CERN's new project, the Large Hadron Collider (LHC), will use superfluid helium as coolant for its high-field superconducting magnets and therefore require large capacity refrigeration at 1.8 K. This may only be achieved by subatmospheric compression of gaseous helium at cryogenic temperature. To stimulate development of this technology, CERN has procured from industry prototype Cold Compressor Units (CCU). This unit is based on a cryogenic axial-centrifugal compressor, running on ceramic ball bearings and driven by a variable-frequency electrical motor operating under low-pressure helium at ambient temperature. The machine has been commissioned and is now in operation. After describing basic constructional features of the compressor, we report on measured performance

Brunovsky, I

Decker, L

Lebrun, P

Löhlein, K

Schustr, P

Tavian, L

Vins, M

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsLHC Hadron Collider LHC Project Report 22A Cryogenic Axial-Centrifugal Compressor For Superfluid Helium RefrigerationL. Decker, K. Löhlein, P. Schustr*, M. Vin_*, I. Brunovsky?, L. Tucek?, Ph. Lebrun? and L. Tavian?AbstractCERN's new project, the Large Hadron Collider (LHC), will usesuperfluid helium as coolant for its high-field superconducting magnetsand therefore require large capacity refrigeration at 1.8 K. This may onlybe achieved by subatmospheric compression of gaseous helium atcryogenic temperature. To stimulate development of this technology,CERN has procured from industry prototype Cold Compressor Units(CCU). This unit is based on a cryogenic axial-centrifugal compressor,running on ceramic ball bearings and driven by a variable-frequencyelectrical motor operating under low-pressure helium at ambienttemperature. The machine has been commissioned and is now inoperation. After describing basic constructional features of thecompressor, we report on measured performance.Linde Kryotechnik, CH-8422 Pfungen, Switzerland* ATEKO,Resslova 13, CS-50010 Hradec Králové, Czech Republic? PBS, CS-59512 Velká Bite_, Czech Republic? LHC Division, CERN, CH-1211 Geneva 23, SwitzerlandICEC 16/ICMC, Kitakyushu, Japan, 20-24 May 1996Administrative SecretariatLHC DivisionCERNCH -1211 Geneva 23SwitzerlandGeneva, 12 July, 1996A Cryogenic Axial-Centrifugal Compressor For Superfluid Helium RefrigerationL. Decker, K. Löhlein, P. Schustr*, M. Vin_*, I. Brunovsky“, L. Tucek“, Ph. Lebrun† and L. Tavian†Linde Kryotechnik, CH-8422 Pfungen, Switzerland* ATEKO,Resslova 13, CS-50010 Hradec Králové, Czech Republic“ PBS, CS-59512 Velká Bite_, Czech Republic† LHC Division, CERN, CH-1211 Geneva 23, SwitzerlandCERN's new project, the Large Hadron Collider (LHC), will use superfluidhelium as coolant for its high-field superconducting magnets and therefore requirelarge capacity refrigeration at 1.8 K. This may only be achieved bysubatmospheric compression of gaseous helium at cryogenic temperature. Tostimulate development of this technology, CERN has procured from industryprototype Cold Compressor Units (CCU). This unit is based on a cryogenic axial-centrifugal compressor, running on ceramic ball bearings and driven by avariable-frequency electrical motor operating under low-pressure helium atambient temperature. The machine has been commissioned and is now inoperation. After describing basic constructional features of the compressor, wereport on measured performance.INTRODUCTIONCERN's new project, the Large Hadron Collider (LHC) [1] now under construction, will use superfluidhelium as coolant for its high-field superconducting magnets and therefore require large capacityrefrigeration at 1.8 K. This may only be achieved by subatmospheric compression of gaseous helium atcryogenic temperature. To stimulate development of this technology, CERN has procured from Linde,with ATEKO and PBS as subcontractors, a prototype Cold Compressor Unit (CCU) with a nominal flow-rate of 18 g/s @ 1 kPa inlet pressure, with a pressure ratio of 3 and with an isentropic efficiency betterthan 60 %. These specifications also meet the need for a booster stage enabling to triple the capacity of anexisting warm pumping unit (WPU) which provides 1.8 K refrigeration to the CERN cryogenic teststation, an upgrade described in a companion paper [2]. The compliance of the system allows to handlelow flow continuously varying over the range from 6 to 18 g/s, with a pressure ratio of 1 to 3 and with anisentropic efficiency better than 0.5. For a given mass-flow, the gas inlet temperature may vary dependingupon the number of test stations in operation. Table 1 summarizes the main specifications of the CCU.Table 1 Main specifications of CCUCapacity Nominal Low-flowHelium flow-rate [g/s] 18 6 to 18Suction Pressure [kPa] 1£  1Discharge pressure [kPa] 3 1 to 3Pressure ratio 3 -Helium suction temperature [K] 3.5 to 4.4 5.3 to 3.5Isentropic efficiency‡  0.6 ‡  0.5BASIC CONSTRUCTIONAL FEATURES    General LayoutThe main constituent of the CCU is a cold box which houses all components at low temperature. On thetop flange, the cold compressor is mounted with vertical axis. A cartridge design allows to remove therotating part of the compressor without breaking the vacuum of the CCU enclosure. Two bayonetconnections to the compressor inlet and outlet constitute the interface with a separate valve box whichcontains two insulation valves and a bypass valve. Figure 1 shows the general layout of the CCU.Figure 1 General layout of the CCU    Cold compressor description   [3]The axial-centrifugal unshrouded wheel with a diameter of 118 mm is fixed to the bearing shaft by atitanium tube giving reduced heat inleaks. Ceramic ball bearings and a variable-frequency electricalmotor drive operate under low-pressure helium at ambient temperature. Motor drive, bearings and wheelcompose the cartridge. The cold part constituted by the diffuser and volute remains inside the CCU coldbox and is supported from the top flange by a thin-walled stainless steel tube.  InstrumentationEfficiency assessment of the cold compressor requires accurate temperature and pressure measurementsat inlet and outlet. Two calibrated germanium temperature sensors and two absolute pressure gauges havebeen chosen to fulfill this requirement. Other standard pressure and temperature measurements allow tomonitor purge, cooldown, pumpdown and warmup. To avoid risk of air inleaks, all warm componentswhich are working below atmospheric pressure are placed in a vacuum guard.COMMISSIONING PROGRESSCommissioning was performed in two steps. During August 1995, pressure and leak tightness tests aswell as piping integrity were checked. Cooldown and warmup were also performed. Initial problems ofelectrical breakdown in very-low pressure helium in the motor housing precluded performance tests.After improving electrical insulation, a second test campaign took place in November 1995 to completecommissioning.RESULTS    Cooldown and warmupThese two operations take a few hours each. Cooldown is produced by the helium flow pumped by theWPU in series with the CCU. During this phase, the two insulation valves are open, the drive speed isfixed to 100 Hz and the bypass valve controls the pressure difference between inlet and outlet of thecompressor. For warmup, the CCU is completely stopped. A small flow of warm helium gas is injected atthe CCU inlet and discharged at the outlet in a recovery line.   Static heat inleaksHeat inleaks were assessed by inlet-outlet temperature difference measurement with the compressorstopped and for different flow rates, yielding a value of 50 W. Due to an increased thickness of the volutesupport tube during manufacturing (to avoid uncertainties from assembly and unexpected problemsduring tests), the heat load is 16 W higher and thus affects the global isentropic efficiency.    Nominal and low-flow operationIn steady-state operation, the compressor controls its suction pressure Pin by adjusting the motor drivespeed N. The inlet pressure then stays stable within 10 Pa, better than the required value of 50 Pa. For agiven mass-flow &m , the volumetric characteristic of the WPU fixes the CCU outlet pressure andconsequently the pressure ratio of the cold compressor. Several measurements were done varying massflow from 6 to 18 g/s and inlet temperature Tin from 3.5 K to 5.3 K. Table 2 summarized the main resultsand figure 2 shows the measured points on the calculated compression field. The operating conditions ofturbomachines are expressed in terms of reduced mass-flow &mr  and speed Nr, defined as follows:& &mr mTinTinPinPin00= × × and Nr N TinTin0= × with subscript 0 for design conditionsTable 2 Nominal and low-flow measurementsPoint &m[g/s]Tin[K]Pin[kPa]Tout[K]PressureratioIsentropicefficiencyN[Hz]Nr[Hz]&mr[g/s]1 8.1 4.42 1 6.55 1.25 0.19 228 203 9.102 10.2 3.85 1 6.09 1.57 0.34 265 252 10.703 12.2 4.68 1 7.74 1.92 0.46 353 306 14.114 14.2 4.34 1 7.60 2.27 0.52 376 338 15.815 16 3.69 1 6.80 2.57 0.54 367 358 16.436 18 3.46 1 6.75 2.95 0.57 381 384 17.907 18 4.17 1 8.16 2.95 0.56 430 394 19.658 18.2 3.31 1 6.52 2.98 0.56 375 385 17.709 18 2.87 0.95 6.03 3.40 0.57 366 404 17.16  Isentropic efficiencyThe measured isentropic efficiency is slightly below specification. The measurement error, whichdepends on those on temperature and pressure, is estimated to –  0.02. The higher static heat inleaks to thecold compressor, which account for a loss of efficiency between 0.03 and 0.06, depending upon theirexact location, may thus explain the difference to the specified efficiency. Possible corrective actions arebeing investigated by the manufacturer. At low flow, the relative proportion of the static heat loads in theloss of efficiency increases and below 14 g/s, it is no longer possible to obtain the specified value of 0.5.   Stall line measurementBy adjusting a bypass valve on the WPU, it was possible to run the cold compressor at constant pressureratio with smaller mass flow. In this way, one can reach and explore the stall limit of the wheel. Figure 2shows the good agreement between the calculated and measured stall limits.267 Hz300 Hz333 Hz367 Hz400 Hz417 Hz11.522.533.546 8 10 12 14 16 18 20 22Reduced mass-flow [g/s]Pres Measurement pointsNominalIso-Nr calculationStall calculationStall measurement123456 789Figure 2 CCU reduced compression field (parameter is reduced speed Nr).   Endurance testIn the course of the second test campaign, the CCU was operated in fully automatic mode, for a total of50 hours without interruption. This test gives confidence in the future use of the CCU but more operatingexperience is required in order to qualify the long-term reliability of the bearings and motor drive.Identical motor and bearings installed in a test rig simulating operating conditions at Linde’s have so fargone through more than 6000 hours of continuous operation without any problem.CONCLUSIONWith the exception of a slightly lower than specified isentropic efficiency, the CCU supplied by Linde,ATEKO and PBS fulfills the technical requirements. Studies to improve its performance are under way.Moreover, additional tests are in progress to qualify long-term performance and reliability of the driveand bearing system.ACKNOWLEDGMENTSThe authors wish to acknowledge contributions of A. Bézaguet, S Claudet, D. Lavielle, and B. Vulliermeto the preparation and execution of the cold compressor tests.REFERENCES1 The LHC Study Group, The Large Hadron Collider, Conceptual Design, CERN ReportAC/95-05(LHC) (1995).2 Benda, V., Dauvergne, J.P., Haug, F., Knoops, S., Lebrun, Ph., Sergo, V., Tavian, L., andVullierme, B., Upgrade Of The CERN Cryogenic Station for Superfluid Helium Testing ofPrototype LHC Superconducting Magnets, paper presented at this conference.3 Schustr, P., Vin_, M., Brunovsky, I. and Tucek, L., Helium Low Temperature compressor, Proc.Kryogenika 96, Prague (1996)

A cryogenic axial-centrifugal compressor for superfluid helium refrigeration

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