Multi-compressor refrigeration system having a common oil equalization line provided with an oil pumping device

Abstract

The multi-compressor refrigeration system comprises a control device and a multi-compressor device (7) comprising at least three compressors (8) which are 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 comprising at least one compressor (8); the suction connections of all the compressors (8) being fluidically connected by a common suction line (16) and the oil sumps of all the compressors (8) being fluidically connected by 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 to control an oil flow between the oil pumping device (27) and at least one compressor (8) of a group associated with the at least one valve (29). Figure 3.

Description

Title of the invention: Multi-compressor refrigeration system having a common oil equalization line provided with an oil pumping device Field of invention

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

[0002] Proper oil balancing is an important issue associated with multiple compressor systems.

[0003] The most common solutions are to provide oil equalization lines between the compressors, oil level sensing means in each compressor and pumps for providing oil flow in the oil equalization lines between the compressors and a controller for controlling the operation of the multi-compressor system. The general idea is that once the oil level sensing means detects a low oil level in a particular compressor, the controller can operate the respective oil pump located in the respective oil equalization line and pump oil from the high oil level compressor to the low oil level compressor.

[0004] Although such a simple solution is sufficient for multi-compressor systems with a relatively small number of compressors (such as 2 or 3), it becomes very difficult and, in particular, expensive to implement such a solution in multi-compressor systems with a higher number of compressors. In particular, the price of the oil pump contributes to the final cost of a multi-compressor system.

[0005] Good oil balancing in a tandem configuration is generally not that difficult to achieve and can be achieved in most cases by adding passive elements (suction washer, restrictor...) in the suction line. This has a limited impact on the pressure drop and the overall efficiency of the refrigeration system. In such cases, there is little need for external actuators like oil pumps.

[0006] However, when dealing with a larger multi-compressor refrigeration system, the situation can change radically. In trio configurations with passive devices integrated in the suction line, the refrigeration system has such a pressure drop that negatively impacts efficiency that the use of external actuators, such as oil pumps, begins to be very useful.

[0007] DE1915174 Al discloses an idea of oil level compensation, where each A compressor in a multi-compressor system has an oil level sensor and an oil pump. A control device triggers oil flow based on the output of the oil level sensors from the compressor with excess oil to the compressor with low oil level. However, such a solution is complex and expensive, as it requires as many pumps as compressors.

[0008] EP0840870 A1 discloses an oil level control apparatus 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 apparatus is capable of maintaining the oil level in a compressor between a predetermined minimum level and a predetermined maximum level. In the event that the oil level in a compressor falls below the predetermined minimum level, oil is added to said compressor from the additional oil reservoir. In the event that the oil level in a compressor exceeds the predetermined maximum level, oil is returned from the compressor to the additional oil reservoir. However, such a solution is also complex and expensive, as it requires an additional oil reservoir and piping.

[0009] CN100516682C discloses the idea that a multiple compressor system is divided in groups and each group has an oil pump, where each group includes at least two compressors. Such an arrangement allows for oil balancing between the groups. However, this does not solve the problem if only one particular compressor in the group needs oil. Therefore, such an arrangement can lead to the creation of excessive oil levels in the compressors.

[0010] JPH04116349A discloses a multi-compressor system with an oil pump capable of changing its transfer direction. However, in this arrangement, the oil pump is at a significant distance from the compressors and therefore the efficiency is affected. Furthermore, this solution does not solve the problem with a high number of compressors in a multi-compressor system. In addition, an additional oil reservoir is required, since direct oil flow between the compressors is not possible. Summary of the invention

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

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

[0013] According to the invention, such a multi-compressor refrigeration system comprises a control device configured to control the operation of the multi-compressor refrigeration system, and a multi-compressor device comprising:

[0014] - at least three compressors which are coupled in parallel, each compressor being equipped with a suction connection, an oil sump and an oil level sensing device operatively connected to the control device and configured to sense an oil level in the respective oil sump, 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 to fluidly connect the suction connections of all compressors of the multi-compressor device,

[0016] - a common oil equalization line configured to fluidly connect the oil sumps of all compressors of the multi-compressor device,

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

[0018] characterized in that the oil pumping device is 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 in that the multi-compressor device further comprises at least one valve located in the common oil equalization line and associated with a group comprising at least two compressors, the at least one valve being operatively connected to the control device 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 the at least one valve,the control device being configured to control operation of the oil pumping device and the at least one valve based on outputs from the oil level sensing devices.

[0019] By means of such an arrangement of the oil pumping device and the at least one valve, it is possible to obtain a simple and inexpensive multi-compressor system with a very limited number of additional equipment. The present The invention allows reliable oil balancing to be achieved without interfering with the common suction line and therefore the common suction line can be designed to be fully optimized for efficiency, and thus the efficiency of the multi-compressor refrigeration system is improved. In addition, 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.

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

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

[0022] According to one embodiment of the invention, each compressor comprises an oil balance connection, the common oil balance line being configured to fluidly connect the oil balance connections of all the compressors of the multi-compressor device.

[0023] According to one embodiment of the invention, the controller is configured to determine a current amount of oil in each compressor based on outputs from the oil level sensing devices.

[0024] According to one embodiment of the invention, the control device is configured to:

[0025] - monitor an oil level in the oil sump of each compressor by through the respective oil level detection device,

[0026] - detect, for example on the basis of a signal emitted by a detection device of oil level, that a low oil level situation, also called a low oil condition, occurs in a respective compressor, if the oil level in said compressor reaches a predetermined low oil level condition, and

[0027] - perform an oil balancing action if a low oil level situation is detected for a compressor, 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 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.

[0028] According to one embodiment of the invention, each oil level detection device is configured to emit 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.

[0029] According to one embodiment of the invention, the predetermined low oil level condition is reached for a compressor if a low level warning signal oil is emitted by the respective oil level detection device.

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

[0031] - the low oil level warning signal emitted by the oil detection device respective oil level is output continuously for a first predetermined time, and for example for at least ten seconds, or

[0032] - the low oil level warning signal emitted by the oil detection device respective oil level has switched, i.e. has been activated and deactivated, more than a predetermined number of times, and for example more than ten times, within a second predetermined time, and for example during the last minute.

[0033] According to one embodiment of the invention, the control device is configured to detect that an excess oil situation occurs in a respective compressor, if the oil level in said compressor reaches a predetermined high oil level condition.

[0034] According to one embodiment of the invention, the oil balancing action comprises 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 for which an excess oil situation has been detected to the oil sump of the compressor for which a low oil situation has been detected.

[0035] According to one embodiment of the invention, the oil balancing action comprises:

[0036] - if oil balancing is required between compressors of different groups, the 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, for which a low oil level situation has not been detected and belonging to a group different from the group comprising 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,

[0037] - if oil balancing is required between compressors of the same group, the 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, for which a low oil level situation has not been detected and belonging to the same group as the compressor 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 of the compressor for which a low oil level situation has been detected, where the temporary oil container is constituted by the oil sump of at least one compressor belonging to a group different from the group comprising the compressor for which a low oil level situation has been detected.

[0038] According to one embodiment of the invention, the control device is configured to determine the quantity of oil to 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.

[0039] According to one embodiment of the invention, the control device is configured to calculate a risk of occurrence of a low oil level in a compressor under a given operating condition and to control the operation of the oil pumping device and the at least one valve on the basis of this calculation.

[0040] According to one embodiment of the invention, the control device is configured to control the oil pumping device and the at least one valve in order to supply more oil to a compressor which has a high risk of occurrence of a low oil level and / or to control the oil pumping device and the at least one valve in order not to take oil from a compressor which has a high risk of occurrence of a low oil level.

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

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

[0043] According to one embodiment of the invention, the at least one oil pump comprises two unidirectional oil pumps, for example two unidirectional volumetric oil pumps, which are connected in parallel.

[0044] According to one embodiment of the invention, the at least one oil pump comprises a bidirectional oil pump, and for example a bidirectional volumetric oil pump.

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

[0046] 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 member and an electromagnetic coupling arranged between the pumping member and the motor. Such an oil pump provides static sealing and eliminates the risk of leaks, and furthermore it is possible for the multi-compressor refrigeration system with such an oil pump to receive appropriate certifications for working in highly hazardous conditions, such as at- explosive spheres.

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

[0048] 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 connection of a respective compressor.

[0049] 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 balance connection provided on a compressor sump of a respective compressor and fluidly 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.

[0050] According to one embodiment of the invention, the 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 an oil flow between the oil pumping device and two respective oil equalization branches of the common oil equalization line which are fluidly connected at said oil equalization line branch. In other words, said three-way valve can control an oil flow between the oil pumping device and two compressors respectively fluidly connected to oil equalization branches which are fluidly connected at said oil equalization line branch. Thus, said three-way valve can control an oil flow to more than one compressor.

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

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

[0053] 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 a two-way valve being operatively connected to the controller 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 controller being configured to control operation of the oil pumping device and the two-way valves based on outputs from the oil level sensing devices.

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

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

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

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

[0058] According to one embodiment of the invention, each oil level detection device is at least partially located in the respective oil pan.

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

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

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

[0062] According to one 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 invention, the compressors may be of a type other than scroll compressors, that is to say that the compressors may in particular be screw compressors or piston compressors.

[0064] According to one embodiment of the invention, the multi-compressor refrigeration system uses a low GWP (global warming potential) refrigerant.

[0065] The present invention also relates to a method of balancing oil in a multi-compressor refrigeration system comprising a controller configured to control operation of the multi-compressor refrigeration system. multiple compressors and a multi-compressor device comprising at least three compressors which are coupled in parallel, each compressor being equipped with a suction connection, an oil sump and an oil level detection device operatively connected to the control device and configured to detect an oil level in the respective oil sump, the oil sumps of all compressors of the multi-compressor device being fluidically connected via a common oil equalization line and the suction connections of all compressors of the multi-compressor device being fluidically connected via a common suction line, the method comprising:

[0066] - the division of the compressors in the multi-compressor device into at least two groups, where each group comprises at least one compressor and at least one group comprises at least two compressors,

[0067] - providing a device for pumping oil into the equalization line of common oil and 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, the oil pumping device comprising at least one oil pump,

[0068] - providing at least one valve located in the oil equalization line common, the 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 the at least one valve,

[0069] - monitoring an oil level in the oil sump of each compressor by through the respective oil level detection device,

[0070] - detecting that a low oil level situation occurs in a compressor, if the oil level in said compressor reaches a predetermined low oil level condition,

[0071] - performing an oil balancing action if a low level situation oil is detected for a compressor, 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 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.

[0072] By having at least one valve, it is possible to accurately draw excess oil from a selected compressor and then pump the oil to a selected compressor with low oil level. In this way, it is possible to maintain the oil level in each compressor to the desired level, which is usually above the minimum oil level and below the maximum oil level.

[0073] According to one embodiment of the invention, the oil balancing action comprises:

[0074] - if oil balancing is required between compressors of different groups, the transfer, by controlling the operation of the oil pumping device and the 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 group different from the group comprising 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,

[0075] - if oil balancing is required between compressors of the same group, the transferring, by controlling the operation of the oil pumping device and the at least one valve, oil from the oil sump of at least one compressor, for which a low oil level situation has not been detected and belonging to the same group as the compressor 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 of the compressor for which a low oil level situation has been detected, where the temporary oil container is constituted by the oil sump of at least one compressor belonging to a group different from the group comprising the compressor for which a low oil level situation has been detected.

[0076] 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 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 returns it to the group with the compressor with a low oil level.

[0077] According to one embodiment of the invention, the method comprises dividing a multi-compressor refrigeration system into two groups.

[0078] According to one embodiment of the invention, the method comprises calculating a risk of occurrence of low oil level in a compressor under a given operating condition, and controlling the operation of the oil pumping device and the at least one valve on the basis of said calculation, for example in order to supply more oil to a compressor which has a high risk of occurrence of low level. oil, and / or controlling the operation of the oil pumping device and valves so as not to take oil from a compressor which has a high risk of low oil level occurrence. Advantageously, the method comprises selecting at least one oil sump, from which oil is pumped for oil transfer, based on said calculation.

[0079] According to one embodiment of the invention, the method comprises transmitting a low oil level warning signal from an oil level detection device and to the control device, if the oil level in the oil sump of the respective compressor reaches a predetermined low oil level value. Brief description of the drawings

[0080] The following detailed description of two embodiments of the invention is best understood when read in conjunction with the accompanying drawings, it being understood, however, that the invention is not limited to the specific embodiments disclosed.

[0081] [Fig.l] is a schematic view of a multi-compressor refrigeration system according to a first embodiment of the invention.

[0082] [Fig.2] is a longitudinal sectional view of a compressor of the refrigeration system multi-compressor management of [Fig.l].

[0083] [Fig. 3] is a schematic view of a multi-compressor device of the multi-compressor refrigeration system of [Fig. 1].

[0084] [Fig.4] is a diagram of an oil balancing process in the refrigeration system multi-compressor management of [Fig.l].

[0085] [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.

[0086] [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.

[0087] [Fig.7] is a schematic view of a multiple compressor device of a multi-compressor refrigeration system according to a fourth embodiment of the invention.

[0088] [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. Detailed description of the invention

[0089] Figures 1 to 3 depict a multi-compressor refrigeration system 2 comprising a refrigerant circulation circuit 3 successively comprising a condenser 4, an electronic expansion valve 5, an evaporator 6 and a multiple compressor device 7 connected in series.

[0090] The multi-compressor device 7 comprises at least three compressors 8 which are coupled in parallel. According to the embodiment shown in [Fig.l], 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 comprises a compressor casing 9 provided with a suction connection 11 configured to supply the respective compressor 8 with refrigerant gas to be compressed and a discharge connection 12 configured to discharge the compressed refrigerant gas. The multi-compressor refrigeration system 2 may use a low GWP (global warming potential) refrigerant as refrigerant gas.

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

[0093] Each compressor 8 also comprises an oil sump 14 arranged at a lower portion of the respective compressor sump 9, and an oil level sensing device 15 located in the respective oil sump 14 and configured to sense an oil level in the respective oil sump 14. Each oil level sensing device 15 may be an oil level sensor or an oil level switch. Advantageously, each oil level sensing device 15 is configured to output a low oil level warning signal if the oil level in the oil sump 14 of the respective compressor 8 reaches a predetermined low oil level value.

[0094] The multi-compressor device 7 further comprises a common suction line 16 configured to fluidically connect the suction connections 11 of all the compressors 8 of the multi-compressor device 7. The common suction line 16 comprises in particular a main suction line 17 and suction branches 18 each connecting the suction line main 17 to the suction connection 11 of a respective compressor 8. The multi-compressor device 7 also comprises a common discharge line 19 configured to fluidically connect the discharge connections 12 of all the compressors 8 of the multi-compressor device 7. The common discharge line 19 comprises 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.

[0095] Furthermore, the multi-compressor device 7 comprises a common oil equalization line 23 configured to fluidly connect the oil sumps 14 of all compressors 8 of the multi-compressor device 7. The common oil equalization line 23 comprises 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 balance 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.

[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. According to the embodiment shown in [Fig.3], one group (named first group) comprises one compressor 8 and the other group (named 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 said two groups is located on a 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. 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 pumped quantity of oil.

[0098] 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 an oil flow between the oil pumping device 27 and the compressor 8 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.

[0099] The multi-compressor refrigeration system 2 further comprises a control device 31 configured to control the operation of the multi-compressor refrigeration system 2, and in particular to control the operation (starting or stopping) of the compressors 8. The control device 31 may for example comprise a microprocessor and a memory.

[0100] The control device 31 is operatively 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 based on outputs from the oil level sensing devices 15.

[0101] [Fig.4] shows a diagram of a process for balancing the oil in the system multi-compressor refrigeration system 2 of [Fig.l].

[0102] The method comprises in particular:

[0103] - monitoring an oil level in the oil sump 14 of each compressor 8 via the respective oil level detection device 15,

[0104] - detection, via the control device 31 and on the basis of a signal emitted by an oil level detection device 15, that a low oil level situation, also called a lack of oil situation, occurs in a respective compressor 8, if the oil level in said compressor 8 reaches a predetermined low oil level condition, and

[0105] - performing an oil balancing action if a low level situation oil level is detected for a compressor 8, the oil balancing action comprising: • if oil balancing is required between compressors of different groups, controlling 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 group different from the group comprising 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, controlling the operation of the oil pumping device 27 and the valves 29 so as to transfer oil from the oil sump 14 from at least 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 group different from the group comprising the compressor 8 for which a low oil level situation has been detected.

[0106] Advantageously, the controller 31 is configured to determine a current amount of oil in each compressor 8 based on outputs from the oil level detection devices 15, and the amount of oil that needs to be transferred to a compressor 8 for which a low oil level situation has been detected to achieve 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 needed for the compressor 8 for which a low oil level situation has been detected.

[0107] 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 emitted 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.

[0108] Advantageously, the method further comprises detecting, via the control device 31 and based on a signal emitted by an oil level detection device 15, that an excess oil situation occurs in a respective compressor 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:

[0109] - if oil balancing is required between compressors 8 of different groups, controlling 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 group different from the group comprising 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,

[0110] - if oil balancing is required between compressors 8 of the same group, controlling 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 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 group different from the group comprising the compressor 8 for which a low oil level situation has been detected.

[0111] 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 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.

[0112] According to one embodiment of the invention, the method further comprises calculating a risk of occurrence of a low oil level in a compressor 8 under a given operating condition, and controlling the operation of the oil pumping device 27 and the valves 29 on the basis of this calculation in order to supply more oil to a compressor 8 which has a high risk of occurrence of a low oil level, and / or controlling the operation of the oil pumping device 27 and the valves 29 in order not to take oil from a compressor 8 which has a high risk of occurrence of a low oil level.

[0113] Several scenarios of the oil balancing process in the multi-compressor refrigeration system 2 of [Fig.l] are disclosed below.

[0114] Scenario 1: Low oil level detected in the first group and excessive oil level detected in the second group

[0115] In scenario 1, there is a need for oil transfer between two different groups of compressors 8, and oil needs to be transferred from the second group, which comprises two compressors 8 (named second and third compressors), to the first group, which comprises only one compressor 8 (named first compressor).

[0116] In such a situation, the oil balancing action comprises 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 particularly 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 said oil sump 14 has reached the predetermined oil level, the oil balancing action comprises stopping the operation of the oil pump 28 and closing the valve 29 associated with the second compressor (or the third compressor).

[0117] It may happen that the amount of oil in the second compressor (or the third compressor) is not sufficient for the first compressor to reach the predetermined oil level. In such a situation, the oil balancing action further comprises the operation of the oil pump 28 and the valves 29 to also transfer oil from the third compressor (or the second compressor) to the first compressor.

[0118] It may also happen that the control device 31 determines a high risk of occurrence of a low oil level for the second compressor (or the third compressor). In such a situation, the oil balancing action will comprise controlling the operation of the oil pump 28 and the valves 29 in order to transfer oil from the other compressor 8 with high oil level, i.e. from the third compressor (or the second compressor).

[0119] It is also contemplated by the present invention to transfer oil from more than one oil pan 14 at the same time.

[0120] Scenario 2: Excessive oil level detected in the first group and low oil level detected in the second group

[0121] In this scenario, oil must be transferred from the first group to the second group. Since the first group only comprises one compressor 8, oil will be transferred from the first compressor to the second compressor and / or the third compressor.

[0122] In such a situation, the oil balancing action comprises 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 particularly 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 comprises stopping the operation of the oil pump 28 and closing the valve 29 associated with the second compressor (and / or the third compressor).

[0123] Scenario 3: Low oil level and excessive oil level detected in the same group

[0124] In this scenario, oil needs to be transferred within the same group. To achieve this, an intermediate step has been introduced. Let's assume that the excessive oil level has been detected in the second compressor and there is the low oil level detected in the third compressor.

[0125] First, the oil balancing action includes controlling the func 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 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 quantity 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.

[0126] Then, the oil balancing action comprises 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 comprises stopping the operation of the oil pump 28 and closing the valve 29 of the third compressor.

[0127] It is also contemplated by the present invention that the controller 31 may 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 comprise controlling, via the controller 31, the oil pump 28 and the valves 29 to transfer additional oil to such a compressor 8 before said compressor 8 reaches said operating condition.

[0128] [Fig.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.

[0129] 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.

[0130] 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 therefore will not be disclosed hereinafter. In this situation, oil can be transferred to / from two compressors 8 at the same time.

[0131] [Fig.6] discloses a multi-compressor refrigeration system 2 according to a third embodiment of the invention which differs from the second embodiment essentially in that the second group comprises three compressors 8.

[0132] 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 therefore will not be disclosed hereinafter. In this situation, oil can be transferred to / from three compressors 8 at the same time.

[0133] [Fig. 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 fluidically connected at said oil equalization line branch 26.

[0134] 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.

[0135] It is also contemplated 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.

[0136] [Fig.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 one-way oil pumps 28, for example two one-way positive displacement oil pumps which are connected in parallel.

[0137] It is also contemplated by the present invention that such a pumping device oil pump 27 may replace the oil pumping devices 27 in other embodiments described above.

[0138] Based on the embodiments described above, one skilled in the art can easily implement the present invention in a multi-compressor refrigeration system 2 comprising any number of compressors 8. In addition, one skilled in the art can easily combine the features described in connection with different embodiments, unless otherwise clearly indicated.

[0139] Of course, the invention is not limited to the embodiments described above as non-limiting examples, but on the contrary it encompasses all embodiments.

Claims

Claims

1. 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 three compressors (8) which are coupled in parallel, each compressor (8) being equipped with a suction connection (11), an oil sump (14) and an oil level detection device (15) operatively connected to the control device (31) and configured to detect an oil level in the respective 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 to fluidically connect the suction connections (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 operatively connected to the control device (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 such that one of said two groups is located on a 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 in that the multi-compressor device (7) further comprises at least one valve (29) located in the common oil equalization line (23) and associated with a group comprising at least two compressors (8),the at least one valve (29) being operatively 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 the at least one valve (29), the control device (31) being configured, to control the operation of the oil pumping device (27) and the at least one valve (29) based on outputs from the oil level sensing devices (15).

2. A multi-compressor refrigeration system (2) according to claim 1, wherein the controller (31) is configured to determine a current amount of oil in each compressor (8) based on outputs from the oil level sensing devices (15).

3. A multi-compressor refrigeration system (2) according to claim 1 or 2, wherein the control device (31) is configured to: - monitor an oil level in the oil sump (14) of each compressor (8) via the respective oil level detection device (15), - detect 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 - perform 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) 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) 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.,

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

5. A multi-compressor refrigeration system (2) according to claims 3 and 4, wherein the oil balancing action comprises 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) for which an excess oil situation has been detected at oil pan (14) of the compressor (8) for which a low oil level situation has been detected.

6. A multi-compressor refrigeration system (2) according to any one of claims 3 to 5, wherein the oil balancing action comprises: - if oil balancing is required between compressors (8) of different groups, 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 situation has not been detected and belonging to a group different from the group comprising 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,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 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, 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 comprising the compressor (8) for which a low oil level situation has been detected.,

7. A multi-compressor refrigeration system (2) according to any one of claims 3 to 6, wherein the controller (31) is configured to determine the amount of oil that is to be transferred to a compressor (8) for which a low oil level situation has been detected to achieve a predetermined oil level in said compressor (8).

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

9. A multi-compressor refrigeration system (2) according to claim 8, wherein the control device (31) is configured to control the oil pumping device (27) and the at least one valve (29) to supply more oil to a compressor (8) that has a high risk of low oil condition occurring and / or to control the oil pumping device (27) and the at least one valve (29) to not draw oil from a compressor (8) that has a high risk of low oil condition occurring.

10. A 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 meter and deliver a quantity of pumped oil.

11. A multi-compressor refrigeration system (2) according to any one of claims 1 to 10, wherein the at least one oil pump (28) comprises two one-way oil pumps which are connected in parallel.

12. A multi-compressor refrigeration system (2) according to any one of claims 1 to 10, wherein the at least one oil pump (28) comprises a bi-directional oil pump.

13. A 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. A 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 located at an oil equalization line branch (26) of the common oil equalization line (23) and configured to control an oil flow between the oil pumping device (27) and two respective oil equalization branches (25) of the common oil equalization line (23) which are fluidically connected at said oil equalization line branch (26).

15. A 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 located in an oil equalization branch (25) of the common oil equalization line (23), which is fluidically connected to a respective compressor, and configured to control an oil flow between the oil pumping device (27) and the respective compressor fluidly connected to said oil equalization branch (25).

16. A 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 of balancing oil in a multi-compressor refrigeration system (2) comprising a controller (31) configured to control operation of the multi-compressor refrigeration system (2) and a multi-compressor device (7) comprising at least three compressors (8) which are coupled in parallel, each compressor (8) being equipped with a suction connection (11), an oil sump (14) and an oil level detection device (15) operatively connected to the controller (31) and configured to detect an oil level in the respective oil sump (14),the oil sumps (14) of all compressors (8) of the multi-compressor device (7) being fluidically connected via a common oil equalization line (23) and the suction connections (11) of all compressors (8) of the multi-compressor device (7) being fluidically 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, where each group comprises at least one compressor (8) and at least one group comprises at least two compressors (8), - providing an 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 oil pumping device (27) and the other of said two groups is located on a second side of the pumping device of oil,the oil pumping device (27) comprising at least one oil pump (28), - providing 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 device, oil pumping (27) and at least one compressor (8) of the group associated with the at least one valve (29), - monitoring an oil level in the oil sump (14) of each compressor (8) via the respective oil level detection device (15), - detecting that a low oil level situation occurs in a compressor (8), if the oil level in said 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) 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) 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.

18. A method according to claim 17, wherein the oil balancing action comprises: - if oil balancing is required between compressors (8) of different groups, transferring, by controlling the operation of the oil pumping device (27) and the at least one valve (29), 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 group different from the group comprising 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, transferring, by controlling the operation of the oil pumping device (27) and the at least one valve (29), 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, 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 comprising the compressor (8) for which a low oil level situation has been detected.

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