Centrifugal Cold Compressors Research Articles

Numerical Simulation of the Influence of Tip Clearance on the Performance of Centrifugal Cold Compressor

To improve the efficiency of the Experimental Advanced Superconducting Tokamak (EAST) subcooled helium cryogenic system, a centrifugal cold compressor (CC) was investigated as an alternative to the oil ring pump. A numerical simulation method was applied to analyse variations in CC performance parameters, pressure ratio and polytropic efficiency. Results indicated that the performance curve of the CC and warm compressor (WC) were similar. The tip clearance at the design point was 0.2 mm and increased to 0.231 mm when the CC was operated at 4 K. This increase in tip clearance resulted in a 0.86% drop in the pressure ratio and a 0.54% drop in polytropic efficiency. To verify the simulation model, experiments were done using cold nitrogen.

Robust control of active magnetic bearing systems for helium centrifugal cold compressors

Centrifugal cold compressors, now being developed at Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, are used to pump gaseous helium from saturated liquid helium tank to obtain super-fluid helium in cryogenic refrigeration systems. Active magnetic bearings (AMB) are replacing traditional oil-fed bearings as the optimal supporting assembly for cold compressors because of their many advantages. In this paper, five degrees of freedom AMBs are developed for the centrifugal cold compressor application. Considering the rotational speed is high, causing the nominal plant model to deviate materially from the precise mathematical model, robust H∞ control methods are proposed to control AMB systems. Experimental results show that H∞ controllers guarantee good dynamic and steady state performance in the closed-loop system at the rotation speed of 50 000 rpm.

Development of a cryogenic performance experiment system for centrifugal cold compressors

Cold compressors are key components in large scale superfluid helium refrigerators. A cryogenic performance experimental system for centrifugal cold compressors is under development; this mainly consists of a warm compressor station, cold compressors, valves, a cold box, a sub-cooling heat exchanger and a sub-atmospheric heater. The flow scheme has been specially designed so that cold compressors with a flow rate less than 30 g/s and inlet temperatures between 3 K and 20 K can be tested. An innovative sub-atmospheric heater has been designed to guarantee good heat transfer efficiency between the helium gas and the wall of the tube. The design and fabrication of the cold box and cold compressors have been completed. The commissioning of the system is foreseen in the near future.

Determination of critical parameters in the EAST subcooled helium system

The cryogenic system at the Experimental Advanced Superconducting Tokamak (EAST) is designed for a capacity of 1050 W/3.5 K + 200 W/4.5 K + 13 g/s liquid helium+ 13 kW/80 K. The 3.5 K subcooled helium is obtained from an oil ring pump. The cryogenic system is now being upgraded with a centrifugal cold compressor that should reduce the helium-tank pressure and temperature. This article discusses the determination of critical parameters for the cold compressor, including the efficiency of heat exchange, the location of the thermal anchor, and the length of the tip clearance between the impeller and shroud. Developing the centrifugal cold compressor is a significant aspect of the EAST cryogenic system upgrades, and the preliminary tests reported herein demonstrate the promise of the retrofit project.

Structure design and simulation research of active magnetic bearing for helium centrifugal cold compressor

Helium centrifugal cold compressors are utilized to pump gaseous helium from saturated liquid helium tank to obtain super-fluid helium in cryogenic refrigeration system, which is now being developed at TIPC, CAS. Active magnetic bearing (AMB) is replacing traditional oil-fed bearing as the optimal supporting assembly for cold compressor because of its many advantages: free of contact, high rotation speed, no lubrication and so on. In this paper, five degrees of freedom for AMB are developed for the helium centrifugal cold compressor application. The structure parameters of the axial and radial magnetic bearings as well as hardware and software of the electronic control system is discussed in detail. Based on modal analysis and critical speeds calculation, a control strategy combining PID arithmetic with other phase compensators is proposed. Simulation results demonstrate that the control method not only stables AMB system but also guarantees good performance of closed-loop behaviour. The prior research work offers important base and experience for test and application of AMB experimental platform for system centrifugal cold compressor.

5-year operation experience with the 1.8 K refrigeration units of the LHC cryogenic system

Since 2009, the Large Hadron Collider (LHC) is in operation at CERN. The LHC superconducting magnets distributed over eight sectors of 3.3-km long are cooled at 1.9 K in pressurized superfluid helium. The nominal operating temperature of 1.9 K is produced by eight 1.8-K refrigeration units based on centrifugal cold compressors (3 or 4 stages depending to the vendor) combined with warm volumetric screw compressors with sub-atmospheric suction. After about 5 years of continuous operation, we will present the results concerning the availability for the final user of these refrigeration units and the impact of the design choice on the recovery time after a system trip. We will also present the individual results for each rotating machinery in terms of failure origin and of Mean Time between Failure (MTBF), as well as the consolidations and upgrades applied to these refrigeration units.

Numerical Study of Unsteady Flow in Centrifugal Cold Compressor

In helium refrigeration system, high-speed centrifugal cold compressor is utilized to pumped gaseous helium from saturated liquid helium tank at low temperature and low pressure for producing superfluid helium or sub-cooled helium. Stall and surge are common unsteady flow phenomena in centrifugal cold compressors which severely limit operation range and impact efficiency reliability. In order to obtain the installed range of cold compressor, unsteady flow in the case of low mass flow or high pressure ratio is investigated by the CFD. From the results of the numerical analysis, it can be deduced that the pressure ratio increases with the decrease in reduced mass flow. With the decrease of the reduced mass flow, backflow and vortex are intensified near the shroud of impeller. The unsteady flow will not only increase the flow loss, but also damage the compressor. It provided a numerical foundation of analyzing the effect of unsteady flow field and reducing the flow loss, and it is helpful for the further study and able to instruct the designing.

Helium Subcooling System for LHD Helical Coils

The helical coils in the Large Helical Device (LHD) are large-scale superconducting magnets for heliotron plasma experiments. The cooling system of the coils was upgraded in 2006 to improve the cryogenic stability of the coils using subcooled helium as the coolant. In the present study, characteristics of the upgraded cooling system have been investigated and the steady-state operating method in which the subcooled helium of 50 g/s at 3.2 K is supplied stably to the coils has been developed. The supplied helium is subcooled via a heat exchanger in a saturated helium bath. The bath pressure and the temperature are reduced by a series of two centrifugal cold compressors. Based on the measured characteristics of the subcooling system, the optimization of the operating method has been performed using an automatic control of the mass flow rate through the cold compressors by the heater. Consequently, the designed mass flow rate and temperature were obtained and stable long-term operations have been achieved. The improvement of the cryogenic stability was also confirmed and the maximum average current of three blocks of the coils has reached up to 11.833 kA.

Operation and Control of Helium Subcooling System of LHD Helical Coils During Change of Rotational Speed of Cold Compressors

Helical coils of the Large Helical Device (LHD) are large scale superconducting magnets for fusion plasma experiments. The cooling system was upgraded in 2006 to improve the cryogenic stability of the coils by subcooling supplied helium. After the upgrade, the subcooled helium of designed mass flow rate of 50 g/s at designed temperature of 3.2 K has been supplied to the coils stably during steady state subcooling operations. The supplied helium is subcooled at a heat exchanger in a saturated helium bath. A series of two centrifugal cold compressors are utilized in the subcooling system to reduce the bath pressure and temperature. It is necessary to operate the cold compressors within a range of appropriate mass flow rate in order to escape from surge and choke. When the rotational speed of the cold compressors increased/decreased, the mass flow rate of the cold compressors varied. Thus, it is important to comprehend the transient performance of the cold compressors in those cases and to control the mass flow rate for the purpose of the safe operations. In the present study, the characteristics of the system were investigated and automatic operating methods with a heater in the bath and a bypass valve were developed during the change of the rotational speed of the cold compressors. Consequently, the reliable subcooling operations of the system have been achieved over 6,000 hours for three years.