TITLE : A multi-compressor system having individual oil balancing lines

Field of the invention

The present invention relates to a multi-compressor system, and in particular to a multi-compressor refrigeration system.

Background of the invention

As known, a refrigeration system comprises a refrigerant circulation circuit successively including a condenser, an expansion device, an evaporator and a multi-compressor system connected in series, the multi-compressor system, also named manifolded compressor system, comprising:

- a plurality of compressors which are parallelly coupled, the plurality of compressors comprising at least three compressors, each compressor including a compressor shell provided with a suction fitting and a discharge fitting,

- a common suction line and inlet connection lines each connecting the common suction line to the suction fitting of a respective compressor,

- a common discharge line and outlet connection lines each connecting the common discharge line to the discharge fitting of a respective compressor,

- a plurality of individual oil balancing lines each fluidly connecting low pressure volumes of two respective compressors of the plurality compressors, at least one compressor of the plurality of compressors comprising two oil balancing line connections formed in the respective compressor shell and each connected to a respective individual oil balancing line, and two of the plurality of compressors comprising a single oil balancing line connection formed in the respective compressor shell and connected to a respective individual oil balancing line, and

- a controller configured to control operation of the multi-compressor system.

When such a multi-compressor system operates with at least one compressor that is off, i.e. not operating, while at least two compressors are running, pressure in the low-pressure volume of the stopped compressor increases considerably and is higher than that of the running compressors, which causes gas to flow from the stopped compressor towards the running compressors through the individual oil balancing lines connected to the stopped compressor.

Such a gas flow, in each of the individual oil balancing lines connected to the stopped compressor, may prevent oil from leaving a running compressor trough the respective individual oil balancing line. Therefore, no oil exchange between the two running compressors would be possible, which could lead to low oil level in one of the running compressors and excessive oil level in the other running compressor. Similar situations may occur, when two compressors are stopped in a multi-compressor system comprising four compressors.

When a compressor, having suffered from a significant oil level drop, is subsequently restarted, the quantity of oil contained in the oil sump thereof may not be sufficient to ensure suitable lubrication of the different moving parts of said compressor, which may damage the integrity of said compressor and thus the relatability of the above-mentioned multicompressor system.

US10495089 discloses a multi-compressor system of the aforementioned type in which the inlet connection lines have specific designs such that, in use, one or several compressors of the multi-compressor system receive(s) more oil, from the the common suction line, than the other compressors of the multi-compressor system. Such a configuration of the multi-compressor system discloses in US10495089 allows part load operation of the multicompressor system with any on/off configuration of the compressors, as in all operating configurations, sufficient oil is available for all running compressors.

However, this special design of the inlet connection lines leads to increased costs for the multi-compressor system.

Summary of the invention

It is an object of the present invention to provide an improved multi-compressor system which can overcome the drawbacks encountered in conventional multi-compressor systems.

Particularly, an object of the present invention is to provide a multi-compressor system with good oil balancing properties at reduced costs.

According to the invention such a multi-compressor system comprises:

- a plurality of compressors which are parallelly coupled, the plurality of compressors comprising at least three compressors, each compressor including a compressor shell provided with a suction fitting and a discharge fitting,

- a common suction line and inlet connection lines each connecting the common suction line to the suction fitting of a respective compressor, the inlet connection lines having identical or similar dimensions,

- a common discharge line and outlet connection lines each connecting the common discharge line to the discharge fitting of a respective compressor, - a plurality of individual oil balancing lines each fluidly connecting low pressure volumes of two respective compressors of the plurality compressors, at least one compressor of the plurality of compressors comprising two oil balancing line connections formed in the respective compressor shell and each connected to a respective individual oil balancing line, and two of the plurality of compressors comprising a single oil balancing line connection formed in the respective compressor shell and connected to a respective individual oil balancing line, and

- a controller configured to control operation of the multi-compressor system, i.e. to control operation of the plurality of compressors and particularly to control which compressor(s) of the plurality of compressors is(are) in operation, the controller being configured to operate the multi-compressor system according to a plurality of predetermined authorized operating configurations and being configured to prevent operation of the multi-compressor system according to a plurality of predetermined forbidden operating configurations.

With the simple suction line design of the present invention, certain configurations of running and stopped compressors during part-load operation of the multi-compressor system have to be avoided, where sufficient oil supply to all running compressors cannot be guaranteed. For example, when the two compressors having a single oil balancing line connection are running and the at least one compressor having two oil balancing line connections is stopped, the enlarged pressure within the compressor shell of the stopped compressor would press oil from the stopped compressor towards the two adjacent running compressors through the individual oil balancing lines. However, no oil exchange between the two running compressors would be possible, which could lead to low oil level in one of the running compressors and excessive oil level in the other running compressor. Particularly, the predetermined authorized operation configurations prevent operation of the multi-compressor system in which sufficient oil supply to all running compressors cannot be guaranteed.

According to the present invention, the predetermined authorized operation configurations comprise specific allowable on/off configurations of said compressors running at the same time, depending on the required load, i.e. the required output capacity. For each number of compressors necessary to fulfil the actual load demand, at least one authorized operation configuration is given in the plurality of predetermined authorized operation configurations.

Therefore, the multi-compressor system according to the present invention does not require a specific suction line design while ensuring good oil balancing properties.

In the present document, “similar dimensions” means that a ratio of any dimension of an inlet connection line to the corresponding dimension of any other inlet connection line is between 0.8 and 1 .2, and advantageously between 0.9 and 1.1. The multi-compressor system may also include one or more of the following features, taken alone or in combination.

Advantageously, each individual oil balancing line is directly communicated with the low pressure volumes of the two respective compressors of the plurality compressors.

According to an embodiment of the invention, each individual oil balancing line is devoid of check valve.

According to an embodiment of the invention, each individual oil balancing line is configured to simultaneously allow a flow of oil and refrigerant gas through said individual oil balancing line.

According to an embodiment of the invention, the compressor shell of each compressor having two oil balancing line connections includes two oil sump ports and two connecting fittings each connected to a respective oil sump port and forming the two respective oil balancing line connections, and the compressor shell of each compressor having a single oil balancing line connection includes one oil sump port and one connecting fitting connected to the respective oil sump port and forming the respective oil balancing line connection.

According to an embodiment of the invention, each compressor of the plurality of compressors has a variable capacity, and for example includes an electric motor having a variable speed.

According to an embodiment of the invention, each compressor of the plurality of compressors has a fixed capacity, and for example includes an electric motor having a fixed speed.

According to an embodiment of the invention, the plurality of compressors comprise both variable capacity compressors and fixed capacity compressors.

According to an embodiment of the invention, each predetermined authorized operating configuration of the plurality of predetermined authorized operating configurations is configured to result in a predetermined output capacity for the multi-compressor system.

According to an embodiment of the invention, the controller is configured to control the plurality of compressors in response to a required output capacity for the multi-compressor system.

According to an embodiment of the invention, the controller is configured to select a predetermined authorized operating configuration, among the plurality of predetermined authorized operating configurations, that corresponds to the required output capacity for the multi-compressor system.

According to an embodiment of the invention, the plurality of predetermined authorized operating configurations includes part-load operation configurations where only one of the compressors having a single oil balancing line connection is stopped while all the other compressors of the plurality of compressors are running.

According to an embodiment of the invention, the plurality of predetermined authorized operating configurations includes part-load operation configurations where the two compressors having a single oil balancing line connection are stopped while all the other compressors of the plurality of compressors are running.

According to an embodiment of the invention, the plurality of predetermined authorized operating configurations includes part-load operation configurations where at least two of the plurality of compressors are running and only one of the two compressors having a single oil balancing line connection is stopped, while the compressor, which has two oil balancing line connections and which is adjacent and coupled, via an oil balancing line, to said compressor having a single oil balancing line connection which is stopped, is stopped.

According to an embodiment of the invention, the plurality of predetermined authorized operating configurations includes a full-load operation configuration where all the compressors of the plurality of compressors are running.

According to an embodiment of the invention, the controller is configured to prevent operation of the multi-compressor system according to a plurality of predetermined forbidden operating configurations.

According to an embodiment of the invention, the plurality of predetermined forbidden operating configurations includes part-load operation configurations where at least two of the plurality of compressors are running and one of said at least two compressors, which are running, is a compressor comprising a single oil balancing line connection, while the compressor, which has two oil balancing line connections and which is adjacent and coupled, via an oil balancing line, to said compressor comprising a single oil balancing line connection which is running, is stopped.

According to an embodiment of the invention, the plurality of predetermined forbidden operating configurations includes part-load operation configurations where one compressor of the at least one compressor comprising two oil balancing line connections is running, while all the others compressors of the plurality of compressors are stopped.

According to an embodiment of the invention, the plurality of predetermined authorized operating configurations includes part-load operation configurations where only one of the plurality of compressors is running and said only one compressor which is running is a compressor comprising a single oil balancing line connection.

The plurality of predetermined authorized operating configurations and the plurality of predetermined forbidden operating configurations may further take into account an equalization of operating time for all compressors of the multi-compressor system. For example, in a multi-compressor system comprising four compressors, the compressors with two oil balancing line connections are the preferred operating compressors of the predetermined authorized operating configurations for part-load with two or three running compressors. Therefore, it is advantageous to preferably use the compressors with single oil balancing line connection for part-load operation with single compressor operation.

According to an embodiment of the invention, each of said compressors includes an oil sump located at a lower portion of the respective compressor shell.

According to an embodiment of the invention, all of the oil balancing line connections are approximately at the same vertical elevation with respect to a bottom of the respective compressor shell. Said configuration ensures proper equalization of the oil levels in the plurality of compressors.

According to an embodiment of the invention, each individual oil balancing line fluidly connects low pressure volumes of two respective adjacent compressors of the plurality compressors.

According to an embodiment of the invention, each of the plurality of individual oil balancing lines includes a tubular connecting part including a first end portion connected to an oil balancing line connection formed in the compressor shell of a respective compressor, a second end portion connected to an oil balancing line connection formed in the compressor shell of a respective compressor, and an intermediate portion located between the respective first and second end portions and fluidly connecting said respective first and second end portions.

According to an embodiment of the invention, each tubular connecting part includes a constant cross section, i.e. which is constant along its length.

According to an embodiment of the invention, each tubular connecting part is a tubular connecting piece.

According to an embodiment of the invention, the first end portion of each tubular connecting part includes an axial end face which is substantially flushed with or recessed from an inner surface of the compressor shell of the respective compressor, and the second end portion of each tubular connecting part includes an axial end face which is substantially flushed with or recessed from the inner surface of the compressor shell of the respective compressor.

According to an embodiment of the invention, each of the plurality of individual oil balancing lines includes:

- a first oil level regulating part fluidly connected to the first end portion of a respective tubular connecting part and protruding into the compressor shell of the respective compressor, and - a second oil level regulating part fluidly connected to the second end portion of a respective tubular connecting part and protruding into the compressor shell of the respective compressor.

According to an embodiment of the invention, the first and second oil level regulating parts and the tubular connecting part of each individual oil balancing lines are made as separate parts, and are secured together, for example by a snap arrangement or by welding or screwing.

According to another embodiment of the invention, the first and second oil level regulating parts and the tubular connecting part of each individual oil balancing lines are manufactured as an integral component.

According to an embodiment of the invention, each compressor of the plurality of compressors is a scroll compressor.

According to an embodiment of the invention, each of the plurality of individual oil balancing lines extends substantially horizontally.

According to an embodiment of the invention, each of the plurality of individual oil balancing lines has a generally V-shape or a generally U-shape.

According to an embodiment of the invention, the individual oil balancing lines are substantially identical.

According to an embodiment of the invention, the plurality of compressors comprises at least four compressors, the plurality of compressors comprising at least two compressors each comprising two oil balancing line connections and two compressors each comprising a single oil balancing line connection.

The present invention also relates to a refrigeration system comprising a refrigerant circulation circuit successively including a condenser, an expansion device, an evaporator and a multi-compressor system according to the present invention connected in series.

The present invention further relates to a method of controlling operation of a multicompressor system, the method comprising:

- providing a multi-compressor system including: a plurality of compressors which are parallelly coupled, the plurality of compressors comprising at least three compressors, each compressor including a compressor shell provided with a suction fitting and a discharge fitting, a common suction line and inlet connection lines each connecting the common suction line to the suction fitting of a respective compressor, the inlet connection lines having identical or similar dimensions, a common discharge line and outlet connection lines each connecting the common discharge line to the discharge fitting of a respective compressor, a plurality of individual oil balancing lines each fluidly connecting low pressure volumes of two respective compressors of the plurality of compressors, at least one compressor of the plurality of compressors comprising two oil balancing line connections formed in the respective compressor shell and each connected to a respective individual oil balancing line, and two of the plurality of compressors comprising a single oil balancing line connection formed in the respective compressor shell and connected to a respective individual oil balancing line,

- preventing operation of the multi-compressor system according to a plurality of predetermined forbidden operating configurations,

- selecting a predetermined authorized operation configuration among a plurality of predetermined authorized operation configurations, and

- operating the multi-compressor system according to the selected predetermined authorized operation configuration.

Brief description of the drawings

The following detailed description of two embodiments of the invention is better understood when read in conjunction with the appended drawings being understood, however, that the invention is not limited to the specific embodiments disclosed.

Figure 1 is a diagrammatic view of a refrigeration system including a multicompressor system according to a first embodiment of the invention.

Figure 2 is a perspective view of the multi-compressor system of figure 1 .

Figure 3 is perspective view of the multi-compressor system of figure 1 in which one compressor is sectioned along a section plan extending through a respective oil sump port and a respective drive shaft.

Figure 4 is a cross section view of the multi-compressor system of figure 2 taken along a horizontal section plan.

Figure 5 is a cross section view of a multi-compressor system view according to a second embodiment of the invention taken along a horizontal section plan.

Detailed description of the invention Figure 1 describes a refrigeration system 2 comprising a refrigerant circulation circuit 3 successively including a condenser 4, an expansion device 5, an evaporator 6 and a multi-compressor system 7 connected in series.

The multi-compressor system 7 includes a plurality of compressors 8 which are parallelly coupled. The plurality of compressors comprises at least three compressors 8, and for example four compressors 8.

Each compressor 8 includes a compressor shell 9 provided with a suction fitting

11 configured to supply the respective compressor 8 with refrigerant gas to be compressed and a discharge fitting 12 configured to discharge compressed refrigerant gas.

Advantageously, each compressor 8 is a scroll compressor, and includes a compression unit 10 disposed inside the respective compressor shell 9 and configured to compress the refrigerant gas supplied by the respective suction fitting 11 . Each compression unit 10 includes a fixed scroll, which is fixed in relation to the respective compressor shell 9, and an orbiting scroll configured to perform an orbiting movement relative to the respective fixed scroll during operation of the respective compressor 8.

Furthermore, each compressor 8 includes a drive shaft 81 which is vertically orientated and which is configured to drive the respective orbiting scroll in an orbital movement, and an electric motor 82 arranged within the respective compressor shell 9 and coupled to the respective drive shaft 81 so as to drive in rotation the respective drive shaft 81 about a rotation axis. Each compressor 8 of the plurality of compressors may have a variable capacity, and may for example include an electric motor 82 having a variable speed. However, each compressor 8 of the plurality of compressors may have a fixed capacity, and for example may include an electric motor 82 having a fixed speed.

Each compressor 8 also includes an oil sump 13 located at a lower portion of the respective compressor shell 9.

The multi-compressor system 7 further includes a common suction line 14 and inlet connection lines 15 each connecting the common suction line 14 to the suction fitting 11 of a respective compressor 8. As shown on figure 1 , the inlet connection lines 15 have identical dimensions and may have identical flow restrictions. Advantageously, the inlet connection lines 15 are substantially identical and extend horizontally.

The multi-compressor system 7 also includes a common discharge line 16 and outlet connection lines 17 each connecting the common discharge line 16 to the discharge fitting

12 of a respective compressor 8. As shown on figure 1 , the outlet connection lines 17 have identical dimensions and may have identical flow restrictions. Advantageously, the outlet connection lines 17 are substantially identical and extend horizontally. Furthermore, the multi-compressor system 7 includes a plurality of individual oil balancing lines 18. Each individual oil balancing line 18 fluidly connects low pressure volumes of two respective compressors 8 of the plurality compressors, and particularly the oil sumps 13 of two respective adjacent compressors 8. Advantageously, the individual oil balancing lines 18 are identical and extend horizontally.

At least one compressor 8, and for example two compressors 8, of the plurality of compressors comprises two oil balancing line connections 19 formed in the respective compressor shell 9 and each connected to a respective individual oil balancing line 18, and two of the plurality of compressors comprise a single oil balancing line connection 19 formed in the respective compressor shell 9 and connected to a respective individual oil balancing line 18. Advantageously, the compressors 8 having two oil balancing line connections 19 are located between the two compressors 8 having a single oil balancing line connection 19. Advantageously, all of the oil balancing line connections 19 are approximately at the same vertical elevation with respect to a bottom of the respective compressor shell 9.

According to the embodiment shown on figures 1 to 4, the compressor shell 9 of each compressor 8 having two oil balancing line connections 19 includes two oil sump ports 20.1 and two connecting fittings 20.2 each connected to a respective oil sump port 20.1 and forming the two respective oil balancing line connections 19, and the compressor shell 9 of each compressor 8 having a single oil balancing line connection 19 includes one oil sump port 20.1 and one connecting fitting 20.2 connected to the respective oil sump port 20.1 and forming the respective oil balancing line connection 19.

Each of the plurality of individual oil balancing lines 18 includes a tubular connecting part 21 having a generally V-shape or a generally U-shape. Each tubular connecting part 21 includes a first end portion 21 .1 connected to an oil balancing line connection 19 formed in the compressor shell 9 of a respective compressor 8, a second end portion 21 .2 connected to an oil balancing line connection 19 formed in the compressor shell 9 of a respective compressor 8, and an intermediate portion 21 .3 located between the respective first and second end portions 21 .1 , 21 .2 and fluidly connecting said respective first and second end portions 21 .1 , 21 .2.

According to the embodiment shown on figures 1 to 4, the first end portion 21 .1 of each tubular connecting part 21 includes an axial end face which is substantially flushed with or recessed from an inner surface 9.1 of the compressor shell 9 of the respective compressor 8, and the second end portion 21.2 of each tubular connecting part 21 includes an axial end face which is substantially flushed with or recessed from the inner surface 9.1 of the compressor shell 9 of the respective compressor 8.

The multi-compressor system 7 also includes a controller 23 configured to control operation of the multi-compressor system 7, i.e. to control operation (starting or stopping) of the plurality of compressors and particularly to control which compressor(s) 8 of the plurality of compressors is(are) in operation. The controller 23 may for example includes a microprocessor and a memory.

Particularly, the controller 23 is configured to operate the multi-compressor system

7 according to a plurality of predetermined authorized operating configurations and is configured to prevent operation of the multi-compressor system 7 according to a plurality of predetermined forbidden operating configurations.

The predetermined authorized operation configurations comprise specific allowable on/off configurations of said compressors 8 running at the same time, depending on the required load, i.e. the required output capacity. For each number of compressors 8 necessary to fulfil the actual load demand, at least one authorized operation configuration is given in the plurality of predetermined authorized operation configurations.

Advantageously, each predetermined authorized operating configuration of the plurality of predetermined authorized operating configurations is configured to result in a predetermined output capacity for the multi-compressor system 7, and the controller 23 is configured to control the plurality of compressors in response to a required output capacity for the multi-compressor system 7. Particularly, the controller 23 is configured to select a predetermined authorized operating configuration, among the plurality of predetermined authorized operating configurations, that corresponds to the required output capacity for the multi-compressor system 7.

The plurality of predetermined authorized operating configurations notably includes:

- part-load operation configurations where only one of the compressors 8 having a single oil balancing line connection 19 is stopped while all the other compressors 8 of the plurality of compressors are running;

- part-load operation configurations where the two compressors 8 having a single oil balancing line connection 19 are stopped while all the other compressors 8 of the plurality of compressors are running;

- part-load operation configurations where at least two of the plurality of compressors are running and only one of the two compressors 8 having a single oil balancing line connection 19 is stopped, while the compressor 8, which has two oil balancing line connections 19 and which is adjacent and coupled, via an oil balancing line, to said compressor

8 having a single oil balancing line connection 19 which is stopped, is stopped;

- a full-load operation configuration where all the compressors 8 of the plurality of compressors are running.

The plurality of predetermined forbidden operating configurations includes: - part-load operation configurations where at least two of the plurality of compressors are running and one of said at least two compressors 8, which are running, is a compressor 8 comprising a single oil balancing line connection 19, while the compressor 8, which has two oil balancing line connections 19 and which is adjacent and coupled, via an oil balancing line, to said compressor 8 comprising a single oil balancing line connection 19 which is running, is stopped;

- part-load operation configurations where one compressor 8 of the at least one compressor comprising two oil balancing line connections 19 is running, while all the others compressors 8 of the plurality of compressors are stopped.

The plurality of predetermined authorized operating configurations and the plurality of predetermined forbidden operating configurations may further take into account an equalization of operating time for all compressors 8 of the multi-compressor system 7. For example, in a multi-compressor system 7 comprising four compressors 8, the compressors 8 with two oil balancing line connections 19 are the preferred operating compressors 8 of the predetermined authorized operating configurations for part-load with two or three running compressors 8. Therefore, it is advantageous to preferably use the compressors 8 with single oil balancing line connection 19 for part-load operation with single compressor operation.

Therefore, advantageously, the plurality of predetermined authorized operating configurations also includes part-load operation configurations where only one of the plurality of compressors is running and said only one compressor 8 which is running is a compressor 8 comprising a single oil balancing line connection 19.

The plurality of predetermined authorized and forbidden operating configurations are summarized in the below table:

[Table 1]

In the above table, CP1 and CP4 correspond respectively to the two compressors 8 each having a single oil balancing line connection 19, CP2 and CP3 correspond to the two compressors 8 each having two oil balancing line connections 19 (CP2 being connected to CP1 via a respective individual oil balancing line 18 and CP3 being connected to CP4 via a respective individual oil balancing line 18), and “ON” and “OFF” correspond respectively to the operating modes of the respective compressor 8. The first column of the table corresponds to the numbers of compressors 8 which are ON, i.e. which are running.

The present invention further relates to a method of controlling operation of the multi-compressor system 7 according to the present invention. Said method comprises:

- providing the multi-compressor system 7,

- requiring an output capacity for the multi-compressor system 7,

- preventing operation of the multi-compressor system 7 according to a plurality of predetermined forbidden operating configurations,

- selecting a predetermined authorized operation configuration, among a plurality of predetermined authorized operation configurations, that corresponds to the required output capacity for the multi-compressor system 7, and

- operating the multi-compressor system 7 according to the selected predetermined authorized operation configuration.

Figure 5 represents a multi-compressor system 7 according to a second embodiment of the invention which differs from the embodiment shown on figures 1 to 4 essentially in that each of the plurality of individual oil balancing lines 18 further includes:

- a first oil level regulating part 24 fluidly connected to the first end portion 21 .1 of a respective tubular connecting part 21 and protruding into the compressor shell 9 of the respective compressor 8, and

- a second oil level regulating part 25 fluidly connected to the second end portion 21.2 of a respective tubular connecting part 21 and protruding into the compressor shell 9 of the respective compressor 8.

According to the embodiment shown on figure 5, the first and second oil level regulating parts 24, 25 and the tubular connecting part 21 of each individual oil balancing lines 18 are made as separate parts, and are secured together, for example by a snap arrangement or by welding or screwing. However, according to an embodiment of the invention not shown on the figures, the first and second oil level regulating parts 24, 25 and the tubular connecting part 21 of each individual oil balancing lines 18 may be manufactured as an integral component.

Of course, the invention is not restricted to the embodiments described above by way of non-limiting examples, but on the contrary it encompasses all embodiments thereof.