KR20180135199A - BOG Reliquefaction System and Method - Google Patents

Abstract

Disclosed is a boil-off gas (BOG) reliquefaction system capable of effectively removing oil mixed in BOG. According to the present invention, the BOG reliquefaction system comprises: a compressor compressing BOG; a second heat exchanger to cool the BOG compressed in the BOG compressor up to a condensation temperature of oil used as lubricant in the compressor through heat exchange; an oil separation means to filter out the condensed oil included in a fluid cooled by the second heat exchange; a first heat exchanger to cool the fluid passing through the oil separation means through heat exchange using BOG, which is not compressed by the compressor, as a refrigerant; and an expansion means to expand the fluid cooled by the first heat exchanger. The fluid primarily used as the refrigerant in the first heat exchanger is secondarily used as the refrigerant in the second heat exchanger to be sent to the compressor and a flow path of the second heat exchanger is greater than that of the first heat exchanger.

Description

{BOG Reliquefaction System and Method}

The present invention relates to a re-liquefaction system and a re-liquefaction system for re-liquefying an evaporation gas by self-heat exchange using the evaporation gas itself as a refrigerant.

In recent years, consumption of liquefied gas such as Liquefied Natural Gas (LNG) has been rapidly increasing worldwide. The liquefied gas obtained by liquefying the gas at a low temperature has an advantage of being able to increase the storage and transport efficiency because the volume becomes very small as compared with the gas. In addition, liquefied natural gas, including liquefied natural gas, can be removed as an eco-friendly fuel with less air pollutant emissions during combustion because air pollutants can be removed or reduced during the liquefaction process.

Liquefied natural gas is a colorless transparent liquid obtained by cooling methane-based natural gas to about -162 ° C and liquefying it, and it has a volume of about 1/600 of that of natural gas. Therefore, when the natural gas is liquefied and transported, it can be transported very efficiently.

However, since the liquefaction temperature of natural gas is a cryogenic temperature of -162 ° C at normal pressure, liquefied natural gas is sensitive to temperature changes and is easily evaporated. As a result, the storage tank storing the liquefied natural gas is subjected to heat insulation, but the external heat is continuously transferred to the storage tank. Therefore, in the transportation of liquefied natural gas, the liquefied natural gas is naturally vaporized continuously in the storage tank, -Off Gas, BOG) occurs. This also applies to other low temperature liquefied gases such as ethane.

Evaporation gas is a kind of loss and is an important issue in transport efficiency. Further, when the evaporation gas accumulates in the storage tank, the internal pressure of the tank may rise excessively, and there is a risk that the tank may be damaged. Accordingly, various methods for treating the evaporative gas generated in the storage tank have been studied. Recently, a method of re-liquefying the evaporated gas and returning it to the storage tank for treating the evaporated gas, a method of returning the evaporated gas to the storage tank And a method of using it as an energy source of a consuming place.

As a method for re-liquefying the evaporation gas, there is a method of re-liquefying the evaporation gas by heat exchange with the refrigerant by providing a refrigeration cycle using a separate refrigerant, and a method of re-liquefying the evaporation gas by using the evaporation gas itself as a refrigerant . Particularly, the system adopting the latter method is called a Partial Re-liquefaction System (PRS).

On the other hand, among the engines used in ships, there are gas fuel engines such as DFDE and ME-GI engines which can use natural gas as fuel.

The DFDE adopts the Otto Cycle, which consists of four strokes, and injects natural gas with a relatively low pressure of about 6.5 bar into the combustion air inlet and compresses the piston as it rises.

The ME-GI engine consists of two strokes and adopts a diesel cycle in which high pressure natural gas near 300 bar is injected directly to the combustion chamber near the piston's top dead center. In recent years, there is a growing interest in ME-GI engines with better fuel efficiency and propulsion efficiency.

1 is a schematic view showing a conventional evaporation gas remelting system.

Referring to FIG. 1, in the conventional evaporation gas remelting system, the evaporation gas generated and discharged from the storage tank storing the liquid cargo is conveyed along the pipe and compressed by the evaporation gas compression unit 10.

The storage tank (T) has sealing and thermal barrier to store liquefied gas such as liquefied natural gas at a cryogenic temperature, but it can not completely block the heat transmitted from the outside, and the evaporation of the liquefied gas is continuous And the tank internal pressure can be raised. In order to prevent excessive increase of the tank pressure due to the evaporated gas and to maintain an appropriate level of internal pressure, the evaporated gas inside the storage tank is discharged to the evaporated gas compression unit 10 Supply.

When the evaporated gas discharged from the storage tank and compressed by the evaporation gas compression unit 10 is referred to as a first stream, the first stream of the compressed evaporative gas is divided into the second stream and the third stream, and the second stream is liquefied To the storage tank T, and the third stream may be configured to be supplied to a gas fuel consuming destination such as a propulsion engine or a power generation engine onboard the ship. In this case, in the evaporation gas compression unit 10, the evaporation gas can be compressed to the supply pressure of the fuel consumption source, and the second stream can be branched through all or a part of the evaporation gas compression unit if necessary. All of the evaporated gas compressed in the third stream may be supplied according to the amount of fuel required by the fuel consumption point, or the entire amount of the compressed evaporated gas may be returned to the storage tank. Gas fuel consumption may include a high pressure gas injection engine (e.g., ME-GI engine developed by MDT) and a low pressure gas injection engine (e.g., Wartsila X-DF engine ), DF Generator, gas turbine, DFDE, and the like.

At this time, the heat exchanger 20 is installed to liquefy the second stream of the compressed evaporated gas, and the evaporated gas generated in the storage tank is used as a cold heat source of the compressed evaporated gas. The compressed evaporated gas, that is, the second stream, which has risen in temperature during the compression process in the evaporated gas compressor, is cooled while passing through the heat exchanger 20, and the evaporated gas generated in the storage tank and introduced into the heat exchanger 20 is heated And is supplied to the evaporation gas compression unit 10.

Since the flow rate of the evaporated gas before compression is greater than the flow rate of the second stream, the second stream of compressed evaporated gas may be at least partially liquefied by receiving cold heat from the evaporated gas before being compressed. Thus, in the heat exchanger, the low-temperature evaporation gas immediately after being discharged from the storage tank is heat-exchanged with the high-pressure evaporation gas compressed by the evaporation gas compression unit to liquefy the high-pressure evaporation gas.

The evaporated gas of the second stream passing through the heat exchanger 20 is further cooled while being decompressed while passing through the expansion means 30 such as an expansion valve or an expander and is supplied to the gas-liquid separator 40. The liquid component, that is, the liquefied natural gas, is returned to the storage tank, and the gaseous component, that is, the evaporation gas is discharged from the storage tank to be separated from the heat exchanger 20 and the evaporation gas Is utilized as a cold / hot supply source that joins the evaporation gas flow to the evaporation gas flow supplied to the compression section (10) or is supplied to the heat exchanger (20) again to heat the evaporation gas in the high pressure state compressed by the evaporation gas compression section . Of course, it may be sent to a gas combustion unit (GCU) or the like, or it may be consumed by sending it to a gas consumption source (including a gas engine). Further expansion means (50) for further depressurizing the gas separated in the gas-liquid separator before joining the evaporative gas stream may be further provided.

According to the conventional evaporation gas re-liquefaction system, the evaporation gas is compressed by the evaporation gas compression unit 10 and the oil mixed in the evaporation gas is condensed in the heat exchanger 20 before the evaporation gas, . ≪ / RTI >

SUMMARY OF THE INVENTION It is an object of the present invention to provide a system and method for liquefying an evaporated gas that can effectively remove oil mixed with evaporated gas compressed by a compressor.

According to an aspect of the present invention, there is provided a compressor for compressing an evaporative gas, A second heat exchanger for exchanging heat between the evaporated gas compressed by the compressor and the condensation temperature of oil used as lubricant in the compressor to cool the evaporated gas; Oil separating means for filtering the condensed oil contained in the fluid cooled by said second heat exchanger; A first heat exchanger which uses the evaporated gas before being compressed by the compressor as a refrigerant and cools the fluid passing through the oil separating means by heat exchange; And an expansion means for expanding the fluid cooled by the first heat exchanger, wherein a fluid used as a first refrigerant in the first heat exchanger is used as a second refrigerant in the second heat exchanger, And the second heat exchanger is larger in flow passage than the first heat exchanger.

The oil separating means may be an oil filter.

The oil separating means may be an oil separator.

The first heat exchanger may be a Micro Channel type heat exchanger.

The first heat exchanger may be a PCHE.

The second heat exchanger may be a Shell & Tube type heat exchanger.

The condensation temperature of the oil used as lubricating oil in the compressor may be 0 to -40 캜.

The temperature of the evaporation gas, which is compressed by the compressor and then sent to the second heat exchanger, may be 100-150 ° C.

The temperature of the fluid sent to the expansion means after being cooled by the first heat exchanger may be between -50 and -120 < 0 > C.

A plurality of the oil filters may be installed in parallel.

The evaporation gas re-liquefaction system may further include a gas-liquid separator provided at the downstream end of the expansion means to separate the liquefied natural gas re-liquefied from the evaporated gas remaining in the gaseous state.

The gaseous evaporation gas separated by the gas-liquid separator may be combined with the evaporated gas discharged from the storage tank and used as the refrigerant of the heat exchanger.

The compressor can compress the evaporation gas to 301 bara.

The compressor may be a multi-stage compressor, the evaporation gas that has undergone a partial compression process of the compressor is partially branched and supplied to the second engine, and the evaporation gas that has undergone the entire compression process by the compressor may be supplied to the first engine .

The first engine may be an ME-GI engine and the second engine may be a DF engine.

According to another aspect of the present invention, there is provided a method of re-liquefying an evaporation gas by compressing an evaporation gas by a compressor, cooling the evaporation gas by heat exchange with the refrigerant before expansion, and expanding the evaporation gas, The condensed oil is separated from the condensed oil by cooling the condensed oil by heat exchange by the second heat exchanger to the condensation temperature of the oil used as the lubricating oil in the compressor to cool the separated fluid further by the first heat exchanger And the second heat exchanger has a larger flow passage than the first heat exchanger.

According to the present invention, since the evaporated gas filtered by the second heat exchanger having the larger flow path than the first heat exchanger is sent to the first heat exchanger, the probability of clogging the flow path of the heat exchanger is remarkably lowered, The advantages of the first heat exchanger can be overcome.

1 is a schematic view showing a conventional evaporation gas remelting system.
FIG. 2 is a schematic view showing a system for evaporating a gas re-liquefaction according to a first preferred embodiment of the present invention.
FIG. 3 is a schematic view illustrating a vaporization gas remelting system according to a second preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. INDUSTRIAL APPLICABILITY The present invention can be applied to various applications such as a ship equipped with an engine using natural gas as fuel and a ship including a liquefied gas storage tank. In addition, the following examples can be modified in various forms, and the scope of the present invention is not limited to the following examples.

The systems for the treatment of the evaporative gas to be described below of the present invention are applicable to all types of vessels and storage vessels equipped with storage tanks capable of storing low temperature liquid cargo or liquefied gas such as FPSO, Structure.

The fluid in each line of the present invention may be in any one of a liquid state, a gas-liquid mixed state, a gas state, and a supercritical fluid state, depending on operating conditions of the system.

FIG. 2 is a schematic view showing a system for evaporating a gas re-liquefaction according to a first preferred embodiment of the present invention.

2, the evaporation gas re-liquefaction system of this embodiment includes a first heat exchanger 110, a second heat exchanger 120, a compressor 200, an oil filter 510, and an expansion means 300 .

The storage tank T stores therein a liquefied gas such as liquefied natural gas or liquefied ethane gas, and discharges the evaporated gas to the outside when the internal pressure exceeds a predetermined pressure. The evaporated gas discharged from the storage tank (T) is sent to the first heat exchanger (110).

The first heat exchanger 110 compresses the refrigerant by the compressor 200 using the evaporated gas discharged from the storage tank T as a refrigerant and then evaporates the refrigerant through the second heat exchanger 120 and the oil filter 510, The gas is cooled by heat exchange. The first heat exchanger 110 may be a micro channel type heat exchanger such as a PCHE (Printed Circuit Heat Exchanger).

The PCHE is a heat exchanger in which a fluid used as a refrigerant and a fluid to be cooled flow in from different directions to perform heat exchange. The temperature range in which heat exchange is possible is as wide as -200 to 900 ° C. and the heat transfer area per unit volume is wide Heat transfer rate. PCHE also has the advantage that it can be used for various fluids such as gas, liquid, and two-phase.

It is preferable that PCHE is used as the first heat exchanger 110 in order to satisfy the required liquefaction amount and re-liquefaction efficiency, considering the amount of the evaporation gas generated in the storage tank T, the temperature range of the fluid, Do.

On the other hand, the PCHE has a disadvantage in that the flow path inside the heat exchanger is very narrow and the flow path can be easily clogged when foreign substances are introduced. Accordingly, the evaporated gas is compressed by the compressor 200, and the mixed oil is cooled in the heat exchanger and condensed, thereby easily blocking the flow path of the PCHE.

In order to compensate for the disadvantage that the flow path is easily blocked while utilizing the advantages of the first heat exchanger 110, the system and method for evaporating the gas re- 2 heat exchanger 120 so that the oil can be filtered out.

The second heat exchanger 120 uses the evaporated gas used as the first refrigerant in the first heat exchanger 110 after being discharged from the storage tank T as the refrigerant and the evaporated gas compressed by the compressor 200 Heat exchange is performed to cool. The second heat exchanger 120 is of a type having a larger flow path than the first heat exchanger 110 and may be a Shell & Tube type heat exchanger.

According to the evaporation gas re-liquefaction system and method of the present embodiment, the evaporated gas compressed by the compressor 200 is first sent to the second heat exchanger 120 having a larger flow path than the first heat exchanger 110, Since the evaporated gas filtered by the oil is sent to the first heat exchanger 110 so that the evaporated gas compressed by the compressor 200 is directly sent to the first heat exchanger 110 for cooling, Lt; / RTI >

The compressor 200 compresses the evaporated gas used as the refrigerant in the first heat exchanger 110 and the second heat exchanger 120 after being discharged from the storage tank T. [ The evaporated gas compressed by the compressor (200) is again sent to the second heat exchanger (120). The compressor 200 of the present embodiment may be a multi-stage compressor including a plurality of compression cylinders connected in series and may be a five-stage compressor including five compression cylinders.

The compressor 200 compresses the evaporating gas to the required pressure of the first engine E1 and the evaporated gas compressed by the compressor 200 is used as the fuel of the first engine E1. The first engine E1 can use approximately 300 bara of evaporative gas as fuel, and the compressor 200 can compress the evaporative gas to approximately 301 bara.

In the case where the compressor 200 is a multi-stage compressor, the evaporation gas that has undergone partial compression by the compressor 200 is partially branched and supplied to the second engine E2, and the evaporation gas May be supplied to the first engine E1.

The first engine E1 may be an ME-GI engine, and the second engine E2 may be a DF engine.

According to the present embodiment, after the evaporated gas compressed by the compressor 200 is cooled by the second heat exchanger 120 to the condensation temperature of the oil used as the lubricating oil in the compressor 200, the condensation temperature of the oil To the oil filter (510).

Since the temperature at which the oil condenses is higher than the temperature at which the evaporation gas is liquefied and the oil is first condensed through the cooling process to block the flow path of the heat exchanger, 2 heat exchanger 120 to cool the oil to a temperature at which the oil is condensed, and then the oil is removed, and the evaporated gas from which the oil is removed is further cooled to a temperature required for re-liquefaction by the first heat exchanger 110.

May be varied depending on the type of oil, the efficiency of the compressor 200, the specifications of the heat exchangers 110 and 120 and the expansion means 300, the operating method of the system, and the like, The temperature of the evaporation gas sent to the oil separator 120 may be approximately 100-150 ° C and the temperature at which the oil condenses, that is, the temperature of the fluid that is cooled by the second heat exchanger 120 and then sent to the oil filter 510 Lt; RTI ID = 0.0 > 0-40 C. < / RTI > In addition, the temperature of the fluid cooled by the first heat exchanger 110 after the oil is filtered by the oil filter 510 may be approximately -50 to -120 캜.

A plurality of oil filters 510 may be installed in parallel so that the evaporating gas can be re-liquefied continuously without interrupting the system operation even when one of the oil filters 510 is in maintenance operation.

The evaporation gas re-liquefaction system of the present embodiment may further include a differential pressure gauge 600 for measuring the pressure difference between the front end and the rear end of the oil filter 510. If the accumulated oil in the oil filter 510 is too much and the flow of the evaporation gas is not smooth, the pressure difference between the front end and the rear end of the oil filter 510 becomes large. Therefore, the pressure difference measured by the differential pressure meter 600 is checked The state of the oil filter 510 can be grasped and the oil filter 510 can be prevented from clogging or the oil path of the first heat exchanger 110 can be prevented from being clogged because the oil is not sufficiently removed by the oil filter 510 have.

The oil-filtered fluid condensed by the oil filter 510 is sent to the first heat exchanger 110 and further cooled and then sent to the expansion means 300.

The expansion means 300 inflates the fluid that has been compressed by the compressor 200 and then passed through the second heat exchanger 120, the oil filter 510, and the first heat exchanger 110 in sequence. The expansion means 300 may be an expansion valve or expansion device such as a line-Thomson valve.

The evaporation gas, which has undergone the compression process by the compressor 200, the cooling process by the second heat exchanger 120 and the first heat exchanger 110, and the expansion process by the expansion means 300, do.

The evaporation-gas re-liquefaction system of this embodiment may further include a gas-liquid separator 400 disposed at the downstream end of the expansion means 300 to separate the liquefied natural gas re-liquefied and the evaporated gas remaining in the gaseous state. The evaporated gas remaining in the gaseous state may contain the flash gas generated when expanded by the expansion means (300).

The liquefied natural gas separated by the gas-liquid separator 400 is sent to the storage tank T and the gaseous vaporized gas separated by the gas-liquid separator 400 is combined with the vaporized gas discharged from the storage tank T Can be used as a refrigerant in the first heat exchanger (110).

FIG. 3 is a schematic view illustrating a vaporization gas remelting system according to a second preferred embodiment of the present invention.

The evaporation gas re-liquefaction system of the second embodiment shown in Fig. 3 differs from the evaporation gas re-liquefaction system of the first embodiment shown in Fig. 1 in that an oil separator 520 is included in place of the oil filter 510, And the difference will be mainly described below. A detailed description of the same components as those of the evaporation gas re-liquefaction system of the first embodiment described above will be omitted. Hereinafter, an apparatus for separating oil from a fluid such as the oil filter 510 and the oil separator 520 is referred to as an oil separator.

3, the evaporation gas re-liquefaction system of the present embodiment includes a first heat exchanger 110, a second heat exchanger 120, a compressor 200, and an expansion means 300, as in the first embodiment. .

However, the evaporation gas re-liquefaction system of this embodiment is different from the first embodiment in that the oil separated by the oil separator 520 is cooled by the second heat exchanger 120 and condensed.

The first heat exchanger 110 uses the evaporation gas discharged from the storage tank T as a refrigerant and is compressed by the compressor 200 and then supplied to the second heat exchanger 120 and the oil The evaporation gas passing through the separator 520 is heat-exchanged and cooled. The first heat exchanger 110 may be a micro channel type heat exchanger such as a PCHE (Printed Circuit Heat Exchanger) as in the first embodiment.

The second heat exchanger 120 is configured such that the refrigerant evaporated from the storage tank T and then used as the first refrigerant in the first heat exchanger 110 is used as the refrigerant, To cool the evaporated gas compressed by the heat exchanger. As in the first embodiment, the second heat exchanger 120 is of a type having a larger flow path than that of the first heat exchanger 110, and may be a Shell & Tube type heat exchanger.

The compressor 200 compresses the evaporation gas used as the refrigerant in the first heat exchanger 110 and the second heat exchanger 120 after being discharged from the storage tank T as in the first embodiment. The evaporated gas compressed by the compressor (200) is again sent to the second heat exchanger (120). The compressor 200 of the present embodiment may be a multi-stage compressor including a plurality of compression cylinders connected in series, as in the first embodiment, and may be a five-stage compressor including five compression cylinders.

The compressor 200 compresses the evaporation gas at the required pressure of the first engine E1 and the evaporated gas compressed by the compressor 200 is used as the fuel of the first engine E1 do. The first engine E1 can use approximately 300 bara of evaporative gas as fuel, and the compressor 200 can compress the evaporative gas to approximately 301 bara.

In the case where the compressor 200 is a multi-stage compressor, as in the first embodiment, some of the evaporation gas that has undergone some compression by the compressor 200 is partially branched and supplied to the second engine E2, The entire evaporation gas having undergone the compression process can be supplied to the first engine E1.

The first engine E1 may be an ME-GI engine, and the second engine E2 may be a DF engine.

According to the present embodiment, after the evaporated gas compressed by the compressor 200 is cooled by the second heat exchanger 120 to the condensation temperature of the oil used as the lubricating oil in the compressor 200, the condensation temperature of the oil To the oil separator (520).

The oil separated by the oil separator 520 is buried in the lower portion of the oil separator 520 and discharges the oil under the oil separator 520 manually or automatically from the oil separator 520. Depending on the operation of the system, the oil under the oil separator 520 may be continuously discharged during the operation of the system, or may be stopped after the operation of the system is stopped.

The temperature of the evaporated gas, which is compressed by the compressor 200 and then sent to the second heat exchanger 120, may be approximately 100-150 ° C and the temperature at which the oil condenses, The temperature of the fluid sent to the oil separator 520 may be approximately 0 to -40 < 0 > C. In addition, the temperature of the fluid cooled by the first heat exchanger 110 after the oil is filtered by the oil separator 520 may be approximately -50 to -120 ° C.

The oil-filtered fluid condensed by the oil separator 520 is sent to the first heat exchanger 110 and further cooled and then sent to the expansion means 300.

The expansion means 300 inflates the fluid that has been compressed by the compressor 200 and then passed through the second heat exchanger 120, the oil separator 520, and the first heat exchanger 110 in sequence. The expansion means 300 may be an expansion valve or expansion device such as a line-Thomson valve.

The evaporation gas, which has undergone the compression process by the compressor 200, the cooling process by the second heat exchanger 120 and the first heat exchanger 110, and the expansion process by the expansion means 300, do.

The evaporation gas re-liquefaction system of this embodiment is provided with a gas-liquid separator 400 disposed at the downstream end of the expansion means 300 to separate the liquefied natural gas that has been re-liquefied and the evaporated gas remaining in the gaseous state . The evaporated gas remaining in the gaseous state may contain the flash gas generated when expanded by the expansion means (300).

The liquefied natural gas separated by the gas-liquid separator 400 is sent to the storage tank T and the gaseous vaporized gas separated by the gas-liquid separator 400 is combined with the vaporized gas discharged from the storage tank T Can be used as a refrigerant in the first heat exchanger (110).

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. It is.

T: Storage tank E1, E2: Engine
10: Evaporative gas compression unit 20, 110, 120: Heat exchanger
200: compressors 30, 50, 300: expansion means
40, 400: gas-liquid separator 510: oil filter
520: Oil separator 600: Differential pressure gauge

Claims (16)

A compressor for compressing the evaporating gas;
A second heat exchanger for exchanging heat between the evaporated gas compressed by the compressor and the condensation temperature of oil used as lubricant in the compressor to cool the evaporated gas;
Oil separating means for filtering the condensed oil contained in the fluid cooled by said second heat exchanger;
A first heat exchanger which uses the evaporated gas before being compressed by the compressor as a refrigerant and cools the fluid passing through the oil separating means by heat exchange; And
And expansion means for expanding the fluid cooled by said first heat exchanger,
The fluid used as the first refrigerant in the first heat exchanger is used as the second refrigerant in the second heat exchanger and then sent to the compressor,
Wherein the second heat exchanger has a larger flow passage than the first heat exchanger. The method according to claim 1,
Wherein the oil separating means is an oil filter. The method according to claim 1,
Wherein the oil separating means is an oil separator. The method according to any one of claims 1 to 3,
Wherein the first heat exchanger is a Micro Channel type heat exchanger. The method of claim 4,
Wherein the first heat exchanger is a PCHE. The method according to any one of claims 1 to 3,
Wherein the second heat exchanger is a Shell & Tube type heat exchanger. The method according to any one of claims 1 to 3,
Wherein the condensing temperature of the oil used as lubricating oil in the compressor is 0 to -40 占 폚. The method according to any one of claims 1 to 3,
Wherein the temperature of the evaporation gas sent to the second heat exchanger after being compressed by the compressor is 100-150 ° C. The method according to any one of claims 1 to 3,
Wherein the temperature of the fluid which is cooled by said first heat exchanger and then sent to said expansion means is between -50 and -120 < 0 > C. The method of claim 2,
Wherein a plurality of the oil filters are installed in parallel. The method according to any one of claims 1 to 3,
Further comprising a gas-liquid separator provided at a downstream end of the expansion means to separate the liquefied natural gas that has been re-liquefied and the evaporated gas remaining in a gaseous state. The method of claim 11,
Wherein the gaseous vaporized gas separated by the gas-liquid separator is combined with the vaporized gas discharged from the storage tank and used as a refrigerant in the heat exchanger. The method according to any one of claims 1 to 3,
Wherein the compressor compresses the evaporation gas to 301 bara. The method according to any one of claims 1 to 3,
Wherein the compressor is a multi-stage compressor,
The evaporating gas, which has undergone a partial compression process of the compressor, is partially branched and supplied to the second engine,
Wherein the evaporation gas that has undergone the entire compression process by the compressor is supplied to the first engine. 15. The method of claim 14,
Wherein the first engine is an ME-GI engine and the second engine is a DF engine. There is provided a method of re-liquefying an evaporated gas by compressing an evaporated gas by a compressor, cooling the evaporated gas before it is compressed by a refrigerant,
Separating the condensed oil after cooling the evaporated gas compressed by the compressor by the second heat exchanger to the condensation temperature of the oil used as lubricating oil in the compressor,
The condensed oil further cooling the separated fluid by the first heat exchanger,
Wherein the second heat exchanger has a larger flow passage than the first heat exchanger.

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