US20160265824A1

US20160265824A1 - Fluid collection assembly

Info

Publication number: US20160265824A1
Application number: US15/034,636
Authority: US
Inventors: Anthony S. Molavi; Michel Grabon; Charbel Rahhal
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2013-11-08
Filing date: 2014-09-05
Publication date: 2016-09-15
Also published as: EP3066401B1; WO2015069373A1; EP3066401A1; CN113294946A; CN105705885A; ES2973831T3

Abstract

A fluid collection assembly includes a vaporizer configured to vaporize a refrigerant to separate the refrigerant from a liquid and a sump configured to collect the liquid in a trough formed by a junction of two diagonal sides of the sump. The sump includes a heating element in the trough.

Description

BACKGROUND OF THE INVENTION

Embodiments of the invention relate to refrigeration, and more particularly to oil reclamation vaporizers for chiller systems.
In refrigeration systems such as chillers, vaporizers are used to separate refrigerant from a refrigerant/lubricant mixture, such as a refrigerant/oil mixture. A vaporizer receives a refrigerant/oil mixture drained from an evaporator, and it is desired to remove the refrigerant from the mixture prior to returning the oil to a compressor to lubricate the compressor. The mixture is run through a vaporizer, where it is exposed to heat to vaporize the refrigerant, separating the refrigerant from the oil, which remains in a liquid state. The oil is drained to an oil sump where further separation of refrigerant from the oil occurs by another heating element, and the vaporized refrigerant is passed to the compressor via a suction line.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the invention include a fluid collection assembly that includes a vaporizer configured to vaporize a refrigerant to separate the refrigerant from another liquid and a sump configured to collect the liquid in a trough formed by a junction of two diagonal sides of the sump. The sump includes a heating element in the trough.
Additional embodiments include a chiller system. The chiller system includes a storage container configured to store a mixture of refrigerant and a liquid and a vaporizer connected to the storage container to receive the mixture of refrigerant and liquid from the first storage container. The vaporizer is configured to vaporize the refrigerant to separate the refrigerant from the liquid. The system further includes a sump configured to collect the liquid in a trough formed by a junction of two diagonal sides of the sump. The sump includes a heating element in the trough to heat the liquid. The system includes a compressor connected to the vaporizer and the sump to receive the refrigerant from the vaporizer and the oil from the sump.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 illustrates a chiller system according to an embodiment of the invention;

FIG. 2A illustrates a perspective view of a vaporizer and sump according to an embodiment of the invention;

FIG. 2B illustrates a cross-section view of the vaporizer and sump according to an embodiment of the invention;

FIG. 2C illustrates a cross-section view of the vaporizer and sump according to an embodiment of the invention;

FIG. 2D illustrates a cross-section view of the vaporizer, sump and reservoir according to an embodiment of the invention;

FIG. 3 illustrates a cross-section view of a vaporizer and sump according to another embodiment of the invention;

FIG. 4 illustrates a cross-section view of a vaporizer and sump according to another embodiment of the invention;

FIG. 5 illustrates a cross-section of view of a vaporizer according to another embodiment of the invention; and

FIG. 6 illustrates an oil rectifier system according to an embodiment of the invention.

The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Vaporizers receive a mixture of liquid refrigerant and oil and separate the refrigerant from the oil by way of a vaporization process. In conventional systems, oil from the vaporizer is transmitted to a sump and to one or more other components for re-use or storage. In a conventional system, an oil separator is usually used for oil separation. However, if the evaporation of refrigerant from the oil is inefficient, the resulting oil has a lower viscosity which makes transmission of the oil difficult, since a certain thickness of the oil is necessary for optimal transmission. Embodiments of the invention include a sump having an angled base and a heater to increase the efficiency of the sump by improving the evaporation of refrigerant in a refrigerant/oil mixture to increase the viscosity of the oil in the sump.
FIG. 1 illustrates a chiller system 100 according to an embodiment of the invention. The chiller system 100 includes a compressor 110, a cooler 120, also referred to as an evaporator 120, a condenser 130, a vaporizer 140 and a sump 150. In operation, the cooler 120 provides a mixture of liquid refrigerant and another liquid to the vaporizer 140. The non-refrigerant liquid may be a lubricant to lubricate mechanical components of the compressor 110. In one embodiment, the non-refrigerant liquid is oil. Accordingly, in the present specification, the non-refrigerant liquid will be referred to as oil, but embodiments of the invention encompass any other type of non-refrigerant liquid capable of performing the required lubricating functions.
The condenser 130 provides a hot gas to the vaporizer 140 to vaporize the liquid refrigerant in the refrigerant/oil mixture. In particular, the vaporizer 140 includes tubing or piping that receives the hot gas from the condenser 130. In one embodiment, the hot gas is refrigerant, and after passing through the vaporizer 140, the now-cooled gas is output to the cooler 120 to exchange heat with the mixture of oil and liquid refrigerant in the cooler 120. The piping in the vaporizer 140 is isolated from the refrigerant/oil mixture in the vaporizer 140, such that the hot gas does not mix with the refrigerant/oil mixture.
After being separated from the oil by a vaporization process, the vapor refrigerant of the refrigerant/gas mixture is transmitted to the compressor 110 via a suction line. The oil of the refrigerant/oil mixture is collected by the sump 150 and flows to a reservoir, before being transmitted, via a pump (not shown), to the compressor 110 to lubricate mechanical components of the compressor 110. In embodiments of the invention, the sump 150 includes a heating element configured to heat the oil in the sump 150 to effectively evaporate refrigerant from the oil and to keep the oil viscous, or to maintain a rich level of viscosity. In the present specification, “rich viscosity” refers to a level of viscosity necessary in oil provided to a compressor or other parts to be lubricated that is sufficient to effectively lubricate the compressor or other parts. In other words, the oil requires a certain minimum thickness or viscosity to be an effective lubricant.
FIG. 2A illustrates a vaporizer and sump assembly 200 according to an embodiment of the invention. The assembly 200 includes a sump 210 and a vaporizer 220. The vaporizer 220 extends through the sump 210 which conserves heat generated by heating elements in one or both of the sump 210 and the vaporizer 220. The sump includes a heating element 213 including a base 213 a and extended portion 213 b. The extended portion 213 b extends along a length of the sump 210 to heat oil in the sump 210. The assembly 200 also includes a reservoir 230 located at an end of the sump 210 to store oil (particularly rich-viscosity oil) collected by the sump 210. The oil may then be selectively transmitted to other devices (such as via a filter, shut-off valve, or pressure regulating valve) or systems via an outlet 231. The sump 210 includes an opening 218 configured to transmit the oil from the sump 210 into the reservoir 230.
FIG. 2B illustrates a cross-section view of the vaporizer and sump assembly 200. The sump 210 includes a housing 211 defining a cavity 212. The sump 210 further includes two diagonal sides 216 and 217 that join at a bottom of the sump 210 to form a trough in which the oil collected by the sump 210 flows or drains. A heating element 213 is located in the trough. The heating element 213 is configured to be immersed in the oil collected by the sump 210. Referring to FIG. 2C, the sides 216 and 217 are diagonal lines with respect to a horizontal axis X. As a result, oil collected by the sump 210 collects at the junction of the sides 216 and 217, which is the low-point of the sump 210.
Referring again to FIG. 2B, since the low-point of the sump 210 is defined by two diagonal sides 216 and 217, a volume of oil required to immerse the heating element 213 is less than if a bottom side of the sump 210 was flat or horizontal. In other words, for a heating element with a height h1, a volume of a fluid having a triangular cross-section that has a height of at least h1 is less than a volume of a fluid having a rectangular cross-section having the same width as the width of the triangle. A smaller volume of oil is required to immerse the heating element 213 using a sump 210 having a triangular lower cross-section than a rectangular lower cross-section of the same height and width; accordingly, the sump 210 operates with a higher efficiency (the entire heating element 213 is immersed down to a lower liquid volume) and maintains a greater level of viscosity of the oil (or a richer viscosity) in the sump 210.
The vaporizer and sump assembly 200 includes the vaporizer 220 located within the sump 210. The vaporizer 220 includes a housing 221 defining a cavity 222. Heating piping 223, which may also be referred to as boiling piping, is located on a bottom side 225 of the vaporizer 220. In addition, a heating element 224 is located on the bottom side 225 of the vaporizer 220 adjacent to the heating piping 223. The heating piping 223 provides a flow path for a heated fluid. In embodiments of the invention, the heated, or boiled, fluid is a gas. In one embodiment, the gas is refrigerant. In one embodiment, the heating element 224 is an electric heater (single or multi-stages).
In operation, the mixture of liquid refrigerant and oil is input to the cavity 222 to flow through the vaporizer 220. In one embodiment, a sufficient volume and flow of the mixture is provided to entirely immerse the heating element 224. The heating piping 223 boils the mixture to vaporize the refrigerant, separating the refrigerant from the oil. The heating element 224 also heats the mixture. The heating element 224 is immersed in the mixture and, together with the heating piping 223, boils the mixture to vaporize the refrigerant. The vaporized refrigerant is transmitted out from the vaporizer 220 via a first flow path 226 and the oil is transmitted out from the vaporizer 220 and into the sump 210 via a second flow path 227. The flow path 226 and its drain port is oriented slightly above all tubing height to assure complete submersion of tube bundles 222 within mixture for maximum level of boiling. The flow paths 226 and 227 may include piping, for example.
The oil flows or drains from the vaporizer 220 to the sump 210 and collects in the trough at the bottom of the sump 210. The oil surrounds and immerses the heating element 213 (single or multi-stages), which heats the oil to further evaporate refrigerant and maintain a high, or rich, viscosity of the oil. Referring to FIG. 2A, the oil flows from the trough of the sump 210 into the reservoir 230 via the opening 218.
Referring to FIG. 2C, the sides 216 and 217 may form any angle θ less than one hundred eighty (180) degrees. For example, the sides 216 and 217 may form an angle θ in a range between around thirty (30) degrees and around one hundred fifty (150) degrees. In one embodiment, the sides 216 and 217 form an angle θ in a range between around forty-five (45) degrees and around one hundred thirty-five (135) degrees. In one embodiment, the sides 216 and 217 form an angle of around ninety (90) degrees.
In one embodiment of the invention, the sides 216 and 217 are substantially straight, meaning that the sides are generally straight while allowing for slight variations in shape due to manufacturing or design considerations. In some embodiments, the sides 216 and 217 may be curved. In one embodiment of the invention, the housing 211 of the sump 210 has a diamond shape, or the shape of a square rotated forty-five (45) degrees. In such an embodiment, the base of the trough is the nadir of the diamond.
Referring to FIG. 2D, an outline of the reservoir 230 is illustrated. The reservoir 230 is located at an end of the sump 210. The opening 218 in the sump 210 permits the flow of oil from the sump 210 into the reservoir 230. The opening 218 has a shape that corresponds to the shape of the sump 210. In particular, the sump 210 has a lower portion having two diagonal sides 216 and 217, and the opening 218 also includes two diagonal sides 214 and 215. In one embodiment, the two diagonal sides 214 and 215 of the opening 218 are substantially parallel to the two diagonal sides 216 and 217, respectively, of the sump 210. In one embodiment, the two diagonal sides 214 and 215 of the opening 218 are flush with inner surfaces of the sump 210, such that a oil is permitted to flow or drain freely from the trough in the sump into the reservoir 230 without traversing any ridge or barrier formed by the diagonal sides 214 and 215 of the opening 218.
The height h2 defining the distance between the opening 218 and a bottom side 232 of the reservoir 230 is designed to provide a minimum level of oil in the reservoir 230. The minimum level of oil may be a minimum amount of lubricant, such as oil, to permit the flow of oil to the compressor 110 of FIG. 1, for example.
While embodiments have been illustrated including a sump 210 having a trough defined by diagonal sides of the sump, and a vaporizer 220 having a substantially rectangular shape with a horizontal bottom, it is understood that embodiments of the invention encompass any configurations of sump and vaporizer. FIG. 3 illustrates a vaporizer and sump assembly 300 according to another embodiment of the invention in which the vaporizer 320 has diagonally-oriented sides 326 and 327 defining a trough. Heating elements 323 including heating piping, an electrical heater, or any other heating elements, are located in the base of the trough and are configured to be submerged by the mixture of refrigerant and oil supplied to the vaporizer 320. In one embodiment, the reservoir 230 is also provided with a heating element.
While embodiments of the invention have been illustrated with a diamond-shaped sump having ninety-degree angles at each corner, it is understood that embodiments of the invention encompass a sump having any shape that includes two sides forming a trough to permit the flow of oil in the trough. The shape may be a diamond, an upside-down triangle, a parachute-type shape having a rounded top and substantially-straight sides, or any other shape that forms a trough at the bottom of the sump.
In addition, referring to FIG. 4, while embodiments have been illustrated with a vaporizer positioned within a sump, embodiments of the invention also encompass a vaporizer and sump assembly 400 having a vaporizer 420 that is separate from the sump 410. As discussed in the embodiments above, the sump 410 includes a housing 411 defining a cavity 412. The sump 410 includes two diagonal sides 416 and 417 forming a trough. A heating element 413 is located in the trough to heat oil flowing through the sump 410. The vaporizer 420 includes a housing 421 defining a cavity 422, and heating piping 423 and a heating element 424 on a bottom side of the vaporizer 420. A mixture of liquid refrigerant and oil is introduced into the vaporizer 420 and the refrigerant is vaporized to separate the refrigerant from the oil. The oil is transmitted from the vaporizer 420 to the sump 410 via the flow path 427, which may be a pipe, for example.
FIG. 5 illustrates a sump 510 according to another embodiment of the invention. The sump 510 includes a housing 511 defining a cavity 512. The sump 510 includes two diagonal sides 516 and 517 forming a trough. A heating element 513 is located in the trough to heat and boil an oil/refrigerant mixture flowing through the sump 510, vaporizing the refrigerant of the oil/refrigerant mixture. In other words, in the embodiment of FIG. 5, the sump 510 acts as both a sump and a vaporizer. In the embodiment illustrated in FIG. 5, the heating element 513 includes one or more pipes 513 a, 513 b, and 513 c. The pipes 513 a, 513 b, and 513 c may be heated by a heating fluid running through the pipes 513 a, 513 b, and 513 c, such as gas from a compressor outlet, liquid from a condenser outlet, or any other heated fluid. The pipes 513 a, 513 b, and 513 c may then heat and boil the oil/refrigerant mixture within the sump 510.
Although the sump 510 of FIG. 5 is illustrated with a diamond shape, embodiments of the invention encompass a sump 510 having any shape, such as cylindrical, semi-cylindrical, triangular, or any other shape.
According to embodiments of the invention, a sump is arranged or provided with a shape to form a trough in the base of the sump to collect oil. The trough may be formed by two diagonal sides of the sump. A heating element, such as an electric heater, is formed in the trough to heat oil collected by the sump. In some embodiments, the vaporizer is located inside the sump. In some embodiments, the vaporizer includes heating piping and another heating element, such as an electric heater, to heat a mixture of refrigerant and the oil. In embodiments of the invention, the oil from the sump is provided to a reservoir, and a shape of an opening from the sump to the reservoir corresponds to a shape of the sump. For example, the opening may include two diagonal sides that are either parallel to or flush with the two diagonal sides of the sump.
Embodiments of the invention provide for an efficient vaporization process and transmittal of oil through a sump to a reservoir by including heating elements, such as electrical heaters, in one or both of a sump and a vaporizer. Positioning the heating element in the trough of the sump having the two diagonal sides that form the trough results in a more efficient heating of a potentially smaller volume of liquid. In addition, forming an opening from the sump to the reservoir in the shape of the trough of the sump results in a more efficient flow or drain of liquid from the sump to the reservoir. In addition, providing an electrical heater in the vaporizer to be immersed in a mixture of refrigerant and liquid results in an efficient heating and boiling of the mixture and an efficient vaporization process.
In some embodiments of the invention, a drain line passage from the vaporizer to the sump is equipped with a solenoid valve, needle valve, a riser drain line loop, or a drain port located above a height of the heating or boiling tubes. In an embodiment in which the vaporizer assembly includes the riser drain line loop or the drain port located above the height of the heating or boiling tubes, the majority of the tubes' surfaces are submerged in the oil/refrigerant mixture in the vaporizer.
In some embodiments of the invention, the sump 510 of FIG. 5, which may be referred to as a rectifier, is used when a quantity of oil to separate from the cooler is low. In other words, a system may include a high-pressure oil separator, and the sump 510 may act as an additional low-pressure oil separator.
FIG. 6 illustrates an evaporator assembly 600 according to an embodiment of the invention. The assembly 600 includes the evaporator 601 and an oil rectifier 602. The evaporator 601 includes an outlet 603 located on the evaporator 601 at a location where a concentration of oil in the oil/refrigerant mix 611 is expected to be high during operation of the assembly 600. For example, in the evaporator 601, the region 611 a, represented by dots, has a greater concentration of oil than the region 611 b. The location of the region 611 a may be pre-calculated or estimated based on expected or designed operating conditions of the evaporator 601, and the outlet 603 may be located on the evaporator 601 at a location corresponding to the region 611 a.
During operation, the oil/liquid mixture 611 exchanges heat with water or another coolant (not shown in FIG. 6 for purposes of clarity) via a heat exchanger, such as a closed-loop heat exchanger that does not mix the water with the oil/gas mixture 611.
The oil/liquid mixture flows out of the outlet 603, through a conduit 604, a solenoid valve 605, and a check valve 606 to the oil rectifier 602. The solenoid valve 605 is controlled, such as by a controller including a processor (not shown) to control the flow of the oil/refrigerant mix into the oil rectifier 602. The check valve 606 prevents a back-flow of fluid into the evaporator 601. The oil rectifier 602 includes a heating element 607 to heat the oil/refrigerant mix. In one embodiment, the heating element 607 is a heat exchanger that receives heated liquid or gas refrigerant from the condenser outlet or the compressor outlet via the conduit 608, subjects the oil/refrigerant mix to heat. In one embodiment, the oil rectifier 602 is located below the outlet 603 to allow gravity to drain the oil/refrigerant mix from the evaporator 601 to the oil rectifier 602.
As the oil/refrigerant mix is heated by the heating element 607, refrigerant is evaporated and returned to the evaporator 601 via the conduit 610. The oil remains in the oil rectifier 602. When a predetermined amount of oil is collected in the oil rectifier 602, the solenoid valve 605 is closed, and the oil is returned to the system, and in particular to the compressor, via the conduit 612. Accordingly, a low oil concentration may be maintained in the evaporator 601.
In one embodiment, sensors 613 are used to detect the state of the oil/refrigerant mix in the oil rectifier 602. The sensors 613 may include one or both of pressure and temperature sensors. The evaporator 601 may also include one or more temperature and pressure sensors 614. In one embodiment, a temperature inside the oil rectifier 602 is compared with a temperature in the evaporator 601. The difference between the two temperatures may then be compared to a predetermined value. The resulting difference may then be used to control whether the oil rectifier 602 is used by turning on the solenoid valve 605, or whether the oil rectifier 602 is turned off by closing the solenoid valve 605. The predetermined value may be a fixed temperature or a function of the heating source temperature (for example, the heated liquid refrigerant introduced via the conduit 608) and the evaporator saturation temperature.
By providing the oil rectifier 602 to decrease the concentration of oil in a flooded evaporator 601, heat transfer in the evaporator 601 may be maintained within predetermined thresholds, oil separation efficiency may be maintained, and oil return from the evaporator 601 to the compressor may be maintained even in low load operating conditions in which a low refrigerant flow rate exists.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

1. A fluid collection assembly, comprising:

a vaporizer configured to vaporize a liquid refrigerant in a mixture of liquid refrigerant and another liquid to separate the vaporized refrigerant from the other liquid; and

a sump configured to collect the other liquid in a trough formed by a junction of two diagonal sides of the sump, the sump including a heating element in the trough.

2. The fluid collection assembly of claim 1, wherein the two diagonal sides that form the trough are substantially straight.

3. The fluid collection assembly of claim 1, wherein the sump has a substantially diamond shape and a base of the trough is in the nadir of the diamond.

4. The fluid collection assembly of claim 1, wherein the vaporizer passes through the sump such that a housing of the vaporizer is surrounded by a housing of the sump.

5. The fluid collection assembly of claim 1, wherein the vaporizer includes a housing including a substantially horizontal bottom side, heating piping adjacent to the bottom side, and an electric heating element located adjacent to the bottom side and the heated piping.

6. The fluid collection assembly of claim 1, wherein the vaporizer includes a trough formed by a junction of two diagonal sides of the vaporizer and a vaporizer heating element in the trough.

7. The fluid collection assembly of claim 1, wherein the heating element is an electric heater extending along a length of the trough.

8. The fluid collection assembly of claim 1, further comprising:

a reservoir at an end of the sump to store the other liquid collected by the sump,

wherein an opening at the end of the sump to allow passage of the other liquid from the sump to the reservoir has a shape that corresponds to the shape of the trough of the sump.

9. The fluid collection assembly of claim 8, wherein the reservoir includes a heating element to heat the other liquid collected by the sump.

10. The fluid collection assembly of claim 1, further comprising:

an evaporator configured to receive a heated mixture of the liquid refrigerant and the other liquid, to cool the mixture, and to transmit the mixture to the vaporizer; and

a liquid rectifier configured to receive the mixture of liquid refrigerant and the other liquid from a port corresponding to a predetermined depth in the evaporator, to heat the mixture, and to return evaporated refrigerant to the evaporator.

11. The fluid collection assembly of claim 10, wherein the liquid rectifier includes an outlet port to output the other liquid separated from the vaporized refrigerant to a compressor.

12. A chiller system, comprising:

a first storage container configured to store a mixture of refrigerant and a liquid;

a vaporizer connected to the first storage container to receive the mixture of refrigerant and liquid from the first storage container, the vaporizer configured to vaporize the refrigerant to separate the refrigerant from the liquid;

a sump configured to collect the liquid in a trough formed by a junction of two diagonal sides of the sump, the sump including a heating element in the trough to heat the liquid; and

a compressor connected to the vaporizer and the sump to receive the refrigerant from the vaporizer and the oil from the sump.

13. The chiller system of claim 12, wherein the two diagonal sides that form the trough are substantially straight.

14. The chiller system of claim 12, wherein the sump has a substantially diamond shape and a base of the trough is in the nadir of the diamond.

15. The chiller system of claim 12, wherein the vaporizer passes through the sump.

16. The chiller system of claim 12, wherein the vaporizer includes a housing including a substantially horizontal bottom side, heating piping adjacent to the bottom side, and an electric heating element located adjacent to the bottom side and the heated piping.

17. The chiller system of claim 16, further comprising:

a condenser configured to generate a hot gas, the condenser connected to the heated piping to run the hot gas through the heated piping to evaporate the refrigerant.

18. The chiller system of claim 12, wherein the vaporizer includes a trough formed by a junction of two diagonal sides of the vaporizer and a vaporizer heating element in the trough.

19. The chiller system of claim 12, wherein the heating element is an electric heater extending along a length of the trough.

20. The chiller system of claim 12, further comprising:

a reservoir at an end of the sump to store the liquid collected by the sump, the reservoir connected to the compressor to supply the liquid to the compressor,

wherein an opening at the end of the sump to allow passage of the liquid from the sump to the reservoir has a shape that corresponds to the shape of the trough of the sump.

21. The chiller system of claim 12, wherein the first storage container is an evaporator configured to cool a mixture of the refrigerant and the liquid, and the chiller system further comprises:

a liquid rectifier configured to receive the mixture of refrigerant and the liquid from a port corresponding to a predetermined depth in the evaporator, to heat the mixture, and to return evaporated refrigerant to the evaporator.

22. The chiller system of claim 21, wherein the liquid rectifier includes an outlet port to output the liquid separated from the refrigerant to a compressor.

23. The chiller system of claim 21, further comprising at least one sensor to detect a first characteristic value of the mixture in the liquid rectifier, to compare the first characteristic value to a second characteristic value of the mixture in the evaporator, to compare a difference between the first and second characteristic values to a predetermined value, and to control a flow of the mixture from the evaporator to the liquid rectifier based on the comparison of the difference between the first and second characteristic values.