Refrigerant circulation pressure control system for reliquefaction system of a ship

Abstract

A refrigerant circulation pressure control system for a reliquefication system of a ship is disclosed. The system includes a heat exchanger that receives boil-off gas from a tank to cool the boil-off gas; a refrigerant circulation that circulates refrigerant for cooling the boil-off gas via a refrigerant circulation line, a refrigerant compressor that compresses the refrigerant that has cooled the boil-off gas and is discharged from the heat exchanger, an expander that expands and cools the refrigerant that has been cooled via the heat exchanger after being compressed by the refrigerant compressor and supplies the refrigerant as the refrigerant of the heat exchanger; a storage tank that stores the refrigerant to be filled in the refrigerant circulation; a refrigerant supply line that is connected from the storage tank to a front end of the refrigerant compressor to replenish the refrigerant in the refrigerant circulation; a refrigerant discharge line that is connected from a rear end of the refrigerant compressor to the storage tank; and a pressure control line that branches from the refrigerant discharge line.

Description

Technical Field

[0001] The present invention relates to a refrigerant cycle pressure control system for a ship's reliquefaction system, and more particularly to a system for controlling the pressure in a refrigerant cycle that circulates via a reliquefaction system that reliquefies evaporative gases generated in the ship. Background Technology

[0002] Natural gas, containing methane as its main component, is favored as an eco-friendly fuel that releases little to no environmental pollutants during combustion. Liquefied natural gas (LNG) is obtained by liquefying natural gas at atmospheric pressure by cooling it to approximately -163°C. Because LNG has a volume approximately 1 / 600th that of natural gas in its gaseous state, it is suitable for long-distance maritime transport. Therefore, natural gas is stored and transported in the easily stored and transportable form of liquefied natural gas.

[0003] Because natural gas is liquefied at a low temperature of -163°C under normal pressure, LNG storage tanks are typically insulated to keep LNG in a liquid state. However, despite being insulated, these tanks have limited ability to block external heat. Therefore, as external heat is continuously transferred to the LNG storage tank, the LNG stored in the tank continuously evaporates naturally during transportation, producing boil-off gas (BOG).

[0004] The continuous generation of vaporized gas in LNG storage tanks increases the internal pressure. If the internal pressure exceeds a predetermined safe pressure, this could lead to an emergency, such as tank rupture. Therefore, safety valves are needed to release the vaporized gas from the tank. However, vaporized gas is a form of LNG loss and a significant issue related to LNG transportation and fuel efficiency. Therefore, various methods are employed to manage the vaporized gas generated in LNG storage tanks.

[0005] Recently, a method for using evaporated gas at fuel demand sites (e.g., ship engines), a method for reliquefying evaporated gas and returning the reliquefied evaporated gas to LNG storage tanks, and a method combining the two have been developed and put into use. Summary of the Invention

[0006] Technical issues

[0007] In reliquefaction cycles used to reliquefy evaporated gases generated in ships, typical available liquefaction methods include processes using a single mixed refrigerant (SMR) cycle and a propane-precooled mixed refrigerant (C3MR) cycle. The C3MR cycle uses propane as the sole refrigerant to cool the natural gas, followed by liquefaction and subcooling using a mixed refrigerant, while the SMR cycle uses a mixed refrigerant composed of multiple components to liquefy the natural gas.

[0008] In this way, both the SMR and C3MR cycles use mixed refrigerants. However, if the composition of the mixed refrigerant changes due to refrigerant leakage during the liquefaction of the evaporated gas, this will lead to poor liquefaction efficiency. Therefore, it is necessary to maintain a constant refrigerant composition by continuously measuring the composition of the mixed refrigerant and replenishing any missing refrigerant components.

[0009] An alternative reliquefaction cycle for reliquefying evaporated gas is a single-cycle liquefaction process using nitrogen refrigerant.

[0010] Although this type of reliquefaction cycle using nitrogen refrigerant is relatively less efficient than that using mixed refrigerant, it is safer due to the inert nature of nitrogen refrigerant and is easier to apply to ships because nitrogen refrigerant does not undergo a phase change.

[0011] In a reliquefaction system using nitrogen refrigerant, refrigerant stored in a storage tank is supplied to the refrigerant cycle of the reliquefaction system, and the load of the reliquefaction system is controlled by replenishing the refrigerant cycle or discharging refrigerant from the refrigerant cycle, depending on load requirements.

[0012] This invention proposes a system that can control the load of a reliquefaction system by smoothly replenishing or discharging refrigerant from the refrigerant cycle, while reducing the use of devices for replenishing nitrogen refrigerant to the refrigerant cycle.

[0013] Technical solutions

[0014] According to one aspect of the present invention, a refrigerant circulation pressure control system for a ship's reliquefaction system includes: a heat exchanger that provides to the reliquefaction system and cools evaporated gas received from a storage tank;

[0015] The refrigerant cycle includes a refrigerant circulation line, a refrigerant compressor, and an expander. The refrigerant circulates through the refrigerant circulation line to cool the evaporating gas in the heat exchanger. The refrigerant compressor is located on the refrigerant circulation line and compresses the refrigerant discharged from the heat exchanger after cooling the evaporating gas in the heat exchanger. The expander expands and cools the refrigerant that has been compressed by the refrigerant compressor and cooled by the heat exchanger, and supplies the expanded and cooled refrigerant to the heat exchanger.

[0016] Storage tanks are used to store refrigerant that will be added to the refrigerant cycle.

[0017] The refrigerant supply line connects the storage tank to the upstream side of the refrigerant compressor to replenish the refrigerant circulation;

[0018] A refrigerant discharge line connects the downstream side of the refrigerant compressor to a storage tank to discharge refrigerant from the refrigerant circulation system into the storage tank; and

[0019] The pressure regulating line branches off from the refrigerant discharge line.

[0020] The refrigerant circulation is depressurized by discharging refrigerant from the refrigerant circulation via a refrigerant discharge line or a pressure regulating line.

[0021] The refrigerant circulation pressure control system may further include: a first valve disposed upstream of the point where the pressure regulating line branches off from the refrigerant discharge line on the refrigerant discharge line; a second valve disposed on the pressure regulating line; and a third valve disposed downstream of the point where the pressure regulating line branches off from the refrigerant discharge line on the refrigerant discharge line, wherein when the refrigerant circulation is depressurized, the refrigerant is first discharged to the storage tank via the refrigerant discharge line, and then discharged to the pressure regulating line when a pressure reversal occurs between the storage tank and the refrigerant circulation.

[0022] The refrigerant circulation pressure control system may further include: a buffer tank, located on the ship and storing utility N2 to be supplied to the ship; a drying and filtering unit, which dries and filters the utility N2 received from the buffer tank to reduce the dew point of the utility N2; and a booster compressor, which compresses the nitrogen refrigerant that has passed through the drying and filtering unit and supplies the compressed nitrogen refrigerant to the storage tank.

[0023] The pressure regulating line can be connected to the buffer tank, which can operate at a lower pressure than the storage tank.

[0024] The pressure regulating line can be connected to the upstream side of the booster compressor.

[0025] The reliquefaction system may include a gas-liquid separator that receives cooled evaporating gas from a heat exchanger and separates the received evaporating gas into a gas phase and a liquid phase; wherein a pressure regulating line may be connected to the gas-liquid separator so that refrigerant discharged from the refrigerant cycle is supplied to the top of the gas-liquid separator to be used as an inert gas to maintain pressure in the gas-liquid separator.

[0026] The storage tank may include a high-pressure tank and a low-pressure tank. The high-pressure tank receives compressed nitrogen refrigerant from the booster compressor and supplies the received nitrogen refrigerant to the upstream side of the refrigerant compressor. The low-pressure tank operates at a lower pressure than the high-pressure tank. A pressure regulating line may be connected to the low-pressure tank.

[0027] The low-pressure tank can be operated using the suction pressure of the booster compressor, and when the low-pressure tank is pressurized by refrigerant discharged to the pressure regulating line, the refrigerant can be discharged from the low-pressure tank to the upstream side of the booster compressor to maintain a constant pressure in the low-pressure tank.

[0028] Beneficial effects

[0029] The refrigerant circulation pressure control system according to the present invention can control the load of the reliquefaction system by smoothly replenishing or discharging refrigerant from the refrigerant circulation.

[0030] Specifically, in the refrigerant cycle pressure control system according to the invention, when the refrigerant cycle is depressurized to reduce the load on the reliquefaction system, the refrigerant in the refrigerant cycle is discharged and first delivered to a storage tank. Then, when no more refrigerant can be delivered to the storage tank due to pressure reversal, the refrigerant is delivered to a buffer tank, a low-pressure tank, or upstream of a booster compressor for reuse in the refrigerant cycle or as blanketing N2 in the gas-liquid separator of the reliquefaction system. This reduces the use of equipment for supplying nitrogen to the ship and thus reduces the power consumption and operating expense (OPEX) associated with the use of such equipment. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of a refrigerant circulation pressure control system for a ship's reliquefaction system according to a basic embodiment of the present invention.

[0032] Figure 2 This is a schematic diagram of a refrigerant circulation pressure control system for a ship's reliquefaction system according to a first embodiment of the present invention.

[0033] Figure 3This is a schematic diagram of a refrigerant circulation pressure control system for a ship's reliquefaction system according to a second embodiment of the present invention.

[0034] Figure 4 This is a schematic diagram of a refrigerant circulation pressure control system for a ship's reliquefaction system according to a third embodiment of the present invention.

[0035] Figure 5 This is a schematic diagram of a refrigerant circulation pressure control system for a ship's reliquefaction system according to a fourth embodiment of the present invention. Detailed Implementation

[0036] To fully understand the operational advantages of the present invention and the objectives achieved by practicing the present invention, reference should be made to the accompanying drawings illustrating preferred embodiments of the invention and their description.

[0037] In the following description, exemplary embodiments of the invention will be described in detail with reference to the accompanying drawings, highlighting their features and effects. It should be noted that throughout the specification and drawings, the same components will be designated by the same reference numerals.

[0038] As used herein, the term "ship" can refer to any type of vessel equipped with liquefied gas storage tanks. For example, a vessel can include self-propelled vessels (e.g., LNG carriers, liquid hydrogen carriers, and LNG regasification vessels, RVs) as well as non-self-propelled floating offshore structures (e.g., floating production storage offloading (FPSO) units and floating storage regasification units, FSRUs).

[0039] Furthermore, embodiments of the present invention can be applied to the reliquefaction cycle of any type of liquefied gas that can be transported in a liquid state by liquefaction at cryogenic temperatures and that may generate evaporating gas during storage. For example, such liquefied gases may include liquefied petrochemical gases, such as liquefied natural gas (LNG), liquefied ethane gas (LEG), liquefied petroleum gas (LPG), liquefied ethylene gas, and liquefied propylene gas. In the following embodiments, LNG, as a typical liquefied gas, will be used as an example to illustrate the present invention.

[0040] Figure 1This is a schematic diagram of a refrigerant circulation pressure control system for a ship's reliquefaction system according to a basic embodiment of the present invention. Figures 2 to 5 These are schematic diagrams of a refrigerant circulation pressure control system for a ship's reliquefaction system, according to the first to fourth embodiments of the present invention.

[0041] Each of the refrigerant cycle pressure control systems according to these embodiments is intended to control the load of a reliquefaction system for a ship by means of the following steps: controlling the pressure in the refrigerant cycle of the reliquefaction system by replenishing the refrigerant cycle with refrigerant from the refrigerant supply unit 300 or discharging refrigerant from the refrigerant cycle.

[0042] A reliquefaction system reliquefies evaporated gas generated from liquefied gases in a ship's storage tanks by compressing and cooling the evaporated gas, and then returns the reliquefied evaporated gas to the storage tanks. The reliquefaction system includes: a compressor for compressing the evaporated gas; a heat exchanger for cooling the compressed evaporated gas; a pressure reducer for reducing the pressure of the evaporated gas cooled by the heat exchanger; and a gas-liquid separator for separating the pressure-reduced evaporated gas into a gas phase and a liquid phase.

[0043] Reference Figures 1 to 5 The refrigerant cycle circulates the refrigerant to cool the evaporating gas in the heat exchanger 100. In the heat exchanger according to these embodiments, the refrigerant CL circulating in the refrigerant cycle and the uncompressed evaporating gas GL introduced from the storage tank to the compressor are used as cold heat sources to cool the compressed evaporating gas RL.

[0044] The refrigerant cycle includes: a refrigerant circulation line CL through which refrigerant circulates; a refrigerant compressor 210, located on the refrigerant circulation line and compressing the refrigerant discharged from the heat exchanger after cooling the evaporating gas; and an expander 200, which expands and cools the refrigerant compressed by the refrigerant compressor and cooled by the heat exchanger, and supplies the expanded and cooled refrigerant to the heat exchanger. The refrigerant compressor and expander can be connected to each other via a common shaft to form a compressor-expander that uses the energy from the expansion of the refrigerant to compress the refrigerant.

[0045] For example, nitrogen (N2) can be used as a refrigerant supplied to the heat exchanger and circulated via the refrigerant circulation line CL.

[0046] The refrigerant supply unit 300 supplies refrigerant to the refrigerant cycle and includes: a drying and filtering unit 330 that receives practical N2 from a buffer tank NT storing practical N2 to be supplied to the ship and dries the received practical N2 to lower its dew point; a booster compressor 320 that receives nitrogen from the drying and filtering unit and compresses the received nitrogen; and a storage tank 310 that receives nitrogen refrigerant from the booster compressor 320, stores the received nitrogen refrigerant in the storage tank 310, and supplies the stored nitrogen refrigerant to the refrigerant cycle.

[0047] As the amount of evaporated gas generated in the storage tank and prepared for reliquefaction changes, the required cooling and heating capacity of the reliquefaction system also changes. Here, the cooling and heating capacity in the refrigerant circulation line and the load on the reliquefaction system can be controlled by changing the mass flow rate of the refrigerant in the refrigerant circulation line using the following steps: supplying some refrigerant to the refrigerant circulation from the refrigerant supply unit or discharging some refrigerant from the refrigerant circulation line, while maintaining the ratio of compression work in the refrigerant compressor to expansion work in the expander at a constant value without adjusting the variable geometry nozzle (VGN) of the expander.

[0048] Therefore, the refrigerant circulation pressure control system according to the present invention includes: a refrigerant supply line SL connecting a storage tank 310 to the upstream side of a refrigerant compressor 210 to replenish refrigerant for refrigerant circulation; and a refrigerant discharge line DL connecting the downstream side of a refrigerant compressor to a storage tank to discharge refrigerant from the refrigerant circulation to the storage tank.

[0049] More specifically, when the cooling and heating required by the reliquefaction system increases, the mass flow rate of refrigerant in the refrigerant cycle increases by supplying some refrigerant from the storage tank via the refrigerant supply line SL to the upstream side of the refrigerant compressor, while maintaining the ratio of compression work to expansion work of the compressor expander at a fixed value without adjusting VGN.

[0050] Conversely, when the required cooling or heating capacity of the reliquefaction system decreases, the mass flow rate of refrigerant in the refrigerant cycle is reduced by discharging some of the refrigerant from the downstream side of the refrigerant compressor via the refrigerant discharge line DL to the storage tank 310. Then, when no more refrigerant can be discharged to the storage tank 310 due to a pressure reversal between the operating pressure of the storage tank and the pressure of the refrigerant discharged from the refrigerant cycle line CL via the refrigerant discharge line DL, the refrigerant cycle is depressurized as much as possible to the required level by discharging the refrigerant to the outside of the refrigerant supply unit via the vent line VL.

[0051] This allows for control of the reliquefaction system load by adjusting the mass flow rate of the refrigerant in the refrigerant cycle. However, each time capacity control is implemented, the discharge of refrigerant to the outside of the unit via the discharge line VL results in refrigerant loss, necessitating refilling of the storage tank. Replenishing the refrigerant cycle requires supplying usable N2 from the onboard buffer tank and operating related equipment (e.g., the drying and filtration unit and the booster compressor), increasing onboard nitrogen consumption and power consumption associated with equipment operation.

[0052] The first to fourth embodiments described below are designed to solve this problem by allowing the nitrogen refrigerant discharged for the purpose of regulating the heating and cooling of the refrigerant cycle to be reused to cool the evaporated gas or to be recirculated for other shipboard uses, thereby reducing shipboard nitrogen consumption and power consumption.

[0053] Therefore, pressure regulating lines PLA, PLb, PLC, or PLd branch off from the refrigerant discharge line DL. First valves V1a, V1b, V1c, or V1d are installed upstream of the point where the pressure regulating lines branch off from the refrigerant discharge line DL on the refrigerant discharge line DL. Second valves V2a, V2b, V2c, or V2d are installed on pressure regulating lines PLA, PLb, PLC, or PLd. Third valves V3a, V3b, V3c, or V3d are installed downstream of the point where the pressure regulating lines branch off from the refrigerant discharge line DL on the refrigerant discharge line DL. Figures 2 to 5 As shown in the image.

[0054] In these embodiments, when the refrigerant cycle is depressurized to reduce the mass flow rate of refrigerant in the refrigerant cycle, first valves V1a, V1b, V1c, or V1d and third valves V3a, V3b, V3c, or V3d are opened to first discharge refrigerant to storage tank 310 via refrigerant discharge line DL. Then, when a pressure reversal occurs between the storage tank and the refrigerant cycle, third valves V3a, V3b, V3c, or V3d are closed and second valves V2a, V2b, V2c, or V2d are opened to discharge refrigerant via pressure regulating lines PLA, PLb, PLc, or PLd. Each embodiment uses refrigerant discharged via pressure regulating lines in a different manner.

[0055] In the refrigerant circulation pressure control system according to the first embodiment (see...) Figure 2In this configuration, the pressure regulating line PLA is connected to the buffer tank NT. The buffer tank operates at a lower pressure than the storage tank. Therefore, when a pressure reversal occurs between the storage tank and the refrigerant cycle during the discharge of refrigerant to the storage tank 310 via the refrigerant discharge line DL, the second valve V2a is opened to discharge refrigerant from the refrigerant cycle line to the buffer tank NT, thereby controlling the pressure in the refrigerant cycle.

[0056] This reduces power consumption by delivering nitrogen refrigerant to a buffer tank for reuse instead of venting it outside the ship.

[0057] In the refrigerant circulation pressure control system according to the second embodiment (see...) Figure 3 In this configuration, the pressure regulating line PLb is connected upstream of the booster compressor 320 of the refrigerant supply unit. In this way, when no more refrigerant can be delivered to the storage tank due to pressure reversal, refrigerant is supplied upstream of the booster compressor 320 instead of being discharged overboard, for compression, storage, and reuse, thereby reducing onboard nitrogen consumption and the power consumption associated with the use of equipment for nitrogen supply.

[0058] In the refrigerant circulation pressure control system according to the third embodiment (see...) Figure 4 In this embodiment, the pressure regulating line PLC is connected to the top of the gas-liquid separator 400 of the reliquefaction system. An inert gas is supplied to the gas-liquid separator to prevent a vacuum from forming within it and to maintain the internal pressure of the separator as the separated liquefied gas is discharged. In this embodiment, nitrogen refrigerant discharged from the refrigerant cycle is supplied to the gas-liquid separator by connecting the pressure control line to the top of the separator. In this way, nitrogen discharged from the refrigerant cycle can be used as a covering N2, thereby reducing onboard nitrogen consumption and the power consumption associated with the use of the equipment for nitrogen supply.

[0059] In the refrigerant circulation pressure control system according to the fourth embodiment (see...) Figure 5 In the storage tank, there are two tanks with different operating pressures, namely high pressure tank 310a and low pressure tank 310b. High pressure tank 310a receives compressed nitrogen refrigerant from the booster compressor and supplies the received nitrogen refrigerant to the upstream side of the refrigerant compressor. Low pressure tank 310b operates at a lower pressure than high pressure tank, and pressure regulating line PLd is connected to low pressure tank 310b.

[0060] The low-pressure tank is operated using the suction pressure of the booster compressor.

[0061] When the refrigerant cycle is depressurized to reduce the mass flow rate of the refrigerant in the refrigerant cycle, the first valve V1d and the third valve V3d are opened to first discharge the refrigerant to the high-pressure tank 310a of the storage tank via the refrigerant discharge line DL, and then when a pressure reversal occurs between the high-pressure tank and the refrigerant cycle, the third valve V3d is closed and the second valve V2d is opened to discharge the refrigerant to the low-pressure tank 310b via the pressure regulating line PLd.

[0062] When the refrigerant discharged into the pressure regulating line pressurizes the low-pressure tank, the refrigerant is discharged from the low-pressure tank 310b to the upstream side of the booster compressor 320 to maintain a constant pressure in the low-pressure tank. The refrigerant discharged from the low-pressure tank can be supplied to the high-pressure tank via the booster compressor to replenish the refrigerant cycle.

[0063] In this way, by delivering refrigerant to a low-pressure tank instead of venting it overboard, and then supplying it upstream of the booster compressor for reuse, nitrogen consumption on board can be reduced, the pressure in the high-pressure tank supplying refrigerant to replenish the refrigerant cycle can be kept constant for stable operation, and the power consumption associated with using the device for nitrogen supply can be reduced.

[0064] Although some embodiments have been described herein, the invention is not limited to the embodiments described above, and as will be apparent to those skilled in the art, the invention can be practiced with various modifications or variations without departing from the spirit of the invention.

Claims

1. A refrigerant circulation pressure control system for a ship's reliquefaction system, comprising: A heat exchanger is provided to the reliquefaction system and cools the evaporated gas received from the storage tank; A refrigerant cycle includes a refrigerant circulation line, a refrigerant compressor, and an expander. The refrigerant circulates through the refrigerant circulation line to cool the evaporating gas in the heat exchanger. The refrigerant compressor is located on the refrigerant circulation line and compresses the refrigerant discharged from the heat exchanger after cooling the evaporating gas in the heat exchanger. The expander expands and cools the refrigerant compressed by the refrigerant compressor and cooled by the heat exchanger, and supplies the expanded and cooled refrigerant to the heat exchanger. Storage tank for storing the refrigerant to be added to the refrigerant cycle; A refrigerant supply line connects the storage tank to the upstream side of the refrigerant compressor to circulate and replenish the refrigerant; A refrigerant discharge line connects the downstream side of the refrigerant compressor to the storage tank to circulate the refrigerant from the refrigerant compressor to the storage tank; as well as The pressure regulating line branches off from the refrigerant discharge line. The refrigerant cycle is depressurized by discharging the refrigerant from the refrigerant cycle via the refrigerant discharge line or the pressure regulating line, and Specifically, during the depressurization of the refrigerant cycle, the refrigerant is first discharged to the storage tank via the refrigerant discharge line, and then discharged to the pressure regulating line when a pressure reversal occurs between the storage tank and the refrigerant cycle. The refrigerant discharged via the pressure regulating line is reused in the refrigerant cycle, for internal ship use, or for covering purposes, rather than being discharged outside the ship.

2. The refrigerant circulation pressure control system according to claim 1 further includes: The first valve is located upstream of the point where the pressure regulating line branches off from the refrigerant discharge line on the refrigerant discharge line; A second valve is installed on the pressure regulating pipeline; as well as The third valve is located downstream of the point where the pressure regulating line branches off from the refrigerant discharge line on the refrigerant discharge line.

3. The refrigerant circulation pressure control system according to claim 2 further includes: A buffer tank, provided in the vessel and storing the usable N2 to be supplied to the vessel; A drying and filtering unit dries and filters the practical N2 received from the buffer tank to reduce the dew point of the practical N2. as well as A booster compressor compresses the nitrogen refrigerant that has passed through the drying and filtering unit and supplies the compressed nitrogen refrigerant to the storage tank.

4. The refrigerant circulation pressure control system according to claim 3, wherein the pressure regulating line is connected to the buffer tank, and the buffer tank operates at a lower pressure than the storage tank.

5. The refrigerant circulation pressure control system according to claim 3, wherein the pressure regulating line is connected to the upstream side of the booster compressor.

6. The refrigerant circulation pressure control system according to claim 3, wherein: The reliquefaction system includes a gas-liquid separator that receives cooled evaporated gas from the heat exchanger and separates the received evaporated gas into a gas phase and a liquid phase; and The pressure regulating line is connected to the gas-liquid separator, such that the refrigerant discharged from the refrigerant cycle is supplied to the top of the gas-liquid separator to serve as an inert gas for maintaining pressure in the gas-liquid separator.

7. The refrigerant circulation pressure control system according to claim 3, wherein: The storage tank includes a high-pressure tank and a low-pressure tank. The high-pressure tank receives compressed nitrogen refrigerant from the booster compressor and supplies the received nitrogen refrigerant to the upstream side of the refrigerant compressor. The low-pressure tank operates at a lower pressure than the high-pressure tank. The pressure regulating line is connected to the low-pressure tank.

8. The refrigerant circulation pressure control system according to claim 7, wherein: The low-pressure tank is operated using the suction pressure of the booster compressor; and, When the low-pressure tank is pressurized by the refrigerant discharged into the pressure regulating line, the refrigerant is discharged from the low-pressure tank to the upstream side of the booster compressor to maintain a constant pressure in the low-pressure tank.

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