KR102010880B1 - Boil-Off Gas Reliquefaction System and Method for Vessel - Google Patents

Abstract

A boil off gas reliquefaction system is disclosed.
The vessel boil-off gas liquefaction system, the first compressor for compressing the boil-off gas; A second compressor installed in parallel with the first compressor and compressing an evaporation gas of another flow which is not sent to the first compressor; A first heat exchanger configured to cool the boil-off gas compressed by the first compressor or the second compressor using a boil-off gas before being compressed by the first compressor or the second compressor as a refrigerant, and heat-cool by heat exchange; A second heat exchanger configured to further heat-exchange and cool the fluid cooled by the first heat exchanger by using an evaporated gas circulating in a refrigerant cycle as a refrigerant; And a first decompression device for depressurizing the fluid cooled by the second heat exchanger, wherein one of the first compressor and the second compressor supplies an evaporation gas to a fuel demand (hereinafter, evaporates to a fuel demand). The compressor for supplying gas is referred to as a 'fuel supply compressor'), and the remaining compressor which does not supply the boil-off gas to the fuel demand among the first compressor and the second compressor supplies the boil-off gas in the refrigerant cycle (hereinafter, The compressor for supplying the boil-off gas to the refrigerant cycle is referred to as a 'compressor cycle compressor.' The second refrigerant cycle is a second pressure for reducing the boil-off gas supplied to the refrigerant cycle after being compressed by the refrigerant cycle compressor. Decompression device; A fourth compressor for compressing the fluid used as the refrigerant in the second heat exchanger after the pressure is reduced by the second pressure reducing device; And a bypass line, which bypasses the second heat exchanger and sends the boil-off gas supplied to the refrigerant cycle after being compressed by the refrigerant cycle compressor, to the second pressure reducing device. After being compressed by ', the boil-off gas supplied to the refrigerant cycle is basically cooled by the second heat exchanger, further reduced in pressure by the second pressure reducing device, cooled, and then supplied to the second heat exchanger again to the refrigerant. If a droplet is likely to occur in the fluid to be used and is sent to the second decompression device, a part or all of the boil-off gas supplied to the refrigerant cycle after being compressed by the 'refrigerant cycle compressor' is stored by the bypass line. Bypass the second heat exchanger is sent to the second decompression device.

Description

Boil-Off Gas Reliquefaction System and Method for Vessel

The present invention relates to a system and method for reliquefying boil-off gas (BOG) produced by natural vaporization of liquefied gas.

Recently, the consumption of liquefied gas such as liquefied natural gas (Liquefied Natural Gas, LNG) is increasing worldwide. Liquefied gas liquefied gas at low temperature has the advantage that the storage and transport efficiency can be improved because the volume is very small compared to the gas. In addition, liquefied gas, including liquefied natural gas can remove or reduce air pollutants during the liquefaction process, it can be seen as an environmentally friendly fuel with less emissions of air pollutants during combustion.

Liquefied natural gas is a colorless and transparent liquid obtained by liquefying natural gas containing methane as a main component at about -163 ℃ and having a volume of about 1/600 compared to natural gas. Therefore, when liquefied and transported natural gas can be transported very efficiently.

However, since the liquefaction temperature of natural gas is a cryogenic temperature of -163 ℃ at normal pressure, liquefied natural gas is sensitive to temperature changes and easily evaporated. As a result, the storage tank storing the liquefied natural gas is insulated. However, since the external heat is continuously transferred to the storage tank, the natural gas is continuously vaporized in the storage tank during the transport of the liquefied natural gas. -Off Gas, BOG) occurs.

Boil-off gas is a kind of loss and is an important problem in transportation efficiency. In addition, when boil-off gas is accumulated in the storage tank, the internal pressure of the tank may be excessively increased, and there is also a risk that the tank may be damaged. Accordingly, various methods for treating the boil-off gas generated in the storage tank have been studied. In recent years, for the treatment of the boil-off gas, a method of re-liquefying the boil-off gas to return to the storage tank, and returning the boil-off gas to the fuel of a ship engine The method used as an energy source of a consumer is used.

As a method for reliquefaction of the boil-off gas, a refrigeration cycle using a separate refrigerant is provided to re-liquefy the boil-off gas by exchanging it with the refrigerant, a method of re-liquefying the boil-off gas itself as a refrigerant without a separate refrigerant, and the like. There is this.

On the other hand, among the engines generally used in ships as a fuel that can use natural gas as a fuel gas engines such as DFDE, X-DF engine, ME-GI engine.

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

The X-DF engine consists of two strokes, uses about 16 bar of natural gas as fuel, and employs an auto cycle.

The ME-GI engine is composed of two strokes and employs a diesel cycle that directly injects high pressure natural gas near 300 bar into the combustion chamber near the top dead center of the piston.

Evaporative gas is reliquefied by using evaporation gas itself as a refrigerant without a separate refrigerant.The evaporated gas compressed by the compressor is cooled by heat-exchanging with the evaporated gas before being compressed by the compressor, and then expanded by a JT valve to evaporate. There is a method of reliquefaction of a part of the gas, and a system employing such a method is called a PRS (Partial Re-liquefaction System).

When there is a large amount of liquefied gas inside the storage tank and the amount of boil-off gas is generated, when the vessel is anchored or when the vessel is operated at a low speed and there is a small amount of boil-off gas used in the engine. However, the PRS alone may not meet the required amount of reliquefaction.

The PRS was improved to re-liquefy the boil-off gas to further cool the boil-off gas by the refrigerant cycle using the boil-off gas itself as a refrigerant. The system employing this method is called MRS (Methane Refrigeration). System).

The present invention is to improve the conventional MRS, to provide a ship boil-off gas reliquefaction system and method, configured to compensate for the problems caused by operating the existing MRS and to more efficiently liquefy the boil-off gas.

According to an aspect of the present invention for achieving the above object, a first compressor for compressing the boil-off gas; A second compressor installed in parallel with the first compressor and compressing an evaporation gas of another flow which is not sent to the first compressor; A first heat exchanger configured to cool the boil-off gas compressed by the first compressor or the second compressor using a boil-off gas before being compressed by the first compressor or the second compressor as a refrigerant, and heat-cool by heat exchange; A second heat exchanger configured to further heat-exchange and cool the fluid cooled by the first heat exchanger by using an evaporated gas circulating in a refrigerant cycle as a refrigerant; And a first decompression device for depressurizing the fluid cooled by the second heat exchanger, wherein one of the first compressor and the second compressor supplies an evaporation gas to a fuel demand (hereinafter, evaporates to a fuel demand). The compressor for supplying gas is referred to as a 'fuel supply compressor'), and the remaining compressor which does not supply the boil-off gas to the fuel demand among the first compressor and the second compressor supplies the boil-off gas in the refrigerant cycle (hereinafter, The compressor for supplying the boil-off gas to the refrigerant cycle is referred to as a 'compressor cycle compressor.' The second refrigerant cycle is a second pressure for reducing the boil-off gas supplied to the refrigerant cycle after being compressed by the refrigerant cycle compressor. Decompression device; A fourth compressor for compressing the fluid used as the refrigerant in the second heat exchanger after the pressure is reduced by the second pressure reducing device; And a bypass line, which bypasses the second heat exchanger and sends the boil-off gas supplied to the refrigerant cycle after being compressed by the refrigerant cycle compressor, to the second pressure reducing device. After being compressed by ', the boil-off gas supplied to the refrigerant cycle is basically cooled by the second heat exchanger, further reduced in pressure by the second pressure reducing device, cooled, and then supplied to the second heat exchanger again to the refrigerant. If a droplet is likely to occur in the fluid to be used and is sent to the second decompression device, a part or all of the boil-off gas supplied to the refrigerant cycle after being compressed by the 'refrigerant cycle compressor' is stored by the bypass line. A ship boil-off gas reliquefaction system is provided, bypassing a second heat exchanger and sent to said second pressure reducing device.

The vessel boil-off liquefaction system, the first valve is installed in the bypass line to control the flow rate and opening and closing of the fluid; And a temperature sensor measuring a temperature of a fluid which is cooled by the second heat exchanger and then sent to the second decompression device, wherein the temperature of the temperature measured by the temperature sensor is a set value. When the temperature is less than or equal to, the more the temperature measured by the temperature sensor is lower than the set value, the more it may be opened.

The ship boil-off gas liquefaction system is installed in a line through which the fluid cooled by the second heat exchanger is sent to the second decompression device, and lowers the droplets contained in the fluid cooled by the second heat exchanger by gravity. Collected on, may further include a suction pipe.

The vessel boil-off liquefaction system may further include a water level sensor installed in the suction pipe, and when the level of the liquid droplets measured by the water level sensor is a predetermined height or more, the liquid collected in the suction pipe may be discharged. .

The refrigerant cycle is closed to connect the 'refrigerant cycle compressor', the second heat exchanger, the second decompression device, the second heat exchanger, the fourth compressor, and the 'refrigerant cycle compressor' again. It can form a loop.

By injecting nitrogen into the refrigerant cycle, a fluid in which evaporated gas and nitrogen are mixed may be used as the refrigerant of the second heat exchanger.

The vessel boil-off liquefaction system is installed on a line for sending the boil-off gas compressed by the first compressor or the second compressor to the first heat exchanger, and is compressed by the first compressor or the second compressor. It may further include a third compressor for further compressing the boil-off gas.

The third compressor may compress the boil-off gas to 150 to 300 bar.

The third compressor may compress the boil-off gas to 150 to 170 bar.

The vessel boil-off gas liquefaction system may further include a gas-liquid separator installed downstream of the first decompression device to separate the liquefied liquefied gas and the boil-off gas remaining in a gaseous state.

The boil-off gas separated by the gas-liquid separator may be combined with the boil-off gas before being used as the coolant in the first heat exchanger and used as the coolant in the first heat exchanger.

According to another aspect of the present invention for achieving the above object, 1) using the boil-off gas as a refrigerant in the first heat exchanger; 2) branching the boil-off gas used as the refrigerant of the first heat exchanger into two streams in step 1); 3) out of the flow branched into two flows in step 2), one flow to the first compressor, the other flow to the second compressor; 4) sending the boil-off gas compressed by the first compressor or the second-compressor to the fuel demand (hereinafter, the compressor for supplying the boil-off gas to the fuel demand is called a 'fuel supply compressor'); 5) using the remaining evaporated gas which is not used in the fuel demand among the evaporated gas compressed by the fuel supply compressor, using the evaporated gas supplied to the first heat exchanger in step 1 as a refrigerant, Cooling by heat exchange with a heat exchanger; 6) sending the evaporated gas compressed by the remaining compressor other than the 'fuel supply compressor' among the first compressor and the second compressor to the refrigerant cycle (hereinafter, the compressor for supplying the boil-off gas by the refrigerant cycle Compressor '); 7) exchanging and cooling the fluid cooled by the first heat exchanger in step 5) using a boil-off gas circulating in the refrigerant cycle as a refrigerant and heat-exchanged by a second heat exchanger; And 8) reducing the fluid cooled by the second heat exchanger in step 7), wherein the boil-off gas circulating in the refrigerant cycle is compressed by the compressor for the refrigerant cycle. Becoming; 5-2) cooling by the second heat exchanger after being compressed in step 5-1); 5-3) a step of reducing the pressure by the second pressure reducing device after cooling in the step 5-2); 5-4) reducing the pressure in step 5-3) and using the refrigerant in the second heat exchanger; And 5-5) being used as a refrigerant of the second heat exchanger in step 5-4) and compressing the same by a fourth compressor. If there is a possibility of an enemy, the part or all of the boil-off gas supplied to the refrigerant cycle after being compressed by the 'refrigerant cycle compressor' to bypass the second heat exchanger by a bypass line to the second decompression device, There is provided a method for re-liquefying boil-off gas.

A temperature sensor may be installed in a line where the fluid cooled by the second heat exchanger or the fluid bypassing the second heat exchanger along the bypass line is sent to the second pressure reducing device, and the temperature measured by the temperature sensor is When the temperature is less than or equal to a predetermined value, part or all of the boil-off gas supplied to the refrigerant cycle after being compressed by the refrigerant cycle compressor may be bypassed to the second heat exchanger and sent to the second pressure reducing device.

After cooling by the second heat exchanger or bypassing the second heat exchanger along the bypass line, droplets included in the fluid sent to the second decompression device may be collected under the suction pipe by gravity.

The boil-off gas compressed by the fuel supply compressor may be further compressed by the third compressor and then sent to the first heat exchanger.

The fluid having undergone the depressurization process of step 8) may be separated into a reliquefied liquefied gas and an evaporated gas remaining in a gaseous state.

After the decompression process of step 8), the separated gaseous evaporated gas may be combined with the evaporated gas before being used as the refrigerant in the first heat exchanger, and then used as the refrigerant in the first heat exchanger.

The refrigerant cycle is closed to connect the 'refrigerant cycle compressor', the second heat exchanger, the second decompression device, the second heat exchanger, the fourth compressor, and the 'refrigerant cycle compressor' again. It can form a loop.

According to another aspect of the present invention for achieving the above object, a first compressor for compressing the boil-off gas; A second compressor installed in parallel with the first compressor and compressing an evaporation gas of another flow which is not sent to the first compressor; A first heat exchanger configured to cool the boil-off gas compressed by the first compressor or the second compressor using a boil-off gas before being compressed by the first compressor or the second compressor as a refrigerant, and heat-cool by heat exchange; A second heat exchanger configured to further heat-exchange and cool the fluid cooled by the first heat exchanger by using an evaporated gas circulating in a refrigerant cycle as a refrigerant; A first decompression device for depressurizing the fluid cooled by the second heat exchanger; And a second decompression device for decompressing boil-off gas used as the refrigerant in the refrigerant cycle.
As the refrigerant of the refrigerant cycle, the boil-off gas compressed by the first compressor or the second compressor, cooled by the second heat exchanger, and decompressed by the second pressure reducing device and cooled is supplied to the second heat exchanger to be a refrigerant. Used,
Prepare a bypass line for bypassing the second heat exchanger to the boil-off gas compressed by the first or the second compressor to the second pressure-reducing device, and measure the temperature of the boil-off gas sent to the second pressure-reducing device. If there is a possibility of an enemy occurring, part or all of the boil-off gas supplied to the refrigerant cycle is diverted to the second heat exchanger by the bypass line to the second decompression device, and the temperature of the refrigerant circulating the refrigerant cycle is There is provided a ship boil-off gas liquefaction system, which is maintained at the lowest in the range where no enemy occurs.

The ship boil-off gas liquefaction system may further include a suction pipe installed in the refrigerant cycle to collect droplets included in the fluid in the lower portion by gravity.

According to the present invention, the temperature of the refrigerant circulating through the refrigerant cycle RC by the bypass line BL1 and the temperature sensor 610 is kept to the lowest in the range where no droplets are generated without installing a lot of additional equipment. Since it is possible to prevent the supercooling of the boil-off gas without significantly affecting the reliquefaction amount and the reliquefaction efficiency, it is possible to prevent the decompression device 420 from being damaged.

In addition, according to an embodiment of the present invention, since the droplets contained in the fluid are collected under the suction pipe 620 by gravity, the second pressure reducing device 420 by the bypass line BL1 and the suction pipe 620. Since it is possible to block the flow into the double, it is possible to more thoroughly prevent the damage of the second decompression device (420).

1 is a schematic diagram of a boil-off gas reliquefaction system according to a preferred embodiment of the present invention.
2 is a graph schematically illustrating a phase change of methane according to temperature and pressure.
3 is a graph showing the temperature value of methane according to the heat flow amount under different pressure, respectively.
4 and 5 are graphs showing the amount of reliquefaction according to the boil-off gas pressure in the PRS (Partial Re-liquefaction System).

Hereinafter, the configuration and operation of the preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Evaporative gas reliquefaction system and method for ships of the present invention can be applied to a variety of applications, such as a ship equipped with an engine using a natural gas as a fuel, a vessel or a liquefied gas storage tank. In addition, the following examples may be modified in many different forms, and the scope of the present invention is not limited to the following examples.

In addition, 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 the operating conditions of the system.

1 is a schematic diagram of a boil-off gas reliquefaction system according to a preferred embodiment of the present invention.

Referring to FIG. 1, the ship boil-off gas reliquefaction system of the present embodiment includes a first heat exchanger 110, a first compressor 210, a second compressor 220, a second heat exchanger 120, and a first pressure reduction. Device 410, and a refrigerant cycle RC.

The first heat exchanger 110 uses the evaporated gas before being compressed by the first compressor 210 or the second compressor 220 as a refrigerant, and the first heat exchanger 110 uses the first compressor 210 or the second compressor 220. Cool the compressed boil-off gas.

The boil-off gas used as the refrigerant in the first heat exchanger 110 may be the boil-off gas discharged from the storage tank (T).

The boil-off gas used as the refrigerant in the first heat exchanger 110 is branched into two flows, one flow is sent to the first compressor 210 and the other flow is sent to the second compressor 220.

The first compressor 210 compresses one stream of the boil-off gas branched into two streams after being used as the refrigerant in the first heat exchanger 110. The first compressor 210 may be a multistage compressor, and a first cooler 310 may be installed downstream of the first compressor 210 to cool the boil-off gas compressed by the first compressor 210.

The second compressor 220 is installed in parallel with the first compressor 210 and sent to the first compressor 210 of the boil-off gas branched into two streams after being used as the refrigerant in the first heat exchanger 110. Compress the rest of the unflowed stream. The second compressor 220 may be a multi-stage compressor, and a second cooler 320 for cooling the boil-off gas compressed by the second compressor 220 may be installed downstream of the second compressor 220.

One of the first and second compressors 210 and 220 supplies the boil-off gas to the fuel demand E, and the other one supplies the boil-off gas to the refrigerant cycle RC. In addition, of the boil-off gas compressed by the compressor for supplying the boil-off gas to the fuel demand (E), surplus boil-off gas not used in the fuel demand (E) is sent to the first heat exchanger (110) to undergo a reliquefaction process. .

The fuel demand (E) is a ME-GI engine using approximately 300 bar of natural gas as fuel, an X-DF engine using approximately 16 bar of natural gas as fuel, and a DF using approximately 6.5 bar of natural gas as fuel. At least one of an engine DFGE, DFDE, and a gas combustion unit (GCU).

When the fuel demand E is an engine, the compressor for supplying fuel to the fuel demand E among the first compressor 210 and the second compressor 220 compresses the boil-off gas at the required pressure of the fuel demand E. Let's do it.

Compressors that supply fuel to the engine should have a redundancy design in case of emergency. Redundancy design is a replacement for another when one cannot be used due to failure, maintenance, etc. Designed to be used.

In the present embodiment, the first compressor 210 and the second compressor 220 may play a role of redundancy with each other. Conventionally, the first compressor 210 and the second compressor 220 are evaporated by one of the first compressor 210 and the second compressor 220. Compresses the gas and supplies it to the fuel demand (E), leaving the other unused in standby (Standby), and immediately when the compressor that supplied the boil-off gas to the fuel demand (E) becomes unavailable, It was operated to supply the boil-off gas to the fuel demand (E) by using the compressor of) state.

However, according to the conventional operation method, the use of the compressor supplying the boil-off gas to the fuel demand (E) is unlikely to occur even though it is unlikely to occur even if the duration is not long. In order to prepare, an extra compressor must be installed and the spare compressor must be kept in a standby state, which causes a problem in that a cost for the actual risks is wasted.

According to this embodiment, when both the first compressor 210 and the second compressor 220 can be used normally, one is used for supplying fuel to the fuel demand E, and the other is a refrigerant cycle. When the compressor used for supplying the evaporation gas to the RC and the compressor supplying the evaporation gas to the fuel demand E cannot be used, the evaporation gas is supplied to the refrigerant cycle RC to reliquefy the reliquefaction amount and the reliquefaction efficiency. To abandon the increase, the boil-off gas is supplied to the fuel demand (E) by a compressor that supplies the boil-off gas to the refrigerant cycle RC.

Therefore, according to the present embodiment, while satisfying the ship regulations requiring the redundancy design, there is an advantage that the re-liquefaction amount and the re-liquefaction efficiency can be increased by utilizing the extra compressor installed for the emergency situation.

The first compressor 210 and the second compressor 220 are preferably compressors of the same performance so as to act as a redundancy with each other.

The second heat exchanger 120 is compressed by the first compressor 210 or the second compressor 220 using the evaporated gas circulating through the refrigerant cycle RC as the refrigerant, and then the first heat exchanger 110. The fluid cooled by the heat exchanger is further cooled by heat exchange. The fluid further cooled by the second heat exchanger 120 is sent to the first pressure reducing device 410.

According to the present embodiment, the boil-off gas is additionally cooled not only in the first heat exchanger 110 but also in the second heat exchanger 120, so that the boil-off gas cooled only by the first heat exchanger 110 is the first pressure reducing device ( Compared to the case where it is sent to 410, since the evaporated gas in a lower temperature is sent to the first decompression device 410, the reliquefaction efficiency and the amount of reliquefaction are increased. The following is a closer look at this.

2 is a graph schematically illustrating a phase change of methane according to temperature and pressure. Referring to FIG. 2, methane is in a supercritical fluid state at a temperature of about −80 ° C. or more and a pressure of about 55 bar or more. That is, in the case of methane, the critical point is about -80 ℃, 55bar state. The supercritical fluid state is a third state different from the liquid state or the gas state.

When the boil-off gas has a temperature lower than the critical point at a pressure above the critical point, it may be in a state similar to a dense supercritical fluid state different from the general liquid state. The state is hereinafter referred to as "high pressure liquid state".

The evaporated gas sent to the first heat exchanger 110 to undergo the reliquefaction process may be in a gaseous state or a supercritical fluid state depending on the degree of compression. When the boil-off gas sent to the first heat exchanger 110 is in a gaseous state, the boil-off gas in the gaseous state passes through the first heat exchanger 110 and the temperature is lowered to form a mixed state of liquid and gas. When the boil-off gas sent to the first heat exchanger 110 is in a supercritical fluid state, the fluid in the supercritical fluid state passes through the first heat exchanger 110 and the temperature is lowered to become a "high pressure liquid state."

The fluid cooled by the first heat exchanger 110 has a lower temperature while passing through the second heat exchanger 120. When the fluid passing through the first heat exchanger 110 is a mixture of liquid and gas, In this case, the fluid in the mixed state of the liquid and gas passes through the second heat exchanger 120 and the temperature is lowered so that the ratio of the liquid becomes a mixed state or becomes a liquid state, and passes through the first heat exchanger 110. When the fluid is in the "high pressure liquid state", the fluid in the "high pressure liquid state" passes through the second heat exchanger 120 and the temperature is lowered to become the "high pressure liquid state" in which the temperature is lower.

Also, even when the fluid passing through the second heat exchanger 120 is in the "high pressure liquid state", the fluid in the "high pressure liquid state" passing through the second heat exchanger 120 passes through the first pressure reducing device 410. As the pressure is lowered, the liquid becomes a liquid state or a mixture of liquid and gas.

Although the boil-off gas is lowered to the same degree (P in FIG. 2) by the first decompression device 410, the temperature is lower than that in the case where the temperature is reduced in pressure (X → X 'in FIG. 2). It can be seen that when the pressure is reduced in the state (Y → Y ′ in FIG. 2), the ratio of the liquid becomes a higher mixed state. In addition, it can be seen that if the temperature can be further lowered, theoretically 100% of the evaporated gas can be reliquefied (Z → Z ′ in FIG. 2). Therefore, when the fluid cooled by the first heat exchanger 110 is further cooled by the second heat exchanger 120 and then sent to the first pressure reducing device 410, the reliquefaction efficiency and the amount of reliquefaction are increased.

The first decompression device 410 decompresses the evaporated gas cooled by the first heat exchanger 110 and the second heat exchanger 120 after being compressed by the first compressor 210 or the second compressor 220. Let's do it. The first pressure reducing device 410 includes all means capable of reducing the evaporated gas to be cooled, and may be an expansion valve such as a Joule-Thomson valve or an expander. In the present embodiment, it is preferable that the first pressure reducing device 410 is an expansion valve. Expansion valves have the advantage of being inexpensive and less prone to failure than inflators.

The compression process by the first compressor 210 or the second compressor 220, the cooling process by the first heat exchanger 110 and the second heat exchanger 120, and the pressure reduction by the first pressure reducing device 410. Partially or all of the evaporated gas undergoes reliquefaction.

The vessel boil-off liquefaction system of this embodiment may further include a gas-liquid separator 500 installed downstream of the first decompression device 410 to separate the liquefied liquefied gas and the boil-off gas remaining in the gas state. .

The liquefied gas separated by the gas-liquid separator 500 may be sent to the storage tank T, and the boil-off gas separated by the gas-liquid separator 500 is an evaporated gas before being used as a refrigerant in the first heat exchanger 110. And may be used as a refrigerant in the first heat exchanger (110).

When the first heat exchanger 110 uses the evaporated gas discharged from the storage tank T as the refrigerant, the evaporated gas separated by the gas-liquid separator 500 joins the evaporated gas discharged from the storage tank T. Can be sent to the first heat exchanger (110).

In addition, the boil-off gas separated by the gas-liquid separator 500 may be separately separated without joining the boil-off gas before being used as the coolant in the first heat exchanger 110 to be used as the coolant in the first heat exchanger 110. In this case, the first heat exchanger 110 may be composed of three flow paths.

When the ship boil-off gas liquefaction system of the present embodiment does not include the gas-liquid separator 500, some or all of the boil-off liquid liquefied boil-off gas may be sent directly from the first decompression device 410 to the storage tank (T).

The refrigerant cycle RC through which the boil-off gas used as the refrigerant in the second heat exchanger 120 circulates includes a second pressure reducing device 420 and a fourth compressor 240.

The boil-off gas, which is compressed by the first compressor 210 or the second compressor 220 and then supplied to the refrigerant cycle RC, is cooled by the second heat exchanger 120 and then sent to the second pressure reducing device 420. Lose.

The second pressure reducing device 420 is an expander including a plurality of blades and further cools the fluid cooled by the second heat exchanger 120 to reduce the pressure. The fluid depressurized by the second pressure reducing device 420 is supplied to the second heat exchanger 120 and used as a refrigerant.

That is, the second heat exchanger 120 is compressed by the first compressor 210 or the second compressor 220 using the fluid decompressed by the second pressure reducing device 420 as a refrigerant, and then the refrigerant cycle ( The boil-off gas supplied to RC) and the fluid cooled by the first heat exchanger 110 are simultaneously cooled by heat exchange.

The fourth compressor 240 compresses the fluid used as the refrigerant in the second heat exchanger 120 to maintain a constant pressure average of the boil-off gas circulating through the refrigerant cycle RC. The fourth compressor 240 forms the second pressure reducing device 420 and the compander C to be driven by the power generated by the second pressure reducing device 420 while inflating the fluid. Downstream), a fourth cooler 340 for cooling the boil-off gas compressed by the fourth compressor 240 may be installed.

According to the ship boil-off gas liquefaction system of the present embodiment, the boil-off gas compressed by the fourth compressor 240 may be sent upstream of the first compressor 210 and may be sent upstream of the second compressor 220. The refrigerant cycle RC may be configured to include the first compressor 210 and the refrigerant cycle may be configured to include the second compressor 220.

When the boil-off gas compressed by the first compressor 210 is supplied to the fuel demand E and the boil-off gas compressed by the second compressor 220 is supplied to the refrigerant cycle RC, the second compressor ( 220, the second heat exchanger 120, the second pressure reducing device 420, the second heat exchanger 120, the fourth compressor 240, and again the refrigerant in the closed loop connecting the second compressor 220 Cycle RC is formed.

When the boil-off gas compressed by the first compressor 210 is supplied to the refrigerant cycle RC and the boil-off gas compressed by the second compressor 220 is supplied to the fuel demand E, the first compressor ( 210, a closed loop refrigerant connecting the second heat exchanger 120, the second pressure reducing device 420, the second heat exchanger 120, the fourth compressor 240, and the first compressor 210 again. Cycle RC is formed.

According to this embodiment, one of the first compressor 210 and the second compressor 220 is selected to supply the boil-off gas to the fuel demand E or the first heat exchanger 110, and the fuel demand E ) Or the refrigerant cycle RC can be circulated by the boil-off gas compressed by the other compressor that does not supply boil-off gas to the first heat exchanger 110, thereby operating the system freely.

In addition, according to the present embodiment, there is an advantage that the re-liquefaction efficiency and re-liquefaction amount of the boil-off gas can be increased by utilizing a redundancy compressor that is not normally used even though it is conventionally installed in a ship.

According to the present embodiment, nitrogen may be injected into the refrigerant cycle RC so that the fluid in which the evaporation gas and nitrogen are mixed circulates through the refrigerant cycle RC and may be used as the refrigerant in the second heat exchanger 120.

The refrigerant cycle RC includes a bypass line BL1 and a temperature sensor 610. The bypass line BL1 is provided with a first valve V1 for controlling the flow rate and opening and closing of the fluid.

The bypass line BL1 has a second boil-off gas, which is compressed by the first compressor 210 or the second compressor 220 and then supplied to the refrigerant cycle RC, bypasses the second heat exchanger 120. It is sent to the decompression device (420). The boil-off gas, which is supplied to the refrigerant cycle RC and bypasses the second heat exchanger 120 along the bypass line BL1, is decompressed by the second pressure reducing device 420 and is refrigerant in the second heat exchanger 120. After used as is sent to the fourth compressor (240).

The lower the temperature of the fluid used as the refrigerant in the second heat exchanger 120 is, the higher the amount of reliquefaction and reliquefaction efficiency is. It is advantageous to cool the heat exchanger 120 and then send it to the second pressure reducing device 420.

However, when the boil-off gas supplied to the refrigerant cycle RC is cooled by the second heat exchanger 120 and then sent to the second pressure reducing device 420, the boil-off gas is supercooled by the second heat exchanger 120. In this case, a droplet may be included in the fluid sent to the second decompression device 420. When the fluid containing the droplets is sent to the second decompression device 420, the droplets may enter the blades rotating at a high speed, and the blades may be damaged.

Accordingly, in the present invention, the fluid cooled by the second heat exchanger 120 or the fluid bypassing the second heat exchanger 120 along the bypass line BL1 is sent to the second pressure reducing device 420. When the temperature sensor 610 is installed and the temperature measured by the temperature sensor 610 is lower than or equal to the set value, the first valve V1 is opened so that the boil-off gas supplied to the refrigerant cycle RC is bypassed. To bypass the second heat exchanger (120).

In addition, the temperature of the refrigerant circulating through the refrigerant cycle RC is adjusted by controlling the opening degree of the first valve V1 as well as opening and closing of the first valve V1 according to the temperature measured by the temperature sensor 610. The first valve V1 is opened more as the temperature measured by the temperature sensor 610 is lower than the set value.

That is, according to the present invention, for the reliquefaction amount and the reliquefaction efficiency, basically, the boil-off gas supplied to the refrigerant cycle RC passes through the second heat exchanger 120 to the second decompression device 420. If the temperature of the refrigerant circulating through the refrigerant cycle RC is too low to cause droplets to occur in the fluid sent to the second decompression device 420, a part of the evaporated gas supplied to the refrigerant cycle RC or All of them bypass the second heat exchanger 120 along the bypass line BL1.

Therefore, according to the present invention, the temperature of the refrigerant circulating through the refrigerant cycle RC by the bypass line BL1 and the temperature sensor 610 can be most easily changed in a range where no droplets are generated without installing a lot of additional equipment. Since it can be kept low, it is possible to prevent the overcooling of the boil-off gas without significantly affecting the amount of reliquefaction and reliquefaction efficiency, thereby preventing the decompression device 420 from being damaged.

The refrigerant cycle RC may include a suction pipe 620.

Even if the temperature of the refrigerant circulating through the refrigerant cycle RC is maintained within a predetermined range by adjusting the opening and closing of the first valve V1 according to the temperature measured by the temperature sensor 610, the second heat exchanger 120 The liquid droplets may be included in the fluid supplied to the second decompression device 420 after being cooled.

The suction pipe 620 is installed in a line in which the fluid that has been bypassed by the second heat exchanger 120 is sent to the second pressure reducing device 420 along the fluid cooled by the second heat exchanger 120 or the bypass line. . The fluid passes through the suction pipe 620, and droplets included in the fluid collect under the suction pipe 620 by gravity, and the gas component rises above the suction pipe 620.

When the present invention includes the suction pipe 620, since the droplet can be blocked by the bypass line BL1 and the suction pipe 620 to the second pressure reducing device 420, the second pressure reducing device 420 Can be more thoroughly prevented.

A water level sensor 630 may be installed in the suction pipe 620, and when the liquid level of the droplet measured by the water level sensor 630 is greater than or equal to a predetermined height, the liquid droplets collected in the suction pipe 620 may be discharged.

The vessel boil-off liquefaction system of this embodiment is installed on the line which sends the boil-off gas compressed by the 1st compressor 210 or the 2nd compressor 220 to the 1st heat exchanger 110, and has the 1st compressor ( It may further include a third compressor 230 for further compressing the boil-off gas compressed by the 210 or the second compressor 220. In addition, a third cooler 330 may be installed downstream of the third compressor 230 to cool the boil-off gas compressed by the third compressor 230.

The reason for further compressing the boil-off gas sent to the first heat exchanger 110 after being compressed by the first compressor 210 or the second compressor 220 by the third compressor 230 may include a reliquefaction process. In order to increase the pressure of the boil-off gas to increase the amount of re-liquefaction and re-liquefaction, the details are as follows.

3 is a graph showing the temperature values of methane according to the heat flow amount under different pressures, and referring to FIG. 3, the higher the pressure of the boil-off gas undergoing the reliquefaction process, the higher the efficiency of self-heat exchange. Self- of self-heat exchange means that the low-temperature evaporated gas itself is used as a refrigerant to heat the high-temperature evaporated gas to be cooled.

FIG. 3 (a) shows the state of each fluid upstream and downstream of the second heat exchanger 120 when the ship boil-off gas reliquefaction system of the present embodiment does not include the third compressor 230. 3 (b) shows the state of each fluid upstream and downstream of the second heat exchanger 120 when the marine boil-off gas reliquefaction system of the present embodiment includes the third compressor 230.

In FIG. 3, the boil-off gas supplied to the refrigerant cycle RC after being compressed by the first compressor 210 or the second compressor 220 is cooled by the second heat exchanger 120, and the second pressure reducing device ( The case where the pressure is reduced by 420 and then supplied to the second heat exchanger 120 is illustrated.

The uppermost graph I of FIGS. 3A and 3B shows the fluid state at the point P1 of FIG. 1 supplied to the second heat exchanger 120, and the lowermost graph L is the second heat exchanger. After passing through the gas 120 and the second pressure reducing device 420 shows the fluid state of the point P2 of FIG. 1 which is supplied back to the second heat exchanger 120 to be used as a refrigerant, and the graph K and The graph J, which is overlaid, shows the fluid state at the point P3 of FIG. 1 supplied to the second heat exchanger 120 after passing through the first heat exchanger 110.

Since the fluid used as the refrigerant loses heat from the heat exchange process and gradually increases in temperature, the graph L proceeds from left to right with time, and the fluid heat-exchanged with the refrigerant cools the heat from the refrigerant during the heat exchange process. As the temperature is getting lower and lower, the graphs I and J progress from right to left over time.

The graph K of the middle part of (a) and (b) of FIG. 3 shows the graph I and the graph J combined. That is, the fluid used as the refrigerant in the second heat exchanger 120 is drawn as a graph L, and the fluid that is cooled by heat exchange with the refrigerant in the second heat exchanger 120 is drawn as a graph K.

When designing a heat exchanger, the temperature of the fluid supplied to the heat exchanger (that is, the fluid at points P1, P2, and P3 in FIG. 1) and the heat flow amount are fixed, and the temperature of the fluid at which the temperature of the fluid used as the refrigerant is cooled. Ensure that the Logarithmic Mean Temperature Difference (LMTD) is as small as possible, while not getting higher (i.e., intersecting graph L and graph K so that graph L does not appear above graph K).

Logical mean temperature difference (LMTD) is a heat exchange method in which the hot fluid and the low temperature fluid are injected in opposite directions and discharged from the opposite side, and the temperature before the low temperature fluid passes through the heat exchanger is tc1 and the low temperature fluid passes through the heat exchanger. After the temperature is tc2, the temperature before the high temperature fluid passes through the heat exchanger is called th1, and the temperature after the high temperature fluid passes through the heat exchanger is called th2, and d1 = th2-tc1, d2 = th1-tc2, and (d2- d1) / ln (d2 / d1), the smaller the logarithmic mean temperature difference, the higher the efficiency of the heat exchanger.

On the graph, the logarithmic mean temperature difference LMTD is represented by the interval between the low temperature fluid (graph L of FIG. 3) used as the refrigerant and the high temperature fluid (graph K of FIG. 3) cooled by heat exchange with the refrigerant. It can be seen that (b) of FIG. 3 shows a narrower interval between the graph L and the graph K than).

The difference is that the initial value of the graph J, the point indicated by the round circle, that is, the pressure of the fluid at the point P3 of FIG. 1 supplied to the second heat exchanger 120 after passing through the first heat exchanger 110 is determined. 3B is higher than that of FIG. 3A.

That is, as a result of the simulation, in the case of FIG. 3A without the third compressor 230, the fluid at the point P3 of FIG. 1 may be approximately −111 ° C. and 20 bar, and the third compressor 230 may be used. In the case of Figure 3 (b) comprising a, the fluid at point P3 of Figure 1 may be approximately -90 ℃, 50 bar, under these initial conditions the heat exchanger so that the logarithm mean temperature difference (LMTD) is the smallest In the design, the efficiency of the heat exchanger is higher in the case of FIG.

In the case of FIG. 3A, when the flow rate of the boil-off gas used as the refrigerant in the second heat exchanger 120 is approximately 6401 kg / h, the refrigerant is heat-exchanged with the refrigerant from the fluid (graph L) used as the refrigerant. The total heat flow delivered to the fluid (graph K) is approximately 585.4 kW and the flow rate of the reliquefied boil-off gas is approximately 3441 kg / h.

In the case of FIG. 3B, when the flow rate of the boil-off gas used as the refrigerant in the second heat exchanger 120 is about 5368 kg / h, the refrigerant is heat-exchanged with the refrigerant from the fluid (graph L) used as the refrigerant. The total heat flow delivered to the fluid (graph K) is approximately 545.2 kW, and the flow rate of the reliquefied boil-off gas is approximately 4325 kg / h.

That is, it can be seen that by increasing the pressure of the boil-off gas including the third compressor 230 undergoing the re-liquefaction process, it is possible to re-liquefy a larger amount of boil-off gas even with less refrigerant.

When the boil-off gas is further compressed by the third compressor 230 to increase the amount of re-liquefaction and re-liquefaction, the second heat exchanger 120 may re-liquefy the entire boil-off gas without further cooling the boil-off gas. And the time required to supply the boil-off gas to the refrigerant cycle RC eventually decreases, thereby fully securing the effect of the redundancy design in case the compressor supplying the boil-off gas to the fuel demand E fails. can do.

On the other hand, when the vessel boil-off liquefaction system of the present embodiment it is advantageous to further include a third compressor 230, and looks at the preferred pressure for the third compressor 230 to compress the boil-off gas as follows.

4 and 5 are graphs showing the amount of reliquefaction according to the boil-off gas pressure in the PRS (Partial Re-liquefaction System). The boil-off gas to be reliquefed means an boil-off gas that is cooled and re-liquefied and named to distinguish it from the boil-off gas used as a refrigerant.

However, FIGS. 4 and 5 are graphs showing the reliquefaction amount according to the evaporation gas pressure in the PRS, and thus the reliquefaction amount is lower than that of the MRS which additionally reliquefies the evaporation gas by the evaporation gas circulating in the refrigerant cycle. . In the case of MRS, approximately 3300 kg of boil-off gas can be reliquefied per hour.

4 and 5, it can be seen that the amount of reliquefaction shows the maximum value when the pressure of the boil-off gas is around 150 to 170 bar, and there is little change in the amount of liquefaction between 150 and 300 bar.

Evaporated gas compressed to the required pressure of the fuel demand (E) by one of the first compressor 210 and the second compressor 220 is sent to the fuel demand (E), and the evaporation gas to the fuel demand (E) Of the boil-off gas compressed by the supplying compressor, the excess boil-off gas not used in the fuel demand (E) is sent to the first heat exchanger 110 to undergo a reliquefaction process. When the third compressor 230 is not included, a part of the boil-off gas compressed at the required pressure of the fuel demand E is sent to the first heat exchanger 110 to undergo a reliquefaction process.

When the fuel demand (E) is an engine using natural gas of 150 bar or more, such as a ME-GI engine using approximately 300 bar of natural gas as fuel, the pressure required by the fuel demand (E) is required. There is no problem in that the excess evaporated gas which is not used in the fuel demand (E) is directly liquefied, but the fuel demand (E) uses approximately 16 bar of natural gas as fuel. In the case of an engine using less than 150 bar of natural gas as a fuel, such as a DF engine or a DF engine (DFGE, DFDE) that uses about 6.5 bar of natural gas as fuel, the third compressor 230 It is desirable to undergo further reliquefaction after further compression by.

In addition, the third compressor 230 preferably compresses the evaporated gas to about 150 to 300 bar, more preferably about 150 to 170 bar, so as to secure the maximum amount of reliquefaction. . In addition, since the amount of the third compressor 230 that compresses the boil-off gas decreases, the energy consumed by the third compressor 230 may be reduced. Thus, the third compressor 230 is more preferably approximately 150 bar. To compress the boil-off gas.

The third compressor 230 is generally sufficient to have approximately half the capacity of the first compressor 210 or the second compressor 220.

On the other hand, when the amount of liquefied gas stored in the storage tank (T) is small and the amount of evaporated gas discharged from the storage tank (T) is small, or when the evaporation gas used as fuel in the fuel demand (E) such as engine For example, when the amount of boil-off gas to be reliquefied is small, the boil-off gas undergoing the re-liquefaction process is cooled only by the first heat exchanger 110, and the second heat exchanger 120 bypasses the first heat exchanger. It may be sent to the decompression device 410 and operated in the same manner as the PRS.

When the evaporated gas undergoing the reliquefaction process is cooled only by the first heat exchanger 110 and the second heat exchanger 120 is bypassed, it is not necessary to supply the refrigerant to the second heat exchanger 120. The boil-off gas may not be supplied to the cycle RC.

The present invention is not limited to the above embodiments, and various modifications or changes may be made without departing from the technical spirit of the present invention, which will be apparent to those of ordinary skill in the art. It is.

T: Storage Tank RC: Refrigerant Cycle
BL1: bypass line V1: first valve
C: Compander E: Fuel demand
110: first heat exchanger 120: second heat exchanger
210: first compressor 220: second compressor
230: third compressor 240: fourth compressor
310: first cooler 320: second cooler
330: third cooler 340: fourth cooler
410: first pressure reducing device 420: second pressure reducing device
500: gas-liquid separator 610: temperature sensor
620: suction pipe 630: water level sensor

Claims (20)

A first compressor for compressing the boil-off gas;
A second compressor installed in parallel with the first compressor and compressing an evaporation gas of another flow which is not sent to the first compressor;
A first heat exchanger configured to cool the boil-off gas compressed by the first compressor or the second compressor using a boil-off gas before being compressed by the first compressor or the second compressor as a refrigerant, and heat-cool by heat exchange;
A second heat exchanger configured to further heat-exchange and cool the fluid cooled by the first heat exchanger by using an evaporated gas circulating in a refrigerant cycle as a refrigerant; And
And a first decompression device for depressurizing the fluid cooled by the second heat exchanger.
Any one of the first compressor and the second compressor supplies the boil-off gas to the fuel demand (hereinafter, the compressor for supplying the boil-off gas to the fuel demand is called a 'fuel supply compressor').
The remaining compressor which does not supply the boil-off gas to the fuel demand among the first compressor and the second compressor supplies the boil-off gas in the refrigerant cycle (hereinafter, the compressor for supplying the boil-off gas in the refrigerant cycle is referred to as a 'compressor cycle compressor'. do.),
The refrigerant cycle,
A second decompression device configured to decompress the boil-off gas supplied to the refrigerant cycle after being compressed by the 'compressor cycle compressor';
A fourth compressor for compressing the fluid used as the refrigerant in the second heat exchanger after the pressure is reduced by the second pressure reducing device; And
And a bypass line for sending the boil-off gas supplied to the refrigerant cycle after being compressed by the refrigerant cycle compressor to bypass the second heat exchanger to the second pressure reducing device.
The boil-off gas supplied to the refrigerant cycle after being compressed by the 'compressor cycle compressor' is basically cooled by the second heat exchanger, further reduced in pressure by the second pressure reducing device, and then cooled again. Supplied to the heat exchanger and used as a refrigerant,
If droplets are likely to occur in the fluid sent to the second decompression device, a portion or all of the boil-off gas supplied to the refrigerant cycle after being compressed by the 'refrigerant cycle compressor' may be transferred to the second line by the bypass line. A boil off gas reliquefaction system, bypassing a heat exchanger and sending it to said second decompression device. The method according to claim 1,
A first valve installed at the bypass line to control flow and opening and closing of the fluid; And
And a temperature sensor for measuring the temperature of the fluid which is cooled by the second heat exchanger and then sent to the second decompression device.
The first valve is opened when the temperature measured by the temperature sensor is less than the set value, more open as the temperature measured by the temperature sensor is lower than the set value, the vessel boil-off gas reliquefaction system. The method according to claim 1,
And a suction pipe installed in a line through which the fluid cooled by the second heat exchanger is sent to the second decompression device to collect droplets contained in the fluid cooled by the second heat exchanger under gravity. Evaporative gas reliquefaction system for ships. The method according to claim 3,
Further comprising a water level sensor installed in the suction pipe,
When the liquid level of the droplets measured by the water level sensor is more than a certain height, to discharge the droplets collected in the suction pipe, the vessel boil-off gas liquefaction system. The method according to any one of claims 1 to 4,
The refrigerant cycle is closed to connect the 'refrigerant cycle compressor', the second heat exchanger, the second decompression device, the second heat exchanger, the fourth compressor, and the 'refrigerant cycle compressor' again. Evaporative gas reliquefaction system for ships, forming a loop. The method according to any one of claims 1 to 4,
Injecting nitrogen into the refrigerant cycle, using a mixture of the boil-off gas and nitrogen as the refrigerant of the second heat exchanger, the vessel boil-off gas liquefaction system. The method according to any one of claims 1 to 4,
A third that is installed on a line for sending the boil-off gas compressed by the first compressor or the second compressor to the first heat exchanger and further compresses the boil-off gas compressed by the first compressor or the second compressor. Further comprising a compressor, marine gaseous liquefaction system. The method according to claim 7,
The third compressor is a boil-off boil-off gas reliquefaction system for compressing the boil-off gas to 150 to 300 bar. The method according to claim 8,
The third compressor is a marine boil-off gas reliquefaction system for compressing the boil-off gas to 150 to 170 bar. The method according to any one of claims 1 to 4,
And a gas-liquid separator installed downstream of the first decompression device to separate the liquefied liquefied gas and the evaporated gas remaining in the gaseous state. The method according to claim 10,
The evaporated gas separated by the gas-liquid separator is combined with the evaporated gas before being used as the refrigerant in the first heat exchanger is used as the refrigerant in the first heat exchanger, marine vaporized gas reliquefaction system. 1) using the boil-off gas as a refrigerant in the first heat exchanger;
2) branching the boil-off gas used as the refrigerant of the first heat exchanger into two streams in step 1);
3) out of the flow branched into two flows in step 2), one flow to the first compressor, the other flow to the second compressor;
4) sending the boil-off gas compressed by the first compressor or the second-compressor to the fuel demand (hereinafter, the compressor for supplying the boil-off gas to the fuel demand is called a 'fuel supply compressor');
5) using the remaining evaporated gas which is not used in the fuel demand among the evaporated gas compressed by the fuel supply compressor, using the evaporated gas supplied to the first heat exchanger in step 1 as a refrigerant, Cooling by heat exchange with a heat exchanger;
6) sending the evaporated gas compressed by the remaining compressor other than the 'fuel supply compressor' among the first compressor and the second compressor to the refrigerant cycle (hereinafter, the compressor for supplying the boil-off gas by the refrigerant cycle Compressor ');
7) exchanging and cooling the fluid cooled by the first heat exchanger in step 5) using a boil-off gas circulating in the refrigerant cycle as a refrigerant and heat-exchanged by a second heat exchanger; And
8) depressurizing the fluid cooled by the second heat exchanger in step 7);
Evaporating gas circulating through the refrigerant cycle,
5-1) compression by the 'refrigerant cycle compressor';
5-2) cooling by the second heat exchanger after being compressed in step 5-1);
5-3) a step of reducing the pressure by the second pressure reducing device after cooling in the step 5-2);
5-4) reducing the pressure in step 5-3) and using the refrigerant in the second heat exchanger; And
5-5) after being used as the refrigerant of the second heat exchanger in step 5-4) and compressed by a fourth compressor;
If there is a risk that droplets may occur in the fluid sent to the second decompression device, a part or all of the boil-off gas supplied to the refrigerant cycle after being compressed by the 'compressor cycle compressor' is transferred to the second heat exchanger by a bypass line. Bypassing the gas and sending it to the second decompression device. The method according to claim 12,
A temperature sensor is installed in a line through which the fluid cooled by the second heat exchanger or the fluid bypassing the second heat exchanger along the bypass line is sent to the second decompression device,
When the temperature measured by the temperature sensor is less than or equal to a set value, a part or all of the boil-off gas supplied to the refrigerant cycle after being compressed by the 'refrigerant cycle compressor' is bypassed by the second heat exchanger to the second A method for reliquefaction of a boil-off boil-off gas sent to a decompression device. The method according to claim 13,
After cooling by the second heat exchanger or bypassing the second heat exchanger along the bypass line, the liquid contained in the fluid sent to the second decompression device is collected under the suction pipe by gravity to re-liquefy the vessel boil-off gas. Way. The method according to claim 12,
And further compressing the boil-off gas compressed by the 'fuel supply compressor' by the third compressor and then sending the boil-off gas to the first heat exchanger. The method according to claim 12,
A method of re-liquefying a ship's boil-off gas for separating a fluid having undergone the decompression process of step 8) into a liquefied liquefied gas and a boil-off gas remaining in a gaseous state. The method according to claim 16,
After the decompression process of step 8) joins the separated gaseous evaporated gas with the evaporated gas before being used as the refrigerant in the first heat exchanger, and then used as a refrigerant in the first heat exchanger, the vessel boil-off gas ash Liquefaction method. The method according to any one of claims 12 to 17, wherein
The refrigerant cycle is closed to connect the 'refrigerant cycle compressor', the second heat exchanger, the second decompression device, the second heat exchanger, the fourth compressor, and the 'refrigerant cycle compressor' again. A method for re-liquefying boil-off gas for ships, forming a loop. A first compressor for compressing the boil-off gas;
A second compressor installed in parallel with the first compressor and compressing an evaporation gas of another flow which is not sent to the first compressor;
A first heat exchanger configured to cool the boil-off gas compressed by the first compressor or the second compressor using a boil-off gas before being compressed by the first compressor or the second compressor as a refrigerant, and heat-cool by heat exchange;
A second heat exchanger configured to further heat-exchange and cool the fluid cooled by the first heat exchanger by using an evaporated gas circulating in a refrigerant cycle as a refrigerant;
A first decompression device for depressurizing the fluid cooled by the second heat exchanger; And
And a second decompression device for decompressing boil-off gas used as the refrigerant in the refrigerant cycle.
As the refrigerant of the refrigerant cycle, the boil-off gas compressed by the first compressor or the second compressor, cooled by the second heat exchanger, and decompressed by the second pressure reducing device and cooled is supplied to the second heat exchanger to be a refrigerant. Used,
Prepare a bypass line for bypassing the second heat exchanger to the boil-off gas compressed by the first or the second compressor to the second pressure-reducing device, and measure the temperature of the boil-off gas sent to the second pressure-reducing device. If the enemy is likely to occur, part or all of the boil-off gas supplied to the refrigerant cycle is bypassed to the second heat exchanger by the bypass line and sent to the second decompression device.
The temperature of the refrigerant circulating through the refrigerant cycle is maintained at the lowest in the range where no droplets occur, the vessel boil-off gas liquefaction system. The method according to claim 19,
And a suction pipe installed in the refrigerant cycle to collect droplets contained in the fluid in the lower portion by gravity.

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