HEAT EXCHANGE APPARATUS The present invention relates to a heat exchange apparatus. It is known that heat exchange apparatuses, such as refrigeration apparatuses, air conditioning apparatuses or heat pumps, have a heat exchange circuit in which an exchange fluid circulates which is adapted to exchange heat between two heat sources at different temperatures. Along the heat exchange circuit there is an evaporator, in which the exchange fluid exchanges heat with the higher-temperature source by transiting from a liquid phase to a vapor phase, and a condenser, in which the exchange fluid exchanges heat with the lower-temperature source, by transiting, at least partially, from the liquid phase to the vapor phase. A compressor is positioned between the outlet of the evaporator and the inlet of the condenser, and compresses the refrigerant fluid, while an expansion valve is interposed between the outlet of the condenser and the inlet of the evaporator, in which the exchange fluid is expanded in order to lower its pressure. Also along the circuit, it is usual to interpose a regenerative exchanger between the evaporator and the condenser, which enables the heat exchange between the vapor-phase exchange fluid exiting from the evaporator and the liquid-phase exchange fluid that exits from the condenser at a higher temperature than the temperature of the exchange fluid exiting from the evaporator. In this manner, the regenerative exchanger makes it possible to increase the supercooling of the liquid-phase exchange fluid exiting from the condenser, before it reaches the expansion valve, and the superheating of the vapor-phase exchange fluid exiting from the evaporator on the suction line of the compressor, with consequent improvement of the efficiency of the apparatus and reduction of the risk of suction into the compressor of exchange fluid that is still in the liquid phase. At present, adjustment of superheating of the exchange fluid, required as a consequence of variations in operation of the compressor, in the presence of variable load conditions, is done by acting on the degree of opening or closure of the expansion valve, on the basis of measured values of the temperature and of the pressure of the exchange fluid exiting from the evaporator. In the known art, the regenerative exchanger generates an additional superheating of the exchange fluid and is not subjected to adjustments with the goal of improving the efficiency of the apparatus. The aim of the present invention is to provide a heat exchange apparatus which is capable of avoiding the drawbacks of the known art in one or more of the above mentioned aspects. Within this aim, an object of the invention is to provide a heat exchange apparatus that makes it possible to ensure an optimal efficiency of the apparatus even under partial loads, by limiting the superheating of the exchange fluid at the evaporator, and to keep the exchange fluid exiting from the evaporator at around the dew point. Furthermore, the present invention sets out to overcome the drawbacks of the background art in a manner that is alternative to any existing solutions. Another object of the invention is to provide a heat exchange apparatus that is highly reliable, is relatively easy to implement, and can be produced at low cost. This aim and these and other objects which will become more apparent hereinafter are achieved by a heat exchange apparatus according to claim 1, optionally provided with one or more of the characteristics of the dependent claims. Further characteristics and advantages of the invention will become better apparent from the description of preferred, but not exclusive, embodiments of the heat exchange apparatus according to the invention, which are illustrated for the purposes of non-limiting example in the accompanying drawings wherein: Figure 1 is a diagram of the apparatus according to the invention in a first embodiment; Figure 2 is a diagram of the apparatus according to the invention, in a second embodiment; Figure 3 is a diagram of the apparatus according to the invention in a third embodiment; Figure 4 is a schematic diagram of a fourth embodiment of the apparatus according to the invention; Figure 5 is a graph of the trend of the flow of the liquid-phase exchange fluid passing through a regenerative exchanger as a function of the degree of opening of a second adjustment valve provided in the apparatus, for the embodiment of Figure 1; Figure 6 is a graph of the trend of the flow of the liquid-phase exchange fluid passing through the regenerative exchanger as a function of the degree of opening of the second adjustment valve, for the embodiment of Figure 3; Figure 7 is a graph of the trend of the flow of the liquid-phase exchange fluid passing through the regenerative exchanger as a function of the degree of opening of the second adjustment valve, for the embodiment of Figure 4; Figure 8 is a general diagram of a portion of the apparatus according to the invention. With reference to the figures, the heat exchange apparatus according to the invention, generally designated by the reference numeral 1, comprises a heat exchange circuit 2 which is passed through by a heat exchange fluid, such as for example refrigerant fluid R134a. In particular, the circuit 2 has an evaporator 3, preferably of the dry expansion type, in which the exchange fluid exchanges heat with a first temperature source, in so doing heating up and thus transiting from the liquid phase to the vapor phase. The evaporator 3 is connected in output to a compressor 4 which receives the vapor-phase exchange fluid and compresses it in order to send it in input to a condenser 5, in which the exchange fluid is cooled, by exchanging heat with a second temperature source, kept at a temperature that is lower than the first source, and thus transiting from the vapor phase to the liquid phase. The output of the condenser 5 is, in turn, connected to the evaporator 3 via the interposition of expansion means 6, the function of which is to lower the pressure of the liquid-phase exchange fluid, before it enters the evaporator 3. The circuit 2 is further provided, between the evaporator 3 and the condenser 5, with at least one regenerative exchanger 7 which enables the heat exchange between the vapor-phase exchange fluid exiting from the evaporator 3 and the liquid-phase exchange fluid exiting from the condenser 5. More specifically, the circuit 2 is provided with at least one liquid line 2a, through which the liquid-phase exchange fluid flows and which connects the outlet of the condenser 5 with the inlet of the evaporator 3 and passes through the regenerative exchanger 7, and at least one steam line 2b, through which the vapor-phase exchange fluid flows and which, in turn, connects the outlet of the evaporator 3 with the inlet of the condenser 5, also passing through the regenerative exchanger 7. As illustrated, the compressor 4 is interposed along the steam line 2b, between the regenerative exchanger 7 and the condenser 5. According to the invention, the circuit 2 is provided, between the outlet of the condenser 5 and the inlet of the evaporator 3, with adjustment valve means for adjusting the flow of the exchange fluid in the circuit 2. Also according to the invention, there are furthermore sensor means, described in more detail below, which make it possible to measure at least one thermodynamic parameter of the exchange fluid at preset points of the circuit 2 in order to allow the calculation of the efficiency of the regenerative exchanger. The sensor means are connected to control means 8, which are constituted, preferably, by a programmable electronic controller. In particular, the control means 8 are configured to calculate, using the measurements supplied by the sensor means, the instantaneous efficiency of the regenerative exchanger 7 and a reference efficiency or set- point efficiency of said regenerative exchanger. The thermal efficiency (ε) of the regenerative exchanger 7 is defined by the general formula, per se known: where: T _Vout is the temperature of the vapor-phase exchange fluid in output from the regenerative exchanger 7, T _Vin is the temperature of the vapor- phase exchange fluid in input to the regenerative exchanger 7, T _Lin is the temperature of the liquid-phase exchange fluid in input to the regenerative exchanger 7 and TLout is the temperature of the liquid-phase exchange fluid in output from the regenerative exchanger 7. The instantaneous efficiency is calculated by the control means 8 by substituting the variables T _in , T _out , T _Lin , T _Vin appearing in the formula with the values measured using the sensor means, while the reference efficiency is instead calculated, again by the control means 8, using the same formula, but requiring that the superheating at the evaporator 3 of the exchange f luid be substantially comprised between 0 and 5 K and, more preferably, between 0 and 3 K, as will be better explained below. The control means 8 furthermore compare the calculated values of the instantaneous efficiency and of the reference efficiency of the regenerative exchanger 7 and calculate the difference between them, and are designed to actuate the adjustment valve means, in order to vary the degree of opening thereof, at least on the basis of the calculated difference between the instantaneous efficiency and the reference efficiency of the regenerative exchanger 7. Advantageously, the expansion means 6 are constituted by at least one expansion valve 9, with adjustable opening, which is interposed along the liquid line 2a between the regenerative exchanger 7 and the evaporator 3. The cited adjustment valve means comprise at least one first adjustment valve which can be constituted by said expansion valve 9. According to possible embodiments of the invention, better described below, the adjustment valve means can also comprise, in addition to the first adjustment valve constituted by the expansion valve 9, at least one second adjustment valve 10, also interposed along the liquid line 2a, which makes it possible, in particular, to vary the flow rate of the liquid-phase exchange fluid that is made to pass through the regenerative exchanger 7. Advantageously, the sensor means supplied for calculating the instantaneous efficiency and the reference efficiency of the regenerative exchanger 7 comprise at least one first temperature sensor 11, arranged along the steam line 2b between the regenerative exchanger 7 and the compressor 4, at least one second temperature sensor 12, arranged along the liquid line 2a upstream of the regenerative exchanger 7, and at least one third temperature sensor 13, arranged along the liquid line 2a downstream of the regenerative exchanger 7. In particular, the first temperature sensor 11 measures the temperature value of the vapor-phase exchange fluid in output from the regenerative exchanger 7 (T _Vout ), which corresponds, basically, to the temperature of the exchange fluid in input to the compressor 4 (T _suction ), the second temperature sensor 12 measures the temperature value of the liquid-phase exchange fluid in input to the regenerative exchanger 7 (TLin), while the third temperature sensor 13 measures the temperature value of the liquid-phase exchange fluid in output from the regenerative exchanger 7 (TLout). Conveniently, the sensor means also comprise a pressure sensor 14 arranged along the steam line 2b, between the evaporator and the compressor 4, for measuring the evaporation pressure value of the exchange fluid exiting from the evaporator 3 (pEVout), with which the control means 8 can calculate the saturation temperature of the exchange fluid in output from the evaporator 3, as will also be better explained below. Optionally, the sensor means can further comprise a fourth temperature sensor 15, arranged along the steam line 2b, between the evaporator 3 and the regenerative exchanger 7, for measuring the temperature of the exchange fluid in input to the regenerative exchanger 7 (T _Vin ), which is equal to the temperature of the exchange fluid in output from the evaporator 3 (T _EVout ). Alternatively, if, for reasons of the apparatus's construction, it should not be possible to install the fourth temperature sensor 15 between the evaporator and the regenerative exchanger 7, as in the case where the regenerative exchanger 7 is integrated with the evaporator 3, the temperature value of the exchange fluid in output from the evaporator 3 and in input to the regenerative exchanger 7 (T _Vin ) can also be estimated by the control means 8 by way of measurements supplied by the other temperature sensors, as will be better described below. It should be noted that, according to possible embodiments of the apparatus according to the invention, on the liquid line 2a there can be a bypass line 2c of the regenerative exchanger 7, which connects a first section of the liquid line 2a, located upstream of the regenerative exchanger 7, with a second section of the liquid line 2a, located downstream of the regenerative exchanger 7. In this case, the second adjustment valve 10 is interposed along the bypass line 2c. Turning now to describe the apparatus according to the invention in more detail, it can be noted, with reference to a first embodiment, shown in Figure 1, that the second adjustment valve 10 can be constituted by a variable-aperture two-way valve, interposed along the bypass line 2c. With this embodiment, the flow rate (ṁL) of the exchange fluid that passes through the regenerative exchanger 7 can be modulated, on the basis of the degree of opening of the second adjustment valve 10, between a minimum value (ṁ _Lmin ), greater than zero, and a maximum value equal to the total flow rate value (ṁ _LTOT ) of the exchange fluid exiting from the condenser 5, as can be observed from the graph of Figure 5. The minimum flow value (ṁ _Lmin ) depends on the configuration of the circuit and in particular on the flow resistances along the liquid line 2a and the bypass line 2c. In a second embodiment, simpler than the first, shown in Figure 2, it can be noted that both the bypass line 2c and the second adjustment valve 10 are absent and only the expansion valve 9 is present to provide the adjustment valve means for adjusting the flow of the exchange fluid. In this case, since the second adjustment valve 10 is not present, the ratio between the flow of the liquid-phase exchange fluid that passes through the regenerative exchanger 7 and the flow of the vapor-phase exchange fluid that passes through the regenerative exchanger 7 is always equal to 1, while, by varying the degree of opening of the expansion valve 9, the only thing that can be varied is the total flow rate of the exchange fluid that circulates inside the circuit 2. According to a preferred embodiment, shown in Figure 3, the second adjustment valve 10 is provided via a variable-aperture three-way valve, which is connected, with two ways, along the liquid line 2a, for example downstream of the regenerative exchanger 7, and, with a third way, to the bypass line 2c. In this embodiment, by varying the degree of opening of the second adjustment valve 10 it is possible to vary the flow (ṁ _L ) of liquid-phase exchange fluid that passes through the regenerative exchanger 7 from zero to the total flow rate value (ṁ _LTOT ) of the exchange fluid exiting from the condenser 5, as the graph of Figure 6 shows. A fourth possible embodiment is shown in Figure 4 and in it the second adjustment valve 10 is provided by a two-way valve interposed along the liquid line 2a, upstream of the regenerative exchanger 7, and there is a bypass line 2c bypassing the regenerative exchanger 7. In this case, by varying the degree of opening of the second adjustment valve 10 it is possible to modulate the flow (ṁ _L ) of the liquid- phase exchange fluid that passes through the regenerative exchanger 7 between zero and a maximum value (ṁ _Lmax ), which is less than the total flow rate value (ṁ _LTOT ) of the exchange fluid exiting from the condenser 5, as the graph of Figure 7 shows. The maximum value of the flow rate of the liquid-phase exchange fluid that can pass through the regenerative exchanger 7 (ṁ _Lmax ) in this embodiment is determined by the flow resistances along the liquid line 2a and along the bypass line 2c. The operation of the apparatus according to the invention is the following. With reference to the embodiment of Figure 1, the flow rate of the liquid-phase exchange fluid that passes through the regenerative exchanger 7 (ṁL) varies between a fraction p and 100% of the total flow rate of the exchange fluid exiting from the condenser 5, according to the degree of opening of the second adjustment valve 10. The sensor means and in particular the temperature sensors 11, 12, 13, 15 and the pressure sensor 14 transmit, to the control means 8, the respective measurement signals IS ₁ , IS ₂ , IS ₃ , IS ₄ , IS ₅ , at each instant i.e. at every time interval of preset length. The control means 8 then calculate, at each instant, the instantaneous efficiency of the regenerative exchanger 7 using the general formula given above for the thermal efficiency of the regenerative exchanger 7, using the measurements taken by the sensor means and, in particular, by the temperature sensors 11, 12, 13, 15, which supply the control means 8 with respectively the temperature of the vapor-phase exchange fluid in output from the regenerative exchanger 7 (T _Vout ), the temperature of the vapor- phase exchange fluid in input to the regenerative exchanger 7 (T _Lin ), the temperature of the liquid-phase exchange fluid in output from the regenerative exchanger 7 (T _Lout ) and the temperature of the vapor-phase exchange fluid in input to the regenerative exchanger 7 (T _Vin ). In particular, in proceeding to calculate the instantaneous efficiency, the control means 8 initially determine, using the general formula given above, using the measurements of the temperature probes 11, 12, 13 and 15, the difference between the temperature of the vapor-phase exchange fluid in output from the regenerative exchanger 7 and the temperature of the vapor- phase exchange fluid in input to the regenerative exchanger 7, as well as the difference between the temperature of the liquid-phase exchange fluid in input to the regenerative exchanger 7 and the temperature of the liquid- phase exchange fluid in output from the regenerative exchanger 7 and to compare the values of the two differences thus calculated, in order to establish which of these two differences is the greatest. More specifically, in the more general case where the greater difference between the two is the difference between the temperature of the vapor-phase exchange fluid in output from the regenerative exchanger 7 and the temperature of the vapor-phase exchange fluid in input to the regenerative exchanger 7, the formula for calculating the instantaneous efficiency used by the control means 8 becomes: where: TVout is the temperature of the vapor-phase exchange fluid in output from the regenerative exchanger 7 and T _Vin is the temperature of the vapor- phase exchange fluid in input to the regenerative exchanger 7. Otherwise, if the greater difference between the two is the difference between the temperature of the liquid-phase exchange fluid in input to the regenerative exchanger 7 and the temperature of the liquid-phase exchange fluid in output from the regenerative exchanger 7, then the formula for calculating the instantaneous efficiency becomes: where: T _Lin is the temperature of the liquid-phase exchange fluid in input to the regenerative exchanger 7 and T _Lout is the temperature of the liquid-phase exchange fluid in output from the regenerative exchanger 7. It should be noted that if it is not possible to obtain the measurement of the fourth temperature probe 15, then the control means 8 proceed to estimate the temperature of the vapor-phase exchange fluid in input to the regenerative exchanger 7 T _Vin using the following formula: which expresses an energy balance within the regenerative exchanger 7 by equalizing the heat exchanged by the vapor-phase exchange fluid inside the regenerative exchanger 7 with the heat exchanged by the liquid-phase exchange fluid, also inside the regenerative exchanger, and from which is obtained: The temperature values to be inserted in the formula shown above to calculate the temperature of the vapor-phase exchange fluid in input to the regenerative exchanger 7 T _Vin , i.e. the values of the temperature of the vapor-phase exchange fluid in output from the regenerative exchanger 7 TVout, of the temperature of the liquid-phase exchange fluid in input to the regenerative exchanger T _Lin and of the temperature of the liquid-phase exchange fluid in output from the regenerative exchanger 7 T _Lout , are, respectively, measured by the temperature probes 11, 12 and 13, while the specific heat values c _pV and c _pL of the vapor-phase exchange fluid and of the liquid-phase exchange fluid at the temperatures of said exchange fluid are determined by the control means 8 from the relations that link these specific heat values cpV and cpL to the temperatures of said exchange fluid, i.e.: which depend on the exchange fluid used and which are per se known. The value of the ratio ṁL/ṁV between the flow rate of the liquid-phase exchange fluid passing through the regenerative exchanger 7 and the flow rate of the vapor-phase exchange fluid passing through the regenerative exchanger 7, also present in the same formula used to calculate the temperature of the vapor-phase exchange fluid in input to the regenerative exchanger 7 T _Vin , is obtained by the control means 8 using the liquid-side characteristic curve of the regenerative exchanger 7, which is obtained from previous measurements once only and which supplies the measurement of the flow resistance on the liquid side of the regenerative exchanger 7, and using the characteristic curve of the second adjustment valve 10, which is supplied by the maker of said valve. In more detail, reference can be made to the general diagram in Figure 8, which shows the liquid line 2a and, more specifically, the section of the latter, indicated with L, that passes through the heat exchanger 7, and which also shows in schematic form the presence of the regenerative exchanger 7 and of the second adjustment valve 10. Also in Figure 8, the branch BP that connects the ends of the section L represents the bypass line 2c. In particular, it must be noted that, generally, the equation that expresses the liquid-side characteristic curve of the regenerative exchanger 7 is constituted by a relation between the flow resistance on the section L of the liquid line 2a that passes through the regenerative exchanger 7 and the product of the "hydraulic characteristic" on the liquid side of the regenerative exchanger 7 and the square of the flow rate of the liquid-phase exchange fluid that passes through said regenerative exchanger. The hydraulic characteristic of the liquid side of the regenerative exchanger 7, C _IHX , can be easily obtained for each model of regenerative exchanger 7 via initial mapping of the characteristic curve with a minimum of three measurement points, i.e. by conducting tests with different flow rates of fluid so as to identify at least three points of the parabola that represents the characteristic curve of the regenerative exchanger 7. The mapping can be carried out with an easily-obtained and easily- managed liquid, such as water, and then scale it to the density of the coolant liquid considered. That is to say, given that CIHX depends on the type of coolant used, if mapping with water results in determining the characteristic C _IHX,WATER , then the value of the sought characteristic is obtained from: where ρ _L is the density of the liquid-phase exchange fluid and ρ _WATER is the density of the liquid water, both calculated in the same temperature interval. More specifically, the hydraulic characteristic of the liquid side of the regenerative exchanger 7, C _IHX , depends not only on the type of coolant used, but also on the geometry of the regenerative exchanger 7 and on the preset interval of temperatures within which the regenerative exchanger is intended to operate, and is defined by the relation: where: ṁ _L is the flow rate of the liquid-phase exchange fluid that passes through the regenerative exchanger 7, C _IHX is the hydraulic characteristic of the liquid side of the regenerative exchanger 7, and Δp _L is the flow resistance through the regenerative exchanger 7 i.e. along the section L of the liquid line 2a that passes through the regenerative exchanger 7. It is also possible to consider the effect of any non-negligible hydraulic resistances that are present, in addition to the second adjustment valve 10, if any, along the liquid line 2a, such as resistances determined by bends, choking and the like, and schematically represented in Figure 8 by the box indicated with C' _L . In particular, if the type and the geometry of the hydraulic resistances are known, their effect is easily determined, in a manner that is per se known, using the theory. For example, for a valve of the on/off type, i.e. with only two working conditions, respectively a completely open condition and a completely closed condition, and therefore without the ability to adjust the degree of opening, its characteristic indicated with K' _L , according to the theory the effect of its hydraulic resistance can be expressed as follows: where: ρ _L is the density of the liquid-phase exchange fluid. If, on the other hand, the type and geometry of the hydraulic resistances are not known, their effect can be measured by way of a preliminary mapping, i.e. by carrying out experimental tests by making different flow rates pass through the hydraulic resistance. By defining the hydraulic characteristic of the liquid side of the regenerative exchanger 7, not connected to the second adjustment valve 10, as the sum, indicated with C _L , of the hydraulic characteristic of the liquid side of the regenerative exchanger 7 C _IHX and the effect C' _L of any non- negligible hydraulic resistances that are present along the liquid line 2a, not including the second adjustment valve 10, i.e.: then the flow resistance on the section L of the liquid line 2a that passes through the regenerative exchanger 7, not connected to the second adjustment valve 10, can be expressed as follows: where: Δp' _L is the flow resistance, not connected to the second adjustment valve 10, on the section L of the liquid line 2a that passes through the regenerative exchanger 7, C _L is the hydraulic characteristic of the section L of the liquid line 2a that passes through the regenerative exchanger 7, not connected to the second adjustment valve 10, and ṁ _L is the flow rate of the liquid-phase exchange fluid that passes through the regenerative exchanger 7. According to the embodiment, along the section L of the liquid line 2a that passes through the regenerative exchanger 7, the second adjustment valve 10 can be absent, as in the embodiments of Figures 1 and 2, or it can be present, as in the embodiment of Figure 4 or as in the embodiment of Figure 3, in which it is present with one of its parts. If the second adjustment valve 10 is present along the section L of the liquid line 2a that passes through the regenerative exchanger 7, then we consider the flow resistance Δp _v,L through the second adjustment valve 10, such that: or: where: Δp _v,L is the flow resistance through the second adjustment valve 10, ρL is the density of the liquid-phase exchange fluid, ṁL is the flow rate of the liquid-phase exchange fluid that passes through the second adjustment valve 10, and K _v,L is a characteristic coefficient of the second adjustment valve 10 which is supplied by the maker of said valve. The above formula, which expresses the value of the flow resistance through the second adjustment valve 10, continues to be valid even if the second adjustment valve 10 is absent, simply by having: both in the embodiment of Figure 2 and in the embodiment of Figure 1, given that the corresponding absence of the adjustment valve 10 is equivalent to assuming the flow resistance through it is nil, which can happen only with a hydraulic resistance of nil and, therefore, with value of the characteristic tending towards infinity. The flow resistance total Δp _L on the section L of the liquid line 2a that passes through the regenerative exchanger 7 is, therefore, equal to the sum of the flow resistance on the section L of the liquid line 2a, not connected to the second adjustment valve 10, that passes through the regenerative exchanger 7 and the flow resistance through the second adjustment valve 10, i.e: and, as a consequence, by substituting in the above equation the previous relations for the flow resistance on the section L of the liquid line 2a that passes through the regenerative exchanger 7, not connected to the second adjustment valve 10, and for the flow resistance through the second adjustment valve 10, and inserting the term ṁ _L , we have: Similarly to the section L of the liquid line 2a, it is necessary to consider the effect of non-negligible hydraulic resistances that are present, in addition to those associated with the second adjustment valve 10, if any, along the bypass line 2c, such as resistances determined by bends, choking and the like, and schematically represented in Figure 8 by the box indicated with C _BP . In particular, if the type and the geometry of the hydraulic resistances are known, their effect is easily determined, in a manner that is per se known, using the theory. For example, for a valve of the on/off type, i.e. with only two working conditions, respectively a completely open condition and a completely closed condition, and therefore without the ability to adjust the degree of opening, its characteristic indicated with K' _BP , according to the theory the effect of its hydraulic resistance can be expressed as follows: where ρ _L is the density of the liquid-phase exchange fluid. If, on the other hand, the type and geometry of the hydraulic resistances are not known, their effect can be measured by way of a preliminary mapping, i.e. by carrying out experimental tests by making different flow rates pass through the hydraulic resistance. The flow resistance on the branch BP of the bypass line 2c, not connected to the second adjustment valve 10, can be expressed as follows: where ṁ _BP is the flow rate of the liquid-phase exchange fluid that passes through the branch BP. According to the embodiment, along the branch BP of the bypass line 2c, the second adjustment valve 10 can be absent, as in the embodiments of Figures 2 and 4, or it can be present, as in the embodiment of Figure 1 or as in the embodiment of Figure 3, in which it is present with one of its parts. If the second adjustment valve 10 is present along the branch BP of the bypass line 2c, then we consider the flow resistance Δp _v,BP through the second adjustment valve 10, such that: where: Δpv,BP is the flow resistance through the second adjustment valve 10, ρ _L is the density of the liquid-phase exchange fluid, ṁ _BP is the flow rate of the liquid-phase exchange fluid that passes through the second adjustment valve 10, and K _v,BP is a characteristic coefficient of the second adjustment valve 10 which is supplied by the producer of said valve. The above formula, which expresses the value of the flow resistance through the second adjustment valve 10, continues to be valid even if the second adjustment valve 10 is absent, simply by having: in the embodiment of Figure 4, given that the corresponding absence of the adjustment valve 10 is equivalent to assuming the flow resistance through it is nil, which can happen only with a hydraulic resistance of nil and, therefore, with value of the characteristic tending towards infinity. The embodiment of Figure 2, by contrast, simulates the absence of the branch BP with a corresponding fixed hydraulic resistance tending towards infinity, i.e., with reference to Figure 8: and therefore the characteristic of the second adjustment valve 10 on the branch BP K _v,BP can assume any value between zero and infinity: The flow resistance on the section L of the liquid line 2a must be equal to the flow resistance in the branch BP of Figure 8, such that: and as a consequence: Considering then that the total flow rate of the exchange fluid ṁ is, by the principle of continuity, equal to the flow rate of the vapor-phase exchange fluid passing through the regenerative exchanger 7 ṁ _V , then the flow rate of the refrigerant fluid through the branch BP is equal to: From the above formulas, we obtain the following general formula which is valid for the general diagram of Figure 8: and this general formula enables the control means 8 to calculate the ratio ṁ _L /ṁ _V between the flow rate of the liquid-phase exchange fluid that passes through the regenerative exchanger 7 and the flow rate of the vapor-phase exchange fluid that passes through the regenerative exchanger 7, and to obtain, from this ratio and the measurements supplied to the control means 8 by the temperature probes 11, 12 and 13, the temperature value of the vapor- phase exchange fluid in input to the regenerative exchanger 7. The general formula shown above for calculating the ratio ṁL/ṁV between the flow rates of the liquid-phase exchange fluid and of the vapor- phase exchange fluid which pass through the regenerative exchanger 7 will take a different form depending on the different embodiments of the apparatus. Turning, in particular, to the case of the first embodiment of Figure 1, since the second adjustment valve 10 is, in this case, absent along the section L of the liquid line, we can consider the second adjustment valve 10 as if it were present along the section L, always in the completely open condition, without offering any resistance, such that: and the formula for calculating the ratio ṁ _L /ṁ _V becomes: From this formula we determine the limit values, minimum and maximum, for the ratio ṁ _L /ṁ _V in the case of the first embodiment. In particular, when, in the first embodiment of Figure 1, the second adjustment valve 10 we have: thus obtaining: When, on the other hand, again in the embodiment of Figure 1, the second adjustment valve is open to the maximum, we have: where K _v,MAX is the characteristic of the second adjustment valve supplied by the producer of the valve, and, as a consequence, the following relation is obtained: which confirms that, in the embodiment of Figure 1, ṁ _L can assume all the values between a minimum value threshold and ṁ _V , as shown by the graph of Figure 5. From the above relations, the control means 8 are able to determine, for the embodiment of Figure 1, the value of the ratio ṁL/ṁV between the flow rates of the liquid-phase exchange fluid and of the vapor-phase exchange fluid which pass through the regenerative exchanger 7 and, from this ratio, the temperature value of the vapor-phase exchange fluid in input to the regenerative exchanger 7 T _Vin , using the formulas given above. At every instant, the control means 8 also calculate the reference efficiency of the regenerative exchanger 7. In particular, for the calculation of the reference efficiency of the regenerative exchanger 7, the control means 8 again use the general formula for the efficiency of the regenerative exchanger 7, but starting from the assumption that the difference ΔT _EVsup between the temperature of the vapor-phase exchange fluid in input to the regenerative exchanger 7 (T _Vin ) and the saturated evaporation temperature (T _EVsat ) of the exchange fluid, i.e. the superheating at the evaporator 3, has a preset reference value or set- point value ΔT _{EVsup,SETPOINT} , chosen in advance and preferably comprised between 0 and 5 K and more preferably between 0 and 3 K. For example, to maximize the benefits of the regenerative exchanger 7 the control means 8 can be programmed to assume that, in the calculation of the reference efficiency, the value of superheating at the evaporator 3 ΔT _EVsup is considered equal to 0 K i.e. that ΔT _{EVsup,SETPOINT} = 0 K. In this manner, the reference efficiency is calculated by the control means 8 using the following formula: with: The saturation temperature T _EVsat that appears in the formulas shown above is, as known, a unique function of the evaporation pressure p _EV . Therefore, in order to determine the reference efficiency, the control means 8 use, in the first embodiment, four parameters: T _Lin , T _Lout , T _Vout , p _EV . It must be noted that, for the same reason that measuring the temperature of the vapor-phase exchange fluid in output from the evaporator and in input to the regenerative exchanger is problematic, measuring the evaporation pressure is also difficult. However, given that in general the regenerative exchanger 7 is designed to minimize the flow resistance on the vapor side (otherwise the advantages of using it are lost), it is possible to use the measurement of the pressure of the vapor-phase exchange fluid downstream of the regenerative exchanger 7 for calculating the reference efficiency, thus positioning the pressure sensor 14 between the regenerative exchanger 7 and the compressor 4, as shown in Figure 1, instead of measuring the pressure of the vapor-phase exchange fluid upstream of the regenerative exchanger 7, with a negligible error in the determination of TEVsat, by virtue of calculating the flow resistance on the steam line 2b. At this point, once the instantaneous efficiency and the reference efficiency are calculated, the control means 8 proceed to adjust the apparatus by following the per se known ε-NTU theory, whereby the efficiency of the exchanger is a function of the flow rate of the vapor-phase exchange fluid that passes through the regenerative exchanger 7, which corresponds, in practice, to the overall flow rate of the exchange fluid in the circuit 2, and to the flow rate of the liquid-phase exchange fluid that passes through the regenerative exchanger 7, which in the embodiment of Figure 1 varies with the extent of opening of the second adjustment valve 10, i.e.: with ṁ controlled by the expansion valve 9 and with ṁL controlled instead, as mentioned, by the second adjustment valve 10. It is thus possible to efficaciously adjust two parameters: the efficiency of the regenerative exchanger 7 and the superheating at the evaporator or the efficiency of the regenerative exchanger 7 and the superheating at the suction line of the compressor 4. More specifically, the control means 8, using the measurement signals received from the sensor means, determine the difference between the actual superheating of the exchange fluid at the outlet of the evaporator 3 or at the inlet of the compressor 4 and the preset reference value for the superheating of the exchange fluid (ΔT _sup – ΔT _sup,SET ) and based on this difference said control means generate a first command signal OS ₁ to control the degree of opening of the expansion valve 9, by way of which the degree of opening of the expansion valve 9 is adjusted so as to ensure that the actual superheating at the evaporator 3 is substantially equal to the preset reference value, i.e. between 0 and 5 K and more preferably between 0 and 3 K. Once the difference between the previously-determined instantaneous efficiency and reference efficiency is calculated, the control means 8, as a function of the difference between the instantaneous efficiency and the reference efficiency, also generate a second command signal OS ₂ to control the degree of opening of the second adjustment valve 10, so as to vary the degree of opening of the second adjustment valve 10 until the value of the instantaneous efficiency of the regenerative exchanger 7 is brought to the value of the reference efficiency. It must be noted that the control means 8 are, conveniently, configured to keep the interaction between the two signals OS ₁ and OS ₂ under control, for the purpose of reaching a condition of stable control. The operation of the embodiments of Figures 3 and 4 is similar to that described for the embodiment of Figure 1, with the exception that the relations that enable the control means 8 to determine the ratio ṁ _L /ṁ _V between the flow rates of the liquid-phase exchange fluid and of the vapor- phase exchange fluid which pass through the regenerative exchanger 7 change, so as to calculate the temperature value of the vapor-phase exchange fluid in input to the regenerative exchanger 7 T _Vin , if the fourth temperature sensor 15 is not present or cannot be installed in the apparatus. In particular, in the embodiment of Figure 3, the control means 8 proceed to calculate the ratio ṁ _L /ṁ _V from the general formula obtained for the diagram of Figure 8, i.e: This formula makes it possible to determine the limit values of the ratio ṁ _L /ṁ _V and, in the embodiment of Figure 3, when the bypass line 2c is closed by acting on the second adjustment valve 10, we have: obtaining from the formula: while, when the section of the liquid line 2a that passes through the regenerative exchanger 7 is closed, again by acting on the second adjustment valve 10, we have: obtaining from the formula: which confirms that, in the embodiment of Figure 3, ṁ _L can assume values between zero and ṁ _V , as shown by the graph of Figure 6. With reference, instead, to the embodiment of Figure 4, to calculate the ratio ṁ _L /ṁ _V between the flow rates of the liquid-phase exchange fluid and of the vapor-phase exchange fluid which pass through the regenerative exchanger 7 we still start from the general formula, considering however that, in this particular case, the bypass line is as if it were always open without offering any resistance. Therefore, in the embodiment of Figure 4, we have: and as a consequence the general formula for calculating the ratio ṁL/ṁV becomes: from which we can obtain the limit values, minimum and maximum, of the ratio ṁ _L /ṁ _V in the embodiment of Figure 4. In particular, in the embodiment of Figure 4, when the second adjustment valve 10 is closed, we have: such that: while, by contrast, when the second adjustment valve 10 is open to the maximum, we have: such that: confirming that, in the embodiment of Figure 4, ṁ _L can assume values between zero and a limit value less than ṁ _V , as shown in the graph of Figure 7. Turning now to the embodiment of Figure 2, in this case the adjustment is simplified, as we can act on the expansion valve 9 only and adjust only the efficiency of the regenerative exchanger 7 on a reference value or set-point value. With this embodiment, the control means 8 operate on the basis of three parameters, since, there being no second adjustment valve 10 to vary the flow of the liquid-phase exchange fluid that passes through the regenerative exchanger 7, the two flow rates of the liquid-phase exchange fluid and of the vapor-phase exchange fluid that pass through the regenerative exchanger 7 are always the same, which is why, in this case, it is always the vapor-phase exchange fluid that has the maximum jump in temperature and, as a consequence, the efficiency of the regenerative exchanger can be determined from: The temperature value of the vapor-phase exchange fluid in input to the regenerative exchanger 7 (T _Vin ) can be calculated by the control means 8 using the formula: using the measurement of the temperature value of the liquid-phase exchange fluid in output from the regenerative exchanger 7 (T _Lout ) provided by the third temperature sensor 13 and considering that in this case ṁ _L /ṁ _V = 1. In fact, if we wished to apply the general formula for calculating the ratio ṁL/ṁV, then, in the embodiment of Figure 2, no second adjustment valve 10 is present along the section of the liquid line 2a that passes through the regenerative exchanger 7, and so it is as if on the section L of the diagram of Figure 8 there were present a hypothetical valve, always open, without offering any resistance, such that: and, furthermore, the bypass line is also absent, such that the hydraulic characteristic of the branch BP is as if it were infinite, i.e: with the consequence that, for any value of CL: such that the formula confirms that, in the embodiment of Figure 2, ṁ _L = ṁ _V . In practice, in this embodiment, the value of the instantaneous efficiency or real efficiency (ε _real ) of the regenerative exchanger 7 is obtained by the control means 8 by way of the measurement, supplied by the pressure sensor 14, of the pressure value of the exchange fluid in output from the evaporator pEVout or, alternatively, in input to the compressor 4 (said pressure value making it possible, if the superheating is nil, to directly obtain T _Vin , otherwise the measurement of TLout), the temperature of the vapor-phase exchange fluid in output from the regenerative exchanger 7 and in input to the compressor 4 TV _out , and the temperature of the liquid-phase exchange fluid in input to the regenerative exchanger 7 T _Lin are also required. In this case too, the reference efficiency or set-point efficiency (εset) is calculated by the control means 8 by setting a preset value for the superheating at the evaporator, as explained previously. Given that, according to the ε-NTU theory, in the embodiment of Figure 2, the efficiency of the regenerative exchanger 7 is a function solely of the flow rate of the exchange fluid that passes through the circuit 2, since this corresponds to the value of the flow rate of the liquid-phase exchange fluid that passes through the regenerative exchanger 7, i.e. ε=ε(ṁ), then in the embodiment of Figure 2, by varying the opening of the expansion valve 9 it is possible to vary the flow rate of the exchange fluid that passes through the regenerative exchanger 7, so that the instantaneous efficiency ε _real reaches the value of the reference efficiency ε _set . In particular, in the embodiment of Figure 2, the control means 8, based on the calculated value of the difference between the instantaneous efficiency and the reference efficiency of the regenerative exchanger 7, generate a command signal OS ₁ to control the degree of opening of the expansion valve 9, so as to be able to vary the flow rate of the exchange fluid that passes through the regenerative exchanger 7 and therefore the efficiency of said regenerative exchanger. More specifically, in the embodiment in Figure 2, if the control means 8 detect that the instantaneous efficiency is less than the reference efficiency, they send a command signal OS ₁ to the expansion valve 9 to reduce the degree of opening of said expansion valve and consequently reduce the flow rate ṁ of the exchange fluid that passes through the circuit 2 and therefore the regenerative exchanger 7, until the value of the instantaneous efficiency is brought to the value of the reference efficiency. Also in the embodiment of Figure 2, if instead the control means 8 detect that the instantaneous efficiency is greater than the reference efficiency, they send a command signal OS ₁ to the expansion valve 9 to increase the degree of opening of said expansion valve, so as to increase as a consequence the flow rate ṁ of the exchange fluid that passes through the circuit 2 and so bring the instantaneous efficiency to the same value as the reference efficiency. In practice it has been found that the invention fully achieves the intended aim and objects and in particular attention is drawn to the fact that the apparatus according to the invention makes it possible to keep the superheating of the exchange fluid at the evaporator within a minimum range, preferably between 0 and 5 K and more preferably between 0 and 3 K. It must also be emphasized that the invention, by virtue of the type of adjustment used, which is based on the calculation of the difference between the instantaneous thermal efficiency of the regenerative exchanger and a reference efficiency of said regenerative exchanger, makes it possible to obtain a more efficient use of the exchange area of the evaporator and of the condenser, in particular under partial workload conditions. This brings a benefit with an increase in the seasonal efficiency of the refrigeration cycle. The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims. Moreover, all the details may be substituted by other, technically equivalent elements. In practice, the materials used, as well as the contingent shapes and dimensions, may be any according to the requirements and to the state of the art. The disclosures in Italian Patent Application No. 102022000003557 from which this application claims priority are incorporated herein by reference. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.