Parallel unit oil return system and parallel unit oil return control method

Abstract

The invention provides a parallel unit oil return system and a parallel unit oil return control method.The parallel unit oil return system comprises an oil separator, an oil return main path connected to the oil separator and a plurality of oil return branches arranged on the oil return main path in parallel, and the oil return branches are connected with compressors in a parallel unit in a one-to-one correspondence mode; the pipe diameters and the pipe lengths of the oil return branches are in positive correlation, so that the oil quantities of the oil return branches are the same or similar; the pipe diameters and the pipe lengths of the multiple oil return branches are in positive correlation, so that the on-way resistance of the oil return branches is equal, it is guaranteed that the oil return amount among the compressors is evenly distributed, the purpose of even oil return is achieved, the structure is simple, and complex components are not needed.

Description

Technical Field

[0001] The present invention belongs to the field of compressors, and more specifically, relates to a parallel unit oil return system and a parallel unit oil return control method. Background Art

[0002] Parallel piston compressor units are widely used in various refrigeration systems, such as commercial refrigeration, air conditioning systems and industrial refrigeration. In these systems, multiple piston compressors are operated in parallel, sharing a condenser and evaporator, and the efficient operation of the system is achieved by coordinating the work of multiple compressors. However, the existing parallel piston compressor units have some problems in oil return, which seriously affect the performance and reliability of the system.

[0003] Specifically, in a parallel piston compressor unit, multiple compressors are running at the same time, and the oil separator needs to handle a large amount of lubricating oil. Existing oil separators are often unable to completely separate oil and refrigerant under high load, resulting in part of the lubricating oil being unable to effectively return to the compressor, thereby affecting the lubrication effect of the compressor. Due to the different loads and operating conditions of each compressor, the oil level in the oil return branch is unevenly distributed, and some compressors may have insufficient oil return, while others may have too much oil return. This uneven oil return phenomenon will lead to significant differences in the lubrication effect between compressors. Some compressors may overheat and wear due to lack of oil, while others may reduce energy efficiency due to excessive oil. A direct reason for the uneven oil return of the parallel unit is that due to the different distances between the oil separators and the different resistances along the oil return branches of each compressor, the oil separated by the oil separator always flows to the branch with the least resistance first, resulting in the compressors close to the oil separator having relatively sufficient oil, while the compressors far from the oil separator have the hidden danger of insufficient lubrication.

[0004] In order to solve the problem of uneven oil distribution, the common solutions are: the first is to add oil balance pipelines and oil tanks. This method requires connecting the oil return branches in parallel, adding a system to ensure that each compressor can effectively return oil. This not only increases the cost of the system, but also increases the difficulty of maintenance and troubleshooting. At the same time, since multiple compressors are running in parallel, the system requires complex control strategies to coordinate the operation of each compressor, which makes system design and debugging more complicated. The second method is to add a proportional control valve to each oil return branch. The proportional control valve is expensive, and a proportional control valve must be added to each compressor. This method has a greater impact on the cost.

[0005] Therefore, how to propose a low-cost solution to the oil return effect of parallel units is a technical problem that needs to be solved urgently in the industry. Summary of the invention

[0006] The purpose of the present invention is to propose a parallel unit oil return system and a parallel unit oil return control method, which can significantly improve the oil return effect of the parallel unit, ensure uniform oil return between compressors, improve the energy efficiency and reliability of the system, reduce operating costs and maintenance difficulty, and provide a guarantee for the stable operation of the refrigeration system.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] The present invention proposes an oil return system for a parallel unit, comprising an oil separator, an oil return main line connected to the oil separator, and a plurality of oil return branches arranged in parallel on the oil return main line, the oil return branches being connected one-to-one with the compressors in the parallel unit, and the pipe diameters and pipe lengths of the plurality of oil return branches being positively correlated so that the oil volumes of the plurality of oil return branches are the same or similar.

[0009] Furthermore, the parallel unit is provided with a first compressor, a second compressor and a third compressor; the multiple oil return branches are respectively a first oil return branch corresponding to the first compressor, a second oil return branch corresponding to the second compressor and a third oil return branch corresponding to the third compressor; wherein, the length of the first oil return branch is greater than the length of the second oil return branch and the length of the third oil return branch, and the diameter of the first oil return branch is greater than the diameter of the second oil return branch and the diameter of the third oil return branch.

[0010] Furthermore, each of the oil return branches is provided with an oil level gauge, a solenoid valve and a ball valve.

[0011] Furthermore, a ball valve is provided on the main oil return line.

[0012] The present invention further proposes a parallel unit oil return control method, which is applied to the parallel unit oil return system as described above. The parallel unit oil return control method comprises:

[0013] Determine whether each compressor in the parallel unit is turned on;

[0014] If so, the on-off of the solenoid valve on the oil return branch corresponding to one compressor is controlled according to the oil level parameter of the oil return branch corresponding to the compressor, or the on-off of the solenoid valve on the oil return branch corresponding to each compressor is controlled according to the oil level parameters of multiple oil return branches corresponding to multiple compressors;

[0015] If not, disconnect the solenoid valve on the oil return branch corresponding to the compressor.

[0016] Further, when only one compressor in the parallel unit is turned on, controlling the on / off of the solenoid valve on the oil return branch corresponding to one compressor according to the oil level parameter of the oil return branch corresponding to the compressor includes:

[0017] Determine whether the current oil level of the oil return branch corresponding to the compressor is within a preset oil level range;

[0018] If yes, close the solenoid valve on the oil return branch corresponding to the compressor;

[0019] If not, disconnect the solenoid valve on the oil return branch corresponding to the compressor.

[0020] Furthermore, if the current oil level of the oil return branch corresponding to the compressor is greater than the preset oil level upper limit, the solenoid valve on the oil return branch corresponding to the compressor is temporarily disconnected and the solenoid valve on the oil return branch corresponding to the compressor is reclosed after the preset disconnection time ends.

[0021] Furthermore, when two compressors in the parallel unit are turned on, controlling the on and off of the solenoid valves on the oil return branches corresponding to the compressors according to the oil level parameters of the multiple oil return branches corresponding to the multiple compressors includes:

[0022] If the current oil level of the first oil return branch corresponding to the first compressor is less than the preset oil level lower limit, and the current oil level of the second oil return branch corresponding to the second compressor is greater than the preset oil level lower limit;

[0023] Close the first solenoid valve on the first oil return branch;

[0024] The second solenoid valve on the second oil return branch is temporarily disconnected and the second solenoid valve on the second oil return branch is reclosed after a preset disconnection time has elapsed.

[0025] Furthermore, when three compressors in the parallel unit are turned on, controlling the on and off of the solenoid valves on the oil return branches corresponding to the compressors according to the oil level parameters of the multiple oil return branches corresponding to the multiple compressors includes:

[0026] If the current oil level of the first oil return branch corresponding to the first compressor is less than the preset oil level lower limit, and the current oil level of the second oil return branch corresponding to the second compressor is greater than the preset oil level lower limit, and the current oil level of the third oil return branch corresponding to the third compressor is greater than the preset oil level lower limit;

[0027] Comparing the current oil level of the second oil return branch with the current oil level of the third oil return branch, obtaining that the current oil level of the third oil return branch is the largest;

[0028] Close the first solenoid valve on the first oil return branch and the second solenoid valve on the second oil return branch;

[0029] The second solenoid valve on the third oil return branch is temporarily disconnected, and the third solenoid valve on the third oil return branch is reclosed after the preset disconnection time ends.

[0030] Furthermore, when at least two compressors in the parallel unit are turned on, if the current oil level of the oil return branches corresponding to all the turned-on compressors is less than the preset oil level lower limit, an oil return abnormality alarm is issued and all the turned-on compressors are controlled to switch to the shutdown state.

[0031] Compared with the prior art, the parallel unit oil return system and parallel unit oil return control method proposed in the present invention have at least the following beneficial effects: the diameters and lengths of the multiple oil return branches arranged in the present invention are positively correlated, so that the resistance along the entire length of each oil return branch is equal, ensuring that the return oil volume between the compressors is evenly distributed, thus achieving the purpose of uniform oil return, and the structure is simple without the need for complex components. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0033] Figure 1 This is a schematic diagram of the structure of the oil return system of the parallel unit proposed by the present invention;

[0034] Figure 2 This is a cross-sectional view of the gradually expanding structure proposed by the present invention;

[0035] Figure 3 A schematic flow chart of the oil return control method for parallel units proposed in the present invention;

[0036] Among them, the figure marks in each figure are:

[0037] 1. Oil separator; 2. Main oil return line; 31. First compressor; 32. Second compressor; 33. Third compressor; 41. First oil return branch line; 42. Second oil return branch line; 43. Third oil return branch line; 51. First oil level gauge; 52. Second oil level gauge; 53. Third oil level gauge; 61. First solenoid valve; 62. Second solenoid valve; 63. Third solenoid valve; 70. Main ball valve; 71. First ball valve; 72. Second ball valve; 73. Third ball valve. DETAILED DESCRIPTION

[0038] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0039] Parallel piston compressor units are widely used in various refrigeration systems, such as commercial refrigeration, air conditioning systems, and industrial refrigeration. In these systems, multiple piston compressors operate in parallel and share a condenser and evaporator. By coordinating the work of multiple compressors, the system can be operated efficiently. However, existing parallel piston compressor units have some problems in oil return, which seriously affect the performance and reliability of the system. Specifically, in parallel piston compressor units, multiple compressors operate simultaneously, and the oil separator needs to handle a large amount of lubricating oil. Existing oil separators often cannot completely separate oil and refrigerant under high load, resulting in part of the lubricating oil being unable to effectively return to the compressor, thereby affecting the lubrication effect of the compressor. Due to the different loads and operating conditions of each compressor, the oil level in the oil return branch is unevenly distributed, and some compressors may have insufficient oil return, while others may have too much oil return. This uneven oil return phenomenon will lead to significant differences in the lubrication effect between compressors. Some compressors may overheat and wear due to lack of oil, while others may reduce energy efficiency due to excessive oil. A direct reason for the uneven oil return of parallel units is that the distances between the oil separators and the compressors are different, and the resistance along the oil return branches of the compressors is also different. The oil separated by the oil separator always flows to the branch with the least resistance, resulting in the compressors close to the oil separator having relatively sufficient oil, while the compressors far from the oil separator have the risk of insufficient lubrication.

[0040] Based on this, the present invention proposes a parallel unit oil return system and a parallel unit oil return control method, which can solve the problems of uneven oil return, high system complexity, low energy efficiency and poor reliability of existing parallel units at a low cost, and improve the overall performance and reliability of the system.

[0041] like Figure 1 As shown, the parallel unit oil return system proposed in the present invention includes an oil separator 1, an oil return main line 2 connected to the oil separator 1, and a plurality of oil return branches arranged in parallel on the oil return main line 2. The oil return branches are connected to the compressors in the parallel unit one by one, and the pipe diameters and pipe lengths of the plurality of oil return branches are positively correlated so that the oil volumes of the plurality of oil return branches are the same or similar.

[0042] It should be understood that the formula for calculating the resistance along the way is: f =f·(L / D)·(v 2 / 2g);

[0043] in:

[0044] h f is the resistance loss along the way (unit: meter water column or meter liquid column);

[0045] f is the friction factor, a dimensionless number whose value depends on the flow state (laminar or turbulent), pipe wall roughness, etc.;

[0046] L is the length of the pipeline (in meters);

[0047] D is the inner diameter of the pipe (in meters);

[0048] v is the average velocity of the fluid (unit: m / s);

[0049] g is the acceleration due to gravity (about 9.81 m / s 2 ).

[0050] From the above-mentioned calculation formula for the resistance along the way, it can be obtained that f and g are both constants, and the flow rate of v in each branch pipe is approximately equal. Usually, the diameters of the oil return branches are equal. Because the lengths of the oil return branches are inconsistent, the resistance along the way of the oil return branches is inconsistent. At this time, the oil separated by the oil separator 1 always flows more to the oil return branch with small resistance along the way, resulting in the compressor close to the oil separator 1 having relatively sufficient oil, while the compressor far from the oil separator 1 has insufficient lubrication, resulting in uneven oil return. In this regard, the present invention uses the inner diameter of the pipeline (also called the diameter) to balance the effect of the pipeline length (also called the pipe length) on the resistance along the way.

[0051] The advantage of the design of the present invention is that the pipe diameters of the multiple oil return branches are positively correlated with the pipe lengths, so that the resistance along the oil return branches is equal, ensuring that the return oil volume between the compressors is evenly distributed, achieving the purpose of uniform oil return, and the structure is simple without the need for complex components.

[0052] In a further preferred embodiment of the present invention, the parallel unit is provided with a first compressor 31, a second compressor 32 and a third compressor 33; the plurality of oil return branches are respectively a first oil return branch 41 corresponding to the first compressor 31, a second oil return branch 42 corresponding to the second compressor 32 and a third oil return branch 43 corresponding to the third compressor 33; wherein, the length of the first oil return branch 41 is greater than the length of the second oil return branch 42 is greater than the length of the third oil return branch 43, and the diameter of the first oil return branch 41 is greater than the diameter of the second oil return branch 42 is greater than the diameter of the third oil return branch 43.

[0053] In practical applications, if the length of the oil return branch is longer, the diameter of the oil return branch will be larger, similar to Figure 2 The gradually expanding structural cross-section shown means that the pipe diameters and pipe lengths of multiple oil return branches are positively correlated, and a single oil return branch has only one pipe diameter and one pipe length.

[0054] The advantage of the design of the present invention is that the lengths and diameters of the three oil return branches are different, and the layout and structure of the three oil return branches are optimized to meet the different distances between the three compressors and the oil separator 1. At the same time, the resistance along the three oil return branches is equal, ensuring that each compressor can effectively return oil and reducing the load on the oil separator 1.

[0055] In a further preferred embodiment of the present invention, each oil return branch line is provided with an oil level gauge, a solenoid valve and a ball valve.

[0056] The advantage of the design of the present invention is that the current oil level of the oil return branch is detected by an oil level gauge, the on and off of the oil return branch is controlled by the opening and closing of the solenoid valve, and a specific part of the oil return branch is controlled and isolated by a ball valve, which is convenient for maintenance.

[0057] In a further preferred embodiment of the present invention, a ball valve is provided on the main oil return line 2 .

[0058] The advantage of the present invention is that a specific portion of the oil return main line 2 can be controlled and isolated by the ball valve, which facilitates maintenance.

[0059] For ease of understanding, the parallel unit oil return system proposed in the present invention is fully introduced below in combination with actual application scenarios.

[0060] like Figure 1 As shown, the parallel unit oil return system includes an oil separator 1, three compressors, an oil return main line 2, and three oil return branches. The three compressors are the first compressor 31, the second compressor 32, and the third compressor 33, and the three oil return branches are the first oil return branch 41, the second oil return branch 42, and the third oil return branch 43.

[0061] Three oil return branches are arranged in parallel, the first oil return branch 41 is connected between the first compressor 31 and the oil separator 1, the second oil return branch 42 is connected between the second compressor 32 and the oil separator 1, and the third oil return branch 43 is connected between the third compressor 33 and the oil separator 1.

[0062] The first oil return branch 41 is provided with a first oil level gauge 51, a first solenoid valve 61 and a first ball valve 71; the second oil return branch 42 is provided with a second oil level gauge 52, a second solenoid valve 62 and a second ball valve 72; the third oil return branch 43 is provided with a third oil level gauge 53, a third solenoid valve 63 and a third ball valve 73, and the main oil return line 2 is provided with a main ball valve 70. Each ball valve is only used during maintenance and is normally closed.

[0063] The length of the first oil return branch 41 is greater than the length of the second oil return branch 42 and greater than the length of the third oil return branch 43, and the diameter of the first oil return branch 41 is greater than the diameter of the second oil return branch 42 and greater than the diameter of the third oil return branch 43. Thus, the resistance along the three oil return branches is balanced from the perspective of the structural design of the oil return branches, ensuring that the return oil volume among the three compressors is evenly distributed.

[0064] It should be understood that in other optional embodiments, the number of oil return branches is not necessarily 3, that is, the number of oil return branches can be flexibly selected according to actual application requirements.

[0065] It should also be understood that the compressor may be a piston compressor or a screw compressor.

[0066] Based on the structural design of the oil return system of the above parallel units, Figure 3 As shown, the parallel unit oil return control method proposed by the present invention includes the following steps:

[0067] Determine whether each compressor in the parallel unit is turned on;

[0068] If so, the on-off of the solenoid valve on the oil return branch corresponding to one compressor is controlled according to the oil level parameter of the oil return branch corresponding to the compressor, or the on-off of the solenoid valve on the oil return branch corresponding to each compressor is controlled according to the oil level parameters of multiple oil return branches corresponding to multiple compressors;

[0069] If not, disconnect the solenoid valve on the oil return branch corresponding to the compressor.

[0070] The advantage of the design of the present invention is that, by introducing an intelligent control method, the oil return strategy is dynamically adjusted according to the switch status of each compressor in the parallel unit and the oil level parameters of at least one oil return branch, thereby improving the energy efficiency and reliability of the system.

[0071] In a further preferred embodiment of the present invention, when only one compressor in the parallel unit is turned on, controlling the on / off of the solenoid valve on the oil return branch corresponding to one compressor according to the oil level parameter of the oil return branch corresponding to the compressor includes:

[0072] Determine whether the current oil level of the oil return branch corresponding to the compressor is within a preset oil level range;

[0073] If yes, close the solenoid valve on the oil return branch corresponding to the compressor;

[0074] If not, disconnect the solenoid valve on the oil return branch corresponding to the compressor.

[0075] It should be noted that the inventor has verified through a large number of experiments that the oil level of the compressor has an optimal range, and this optimal range is set as the preset oil level range. If the oil level is higher than the preset upper limit, there is too much lubricating oil and heat exchange is affected. If the oil level is lower than the preset lower limit, there is insufficient lubricating oil, which makes the compressor operation risky.

[0076] The advantage of the design of the present invention is that when only one compressor in the parallel unit is turned on, the on and off of the oil return branch corresponding to the compressor is controlled according to the current oil level of the oil return branch corresponding to the compressor and the preset oil level range, thereby adjusting the oil level of the oil return branch corresponding to the compressor, which is beneficial to ensure the normal operation of the compressor.

[0077] In a further preferred embodiment of the present invention, if the current oil level of the oil return branch corresponding to the compressor is greater than the preset oil level upper limit value, the solenoid valve on the oil return branch corresponding to the compressor is temporarily disconnected and the solenoid valve on the oil return branch corresponding to the compressor is reclosed after the preset disconnection time.

[0078] It should be noted that the preset disconnection time can be flexibly set according to actual application requirements. For example, the preset disconnection time is 5 seconds.

[0079] The advantage of such a design of the present invention is that when the current oil level of the oil return branch corresponding to the started compressor is greater than the preset oil level upper limit, the oil return branch corresponding to the started compressor can be controlled to be disconnected for a period of time and then reconnected, and cyclic detection and judgment can be performed, so as to more accurately realize the regulation of the oil level of the oil return branch corresponding to the started compressor, which is more conducive to ensuring that the started compressor can operate normally.

[0080] In a further preferred embodiment of the present invention, when two compressors in the parallel unit are turned on, controlling the on and off of the solenoid valves on the oil return branches corresponding to the compressors according to the oil level parameters of the multiple oil return branches corresponding to the multiple compressors includes:

[0081] If the current oil level of the first oil return branch corresponding to the first compressor is less than the preset oil level lower limit, and the current oil level of the second oil return branch corresponding to the second compressor is greater than the preset oil level lower limit;

[0082] Close the first solenoid valve on the first oil return branch;

[0083] The second solenoid valve on the second oil return branch is temporarily disconnected and the second solenoid valve on the second oil return branch is reclosed after the preset disconnection time ends.

[0084] The advantage of the design of the present invention is that when two compressors in the parallel unit are started, the on and off of the oil return branches corresponding to the two compressors can be controlled according to the current oil level of the oil return branches corresponding to the two compressors that have been started and the preset oil level range, so as to adjust the oil level of the oil return branches corresponding to all the compressors that have been started, thereby ensuring that all the compressors that have been started can operate normally; at the same time, the oil return branch with a larger current oil level can be effectively used to balance the oil level of the oil return branch with a smaller current oil level.

[0085] In a further preferred embodiment of the present invention, when three compressors in the parallel unit are turned on, controlling the on and off of the solenoid valves on the oil return branches corresponding to the compressors according to the oil level parameters of the multiple oil return branches corresponding to the multiple compressors includes:

[0086] If the current oil level of the first oil return branch corresponding to the first compressor is less than the preset oil level lower limit, and the current oil level of the second oil return branch corresponding to the second compressor is greater than the preset oil level lower limit, and the current oil level of the third oil return branch corresponding to the third compressor is greater than the preset oil level lower limit;

[0087] Compare the current oil level of the second oil return branch with the current oil level of the third oil return branch, and find that the current oil level of the third oil return branch is the largest;

[0088] Close the first solenoid valve on the first oil return branch and the second solenoid valve on the second oil return branch;

[0089] Temporarily disconnect the second solenoid valve on the third oil return branch, and reclose the third solenoid valve on the third oil return branch after the preset disconnection time ends.

[0090] The advantage of the design of the present invention is that when three compressors in the parallel unit are turned on, the on and off of the oil return branches corresponding to the three compressors can be controlled according to the current oil level of the oil return branches corresponding to the three compressors that have been turned on and the preset oil level range, so as to adjust the oil level of the oil return branches corresponding to all the compressors that have been turned on, thereby ensuring that all the compressors that have been turned on can operate normally; at the same time, the oil return branch with the largest current oil level can be effectively used to balance the oil level of the oil return branch with a relatively small current oil level.

[0091] In a further preferred embodiment of the present invention, when at least two compressors in the parallel unit are turned on, if the current oil level of the oil return branches corresponding to all the turned-on compressors is less than the preset oil level lower limit, an oil return abnormality alarm is issued and all the turned-on compressors are controlled to switch to a shutdown state.

[0092] The advantage of the design of the present invention is that when at least two compressors in a parallel unit are turned on, but the current oil levels of the oil return branches corresponding to all the turned-on compressors are not within the preset oil level range, it reflects that the parallel unit has oil leakage, and an abnormal oil return alarm is issued in time to protect each compressor.

[0093] For ease of understanding, the following takes the parallel unit oil return system including an oil separator 1, three compressors, a main oil return line 2, and three oil return branches as an example, and fully introduces the parallel unit oil return control method applied to the parallel unit oil return system.

[0094] In the parallel unit oil return system, the three compressors are the first compressor 31, the second compressor 32 and the third compressor 33, and the three oil return branches are the first oil return branch 41, the second oil return branch 42 and the third oil return branch 43. When the loads of the compressors in the parallel unit are the same, the demand for oil return is the same, and the oil return is evenly divided at this time; in the actual operation of the parallel unit, there are inevitably differences in the loads of the compressors, and there are also differences in consistency between the compressors, so a control module is required to regulate the oil level of each oil return branch.

[0095] The oil return control method of the parallel unit is as follows:

[0096] Determine whether each compressor in the parallel unit is turned on;

[0097] If all three compressors are shut down, the solenoid valves on the three oil return branches will be disconnected, thereby disconnecting the three oil return branches.

[0098] If only one of the three compressors is turned on, that is, the first compressor 31 is turned on, the second compressor 32 and the third compressor 33 are both turned off.

[0099] Then the second solenoid valve 62 and the third solenoid valve 63 are controlled to be disconnected, thereby disconnecting the second oil return branch 42 and the third oil return branch 43 .

[0100] It is then determined whether the current oil level of the first oil return branch 41 is within the preset oil level range. If so, since the first solenoid valve 61 is closed, there is no need to adjust the first solenoid valve 61, and the first oil return branch 41 is maintained connected; if not and the current oil level of the first oil return branch 41 is greater than the preset oil level upper limit, the first solenoid valve 61 is controlled to be disconnected for 5 seconds, and after 5 seconds, the first solenoid valve 61 is controlled to be closed, and a cycle detection and judgment are performed.

[0101] If two of the three compressors are turned on, that is, the first compressor 31 and the second compressor 32 are both turned on, the third compressor 33 is turned off.

[0102] The third solenoid valve 63 is controlled to be disconnected, thereby disconnecting the third oil return branch 43.

[0103] It is then determined whether the current oil level of the first oil return branch 41 is within a preset oil level range;

[0104] If yes, since the first solenoid valve 61 and the second solenoid valve 62 are closed, there is no need to adjust the first solenoid valve 61 and the second solenoid valve 62, and the first oil return branch 41 and the second oil return branch 42 are kept connected;

[0105] If not and the current oil level of the first oil return branch 41 is greater than the preset oil level upper limit, the first solenoid valve 61 is controlled to be disconnected for 5 seconds, and after 5 seconds, the first solenoid valve 61 is controlled to be closed again, and the second solenoid valve 62 does not need to be adjusted, and the second oil return branch 42 is kept connected;

[0106] If not and the current oil level of the first oil return branch 41 is less than the preset oil level lower limit, then determine whether the current oil level of the second oil return branch 42 is greater than the preset oil level lower limit. If not, the system has oil leakage or the like, and an oil return abnormality alarm is issued, and the first compressor 31 and the second compressor 32 are controlled to shut down; if so, the second solenoid valve 62 is controlled to be disconnected for 5 seconds, and after 5 seconds, the second solenoid valve 62 is controlled to be closed, and a cycle detection and judgment are performed.

[0107] If all three compressors are turned on, first determine whether the current oil level of the first oil return branch 41 is within the preset oil level range;

[0108] If yes, there is no need to adjust the first solenoid valve 61, the second solenoid valve 62 and the third solenoid valve 63, and the first oil return branch 41, the second oil return branch 42 and the third oil return branch 43 are kept connected;

[0109] If not and the current oil level of the first oil return branch 41 is greater than the preset oil level upper limit, the first solenoid valve 61 is controlled to be disconnected for 5 seconds, and after 5 seconds, the first solenoid valve 61 is controlled to be closed without adjusting the second solenoid valve 62 and the third solenoid valve 63, and the second oil return branch 42 and the third oil return branch 43 are kept connected;

[0110] If not and the current oil level of the first oil return branch 41 is less than the preset oil level lower limit, then determine whether the current oil levels of the second oil return branch 42 and the third oil return branch 43 are both greater than the preset oil level lower limit. If not, the system has oil leakage or other phenomena, and an oil return abnormality alarm is issued, and the first compressor 31, the second compressor 32 and the third compressor 33 are controlled to shut down; if yes, and the current oil level of the third oil return branch 43 is greater than the current oil level of the second branch, the third solenoid valve 63 is controlled to be disconnected for 5 seconds. After 5 seconds, the third solenoid valve 63 is controlled to be closed and a cycle detection and judgment is performed without adjusting the second solenoid valve 62, and the second oil return branch 42 is maintained connected.

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A parallel unit oil return system, characterized in that: It includes an oil separator, a main oil return line connected to the oil separator, and a plurality of oil return branches arranged in parallel on the main oil return line. The oil return branches are connected one-to-one with the compressors in the parallel unit, and the pipe diameters and pipe lengths of the plurality of oil return branches are positively correlated so that the oil volumes of the plurality of oil return branches are the same or similar.

2. The parallel unit oil return system according to claim 1, characterized in that: The parallel unit is provided with a first compressor, a second compressor and a third compressor; the multiple oil return branches are respectively a first oil return branch corresponding to the first compressor, a second oil return branch corresponding to the second compressor and a third oil return branch corresponding to the third compressor; wherein, the length of the first oil return branch is greater than the length of the second oil return branch and the length of the third oil return branch, and the diameter of the first oil return branch is greater than the diameter of the second oil return branch and the diameter of the third oil return branch.

3. The parallel unit oil return system according to claim 1, characterized in that: Each of the oil return branches is provided with an oil level gauge, a solenoid valve and a ball valve.

4. The parallel unit oil return system according to claim 1, characterized in that: A ball valve is arranged on the main oil return line.

5. A parallel unit oil return control method, characterized in that: Applied to the parallel unit oil return system according to any one of claims 1 to 4, the parallel unit oil return control method comprises: Determine whether each compressor in the parallel unit is turned on; If so, the on-off of the solenoid valve on the oil return branch corresponding to one compressor is controlled according to the oil level parameter of the oil return branch corresponding to the compressor, or the on-off of the solenoid valve on the oil return branch corresponding to each compressor is controlled according to the oil level parameters of multiple oil return branches corresponding to multiple compressors; If not, disconnect the solenoid valve on the oil return branch corresponding to the compressor.

6. The parallel unit oil return control method according to claim 5, characterized in that: When only one compressor in the parallel unit is turned on, controlling the on / off of the solenoid valve on the oil return branch corresponding to one compressor according to the oil level parameter of the oil return branch corresponding to the compressor includes: Determine whether the current oil level of the oil return branch corresponding to the compressor is within a preset oil level range; If yes, close the solenoid valve on the oil return branch corresponding to the compressor; If not, disconnect the solenoid valve on the oil return branch corresponding to the compressor.

7. The parallel unit oil return control method according to claim 6, characterized in that: If the current oil level of the oil return branch corresponding to the compressor is greater than the preset oil level upper limit, the solenoid valve on the oil return branch corresponding to the compressor is temporarily disconnected and the solenoid valve on the oil return branch corresponding to the compressor is reclosed after the preset disconnection time ends.

8. The parallel unit oil return control method according to claim 5, characterized in that: When two compressors in the parallel unit are turned on, the on-off of the solenoid valves on the oil return branches corresponding to the compressors is controlled according to the oil level parameters of the multiple oil return branches corresponding to the multiple compressors, including: If the current oil level of the first oil return branch corresponding to the first compressor is less than the preset oil level lower limit, and the current oil level of the second oil return branch corresponding to the second compressor is greater than the preset oil level lower limit; Close the first solenoid valve on the first oil return branch; The second solenoid valve on the second oil return branch is temporarily disconnected and the second solenoid valve on the second oil return branch is reclosed after a preset disconnection time has elapsed.

9. The parallel unit oil return control method according to claim 5, characterized in that: When three compressors in the parallel unit are turned on, the on-off of the solenoid valves on the oil return branches corresponding to the compressors is controlled according to the oil level parameters of the multiple oil return branches corresponding to the multiple compressors, including: If the current oil level of the first oil return branch corresponding to the first compressor is less than the preset oil level lower limit, and the current oil level of the second oil return branch corresponding to the second compressor is greater than the preset oil level lower limit, and the current oil level of the third oil return branch corresponding to the third compressor is greater than the preset oil level lower limit; Comparing the current oil level of the second oil return branch with the current oil level of the third oil return branch, obtaining that the current oil level of the third oil return branch is the largest; Close the first solenoid valve on the first oil return branch and the second solenoid valve on the second oil return branch; The second solenoid valve on the third oil return branch is temporarily disconnected, and the third solenoid valve on the third oil return branch is reclosed after the preset disconnection time ends.

10. The parallel unit oil return control method according to claim 5, characterized in that: When at least two compressors in the parallel unit are turned on, if the current oil level of the oil return branches corresponding to all the turned-on compressors is less than the preset oil level lower limit, an oil return abnormality alarm is issued and all the turned-on compressors are controlled to switch to the shutdown state.

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