EP4632291A1

EP4632291A1 - Vapor compression refrigeration plant

Info

Publication number: EP4632291A1
Application number: EP25169299.2A
Authority: EP
Inventors: Giuseppe Vitri
Original assignee: Rivacold SRL
Current assignee: Rivacold SRL
Priority date: 2024-04-09
Filing date: 2025-04-08
Publication date: 2025-10-15

Abstract

Vapor compression refrigeration plant with refrigerant fluid consisting of carbon dioxide or mixtures thereof and of the type comprises a fluid circulation circuit provided with: at least one compressor group (21, 54); a first heat exchanger (22) having at least one outflow branch (35) to the inlet of which the outlet of at least one compressor group (21, 54) is connected, where this first exchanger (22) is assigned to the transfer of heat from the fluid to a source or user; a second heat exchanger (23) having an overheating branch (31) and a heat transfer branch (32) in mutual thermal connection, where an inlet of the heat transfer branch (32) is connected to the outlet of the outflow branch (35) of the first exchanger (22), where an outlet of the overheating branch (31) of the second exchanger (23) is connected to at least one compressor group (21, 54); a receiving element (28) provided with an inlet (44) for the fluid connected at least to the outlet of the heat transfer branch (32) of the second exchanger (23) and provided with a liquid outlet (45) and a vapor outlet (46), the latter connected to the inlet of the overheating branch (31) of the second exchanger (23) and/or to at least one compressor group (21, 54); a lamination device (24) an inlet of which is connected to the liquid outlet (45) of the receiving element (28); an evaporator (25) having at least one evaporation branch (38) an inlet of which is connected to an outlet of the lamination device (24), the outlet of such evaporation branch (38) is connected to the inlet of the receiving element (28) or to the inlet of the overheating branch (31) of the second exchanger (23), where this evaporator (25) is assigned to the transfer of heat from a source or user to the fluid. The outlet of the outflow branch (35) of the first exchanger (22) is connected to the inlet of the heat transfer branch (32) of the second exchanger (23) and to the inlet (44) of the receiving element (28) by means of a remote control valve (26) having an inlet (41) connected to said outlet of the outflow branch (35), a first outlet (42) connected to said inlet (44) of the receiving element (28), and a second outlet (43) connected to said inlet of the heat transfer branch (32).

Description

The present invention falls within the field of the refrigeration plants, systems or circuits, and refers to a refrigeration plant, in particular operating according to a transcritical refrigeration cycle.
Refrigeration plants that use carbon dioxide (CO2) as a refrigerant and in which the cycle also includes phases in the transcritical state are known. Generally, this type of circuit includes at least one or more compressors, a heat exchanger and an evaporator.
A disadvantage of known refrigeration plants is that they do not operate equally efficiently under all operating conditions, for example when the ambient temperatures or the required refrigeration power vary, thus worsening the efficiency of the plant.
Another disadvantage of known plants is that the compressor can suck in liquid refrigerant fluid, with the consequent risk of damaging the compressor itself.
Another disadvantage of some known plants is that they are not always able to guarantee the correct circulation of the compressor's lubricating oil.
An object of the present invention is to propose a vapor compression refrigeration plant in which the operation is adjustable to maintain the efficiency of the plant unchanged at all operating conditions.
Another object of the present invention is to propose a vapor compression refrigeration plant in which the risk of liquid refrigerant being sucked into the compressor is eliminated.
A further object of the present invention is to propose a vapor compression refrigeration plant in which the oil in the refrigerant fluid is efficiently recovered and returned to the compressor for the correct and robust operation of the latter.
Another object of the present invention is to propose a vapor compression refrigeration plant provided with sensors, probes, actuators and with control or controls and specific data connection among the controls or the modules of the control to allow a controlled operation of the plant providing an improved efficiency and energy saving.
The solution object of the invention applies to refrigeration plants and, more generally, to heat pumps with a vapor compression cycle that use carbon dioxide (CO2) as a refrigerant, which is considered a natural refrigerant for the development of increasingly sustainable systems as it is non-toxic, non-flammable and non-polluting.
This type of refrigerant requires special circuits compared to classic hydrofluorocarbon (HFC) refrigerants since it can operate in transcritical mode.
To obtain efficient transcritical CO2 systems, it is necessary to operate by reducing inefficiencies and implementing particular solutions. One of these solutions consists in having an evaporation pressure-temperature parameters pair as close as possible to the temperature of the fluid to be treated on the evaporator side.
The circuits of interest are those in which the evaporator works with low overheating, or of the flooded type and therefore with a vapor title (intended as the ratio between the mass of the vapor fraction and the total mass of the fluid) less than 1 at the outlet of the evaporator itself.
These solutions, which are known, require measures to ensure that the compressor receives overheated vapor within well-imposed limits between 5 K and 30 K to avoid compressor failure.
The purpose of the solution of the invention is twofold:

protect the compressor from liquid returns through the return of incoming overheated vapor;
allow the increase in efficiency by means of other members.

More specifically, it involves the insertion in a circuit as described above of an exchanger that has the purpose of overheating the gas before entering the compressor, removing heat from the refrigerant fluid exiting the heat exchanger or gas cooler. A peculiarity of the invention consists of the insertion of two flow management modes by injecting liquid and diverting the refrigerant by means of a three-way valve, appropriately sized, which allows controlling the overheating of the compressor in all operating conditions of the system and ensuring the return of the oil to the compressor.
The three-way valve diverts the flow rate exiting the gas cooler (which is a hot fluid) towards said exchanger. A greater or lesser flow rate of the hot fluid in the exchanger will respectively increase or reduce the overheating of the refrigerant that the compressor is sucking (cold fluid).
By controlling the overheating, the three-way valve will allow maintaining constant and stable conditions in all the most diverse working conditions.
The three-way valve must be of the modulating type with a passage through it that allows a relationship as linear as possible between the opening percentage and the percentage of diverted flow. Not all valves have this functionality in the CO2 field.
In addition, there is a liquid injection valve, for example operated by a solenoid, which by means of the control the same has two purposes:

if the overheating rises beyond acceptable measure due to some particular condition, liquid injection is triggered;
since the oil would not return to the compressor, a liquid injection is necessary, inside which there is mixed oil.

To allow the injection, there is a localized leak in order to have a lower pressure in that point of the receiver element. This is possible because the restriction increases the speed of the main fluid generating a localized pressure drop. This pressure will be such as to allow the recall of liquid refrigerant in the main suction. Without this system it would not be possible to inject oil since the pressure on the receiver is the same as that of the compressor suction.
The prior documents CN1265136 , US2019/257562 , US2021/055020 , US2018/274821 , DE102019111309 , JP2004324936 and US2011/041527 refer to prior refrigeration plants. None of these documents, considered in combination with the others or with the normal knowledge available to the skilled in the sector, suggests the adoption of the combinations of characteristics claimed in order to reduce electrical consumption and increase the efficiency of the plant which represents the technical problem underlying the invention.
The characteristics of the invention are highlighted below with particular reference to the attached drawings in which:

figure 1 illustrates a schematic view of a first embodiment of the vapor compression refrigeration plant object of the present invention;
figure 2 illustrates a schematic pressure-enthalpy diagram of the refrigeration cycle carried out by the plant of figure 1;
figure 3 illustrates a schematic view of a second embodiment of the vapor compression refrigeration plant object of the present invention;
figure 4 illustrates a schematic pressure-enthalpy diagram of the refrigeration cycle carried out by the plant of figure 3;
figure 5 illustrates a schematic view of a variant of the refrigeration plant of figure 3;
figure 6 illustrates a schematic pressure-enthalpy diagram of the refrigeration cycle carried out by the plant of figure 5;
figures 7 and 8 illustrate respective diagrams.

With reference to figure 1, numeral 100 indicates the vapor compression refrigeration plant that is the object of the present invention.
The refrigerant fluid consists of carbon dioxide or a mixture comprising carbon dioxide, and is suitable for use in refrigeration circuits operating in the transcritical regime of the fluid itself. Such refrigeration circuits are particularly suitable for plants with low overheat or flooded evaporators and operating with carbon dioxide or mixtures thereof.
The plant 100 is of the type comprising a fluid circulation circuit, and in its preferred embodiment is provided with the following basic components:

a compressor group, preferably comprising a flow compressor 21 or more flow compressors 21 associated in series and/or in parallel;
a first heat exchanger 22 having at least one outflow branch 35 to the inlet of which the outlet of the compressor group 21 is connected, where such first exchanger 22 is assigned for the transfer of heat from the refrigerant fluid to a source or user;
a second heat exchanger 23 having an overheating branch 31 and a heat transfer branch 32 independent but in mutual thermal connection, where an inlet of the heat transfer branch 32 is connected to the outlet of the outflow branch 35 of the first exchanger 22, and where an outlet of the overheating branch 31 of the second exchanger 23 is connected at the inlet of the compressor group 21;
a receiving element 28 provided with a respective inlet 44 for the refrigerant fluid connected to the outlet of the heat transfer branch 32 of the second exchanger and provided with a respective liquid outlet 45 and a vapor outlet 46, the latter connected at the inlet of the overheating branch 31 of the second exchanger 23;
a lamination device 24 an inlet of which is connected to the liquid outlet 45 of the receiving element 28;
an evaporator 25 assigned for the transfer of heat from a source or user external to the circuit to the refrigerant fluid, and having an evaporation branch 38 an inlet of which is connected to an outlet of the lamination device 24, where the outlet of such evaporation branch 38 is connected at the inlet of the receiving element.

In particular, the outlet of the outflow branch 35 of the first exchanger 22 is connected to the inlet of the heat transfer branch 32 of the second exchanger 23 and to the inlet 44 of the receiving element 28 by means of a remote-controlled valve 26 having an inlet 41 connected to said outlet of the outflow branch 35, a first outlet 42 connected to said inlet 44 of the receiving element 28, and a second outlet 43 connected to said inlet of the heat transfer branch 32.
A control unit 27, also referred to as control C1 of the plant 100, operates the valve 26 to control the state of the fluid at the inlet of the compressor group 21.
It should be noted that in the context of this description the concepts of "connection" or "linkage" between two or more hydraulic components indicate a flow connection or hydraulic connection, direct or indirect, between the components themselves, which therefore make up the same hydraulic circuit for the circulation of the refrigerator fluid. Such connections can be considered adiabatic, or in any case affected by heat exchanges or pressure drops of negligible amount.
The valve 26 is a three-way valve with electromechanical operation with proportional-integrative-derivative (PID) control or other type of control with or without feedback, where the flow of refrigerant fluid entering the inlet 41 can be directed entirely towards the first outlet 42, entirely towards the second outlet 43, or can be divided into variable and adjustable proportions between the two extreme conditions, i.e. between the first outlet 42 and the second outlet 43.
Preferably the plant 100 comprises a temperature sensor 61 and a pressure sensor 62 installed immediately upstream of the inlet of the flow compressor or compressors 21 and assigned to measure the temperature and pressure of the refrigerant fluid at that point, respectively.
The temperature sensor 61 and pressure sensor 62 are in electronic data connection, wired or wireless, with the control unit 27.
The control unit 27 is of the programmable digital electronic type, and is programmed with instructions that, when executed by the control unit 27 itself, cause it to at least command the operation of the valve 26 based on the temperature and pressure data provided by said temperature sensor 61 and pressure sensor 62, as better specified below.
A variant of the plant 100 provides for the use of only one of the temperature 61 and pressure 62 sensors, deriving or calculating the remaining quantity on the basis of other operating parameters of the plant 100 and/or on the basis of known relations or state diagrams relating to the particular refrigerant fluid chosen.
The flow compressor or compressors 21, and more generally all the compressors that are or can be included in the plant 100, are preferably rotary, piston, scroll, or similar compressors, assigned for the circulation of the refrigerant fluid, and for their correct operation it is essential that the fluid at the inlet thereof is exclusively in the gaseous state and does not contain a fraction in the liquid state. For their correct operation, the compressor groups also need lubricant or oil, a part of which can mix with the refrigerant fluid and enter the circuit.
The speed of the flow compressors 21 is preferably adjustable and managed by a digital controller.
The second exchanger 23 is a heat exchanger in which the overheating branch 31 is thermally coupled to the heat transfer branch 32, for example by means of coupled plates or cocurrent or countercurrent coils, so that the hotter refrigerant fluid flowing upstream in the heat transfer branch 32 transfers heat to the refrigerant fluid flowing downstream in the overheating branch 31.
The receiving element 28, or so-called liquid/vapor separator, is a tank designed to separate the liquid fraction from the gaseous fraction of the refrigerant fluid flow: the first fraction proceeds towards the liquid outlet 45, which is obtained in the lower part of the receiving element 28; the second fraction proceeds towards the vapor outlet 46, which is arranged in the upper part of the receiving element 28.
The lamination device 24 is a restriction, a capillary, or preferably a thermostatic valve or other lamination device whose opening is proportional and adjustable, preferably with electronic control.
Preferably, the lamination device 24 is of the electromechanically operated type and therefore controllable, and is controlled by a controller 70, also indicated as C4 or control VT, as a function of the temperature and pressure detected by respective temperature sensor 64 and pressure sensor 65 installed downstream of the outlet of the evaporation branch 38; alternatively or in addition, such control is carried out as a function of the pressures detected by respective pressure sensors 65, 68 installed downstream of the outlet of the evaporation branch 38 and in the receiving element 28 respectively.
The plant is provided with a first data and/or signal line 63 for mutual connection of the control VT 70, i.e. C4, and the control unit C1, identified by reference 27, which operates the valve 26 to allow more accurate control and further improvement of the efficiency of the plant as better explained below.
The plant is also provided with a second data and/or signal line 71 for mutual connection of a C2 control 72 of the compressor, and of the C1 control unit 27. These data lines first 63 and second 71 allow the optimization of the compressor set point thanks to specific optimization algorithms resident in the controls C1, C2 and C4, identified by references 27, 72 and 70, which can thus operate synergistically.
The C2 control 72 is further connected to a set of sensors and or probes of the plant, for instance the temperature (STOUT, STIN) sensors at the inlet and outlet of the further branch for the secondary fluid of the first exchanger 22, the pressure sensor 62 at the inlet of the flow compressors 21, the pressure (BPR) sensor 68 installed in the receiving element 28, and/or other of the plant for receiving respective data for controlling at least one of the compressor group (21, 54).
The first exchanger 22 is a heat exchanger, or so-called "gas cooler", in which the flow of refrigerant fluid releases heat and undergoes cooling between the inlet and outlet of the outflow branch 35. This first exchanger 22 is preferably also provided with a further branch, independent of the outflow branch 35 but thermally coupled to it, in which a second fluid flows, for example water or air, from a thermal source or user for the transfer of heat from the refrigerant fluid of the outflow branch 35 to the second fluid itself, thus achieving the thermal exchange between the two fluids.
The evaporator 25 is a heat exchanger in which the flow of refrigerant fluid acquires heat and undergoes a phase change and possibly a temperature increase between the inlet and outlet of the evaporation branch 38. This evaporator 25 is preferably also provided with a further branch, independent of the evaporation branch 38 but thermally associated with it, in which a branch flows for a third fluid of a user or a thermal source for the transfer of heat from the third fluid itself to the refrigerant fluid of the evaporation branch 38. For example, the third fluid is water and the further branch of the evaporator 25 is integrated into a cooling circuit of a building.
Alternatively or in addition, the further branches just mentioned of the first exchanger 22 and of the evaporator 25 can be identified in a surface, coil or other element suitable for the heat exchange of the respective components in contact with the user or thermal source. The outflow branch 35, the overheating branch 31, the heat transfer branch 32 and the evaporation branch 38 may also optionally consist of respective bundles of two or more parallel branches, to increase the contact surface with the fluid and facilitate heat exchange.
The inlet 44 of the receiving element 28 is preferably connected to the outlets respectively of the outflow branch 35 of the first exchanger 22, of the heat transfer branch 32 of the second exchanger 23, and of the evaporation branch 38 of the evaporator 25 by means of an ejector element 29.
More precisely, the ejector element 29 is provided with a main inlet 47, a secondary inlet 48 and an outlet 49. The main inlet 47 is connected downstream of the first outlet 42 of the valve 26 and of the outlet of the heat transfer branch 32 of the second exchanger 23, which are preferably connected before the main inlet 47 itself; the secondary inlet 48 is connected downstream of the outlet of the evaporation branch 38 of the evaporator 25; the outlet 49 of the ejector element 29 is connected upstream of the inlet 44 of the receiving element 28.
The ejector element 29 is preferably of the type in which the introduction of the fluid flow from the evaporation branch 38 of the evaporator 25, i.e. the flow from the secondary inlet 48, into the main fluid flow, i.e. the flow from the main inlet 47, is continuously adjustable. More specifically, the ejector element 29 is of the type with a control or back pressure valve or group of valves each comprising an adjustable restriction between the main inlet 47 and the outlet 49, where the secondary inlet 48 is inserted into this restriction: the main flow in turn recalls the secondary flow, the latter being determined by the amount of the main flow.
The regulation of the opening of the restriction occurs in a closed loop by means of a digital controller (for example PID type) as a function of the pressure measured between the outlet of the compressor group of the flow compressors 21 and the inlet 41 of the valve 26, i.e. at the inlet or outlet of the outflow branch 35 of the first exchanger 22, preferably at the inlet of the outflow branch 35, and of the temperature measured immediately downstream of the outlet of the outflow branch 35. These measurements are carried out by means of respective temperature sensor 66 and pressure sensor 67. From the data of said temperature, an optimal value of the pressure is calculated upstream of the ejector element 29, i.e. in the so-called high pressure portion of the refrigeration circuit of the plant 100: if the data of said measured pressure is higher or lower than the optimal value, the opening of the restriction is respectively widened or narrowed.
Since the fluid pressure at the outlet of the evaporation branch 38 is generally lower than the fluid pressure at the inlet 44 of the receiving element 28, to ensure that the fluid can flow from the evaporator 25 to the receiving element 28, the plant 100 preferably also comprises a check valve 50 inserted in an interposed position in the circuit between the outlet of the evaporation branch 38 of the evaporator 25 and the secondary inlet 48 of the ejector element 29.
Preferably, the plant 100 also comprises a depression member 56 provided with a main inlet and an outlet inserted along the connection between the vapor outlet 46 of the receiving element 28 and the inlet of the overheating branch 31 of the second exchanger 23. Said main inlet and outlet of the depression member 56 are connected by a constriction into which a secondary inlet of the depression member 56 flows, which in turn is connected, via a branch carrying an injection valve 57, at the liquid outlet 45 of the receiving element 28.
The injection valve 57 is electrically actuated in a bistable or preferably proportional manner, for example by means of a solenoid or other electromechanical actuator, and is timed to open according to a predefined work cycle, or is electronically controllable by a digital controller such as a PID controller. When the injection valve 57 is open, part of the fluid flow that exits (in the liquid state) from the receiving element 28 enters the branch that carries the injection valve 57 itself, this branch being connected to the connection between the liquid outlet 45 and the lamination device 24; said part of the fluid flow exits at or near the constriction of the depression member 56 and mixes with the flow of gaseous fluid coming from the vapor outlet 46.
The depression member 56 and the injection valve 57 therefore allow the operating oil of the compressor groups diffused in the refrigerant fluid to be recirculated, to return it to the compressors via the flow entering them.
Preferably, the control unit 27 is connected to all the sensors and performs the function of controller of all the valves, of the lamination device 24 and of the compressor groups of the plant 100, integrating and implementing for each controllable valve predefined algorithms and/or circuits for the detection and analysis of the data from the sensors of interest and for the control of the respective electromechanical actuators.
In particular:

the control function of the valve 26 requires that the control unit 27 receives at the entrance port the temperature and pressure data detected respectively by the temperature sensor 61 and the pressure sensor 62 at the inlet of the flow compressors 21, then calculates from said pressure data the corresponding saturation temperature, i.e. the temperature of complete evaporation of the refrigerant fluid; subsequently the overheating of the fluid is calculated, understood as the difference between the temperature data and the saturation temperature; then the overheat is compared with a predetermined overheat value: if the overheat is greater than the predetermined value, then the control unit 27 commands the valve 26 to divert the fluid flow more towards the first outlet 42, otherwise if the overheat is less than the predetermined value the control unit 27 commands the valve 26 to divert the fluid flow more towards the second outlet 43, so that the warmer fluid in the heat transfer branch 32 releases heat and overheats the fluid in the overheating branch 31;
the control function of the flow compressors 21 provides for modulating their speed, and therefore their flow rate, as a function of the pressure at the inlet of the compressors 21 themselves, the pressure in the receiving element 28 (measured by the appropriate pressure sensor 68), the inlet temperature and/or the outlet temperature of the secondary fluid of the user of the first exchanger 22, to reach or maintain a desired value for one or more of these parameters;
the control function of the lamination device 24 provides for calculating the saturation temperature of the fluid from the pressure measured by the sensor 65 at the outlet of the evaporation branch 38, calculating the overheating by subtracting this saturation temperature from that measured by the sensor 64 installed at the same point, and increasing or reducing the opening of the lamination device 24 depending on whether this overheating is respectively greater or less than a predefined and small optimal value; alternatively, in cycles in which overheating is not expected, the opening of the lamination device 24 s regulated as a function of the difference between the pressures in the receiving element 28 and at the outlet of the evaporation branch 38, where this difference is compared with an optimal reference value;
the control function of the injection valve 57 generally provides that the latter can be opened only on condition that the compressor group of the flow compressors 21 is in operation; the opening can be regulated according to a predetermined timing cycle and/or as a function of the temperature and pressure at the inlet of the flow compressors 21, similarly to the control of the valve 26.

The presence of the valve 26 and its control as described advantageously allow to ensure that the fluid enters the compressor group completely in gaseous form and with a overheat equal to or very close to the predetermined value, which is the overheat value of the fluid for which the compressor group operates with maximum efficiency.
Control C2 is therefore preferably connected to the temperature sensors of the secondary fluid connections of the first exchanger 22, the pressure sensor 62, the pressure sensor 68 installed in the receiver element 28 and C1 control 27 and to the compressor unit 21.
The C3 control is preferably connected to the temperature sensors 66 and pressure sensors 67 and to the ejector element 29.
Figure 2 schematically illustrates the refrigerant fluid cycle in the plant 100 just described, in a diagram having the enthalpy h along the horizontal axis and the pressure along the vertical axis. The points along the diagram indicated by numbers 1-11 correspond to the state of the fluid measured at steady state in respective points of the circuit numbered consistently in the schematic representation of the plant 100 in figure 1.
With reference to figures 1 and 2, the operation of the plant 100 in the preferred embodiment described provides that the refrigerant fluid enters the flow compressor 21 in the state of overheated gas (point 1), which performs work to compress the fluid, which at the outlet of the flow compressor 21 has gained in pressure and temperature by passing to the supercritical gaseous state (point 2).
As it passes through the outflow branch 35, the fluid releases heat to the outside of the plant 3 and decreases in temperature, maintaining its pressure unchanged, until it exits the first exchanger 22 (point 3). The passage through the valve 26 does not cause changes in the state of the fluid. The fraction of fluid that eventually passes through the heat transfer branch 32 releases heat to the fluid that simultaneously flows in the overheating branch 31 and therefore decreases in temperature (point 4); when this fraction of fluid reunites with the remaining fraction at the connection downstream of the first outlet 42 of the valve 26 (point 5), the overall temperature of the flow has dropped with respect to the inlet 41 of the valve 26, but the pressure is unchanged and the fluid is still in the transcritical zone of the diagram. The ejector element 29 causes a sudden expansion of the fluid, with a consequent decrease in pressure downstream of its outlet 49 (point 6).
Following the separation of the liquid fraction from the gaseous fraction of the fluid in the receiving element 28, the fluid in the liquid state proceeds through the liquid outlet 45 (point 7). As it passes through the lamination device 24, the fluid undergoes a further drop in pressure, with consequent partial evaporation and expansion (point 8). In the evaporation branch 38, the fluid acquires heat from the user to be cooled and evaporates by expanding (point 9); more precisely, the vapor content at the outlet of the evaporator 25 is less than 1, i.e. the fluid has not completely evaporated and therefore does not undergo any overheating. The main flow in the ejector element 29 draws the partially evaporated fluid up to the inlet 44 of the receiving element 28.
At the same time, i.e. in parallel, the gaseous fraction of the fluid proceeds through the vapor outlet 46 of the receiving element 28, passes through the depression member 56 and reaches the inlet of the overheating branch 31 of the second exchanger 23, where:

if the injection valve 57 is closed, the fluid is in the so-called dry saturated vapor state, i.e. it is a gas with vapor content equal to 1 and temperature equal to the evaporation temperature relative to the corresponding pressure (point 10);
if the injection valve 57 is open, part of the liquid fraction exiting the receiving element 28 is introduced into the gas flow at the depression member 56, and since the two gaseous and liquid fractions have the same temperature and pressure, these quantities remain unchanged, but the fluid entering the second exchanger 23 is in the so-called wet saturated vapor state, i.e. it contains a liquid part (point 11).

In any case, when crossing the overheating branch 31, the fluid subtracts from the fluid that simultaneously flows in the heat transfer branch 32 a quantity of heat sufficient to evaporate completely and overheat (point 1) before re-entering the flow compressor 21. In this way, the possibility that even just a minimal fraction of fluid may re-enter the compressor group in the liquid state, with the consequent risks of damage to the same, is advantageously avoided. With the injection valve 57 open, moreover, the oil or lubricant can thus return to the flow compressor 21.
The data and/or signal line 63 of mutual connection of the VT control 70, i.e. C4, and the C1 control unit 27, that is that operates the valve 26, as seen, ensures the possibility of a more accurate control and a further improvement of the efficiency of the plant as better explained below, synergistically operating with the C2 control 72, thanks to the second data line 71.
It is important to note that the embodiment of figure 1, in addition to avoid feeding the compressor with the liquid, guarantees an unexpected reduction in electrical consumption and a maximization of efficiency.
One of the possibilities is that the compressor works with a lower compression ratio, obviously with the same external working conditions.
Specifically, to obtain a lower compression ratio, the invention allows to act on the evaporation pressure allowing it to be raised which means a higher suction pressure which leads to a lower compression ratio and, ultimately, greater efficiency.
The invention allows to have the same desired external conditions and in the specific case the same temperature of the cell or water or fluid to be treated.
Basically, there is heat exchange if there is a temperature jump according to an evaporator temperature profile of the type in figure 7. To have heat exchange DT1 and DT2 cannot be zeroized and the present invention allows the adoption of the mode of figure 8.
The secondary fluid will have the same performance but the plant of figure 1, and the other plants of this document and in general the plants made according to the present invention, allow the expansion to be carried out at a higher evaporation temperature, therefore with a higher evaporation pressure and with a lower compression ratio and therefore with lower energy consumption.
The same principle is easily detectable in the embodiments of figures 1-6 and is applied in these conditions at low overheating intended 1-2K or null therefore with liquid still exiting the evaporator.
The three-way valve 26 remotely controlled by the C1 control 27 is therefore governed by the desired overheating compressor and as a function of the overheating on the EVAP evaporator 38 to obtain maximum system efficiency; this is also thanks to the information exchanged with the compressor C2 control 72 by means of the second data line 71 which allows synergic cooperation between such C1 and C2 controls 27, 72.
The lower the evaporator overheating SHE (SHE calculated as the difference between the temperature of the STE sensor 64 and the conversion into saturation temperature of the pressure BPE 65) the greater the efficiency of the system is, where this overheating is controlled by the C4 control.
The lower the overheating SHE the greater the gas diverted onto the ECO plate 23 by the three-way valve (3V) 26 to obtain a correct overheating SH on the compressor (SH calculated as the difference between the temperature detected by the sensor 61 and the conversion into temperature of the pressure detected by the sensor 62) where the overheating control is obviously carried out by the C1 control 27 which also provides for maximizing efficiency.
Therefore, the SH set will be a function of the desired efficiency and the overheat imposed on the evaporator plate 38.
With specific reference now to figures 3 and 5, in a second embodiment of the plant 100 the outlet of the evaporation branch 38 of the evaporator 25 is directly connected at the inlet of the overheating branch 31 of the second exchanger 23.
In this second embodiment, the plant 100 comprises a compressor group having one or more flow compressors 21, the first heat exchanger 22 and the second heat exchanger 23, the receiving element 28, the lamination device 24, the evaporator 25, and the three-way valve 26, i.e. the same basic components listed for the first embodiment, connected in a similar way along the same refrigerant fluid circulation circuit.
Preferably, instead of the ejector element 29, the plant 100 also comprises a pressure reducer 51, such as a back pressure valve, inserted in the connection between the inlet 44 of the receiving element 28 and a fitting between the first outlet 42 of the valve 26 and the outlet of the heat transfer branch 32 of the second exchanger 23.
The connection between the vapor outlet 46 of the receiving element 28 and the inlet of the overheating branch 31 of the second exchanger 23 preferably has a back pressure valve 53, also called control, "backpressure" or "flash gas" valve, controllable as a function of the pressure in the receiving element 28.
Figure 4 illustrates in a pressure-enthalpy diagram the refrigerant fluid cycle in the plant 100 of the second embodiment just described, in which the points numbered 1-12 indicate the state of the fluid at steady state at the correspondingly numbered points in the diagram of the plant 100 of figure 3.
With reference to figures 3 and 4, in this embodiment the refrigerant fluid cycle proceeds as for the preferred embodiment between the inlet of the flow compressor 21 (point 1) and the connection between the first outlet 42 of the valve 26 and the outlet of the heat transfer branch 32 of the second exchanger 23 (point 5). The pressure reducer 51 causes a sudden expansion of the fluid, which loses pressure (point 6).
The liquid fraction of the fluid exiting from the liquid outlet 45 (point 7) passes through the lamination device 24 and evaporates partially, decreasing in pressure (point 8), then passes through the evaporation branch 38 of the evaporator 25 where it acquires heat and expands, evaporating completely and with a slight overheating (point 9), or evaporating almost completely and without any overheating (point 9' in figure 4). In parallel, the gaseous fraction exiting from the receiving element 28 (point 10) passes through the back pressure valve 53 (point 12) and, through the connection with the section of the circuit coming from the outlet of the evaporation branch 38, mixes with the fluid exiting from the evaporator 25 and then enters the overheating branch 31 (point 12), where the fluid acquires the heat necessary to evaporate completely into overheated gas (point 1).
In a variant illustrated in figure 5 for the second embodiment, but also applicable to the preferred embodiment of the plant 100, this also comprises at least one additional compressor group 54 operating in parallel with the flow compressor 21, having an inlet directly connected to the vapor outlet 46 of the receiving element 28 and an outlet connected to the inlet of the outflow branch 35 of the first exchanger 22. The additional compressor group 54 is designed to increase the efficiency of the plant 100, being supplied with gas at a higher pressure than the gas downstream of the evaporator 25.
The plant of figure 5 differs from that of figure 3 by adopting a depression element 58 provided with a main inlet and an outlet inserted in the connection between the vapor outlet 46 of the receiving element 28 and the inlet of the compressor group 54, where said main inlet and outlet of the depression element 58 are connected by a constriction in which flows into a secondary inlet connected, via a respective injection valve 59, to the liquid outlet 45 of the receiving element 28, where such injection valve 59 is controlled by an oil injection controller C7.
With reference to the corresponding operating cycle illustrated in figure 6, in this variant part of the gaseous fraction of the fluid exiting the receiving element 28 is directed to the additional compressor group 54 (point 1p), and once compressed (point 2p) it joins the fluid compressed by the flow compressor 21 (point 2a) to enter the outflow branch 35 of the first exchanger 22.
With reference to figures 1, 3 and 5, the main control functions of the plant 100 are implemented by algorithms that operate on the basis of the parameters reported below:

the valve 26 is controlled as a function of the variable SH of the overheating of the fluid entering the flow compressors 21 and of a corresponding desired value SHset, where SH = STL - TSAT(BPL) (control C1), with TSAT indicating the saturation temperature of the fluid as a function of a given pressure and used to maximise the efficiency.
the flow compressors 21 are controlled as a function of one or more of the variables BPL, BPR, STIN, STOUT and of corresponding desired values (control C2) and utilized to maximize the efficiency;
the high pressure in the circuit section upstream of the ejector element 29, and therefore the opening of the ejector element 29 itself, is controlled as a function of the variable BPH and of a corresponding desired value POTT = f(STGC) (control C3);
the lamination device 24 is controlled as a function of the variable SHE of the fluid overheating at the outlet of the evaporator 25 and of a corresponding desired value SHEset, where SHE = STE - TSAT(BPE), or is controlled as a function of the variable DPE and of a corresponding desired value DPset, where DPE = BPR - BPE (control C4);
the injection valve 57 is controlled as a function of the variable SH and the corresponding SHset, or is controlled as a function of the state of the compressor group of the flow compressors 21 and of a predetermined work cycle or duty cycle Tinj (control C5);
the back pressure valve 53 and/or the compressor group of the auxiliary compressors 54 are controlled as a function of the variable BPR and of a corresponding value Pset of the desired pressure in the receiving element 28 (control C6);

It should be noted that the invention, also thanks to the contribution of the first data and/or signal line 63 of mutual connection of the VT control 70 and the C1 control unit 27 and to the contribution of the second data and/or signal line 71 of mutual connection of the C1 and C2 controls 27, 72, allows to optimize the operating parameters and provides an unexpected increase in efficiency.
The controls and controllers C1-C7 mentioned in the description are programmable modules and can be carried out in a single programmable digital computer provided with input ports for the signals provided by the sensors and detectors, output ports for the actuation commands of the remotely controlled active elements of the system and data connections between the modules. Alternatively, the modules can be obtained in separate elements or computers, for example the C7 control can be of the stand alone type.

Claims

Vapor compression refrigeration plant with refrigerant fluid consisting of carbon dioxide or mixtures thereof and of the type comprising a fluid circulation circuit provided with:
- at least one compressor group (21, 54);

- a first heat exchanger (22) having at least one outflow branch (35) to the inlet of which the outlet of at least one compressor group (21, 54) is connected, where this first exchanger (22) is assigned to the transfer of heat from the fluid to a source or user;

- a second heat exchanger (23) having an overheating branch (31) and a heat transfer branch (32) in mutual thermal connection, where an inlet of the heat transfer branch (32) is connected to the outlet of the outflow branch (35) of the first exchanger (22), where an outlet of the overheating branch (31) of the second exchanger (23) is connected to at least one compressor group (21, 54);

- a receiving element (28) provided with an inlet (44) for the fluid connected at least to the outlet of the heat transfer branch (32) of the second exchanger (23) and provided with a liquid outlet (45) and a vapor outlet (46), the latter connected to the inlet of the overheating branch (31) of the second exchanger (23) and/or to at least one compressor group (21, 54);

- a lamination device (24) an inlet of which is connected to the liquid outlet (45) of the receiving element (28);

- an evaporator (25) having at least one evaporation branch (38) an inlet of which is connected to an outlet of the lamination device (24), the outlet of such evaporation branch (38) is connected to the inlet of the receiving element (28) or to the inlet of the overheating branch (31) of the second exchanger (23), where this evaporator (25) is assigned to the transfer of heat from a source or user to the fluid;
said plant (100) being characterized in that the outlet of the outflow branch (35) of the first exchanger (22) is connected to the inlet of the heat transfer branch (32) of the second exchanger (23) and to the inlet (44) of the receiving element (28) by means of a remote control valve (26) having an inlet (41) connected to said outlet of the outflow branch (35), a first outlet (42) connected to said inlet (44) of the receiving element (28), and a second outlet (43) connected to said inlet of the heat transfer branch (32), and in comprising a C1 control unit (27) which operates the valve (26) for controlling the state of the fluid at the inlet of at least one compressor group (21, 54) and in comprising a C4 VT control (70) controlling the lamination device (24) and connected to respective temperature sensor (64) and pressure sensor (65) installed downstream of the outlet of the evaporation branch (38) and/or connected to respective pressure sensors (65, 68) installed downstream of the outlet of the evaporation branch (38) and in the receiving element (28) respectively; and in comprising a C2 control (72) controlling at least one of the compressor group (21, 54) and connected to a set of sensors and or probes of the plant and in that it comprises a first data and/or signals line (63) for mutual connection of the VT C4 control (70), and of the C1 control unit (27), and by comprising a second data and/or signals line (71) for mutual connection of the C2 control (72) and of the C1 control unit (27) to allow a more accurate control and further improvement of the efficiency of the plant.
Plant according to claim 1 characterized in that the at least one compressor group (21, 54) comprises one or more flow compressors (21) whose respective inlets are connected to the outlet of the overheating branch (31) of the second exchanger (23), and in that it comprises at least one between temperature (61) and pressure (62) sensors of the fluid immediately upstream of the inlets of the flow compressors (21) and in data connection with the control unit (27), which is of the programmable digital type and which controls the operation of the valve (26) of the proportional type based on the temperature and/or pressure data provided by such temperature (61) and pressure (62) sensors.
Plant according to claim 1 or 2 characterized in that the first exchanger (22) is also provided with a further branch for a second fluid of a thermal source or user for the transfer of heat from the refrigerant fluid of the outflow branch (35) to the second fluid, and in that the evaporator (25) is also provided with a further branch for a third fluid of a user or a thermal source for the transfer of heat from the third fluid to the refrigerant fluid of the evaporation branch (38).
Plant according to any one of the preceding claims characterized in that the inlet (44) of the receiving element (28) is connected to the outlets of the outflow branch (35) of the first exchanger (22), of the heat transfer branch (32) of the second exchanger (23) and of the evaporation branch (38) of the evaporator (25) by means of an ejector element (29) a main inlet (47) of which is connected to the first outlet (42) of the valve (26) and to said outlet of the heat transfer branch (32), and a secondary inlet (48) of which is connected to said outlet of the evaporation branch (38); where an outlet (49) of the ejector element (29) is connected to such inlet (44) of the receiving element (28).
Plant according to claim 4 characterized in that the ejector element (29) is of the type in which the introduction of the fluid flow from the secondary inlet (48) into the fluid flow from the main inlet (47) is adjustable continuously, and such regulation occurs by means of a controller based on the pressure measured at the inlet or the outlet of the outflow branch (35) outlet of the first exchanger (22) and the temperature measured immediately downstream of the outflow branch (35) outlet.
Plant according to claim 4 or 5 characterized in that it further comprises a check valve (50) placed between the outlet of the evaporation branch (38) of the evaporator (25) and the secondary inlet (48) of the ejector element (29).
Plant according to any one of claims 3-6 characterized in comprising a depression member (56) provided with a main inlet and an outlet inserted in the connection between the vapor outlet (46) of the receiving element (28) and the inlet of the overheating branch (31) of the second exchanger (23), where said main inlet and outlet of the depression member (56) are connected by a choke into which flows a secondary inlet connected, by means of a injection valve (57), to the liquid outlet (45) of the receiving element (28), where such injection valve (57) is timed or can be controlled by a controller.
Plant according to claim 3 characterized in that it further comprises a pressure reducer (51) inserted in the connection between the inlet (44) of the receiving element (28) and the two outlets, the first (42) of the valve (26) and the one of the heat transfer branch (32) of the second exchanger (23).
Plant according to claim 8 characterized in that the connection between the vapor outlet (46) of the receiving element (28) and the inlet of the overheating branch (31) of the second exchanger (23) has a back pressure valve (53).
Plant according to any one of the preceding claims characterized in that the at least one compressor group (21, 54) comprises one or more parallel auxiliary compressors (54) each having the respective inlet connected to the vapor outlet (46) of the receiving element (28) and the respective outlet connected to the inlet of the outflow branch (35) of the first exchanger (22).
Plant according to any one of the preceding claims characterized in that the lamination device (24) is of the controllable type and is controlled by a controller based on the temperature and/or pressure detected by respective sensors downstream of the outlet of the evaporation branch (38), or based on the pressures detected by respective sensors downstream of the evaporation branch (38) and in the receiving element (28).
System according to any of the preceding claims characterized in that it comprises a depression element (58) provided with a main inlet and an outlet inserted in the connection between the vapor outlet (46) of the receiving element (28) and the additional compressor group inlet (54), where said main inlet and outlet of the depression element (58) are connected by a constriction into which a secondary inlet of the depression element (58) flows, connected, by means of a respective injection valve (59), to the liquid outlet (45) of the receiving element (28), where said injection valve (59) is controlled by an oil injection C7 controller.