Ship Evaporated Gas Re - liquefaction System and Evaporated Gas Re - liquefaction Method

Abstract

A ship evaporation gas reliquefaction system is disclosed. The ship evaporation gas reliquefaction system of the present invention includes a compressor that is installed in the ship and compresses evaporation gas generated in a storage tank in which liquefied gas is stored, a reliquefaction line that connects the compressor and the storage tank and reliquefies the compressed gas and transfers it to the storage tank, a heat exchanger that is installed in the reliquefaction line and cools the supplied compressed gas, a first refrigerant compression unit that compresses the refrigerant discharged after cooling the compressed gas in the heat exchanger, a second refrigerant compression unit that additionally compresses the refrigerant compressed in the first refrigerant compression unit, and a medium expansion unit that expands and cools the refrigerant compressed in the first and second refrigerant compression units. The refrigerant compressed in the first and second refrigerant compression units is pre-cooled in the heat exchanger, expanded and cooled in the refrigerant expansion unit, and supplied as a cold heat source for the heat exchanger, and at least one of the first refrigerant compression unit and the second refrigerant compression unit is driven by the expansion energy of the refrigerant transferred from the refrigerant expansion unit. [Figure 1]

Description

Technical Field

[0001] The present invention relates to a boil-off gas re-liquefaction system and a boil-off gas re-liquefaction method for re-liquefying boil-off gas (BOG) generated from liquefied gas stored in a storage tank of a ship and recovering it into the storage tank.

Background Art

[0002] Natural gas, which mainly consists of methane and emits almost no environmental pollutants during combustion, has attracted attention as an environmentally friendly fuel. Liquefied natural gas (LNG) is obtained by cooling natural gas to about -163°C at atmospheric pressure for liquefaction. Compared with gaseous natural gas, its volume is reduced to about 1 / 600, making it very suitable for long-distance transportation by sea. Therefore, natural gas is mainly stored and transported in the form of liquefied natural gas, which is easy to store and transfer.

[0003] Since the liquefaction point of natural gas is an extremely low temperature of about -163°C at atmospheric pressure, it is common to perform heat insulation treatment to maintain the liquid state of LNG in the LNG storage tank. However, even with heat insulation treatment on the LNG storage tank, there is a limit to blocking external heat. Due to the continuous transfer of external heat to the LNG storage tank, LNG continuously vaporizes naturally in the LNG storage tank during the LNG transportation process, generating boil-off gas (BOG).

[0004] When boil-off gas is continuously generated in the LNG storage tank, it causes the internal pressure of the LNG storage tank to rise. If the internal pressure of the storage tank exceeds the set safety pressure, there is a risk of emergency situations such as tank rupture. Therefore, it is necessary to use a safety valve to discharge the boil-off gas outside the storage tank. However, since boil-off gas is an important issue in the transportation efficiency and fuel efficiency of LNG as one of the LNG losses, various methods for treating boil-off gas generated in the storage tank are used.

[0005] In recent years, methods such as using evaporation gas at fuel demand destinations such as a ship's engine, re-liquefying evaporation gas and recovering it into a storage tank, or using a combination of these two methods have been developed and applied.

Summary of the Invention

Problems to be Solved by the Invention

[0006] When applying a re-liquefaction cycle to re-liquefy evaporation gas on a ship, typical liquefaction methods include, for example, processes using an SMR cycle or a C3MR cycle. The C3MR cycle (Propane-precooled Mixed Refrigerant Cycle) is a process of cooling natural gas using a single refrigerant of propane and then liquefying and subcooling it using a mixed refrigerant. The SMR cycle (Single Mixed Refrigerant Cycle) is a process of liquefying natural gas using a mixed refrigerant composed of a plurality of components.

[0007] These SMR cycles and C3MR cycles involve a process of using a mixed refrigerant. As the liquefaction process progresses, refrigerant leakage occurs, and the composition ratio of the mixed refrigerant changes, resulting in a decrease in liquefaction efficiency. Therefore, it is necessary to maintain the composition of the refrigerant by continuously measuring the composition ratio of the mixed refrigerant and filling the insufficient refrigerant components.

[0008] As another method of the re-liquefaction cycle for re-liquefying evaporation gas, there is a single-cycle liquefaction process using a nitrogen refrigerant.

[0009] Although the nitrogen refrigerant has a lower cooling efficiency compared to the cycle using a mixed refrigerant, it has the advantages of high safety because the refrigerant is an inert substance, and it is easy to apply to ships because there is no phase change of the refrigerant.

[0010] The present invention proposes a system that can configure a device without restrictions and smoothly adjust the load of a re-liquefaction system even when the evaporation gas to be re-liquefied changes from a low flow rate to a high flow rate in a re-liquefaction cycle using a separate refrigerant.

Means for Solving the Problems

[0011] To solve the above problems, the present invention provides a compressor provided on a ship for compressing evaporation gas generated in a storage tank storing liquefied gas, a re-liquefaction line connecting the compressor and the storage tank for re-liquefying the compressed gas compressed by the compressor and transferring it to the storage tank, a heat exchanger provided on the re-liquefaction line for cooling the supplied compressed gas, a first refrigerant compression unit for compressing the refrigerant discharged from the heat exchanger after cooling the compressed gas in the heat exchanger, a second refrigerant compression unit for additionally compressing the refrigerant compressed by the first refrigerant compression unit, and a refrigerant expansion unit for expanding and cooling the refrigerant supplied to the heat exchanger after being compressed by the first refrigerant compression unit and the second refrigerant compression unit. After pre-cooling the refrigerant compressed by the first refrigerant compression unit and the second refrigerant compression unit in the heat exchanger, it is expanded and cooled by the refrigerant expansion unit and supplied to the heat exchanger as a cold heat source. At least one of the first refrigerant compression unit and the second refrigerant compression unit is driven by the expansion energy of the refrigerant transmitted from the refrigerant expansion unit. A ship evaporation gas re-liquefaction system is provided.

[0012] Further, it is preferable that the heat exchanger exchanges heat among four flows, namely, uncompressed evaporation gas supplied from the storage tank to the compressor, compressed gas compressed by the compressor, refrigerant compressed by the first refrigerant compression unit and the second refrigerant compression unit, and refrigerant expanded and cooled by the refrigerant expansion unit.

[0013] Further, it is preferable that the second refrigerant compression unit is driven by the expansion energy of the refrigerant transmitted from the refrigerant expansion unit to compress the refrigerant.

[0014] Further, it is preferable that the first refrigerant compression unit is driven by the expansion energy of the refrigerant transmitted from the refrigerant expansion unit to compress the refrigerant.

[0015] Further, it is preferable to further include a refrigerant circulation line in which the first refrigerant compression unit, the second refrigerant compression unit, and the refrigerant expansion unit are provided to circulate the refrigerant supplied to the heat exchanger, and a refrigerant inventory unit that supplies the refrigerant circulating in the refrigerant circulation line.

[0016] Further, it is preferable to further include a refrigerant supply line that connects the refrigerant inventory unit and the upstream of the first refrigerant compression unit of the refrigerant circulation line to replenish the refrigerant circulation line with refrigerant, and a refrigerant discharge line that connects the downstream of the second refrigerant compression unit of the refrigerant circulation line and the refrigerant inventory unit to discharge the refrigerant from the refrigerant circulation line.

[0017] Further, when the amount of the compressed gas re-liquefied via the re-liquefaction line increases, it is preferable to replenish the refrigerant circulation line with refrigerant via the refrigerant supply line, and when the amount of the compressed gas re-liquefied decreases, to discharge the refrigerant from the refrigerant circulation line via the refrigerant discharge line to adjust the load of the re-liquefaction cycle.

[0018] Further, it is preferable to further include an evaporation gas supply line that connects the storage tank and the compressor via the heat exchanger, and a pre-heater that heats all or part of the evaporation gas that branches from the upstream of the heat exchanger of the evaporation gas supply line and then merges into the upstream of the heat exchanger of the evaporation gas supply line downstream of this branch point.

[0019] Further, it is preferable that the evaporation gas compressed by the compressor is supplied as fuel to a ship's engine or generator, and the evaporation gas not supplied as fuel is re-liquefied via the re-liquefaction line.

[0020] Also, in order to solve the above problems, the present invention compresses evaporation gas generated from a storage tank of a ship with a compressor, cools and reliquefies the compressed gas compressed by the compressor by heat exchange with a refrigerant in a heat exchanger, and after cooling the compressed gas in the heat exchanger, the refrigerant discharged from the heat exchanger is compressed by a first refrigerant compression unit and a second refrigerant compression unit, pre-cooled in the heat exchanger, cooled by expansion in a refrigerant expansion unit, and supplied as a cold heat source to the heat exchanger and circulated. At least one of the first refrigerant compression unit and the second refrigerant compression unit is driven by the expansion energy of the refrigerant transmitted from the refrigerant expansion unit. A method for re-liquefying evaporation gas of a ship is provided.

[0021] Further, in the heat exchanger, it is preferable that four flows of uncompressed evaporation gas supplied from the storage tank to the compressor, compressed gas compressed by the compressor, refrigerant compressed by the first refrigerant compression unit and the second refrigerant compression unit, and refrigerant cooled by expansion in the refrigerant expansion unit are heat-exchanged.

[0022] Also, when the amount of the compressed gas to be reliquefied increases, it is preferable to replenish the refrigerant from the refrigerant inventory unit upstream of the first refrigerant compression unit, and when the amount of the compressed gas to be reliquefied decreases, a part of the refrigerant is discharged from the downstream of the second refrigerant compression unit to the refrigerant inventory unit to adjust the load of the re-liquefaction cycle.

Advantages of the Invention

[0023] In the re-liquefaction cycle using a separate refrigerant, the present invention can configure the device without restrictions even when the evaporation gas to be re-liquefied changes from a low flow rate to a high flow rate or from an alternating flow rate to a constant flow rate, enabling a low-capacity design. When a large-capacity refrigerant cycle is required, the capacity can be increased by replacing each device with a larger-sized one or configuring them in parallel. Since the redundancy (double design) required for a ship can be selectively considered for each device, the cost can be reduced. Also, by using the expansion energy of the refrigerant in the refrigerant expansion unit for compressing the refrigerant, the power consumption required for compressing the refrigerant can be reduced.

[0024] Furthermore, while replenishing or discharging the refrigerant according to the cooling requirement of the reliquefaction system, the load of the reliquefaction cycle can be smoothly adjusted.

Brief Description of the Drawings

[0025]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0026] To explain the operational advantages of the present invention and the objectives achieved by the embodiments of the present invention, the embodiments of the present invention will be described with reference to the accompanying drawings and the content described in the accompanying drawings.

[0027] Hereinafter, with reference to the accompanying drawings, the embodiments of the present invention will be described in detail in terms of their configuration and operation. It should be noted that the reference numerals of the components in each drawing are denoted by the same numerals as much as possible for the same components shown in other drawings.

[0028] The ships of the embodiments of the present invention described below include all types of ships provided with storage tanks for storing liquefied gas. Typically, ships with self-propelling capabilities such as LNG carriers, LNG bunkering vessels, liquid hydrogen carriers, and LNG regasification vessels, as well as offshore floating structures without propulsion capabilities such as LNG FPSOs (Floating Production Storage Offloading) and LNG FSRUs (Floating Storage Regasification Unit) are included.

[0029] In addition, this embodiment can liquefy gas at low temperature for transportation and can be applied to the reliquefaction cycle of all types of liquefied gas that generates evaporation gas in the storage state. The liquefied gas includes, for example, liquefied petrochemical gases such as LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), Liquefied Ethylene Gas, and Liquefied Propylene Gas. However, in the embodiments described below, the application of LNG, which is one of the typical liquefied gases, will be described as an example.

[0030] FIG. 1 schematically shows the evaporation gas reliquefaction system of a ship according to the first embodiment of the present invention. FIG. 2 schematically shows the evaporation gas reliquefaction system of a ship according to the second embodiment of the present invention.

[0031] In the reliquefaction system of this embodiment, the evaporation gas generated from the liquefied gas stored in the storage tank T of the ship is compressed, cooled, reliquefied, and returned to the storage tank. The reliquefaction system includes a compressor 100 that compresses the evaporation gas and a heat exchanger 200 that cools the evaporation gas compressed by the compressor 100. Although not shown, it further includes a pressure reducing device (not shown) that reduces the pressure of the evaporation gas cooled by the heat exchanger 200 and a gas-liquid separator (not shown) that separates the gas-liquid of the evaporation gas reduced in pressure by the pressure reducing device.

[0032] The compressor 100 compresses the evaporation gas discharged from the storage tank T via the evaporation gas supply line GL. The compressor 100 compresses the evaporation gas to the fuel supply pressure of a ship's engine or generator (not shown). For example, when a DF engine is provided, the evaporation gas is compressed to a pressure of 5.5 barg, when an X-DF engine is provided, it is compressed to 15 barg, and when an ME-GI engine is provided, it is compressed to 300 barg. The compressed evaporation gas is supplied as fuel for a ship's engine or generator (not shown). In addition, the evaporation gas not supplied as fuel is supplied to the heat exchanger 200 via the reliquefaction line RL, cooled, and reliquefied.

[0033] The evaporated gas cooled by the heat exchanger 200 is adiabatically expanded or isentropically expanded by decompression with a decompression device (not shown), further cooled, and then separated into gas and liquid by a gas-liquid separator (not shown) and recovered into the storage tank.

[0034] When extremely low temperature evaporated gas of about -130°C to -100°C is directly supplied from the storage tank T to the heat exchanger 200 in the normal temperature state at the initial stage of startup when the re-liquefaction system that has been stopped is just started up, or to the heat exchanger 200 before sufficient cool-down is performed, considerable thermal stress is applied, which may cause damage to the heat exchanger 200. In the present embodiment, in order to prevent this, all or part of the evaporated gas is branched upstream of the heat exchanger 200 in the evaporated gas supply line GL, heated, and then a branch line for supplying it upstream of the heat exchanger in the evaporated gas supply line GL is provided, and a preheater 300 for heating the evaporated gas is provided in the branch line. This minimizes the thermal stress applied to the heat exchanger 200 and prevents damage to the device.

[0035] Even during normal operation of the re-liquefaction system, when there is a possibility that thermal stress is generated in the heat exchanger 200 due to a change in the state of the storage tank T and a change in the temperature of the evaporated gas, all or part of the evaporated gas supplied from the storage tank T to the compressor 100 may be heated by the preheater 300 and then supplied to the heat exchanger 200 as necessary.

[0036] The refrigerant cycle is configured such that the refrigerant that cools the evaporated gas in the heat exchanger 200 circulates. The heat exchanger 200 of the present embodiment cools the compressed evaporated gas RL using each refrigerant CLa, CLb circulating in the refrigerant cycle and the uncompressed evaporated gas GL supplied from the storage tank T to the compressor 100 as heat sources.

[0037] Examples of the refrigerant supplied to the heat exchanger 200 by circulating in each refrigerant circulation line CLa, CLb include nitrogen (N 2 )

[0038] The refrigerant cycle includes each refrigerant circulation line CLa, CLb through which the refrigerant circulates, and a first refrigerant compression part 400a, 400b and a second refrigerant compression part 450a, 450b which are respectively provided in each refrigerant circulation line CLa, CLb, compress the refrigerant discharged from the heat exchanger 200 after cooling the evaporation gas in the heat exchanger 200, and a refrigerant expansion part 500a, 500b which cools the refrigerant compressed by these first and second refrigerant compression parts by expansion and supplies it as a cold heat source of the heat exchanger 200. The refrigerant compressed by the first refrigerant compression part 400a, 400b and the second refrigerant compression part 450a, 450b is precooled in the heat exchanger 200 and then cooled by expansion in the refrigerant expansion part 500a, 500b and circulated as the refrigerant of the heat exchanger 200. Therefore, in the heat exchanger 200, four flows of the compressed gas compressed by the compressor 100, the uncompressed evaporation gas supplied to the compressor 100, the refrigerant cooled by expansion in the refrigerant expansion part 500a, 500b, and the refrigerant compressed by the first refrigerant compression part 400a, 400b and the second refrigerant compression part 450a, 450b are heat-exchanged.

[0039] In the present embodiment, at least one of the first refrigerant compression part 400a, 400b and the second refrigerant compression part 450a, 450b and the refrigerant expansion part 500a, 500b are axially connected, and the expansion energy of the refrigerant can be used for the compression of the refrigerant.

[0040] In the first embodiment shown in FIG. 1, a second refrigerant compression part 450a that additionally compresses the refrigerant compressed by the first refrigerant compression part 400a is driven by the expansion energy of the refrigerant being transmitted from the refrigerant expansion part 500a. The first refrigerant compression part 400a is driven by the supply of external energy, that is, electric power. Further, the first refrigerant compression part 400a may include a plurality of compressors 410a, 420a as necessary.

[0041] By configuring the first refrigerant compression unit 400a driven by external power and the second refrigerant compression unit 450a driven by the refrigerant expansion energy of the refrigerant expansion unit separately in this way, a low-capacity design is possible. When a large-capacity refrigerant cycle is required, the capacity can be increased by replacing each device with a larger-sized one or configuring them in parallel. Since the redundancy (double design) required for a ship can be selectively considered for each device, the cost can be reduced. Also, by using the refrigerant expansion energy of the refrigerant expansion unit 500a for refrigerant compression, the power consumption required for refrigerant compression can be reduced.

[0042] In the second embodiment shown in FIG. 2, the refrigerant discharged from the heat exchanger 200 after being cooled by expansion in the refrigerant expansion unit 500b and used as the cold heat source of the heat exchanger 200 is supplied to the first refrigerant compression unit 400b driven by the transmission of the refrigerant expansion energy from the refrigerant expansion unit 500b and compressed. After that, it is additionally compressed by the second refrigerant compression unit 450b driven by external energy and supplied to the heat exchanger 200 for precooling. The precooled refrigerant is cooled by expansion in the refrigerant expansion unit 500b and supplied again as the cold heat source of the heat exchanger 200 to circulate. This is different from the system of the first embodiment in that the refrigerant expansion unit 500b transmits the refrigerant expansion energy to the first refrigerant compression unit 400b. Also in the second embodiment, similar to the first embodiment, a low-capacity design is possible. When a large-capacity refrigerant cycle is required, the capacity can be increased by replacing each device with a larger-sized one or configuring them in parallel.

[0043] Moreover, this embodiment includes a refrigerant inventory unit RS to which the refrigerant circulating in the refrigerant circulation lines CLa and CLb is supplied. Also, refrigerant supply lines SLa and SLb that connect the refrigerant inventory unit RS and the upstream of the first refrigerant compression units 400a and 400b of the refrigerant circulation lines are provided to replenish the refrigerant into the refrigerant circulation lines CLa and CLb, and refrigerant discharge lines ELa and ELb that connect the downstream of the second refrigerant compression units 450a and 450b of the refrigerant circulation lines and the refrigerant inventory unit RS are provided to discharge the refrigerant in the refrigerant circulation lines CLa and CLb.

[0044] By replenishing the refrigerant from the refrigerant inventory section RS or discharging a part of the refrigerant to the refrigerant inventory section RS to change the mass flow rate of the refrigerant, the cooling and heating capacity of the refrigerant circulation lines CLa and CLb can be adjusted, and the load of the re-liquefaction cycle can be adjusted.

[0045] More specifically, when the required amount of cooling and heating of the re-liquefaction cycle increases due to an increase in the amount of compressed gas to be re-liquefied via the re-liquefaction line RL, the refrigerant in the refrigerant circulation lines CLa and CLb is replenished to the upstream (low-pressure section) of the first refrigerant compression sections 400a and 400b via the refrigerant supply lines SLa and SLb, so that the mass flow rate of the refrigerant circulating in the refrigerant circulation lines CLa and CLb can be increased. Conversely, when the required amount of cooling and heating of the re-liquefaction cycle decreases due to a decrease in the amount of compressed gas to be re-liquefied, a part of the refrigerant in the refrigerant circulation lines CLa and CLb is discharged from the downstream (high-pressure section) of the second refrigerant compression sections 450a and 450b to the refrigerant inventory section RS via the refrigerant discharge lines ELa and ELb, so that the mass flow rate of the refrigerant circulating in the refrigerant circulation lines CLa and CLb can be decreased. By such a method, the load of the re-liquefaction cycle is adjusted.

[0046] It is obvious to those skilled in the technical field to which the present invention pertains that the present invention is not limited to the above-described embodiments, and various changes or modifications can be made without exceeding the technical gist of the present invention.

Claims

1. A compressor provided on a ship for compressing evaporation gas generated in a storage tank storing liquefied gas; and, A re-liquefaction line connecting the compressor and the storage tank for re-liquefying the compressed gas compressed by the compressor and transferring it to the storage tank; and, A heat exchanger provided in the re-liquefaction line for cooling the compressed gas; and, A first refrigerant compression section for compressing the refrigerant discharged from the heat exchanger after cooling the compressed gas in the heat exchanger; and, A second refrigerant compression section for additionally compressing the refrigerant compressed by the first refrigerant compression section; and, A refrigerant expansion section for expanding and cooling the refrigerant supplied to the heat exchanger after being compressed by the first refrigerant compression section and the second refrigerant compression section: and, A refrigerant circulation line provided with the first refrigerant compression section, the second refrigerant compression section, and the refrigerant expansion section for circulating the refrigerant supplied to the heat exchanger; and, A refrigerant inventory section for supplying the refrigerant circulating in the refrigerant circulation line: and, A refrigerant supply line connecting the refrigerant inventory section and the upstream side of the first refrigerant compression section of the refrigerant circulation line for replenishing the refrigerant circulation line with refrigerant; and, A refrigerant discharge line connecting the downstream side of the second refrigerant compression section of the refrigerant circulation line and the refrigerant inventory section for discharging the refrigerant from the refrigerant circulation line: and comprising, After pre-cooling the refrigerant compressed by the first refrigerant compression section and the second refrigerant compression section in the heat exchanger, expanding and cooling it in the refrigerant expansion section, and supplying it to the heat exchanger as a cold heat source, At least one of the first refrigerant compression section and the second refrigerant compression section is driven by the expansion energy of the refrigerant transmitted from the refrigerant expansion section, When the amount of compressed gas re-liquefied via the re-liquefaction line increases, refrigerant is replenished into the refrigerant circulation line via the refrigerant supply line, and when the amount of compressed gas re-liquefied decreases, refrigerant is discharged from the refrigerant circulation line via the refrigerant discharge line, and by changing the mass flow rate of the refrigerant circulating in the refrigerant circulation line, the cold heat amount of the refrigerant circulation line is adjusted, and the load of the re-liquefaction cycle is adjusted, characterized in that, An evaporation gas re-liquefaction system for a ship.

2. The heat exchanger is characterized by heat-exchanging four flows, namely, uncompressed evaporation gas supplied from the storage tank to the compressor, compressed gas compressed by the compressor, refrigerant compressed by the first refrigerant compression section and the second refrigerant compression section, and refrigerant expanded and cooled by the refrigerant expansion section. The evaporation gas re-liquefaction system for a ship according to claim 1.

3. The second refrigerant compression section is driven by the expansion energy of the refrigerant transmitted from the refrigerant expansion section to compress the refrigerant. The evaporation gas re-liquefaction system for a ship according to claim 2.

4. The first refrigerant compression section is driven by the expansion energy of the refrigerant transmitted from the refrigerant expansion section to compress the refrigerant. The evaporation gas re-liquefaction system for a ship according to claim 2.

5. An evaporation gas supply line connecting the storage tank and the compressor via the heat exchanger; and A preheater that branches from upstream of the heat exchanger in the evaporation gas supply line and heats all or part of the evaporation gas that merges into the upstream of the heat exchanger in the evaporation gas supply line downstream of this branch point. The evaporation gas re-liquefaction system for a ship according to claim 1.

6. The evaporation gas compressed by the compressor is supplied as fuel to a ship's engine or generator, and the evaporation gas not supplied as fuel is re-liquefied via the re-liquefaction line. The evaporation gas re-liquefaction system for a ship according to claim 1.

7. Evaporation gas generated from a storage tank of a ship is compressed by a compressor, and the compressed gas compressed by the compressor is cooled and re-liquefied by heat exchange with a refrigerant in a heat exchanger. After the compressed gas is cooled by the heat exchanger, the refrigerant discharged from the heat exchanger is compressed by the first refrigerant compression section and the second refrigerant compression section, pre-cooled by the heat exchanger, cooled by expansion in the refrigerant expansion section, and supplied as a cold heat source to the heat exchanger and circulated. At least one of the first refrigerant compression section and the second refrigerant compression section is driven by the expansion energy of the refrigerant transmitted from the refrigerant expansion section. When the amount of the compressed gas to be reliquefied increases, refrigerant is replenished from the refrigerant inventory section upstream of the first refrigerant compression section, and when the amount of the compressed gas to be reliquefied decreases, a part of the refrigerant is discharged from downstream of the second refrigerant compression section to the refrigerant inventory section, and by changing the mass flow rate of the refrigerant in the refrigerant circulation line through which the refrigerant circulates, the amount of cooling heat in the refrigerant circulation line is adjusted, and the load of the reliquefaction cycle is adjusted. A method for re-liquefying evaporation gas of a ship.

8. In the heat exchanger, four flows, namely, uncompressed evaporation gas supplied from the storage tank to the compressor, compressed gas compressed by the compressor, refrigerant compressed by the first refrigerant compression section and the second refrigerant compression section, and refrigerant cooled by expansion in the refrigerant expansion section, are heat-exchanged. The method for re-liquefying evaporation gas of a ship according to Claim 7.

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