Multi-compressor refrigeration system with common oil equalization line provided with oil pumping equipment

Abstract

The multi-compressor refrigeration system comprises a controller and a multi-compressor device (7) comprising at least three compressors (8) coupled in parallel, each compressor (8) being equipped with an oil sump and an oil level detection device (15), the compressors (8) being divided into two groups, each group comprising at least one compressor (8); the suction connections (11) of all compressors (8) are fluidly connected via a common suction line (16) and the oil grooves of all compressors (8) are fluidly connected via a common oil equalization line (23). The multi-compressor device (7) comprises an oil pumping device (27) located in the common oil equalization line (23) and between the two groups and at least one valve (29) located in the common oil equalization line (23) and configured for controlling an oil flow between the oil pumping device (27) and the at least one compressor (8) of the group associated with the at least one valve (29).

Description

Technical Field

[0001] The present invention relates to a multi-compressor refrigeration system and a method for balancing oil in a multi-compressor refrigeration system. Background Art

[0002] Good oil balance is an important issue associated with multi-compressor systems.

[0003] The most common solution includes providing an oil equalization line between the compressors, an oil level detection device in each compressor, and a pump for providing oil flow in the oil equalization line between the compressors, and a controller for controlling the operation of the multi-compressor system. The general idea is that once the oil level detection device will detect a low oil level in a particular compressor, the controller can operate a corresponding oil pump in the corresponding oil equalization line and pump some oil from the compressor with the high oil level to the compressor with the low oil level.

[0004] While this simple solution is sufficient for a multi-compressor system with a relatively small number of compressors, such as 2 or 3, implementing this solution into a multi-compressor system with a larger number of compressors becomes very difficult and, in particular, becomes more expensive. The price of the oil pump in particular has a great influence on the final cost of the multi-compressor system.

[0005] A good oil balance in a double series configuration is usually not too difficult to achieve and in most cases can be achieved by adding passive features in the suction line (suction gasket, flow restrictor…). This has a limited impact on the pressure drop and the overall refrigeration system efficiency. In this case, external actuators or actuators like oil pumps are less needed.

[0006] However, when it comes to larger, multi-compressor refrigeration systems, the situation can change dramatically. In a triple configuration with passive devices embedded in the suction line, this pressure drop in the refrigeration system has a negative impact on efficiency and the use of external actuators or actuators (such as oil pumps) starts to become very valuable.

[0007] DE1915174 A1 discloses an idea for oil level compensation, in which each compressor in a multi-compressor system has an oil level sensor and an oil pump. A controller drives or actuates the oil flow from the compressor with excess oil to the compressor with low oil level based on the output from the oil level sensor. However, this solution is complex and expensive because it requires as many pumps as compressors.

[0008] EP0840870 A1 discloses an oil level control device for controlling the oil level in each compressor of a multi-compressor system. In particular, each compressor is fluidically connected to an additional oil reservoir, and the oil level control device is capable of maintaining the oil level in the compressor between a predetermined minimum level and a predetermined maximum level. In the event that the oil level in the compressor drops below the predetermined minimum level, oil is added to the compressor from the additional oil reservoir. In the event that the oil level in the compressor exceeds the predetermined maximum level, oil is returned from the compressor to the additional oil reservoir. However, this solution is also complex and expensive because it requires an additional oil reservoir and piping.

[0009] CN100516682C discloses the following concept: a multi-compressor system is divided into groups, and each group has an oil pump, wherein each group includes at least two compressors. This arrangement allows oil to be balanced between the groups. However, it does not solve the problem if only certain compressors in the group need oil. Therefore, this arrangement may lead to excessive oil levels in the compressors.

[0010] JPH04116349A discloses a multi-compressor system with an oil pump capable of switching its delivery direction. However, in this arrangement, the distance between the oil pump and the compressor is large, and thus affects efficiency. In addition, this solution does not solve the problem of a large number of compressors in a multi-compressor system. In addition, since direct oil flow between compressors is not feasible, an additional oil tank is required. Summary of the invention

[0011] An object of the present invention is to provide an improved multi-compressor refrigeration system which is capable of overcoming the disadvantages encountered in conventional multi-compressor refrigeration systems.

[0012] In particular, the object of the invention is to provide a simple, reliable and inexpensive multi-compressor refrigeration system while ensuring reliable oil balancing even if the multi-compressor system comprises a large number of compressors.

[0013] According to the present invention, the multi-compressor refrigeration system comprises a controller and a multi-compressor device, wherein the controller is configured to control the operation of the multi-compressor refrigeration system, and the multi-compressor device comprises:

[0014] - at least three compressors connected in parallel, each compressor being equipped with a suction connection, an oil sump and an oil level detection device, the oil level detection device being operatively connected to the controller and configured for detecting the oil level in the respective oil sump, wherein 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;

[0015] - a common suction line configured for fluidically connecting the suction connections of all compressors of the multi-compressor installation;

[0016] - a common oil equalization line configured for fluidly connecting the oil sumps of all compressors of the multi-compressor installation;

[0017] - an oil pumping device located in the common oil equalization line and operatively connected to the controller, the oil pumping device comprising at least one oil pump,

[0018] Characterized in that the oil pumping device is located between two groups, so that one of the two groups is located on a first side of the oil pumping device, and the other of the two groups is located on a second side of the oil pumping device, and wherein the multi-compressor device also includes at least one valve, the at least one valve is located in the common oil equalization pipeline and is associated with a group including at least two compressors, the at least one valve is operably connected to the controller, and the at least one valve is configured to control the oil flow in the common oil equalization pipeline and between the oil pumping device and at least one compressor of the group associated with the at least one valve, and the controller is configured to control the operation of the oil pumping device and the at least one valve based on the output from the oil level detection device.

[0019] Due to this arrangement of the oil pumping device and the at least one valve, a simple and inexpensive multi-compressor system with a very limited number of additional equipment can be obtained. The invention allows a reliable oil balance to be obtained without disturbing the common suction line, and thus the common suction line can be designed to be fully optimized for efficiency, thereby 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 beneficial to replace the oil pump with a valve.

[0020] The multi-compressor refrigeration system may further include one or more of the following features, alone or in combination.

[0021] According to an embodiment of the invention, the compressors in the multi-compressor device are divided into two groups.

[0022] According to an embodiment of the invention, each compressor comprises an oil balancing connection, the common oil balancing line being configured for fluidly connecting the oil balancing connections of all compressors of the multi-compressor arrangement.

[0023] According to an embodiment of the invention, the controller is configured to determine a current amount of oil in each compressor based on an output from the oil level detection device.

[0024] According to an embodiment of the present invention, the controller is configured to:

[0025] - monitoring the oil level in the oil tank of each compressor by means of a corresponding oil level detection device;

[0026] - detecting the occurrence of a low oil level condition (also referred to as a lack of oil condition) in the respective compressor, e.g. based on a signal output from an oil level detection device, if the oil level in the respective compressor reaches a predetermined low oil level condition; and

[0027] -If a low oil level condition is detected for a compressor, performing an oil balancing action, the oil balancing action comprising controlling the operation of the oil pumping device and the at least one valve so as to transfer oil from the oil sump of at least one compressor of the multi-compressor device in which a low oil level condition has not been detected to the oil sump of a compressor in which a low oil level condition has been detected.

[0028] According to an embodiment of the present 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 corresponding compressor reaches a predetermined low oil level value.

[0029] According to an embodiment of the present invention, if a low oil level warning signal is output from a corresponding oil level detection device, a predetermined low oil level condition is reached for the compressor.

[0030] According to an embodiment of the present invention, a predetermined low oil level condition is reached for the compressor if:

[0031] - the low oil level warning signal output from the corresponding oil level detection device is continuously output for a first predetermined time, and for example for at least 10 seconds; or

[0032] - The low oil level warning signal output from the corresponding oil level detection device has been switched (ie, has been switched on and off) more than a predetermined number of times (and, for example, more than ten times) within the second predetermined time (and, for example, during the last minute).

[0033] According to an embodiment of the present invention, the controller is configured to detect that an oil excess condition occurs in the corresponding compressor if the oil level in the corresponding compressor reaches a predetermined high oil level condition.

[0034] According to an embodiment of the present invention, the oil balancing action includes controlling the operation of the oil pumping device and the at least one valve so as to transfer oil from the oil tank of at least one compressor of the multi-compressor device in which an oil excess condition has been detected to the oil tank of a compressor in which a low oil level condition has been detected.

[0035] According to an embodiment of the present invention, the oil balancing action includes:

[0036] - if oil balancing between compressors from different groups is required, controlling the operation of the oil pumping device and the at least one valve so as to transfer oil from the oil sump of at least one compressor in which a low oil level condition has not been detected and which belongs to a group different from the group including the compressor in which a low oil level condition has been detected, to the oil sump of the compressor in which a low oil level condition has been detected;

[0037] -If oil balancing between compressors from the same group is required, controlling the operation of the oil pumping device and the at least one valve so as to transfer oil from the oil sump of at least one compressor in which a low oil level situation has not been detected and which belongs to the same group as the compressor in 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 of the compressor in which a low oil level situation has been detected, wherein the temporary oil container is constituted by the oil sump of at least one compressor belonging to a group different from the group including the compressor in which a low oil level situation has been detected.

[0038] According to an embodiment of the invention, the controller is configured to determine an amount of oil that must be delivered to a compressor in which a low oil level condition has been detected in order to reach a predetermined oil level in the compressor.

[0039] According to an embodiment of the invention, the controller is configured for calculating a risk of low oil level occurring in the compressor under given operating conditions and controlling the operation of the oil pumping device and the at least one valve based on the calculation.

[0040] According to an embodiment of the present invention, the controller is configured to: control the oil pumping device and the at least one valve so as to transfer more oil to the compressor with a high risk of low oil level; and / or control the oil pumping device and the at least one valve so as not to extract oil from the compressor with a high risk of low oil level.

[0041] According to an embodiment of the invention, the at least one oil pump comprises a positive displacement oil pump configured to measure and output a pumped oil amount.

[0042] According to an embodiment of the present invention, the oil pump is a variable speed oil pump.

[0043] According to an embodiment of the present invention, the at least one oil pump comprises two unidirectional oil pumps connected in parallel, such as two unidirectional positive displacement oil pumps.

[0044] According to an embodiment of the present invention, the at least one oil pump comprises a bidirectional oil pump, and is, for example, a bidirectional positive displacement oil pump.

[0045] According to an embodiment of the present invention, the oil pumping device comprises only one bidirectional oil pump. Such a configuration of the multi-compressor refrigeration system simplifies the control of the multi-compressor refrigeration system and significantly reduces its manufacturing cost.

[0046] According to an 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 member, and an electromagnetic coupling arranged between the pumping member and the motor. Such an oil pump provides a static seal and eliminates the risk of leakage, and furthermore a multi-compressor refrigeration system having such an oil pump can receive appropriate certification for working in highly hazardous conditions such as explosive atmospheres.

[0047] According to an embodiment of the invention, the at least one valve comprises a normally closed solenoid valve.

[0048] According to an embodiment of the present invention, the common suction line comprises a main suction line and various suction branches, each suction branch connecting the main suction line to a suction connection of a corresponding compressor.

[0049] According to an embodiment of the present invention, the common oil equalization line comprises a main oil equalization line and respective oil equalization branches, each oil equalization branch connecting the main oil equalization line to the oil sump of the corresponding compressor, and in particular to an oil equalization connection provided on the compressor housing of the corresponding compressor and fluidically connected to the corresponding oil sump. In particular, each oil equalization branch is connected to the main oil equalization line at the oil equalization line bifurcation.

[0050] According to an embodiment of the present invention, the at least one valve comprises a three-way valve, the three-way valve is located at the oil equalization line bifurcation of the common oil equalization line, and the three-way valve is configured to control the oil flow between two corresponding oil equalization branches of the common oil equalization line and the oil pumping device that are fluidly connected at the oil equalization line bifurcation. In other words, the three-way valve can control the oil flow between two compressors that are fluidly connected at the oil equalization line bifurcation and are respectively fluidly connected to the oil equalization branches and the oil pumping device. Therefore, the three-way valve can control the oil flow to more than one compressor.

[0051] According to an embodiment of the invention, the at least one valve comprises a two-way valve, the two-way valve being located in an oil equalization branch of the common oil equalization line fluidically connected to the respective compressor, and the two-way valve being configured to control the oil flow between the oil pumping device and the respective compressor fluidically connected to the oil equalization branch. In other words, the two-way valve can control the oil flow to only one compressor.

[0052] According to an embodiment of the invention, each group comprises at least two compressors.

[0053] According to the embodiment of the present invention, the multi-compressor device includes a plurality of two-way valves, each of which is located in a corresponding oil equalization branch of the common oil equalization pipeline, each of which is operably connected to the controller, and each of which is configured to control the oil flow between the oil pumping device and the corresponding compressor fluidically connected to the oil equalization branch, and the controller is configured to control the operation of the oil pumping device and the two-way valve based on the output from the oil level detection device.

[0054] According to the embodiment of the present invention, each oil equalization branch is provided with a respective two-way valve.

[0055] According to an embodiment of the invention, at least one compressor of the multi-compressor device comprises at least two oil level detection devices.

[0056] According to an embodiment of the present invention, the oil level detection device of each compressor includes an oil level sensor or an oil level switch.

[0057] According to an embodiment of the invention, each compressor of the multi-compressor device is a scroll compressor.

[0058] According to an embodiment of the invention, each oil level detection device is at least partially located in a corresponding oil sump.

[0059] According to an embodiment of the invention, each compressor comprises a compressor housing, and the respective oil sump is arranged in a lower part of said compressor housing.

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

[0061] According to an embodiment of the invention, at least one compressor of the multi-compressor device is a variable speed compressor.

[0062] According to an embodiment of the invention, at least one compressor of the multi-compressor device is a fixed speed compressor.

[0063] According to other embodiments of the present invention, the compressor may be of another type than a scroll compressor, ie the compressor may in particular be a screw compressor or a piston compressor.

[0064] According to an embodiment of the present invention, the multi-compressor refrigeration system uses a low GWP (Global Warming Potential) refrigerant.

[0065] The present invention also relates to a method for balancing oil in a multi-compressor refrigeration system, the multi-compressor refrigeration system comprising a controller and a multi-compressor device, the controller being configured to control the operation of the multi-compressor refrigeration system, the multi-compressor device comprising at least three compressors connected in parallel, each compressor being equipped with a suction connection, an oil sump and an oil level detection device, the oil level detection device being operably connected to the controller and being configured to detect the oil level in the corresponding oil sump, the oil sump of all compressors of the multi-compressor device being fluidly connected by a common oil balancing line, and the suction connections of all compressors of the multi-compressor device being fluidly connected by a common suction line, the method comprising:

[0066] - dividing the compressors in the multi-compressor device into at least two groups, wherein each group comprises at least one compressor and at least one group comprises at least two compressors;

[0067] - an oil pumping device is arranged in the common oil equalization line and between the two groups, so that one of the two groups is located on a first side of the oil pumping device and the other of the two groups is located on a second side of the oil pumping device, the oil pumping device comprising at least one oil pump;

[0068] - providing at least one valve in the common oil equalization line, the at least one valve being associated with a group comprising at least two compressors and being configured for controlling the oil flow in the common oil equalization line and between the oil pumping device and at least one compressor of the group associated with the at least one valve;

[0069] - monitoring the oil level in the oil tank of each compressor by means of a corresponding oil level detection device;

[0070] - detecting the occurrence of a low oil level condition in said compressor if said oil level in the compressor reaches a predetermined low oil level condition;

[0071] -If a low oil level condition is detected for a compressor, performing an oil balancing action, the oil balancing action comprising controlling the operation of the oil pumping device and the at least one valve so as to transfer oil from the oil sump of at least one compressor of the multi-compressor device in which a low oil level condition has not been detected to the oil sump of a compressor in which a low oil level condition has been detected.

[0072] Thanks to the presence of at least one valve, excess oil can be accurately extracted from selected compressors and then pumped to selected compressors with low oil levels. In this way, the oil level in each compressor can be maintained at a desired oil level, which is generally above a minimum oil level and below a maximum oil level.

[0073] According to an embodiment of the present invention, the oil balancing action includes:

[0074] - if oil balancing between compressors from different groups is required, transferring oil from the oil sump of at least one compressor in which a low oil level condition has not been detected and which belongs to a group different from the group including the compressor in which a low oil level condition has been detected, to the oil sump of the compressor in which a low oil level condition has been detected, by controlling the operation of the oil pumping device and the at least one valve;

[0075] -If oil balancing between compressors from the same group is required, oil is transferred from the oil sump of at least one compressor in which a low oil level situation has not been detected and which belongs to the same group as the compressor in which a low oil level situation has been detected to a temporary oil container by controlling the operation of the oil pumping device and the at least one valve, and then transferred from the temporary oil container to the oil sump of the compressor in which a low oil level situation has been detected, wherein the temporary oil container is composed of the oil sump of at least one compressor belonging to a group different from the group including the compressor in which a low oil level situation has been detected.

[0076] In this way, the oil in a multi-compressor refrigeration system can be balanced without an additional external oil tank. When the compressor with low oil level and the compressor with excess oil level are in different groups, the oil pumping device located between these groups simply pumps the oil from the group with excess oil to the group lacking oil. When the compressor with low oil level and the compressor with excess oil level are in the same group, the oil pump first pumps the excess oil from the group to the other group (which acts like a temporary oil tank), and then returns to the group of the compressor with low oil level.

[0077] According to an embodiment of the present invention, the method includes dividing the multi-compressor refrigeration system into two groups.

[0078] According to an embodiment of the invention, the method further comprises: calculating the risk of low oil level occurring in the compressor under given operating conditions, and controlling the operation of the oil pumping device and at least one valve based on the calculation, for example in order to transfer more oil to the compressor with a high risk of low oil level occurring; and / or controlling the operation of the oil pumping device and the valve in order not to extract oil from the compressor with a high risk of low oil level occurring. Advantageously, the method comprises: selecting at least one oil sump based on the calculation, from which oil is pumped for oil delivery.

[0079] According to an embodiment of the present invention, the method includes outputting a low oil level warning signal from the oil level detection device and to the controller if the oil level in the oil sump of the corresponding compressor reaches a predetermined low oil level value. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] The following detailed description of two embodiments of the invention will be better understood when read in conjunction with the appended drawings understood however that the invention is not limited to the specific embodiments disclosed.

[0081] Figure 1 is a schematic diagram of a multi-compressor refrigeration system according to a first embodiment of the present invention.

[0082] Figure 2 yes Figure 1 A longitudinal cross-sectional view of a compressor of a multi-compressor refrigeration system.

[0083] Figure 3 yes Figure 1 Schematic diagram of a multi-compressor device of a multi-compressor refrigeration system.

[0084] Figure 4 It is used for balance Figure 1 Schematic diagram of the oil method in a multi-compressor refrigeration system.

[0085] Figure 5 is a schematic diagram of a multi-compressor device of a multi-compressor refrigeration system according to a second embodiment of the present invention.

[0086] Figure 6 is a schematic diagram of a multi-compressor device of a multi-compressor refrigeration system according to a third embodiment of the present invention.

[0087] Figure 7 is a schematic diagram of a multi-compressor device of a multi-compressor refrigeration system according to a fourth embodiment of the present invention.

[0088] Figure 8 is a schematic diagram of a multi-compressor device of a multi-compressor refrigeration system according to a fifth embodiment of the present invention. DETAILED DESCRIPTION

[0089] Figures 1 to 3 A multi-compressor refrigeration system 2 is described, which comprises a refrigerant circulation circuit 3, which in turn comprises a condenser 4, an electronic expansion valve 5, an evaporator 6 and a multi-compressor device 7 connected in series.

[0090] The multi-compressor device 7 comprises at least three compressors 8 connected in parallel. Figure 1 In the embodiment shown, 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. Each compressor 8 may for example be a variable speed compressor.

[0091] Each compressor 8 includes a compressor housing 9, which is provided with a suction connection 11 and a discharge connection 12, wherein the suction connection 11 is configured to supply the refrigerant gas to be compressed to the corresponding compressor 8, and the discharge connection 12 is configured to discharge the compressed refrigerant gas. The multi-compressor refrigeration system 2 can use a low GWP (Global Warming Potential) refrigerant as the refrigerant gas.

[0092] Advantageously, each compressor 8 is a scroll compressor and comprises a compression unit 13, which is arranged inside the corresponding compressor housing 9 and is configured to compress the refrigerant gas supplied by the corresponding suction connection 11. Each compression unit 13 comprises a fixed scroll and an orbiting scroll, which is fixed relative to the corresponding compressor housing 9 and is configured to perform an orbiting motion relative to the corresponding fixed scroll during operation of the corresponding compressor 8. However, according to another embodiment of the present invention, the compressor 8 can be of other types than a scroll compressor, and in particular can be a screw compressor or a piston compressor.

[0093] Each compressor 8 further comprises an oil tank 14 arranged at the lower part of the corresponding compressor housing 9 and an oil level detection device 15, the oil tank 14 being arranged in the corresponding oil tank 14 and being configured to detect the oil level in the corresponding oil tank 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 tank 14 of the corresponding compressor 8 reaches a predetermined low oil level value.

[0094] The multi-compressor device 7 further comprises a common suction line 16, which is configured to fluidically connect the suction connections 11 of all compressors 8 of the multi-compressor device 7. In particular, the common suction line 16 comprises a main suction line 17 and suction branches 18, each suction branch 18 connecting the main suction line 17 to the suction connection 11 of a corresponding compressor 8. The multi-compressor device 7 further comprises a common discharge line 19, which is configured to fluidly connect the discharge connections 12 of all compressors 8 of the multi-compressor device 7. In particular, the common discharge line 19 comprises a main discharge line 21 and a discharge branch 22, each discharge branch 22 connecting the main discharge line 21 to the discharge connection 12 of a corresponding compressor.

[0095] Furthermore, the multi-compressor device 7 comprises a common oil equalization line 23, which is configured for fluidly connecting the oil sumps 14 of all compressors 8 of the multi-compressor device 7. In particular, the common oil equalization line 23 comprises a main oil equalization line 24 and oil equalization branches 25, each oil equalization branch 25 connecting the main oil equalization line 24 to the oil sumps 14 of the respective compressor 8, and in particular to an oil equalization connection 26 provided on the compressor housing 9 of the respective compressor 8 and fluidly connected to the respective oil sumps 14. In particular, each oil equalization branch 25 is connected to the main oil equalization line 24 at the oil equalization line bifurcation 26.

[0096] The compressors in the multi-compressor device 7 are divided into two groups, each group comprising at least one compressor 8, and at least one group comprising at least two compressors 8. Figure 3 In the embodiment shown, one group (referred to as the first group) comprises one compressor 8 and the other group (referred to as the second group) comprises two compressors 8 .

[0097] The multi-compressor device 7 further comprises an oil pumping device 27 located in the common oil equalization line 23 and between the two groups, such that one of the two groups is located on a first side of the oil pumping device 27 and the other of the two groups is located on a second side of the oil pumping device 27. Figure 3 In the embodiment shown, 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 output the amount of oil pumped.

[0098] according to Figure 3 In the embodiment shown, the multi-compressor device 7 further comprises two valves 29 located in the common oil equalization line 23 and associated with a group comprising two compressors 8. Advantageously, each valve 29 is a two-way normally closed solenoid valve and is located in the oil equalization branch 25 of the common oil equalization line 23 and therefore downstream of the respective compressor 8. Thus, each valve 29 is configured for controlling the oil flow between the oil pumping device 27 and the compressor 8 fluidically connected to the respective oil equalization branch 25. In other words, each valve 29 is configured for controlling the fluid communication between the oil pumping device 27 and the respective compressor 8.

[0099] The multi-compressor refrigeration system 2 further comprises a controller 31, which is configured to control the operation of the multi-compressor refrigeration system 2, and in particular to control the operation (start or stop) of the compressor 8. The controller 31 may comprise, for example, a microprocessor and a memory.

[0100] The controller 31 is operably connected to each oil level detection device 15 , the oil pumping device 27 and each valve 29 , and in particular is configured to control the operation of the oil pumping device 27 and the valve 29 based on output from the oil level detection device 15 .

[0101] Figure 4 Shown for balance Figure 1 Schematic diagram of the oil method of a multi-compressor refrigeration system 2.

[0102] In particular, the method comprises:

[0103] - monitoring the oil level in the oil sump 14 of each compressor 8 by means of a corresponding oil level detection device 15;

[0104] - if the oil level in the corresponding compressor 8 reaches a predetermined low oil level condition, the occurrence of a low oil level condition (also referred to as a lack of oil condition) in said compressor 8 is detected by the controller 31 and based on the signal output from the oil level detection device 15; and

[0105] - If a low oil level condition is detected for the compressor 8, an oil balancing action is performed, which includes:

[0106] if oil balancing between compressors from different groups is required, controlling the operation of the oil pumping device 27 and the valve 29 so as to transfer oil from the oil sump 14 of at least one compressor 8 in which a low oil level condition has not been detected and which belongs to a group different from the group including the compressor 8 in which a low oil level condition has been detected, to the oil sump 14 of the compressor 8 in which a low oil level condition has been detected;

[0107] If oil balancing is required between compressors 8 from the same group, the operation of the oil pumping device 27 and the valve 29 is controlled so that oil is transferred from the oil tank 14 of at least one compressor 8 in which a low oil level condition has not been detected and which belongs to the same group as the compressor 8 in which a low oil level condition has been detected to a temporary oil container, and then transferred from the temporary oil container to the oil tank 14 of the compressor 8 in which a low oil level condition has been detected, wherein the temporary oil container is composed of the oil tank 14 of at least one compressor 8 belonging to a group different from the group including the compressor 8 in which a low oil level condition has been detected.

[0108] Advantageously, the controller 31 is configured to determine the current amount of oil in each compressor 8 based on the output from the oil level detection device 15, and the amount of oil that must be delivered to the compressor 8 where a low oil level condition has been detected in order to reach a predetermined oil level in said compressor 8. Based on this, the controller 31 can operate the oil pumping device 27 to pump the exact amount of oil required for the compressor 8 where a low oil level condition has been detected.

[0109] According to an embodiment of the present invention, if a low oil level warning signal is output from the corresponding oil level detection device 15, that is, if the oil level in the oil tank 14 of the corresponding compressor 8 reaches a predetermined low oil level value, the predetermined low oil level condition is reached for the compressor 8.

[0110] Advantageously, the method further comprises: if the oil level in the corresponding compressor 8 reaches a predetermined high oil level condition, detecting, by the controller 31 and based on the signal output from the oil level detection device 15, that an oil excess condition occurs in the compressor 8. According to this embodiment of the invention, the oil balancing action comprises:

[0111] - if oil balancing between compressors 8 from different groups is required, controlling the operation of the oil pumping device 27 and the valve 29 so as to transfer oil from the oil sump 14 of at least one compressor 8 in which an oil excess condition has been detected and which belongs to a group different from the group including the compressor 8 in which a low oil level condition has been detected, to the oil sump 14 of the compressor 8 in which a low oil level condition has been detected;

[0112] -If oil balancing is required between compressors 8 from the same group, the operation of the oil pumping device 27 and the valve 29 is controlled so as to transfer oil from the oil sump 14 of at least one compressor 8 in which an oil excess situation has been detected and which belongs to the same group as the compressor 8 in 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 in which a low oil level situation has been detected, wherein the temporary oil container is constituted by the oil sump 14 of at least one compressor 8 belonging to a group different from the group including the compressor 8 in which a low oil level situation has been detected.

[0113] According to an embodiment of the present invention, if a high oil level warning signal is output from the corresponding oil level detection device 15, that is, if the oil level in the oil tank 14 of the corresponding compressor 8 reaches a predetermined high oil level value, the predetermined high oil level condition is reached for the compressor 8.

[0114] According to an embodiment of the present invention, the method also includes: calculating the risk of low oil level occurring in the compressor 8 under given operating conditions, and controlling the operation of the oil pumping device 27 and the valve 29 based on the calculation so as to transfer more oil to the compressor 8 with a high risk of low oil level occurring; and / or controlling the operation of the oil pumping device 27 and the valve 29 so as not to extract oil from the compressor 8 with a high risk of low oil level occurring.

[0115] The following discloses a method for balancing Figure 1 Several scenarios of the oil method in a multi-compressor refrigeration system 2.

[0116] Scenario 1: Low oil level detected in the first group and excessive oil level detected in the second group

[0117] In scenario 1, oil transfer is required between two different groups of compressors 8, and oil should be transferred from the second group including two compressors 8 (referred to as the second and third compressors) to the first group including only one compressor 8 (referred to as the first compressor).

[0118] In this case, the oil balancing action includes opening the valve 29 associated with the second compressor (or the third compressor) and operating the oil pump 28 to start pumping oil in the direction from the second group to the first group, and more specifically, pumping oil in the direction from the second compressor (or the 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 the oil sump 14 has reached a 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 (or the third compressor).

[0119] It may happen that the amount of oil in the second compressor (or the third compressor) is insufficient to bring the first compressor to the predetermined oil level. In this case, the oil balancing action additionally comprises: operating the oil pump 28 and the valve 29 to deliver oil also from the third compressor (or the second compressor) to the first compressor.

[0120] It may also happen that the controller 31 determines that there is a high risk of low oil level for the second compressor (or the third compressor). In this case, the oil balancing action will include controlling the operation of the oil pump 28 and the valve 29 so as to deliver oil from another compressor 8 with a high oil level, i.e., delivering oil from the third compressor (or the second compressor).

[0121] The present invention also contemplates delivering oil from more than one oil sump 14 simultaneously.

[0122] Scenario 2: Excess oil level detected in the first group and low oil level detected in the second group

[0123] In this case, oil should be transferred from the first group to the second group.Since the first group comprises only one compressor 8, oil is transferred from the first compressor to the second compressor and / or the third compressor.

[0124] In this case, 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 start pumping oil in the direction from the first group to the second group, and more specifically, in the direction 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 the oil sump 14 has reached a 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).

[0125] Scenario 3: Low oil level and excess oil level detected in the same group

[0126] In this case, the oil should be delivered within the same group. To achieve this, an intermediate step is introduced. It is assumed that an excess oil level has been detected in the second compressor and a low oil level has been detected in the third compressor.

[0127] Firstly, the oil balancing action comprises controlling the operation of the oil pump 28 and the valve 29 associated with the second and third compressors in order to transfer excess oil from the second compressor to a temporary oil reservoir constituted by the oil sump 14 of the first compressor. In particular, the oil balancing action comprises 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 in particular from the oil sump 14 of the second compressor to the oil sump 14 of the first compressor. Once a sufficient amount of oil has been transferred to the oil sump 14 of the first compressor, the oil balancing action comprises stopping the operation of the oil pump 28 and closing the valve 29 associated with the second compressor.

[0128] Then, 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, i.e., 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 the oil sump 14 has reached a 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.

[0129] The present invention also contemplates that the controller 31 may operate the oil pump 28 and the valve 29 to deliver oil to the compressor 8 even if a low oil level condition is not detected for the compressor 8 at this time, because the compressor 8 is about to reach a given operating condition in which the compressor 8 has a high risk of a low oil level. In this case, the method may include: controlling the oil pump 28 and the valve 29 by the controller 31 to deliver some additional oil to the compressor 8 before such compressor 8 reaches the operating condition.

[0130] Figure 5 A multi-compressor refrigeration system 2 according to a second embodiment of the present invention is disclosed, and the essential difference between the second embodiment and the first embodiment is that the multi-compressor device 7 has four compressors 8, and each of the first group and the second group includes two compressors 8, and each oil balancing branch 25 is equipped with a respective corresponding valve 29, and is particularly equipped with a double normally closed solenoid valve 29.

[0131] On one side of the oil pump 28 there are a first compressor and a second compressor belonging to the first group, and on the other side of the oil pump 28 there are a third compressor and a fourth compressor belonging to the second group.

[0132] The oil balancing operation for the multi-compressor refrigeration system 2 according to the second embodiment of the present invention is very similar to the oil balancing operation for the first embodiment of the present invention described above, and therefore will not be disclosed hereinafter. In this case, oil can be delivered to / from two compressors 8 at the same time.

[0133] Figure 6 A multi-compressor refrigeration system 2 according to a third embodiment of the invention is disclosed, which substantially differs from the second embodiment in that the second group comprises three compressors 8 .

[0134] The oil balancing operation for the multi-compressor refrigeration system 2 according to the third embodiment of the present invention is very similar to the oil balancing operation for the first embodiment of the present invention described above, and therefore will not be disclosed hereinafter. In this case, oil can be delivered to / delivered from three compressors 8 at the same time.

[0135] Figure 7 A multi-compressor refrigeration system 2 according to a fourth embodiment of the present invention is disclosed, which is substantially different from the first embodiment in that the multi-compressor device 7 comprises only one valve 29 (instead of two double normally closed solenoid valves), and the valve 29 is a three-way valve located at an oil equalizing line bifurcation 26 of a common oil equalizing line 23. In particular, the three-way valve is configured to control the oil flow between two oil equalizing branches 25 of the common oil equalizing line 23 and an oil pumping device 27 fluidly connected at the oil equalizing line bifurcation 26.

[0136] Replacing two two-way valves with a three-way valve simplifies the multi-compressor refrigeration system 2 according to the present invention and also reduces the overall cost.

[0137] The present invention also contemplates that a three-way valve is used instead of a two-way valve in the second and third embodiments of the present invention described above.

[0138] Figure 8 A multi-compressor refrigeration system 2 according to a fifth embodiment of the present invention is disclosed. The essential difference between the fifth embodiment and the fourth embodiment is that the oil pumping device 27 includes two unidirectional oil pumps 28 (instead of a bidirectional oil pump), for example, two unidirectional volumetric oil pumps, and the two unidirectional oil pumps 28 are connected in parallel.

[0139] The present invention also contemplates that such an oil pumping device 27 may replace the oil pumping devices 27 in the other embodiments described above.

[0140] Based on the above embodiments, those skilled in the art can easily implement the present invention into a multi-compressor refrigeration system 2 including any number of compressors 8. In addition, unless explicitly stated otherwise, those skilled in the art can easily combine features described with respect to different embodiments.

[0141] Of course, the invention is not limited to the embodiments described above by way of non-limiting examples, but on the contrary, the invention encompasses all its embodiments.

Claims

1. A multi-compressor refrigeration system (2), the multi-compressor refrigeration system (2) comprising a controller (31) and a multi-compressor device (7), the controller (31) being configured to control the operation of the multi-compressor refrigeration system (2), the multi-compressor device (7) comprising: - at least three compressors (8) connected in parallel, each compressor (8) being equipped with a suction connection (11), an oil sump (14) and an oil level detection device (15), the oil level detection device (15) being operatively connected to the controller (31) and being configured to detect the oil level in the corresponding oil sump (14), wherein 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); - a common suction line (16) configured for fluidically connecting the suction connections (11) of all compressors (8) of the multi-compressor device (7); - a common oil equalization line (23) configured for fluidly connecting the oil sumps (14) of all compressors (8) of the multi-compressor device (7); - an oil pumping device (27) located in the common oil equalization line (23) and operatively connected to the controller (31), the oil pumping device (27) comprising at least one oil pump (28), Characterized in that the oil pumping device (27) is located between two groups, so that one of the two groups is located on a first side of the oil pumping device (27) and the other of the two groups is located on a second side of the oil pumping device (27), and wherein the multi-compressor device (7) also includes at least one valve (29), the at least one valve (29) is located in the common oil equalization pipeline (23) and is associated with a group including at least two compressors (8), the at least one valve (29) is operably connected to the controller (31), and the at least one valve (29) is configured to control the oil flow in the common oil equalization pipeline (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), and the controller (31) is configured to control the operation of the oil pumping device (27) and the at least one valve (29) based on the output from the oil level detection device (15).

2. The multi-compressor refrigeration system (2) according to claim 1, wherein: The controller (31) is configured to determine the current amount of oil in each compressor (8) based on the output from the oil level detection device (15).

3. The multi-compressor refrigeration system (2) according to claim 1 or 2, wherein: The controller (31) is configured to: - monitoring the oil level in the oil tank (14) of each compressor (8) by means of a corresponding oil level detection device (15); - detecting the occurrence of a low oil level condition in the corresponding compressor (8) if the oil level in the corresponding compressor (8) reaches a predetermined low oil level condition; and -If a low oil level condition is detected for a compressor (8), an oil balancing action is performed, the oil balancing action comprising controlling the operation of the oil pumping device (27) and the 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) in which the low oil level condition has not been detected to the oil sump (14) of a compressor (8) in which the low oil level condition has been detected.

4. The multi-compressor refrigeration system (2) according to any one of claims 1 to 3, wherein: The controller (31) is configured to detect an oil excess condition in the corresponding compressor (8) if the oil level in the corresponding compressor (8) reaches a predetermined high oil level condition.

5. The multi-compressor refrigeration system (2) according to claim 3 or 4, wherein: The oil balancing action includes controlling the operation of the oil pumping device (27) and the at least one valve (29) to transfer oil from the oil sump (14) of at least one compressor (8) of the multi-compressor device (7) in which an oil excess condition has been detected to the oil sump (14) of a compressor (8) in which a low oil level condition has been detected.

6. The multi-compressor refrigeration system (2) according to any one of claims 3 to 5, wherein: The oil balancing action includes: - if oil balancing between compressors (8) from different groups is required, controlling the operation of the oil pumping device (27) and the at least one valve (29) so as to transfer oil from the oil sump (14) of at least one compressor (8) for which a low oil level condition has not been detected and which belongs to a group different from the group including the compressor (8) for which a low oil level condition has been detected, to the oil sump (14) of the compressor (8) for which a low oil level condition has been detected; - If oil balancing is required between compressors (8) from the same group, controlling the operation of the oil pumping device (27) and the at least one valve (29) so as to transfer oil from the oil sump (14) of at least one compressor (8) in which a low oil level situation has not been detected and which belongs to the same group as the compressor (8) in 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) in which a low oil level situation has been detected, wherein the temporary oil container is constituted by the oil sump (14) of at least one compressor (8) belonging to a group different from the group including the compressor (8) in which a low oil level situation has been detected.

7. The multi-compressor refrigeration system (2) according to any one of claims 3 to 6, wherein: The controller (31) is configured to determine an amount of oil that must be delivered to a compressor (8) where a low oil level condition has been detected in order to reach a predetermined oil level in the compressor (8).

8. The multi-compressor refrigeration system (2) according to any one of claims 1 to 7, wherein: The controller (31) is configured to calculate the risk of low oil level occurring in the compressor (8) under given operating conditions and to control the operation of the oil pumping device (27) and the at least one valve (29) based on the calculation.

9. The multi-compressor refrigeration system (2) according to claim 8, wherein: The controller (31) is configured to: control the oil pumping device (27) and the at least one valve (29) so as to transfer more oil to the compressor (8) with a high risk of experiencing a low oil level; and / or control the oil pumping device (27) and the at least one valve (29) so as not to extract oil from the compressor (8) with a high risk of experiencing a low oil level.

10. The multi-compressor refrigeration system (2) according to any one of claims 1 to 9, wherein: The at least one oil pump (28) comprises a positive displacement oil pump configured to measure and output an amount of oil pumped.

11. The multi-compressor refrigeration system (2) according to any one of claims 1 to 10, wherein: The at least one oil pump (28) includes two one-way oil pumps connected in parallel.

12. The multi-compressor refrigeration system (2) according to any one of claims 1 to 10, wherein: The at least one oil pump (28) comprises a bidirectional oil pump.

13. The multi-compressor refrigeration system (2) according to any one of claims 1 to 12, wherein: The at least one valve (29) comprises a normally closed solenoid valve.

14. The multi-compressor refrigeration system (2) according to any one of claims 1 to 13, wherein: The at least one valve (29) comprises a three-way valve, which is located at an oil equalization line bifurcation (26) of the common oil equalization line (23), and is configured to control the oil flow between two corresponding oil equalization branches (25) of the common oil equalization line (23) and the oil pumping device (27) which are fluidly connected at the oil equalization line bifurcation (26).

15. The multi-compressor refrigeration system (2) according to any one of claims 1 to 14, wherein: The at least one valve (29) comprises a two-way valve, which is located in an oil equalization branch (25) of the common oil equalization line (23) fluidly connected to the corresponding compressor, and the two-way valve is configured to control the oil flow between the oil pumping device (27) and the corresponding compressor fluidly connected to the oil equalization branch (25).

16. The multi-compressor refrigeration system (2) according to any one of claims 1 to 15, wherein: At least one compressor (8) of the multi-compressor device (7) comprises at least two oil level detection devices (15).

17. A method for balancing oil in a multi-compressor refrigeration system (2), the multi-compressor refrigeration system (2) comprising a controller (31) and a multi-compressor device (7), the controller (31) being configured to control the operation of the multi-compressor refrigeration system (2), the multi-compressor device (7) comprising at least three compressors (8) connected in parallel, each compressor (8) being equipped with a suction connection (11), an oil sump (14) and an oil level detection device (15), the oil level detection device (15) being operably connected to the controller (31) and being configured to detect the oil level in the corresponding oil sump (14), the oil sump (14) of all compressors (8) of the multi-compressor device (7) being fluidly connected via a common oil balancing line (23), and the suction connections (11) of all compressors (8) of the multi-compressor device (7) being fluidly connected via a common suction line (16), the method comprising: - dividing the compressors (8) in the multi-compressor device (7) into at least two groups, wherein each group comprises at least one compressor (8) and at least one group comprises at least two compressors (8); - an oil pumping device (27) is arranged in the common oil equalization line (23) and between the two groups, so that one of the two groups is located on a first side of the oil pumping device (27) and the other of the two groups is located on a second side of the oil pumping device, the oil pumping device (27) comprising at least one oil pump (28); - providing at least one valve (29) in the common oil equalization line (23), the at least one valve (29) being associated with a group of at least two compressors (8) and being configured to control the 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); - monitoring the oil level in the oil tank (14) of each compressor (8) by means of a corresponding oil level detection device (15); - detecting the occurrence of a low oil level condition in the compressor (8) if the oil level in the compressor (8) reaches a predetermined low oil level condition; -If a low oil level condition is detected for a compressor (8), an oil balancing action is performed, the oil balancing action comprising controlling the operation of the oil pumping device (27) and the 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) in which the low oil level condition has not been detected to the oil sump (14) of a compressor (8) in which the low oil level condition has been detected.

18. The method according to claim 17, wherein: The oil balancing action includes: - if oil balancing between compressors (8) from different groups is required, transferring oil from the oil sump (14) of at least one compressor (8) in which a low oil level condition has not been detected and which belongs to a group different from the group including the compressor (8) in which a low oil level condition has been detected, to the oil sump (14) of the compressor (8) in which a low oil level condition has been detected, by controlling the operation of the oil pumping device (27) and the at least one valve (29); -If oil balancing is required between compressors (8) from the same group, oil is transferred from the oil sump (14) of at least one compressor (8) in which a low oil level situation has not been detected and which belongs to the same group as the compressor (8) in which a low oil level situation has been detected to a temporary oil container by controlling the operation of the oil pumping device (27) and the at least one valve (29), and then transferred from the temporary oil container to the oil sump (14) of the compressor (8) in which a low oil level situation has been detected, wherein the temporary oil container is composed of the oil sump (14) of at least one compressor (8) belonging to a group different from the group including the compressor (8) in which a low oil level situation has been detected.

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