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Performance of cold compressors in a cooling system of an R&D superconducting coil cooled with subcooled helium
Fusion Engineering and Design 81 (2006) 2617–2621
Performance of cold compressors in a cooling system of an R&D superconducting coil cooled with subcooled helium S. Hamaguchi ∗ , S. Imagawa, N. Yanagi, K. Takahata, R. Maekawa, T. Mito National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan Available online 19 October 2006
Abstract The helical coils of large helical device (LHD) have been operated in saturated helium at 4.4 K and plasma experiments have been carried out at magnetic fields lower than 3 T for 8 years. Now, it is considered that the cooling system of helical coils will be improved to enhance magnetic fields in 2006. In the improvement, the helical coils will be cooled with subcooled helium and the operating temperature of helical coils will be lowered to achieve the designed field of 3 T and enhance cryogenic stabilities. Two cold compressors will be used in the cooling system of helical coils to generate subcooled helium. In the present study, the performance of cold compressors has been investigated, using a cooling system of R&D coil, to apply cold compressors to the cooling system of helical coils. Actual surge lines of cold compressors were observed and the stable operation area was obtained. Automatic operations were also performed within the area. In the automatic operations, the suitable pressure of a saturated helium bath, calculated from the rotation speed of the 1st cold compressor, was regulated by bypass valve. From these results, stable operations will be expected in the cooling system of helical coils. © 2006 Elsevier B.V. All rights reserved. Keywords: Subcooled helium; Cold compressor; Surge; Stable operation; Automatic control
1. Introduction The helical coils of the large helical device (LHD) have been operated by saturated helium at the temperature of 4.4 K and the pressure of 120 kPa [1]. So far, plasma experiments have been carried out at the magnetic field lower than 3 T successfully [2]. However, the operating field has not reached the nominal field of 3 T [3]. So, it is considered that the helical coils of ∗ Corresponding author. Tel.: +81 572 58 2116; fax: +81 572 58 2616. E-mail address: [email protected] (S. Hamaguchi).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.052
LHD will be cooled with subcooled helium to achieve the field of 3 T and enhance the cryogenic stability. To use subcooled helium as a coolant for the helical coils of LHD, an R&D superconducting coil, wound with superconductors of the helical coils, was tested in subcooled helium [4]. From the results, it is expected that the designed magnetic field of 3 T will be achieved in case of LHD if the helical coils will be cooled down with subcooled helium [5]. Two cold compressors were installed in the cooling system of the R&D coil to lower supplied helium temperature [6]. It is very important that these two cold compressors are operated stably and safely. If the cold compressors enter the surge region
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S. Hamaguchi et al. / Fusion Engineering and Design 81 (2006) 2617–2621
and then trips occur, there is every possibility of damaging equipments and reducing machine time for plasma experiments of LHD [7]. In the present study, the performance of cold compressors was investigated in the cooling system of the R&D coil for stable and safe operations, especially a surge limit of cold compressors. A control method for automatic operations was also tested, in order to operate cold compressors without a surge. In this paper, the control method of cold compressors to operate stably and safely will be discussed.
Table 1 Main parameters of cooling system for R&D test facility Supplied subcooled He Mass flow Pressure Inlet temperature
5.0–10.0 g/s 120 kPa 3.1 K
Cold compressors Inlet pressure Inlet temperature Outlet pressure Outlet temperature Overall pressure ratio Mass flow
23.0 kPa 3.0 K 120 kPa 7.39 K 5.2 15.9 g/s
2. Experimental setup Fig. 1 shows a flow diagram of the cooling system of the R&D coil. The cooling system consists of a saturated helium bath with a heat exchanger to generate subcooled helium, two centrifugal cold compressors with gas foil bearings, a bypass valve to adjust mass flow of the cold compressors, a current lead tank above the R&D coil and heaters to control the level of liquid helium. The saturated helium bath was evacuated by two cold compressors in series, from 120 kPa (4.4 K) to 23 kPa (3.0 K). The pressure in the saturated helium bath and the mass flow rate of the cold compressors were regulated by bypass valve, during cooling down
and warming up. In the steady state at 3.0 K, the inlet pressure and temperature of 1st cold compressor are 23 and 3.0 K while the outlet pressure and temperature of 2nd cold compressor are 120 kPa and 7.39 K, respectively. The overall pressure ratio is 5.2. The mass flow of the cold compressors is 15.9 g/s. Supplied helium was subcooled up to 3.1 K at the heat exchanger in the saturated helium bath. The subcooled helium was supplied from the bottom of the R&D coil and then flowed out to the current lead tank above. The mass flow rate of subcooled helium was 5–10 g/s throughout the present experiments. These parameters of the cooling system are summarized in Table 1.
Fig. 1. Flow diagram of the cooling system for the R&D coil.
S. Hamaguchi et al. / Fusion Engineering and Design 81 (2006) 2617–2621
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Fig. 2. Performance of the cold compressors under the various conditions.
Generally speaking, cold compressors enter the surge region in the case of low mass flow or high pressure ratio, while they enter the choke region in the case of high mass flow or low pressure ratio. Since a surge causes an intense oscillation of pressure, it is very dangerous in particular. The cold compressors should be operated as the surge region is avoided. It is necessary
to comprehend the surge region of cold compressors exactly for stable and safe operations [8]. So, the performance of the cold compressors was examined under various pressure ratios and reduced mass flow. Fig. 2 shows a map of the performance of the cold compressors under the various conditions. The left figure is the experimental results of the 1st cold compressor and the right one is that of the 2nd cold compressor. Open marks express working points of the cold compressors. Solid lines are actual surge lines obtained from the present experiments, while dotted lines are nominal surge lines estimated from preliminary examinations with air at room temperature. It was found that the actual surge lines shifted to the left with respect to the estimated surge lines. Therefore, the wide area on the
Fig. 3. Schematic of cooling system in R&D test facility to generate subcooled helium.
Fig. 4. Relationship between rotation speed of 1st cold compressor and pressure/pressure ratio.
3. Experimental results 3.1. Operation area
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Fig. 5. History of performance of the cold compressors during an automatic operation.
map can be used in order to operate the cold compressors stably and safely. In the present examinations, both cold compressors have never entered the surge region or the choke region simultaneously. It means that the area for stable operation can be enlarged if the relationship of the rotation speed between the 1st cold compressor and the 2nd cold compressor is optimized. 3.2. Automatic control Cold compressors should be operated without a surge in order to avoid machine trips and damages. So, automatic operations were performed to control the cold compressors within a stable area on the map [9] shown in Fig. 2. Fig. 3 shows a schematic of a subcooling system for the R&D test facility. The saturated pressure in the saturated helium bath as the inlet pressure of the 1st cold compressor, Pin, and the pressure between the 1st cold compressor and the 2nd cold compressor, Pm, were measured throughout operations to comprehend the status of the subcooling system. These pressures and mass flow could be regulated by rotation speeds of cold compressors, a heater in the saturated helium bath and a bypass valve. In the present subcooling system, the rotation speed of the 2nd cold compressor was linked with that of the 1st cold compressor to simplify operations. Relationships between a rotation speed of the 1st cold compressor and these pressures during steady state operations are plotted in Fig. 4. The pressure of a
saturated helium bath decreases monotonously, as the rotation speed of the 1st cold compressor increases. In the present automatic operations, the suitable pressure of the saturated helium bath which was calculated from the rotation speed of the 1st cold compressor using the relationship in Fig. 4, was regulated by the bypass valve. Fig. 5 shows the history of status of the cold compressors during an automatic operation. The operation time for cooling down or warming up was 20 min. The automatic operation could be completed within a stable area on the map. The constant rotation speeds of both cold compressors were used during the steady state operation, because the subcooling system became unstable easily while the rotation speed was varied. Stable operations of the cooling system for the helical coils will be expected to apply these control methods to the cooling system for the helical coils.
4. Conclusions The performance of the cold compressors was tested in the cooling system of the R&D test facility to improve the cooling system of the helical coils. The actual surge line of the cold compressors was comprehended. As a result, we can use a wide range to operate the cold compressors stably. It is also expected that the range will be enlarged if the relationship of the rotation speed between the 1st cold compressor and the 2nd cold compressor is optimized. The cold compressors could
S. Hamaguchi et al. / Fusion Engineering and Design 81 (2006) 2617–2621
be operated stably during cooling down and warming up with the present automatic method in which the saturated bath pressure was regulated by the bypass valve.
Acknowledgements The authors would like to thank Mr. S. Yoshinaga, Mr. K. Kurihara and Mr. T. Shinba for their useful suggestions and also appreciate the great helps of Mr. S. Moriuchi, Mr. H. Sekiguchi and Mr. H. Kitano to operate the cooling system of the R&D test facility. The present work was partially supported by the Ministry of Education, Science, Sports and Culture of Japan, Grant-in-Aid for Science Research, 14750151.
References [1] A. Iiyoshi, A. Komori, A. Ejiri, M. Emoto, H. Funaba, M. Goto, et al., Overview of the large helical device project, Nucl. Fusion 39 (9Y) (1999) 1245–1256.
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[2] O. Motojima, H. Yamada, N. Ashikawa, M. Emoto, H. Funaba, M. Goto, et al., Recent development of LHD experiment, J. Plasma Fusion Res. 5 (2003) 22–27. [3] S. Imagawa, N. Yanagi, T. Mito, H. Chikaraishi, S. Hamaguchi, H. Sekiguchi, et al., Excitation properties and cryogenic stability of helical coils for the LHD, IEEE Trans. Appl. Supercond. 11 (2001) 1889–1892. [4] S. Hamaguchi, S. Imagawa, N. Yanagi, Y. Hishinuma, T. Mito, K. Takahata, et al., Thermal hydraulic characteristics of superconducting coil cooled by subcooled He I, IEEE Trans. Appl. Supercond. 14 (2) (2004) 1439–1442. [5] S. Imagawa, N. Yanagi, Y. Hishinuma, T. Mito, K. Takahata, H. Chikaraishi, et al., Results of stability test in subcooled helium for the R&D coil of the LHD helical coil, IEEE Trans. Appl. Supercond. 14 (2) (2004) 1511–1514. [6] B. Gravil, B. Jager, Experimental with a large scale He II refrigeration system at Tore Supra, Adv. Cryog. Eng. 43 (1998) 571–579. [7] A. Martinez, A.L. Klebaner, J.N. Makara, J.C. Theilacker, Surge recovery techniques for the TEVATRON cold compressors, Adv. Cryog. Eng. 51 (2006) 1384–1391. [8] A. B´ezaguet, P. Lebrun, L. Tavian, Performance assessment of industrial prototype cryogenic helium compressors for the Large Hadron Collider, in: Proceedings of ICEC17, 1999, pp. 145–148. [9] H. Asakura, A. K¨undig, Control considerations of multi stage cold compression systems in large helium refrigeration plants, in: Proceedings of ICEC19, 2003, pp. 681–686.
Performance of cold compressors in a cooling system of an R&D superconducting coil cooled with subcooled helium S. Hamaguchi ∗ , S. Imagawa, N. Yanagi, K. Takahata, R. Maekawa, T. Mito National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan Available online 19 October 2006
Abstract The helical coils of large helical device (LHD) have been operated in saturated helium at 4.4 K and plasma experiments have been carried out at magnetic fields lower than 3 T for 8 years. Now, it is considered that the cooling system of helical coils will be improved to enhance magnetic fields in 2006. In the improvement, the helical coils will be cooled with subcooled helium and the operating temperature of helical coils will be lowered to achieve the designed field of 3 T and enhance cryogenic stabilities. Two cold compressors will be used in the cooling system of helical coils to generate subcooled helium. In the present study, the performance of cold compressors has been investigated, using a cooling system of R&D coil, to apply cold compressors to the cooling system of helical coils. Actual surge lines of cold compressors were observed and the stable operation area was obtained. Automatic operations were also performed within the area. In the automatic operations, the suitable pressure of a saturated helium bath, calculated from the rotation speed of the 1st cold compressor, was regulated by bypass valve. From these results, stable operations will be expected in the cooling system of helical coils. © 2006 Elsevier B.V. All rights reserved. Keywords: Subcooled helium; Cold compressor; Surge; Stable operation; Automatic control
1. Introduction The helical coils of the large helical device (LHD) have been operated by saturated helium at the temperature of 4.4 K and the pressure of 120 kPa [1]. So far, plasma experiments have been carried out at the magnetic field lower than 3 T successfully [2]. However, the operating field has not reached the nominal field of 3 T [3]. So, it is considered that the helical coils of ∗ Corresponding author. Tel.: +81 572 58 2116; fax: +81 572 58 2616. E-mail address: [email protected] (S. Hamaguchi).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.052
LHD will be cooled with subcooled helium to achieve the field of 3 T and enhance the cryogenic stability. To use subcooled helium as a coolant for the helical coils of LHD, an R&D superconducting coil, wound with superconductors of the helical coils, was tested in subcooled helium [4]. From the results, it is expected that the designed magnetic field of 3 T will be achieved in case of LHD if the helical coils will be cooled down with subcooled helium [5]. Two cold compressors were installed in the cooling system of the R&D coil to lower supplied helium temperature [6]. It is very important that these two cold compressors are operated stably and safely. If the cold compressors enter the surge region
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S. Hamaguchi et al. / Fusion Engineering and Design 81 (2006) 2617–2621
and then trips occur, there is every possibility of damaging equipments and reducing machine time for plasma experiments of LHD [7]. In the present study, the performance of cold compressors was investigated in the cooling system of the R&D coil for stable and safe operations, especially a surge limit of cold compressors. A control method for automatic operations was also tested, in order to operate cold compressors without a surge. In this paper, the control method of cold compressors to operate stably and safely will be discussed.
Table 1 Main parameters of cooling system for R&D test facility Supplied subcooled He Mass flow Pressure Inlet temperature
5.0–10.0 g/s 120 kPa 3.1 K
Cold compressors Inlet pressure Inlet temperature Outlet pressure Outlet temperature Overall pressure ratio Mass flow
23.0 kPa 3.0 K 120 kPa 7.39 K 5.2 15.9 g/s
2. Experimental setup Fig. 1 shows a flow diagram of the cooling system of the R&D coil. The cooling system consists of a saturated helium bath with a heat exchanger to generate subcooled helium, two centrifugal cold compressors with gas foil bearings, a bypass valve to adjust mass flow of the cold compressors, a current lead tank above the R&D coil and heaters to control the level of liquid helium. The saturated helium bath was evacuated by two cold compressors in series, from 120 kPa (4.4 K) to 23 kPa (3.0 K). The pressure in the saturated helium bath and the mass flow rate of the cold compressors were regulated by bypass valve, during cooling down
and warming up. In the steady state at 3.0 K, the inlet pressure and temperature of 1st cold compressor are 23 and 3.0 K while the outlet pressure and temperature of 2nd cold compressor are 120 kPa and 7.39 K, respectively. The overall pressure ratio is 5.2. The mass flow of the cold compressors is 15.9 g/s. Supplied helium was subcooled up to 3.1 K at the heat exchanger in the saturated helium bath. The subcooled helium was supplied from the bottom of the R&D coil and then flowed out to the current lead tank above. The mass flow rate of subcooled helium was 5–10 g/s throughout the present experiments. These parameters of the cooling system are summarized in Table 1.
Fig. 1. Flow diagram of the cooling system for the R&D coil.
S. Hamaguchi et al. / Fusion Engineering and Design 81 (2006) 2617–2621
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Fig. 2. Performance of the cold compressors under the various conditions.
Generally speaking, cold compressors enter the surge region in the case of low mass flow or high pressure ratio, while they enter the choke region in the case of high mass flow or low pressure ratio. Since a surge causes an intense oscillation of pressure, it is very dangerous in particular. The cold compressors should be operated as the surge region is avoided. It is necessary
to comprehend the surge region of cold compressors exactly for stable and safe operations [8]. So, the performance of the cold compressors was examined under various pressure ratios and reduced mass flow. Fig. 2 shows a map of the performance of the cold compressors under the various conditions. The left figure is the experimental results of the 1st cold compressor and the right one is that of the 2nd cold compressor. Open marks express working points of the cold compressors. Solid lines are actual surge lines obtained from the present experiments, while dotted lines are nominal surge lines estimated from preliminary examinations with air at room temperature. It was found that the actual surge lines shifted to the left with respect to the estimated surge lines. Therefore, the wide area on the
Fig. 3. Schematic of cooling system in R&D test facility to generate subcooled helium.
Fig. 4. Relationship between rotation speed of 1st cold compressor and pressure/pressure ratio.
3. Experimental results 3.1. Operation area
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Fig. 5. History of performance of the cold compressors during an automatic operation.
map can be used in order to operate the cold compressors stably and safely. In the present examinations, both cold compressors have never entered the surge region or the choke region simultaneously. It means that the area for stable operation can be enlarged if the relationship of the rotation speed between the 1st cold compressor and the 2nd cold compressor is optimized. 3.2. Automatic control Cold compressors should be operated without a surge in order to avoid machine trips and damages. So, automatic operations were performed to control the cold compressors within a stable area on the map [9] shown in Fig. 2. Fig. 3 shows a schematic of a subcooling system for the R&D test facility. The saturated pressure in the saturated helium bath as the inlet pressure of the 1st cold compressor, Pin, and the pressure between the 1st cold compressor and the 2nd cold compressor, Pm, were measured throughout operations to comprehend the status of the subcooling system. These pressures and mass flow could be regulated by rotation speeds of cold compressors, a heater in the saturated helium bath and a bypass valve. In the present subcooling system, the rotation speed of the 2nd cold compressor was linked with that of the 1st cold compressor to simplify operations. Relationships between a rotation speed of the 1st cold compressor and these pressures during steady state operations are plotted in Fig. 4. The pressure of a
saturated helium bath decreases monotonously, as the rotation speed of the 1st cold compressor increases. In the present automatic operations, the suitable pressure of the saturated helium bath which was calculated from the rotation speed of the 1st cold compressor using the relationship in Fig. 4, was regulated by the bypass valve. Fig. 5 shows the history of status of the cold compressors during an automatic operation. The operation time for cooling down or warming up was 20 min. The automatic operation could be completed within a stable area on the map. The constant rotation speeds of both cold compressors were used during the steady state operation, because the subcooling system became unstable easily while the rotation speed was varied. Stable operations of the cooling system for the helical coils will be expected to apply these control methods to the cooling system for the helical coils.
4. Conclusions The performance of the cold compressors was tested in the cooling system of the R&D test facility to improve the cooling system of the helical coils. The actual surge line of the cold compressors was comprehended. As a result, we can use a wide range to operate the cold compressors stably. It is also expected that the range will be enlarged if the relationship of the rotation speed between the 1st cold compressor and the 2nd cold compressor is optimized. The cold compressors could
S. Hamaguchi et al. / Fusion Engineering and Design 81 (2006) 2617–2621
be operated stably during cooling down and warming up with the present automatic method in which the saturated bath pressure was regulated by the bypass valve.
Acknowledgements The authors would like to thank Mr. S. Yoshinaga, Mr. K. Kurihara and Mr. T. Shinba for their useful suggestions and also appreciate the great helps of Mr. S. Moriuchi, Mr. H. Sekiguchi and Mr. H. Kitano to operate the cooling system of the R&D test facility. The present work was partially supported by the Ministry of Education, Science, Sports and Culture of Japan, Grant-in-Aid for Science Research, 14750151.
References [1] A. Iiyoshi, A. Komori, A. Ejiri, M. Emoto, H. Funaba, M. Goto, et al., Overview of the large helical device project, Nucl. Fusion 39 (9Y) (1999) 1245–1256.
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[2] O. Motojima, H. Yamada, N. Ashikawa, M. Emoto, H. Funaba, M. Goto, et al., Recent development of LHD experiment, J. Plasma Fusion Res. 5 (2003) 22–27. [3] S. Imagawa, N. Yanagi, T. Mito, H. Chikaraishi, S. Hamaguchi, H. Sekiguchi, et al., Excitation properties and cryogenic stability of helical coils for the LHD, IEEE Trans. Appl. Supercond. 11 (2001) 1889–1892. [4] S. Hamaguchi, S. Imagawa, N. Yanagi, Y. Hishinuma, T. Mito, K. Takahata, et al., Thermal hydraulic characteristics of superconducting coil cooled by subcooled He I, IEEE Trans. Appl. Supercond. 14 (2) (2004) 1439–1442. [5] S. Imagawa, N. Yanagi, Y. Hishinuma, T. Mito, K. Takahata, H. Chikaraishi, et al., Results of stability test in subcooled helium for the R&D coil of the LHD helical coil, IEEE Trans. Appl. Supercond. 14 (2) (2004) 1511–1514. [6] B. Gravil, B. Jager, Experimental with a large scale He II refrigeration system at Tore Supra, Adv. Cryog. Eng. 43 (1998) 571–579. [7] A. Martinez, A.L. Klebaner, J.N. Makara, J.C. Theilacker, Surge recovery techniques for the TEVATRON cold compressors, Adv. Cryog. Eng. 51 (2006) 1384–1391. [8] A. B´ezaguet, P. Lebrun, L. Tavian, Performance assessment of industrial prototype cryogenic helium compressors for the Large Hadron Collider, in: Proceedings of ICEC17, 1999, pp. 145–148. [9] H. Asakura, A. K¨undig, Control considerations of multi stage cold compression systems in large helium refrigeration plants, in: Proceedings of ICEC19, 2003, pp. 681–686.