DK2663817T3

DK2663817T3 - COOLING SYSTEM AND PROCEDURE FOR OPERATING A COOLING SYSTEM.

Info

Publication number: DK2663817T3
Application number: DK11701483.7T
Authority: DK
Inventors: Hans-Joachim Huff; Tobias Sienel
Original assignee: Carrier Corp
Priority date: 2011-01-14
Filing date: 2011-01-14
Publication date: 2019-01-28
Also published as: EP2663817A1; EP2663817B1; CN103282729B; CN103282729A; WO2012095186A1; US20130283833A1

Description

DESCRIPTION

[0001] The invention relates to a refrigeration system and a method of operating a refrigeration system.

[0002] Conventional refrigeration systems, for instance in supermarkets, typically include both a medium temperature refrigeration sales furniture, wherein for example food items such as fruits, vegetables, and beverages are stored and cooled, and a low temperature/freezing sales furniture where food items are stored and kept in a frozen condition.

[0003] From the WO 2006015741 a refrigeration circuit is known employing a medium temperature compressor set and a low temperature compressor set. The refrigeration circuit further comprises in the direction of flow, a condenser, a collector, a pressure-relief device, arranged before an evaporator, an evaporator and a single-stage compressor unit. An intermediate pressure-relief device is arranged between the condenser/gas cooler and the collector. Furthermore, a method for operating such refrigeration circuit is disclosed.

[0004] It would be beneficial to provide an alternative, efficient refrigeration system providing both low temperature and medium temperature refrigeration.

[0005] WO 2007/053149 discloses a combined medium and low temperature refrigeration circuit for circulating a refrigerant in a predetermined flow direction, comprising in flow direction (a) a heat-rejecting heat exchanger; (b) a medium temperature refrigeration consumer; (c) a medium pressure vapor separator having a vapor portion and a liquid portion connected to the medium temperature refrigeration consumer; (d) a low temperature refrigeration consumer connected to the liquid portion of the medium pressure vapor separator; and (e) a compressor unit having an inlet connected to the vapor portion of the medium pressure vapor separator and the low temperature refrigeration consumer. US3766745 discloses another combined medium and low temperature refrigeration circuit.

[0006] Exemplary embodiments of the invention include a refrigeration system as according to claim 1.

[0007] Moreover, exemplary embodiments of the invention include a method for operating a refrigeration system as according to claim 15.

[0008] Exemplary embodiments of the invention will be described in greater detail below taking reference to the accompanying drawings.

Fig. 1 shows a schematic view of a refrigeration system according to exemplary embodiment of the invention.

Fig. 2 shows an embodiment of an oil management system in the refrigeration system of Fig. 1.

Fig. 3 shows a further embodiment of an oil management system in the refrigeration system of Fig. 1.

Fig. 4 shows a further embodiment of an oil management system in the refrigeration system of Fig. 1.

Fig. 5 shows a further embodiment of an oil management system in the refrigeration system of Fig. 1.

Fig. 6 shows a further embodiment of an oil management system in the refrigeration system of Fig. 1.

Fig. 7 shows a further embodiment of an oil management system in the refrigeration system of Fig. 1.

Fig. 8 shows a further embodiment of an oil management system in the refrigeration system of Fig. 1.

[0009] The refrigeration system 2 shown in Fig. 1 comprises, in flow direction of a refrigerant circulating therein, a gas cooler/condenser 4, an intermediate pressure expansion device 6, a refrigerant collecting container 8 as well as a normal refrigeration branch 10 and a freezing branch 18 arranged in parallel.

[0010] In the condenser/gas cooler 4, the refrigerant is cooled down against a secondary medium. In the exemplary embodiment of Fig. 1, the secondary medium is air as for example ambient air of the outside of a supermarket. Other secondary media, such as water, sole or air with water particles, may also be used. In the case of CO2 being the refrigerant and the refrigeration system being operated in the transcritical mode, the condenser/gas cooler 4 is referred to as gas cooler, as the refrigerant leaves the condenser/gas cooler 4 in gaseous phase. For other refrigerants and/or a sub-critical operation, a condensation takes place in the condenser/gas cooler 4, such that this refrigerant circuit element is referred to as condenser. In this case, the relatively warm refrigerant enters the gas cooler/condenser 4 in gaseous phase, is cooled down below the dew-point of the refrigerant and leaves the gas cooler/condenser 4 in liquid phase.

[0011] The intermediate pressure expansion device 6 located between the gas cooler/condenser 4 and the refrigerant collecting container 8 expands the high pressure refrigerant, that has been cooled by the condenser/gas cooler 4 to an intermediate pressure.

[0012] The refrigerant as it leaves the condenser/gas cooler 4 has a typical pressure level from 10 to 120 bar (10 * 105 to 120 * 105Pa), particularly 60 to 100bar (60 * 105 to 100 * 105 Pa) and more particularly 80bar (80 * 105 Pa) and is then expanded to an intermediate pressure of 5 to 40 bar (5 * 105 to 40 * 105Pa), particularly 25 to 35bar (25 * 105 to 35 * 105Pa) and more particularly 30bar (30 * 105 Pa). By using an intermediate pressure expansion device 6, the lines and the equipment downstream the intermediate pressure expansion device 6 can be designed for a lower pressure to reduce costs and increase efficiency of the refrigeration system 2. The pressure of the refrigerant after it has been expanded in the intermediate expansion device 6 is still below the evaporation-pressure of the refrigerant, so that it remains in the liquid phase.

[0013] As can be seen in Fig. 1, the refrigerant line coming from the intermediate pressure expansion device 6 connects to an upper portion of the refrigerant collecting container 8, that has a lower liquid space portion, in which liquid refrigerant collects and an upper gaseous refrigerant portion, in which gaseous refrigerant collects.

[0014] The refrigeration system 2 further comprises a normal refrigeration branch 10 and a freezing branch 18 arranged in parallel.

[0015] The refrigerant line coming from the refrigerant collecting container 8 splits up into the normal refrigeration branch 10 and the freezing branch 18 both connecting the refrigerant collecting container 8 to the condenser/gas cooler 4.

[0016] The normal refrigeration branch 10 comprises in flow direction of the refrigerant, a first expansion device 12, a first evaporator 14 and a compressor unit 16. In the present nonlimiting embodiment the compressor unit 16 consists of three compressors arranged in parallel.

[0017] In the first expansion device 12 of the normal refrigeration branch 10 the refrigerant is further expanded to a pressure between 30 and 35 bar (30 * 105 to 35 * 105Pa), particularly 32 bar (32 * 105Pa), and thereby cooled down to a desired refrigeration temperature between -1 and 5 °C (272,15 and 278,15K).

[0018] Downstream of the first expansion device 12 of the normal refrigeration branch 10, there is provided a first evaporator 14 in which the refrigerant is heated against the environment, which thereby is cooled.

[0019] The evaporators 14 and 22 as described in the invention can be referred to as medium and low temperature cold consumers that can be formed as refrigerating furnitures, like refrigerating cabinets, refrigerating chests or refrigerating islands, or as refrigerating rooms, that provide goods arranged therein with the desired refrigeration. The first evaporator 14 of the normal refrigeration branch 10 can for example be a normal temperature food refrigerator used in a supermarket to cool down foods to a temperature from 0 to 5 °C (273,15 to 278,15K).

[0020] The heated refrigerant exits the first evaporator 14 in a gaseous phase and flows to the suction side of the first compressor unit 16 of the normal refrigeration branch 10, in which the gaseous refrigerant is compressed to high pressure and led back to the gas cooler/condenser 4.

[0021] The freezing branch 18 connects the refrigerant collecting container 8 to the condenser/gas cooler 4 and comprises in flow direction of the refrigerant, a second expansion device 20, a second evaporator 22, a first compressor unit 24 and a second compressor unit 26. The first and the second compressor units 24 and 26 of the freezing branch 18 are connected in series.

[0022] Similar to the normal refrigeration branch 10, the liquid refrigerant in the freezing branch 18 is expanded in the second expansion device 20 and evaporated in the second evaporator 22 arranged downstream of the second expansion device 20. Compared to the normal refrigeration branch 10, the refrigerant is expanded to a lower pressure in the second expansion device 20 in a range of 10 to 20 bar (10 * 105 to 20 * 105Pa), particularly 15 bar (15 * 105 Pa), to achieve a lower temperature in the second evaporator 22 of approximately -20 to -40°C (253,15 to233,15K), particularly -30 °C (243,15K).

[0023] The second evaporator 22 of the freezing branch 18 can for example be a freezer to freeze foods to a temperature up to -40°C (233,15K).

[0024] Leaving the second evaporator 22 in a gaseous phase the refrigerant flows to the suction side of the first compressor unit 24 of the freezing branch 18 consisting of three compressors arranged in parallel, in the present non-limiting embodiment of the invention.

[0025] In the first compressor unit 24 of the freezing branch 18, the gaseous refrigerant is compressed to a medium pressure level below or equal to the pressure of the line entering the gas cooler/condenser 4.

[0026] Downstream of the first compressor unit 24 of the freezing branch 18, there is provided a second compressor unit 26 consisting in the present non-limiting embodiment of two compressors arranged in parallel. The second compressor unit 26 of the freezing branch 18 is referred to as a booster compressor that is frequency controlled to match the mass flow in line 34 connecting the first and second compressor units 24 and 26. In the second compressor unit 26 the refrigerant is compressed to a high pressure level complying to the pressure at the exit of the compressor unit 16 of the normal refrigeration branch 10. Atypical pressure level at the high pressure side of the system between the second compressor unit 26 of the freezing branch 18, the compressor unit 16 of the normal refrigeration branch 10 and the intermediate expansion device 6 is 20 to 120 bar (20 * 105 to 120 * 105Pa), particularly 50 to 100bar (50 * 105 to 100 * 105Pa) and more particularly 80 bar (80 * 105 Pa).

[0027] Leaving the second compressor unit 26 of the freezing branch 18, the gaseous refrigerant enters the gas cooler/condenser 4 together with the gaseous refrigerant coming from the compressor unit 16 of the normal refrigeration branch 10.

[0028] The compression of the gaseous refrigerant of the freezing branch 18 is accomplished in two stages consisting of medium stage compression in the first compressor unit 24 and a high compression in the second compressor unit 26.

[0029] According to a discovery of the inventor, the two-stage compression of the refrigerant of the freezing branch 18 leads to a lower number of compressors needed to compress the refrigerant from a low pressure level at the outlet oft the second evaporator 22 to a high pressure level as needed at the inlet of the gas cooler/condenser 4. The number of compressors that can be reduced by the refrigeration system of the present invention comprises the compressors building the first and second compressor units 24 and 26 of the freezing branch 18. The number of compressors depicted in the Fig. is only of exemplary kind.

[0030] Attaching to the upper gas space portion of the refrigerant collecting container 8 and leading to the line 34 connecting the first and second compressor units 24 and 26 of the freezing branch 18, there is provided a flash gas line 28 by means of which flash gas from the refrigerant collecting container 8, in particular from its upper gas space portion can be sucked off by the compressor unit 26.

[0031] The lower liquid space of the refrigerant collecting container is connected to the normal refrigeration branch 10 and to the freezing branch 18 to ensure that only liquid refrigerant is supplied to the expansion devices 12 and 20 and to the evaporators 14 and 22.

[0032] By connecting the refrigeration collecting container 8 to the suction side of the second compressor unit 26 of the freezing branch 18, the pressure of the refrigerant collecting container 8 is controlled by the pressure of the second compressor unit 26. Hence, the exemplary refrigeration system does not require an economizer stage comprising additional regulating valves to control the medium and high discharge pressure, as used in conventional refrigeration systems. That results in lower vibration of the present refrigeration system due to less valves installed in the system that have to be opened and closed.

[0033] Moreover, the pressure of the refrigerant collecting container 8 can be controlled more precisely which increases the overall system efficiency. A more constant pressure of the refrigerant collecting container 8 also reduces instabilities at the evaporators 14 and 22 and expansion valves levels 12 and 20 allowing for a more stable overall system characteristic.

[0034] Fig. 2 shows an embodiment of an oil management system 31 in a refrigeration system 2 as described in Fig. 1. A simplified detail of the refrigeration system 2 of Fig. 1 is shown in Fig. 2, wherein only the compressor units consisting of the compressor unit 16 of the normal refrigeration branch 10 and the first and second compressor units 24 and 26 of the freezing branch 18 and the lines around those elements can be seen.

[0035] The first and second compressor units 24 and 26 of the freezing branch 18 are connected by the line 34 which is also connected to the gas space of the refrigerant collecting container 8 via the flash gas line 28. The flash gas line 28 connecting the gas space of the refrigerant collecting container 8 to the line connecting the first compressor unit 24 to the second compressor unit 26 of the freezing branch 18 only contains gaseous refrigerant that does not contain any oil.

[0036] In Fig. 2, there is provided an oil balance line 30 equipped with a valve 32, for example a solenoid valve 32 connecting the oil sumps of the compressor unit 10 of the normal refrigeration branch 10 to oil sumps of the first compressor unit 24 of the freezing branch 18. By means of this oil balance line 30, the oil levels within the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 and the oil sumps of the first compressor unit 24 of the freezing branch 18 can be balanced.

[0037] The valve 32 can for example be pressure controlled so as to open and close the oil balance line 30 at a certain pressure difference between the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 and the oil sumps of the first compressor unit 24 of the freezing branch 18. If the solenoid valve is switched open, the oil from the oil sumps of the compressor unit 16 is fed to the oil sumps of the first compressor unit 24 by the pressure difference, and by the increased oil rate within the refrigerant in the first compressor unit 24 and the refrigerant exiting the same, the oil level in the second compressor unit 26 will be raised as well.

[0038] In operation of the exemplary refrigeration system 2 oil generally collects at the compressor unit 16 of the normal refrigeration branch 10 whereas the compressor units 24 and 26 of the freezing branch 18 are not sufficiently supplied with oil. Especially, the oil rate of the refrigerant at the inlet of the second compressor unit 26 of the freezing branch 18 is relatively low due the fact that flash gas, not containing any oil, from the gas space of the refrigerant collection container 8 is led to the inlet of the second compressor unit 26.

[0039] The valve 32 can further be controlled by a control unit (not shown) monitoring the oil levels in the oils sumps of the first and second compressor units 24 and 26 of the freezing branch 18 and the compressor unit 16 of the normal refrigeration branch. The valve 32 can be switch open by the control unit if the oil level of one of the oil sumps of the first or second compressor units 24 and 26 falls below a predetermined value or if the oil level of the oil sumps of the compressor unit 16 of the normal refrigeration branch exceeds a predetermined value. Alternatively or additionally, the valve 32 can be switched open at predetermined time intervals independently from the oil levels in the oil sumps of the compressors 16, 24 and 26. The time interval can be different for every solenoid valve and can depend on the change of the oil levels over the time in each sump.

[0040] By an oil balance line 30 of the present embodiment, low oil levels of the oil sumps of the first and second compressor units 24, 26 of the freezing branch 18 and too high oil levels of the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 are reliably avoided and therefore defects of those compressors being caused by insufficient lubrication or by oil hits can be considerably reduced. The valve 32 can be controlled to fed the oil either continuously or intermittently to the respective oil sumps and allows to adjust the oil flow rate depending on the oil demand of the compressors.

[0041] Fig. 3 shows an embodiment of an oil management system 33 in a refrigeration system 2 as described in Fig. 1. A simplified detail of the refrigeration system 2 of Fig. 1 is shown in Fig. 3, wherein only the compressor units consisting of the compressor unit 16 of the normal refrigeration branch 10 and the first and second compressor units 24 and 26 of the freezing branch 18 and the lines around those elements can be seen.

[0042] An oil balance line 35 connects the oil sumps of the compressing unit 16 of the normal refrigeration branch 10 to the suction side of the first compressor unit 24, particularly to the suction line of the first compressor unit 24 at a point before it divides up into the separate lines leading to each of the compressors of the compressor unit 24. The oil balance line 35 is provided with a valve 32, for example a solenoid valve 32.

[0043] By means of this oil balance line 35, the oil levels within the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 and the oil rate of the first compressor unit 24 of the freezing branch 18 can be balanced.

[0044] The valve 32 can for example be pressure controlled so as to open and close the oil balance line 30 at a certain pressure difference between the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 and the suction side of the first compressor unit 24 of the freezing branch 18. If the valve 32 is switched open, the oil from the oil sumps of the compressor unit 16 is fed to the suction side of the first compressor unit 24 by the pressure difference, and by the increased oil rate within the refrigerant in the first compressor unit 24 and within the refrigerant exiting the same, the oil level in the second compressor unit 26 will be raised as well.

[0045] In operation of the exemplary refrigeration system 2 oil generally collects at the compressor unit 16 of the normal refrigeration branch 10 whereas the compressor units 24 and 26 of the freezing branch 18 are not sufficiently supplied with oil. Especially, the oil rate of the refrigerant at the inlet of the second compressor unit 26 of the freezing branch 18 is relatively low due the fact that flash gas, not containing any oil, from the gas space of the refrigerant collection container 8 is led to the inlet of the second compressor unit 26.

[0046] The valve 32 can further be controlled by a control unit (not shown) monitoring the oil rates of the first and second compressor units 24 and 26 and, respectively, the oil levels in the oils sumps of the first and second compressor units 24 and 26 of the freezing branch 18 and the compressor unit 16 of the normal refrigeration branch. The valve 32 can be switched open by the control unit if the oil level of one of the oil sumps of the first or second compressor units 24 and 26 falls below a predetermined value or if the oil level of the oil sumps of the compressor unit 16 of the normal refrigeration branch exceeds a predetermined value. Alternatively or additionally, the valve 32 can be switched open at predetermined time intervals independently from the oil levels in the oil sumps of the compressors 16, 24 and 26. The time interval can be different for every valve and can depend on the change of the oil levels over the time in each sump.

[0047] By an oil balance line 35 of the present embodiment, low oil levels of the oil sumps of the first and second compressor units 24, 26 of the freezing branch 18 and too high oil levels of the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 are reliably avoided and therefore defects of those compressors being caused by insufficient lubrication or by oil hits can be considerably reduced. The valve 32 can be controlled to feed the oil either continuously or intermittently and allows to adjust the oil flow rate depending on the oil demand of the compressors.

[0048] Fig. 4 shows an embodiment of an oil management system 36 in a refrigeration system as described in Fig. 1, comprising the elements already shown in Fig 2, as the first and second compressor units 24, 26 of the freezing branch 18 and the compressor unit 16 of the normal refrigeration branch 10 and the conduits 34 and 28.

[0049] As can be seen in Fig. 3 an oil balance line 38 connects the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 to the outlet of the first compressor unit 24 of the freezing branch 18. The oil balance line 38 further comprises a valve 40 by means of which the oil balance line 38 can be switched open or closed. To balance the oil flow between the compressor unit 16 of the normal refrigeration branch 10 and the outlet of the compressor unit 24 of the freezing branch 18, the valve 40 is for example controlled by the oil level in the oil sumps of the second compressor unit 26 of the freezing branch 18. If the oil level in the oil sumps of the second compressor unit 26 falls below a certain value, the valve 40 is switched open to allow oil from the oil sumps of the compressor unit 16 to flow to the outlet of the second compressor unit 26.

[0050] In a further embodiment, the valve 40 is controlled by the oil level in the first compressor units 24 of the freezing branch 18. To avoid a too low oil level in the oil sumps of the first compressor unit 24, the valve 40 is opened if the oil level of the oil sumps of the first compressor unit 24 falls below a certain value to feed oil to the outlet of the compressor unit 24 which is connected to the suction side of the compressor unit 26 by the line 34.

[0051] Another possibility of balancing the oil comprises detecting the oil rate in the line 34 connecting the first and second compressor units 24 and 26 of the freezing branch 18. By monitoring the oil rate in the line 34 and opening the valve if the oil rate falls below a certain value, sufficient oil supply to the compressor unit 26 is ensured.

[0052] An oil balance line 38 as described in this embodiment is especially beneficial if the compressor unit 24 is always sufficiently supplied with oil. Therefore, oil from the compressor unit 16 is supplied to the compressor unit 26 only due to the fact that the flash gas is fed to the inlet thereof.

[0053] A further embodiment of an oil management system 42 in a refrigeration system 2 is shown in Fig. 5 which also depicts the compressor units 26, 24 and 16, wherein the first and second compressor units 24 and 26 of the freezing branch 18 are connected by the line 34 to which the flash gas line 28 is connected.

[0054] In the embodiment shown in Fig. 5, there is provided an oil balance line 44 comprising a valve 46 and connecting the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 to the oil sumps of the first compressor unit 24 of the freezing branch 18. An additional oil balance line 48 comprising an additional valve 50 branches off the oil balance line 44 and connects the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 to the oil sumps of the second compressor unit 26 of the freezing branch 18.

[0055] For the oil balancing and re-feeding operation the valves 46 and 50 can both be switched open to allow an oil flow from the oil sumps of the compressor unit 16 to the oil sumps of the first and second compressor units 24 and 26 of the freezing branch 18.

[0056] The valves 46 and 50 can for example be controlled by the pressure of the oil sumps of the first and second compressor units 24 and 26 so as to allow for an oil flow to said oil sumps in case the pressure of the oil sumps falls below a certain value.

[0057] Further, the valves 46 and 50 can be controlled by the pressure difference between the oil sumps of the compressor units. If for example the pressure difference between the oil sumps of the compressor unit 16 and the oil sumps of the compressor unit 24 exceed a certain value, the valve 46 is switched open. That also applies to the valve 50 which is switched open if the pressure difference between the oil sumps of the compressor unit 16 and the oil sumps of the compressor unit 26 exceeds a certain value.

[0058] A benefit of this embodiment is that the valves 46 and 50 and thereby the oil flow in the oil balance lines 44 and 48 can be controlled independently so as to ensure an oil flow from the oil sumps of the compressor unit 16 to the oil sumps of the compressor units 24 and 26 depending on their individual oil demand.

[0059] Fig. 6 shows a further embodiment of an oil management system 52 comprising every element already shown in Fig. 5, except for the oil balance line 44 and the corresponding valve 46. In contrast to Fig. 5, an oil balance line 54 comprising a valve 56 is connected to the outlet of the first compressor unit 24 of the freezing branch 18.

[0060] Thereby, the oil is fed directly into the gaseous refrigerant in line 34 connecting the first compressor unit 24 to the second compressor unit 26 of the freezing branch 18.

[0061] A further embodiment of an oil management system 58 in a refrigeration system 2 is shown in Fig. 7 which also depicts the compressor units 26, 24 and 16, wherein the first and second compressor units 24 and 26 of the freezing branch 18 are connected by the line 34 to which the flash gas line 28 is connected. The embodiment in Fig. 6 comprises an oil balance line 60 connecting the oil sumps of the compressor unit 16 of the normal refrigeration branch 10 to the oil sumps of the compressor unit 24. Furthermore, an oil balance line 64 is provided in the embodiment of Fig. 5 connecting the oil sumps of the compressor unit 16 to the outlet of the first compressor unit 24. Each of the oil balance lines 60 and 64 comprises a valve 62 and 66 respectively. Controlling the valves 62 and 66 can be performed depending on the parameters already mentioned in the description of the preceding figures as for example the pressure difference between the oil sumps of the compressor unit 16 and the oil sumps of the compressor unit 24 and/or compressor unit 26, the pressure in the oil sumps of the compressor units 24 and 26 and the oil rate in the refrigerant in line 34.

[0062] Fig. 8 shows a further embodiment of an oil management system 68 containing every element described in Fig. 6 and an additional oil balance branch 70 with an additional valve 72 arranged therein. The oil balance branch 70 connects the oil sumps of the second compressor unit 26 of the freezing branch 18 to the oil balance branches 60 and 64 leading to the oil sumps and the outlet of the first compressor unit 24 of the freezing branch 18 and to the oil sumps of the compressor unit 16 of the normal refrigeration branch 10.

[0063] In the embodiment of Fig. 8, three locations at which a lack of oil can occur are connected to the oil sump of the compressor unit 16 of the normal refrigeration branch. By measuring the oil content in the refrigerant in line 34 and the oil levels of the oil sumps of the first and second compressor units 24 and 26 of the freezing branch 18 and controlling the valves 62, 66 and 72 accordingly, oil can be directed to each of the three locations depending on their individual demand.

[0064] While the refrigerating circuit and the corresponding method according to exemplary embodiments, as described above, is generally suitable for a wide variety of refrigerants, carbondioxide (CO2) is particularly well suited.

[0065] According to an exemplary embodiment, as described above with respect to Fig. 1, the two-stage compression of the refrigerant of the freezing branch leads to a lower number of compressors needed to compress the refrigerant from a low pressure level at the outlet oft the second evaporator to a high pressure level as needed at the inlet of the gas cooler/condenser. Moreover, the exemplary refrigeration system does not require an economizer stage comprising additional regulating valves to control the medium and high discharge pressure, as used in conventional refrigeration systems. That results in lower vibration of the present refrigeration system due to less valves installed in the system that have to be opened and closed.

[0066] Further, a more constant pressure of the refrigerant collecting container is achieved, which also reduces instabilities at the evaporators and expansion valves levels and allows for a more stable overall system characteristic.

[0067] By exemplary embodiments of the invention, as described with respect to Figs. 2 to 8, the oil levels of the medium temperature compressors of compressor unit and the low temperature compressors of compressor units are balanced automatically. The excess amount of oil that regularly collects at the oil sumps of the compressor unit of the normal refrigeration branch is automatically fed to the low temperature compressor side, in particular to the oil sumps of the second compressor unit and if applicable also to the first compressor unit of the freezing branch. Thereby, too low oil levels at the freezing temperature compressor side and too high oil levels at the normal refrigeration temperature compressor side are reliably avoided and therefore defects of those compressors being caused by insufficient lubrication can be considerably reduced.

[0068] According to an embodiment, the refrigeration system further comprises an oil balance line connecting at least one oil sump of the compressor unit of the normal refrigeration branch to at least one of the compressors of the first and second compressor units of the freezing branch.

[0069] Thereby, it is ensured that oil can flow from the compressor unit of the normal refrigeration branch to at least one of the compressors of the first and second compressor units of the freezing branch. The inventor has discovered that normally the first and second compressors of the freezing branch tend to have a too low oil level in their oil sumps and that especially the second compressor of the freezing branch is not sufficiently supplied with oil due to the fact that the flash gas stream coming from the refrigerant collecting container does not contain any oil so that the combined refrigerant flow at the suction side of the compressor unit does not contain sufficient oil for lubrication of the compressor unit.

[0070] According to an embodiment, the oil balance line connects at least one oil sump of the compressor unit of the normal refrigeration branch to at least one oil sump of the first compressor unit of the freezing branch.

[0071] Thus, an oil flow to the sumps of the first compressor unit of the freezing branch is allowed which leads to sufficient oil supply thereof, as well as a higher oil rate in the discharge gas which, combined with the flash gas, is led to the second compressor unit of the freezing branch. Hence, both the first and second compressor units are supplied with oil.

[0072] According to an embodiment, the oil balance line connects at least one oil sump of the compressor unit of the normal refrigeration branch to the suction side of the first compressor unit of the freezing branch.

[0073] Thus, an oil flow to the suction side of the first compressor unit of the freezing branch is allowed which leads to sufficient oil supply thereof, as well as a higher oil rate in the discharge gas which, combined with the flash gas, is led to the second compressor unit of the freezing branch. Hence, both the first and second compressor units are supplied with oil.

[0074] According to an embodiment, the oil balance line can be switched open or closed by means of at least one valve, for example a solenoid valve arranged therein.

[0075] Since the oil level of the oil sumps of the compressor units in the system depend on varying system parameters, as for example ambient temperature and refrigerating capacity, oil does not need to be fed continuously to the first and second compressor units of the freezing branch. A valve arranged in the oil balance line allows to adjust the oil flow rate to the demand for oil of the compressor units.

[0076] According to an embodiment, the refrigeration system further comprises a control unit configured to switch the valve in the oil balance line open if the oil level in the second compressor unit of the freezing branch falls below a predetermined threshold value.

[0077] Monitoring the oil level in the second compressor unit, where a lack of oil most likely occurs, ensures sufficient oil supply since the oil flow rate can be adjusted to the oil level in the oil sumps.

[0078] According to an embodiment, the refrigeration system, further comprises a control unit configured to switch a valve in an oil balance line open if the oil level in the first compressor unit of the freezing branch falls below a predetermined threshold value. Thereby, a too low oil level in the first compressor unit of the freezing branch is avoided and the oil flow rate can be adjusted to the oil demand of thereof.

[0079] According to an embodiment, the refrigeration system further comprises a control unit configured to switch a valve in an oil balance line open if the oil level in the compressor unit of the normal refrigeration branch exceeds a predetermined threshold value.

[0080] In operation, oil generally collects in the oil sumps of the compressor unit of the normal refrigeration branch. To avoid a too high oil level in the oil sumps of the compressor unit, a monitoring device is arranged in the oil sumps to measure the oil level thereof and transmit a signal to the control unit if the oil level exceeds a predetermined level. Thereby, the oil sumps of the compressor unit can be designed for smaller volumes and additional elements as for instance overflow valves and refrigerant collectors are not necessary.

[0081] According to an embodiment, the refrigeration system comprises a control unit configured to switch a valve in an oil balance line open if the oil level in the compressor unit of the normal refrigeration branch exceeds a predetermined threshold value and/ or if the oil level in the second compressor unit of the freezing branch falls below a predetermined level. Hence, the valve is controlled by two parameters and the oil flow rate can be adjusted depending on the oil levels in both the compressor units.

[0082] According to an embodiment, the refrigeration system comprises a control unit configured to switch a valve in an oil balance line open if the oil level in the compressor unit of the normal refrigeration branch exceeds a predetermined threshold value and/ or if the oil level in the second compressor unit of the freezing branch falls below a predetermined level and I or if the oil level in the first compressor unit of the freezing branch falls below a predetermined threshold value.

[0083] In this embodiment the valves in the oil balance lines connecting the compressor units of the refrigeration system can be controlled by a control unit to which parameter values concerning the oil level or oil pressure in the oil sumps or the oil content in the refrigerant are transmitted. The control unit can switch the valves open or close to allow an oil flow through the respective oil balance line leading to the oil sump where oil is needed.

[0084] According to an embodiment, a valve is configured to be opened after a predetermined time interval. Thus, there are no monitoring devices and control units needed in the refrigeration system to adjust the oil flow to the demand of the respective compressor units.

[0085] This embodiment offers a more simple and cost efficient way to feed oil intermittently to the oil sumps of the compressor units. The time interval, at which the valves are opened or closed, has to comply to the change of the oil levels in the oil sumps over the time for a certain operation condition of the refrigeration system. The time intervals can be different for every valve to control the oil flow in the respective oil balance line.

[0086] According to an embodiment, an oil balance line connects the at least one oil sump of the compressor unit of the normal refrigeration branch to an outlet of the first compressor unit of the freezing branch, said outlet being part of the first compressor unit.

[0087] This results in a higher oil content in the line connecting the first and the second compressor unit of the freezing branch. Thus, sufficient oil supply to the second compressor unit of the freezing branch is ensured.

[0088] According to an embodiment, the oil balance line further connects at least one oil sump of the second compressor unit of the freezing branch to the at least one oil sump of the compressor unit of the normal refrigeration branch and the at least one of the compressors of the first compressor unit of the freezing branch.

[0089] In this embodiment all of the compressor units of the refrigeration system are connected to allow an oil exchange between the compressor units in case of too high or too low oil levels in any of the compressor units.

[0090] According to an embodiment, the oil balance line connects the at least one oil sump of the compressor unit of the normal refrigeration branch and the at least one oil sump of the second compressor unit of the freezing branch to at least one oil sump of the first compressor unit of the freezing branch.

[0091] Thereby, the oil sumps of all the compressor units in the refrigeration system are connected to each other and oil may be fed to any of the oil sumps of the compressor units depending on their demand.

[0092] According to an embodiment, the oil balance line connects the at least one oil sump of the compressor unit of the normal refrigeration branch and the at least one oil sump of the second compressor unit of the freezing branch to an outlet of the first compressor unit of the freezing branch.

[0093] In case of an oil-shortage in the second compressor unit of the freezing branch oil can be supplied directly to the oil sumps of the compressor unit and to the outlet of the first compressor unit of the freezing branch so that the oil in the refrigerant flowing from the first to the second compressor unit is enriched with oil. That results in a higher oil content of the combined refrigerant flow from the refrigerant collecting container and from the first compressor unit at the inlet of the second compressor unit. This embodiment ensures an adequate oil level in the oil sumps and a sufficient oil content of the refrigerant at the inlet of the second compressor unit simultaneously.

[0094] According to an embodiment, the second compressor unit of the freezing branch is frequency controlled to match the mass flow from the first compressor unit of the freezing branch and from the flash gas line.

[0095] This arrangement offers a more stable and energy efficient refrigeration system compared to the standard system. Since the pressure of the refrigerant collecting container is controlled by the frequency and hence by the capacity of the second compressor unit of the freezing branch, this pressure can be controlled more precisely. As a result, the overall system efficiency is increased and a more constant pressure in the refrigerant collecting container is achieved which reduces instabilities at the evaporators and the expansion valves connected to the refrigerant collecting container, allowing for a more stable overall system characteristic.

[0096] According to an embodiment, the refrigerant is CO2 or a CO2 blend.

[0097] Compared to conventional refrigerants CO2 has important environmental friendly characteristics as for example being non-flammable and non-ozone depleting. Its physical properties are highly favourable for cooling, refrigeration and heating purposes, having a high volumetric cooling capacity. Due to its operation at pressures of up to 130 bar (130-105 Pa), systems require highly resistant components that have been already developed to serial production in many sectors.

[0098] According to an embodiment, the first evaporator and the second evaporator are cooling devices of a supermarket.

[0099] According to an embodiment, the normal refrigeration branch is parallel to the freezing branch and the compressor unit of the normal refrigeration branch is parallel to the first and second compressor units of the freezing branch.

[0100] Thereby, a low temperature freezing branch and a normal refrigeration temperature branch are realized wherein the discharge of the compressor unit of the normal refrigeration temperature branch is not led to the inlet of the first and second compressor units of the freezing temperature branch. Moreover, the proposed system does not require an economizer stage as used in standard refrigeration systems which leads to lower vibration due to the fact that less valves are needed to control the discharge of the refrigerant collecting container.

[0101] The automatic oil balancing and oil re-feeding provided by the particular embodiments, as described above, can be realized easily and cost-efficiently for all booster systems no matter what performance. The proposed refrigeration system has been shown to be more energy efficient compared to the standard system especially at ambient conditions above 30°C (300,15K) which is because the amount of flash gas increases at high temperatures and the standard systems. The proposed system can reduce the annual energy consumption by 1 % to 4% below the energy consumption of the standard system.

[0102] Additionally, thermodynamic losses occurring in the standard system by using an economizing stage, can be prevented in the present invention. Furthermore, in the proposed system, the compressor unit of the normal refrigeration branch requires a smaller suction volume than the compressor in the standard system which also leads to a smaller total suction volume of the system. Hence, the number of compressors used in the proposed system can be reduced.

[0103] All the advantages and the embodiments that have been described with respect to the refrigerating circuit also hold true for the corresponding method. These advantages and embodiments are herewith explicitly disclosed also in terms of corresponding method steps, however without repeating them again.

[0104] 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 essential 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.

REFERENCES CITED IN THE DESCRIPTION

This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description • W02006015741A [00031 • W02007053149A [00051 • US3766745A [00051

Claims

COOLING SYSTEM AND PROCEDURE FOR OPERATING A COOLING SYSTEM

A cooling system (2) comprising: a condenser / gas cooler (4), an intermediate expansion device (6) and a refrigerant collection vessel (8); a normal cooling branch (10) connecting the refrigerant collection vessel (8) with the condenser / gas cooler (4), said normal cooling branch (10) comprising a first expansion device (12), a first evaporator (14) at a medium cooling temperature and a compressor unit (16) in the normal cooling branch (10); a freezer branch (18) connecting the refrigerant collection vessel (8) to the condenser / gas cooler (4), said freezer branch (18) comprising a second expansion device (20), a second evaporator (22) at a low cooling temperature and a first compressor unit (24) and a second compressor unit (26) in the freezing branch (18), the first and second compressor units (24, 2.6) in the freezing branch (18) being connected in series, the normal cooling branch (10) being parallel to the freezing branch (18) providing a flash gas line (28) connecting the gas space in the refrigerant collection vessel (8) with the line connecting the first compressor unit (24) to the second compressor unit (26) in the freezer branch (18); and further comprising refrigerant pipelines for connecting the elements and for circulating a refrigerant therethrough.

A cooling system according to claim 1, further comprising an oil equalization line (30) connecting at least one oil sump in the compressor unit (16) of the normal cooling branch (10) to at least one of the compressors in the first and second compressor units (24, 26) in the freezer branch. (18).

A cooling system according to claim 2, wherein the oil equalization line (30) connects at least one oil sump in the compressor unit (16) in the normal cooling branch (10) with at least one oil sump in the first compressor unit (24) in the freezer branch (18).

The cooling system of claim 2, wherein the oil equalization line (35) connects at least one oil sump in the compressor unit (16) of the normal cooling branch (10) with the suction side of the first compressor unit (24) in the freezer branch (18).

A cooling system according to any one of claims 2 to 4, wherein the oil equalization line (30) can be changed to open or closed by at least one valve (32) disposed therein.

A cooling system according to any one of claims 2 to 5, comprising a control unit configured to switch the valve (32) of the oil equalizer line (30) to open if the oil level of the second compressor unit (26) in the freezer branch (18) ) falls below a predetermined threshold and / or if the oil level in the first compressor unit (24) in the freezer branch (18) falls below a predetermined threshold and / or if the oil level in the compressor unit (16) in the normal cooling branch (10) exceeds a predetermined threshold and / or if the oil level in the compressor unit (16) in the normal cooling branch (10) exceeds a predetermined threshold value, and / or if the oil level in the first compressor unit (24) in the freezer branch (18) falls below a predetermined threshold value.

A cooling system according to any one of claims 2 to 6, wherein the valve (32) is configured to be opened after a predetermined time interval.

Cooling system according to any of claims 2 to 7, wherein the oil equalization line (38) connects at least one oil sump in the compressor unit (16) in the normal cooling branch (10) with an outlet in the first compressor unit (24) in the freezer branch (18).

A cooling system according to any one of claims 2 to 8, wherein the oil equalization line (44, 48; 48, 54) connects at least one oil sump in the compressor unit (16) in the normal cooling branch (10) with at least one oil sump in the other compressor unit (26). in the freezer branch (18) and with at least one of the compressors in the first compressor unit (24) in the freezer branch (18).

A cooling system according to any one of claims 2 to 9, wherein the oil equalization line (44) is connected to at least one oil sump in the first compressor unit (24) in the freezer branch (18).

A cooling system according to any one of claims 2 to 10, wherein the oil equalization line (54) is connected to an outlet in the first compressor unit (24) in the freezer branch (18).

A cooling system according to any one of the preceding claims, wherein the second compressor unit (26) in the freezer branch (18) is frequency controlled to match the mass flow from the first compressor unit (24) in the freezer branch (18) and from the flash gas line (28).

A refrigeration system according to any one of the preceding claims, wherein the refrigerant is CO2 and / or where the first evaporator (14) and the second evaporator (22) are located in refrigerated furniture or refrigeration / freezer rooms in a supermarket.

A cooling system according to any one of the preceding claims, wherein the compressor unit (16) in the normal cooling branch (10) is parallel to the first and second compressor units (24, 26) in the freezer branch (18).

A method of operating a cooling system (2), comprising: a condenser / gas cooler (4), an intermediate expansion device (6), and a refrigerant collection vessel (8), the method comprising the steps of: operating a normal cooling branch (10) between the refrigerant collection vessel (8) for the condenser / gas cooler (4), said normal cooling branch (12) comprising a first expansion device (12), a first evaporator (14) at a medium cooling temperature and a compressor unit (16) in the normal cooling branch (10); operating a freezer branch (18) between the refrigerant collection vessel (8) for the condenser / gas cooler (4), said freezer branch (18) comprising a second expansion device (20), a second evaporator (22) at a low cooling temperature, and a first compressor unit (24) and a second compressor unit (26) in the freezer branch (18), the first and second compressor unit (24, 26) in the freezer branch (18) being connected in series, the normal cooling branch (10) being parallel to the freezer branch (18); and supplying flash gas from the gas compartment of the refrigerant collection vessel (8) to the conduit connecting the first compressor unit (2.4) to the second compressor unit (26) in the freezer branch (18) by means of a flash gas line (28).