Method of managing oil in multi-compressor refrigeration system using oil level detection device

Abstract

A method for managing oil in a multi-compressor refrigeration system in operation, the multi-compressor refrigeration system comprising a controller and a multi-compressor device, the method comprising: monitoring, by a corresponding oil level detection device, an oil level in the oil sump of each compressor; if the oil level in the running compressor reaches a preset threshold condition, detecting that the oil shortage condition occurs in the running compressor; if an oil shortage condition is detected for the compressors in operation, performing an oil management action based on a change in on / off configuration of the compressors in the multi-compressor apparatus, and including: performing an oil balancing action if more than half of the total number of compressors is in operation; or if half or less of the total number of compressors is operating, an oil return action is performed.

Description

Technical Field

[0001] The present invention relates to a method of managing oil in a multi-compressor refrigeration system using an oil level detection device. Background Art

[0002] CN212867870U discloses a refrigerant compressor, which includes a compressor housing, an oil tank and an oil level sensor device, wherein the oil tank is arranged in the lower part of the compressor housing, and the oil level sensor device is positioned in the lower part of the compressor housing and is configured to detect the oil level in the oil tank of the refrigerant compressor.

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

[0004] Between the low oil level warning and stopping the compressor, some action is recommended to avoid shutting down the compressor. There are active and passive oil management methods.

[0005] Active oil management methods monitor the refrigeration system in real time and perform oil management by using additional valves and / or oil tanks. The control of the operation is valve-based. Such solutions are complex and expensive.

[0006] Passive oil management methods perform oil management operations in a periodic or substantially predetermined manner, regardless of the actual state of the multi-compressor refrigeration system. However, such schemes can significantly affect the efficiency of the multi-compressor refrigeration system because the oil management operations are performed even when they are not necessary.

[0007] Furthermore, a detected low oil level may be due to an oil backflow problem (oil trapped in the refrigeration system) or due to an oil balance problem between compressors. Depending on the cause of the low oil level, different actions are recommended.

[0008] EP1605212 discloses an air conditioner and a method for performing an oil balancing operation in the air conditioner. When it is detected that the oil level in the oil reservoir of a specific one of the compressors is lower than a predetermined level, oil balancing can be performed. The compressors are connected by an oil balancing pipe to supply excess oil in each compressor to the remaining compressors. An oil balancing pipe opening / closing valve is arranged in the oil balancing pipe. The method for performing the oil balancing operation involves: opening a valve to supply oil from one compressor oil tank to another compressor oil tank, and pressurizing the lubricating oil by the discharge pressure of one compressor, and supplying the pressurized lubricating oil to the other compressor.

[0009] However, this special design of the oil equalization pipe leads to an increase in the cost of the multi-compressor equipment. Summary of the invention

[0010] It is an object of the present invention to provide an improved method for managing oil in an operating multi-compressor refrigeration system which overcomes the disadvantages encountered in conventional multi-compressor refrigeration systems.

[0011] In particular, an object of the present invention is to provide a method for managing oil in an operating multi-compressor refrigeration system which is simple and easy to implement and which is capable of providing a robust or robust prevention of an operating compressor from being in an oil starved situation, particularly if oil is trapped in the refrigeration system outside the compressor manifold.

[0012] According to the present invention, there is provided a method for managing oil in a running multi-compressor refrigeration system, the multi-compressor refrigeration system comprising a controller and a multi-compressor device, the controller being configured to control the operation of the multi-compressor refrigeration system, the multi-compressor device comprising compressors connected in parallel, the total number of compressors in the multi-compressor device being at least two, and each compressor being equipped with an oil tank and an oil level detection device, the oil level detection device being operably connected to the controller and configured to detect the oil level in a corresponding oil tank, the oil tanks being connected via a common oil balance line, the method comprising:

[0013] - monitoring the oil level in the oil tank of each compressor by means of a corresponding oil level detection device;

[0014] - if the oil level in the operating compressor reaches a predetermined threshold condition, an oil shortage condition in the operating compressor is detected, e.g. by the controller and e.g. based on a signal output from an oil level detection device;

[0015] - if a lack of oil condition is detected for a running compressor, performing an oil management action, the oil management action being based on a change in the on / off configuration of the compressors in the multi-compressor installation, and the oil management action comprising:

[0016] If the total number of compressors in the multi-compressor device is 2 to 4, then:

[0017] performing an oil balancing action if more than half of the total number of compressors are operating, wherein the oil balancing action includes balancing oil between the compressors;

[0018] performing an oil return action if half or less of the total number of compressors are operating, wherein the oil return action comprises recovering oil from the multi-compressor refrigeration system (i.e., from outside the multi-compressor device) to the multi-compressor device,

[0019] If the total number of compressors in the multi-compressor installation is 5 or more, then:

[0020] performing said oil balancing action if more than half of said total number of compressors are operating;

[0021] If half or less of the total number of compressors are operating and at least two compressors are operating, performing the oil balancing action and / or the oil return action;

[0022] If only one compressor is running, the oil return action is performed.

[0023] The method according to the invention combines robustness or robustness prevention of running compressors in oil starvation situations, since it monitors the operation of the compressors in real time (claimed to have the advantages of active oil management) and is easy to implement, since it does not require any additional valves, oil balancing lines or oil tanks (claimed to have the advantages of passive oil management). Furthermore, the method according to the invention confers improved efficiency to multi-compressor refrigeration systems, since it performs oil management only when necessary.

[0024] The method may also include one or more of the following features, alone or in combination.

[0025] According to an embodiment of the present invention, the method further comprises: determining the number of operating compressors in the multi-compressor device, for example by a controller.

[0026] According to an embodiment of the present invention, the method further comprises: determining the total number of compressors in the multi-compressor device, for example by a controller.

[0027] Advantageously, the method further comprises determining, for example by a controller, a percentage of compressors in the multi-compressor arrangement that are in operation.

[0028] According to an embodiment of the present invention, the method further includes:

[0029] - If the oil level in the oil sump of the corresponding compressor reaches a predetermined low oil level value, a low oil level warning signal is output from the oil level detection device and output to the controller.

[0030] According to an embodiment of the present invention, the predetermined threshold condition is met if the following conditions occur:

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

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

[0033] Thanks to the above requirements, false fault detection of oil shortage in a running compressor can be prevented, and the efficiency and reliability of a multi-compressor refrigeration system can be further improved.

[0034] According to an embodiment of the present invention, the oil balancing action includes: maintaining the operation of the running compressor in which the oil shortage situation has been detected, and stopping the operation of other running compressors or all other running compressors (i.e., (one or more) other running compressors in which the oil shortage situation has not been detected). In particular, if all compressors are running and an oil shortage situation is detected for one of the running compressors, it is assumed that there is a problem with oil balancing (i.e., there is excess oil in at least one of the running compressors in which the oil shortage situation has not been detected). In order to overcome this problem and balance the oil level in all compressors, the controller stops the operation of (one or more) other running compressors. Thanks to this action, the running compressor in which the oil shortage situation has been detected can draw more oil from the multi-compressor device and thus increase the oil level in the corresponding oil tank. In other words, the pressure of the oil-free surface of the stopped compressor increases (the pressure drop decreases), and its potential excess oil is transported to the adjacent compressors through the common oil balancing line.

[0035] According to an embodiment of the present invention, if all compressors are running, the oil balancing action includes: maintaining the operation of the running compressor in which the oil shortage situation has been detected, and stopping the operation of the other running compressors or all other running compressors (i.e., (one or more) other running compressors in which the oil shortage situation has not been detected).

[0036] According to an embodiment of the present invention, the oil balancing action includes: stopping the operation of the running compressors in which the oil shortage has been detected, and starting the operation of the idle compressors (i.e., the idle compressors in which the oil shortage has not been detected) so as to maintain the workload of the multi-compressor refrigeration system substantially the same. In particular, if not all compressors are running, and an oil shortage is detected for one of the running compressors, it is assumed that there is a problem with the oil balance (i.e., there is an excess of oil in the compressors in which the oil shortage has not been detected). In order to overcome this problem and balance the oil level in all compressors, the controller stops the operation of the running compressors in which the oil shortage has been detected, and maintains the operation of the (one or more) running compressors in which the oil shortage has not been detected, and starts the operation of the idle compressors in which the oil shortage has not been detected. Thanks to this action, the multi-compressor refrigeration system maintains the required workload, balances the oil between the compressors, and protects the compressors in the oil shortage, and allows the compressors in the oil shortage to recover some oil. In fact, a suction bypass is created through the common oil balancing line, so some refrigerant loaded with oil will still enter the compressor and fill the oil into the compressor oil tank.

[0037] According to an embodiment of the present invention, if not all compressors are running, and therefore if at least one compressor is idle, the oil balancing action includes: stopping the operation of the running compressors for which the oil shortage situation has been detected, and starting the operation of the idle compressors (i.e., the idle compressors for which the oil shortage situation has not been detected) so as to maintain the workload of the multi-compressor refrigeration system substantially consistent.

[0038] According to an embodiment of the present invention, the oil balancing action includes: stopping the operation of all compressors. By doing so, the oil level between the compressors can be balanced by utilizing the principle of the communicating containers.

[0039] According to an embodiment of the present invention, the oil return action includes: increasing the workload of the multi-compressor refrigeration system. By increasing the workload of the multi-compressor refrigeration system, oil pressurization can be provided, and oil can be returned to the multi-compressor device and thereby to the compressor. In fact, due to the increase in workload, the demand for oil supply increases, and oil can be drawn from a more distant part of the multi-compressor refrigeration system.

[0040] According to an embodiment of the present invention, if the total number of compressors in the multi-compressor is equal to 2, the oil return action includes: maintaining the operation of the running compressors for which the lack of oil condition has been detected, and starting the operation of the idle compressors (i.e., the idle compressors for which the lack of oil condition has not been detected). Therefore, if the total number of compressors in the multi-compressor is equal to 2, and if one compressor is idle, the oil return action includes: maintaining the operation of the running compressors for which the lack of oil condition has been detected, and starting the operation of the idle compressors. In particular, if only one of the two compressors is running (50% load), and a lack of oil condition is detected for the one running compressor, it is assumed that there is a problem with the oil returning from the refrigeration system to the compressor (the oil is trapped in the refrigeration system outside the multi-compressor device). In order to overcome this problem and return the oil to the multi-compressor device, and in particular to the compressor for which the lack of oil condition has been detected, the controller starts the operation of the idle compressor. Thanks to this action, oil pressurization can be provided, and the oil can be returned to the multi-compressor device, and in particular to the running compressor for which the lack of oil condition has been detected.

[0041] According to an embodiment of the present invention, if the total number of compressors in the multi-compressor is greater than 2, the oil return action includes: stopping the operation of the running compressor in which the oil shortage has been detected, and starting the operation of at least two idle compressors, and for example starting the operation of all idle compressors, i.e. starting the operation of (one or more) idle compressors in which the oil shortage has not been detected. In particular, if only one compressor is running, and all other compressors are idle, and an oil shortage is detected for the running compressor, it is assumed that there is a problem about the oil returning from the refrigeration system to the multi-compressor device (the oil is trapped in the refrigeration system outside the multi-compressor device). In order to overcome this problem and return the oil to the compressor, the controller stops the running compressor in which the oil shortage has been detected, and starts all other compressors. Thanks to this action, oil boosting can be provided, and the oil can be returned to the multi-compressor device, and in particular to the running compressor in which the oil shortage has been detected.

[0042] According to an embodiment of the present invention, if the total number of compressors in the multi-compressor is greater than 2, and if only one compressor is running and all other compressors are idle, the oil return action includes: stopping the operation of the running compressor in which the oil shortage condition has been detected, and starting the operation of all idle compressors.

[0043] According to an embodiment of the present invention, the oil return action includes: opening an electronic expansion valve of the multi-compressor refrigeration system to increase the suction mass flow of the multi-compressor device. By increasing the suction mass flow, oil can be drawn from a further part of the refrigeration system and returned to the multi-compressor device, and in particular to the operating compressor for which the oil shortage condition has been detected.

[0044] According to an embodiment of the present invention, the oil return action additionally includes: opening an electronic expansion valve of the multi-compressor refrigeration system to increase the suction mass flow of the multi-compressor device. Therefore, by combining the increase in suction mass flow with the increase in the workload of the multi-compressor refrigeration system, oil pressurization can be enhanced.

[0045] According to an embodiment of the present invention, the method further comprises: if after a third predetermined time period (e.g., two minutes) has passed since the oil management action was initiated, a lack of oil condition is still detected for the compressor for which the lack of oil condition has been previously detected, then stopping the operation of all compressors. In other words, the method further comprises: if the predetermined threshold condition remains in the corresponding compressor for a third predetermined time (e.g., two minutes) since the oil management action was initiated, then stopping the operation of all compressors. In particular, if the lack of oil condition still exists after the oil management action has been performed, it is assumed that there is a lack of oil trip or trip condition, and the controller stops the operation of the entire refrigeration system for safety reasons.

[0046] According to an embodiment of the present invention, the method further comprises: if the oil shortage situation disappears before the third predetermined time period has passed, returning to the monitoring step.

[0047] According to an embodiment of the present invention, the oil level detection device of each compressor includes an oil level sensor or an oil level switch.

[0048] According to an embodiment of the present invention, the method further comprises providing a low oil level warning if a lack of oil condition is detected for the operating compressor.

[0049] According to an embodiment of the invention, each compressor of the multi-compressor device is a scroll compressor.

[0050] According to an embodiment of the invention, each oil level detection device is at least partially located in a corresponding oil sump.

[0051] According to an embodiment of the invention, each compressor comprises a compressor housing, and the respective oil sump is arranged in a lower part of said compressor housing.

[0052] According to an embodiment of the present invention, if the total number of compressors in the multi-compressor device is 5 or more, and half or less of the total number of compressors are running and at least two compressors are running, the oil management action comprises:

[0053] - if the total pipe length of the multi-compressor refrigeration system is less than a predetermined length, performing the oil balancing action;

[0054] -If the total pipeline length of the multi-compressor refrigeration system is greater than the predetermined length, the oil return action is performed.

[0055] According to an embodiment of the present invention, the predetermined length of the pipeline is 10 meters.

[0056] The present invention also relates to a multi-compressor refrigeration system, comprising:

[0057] - a controller configured to control the operation of the multi-compressor refrigeration system;

[0058] a multi-compressor arrangement comprising compressors connected in parallel, the total number of compressors in the multi-compressor arrangement being at least two, and each compressor being provided with an oil sump and an oil level detection device, the oil level detection device being operatively connected to the controller and configured for detecting the oil level in the respective oil sump, the oil sump being connected via a common oil balancing line,

[0059] Wherein, the controller is configured to:

[0060] - monitoring the oil level in the oil tank of each compressor by means of a corresponding oil level detection device;

[0061] - if the oil level in the operating compressor reaches a predetermined threshold condition, detecting the occurrence of an oil shortage in the operating compressor based on a signal output from a corresponding oil level detection device;

[0062] - if a lack of oil condition is detected for a running compressor, performing an oil management action, the oil management action being based on a change in the on / off configuration of the compressors in the multi-compressor installation, and the oil management action comprising:

[0063] If the total number of compressors in the multi-compressor device is 2 to 4, then:

[0064] performing an oil balancing action if more than half of the total number of compressors are operating, wherein the oil balancing action includes balancing oil between the compressors;

[0065] performing an oil return action if half or less of the total number of compressors are operating, wherein the oil return action comprises recovering oil from the multi-compressor refrigeration system to the multi-compressor device,

[0066] If the total number of compressors in the multi-compressor installation is 5 or more, then:

[0067] performing said oil balancing action if more than half of said total number of compressors are operating;

[0068] If half or less of the total number of compressors are operating and at least two compressors are operating, performing the oil balancing action and / or the oil return action;

[0069] If only one compressor is running, the oil return action is performed.

[0070] According to an embodiment of the present invention, the multi-compressor refrigeration system further comprises a refrigerant circulation circuit, which sequentially comprises a condenser, an electronic expansion valve, an evaporator and a multi-compressor device connected in series.

[0071] According to an embodiment of the present invention, the multi-compressor refrigeration system has no on / off valve and / or oil tank.

[0072] According to an embodiment of the present invention, the compressor may be a fixed speed compressor or a variable speed compressor.

[0073] According to other embodiments of the present invention, the compressor may be of another type than a scroll compressor, ie the compressor may in particular be a screw compressor or a piston compressor. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] The following detailed description of two embodiments of the invention may be better understood when read in conjunction with the appended drawings whereby the invention is understood not to be limited to the particular embodiments disclosed.

[0075] Figure 1 is a schematic diagram of a multi-compressor refrigeration system according to the present invention.

[0076] Figure 2 yes Figure 1 A longitudinal cross-sectional view of a compressor of a multi-compressor refrigeration system.

[0077] Figure 3 is a perspective view of a multi-compressor device of a multi-compressor refrigeration system according to another embodiment of the present invention.

[0078] Figure 4 It is management Figure 1 Illustration of the oil method in a multi-compressor refrigeration system.

[0079] Figure 5 yes Figure 4 An illustration of a first embodiment of oil management actions of a method wherein the total number of compressors in a multi-compressor refrigeration system is 2 to 4.

[0080] Figure 6 yes Figure 4 An illustration of a second embodiment of the oil management actions of the method, wherein the total number of compressors in the multi-compressor refrigeration system is 2 to 4;

[0081] Figure 7 yes Figure 4 An illustration of an embodiment of oil management actions of a method wherein the total number of compressors in a multi-compressor refrigeration system is five or more. DETAILED DESCRIPTION

[0082] Figure 1 and Figure 2 A multi-compressor refrigeration system 2 is described, which comprises a refrigerant circulation circuit 3, which in turn comprises a condenser 4, an electronic expansion valve 5, an evaporator 6 and a multi-compressor device 7 connected in series.

[0083] The multi-compressor device 7 comprises a plurality of compressors 8 connected in parallel. Figure 1 In the illustrated embodiment, the plurality of compressors comprises three compressors 8 . However, the plurality of compressors may comprise only two compressors 8 or more than three compressors 8 , and for example four compressors 8 .

[0084] Each compressor 8 includes a compressor housing 9 provided with a suction connection 11 configured to supply the refrigerant gas to be compressed to the corresponding compressor 8 and a discharge connection 12 configured to discharge the compressed refrigerant gas.

[0085] Advantageously, each compressor 8 is a scroll compressor and comprises a compression unit 13, which is placed inside the corresponding compressor housing 9 and is configured to compress the refrigerant gas supplied by the corresponding suction connection 11. Each compression unit 13 comprises a fixed scroll, which is fixed relative to the corresponding compressor housing 9, and an orbiting scroll, which is configured to perform an orbiting movement relative to the corresponding fixed scroll during operation of the corresponding compressor 8.

[0086] Each compressor 8 further comprises an oil tank 14 arranged at the lower part of the corresponding compressor housing 9 and an oil level detection device 15, the oil tank 14 being arranged in the corresponding oil tank 14 and being configured to detect the oil level in the corresponding oil tank 14. Each oil level detection device 15 may be an oil level sensor or an oil level switch. Advantageously, each oil level detection device 15 is configured to output a low oil level warning signal if the oil level in the oil tank 14 of the corresponding compressor 8 reaches a predetermined low oil level value.

[0087] The multi-compressor device 7 further comprises a common suction line 16 and individual inlet connecting lines 17, each of which connects the common suction line 16 to the suction connection 11 of the corresponding compressor 8. The multi-compressor device 7 further comprises a common discharge line 18 and individual outlet connecting lines 19, each of which connects the common discharge line 18 to the discharge connection 12 of the corresponding compressor 8.

[0088] Furthermore, the multi-compressor device 7 comprises a common oil balancing line 20.1 and individual balancing connecting lines 20.2 (also referred to as balancing branch lines), each balancing connecting line 20.2 connecting the common oil balancing line 20.1 to an oil balancing connection 20.3 provided on the compressor housing 9 of the corresponding compressor 8. The common oil balancing line 20.1 and the balancing connecting lines 20.2 are particularly configured for fluidly connecting the oil sumps 14 of the compressors 8 and thereby allowing oil to flow between the individual compressors 8 and balancing the oil levels within the individual compressors 8.

[0089] The multi-compressor refrigeration system 2 also includes a controller 21, which is configured to control the operation of the multi-compressor refrigeration system 2, and in particular to control the operation (start or stop) of the compressor 8, and to control which compressor or compressors 8 are operated. The controller 21 may include, for example, a microprocessor and a memory. Each oil level detection device 15 is particularly operatively connected to the controller 21.

[0090] Figure 4 Shows management Figure 1 Illustration of the oil method in a multi-compressor refrigeration system 2.

[0091] In particular, the method comprises:

[0092] - monitoring the oil level in the oil sump 14 of each compressor 8 by means of a corresponding oil level detection device 15;

[0093] - if the oil level in the operating compressor 8 reaches a predetermined threshold condition, the occurrence of oil shortage in the operating compressor 8 is detected by the controller 21 based on the signal output from the oil level detection device 15;

[0094] - if a lack of oil situation is detected for an operating compressor 8, performing an oil management action based on a change of the on / off configuration of the compressors 8 in the multi-compressor installation 7; and

[0095] - if after a third predetermined period of time (e.g. two minutes) has elapsed since the initiation of the oil management action, a lack of oil condition is still detected for a compressor 8 for which a lack of oil condition had previously been detected, stopping the operation of all compressors 8; or

[0096] - If the oil shortage situation disappears before a third predetermined period of time has elapsed from the initiation of the oil management action, returning to the monitoring step.

[0097] In particular, if the lack of oil condition still exists after the oil management action has been performed, it is assumed that a lack of oil trip or trip condition exists, and the controller 21 stops the operation of the entire multi-compressor refrigeration system 2 for safety reasons. However, if the lack of oil condition disappears, it means that the oil management action has been successful and the oil sump 14 of the compressor 8 for which the lack of oil condition had previously been detected now includes sufficient oil.

[0098] Advantageously, the predetermined threshold condition is met if:

[0099] - the low oil level warning signal output from the oil level detection device 15 is continuously output for a first predetermined time, for example for at least 10 seconds; or

[0100] - The low oil level warning signal output from the oil level detection device 15 has been switched (ie, has been switched on and off) more than a predetermined number of times (and, for example, more than ten times) within the second predetermined time (and, for example, during the last minute).

[0101] Thanks to the above requirements, it is possible to prevent erroneous fault detection of the oil shortage situation of the compressor 8 in operation.

[0102] Depending on the actual operating conditions of the multi-compressor refrigeration system 2, the oil management actions may take different forms.

[0103] Figure 5 A diagram showing a first embodiment of an oil management action is shown.

[0104] Oil management actions include the following steps:

[0105] - determining the total number of compressors 8 in the multi-compressor device 7 by means of the controller 21;

[0106] - determining the number of compressors 8 in operation in the multi-compressor installation 7 by means of the controller 21;

[0107] - determining, by means of the controller 21, the percentage of compressors 8 in operation in the multi-compressor installation 7;

[0108] - if more than half of the total number of compressors 8 are operating, performing an oil balancing action, wherein the oil balancing action comprises balancing the oil between the individual compressors 8;

[0109] - If half or less of the total number of compressors 8 are operating, an oil return action is performed, wherein the oil return action comprises recovering oil from the multi-compressor refrigeration system 2 (ie from outside the multi-compressor device 7) to the multi-compressor device 7.

[0110] In particular, if all compressors 8 are running (i.e., if 100% of the compressors are running), the oil balancing action includes maintaining the operation of the running compressors for which the oil shortage condition has been detected, and stopping the operation of (one or more) other running compressors 8 (i.e., (one or more) other running compressors for which the oil shortage condition has not been detected).

[0111] However, if not all compressors 8 are operating (i.e., the percentage of operating compressors is different from 100%), and therefore if at least one compressor 8 is idle, the oil balancing action includes: stopping the operation of the operating compressors 8 for which the oil shortage condition has been detected, and starting the operation of the idle compressors 8 so as to maintain the workload of the multi-compressor refrigeration system 2 substantially uniform.

[0112] according to Figure 5 In the illustrated embodiment, the oil return action includes increasing the workload of the multi-compressor refrigeration system 2. By increasing the workload of the multi-compressor refrigeration system 2, an oil boost may be provided, and oil may be returned to the multi-compressor device 7, and in particular to the compressor 8 for which an oil shortage condition has been detected. In effect, due to the increase in workload, the demand on the oil supply increases, and oil may be drawn from a more remote portion of the multi-compressor refrigeration system 2.

[0113] This increase in the workload of the multi-compressor refrigeration system 2 can be achieved in various ways.

[0114] For example, if the total number of compressors 8 in the multi-compressor is equal to 2, the oil return action includes: maintaining the operation of the running compressors 8 that have detected the oil shortage situation, and starting the operation of the idle compressors 8 (i.e., the idle compressors 8 that have not detected the oil shortage situation).

[0115] However, if the total number of compressors 8 in the multi-compressor is greater than 2, the oil return action includes stopping the operation of the running compressor 8 for which the oil shortage condition has been detected, and starting the operation of all idle compressors 8 .

[0116] The oil return action may additionally include opening the electronic expansion valve 5 of the multi-compressor refrigeration system 2 so as to increase the suction mass flow of the multi-compressor device 7. By increasing the suction mass flow, oil may be drawn from a remote portion of the multi-compressor refrigeration system 2 and may be returned to the multi-compressor device 7, and in particular to the operating compressor 8 where the lack of oil condition has been detected. Thus, by combining the increase in suction mass flow with the increase in the workload of the multi-compressor refrigeration system 2, oil pressurization is enhanced.

[0117] Figure 6 A diagram showing a second embodiment of oil management action is shown, which is similar to Figure 5 The difference of the first embodiment shown is mainly that the oil return action consists in increasing the suction mass flow of the multi-compressor device 7 instead of increasing the workload of the multi-compressor refrigeration system 2. This increase of the suction mass flow of the multi-compressor device 7 can be obtained by opening the electronic expansion valve 5 of the multi-compressor refrigeration system 2.

[0118] Several scenarios of this method are disclosed below.

[0119] Scenario 1: Two parallel compressors and 100% load

[0120] If both compressors 8 in the multi-compressor refrigeration system 2 are running, and a lack of oil condition is detected for one of the running compressors 8 (e.g., the first compressor), it is assumed that there is a problem with the oil balance (excess oil in the second compressor). In order to overcome this problem and balance the oil levels in the various compressors 8, the controller 21 stops the operation of the other running compressor 8 (the second compressor) in which the lack of oil condition is not detected. Thanks to this action, the first compressor can draw more oil from the multi-compressor device 7 and thus increase the oil level in the corresponding oil sump 14.

[0121] Scenario 2: Two parallel compressors and 50% load

[0122] If only one of the two compressors 8 (e.g., the first compressor) is running (50% load), and a lack of oil condition is detected for this running compressor 8 (the first compressor), it is assumed that there is a problem with oil flowing back from the multi-compressor refrigeration system 2 to the compressor 8 (the oil is trapped in the system outside the multi-compressor device 7). In order to overcome this problem and return the oil to the multi-compressor device 7, and in particular to the compressor 8 for which the lack of oil condition has been detected, the controller 21 starts the operation of the idle compressor 8 (the second compressor). Thanks to this action, oil pressurization can be provided, and oil can flow back to the multi-compressor device 7, and in particular to the running compressor 8 for which the lack of oil condition has been detected.

[0123] In another embodiment of the present invention, in order to overcome the problem of oil backflow from the multi-compressor refrigeration system 2 to the compressor 8, the controller 21 can open the electronic expansion valve 5 of the refrigeration system to increase the suction mass flow. Opening the electronic expansion valve 5 of the refrigeration system to increase the suction mass flow can also be additionally applied to start the operation of the idle compressor 8 to further enhance the oil pressurization.

[0124] Scenario 3: Three compressors and 100% load

[0125] If all compressors 8 are running and a lack of oil condition is detected for one of the running compressors 8 (e.g., the first compressor), it is assumed that there is a problem with the oil balance (excess oil in the second compressor and / or the third compressor). In order to overcome this problem and balance the oil levels in the various compressors 8, the controller 21 maintains the operation of the first compressor and stops the operation of all other compressors 8 (the second compressor and the third compressor). Thanks to this action, the first compressor can draw more oil from the multi-compressor device 7 and thus increase the oil level in the corresponding oil sump 14.

[0126] Scenario 4: Three compressors and 66% load

[0127] If two compressors 8 are running (e.g., the first compressor and the second compressor), and one compressor 8 is idle (e.g., the third compressor), and an oil shortage condition is detected for one of the running compressors 8 (e.g., the first compressor), it is assumed that there is a problem with oil balance (excess oil is present in the second compressor and the third compressor). In order to overcome this problem and balance the oil levels in the various compressors 8, the controller 21 changes the on / off configuration of the multi-compressor refrigeration system 2 while maintaining the same workload. Specifically, the controller 21 stops the operation of the first compressor, maintains the operation of the other running compressor 8 (the second compressor), and starts the operation of the idle compressor 8 (the third compressor). Thanks to this action, the multi-compressor refrigeration system 2 maintains the required workload and balances the oil levels in the various compressors.

[0128] Scenario 5: Three compressors and 33% load

[0129] If only one compressor 8 is running (e.g., the first compressor), while all other compressors 8 are idle (e.g., the second and third compressors), and a lack of oil condition is detected for the running compressor 8 (the first compressor), it is assumed that there is a problem with the oil flowing back from the multi-compressor refrigeration system 2 to the individual compressors 8. In order to overcome this problem and return the oil to the individual compressors, the controller 21 stops the running compressor 8 (the first compressor) for which the lack of oil condition has been detected, and starts all other compressors 8 (the second and third compressors). Thanks to this action, oil pressurization can be provided, and oil can flow back to the multi-compressor device 7, and in particular to the first compressor.

[0130] Scenario 6: Five compressors and 40% load

[0131] If only two compressors 8 are running, while all other compressors 8 are idle, and an oil shortage condition is detected for one of the running compressors 8, it is assumed that there may be a problem with oil balancing or a problem with oil return. To overcome this problem, the controller 21 may first perform an oil balancing action for a fourth predetermined period of time. If the oil shortage condition does not disappear for one of the running compressors 8 before the fourth predetermined period of time has passed, the controller 21 may then perform an oil return action. The oil balancing action and the oil return action may be performed as described above for other scenarios.

[0132] Generally and as Figure 7 As shown, if the total number of compressors 8 in the multi-compressor device 7 is 5 or more, half or less of the total number of compressors 8 are in operation, and the number of compressors 8 in operation is at least 2, then performing an oil balancing action and / or an oil return action may be beneficial. In particular, in the case of a "short pipe" system (i.e., for a multi-compressor refrigeration system 2 with a total pipe length less than a predetermined length), an oil balancing action is preferred, and for a "long pipe" ("split") system (i.e., for a multi-compressor refrigeration system 2 with a total pipe length greater than a predetermined length), an oil return action is preferred.

[0133] A "short pipe" system refers to an "integrated" system in which all components of the refrigerant circuit (evaporator, condenser, compressor and expansion valve) are located on the same compact frame. The circuit size between the evaporator and the compressor is about 2 meters, and the circuit size between the condenser and the compressor is also about 2 meters.

[0134] A "long pipe" ("split") system refers to a system with several smaller units connected by refrigerant pipes within the refrigeration system. In other words, in a "long pipe" system, at least one heat exchanger unit (evaporator or condenser) is located separately from the unit with the compressor. The distance between the individual units can be up to 30 meters.

[0135] In another embodiment of the present invention, in order to overcome the problem of oil backflow from the multi-compressor refrigeration system 2 to the compressor 8, the controller 21 can open the electronic expansion valve 5 of the refrigeration system to increase the suction mass flow. Opening the electronic expansion valve 5 of the refrigeration system to increase the suction mass flow can also be additionally applied to start the operation of the idle compressor 8, thereby further enhancing the oil pressurization.

[0136] Regardless of the scenario, if the oil shortage condition persists for two minutes after the oil management action is initiated, an oil shortage trip or trip condition exists and the controller 21 stops the operation of the entire multi-compressor refrigeration system 2 by stopping the operation of all compressors 8.

[0137] In situations other than those described above, no oil management actions are required.

[0138] Of course, the invention is not limited to the embodiments described above by way of non-limiting examples, but on the contrary, the invention encompasses all embodiments thereof.

Claims

1. A method for managing oil in a running multi-compressor refrigeration system (2), the multi-compressor refrigeration system (2) comprising a controller (21) and a multi-compressor device (7), the controller (21) being configured to control the operation of the multi-compressor refrigeration system (2), the multi-compressor device (7) comprising compressors (8) connected in parallel, the total number of compressors (8) in the multi-compressor device (7) being at least two, and each compressor (8) being equipped with an oil sump (14) and an oil level detection device (15), the oil level detection device (15) being operably connected to the controller (21) and being configured to detect the oil level in the corresponding oil sump (14), the oil sump (14) being connected via a common oil balance line (20.1), the method comprising: - monitoring the oil level in the oil tank (14) of each compressor (8) by means of a corresponding oil level detection device (15); - if the oil level in the operating compressor (8) reaches a predetermined threshold condition, an oil shortage condition is detected in the operating compressor (8); - if a lack of oil condition is detected for a running compressor (8), an oil management action is performed, the oil management action being based on a change in the on / off configuration of the compressors (8) in the multi-compressor device (7), and the oil management action comprising: If the total number of compressors (8) in the multi-compressor device (7) is 2 to 4, then: performing an oil balancing action if more than half of the total number of compressors (8) are operating, wherein the oil balancing action comprises balancing the oil between the compressors (8); If half or less of the total number of compressors (8) are in operation, an oil return action is performed, wherein the oil return action includes recovering oil from the multi-compressor refrigeration system (2) to the multi-compressor device (7), If the total number of compressors (8) in the multi-compressor device (7) is 5 or more, then: performing said oil balancing action if more than half of said total number of compressors (8) are operating; If half or less of the total number of compressors (8) are in operation and at least two compressors (8) are in operation, performing the oil balancing action and / or the oil return action; If only one compressor (8) is running, the oil return action is performed.

2. The method according to claim 1, further comprising: - If the oil level in the oil tank (14) of the corresponding compressor (8) reaches a predetermined low oil level value, a low oil level warning signal is output from the oil level detection device (15) and output to the controller (21).

3. The method according to claim 2, wherein: The predetermined threshold condition is met if the following conditions occur: - the low oil level warning signal output from the oil level detection device (15) is continuously output for a first predetermined time; or - the low oil level warning signal output from the oil level detection device (15) has been switched more than a predetermined number of times within a second predetermined time.

4. The method according to any one of claims 1 to 3, wherein: The oil balancing action includes maintaining the operation of the operating compressor (8) in which the oil shortage condition has been detected, and stopping the operation of the other operating compressors (8) or all other operating compressors (8).

5. The method according to any one of claims 1 to 3, wherein: The oil balancing action includes: stopping the operation of the running compressor (8) in which the oil shortage condition has been detected, and starting the operation of the idle compressor (8) so as to maintain the working load of the multi-compressor refrigeration system (2) substantially consistent.

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

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

8. The method according to any one of claims 1 to 7, wherein: If the total number of compressors (8) in the multi-compressor is equal to 2, the oil return action includes: maintaining the operation of the running compressor (8) for which the oil shortage condition has been detected, and starting the operation of the idle compressor (8).

9. The method according to any one of claims 1 to 7, wherein: If the total number of compressors (8) in the multi-compressor is greater than 2, the oil return action includes stopping the operation of the running compressor (8) in which the oil shortage condition has been detected, and starting the operation of at least two idle compressors (8).

10. The method according to claim 8 or 9, wherein: The oil return action additionally includes: opening the electronic expansion valve (5) of the multi-compressor refrigeration system (2) to increase the suction mass flow of the multi-compressor device (7).

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

12. The method according to any one of claims 1 to 11, further comprising: - If, after a third predetermined period of time has elapsed from the initiation of said oil management action, a lack of oil condition is still detected for a compressor (8) for which a lack of oil condition has previously been detected, the operation of all compressors (8) is stopped.

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

14. The method according to any one of claims 1 to 13, further comprising: If a low oil condition is detected for a running compressor (8), a low oil level warning is provided.

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

16. A multi-compressor refrigeration system (2), comprising: - a controller (21), the controller (21) being configured to control the operation of the multi-compressor refrigeration system (2); - a multi-compressor device (7), the multi-compressor device (7) comprising compressors (8) connected in parallel, the total number of compressors (8) in the multi-compressor device (7) being at least two, and each compressor (8) being equipped with an oil sump (14) and an oil level detection device (15), the oil level detection device (15) being operatively connected to the controller (21) and being configured to detect the oil level in the respective oil sump (14), the oil sump (14) being connected via a common oil balancing line (20.1), Wherein, the controller (21) is configured to: - monitoring the oil level in the oil tank (14) of each compressor (8) by means of a corresponding oil level detection device (15); - if the oil level in the operating compressor (8) reaches a predetermined threshold condition, detecting the occurrence of an oil shortage in the operating compressor (8) based on a signal output from a corresponding oil level detection device (15); - if a lack of oil condition is detected for a running compressor (8), an oil management action is performed, the oil management action being based on a change in the on / off configuration of the compressors (8) in the multi-compressor device (7), and the oil management action comprising: If the total number of compressors (8) in the multi-compressor device (7) is 2 to 4, then: performing an oil balancing action if more than half of the total number of compressors (8) are operating, wherein the oil balancing action comprises balancing the oil between the compressors (8); If half or less of the total number of compressors (8) are in operation, an oil return action is performed, wherein the oil return action includes recovering oil from the multi-compressor refrigeration system (2) to the multi-compressor device (7), If the total number of compressors (8) in the multi-compressor device (7) is 5 or more, then: performing said oil balancing action if more than half of said total number of compressors (8) are operating; If half or less of the total number of compressors (8) are in operation and at least two compressors (8) are in operation, performing the oil balancing action and / or the oil return action; If only one compressor (8) is running, the oil return action is performed.

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