EP2313711B1

EP2313711B1 - Refrigerating circuit

Info

Publication number: EP2313711B1
Application number: EP09776836.0A
Authority: EP
Inventors: Christian Douven; Bernd Heinbokel; Markus Hafkemeyer
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2008-07-07
Filing date: 2009-06-25
Publication date: 2013-07-24
Anticipated expiration: 2029-06-25
Also published as: EP2313711A1

Description

The present invention relates to a refrigerating circuit.
Refrigerating circuits circulate a refrigerant within at least one cooling loop comprising at least one refrigeration consumer, at least one compressor stage connected to the refrigeration consumer via a suction line, and at least one one heat dissipating heat exchanging unit connected to the compressor unit via a pressure line.
The refrigeration consumer consumes enthalpy of heat for cooling for cooling at least one cooling load and comprises at least one evaporator stage which evaporates refrigerant thereby depriving the the evaporator stage's environment of heat.
The heat dissipating heat exchanging unit comprises a condenser/gas cooler unit and produces heat which is discharged to the environment of the heat dissipating heat exchanging unit. Depending on whether the refrigerating circuit is operated in a subcritical operation mode or in a supercritical (or transcritical) operation mode, the condenser/gas cooler unit works as a condenser unit condensing the evaporated refrigerant, or works as a gas cooler unit cooling the refrigerant from a temperature above its critical temperature to a lower temperature. Particularly, in case of supercritical operation, the heat dissipating heat exchanging unit also comprises an expansion means adapted to expand the refrigerant to a pressure below its critical pressure, and therefore allowing the refrigerant to condense. Often the condensed refrigerant is supplied to a receiver in which a gaseous fraction of refrigerant is separated from the liquid fraction of refrigerant. Supercritical operation commonly takes place in case a CO₂ refrigerant is used, particularly under high load conditions as they occur on warm summer days.
Flow of refrigerant within the refrigerating circuit is driven by the compressor stage having a suction side connected to the refrigeration consumer and a pressure side connected to the heat dissipating heat exchanging unit.
Refrigeration consumers may e.g. comprise refrigerating sales furnitures placed at various locations in a supermarket for presenting goods at cooling temperatures, i.e. at temperatures below ambient temperature. In a refrigeration circuit, a plurality of such refrigeration consumers may be provided within on or more cooling loops, each cooling loop providing a respective cooling temperature (e.g. a normal or medium temperature cooling loop with temperatures around -10 °C or a low temperature cooling loop with temperatures around -37 °C). In such installations the refrigeration consumers within each cooling loop may be connected to a respective compressor stage comprising a single or a plurality of compressors. The cooling loops may have an arrangement in with the compressor stage of a low temperature cooling loop (in the following: low temperature loop compressor stage) has its pressure side connected to the suction side of the compressor stage of a medium temperature cooling loop (in the following: medium temperature loop compressor stage).
Typical refrigerating systems, as are used e.g. in supermarkets, only comprise a central heat dissipating heat exchanging unit to which is supplied refrigerant in at least partly evaporated form from the respective pressure sides of compressor stages assigned to a medium or high temperature cooling loop. To effectively discharge heat produced by condensation of refrigerant the heat dissipating heat exchanging unit is typically located outside the building, e.g. on the roof of the building.
Typically the thermal stress on the devices located on the pressure side of a compressor stage is relatively high. This particularly holds for the condenser/gas cooler unit of the heat dissipating heat exchanging unit, since it is supplied with refrigerant having high pressure and temperature from all compressor stages, either directly from compressor stages assigned to a medium temperature loop or, e. g. in a booster installation, also indirectly from compressor stages assigned to a low temperature cooling loop via respective medium temperature loop compressor stages.
High thermal stress may also arise for the compressors of a medium temperature cooling loop compressor stage connected to a low temperature cooling loop, e. g. in a booster installation. Under certain conditions such compressors are supplied with considerable amounts of refrigerant from the pressure lines of compressor stages assigned to a low temperature cooling loop. Typically such refrigerant may be at rather high temperature, reducing efficiency of the downstream compressor and moreover leading to stress for the mechanical parts of such compressor. Moreover, the increased temperature at the suction side of the downstream processor will also lead to increased temperature at its pressure side, and therefore higher temperature of the refrigerant entering the condenser/gas cooler.
Significant thermal stresses, as outlined above, applied to elements of a heat exchanging unit or compressor stage exposed to refrigerant (e.g. heat exchanging plates of the condenser/gas cooler unit, blades/vanes of a compressor) reduces lifetime of that elements.
Thermal stress applied to the elements of a downstream compressor stage and/or condenser/gas cooler unit could be reduced by desuperheating the refrigerant (essentially depriving the refrigerant from enthalpy of heat, thereby typically reducing the temperature of the refrigerant) flowing in the pressure line, already before it enters the downstream compressor stage or condenser/gas cooler unit.
It would be beneficial to provide a refrigerating circuit having the capability of desuperheating refrigerant flowing in a pressure line already before it enters a downstream device.
WO96/34237 discloses a refrigerating circuit according to the preamble of claim 1.
Exemplary embodiments of the invention comprise a refrigerating circuit according to claim 1.
Exemplary embodiments of the invention will be described in greater detail below taking reference to the accompanying drawings.

Fig. 1: shows in schematic and simplified form a portion of a refrigerating circuit according to an exemplary embodiment, the portion extending between a compressor stage and a condenser/gas cooler unit;
Fig. 2: shows a detail designated by A in fig. 1; and
Fig. 3: shows a further exemplary embodiment in the form of a booster refrigerating circuit having a medium temperature loop and a low temperature loop.

Fig. 1 shows in schematic and simplified form a portion of a refrigerating circuit, generally designated by 10, according to an exemplary embodiment. The refrigerating circuit 10 circulates a substantially liquid refrigerant (not shown) to a number of refrigeration consumers (not shown) in which the refrigerant is at least partly evaporated, and further from the refrigeration consumers to a compressor stage 12, and further from the compressor stage 12 to a heat dissipating heat exchanging unit 14 formed by a condenser/gas cooler unit 16 connected via a pressure reduction valve (not shown) in series to a receiver 18. The receiver temporarily stores liquid refrigerant, and provides, if necessary, separation of a gaseous and liquid fraction of the refrigerant. From the receiver the refrigerant is supplied in essentially liquid form via a liquid line 20 to the refrigeration consumers.
Flow of refrigerant within the refrigerating circuit 10 is driven by the compressor stage 12 including a plurality of compressor units 12a, 12b, 12c, 12d connected in parallel to each other between a suction line 22 of the compressor stage 12, the suction line 22 leading to the refrigeration consumers, and a pressure line 24 leading to the heat dissipating heat exchanging unit 14. Accordingly, each of the compressor units 12a, 12b, 12c, and 12d has a suction side and a pressure side. The pressure sides of all compressor units 12a, 12b, 12c and 12d are connected to an inlet side 14a of the heat dissipating heat exchanging unit 14, particularly to an inlet side of the condenser/gas cooler unit 16.
The condenser/gas cooler unit 16 works against air as medium to which heat released from the refrigerant is transferred when refrigerant is passed through the condenser/gas cooler unit 16 with no significant pressure loss. When operated in a subcritical operation mode (an operation mode in which the refrigerant, when entering the condenser/gas cooler unit 16, has a pressure below its critical pressure) the condenser/gas cooler unit 16 essentially works as a condenser transferring the refrigerant from gaseous phase into liquid phase. When operated in a supercritical (also called transcritical) operation mode (an operation mode in which the refrigerant, when entering the condenser/gas cooler unit 16, has a pressure above its critical pressure) the refrigerant cannot undergo a transition from the gaseous phase to the liquid phase, since there is no substantial pressure drop of the refrigerant when passing the condenser/gas cooler unit 16. Hence, the condenser/gas cooler unit 16 therefore works as a gas cooler. The invention uses CO₂ as a refrigerant, the refrigerating circuit will thus typically operate in the subcritical operation mode or the supercritical operation mode, depending on load conditions.
The refrigerating circuit 10 further includes a liquid refrigerant return line 26 connecting the outlet side 14b of the heat dissipating heat exchanging unit 14 to the pressure line 24. The liquid refrigerant return line 26 includes a valve 44 for opening/closing the return line 26 and/or for controlling flow of liquid refrigerant through the return line 26. Valve 44 is controlled by a thermostat unit 46 provided such as to sense temperature of the refrigerant in the pressure line 24, and thereby determine a superheating state of the refrigerant in the pressure line 24. Flow of liquid refrigerant in the return line 26 is controlled accordingly, such as to provide sufficient desuperheating of the refrigerant passing through the pressure line 24, before the refrigerant enters the condenser/gas cooler unit 14. The liquid refrigerant return line 26 opens at a downstream side of the receiver 18.
The pressure line 24 includes pressure reduction means 28 formed such as to locally reduce pressure of the refrigerant in the pressure line 24 from an operational pressure p_c to a reduced pressure p₁. The reduced pressure p₁ is smaller than the operational pressure p_c to such an extent as to provide a sufficiently steep pressure gradient between a position at which the liquid refrigerant return line 26 opens into liquid line 20 extending from the outlet side 14b of the heat dissipating heat exchanging unit 14, and a position at which the return line 26 opens into the pressure line 24, to allow injection of liquid refrigerant into the pressure line 24.
As can be seen most clearly in fig. 2, the pressure reduction means 28 includes a venturi structure 30. The venturi structure 30 is formed such as to locally reduce the cross section of the pressure line 24. The venturi structure 30 has an inlet side 32 and an outlet side 34 defining the cross section of the pressure line 24 upstream and downstream of the venturi structure 30, respectively. The inlet side 32 and the outlet side 34 define essentially the same cross section, and therefore the refrigerant will have essentially identical pressures p_c and p_c' upstream and downstream of the venturi structure 30.
The venturi structure 30 further has an intermediate portion 36 located between the inlet side 32 and the outlet side 34. The intermediate portion 36 includes a first conically tapered portion 38 extending from the inlet side 32 towards an axial inner side thereof, and a second tapered portion 40 extending from the outlet side 34 towards an axial inner side thereof. Both conically tapered portions 38, 40 taper towards the axial inner sides thereof. The axial inner sides of the first and second tapered portions 38, 40 thus define the reduced cross section of the pressure line 24 in the region of the venturi structure 30.
The first and second tapered portions 38, 40 have an essentially identical extension in axial direction. The axial inner sides of the first and second tapered portions 38, 40 are connected by a non-tapered central portion 42. The liquid refrigerant return line 26 opens into this cylindrical central portion 42. The cylindrical central portion 42 essentially has the same extension in axial direction as have the first and second tapered portions 38, 40.
The refrigerating circuit 10 is adapted to circulate a CO₂ refrigerant.
Fig. 3 shows, as a further example of a refrigerating circuit according to an embodiment, a CO₂ refrigeration circuit 10 for circulating a CO₂ refrigerant in a predetermined flow direction. The refrigeration circuit 10 comprises a heat dissipating heat exchanger unit 16 which is with a CO₂ refrigerant a gas cooler in the supercritical operational mode and a condensor in the subcritical operation mode. A condenser/gas cooler outlet line 52 connects the condenser/gas cooler 16 via an intermediate expansion device 54 to a receiver 18. While the pressure of the refrigerant can be up to 120 bar and is typically approximately 85 bar in "summer mode" and approximately 45 bar in "winter mode" in the condenser/gas cooler 16 and its outlet line 52, the intermediate expansion device 54 reduces the pressure to between 30 and 40 bar and preferably 36 bar. This intermediate pressure typically will be independent from "winter mode" and "summer ode". The receiver 18 collects and separates gaseous and liquid refrigerant in a gaseous and liquid receiver portion 18a and 18b, respectively.
A liquid line 20 opens from the liquid portion 18b of the receiver 18, and connects the liquid portion 18b of the receiver 18 with the refrigeration- consumers 56 and 58 of a medium temperature loop 60 and a low temperature loop 62. Particularly, the liquid line 20 bifurcates into a low temperature branch line 64 and a medium temperature branch line 66. The low and medium temperature loops 60 and 62 each comprise at least one low temperature and medium temperature, respectively, refrigeration consumer 56, 58. The refrigeration consumers 56 and 58 each comprise an expansion device 68, 70 and an evaporator 72, 74.
The medium temperature loop 60 closes through the suction line 76 leading to inlets of compressors 12b, 12c, 12d of a compressor stage 12 of the medium temperature loop 60 and a first pressure line portion 24a which connects the outlet of the compressors 12b, 12c,12d with the inlet of the condenser/gas cooler 16. The pressure at the inlet of the medium temperature loop compressors 12b, 12c, 12d is typically between 20 and 30 bar and approximately 26 bar which results in a temperature of the refrigerant of approximately -10 °C in the refrigeration consumer(s) 56 of the medium temperature loop 60.
In the low temperature loop 62 the low temperature suction line 78 connects the low temperature refrigeration consumer(s) 58 with the inlets of compressors 80a, 80b, 80c of the low temperature loop compressor stage 80. A second pressure line portion 24b returns the low temperature loop refrigerant to the inlet of the medium temperature loop compressor stage 12. While the pressure at the inlet of the low temperature loop compressor stage 80 is typically between 8 and 20 bar, and preferably approximately 12 bar which results in a temperature of the refrigerant of approximately -37 °C in the refrigeration consumer(s) 58 of the low temperature loop 62, the pressure at the outlet thereof, and therefore also in the second pressure line portion 24b, is approximately at about the same level as the inlet pressure of the medium temperature loop compressor stage 12. The low temperature loop 62 subsequently closes through the common loop portion with the medium temperature loop 60, i.e. medium temperature loop compressor stage 12, first pressure line portion 24a, condenser/gas cooler unit 16, intermediate expansion device 54, receiver 18 and liquid line 20. In this embodiment the pressure line is made up of two portions at different pressures, a first pressure line portion 24a at high pressure and a second pressure line portion 24b at medium pressure, lower than the pressure in the first pressure line portion 24a.
A flash gas line 84 is connected with the gaseous portion 18a of the receiver 18. The flash gas line 18a taps flash gas which is substantially at the saturation pressure, i.e. at least near the 2-phase state thereof. The flash gas line 84 leads the flash gas via a flash gas expansion device, for example a flash gas valve 86, and an internal heat exchanger 88 which is connected to the liquid line 20 in heat exchange relationship with liquid refrigerant and returns it to the inlet or suction of the low temperature loop compressor stage 80. Accordingly, the flash gas which is at the intermediate pressure of approximately 36 bar in the receiver 18 is expanded to approximately 12 bar at the inlet to the low temperature loop compressor stage 80. The respective cooling capacity, i.e. heat from the liquid refrigerant, will substantially be transferred to the liquid refrigerant in the internal heat exchanger 88 and increases the cooling or refrigeration capacity thereof.
This transfer of heat to the flash gas refrigerant increases the temperature thereof and insures that the initially 2-phase state flash gas is fully dry and superheated before feeding into the suction side or inlet of the low temperature compressor stage 80. The internal heat exchanger 88 can be in the liquid line 20 resulting in an increase of the refrigeration capacity of the liquid for the medium temperature and the low temperature loops 60 and 62, but can also be in any of the branch lines 64 and 66 so that the refrigeration capacity merely for this loop 60 or 62 will be increased. It is also possible to provide a switch-over valve (not shown) in the flash gas line 84 subsequent to the internal heat exchanger 88, and an alternative flash gas line (not shown) which connects the switch-over valve and thus the internal heat exchanger 88 to the inlet or suction of the medium temperature compressor stage 12. By switching over between flowing the flash gas to the inlet of the low temperature compressor stage 80 and the inlet of the medium temperature compressor stage 12 the increase of the refrigeration capacity can be controlled in a wide range.
The flash gas valve 86 can be a thermal expansion device and can be a controllable valve of the type as known to the skilled person. It can particularly be an electronically controlled valve or a mechanically controlled valve. It can be a thermal expansion valve TXV or an electronic expansion valve EXV.
A control 90 is provided for controlling the flash gas valve 86. The control 90 can be separate or part of the overall refrigeration circuit control. The control can also be integrated with the flash gas valve 86. A monitoring device 92 which includes a temperature sensor 94 and a pressure sensor 96 is connected via line 98 to the control 90. The control 90 is adapted to control the flow of flash gas through the internal heat exchanger 88, for example dependent on the desired refrigeration capacity increase in the liquid refrigerant or dependent of the superheat condition of the flash gas. The control 90 can also be adapted to control the above mentioned switch-over valve.
Further sub-cooling is provided for the high-pressure refrigerant in the outlet line 52 of the condenser/gas cooler 16. Therefore, a portion of the refrigerant is diverted through high-pressure expansion valve 100 and high-pressure heat exchanger 102 for sub-cooling the remainder of the refrigerant. Line 104 returns the diverted portion of the refrigerant to the inlet of the compressor 12a. The inlet of compressor 12a can be at the same pressure level as the remaining compressors 12b, 12c, 12d of the compressor stage 12 or at a different, i.e. higher or lower, level.
Depending on the load conditions of the low temperature loop 62, a considerable amount of refrigerant at comparatively high temperature is delivered via second pressure line portion 24b of the low temperature loop compressor stage 80 to the inlet (i.e. suction side) of the medium temperature loop compressor stage 12. Therefore, under conditions of high cooling load in the low temperature cooling loop 62, thermal stress applied to the compressors 12b, 12c, and 12d of the medium temperature loop compressor stage 12 is considerably high. To reduce such thermal stress, at least under high load conditions as described above, a liquid refrigerant return line 26 is branching off from the liquid line 20. The liquid refrigerant return line 26 opens into the second pressure line portion 24b at a position in between the pressure side of low temperature loop compressor stage 80 and the suction side of medium temperature loop compressor stage 12. Via liquid refrigerant return line 26 liquid refrigerant can be injected into the second pressure line portion 24b, thereby evaporating in the second pressure line portion 24b and desuperheating refrigerant flowing in the second pressure line portion 24b from the pressure side of low temperature loop compressor stage 80 to the medium temperature loop compressor stage 12. The pressure of liquid refrigerant at the liquid outlet of receiver 18 typically is higher than the pressure of refrigerant in the second pressure line portion. As an example, a typical pressure in liquid line 20 is between 30 and 40 bar (35 bar), while a typical pressure in second pressure line portion 24b is between 25 and 30 bar. Therefore a positive pressure gradient exists between the liquid line 20 and the second pressure line portion 24b allowing injection of liquid refrigerant without any further provisions to manipulate pressure of refrigerant.
The liquid refrigerant return line 26 includes a valve 44 adapted to open/close the liquid refrigerant return line 26, or control of refrigerant passing liquid refrigerant return line. Valve 44 is controlled by a thermostat unit 46 provided such as to sense temperature of the refrigerant in the second pressure line portion 24b, and thereby determine a superheating state of the refrigerant in the second pressure line portion 24b. Flow of liquid refrigerant in the return line 26 is controlled accordingly, such as to provide sufficient desuperheating of the refrigerant passing through the second pressure line portion 24b, before the refrigerant enters the medium temperature loop compressor stage 12.
The embodiments described before provide a refrigerating circuit having the capability of reducing the temperature of refrigerant (or desuperheating the refrigerant) leaving the compressor stage already before it enters a downstream device, in particular a condenser/gas cooler unit, or a downstream compressor stage.
The refrigerating circuit is adapted to circulate, in operation, a refrigerant in a predetermined flow direction, and comprises a compressor stage, the compressor stage having a suction side and a pressure side, and at least one heat dissipating heat exchanging unit connected to the pressure side of the compressor stage via a pressure line, the heat dissipating heat exchanging unit having an inlet side and an outlet side and including a condenser/gas cooler unit. A liquid refrigerant return line connects the outlet side of the heat dissipating heat exchanging unit to the pressure line.
At the outlet side of the heat dissipating heat exchanging unit the refrigerant to a large part has undergone transition from a gaseous state into a liquid state, and therefore essentially liquid refrigerant is flowing in the return line. Injecting such liquid refrigerant into the pressure line (in which essentially refrigerant in a gaseous state is flowing) will lead to evaporation of the injected liquid refrigerant, and thereby desuperheat the refrigerant flowing in the pressure line before entering any downstream device.
As a positive effect, the thermal stress to the downstream devices, e. g. condenser plates of the condenser/gas cooler unit or parts of a downstream compressor stage, can be reduced significantly by evaporation of the liquid refrigerant injected into the pressure line. This increases the serviceable lifetime of the condenser elements substantially.
In a first embodiment, to reduce thermal stress applied to the heat dissipating, particularly to the condenser/gas cooler unit, it is conceivable to directly desuperheat refrigerant flowing in the pressure line opening into the inlet side of the heat dissipating heat exchanging unit. Basically such desuperheating can be achieved by injecting liquid refrigerant into the pressure line connecting the compressor stage and the heat dissipating exchanging unit. However, this requires to provide liquid refrigerant at a pressure sufficiently larger than the pressure of refrigerant in this pressure line. As a matter of fact, the pressure of refrigerant in the pressure line opening into the inlet side of the heat dissipating heath exchanging unit typically is not less than the pressure of liquid refrigerant available downstream of the heat dissipating heat exchanging unit: In subcritical operation, pressure of liquid refrigerant leaving the condenser/gas cooler is about the same or slightly less than pressure of (essentially gaseous) refrigerant entering the condenser/gas cooler unit. In supercritical operation, refrigerant can only condense after having been expanded downstream of the condenser/gas cooler unit, and thus pressure of refrigerant leaving the heat dissipating heat exchanging unit in liquid form is significantly lower than pressure of refrigerant in the pressure line. It would require considerable efforts (e.g. provision of an additional compressor or pumping unit) to bring such liquid refrigerant to a pressure sufficiently high to inject the liquid refrigerant into the pressure line, these efforts rendering the option of injecting liquid refrigerant into the pressure line inefficient.
Hence, it would further be beneficial to provide a refrigerating circuit having the capability of desuperheating refrigerant flowing in the pressure line opening into the heat dissipating heat exchanging unit already before it enters the condenser/gas cooler unit of such heat dissipating heat exchanging unit. Beneficially such injecting should not require much additional installation efforts.
In a particular embodiment, the pressure line includes pressure reduction means for locally reducing pressure of the refrigerant in the pressure line from an operational pressure to a reduced pressure smaller than the operational pressure, and the liquid refrigerant return line opens into the pressure line at a position in which the refrigerant has the reduced pressure.
Provision of the pressure reduction means allows to create locally, i.e. in a predetermined portion of the pressure line, a reduced pressure of refrigerant compared to the operational pressure of refrigerant in the pressure line, and therefore allows, in case it is necessary due to an adverse pressure gradient between operational pressure in the pressure line and liquid pressure, to create a favourable pressure gradient of refrigerant at a higher pressure at the outlet side of the heat dissipating heat exchanging unit (this refrigerant leaving the heat dissipating heat exchanging unit being, in both subcritical and supercritical operation, essentially in the liquid phase), and refrigerant in the pressure line being locally at a lower pressure than operational pressure. Connecting the higher pressure side and the lower pressure side will thus allow to inject essentially liquid refrigerant into the pressure line, without the need to provide any special installations to drive flow of refrigerant against an adverse pressure gradient that would normally exist between the pressure of essentially liquid refrigerant at the outlet of the heat dissipating heat exchanging unit and the operational pressure of refrigerant in the pressure line. As the pressure is reduced in the pressure line only locally, and there is no need to actively drive any return flow of fluid, there is essentially no adverse effect on the efficiency of the refrigerating circuit.
The pressure reduction means may include a venturi structure (e.g. a venturi pipe or venturi nozzle). The venturi structure may be formed such as to locally reduce the cross section of the pressure line, i.e to reduce the cross section of the pressure line in a predetermined axial portion thereof. As the flow of refrigerant in the pressure line approximately will have the characteristics of flow of an incompressible fluid, a reduction of cross section of the pressure line, as achieved by a venturi structure will lead to an increase of the velocity in the flow of refrigerant passing the portion having reduced cross section. According to Bernoulli's law, this will lead to a corresponding reduction in static pressure of refrigerant passing the portion having reduced cross section, since part of the energy is to be converted in additional kinetic energy, i.e. dynamic pressure. To allow safe injection of fluid, the venturi structure may be designed such as to locally reduce pressure in the pressure line to a pressure about 1 or 2 bar lower than the pressure of liquid refrigerant to be injected.
A venturi structure can be created in the pressure line almost without any material efforts. All that it is necessary is to form the walls of the pressure line with a corresponding restriction or constriction, or to insert a restricting or constricting structure into the pressure line. Such venturi structure can easily be added to already existing installations, thus providing a simple and cheap possibility to modernize older installations.
The venturi structure may have an inlet side and an outlet side defining the cross section of the pressure line upstream and downstream of the venturi structure, respectively. To achieve a nearly same operational pressure of the refrigerant upstream and downstream of the venturi structure, the inlet side and the outlet side of the venturi structure may define essentially the same cross section.
The venturi structure may have an intermediate portion located between an inlet side and an outlet side thereof. The intermediate portion may include a first tapered portion extending from the inlet side and a second tapered portion extending from the outlet side. Inner axial ends of the first and second tapered portions may define the reduced cross section of said pressure line. As an example, the first and second tapered portions may be tapered conically from an annular portion with larger diameter formed at the inlet side and outlet side, respectively, towards an annular portion with smaller diameter formed at the inner side, respectively. The radius of the annular portion formed at the inner sides then defines the cross section of the pressure line in the venturi structure.
In an example, the first and said second tapered portions may have an essentially identical extension in axial direction. The first and second tapered portions will then have an identical inclination with respect to the axial direction of the pressure line; and thus flow conditions at the inlet end of the venturi structure will be identical to flow conditions at the outlet end.
Further, the inner axial ends of the first and second tapered portions may be connected by a non-tapered central portion. In this case, the liquid refrigerant return line may open into this central portion. The non-tapered portion extends essentially in longitudinal direction of the pressure line, and in case of the first and second tapered portions having annular inner axial ends forms has a cylindrical shape. In an embodiment the central portion may have essentially the same extension in axial direction as said first and/or second tapered portions.
According to the invention the refrigerating circuit uses CO₂ as refrigerant. The operational pressure of such refrigerant in the pressure line may e.g. be between 20 and 150 bar, particularly at least temporarily as high as 125 bar (this value applying to the most downstream portion of the pressure line extending between a most downstream compressor stage and the heat dissipating heat exchanging unit). The critical pressure of CO₂ (pressure above which there cannot exist a liquid phase of CO₂) is about 70 bar, and hence in case of a CO₂ refrigerant, the above pressures in the pressure line, at least in case of the maximum pressures between 100 and 125 bar, as they may occur under high refrigerating loads (e.g. on warm summer days), lead to supercritical operation of the refrigerating circuit in the portion between the most downstream (medium temperature loop) compressor stage and heat dissipating heat exchanging unit. For a CO₂ refrigerant, in supercritical operation mode a suitable reduction in pressure of the refrigerant downstream of the condenser/gas cooler unit may be a reduction to about 35 to 36 bar. Injecting liquid refrigerant into the most downstream portion of the pressure line will be possible if the pressure reduction means provided in the pressure line is be adapted such as to reduce the pressure of refrigerant locally to a pressure below 35 to 36 bar.
In an embodiment, the refrigerating circuit includes a single compressor stage, and the pressure line has only one portion extending between the pressure side of the compressor stage and the inlet side of the heat dissipating heat exchanging unit. As the pressure reduction means provides for reduction of the refrigerant pressure in the pressure line, it is easily possible to return liquid refrigerant from the outlet side of the heat dissipating heat exchanging unit to the pressure line connected to the inlet side of the same heat dissipating heat exchanging unit. This particularly applies in case the refrigerating circuit is operated in a subcritical mode, as in this case the pressure drop between the inlet side of the heat dissipating heat exchanging unit (corresponding to the operational pressure in the pressure line) and the outlet side of the heat dissipating heat exchanging unit (corresponding to the pressure in the liquid refrigerant return line) will be small, typically at most several bar.
According to the above pressure values, under supercritical operation of a single compressor stage refrigerating circuit, as it might occur in case a CO₂ refrigerant is used, injection of liquid refrigerant into the pressure line will still be possible, if the pressure reduction means is designed such as to provide, in operation, a substantial reduction in pressure. As an example a ratio between the reduced pressure and the operational pressure of the refrigerant in the pressure line below 0.4 should be sufficient.
In supercritical operation, pressure in the pressure line may be as high as twice the critical pressure of the refrigerant, and therefore, to allow the refrigerant to change into the liquid phase, the refrigerant will have to be expanded after passing the condenser/gas cooler unit such as to reduce the pressure of the refrigerant by a factor of more than two. The ratio of the pressure of the - substantially liquid - refrigerant at the outlet side of the heat dissipating heat exchanging unit to the operational pressure of the - substantially gaseous - refrigerant in the pressure line thus may be 0.4 or even smaller. Reducing the pressure in the pressure line locally to a value more than 2.5 times smaller than the operational pressure in the pressure line will thus allow to create a sufficient pressure gradient in the liquid refrigerant return line to inject liquid refrigerant into the pressure line.
According to the invention, the refrigerating circuit includes two compressor stages in a so-called booster arrangement: a first compressor stage assigned to a medium temperature cooling loop, and second compressor stage assigned to a low temperature cooling loop). In such arrangement the pressure line will connect the pressure side of the second compressor stage indirectly via the first compressor stage to the inlet side of the heat dissipating heat exchanging unit. The pressure line thus will include two portions, a first pressure line portion opening at opposite sides into the first compressor stage and the heat dissipating heat exchanging unit, respectively, and a second pressure line portion opening at opposite sides into the second compressor stage and the first compressor stage, respectively. The liquid refrigerant return line will the open into the second pressure line portion.
In a specific embodiment the refrigeration circuit may be adapted for circulating a CO₂ refrigerant in a predetermined flow direction, and may comprise in flow direction a heat dissipating heat exchanger, a receiver having a liquid portion and a flash gas portion, and subsequent to the receiver a medium temperature loop and a low temperature loop, wherein the medium and low temperature loops each comprise in flow direction an expansion device, an evaporator and a compressor stage. The refrigeration circuit may further comprise a liquid line connecting the liquid portion of the receiver with at least one of the medium and low temperature loops, and a liquid refrigerant return line branching off the liquid line.
The liquid refrigerant return line may further include a valve for opening and closing the liquid refrigerant return line and/or for adjusting flow of refrigerant flowing through the return line. This valve may be controlled, e.g. by a thermostat unit, according to a superheating state of the refrigerant in the pressure line. This superheating state of the refrigerant can be controlled, e.g., by sensing temperature of the refrigerant in the pressure line.
Besides the condenser/gas cooler unit, the heat dissipating heat exchanging unit may further include a receiver connected in series to the condenser/gas cooler unit. This receiver may have an upstream side connected to an outlet side of the condenser/gas cooler unit, and the liquid refrigerant return line may open at a downstream side of the receiver. Particularly in case of supercritical operation of the refrigerating circuit, the receiver may be connected to the outlet side of the condenser/gas cooler unit via a pressure reduction valve reducing the pressure of the refrigerant to a value well below its critical pressure (e.g. in case of a CO₂ refrigerant, to a pressure of about 35 to 36 bar well below the critical pressure), therefore allowing the superheated refrigerant leaving the condenser/gas cooler to condense after having passed the pressure reduction valve.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt the particular situation or material to the teachings of the invention without departing from the scope thereof. Therefore it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

Refrigerating circuit (10) adapted to circulate, in operation, a CO₂ refrigerant in a predetermined flow direction,
said refrigerating circuit (10) comprising
a compressor stage (12; 80), said compressor stage (12; 80) having a suction side and a pressure side, and

at least one heat dissipating heat exchanging unit (14) connected to said pressure side of said compressor stage (12; 80) via a pressure line (24; 24a, 24b), said heat dissipating heat exchanging unit (14) having an inlet side (14a) and an outlet side (14b) and including a condenser/gas cooler unit (16), and

a liquid refrigerant return line (26) connecting said outlet side of said heat dissipating heat exchanging unit (14) to said pressure line (24; 24a, 24b),
characterised in that said refrigerating circuit (10) includes a first compressor stage (12) assigned to a medium temperature cooling loop (60), and a second compressor stage (80) assigned to a low temperature cooling loop (62), and said pressure line (24a, 24b) connects the pressure side of said second compressor stage (80) via said first compressor stage (12) to said inlet side of said heat dissipating heat exchanging unit (14), and
wherein said pressure line has a first pressure line portion (24a) connecting said first compressor stage (12) and said heat dissipating heat exchanging unit (14), and a second pressure line portion (24b) connecting said second compressor stage (80) and said first compressor stage (12), said liquid refrigerant return line (26) opening into said second pressure line portion (24b).
Refrigerating circuit (10) according to claim 1,
wherein said pressure line (24) includes pressure reduction means (28) for locally reducing pressure of said refrigerant in said pressure line (24) from an operational pressure (p_c, p_c') to a reduced pressure (p₁) smaller than said operational pressure (p_c, p_c'), and said liquid refrigerant return line (26) opens into said pressure line (24) at a position in which said refrigerant has said reduced pressure (p₁).
Refrigerating circuit (10) according to claim 2,
said pressure reduction means (28) includes a venturi structure (30), said venturi structure (30) being formed such as to locally reduce the cross section of said pressure line (24).
Refrigerating circuit (10) according to claim 3,
wherein said venturi structure (30) has an inlet side (32) and an outlet side (34) defining the cross section of said pressure line (24) upstream and downstream of said venturi structure (30), respectively, said inlet side (32) and said outlet side (34) defining essentially the same cross section.
Refrigerating circuit (10) according to claim 3 or 4,
wherein said venturi structure (30) has an intermediate portion (36) located between an inlet side (32) and an outlet side (34) thereof, said intermediate portion including a first tapered portion (38) extending from said inlet side (32), and a second tapered portion (40) extending from said outlet side (34), axial inner sides of said first and second tapered portions (38, 40) defining said reduced cross section of said pressure line (24), said first and said second tapered portions (38, 40) having an essentially identical extension in axial direction.
Refrigerating circuit (10) according to claim 5,
wherein said axial inner sides of said first and second tapered portions (38, 40) are connected by a non-tapered central portion (42), said liquid refrigerant return line (26) opening into said central portion (42).
Refrigerating circuit (10) according to claim 6,
wherein said central portion (42) essentially has the same extension in axial direction as said first and/or second tapered portions (38, 40).
Refrigerating circuit (10) according to any of claims 1 to 7,
wherein said refrigerating circuit (10) includes a single compressor stage (12).
Refrigerating circuit (10) according to any of claims 1 to 8,
wherein said liquid refrigerant return line (26) includes a valve (44), said valve being controlled according to a superheating state of said refrigerant in said pressure line (24; 24a, 24b).
Refrigerating circuit (10) according to any of claims 1 to 9,
wherein said valve is controlled by a thermostat unit (46).
Refrigerating circuit (10) according to any of claims 1 to 10,
wherein said heat dissipating heat exchanging unit (14) includes a receiver (18) connected in series to said condenser/gas cooler unit (16), said receiver (18) having an upstream side connected to an outlet side of said condenser/gas cooler unit (16), said liquid refrigerant return line (26) opening at a downstream side of said receiver (18).