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In a 4K Gifford-McMahon (GM) cryocooler, the performance of the second stage is strongly dependent on the efficiency of the second stage regenerator. In order to improve the efficiency of the second stage regenerator, the internal dynamic parameters are analyzed by numerical simulation. It is found that the performance of a 4K GM cryocooler can be improved by shifting the temperature profile in the second stage regenerator to a higher level. The temperature profile in the second stage regenerator with an artificially reduced heat capacity of the regenerator material at the warm end is analyzed. Based on the simulation results, a novel and simple way, which is to replace the regenerator material at the warm end with a material having a lower heat capacity, is proposed. By reducing the heat capacity to 20 % of lead, the PV power is increased by 7.9 W at the first stage and 1.8 W at the second stage. Correspondingly, the cooling capacity is increased by 9.0 W at the first stage and 0.13 W at the second stage. However, if the heat capacity is further reduced to 10 % of lead, the effect is reduced. The P-V power is increased by only 1.9 W at the first stage and 0.1 W at the second stage. The enthalpy flux from the regenerator is increased by 0.7 W at the first stage and 0.1 W at the second stage. Accordingly, the cooling capacity is increased by only 2.2 W at the first stage and 0.01 W at the second stage. The simulation results will be reported in this paper. [ABSTRACT FROM AUTHOR]

The compressor system used for the Jefferson Lab (JLab) 12 GeV upgrade, also known as the CHL-2 compressor system, incorporates many design changes to the typical compressor skid design to improve the efficiency, reliability and maintainability from previous systems. These include a considerably smaller bulk oil separator design that does not use coalescing elements/media, automated control of cooling oil injection based on the helium discharge temperature, a helium after-cooler design that is designed for and promotes coalescing of residual oil and a variable speed bearing oil pump to reduce oil bypass. The CHL-2 helium compression system has five compressors configured with four pressure levels that supports the three pressure levels in the cold box. This paper will briefly review several of these improvements and discuss some of the recent commissioning results. [ABSTRACT FROM AUTHOR]

A one-dimensional theoretical model is developed to describe the transient state phenomena in a low cooling standing wave Thermoacoustic Refrigerator. The model is based on the Linear Theory of Thermoacoustics. It uses Implicit Finite Difference method to calculate the temporal evolution of temperature and steady state temperature distribution in the refrigerator stack and the resonator. The cold temperatures predicted by the model are compared with those obtained by techniques given in literature, and show a very good match. Due to simplistic assumptions in the model and its one-dimensional nature, the cooldown rates are shown to be very fast compared to other experimental findings in literature. It is also seen that the resonator takes a long time to cool down compared to the stack. [ABSTRACT FROM AUTHOR]

The Pulse tube refrigerator (PTR) driven by a Thermoacoustic prime mover (TAPM) can completely eliminate mechanically moving components and serve as a simple and reliable refrigeration system. This uses environmentally friendly working fluids and can utilize waste heat for its operation of converting thermal energy into acoustic energy. The conversion efficiency depends on the design of TAPM as well as the working fluid. The performance of TAPM can be made better by the proper design of TAPM and by the right choice of the working fluid. The Prandtl number (Pr) of the latter will indicate the extent to which thermoacoustic effects can be expected from the prime mover. The lower the Prandtl number of the working fluid, the higher is the thermoacoustic effect. The Prandtl numbers between 0.2 and 0.66 can be realized with gas mixtures of heavy and light gases. Here, we present the experimental studies carried out on the TAPM with different gas mixtures to understand the effect of Prandtl number on the performance of these systems. The studies show that the best performance of TAPM is seen for the gas mixture with the lowest Pr. Although pure argon leads to the highest pressure amplitude, it needs the highest temperature difference across the stack and hence is not suitable for practical applications. The experimental studies indicate that the gas mixture of 60% helium and 40% argon leads to the best performance of the system. [ABSTRACT FROM AUTHOR]

Fermi National Accelerator Laboratory (FNAL) is in the process of re-commissioning a vintage CTI-4000 liquid helium coldbox, initially supplied by CTI-Cryogenics/Sulzer to Los Alamos in 1979. The coldbox was originally designed as a liquid helium refrigerator with capacity of ~1200 W at nominal 4-K. The process utilized LN2 precooling, in-series operation of two centrifugal gas bearing turboexpanders and final Joule-Thomson (J-T) expansion. At FNAL, the coldbox will be utilized as a liquefier to support 2-K operations. A process model was developed to aid in the upgrade decisions and used to determine the nominal capacity of the liquefier. Capacity upgrades are achieved by safely utilizing the internal LN2 precooler, the addition of a 3-inch reciprocating wet expansion engine and increasing the overall process pressure by recertifying two limiting pressure vessels to a higher MAWP. [ABSTRACT FROM AUTHOR]

Two helium refrigerators, each rated for 3.5 KW at 20 K, are used at NASA’s Johnson Space Center (JSC) in Building No. 32 to provide cryogenic-pumping within two large thermal-vacuum chambers. These refrigerators were originally commissioned in 1996. New changes to the controls of these refrigerators were recently completed. This paper describes some of the control issues that necessitated the controls change-over. It will describe the modifications and the new process control which allows the refrigerators to take advantage of the Ganni Cycle “floating pressure” control technology. The controls philosophy change-over to the floating pressure control technology was the first application on a helium gas refrigeration system. Previous implementations of the floating pressure technology have been on 4 K liquefaction and refrigeration systems, which have stored liquid helium volumes that have level indications used for varying the pressure levels (charge) in the system for capacity modulation. The upgrades have greatly improved the performance, stability, and efficiency of these two refrigerators. The upgrades have also given the operators more information and details about the operational status of the main components (compressors, expanders etc.) of the refrigerators at all operating conditions (i.e. at various loads in the vacuum chambers). The performance data of the two systems, pre and post upgrading are presented. [ABSTRACT FROM AUTHOR]

The constant pressure ratio process, as implemented in the floating pressure—Ganni cycle, is a new variation to prior cryogenic refrigeration and liquefaction cycle designs that allows for optimal operation and design of helium refrigeration systems. This cycle is based upon the traditional equipment used for helium refrigeration system designs, i.e., constant volume displacement compression and critical flow expansion devices. It takes advantage of the fact that for a given load, the expander sets the compressor discharge pressure and the compressor sets its own suction pressure. This cycle not only provides an essentially constant system Carnot efficiency over a wide load range, but invalidates the traditional philosophy that the (‘TS’) design condition is the optimal operating condition for a given load using the as-built hardware. As such, the Floating Pressure-Ganni Cycle is a solution to reduce the energy consumption while increasing the reliability, flexibility and stability of these systems over a wide operating range and different operating modes and is applicable to most of the existing plants. This paper explains the basic theory behind this cycle operation and contrasts it to the traditional operational philosophies presently used. [ABSTRACT FROM AUTHOR]

As part of the planned Jefferson Laboratory’s electron accelerator (CEBAF) power upgrade, ten additional superconducting RF cryomodules will be added to its accelerator linacs. Although physically the same size as each of the original 40 linac cryomodules, each new cryomodule will have approximately 4 times the acceleration power. To support the additional cryomodule heat loads generated, the existing 2 K, 4600 W Central Helium Liquefier (CHL) plant capacity will be doubled to a total of 9200 W at 2 K plus 24,000 W at 35 K for shield loads. The specified base line process cycle has been modeled after the laboratory’s “Ganni Helium Cycle” process technology. In addition, a fourth physics experimental “Hall D” will be constructed which will have an additional stand alone 200 W at 4 K helium cryogenic plant. In October of 2008, Jefferson Laboratory (JLab) received approval for project “Critical Decision 3” construction phase status from the US Department of Energy. An overview of the planned cryogenic systems and current design baseline as it relates to operations will be presented. [ABSTRACT FROM AUTHOR]

In 2007, we developed a prototype refrigerator with a small turbo-expander to provide adequate cooling power (2 kW at 70 K) for HTS (High Temperature Superconductor) power machines. The reverse-Brayton cycle with neon gas as a working fluid was adopted in the refrigerator. The prototype refrigerator does not have enough COP (Coefficient of Performance) for practical HTS applications, and the purpose of this study is to research the information required for designing a new neon refrigerator with improved performance. We take the same refrigeration cycle and working fluid as the prototype one adopted, but a lower process pressure of 1 MPa/0.5 MPa is chosen instead of 2 MPa/1 MPa. The lower process pressure is required by the turbo-compressor design and the refrigeration process is analyzed by using a newly developed process simulator. Also, a heat-exchanger configuration is studied to make the refrigerator size small. The new refrigerator will have a cooling power of 2.5 kW at 65 K, and a COP of 0.06 at 80 K. [ABSTRACT FROM AUTHOR]

The Qweak experiment at Jefferson Lab (JLab) is a 3000 W hydrogen target scheduled for the summer of 2010 and running for two years until the planned shut-down for 12 GeV. The End Station Refrigerator (ESR) supports the three experiment halls, two of which may normally have a hydrogen target. The refrigerator for the ESR is a CTI/Helix 1500 W 4.5-K refrigerator nominally capable of supporting a 1250 W target load at 12 bar and 15-K (plus 1100 W of 4.5-K refrigeration). As such, this refrigerator is not capable of supporting the Qweak experiment target load in its present condition. Additionally, since the installation of an ambient air vaporizer for a single use, two week run duration of a high target load in the summer of 2003 there has been a consistent usage of the Central Helium Liquefier’s (CHL’s) 3 bar 4.5-K helium, supplied via an existing transfer-line to the ESR, for other high target loads. By the fall of 2004, it was apparent that this continued use of CHL’s supercritical helium was routinely being sought by the hall experimenters. As such, a method of refrigeration recovery was proposed to reduce the support required of CHL for these high target loads, including the anticipated Qweak experiment, while utilizing the recovered CHL refrigeration from the target to increase ESR’s 12 bar 15-K capacity. [ABSTRACT FROM AUTHOR]

It is not uncommon for helium refrigerators using liquid nitrogen (LN) pre-cooling to exhibit nitrogen usage significantly higher than estimated by the design. Often this is attributed to an underperforming 300 to 80-K helium-nitrogen heat exchanger(s) rather than the 300 to 80-K helium-helium heat exchanger(s), even though the return helium stream(s) exiting the cold box at the warm (ambient) end are significantly colder than the design goals. Furthermore, typically there is an specious emphasis on the size of the 300 to 80-K helium-nitrogen heat exchanger(s) to ensure the exiting nitrogen is as close to ambient temperature as possible, presumably to reduce the LN usage. However, in fact, it is the performance resulting from the size and the flow distribution of the 300 to 80-K helium-helium heat exchangers which has the prevalent influence on the nitrogen consumption. This paper examines the sensitivity of the key parameters in the design and operation of a refrigerator LN pre-cooler section affecting the LN consumption. [ABSTRACT FROM AUTHOR]

The flexure-bearing technology of Thales Cryogenics BV is being used in an extensive range of pulse-tube and Stirling cryocoolers. Compressors based on this flexure-bearing technology have a high reliability, long lifetime, and low vibrations. They are therefore the compressors of choice for high-end cryocoolers. Recently, the range of compressors has been extended with a larger compressor, capable of delivering more than 300 W of mechanical power. Apart from the above-mentioned characteristics, this particular compressor model has a high efficiency of more than 85% at its rated performance. In this paper, the design validation of this compressor is presented. This includes the verification of the individual loss mechanisms present in linear motor driven compressors. Furthermore, the potential of this compressor is demonstrated by its application in two cryocoolers. First the performance of a large, 15 W pulse tube (LPT9710) is presented. The combination of the large compressor with a newly designed pulse tube has resulted in a cooler with an efficiency of up to 15% of Carnot. Finally, measurements on a combination of the large compressor with a 20 mm Stirling cold finger are presented to demonstrate the potential performance of a—yet to be developed—large-capacity Stirling cold finger. [ABSTRACT FROM AUTHOR]

Sumitomo Heavy Industries, Ltd. (SHI) has been developing 4 K pulse tube cryocoolers for cooling MRI Magnet Systems, Small Superconducting Magnets, SQUIDs, X-ray detectors, etc. Since 2002, a joint team at SHI-Tanashi and SHI-APD Cryogenics has been developing high efficiency, high cooling capacity and high reliability 4 K pulse tube cryocoolers. Much effort has been expended to improve cooling capacity, reliability, and temperature stability. A typical cooling capacity of 1.2 W @ 4.19 K at the second stage and 50 W @ 44.8 K at the first stage has been achieved with 7.2 kW compressor input power. The pulse tube reliability and temperature stability have been improved by improvements in the valve design, and the system reliability has been improved by developing a new fail safe compressor. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The investigation on the performance of a traveling-wave thermoacoustic refrigerator driven by a self-made linear compressor is presented in this paper. The linear compressor is of moving-coil type, which has a moving coil, a piston with about 100 square centimeters cross section area, and a flexure bearing unit for piston support. The operating frequency can be adjusted to achieve a resonant status in order that the compressor can work with high electroacoustic efficiency. The thermoacoustic refrigerator operates on traveling-wave mode with acoustic power recovery. In the experiments, influence of different working frequencies on electroacoustic efficiency, lowest temperature, cooling power and COP is investigated. So far in the experiment with helium as working fluid, a lowest temperature of -29 °C is obtained, when the mean and oscillating pressures are 1.0MPa and 0.042MPa respectively and the temperature of room-temperature heat exchanger is kept around 15°C. And a maximum cooling power of about 28.5W@0°C is achieved under 1.5MPa mean pressure and 0.049MPa oscillating pressure. Besides, the performance of the linear compressor itself is also investigated, which is important for a reasonable evaluation of the refrigerator performance. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Large scale collider facilities will make intensive use of superconducting magnets, operating below 2.0 K. This dictates high-capacity refrigeration systems operating below 2.0 K. These systems, making use of cryogenic centrifugal compressors in a series arrangement with room temperature screw compressors will be coupled to a refrigerator, providing a certain power at 4.5 K. A first Air Liquide Cold Compression System (CCS) unit was built and delivered to CERN in 2001. Installed at the beginning of 2002, it was commissioned and tested successfully during year 2002. A series of four sets of identical CCS were then tested in 2004. Another set of four cryogenic centrifugal compressors (CCC) has been delivered to Thomas Jefferson National Accelerator Facility (JLAB) for the Spallation Neutron Source (SNS) in 2002. These compressors were tested and commissioned from December 2004 to July 2005. The experimental results obtained with these systems will be presented and discussed: the characteristics of the CCC will be detailed. The principles of control for the CCC in series will be detailed. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

An ongoing program at Brookhaven National Laboratory (BNL) consists of improving the efficiency of the Relativistic Heavy Ion Collider (RHIC) cryogenic system and reducing its power consumption. Phase I and II of the program addressed plant operational improvements and modifications that resulted in substantial operational cost reduction and improved system reliability and stability, and a compressor input power reduction of 2 MW has been demonstrated. Phase III, now under way, consists of plans for further increasing the efficiency of the plant by adding a load “wet” turbo-expander and its associated heat exchangers at the low temperature end of the plant. This additional stage of cooling at the coldest level will further reduce the required compressor flow and therefore compressor power input. This paper presents the results of the plant characterization, as it is operating presently, as well as the results of the plant simulations of the various planned upgrades for the plant. The immediate upgrade includes the changes associated with the load expander. The subsequent upgrade will involve the resizing of expander 5 and 6 to increase their efficiencies. The paper summarizes the expected improvement in the plant efficiency and the overall reduction in the compressor power. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

An analysis of the Claude form of an idealized helium liquefier for the minimum input work reveals the ‘Carnot Step’ for helium refrigerator cycles. As the ‘Carnot Step’ for a multi-stage polytropic compression process consists of equal pressure ratio stages; similarly for an idealized helium liquefier the ‘Carnot Step’ consists of equal temperature ratio stages for a given number of expansion stages. This paper presents the analytical basis and some useful equations for the preliminary examination of existing and new Claude helium refrigeration cycles. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

At CERN a new helium refrigeration plant specified for a cooling capacity of 800 W at 4.45 K, 4500 W between 60 K and 80 K and 4 g/s liquefaction simultaneously has been installed and commissioned. In a first operation phase, the plant will provide the refrigeration capacity for the cool down and the operation of the superconducting solenoid of the CMS experiment during the magnet tests at the surface of the LHC interaction point 5. On completion of the tests a part of the refrigeration plant will be dismantled and moved to its final underground position next to the CMS experimental cavern. This paper compiles the commissioning activities starting from the compressor station up to the intermediate cryostat interfacing the refrigerator cold box with the magnet thermosiphon cooling system. It reports the problems encountered and the lessons learned during the commissioning phase and summarizes the results of the refrigeration power measurements in the transient cool-down and the steady-state operation modes. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]