EP2807431A1

EP2807431A1 - Cascade refrigeration system

Info

Publication number: EP2807431A1
Application number: EP13701840.4A
Authority: EP
Inventors: Wissam Rached
Original assignee: Arkema France SA
Current assignee: Arkema France SA
Priority date: 2012-01-26
Filing date: 2013-01-08
Publication date: 2014-12-03
Anticipated expiration: 2033-01-08
Also published as: WO2013110866A1; FR2986309B1; EP2807431B1; JP5901797B2; JP2015505029A; US20140352336A1; CN104067072A; KR20140123968A; FR2986309A1

Abstract

The invention relates to a method for cooling a fluid or a body by means of at least a first vapour compression circuit (10) containing a first heat transfer fluid and at least a second vapour compression circuit (20) containing a second heat transfer fluid, the method comprising: - 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. The invention also relates to an installation suited to implementing this method.

Description

CASCADE REFRIGERATION SYSTEM

FIELD OF THE INVENTION

The present invention relates to a cascade refrigeration system designed to operate optimally, and a refrigeration method implemented in this system.

TECHNICAL BACKGROUND

Refrigeration systems are generally based on a thermodynamic cycle including the vaporization of a fluid at low pressure (in which the fluid absorbs heat); compressing the vaporized fluid to a high pressure; condensing the vaporized fluid into a high pressure liquid (in which the fluid emits heat); and the expansion of the fluid to complete the cycle.

The choice of a heat transfer fluid (which may 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 considered on the environment. In particular, chlorinated compounds (chlorofluorocarbons and hydrochlorofluorocarbons) have the disadvantage of damaging the ozone layer. Thus, non-chlorinated compounds such as hydrofluorocarbons, fluoroethers and fluoroolefins are now generally preferred.

Another environmental constraint is that of global warming potential (GWP). It is therefore essential to develop heat transfer compositions with as low a GWP as possible and good energy performance.

Some particular refrigeration systems rely on the use of several refrigeration circuits, including two circuits, coupled together, namely a high temperature circuit and a low temperature circuit: these systems are called "cascading". Both circuits generally include different heat transfer fluids.

A cascading system has a number of security benefits. In particular, one can use, for cost issues or performance, some heat transfer fluid in the high temperature circuit, and use another less flammable or less toxic heat transfer fluid in the low temperature circuit. Thus, the total charge of the most flammable or toxic heat transfer fluid is minimized, and this most flammable or toxic heat transfer fluid is confined to an unconfined zone and / or a risk-free zone. contact with the public or staff in the event of a leak.

For example, carbon dioxide is a very advantageous heat transfer fluid because of its non-flammability, as well as from an environmental point of view. But because of its low critical point, it is generally less efficient than a traditional heat transfer fluid (hydrocarbon, hydrofluorocarbon ...). An optimal solution may be to use a cascade system containing carbon dioxide in the low temperature circuit and a conventional heat transfer fluid in the high temperature circuit.

WO 2008/150289 and WO 201 1/056824 provide examples of cascade refrigeration systems.

Theoretical analysis of a CO2-NH3 cascade refrigeration system for cooling applications at low temperatures, from Dopazo et al. in Applied Thermal Engineering 29: 1577-1583 (2009) as well as the article Experimental Investigation on the Performance of NH3 / CO2 Cascade Refrigeration System with twin-screw compressor, 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.

However, there is still a need to improve the efficiency and performance of cascade refrigeration systems, and in particular a need to minimize the overall energy consumption of these systems as well as the associated environmental impact.

SUMMARY OF THE INVENTION

The invention firstly relates to a method of cooling a fluid or a body by means of at least a first vapor compression circuit containing a first heat transfer fluid and at least a second heat transfer circuit. vapor compression containing a second heat transfer fluid, the method comprising:

in the first vapor compression circuit: ^■ at least partial evaporation of the first heat transfer fluid by heat exchange 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 heat exchange with the second heat transfer fluid;

^■ the expansion of the first heat transfer fluid;

in the second vapor compression circuit:

^■ at least partial evaporation of the second heat transfer fluid by heat exchange 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 heat exchange with an external medium;

^■ the expansion of the second heat transfer fluid; the method further comprising:

- measurement of the temperature of the external environment; and

the adjustment of the temperature of the second heat transfer fluid to evaporation, as a function of the temperature of the external medium.

According to one embodiment, the adjustment of the temperature of the second heat transfer fluid to evaporation is carried out continuously or is performed at least once an hour.

According to one embodiment, the method comprises the detection of variations of the temperature of the external medium, and the adjustment of the temperature of the second heat transfer fluid to evaporation comprises an increase in the temperature of the second transfer fluid of the medium. heat to 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 to evaporation if a decrease in the temperature of the external medium is detected.

According to one embodiment, the method comprises calculating an optimum evaporation temperature as a function of the measurement of the temperature of the external medium. According to one embodiment, the temperature of the second heat transfer fluid at evaporation is adjusted to the optimum evaporation temperature.

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 the second vapor compression circuit is maximum.

According to one embodiment, the optimum evaporation temperature is defined by the formula T _op t = A x T _ex t + B, in which T _ex t is the temperature of the external medium in degrees Celsius, A is a dimensionless constant , and B is a constant in degrees Celsius.

According to one embodiment, the constant A is from 0.3 to 0.6, preferably from 0.4 to 0.45; and the constant B is from -50 ° C to 0 ° C, preferably from -30 ° C to -20 ° C.

According to one embodiment, the fluid or body is cooled to a temperature of -50 to -15 ° C, preferably -40 to -25 ° C.

According to one embodiment:

the first heat transfer fluid is chosen from carbon dioxide, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and mixtures thereof, and preferably is carbon dioxide; and or

the second heat transfer fluid is chosen from ammonia, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and mixtures thereof, preferably tetrafluoropropene, and more preferably is 2,3,3,3-tetrafluoropropene or 1,3,3,3-tetrafluoropropene.

According to one embodiment, the compression of the second heat transfer fluid is carried out by one or more compressors, and the adjustment of the temperature of the second heat transfer fluid to evaporation is performed by adjusting said compressors.

According to one embodiment, the adjustment of said compressors comprises an adjustment of the speed of rotation of the compressors, or is performed by starting and stopping successive compressors.

According to one embodiment, the method is a method of cooling a compartment containing products, preferably foods, frozen or frozen. The invention furthermore relates to a cooling installation for a fluid or a body, comprising at least:

a first vapor compression circuit containing a first heat transfer fluid;

a second vapor compression circuit containing a second heat transfer fluid;

a cascade heat exchanger adapted to exchange heat between the first heat transfer fluid and the second heat transfer fluid;

the first vapor compression circuit comprising:

a first evaporator adapted to exchange heat between the first heat transfer fluid and said fluid or body;

one or more first compressors;

a first expander;

the second vapor compression circuit comprising:

one or more second compressors;

a second condenser adapted to exchange heat between the second heat transfer fluid and an external medium;

- a second regulator;

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.

According to one embodiment, the installation further comprises a module for calculating an optimum evaporation temperature as a function of the measurement of the temperature of the external medium.

According to one embodiment, the means for adjusting the evaporation temperature in the cascade heat exchanger are adapted to adjust the evaporation temperature in the cascade heat exchanger to the optimum evaporation temperature.

According to one embodiment, the optimum evaporation temperature is defined by the formula T _op t = A x T _ex t + B, in which T _ex t is the temperature of the outer medium in degrees Celsius, A is a dimensionless constant, and B is a constant in degrees Celsius.

According to one embodiment, the installation is adapted to cool the body or the fluid to a temperature of -50 to -15 ° C, preferably -40 to -25 ° C.

According to one embodiment:

According to one embodiment, the means for adjusting the evaporation temperature in the cascade heat exchanger comprise means for adjusting the second compressors.

According to one embodiment, the adjustment means of the second compressors are adapted to adjust the speed of rotation of the second compressors, or are adapted to turn on and off successively the second compressors.

According to one embodiment, the installation comprises a compartment adapted to receive products, preferably food, frozen or frozen.

The present invention makes it possible to meet the needs felt in the state of the art. In particular, it provides refrigeration processes and related facilities in which overall energy consumption and environmental impact are minimized.

This is accomplished by adjusting the evaporation temperature of the heat transfer fluid of the high temperature circuit as a function of the outside temperature (room temperature). It has been discovered that such an adjustment optimizes the overall performance of the system. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a diagram of an installation according to the invention.

FIG. 2 is a graph showing: (1) the evolution of the ambient temperature during a typical day taken for example (white circles, left ordinate axis, values in degrees Celsius); and (2) an example of a conventional evolution of the cooling capacity necessary for storing frozen food on this typical day (black squares, right y-axis, values in kW); and this, according to the hours of the day (axis of the abscissas).

FIG. 3 is a graph illustrating the optimal evaporation temperature (in degrees Celsius, ordinate axis) versus ambient temperature (in degrees Celsius, x-axis) for a cascade refrigeration system in which the transfer fluid High temperature circuit heat is: (1) HFO-1234yf (white squares); or (2) HFO-1234ze (black circles).

FIG. 4 is a graph illustrating the total energy consumption of a refrigeration system during a typical day, in kWh, depending on whether the refrigeration system is according to the invention (gray bars, evaporation temperature of the cooling fluid). heat transfer at high temperature adjusted according to the ambient temperature) or is a conventional system (black bars, evaporation temperature of the heat transfer fluid at high temperature set at -10 ° C.). The two data series 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.

Figure 5 is a graph illustrating the TEWI index of a cascading refrigeration system on a typical day in different cases: conventional refrigeration system and HFO-1234yf in the high temperature circuit (R1234yf bar); refrigeration system according to the invention and HFO-1234yf in the high temperature circuit (R1234yf opti bar); conventional refrigeration system and HFO-1234ze in the high temperature circuit (R1234ze bar); refrigeration system according to the invention and HFO-1234ze in the high temperature circuit (R1234ze opti bar). The values correspond to the percentage of TEWI index relative 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 the invention is a system in which the evaporation temperature of the heat transfer fluid at high temperature is adjusted according to the ambient temperature.

Figure 6 is a graph equivalent to that of Figure 4, but with a conventional system where the evaporation temperature of the high temperature heat transfer fluid is set at -18 ° C.

Figure 7 is a graph equivalent to that of Figure 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 OF THE INVENTION

The invention is now described in more detail and without limitation in the description which follows.

By "heat transfer compound" or "heat transfer fluid" (or refrigerant), is meant a compound, respectively a fluid, capable of absorbing heat by evaporating at low temperature and low pressure and to reject heat by condensing at high temperature and high pressure, in a vapor compression circuit. In general, a heat transfer fluid may comprise one, two, three or more than three heat transfer compounds.

By "heat transfer composition" is meant a composition comprising a heat transfer fluid and optionally one or more additives which are not heat transfer compounds for the intended application.

The invention relates to cooling installations of a fluid or a body, as well as the associated cooling methods. These installations may be stationary or mobile air-conditioning installations or, preferably, refrigeration and / or freezing and / or cryogenic units, stationary or mobile.

With reference to FIG. 1, according to one embodiment, the installation according to the invention comprises a first vapor compression circuit 10 (or low temperature circuit), which contains a first heat transfer fluid, and a second vapor compression circuit 20 (or high temperature circuit), which contains a second heat transfer fluid. A cascade heat exchanger 30 (or evapo-condenser, or refrigerant-refrigerant heat exchanger) ensures the caloric coupling between the two vapor compression circuits.

The first vapor compression circuit 10 comprises at least a first evaporator 1 1, at least a first compressor 12 and at least a first expander 14. Between the first compressor 12 and the first expander 14, the circuit passes through the exchanger. heat cascade 30, which plays the role of condenser for this first circuit (first condenser).

Fluid lines are provided between all elements of the circuit.

The vapor compression circuit 10 operates according to a conventional vapor compression cycle. The cycle comprises changing the state of the first heat transfer fluid from a liquid phase (or diphasic liquid / vapor) to a vapor phase at a relatively low pressure (in the first evaporator 11), and then compressing the fluid. in the vapor phase to a relatively high pressure (in the first compressor 12), the change of state (condensation) of the heat transfer fluid from the vapor phase to the liquid phase at a relatively high pressure (in the exchanger cascade of heat 30) and reducing the pressure to restart the cycle (in the first expander 14).

The second vapor compression circuit 20 comprises at least one second compressor 22a, 22b, 22c, at least one second condenser 23 and at least one second expander 24.

Between the second expander 24 and the second compressor 22a, 22b, 22c, the circuit passes through the cascade heat exchanger 30, which acts as an evaporator for this second circuit (second evaporator).

Fluid lines are provided between all elements of the circuit.

The second vapor compression system 20 operates in a similar manner to the first.

An accumulator 27 may be provided in the circuit to form a fluid reservoir in the liquid state. The level of the liquid in the accumulator varies according to the need of the installation according to the conditions of use.

The first heat transfer fluid receives heat from the fluid or the body to be cooled in the first evaporator January 1. For example, when the body to be cooled consists of one or more products (including food products) frozen or deep-frozen, this body can be arranged in a compartment of which at least a portion of the walls are in direct contact with the first evaporator 1 1 (or at least part of whose walls belong to the first evaporator January 1).

Alternatively, the heat exchange between the fluid or body to be cooled and the first heat transfer fluid can be performed via an auxiliary circuit containing a coolant such as air or a glycol compound for example (with or without change state).

The first heat transfer fluid in turn gives off heat to the second heat transfer fluid in the cascade heat exchanger 30 which couples between the two circuits. Heat transfer from the first heat transfer fluid to the second heat transfer fluid induces 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.

Finally, the second condenser 23 allows the second heat transfer fluid to give heat to the outside environment. The external medium is preferably the surrounding air.

The exchange of heat between the second heat transfer fluid and the external medium can be carried out either directly or via a heat transfer fluid auxiliary circuit (with or without a change of state).

As compressors, in the aforementioned circuits, it is possible to use centrifugal compressors with one or more stages or centrifugal mini-compressors. Rotary, piston or screw compressors can also be used. The compressors may be driven by an electric motor or by a gas turbine (for example powered by the exhaust gas of a vehicle, for mobile applications) or by gearing.

As heat exchangers for the implementation of the invention, it is possible to use co-current heat exchangers or, preferably, countercurrent heat exchangers. It is also possible to use microchannel exchangers.

Each equipment (condenser, expansion valve, evaporator, compressor) may be constituted by one unit or by several units arranged in series and / or in parallel. When several parallel units are used, as is the case for the second compressors 22a, 22b, 22c in FIG. 1, provision is made if necessary for a distributor 25 and a manifold 26 to distribute the fluid in the different units and to collect the fluid from different units.

It is also possible to provide a plurality of first vapor compression circuits (low temperature) coupled to a single second vapor compression circuit (high temperature), or several second vapor compression circuits (high temperature) coupled to a single first vapor compression circuit (low temperature).

The first heat transfer fluid is preferably selected from carbon dioxide, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and mixtures thereof. It can especially be carbon dioxide.

The second heat transfer fluid is preferably chosen from ammonia, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and mixtures thereof. It may especially be tetrafluoropropene, and more particularly preferably 2,3,3,3-tetrafluoropropene (HFO-1234yf) or 1,1,3,3,3-tetrafluoropropene (HFO-1234ze), in cis form. or trans or as a mixture of cis and trans forms.

According to one embodiment, the first heat transfer fluid is carbon dioxide, and the second heat transfer fluid is HFO-1234yf.

In another embodiment, the first heat transfer fluid is carbon dioxide, and the second heat transfer fluid is HFO-1234ze.

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 about 56% HFO-1234yf.

A mixture of HFO-1234ze and HFC-134a, which is preferably a binary mixture, and which preferably comprises from 50 to 65% HFO-1234ze, and most preferably about 58% 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 about 50% HFO-1234yf.

A mixture of HFO-1234yf, HFO-1234ze and HFC-134a, which preferably is 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% HFO-1234yf. - A mixture of HFO-1234yf and ammonia, which is preferably a binary mixture, and which preferably comprises 15 to 30% 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% HFC-134a, from 2 to at 20% HFC-152a, and 65-96% HFO-1234yf.

A mixture of HFO-1234ze, HFC-134a and HFO-1336mzz (1, 1, 1, 4,4,4-hexafluorobut-2-ene), which is preferably a ternary mixture.

In the ranges above, the proportions of the various compounds are mass proportions.

Various additives may be added to the heat transfer fluids within the scope of the invention in the vapor compression circuits. It may especially be lubricants, stabilizers, surfactants, tracer agents, fluorescent agents, odorants and solubilizing agents.

The stabilizer (s), when present, preferably represent at most 5% by weight in the heat transfer composition.

Among the stabilizers, there may be mentioned in particular nitromethane, ascorbic acid, terephthalic acid, azoles such as azole tolut or benzotriazole, phenol compounds such as tocopherol, hydroquinone, t-butyl hydroquinone, 2,6-di-tert-butyl-4-methylphenol, epoxides (optionally fluorinated or perfluorinated alkyl or alkenyl or aromatic) such as n-butylglycidyl ether, hexanedioldiglycidyl ether, allylglycidyl ether, butylphenylglycidyl ether, phosphites, phosphonates thiols and lactones.

As tracer agents (which can be detected) mention may be made of deuterated or non-deuterated hydrofluorocarbons, deuterated hydrocarbons, perfluorocarbons, fluoroethers, brominated compounds, iodinated compounds, alcohols, aldehydes, ketones, nitrous oxide and combinations thereof. The tracer agent is different from the one or more heat transfer compounds composing the heat transfer fluid.

As solubilizing agents, mention may be made of hydrocarbons, dimethyl ether, polyoxyalkylene ethers, amides, ketones, nitriles, chlorocarbons, esters, lactones, aryl ethers, fluoroethers and magnesium compounds. 1-trifluoroalkanes. The solubilizing agent is different from the one or more heat transfer compounds composing the heat transfer fluid. As fluorescent agents, mention may be made of naphthalimides, perylenes, coumarins, anthracenes, phenanthracenes, xanthenes, thioxanthenes, naphthoxanhthenes, fluoresceins and derivatives and combinations thereof.

As odorants, mention may be made of alkyl acrylates, allyl acrylates, acrylic acids, acrylresters, alkyl ethers, alkyl esters, alkynes, aldehydes, thiols, thioethers, disulfides, allyl isothiocyanates and alkanoic acids. , amines, norbornenes, norbornene derivatives, cyclohexene, heterocyclic aromatic compounds, ascaridole, o-methoxy (methyl) phenol and combinations thereof.

As lubricants or lubricating oils, it is possible in particular to choose compounds chosen from oils of mineral origin, silicone oils, paraffins of natural origin, naphthenes, synthetic paraffins, alkylbenzenes and poly-alpha olefins. , polyol esters, polyalkylene glycols and / or polyvinyl ethers. Polyol esters and polyvinyl ethers are preferred. Polyalkylene glycols are particularly preferred.

The invention is particularly suitable for fluids or bodies to be cooled to a temperature of -50 to -15 ° C, preferably -40 to -25 ° C. The temperature of the external medium typically ranges from -10 to 50 ° C, especially from 0 to 40 ° C, and most preferably from 10 to 35 ° C.

The evaporation temperature of the first heat transfer fluid (temperature in the first evaporator 11) is preferably -60 to -20 ° C, more preferably -50 to -25 ° C.

The condensation temperature of the second heat transfer fluid (temperature in the second condenser 23) depends on the outside temperature, and is typically 20 to 60 ° C, more preferably 20 to 45 ° C. It may for example be + 10 ° C relative to the outside temperature.

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 the same heat exchanger. It may for example be + 5 ° C with respect to said evaporation temperature.

The invention furthermore provides a device for measuring the temperature of the external medium 41 as well as means for adjusting the temperature. in the cascade heat exchanger 30 as a function of the temperature of the external medium being measured.

It has been found by the inventors that the overall performance of the installation is optimal (that is to say that the energy consumption is minimal, 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 according to the outside temperature. The higher the outside temperature, the higher the temperature of the second heat transfer fluid in the cascaded heat exchanger must be, for better efficiency, and vice versa.

According to a preferred embodiment, the evaporation temperature in the cascade heat exchanger 30 is adjusted to an optimum evaporation temperature, which is determined by a calculation module, as a function of the temperature of the external medium which is measured.

The optimum evaporation temperature is preferably defined as the evaporation temperature in the cascade heat exchanger 30 for which the overall performance coefficient of the plant is at a maximum, or for which the overall energy consumption of the installation is minimal (for a given cooling capacity and / or for a given cooling temperature of the fluid or cooled body).

For a given installation, the optimum evaporation temperature can be determined either by directly using the data provided in Example 1 below in connection with Figure 3; either by performing a calculation similar to that presented in Example 1 below, for the installation in question; either experimentally or empirically, by measuring the energy consumption of the installation for different evaporation temperatures of the high temperature circuit, and establishing the correlation with respect to the outside temperature.

Means for determining the optimum evaporation temperature may be included in the installation. Alternatively, and preferably, the function connecting the optimal evaporation temperature to the outside temperature is previously determined, then only this function is incorporated in the above-mentioned calculation module.

The evaporation temperature in the cascade heat exchanger 30 can also be adjusted to a temperature different from the optimum evaporation temperature, to accommodate other constraints. For example, it may be appropriate to limit possible variations in temperature in the cascaded heat exchanger 30 at a certain Ti-T ₂ temperature range. In this case, the evaporation temperature in the cascade heat exchanger 30 is adjusted to the optimum evaporation temperature if it belongs to the Ti-T _{2 range} , or it is adjusted to the temperature Ti if the optimum evaporation temperature is lower than Ti, and finally it is adjusted to the temperature T ₂ if the optimum evaporation temperature is greater than T ₂ .

Many other variations are possible. In particular, a delayed adjustment or a hysteretic adjustment of the evaporation temperature in the cascaded heat exchanger 30 may be provided depending on the temperature of the external medium, in order to avoid adjustments that are too frequent or too abrupt.

In general, the optimum evaporation temperature is an increasing function of the temperature of the external environment. Therefore, it is desirable that, when an increase in the temperature of the external medium is detected, the evaporation temperature in the cascade heat exchanger 30 is increased, and that when a decrease in the temperature of the external environment is detected, the evaporation temperature in the cascade heat exchanger 30 is decreased. Or according to another embodiment, the adjustment is such that, for all temperatures Ti and T ₂ of the external medium given with T ₂ > Ti, the evaporation temperature in the cascade heat exchanger 30 is respectively adjusted to TV and T ₂ 'temperatures with T ₂ ' greater than or equal to TV.

The adjustment of the evaporation temperature in the cascade heat exchanger 30 can be achieved by adjusting the second compressors 22a, 22b, 22c. For example, the means for adjusting the evaporation temperature 42 in the cascade heat exchanger 30 may comprise means for adjusting the speed of rotation of the second compressors 22a, 22b, 22c, or means for successive starting and stopping of the second compressors 22a, 22b, 22c.

The evaporation temperature adjustment in the cascade heat exchanger 30 can be carried out either continuously or at separate times, and for example at regular time intervals (every 15, 30, 45 minutes or 60 minutes, etc.). The temperature adjustment can also be carried out with reference to an average of the temperature of the external medium measured over a certain period, for example over 10 minutes, 30 minutes or 1 hour. EXAMPLE

The following examples illustrate the invention without limiting it. Example 1 - Demonstration of an optimal evaporation temperature

Figure 2 provides a typical example of the variation of the ambient temperature (ambient temperature) during a day, as well as a typical example of the cooling capacity requirements during this day, in order to refrigerate compartments containing products. frozen or frozen in a supermarket type store.

The refrigeration plant is of the type shown schematically in Figure 1. The low temperature circuit contains carbon dioxide, and the high temperature circuit contains HFO-1234yf or HFO-1234ze.

For the low temperature circuit, the evaporation temperature is -40 ° C, the superheat is 25 ° C, and the subcooling is 5 ° C. The compressor is a screw compressor with an isentropic efficiency according to the following equation: η _ί80 = 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 CO2 / NH3 Cascade Refrigeration Systems by Tzong-Shring et al in International Journal of Refrigeration, Vol 29, No. 7, 2006 p.1 100-1 108).

The condensation temperature is 5 ° C higher than the evaporation temperature in the high temperature circuit.

With regard to the high temperature circuit, the evaporation temperature is either set at a constant value (-10 ° C or -18 ° C), or is variable depending on the outside temperature. Overheating is 25 ° C, subcooling is 5 ° C. The compressor is a screw compressor with an isentropic efficiency using the following equation: r | i _S0 = 0.00060079 τ ^{2 to} 0.03002352 0.90880781 + τ (reference: 2008 ASHRAE Handbook, HVAC System and equipments, Chapter 37 , p.22, Twin screw compressor, Figure 34). The condensation temperature is 10 ° C higher than the outside temperature

With the evaporation temperature as a parameter (evaporation temperature in the high temperature stage) the coefficient of performance (COP) is optimized according to the ambient temperature. The COP value for the entire installation has the following formula: (in which COP1 and COP2 are the performance coefficients of low temperature and high temperature circuits).

The correlation between the ambient temperature (T _ex t) and the optimum temperature of evaporation in the high temperature circuit (T _op t) is visible in FIG.

The trend equations are very similar for the two refrigerants tested:

- For HFO-1234yf, T _opt = 0.441 1 T _ext - 26.549 (in degrees Celsius).

- For the HFO-1234ze, T _opt = 0.4208 ^χ T _ext - 26.107 (in degrees Celsius).

Example 2 - Gains Offered by the Invention

In this example, the optimum evaporation temperature demonstrated in Example 1 is used to achieve energy savings.

Thus, the graph of FIG. 4 illustrates the comparison between: (1) the overall energy consumption of the installation operating according to the invention (in gray), that is to say with a temperature adjustment of evaporation of the high temperature circuit to its optimum value as a function of the ambient temperature (as determined in FIG. 3), hour by hour, assuming that the temperature changes during the day according to the curve of FIG. 2; and (2) the overall energy consumption of the same system operating in the traditional way (in black), with an evaporation temperature in the constant high temperature circuit equal to -10 ° C (this is the most usually retained).

The graph in Figure 5 illustrates a comparison between the same two situations, but compared to the TEWI index (equivalent global climate change impact index) as defined in Annex B of EN 378-1: 2008 + A1: 2010. In the graph, the indices are reduced to a reference value of 100 for the installation operating in a traditional manner with HFO-1234yf in the high temperature circuit. The graphs of FIGS. 6 and 7 are similar to those of FIGS. 4 and 5, except that the operation operating in the traditional manner operates with an evaporation temperature in the constant high temperature circuit equal to -18 ° C. instead of -10 ° C.

It has also been verified that the invention makes it possible to predict correctly

(especially based on the graph of Figure 3) the daily energy consumption in case of ambient temperatures is warmer or colder than those of the typical day of Figure 2.

Claims

A method of cooling a fluid or a body by means of at least a first vapor compression circuit (10) containing a first heat transfer fluid and at least a second vapor compression circuit (20) ) containing a second heat transfer fluid, the method comprising:

in the first vapor compression circuit (10):

^■ at least partial evaporation of the first heat transfer fluid by heat exchange with said fluid or body;

^■ the compression of the first heat transfer fluid;

^■ the expansion of the first heat transfer fluid;

in the second vapor compression circuit (20):

^■ the compression of the second heat transfer fluid;

^■ the expansion of the second heat transfer fluid;

the method further comprising:

- measurement of the temperature of the external environment; and

The method of claim 1, wherein adjusting the temperature of the second heat transfer fluid to the evaporation is carried out continuously or is carried out at least once an hour.

A method according to claim 1 or 2 including detecting changes in the temperature of the external medium, and wherein adjusting the temperature of the second heat transfer fluid to evaporation comprises increasing the temperature of the second fluid of heat transfer to 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 to evaporation if a decrease in the temperature of the external medium is detected.

Process according to one of Claims 1 to 3, comprising the calculation of an optimum evaporation temperature as a function of the measurement of the temperature of the external medium.

The method of claim 4, wherein the temperature of the second heat transfer fluid at evaporation is adjusted to the optimum evaporation temperature.

A method according to claim 4 or 5, wherein the optimum evaporation temperature corresponds to the evaporation temperature for which the overall coefficient of performance of the first vapor compression circuit and the second vapor compression circuit is maximum.

Process according to one of Claims 4 to 6, in which the optimum evaporation temperature is defined by the formula T _op t = A x Text + B, in which T _ex t is the temperature of the external medium in degrees Celsius, A is a dimensionless constant, and B is a constant in degrees Celsius.

The method of claim 7, wherein the constant A is from 0.3 to 0.6, preferably from 0.4 to 0.45; and the constant B is from -50 ° C to 0 ° C, preferably from -30 ° C to -20 ° C. Process according to one of claims 1 to 8, wherein the fluid or body is cooled to a temperature of -50 to -15 ° C, preferably -40 to -25 ° C.

Process according to one of Claims 1 to 9, in which:

the second heat transfer fluid is chosen from ammonia, hydrocarbons, hydrofluorocarbons, ethers, hydrofluoroethers, fluoroolefins and mixtures thereof, preferably is tetrafluoropropene, and more preferably is 2,3,3,3-tetrafluoropropene or 1,3,3,3-tetrafluoropropene.

A method according to one of claims 1 to 10, wherein the compression of the second heat transfer fluid is effected by one or more compressors (22a, 22b, 22c), and adjusting the temperature of the second heat transfer fluid. Evaporating heat is effected by adjusting said compressors (22a, 22b, 22c).

A method according to claim 11, wherein the adjustment of said compressors (22a, 22b, 22c) comprises an adjustment of the rotational speed of the compressors (22a, 22b, 22c), or is performed by successive starts and stops of the compressors (22a, 22b, 22c). compressors (22a, 22b, 22c).

Method according to one of claims 1 to 12, which is a method of cooling a compartment containing products, preferably foods, frozen or frozen.

14. Cooling plant for a fluid or a body, comprising at least: a first vapor compression circuit (10) containing a first heat transfer fluid;

a second vapor compression circuit (20) containing a second heat transfer fluid;

a cascade heat exchanger (30) adapted to exchange heat between the first heat transfer fluid and the second heat transfer fluid;

the first vapor compression circuit (10) comprising:

a first evaporator (1 1) adapted to exchange heat between the first heat transfer fluid and said fluid or body;

one or more first compressors (12);

a first expander (14);

the second vapor compression circuit (20) comprising:

one or more second compressors (22a, 22b, 22c);

a second condenser (23) adapted to exchange heat between the second heat transfer fluid and an external medium;

a second expander (24);

the installation also comprising:

a device for measuring the temperature of the external environment

(41); and

means for adjusting the evaporation temperature

(42) in the cascade heat exchanger (30), depending on the measurement of the ambient temperature.

Installation according to claim 14, further comprising a module for calculating an optimum evaporation temperature as a function of the measurement of the temperature of the external environment.

Plant according to claim 15, wherein the means for adjusting the evaporation temperature (42) in the cascade heat exchanger (30) are adapted to adjust the evaporation temperature in the cascade heat exchanger (30) at the optimum evaporation temperature.

17. Installation according to claim 15 or 16, wherein the optimum evaporation temperature corresponds to the evaporation temperature for which the overall coefficient of performance of the first vapor compression circuit and the second vapor compression circuit is maximum.

18. Installation according to one of Claims 15 to 17, in which the optimum evaporation temperature is defined by the formula T _op t = A x Text + B, in which T _ex t is the temperature of the external medium in degrees Celsius , A is a dimensionless constant, and B is a constant in degrees Celsius.

19. Installation according to claim 18, wherein the constant A is from 0.3 to 0.6, preferably from 0.4 to 0.45; and the constant B is from -50 ° C to 0 ° C, preferably from -30 ° C to -20 ° C.

20. Installation according to one of claims 14 to 19, adapted to cool the body or the fluid at a temperature of -50 to -15 ° C, preferably -40 to -25 ° C.

21. Installation according to one of claims 14 to 20, wherein:

22. Installation according to one of claims 14 to 21, wherein the means for adjusting the evaporation temperature (42) in the cascade heat exchanger (30) comprise means for adjusting the second compressors (22a). , 22b, 22c). Installation according to claim 22, wherein the adjustment means of the second compressors (22a, 22b, 22c) are adapted to adjust the speed of rotation of the second compressors (22a, 22b, 22c), or are adapted to start and stop successively the second compressors (22a, 22b, 22c).

24. Installation according to one of claims 14 to 23, comprising a compartment adapted to receive products, preferably food, frozen or frozen.