SYSTEM AND METHOD FOR THE PRODUCTION AND SUPPLY OF A DENSIFIED LIQUID OXYGEN PRODUCT

Abstract

A system and method for the liquefaction and densification of oxygen for use in space vehicle applications is provided that uses high pressure air or synthetic air as the refrigerant source. The disclosed system and method employs a heat exchanger arrangement comprising a first heat exchange device configured to liquefy the high pressure gaseous oxygen stream and at least a portion of the high pressure gaseous air stream via indirect heat exchange with a refrigerant stream to yield a liquid oxygen stream and a liquid air stream. The heat exchanger arrangement also includes a second heat exchange device configured to densify the liquified oxygen stream via indirect heat exchange with the liquid air stream which yields the densified liquid oxygen and a cold vaporized air stream. The refrigerant stream comprises a mixture of the exhaust streams from one or more turbines with the cold vaporized air stream.

Claims

1. A system for production of a densified, liquid oxygen stream from a high pressure gaseous oxygen stream, the system comprises: a heat exchanger arrangement comprising a first heat exchange device configured to liquefy the high pressure gaseous oxygen stream and at least one high pressure gaseous air stream having a pressure greater than or equal to about 10 bar (a) via indirect heat exchange with a refrigerant stream to yield a liquid oxygen stream and a liquid air stream and a second heat exchange device configured to densify the liquified oxygen stream via indirect heat exchange with the liquid air stream to yield a densified liquid oxygen stream and a cold vaporized air stream; one or more turbines configured to expand a diverted portion of the at least one high pressure gaseous air stream to yield one or more exhaust streams; wherein the refrigerant stream comprises a mixture of the one or more exhaust streams and the cold vaporized air stream.

2. The system for production of the densified, liquid oxygen stream of claim 1, wherein the one or more turbines further comprise: a first turbine configured to expand a diverted portion of the at least one high pressure gaseous air stream to yield a first exhaust stream; and a second turbine configured to further expand the first exhaust stream to yield a second exhaust stream; wherein the refrigerant stream comprises a mixture of the second exhaust stream and the cold vaporized air stream.

3. The system for production of the densified, liquid oxygen stream of claim 2, wherein the first heat exchange device is further configured to pre-cool the high pressure air stream and the first turbine is configured to expand the pre-cooled high pressure air stream to yield the first exhaust stream.

4. The system for production of the densified, liquid oxygen stream of claim 2, wherein the first heat exchange device is further configured to partially warm the first exhaust stream and the second turbine is configured to expand the partially warmed, first exhaust stream to yield the second exhaust stream.

5. The system for production of the densified, liquid oxygen stream of claim 1, further comprising one or more auxiliary compressors configured to compress the high pressure air stream to a pressure greater than or equal to a pressure greater than or equal to about 50.5 bar (a).

6. The system for production of the densified, liquid oxygen stream of claim 5, wherein the work provided by the one or more turbines is equal to or greater than the work required by the one or more auxiliary compressors.

7. The system for production of the densified, liquid oxygen stream of claim 6, further comprising an integral gear machine operatively coupling the one or more turbines and the one or more auxiliary compressors.

8. The system for production of the densified, liquid oxygen stream of claim 6, wherein the cold vaporized air stream is at a subambient pressure and the system further comprises a vacuum pump configured to pressurize the cold vaporized air stream.

9. The system for production of the densified, liquid oxygen stream of claim 8, wherein the work provided by the one or more turbines is greater than 90% of the work required by the one or more auxiliary compressors and the vacuum pump.

10. The system for production of the densified, liquid oxygen stream of claim 2, further comprising: a third turbine configured to expand another diverted portion of the at least one high pressure gaseous air stream to yield a third exhaust stream; and wherein the second turbine is further configured to further expand a mixture of the first exhaust stream and the third exhaust stream to yield the second exhaust stream.

11. The system for production of the densified, liquid oxygen stream of claim 10, wherein the first heat exchange device is further configured to partially warm the first exhaust stream and the third exhaust stream and the second turbine is configured to expand a mixture of the partially warmed, first exhaust stream and partially warmed, third exhaust stream to yield the second exhaust stream.

12. The system for production of the densified, liquid oxygen stream of claim 1, wherein the first heat exchange device, the second heat exchange device, the one or more turbines are disposed proximate a space vehicle launch platform at a launch facility.

13. The system for production of the densified, liquid oxygen stream of claim 12, wherein the high pressure gaseous oxygen and the at least one high pressure gaseous air stream are supplied to the launch facility via a pipeline from an air separation unit located within 2 kilometers of the launch facility.

14. The system for production of the densified, liquid oxygen stream of claim 13, wherein the at least one high pressure gaseous air stream comprises a synthetic air stream comprising a mixture of oxygen and nitrogen from the air separation unit.

15. The system for production of the densified, liquid oxygen stream of claim 12, further comprising a storage tank and wherein and the densified, liquid oxygen stream is stored in the storage tank for use as an oxidant for a space vehicle propulsion system.

16. A method for production of a densified, liquid oxygen stream from a high pressure gaseous oxygen stream, the method comprising the steps of: liquefying the high pressure gaseous oxygen stream and at least one high pressure gaseous air stream having a pressure greater than or equal to about 10 bar (a) via indirect heat exchange with a refrigerant stream in a heat exchanger arrangement to yield a liquid oxygen stream and a liquid air stream; densifying the liquified oxygen stream via indirect heat exchange with the liquid air stream in the heat exchange arrangement to yield the densified liquid oxygen stream and a cold vaporized air stream; wherein the heat exchange arrangement comprises a first heat exchange device configured to liquefy the high pressure gaseous oxygen stream and at least one high pressure gaseous air stream and a second heat exchange device configured to densify the liquified oxygen stream; expanding a diverted portion of the at least one high pressure gaseous air stream and in one or more turbines to yield one or more exhaust streams; mixing the one or more exhaust streams and the cold vaporized air stream to yield the refrigerant stream.

17. The method for production of the densified, liquid oxygen stream of claim 16, wherein the one or more turbines further comprise: a first turbine configured to expand a diverted portion of the at least one high pressure gaseous air stream to yield a first exhaust stream; and a second turbine configured to expand the first exhaust stream to yield a second exhaust stream; wherein the refrigerant stream comprises a mixture of the second exhaust stream and the cold vaporized air stream.

18. The method for production of the densified, liquid oxygen stream of claim 17, further comprising the step of pre-cooling the at least one high pressure air stream in the first heat exchanger, and wherein the step of expanding the diverted portion of the at least one high pressure gaseous air stream in the first turbine further comprises expanding the pre-cooled at least one high pressure air stream to yield the first exhaust stream.

19. The method for production of the densified, liquid oxygen stream of claim 17, further comprising the step of partially warming the first exhaust stream, and wherein the step of expanding the first exhaust stream further comprises expanding the partially warmed, first exhaust stream to yield the second exhaust stream.

20. The method for production of a densified, liquid oxygen stream of claim 16, further comprising the step of compressing the at least one high pressure air stream in one or more auxiliary compressors to a pressure greater than or equal to about 50.5 bar (a).

21. The method for production of the densified, liquid oxygen stream of claim 20, wherein the work provided by the one or more turbines is equal to or greater than the work required by the one or more auxiliary compressors.

22. The method for production of the densified, liquid oxygen stream of claim 20, wherein the cold vaporized air stream is at a subambient pressure and the method further comprises the step of pressurizing the cold vaporized air stream with a vacuum pump, and wherein the work provided by the one or more turbines is greater than 90% of the work required by the one or more auxiliary compressors and the vacuum pump.

23. The method for production of the densified, liquid oxygen stream of claim 17, further comprising the steps of: expanding a second diverted portion of the high pressure gaseous air stream in a third turbine to yield a third exhaust stream; and mixing the first exhaust stream and the third exhaust stream in the first heat exchange device; and wherein the step of expanding the first exhaust stream in the second turbine further comprises further expanding the mixture of the first exhaust stream and the third exhaust stream in the second turbine to yield the second exhaust stream.

24. The method for production of the densified, liquid oxygen stream of claim 23, further comprising the steps of partially warming the first exhaust stream and the third exhaust stream in the first heat exchanger, and wherein the step of further expanding the first exhaust stream in the second turbine further comprises further expanding the mixture of the first exhaust stream and the third exhaust stream in the second turbine to yield the second exhaust stream.

25. The method for production of the densified, liquid oxygen stream of claim 16, further comprising the step of suppling the at least one high pressure gaseous oxygen and the high pressure gaseous air stream via a pipeline from an air separation unit to a launch facility located within 2 kilometers of the air separation unit.

26. The method for production of the densified, liquid oxygen stream of claim 25, wherein the at least one high pressure gaseous air stream comprises a stream of purified air from the air separation unit.

27. The method for production of the densified, liquid oxygen stream of claim 25, wherein the at least one high pressure gaseous air stream comprises a stream of purified synthetic air comprising a mixture of oxygen and nitrogen produced by the air separation unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] It is believed that the claimed system and method will be better understood when taken in connection with the accompanying drawings in which:

[0011] FIG. 1 shows a schematic illustration of the present arrangement for the production of a densified, liquid oxygen stream for use in a space vehicle launch from a launch facility using a high pressure gaseous oxygen stream and a high pressure air stream sourced from a nearby air separation unit;

[0012] FIG. 2 shows a schematic of an embodiment of a system and method for the liquefaction of a high pressure gaseous oxygen stream and densification of the resulting liquid oxygen stream using a high pressure air stream as a refrigerant; and

[0013] FIG. 3 shows a schematic of an alternate embodiment of a system and method for the liquefaction of a high pressure gaseous oxygen stream and densification of the resulting liquid oxygen stream using a high pressure air stream as a refrigerant.

DETAILED DESCRIPTION

[0014] The present system and method for the production of the densified, liquid oxygen stream incorporates several innovative and/or advantageous features when compared to conventional oxygen densification systems and processes currently used in many space vehicle launch applications. The first advantageous feature involves piping high pressure gaseous oxygen stream and a stream of high pressure purified air from a nearby air separation unit to the liquefaction site, preferably at the launch facility. The high-pressure purified air serves to provide the necessary pressure energy to facilitate liquid oxygen production and densification. This refrigeration necessary for the oxygen liquefaction and densification is preferably generated by gas expansion refrigeration process using diverted portions of the high pressure purified air stream. A second innovative aspect of the present system and method employs the integration of the oxygen liquefaction process with the oxygen densification process. The use of a cryogenic liquefaction process using the high pressure purified air stream enables the direct on-site integration of the densification process.

[0015] Another innovative aspect of the present system and method involves the use of an integral gear combined expansion-compression device, such as a compander or bridge machine (i.e. BRIM) that actually consumes negligible power. Such a compander or bridge machine would serve to both recoup the energy or work from the gas-expansion of the high pressure purified air streams and as means to redistribute the power to associated compression of the high pressure purified air streams. In this way, little or no external power needs to be brought to the liquefaction site. Yet another innovative or advantageous aspect involves the use of multiple turbo-expanders, operatively coupled to the integral gear machine to facilitate a high efficiency liquefaction of the high pressure gaseous oxygen stream. It has been found that the use of three turbo-expanders in addition to one or more auxiliary feed air compressors operatively coupled in a three pinion compander or bridge machine enables oxygen liquefaction at high efficiency.

[0016] Turning now to FIG. 1, there is shown a highly simplified depiction of the general processing scheme for the production and supply of a densified, liquid oxidant to a launch facility for use in a space vehicle launch. As seen therein, a high pressure gaseous oxygen stream 20 is produced by an air separation unit 10 located in proximity to a space vehicle launch facility 30. The high pressure gaseous oxygen stream 20 is delivered to the space vehicle launch facility 30 via an oxygen pipeline. Optionally, a high pressure air stream 25 or synthetic air stream, comprised of a mixture of oxygen and nitrogen from the air separation unit, may also be directed via a pipeline from the air separation unit 10 to the space vehicle launch facility 30. The high pressure air stream 25 is preferably delivered via pipeline to the launch facility 30 at a pressure equal to or greater than 10 bar (a) and preferably at a pressure greater than 20 bar (a). Preferably, the air separation unit 10 is located within 2 kilometers of the launch facility 30. The high pressure gaseous oxygen stream 20 and the high pressure air stream 25 are directed to a combined liquefaction and densification system 40 where the high pressure gaseous oxygen stream 20 is liquefied and the resulting liquid oxygen stream 44 is densified as described in more detail below with reference to FIGS. 2 and 3. Preferably, the high pressure gaseous oxygen stream 20 may be further compressed at the launch facility and delivered to the liquefaction process in a supercritical state. The resulting product from the combined liquefaction and densification system/process is a densified liquid oxygen product 65 may be stored in a storage tank 35 and used in rocket launch applications from one or more launch pads 33 located at the launch facility 30. A low pressure stream of waste air 60 from the liquefaction and densification process/system may be vented to the atmosphere or optionally recycled back to the air separation unit or used in other applications for clean dry air.

[0017] The air separation unit 10 shown in FIG. 1 is equipped with the conventional warm-end processing equipment 14 and cold-end processing equipment 16. The warm-end processing equipment 14 within the air separation unit 10 typically comprises a main feed air compression train and air pre-purification units configured to substantially compress and pre-purify an incoming feed air stream 12, together with one or more turbine air circuits and/or booster air circuits. A portion of the substantially compressed and pre-purified air 15 is extracted as a high pressure purified air stream from the warm end processing equipment 15 within the air separation unit 10. The cold-end processing equipment 16 within the air separation unit 10 typically comprises a main or primary heat exchanger arrangement and a distillation column system that are typically housed in one or more insulated cold boxes. The cold-end processing equipment 16 within the air separation unit 10 is configured to cool a portion of the substantially compressed and purified air 15 and then fractionally distill or separate the cooled air in the distillation column system to yield a plurality of products, including a high-pressure gaseous oxygen stream 20 and perhaps liquid nitrogen, liquid oxygen or argon products.

[0018] In the arrangement depicted in FIG. 1, liquid storage tanks are also preferably located at the launch facility 30 to store the densified liquid oxygen and optionally any liquid air produced within the combined liquefaction and densification system or other liquid products such as liquid nitrogen or liquid fuels such as methane needed at the launch facility 30. Although not shown, the liquefaction and densification system and/or components thereof may be disposed on moveable platforms that can be aggregated at a central location within the launch facility 30 or can be readily moved to a location proximate a space vehicle launch pad 33 or platform within the launch facility 30.

[0019] Turning now to FIG. 2, the illustrated liquefaction and densification system 40 and associated method receives a stream of high pressure gaseous oxygen 20 preferably at a pressure greater than or equal to 40 bar (a) and more preferably at a pressure greater than about 60 bar (a). The high pressure gaseous oxygen stream 20 is directed to a first heat exchanger device 45 or heat exchanger core configured to cool and condense the high pressure gaseous oxygen stream 20 via indirect heat exchange with a refrigerant stream to yield a liquid oxygen stream 44. The liquid oxygen stream 44 is then directed to a second heat exchange device 55 configured to subcool or densify the resulting liquid oxygen stream 44 via indirect heat exchange with a liquid air stream 54 to yield the densified liquid oxygen stream 65 and a cold vaporized air stream 56.

[0020] The illustrated liquefaction and densification system 40 and associated methods also receives one or more streams of high pressure gaseous air 25 having a pressure preferably greater than or equal to about 10 bar (a), more preferably at a pressure greater than about 20 bar (a), and most preferably at a pressure equal to or greater than about 25 bar (a). The high pressure gaseous air stream 25 may be further compressed in one or more auxiliary compressors 41 to a pressure at or near the supercritical pressure. The further compressed high pressure air stream is aftercooled in aftercooler 42 and directed to the first heat exchanger device 45 or heat exchanger core. A first fraction of the further compressed high pressure air stream 43 is pre-cooled or partially cooled and then diverted as stream 46 to a first turbine 47 or warm turbo-expander. The first exhaust stream 48 exiting this first turbine 47 will typically be near saturation at a pressure preferably in the range of 5 bar (a) to 10 bar (a). The first exhaust stream 48 is partially warmed in the first heat exchange device 45 and the warmed stream 49 is directed to a second turbine 50 or cold turbo-expander which serves to further expand the warmed first exhaust to a pressure marginally above ambient or preferably about 1.2 bar (a) to yield a second exhaust stream 51. The second exhaust stream 51 is directed back to the first heat exchange device 45 as part of the refrigeration stream.

[0021] Another fraction of the further compressed high pressure air stream is fully cooled to a point where it is substantially liquefied. The liquefied air stream 52 is then depressurized via an expansion valve 53 and/or liquid turbine to a sub-ambient pressure. The depressurized stream 54 is then substantially vaporized in a vacuum within the second heat exchange device 55 or second heat exchange core against the liquid oxygen stream 44. The resulting cold vaporized air stream 56 is pulled through a vacuum pump 57 and repressurized to a pressure just above ambient pressure or preferably to a pressure of about 1.2 bar (a). The repressurized cold vaporized air stream 58 is directed back to the first heat exchange device 45 as part of the refrigeration stream. As such, the refrigeration stream used to liquefy the high pressure gaseous oxygen stream 20 and liquefy a portion or fraction of the high pressure air stream 52 comprises a mixture of the one or more exhaust streams 51 and the cold vaporized air stream 58.

[0022] The work resulting from expansion of a portion of the high pressure air stream in the first and second turbines is preferably directed to an integral gear compression-expansion device so that this work may be recouped for use in the further compression of the incoming or feed high pressure air stream in the one or more auxiliary compressors 41 and/or the vacuum pump 57. Ideally, the further compression of the feed high pressure air stream in the one or more auxiliary compressors 41 and re-pressurization of the sub-ambient cold vaporization air stream will be defined such that an energy balance within the integral gear compression-expansion device may be established without the import or export of additional power. In practice, however, the work provided by the one or more turbines 47, 50 should be greater than 90% of the work required by the one or more auxiliary compressors 41 and the vacuum pump 57.

[0023] The schematic and process flow diagram depicted in FIG. 3 is very similar to the schematic and process flow diagram of FIG. 2 described above and, for sake of brevity, much of the descriptions of the detailed arrangements will not be repeated. Rather, the following discussion will focus on the differences and additions depicted in the process flow diagram of FIG. 3 when compared to the process flow diagram depicted in FIG. 2.

[0024] In the embodiment of FIG. 3, the high pressure gaseous oxygen stream 20 preferably at a pressure greater than or equal to 40 bar (a) and more preferably at a pressure greater than about 60 bar (a) is cooled and condensed within the first heat exchange device 45, and the resulting liquid oxygen stream 44 is then subcooled in the second heat exchange device 55 via indirect heat exchange with a liquid air stream 54 to yield the densified liquid oxygen stream 65 and a cold vaporized air stream 56.

[0025] The key difference between the embodiment of FIG. 2 and the embodiment of FIG. 3 is that the high pressure gaseous air stream 25 is further compressed to a supercritical pressure and then split into three fractions. Similar to the embodiment of FIG. 2, a first fraction of the further compressed high pressure air stream 46 is diverted to a first turbine 47 where it is expanded to produce a first exhaust stream 48. A second fraction of the further compressed high pressure air stream is fully cooled in the first heat exchange 45 device to a point where it is substantially liquefied. The liquefied air stream 52 is then depressurized via an expansion valve 53 and/or liquid turbine to a sub-ambient pressure. A third fraction of the further compressed high pressure air stream is cooled in the first heat exchange device 45 and the cooled stream 66 is diverted to a third turbine 67 where it is expanded to produce a third exhaust stream 68.

[0026] The first exhaust stream 48 and the third exhaust stream 68 are both warmed in the first heat exchange device 45 possibly in different warming passages within the first heat exchange device 45 and then mixed. The resulting mixed exhaust stream 49 is then directed to the second turbine 50 or cold turbo-expander which serves to further expand the mixed exhaust stream to a pressure marginally above ambient or preferably about 1.2 bar (a) to yield the second exhaust stream 51. The second exhaust stream 51 is then directed back to the cold end of the first heat exchange device 45 as part of the refrigeration stream. Note that in this embodiment, the temperature of the third turbine exhaust 68 is comparable to the temperature of the second turbine exhaust 51.

[0027] Similar to the embodiment of FIG. 2, the resulting cold vaporized air stream 56 in the embodiment of FIG. 3 is pulled through the vacuum pump 57 and repressurized to a pressure of about 1.2 bar (a). The repressurized cold vaporized air stream 58 may be combined with the low-pressure exhaust stream 51 of the second turbine 50 as the refrigeration stream. The combined refrigeration stream is then warmed to ambient temperature, with the resulting low pressure air stream 60 preferably being either vented or recycled back to the air separation unit 10.

INDUSTRIAL APPLICABILITY

[0028] The most obvious technical advantage realized by the present system and method for production of a densified, liquid oxygen stream from a pipeline supplied high pressure gaseous oxygen stream using pipeline supplied high pressure air streams as the refrigerant would be the elimination of the need to truck liquid oxygen and/or liquid nitrogen to the launch facility. Furthermore, there would be no need to have an additional refrigeration system for densification of liquid oxygen. By integrating the oxygen densification with the liquefaction using high pressure air streams, the disposable use of liquid nitrogen in conventional oxygen densification processes will be substantially reduced, if not eliminated. The use of a compander or bridge machine to recoup gas expansion power from the turbine expansion of the high pressure air streams would enable both a high efficiency liquefaction process and would potentially eliminate the capital associated with building out the high voltage power supply at the launch facility for oxygen liquefaction and densification purposes.

[0029] In a space vehicle launch applications requiring 1000+ mtpd of liquid oxygen, the external liquid nitrogen supply is typically about 0.4 (LN2/LO2). It is anticipated that if this requirement of liquid nitrogen were to be eliminated, the potential power savings realized by the air separation unit is on the order of about 10 MW. The reduction of liquid nitrogen demand from the air separation unit will also substantially simplify the air separation unit and air separation cycle design necessary to supply the high-pressure oxygen without supply of liquid nitrogen. A true gas only air separation unit designed to produce mainly high pressure gaseous oxygen stream and high pressure air streams required by the present system and method will likely require only a single turbine and a booster air compressor. The capital cost savings in the sir separation unit will likely be several million dollars compared to a dedicated integrated air separation and liquefaction plant currently used to supply liquid oxygen and liquid nitrogen to space application customers.

[0030] While the present systems and methods for the production of a densified liquid oxygen product has been described with reference to one or more preferred embodiments, it is understood that numerous variations, additions, changes, and omissions can be made without departing from the spirit and scope of the present systems and methods as set forth in the appended claims.

[0031] For example, with respect the air compression, dedicated air compression for refrigeration production is possible. The main air compression and associated pre-purification may reside at either or both locations, namely the air separation unit or the launch facility. Also, the main air compression as well as the auxiliary compression may be arranged in a serial compression arrangements or a parallel compression arrangements depending on the flow and pressure requirements from the space application customer together with any secondary requirements such as supply of merchant liquid or the need for liquid air energy storage. The air pre-purification demands may also be partitioned between the air separation unit and the launch facility. For instance, high pressure gaseous air supplied to the launch facility via pipeline may initially be dehydrated via absorption at the air separation unit and then piped to the launch facility where it may be further dried to a cryogenic ready condition.

[0032] In order to increase the liquid air yield fraction, a liquid turbine or dense phase expander may be employed to reduce the subsequent flash loss of the liquid air. Such a machine may be configured in series with subsequent valving to fully reduce the liquid air pressure prior to entry into the second heat exchange device for densification.

[0033] Also, while the liquefaction of supercritical oxygen at pressures>50.5 bar (a) is thermodynamically preferred, it is possible to configure the above-described system and method for lower oxygen pressures. In addition, it is possible to further compress the oxygen at the ASU or at the Launch pad. Such compression could be integrated into the integral gear machine.

[0034] The process may be operated without integral liquid oxygen densification or the process could be operated with intermittent liquid oxygen densification in which case the non-densified, liquified oxygen is simply stored in one or more storage tanks at the launch facility for subsequent densification or use. It is also conceivable to add a supplemental or alternative densification means using a different densification refrigerant such as rare gases, argon, or even liquid nitrogen. It is also possible to incorporate methane densification along with oxygen densification into the densification process Alternatively, one may even design the current liquefaction process to also liquefy a small stream of gaseous nitrogen from the air separation unit in conjunction using the high pressure air stream as the refrigerant, with the resulting liquid nitrogen subsequently stored in storage tanks or employed for supplemental densification or densification of methane.

[0035] Although it is highly advantageous to integrate the subject gas expanders into a common, single BRIM type machine, this need not be the case. Alternatively, the subject turbines or turbo-expanders may be connected to high speed generators or the power can be dissipated to brake. It should be noted that the vacuum compressor could also be integrated into the BRIM type machine. If power is available, the BRIM could employ a motor (or a generator). It is also possible to configure the auxiliary compressors as a dedicated booster compressors and operatively couple one or more of the auxiliary compressors to one or more of the turbines.