Showing papers in "Advances in cryogenic engineering in 2008"

Development of a 4.5 k pulse tube cryocooler for superconducting electronics

Abstract: Lockheed Martin's (LM) Advanced Technology Center (ATC) has developed a four stage pulse tube cryocooler (stirling-type pulse tube system) to provide cooling at 4.5 K for superconducting digital electronics communications programs. These programs utilize superconducting niobium integrated circuits [1, 2]. A prior ATC 4 stage unit has provided cooling to 3.8 K. [3] The relatively high cooling loads for the present program led us to a new design which improves the 4.5 K power efficiency over prior systems. This design includes a unique pulse tube approach using both He-3 and He-4 working gas in two compression spaces. The compressor utilizes our standard moving magnet linear motor, clearance seal and flexure bearing system. The system is compact, lightweight and reliable and utilizes our aerospace cooler technology to provide unlimited lifetime. The unit is a proof of concept, but the construction is at an engineering model level. Follow on activities for improvements of performance and more compact packaging and future production for ground based communication systems is anticipated.This paper presents the experimental results at various cooling conditions.Primary results are shown for HYPRES cooling requirements and data is also included at lower cooling loads that may be required for future space missions. The system provides a maximum of 42 mW @ 4.5 K and a no load temperature of 3 K.The majority of this work was subcontracted by HYPRES and funded by the Army and Navy. A small part of this effort to obtain data at lower cooling loads (1-10 mW @ 4.5 K) was funded by LM internal funds.

Calculated regenerator performance at 4 k with helium-4 and helium-3

Abstract: The helium-4 working fluid in regenerative cryocoolers operating with the cold end near 4 K deviates considerably from an ideal gas. As a result, losses in the regenerator, given by the time-averaged enthalpy flux, are increased and are strong functions of the operating pressure and temperature. Helium-3, with its lower boiling point, behaves somewhat closer to an ideal gas in this low temperature range and can reduce the losses in 4 K regenerators. An analytical model is used to find the fluid properties that strongly influence the regenerator losses as well as the gross refrigeration power. The thermodynamic and transport properties of helium-3 were incorporated into the latest NIST regenerator numerical model, known as REGEN3.3, which was used to model regenerator performance with either helium-4 or helium-3. With this model we show how the use of helium-3 in place of helium-4 can improve the performance of 4 K regenerative cryocoolers. The effects of operating pressure, warm-end temperature, and frequency on regenerators with helium-4 and helium-3 are investigated and compared. The results are used to find optimum operating conditions. The frequency range investigated varies from 1 Hz to 30 Hz, with particular emphasis on higher frequencies.

Stirling-Type Pulse Tube Cryocooler with 1KW of Refrigeration at 77K

Abstract: A large capacity Stirling-type pulse tube cryocooler has been successfully developed by Praxair Inc. Performance testing of both the prototype and initial production models of the pulse tube cryocooler has verified a refrigeration capacity of 1kW at 77K using a 20kW dual-opposed pressure wave generator from CFIC Inc. These results were obtained using nitrogen subcooling test methods. The cryocooler design incorporates sophisticated geometry to successfully minimize streaming other and loss mechanisms which have limited the performance of previous large pulse tube cryocooler designs. The intended application of this unit is for the commercial HTS (High Temperature Superconductivity) market. The current design will be deployed for field testing in Columbus, OH where reliability performance will be measured in an operating HTS application.

Cryogenic propellant boil-off reduction system

Abstract: Lunar missions under consideration would benefit from incorporation of high specific impulse propellants such as LH2 and LO2, even with their accompanying boil-off losses necessary to maintain a steady tank pressure This paper addresses a cryogenic propellant boil-off reduction system to minimize or eliminate boil-off Concepts to do so were considered under the In-Space Cryogenic Propellant Depot Project Specific to that was an investigation of cryocooler integration concepts for relatively large depot sized propellant tanks One concept proved promising—it served to efficiently move heat to the cryocooler even over long distances via a compressed helium loop The analyses and designs for this were incorporated into NASA Glenn Research Center's Cryogenic Analysis Tool That design approach is explained and shown herein Analysis shows that, when compared to passive only cryogenic storage, the boil-off reduction system begins to reduce system mass if durations are as low as 40 days for LH2, and 14 days

Cryogenic Design of the Katrin Source Cryostat

Abstract: The KATRIN experiment will measure the mass of electron antineutrinos with a sensitivity of 0.2 eV/c2 by the precise measurement of the tritium β spectrum. A key component is the Windowless Gaseous Tritium Source (WGTS), which will deliver 1011 β decay electrons per second. The WGTS consists in its centre of a 10 m long beam tube operated at 30 K, which is surrounded by a series of superconducting solenoids. Molecular tritium is injected through a central injection chamber and pumped at either beam tube end. The tritium density profile must have a stability of 10−3 to limit systematic errors, yielding stringent requirements on the beam tube temperature homogeneity (±30 mK) and stability (±30 mK/h). This shall be achieved with a design, where the thermal radiation heat from the vacuum pumps is almost entirely absorbed by LN2 and He coolers on the pump ports, while the beam tube temperature is stabilized with saturated Ne in a thermosiphon. Starting from a functional description, we explain the cryogenic de...

Damping of intrinsic temperature oscillations in a 4 k pulse tube cooler by means of rare earth plates

Abstract: Regenerative cryocoolers show oscillations of the refrigeration temperature that result from the periodic expansion of the working fluid (He) In case of cryogen-free operation of sensitive superconducting devices the temperature oscillations can be rather disturbing because of the associated variation of critical device parameters The oscillations can be damped by increasing the thermal mass attached to the cold end Here we report on the damping of temperature oscillations by making use of the high specific heat of rare-earth alloys, such as ErNi Tests were performed on an in-house made 4 K pulse tube cooler The damping is accomplished by ErNi plates of different thickness that were installed between the 4 K stage and the device mounting platform With a 36 mm thick ErNi plate the temperature oscillation at 4 K was reduced by a factor of 19 from 275 mK to 145 mK, which is sufficiently low to operate an AC Josephson voltage standard

Economic analysis of mixed-refrigerant cycle and nitrogen expander cycle in small scale natural gas liquefier

Abstract: Two types of natural gas liquefaction processes, mixed-refrigerant cycle and nitrogen expander cycle were simulated. Their process parameters were optimized and compared. Their economic characteristics were analyzed. Although the mixed-refrigerant liquefaction process is more complicated than nitrogen expander cycle, its energy consumption is only 46% of the nitrogen expander cycle. The operation costs of mixed-refrigerant process are lower than those of nitrogen expander cycle, so the process is more competitive. The energy consumption of the optimized mixed-refrigerant cycle reaches the level of propane pre-cooled mixed-refrigerant process, which is usually used in base-load natural gas liquefaction systems and is the lowest of the mixed-refrigerant process. The process is comparatively simple, consumes less energy and has economic benefits, so the mixed-refrigerant process is the preferred choice for small-scale natural gas liquefaction device.

Optimal Pulse-Tube Design Using Computational Fluid Dynamics

Abstract: Over the past few decades, the pulse-tube cryocooler has advanced from a curiosity to one of the most attractive systems for providing reliable cryogenic cooling; it is now used in aerospace, medical and superconductor applications. This technology development has been enabled by the simulation tools that are available for regenerator, compressor, and inertance tube design. However, a dedicated design tool for the pulse-tube component in a pulse-tube cryocooler and the associated flow transitions between the pulse tube and the regenerator and the pulse tube and the inertance network is not currently available.This paper describes the development of a two-dimensional, axisymmetric computational fluid dynamic (CFD) model of the pulse-tube and its associated flow transitions. The model is implemented in the commercial CFD package FLUENT. The CFD simulations are sufficient to calculate and delineate the various loss mechanisms; these are reported as a percentage of the acoustic power that is present at the co...

Characteristics of mgb2 sensor for detecting level of liquid hydrogen

Abstract: The superconductive magnesium diboride (MgB2) sensor for detecting the level of liquid hydrogen has attracted attention as a new senor that has high resolution and good reproducibility. The MgB2 sensor utilizes the difference in the electric resistance between the liquid and vapor phases of hydrogen. In this study, external-heating-type MgB2 sensors are fabricated by the in situ method, and their level-detecting characteristics, e.g., the heater input dependence and the linearity of the detected level, are studied by comparison with the actual level of liquid hydrogen in a optical cryostat. It is shown that the MgB2 sensor demonstrates good reproducibility, a short response time of about one second, no hysteresis, and accurate operation even at low liquid levels.

Using high temperature superconducting leads in a magnetic field

Abstract: HTS leads are increasingly used on superconducting magnets. In most cases the magnet leads are shielded from the magnetic field by iron or the magnet is actively shielded, so that the stray field at HTS leads is low. There are magnets where the HTS leads must be located in a magnetic field. The two general types of HTS leads that are commercially available are either leads fabricated from bulk HTS materials or leads fabricated from oriented HTS materials that have one or two planes of favorable current density. This paper will discuss how two types of leads are affected by magnetic field. The warm end temperature of the HTS lead is a key lead performance factor.

The performance of gas filled multilayer insulation

Abstract: The NASA Exploration Program is currently planning to use liquid oxygen, methane and hydrogen for propulsion in future spacecraft for the human exploration of the Moon and Mars. This will require the efficient long term, on-orbit storage of these cryogens. Multilayer insulation (MLI) will be critical to achieving the required thermal performance since it has much lower heat transfer than any other insulation when used in a vacuum. However, the size and mass constraints of these propulsion systems will not allow a structural shell to be used to provide vacuum for the MLI during ground hold and launch. One approach is to purge the MLI during ground hold with an inert gas which is then vented during launch ascent and on-orbit. In this paper, we report on experimental tests and modeling that we have done on MLI used to insulate a cryogenic tank. These include measurements of the heat transfer of gas filled insulation, evacuated insulation and during the transition in between.

Pressure drop of cable-in-conduit conductors with different void fraction

Abstract: A number of pressure drop measurements have been carried out at CRPP on single channel Cable-In-Conduit Conductors (CICC's) with two different cabling patterns, using water at room temperature. The cables have been compacted three times to reduce the void fraction in the range of 25–35%. The longitudinal bundle friction factor has been deduced from measurements of pressure drop and mass flow rate at each stage of compaction. The experimental results are compared with correlations proposed by Katheder [1] and others resulting from the porous media analogy model [2].

A low cost pressure wave generator using diaphragms

Abstract: The high cost of Pressure Wave Generators (PWGs) is a major barrier to the more widespread use of high-efficiency pulse tube and Stirling cryocoolers This paper describes the development and testing of a low-cost industrial-style PWG which employs metal diaphragms The use of diaphragms removes the need for rubbing or clearance seals, and eliminates contamination problems by hermetically separating the gas circuit and the lubricated driving mechanism A conventional low-cost electric motor is used for power input, via a novel high-efficiency kinematic linkage A first prototype of the diaphragm PWG produced 32 kW of PV power with a measured electro-acoustic efficiency of 72% Accelerated testing predicts a diaphragm life time in excess of 40,000 hours An additional advantage of the use of diaphragms is the ability to directly cool the gas in the compression space This eliminates or significantly reduces the requirement for an after cooler, and further decreases the cost of the whole cryocooler system A pulse tube cryocooler has been successfully run at Industrial Research Ltd to 59K with the diaphragm PWG and no aftercooler Another pulse tube cryocooler with the diaphragm PWG is undergoing development at Cryomech, the results of which will be given in another presentation

Numerical modeling of propellant boil-off in a cryogenic storage tank

Abstract: A numerical model to predict boil-off of stored propellant in large spherical cryogenic tanks has been developed. Accurate prediction of tank boil-off rates for different thermal insulation systems was the goal of this collaboration effort. The Generalized Fluid System Simulation Program, integrating flow analysis and conjugate heat transfer for solving complex fluid system problems, was used to create the model. Calculation of tank boil-off rate requires simultaneous simulation of heat transfer processes among liquid propellant, vapor ullage space, and tank structure. The reference tank for the boil-off model was the 850,000 gallon liquid hydrogen tank at Launch Complex 39B (LC- 39B) at Kennedy Space Center, which is under study for future infrastructure improvements to support the Constellation program. The methodology employed in the numerical model was validated using a sub-scale model and tank. Experimental test data from a 1/15th scale version of the LC-39B tank using both liquid hydrogen and liquid nitrogen were used to anchor the analytical predictions of the sub-scale model. Favorable correlations between sub-scale model and experimental test data have provided confidence in full-scale tank boil-off predictions. These methods are now being used in the preliminary design for other cases including future launch vehicles

The cost of helium refrigerators and coolers for superconducting devices as a function of cooling at 4 k

Abstract: This paper is an update of papers written in 1991 and in 1997 by Rod Byrns and this author concerning estimating the cost of refrigeration for superconducting magnets and cavities. The actual costs of helium refrigerators and coolers (escalated to 2007 dollars) are plotted and compared to a correlation function. A correlation function between cost and refrigeration at 4.5 K is given. The capital cost of larger refrigerators (greater than 10 W at 4.5 K) is plotted as a function of 4.5-K cooling. The cost of small coolers is plotted as a function of refrigeration available at 4.2 K. A correlation function for estimating efficiency (percent of Carnot) of both types of refrigerators is also given.

Heat transfer efficiency of kleemenko cycle heat exchangers

Abstract: During the past decade Kleemenko cycle coolers have been developed to operate at temperatures between 70 K and 180 K using throttle expansion of multi-component refrigerants They have demonstrated remarkable reliability—continuous, maintenance-free operation now approaching 100,000 hrs, and the simplicity of their design and use of low cost components has kept their cost at a small fraction of that of any other cryocoolers However, little attention has been focused on optimizing the efficiency of these coolers The problem has been the lack of an adequate understanding of the heat transfer of such multi-component mixtures to the walls of the heat exchanger and to a lesser degree an understanding of the pressure drop along the exchangers These factors have limited the ability to design more efficient, low cost exchangers needed for improved cryocooler performance Recently we have made progress in this area allowing calculation of the heat transfer factors under the conditions of use of these exchangers in good agreement with experiment We describe this work and identify some remaining issues

A new correlation of friction factor for oscillating flow regenerator operating at high frequencies

Abstract: Regenerator plays an important role on the performance of low-power cryocoolers, in particular at high operating frequencies. Many works have revealed that the friction factors under unidirectional steady flow conditions are unsuccessful in predicting flow characteristics of regenerators typically operating at oscillating flow conditions. Recent research has been conducted, using both theoretical analyses and experimental measurements, either to correlate the conventional friction factor by introducing additional parameters or to develop new flow models to overcome the shortcoming of the steady-flow friction factor. However, validation and application of these results for cryocooler regenerators are still questionable because of the complex and randomly oriented matrix geometry of regenerators. In this paper, we will first summarize typical experimental results and correlations on the friction factor of regenerators, at different operating frequencies, at room and cryogenic temperatures. The comparison of those friction factor data will then be presented to clarify the reason for their difference. Finally, a new correlation of friction factor for oscillating flow regenerator, in terms of two non-dimensional parameters, will be presented.

The nicmos cooling system—5 years of successful on-orbit operation

Abstract: The NICMOS Cooling System consists of a closed-loop turbo-Brayton cryocooler coupled with a cryogenic circulator that provides refrigeration to the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope (HST). The cryocooler heat is rejected to space through a capillary pumped loop connected to radiators mounted on the side of the telescope. The system was deployed and integrated with NICMOS by astronauts during STS–109 (Space Shuttle Columbia) in March 2002. It has operated nearly continuously without performance degradation since that time, maintaining NICMOS detectors at a constant temperature of 77 K. Miniature, high-speed turbomachines are used in the cryocooler and the circulator loop to provide vibration-free, long-life operation. A small centrifugal compressor and miniature turboalternator are key elements of the closed loop cryocooler. A miniature cryogenic centrifugal circulator in a separate pressurized neon loop transports heat from the NICMOS instrument to the cryocooler interface heat exchanger. This paper describes the development of the system, key operational features, ground and orbital tests prior to its deployment, and operational results during its five-year operational history on orbit.

Bubble Point Measurements with Liquid Methane of a Screen Capillary Liquid Acquisition Device

Abstract: Liquid acquisition devices (LADs) can be utilized within a propellant tank in space to deliver single-phase liquid to the engine in low gravity. One type of liquid acquisition device is a screened gallery whereby a fine mesh screen acts as a “bubble filter” and prevents the gas bubbles from passing through until a crucial pressure differential condition across the screen, called the bubble point, is reached. This paper presents data for LAD bubble point data in liquid methane (LCH4) for stainless steel Dutch twill screens with mesh sizes of 325×2300 and 200×1400 wires per inch. Data is presented for both saturated and sub-cooled LCH4, and is compared with predicted values.

High capacity two-stage coaxial pulse tube cooler

Abstract: The High Capacity Cryocooler Qualification unit (HCCQ) provides large capacity cooling at both 35 K and 85 K for space applications in which focal planes and optics require cooling. The compressor is scaled from the High Energy Cryocooler (HEC) compressor and is capable of using input powers up to 700 W. The two coaxial pulse tube cold heads are integrated with the compressor into an integral cryocooler. A thermal strap between the cold heads improves efficiency and can be positioned to provide cooling for a wide range of applied loads. The cooler will be acceptance tested at space qualification levels that include thermal performance mapping over a range of reject temperatures and power levels and launch vibration testing.

Testing of a vacuum insulated flexible line with flowing liquid nitrogen during the loss of insulating vacuum

Abstract: Long length vacuum insulated lines are used to carry flowing liquid nitrogen in several high temperature superconducting cable projects. An important, but rare, failure scenario is the abrupt or catastrophic loss of the thermal insulating vacuum producing a rapid increase in heat transfer to the liquid nitrogen stream. In this experimental investigation, a vacuum superinsulated 3 inch by 5 inch nominal pipe size (NPS) (88.9 mm by 141.3 mm) flexible cryostat is subjected to an abrupt loss of vacuum in order to measure the thermal response of a flowing liquid nitrogen stream and the temperature response of the cryostat. The measured outlet stream temperature has a slight peak shortly after the loss of vacuum incident and decreases as the cryostat warms up. The heat loads measured before and after the vacuum loss event are reported. Measurements of the temperatures in the multi-layer superinsulation are also discussed.

MODELING AND EXPERIMENTS ON FAST COOLDOWN OF A 120 Hz PULSE TUBE CRYOCOOLER

Abstract: High frequency operation of a pulse tube cryocooler leads to reduced regenerator volume, which results in a reduced heat capacity and a faster cooldown time. A pulse tube cryocooler operating at a frequency of 120 Hz and an average pressure of 3.5 MPa achieved a no-load temperature of 50 K. The cooling power at 80 K was about 3.35 W with a cooldown time from 285 K to 80 K of about 5.5 minutes, even though the additional thermal mass at the cold end due to flanges, screws, heater, and thermometer was 4.2 times that of the regenerator. This fast cooldown is about two to four times faster than that of typical pulse tube cryocoolers and is very attractive to many applications. In this study we measure the cooldown time to 80 K for different cold-end masses and extrapolate to zero cold-end mass. We also present an analytical model for the cooldown time for different cold-end masses and compare the results with the experiments. The model and the extrapolated experimental results indicate that with zero cold-end mass the cooldown time to 80 K with this 120 Hz pulse tube cryocooler would be about 32 s.

Development of a low cost high frequency pulse tube cryocooler

Abstract: In cooperation with Industrial Research Ltd (IRL), Cryomech, Inc. is developing a low cost high frequency pulse tube cryocooler. The valveless compressor, developed at IRL, employs two S.S. diaphragms and a novel kinematics driven mechanism. The pulse tube cold head has co-axial configuration. It is separated from the compressor with a SS flexible line of 1 meter long. The test results demonstrate a very small orientation effect of the cold head (<3 K at any orientation). This pulse tube cryocooler provides flexibility for user's integration. It can provide 108W at 77K with an electric input power of 3.7 kW in the primary test.

Dry dilution refrigerator with high cooling power

Abstract: We present the construction concept and cooling capacity measurements of a 3,4He dilution refrigerator (DR), which was pre-cooled by a commercial pulse tube refrigerator (PTR). No cryogens are needed for the operation of this type of cryostat. The condensation of the helium mash was done in an integrated Joule-Thomson circuit, which was part of the dilution unit. The composition of the dilution unit was standard, but its components (still, heat exchangers, mixing chamber) were designed for high 3He flow. For thermometry, calibrated RuO chip resistance thermometers were available.In order to condense the mixture before an experiment, the fridge was operated like a Joule-Thomson liquefier with a relatively high inlet pressure (4 bar), where the liquid fraction of the circulating 3,4He mixture was accumulated in the dilution unit. The condensation took about 2 hours, and after 2 more hours of running, the temperature of the mixing chamber approached its minimum temperature of 10 mK. The maximum flow rate of the fridge was 1 mmol/s, and the refrigeration capacity of the mixing chamber was 700 μW at 100 mK.High cooling capacity, ease of operation and reliability distinguish this type of milli-Kelvin cooler.

Operation of superconducting digital receiver circuits on 2-stage gifford-mcmahon cryocooler

Abstract: We have demonstrated the full operation of digital RF receivers on a HYPRES-designed cryostat, which couples an Nb-based superconducting Rapid Single Flux Quantum (RSFQ) chip to a commercially available 100 mW 4 Kelvin Gifford-McMahon cryocooler. A field demonstration of digitization of a satellite-received signal at 7.7 GHz is described. The digital receiver chip is a 1 cm×1 cm chip comprising ∼11,000 Josephson junctions (JJ). The measured cryogenic thermal and magnetic environments of the chip are discussed and their impact on performance. Reception and direct digital conversion of real signals in the HF-, VHF-, and X-bands including signals from military satellite antennae shows the potential of this system to replace large and power-hungry multiple analog receivers.

Performance of heat exchanger models in upside-down orientation for the use in hts current leads for w7-x

Abstract: The superconducting magnet system of the W7-X stellarator requires 7 pairs of current leads designed for a maximum current of 182 kA Special design feature is the installation of the current leads in upside-down orientation, ie, the low temperature end is at the top and the room temperature end at the bottom side This could lead to the problem of free convection inside the heat exchanger (HEX) due to density gradients of the helium between 45 and 300 K The free convection would lead to a higher amount of He mass flow rate required for cooling To overcome the problem it was decided to use HTS-material The current lead can then be cooled with 50 K helium resulting in a drastically reduced density gradient between 50 K and 300 KUp to now it is hard to predict to what degree this effect would be in a current lead heat exchanger In the paper the performance of HEX mock-ups built of different HEX types will be described The tests were performed with helium cooling of various temperatures in normal and upside-down orientation These results will be used to select the HEX type for the W7-X current leads

Vacuum-Insulated, Flexible Cryostats for Long HTS Cables: Requirements, Status, and Prospects.

Abstract: Several high temperature superconducting (HTS) cable demonstration projects have begun operation on the electric grid in the last few years with the liquid nitrogen-cooled, three-phase cable contained in one or more vacuum-insulated, flexible cryostats with lengths up to ∼600 meters. These grid demonstration projects are realistic prototypes of the anticipated commercial market which will require superconducting cable lengths in the multiple kilometer range with the vacuum-jacketed cryostats in underground ducts providing acceptable thermal insulation for up to decades. The current state-of-the art for flexible cryostats (installation constraints, heat loads with a good and degraded vacuum, impact of cable bends, getter performance and lifetime, weld and accessory reliability) is discussed. Further development needed to meet the challenging commercial HTS cable application is outlined.

Mid infrared instrument (miri) cooler subsystem prototype demonstration

Abstract: The Cooler Subsystem for the Mid InfRared Instrument (MIRI) of the James Webb Space Telescope (JWST) features a 6 Kelvin Joule-Thomson (JT) cooler pre-cooled by a three-stage Pulse Tube (PT) cryocooler to provide 65 mW of cooling at the instrument. MIRI's 6 Kelvin cooling load, directly behind the primary mirror of JWST, is remote from the location of the compressors and pre-cooler. This distance, and the parasitic heat load on the refrigerant lines spanning it, is accommodated by the design. The effort during 2006 and the first part of 2007 has focused on the demonstration of a MIRI Cooler prototype in the relevant environment, required to achieve Technology Readiness Level 6 (TRL 6) as defined by NASA[1]. The tests that have been used to successfully demonstrate TRL 6: launch vibration and cooler performance in the relevant thermal-vacuum environment, will be discussed.

Thermal conductivity of powder insulations below 180 k

Abstract: We have measured the thermal conductivity of aerogel beads and glass microspheres at average temperatures ranging from 30 K to 180 K The measuring device consists of two closed, concentric cylinders suspended inside of a vacuum insulated cryostat The insulation being tested occupies the annular space between the cylinders A single stage Gifford-McMahon cryocooler, thermally anchored to the outer cylinder, cools the apparatus to a desired temperature range A heater mounted on the inner cylinder generates uniform heat flux through the insulating material between the two cylinders During each measurement, a temperature difference of roughly 10 K across the insulation is maintained Fourier's law of heat conduction is used to relate the temperature difference between the two cylinders and the applied heating power to a bulk effective thermal conductivity of the powder insulation Data were collected for aerogel beads between 30 K and 80 K and for glass bubbles between 30 K and 180 K Results are compared to data from the literature

Development of jt Coolers Operating at Cryogenic Temperatures with Nonflammable Mixed Refrigerants

Abstract: Throttle cycle coolers, operating with mixed refrigerants have been already used at refrigeration temperatures down to 70 K An industrial single-stage, oil lubricated compressor can be used to provide both, a relatively low cost and long operational time Application of nonflammable mixed refrigerants is important for safety, transportation, service and maintenance However, it is more difficult to provide a required refrigeration performance and long-term reliability due to properties of nonflammable components The paper presents experimental and modeling data for single-stage coolers operating in a temperature range of 80 to 150 K The nonflammable components were selected to develop mixed refrigerants for the desired temperature ranges A gas refrigerant supply technology is used for a single-stage cooler design The minimal achievable temperature is restricted by the freezing point of the mixture Selected gas refrigerant technology restricts the maximal refrigeration capacity However, it allows a compromise between stable, long-term reliable operation and simplicity of equipment design Stability of operation of small-scale, highly reliable coolers has been proven by the data accumulated over more than one year of testing