Thermodynamic system for bog compressor

Abstract

A thermodynamic system for a BOG compressor. A gas outlet end of a second-stage compressor (5) or a first-stage compressor (3) is communicated with a second gas inlet (23) of a preheater (2). A second gas outlet (24) of the preheater (2) is communicated with a gas inlet of an end cooler (6) or an interstage cooler (4). High-temperature exhaust gas at the gas outlet end of the second-stage compressor (5) or the first-stage compressor (3) serves as a heat source to preheat low-temperature BOG at a first gas inlet (21) of the preheater (2), such that the temperature of BOG entering the gas inlet end of the first-stage compressor (3) reaches a room temperature range.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of BOG compression, and more specifically, to a thermodynamic system for a BOG compressor.BACKGROUND

[0002] As a high-quality and efficient clean energy source, LNG (liquefied natural gas) plays an increasingly important role in optimizing China's energy consumption structure, controlling greenhouse gas emissions, and improving the atmospheric environment, etc. In recent years, the construction of the LNG industry has developed vigorously. LNG trade is becoming a new hot spot in the global energy market. In order to ensure the diversification of energy supply and improve the structure of energy consumption, some large energy-consuming countries pay more and more attention to the import of LNG. Japan, South Korea, the United States, Europe are building large-scale LNG receiving terminals and forming LNG transport fleets. Global oil and gas giants have also turned their new profit growth points to the LNG business. LNG will become the next globally sought-after energy commodity after oil.TECHNICAL ISSUE

[0003] During the ship's bunkering, transportation or loading and unloading of LNG, due to the intrusion of external heat and the rocking of the ship, part of the LNG in the cargo tank easily evaporates to form low-temperature BOG (boil-off gas). The generation of BOG will increase the pressure inside the cargo tanks, and excessive pressure will damage the structure of the cargo tanks. Therefore, it is necessary to handle the BOG. LNG is a flammable, explosive and easily leaking substance, so the ability of a ship to safely and efficiently store, transport and utilize LNG depends on the proper handling of the BOG generated during the process. For the treatment of BOG, whether it is early fuel utilization or reliquefaction systems in recent years, low-temperature BOG needs to be compressed. However, BOG is an ultra-low-temperature (can be as low as -140°C) vaporated gas, and this property poses strict requirements for downstream processing equipment.

[0004] The BOG compressor unit is a special mechanical device that can directly inhale ultra-low temperature gas. For ultra-low temperature working environments, the traditional solution is to match the compressor with low-temperature materials. However, this solution has high manufacturing costs. Meanwhile, due to the relatively large temperature fluctuation range of BOG and the significant temperature increase during the compression process, the above-mentioned temperature change poses huge challenges to the design and manufacture of the compressor.TECHNICAL SOLUTION

[0005] The purpose of the present application is to provide a thermodynamic system for a BOG compressor, the thermodynamic system for the BOG compressor unit is capable of solving the problems of high manufacturing cost due to the utilization of low-temperature materials and the difficulty in designing and manufacturing due to the temperature fluctuation in the working conditions of BOG compressor unit.

[0006] In order to solve the above mentioned problems, the present application provides a thermodynamic system for a BOG compressor unit, the thermodynamic system for the BOG compressor unit comprises an inlet preheater, a first-stage compressor, an interstage cooler, a second-stage compressor, and an end cooler arranged in sequence; the preheater comprises a first inlet and a first outlet connected to each other, and a second inlet and a second outlet connected to each other, and the first outlet is connected to the inlet end of the first-stage compressor; the outlet end of the first-stage compressor is connected to the inlet of the interstage cooler, and the outlet of the interstage cooler is connected to the inlet end of the second-stage compressor; the outlet end of the second-stage compressor is connected to the inlet of the end cooler through a first branch pipe, the outlet end of the second-stage compressor is connected to the second inlet of the preheater through a second branch pipe, the second outlet of the preheater is connected to the inlet of the end cooler through a third branch pipe, and the outlet of the end cooler is connected to the user pipe.

[0007] Further, a first control valve is provided on the second branch pipe.

[0008] Further, the outlet of the interstage cooler is further connected to the inlet end of the first-stage compressor through a fourth branch pipe; a second control valve is further provided on the fourth branch pipe; the outlet of the end cooler is further connected to the inlet end of the first-stage compressor through a fifth branch pipe; the fifth branch pipe is further provided with a third control valve.

[0009] Further, the thermodynamic system for the BOG compressor unit further comprises: a main cooling medium inlet and a main cooling medium outlet; the cooling medium inlet of the end cooler and the cooling medium inlet of the interstage cooler are both connected to the main cooling medium inlet; the cooling medium outlet of the end cooler and the cooling medium outlet of the interstage cooler are both connected to the main cooling medium outlet.

[0010] Further, the first-stage compressor and the second-stage compressor have a cooling jacket respectively, the inlet of the cooling jacket of the first-stage compressor and the inlet of the cooling jacket of the second-stage compressor are both connected to the main cooling medium inlet; the outlet of the cooling jacket of the first-stage compressor and the outlet of the cooling jacket of the second-stage compressor are both connected to the main cooling medium outlet.

[0011] Further, the temperature of the cooling medium at the main cooling medium inlet ranges from 20 °C to 36 °C, and the temperature of the cooling medium at the main cooling medium outlet ranges from 25 °C to 44 °C.

[0012] Further, the cooling medium at the main cooling medium inlet comprises: at least one of water, lubricating oil and antifreeze and the combination thereof.

[0013] Further, the pressure of the BOG before entering the first inlet of the preheater ranges from 50 kPaA to 160 kPaA, and the temperature ranges from - 140 °C to 30 °C, the volume fraction of methane in the BOG ranges from 80% to 100%, and the temperature of BOG after preheating by the preheater ranges from - 30 °C to 30 °C.

[0014] Further, the pressure of the BOG compressed by the first-stage compressor ranges from 300 kPaA to 600kPaA, and the temperature ranges from 120 °C to 200 °C, and the temperature of the BOG cooled by the interstage cooler ranges from 40 to 45°C.

[0015] Further, the pressure of the BOG compressed by the second-stage compressor ranges from 650 kPaA to 1450kPaA, and the temperature ranges from 120 °C to 200 °C; the temperature of the BOG, merged by the BOG transmitted through the third branch pipe to the inlet of the end cooler and the BOG transmitted through the first branch pipe to the inlet of the end cooler, ranges from 75 °C to 200 °C, and the temperature of the BOG after being cooled by the end cooler ranges from 40 °C to 45 °C.

[0016] In order to solve the above mentioned problems, the present application provides a thermodynamic system for a BOG compressor, the thermodynamic system for the BOG compressor unit comprises an inlet preheater, a first-stage compressor, an interstage cooler, a second-stage compressor, and an end cooler arranged in sequence; the preheater comprises a first inlet and a first outlet connected to each other, and a second inlet and a second outlet connected to each other, the system inlet is connected to the first inlet of the preheater, the first outlet is connected to the inlet end of the first-stage compressor, the outlet end of the first-stage compressor is connected to the inlet of the interstage cooler through a first branch pipe, and the air outlet end of the first-stage compressor is connected to the second inlet of the preheater through a second branch pipe, the second outlet of the preheater is connected to the inlet of the interstage cooler through a third branch pipe, the outlet of the interstage cooler is connected to the inlet end of the second-stage compressor, the outlet end of the second-stage compressor is connected to the inlet of the end cooler, and the outlet of the end cooler is connected to the user pipe.

[0017] Further, a first control valve is provided on the second branch pipe.

[0018] Further, the outlet of the interstage cooler is further connected to the inlet end of the first-stage compressor through a fourth branch pipe; a second control valve is further provided on the fourth branch pipe; the outlet of the end cooler is further connected to the inlet end of the first-stage compressor through a fifth branch pipe; the fifth branch pipe is further provided with a third control valve.

[0019] Further, the thermodynamic system for the BOG compressor unit further comprises: a main cooling medium inlet and a main cooling medium outlet; the cooling medium inlet of the end cooler and the cooling medium inlet of the interstage cooler are both connected to the main cooling medium inlet; the cooling medium outlet of the end cooler and the cooling medium outlet of the interstage cooler are both connected to the main cooling medium outlet.

[0020] Further, both the first-stage compressor and the second-stage compressor have a cooling jacket respectively, the inlet of the cooling jacket of the first-stage compressor and the inlet of the cooling jacket of the second-stage compressor are both connected to the main cooling medium inlet; the outlet of the cooling jacket of the first-stage compressor and the outlet of the cooling jacket of the second-stage compressor are both connected to the main cooling medium outlet.BENEFICIAL EFFECT

[0021] The thermodynamic system for the BOG compressor unit in the present application features a simple process, a compact footprint, flexible use and low equipment investment. The outlet end of the second-stage compressor or the first-stage compressor in the present application is connected to the second inlet of the preheater through the second branch pipe, and the second outlet of the preheater is connected to the inlet of the end cooler or the interstage cooler through the third branch pipe. The high-temperature exhaust gas at the outlet end of the second-stage compressor or the first-stage compressor in the thermodynamic system for the BOG compressor unit is used as a heat source to preheat the low-temperature BOG at the first inlet of the preheater, so that the temperature of the BOG entering the inlet end of the first-stage compressor reaches the normal temperature range, shrinking the working temperature range of the compressor. Thereby, it avoids the difficulty of material selection and structure design of the main unit of the compressor and its flow passage components due to the low-temperature working conditions, avoids the use of low-temperature materials to manufacture the compressor in the existing art, reduces the manufacturing cost of the compressor, avoids the difficulty of the design and manufacture of the main unit of the compressor caused by the huge temperature change factors during the BOG compression in the existing art, simplifies the structure of the compressor, and avoids the ice blockage problem caused by the use of liquid cooling medium.

[0022] The outlet end of the second-stage compressor or the first-stage compressor in the present application is connected to the second inlet of the preheater through the second branch pipe, and the second outlet of the preheater is connected to the inlet of the end cooler or the interstage cooler through the third branch pipe. By utilizing the self-cooling capacity of the BOG at the inlet, the temperature of the BOG entering the inlet of the end cooler or the interstage cooler is reduced, thereby reducing the consumption of the cooling medium in the end cooler or the interstage cooler and thus reducing energy consumption.BRIEF DESCRIPTION OF THE DRAWINGS

[0023] To describe technical solutions in embodiments of the present application more clearly, the following briefly introduces the accompanying drawings for describing the embodiments. It is clear that the accompanying drawings in the following descriptions show merely some embodiments of this application, and a person skilled in the art may still derive other drawings from these accompanying drawings without creative efforts. FIG. 1 is a schematic diagram of a structure of a thermodynamic system for a BOG compressor, according to embodiment 1 of the present application; FIG. 2 is a schematic diagram of a structure of a thermodynamic system for a BOG compressor, according to embodiment 3 of the present application.

[0024] The meanings of the reference numbers are as follows: 100-thermodynamic system for the BOG compressor; 1-inlet; 2-preheater; 3-first-stage compressor; 4-interstage cooler; 5-second-stage compressor; 6-end cooler; 7-first branch pipe; 8-second branch pipe; 9-third branch pipe; 10-user pipe; 11-main cooling medium inlet; 12-main cooling medium outlet; 13-fourth branch pipe; 14-fifth branch pipe; 15-cooling jacket; 21-first inlet; 22-first outlet; 23-second inlet; 24-second outlet; 81-first control valve; 131-second control valve; 141-third control valve. DETAILED WAYS

[0025] The following will describe the preferred embodiments of this application in detail in combination with the accompanying drawings in the specification, so as to completely introduce the technical content of this application to the technicians in the relevant field, prove by way of example that this application can be implemented, make the technical content disclosed in this application clearer, and make it easier for the technicians in the relevant field to understand how to implement this application. However, the present application can be embodied by many different forms of embodiments, and the scope of protection of the present application is not limited to the embodiments mentioned in the text, and the description of the embodiments below is not intended to limit the scope of the present application.

[0026] The directional terms mentioned herein, such as "up", "down", "front", "back", "left", "right", "inside", "outside", "side", etc., are only the directions in the accompanying drawings, and the directional terms used herein are for explaining and illustrating the present application, and are not intended to limit the scope of protection of the present application.

[0027] In the accompanying drawings, components with the same structure are denoted by the same numerical labels, and systems with similar structures or functions at various places are denoted by similar numerical labels. In addition, for the convenience of understanding and description, the size and thickness of each system shown in the accompanying drawings are shown arbitrarily. This application does not limit the size and thickness of each system.Embodiment 1

[0028] This embodiment provides a thermodynamic system for a BOG compressor unit 100, the thermodynamic system for the BOG compressor unit 100 is mainly configured for handling scenarios of BOG during the supply of gas to a ship's main engine, loading and unloading of LNG liquid cargoes, and refueling of LNG fuel.

[0029] As shown in FIG. 1, the thermodynamic system for the BOG compressor unit 100 comprises an inlet 1, a preheater 2, a first-stage compressor 3, an interstage cooler 4, a second-stage compressor 5 and an end cooler 6 arranged in sequence. Specifically, the inlet 1, the preheater 2, the first-stage compressor 3, the interstage cooler 4, the second-stage compressor 5 and the end cooler 6 are installed on a compressor skid package.

[0030] In this case, both the first-stage compressor 3 and the second-stage compressor 5 comprises at least one of reciprocating compressors, centrifugal compressors and screw compressors and the combination thereof. Since the screw compressor does not introduce impurity components from liquid injection and can achieve a relatively high pressure ratio, both the first-stage compressor 3 and the second-stage compressor 5 in this embodiment prefer screw compressors.

[0031] In this case, the preheater 2, the interstage cooler 4 and the end cooler 6 all comprise at least one of shell-and-tube heat exchangers, plate-fin heat exchangers, plate heat exchangers, plate-shell heat exchangers, and tube-and-fin heat exchangers and the combination thereof.

[0032] Among them, the preheater 2 comprises a first inlet 21, a first outlet 22, a second inlet 23 and a second outlet 24. The first inlet 21 and the first outlet 22 are connected to each other, and the second inlet 23 and the second outlet 24 are connected to each other, the first inlet 21 is not connected to the second inlet 23. In other words, the BOG input through the first inlet 21 can only be transmitted from the first outlet 22 to the inlet end of the first-stage compressor 3, and the BOG input through the second inlet 23 can only be transmitted from the second outlet 24 to the inlet of the end cooler 6.

[0033] Among them, the inlet 1 is connected to the first inlet 21. The first outlet 22 is connected to the inlet end of the first-stage compressor 3. In this case, the BOG from the LNG liquid cargo tank is transmitted from the inlet 1 to the first inlet 21 of the preheater 2. After being heated to normal temperature by the preheater 2, it is transmitted through the first outlet 22 to the inlet end of the first-stage compressor 3. The pressure of the BOG before entering the first inlet of the preheater ranges from 50 kPaA to 160 kPaA, and the temperature ranges from -140 °C to 30 °C, the volume fraction of methane in the BOG ranges from 80% to 100%, and the temperature of BOG after preheating by the preheater ranges from -30 °C to 30 °C.

[0034] Among them, the outlet end of the first-stage compressor 3 is connected to the inlet of the interstage cooler 4. The BOG that enters the first-stage compressor 3 from its inlet end is compressed by the first-stage compressor 3. The BOG that has been heated up after being compressed by the first-stage compressor 3 is transmitted from the outlet end of the first-stage compressor 3 to the inlet of the interstage cooler 4 and then enters the interstage cooler 4 to be cooled down. After being compressed by the first-stage compressor 3, the pressure of the BOG ranges from 300 kPaA to 600 kPaA, and the temperature ranges from 120 °C to 200 °C. The temperature of the BOG after being cooled by the interstage cooler 4 ranges from 40 °C to 45 °C.

[0035] In this regard, the outlet of the interstage cooler 4 is connected to the inlet end of the second-stage compressor 5. The BOG at the outlet of the interstage cooler 4 enters the second-stage compressor 5 from its inlet end and is then compressed by the second-stage compressor 5. After being compressed by the second-stage compressor 5, the pressure of the BOG ranges from 650 kPaA to 1450 kPaA, and the temperature ranges from 120 °C to 200 °C.

[0036] In this case, the outlet end of the second-stage compressor 5 is connected to the inlet of the end cooler 6 through the first branch pipe 7. The outlet end of the second-stage compressor 5 is also connected to the second inlet 23 of the preheater 2 through the second branch pipe 8. The second outlet 24 of the preheater 2 is connected to the inlet of the end cooler 6 through the third branch pipe 9. The BOG transmitted by the third branch pipe 9 to the inlet of the end cooler 6 and the BOG transmitted by the first branch pipe 7 to the inlet of the end cooler 6 converge and then enter the end cooler 6 to be cooled down. The temperature of the BOG, merged by the BOG transmitted through the third branch pipe to the inlet of the end cooler and the BOG transmitted through the first branch pipe to the inlet of the end cooler, ranges from 75 °C to 200 °C, and the temperature of the BOG after being cooled by the end cooler ranges from 40 °C to 45 °C.

[0037] In this embodiment, the outlet end of the second-stage compressor 5 is connected to the second inlet 23 of the preheater 2 through the second branch pipe 8, and the second outlet 24 of the preheater 2 is connected to the inlet of the end cooler 6 through the third branch pipe 9. The high-temperature exhaust gas at the outlet end of the second-stage compressor 5 in the thermodynamic system for the BOG compressor unit 100 is used as a heat source to preheat the low-temperature BOG at the first inlet 21 of the preheater 2, which saves the consumption of the BOG of the preheater 2 and enables the temperature of the BOG entering the inlet end of the first-stage compressor 3 to reach the normal temperature range. The working temperature range of the main unit of the compressor is shrinked, thereby avoiding the difficulty of material selection and structure design of the main unit of the compressor and its flow passage components due to the low-temperature working conditions. It also avoids using low-temperature materials to manufacture the compressor in the existing art, reduces the manufacturing cost of the compressor, avoids the difficulty of the design and manufacture of the main unit of the compressor caused by the huge temperature change factors during the BOG compression in the existing art, simplifies the structure of the compressor, and avoids the ice blockage problem caused by the use of liquid cooling medium.

[0038] In this embodiment, the outlet end of the second-stage compressor 5 is connected to the second inlet 23 of the preheater 2 through the second branch pipe 8, and the second outlet 24 of the preheater 2 is connected to the inlet of the end cooler 6 through the third branch pipe 9. By utilizing the self-cooling capacity of the BOG at the inlet 1, the temperature of the BOG entering the inlet of the end cooler 6 is reduced, thereby reducing the consumption of the cooling medium of the end cooler 6 and thus reducing energy consumption.

[0039] Among them, a first control valve 81 is provided on the second branch pipe 8. The first control valve 81 can be regulated according to the temperature of the BOG at the inlet 1 and the performance of the preheater 2, and then the amount of the BOG entering the preheater 2 through the second branch pipe 8 can be adjusted, and further the temperature of the BOG entering the first-stage compressor 3 can be adjusted. The logic is simple and the control is reliable. Meanwhile, excessive preheating of the BOG can be avoided. Specifically, the amount of the BOG entering the preheater 2 through the second branch pipe 8 can account for 0-100% of the total exhaust volume at the outlet end of the second-stage compressor 5.

[0040] In this case, the outlet of the end cooler 6 is connected to the user pipe. The BOG that has been cooled down by the end cooler 6 is transported from the outlet of the end cooler 6 to the downstream user pipe 10.

[0041] Among them, the outlet of the interstage cooler 4 is also connected to the inlet end of the first-stage compressor 3 through the fourth branch pipe 13. A second control valve 131 is also provided on the fourth branch pipe 13, and the second control valve 131 is configured to control the amount of the BOG entering the inlet end of the first-stage compressor 3. Thereby, the adjustment range of the first-stage compressor 3 can be increased, it can adapt to the large fluctuations of the inlet BOG working conditions, improve the long-term stability of the operation of the thermodynamic system for the BOG compressor unit 100, and provide operating margin under complex working conditions such as starting up with nitrogen.

[0042] Among them, the outlet of the end cooler 6 is also connected to the inlet end of the first-stage compressor 3 through the fifth branch pipe 14. A third control valve 141 is also provided on the fifth branch pipe 14, and the third control valve 141 is configured to control the amount of the BOG entering the inlet end of the first-stage compressor 3. Thereby, the adjustment range of the first-stage compressor 3 can be increased, it can adapt to the large fluctuations of the inlet BOG working conditions, improve the long-term stability of the operation of the thermodynamic system for the BOG compressor unit 100, and provide operaating margin under complex working conditions such as starting up with nitrogen.

[0043] In this case, the thermodynamic system for the BOG compressor unit 100 further comprises a main cooling medium inlet 11 and a main cooling medium outlet 12.

[0044] Among them, the cooling medium inlet of the end cooler 6 is connected to the main cooling medium inlet 11, and the cooling medium outlet of the end cooler 6 is connected to the main cooling medium outlet. The cooling medium enters the end cooler 6 through the cooling medium inlet of the end cooler 6 to cool the BOG, and then enters the main cooling medium outlet through the cooling medium outlet of the end cooler 6 for cyclic use, so as to reduce the consumption of the cooling medium.

[0045] Among them, the cooling medium inlet of the interstage cooler 4 is connected to the main cooling medium inlet 11, and the cooling medium outlet of the interstage cooler 4 is connected to the main cooling medium outlet 12. The cooling medium enters the interstage cooler 4 through the cooling medium inlet of the interstage cooler 4 to cool the BOG, and then enters the main cooling medium outlet through the cooling medium outlet of the interstage cooler 4 for cyclic use, so as to reduce the consumption of the cooling medium.

[0046] Among them, both the first-stage compressor 3 and the second-stage compressor 5 have a cooling jacket 15 respectively. The inlet of the cooling jacket 15 of the first-stage compressor 3 is connected to the main cooling medium inlet 11, and the outlet of the cooling jacket 15 of the first-stage compressor 3 is connected to the main cooling medium outlet 12. The cooling medium enters the first-stage compressor 3 through the inlet of the cooling jacket 15 of the first-stage compressor 3 to cool the BOG, and then enters the main cooling medium outlet through the outlet of the cooling jacket 15 of the first-stage compressor 3 for cyclic use, reducing the consumption of the cooling medium. By using the cooling medium to cool the casing of the first-stage compressor 3, the temperature of the BOG is synchronously reduced during the compression process, making the compression process as close as possible to an isothermal compression process, improving the isentropic efficiency of the first-stage compressor 3 and reducing the compression power consumption.

[0047] Among them, the inlet of the cooling jacket 15 of the second-stage compressor 5 is connected to the main cooling medium inlet 11, and the outlet of the cooling jacket 15 of the second-stage compressor 5 is connected to the main cooling medium outlet 12. The cooling medium enters the second-stage compressor 5 through the inlet of the cooling jacket 15 of the second-stage compressor 5 to cool the BOG, and then enters the main cooling medium outlet through the outlet of the cooling jacket 15 of the second-stage compressor 5 for cyclic use, reducing the consumption of the cooling medium. By using the cooling medium to cool the casing of the second-stage compressor 5, the temperature of the BOG is synchronously reduced during the compression process, making the compression process as close as possible to an isothermal compression process, improving the isentropic efficiency of the second-stage compressor 5 and reducing the compression power consumption.

[0048] In this case, the temperature of the cooling medium at the main cooling medium inlet ranges from 20 °C to 36 °C, and the temperature of the cooling medium at the main cooling medium outlet ranges from 25 °C to 44 °C.

[0049] In this case, the cooling medium at the main cooling medium inlet comprises: at least one of water, lubricating oil and antifreeze and the combination thereof. In this embodiment, the cooling medium at the main cooling medium inlet 11 is water.

[0050] The thermodynamic system for the BOG compressor in this embodiment has a simple process, a compact footprint, flexible use and low equipment investment. It enables conventional compressors to directly handle low-temperature gases, which is of great significance for reducing the investment cost of the equipment and improving the indenpendence of the LNG equipment chain.

[0051] In this embodiment, the BOG from the LNG liquid cargo tank is transmitted from the inlet 1 to the first inlet 21 of the preheater 2. After being heated to normal temperature by the preheater 2, it is transmitted through the first outlet 22 to the inlet end of the first-stage compressor 3. Before entering the first inlet 21 of the preheater 2, the pressure of the BOG is 106 kPaA, the temperature is -90 °C, and the volume fraction of methane in the evaporated gas is 100%. After being preheated by the preheater 2, the temperature range of the BOG is -20 °C. The BOG that enters the first-stage compressor 3 from its inlet end is compressed by the first-stage compressor 3. After being compressed by the first-stage compressor 3, the pressure of the BOG is 500 kPaA and the temperature is 135 °C. The BOG that has been heated up after being compressed by the first-stage compressor 3 is transmitted from the outlet end of the first-stage compressor 3 to the inlet of the interstage cooler 4 and then enters the interstage cooler 4 to be cooled down. After being cooled by the interstage cooler 4, the temperature range of the BOG is 40 °C. The BOG at the outlet of the interstage cooler 4 enters the second-stage compressor 5 from its inlet end and is then compressed by the second-stage compressor 5. After being compressed by the second-stage compressor 5, the pressure of the BOG is 1400 kPaA and the temperature is 161 °C. The outlet end of the second-stage compressor 5 is connected to the inlet of the end cooler 6 through the first branch pipe 7, and the outlet end of the second-stage compressor 5 is also connected to the second inlet 23 of the preheater 2 through the second branch pipe 8. The second outlet 24 of the preheater 2 is connected to the inlet of the end cooler 6 through the third branch pipe 9. Among them, the amount of the BOG entering the preheater 2 through the second branch pipe 8 accounts for 95% of the total exhaust volume at the outlet end of the second-stage compressor 5. The temperature of the BOG after the BOG transmitted by the third branch pipe 9 to the inlet of the end cooler 6 and the BOG transmitted by the first branch pipe 7 to the inlet of the end cooler 6 converge is 81 °C. After being cooled by the end cooler 6, the temperature of the BOG is 40 °C, and the pressure of the BOG entering the downstream user pipe is 1350 kPaA.Embodiment 2

[0052] This embodiment comprisess most of the technical features of embodiment 1. The difference between this embodiment and embodiment 1 lies in the following aspects: In this embodiment, the BOG from the LNG liquid cargo tank is transmitted from the inlet 1 to the first inlet 21 of the preheater 2. After being heated to normal temperature by the preheater 2, it is transmitted through the first outlet 22 to the inlet end of the first-stage compressor 3. Before entering the first inlet 21 of the preheater 2, the pressure of the BOG is 95 kPaA, the temperature is -15 °C, and the volume fraction of methane in the evaporated gas is 80%. After being preheated by the preheater 2, the temperature of the BOG is -15 °C. The BOG that enters the first-stage compressor 3 from its inlet end is compressed by the first-stage compressor 3. After being compressed by the first-stage compressor 3, the pressure of the BOG is 485 kPaA and the temperature is 157 °C. The BOG that has been heated up after being compressed by the first-stage compressor 3 is transmitted from the outlet end of the first-stage compressor 3 to the inlet of the interstage cooler 4 and then enters the interstage cooler 4 to be cooled down. After being cooled by the interstage cooler 4, the temperature of the BOG is 40 °C. The BOG at the outlet of the interstage cooler 4 enters the second-stage compressor 5 from its inlet end and is then compressed by the second-stage compressor 5. After being compressed by the second-stage compressor 5, the pressure of the BOG is 700 kPaA and the temperature is 82 °C. The entire outlet end of the second-stage compressor 5 is connected to the inlet of the end cooler 6 through the first branch pipe 7, that is, the amount of the BOG entering the preheater 2 through the second branch pipe 8 accounts for 0 of the total exhaust volume at the outlet end of the second-stage compressor 5. After being cooled by the end cooler 6, the temperature of the BOG is 40 °C, and the pressure of the BOG entering the downstream user pipe is 650 kPaA.Embodiment 3

[0053] As shown in FIG. 2, this embodiment comprises most of the technical features of embodiment 1. The difference between this embodiment and embodiment 1 lies in the following aspects: In this embodiment, the outlet end of the first-stage compressor 3 is connected to the inlet of the interstage cooler 4 through the first branch pipe 7. The outlet end of the first-stage compressor 3 is also connected to the second inlet 23 of the preheater 2 through the second branch pipe 8. The second outlet 24 of the preheater 2 is connected to the inlet of the interstage cooler 4 through the third branch pipe 9. The BOG transmitted by the third branch pipe 9 to the inlet of the interstage cooler 4 and the BOG transmitted by the first branch pipe 7 to the inlet of the interstage cooler 4 converge and then enter the interstage cooler 4 to be cooled down. The outlet of the interstage cooler 4 is connected to the inlet end of the second-stage compressor 5, the outlet of the second-stage compressor 5 is connected to the inlet of the end cooler 6, and the outlet of the end cooler 6 is connected to the user pipe 10.

[0054] In this embodiment, the outlet end of the first-stage compressor 3 is connected to the second inlet 23 of the preheater 2 through the second branch pipe 8, and the second outlet 24 of the preheater 2 is connected to the inlet of the interstage cooler 4 through the third branch pipe 9. The high-temperature exhaust gas at the outlet end of the first-stage compressor 3 in the thermodynamic system for the BOG compressor unit 100 is used as a heat source to preheat the low-temperature BOG at the first inlet 21 of the preheater 2, which saves the consumption of the BOG of the preheater 2 and enables the temperature of the BOG entering the inlet end of the first-stage compressor 3 to reach the normal temperature range. It shrinks the working temperature range of the main unit of the compressor, thereby avoiding the the difficulty of material selection and structure of the main unit of the compressor and its flow passage components due to the low-temperature working conditions. It also avoids using low-temperature materials to manufacture the compressor in the prior art, reduces the manufacturing cost of the compressor, avoids the difficulty of the design and manufacture of the main unit of the compressor caused by the huge temperature change factors during the BOG compression process in the prior art, simplifies the structure of the compressor, and avoids the ice blockage problem caused by the use of liquid heat transfer medium.

[0055] In this embodiment, the outlet end of the first-stage compressor 3 is connected to the second inlet 23 of the preheater 2 through the second branch pipe 8, and the second outlet 24 of the preheater 2 is connected to the inlet of the interstage cooler 4 through the third branch pipe 9. By utilizing the self-cooling capacity of the BOG at the inlet 1, the temperature of the BOG entering the inlet of the interstage cooler 4 is reduced, thereby reducing the consumption of the cooling medium of the interstage cooler 4 and thus reducing the energy consumption.

[0056] The above embodiments are intended to illustrate the technical solution of this application rather than limiting it. Although detailed descriptions have been provided with reference to the aforementioned embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the aforementioned embodiments, or equivalent substitutions can be made for some technical features. Such modifications or substitutions should not depart from the essence, spirit, and scope of the technical solutions of the embodiments of the present application. The above has provided a detailed introduction to the thermodynamic system for the BOG compressor provided by the present application. Specific examples have been applied in this article to expound the principle and implementation modes of this application. The descriptions of the above embodiments are only used to help understand the methods and core ideas of this application. Meanwhile, for the technicians in the relevant field, there will be changes in specific implementation modes and application scopes based on the ideas of this application. To sum up, the content of this specification should not be understood as a limitation on this application.

Claims

1. A thermodynamic system for a BOG compressor, wherein the thermodynamic system for the BOG compressor comprises an inlet, a preheater, a first-stage compressor, an interstage cooler, a second-stage compressor, and an end cooler arranged in sequence; wherein the preheater comprises a first inlet and a first outlet connected to each other, and a second inlet and a second outlet connected to each other, the first inlet and the second inlet are not connected, the inlet is connected to the first inlet, and the first outlet is connected to the inlet end of the first-stage compressor; the outlet end of the first-stage compressor is connected to the inlet of the interstage cooler, and the outlet of the interstage cooler is connected to the inlet end of the second-stage compressor; the outlet end of the second-stage compressor is connected to the inlet of the end cooler through a first branch pipe, the outlet end of the second-stage compressor is connected to the second inlet of the preheater through a second branch pipe, the second outlet of the preheater is connected to the inlet of the end cooler through a third branch pipe, and the outlet of the end cooler is connected to the user pipe.

2. The thermodynamic system for the BOG compressor according to claim 1, wherein a first control valve is provided on the second branch pipe.

3. The thermodynamic system for the BOG compressor according to claim 1, the outlet of the interstage cooler is further connected to the inlet end of the first-stage compressor through a fourth branch pipe; a second control valve is further provided on the fourth branch pipe; the outlet of the end cooler is further connected to the inlet end of the first-stage compressor through a fifth branch pipe; the fifth branch pipe is further provided with a third control valve.

4. The thermodynamic system for the BOG compressor according to claim 1, wherein the thermodynamic system for the BOG compressor further comprises: a main cooling medium inlet and a main cooling medium outlet; the cooling medium inlet of the end cooler and the cooling medium inlet of the interstage cooler are both connected to the main cooling medium inlet; the cooling medium outlet of the end cooler and the cooling medium outlet of the interstage cooler are both connected to the main cooling medium outlet.

5. The thermodynamic system for the BOG compressor according to claim 4, wherein the first-stage compressor and the second-stage compressor have a cooling jacket respectively, the inlet of the cooling jacket of the first-stage compressor and the inlet of the cooling jacket of the second-stage compressor are both connected to the main cooling medium inlet; the outlet of the cooling jacket of the first-stage compressor and the outlet of the cooling jacket of the second-stage compressor are both connected to the main cooling medium outlet.

6. The thermodynamic system for the BOG compressor according to claim 4, wherein the temperature of the cooling medium at the main cooling medium inlet ranges from 20 °C to 36 °C, and the temperature of the cooling medium at the main cooling medium outlet ranges from 25 °C to 44 °C.

7. The thermodynamic system for the BOG compressor according to claim 4, wherein the cooling medium at the main cooling medium inlet comprises: at least one of water, lubricating oil and antifreeze and the combination thereof.

8. The thermodynamic system for the BOG compressor according to claim 1, wherein the pressure of the BOG before entering the first inlet of the preheater ranges from 50 kPaA to 160kPaA, and the temperature ranges from -140 °C to 30 °C, the volume fraction of methane in the BOG ranges from 80% to 100%, and the temperature of BOG after preheating by the preheater ranges from -30 °C to 30 °C.

9. The thermodynamic system for the BOG compressor according to claim 1, wherein the pressure of the BOG compressed by the first-stage compressor ranges from 300 kPaA to 600kPaA, and the temperature ranges from 120 °C to 200 °C, and the temperature of the BOG cooled by the interstage cooler ranges from 40 to 45°C.

10. The thermodynamic system for the BOG compressor according to claim 1, wherein the pressure of the BOG compressed by the second-stage compressor ranges from 650 kPaA to 1450kPaA, and the temperature ranges from 120 °C to 200 °C; the temperature of the BOG, merged by the BOG transmitted through the third branch pipe to the inlet of the end cooler and the BOG transmitted through the first branch pipe to the inlet of the end cooler, ranges from 75 °C to 200 °C, and the temperature of the BOG after being cooled by the end cooler ranges from 40 °C to 45 °C.

11. A thermodynamic system for a BOG compressor, wherein the thermodynamic system for the BOG compressor comprises an inlet, a preheater, a first-stage compressor, an interstage cooler, a second-stage compressor, and an end cooler arranged in sequence; wherein the preheater comprises a first inlet and a first outlet connected to each other, and a second inlet and a second outlet connected to each other, the first inlet and the second inlet are not connected, the inlet is connected to the first inlet, the first outlet is connected to the inlet end of the first-stage compressor, the outlet end of the first-stage compressor is connected to the inlet of the interstage cooler through a first branch pipe, and the air outlet end of the first-stage compressor is connected to the second inlet of the preheater through a second branch pipe, the second outlet of the preheater is connected to the inlet of the interstage cooler through a third branch pipe, the outlet of the interstage cooler is connected to the inlet end of the second-stage compressor, the outlet end of the second-stage compressor is connected to the inlet of the end cooler, and the outlet of the end cooler is connected to the user pipe.

12. The thermodynamic system for the BOG compressor according to claim 11, wherein a first control valve is provided on the second branch pipe.

13. The thermodynamic system for the BOG compressor according to claim 11, the outlet of the interstage cooler is further connected to the inlet end of the first-stage compressor through a fourth branch pipe; a second control valve is further provided on the fourth branch pipe; the outlet of the end cooler is further connected to the inlet end of the first-stage compressor through a fifth branch pipe; the fifth branch pipe is further provided with a third control valve.

14. The thermodynamic system for the BOG compressor according to claim 11, wherein the thermodynamic system for the BOG compressor further comprises: a main cooling medium inlet and a main cooling medium outlet; the cooling medium inlet of the end cooler and the cooling medium inlet of the interstage cooler are both connected to the main cooling medium inlet; the cooling medium outlet of the end cooler and the cooling medium outlet of the interstage cooler are both connected to the main cooling medium outlet.

15. The thermodynamic system for the BOG compressor according to claim 14, wherein the first-stage compressor and the second-stage compressor have a cooling jacket respectively, the inlet of the cooling jacket of the first-stage compressor and the inlet of the cooling jacket of the second-stage compressor are both connected to the main cooling medium inlet; the outlet of the cooling jacket of the first-stage compressor and the outlet of the cooling jacket of the second-stage compressor are both connected to the main cooling medium outlet.

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