Method of oil management in a multi-compressor refrigeration system using oil level sensing devices

Abstract

The method for managing oil in a running multi-compressor refrigeration system comprising a controller and a multi-compressor device, comprises monitoring an oil level in the oil sump of each compressor by the respective oil level detection device; detecting that an oil shortage situation occurs in a running compressor, if the oil level in said running compressor reaches a predetermined threshold condition;performing an oil management action if an oil shortage situation is detected for a running compressor, the oil management action being based on a change in the ON / OFF configurations of the compressors in the multi-compressor device and including performing an oil balancing action, if more than half of the total number of compressors are running, or performing an oil return action, if half or less of the total number of compressors are running. Figure 4;

Description

Title of the invention: Method for managing oil in a multi-compressor refrigeration system using oil level sensing devices Field of invention

[0001] The present invention relates to a method of managing oil in a multi-compressor refrigeration system using oil level sensing devices. Prior art

[0002] Document CN212867870U discloses a refrigeration compressor comprising a compressor housing, an oil sump arranged in a lower portion of the compressor housing, and an oil level sensor arrangement positioned in the lower portion of the compressor housing and configured to detect an oil level in the oil sump of the refrigeration compressor.

[0003] Document FR3124554A1 relates to a refrigeration system comprising a refrigeration compressor with an oil level sensor arrangement, and a controller configured to control the operation of the refrigeration compressor based on the oil level detected by the oil level sensor arrangement of the refrigeration compressor. The oil level sensor arrangement is configured to provide a low oil level warning signal to the controller if the oil level in the oil sump reaches a predetermined low oil level. The controller is configured to stop the refrigeration compressor, for example without or with a predetermined time delay, if a low oil level warning signal is provided to the controller.

[0004] Between a low oil level warning and a compressor shutdown, it is recommended to take certain actions to prevent the compressor from shutting down. There are active and passive oil management methods.

[0005] Active oil management methods monitor the refrigeration system in real time and oil management is performed using additional valves and / or oil reservoirs. Operation control is based on the valves. Such solutions are complex and expensive.

[0006] Passive oil management methods perform oil management operations in a periodic or fundamentally predetermined manner, regardless of the actual state of the multi-compressor refrigeration system. However, such solutions significantly influence the efficiency of the multi-compressor refrigeration system, because they perform oil management operations even if they are not necessary.

[0007] Additionally, a detected low oil level could result from an oil return problem (oil is trapped in the refrigeration system) or an oil balancing problem between compressors. Depending on the reason for the low oil level, different actions are recommended.

[0008] Document EP1605212 discloses an air conditioner and a method of performing an oil equalization operation in the air conditioner. Oil equalization may be performed when it is detected that an oil level in the oil reservoir of a particular one of the compressors is below a predetermined level. The compressors are connected by an oil equalization tube to route excess oil in each of the compressors to the remaining compressors. An oil equalization tube opening / closing valve is arranged in the oil equalization tube. A method of performing an oil equalization operation involves opening valves to supply oil from one compressor oil sump to another and pressurizing the lubricating oil by the discharge pressure of one compressor and routing the pressurized lubricating oil to another compressor.

[0009] However, this special design of the oil equalization tubes results in increased costs for the multi-compressor device. Summary

[0010] An object of the present invention is to provide an improved method of oil management in an operating multi-compressor refrigeration system that can overcome the disadvantages encountered in conventional multi-compressor refrigeration systems.

[0011] In particular, an object of the present invention is to provide an oil management method in a refrigeration system with multiple compressors in operation which is simple and easy to implement, which can effectively prevent operation of a compressor in an oil starvation situation, particularly if oil is trapped in the refrigeration system outside the compressor header.

[0012] According to the invention, there is provided a method for managing oil in an operating multi-compressor refrigeration system comprising a controller configured to control operation of the multi-compressor refrigeration system and a multi-compressor device comprising compressors that are coupled in parallel, wherein a total number of compressors in the multi-compressor device is at least two and each compressor is equipped with an oil sump and an oil level sensing device operatively connected to the controller and configured to sense an oil level of oil in the respective oil pan, the oil pans being connected by a common oil balance line, the method comprising:

[0013] - monitoring an oil level in the oil sump of each compressor via the respective oil level detection device,

[0014] - detection, for example by the control device and for example on the base of a signal emitted by an oil level detection device, due to the fact that an oil shortage situation occurs in a running compressor, if the oil level in said running compressor reaches a predetermined threshold condition,

[0015] - carrying out an oil management action if an oil shortage situation is detected for a running compressor, the oil management action being based on a change in the ON / OFF configurations of the compressors in the multi-compressor device and comprising:

[0016] if the total number of compressors in the multi-compressor device is between 2 and 4: • carrying out an oil balancing action, if more than half of the total number of compressors are running, where the oil balancing action includes oil balancing between the compressors, • performing an oil return action, if half or less of the total number of compressors are operating, where the oil return action includes recovering oil from the multi-compressor refrigeration system, i.e. from outside the multi-compressor device, to the multi-compressor device,

[0017] if the total number of compressors in the multi-compressor device is greater than or equal to 5: • carrying out the oil balancing action, if more than half of the total number of compressors are running, • carrying out the oil balancing action and / or the oil return action, if half or less of the total number of compressors are running and at least two compressors are running, • carrying out the oil return action, if only one compressor is running.

[0018] The method according to the present invention combines effective prevention of compressor operation in oil starvation situations, since it monitors compressor operation in real time (claiming the advantages of active oil management) with ease of implementation, since it does not require any additional valves, oil balance lines or oil reservoirs (claiming the advantages of passive oil management). In addition, the method according to the present invention provides improved efficiency to the oil management system. multi-compressor refrigeration, since it only manages oil when necessary.

[0019] The method may also comprise one or more of the following features, taken alone or in combination.

[0020] According to one embodiment of the invention, the method further comprises determining, for example by the control device, the number of compressors running in the multi-compressor device.

[0021] According to one embodiment of the invention, the method further comprises determining, for example by the control device, the total number of compressors in the multi-compressor device.

[0022] Advantageously, the method further comprises determining, for example by the control device, the percentage of compressors running in the multi-compressor device.

[0023] According to one embodiment of the invention, the method further comprises:

[0024] - the emission of a low oil level warning signal by a device oil level detection and to the control device, if the oil level in the oil sump of the respective compressor reaches a predetermined low oil level value.

[0025] According to one embodiment of the invention, the predetermined threshold condition is reached if:

[0026] - the low oil level warning signal emitted by a detection device oil level indicator is emitted continuously for a first predetermined time, and for example for at least ten seconds, or

[0027] - the low oil level warning signal emitted by a detection device oil level switch has switched, i.e. has been activated and deactivated, more than a predetermined number of times, and for example more than ten times, during a second predetermined time, and for example during the last minute.

[0028] By means of the above requirements, erroneous detection of an oil shortage situation in the running compressor can be prevented and the efficiency and reliability of the multi-compressor refrigeration system can be further improved.

[0029] According to one embodiment of the invention, the oil balancing action comprises maintaining the operation of the running compressor for which an oil shortage situation has been detected and stopping the operation of the other running compressor or all other running compressors, i.e. the other running compressor(s) for which an oil shortage situation has not been detected. In particular, if all compressors are running and an oil shortage situation is detected for one of the running compressors, it is assumed that there is has an oil balancing problem (i.e., there is excess oil in at least one of the running compressors for which a low oil situation has not been detected). In order to overcome this and balance the oil level in all compressors, the control device stops the operation of the other running compressor(s). Through this action, the running compressor, for which a low oil situation has been detected, can withdraw more oil from the multi-compressor device and thus increase the oil level in the respective oil sump. In other words, the pressure of the free oil surface of the stopped compressor increases (reduced pressure drop) and its potential excess oil quantity is sent to an adjacent compressor via the common oil balancing line.

[0030] According to one embodiment of the invention, if all the compressors are running, the oil balancing action comprises maintaining the operation of the running compressor for which an oil shortage situation has been detected and stopping the operation of the other running compressor or all the other running compressors, i.e. the other running compressor(s) for which an oil shortage situation has not been detected.

[0031] According to one embodiment of the invention, the oil balancing action comprises stopping the operation of the running compressor for which an oil shortage situation has been detected and starting the operation of a stopped compressor, i.e., a stopped compressor for which an oil shortage situation has not been detected, in order to maintain the workload of the multi-compressor refrigeration system substantially the same. In particular, if the compressors are not all running and an oil shortage situation is detected for one of the running compressors, it is assumed that there is an oil balancing problem (i.e., there is excess oil in the compressors for which an oil shortage situation has not been detected).In order to overcome this and balance the oil level in all compressors, the control device stops the operation of the running compressor for which an oil shortage situation has been detected, keeps the operation of the running compressor(s) for which an oil shortage situation has not been detected and starts the operation of a stopped compressor for which an oil shortage situation has not been detected. With this action, the multi-compressor refrigeration system maintains the required workload, balances the oil between the compressors and protects the compressor in an oil shortage situation and allows the compressor in an oil shortage situation to recover some oil. In fact, a suction bypass is created by the common oil balance line, so some of the oil-laden refrigerant will still come inside this com- . presser and fill the compressor oil pan with oil.

[0032] According to one embodiment of the invention, if the compressors are not all running, and therefore if at least one compressor is stopped, the oil balancing action comprises stopping the operation of the running compressor for which an oil shortage situation has been detected and starting the operation of a stopped compressor, i.e. a stopped compressor for which an oil shortage situation has not been detected, in order to maintain the workload of the multi-compressor refrigeration system substantially identical.

[0033] According to one embodiment of the invention, the oil balancing action comprises stopping the operation of all the compressors. In doing so, it is possible to use the principle of communicating containers in order to balance the oil level between the compressors.

[0034] According to one embodiment of the invention, the oil return action comprises increasing the workload of the multi-compressor refrigeration system. By increasing the workload of the multi-compressor refrigeration system, an oil boost can be provided and the oil can return to the multi-compressor device and thus to the compressors. Indeed, due to the increase in workload, the demand for the oil supply increases, and the oil can be removed from the more distant parts of the multi-compressor refrigeration system.

[0035] According to one embodiment of the invention, if the total number of compressors in the multiple compressors is equal to two, the oil return action comprises maintaining the operation of the running compressor for which an oil shortage situation has been detected and starting the operation of the stopped compressor, i.e., the stopped compressor for which an oil shortage situation has not been detected. Therefore, if the total number of compressors in the multiple compressors is equal to two and one compressor is stopped, the oil return action comprises maintaining the operation of the running compressor for which an oil shortage situation has been detected and starting the operation of the stopped compressor.In particular, if only one of the two compressors is running (50% load) and an oil shortage situation is detected for one of the running compressors, it is assumed that there is a problem with the oil return from the refrigeration system to the compressors (oil is trapped in the refrigeration system outside the multi-compressor device). In order to overcome this and return the oil to the multi-compressor device, and in particular to the compressor for which an oil shortage situation has been detected, the control device starts the operation of the stopped compressor. Through this action, an oil boost can be provided and the oil can return to the device. multiple compressors and in particular to the running compressor for which an oil shortage situation has been detected.

[0036] According to one embodiment of the invention, if the total number of compressors in the multiple compressors is greater than two, the oil return action comprises stopping the operation of the running compressor for which an oil shortage situation has been detected and starting the operation of at least two stopped compressors, and for example all the stopped compressors, i.e. the stopped compressor(s) for which an oil shortage situation has not been detected. In particular, if only one compressor is running and all the other compressors are stopped and an oil shortage situation is detected for the running compressors, it is assumed that there is a problem with oil return from the refrigeration system to the multiple compressor device (oil is trapped in the refrigeration system outside the multiple compressor device).In order to overcome this and return the oil to the compressors, the control device stops the running compressor for which an oil shortage situation has been detected and starts all other compressors. By this action, an oil boost can be provided and the oil can return to the multi-compressor device and in particular to the running compressor for which an oil shortage situation has been detected.

[0037] According to one embodiment of the invention, if the total number of compressors in the multiple compressors is greater than two and if only one compressor is running and all other compressors are stopped, the oil return action comprises stopping the operation of the running compressor for which an oil shortage situation has been detected and starting the operation of all stopped compressors.

[0038] According to one embodiment of the invention, the oil return action comprises opening an electronic expansion valve of the multi-compressor refrigeration system in order to increase the suction mass flow rate of the multi-compressor device. By increasing the suction mass flow rate, oil can be removed from the most remote parts of the refrigeration system and returned to the multi-compressor device and in particular to the running compressor for which an oil shortage situation has been detected.

[0039] According to one embodiment of the invention, the oil return action further comprises opening an electronic expansion valve of the multi-compressor refrigeration system in order to increase the suction mass flow rate of the multi-compressor device. Thus, by combining the increase in the suction mass flow rate and the increase in the workload of the multi-compressor refrigeration system, the oil boost is improved.

[0040] According to one embodiment of the invention, the method further comprises stopping the operation of all compressors, if, after the lapse of a third predetermined time period, for example two minutes, since the start of the oil management action, an oil shortage situation is still detected for the compressor for which an oil shortage situation was previously detected. In other words, the method further comprises stopping the operation of all compressors, if the predetermined threshold condition remains in the respective compressor for a third predetermined time, for example two minutes, since the start of the oil management action.In particular, if the oil shortage situation persists after the oil management action is performed, it is assumed that there is an oil shortage trigger situation and the control device stops the operation of the entire refrigeration system for safety reasons.

[0041] According to one embodiment of the invention, the method further comprises returning to the monitoring step, if the oil shortage situation disappears before the third predetermined time period has elapsed.

[0042] According to one embodiment of the invention, the oil level detection device of each compressor comprises an oil level sensor or an oil level switch.

[0043] According to one embodiment of the invention, the method further comprises providing a low oil level warning, if an oil shortage situation is detected for a running compressor.

[0044] According to one embodiment of the invention, each compressor of the multi-compressor device is a scroll compressor.

[0045] According to one embodiment of the invention, each oil level detection device is at least partially located in the respective oil pan.

[0046] According to one embodiment of the invention, each compressor comprises a compressor housing, and the respective oil pan is arranged in a lower portion of said compressor housing.

[0047] According to one embodiment of the invention, if the total number of compressors in the multi-compressor device is greater than or equal to 5 and half or less of the total number of compressors are running and at least two compressors are running, the oil management action comprises:

[0048] - performing the oil balancing action if a total piping length of the multi-compressor refrigeration system is less than a predetermined length,

[0049] - carrying out the oil return action if the total piping length of the multi-compressor refrigeration system is greater than the predetermined length.

[0050] According to one embodiment of the invention, the predetermined length of the piping is 10 meters.

[0051] The present invention also relates to a multi-compressor refrigeration system comprising:

[0052] - a control device configured to control the operation of the multi-compressor refrigeration system,

[0053] - a multiple compressor device comprising compressors which are coupled in parallel, a total number of compressors in the multi-compressor device being at least two and each compressor being equipped with an oil sump and an oil level sensing device operatively connected to the control device and configured to sense an oil level in the respective oil sump, the oil sumps being connected by a common oil balance line,

[0054] wherein the control device is configured to:

[0055] - monitor an oil level in the oil sump of each compressor via the respective oil level detection device,

[0056] - detect, on the basis of a signal emitted by a level detection device respective oil level, that an oil shortage situation occurs in a running compressor, if the oil level in said running compressor reaches a predetermined threshold condition,

[0057] - perform an oil management action if an oil shortage situation is detected for a running compressor, the oil management action being based on a change of the ON / OFF configurations of the compressors in the multi-compressor device and comprising:

[0058] if the total number of compressors in the multi-compressor device is between 2 and 4: • carrying out an oil balancing action, if more than half of the total number of compressors are running, where the oil balancing action includes oil balancing between the compressors, • performing an oil return action, if half or less of the total number of compressors are operating, where the oil return action includes recovering oil from the multi-compressor refrigeration system to the multi-compressor device,

[0059] if the total number of compressors in the multi-compressor device is greater than or equal to 5: • carrying out the oil balancing action, if more than half of the total number of compressors are running, • carrying out the oil balancing action and / or the return action of oil, if half or less of the total number of compressors are running and at least two compressors are running, • carrying out the oil return action, if only one compressor is running.

[0060] According to one embodiment of the invention, the multi-compressor refrigeration system further comprises a refrigerant circulation circuit successively comprising a condenser, an electronic expansion valve, an evaporator and the multi-compressor device connected in series.

[0061] According to one embodiment of the invention, the multi-compressor refrigeration system is devoid of opening / closing valves and / or oil reservoirs.

[0062] According to one embodiment of the invention, the compressors may be fixed speed compressors or variable speed compressors.

[0063] According to other embodiments of the invention, the compressors may be of a type other than scroll compressors, i.e. the compressors may be particularly screw compressors or piston compressors. Brief description of the drawings

[0064] The following detailed description of two embodiments of the invention is best understood when read in conjunction with the accompanying drawings, it being understood, however, that the invention is not limited to the specific embodiments disclosed.

[0065] [Fig.l] is a schematic view of a multi-compressor refrigeration system according to the invention.

[0066] [Fig.2] is a longitudinal sectional view of a compressor of the multi-compressor refrigeration system of [Fig.l].

[0067] [Fig. 3] is a perspective view of a multi-compressor device of a multi-compressor refrigeration system according to another embodiment of the invention.

[0068] [Fig.4] is a diagram of an oil management process in the multi-compressor refrigeration system of [Fig.l].

[0069] [Fig.5] is a diagram of a first embodiment of an oil management action of the method of [Fig.4] for a total number of compressors in the multi-compressor refrigeration system ranging from 2 to 4.

[0070] [Fig.6] is a diagram of a second embodiment of the oil management action of the method of [Fig.4] for a total number of compressors in the multi-compressor refrigeration system ranging from 2 to 4,

[0071] [Fig.7] is a diagram of one embodiment of the oil management action of the method of [Fig.4] for a total number of compressors in the multi-compressor refrigeration system greater than or equal to 5. Detailed description of the invention

[0072] Figures 1 and 2 describe a multi-compressor refrigeration system 2 comprising a refrigerant circulation circuit 3 successively comprising a condenser 4, an electronic expansion valve 5, an evaporator 6 and a multiple-compressor device 7 connected in series.

[0073] The multi-compressor device 7 comprises a plurality of compressors 8 which are coupled in parallel. According to the embodiment shown in [Fig.l], the plurality of compressors comprises three compressors 8. However, the plurality of compressors may comprise only two compressors 8 or more than three compressors 8 and for example four compressors 8.

[0074] Each compressor 8 comprises a compressor casing 9 provided with a suction connection 11 configured to supply the respective compressor 8 with refrigerant gas to be compressed and a discharge connection 12 configured to discharge the compressed refrigerant gas.

[0075] Advantageously, each compressor 8 is a scroll compressor, and comprises a compression unit 13 arranged inside the respective compressor housing 9 and configured to compress the refrigerant gas supplied by the respective suction connection 11. Each compression unit 13 comprises a fixed scroll, which is fixed relative to the respective compressor housing 9, and an orbiting scroll configured to perform an orbital movement relative to the respective fixed scroll during operation of the respective compressor 8.

[0076] Each compressor 8 also comprises an oil sump 14 arranged at a lower portion of the respective compressor sump 9, and an oil level sensing device 15 located in the respective oil sump 14 and configured to sense an oil level in the respective oil sump 14. Each oil level sensing device 15 may be an oil level sensor or an oil level switch. Advantageously, each oil level sensing device 15 is configured to output a low oil level warning signal if the oil level in the oil sump 14 of the respective compressor 8 reaches a predetermined low oil level value.

[0077] The multi-compressor device 7 further comprises a common suction line 16 and inlet connection lines 17 each connecting the common suction line 16 to the suction connection 11 of a respective compressor 8. The multi-compressor device 7 also comprises a common discharge line 18 and outlet connection lines 19 connecting each the common discharge line 18 to the discharge connection 12 of a respective compressor 8.

[0078] Furthermore, the multi-compressor device 7 comprises a common oil balance line 20.1 and balance connection lines 20.2, also called balance bypass lines, each connecting the common oil balance line 20.1 to an oil balance connection 20.3 provided on the compressor housing 9 of a respective compressor 8. The common oil balance line 20.1 and the balance connection lines 20.2 are particularly configured to fluidly connect the oil housings 14 of the compressors 8, and thus allow oil to flow between the compressors 8 and balance the oil levels in the compressors 8.

[0079] The multi-compressor refrigeration system 2 further comprises a control device 21 configured to control the operation of the multi-compressor refrigeration system 2, and in particular to control the operation (start or stop) of the compressors 8 and to control which compressor(s) 8 is (are) in operation. The control device 21 may for example comprise a microprocessor and a memory. Each oil level detection device 15 is particularly operatively connected to the control device 21.

[0080] [Fig.4] shows a diagram of an oil management process in the multi-compressor refrigeration system 2 of [Fig.l].

[0081] The method particularly comprises:

[0082] - monitoring an oil level in the oil sump 14 of each compressor 8 via the respective oil level detection device 15,

[0083] - detection, by the control device 21 and on the basis of a signal emitted by a oil level detection device 15, due to the fact that an oil shortage situation occurs in a running compressor 8, if the oil level in said running compressor 8 reaches a predetermined threshold condition,

[0084] - performing an oil management action if an oil shortage situation is detected for a running compressor 8, the oil management action being based on a change in the ON / OFF configurations of the compressors 8 in the multi-compressor device 7, and

[0085] - stopping the operation of all the compressors 8, if, after the elapse of a third predetermined time period, for example two minutes, since the start of the oil management action, the oil shortage situation is still detected for the compressor 8 for which the oil shortage situation was previously detected, or

[0086] - return to the monitoring step, if the oil shortage situation disappears before the elapse of the third predetermined time period since the start of the oil management action.

[0087] Particularly, if the oil shortage situation persists after the oil management action is performed, it is assumed that there is an oil shortage trigger situation and the control device 21 stops the operation of the entire multi-compressor refrigeration system 2 for safety reasons. However, if the oil shortage situation disappears, this means that the oil management action has been successful and the oil sump 14 of the compressor 8, for which the oil shortage situation was previously detected, now includes sufficient oil.

[0088] Advantageously, the predetermined threshold condition is reached if:

[0089] - the low oil level warning signal emitted by a detection device oil level 15 is emitted continuously for a first predetermined time, and for example for at least ten seconds, or

[0090] - the low oil level warning signal emitted by a detection device oil level sensor 15 has switched, i.e. has activated and deactivated, more than a predetermined number of times, and for example more than ten times, during a second predetermined time, and for example during the last minute.

[0091] By means of the above requirements, erroneous detection of an oil shortage situation in the running compressor 8 can be prevented.

[0092] Depending on the actual operating condition of the multi-compressor refrigeration system 2, the oil management action may take different forms.

[0093] [Fig.5] shows a diagram of a first embodiment of the oil management action.

[0094] The oil management action includes the following steps:

[0095] - determination, by the control device 21, of the total number of compressors 8 in the multiple compressor device 7,

[0096] - determination, by the control device 21, of the number of compressors in step 8 in the multi-compressor device 7,

[0097] - determination, by the control device 21, of the percentage of compressors running 8 in the multi-compressor device 7,

[0098] - performing an oil balancing action, if more than half of the total number of compressors 8 are in operation, where the oil balancing action includes balancing oil between the compressors 8,

[0099] - performing an oil return action, if half or less of the total number of compressors 8 are in operation, where the oil return action includes recovering oil from the multi-compressor refrigeration system 2, i.e. from outside the multi-compressor device 7, to the compressor device multiples 7.

[0100] Particularly, if all the compressors 8 are running (i.e. if 100% of the compressors are running), the oil balancing action comprises maintaining the operation of the running compressor for which an oil shortage situation has been detected and stopping the operation of the other running compressor(s) 8, i.e. the other running compressor(s) for which an oil shortage situation has not been detected.

[0101] However, if the compressors 8 are not all running (i.e. the percentage of running compressors is different from 100%), and therefore if at least one compressor 8 is stopped, the oil balancing action comprises stopping the operation of the running compressor 8 for which an oil shortage situation has been detected and starting the operation of a stopped compressor 8 in order to maintain the workload of the multi-compressor refrigeration system 2 substantially identical.

[0102] According to the embodiment shown in [Fig.5], the oil return action comprises increasing the workload of the multi-compressor refrigeration system 2. By increasing the workload of the multi-compressor refrigeration system 2, an oil boost can be provided and the oil can return to the multi-compressor device 7 and in particular to the compressor 8 for which an oil shortage situation has been detected. Indeed, due to the increase in the workload, the demand for the oil supply increases, and the oil can be removed from the most distant parts of the multi-compressor refrigeration system 2.

[0103] Such an increase in the workload of the multi-compressor refrigeration system 2 could be achieved in various ways.

[0104] For example, if the total number of compressors 8 in the multiple compressors is equal to two, the oil return action comprises maintaining the operation of the running compressor 8 for which an oil shortage situation has been detected and starting the operation of the stopped compressor 8, i.e. the stopped compressor 8 for which an oil shortage situation has not been detected.

[0105] However, if the total number of compressors 8 in the multiple compressors is greater than two, the oil return action includes stopping the operation of the running compressor 8 for which an oil shortage situation has been detected and starting the operation of all the stopped compressors 8.

[0106] The oil return action may further comprise opening an electronic expansion valve 5 of the multi-compressor refrigeration system 2 to increase the suction mass flow rate of the multi-compressor device 7. By increasing the suction mass flow rate, oil may be removed from the parts further from the multi-compressor refrigeration system 2 and returned to the multi-compressor device 7 and in particular to the running compressor 8 for which an oil shortage situation has been detected. Thus, by combining the increase in the suction mass flow rate and the increase in the workload of the multi-compressor refrigeration system 2, the oil boosting is improved.

[0107] [Fig.6] shows a diagram of a second embodiment of the oil management action which differs from the first embodiment shown in [Fig.5] essentially in that the oil return action comprises, instead of increasing the workload of the multi-compressor refrigeration system 2, increasing the suction mass flow rate of the multi-compressor device 7. Such an increase in the suction mass flow rate of the multi-compressor device 7 can be achieved by opening the electronic expansion valve 5 of the multi-compressor refrigeration system 2.

[0108] Several scenarios of the present method are disclosed below.

[0109] Scenario 1: Compressors in tandem and 100% load

[0110] If both compressors 8 in the multi-compressor refrigeration system 2 are running and an oil shortage situation is detected for one of the running compressors 8 (e.g., the first compressor), it is assumed that there is an oil balancing problem (there is excess oil in the second compressor). In order to overcome this and balance the oil levels in the compressors 8, the control device 21 stops the operation of the other running compressor 8 (the second compressor), where the oil shortage situation has not been detected. By this action, the first compressor can remove more oil from the multi-compressor device 7 and thus increase the oil level in the respective oil sump 14. [YES] Scenario 2: Compressors in tandem and 50% load

[0112] If only one of the two compressors 8 (e.g. the first compressor) is running (50% load) and an oil shortage situation is detected for this running compressor 8 (the first compressor), it is assumed that there is a problem with the oil return from the multi-compressor refrigeration system 2 to the compressors 8 (oil is trapped in the system outside the multi-compressor device 7). In order to overcome this and return the oil to the multi-compressor device 7, and in particular to the compressor 8 for which an oil shortage situation has been detected, the control device 21 starts the operation of the stopped compressor 8 (the second compressor). By this action, an oil boost can be provided and the oil can return to the multi-compressor device 7 and in particular to the running compressor 8 for which an oil shortage situation has been detected. low oil situation has been detected.

[0113] In another embodiment of the invention, in order to overcome the problem of oil return from the multi-compressor refrigeration system 2 to the compressors 8, the controller 21 may open the electronic expansion valve 5 of the refrigeration system to increase the suction mass flow rate. Opening the electronic expansion valve 5 of the refrigeration system to increase the suction mass flow rate may also be applied in addition to starting the operation of the stationary compressor 8, to further improve the oil boosting.

[0114] Scenario 3: Compressors in Trio and 100% load

[0115] If all compressors 8 are running and an oil shortage situation is detected for one of the running compressors 8 (e.g., the first compressor), it is assumed that there is an oil balancing problem (there is excess oil in the second compressor and / or the third compressor). In order to overcome this and balance the oil levels in the compressors 8, the control device 21 maintains the operation of the first compressor and stops the operation of all other compressors 8 (the second compressor and the third compressor). By this action, the first compressor can remove more oil from the multi-compressor device 7 and thus increase the oil level in the respective oil sump 14.

[0116] Scenario 4: Compressors in Trio and 66% load

[0117] If two compressors 8 are running (e.g., the first compressor and the second compressor) and one compressor 8 is stopped (e.g., the third compressor) and a low oil condition is detected for one of the running compressors (e.g., the first compressor), it is assumed that there is an oil balance problem (there is excess oil in the second compressor and the third compressor). In order to overcome this and balance the oil levels in the compressors 8, the controller 21 changes the ON / OFF configuration of the multi-compressor refrigeration system 2 while maintaining the same workload. Specifically, the controller 21 stops the operation of the first compressor, keeps the operation of the other running compressor 8 (the second compressor), and starts the operation of the stopped compressor 8 (the third compressor).By this action, the multi-compressor refrigeration system 2 maintains the required workload and balances the oil levels in the compressors.

[0118] Scenario 5: Compressors in Trio and 33% load

[0119] If only one compressor 8 is running (e.g., the first compressor) and all other compressors 8 are stopped (e.g., the second compressor and the third compressor) and an oil shortage situation is detected for the running compressor 8 (the first compressor), it is assumed that there is a problem with oil return from the multi-compressor refrigeration system 2 to the compressors 8. In order to overcome this and return the oil to the compressors, the control device 21 stops the running compressor 8 for which an oil shortage situation has been detected (the first compressor) and starts all the other compressors 8 (the second compressor and the third compressor). By this action, an oil boost can be provided and the oil can return to the multi-compressor device 7 and in particular to the first compressor. Scenario 6: Five compressors and 40% load

[0120] If only two compressors 8 are running and all other compressors 8 are stopped and an oil shortage situation is detected for one of the running compressors 8, it is assumed that there could be an oil balancing problem or an oil return problem. In order to overcome this, the controller 21 may first perform the oil balancing action for a fourth predetermined time period. If the oil shortage situation does not disappear before the fourth predetermined time period for one of the running compressors 8 has elapsed, the controller 21 may then perform the oil return action. The oil balancing action and the oil return action may be performed as described above in connection with other scenarios.

[0121] In general and as shown in [Fig. 7], if the total number of compressors 8 in the multi-compressor device 7 is greater than or equal to five, half or less of the total number of compressors 8 are running and the number of running compressors 8 is at least 2, it may be advantageous to perform the oil balancing action and / or the oil return action. In particular, the oil balancing action is preferable in the case of "short piping" systems (i.e. for the multi-compressor refrigeration system 2 having a total piping length less than a predetermined length) and the oil return action is preferable for "long piping" ("multi-block") systems (i.e. for the multi-compressor refrigeration system 2 having a total piping length greater than a predetermined length).

[0122] The "short pipe" system refers to "packaged" systems with all the components of the refrigerant circuit (an evaporator, a condenser, a compressor and an expansion valve) located on the same compact chassis. The circuit dimension between the evaporator and the compressor is approximately 2 meters and the circuit dimension between the condenser and the compressor is also approximately 2 meters.

[0123] "Long pipe" ("multi-block") system refers to systems with several smaller units in a refrigeration system connected by a pipe of refrigerant. In other words, in "long pipe" systems, at least one heat exchanger unit (an evaporator or condenser) is located separately from the unit with a compressor. The distance between the units can be up to 30 meters.

[0124] In another embodiment of the invention, in order to overcome the problem of oil return from the multi-compressor refrigeration system 2 to the compressors 8, the controller 21 may open the electronic expansion valve 5 of the refrigeration system to increase the suction mass flow rate. Opening the electronic expansion valve 5 of the refrigeration system to increase the suction mass flow rate may also be applied in addition to starting the operation of the stationary compressors 8, to further improve the oil boosting.

[0125] Regardless of the scenarios, if, after the start of the oil management action, the oil shortage situation persists for two minutes, then there is an oil shortage trigger situation and the controller 21 stops the operation of the entire multi-compressor refrigeration system 2 by stopping the operation of all the compressors 8.

[0126] In scenarios other than those described above, no oil management action is necessary.

[0127] Of course, the invention is not limited to the embodiments described above as a non-limiting example, but on the contrary it encompasses all the corresponding embodiments.

Claims

1. Claims A method for managing oil in an operating multi-compressor refrigeration system (2) comprising a control device (21) configured to control the operation of the multi-compressor refrigeration system (2) and a multi-compressor device (7) comprising compressors (8) which are coupled in parallel, a total number of compressors (8) in the multi-compressor device (7) being at least two and each compressor (8) being equipped with an oil sump (14) and an oil level detection device (15) operatively connected to the control device (21) and configured to detect an oil level in the respective oil sump (14), the oil sumps (14) being connected by a common oil balance line (20.1), the method comprising: - monitoring an oil level in the oil sump (14) of each compressor (8) via the respective oil level detection device (15), - detecting that an oil shortage situation occurs in a running compressor (8), if the oil level in said running compressor (8) reaches a predetermined threshold condition, - performing an oil management action if an oil shortage situation is detected for a running compressor (8), the oil management action being based on a change of the ON / OFF configurations of the compressors (8) in the multi-compressor device (7) and comprising: if the total number of compressors (8) in the multi-compressor device (7) is between 2 and 4: • performing an oil balancing action, if more than half of the total number of compressors (8) are running, the oil balancing action comprising balancing oil between the compressors (8), • performing an oil return action, if half or less of the total number of compressors (8) are running, the oil return action comprising recovering oil from the multi-compressor refrigeration system (2) to the multi-compressor device (7), if the total number of compressors (8) in the multi-compressor device (7) is greater than or equal to 5: • performing the oil balancing action, if more than half of the total number of compressors (8) are running, • performing the oil balancing action and / or the oil return action, if half or less of the total number of compressors (8) are running and at least two compressors (8) are running, • performing the oil return action, if only one compressor (8) is running.

2. A method according to claim 1, further comprising: - issuing a low oil level warning signal by an oil level detection device (15) and to the control device (21), if the oil level in the oil sump (14) of the respective compressor (8) reaches a predetermined low oil level value.

3. A method according to claim 2, wherein the predetermined threshold condition is reached if: - the low oil level warning signal emitted by an oil level detection device (15) is continuously emitted for a first predetermined time, or - the low oil level warning signal emitted by an oil level detection device (15) has switched more than a predetermined number of times during a second predetermined time.

4. A method according to any one of claims 1 to 3, wherein the oil balancing action comprises maintaining operation of the running compressor (8) for which an oil shortage condition has been detected and stopping operation of the other running compressor (8) or all other running compressors (8).

5. A method according to any one of claims 1 to 3, wherein the oil balancing action comprises stopping the operation of the running compressor (8) for which an oil shortage condition has been detected and starting the operation of a stopped compressor (8) in order to maintain the workload of the multi-compressor refrigeration system (2) substantially the same.

6. A method according to any one of claims 1 to 3, wherein the oil balancing action comprises stopping the operation of all the compressors (8).

7. A method according to any one of claims 1 to 6, wherein the oil return action comprises increasing the workload of the multi-compressor refrigeration system (2).

8. A method according to any one of claims 1 to 7, wherein, if the total number of compressors (8) in the multiple compressors is two, the oil return action comprises maintaining operation of the running compressor (8) for which an oil shortage condition has been detected and starting operation of the stopped compressor (8).

9. A method according to any one of claims 1 to 7, wherein, if the total number of compressors (8) in the multiple compressors is greater than two, the oil return action comprises stopping the operation of the running compressor (8) for which an oil shortage condition has been detected and starting the operation of at least two stopped compressors (8).

10. A method according to claim 8 or 9, wherein the oil return action further comprises opening an electronic expansion valve (5) of the multi-compressor refrigeration system (2) to increase the suction mass flow rate of the multi-compressor device (7).

11. A method according to any one of claims 1 to 6, wherein the oil return action comprises opening an electronic expansion valve (5) of the multi-compressor refrigeration system (2) to increase the suction mass flow rate of the multi-compressor device (7).

12. A method according to any one of claims 1 to 11, further comprising: - stopping the operation of all the compressors (8), if, after the lapse of a third predetermined time period since the start of the oil management action, an oil shortage situation is still detected for the compressor (8) for which an oil shortage situation was previously detected.

13. A method according to any one of claims 1 to 12, wherein the oil level detection device (15) of each compressor (8) comprises an oil level sensor or a level switch of oil.

14. A method according to any one of claims 1 to 13, further comprising providing a low oil level warning, if an oil shortage situation is detected for a running compressor (8).

15. A method according to any one of claims 1 to 14, wherein, if the total number of compressors (8) in the multi-compressor device (7) is greater than or equal to 5 and half or less of the total number of compressors (8) are running and at least two compressors (8) are running, the oil management action comprises: - performing the oil balancing action if a total piping length of the multi-compressor refrigeration system (2) is less than a predetermined length, - performing the oil return action if the total piping length of the multi-compressor refrigeration system (2) is greater than the predetermined length.

16. A multi-compressor refrigeration system (2) comprising: - a controller (21) configured to control operation of the multi-compressor refrigeration system (2), - a multi-compressor device (7) comprising compressors (8) which are coupled in parallel, a total number of compressors (8) in the multi-compressor device (7) being at least two and each compressor (8) being equipped with an oil sump (14) and an oil level detection device (15) operatively connected to the controller (21) and configured to detect an oil level in the respective oil sump (14), the oil sumps (14) being connected by a common oil balance line (20.1), wherein the control device (21) is configured to: - monitor an oil level in the oil sump (14) of each compressor (8) via the respective oil level detection device (15), - detect, based on a signal emitted by a respective oil level detection device (15), that an oil shortage situation occurs in a running compressor (8), if the oil level in said running compressor (8) reaches a predetermined threshold condition, - perform an oil management action if an oil shortage situation. oil is detected for a running compressor (8), the oil management action being based on a change of the ON / OFF configurations of the compressors (8) in the multi-compressor device (7) and comprising: if the total number of compressors (8) in the multi-compressor device (7) is between 2 and 4: • performing an oil balancing action, if more than half of the total number of compressors (8) are running, the oil balancing action comprising balancing oil between the compressors (8), • performing an oil return action, if half or less of the total number of compressors (8) are running, the oil return action comprising recovering oil from the multi-compressor refrigeration system (2) to the multi-compressor device (7), if the total number of compressors (8) in the multi-compressor device (7) is greater than or equal to 5: • carrying out the oil balancing action, if more than half of the total number of compressors (8) are running, • performing the oil balancing action and / or the oil return action, if half or less of the total number of compressors (8) are running and at least two compressors (8) are running, • carrying out the oil return action, if only one compressor (8) is running.

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