Method for managing oil in a multi-compressor refrigeration system and a multi-compressor refrigeration system having a common oil equalization line equipped with an oil pumping device

Abstract

An oil management method in a multi-compressor refrigeration system comprising a control device; a multi-compressor device (7) comprising at least two compressors (8) which are coupled in parallel, each compressor (8) being equipped with a suction connection and an oil sump, the suction connections of all the compressors (8) being fluidly connected by a common suction line (16) and the oil sumps of all the compressors (8) being fluidly connected by a common oil equalization line (23); and an oil pumping device (27) comprising an oil pump (28) located in the common oil equalization line (23).The method includes monitoring an operating parameter of the oil pump (28); and performing an oil return action if the operating parameter of the oil pump (28) reaches a predetermined value, where the oil return action includes recovering oil from the multi-compressor refrigeration system to the multi-compressor device (7). Figure 3.

Description

Title of the invention: Method for managing oil in a multi-compressor refrigeration system and multi-compressor refrigeration system having a common oil equalization line equipped with an oil pumping device. Field of the invention

[0001] The present invention relates to a multi-compressor refrigeration system and to a method of oil management in a multi-compressor refrigeration system. Previous art

[0002] Proper oil balancing is an important issue related to multi-compressor systems.

[0003] Proper oil balancing in a tandem configuration is generally not so difficult to achieve, and can be obtained in most cases by adding passive elements (suction washer, restrictor, etc.) to the suction line. This solution has a limited impact on the pressure drop and the overall efficiency of the refrigeration system.

[0004] However, when dealing with a larger multi-compressor refrigeration system, the situation can change dramatically. In trio configurations with passive devices integrated into the suction line, the refrigeration system experiences such a significant pressure drop that it negatively impacts efficiency.

[0005] Common solutions for balancing the oil in a multi-compressor refrigeration system comprising at least three compressors include providing oil equalization lines between the compressors, oil level detection means in each compressor, pumps to provide oil flow in the oil equalization lines between the compressors, and a control device to control the operation of the multi-compressor system. The general idea is that once the oil level detection means detects a low oil level in a particular compressor, the control device can operate the respective oil pump located in the respective oil equalization line and pump some oil from the compressor with the high oil level to the compressor with the low oil level.

[0006] However, a low oil level detected in a compressor of a multi-compressor system could result from an oil return problem (oil is trapped in the refrigeration system).

[0007] Therefore, even if the multi-compressor system is equipped with an oil balancing device (and in particular an active oil balancing device), an oil balancing problem may occur if oil is trapped in the refrigeration system.

[0008] From a design perspective, multi-compressor refrigeration systems with large-diameter system pipes are advantageous for system efficiency. However, due to the low flow velocity in large-diameter system pipes, such multi-compressor refrigeration systems require additional, costly oil separators to limit situations of low oil levels in the compressors and, in particular, oil trapped in the refrigeration system. On the other hand, reducing the diameter (and thus avoiding the use of oil separators) leads to high pressure drops and therefore reduces system efficiency. Summary of the invention

[0009] An object of the present invention is to provide an improved method of oil management in a running multi-compressor refrigeration system that can overcome the disadvantages encountered in conventional multi-compressor refrigeration systems.

[0010] In particular, an object of the present invention is to provide a method of oil management in a running multi-compressor refrigeration system which is simple and easy to implement, which can provide robust prevention of running compressor in oil shortage situations, particularly if oil is trapped in the refrigeration system outside the multi-compressor device.

[0011] To this end, the present invention relates to a method for managing oil in a multi-compressor refrigeration system comprising a control device configured to control the operation of the multi-compressor refrigeration system and a multi-compressor device comprising at least two compressors connected in parallel, each compressor being equipped with a suction connection and an oil sump, the oil sumps of all the compressors in the multi-compressor device being fluidly connected by a common oil equalization line and the suction connections of all the compressors in the multi-compressor device being fluidly connected by a common suction line, wherein the multi-compressor refrigeration system further comprises an oil pumping device located in the common oil equalization line and functionally connected to the control device, the oil pumping device comprising at least one oil pump and being configured to perform an oil balancing action between the oil sumps of the multi-compressor device, the method comprising:

[0012] - monitoring an operating parameter of at least one oil pump, for example, under partial load conditions of the multi-compressor refrigeration system,

[0013] - the implementation of an oil return action if the operating parameter of at least one oil pump reaches a predetermined value, where the oil return action includes the recovery of oil from the multi-compressor refrigeration system, i.e. from outside the multi-compressor device, to the multi-compressor device.

[0014] In particular, if a significant amount of oil is trapped outside the multi-compressor device, there is insufficient oil in at least one oil sump of the multi-compressor device. Consequently, when the multi-compressor refrigeration system starts operating, at least one oil pump is activated to balance the oil between the compressor oil sumps. However, since there is insufficient oil, the at least one oil pump cannot perform oil balancing.

[0015] Sometimes, when a significant amount of oil is trapped outside the multi-compressor device, the amount of oil contained within the device may be insufficient to maintain the correct oil level in each oil sump, and a low oil level situation may be detected in several oil sumps simultaneously. It may then be necessary for the oil pump to supply a significant amount of oil to all the oil sumps with a detected low oil level in order to overcome the low oil level situation. The standard oil flow rate of the oil pump used in such a multi-compressor refrigeration system imposes certain limitations, particularly if it is necessary to supply oil quickly and efficiently to all the respective oil sumps.In the case where at least one oil pump is a variable-speed oil pump, the variable-speed oil pump can increase its rotational speed (and therefore its oil flow rate) by attempting to perform an oil balancing action. If said oil flow rate is the operating parameter of the at least one oil pump that is monitored by the method according to the present invention, then an increase in the oil flow rate beyond a predetermined value, for example, beyond said standard oil flow rate, can be identified as an indication that oil is trapped outside the multi-compressor device, and an oil return action can be performed based on said increase in oil flow rate. Once a sufficient quantity of oil is returned to the multi-compressor device, appropriate oil balancing can be performed. The use of the pump... Variable speed oil and its flow rate as the operating parameter allows the multi-compressor refrigeration system to be much more responsive to the detected situation.

[0016] Sometimes, when a significant amount of oil is trapped outside the multi-compressor device, the amount of oil contained in the oil sump of each compressor, for each of which a low oil situation has not been detected, may be insufficient to transfer a sufficient amount of oil to the oil sump of the compressor for which a low oil situation has been detected.In this case, at least one oil pump can, for example, be started to transfer a first quantity of oil from a first compressor, for which a low oil level situation has not been detected, until a low oil level is detected in said first compressor; then at least one oil pump can be started to transfer a second quantity of oil from a second compressor, for which a low oil level situation has not been detected, until a low oil level is detected in said second compressor; and finally at least one oil pump can be started to transfer a third quantity of oil from a third compressor, for which a low oil level situation has not been detected.

[0017] In the case where the starting frequency of at least one oil pump (which corresponds to a number of times, in a predetermined period of time, that at least one oil pump has been started to perform an oil balancing action) is the operating parameter of the at least one oil pump which is monitored by the method according to the present invention, then if the starting frequency of at least one oil pump reaches the predetermined value, it can be deduced that the oil is trapped outside the multi-compressor device and an oil return action can be carried out on the basis of said starting frequency.

[0018] Therefore, by monitoring an operating parameter of at least one oil pump, the method according to the present invention can easily detect when oil is trapped in the refrigeration system outside the multi-compressor device and perform an oil return action only if necessary.

[0019] In conclusion, the method according to the present invention makes it possible to combine robust compressor prevention in low-oil situations, since it monitors the operation of at least one oil pump and ensures oil return action if necessary, with minimal pressure drop even under full load conditions, using large-diameter system pipes (large-diameter system pipes are advantageous in terms of efficiency). of the system), without needing an oil separator (the absence of an oil separator contributes to the low cost of a system).

[0020] The process may also include one or more of the following features, taken alone or in combination.

[0021] According to one embodiment of the invention, the oil return action comprises increasing the workload of the multi-compressor refrigeration system, for example, for a predetermined time. By increasing the workload of the multi-compressor refrigeration system, oil can be supercharged, and the oil can return to the multi-compressor device and thus to the compressors. Indeed, due to the increased workload, the demand for oil increases, and oil can be drawn from the most distant parts of the multi-compressor refrigeration system.

[0022] According to one embodiment of the invention, the oil return action includes controlling the multi-compressor refrigeration system to operate, for example for a predetermined time, at full workload.

[0023] According to one embodiment of the invention, the oil return action includes controlling the multi-compressor refrigeration system to restart, for example for a predetermined time, the multi-compressor device with a different operating configuration.

[0024] According to one embodiment of the invention, the oil return action includes changing the ON / OFF configurations of the compressors in the multi-compressor device.

[0025] According to one embodiment of the invention, each compressor is equipped with an oil level detection device functionally connected to the control device and configured to detect an oil level in the respective oil sump, the method further comprising:

[0026] - monitoring the oil level in the oil sump of each compressor by the respective oil level detection device,

[0027] - the detection of the fact that a low oil level situation, also called A low oil situation occurs in a running compressor if the oil level in said running compressor reaches a predetermined low oil level condition.

[0028] - performing an oil balancing action if a low level situation When oil is detected for a compressor, the oil balancing action includes controlling the operation of the oil pumping device to transfer oil from the oil sump to at least one compressor in the multi-compressor system for which a low oil level situation has not been detected. detected in the compressor oil sump where a low oil level situation was detected.

[0029] According to one embodiment of the invention, the method comprises calculating the risk of low oil levels occurring in a compressor under a given operating condition, and controlling the operation of the oil pumping device and at least one valve based on said calculation, for example, to supply more oil to a compressor that presents a high risk of low oil levels occurring, and / or controlling the operation of the oil pumping device and valves so as not to draw oil from a compressor that presents a high risk of low oil levels occurring. Advantageously, the method comprises selecting at least one oil sump, from which oil is pumped for oil transfer, based on said calculation.

[0030] According to one embodiment of the invention, the method comprises emitting a low oil level warning signal from an oil level detection device to the control device, if the oil level in the oil sump of the respective compressor reaches a predetermined low oil level value.

[0031] According to one embodiment of the invention, each oil level detection device is configured to output a low oil level warning signal if the oil level in the oil sump of the respective compressor reaches a predetermined low oil level value.

[0032] According to one embodiment of the invention, the predetermined low oil level condition is reached for a compressor if a low oil level warning signal is delivered at the output by the respective oil level detection device.

[0033] According to one embodiment of the invention, the predetermined low oil level condition is reached for a compressor if:

[0034] - the low oil level warning signal delivered at the output by the device The respective oil level detection signal is continuously output for a predetermined initial time, for example for at least ten seconds, or

[0035] - the low oil level warning signal delivered at the output by the device respective oil level detection has changed, i.e. turned on and off, more than a predetermined number of times, and for example more than ten times, in a second predetermined time, and for example during the last minute.

[0036] According to one embodiment of the invention, if the total number of compressors in the multi-compressor device is equal to two, the oil return action includes maintaining the operation of the running compressor for which a low oil level situation has been detected and starting the operation of the idle compressor, i.e., the inactive compressor for which a situation No low oil level was detected. Therefore, if the total number of compressors in the multi-compressor system is two, and one compressor is idle (50% load), the oil return action involves maintaining the operation of the running compressor for which a low oil level situation has been detected and starting the operation of the idled compressor. Through this action, oil can be supercharged, and oil can return to the multi-compressor system, specifically to the running compressor for which a low oil level situation has been detected.

[0037] According to one embodiment of the invention, if the total number of compressors in the multi-compressor device is greater than two, the oil return action includes starting the operation of at least two inactive compressors, i.e., the inactive compressor(s) for which a low oil level situation has not been detected. Through this action, oil can be supercharged and the oil can return to the multi-compressor device, and in particular to the running compressor for which a low oil level situation has been detected.

[0038] According to one embodiment of the invention, if the total number of compressors in the multi-compressor device is greater than two and if only one compressor is running and all the other compressors are inactive, the oil return action includes starting the operation of all the inactive compressors.

[0039] According to one embodiment of the invention, the oil return action includes stopping the operation of the running compressor for which a low oil level situation has been detected.

[0040] According to one embodiment of the invention, the multi-compressor device comprises at least three compressors that are coupled in parallel, and the method further comprises:

[0041] - the division of the compressors in the multiple compressor device into at least two groups, where each group includes at least one compressor and at least one group includes at least two compressors,

[0042] - the supply of the oil pumping device in the oil equalization line common and between two groups such that one of said two groups is located on one side of the oil pumping device and the other of said two groups is located on a second side of the oil pumping device,

[0043] - the supply of at least one valve located in the oil equalization line common, at least one valve being associated with a group comprising at least two compressors and being configured to control an oil flow in the common oil equalization line and between the oil pumping device and at least one compressor of the group associated with at least one valve,

[0044] the oil balancing action comprising the control of the operation of the oil pumping device and at least one valve so as to transfer oil from the oil sump of at least one compressor of the multi-compressor device for which a low oil level situation has not been detected to the oil sump of the compressor for which a low oil level situation has been detected.

[0045] Thanks to the presence of at least one valve, it is possible to precisely draw excess oil from a selected compressor and then pump the oil to a selected compressor with a low oil level. In this way, it is possible to maintain the oil level in each compressor at the desired level, which is generally above the minimum oil level and below the maximum oil level.

[0046] According to one embodiment of the invention, the oil balancing action comprises:

[0047] - if oil balancing is required between compressors from different groups, the transfer, by controlling the operation of the oil pumping device and at least one valve, of oil from the oil sump of at least one compressor, for which a low oil level situation has not been detected and belonging to a different group from the group including the compressor for which a low oil level situation has been detected, to the oil sump of the compressor for which a low oil level situation has been detected,

[0048] - if oil balancing is required between compressors in the same group, the transfer, by controlling the operation of the oil pumping device and at least one valve, of oil from the oil sump of at least one compressor, for which a low oil situation has not been detected and belonging to the same group as the compressor for which a low oil situation has been detected, to a temporary oil container and then from the temporary oil container to the oil sump of the compressor for which a low oil situation has been detected, where the temporary oil container is constituted by the oil sump of at least one compressor belonging to a different group from the group including the compressor for which a low oil situation has been detected.

[0049] In this way, it is possible to balance the oil in the multi-compressor refrigeration system without additional external oil reservoirs. When a compressor with a low oil level and a compressor with an excessive oil level are in different groups, the oil pumping device, which is located between the groups, simply pumps the oil from the group with excess oil to the group with a lack of oil. When a compressor with a low oil level and a compressor with an excessive oil level are in the same group, the oil pump first pumps the excess oil from that group to the other group (the other group acts as a temporary oil reservoir), and then brings it back to the group with the compressor at low oil level.

[0050] According to one embodiment of the invention, the method comprises dividing a multi-compressor refrigeration system into two groups.

[0051] According to one embodiment of the invention, the method further comprises:

[0052] - determining the current quantity of oil in each compressor on the output base from oil level detection devices.

[0053] According to one embodiment of the invention, the method further comprises:

[0054] - determining the quantity of oil that must be transferred to a compressor for which a low oil level situation has been detected in order to reach a predetermined oil level in said compressor.

[0055] 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. Thanks to the increased suction mass flow rate, oil can be drawn from more distant parts of the refrigeration system and returned to the multi-compressor device, and in particular to the operating compressor for which a low oil level situation has been detected.

[0056] According to one embodiment of the invention, the oil return action further comprises opening the 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 suction mass flow rate and the increase in the workload of the multi-compressor refrigeration system, the oil supercharging is improved.

[0057] According to one embodiment of the invention, the operating parameter of at least one oil pump, which is monitored, is a starting frequency of at least one oil pump to perform an oil balancing action, said starting frequency corresponding to a number of times, in a predetermined period of time, that at least one oil pump has been started to perform an oil balancing action.

[0058] According to one embodiment of the invention, the predetermined time period is between 1 minute and 5 minutes, and is in particular 3 minutes.

[0059] According to one embodiment of the invention, the predetermined value of the operating parameter of at least one oil pump is at least 2 starts of at least one oil pump in the predetermined time period.

[0060] According to one embodiment of the invention, the operating parameter of at least one oil pump, which is monitored, is an oil flow rate of at least one oil pump.

[0061] According to one embodiment of the invention, at least one oil pump comprises a variable speed oil pump.

[0062] According to one embodiment of the invention, the predetermined value of the operating parameter, which is the oil flow rate of the variable speed oil pump, is between 4 L / min and 10 L / min, in particular 6 L / min.

[0063] The present invention also relates to a multi-compressor refrigeration system comprising a control device configured to control the operation of the multi-compressor refrigeration system, and a multi-compressor device comprising:

[0064] - at least two compressors which are connected in parallel, each compressor being equipped with a suction fitting and an oil sump,

[0065] - a common suction line configured to fluidly connect the suction connections for all compressors in the multi-compressor system,

[0066] - a common oil equalization line configured to fluidly connect the oil sumps of all the compressors in the multi-compressor system,

[0067] - an oil pumping device located in the oil equalization line common and functionally connected to the control device, the oil pumping device comprising at least one oil pump and configured to perform an oil balancing action between the oil sumps of the multi-compressor device,

[0068] in which the control device is configured to:

[0069] - monitor an operating parameter of at least one oil pump, for example, under partial load conditions of the multi-compressor refrigeration system,

[0070] - perform an oil return action if the operating parameter of the less an oil pump reaches a predetermined value, where the oil return action includes the recovery of oil from the multi-compressor refrigeration system, i.e. from outside the multi-compressor device, to the multi-compressor device.

[0071] According to one embodiment of the invention, each compressor is equipped with an oil level detection device functionally connected to the control device and configured to detect an oil level in the respective oil sump, the control device being configured to:

[0072] - monitor the oil level in the oil sump of each compressor by the respective oil level detection device,

[0073] - detect, for example on the basis of a signal delivered as output by a device oil level detection, that a low oil level situation occurs in a respective compressor, if the oil level in said compressor reaches a predetermined low oil level condition, and

[0074] - perform an oil balancing action if a low oil level situation is detected for a compressor, the oil balancing action includes controlling the operation of the oil pumping device so as to transfer oil from the oil sump of at least one compressor of the multi-compressor device for which a low oil level situation has not been detected to the oil sump of the compressor for which a low oil level situation has been detected.

[0075] According to one embodiment of the invention, the multi-compressor device comprises at least three compressors which are coupled in parallel and the compressors in the multi-compressor device are divided into at least two groups, each group comprising at least one compressor and at least one group comprising at least two compressors, the oil pumping device being located between two groups such that one of said two groups is located on a first side of the oil pumping device and the other of said two groups is located on a second side of the oil pumping device, and the multi-compressor device further comprising at least one valve located in the common oil equalization line and associated with a group comprising at least two compressors,at least one valve being functionally connected to the control device and configured to control oil flow in the common oil equalization line and between the oil pumping device and at least one compressor of the group associated with at least one valve, the control device being configured to control the operation of the oil pumping device and at least one valve based on outputs from the oil level detection devices.

[0076] Thanks to such an arrangement of the oil pumping device and at least one valve, it is possible to obtain a simple and inexpensive multi-compressor system with a very limited number of additional components. The present invention makes it possible to achieve reliable oil balancing without interfering with the common suction line, and consequently, the common suction line can be designed to be fully optimized for efficiency, thus improving the efficiency of the multi-compressor refrigeration system. Furthermore, the cost of a valve is much lower than the cost of an oil pump, so it is advantageous to replace oil pumps with valves.

[0077] According to one embodiment of the invention, the compressors in the multi-compressor device are divided into two groups.

[0078] According to one embodiment of the invention, the control device is configured to control the operation of the oil pumping device and at least one valve based on outputs from oil level detection devices.

[0079] According to one embodiment of the invention, the control device is configured to determine a current quantity of oil in each compressor based on outputs from the oil level detection devices.

[0080] According to one embodiment of the invention, the control device is configured to determine the quantity of oil that must be transferred to a compressor for which a low oil level situation has been detected in order to reach a predetermined oil level in said compressor.

[0081] According to one embodiment of the invention, at least one oil pump comprises a volumetric oil pump configured to measure and deliver at the outlet a quantity of pumped oil.

[0082] According to one embodiment of the invention, at least one oil pump comprises two unidirectional oil pumps, for example two unidirectional volumetric oil pumps, which are connected in parallel.

[0083] According to one embodiment of the invention, at least one oil pump comprises a bidirectional oil pump, and for example a bidirectional volumetric oil pump.

[0084] According to one embodiment of the invention, the oil pumping device comprises only a single bidirectional oil pump. Such a configuration of a multi-compressor refrigeration system facilitates the control of the multi-compressor refrigeration system and significantly reduces its manufacturing cost.

[0085] According to one embodiment of the invention, the oil pump is an electromagnetic oil pump, and for example, a bidirectional electromagnetic positive displacement oil pump. Advantageously, the electromagnetic oil pump comprises a motor, a pumping element, and an electromagnetic coupling arranged between the pumping element and the motor. Such an oil pump provides static sealing and eliminates the risk of leaks, and furthermore, it is possible for a multi-compressor refrigeration system with such an oil pump to obtain appropriate certifications for operation in highly hazardous conditions, such as explosive atmospheres.

[0086] According to one embodiment of the invention, at least one valve comprises a normally closed solenoid valve.

[0087] According to one embodiment of the invention, the common suction line comprises a main suction line and suction branches each connecting the main suction line to the suction fitting of a respective compressor.

[0088] According to one embodiment of the invention, the common oil equalization line comprises a main oil equalization line and oil equalization branches, each connecting the main oil equalization line to the oil sump of a respective compressor, and in particular to an oil balancing connection provided on a compressor sump of a respective compressor and fluidically connected to the respective oil sump. Each oil equalization branch is in particular connected to the main oil equalization line at an oil equalization line branch.

[0089] According to one embodiment of the invention, at least one valve comprises a three-way valve located at an oil equalization line branch of the common oil equalization line and configured to control oil flow between the oil pumping device and two respective oil equalization branches of the common oil equalization line that are fluidly connected at said oil equalization line branch. In other words, said three-way valve can control oil flow between the oil pumping device and two compressors respectively fluidly connected to oil equalization branches that are fluidly connected at said oil equalization line branch. Thus, said three-way valve can control oil flow to more than one compressor.

[0090] According to one embodiment of the invention, at least one valve comprises a two-way valve located in an oil equalization branch of the common oil equalization line, which is fluidly connected to a respective compressor, and configured to control an oil flow between the oil pumping device and the respective compressor fluidly connected to said oil equalization branch. In other words, said two-way valve can control an oil flow to a single compressor.

[0091] According to one embodiment of the invention, each group comprises at least two compressors.

[0092] According to said embodiment of the invention, the multi-compressor device comprises a plurality of two-way valves each located in a respective oil equalization branch of the common oil equalization line, each two-way valve being functionally connected to the control device and being configured to control an oil flow between the oil pumping device and a respective compressor fluidly connected to said oil equalization branch, the control device being configured to control the operation of the oil pumping device and the two-way valves on the basis of outputs from the oil level sensing devices.

[0093] According to said embodiment of the invention, each oil equalization branch is provided with a respective two-way valve.

[0094] According to one embodiment of the invention, each oil level detection device is at least partially located in the respective oil sump.

[0095] According to one embodiment of the invention, each compressor comprises a compressor housing, and the respective oil housing is arranged in a lower part of said compressor housing.

[0096] According to one embodiment of the invention, the multi-compressor refrigeration system further comprises a refrigerant circulation circuit comprising successively a condenser, an electronic expansion valve, an evaporator and the multi-compressor device connected in series.

[0097] According to one embodiment of the invention, at least one compressor of the multi-compressor device is a variable speed compressor.

[0098] According to one embodiment of the invention, at least one compressor of the multi-compressor device is a fixed-speed compressor.

[0099] According to one embodiment of the invention, each compressor of the multi-compressor device is a scroll compressor. Brief description of the drawings

[0100] The following detailed description of several 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.

[0101] Fig. 1 is a schematic view of a multi-compressor refrigeration system according to a first embodiment of the invention.

[0102] Fig. 2 is a longitudinal cross-sectional view of a compressor of the multi-compressor refrigeration system of Fig. 1.

[0103] The [Fig.3] is a schematic view of a multi-compressor device of the multi-compressor refrigeration system of the [Fig.1].

[0104] The [Fig.4] is a diagram of an oil management process in the multi-compressor refrigeration system of the [Fig.1].

[0105] Fig. 5 is a schematic view of a multi-compressor device of a multi-compressor refrigeration system according to a second embodiment of the invention.

[0106] Fig. 6 is a schematic view of a multi-compressor device of a multi-compressor refrigeration system according to a third embodiment of the invention.

[0107] Fig. 7 is a schematic view of a multi-compressor device of a multi-compressor refrigeration system according to a fourth embodiment of the invention.

[0108] Fig. 8 is a schematic view of a multi-compressor device of a multi-compressor refrigeration system according to a fifth embodiment of the invention.

[0109] Fig. 9 is a schematic view of a multi-compressor device of a multi-compressor refrigeration system according to a sixth embodiment of the invention. Detailed description of the invention

[0110] Figures 1 to 3 describe a multi-compressor refrigeration system 2, according to a first embodiment of the invention, comprising a refrigerant circulation circuit 3 comprising successively a condenser 4, an electronic expansion valve 5, an evaporator 6 and a multi-compressor device 7 connected in series.

[0111] The multi-compressor device 7 comprises at least two compressors 8 which are connected in parallel. According to the embodiment shown in [Fig. 1], the multi-compressor device 7 comprises three compressors 8. However, the multi-compressor device 7 may comprise more than three compressors 8 and, for example, four or five compressors 8, or only two compressors. Each compressor 8 may, for example, be a variable-speed compressor.

[0112] Each compressor 8 comprises a compressor housing 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. The multi-compressor refrigeration system 2 can use a refrigerant with a low GWP (global warming potential) as the refrigerant gas.

[0113] Advantageously, each compressor 8 is a scroll compressor, and comprises a compression unit 13 disposed 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 volute, which is fixed relative to the respective compressor housing 9, and an orbiting volute configured to perform an orbital movement relative to the respective fixed volute during the operation of the respective compressor 8. However, according to another embodiment of the invention, the compressors 8 may be of a different type than scroll compressors, and may in particular be screw compressors or piston compressors.

[0114] Each compressor 8 also includes an oil sump 14 arranged at the lower part of the respective compressor housing 9, and an oil level detection device 15 located in the respective oil sump 14 and configured to detect an oil level in the respective oil sump 14. Each oil level detection device 15 may be an oil level sensor or an oil level switch. Advantageously, each oil level detection 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.

[0115] The multi-compressor device 7 further includes a common suction line 16 configured to fluidly connect the suction connections 11 of all the compressors 8 of the multi-compressor device 7. The common suction line 16 includes, in particular, a main suction line 17 and suction branches 18, each connecting the main suction line 17 to the suction connection 11 of a respective compressor 8. The multi-compressor device 7 also includes a common discharge line 19 configured to fluidly connect the discharge connections 12 of all the compressors 8 of the multi-compressor device 7. The common discharge line 19 includes, in particular, a main discharge line 21 and discharge branches 22, each connecting the main discharge line 21 to the discharge connection 12 of a respective compressor.

[0116] Furthermore, the multi-compressor device 7 includes a common oil equalization line 23 configured to fluidly connect the oil sumps 14 of all the compressors 8 of the multi-compressor device 7. The common oil equalization line 23 includes, in particular, a main oil equalization line 24 and oil equalization branches 25, each connecting the main oil equalization line 24 to the oil sump 14 of a respective compressor 8, and in particular to an oil balancing connection 26 provided on the compressor sump 9 of a respective compressor 8 and fluidly connected to the respective oil sump 14. Each oil equalization branch 25 is, in particular, connected to the main oil equalization line 24 at an oil equalization line branch 26.

[0117] The compressors in the multi-compressor device 7 are divided into two groups. According to the embodiment shown in [Fig. 3], one group (called the first group) comprises one compressor 8 and the other group (called the second group) comprises two compressors 8.

[0118] The multi-compressor device 7 further includes an oil pumping device 27 located in the common oil equalization line 23 and between the two groups such that one of said two groups is located on one side of the oil pumping device 27 and the other of said two groups is located on the other side of the oil pumping device 27. According to the embodiment shown in [Fig. 3], the oil pumping device 27 comprises an oil pump 28, such as a bidirectional positive displacement oil pump and, for example, a bidirectional electromagnetic positive displacement oil pump. Advantageously, the oil pump 28 is configured to measure and deliver a quantity of pumped oil.

[0119] According to the embodiment shown in [Fig.3], the multi-compressor device 7 further comprises two valves 29 located in the common oil equalization line 23 and associated with the group comprising two compressors 8. Advantageously, each valve 29 is a normally closed two-way solenoid valve and is located in an oil equalization branch 25 of the common oil equalization line 23 and therefore downstream of the respective compressor 8. Each valve 29 is thus configured to control the flow of oil between the oil pumping device 27 and the compressor 8, which is fluidly connected to the respective oil equalization branch 25. In other words, each valve 29 is configured to control the fluid communication between the oil pumping device 27 and the respective compressor 8.

[0120] The multi-compressor refrigeration system 2 further includes a control device 31 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. The control device 31 may, for example, include a microprocessor and a memory.

[0121] The control device 31 is functionally connected to each oil level sensing device 15, the oil pumping device 27 and each valve 29, and is particularly configured to control the operation of the oil pumping device 27 and the valves 29 on the basis of outputs from the oil level sensing devices 15.

[0122] [Fig.4] shows a diagram of an oil management process in the multi-compressor refrigeration system 2 of [Fig.1].

[0123] The process includes in particular:

[0124] - monitoring the oil level in the oil sump 14 of each compressor 8 by the respective oil level detection device 15,

[0125] - detection, by the control device 31 and on the basis of a signal delivered output by an oil level detection device 15, that a low oil level situation, also called an oil shortage situation, occurs in a respective compressor 8, if the oil level in said compressor 8 reaches a predetermined low oil level condition, and

[0126] - performing an oil balancing action if a low level situation Oil is detected for compressor 8, the oil balancing action includes: • if oil balancing is required between compressors from different groups, the control of the operation of the oil pumping device 27 and the valves 29 so as to transfer oil from the oil sump 14 of at least one compressor 8, for which a low oil level situation has not been detected and belonging to a different group than the group including the compressor 8 for which a low oil level situation has been detected, to the oil sump 14 of the compressor 8 for which a low oil level situation has been detected, • if oil balancing is required between compressors 8 of the same group, the control of the operation of the oil pumping device 27 and the valves 29 so as to transfer oil from the oil sump 14 of at least one compressor 8, for which a low oil situation has not been detected and belonging to the same group as the compressor 8 for which a low oil situation has been detected, to a temporary oil container and then from the temporary oil container to the oil sump 14 of the compressor 8 for which a low oil situation has been detected, where the temporary oil container is constituted by the oil sump 14 of at least one compressor 8 belonging to a different group from the group including the compressor 8 for which a low oil situation has been detected.

[0127] Advantageously, the control device 31 is configured to determine a current quantity of oil in each compressor 8 based on outputs from the oil level detection devices 15, and the quantity of oil that must be transferred to a compressor 8 for which a low oil level situation has been detected to reach a predetermined oil level in said compressor 8. On this basis, the control device 31 can operate the oil pumping device 27 to pump the exact quantity of oil required for the compressor 8 for which a low oil level situation has been detected.

[0128] According to one embodiment of the invention, the predetermined low oil level condition is reached for a compressor 8 if a low oil level warning signal is delivered at the output by the respective oil level detection device 15, i.e. if the oil level in the oil sump 14 of the respective compressor 8 reaches the predetermined low oil level value.

[0129] Advantageously, the method further comprises the detection, by the control device 31 and on the basis of a signal delivered at the output by an oil level detection device 15, that an excess oil situation occurs in a compressor respective 8, if the oil level in said compressor 8 reaches a predetermined high oil level condition. According to such an embodiment of the invention, the oil balancing action consists of:

[0130] - if oil balancing is required between 8 compressors from different groups, to control the operation of the oil pumping device 27 and the valves 29 so as to transfer oil from the oil sump 14 of at least one compressor 8, for which an excess oil situation has been detected and belonging to a different group from the group including the compressor 8 for which a low oil level situation has been detected, to the oil sump 14 of the compressor 8 for which a low oil level situation has been detected,

[0131] - if oil balancing is required between compressors 8 of the same group, control the operation of the oil pumping device 27 and the valves 29 so as to transfer oil from the oil sump 14 of at least one compressor 8, for which an excess oil situation has been detected and belonging to the same group as the compressor 8 for which a low oil situation has been detected, to a temporary oil container and then from the temporary oil container to the oil sump 14 of the compressor 8 for which a low oil situation has been detected, where the temporary oil container is constituted by the oil sump 14 of at least one compressor 8 belonging to a different group from the group comprising the compressor 8 for which a low oil situation has been detected.

[0132] According to one embodiment of the invention, the predetermined high oil level condition is reached for a compressor 8 if a high oil level warning signal is delivered at the output by the respective oil level detection device 15, i.e. if the oil level in the oil sump 14 of the respective compressor 8 reaches a predetermined high oil level value.

[0133] According to one embodiment of the invention, the method further comprises calculating the risk of a low oil level occurring in a compressor 8 under a given operating condition, and controlling the operation of the oil pumping device 27 and the valves 29 based on this calculation in order to supply more oil to a compressor 8 which has a high risk of a low oil level occurring, and / or controlling the operation of the oil pumping device 27 and the valves 29 so as not to draw oil from a compressor 8 which has a high risk of a low oil level occurring.

[0134] Several scenarios of the oil management process in the multi-compressor refrigeration system 2 of [Fig.1] are disclosed below.

[0135] Scenario 1: A low oil level detected in the first group and an excessive oil level detected in the second group

[0136] In scenario 1, there is a need to transfer oil between two different groups of compressors 8, and the oil must be transferred from the second group, which includes two compressors 8 (named second and third compressors), to the first group, which includes only one compressor 8 (named first compressor).

[0137] In such a situation, the oil balancing action includes opening the valve 29 associated with the second (or third) compressor and operating the oil pump 28 to begin pumping oil from the second group to the first group, and more specifically from the second (or third) compressor in the second group to the first compressor in the first group. Once the oil level detection device 15 located in the oil sump 14 of the first compressor detects that the oil level in said oil sump 14 has reached the predetermined oil level, the oil balancing action includes stopping the operation of the oil pump 28 and closing the valve 29 associated with the second (or third) compressor.

[0138] It may happen that the quantity of oil in the second (or third) compressor is insufficient for the first compressor to reach the predetermined oil level. In such a situation, the oil balancing action further includes the operation of the oil pump 28 and the valves 29 to also transfer oil from the third (or second) compressor to the first compressor.

[0139] It may also happen that the control device 31 determines a high risk of a low oil level occurring in the second (or third) compressor. In such a situation, the oil balancing action will include controlling the operation of the oil pump 28 and the valves 29 in order to transfer oil from the other compressor 8 with a high oil level, i.e., from the third (or second) compressor.

[0140] The present invention also envisages transferring oil from more than one oil sump 14 at the same time.

[0141] Scenario 2: An excessive oil level detected in the first group and a low oil level detected in the second group

[0142] In this scenario, the oil must be transferred from the first group to the second group. Since the first group comprises only one compressor 8, the oil will be transferred from the first compressor to the second compressor and / or the third compressor.

[0143] In such a situation, the oil balancing action includes opening the valve 29 associated with the second compressor (and / or the third compressor) and operating the oil pump 28 to begin pumping oil into the direction from the first group to the second group, and more specifically from the first compressor in the first group to the second compressor (and / or the third compressor) in the second group. Once the oil level detection device 15 located in the oil sump 14 of the second compressor (and / or the third compressor) detects that the oil level in said oil sump 14 has reached the predetermined oil level, the oil balancing action includes stopping the operation of the oil pump 28 and closing the valve 29 associated with the second compressor (and / or the third compressor).

[0144] Scenario 3: Low oil level and excessive oil level detected in the same group

[0145] In this scenario, oil must be transferred within the same group. To achieve this, an intermediate step has been introduced. Suppose that an excessive oil level has been detected in the second compressor and that a low oil level has been detected in the third compressor.

[0146] First, the oil balancing action includes controlling the operation of the oil pump 28 and the valves 29 associated with the second and third compressors in order to transfer excess oil from the second compressor to a temporary oil reservoir formed by the oil sump 14 of the first compressor. In particular, the oil balancing action includes opening the valve 29 associated with the second compressor and operating the oil pump 28 to pump oil from the second group to the first group, and specifically from the oil sump 14 of the second compressor to the oil sump 14 of the first compressor. Once a sufficient quantity of oil has been transferred to the oil sump 14 of the first compressor, the oil balancing action includes stopping the operation of the oil pump 28 and closing the valve 29 associated with the second compressor.

[0147] Next, the oil balancing action includes opening the valve 29 associated with the third compressor and operating the oil pump 28 to pump oil from the first group to the second group, namely from the oil sump 14 of the first compressor to the oil sump 14 of the third compressor. Once the oil level detection device 15 located in the oil sump 14 of the third compressor detects that the oil level in said oil sump 14 has reached the predetermined oil level, the oil balancing action includes stopping the operation of the oil pump 28 and closing the valve 29 of the third compressor.

[0148] It is also envisaged by the present invention that the control device 31 can operate the oil pump 28 and the valves 29 to transfer oil to a compressor 8 even if a low oil level situation has not been detected for said compressor 8 at that time because said compressor 8 is about to reach a given operating condition where there is a high risk of a low oil level occurring. In such a situation, the method may include the control, by the control device 31, of the oil pump 28 and the valves 29 to transfer additional oil to such compressor 8 before said compressor 8 reaches said operating condition.

[0149] Returning to [Fig. 1], the oil circulates together with the refrigerant throughout the multi-compressor refrigeration system 2, so that the oil is also present, for example, in the condenser 4 and in the evaporator 6. During normal operation of the multi-compressor refrigeration system 2, the oil returns to the oil sumps 14. However, in some cases, the oil may be trapped outside the oil sumps 14 and the multi-compressor device 7, for example in the condenser 4 or in the evaporator 6. This is mainly the case when the multi-compressor refrigeration system 2 is operating under a partial load condition.

[0150] If oil is trapped outside the multi-compressor device 7 and / or the oil sumps 14, there is not a sufficient quantity of oil in at least one oil sump 14. When the multi-compressor refrigeration system 2 initiates an oil balancing action for the compressor 8 for which a low oil level situation has been detected, the oil pump 28 is activated to balance the oil between the oil sumps 14. However, since there is not a sufficient quantity of oil, the oil pump 28 is unable to perform this oil balancing action. This inability of the oil pump 28 to perform the oil balancing action can be detected by monitoring, via the control device 31, at least one operating parameter of the oil pump 28.

[0151] Thus, the process advantageously comprises:

[0152] - monitoring an operating parameter of the oil pump 28, for example in a partial load condition of the multi-compressor refrigeration system 2, and

[0153] - the implementation of an oil return action if the operating parameter of the oil pump 28 reaches a predetermined value, where the oil return action includes the recovery of oil from the multi-compressor refrigeration system 2, i.e. from outside the multi-compressor device 7, to the multi-compressor device 7.

[0154] Sometimes, when a significant amount of oil is trapped outside the multi-compressor device 7, the amount of oil contained within the multi-compressor device 7 may be insufficient to ensure a correct oil level in each oil sump 14, and a low oil level situation may be detected. in several oil sumps 14 simultaneously. Then, it may be necessary for the oil pump 28 to supply a significant quantity of oil to all the oil sumps 14 with a detected low oil level in order to overcome the low oil situation. The standard oil flow rate of the oil pump 28 used in such a multi-compressor refrigeration system, which is generally 2 L / min to 4 L / min, imposes certain limitations, especially if it is necessary to supply oil quickly and efficiently to all the respective oil sumps 14. In the case where the oil pump 28 is a variable-speed oil pump, the variable-speed oil pump can increase its rotational speed (and therefore its oil flow rate, for example up to 6 L / min) in an attempt to achieve a more efficient oil balancing action.If the oil flow rate is the operating parameter of the oil pump 28 that is monitored by the method according to the present invention, then an increase in the oil flow rate beyond the predetermined value, for example above 4 L / min, can be identified as an indication that oil is trapped outside the multi-compressor device 7, and an oil return action can be performed based on said increase in oil flow rate. Once a sufficient quantity of oil is returned to the multi-compressor device 7, the oil balancing action can be performed for the compressor 8 for which a low oil level situation was previously detected. For example, the predetermined value of the oil flow rate of the oil pump 28 can be between 4 L / min and 10 L / min, in particular 6 L / min.

[0155] Sometimes, when a significant amount of oil is trapped outside the multi-compressor device 7, the amount of oil contained in the oil sump 14 of each compressor 8, for each of which a low oil situation has not been detected, may be insufficient to transfer a sufficient amount of oil to the oil sump 14 of the compressor 8 for which a low oil situation has been detected.In this case, the oil pump 28 can, for example, be started to transfer a first quantity of oil from a first compressor, for which a low oil level situation has not been detected, until a low oil level is detected in said first compressor, then the oil pump 28 can be started to transfer a second quantity of oil from a second compressor, for which a low oil level situation has not been detected, until a low oil level is detected in said second compressor, and finally the oil pump 28 can be started to transfer a third quantity of oil from a third compressor, for which a low oil level situation has not been detected.

[0156] In the case where the starting frequency of the oil pump 28 (which corresponds to a number of times, in a predetermined period of time, that the oil pump The operating parameter of the oil pump 28, which is monitored by the method according to the present invention, is the predetermined value. If the starting frequency of the oil pump 28 reaches the predetermined value, it can be deduced that oil is trapped outside the multi-compressor device 7, and an oil return action can be performed based on said starting frequency. For example, the predetermined time period can be from 2 to 5 minutes, and the predetermined value of the operating parameter can be at least 2 starts of the oil pump 28 within the predetermined time period.

[0157] Therefore, by monitoring an operating parameter of the oil pump 28, the method according to the present invention can easily detect when oil is trapped in the multi-compressor refrigeration system 2 outside the multi-compressor device 7 and perform an oil return action only if necessary.

[0158] According to one embodiment of the invention, the oil return action comprises increasing the workload of the multi-compressor refrigeration system 2, for example, for a predetermined period. 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 thus to the oil sumps 14 of the compressors 8. Indeed, due to the increased workload, the demand for oil supply is increased, and oil can be drawn from more distant parts of the multi-compressor refrigeration system 2.

[0159] According to one embodiment of the invention, the oil return action includes controlling the multi-compressor refrigeration system 2 to operate, for example for a predetermined time, at full workload, i.e. in a condition in which all the compressors 8 of the multi-compressor device 7 are operating.

[0160] According to one embodiment of the invention, the oil return action includes controlling the multi-compressor refrigeration system 2 to restart, for example for a predetermined time, the multi-compressor device 7 with a different operating configuration, i.e. by changing the ON / OFF configurations of the compressors 8 in the multi-compressor device 7.

[0161] For example, if the total number of compressors 8 in the multiple compressor device 7 is greater than two, the oil return action may include stopping the operation of the running compressor for which a low oil level situation has been detected and starting the operation of at least two inactive compressors, that is, the inactive compressor(s) for which a low oil level situation has not been detected. Thanks to this action, oil can be supercharged and the oil can return to the multi-compressor device 7 and particularly to the running compressor for which a low oil level situation has been detected.

[0162] According to another embodiment of the invention, if the total number of compressors in the multi-compressor device is greater than two and if only one compressor is running and all the other compressors are inactive, the oil return action may include stopping the operation of the running compressor for which a low oil level situation has been detected and starting the operation of all the inactive compressors.

[0163] According to another embodiment of the invention, the oil return action includes opening the electronic expansion valve 5 of the multi-compressor refrigeration system 2 in order to increase the suction mass flow rate of the multi-compressor device 7. Thanks to the increase in the suction mass flow rate, oil can be taken from more distant parts of the multi-compressor refrigeration system 2 and returned to the multi-compressor device 7 and in particular to the running compressor for which a low oil level situation has been detected.

[0164] Figure 5 discloses a multi-compressor refrigeration system 2 according to a second embodiment of the invention which differs from the first embodiment essentially in that the multi-compressor device 7 has four compressors 8 and each of the first and second groups comprises two compressors 8, and in that each of the oil equalization branches 25 is equipped with a respective valve 29, and in particular with a normally closed two-way solenoid valve 29.

[0165] On one side of the oil pump 28 are the first compressor and the second compressor which belong to the first group, and on the other side of the oil pump 28 are the third compressor and the fourth compressor which belong to the second group.

[0166] The oil balancing operation for the multi-compressor refrigeration system 2 according to the second embodiment of the invention is very similar to that described above for the first embodiment of the invention, and will therefore not be disclosed hereafter. In this situation, the oil can be transferred simultaneously to / from two compressors 8.

[0167] Figure 6 discloses a multi-compressor refrigeration system 2 according to a third embodiment of the invention which differs from the second embodiment of essentially the second group comprises three 8 compressors.

[0168] The oil balancing operation for the multi-compressor refrigeration system 2 according to the third embodiment of the invention is very similar to that described above for the first embodiment of the invention, and will therefore not be disclosed hereafter. In this situation, the oil can be transferred simultaneously to / from three compressors 8.

[0169] Figure 7 discloses a multi-compressor refrigeration system 2 according to a fourth embodiment of the invention which differs from the first embodiment essentially in that the multi-compressor device 7 comprises a single valve 29 (instead of two normally closed two-way solenoid valves) and said valve 29 is a three-way valve located at an oil equalization line branch 26 of the common oil equalization line 23. The three-way valve is particularly configured to control an oil flow between the oil pumping device 27 and the two oil equalization branches 25 of the common oil equalization line 23 which are fluidly connected at said oil equalization line branch 26.

[0170] Replacing two two-way valves with one three-way valve simplifies the multi-compressor refrigeration system 2 according to the invention and also reduces the overall cost.

[0171] It is also envisaged by the present invention to replace the two-way valves with three-way valves in the second and third embodiments of the invention described above.

[0172] Figure 8 discloses a multi-compressor refrigeration system 2 according to a fifth embodiment of the invention which differs from the fourth embodiment essentially in that the oil pumping device 27 comprises (instead of a bidirectional oil pump) two single-way oil pumps 28, for example two single-way positive displacement oil pumps which are connected in parallel.

[0173] It is also envisaged by the present invention that such an oil pumping device 27 may replace the oil pumping devices 27 in other embodiments described above.

[0174] Based on the embodiments described above, a person skilled in the art can easily implement the present invention in a multi-compressor refrigeration system 2 comprising any number of compressors 8. Furthermore, a person skilled in the art can easily combine the described features in relation to different embodiments, unless otherwise clearly stated.

[0175] Figure 9 discloses a multi-compressor refrigeration system 2 according to a sixth embodiment of the invention which differs from the first embodiment essentially in that the multi-compressor device 7 comprises only two compressors 8, one for each group of compressors, and does not include a valve 29.

[0176] According to this embodiment of the invention, if only one compressor is running (50% load), the oil return action comprises maintaining the operation of the running compressor for which a low oil level situation has been detected and starting the operation of the inactive compressor, i.e., the inactive compressor for which a low oil level situation has not been detected. Through this action, oil can be supercharged and the oil can return to the multi-compressor device 7 and particularly to the running compressor for which a low oil level situation has been detected.

[0177] It is understood that the described process is based directly on monitoring an operating parameter of the oil pump, which operation may itself be based on monitoring signals from oil level detection devices provided on the compressors 8. This makes this process indirectly dependent on the signals from said oil level detection devices.

[0178] Of course, the invention is not limited to the embodiments described above by way of non-limiting examples, but on the contrary encompasses all embodiments.

Claims

Demands

1. A method for managing oil in a multi-compressor refrigeration system (2) comprising a control device (31) configured to control the operation of the multi-compressor refrigeration system (2) and a multi-compressor device (7) comprising at least two compressors (8) that are coupled in parallel, each compressor (8) being equipped with a suction connection (11) and an oil sump (14), the oil sumps (14) of all the compressors (8) of the multi-compressor device (7) being fluidly connected by a common oil equalization line (23) and the suction connections of all the compressors of the multi-compressor device (7) being fluidly connected by a common suction line,in which the multi-compressor refrigeration system (2) further comprises an oil pumping device (27) located in the common oil equalization line (23) and functionally connected to the control device (31), the oil pumping device (27) comprising at least one oil pump (28) and being configured to perform an oil balancing action between the oil sumps (14) of the multi-compressor device (7), the method comprising: - monitoring an operating parameter of at least one oil pump (28), - performing an oil return action if the operating parameter of at least one oil pump (28) reaches a predetermined value, wherein the oil return action comprises the recovery of oil from the multi-compressor refrigeration system (2) to the multi-compressor device (7).

2. A method according to claim 1, wherein the oil return action includes increasing the workload of the multi-compressor refrigeration system (2).

3. A method according to claim 2, wherein the oil return action includes controlling the multi-compressor refrigeration system (2) to operate at full workload.

4. A method according to any one of claims 1 to 3, wherein the oil return action comprises the system control multi-compressor refrigeration (2) to restart the multi-compressor device (7) with a different operating configuration.

5. A method according to claim 4, wherein the oil return action includes changing the ON / OFF configurations of the compressors (8) in the multi-compressor device (7).

6. A method according to any one of claims 1 to 5, wherein each compressor (8) is equipped with an oil level detection device (15) functionally connected to the control device (21) and configured to detect an oil level in the respective oil sump (14), the method further comprising: - monitoring an oil level in the oil sump (14) of each compressor (8) by the respective oil level detection device (15), - detecting that a low oil level situation occurs in a running compressor (8), if the oil level in said running compressor (8) reaches a predetermined low oil level condition, - performing an oil balancing action if a low oil level situation is detected for a compressor (8),the oil balancing action comprising controlling the operation of the oil pumping device (27) so as to transfer oil from the oil sump (14) of at least one compressor (8) of the multi-compressor device (7) for which a low oil level situation has not been detected to the oil sump (14) of the compressor (8) for which a low oil level situation has been detected.

7. A method according to claim 6, wherein the multi-compressor device (7) comprises at least three compressors (8) which are coupled in parallel, the method further comprising: - dividing the compressors (8) in the multi-compressor device (7) into at least two groups, where each group comprises at least one compressor (8) and at least one group comprises at least two compressors (8), - supplying the oil pumping device (27) in the common oil equalization line (23) and between two groups such that one of said two groups is located on a first side of the

8. oil pumping device (27) and the other of said two groups is located on a second side of the oil pumping device, - the supply of at least one valve (29) located in the common oil equalization line (23), the at least one valve (29) being associated with a group comprising at least two compressors (8) and being configured to control an oil flow in the common oil equalization line (23) and between the oil pumping device (27) and at least one compressor (8) of the group associated with the at least one valve (29), the oil balancing action including the control of the operation of the oil pumping device (27) and at least one valve (29) so as to transfer oil from the oil sump (14) of at least one compressor (8) of the multi-compressor device (7) for which a low oil level situation has not been detected to the oil sump (14) of the compressor (8) for which a low oil level situation has been detected. A method according to claim 7, wherein the oil balancing action comprises: - if oil balancing is required between compressors (8) from different groups, the transfer, by controlling the operation of the oil pumping device (27) and at least one valve (29), of oil from the oil sump (14) of at least one compressor (8), for which a low oil level situation has not been detected and belonging to a different group than the group including the compressor (8) for which a low oil level situation has been detected, to the oil sump (14) of the compressor (8) for which a low oil level situation has been detected, - if oil balancing is required between compressors (8) of the same group, the transfer, by controlling the operation of the oil pumping device (27) and at least one valve (29), of oil from the oil sump (14) of at least one compressor (8), for which a low oil level situation has not been detected and belonging to the same group as the compressor (8) for which a low oil level situation has been detected, to a temporary oil container and then from the temporary oil container to the oil sump (14) of the compressor (8) for which a low oil level situation has been detected, where the temporary oil container is constituted by the oil sump (14) of at least one compressor (8) belonging to a different group from the group including the compressor (8) for which a low oil level situation was detected.

9. A method according to any one of claims 6 to 8, further comprising: - determining a current quantity of oil in each compressor (8) based on outputs from the oil level detection devices (15).

10. A method according to any one of claims 6 to 9, further comprising: - determining a quantity of oil to be transferred to a compressor (8) for which a low oil level situation has been detected in order to reach a predetermined oil level in said compressor (8).

11. A method according to any one of claims 1 to 10, wherein the operating parameter of at least one oil pump (28), which is monitored, is a starting frequency of at least one oil pump (28) to perform an oil balancing action, said starting frequency corresponding to a number of times, in a predetermined period of time, that at least one oil pump (28) has been started to perform an oil balancing action.

12. A method according to claim 11, wherein the predetermined time period is between 1 minute and 5 minutes.

13. A method according to claim 11 or 12, wherein the predetermined value of the operating parameter of at least one oil pump (28) is at least 2 starts of at least one oil pump (28) in the predetermined time period.

14. A method according to any one of claims 1 to 13, wherein at least one oil pump (28) comprises a variable speed oil pump.

15. A method according to claim 14, wherein the operating parameter of at least one oil pump (28), which is monitored, is an oil flow rate from at least one oil pump (28).

16. A method according to claim 15, wherein the predetermined value of the operating parameter, which is the oil flow rate, is between 4 L / min and 10 L / min.

17. A multi-compressor refrigeration system (2) comprising a control device (31) configured to control the

18. operation of the multi-compressor refrigeration system (2), and a multi-compressor device (7) comprising: - at least two compressors (8) which are coupled in parallel, each compressor (8) being equipped with a suction connection (11) and an oil sump (14), - a common suction line (16) configured to fluidly connect the suction fittings (11) of all the compressors (8) of the multi-compressor device (7), - a common oil equalization line (23) configured to fluidly connect the oil sumps (14) of all the compressors (8) of the multi-compressor device (7), - an oil pumping device (27) located in the common oil equalization line (23) and functionally connected to the control device (31), the oil pumping device (27) comprising at least one oil pump (28) and being configured to perform an oil balancing action between the oil sumps of the multi-compressor device (7), in which the control device (21) is configured to: - monitor an operating parameter of at least one oil pump (28), - to perform an oil return action if the operating parameter of at least one oil pump (28) reaches a predetermined value, where the oil return action comprises the recovery of oil from the multi-compressor refrigeration system (2) to the multi-compressor device (7). Multi-compressor refrigeration system (2) according to claim 17, wherein each compressor (8) is equipped with an oil level detection device (15) functionally connected to the control device (31) and configured to detect an oil level in the respective oil sump (14), the control device (31) being configured to: - monitor the oil level in the oil sump (14) of each compressor (8) using the respective oil level detection device (15), - detect that a low oil level situation occurs in a respective compressor (8), if the oil level in said compressor (8) reaches a predetermined low oil level condition, and

19. - to carry out an oil balancing action if a low oil situation is detected for a compressor (8), the oil balancing action including the control of the operation of the oil pumping device (27) so as to transfer oil from the oil sump (14) of at least one compressor (8) of the multi-compressor device (7) for which a low oil situation has not been detected to the oil sump (14) of the compressor (8) for which a low oil situation has been detected. Multi-compressor refrigeration system (2) according to claim 18, wherein the multi-compressor device (7) comprises at least three compressors (8) which are coupled in parallel and the compressors (8) in the multi-compressor device (7) are divided into at least two groups, each group comprising at least one compressor (8) and at least one group comprising at least two compressors (8), the oil pumping device (27) being located between two groups such that one of said two groups is located on one first side of the oil pumping device (27) and the other of said two groups is located on a second side of the oil pumping device (27), and the multi-compressor device (7) further comprising at least one valve (29) located in the common oil equalization line (23) and associated with a group comprising at least two compressors (8),at least one valve (29) being functionally connected to the control device (31) and being configured to control an oil flow in the common oil equalization line (23) and between the oil pumping device (27) and at least one compressor (8) of the group associated with at least one valve (29), the control device (31) being configured to control the operation of the oil pumping device (27) and at least one valve (29) based on outputs from the oil level sensing devices (15).