ES2729981T3 - Method and system to control the overheating of compression refrigeration cycles with a recuperator - Google Patents

Abstract

A method for controlling overheating in a refrigeration cycle system that operates by compressing a refrigerant fluid with recuperator (4), said system comprising a compressor (1) having an inlet and an outlet, an evaporator (6) which it has an inlet (10) and an outlet (9), a thermal expansion valve (5) coupled to said inlet (10) of said evaporator (6) and the recuperator (4) having an inlet (9) and an outlet , characterized in that said method stabilizes, at least locally, the operation of said evaporator (6) by executing a stabilization algorithm for the operation of the evaporator (6), in the following order, at least the steps of: calculating the evaporation temperature ( T4) from the pressure (P1) measured in the evaporator; calculate the thermal power absorbed by the evaporator (6) based on the difference between the air temperature (T2) and the evaporation temperature (T4), multiplied by the equivalent conductance; calculate the enthalpy of the refrigerant at the inlet (10) of the thermal expansion valve (5) based on its measured temperature (T3); calculate the desired enthalpy at the outlet (9) of the evaporator (6) based on the desired vaporization ratio at the outlet (9) of the evaporator (6) and the measured evaporation pressure P1; calculate the refrigerant flow that provides a required enthalpy change between the inlet (10) and the outlet (9) of the evaporator (6) given the estimated thermal power per inversion, knowing the upstream (P2) and downstream (P1) pressure ) of the thermal expansion valve (5) and the density of the liquid refrigerant at the inlet (10), which is a function of the measured temperature (T3), the characteristic discharge rule of the valve (5) to calculate the opening necessary to achieve the calculated flow and send the calculated opening signal of said valve to an actuator of said thermal expansion valve (5).

Description

DESCRIPTION

Method and system to control the overheating of compression refrigeration cycles with a recuperator

[0001] The present invention relates, in a first aspect thereof, to a method of controlling overheating or overheating in compression cooling cycles with a regenerative heat exchanger.

[0002] In a second aspect thereof, the present invention relates to a system for the implementation of the method of the invention.

BACKGROUND OF THE INVENTION

[0003] As is known, the objective of the control system in compression refrigeration cycles is to maximize the thermal power taken by the evaporator of a fluid that carries heat, normally the air from a cooling chamber of an air-conditioned area, while avoiding sending a two-phase mixture to the compressor, which could damage it.

[0004] In conventional refrigeration cycles, the evaporator terminal section acts as a superheater, to send dry superheated steam to the compressor.

[0005] In particular, the control objective is achieved with a simple feedback operation, where the level of overheating is measured at the evaporator outlet and the opening of the thermal expansion valve at the evaporator inlet is adequately modulated so that the overheating remains around an appropriate value "ADJUSTMENT VALUE" or reference point. The latter must be high enough to guarantee the absence of liquid drops at the compressor inlet, with a safety margin during the transients, but at the same time it must be restricted to avoid high gas supply temperatures. In any case, the presence of the overheating section forces the evaporator to operate at a temperature and, therefore, at a pressure, lower than what would be possible in the absence of the section, penalizing the cooling efficiency.

[0006] It is also known that it is possible to significantly improve this efficiency by using the residual heat of the high temperature coolant at the condenser outlet to overheat the coolant in a heat recuperator separated from the evaporator. In this way, it is possible to reduce the temperature difference between the evaporation fluid and the air to be cooled at will, as long as the surface of convective exchange and / or coefficient between the two is increased, thus improving the thermodynamic efficiency of the cycle.

[0007] In this engineering or structural configuration of the plant, the first control objective is to maintain the vaporization ratio at the evaporator outlet at a predetermined value of less than one, possibly at a value close to that of drying to ensure the best possible use of heat exchange surface. The second objective is to ensure sufficient overheating of the refrigerant fluid at the outlet of the recuperator, that is, at the inlet of the compressor to avoid damaging it.

[0008] The simplest method to achieve this objective is to use the same control strategy used in conventional cycles, namely to measure the level of steam overheating at the recuperator outlet and to use a feedback control that acts on the opening of the thermal expansion valve to carry it and keep it at an approximate set point value, corresponding to the optimum conditions of the cycle conditioning.

[0009] Unfortunately, this configuration tends to exhibit very easily unstable behavior, characterized by strong oscillations.

[0010] In particular, it is very difficult, if not impossible, to design a control rule that guarantees stable behavior in all possible operating conditions of the system.

[0011] The origin of this difficulty is due to the combination of three phenomena. The first is the propagation delay between variations in the thermal expansion valve flow rate and the corresponding variations in the vaporization ratio at the evaporator outlet.

[0012] The second phenomenon is the extreme nonlinearity of the relationship between the vaporization ratio at the evaporator outlet and, therefore, at the recuperator inlet, and the level of overheating at the recuperator outlet that is used for feedback. This nonlinearity is due to the strong dependence on the vaporization ratio of the convective exchange coefficient in the inlet section of the recuperator, which provides the final evaporation section for the cooling fluid.

[0013] The third phenomenon is given by the additional coupling introduced in the process by the recuperator, whereby even a moderate increase in the vaporization ratio at the entrance leads to a sharp drop in the coefficient of exchange and, therefore, a drop in heat taken from the hot side, that is, an increase in temperature on the hot side, which implies an increase in the vaporization ratio downstream of the thermal expansion valve and, therefore, an additional increase in the vaporization ratio at the evaporator outlet.

[0014] This positive feedback mechanism is destabilizing and results in hysteresis phenomena that have been experimentally confirmed.

[0015] In conclusion, only the overheating measurement downstream of the recuperator is inadequate to implement a feedback setting that is reliable and stable in all process operating conditions, unless it is integrated with other measurements. EP 2765370 A1 describes a method for controlling overheating in a refrigeration cycle system according to the preamble of claim 1 and a system for controlling overheating in a refrigeration cycle.

SUMMARY OF THE INVENTION

[0016] The aim of the present invention is, therefore, to provide a method for controlling overheating in compression refrigeration cycles with a regenerative heat exchanger that allows stabilizing the vaporization ratio at the evaporator outlet in an indirect manner. .

[0017] Within the objective mentioned above, a main object of the present invention is to provide a method of the indicated type that provides the stabilization sought by measuring and evaluating at least the following parameters:

- temperature of the air to cool;

- flow temperature upstream of the thermal expansion valve;

- pressure upstream of the thermal expansion valve;

- evaporation pressure.

[0018] Another object of the present invention is to provide a method of the indicated type that can be implemented with extremely simple and economical instrumentation, in particular a single pressure sensor in the low pressure line of the circuit, positioned at the evaporator outlet , as well as a single temperature sensor and a single pressure sensor, both placed upstream of the thermal expansion valve.

[0019] A further object of the present invention is to provide a method of the indicated type that works on the basis of a new and novel control and regulation algorithm capable of stabilizing the operation of the cooling system by directly compensating the effects of: changes in temperature of the air to cool; changes in the condensation pressure upstream of the thermal expansion valve; changes in the flow and temperature of the condenser coolant; and changes in evaporation pressure and heat exchange of the recuperator with subsequent variations in the enthalpy content of the refrigerant at the outlet of the recuperator and at the inlet of the thermal expansion valve.

[0020] A further object of the present invention is to provide a method of the indicated type that is extremely effective not only for local stabilization of the system, but also for large disturbances involving the entire system.

[0021] A further object of the present invention is to provide a method of the indicated type that can easily be adapted to handle possible failures of the fans serving the evaporators.

[0022] A further object of the present invention is to provide a control and regulation system to implement the method of the invention, it being possible to configure this system as a modular device applicable to any cooling chamber or room with air conditioning and / or area similar, whether of new construction or even of a pre-existing type, this system guaranteeing, thanks to the implementation of the method of the invention, the maximization of the thermal power taken from the heat-carrying refrigerant by the evaporator, while avoiding shipping of a two-phase mixture to the system compressor, which could damage it.

[0023] The last, but not least, object of the present invention is to provide a refrigeration cycle control and regulation system that can be constructed from materials / components readily available in the market with reliable operation, as well as costs economically competitive

[0024] According to the present invention, the objective and the aforementioned objects, as well as other objects, which will appear clearer later on, are achieved by a method for controlling overheating in a refractive cycle system that works by compressing a fluid. refrigerant with a regenerative heat exchanger according to claim 1.

[0025] Other features of the method of the invention are defined in the dependent claims.

[0026] The object and objects mentioned above are also achieved by a system to implement the method of the invention as set forth in the appended claims regarding the system.

BRIEF DESCRIPTION OF THE DRAWINGS.

[0027] Other features and advantages of the method and system according to the present invention will become more apparent from the following detailed description of its presently preferred embodiments, shown by way of indicative and non-limiting example in the accompanying drawings, in which:

FIG. 1 contains the pseudocode of the stabilization algorithm for the vaporization ratio at the outlet 9 of the evaporator 6 at a value close to that required;

FIG. 2 is a block diagram of the overall configuration of the control, regulation and stabilization system, which constitutes the subject of the present invention; and

FIG. 3 shows the arrangement of a refrigeration cycle system with recuperator, in which the method and system of the invention is used.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] An arrangement of the refrigeration cycle system with recuperator referred to in the method of the invention is shown in Figure 3. The system comprises a compressor 1, a condenser 2, a receiver 3 for liquid refrigerant, a heat recuperator or regenerative heat exchanger 4, a thermal expansion valve 5, an evaporator 6 and a transmission line 8 to the valve 5, for the signal detected by the sensor 7.

[0029] When the system is in operation, the refrigeration gas is compressed in the compressor 1, then enters the condenser 2 where it changes state, going into a liquid phase, after which it is sent to the liquid receiver 3. The liquid that leaves the receiver 3 is sent to the recuperator 4, where it is cooled by the gas leaving the evaporator 6. From the recuperator 4, the liquid is made to enter the thermal expansion valve 5, where the adiabatic expansion occurs, which forms a mixture of liquid / gas that enters the evaporator 6. In the latter, there is a change of state from the liquid phase to the gas phase which, in turn, enters the recuperator 4. The cycle ends with the shipment of the gas from recuperator 4 to compressor 1.

[0030] As mentioned, the main problem in the regulation of the refrigeration cycle with the recuperator 4 is that the variations in the vaporization ratio of the evaporator 6 occur at its outlet 9 with a great delay with respect to the conditions at the entrance 10, but this variation is not directly detectable because a two-phase mixture that is in almost isobaric and isothermal conditions passes through the evaporator and there are no reliable and economical methods to measure the vaporization ratio of this mixture.

[0031] Vaporization variations occur, even brutally, only in recuperator 4, where latent heat is transformed into sensible heat, but when this occurs, it is too late to act on the thermal expansion valve 5 without causing oscillations and instability in the operation of the process.

[0032] The determination of the level of steam overheating at the inlet of compressor 1, which must be controlled, in any case would require measuring:

- the pressure P1 of the steam at the outlet of the recuperator 4, necessary to calculate the corresponding saturation temperature; and

- the temperature T1 of the steam measured by the sensor 7 at the outlet of the recuperator 4.

[0033] In any case, the temperature T2 of the air at the inlet of the evaporator 6 must also be measured, to allow its regulation when turning on and off the compressor 1.

[0034] Therefore, the fundamental idea of the method of the invention was to introduce additional measurements of the process that directly stabilize the ratio of the liquid / gas mixture at the evaporator outlet, which is defined as the ratio between the mass of the gas phase and the total mass. In particular, these additional measures refer to:

- the temperature T3 of the refrigerant fluid upstream of the thermal expansion valve 5; and

- the pressure P2 of the fluid upstream of the thermal expansion valve 5.

[0035] In principle, the implementation of the proposed method would also require measuring the evaporation pressure inside the evaporator 6. To keep instrumentation costs low, given the relatively small head losses in the low pressure line of the circuit, it is possible to ignore these losses of load and to use only a pressure sensor P1 in this line, located at the outlet of the recuperator 4 at point 11. In this way, the additional instrumentation that needs to be installed in the circuit is reduced to a single temperature sensor T3 and a single pressure sensor P2, both positioned upstream of the thermal expansion valve 5 at point 10.

[0036] It is also necessary to know:

- the nominal discharge coefficient K ^{v, nom} and the characteristic opening curve f (0 ^v ) of the thermal expansion valve 5, so that the effective discharge coefficient based on the opening 0 ^v is given by K ^v = K ^{v, nom} ■ f (0 ^v ). The value of the function f () varies from zero (fully closed valve) to 1 (fully open valve). Both data elements can be deduced from data provided by the valve manufacturer and / or measured experimentally by bench tests.

- the equivalent thermal conductance G of the evaporator 6, that is, the factor which, multiplied by the temperature jump AT between the evaporation temperature of the liquid at the outlet of the evaporator 6 at the pressure P1 and the temperature T2 of the air to be cooled, produces the thermal power absorbed by the evaporator 6, Q = G ■ AT; This value can be obtained from the design data of the evaporator and / or experimental measurements in a thermostatic cell, and can depend on the number and speed of the active fans that feed the evaporator.

- the specific enthalpy curves of the liquid and the saturated vapor h ^ls (p) and h ^vs (p) depending on the refrigerant pressure, the density curve of the saturated liquid as a function of the saturation temperature p ^ls (T), the saturation temperature curve as a function of the pressure of the refrigerant T ^s (p), as well as the average specific heat c ^p of the refrigerant in the subcooled liquid state. These curves are known in the literature for all industrially used refrigerants and can be approximated by polynomials or other interpolation formulas easily implemented in industrial controllers.

[0037] According to the present invention, the method of the invention operates based on an innovative control algorithm, capable of stabilizing the vaporization ratio at the outlet of the evaporator 6 and based on the following process steps, represented by the pseudocode in Figure 1.

[0038] In a first step S1, the evaporation temperature T ^ev is calculated from the pressure P1 measured at the outlet of the recuperator at point 7, which is approximately equal to the pressure inside the evaporator, according to the formula :

(S1) T ^ = Ts (P1)

where Ts represents the saturation temperature of the saturated steam at the pressure P1.

[0039] In a successive operation stage S2, the thermal power Q absorbed by the evaporator 6 is calculated based on the difference between the measured air temperature T2 and the evaporation temperature Tev multiplied by the equivalent thermal conductance G, according with the formula:

[0040] In a successive step S3, the specific enthalpy h¡n of the refrigerant at the inlet of the thermal expansion valve 5 is measured at point 10, depending on its measured temperature T3, according to the formula:

where hls is the enthalpy of the saturated liquid at the pressure P1 and cp is the specific heat at constant pressure.

[0041] In a successive step S4, the desired enthalpy hu at the outlet 9 of the evaporator 6 is calculated based on the desired vaporization ratio xv ° at the outlet of the evaporator 6 and the evaporation pressure measured P1, according to the formula :

where hvs is the enthalpy of saturated steam at pressure P1.

[0042] In a successive step S5, given the estimated thermal power Q, the refrigerant flow mr that causes the required enthalpy change between the inlet 10 and the outlet 9 of the evaporator 6 is calculated according to the formula:

[0043] In a successive step S6, knowing the upstream pressure P2 and downstream P1 of the thermal expansion valve 5 and the density of the coolant at the inlet 10, which is a function of the measured temperature T3, it is then calculated the necessary discharge coefficient of the valve 5 to achieve the flow calculated in step S5. In this regard, it may be appropriate to introduce a correction dp that takes into account additional load losses in distributor 12, in accordance with the formula:

(S6) Kv = mr / sqrt (pis (T3) * (P2 - P1 -dp))

where the sqrt () function calculates the square root, Kv is the discharge constant of the thermal expansion valve and pee is the density of the saturated liquid at the temperature T3.

[0044] In the last step S7, the corresponding valve opening is calculated, using the inverse function of the opening characteristic. The corresponding signal is sent to the thermal expansion valve actuator, according to the formula:

where 9y is the valve opening level and f-1 is an inverse function of the nominal Kv / Kv ratio.

[0045] The steps of the algorithm that regulate the method of the invention and have been briefly described above allow to achieve, in normal operation under nominal conditions, as the applicants have demonstrated, the desired vaporization ratio of the liquid / gas mixture to the evaporator outlet 9 6.

[0046] In particular, stabilization is achieved by directly compensating, in accordance with the method of the invention, the effect of the following phenomena:

- change in air temperature T2;

- change in the condensation pressure P2 upstream of the thermal expansion valve 5, possibly caused by changes in the flow and temperature of the condenser cooling fluid;

- changes in evaporation pressure P1;

- changes in the heat exchange in the recuperator 4, with the consequent variations in the enthalpy content of the refrigerant that leaves the recuperator and enters the thermal expansion valve.

[0047] In particular, an expert in the field will understand that offsetting the last two effects eliminates the destabilizing positive feedback of the prior art mentioned above.

[0048] Experimental studies conducted by Applicants have shown that the method described above is effective in stabilizing the system.

[0049] The base algorithm described above, on which the method of the invention is constructed, is capable of stabilizing the operation of the system. However, the uncertainty in the values of the parameters, in particular the equivalent conductance of the evaporator and the measurement errors, can result in an operation of stabilization of the evaporator at real values of the vaporization ratio at the outlet of the evaporator 6 and of overheating at the inlet of compressor 1 that are significantly different from those required.

[0050] As it is not possible to measure the value of the vaporization ratio at the evaporator outlet directly, by introducing a direct feedback, a provision is made instead, according to the invention, to use the control diagram shown in the Fig. 2. Block S represents the algorithm described above, where X ° ev represents the vaporization ratio x ° v required at the evaporator outlet 6. Block R represents a conventional proportional-integral-derivative controller (PID). The block P represents the process to be controlled, where Tmv and Pmv correspond to the temperature and pressure upstream of the valves T3 and P2, Pev corresponds to the evaporation pressure P1 and Tvr corresponds to the steam temperature at the outlet of the recuperator T4 . The DT block represents the calculation of the superheat level based on the pressure and temperature values measured at the recuperator outlet at point 7, which can be calculated with the formula DT = T4 - Ts (P1).

[0051] According to this global control diagram, the feedback circuit for the superheat level at the output of the recuperator 4 is implemented by a PID controller, which acts on the value of the vaporization ratio required at the evaporator output. 6 using it as a virtual control variable. The control algorithm described above establishes the opening of the thermal expansion valve 5 to obtain this vaporization ratio.

[0052] Given the presence of a significant delay between the moment at which the required vaporization ratio is changed and the moment at which the overheating reacts, due to the dynamics of the evaporator, it is necessary that this feedback be configured with a small band of step to avoid the onset of oscillations or instability. The parameter values of the PlD controller can be easily configured by an expert operator in the installation phase, considering the system resulting from the connection of the S and P blocks as the system to be controlled. This task can also be carried out by means of an appropriate self-calibration algorithm chosen from those available in the market or in the literature. Experimental studies conducted by Applicants have shown that this activity configuration is not critical and that a fixed calibration of the parameters is sufficient to guarantee the correct operation of the system in all possible operating conditions.

[0053] An expert in the field will understand that this strategy is fundamentally different from the standard, in which the block S in Figure 2 is absent and the feedback on the level of overheating performed by the PID controller acts directly on the opening of The valve. In particular, as already noted, the additional feedback of the signals P1, P2 and T3, carried out by the block S, efficiently performs the function of stabilizing and regulating the operation of the process. This facilitates the operation and configuration of the PID controller, which has the sole task of introducing a correction to the value required for the vaporization ratio x ° ^v , to obtain, in normal operation, the value of the overheating level required in point 7 of the circuit, despite the uncertainty regarding the values of the process parameters.

[0054] With respect to the starting transient, it is assumed that it starts from conditions in which the evaporator is empty or, in any case, at a very low pressure corresponding to a minimum refrigerant content. When the compressor is started, the thermal expansion valve is initially opened at a constant value that can be adapted according to the evaporation pressure, to take into account the different operating conditions at different temperatures. Then there is a waiting period until the pressure exceeds a threshold value, determined based on the reduced saturation pressure in a timely range. Once the threshold is exceeded, the previously defined control-regulation method-algorithm is started.

[0055] Also in this case, experimental tests have shown that in this way, the superheating sub-pulses at the compressor inlet with respect to the set point value are maintained within a few degrees, so a value of around 10-15 degrees for this setpoint ensures a wide safety margin.

[0056] When the compressor must be stopped, for example, depending on the thermostatic control of the air temperature, it is first necessary to completely close the thermal expansion valve, in order to empty the evaporator; The compressor can be stopped when the pressure drops below an appropriate minimum threshold.

[0057] If this sequence is not feasible, since the control of the compressor is independent of that of the thermal expansion valve, it is preferable to modify the starting procedure in this way: when starting the compressor, the valve remains closed until the pressure falls below the minimum threshold, after which the sequence described above is followed.

[0058] Starting with an empty evaporator serves to guarantee the repeatability of the maneuver, in particular with respect to potentially dangerous subheats of overheating, which may lead to the temporary presence of liquid in the intake of the compressor, with the consequent mechanical damage.

[0059] According to an additional feature of the invention, the method and the algorithm used therein can be advantageously adapted to handle fan failures (not shown) that serve the evaporators.

[0060] In the case of an evaporator with a single fan, the only possible strategy is to identify the fault condition by introducing a lower overheating threshold, far enough from the set reference point to avoid false alarms, but at the same time high enough to prevent the entry of two-phase fluid into the compressor.

[0061] When this threshold is exceeded, the thermal expansion valve closes immediately to avoid flooding the evaporator and the two-phase fluid entering the compressor.

[0062] It should be taken into account that, in this case, it is not possible to continue operating the system in degraded conditions.

[0063] In the case of systems with N multiple fans, it can be assumed that the fault only affects one of them, and in this case it may make sense to continue operating the system in degraded conditions without stopping it, waiting for maintenance operations. Therefore, it is necessary to set a lower overheating alarm threshold and an even lower one to stop.

[0064] When the first threshold is exceeded, the controller will initially assume that the functionality of one of the N fans has been lost. In a first approximation, this implies a reduction by a factor of 1 / N of the equivalent conductance G of the evaporator. Therefore, it is possible to modify this parameter in the stabilization algorithm within block S in Fig. 2 in this regard.

[0065] It should be borne in mind that the immediate effect of this modification will be a drop in the estimated heat flux Q, which will be followed by an immediate drop in the flow required for the thermal expansion valve m ^r , which will take carried out immediately by a reduction in the opening of the valve 0v, in order to maintain the thermal equilibrium of the evaporator.

[0066] If the fault is really due to the loss of a single fan, the system will stabilize under the required conditions of overheating and vaporization, obviously with a cooling power reduced by a factor of 1 / N.

[0067] This operational condition can be reported to the supervising operator, so that the latter activates the maintenance procedure, which is not necessarily immediate.

[0068] In contrast, if overheating continues to fall and falls below the stop threshold, then it is necessary to completely close the thermal expansion valve and stop the compressor once the evaporator is empty.

[0069] The possibility described above of adapting the method of the invention to handle faults in the fans that serve the evaporators also constitutes an important aspect of the present invention, which is not deductible in any way from the control systems or corresponding refrigeration of the prior art.

[0070] It can be seen from the foregoing that the invention fully achieves the objective and the intended objects.

Claims (7)

i. A method for controlling overheating in a refrigeration cycle system that operates by compressing a refrigerant fluid with recuperator (4), said system comprising a compressor (1) having an inlet and an outlet, an evaporator (6) which it has an inlet (10) and an outlet (9), a thermal expansion valve (5) coupled to said inlet (10) of said evaporator (6) and the recuperator (4) having an inlet (9) and an outlet , characterized in that said method stabilizes, at least locally, the operation of said evaporator (6) by executing a stabilization algorithm for the operation of the evaporator (6), in the following order, at least the steps of: calculating the evaporation temperature ( T4) from the pressure (P1) measured in the evaporator; calculate the thermal power absorbed by the evaporator (6) based on the difference between the air temperature (T2) and the evaporation temperature (T4), multiplied by the equivalent conductance; calculate the enthalpy of the refrigerant at the inlet (10) of the thermal expansion valve (5) based on its measured temperature (T3); calculate the desired enthalpy at the outlet (9) of the evaporator (6) based on the desired vaporization ratio at the outlet (9) of the evaporator (6) and the measured evaporation pressure P1; calculate the refrigerant flow that provides a required enthalpy change between the inlet (10) and the outlet (9) of the evaporator (6) given the estimated thermal power per inversion, knowing the upstream (P2) and downstream (P1) pressure ) of the thermal expansion valve (5) and the density of the liquid refrigerant at the inlet (10), which is a function of the measured temperature (T3), the characteristic discharge rule of the valve (5) to calculate the opening necessary to achieve the calculated flow and send the calculated opening signal of said valve to an actuator of said thermal expansion valve (5). 2. A method according to claim 1, characterized in that it includes an additional step for limiting the opening and closing speed of said thermal expansion valve to a maximum value compatible with the maximum opening and closing speed of said actuator. 3. A method according to claim 1, characterized in that it comprises an additional corrective step consisting of feedback, with a reduced pitch band, an overheating value at said inlet of said compressor, said overheating value acting on a value of the ratio of vaporization required, used as virtual variable of said evaporator. 4. A method according to any of the preceding claims, characterized in that said evaporator comprises an evaporator with a single fan and said method includes an additional step of identifying a possible failure state of said single fan by introducing a lower overheating threshold sufficiently distant from a set point to avoid false alarms, but at the same time high enough to prevent the discharge of biphasic liquid into said compressor, immediately closing the thermal expansion valve and preventing said cooling system from continuing to operate when the lower threshold is exceeded. A method according to any of the preceding claims, characterized in that said evaporator comprises a plurality of fans, said method comprising, in the case where only one of said fans fails, the step of establishing a lower overheating alarm threshold and a corresponding lower stop threshold of said system to allow said system to continue operating in degraded conditions, without going into a stop state, waiting for maintenance operations not necessarily immediate on said defective fan to restore said system to a state of total operability. 6. A system for performing the method according to any of the preceding claims, the system comprising at least one logical block (S) based on the vaporization ratio x ° v required at the evaporator outlet (6) to implement a stabilization algorithm comprising, in the following order, at least the steps of: calculating the evaporation temperature (T4) from the pressure (P1) measured in the evaporator; calculate the thermal power absorbed by the evaporator (6) based on the difference between the air temperature (T2) and the evaporation temperature (T4), multiplied by the equivalent conductance; calculate the enthalpy of the refrigerant at the inlet (10) of the thermal expansion valve (5) based on its measured temperature (T3); calculate the desired enthalpy at the outlet (9) of the evaporator (6) based on the desired vaporization ratio at the outlet (9) and the measured evaporation pressure P1; calculate the refrigerant flow that provides a required enthalpy change between the inlet (10) and the outlet (9) of the evaporator (6) given the estimated thermal power per inversion, knowing the upstream (P2) and downstream (P1) pressure ) of the thermal expansion valve (5) and the density of the liquid refrigerant at the inlet (10), which is a function of the measured temperature (T3), the characteristic discharge rule of the valve (5) to calculate the opening necessary to achieve the calculated flow; and sending the calculated opening signal of said valve to an actuator of said thermal expansion valve (5), at least one logical block (R) representing a proportional integral-derivative regulation type PID that generates the desired vaporization value in the outlet (9) of the evaporator (6) at least one logic block (S), and at least one logic block (DT) that calculates the level of overheating based on the pressure and temperature of the steam at the outlet of said recuperator ( 4) and sends said superheat level to said at least one logic block (R). A system according to claim 6, characterized in that said stabilization algorithm generated by said block (S) provides, in normal operation and in optimal conditions, the desired vaporization ratio at the outlet of said evaporator and the desired stabilization by directly compensating the less: the change in air temperature, the change in condensation pressure upstream of the thermal expansion valve, the change in evaporation pressure; and the change in heat exchange in the recuperator, with consequent variations in the enthalpy content of the refrigerant at the outlet of the recuperator and at the inlet of the thermal expansion valve.