensation stage by a a cold source produced by a vapor compression system and the liquid thus condensed is then stored in the storage tank; and one proceeds to said supply of CO ₂ from said second source into the storage tank where it is mixed with CO ₂ from the evaporator (s) or said condensation step.

Advantageously, the amount of CO ₂ present in the circuit is between two predetermined operating limits.

As will be clear from the above, it is thus possible to maintain production, by ensuring the continuity of the cooling capacity, by injecting into the CO ₂ circuit from the mechanical system a CO _{2 supply.} liquid from a storage that is connected to the "mechanical" CO ₂ distribution network, an equivalent amount (or at least substantially equivalent) to that of CO ₂ of cryogenic origin injected into the circuit being then rejected, by example after the evaporation of the fluid in the evaporator of the cooling system of the thermal load considered (for example a freezer).

As will be seen below, the important point is to ensure that the amount of CO ₂ present in the circuit does not increase to infinity without control (because of the contribution of CO2 of cryogenic origin) , which would be unimaginable, on the contrary the amount of CO ₂ present in the circuit remains substantially constant or at least remains between two acceptable and predetermined operating limits. It is therefore necessary to evacuate towards the outside of the circuit a quantity of CO2 substantially equivalent to that which has been admitted into the circuit and which was of cryogenic origin.

It is thus possible according to the present invention to adopt one or each of the following combined solutions:

the whole of the fluid obtained at the outlet of the evaporator (s) (from the installation using cold to cool a heat load) can be sent to a non-cryogenic storage tank (tank from which the evaporators are fed), and realizes the reservoir extraction and evacuation of the flow to be discharged to the outside;

it is also possible, directly at the outlet of the evaporator (s), to separate part of the flow from the evaporator (s) for evacuate it, the remainder being sent either to the non-cryogenic storage tank, or to the mechanical cold cycle (compression step or the condensation step).

Thus in summary, the useful cold is produced by the evaporation of a CO ₂ flow rate, resulting from the mixing of a flow rate from a cryogenic storage of CO ₂ and a CO ₂ flow rate obtained by a mechanical system. .

The "global" CO ₂ flow produces useful refrigeration in an evaporator located at the facility using cold to cool a load.

As mentioned above, after evaporation, the total flow of CO ₂ is separated into two flow rates, one substantially equal to the flow rate that came from the cryogenic storage, the other equal to the flow rate that came from the mechanical cold.

As a preference, the regulation of the mixed cryogenic CO ₂ flow rate and of the separated CO2 flow rate is carried out respectively by the opening of a supply valve from the storage and by the opening of the CO extraction valve. ₂ . These valves are controlled for example:

• by measuring the level of liquid CO ₂ in the tank that feeds the evaporator;

• by the CO ₂ evaporation pressure or temperature

The flow rate which corresponds substantially to that which came from the cryogenic storage is evacuated directly into the atmosphere or indirectly through an oil separation phase if necessary.

The flow corresponding to that which came from the mechanical cold is recondensed by the mechanical system.

The coupling of the two circuits can be achieved in different ways, and in particular: 1. In the case where the CO? of the mechanical system is the fluid friqoriqène:

The mixture of the two CO ₂ flows can be achieved:

in the circuit upstream of the evaporator at the evaporation pressure, for example in the low pressure bottle containing the liquid CO2 produced by the mechanical cold,

in the circuit upstream of the expansion valve of the mechanical system, between the condenser and the expansion valve.

The separation of the two flow rates after the evaporation in the evaporator can be carried out:

- in the circuit just after the evaporator;

in the low pressure bottle from which the CO2 vapors are re-compressed by the compressor of the mechanical cooling system.

2. In the case where the CO? is the refrigerant fluid of the mechanical system

The mixture of the two CO ₂ flows can be achieved:

in the storage cylinder of the CO ₂ fridge-carrier which is at the evaporation pressure;

- between the evaporator and the low pressure CO ₂ storage cylinder.

- in the circuit just after the evaporator;

- In the low pressure bottle in which the CO ₂ re-condensed by the mechanical cold is accumulated. As mentioned above, the coupling of the two circuits must be regulated so that the amount of CO ₂ present in the closed circuit remains substantially constant or at least between two acceptable operating limits.

The mixed cryogenic CO ₂ flow rate regulation and the separated CO ₂ flow rate are achieved, for example, by the opening of a supply valve from the storage and by the opening of the CO ₂ extraction valve. . These valves can be ordered for example either:

• by the CO ₂ evaporation pressure or temperature.

Other features and advantages of the invention will become apparent from the examples below.

- Figure 1 is a schematic representation of an example of a mechanical cold system (two sets of compression);

- Figure 2 is a schematic representation of a cryogenic cold system;

- Figure 3 is a schematic representation of an installation suitable for the implementation of the invention;

FIG. 4 is a schematic representation of a second installation that is suitable for implementing the invention (mixing of the fluids coming from the open circuit using CO ₂ as a cryogenic fluid and of the closed circuit also using CO ₂ as a refrigerant fluid; -carrier).

- Figure 5 is a schematic representation of a third installation suitable for the implementation of the invention;

- Figure 6 is a schematic representation of a 4th installation suitable for the implementation of the invention.

The following elements are recognized in FIG.

by the reference 1: a thermal load, for example a freezer; the thermal load can be spread over several uses each having an evaporator (2) where the CO ₂ produces a cooling effect by evaporation.

- In 2: the CO ₂ heat exchanger / evaporator delivering the cold to the thermal load by evaporation of CO ₂ and via an intermediate fluid (usually air), the evaporation of CO ₂ typically takes place between 5, 2 bar and 26.5 bar and preferably between 5.5 bar and 10 bar.

- In 3: a storage tank for CO ₂ in the liquid phase; its operating pressure is that of the CO ₂ evaporator, with the pressure drops close to it. - In 4: a CO ₂ circulation pump for feeding the evaporator or evaporators.

- in 5 and 6: circuit breaker valves.

- In 7: a low-temperature vapor compression system using CO ₂ as a refrigerant (the CO ₂ vapors are compressed by the compressor 9 and are condensed in an evapo-condenser exchanger 10, at a temperature typically between -2O ⁰ C and -1O ⁰ C. Preferably the liquid obtained undergoes a loss of pressure in an expansion member 11 where a fraction of CO ₂ vaporizes allowing cooling of the flow of CO ₂ between -56 ⁰ C and -1O ⁰ C The CO ₂ obtained is accumulated in the tank 3 at the low pressure of the circuit.

in 8: a high-temperature vapor compression system using a refrigerant such as ammonia (NH3), or HFC R404, R410, or any other fluid adapted to condense CO ₂ by vaporizing (between -30 ^{° C);} C and -5 ⁰ C preferably) and condensing at the high pressure of the circuit between 15 ⁰ C and 45 ⁰ C preferably.

- In 9: a compressor sucking the CO ₂ vapors in the tank 3 to compress at the high pressure of condensation.

- At 10: a heat exchanger to condense the CO ₂ by heat transfer to the high temperature circuit (the CO ₂ condensing between -28 ⁰ C and -10 ⁰ C preferably).

in 11: an expansion element between the high pressure and the low pressure of the CO ₂ low temperature circuit. - At 12: a refrigerant vapor compressor of the high temperature circuit.

- At 13: a heat exchanger for condensing the refrigerant of the high temperature circuit with the ambient air or another cold source (for example a cold water network); the condensation of the fluid is typically between 15 ⁰ C and 45 ⁰ C preferably.

- At 14: an expansion element between the high pressure and the low pressure of the high temperature circuit.

It is then known that the thermal load to be cooled (1) is cooled by a cascade vapor compression cycle with CO2 at low temperature as the refrigerant. The advantage of a cascade is to achieve a high energy efficiency when the total temperature difference between the low temperature evaporator and the high temperature condenser is high. The evaporation temperature of the CO ₂ is adjusted for the use of the required cooling between -56 ^° C. and -1 ^° C.

The heat exchange between the CO ₂ condenser and the evaporation of the high pressure circuit takes place at an optimized temperature depending on the refrigerant of the high temperature circuit and the total temperature difference and is generally between -28 ⁰ C and -5 ⁰ C. The CO ₂ in the low temperature circuit circulates in closed circuit.

We now recognize in Figure 2 the following elements:

- By reference 20: a cryogenic CO ₂ storage tank at a pressure typically between 15 and 30 bar preferably.

- At 21: a valve regulating the flow of liquid CO ₂ . - At 22: an expansion member passing the CO ₂ from the storage pressure to the operating pressure in the evaporator ie typically between 5.2 bar and 26.5 bar and preferably between 5.5 bar and 10 bar. bar.

The CO ₂ tank is used to supply one or more evaporators for cooling the thermal load or charges. The flow rate (s) are regulated according to a temperature or a pressure. The CO ₂ evaporates in an evaporator or several between -56 ⁰ C and -1O ⁰ C. The vaporized CO ₂ is released into the atmosphere via an extraction duct. Fig. 3 is a schematic representation of one embodiment of the present invention.

The heat load or charges are cooled by the evaporation of CO2 in one or more evaporators 2. The liquid CO ₂ feeding the evaporator (s) is supplied by a cascade system 31 with vapor compression comparable to that described in the of FIG. 1, and by a cryogenic storage of CO ₂ 32. The two liquid CO ₂ supply means are connected to the accumulation tank 3 of the low temperature circuit of the cascade system where the two CO ₂ streams are mixed together. . The CO ₂ resulting from the condensation of the low temperature circuit is expanded in the member 11 and accumulates in the tank 3.

The CO ₂ of the cryogenic storage is regulated in flow by the valve 21 and is expanded by the member 22 at the pressure of the tank 3.

The circulation pump 4 is used to supply the evaporator (s) cooling the thermal load (s). The pump must be sized to circulate a flow equal to the sum of the CO2 flow rates provided by the cryogenic storage 32 and the low temperature compression circuit 7. The same is true for the evaporator or evaporators 2 which are dimensioned at the same time. help from the sum of CO ₂ flows. When the compression system reaches its liquefied CO ₂ flow rate limits and the cooling capacity thus produced by the evaporation of the CO ₂ thus produced is not sufficient to cool the heat load, the additional cooling power is supplied by the flow of CO2. CO ₂ from cryogenic storage 32. The quantity of CO ₂ injected into the tank 3 must be evacuated after evaporation of the liquid by means of an extraction circuit 33.

The CO ₂ flow rate provided by the storage 32 can provide from 0 to 100% of the cooling capacity related to the heat load or charges. Preferably, the CO ₂ coming from the storage will make it possible to supplement the cooling capacity of the compression system during peak production so as not to oversize the latter. During compression system shutdowns, for maintenance or breakdown, the CO ₂ of the cryogenic storage can ensure 100% of the refrigeration requirements, which prevents the production stoppage. The operating pressures and temperatures of the compression cascade system and the cryogenic storage are for example comparable to those already indicated with reference to FIGS. 1 and 2 above.

Figure 4 illustrates another example of implementation of the invention. The heat load or charges are cooled by one or two evaporators using CO ₂ . The liquid CO ₂ is provided, on the one hand, by the liquefaction of all or part of the CO ₂ vapors from the evaporator (s) 2, condensation produced in the exchanger 41, and on the other hand by the CO ₂ from cryogenic storage. The CO ₂ flowing through the evaporator (s) and the condenser 41 is called a "coolant" fluid.

A cold production system 40 (compression system using CO ₂ or other refrigerants in cascade or not) allows the liquefaction of CO ₂ from or evaporators 2. The liquefaction of CO ₂ can take place in a exchanger separate storage tank 3 (as is the case of this Figure 4) or in this tank via an exchanger (as is the case in the context of Figure 6).

The use of a coolant circuit decouples the compression system that can be installed in a technical room remote from the installation cooling the thermal load. The operation of the coupling of the CO ₂ refrigerant-carrier circuit and the

CO ₂ of the cryogenic storage is similar to that of FIG. 3. A reserve 3 allows the mixing of the two streams of CO ₂ and two extraction lines 42 make it possible to extract a quantity of CO ₂ equal to that coming from the cryogenic storage and to evacuate it to the outside ambient air. This extraction is carried out downstream of the evaporator 2 (on the line returning to the tank 3) and / or directly on the tank 3. This second case forces the condenser 41 not to completely condense the flow of CO ₂ .

The typical operating pressures and temperatures of such an operating installation of the evaporator 2 are comparable to those described with reference to FIGS. 1 and 2.

In the context of FIG. 5, the compression system producing the cooling effect to totally or partially cool the heat load is consisting of the compressor 56, a condenser 57, a high pressure tank 58, an expansion member 54 and one (or more) evaporators 50. The refrigerant is CO2.

The cryogenic storage of CO2 51 is connected to the storage tank 58 via a pipe equipped with a flow control valve 52 and an expansion device 53. The CO ₂ mixes with that of the system in the reservoir 58.

The device operates in the modes previously described, the CO ₂ of the cryogenic storage being able to provide from 0 to 100% of the refrigerating needs but preferably is used to supplement the cooling capacity of the compression system during peak production or shutdown of this system. latest.

The CO ₂ coming from the cryogenic storage is vaporized partially by the expansion in the member 53. The vapor is evacuated from the reservoir 58 by an extraction line 59. The liquid CO ₂ accumulated in the reservoir 58 is expanded in the expansion 54 and is evaporated in the evaporator 50. Between the evaporator and the compressor 56, an extraction (55) CO ₂ vapor is installed to reject the CO ₂ introduced by the cryogenic storage. The extractions 55 and 59 are adjusted so that the amount of CO ₂ extracted is equal to that of the CO ₂ introduced by the cryogenic storage. The condenser 57 is cooled by a compression system forming a cascade as explained for Figure 3 and not re-detailed here.

As can be seen, one of the main differences between the mode of Figure 3 and that of Figure 5 lies in the position of the CO ₂ storage tank which can be at low pressure (Figure 3) or at the top. pressure (Fig. 5) of the CO ₂ compression system. If the device of Figure 3 is advantageous for simplifying the regulation of CO ₂ extraction, the device of Figure 5 avoids the dispensing pump.

Claims

1. A method of refrigeration of one or more heat loads in which there is a first source of CO ₂ from a mechanical cooling system, and where one or more evaporators are fed from this first source so to carry out the evaporation of the CO ₂ and thereby cool said one or more heat loads, characterized in that there is a second source of CO ₂ , constituted by a cryogenic storage of CO ₂ , and in that a supply of CO _{2 is made} from said second source, so that the flow of fluid supplying the evaporator or evaporators is a mixture of liquid CO ₂ from said first source and liquid CO ₂ from said second source.

2. Refrigeration method according to claim 1, characterized in that the mechanical cooling system proceeds with the steps of:

- CO ₂ compression in the vapor state;

condensation in a CO ₂ exchanger thus compressed;

- relaxation of CO ₂ and sub-cooled;

storage of CO ₂ thus expanded in a storage tank; and in that the following measures are implemented:

- CO ₂ is extracted from the storage tank to supply said one or more evaporators;

an amount of CO ₂ substantially equivalent to that corresponding to said CO ₂ supply is delivered to the outside in one or each of the following ways: i) the fluid or a part of the fluid obtained at the outlet of the evaporator (s) is directed towards said storage tank and is made from this reservoir the extraction and evacuation of said CO ₂ flow to be discharged to the outside; j) the part of the fluid obtained at the outlet of the evaporator (s) is separated and evacuated to the outside, the remainder being directed either to said storage tank or to said compression step;

said supply of CO ₂ from said second source is carried out in said storage tank where it is mixed with CO ₂ originating from said evaporator or said expansion step.

3. refrigeration process according to claim 1 characterized in that it is discharged to the outside a quantity of CO ₂ substantially equivalent to that corresponding to said CO _{2 supply} according to one or each combination of the following ways: i) the fluid or a portion of the fluid obtained at the outlet of the evaporator (s) is directed towards a storage tank and is made from this reservoir the extraction and evacuation of said CO ₂ flow to be discharged to the outside; j) the part of the fluid obtained at the outlet of the evaporator (s) is separated and evacuated to the outside, the remainder being directed towards either said storage tank or to a condensation stage by a a cold source produced by a vapor compression system and the liquid thus condensed is then stored in the storage tank; and in that one carries out said supply of CO ₂ from said second source in the storage tank where it is mixed with CO ₂ from the evaporator (s) or said condensation step.

4. Refrigeration method according to one of the preceding claims, characterized in that the amount of CO ₂ present in the circuit is between two predetermined operating limits.