Method for operating a reliquefaction system

Abstract

A method for improving reliability and availability of a cryogenic fluid reliquefaction system is provided. The cryogenic fluid reliquefaction system can include at least N subcoolers, the N subcoolers including a motor and compressor and at least one variable speed. The cryogenic fluid reliquefaction system can include N-1 variable speed systems shared between the motor and compressor in the case where N equals 2, or N-2 variable speed systems shared between the motor and compressor in the case where N is greater than 2. The cryogenic fluid reliquefaction system can include providing liquid cryogenic fluid to two different liquid cryogenic fluid users using two different primary cryogenic tanks with a common subcooler and recirculation loop, where pressures in the two different primary cryogenic tanks are controlled using pressure controllers acting on two different subcooled liquid cryogenic fluid valves. And / or, the cryogenic fluid reliquefaction system can include providing refrigeration to at least one liquid cryogenic fluid user from two or more subcooling systems in a lead-lag arrangement.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit of priority to U.S. Provisional Patent Applications Nos. 63 / 021,868, 63 / 021,880 and 63 / 021,889, filed May 8, 2020, pursuant to 35 U.S. SC §119(a) and (b), the entire contents of which are incorporated herein by reference. Background Technology

[0003] In certain applications, cryogenic liquid streams (such as liquid nitrogen) can be used for cooling purposes. In this case, the liquid nitrogen is typically at least partially vaporized, and this nitrogen vapor needs to be recondensed to avoid the loss of nitrogen products and cooling energy (refrigeration). A typical method for recondensing such a stream is to cool the gas and extract a certain amount of enthalpy until liquefaction is complete. Enthalpy extraction is usually achieved through indirect heat exchange with another fluid, which typically undergoes various steps of compression, cooling, and pressure reduction in valves and / or turbines.

[0004] A typical alternative solution is to mix the gaseous stream with a subcooled liquid, allowing direct heat exchange between the gas and the subcooled liquid to condense the gaseous stream. This mixing can usually be achieved in the vapor phase of a tank. Summary of the Invention

[0005] A method is provided for improving the reliability and availability of a cryogenic fluid reliquefaction system. The reliquefaction system may include: at least N subcoolers, each subcooler comprising a motor and a compressor with design capacity; and at least one variable speed system for controlling the speed of at least one motor. The reliquefaction system may include: N-1 variable speed systems shared between the motor and compressor when N equals 2, or N-2 variable speed systems shared between the motor and compressor when N is greater than 2. The method includes: connecting the reliquefaction system to a cryogenic fluid user, the cryogenic fluid user being subsequently supplied with cryogenic fluid; vaporizing the cryogenic fluid within the cryogenic fluid user; and returning the vaporized cryogenic fluid to a main cryogenic tank. Specifically, when a first motor and a first compressor with variable speed systems are at or near design capacity, the first motor disengages from the variable speed system and connects to the existing power grid, thereby releasing the variable speed system, which then engages with a second motor and a second compressor, and the second motor and second compressor are then started.

[0006] The reliquefaction system may include: using a common subcooler and recirculation loop to supply cryogenic fluid to two different cryogenic fluid users from two different main cryogenic tanks, wherein the pressure in the two different main cryogenic tanks is controlled using pressure controllers acting on two different subcooled cryogenic fluid valves. Alternatively, the reliquefaction system may include: supplying cooling to at least one cryogenic fluid user from two or more subcooling systems arranged in a lead-lag configuration, wherein the pressure in the main cryogenic tanks is controlled using pressure controllers acting on the outlet valves of each subcooler outlet valve. Attached Figure Description

[0007] To further understand the nature and purpose of the invention, reference should be made to the following detailed description in conjunction with the accompanying drawings, in which similar elements are given the same or similar reference numerals, and in the drawings:

[0008] Figure 1 This is a schematic diagram of a basic overall system according to an embodiment of the present invention.

[0009] Figure 2 This is a schematic diagram showing details of the cryogenic fluid user and the main cryogenic tank of a dual-unit system according to an embodiment of the present invention.

[0010] Figure 3 This is a schematic diagram illustrating details of the subcooling system of a dual-unit system according to an embodiment of the present invention.

[0011] Figure 4 This is a schematic diagram of a turbo Brayton system according to an embodiment of the present invention.

[0012] Figure Labels

[0013] 102 = Main Cryogenic Tank

[0014] 103 = Liquid cryogenic fluid flow

[0015] 104 = Cryogenic fluid flow undergoing vaporization

[0016] 105 = Exhaust valve

[0017] 106 = Supercooler

[0018] 107 = Warm Recirculating Flow

[0019] 108 = Supercooled recirculation flow

[0020] 109 = Recirculation control valve

[0021] 110 = Recirculation pump

[0022] 111 = Liquid Buffer Tank

[0023] 112 = Buffer tank transport flow

[0024] 113 = Buffer tank transfer control valve

[0025] 114 = Liquid cryogenic fluid (in the main cryogenic tank)

[0026] 115 = Cryogenic fluid vapor (in the main cryogenic tank)

[0027] 116 = Cryogenic Liquid Fluid User

[0028] 117 = External liquid cryogenic fluid source

[0029] 118 = Supercooler bypass line

[0030] 119 = First pressure transmitter (in the main cryogenic tank)

[0031] 120 = First Peripheral Interface Controller

[0032] 122 = Second pressure transmitter (in the supercooler bypass line)

[0033] 123 = Second Peripheral Interface Controller

[0034] 124 = Third Peripheral Interface Controller

[0035] 125 = Bypass control valve

[0036] 102A = First Cryogenic Tank

[0037] 102B = Second Cryogenic Tank

[0038] 103A = First-state cryogenic fluid flow

[0039] 103B = Second liquid cryogenic fluid flow

[0040] 104A = Cryogenic fluid flow of the first vaporization

[0041] 104B = Cryogenic fluid flow of the second vaporization

[0042] 105A = First exhaust valve

[0043] 105B = Second exhaust valve

[0044] 106A = First subcooler

[0045] 106B = Second subcooler

[0046] 107 = Warm Recirculating Flow

[0047] 107A = First warm recirculation flow section

[0048] 107B = Second warm recirculation flow section

[0049] 108 = Supercooled recirculation flow

[0050] 108A = First subcooled recirculation section

[0051] 108B = Second subcooled recirculation section

[0052] 109A = First recirculation control valve

[0053] 109B = Second recirculation control valve

[0054] 110A = First recirculation pump

[0055] 110B = Second recirculation pump

[0056] 110C = Transfer Pump

[0057] 114A = Liquid cryogenic fluid (in the first cryogenic tank)

[0058] 114B = Liquid cryogenic fluid (in the second cryogenic tank)

[0059] 115A = Cryogenic fluid vapor (in the second cryogenic tank)

[0060] 115B = Cryogenic fluid vapor (in the second cryogenic tank)

[0061] 116A = First cryogenic liquid fluid user

[0062] 116B = Second cryogenic liquid fluid user

[0063] 119A = First pressure transmitter (inside the main cryogenic tank)

[0064] 119B = Second pressure transmitter (inside the main cryogenic tank)

[0065] 120 = First Peripheral Interface Controller

[0066] 122 = Second pressure transmitter (in the supercooler bypass line)

[0067] 123 = Second Peripheral Interface Controller

[0068] 124 = Third Peripheral Interface Controller

[0069] 125 = Bypass control valve

[0070] 126A = First booster coil

[0071] 126B = Second booster coil

[0072] 127A = First Pressure Controller

[0073] 127B = Second Pressure Controller

[0074] 128 = Reflux conduit

[0075] 129 = Warm cryogenic liquid return flow

[0076] 130 = Warm recirculated feed stream

[0077] 130A = First part of warm recirculated feed stream

[0078] 130B = Second part of warm recirculated feed stream

[0079] 131 = Warm cryogenic liquid supply line

[0080] 132 = Cooling water supply pipeline

[0081] 133 = Cooling water return pipeline Detailed Implementation

[0082] The following describes illustrative embodiments of the invention. While the invention is susceptible to various modifications and alternatives, specific embodiments thereof have been shown by way of example in the accompanying drawings and described in detail herein. However, it should be understood that the description of specific embodiments herein is not intended to limit the invention to the specific forms disclosed, but rather is intended to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

[0083] Of course, it will be understood that in the development of any such practical embodiment, many implementation-specific decisions must be made to achieve the developer’s specific goals (e.g., compliance with system-related and business-related constraints), which will vary depending on the implementation. Furthermore, it will be understood that such development work can be complex and time-consuming, but remains a common assurance to those skilled in the art who will obtain the benefits of this disclosure.

[0084] For simplicity, the following is a simplified description of the basic operation of the system, which has a cryogenic tank and a subcooler, such as... Figure 1 The component numbers shown are generic, but those skilled in the art will recognize that the description applies equally to either Unit 1 (A) or Unit 2 (B). Operational details involving two cryogenic tanks and / or two subcoolers are given below.

[0085] The following system describes the use of liquid nitrogen, but those skilled in the art will recognize that any suitable cryogenic fluid (oxygen, methane, etc.) can be used with the same concept, depending on the temperature level required to cool the target system.

[0086] Liquid nitrogen 114 is stored in the main cryogenic tank 102 under saturated conditions (pressure P1). Nitrogen vapor 115 occupies the headspace of the main cryogenic tank 102. During normal operation, a portion of the liquid nitrogen 114 is extracted from the main cryogenic tank 102 and sent to the liquid nitrogen user 116. The liquid nitrogen user 116 will use the liquid nitrogen stream 103 for internal cooling purposes. Therefore, the liquid nitrogen stream 103 will be vaporized, and the vaporized nitrogen stream 104 will be recycled back to the main cryogenic tank 102.

[0087] Simultaneously, a portion of liquid nitrogen 114 is extracted from the main cryogenic tank 102 as a warm recirculation stream 107 and sent to the recirculation pump 110. The pressurized liquid nitrogen then enters the supercooler 106. The supercooler 106 cools the liquid nitrogen by at least a few degrees Celsius. The subcooled recirculation stream 108 is then sent back to the main cryogenic tank 102, where it is introduced as a spray into the vapor phase 115. Upon contact with the subcooled liquid, the vaporized nitrogen stream 104 returned from the liquid nitrogen user 116 is cooled and condensed back into saturated liquid 114.

[0088] The main cryogenic tank 102 may include a first pressure transmitter 119. The first pressure transmitter 119 may interface with one or more peripheral interface controllers (PICs). A first PIC 120 is functionally connected to the first pressure transmitter 119 and the recirculation control valve 109. A subcooler bypass line 118 is fluidly connected to the warm recirculation flow 107 and the subcooled recirculation flow 108, thereby allowing at least a portion of the pressurized recirculation flow exiting the recirculation pump 110 to bypass the subcooler 106. The subcooler bypass line 118 may include a second pressure transmitter 122. The second pressure transmitter 122 may interface with one or more PICs. A second PIC 123 is functionally connected to both the second pressure transmitter 122 and the bypass control valve 125. A third PIC 124 is functionally connected to both the second pressure transmitter 122 and the recirculation pump 110.

[0089] The pressure inside the main cryogenic tank 102 is primarily controlled by the recirculation control valve 109 on the subcooled recirculation flow 108 that exits the supercooler 106.

[0090] The reliquefaction system also includes a liquid buffer tank 111, a buffer tank transfer flow 112, and a buffer tank transfer control valve 113. The liquid buffer tank 111 can be refilled as needed from an external liquid nitrogen source 117 (such as a liquid nitrogen truck trailer (not shown)).

[0091] In the following embodiments, for ease of explanation and to avoid unnecessary confusion, a system with two units (Unit A and Unit B) is described. Those skilled in the art will recognize that the same method can be readily applied to three or more units if such design considerations are required.

[0092] Figure 2 and Figure 3 The diagram schematically illustrates one embodiment of the invention. The reliquefaction system includes a first cryogenic tank 102A, a second cryogenic tank 102B, a first liquid nitrogen stream 103A, a second liquid nitrogen stream 103B, a first vaporized nitrogen stream 104A, a second vaporized nitrogen stream 104B, a first exhaust valve 105A with fluid attached to the first vaporized nitrogen stream 104A, and a second exhaust valve 105B with fluid attached to the second vaporized nitrogen stream 104B.

[0093] The reliquefaction system also includes a first subcooler 106A, a second subcooler 106B, a warm recirculation flow 107, a first warm recirculation flow section 107A, a second warm recirculation flow section 107B, a subcooled recirculation flow 108, a first subcooled recirculation flow section 108A, a second subcooled recirculation flow section 108B, a first recirculation control valve 109A, a second recirculation control valve 109B, a first recirculation pump 110A, a second recirculation pump 110B, and a third recirculation pump 110C.

[0094] The first subcooler 106A and the second subcooler 106B, as well as any potential additional subcoolers, can be cooled by the cooling water supply line 132 and the cooling water return line 133.

[0095] In one embodiment of the invention, the cryogenic liquid is subcooled using two or more subcooling systems connected in parallel. These two or more subcooling systems may have similar or different cooling capacities. The purpose of using parallel subcooling systems is to improve the overall availability of the equipment and to increase the cooling capacity of the reliquefaction equipment.

[0096] The pressure in the first cryogenic tank 102B is maintained between two ideal maximum values ​​by the first booster coil 126A (when the pressure reaches a predetermined minimum threshold) and the first vent valve 105A (when the pressure reaches a predetermined maximum threshold).

[0097] The first and second booster coils 126A / B are well known in the art. The first and second booster coils are typically ambient temperature vaporizers that use heat from the environment to vaporize a small amount of cryogenic liquid 114A in the tank. This small amount of vaporized liquid is then returned to the tank to maintain or increase the internal pressure as needed.

[0098] The pressure in the first cryogenic tank 102B is controlled to a constant value, which is set between a predetermined minimum pressure value and a predetermined maximum pressure value as defined above. This pressure control is ensured by pressure controllers 127A / B acting on the subcooler outlet valves 109A / B. A lead-lag control scheme is implemented so that the cooling capacity of the next subcooler only increases when the outlet valve of the preceding subcooler is fully open or nearly fully open.

[0099] As used herein, the term “at or near design capability” is defined as being within 80% of the design capability, preferably within 90% of the design capability, and more preferably within 95% of the design capability.

[0100] As used herein, the term "lead-lag system" is defined as a system in which a "lag" device is activated and used to meet the system demand when the system demand exceeds the design capacity of a single unit, and when the "lead" device is at or near its design capacity. Such "lead-lag" systems are well known in the art.

[0101] As used herein, the term “fully open or nearly fully open” is defined for the valve as being within 80% of the fully open position, preferably within 90% of the fully open position, and more preferably within 95% of the fully open position.

[0102] Each subcooler controls the temperature of the subcooled cryogenic liquid at its respective outlet. The temperature setpoint can be the same for each subcooler and should be a few degrees Celsius (typically 10 degrees Celsius lower) below the saturation temperature of the associated cryogenic tank 102A / B.

[0103] To properly balance the flow rate through each subcooler, the following factors must be considered:

[0104] - The cooling task of this subcooler (typically related to the speed of the turbine machine),

[0105] - The difference between the temperature downstream of the supercooler and the temperature setpoint.

[0106] A subcooler used at full or near full load (with an outlet temperature above the setpoint and other subcoolers still under additional load) will receive excessive flow compared to other subcoolers. A specific controller acting on the valve downstream of this subcooler at its maximum or near maximum opening will allow for a reduction in flow to that subcooler under these specific conditions.

[0107] In another embodiment of the invention, during rated operating conditions, liquid nitrogen from the first cryogenic tank 102A is subcooled in one or more parallel subcooling units. The subcooled nitrogen is then sprayed into the first cryogenic tank 102A and mixed with superheated vapor 104A from the first liquid cryogenic fluid user 116A.

[0108] The second cryogenic tank 102B is maintained and operates at a higher pressure than the first cryogenic tank 102A. The first liquid cryogenic fluid flow 103A from the first cryogenic tank 102A is colder than the second liquid cryogenic fluid flow 103B and is pumped to the pressure of the second cryogenic tank 102B, where it mixes with superheated vapor 104B from the second liquid cryogenic fluid user 116B.

[0109] Liquid cryogenic fluid 114B from the second cryogenic tank 102B is transferred back to the first cryogenic tank 102A via the return conduit 128 to maintain the level of liquid cryogenic fluid 114A in the first cryogenic tank 102A. The system can still operate at both temperature levels if the subcoolers 106A / B are reduced or stopped. The liquid cryogenic fluid stream 103A / B, which is being vaporized by the liquid cryogenic fluid users 116A / B, is then discharged through the vent valves 105A / B.

[0110] If the inventory of liquid cryogenic fluid 114A is close to the lower limit of the first cryogenic tank 102A, then liquid cryogenic fluid 114B from the second cryogenic tank 102B can be used to supply the first cryogenic tank 102A. If the inventory of liquid cryogenic fluid 114B is close to the lower limit of the second cryogenic tank 102B, then liquid cryogenic fluid 114A from the first cryogenic tank 102A can be used by transfer pump 110C to supply the second cryogenic tank 102B through the warm cryogenic liquid supply line 131.

[0111] Additional subcoolers 106 can be added to the system described above. Each subcooler 106A / B can be configured to provide the same cooling temperature, if needed. Increasing the number of subcooling units contributes to the overall capacity and availability of the system.

[0112] In one embodiment, each subcooler 106A / B can be disconnected from the first cryogenic tank 102A and connected only to the second cryogenic tank 102B. This configuration allows for greater efficiency of the overall cooling system when one of the cryogenic tanks 102A / B is operating at a higher temperature than the other.

[0113] Go to Figure 4This demonstrates a typical turbo-Brayton cycle. A turbo-Brayton is a cryogenic refrigeration device, typically operating between -100°C and -273°C, and is therefore cryogenic. A turbo-Brayton is a closed-loop operating circuit containing a working fluid circulating at a cryogenic temperature. The cooled working fluid exchanges heat with a warm recirculated flow 107 to extract heat from the warm recirculated flow via a subcooler heat exchanger 408.

[0114] The working circuit includes the following components arranged in series: a first compression stage 403, a second compression stage 405 (preferably isentropic or substantially isentropic), an intercooler 404, an aftercooler 406, a recuperative heat exchanger 407 (preferably isobaric or substantially isobaric) for cooling the fluid, a turbine expander 409 (preferably isentropic or substantially isentropic) and recuperative heat exchanger 407 for expanding the fluid, and a subcooler heat exchanger 408 (preferably isobaric or substantially isobaric) for heating the fluid.

[0115] A typical turbo Brayton system includes a first motor 401 and a second motor 402, which are preferably electric, for driving a first compression stage 403 and a second compression stage 405, respectively. A turboexpander 409 typically includes a radial turbine that drives the first motor 401. More precisely, the turboexpander 409 assists the first motor 401 in driving the first compression stage.

[0116] Therefore, the device uses two motors 401 / 402, and the second motor 402 drives the second compression stage 405 only at one of its ends. The second compression stage 405 is located downstream of the first compression stage 403 (downstream refers to the circulation direction of the working fluid in the loop 10).

[0117] This novel structure allows for the distribution of the total enthalpy increment Δhs across the two compression stages, and thus enables a reduction in the enthalpy increment Δhs of one stage and an increase in the specific speed of the compression stage to approach the optimal specific speed of each compressor.

[0118] The total enthalpy increment Δhs is distributed between the two compression stages 403 / 405, thereby enabling the specific speed of the compression stages to increase and approach or reach the optimal specific speed. Due to this novel structure, the two compression stages 403 / 405 can operate at or near the optimal specific speed (instead of only the first stage as in the prior art).

[0119] In one operating mode, the two motors 401 / 402 are identical, their speeds are the same, and the specific speeds of the two compressors 403 / 405 are the same and optimal.

[0120] In another operating mode, the two compression stages 403 / 405 can be controlled by variable speed motors and operate at different speeds to operate close to or at the optimal specific speed even when the mechanical power and / or rotational speed of the two motors 401 / 402 differ. The compression ratio of the two compression stages 403 / 405 can be selected to make the specific speed of the two compression stages as close as possible to the optimal value.

[0121] Back Figure 2 and Figure 3 In another embodiment of the invention, two or more subcooling systems connected in parallel can be used to subcool the cryogenic liquid. These two or more subcooling systems may have similar or different cooling capacities. The purpose of using parallel subcooling systems is to improve the overall availability of the equipment and increase its cooling capacity.

[0122] As discussed above regarding the Brayton turbine system, some subcooler systems require a variable speed system during startup and / or to effectively control the operation of their turbine machinery between reduced and high loads. In systems with multiple subcoolers of the same size, it is conceivable that a variable speed system be shared among these different subcoolers. When one subcooler starts up, the variable speed system is used to control its speed increase. When the load reaches near the subcooler's maximum cooling capacity and requires additional cooling, the variable speed system associated with that subcooler is switched to another subcooler to be started, and the subcooler nearing maximum cooling capacity is connected to the power supply network before the variable speed system switchover. During this transition, the subcooler system will not be able to adapt to the load change; therefore, some liquid nitrogen from the first cryogenic tank 102B will be used to compensate for this insufficient cooling supply from the subcooler.

[0123] With two subcoolers installed, a single transmission system can be shared between each turbine in each subcooler. When N equals 2, only one transmission system exists (i.e., N-1). When N is greater than 2, one or more transmission systems exist (i.e., N-2). These transmission systems can be shared between the turbine expanders of the N subcoolers.

[0124] As a non-limiting example, when N=3, there will be one variable-speed system and two constant-speed systems. In this case, when turbine expander 1 (with the variable-speed system) is at or near its design capacity, turbine expander 2 (the next in the series) will be started and increase in speed. Once the speed is reached, the variable-speed system will switch from turbine expander 1 to turbine expander 2, and turbine expander 1 will now operate at a constant speed. As used herein, the term "at or near design capacity" is defined as meaning within 80% of the design capacity, preferably within 90% of the design capacity, and more preferably within 95% of the design capacity.

[0125] It should be understood that many additional changes in details, materials, steps, and arrangements of parts that have been described herein to explain the essence of the invention can be made by those skilled in the art within the principles and scope of the invention as set forth in the appended claims. Therefore, the invention is not intended to be limited to the specific embodiments given in the examples above.

Claims

1. A method for improving the reliability and availability of a cryogenic fluid reliquefaction system, the cryogenic fluid reliquefaction system comprising: At least N subcoolers, which include motors and compressors with design capabilities, where N is greater than or equal to 2; and at least one speed control system for controlling the speed of at least one motor, and When N equals 2, there are N-1 speed-changing systems shared between these motors and these compressors, or When N is greater than 2, N-2 speed transmission systems are shared between these motors and these compressors. The method includes: The reliquefaction system is connected to a cryogenic fluid user, who is then supplied with cryogenic fluid. The cryogenic liquid is vaporized within the user's liquid cryogenic fluid. The vaporized cryogenic liquid is returned to the main cryogenic tank. in, When the first motor and the first compressor with the variable speed system are within 80% of their design capacity, ○ The first motor is disengaged from the transmission system and connected to the existing power grid, thereby releasing the transmission system. ○ This transmission system is engaged with the second motor and the second compressor. Then start the second motor and the second compressor.

2. The method as described in claim 1, wherein, The reliquefaction system includes at least one of a main cryogenic tank, a subcooler, and a recirculation pump.

3. The method as described in claim 1, wherein, The cryogenic liquid is selected from the group consisting of nitrogen, helium, argon, oxygen, krypton, xenon, carbon dioxide, methane, ethane, propane, hydrogen, and combinations thereof.

4. A method for improving the reliability and availability of a cryogenic fluid reliquefaction system, comprising: The reliquefaction system is connected to at least one cryogenic fluid user, who is then supplied with cryogenic fluid. The cryogenic liquid is vaporized within the user's liquid cryogenic fluid. The vaporized cryogenic liquid is returned to at least one main cryogenic tank. in, A common subcooler and recirculation loop are used to supply cryogenic fluid to two different cryogenic fluid users from two different main cryogenic tanks, wherein the pressure in the two different main cryogenic tanks is controlled by pressure controllers acting on two different subcooled cryogenic fluid valves, and / or Cooling is supplied to at least one user of liquid cryogenic fluid from two or more subcooling systems arranged in a lead-lag configuration, wherein the pressure in the main cryogenic tank is controlled using a pressure controller acting on the outlet valve of each subcooler.

5. The method of claim 4, wherein, The reliquefaction system includes at least one main cryogenic tank, at least one subcooler, and at least one recirculation pump.

6. The method of claim 4, wherein, The cryogenic liquid is selected from the group consisting of nitrogen, helium, argon, oxygen, krypton, xenon, carbon dioxide, methane, ethane, propane, hydrogen, and combinations thereof.

7. The method of claim 4, wherein, If two or more reliquefaction systems are used, they operate at the same temperature level.

8. The method of claim 4, wherein, If two or more reliquefaction systems are used, these two or more reliquefaction systems operate at different temperature levels.

9. The method of claim 4, wherein, These subcooling systems include a downstream fluid temperature and a fluid temperature setpoint, and wherein the flow rate is balanced among these subcoolers by using the cooling task of these subcoolers and the difference between the downstream fluid temperature of the subcooler and the fluid temperature setpoint.

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