Refrigeration system and method The present invention relates to a refrigeration system. Such a system may be used, for example, for liquefaction of gaseous hydrocarbon fuels, such as natural gas.

Generally, gaseous hydrocarbon fuels, such as natural gas are liquefied to reduce their volume for easier transportation and storage. The liquefaction involves a refrigeration process, wherein a refrigerant fluid, typically comprising nitrogen especially for smaller liquid natural gas units and floating liquid natural gas systems, is circulated in a refrigeration cycle. A typical refrigeration cycle involves compressing the refrigerant in successive compressor stages, partially cooling the refrigerant at a relatively constant pressure and then expanding the refrigerant in one or more expander stages resulting in a temperature drop of the refrigerant. The cooled refrigerant and the gaseous fuel are passed through a cryogenic heat exchanger, wherein the refrigerant absorbs heat from the gaseous fuel resulting in liquefaction of the gaseous fuel. The refrigerant exiting the heat exchanger is passed back to the compressor stages, whereby the above cycle is repeated.

Some existing refrigeration systems are known to use

integrally geared compressor stages for successive

compression of the refrigerant, along with turboexpanders for expanding the refrigerant. However, in such systems, the axial load or thrust on the transmission gear train lead to increased vibration of the transmission gear train, loss of power and a reduction in overall efficiency of the system. Further, since the axial load or thrust increases with increase in the number of compressor stages, this leads to a limitation of the total number compressor stages, thus limiting the refrigerating capacity of the system. The objective of the present invention is to minimize load on the transmission gear train for refrigeration systems involving geared compressor stages and one or more

turboexpanders .

The above object is achieved by the system according to claim 1 and the method according to claim 9.

The underlying idea of the present invention is to compensate loads on the transmission (i.e., the gear train) of

integrally geared compressor stages by directly connecting one of the compressors to the drive and directly connecting one or more other compressor to a respective turboexpander . This has the advantage that power consumption of a compressor and the power output of the respective turboexpander

connected thereto now compensate each other, so that the transmission has to transmit only the difference. This leads to reduced power loss and greater overall efficiency of the system.

In one example embodiment, the proposed system includes a plurality of compressors in addition to said first

compressor, operable for compressing said refrigerant fluid in successive stages of compression, and a plurality of turboexpanders operable to expand portions of the refrigerant fluid downstream of said compressors, said system further characterized in that

- each of said plurality of turboexpanders is directly connected to one of the compressors of said plurality of compressors by a respective shaft, each of said plurality of turboexpanders operable to drive the respective compressor directly connected thereto by a mechanical power output resultant of said expansions, and

- said shafts are drivingly coupled by the transmission gear train. Thus any number of compressor trains can be used for increase in the refrigerating capacity of the system, without a substantial increase in load on the transmission, due to the power compensation as mentioned above. In an advantageous embodiment, the proposed system is further characterized in that one or more turboexpanders are operable such that the mechanical power output of each of said one or more turboexpanders balances power consumption by a

respective compressor directly connected thereto. This leads to a further reduction in the power transmitted by the transmission, which reduces mechanical load on the

transmission .

In a preferred embodiment, to further compensate axial load or thrust on the transmission, the proposed system is further characterized in that each compressor and the drive or turboexpander connected thereto are arranged on opposite sides of said transmission gear train.

In a preferred embodiment, the proposed system is further characterized in that one or more of said turboexpanders comprise inlet guide vanes, said inlet guide vanes being adjustable to regulate flow of the refrigerant fluid through a respective compressor directly connected thereto. This provides simplicity to the refrigeration process wherein the refrigerating capacity of the system can be regulated by adjusting the inlet guide vanes, obviating the need for cut- off valves and complicated piping.

In an exemplary embodiment, said drive comprises a gas turbine. In a further embodiment, a starter-helper motor is drivingly coupled to said transmission gear train. The starter-helper motor can be used to facilitate starting of the gas turbine drive and to further boost the rated power output of the gas turbine drive at higher ambient

temperatures prevailing at most gas liquefaction plants. The present invention is further described hereinafter with reference to illustrated embodiments shown in the

accompanying drawings, in which: FIG 1 is a block diagram of a refrigeration system involving two compressors and an expander, according to one embodiment of the present invention, and FIG 2 is a block diagram of a refrigeration system involving three compressors and two expanders, according to another embodiment of the present invention.

Embodiments of the present invention provide a refrigeration system involving integrally geared compressors with expander stages used for circulating a refrigerant fluid, wherein the system has an arrangement that results in reduced power transmission and compensation of axial load or thrusts on the gear train. Embodiments of the present invention illustrated below deal with a refrigeration system used in liquefaction of a gaseous hydrocarbon, more particularly, to liquefaction of natural gas. However, it is to be understood that the underlying inventive principle may used for any other

application .

Referring now to FIG 1 is illustrated a refrigeration system 1 for circulating a refrigerant fluid, typically comprising nitrogen, in accordance with one embodiment of the present invention. The system 1 includes a plurality of compressors, in this example, a first compressor 2a and a second

compressor 2b, and includes at least one turboexpander 4a. The first compressor 2a is directly connected to a drive 3 by means of a first shaft 5a. The second compressor 2b is directly connected to the turboexpander 4a by means of a second shaft 5b. The first shaft 5a and the second shaft 5b are drivingly coupled by a transmission gear train 6. The drive 3 provides power to the transmission gear train 6. In the illustrated embodiment, the drive 3 includes a gas turbine. Alternately, the drive 3 may include an electrical motor. The compressors 2a and 2b compress a stream 10 of the refrigerant fluid in successive stages of compression.

Downstream of the compressors 2a and 2b, a stream lOi of the refrigerant fluid is partially cooled and subsequently expanded by the turboexpander 4a, which resultantly produces a mechanical power output, which, in turn, is used to drive the second compressor 2b connected to the turboexpander 4a. The proposed arrangement has the advantage that power

consumption of the compressor 2b and the power output of the turboexpander 4a now compensate each other, so that the gear train 6 has to transmit only the difference. This leads to reduced power loss and greater overall efficiency of the system 1.

In a preferred embodiment, the turboexpander 4a is operated such that the mechanical power output of the turboexpander 4a balances the power consumption of the second compressor 2b, i.e. the power output of the turboexpander 4a and the power consumption of the second compressor 2b are substantially equal. Since the power transmitted by gear train 6 from the second shaft 5b is a difference of the power output of the turboexpander 4a and the power consumed by the second

compressor 2b, the above arrangement leads to a significant reduction in the power transmitted by the gear train 6, which reduces mechanical load on the gear train 6. Further

preferably, as shown, the first compressor 2a and the drive 3 are arranged on opposite sides of the transmission gear train 6, and the second compressor 2b and the expander 4a are arranged on opposite sides of the transmission gear train 6. The above arrangement has the advantage that the axial load or thrust on the gear train 6 by compressors 2a and 2b are respectively compensated by the drive 3 and the turboexpander 4a.

In the illustrated embodiment, a starter-helper motor 9 is drivingly coupled to the gear train 6. Advantageously, the starter-helper motor 9 can be used to facilitate starting of the gas turbine 3 and to further boost the rated power output of the gas turbine 3 at higher ambient temperatures

prevailing at most gas liquefaction plants. In an advantageous embodiment of the present invention, the turboexpander 4a includes adjustable inlet guide vanes 8c to control power output of the turboexpander 4a, and, in turn, the flow of refrigerant fluid through the second compressor 2b. Also, the flow of refrigerant fluid through the first compressor 2a may be controlled by adjustable inlet guide vanes 8c provided on the suction end of the compressor 2a. Advantageously, the inlet guide vanes 8a and 8c may

controlled, individually, or in combination for regulating the refrigerant flow rate through the compressors and hence, the regulating the refrigerating capacity of the system 1. This provides simplicity to the refrigeration process

obviating the need for cut-off valves and complicated piping. Referring to FIG 1, in operation, a stream 10 of the

refrigerant fluid is compressed in a first stage of

compression by the first compressors 2a. Optionally, the compressed stream 10a of the refrigerant fluid flowing out of the first compressor 2a is cooled by a first aftercooler 7a and a cooled stream 10b of the refrigerant fluid is further compressed in a second stage of compression by the second compressor 2b. The compressed refrigerant fluid 10c flowing out of the second compressor 2b is optionally cooled by a second aftercooler 7b. The refrigerant fluid stream lOg flowing out of the aftercooler 7b is partially cooled in a heat exchanger 11, against a low temperature, low pressure returning stream lOj of the refrigerant fluid. The partially cooled refrigerant fluid stream lOi is expanded by the turboexpander 4a, resulting in a drop in temperature and pressure of the refrigerant fluid. As mentioned earlier, expansion of the refrigerant fluid by the turboexpander 4a produces mechanical power, which is transmitted to the second compressor 2b through the shaft 5b. The refrigerant fluid stream lOj exiting the expander at low temperature and pressure is passed to the heat exchanger 11, wherein it absorbs heat from the stream lOg of refrigerant fluid

downstream of the compressor stages to partially cool the refrigerant fluid prior to expansion by the turboexpander 4a. For liquefaction of natural gas, a stream 13 of natural gas is passed through the heat exchanger 11, wherein it is cooled and subsequently liquefied by heat transfer to the

refrigerant fluid stream lOj . Although not explicitly shown, liquefaction may be achieved by cooling the stream 13 of natural gas over multiple stages.

In this example a gas/liquid stream 13a of natural gas coming from the heat exchanger 11 is passed into the separator 12. A separation device 14 separates gas 13b and liquid 13d

(heavy hydro carbons) . The precooled gas 13b (mainly methane) is passed again into heat exchanger 11 for a further stage of cooling via heat transfer to the refrigerant stream

lOj, whereby a stream of liquefied natural gas (13C) exits the heat exchanger 11, which may be subsequently passed to a storage tank (not shown) . The gaseous components are

submitted to a flare connection 13e. The refrigerant stream 10 exiting the heat exchanger 11 re-enters the first stage compressor 2a and the above cycle is repeated.

The present invention may also be used for refrigeration systems having more than two compressors, and multiple turboexpanders , for increased refrigerating capacity. In such a case, the first compressor stage may be directly connected to the drive by a shaft, and each of the other compressors directly connected to a respective turboexpander via separate shafts, the shafts being drivingly coupled by a transmission gear train. FIG 2 illustrates an example of a refrigeration system 1 having three compressors 2a, 2b and 2c and two expanders 4a and 4b. The arrangement of the compressors 2a and 2b with respect to the drive 3 and the first

turboexpander 4a are similar to that of the earlier mentioned embodiment (FIG 1) . Additionally herein (FIG 2), a third compressor 2c is directly connected to a second turboexpander 4b by a third shaft 5c. The shafts 5a, 5b and 5c are

drivingly coupled by the gear train 6. Herein the power consumptions by the compressors 2b and 2c and the power outputs of the turboexpanders 4a and 4b respectively

compensate each other, leading to reduced load on the gear train 6. Further in a preferred embodiment, each of the turboexpanders 4a and 4b is operated such that their

mechanical power output balance (i.e., are substantially equal to) the power consumption of the respective compressors 2b and 2c, thereby significantly reducing the power to be transmitted by the gear train 6. Further preferably, as shown, each of the compressors and the corresponding

turboexpander/drive are arranged on opposite sides of the transmission gear train 6 for compensation of axial load or thrusts on the gear train 6. Further, similar to the earlier illustrated embodiment, refrigerant fluid flow and hence the refrigerating capacity of the system 1 may be regulated by controlling, individually or in combination, adjustable inlet guide vanes 8a, 8b and 8c provided respectively on the inlets of the turboexpanders 4a and 4b and the compressor 2a.

In operation of the system 1 of FIG 2, a stream 10 of the refrigerant fluid is compressed in a first stage of

compression by the first compressors 2a. Optionally, the compressed stream 10a of the refrigerant fluid flowing out of the first compressor 2a is cooled by a first aftercooler 7a and a cooled stream 10b of the refrigerant fluid is further compressed in a second stage of compression by the second compressor 2b. The compressed refrigerant fluid 10c flowing out of the second compressor 2b is optionally cooled by a second aftercooler 7b. The refrigerant fluid stream lOd flowing out of the aftercooler 7b is further compressed in a third stage of compression by the third compressor 2c. The compressed refrigerant fluid lOe flowing out of the third compressor 2c is optionally cooled by a third aftercooler 7c. The refrigerant fluid stream lOf flowing out of the third aftercooler 7c is divided into two stream portions lOg and lOh and passed into the heat exchanger 11. The first divided stream lOg is partially cooled in the heat exchanger 11 against a low temperature, low pressure returning stream 10η of the refrigerant fluid. The stream lOg exits the heat exchanger as a partially cooled refrigerant stream lOi, which is expanded by the turboexpander 4a to result in a drop in temperature and pressure of the refrigerant. The second divided stream lOh is further cooled in the heat exchanger 11 against the low temperature, low pressure returning stream 10η of the refrigerant fluid. The stream lOh exits the heat exchanger as a further cooled refrigerant stream 10k, which is expanded by the turboexpander 4b to result in a further drop in temperature and pressure of the refrigerant. The refrigerant streams lOj and 101 exiting the turboexpanders 4a and 4b respectively are then merged into a low temperature, low pressure refrigerant stream in the heat exchanger 11, which is further used to for liquefaction of a stream 13 of natural gas described above.

Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. For example, it may be appreciated by one skilled in the art that embodiments of the present invention may also include

refrigeration systems having more than three compressors and more than two turboexpanders, as may be necessary, for example, for providing a required refrigerating capacity. It is therefore contemplated that all such embodiments are within the scope of the present invention as defined by the below-mentioned patent claims.