An efficient continuous flow helium cooling unit for mossbauer experiments

A Mossbauer continuous flow cooling unit for use with liquid helium over the temperature range 4.2–300 K is described. The cooling unit can be used for either absorber or source studies in the horizontal plane and it is positioned directly on top of a helium storage vessel. The helium transfer line forms an integral part of the cooling unit and feeds directly into the storage vessel so that helium losses are kept to the minimum. The helium consumption is 0.12 / h^-1 itat 4.2 K decreasing to 0.055 / h-1 at 40 K. The unit is top loading and the exchange gas cooled samples can be changed easily and quickly.     

发射场氦气现场提纯技术

液氢贮箱在加注液氢前首先进行氮气置换 ,合格后再进行氦气置换 ,置换后排出的氮氦混合气混合贮存 ,混合气中He纯度为 91 6 6 % ,其余主要为氮气。结合中心现有条件 ,提出了利用高压低温冷凝法和低温吸附法相结合的方法对回收的氦气进行现场提纯 ,可以生产纯度大于 99 995 %的氦气。     

超临界氦的储存及排放

叙述了超临界氦储存过程中存在的热分层现象,研究了超临界氦储槽的有效容积,日蒸发率,填充率对储槽气枕空间内压力和温度变化的影响,给出了估算增压排放液氦所需气量的方法和实例。     

Cryogenic leak test procedure for a large superconducting cyclotron helium vessel

A procedure for testing the vacuum integrity of very large stainless steel weldments used at cryogenic temperatures has been developed at Michigan State University. This development, which uses large quantities of liquid nitrogen, is a modification of a technique commonly applied to small devices and involves cooling the cryostat's liquid helium vessel (bobbin) to liquid nitrogen temperature, and then proceeding immediately with leak testing. This method was applied to the K800 superconducting magnet helium vessel, which seemed leak tight at room temperature, but was found to have an easily detectable helium leak when cooled. After repairing the leak, retesting revealed no leaks, where upon the K800 cryostat construction was completed; i.e. the bobbin was wrapped with superinsulation, a liquid nitrogen radiation shield was added, and the assembly was inserted into the vacuum jacket. The final leak test occurred when the cryostat was cooled to liquid helium temperature and was found to be helium leak tight.     

1000L卧式液氦供气杜瓦的研制

低温液体储运具有效率高，供气质量好，压力稳定，供气简便及使用安全的特点，液氦供气杜瓦不仅具有常规杜瓦的要求，而且气压可控，本文主要论述了1000L液氦供气杜瓦的工作原理和结构设计，及主要研制工艺和试验情况，介绍了液氦供气系统的特点，产品于1997年9月投入正常使用，全部性能指标达到设计要求，液氦日蒸发率为0．9％。     

Demonstration of Transfer of Liquid Helium Using a Pump System with MgB2 Wires

An electric pump composed of an MgB₂ motor is combined with a superconducting level sensor using a thin CuNi-sheathed MgB₂ wire to demonstrate the transfer of liquid helium. An impeller is attached to the lower end of a rotating shaft of the MgB₂ motor and covered with an outer casing to form a kind of centrifugal pump. After that, the MgB₂ motor with the impeller is located vertically inside a cryostat with an infill of the liquid helium. Another glass Dewar vessel is also prepared to receive the liquid helium transferred from the cryostat containing the MgB₂ motor. The MgB₂ sensor is used not only to detect the level of the liquid helium but also to control the electric pump on the basis of its pre-estimated calibration curve. By using the assembled pump system, the liquid helium is successfully transferred from the cryostat to the glass Dewar vessel via a transfer tube. The transfers of liquid hydrogen with the identical pump system will be carried out in the near future.     

Theoretical Analysis and Procedure for the Calculation of Thermophysical Processes Occurring in a Cryogenic Vessel under Conditions of Nonvented Storage

A theoretical analysis is made of the processes which define the rate of pressure rise in a vessel with cryogenic liquid under conditions of nonvented storage. Different methods of the calculation of pressure are treated. The determining part played by thermal stratification in the liquid phase is demonstrated. A new procedure is suggested for the calculation of pressure and thermal stratification in a liquid.     

Electro-magnetic stress-induced degradation of insulation vacuum of a large cryo-magnetic system

In superconducting magnets, the cold mass is placed in a vacuum vessel to reduce heat load to the liquid helium system. Helium leaks into the vacuum vessel can degrade the insulation vacuum, which can, in turn, cause an increase in the heat load to the liquid helium system. These leaks are called cold leaks, as they show up when the coil is cooled with liquid helium. K500 superconducting cyclotron magnet at Variable Energy Cyclotron Centre, Kolkata has such cold leaks in the helium vessel that developed during cool down. The leak rate increases with the increase of current in the superconducting coils. This paper describes a series of experiments carried out on the superconducting cyclotron magnet to find the level of degradation of insulation vacuum and measure the increase in heat load with magnet current. The leak rate was also measured and the leak size was estimated analytically. Detail magneto-structural analysis was done using Finite Element Method (FEM) to identify highly stressed zones in the helium vessel and found out that highly stressed zones coincide with the weld zones. The magneto-structural stress was applied on an estimated size of single crack and found that crack tip stress could reach beyond elastic limit of the material. We can predict that the full design current may be unachievable in this situation. Mitigation of increased heat load was also done using an additional vacuum pump for the insulation vacuum space.     

Precise Measurements of the Vapor-Liquid Equilibria (VLE) of HFC-32/134a Mixtures Using a New Apparatus

A new apparatus for precise measurements of the vapor-liquid equilibria of mixtures by the circulation method has been developed. This apparatus has two special components: a high-stability temperature control system and a helium pressurization system. The temperature in the liquid bath surrounding the sample cell is kept constant within ±0.5mK. The helium pressurization system increases the pressure of the sampled mixture when measuring the compositions at low temperatures by gas chromatography. With these components, the uncertainty in measuring the vapor-liquid equilibria has been reduced. Using this apparatus, the vapor-liquid equilibria of HFC-32/134a mixtures were measured in a temperature range of 263.15 to 293.15K. These results are in good agreement with the calculated results from REFPROP (Ver. 6.01) with a relative pressure difference of about 2%.     

Visualisation of Convective Flow Patterns in Liquid Helium

The advantages of using liquid helium as the investigative fluid in Rayleigh-Bénard experiments are reviewed. A low temperature shadowgraphy apparatus is described that permits convective flow pattern visualisation in liquid helium, thus overcoming the main disadvantage until now of using quantum fluids. The factors involved in maximising the optical resolution for both the shadowgraph and schlieren methods are examined for several fluids and this discussion is applied to the low temperature apparatus in assessing its performance. Some preliminary results on pattern formation in liquid helium are presented.     

Boiling phenomena in pressurized He II confined to a channel

Results from an experiment to study boiling phenomena in a channel containing pressurized He II are presented as a function of temperature, pressure and orientation with respect to gravity. The experimental apparatus is made of glass to allow visual observations and high-speed motion pictures to be made of boiling events. Visual data are combined with temperature and pressure measurements to characterize the boiling behaviour and to group the phenomena into boiling regimes. Results are presented in the form of heat transfer regime maps, temperature and pressure traces during boiling, and sketches of the helium vapour and liquid in the channel. One unexpected result is the observation of a macroscopic region of He I with He II below and vapour above. The two interfaces are clearly visible. Another unanticipated result is that the boiling phenomena are often characterized by the periodic production, growth and collapse of vapour even though the heat input is constant.     

低温容器无损贮存规律

编制了计算低温容器无损贮存规律的程序,就６．５ｍ＾３目标低温容器,计算出了初始充满率与无损贮存时间,容器内压力上升速度,日蒸发率与无损贮存及容器的工作压力与无损贮存时间关系,对低温容器的实际使用有指导意义。     

Helium cryostats for physical studies in space

The design of liquid helium cryostats intended to operate on-board space apparatus is described. The distinctive features of such cryostats are the presence of a phase separator preventing the ejection of liquid from the helium space, and a special neck support to reduce gas convection in it when the cryostat alters its position in space. Results are given of tests of the designs developed, both under laboratory conditions and in earth satellites.     

Dynamic modeling of a liquid helium cryostat at the Canadian Light Source

This work includes the creation and validation of a computer model of a liquid helium cryostat located at the Canadian Light Source (CLS) in Saskatoon, Canada. This cryostat contains a superconducting radio frequency (RF) cavity, and requires careful pressure and level modulation to ensure proper RF control. A detailed mathematical model of the cryostat is generated based on gas and liquid mass balances for a boiling vessel, along with pressure-volume-temperature relations. The model is discretized and solved, and model results are compared with experimental data taken from the actual cryostat at the CLS to determine the accuracy of the simulation. The model is found to reasonably represent the cryostat at the CLS from a process perspective.     

新型低温高压液氢容器的夹套预冷计算

提出了一种新型的低温高压液氢容器结构,并对其中的夹套结构进行了工程设计、预冷分析及预冷计算,从而说明采用夹套结构可以节省昂贵的液氢,此液氢容器结构也适用于贮存液氧。     

Vapor Pressure of Water at Its Triple Point

The vapor pressure of water at its triple point was measured with exceptionally high accuracy by realizing it with a special apparatus and measuring the pressure with the NBS precision mercury manometer. The vapor pressure apparatus had a system for circulating the liquid water. Actual triple point conditions were established with a thin sheet of freshly distilled liquid flowing down over an exposed mantle of ice frozen on a vertical well. This technique reduced non-volatile contaminants and the vapor was repeatedly pumped to remove accumulated volatile contaminants. A diaphragm pressure transducer was used to separate the water vapor from the helium used to transmit the pressure to the manometer. The value found for the vapor pressure of water at its triple point was 611.657 Pa with an uncertainty of ± 0.010 Pa from random errors, computed at 99 percent confidence limits. The systematic errors are estimated to be insignificant relative to the random errors.     

失超引起的气泡对超导体绝缘特性的影响

综述了超导设备在失超情况下液氦中气泡的形成,以及气泡对超导设备的绝缘特性的影响,分析了在均匀和不均匀电场中热传导率、压强及电极表面状况对击穿特性的影响,并探讨了提高失超情况下液氦击穿电压的方法.     

Explosion caused by flashing liquid in a process vessel

An explosion occurred at a polyvinyl chloride (PVC) resin manufacturing plant. The explosion originated at an atmospheric storage vessel when it received a slurry discharge from a suspension polymerization reactor. The pressure rise caused by the uncontrolled flashing of superheated liquid vinyl chloride resulted in the complete separation of the roof from the tank shell. A cloud of vinyl chloride vapor was released and ignited resulting in a vapor cloud explosion. The accident caused significant property damage but no serious injuries. An investigation was conducted to determine the causes of the accident. It was discovered that the facility had experienced numerous overpressure incidents in the atmospheric storage vessels used as slurry tanks. Many of these incidents resulted in modest structural damage to these slurry tanks. It was determined by Exponent that the rapid flashing of residual liquid monomer present in the product slurry stream caused the earlier overpressure incidents. The facility operator did not adequately investigate or document these prior overpressure events nor did it communicate their findings to the operating personnel. Thus, the hazard of flashing liquid vinyl chloride was not recognized. The overpressure protection for the slurry tanks was based on a combination of a venting system and a safety instrumentation system (SIS). The investigation determined that neither the venting system nor the SIS was adequate to protect the slurry tank from the worst credible overpressure scenario. Fundamentally, this is because the performance objectives of the venting system and SIS were not clearly defined and did not protect against the worst credible overpressure scenario. The lessons learned from this accident include: use prior incident data for recognizing process hazards; identify targets vulnerable to these hazards; explicitly define performance objectives for safeguards to protect against the worst credible overpressure scenario. The ultimate lesson learned here is that a liquid trapped under pressure above its normal boiling point represents an overpressure hazard. To avoid exceeding the design pressure of the receiving vessel, the superheated liquid must be discharged slowly so that the vapor production rate caused by flashing does not exceed the venting rate of the receiving vessel.     

卧式高真空多层绝热低温容器无损贮存规律

介绍了卧式低温容器的传热特点,以及低温液体无损贮存的传热模型.通过2m3卧式高真空多层绝热低温容器在90%、85%和80%初始充满率下的静态无损贮存试验,拟合现有的传热模型,对升压过程中不同规律的3个阶段进行了分析,得到了第一、第三阶段升压的初步规律.     

AMS-02超导磁体中超流氦加注过程研究

论述了AMS-02超导磁体中超流氦的加注过程,进行了各个环节压降和温降的计算分析,并用图表表示了在不同质量流量加注超流氦工况下管路系统中氦工质的速度、压力、温度和含气量的变化情况.计算结果表明,氦工质从液氦主杜瓦加注到磁体杜瓦的过程中,其压力和温度不断降低,而含气量不断增加.同时也表明在节流阀VVP9中实现了常流氦向超流氦的转变和质量流量的控制.