This invention relates to a liquid helium circulation system and transfer lines used with the system. More specifically, it relates to a liquid helium circulation system used in a brain magnetism measurement system that condenses helium gas evaporating from its liquid helium reservoir, where a magnetoencephalography is disposed in an extreme low temperature environment, and to the transfer line used with the system that returns the condensed helium back to the liquid helium reservoir. Besides brain magnetism measurement systems, the liquid helium circulation system and transfer lines are also usable with magnetocardiographs and magnetic resonance imaging (MRI) systems, and in studying and evaluating the properties of a variety of materials at extreme low temperatures.
Brain magnetism measurement systems to detect magnetic fields generated by human brains are under development. These systems use super-conducting quantum interference devices (SQUIDs) capable of measuring brain activities with a high space-time resolution and without harming the organs. The SQUID operates in a refrigerated state, emerged in a liquid helium filled insulated reservoir.
With most conventional liquid helium reservoirs in those systems, the helium gas evaporating from the reservoir is released into the atmosphere. This waste of helium in large quantity makes the systems economically disadvantageous because helium is so expensive. Moreover, as the liquid helium in the reservoir is consumed, it must be replenished with fresh liquid helium from a commercial cylinder. The replenishment presents problems, for example, the process becomes extremely troublesome, or that outsourcing costs are substantial.
Accordingly, there is movement to develop liquid helium circulation systems, which may recover and condense the helium gas evaporating from the reservoir in its entirety and recirculate it back to the reservoir.
Referring to FIG. 4, described below, is the schematic configuration of a type of such a liquid helium circulation system. A magnetoencephalography is disposed in a liquid helium reservoir 101, a drive pump 102 recovers the helium gas which vaporized from inside reservoir 101; a dryer 103 dehydrates the recovered helium gas, a flow regulating valve 104; a purifier 105; an auxiliary refrigerator 106; a first heat exchanger 107 for auxiliary refrigerator 106; a condensing refrigerator 108 and a condensing heat exchanger 109 of condensing refrigerator 108 are also present. The helium gas is boiled off from the liquid helium reservoir 101 and whose gaseous temperature is raised to about 300xe2x80x2 Kelvin N is suctioned with drive pump 102, and sent through dryer 103 and purifier 105 to auxiliary refrigerator 106, where it is cooled down to about 40xe2x80x2 K and condensed. The liquid helium is sent to condensing refrigerator 108, where it is further cooled down to about 4xe2x80x2 K as it passes condensing heat exchanger 109. Finally, the extreme low temperature liquid helium is supplied to liquid helium reservoir 101 through transfer line 110.
This prototype helium circulation system is basically a system to recover and recycle all the helium gas evaporating from the liquid helium reservoir. Compared with other conventional or similar systems, whose vaporized helium is released into the air or recovered in a gas bag or the like for reprocessing, it consumes a remarkably smaller quantity of helium, promising benefits of economy and efficiency, which has been spurring recent efforts to put to practical use. In addition, the added feature of the new system to reliably refill fresh liquid helium would make maintenance of the measurement system easier as a whole.
Nevertheless, the new circulation system as above-mentioned is not free from necessary improvements as follows:
While liquid helium is an indispensable medium to keep a SQUID in the refrigerated state, a huge amount of electric energy has to be consumed to run the refrigerator to liquify helium gas. In addition, a large volume of water is required to cool the compression pump of the refrigerator. Furthermore, as the liquefied helium is transferred from the refrigerator to the liquid helium reservoir through the transfer line, it is difficult to isolate it completely from high-temperature parts, causing a large portion of it to become vaporized, resulting in a poor transfer rate. Accordingly, the operating cost as well as insulation measures are expensive comparable to that in the case of allowing the gas to escape into the air. An economical version of liquid circulation system overcoming such problems needs to be developed.
With the above-mentioned background considerations, the inventor has developed the idea of this invention from the phenomena that the quantity of heat (enthalpy) required to raise the temperature of helium gas from about 40 K to about 300 K is much higher than the vaporization heat required for the phase change from liquid helium to gaseous helium at about 4xc2x0 K, and that while the energy required to cool down high-temperature helium to low-temperature helium is moderate, substantial energy is required to liquefy low-temperature helium gas.
Namely, this invention offers a new type of liquid helium circulation system as a solution to the problems conventional circulation systems have had as above-mentioned. With this invention, high-temperature helium gas as high as 300xc2x0 K boiling off from the liquid helium reservoir is recovered, cooled down to about 40xc2x0 K a temperature within the easy reach of a refrigerator, and supplied to the upper part in said reservoir. Also, low-temperature helium gas, for example, approximately 10xc2x0 K, near the surface of liquid helium inside the reservoir is recovered and liquefied at about 4xc2x0 K and supplied back to the reservoir. In this manner, the inventory of liquid helium inside the reservoir is easily replenished by as much as is lost by evaporation.
In one embodiment, a liquid helium circulation system having a liquid helium reservoir and a refrigerator that cools down and liquefies helium gas evaporating from the reservoir, and being capable of returning refrigerated helium gas or liquefied helium to the reservoir. One line routes the high-temperature helium gas heated from up inside the reservoir to the refrigerator where it is cooled down, and returns the refrigerated helium to the upper part of the reservoir. It is also characteristic of another line to route low-temperature helium gas in the vicinity of the surface of liquid helium inside the reservoir to the refrigerator where it is liquefied, and returns the liquefied helium to the reservoir.
Also, a liquid helium circulation system characteristic of two pipelines, one connecting the refrigerator and the upper part in said reservoir, another that supplies the low-temperature gas to the refrigerator where it is liquefied, and returns the liquefied helium to the reservoir disposed in the same conduit pipe whose periphery is insulated with a vacuum layer.
Also, a liquid helium circulation system characteristic of a triple-pipe construction has a line that supplies liquid helium at the center, another line that supplies low-temperature helium gas to the refrigerator around the central pipe, and an outermost line that supplies helium gas refrigerated by the refrigerator.
Also, a liquid helium circulation system characteristic of three lines has one line that supplies liquid helium, another line that supplies low-temperature helium gas to the refrigerator, and another line that supplies helium gas refrigerated by the refrigerator. All three lines are disposed in parallel with one another.
Also, a liquid helium circulation system characteristic of three lines has each line having its own surrounding vacuum layer.
Also, a liquid helium circulation system characteristic of two lines, one connecting between the refrigerator and the upper part of the reservoir, and another that supplies low-temperature gas to the refrigerator where it is liquefied, and returns the liquefied helium to said reservoir are disposed separately from one another and each one isolated with a vacuum layer.
Also, a liquid helium circulation system characteristic of a structure that enables the liquid helium liquefied by said refrigerator to be surrounded with low-temperature helium gas and thus isolated from high-temperature parts as it is transported to said reservoir.
Also, a liquid helium circulation system characteristic of a feature that makes it possible to liquefy part of said high-temperature helium gas and supplies the liquefied helium to said refrigerator.
Also, a liquid helium circulation system characteristic of a gas-liquid separator that the liquid helium liquefied by said refrigerator passes through as it is supplied to said reservoir.
Also, in a process to recover helium gas boil-off from a liquid helium reservoir, cool down or liquefy said helium gas and return it to the liquid helium reservoir, a liquid helium circulation method characteristic of supplying high-temperature helium gas heated up inside said liquid helium reservoir to a refrigerator, where it is liquefied, and the liquefied helium to the upper part in said reservoir, and also supplying low-temperature helium gas in the vicinity of the surface of the liquid helium inside said liquid helium reservoir to a refrigerator, where it is liquefied, and the liquefied helium to said reservoir.
Also, a liquid helium circulation method to protect said liquid helium, while being supplied to said liquid helium reservoir, with either low-temperature helium gas or refrigerated helium gas from direct contact with high-temperature parts.
Also, a transfer line, characteristic of its construction, consisting of a line that supplies liquid helium, a line that supplies low-temperature helium gas, and a line that supplies refrigerated helium gas of a temperature higher than that of said low-temperature helium, with each line surrounded by a vacuum layer and all lines disposed inside a conduit whose outer surface is insulated with a vacuum layer.
Also, a transfer line having a triple-pipe design consisting of a line that supplies liquid helium at the center, an intermediate line that supplies low temperature helium gas, and an outermost line that supplies refrigerated helium gas of a temperature higher than that of the low-temperature helium gas, with each line surrounded by a vacuum layer.
With the liquid helium circulation system according to this invention, it is possible to minimize liquid helium boil-off from the liquid helium reservoir because the enthalpy of refrigerated helium gas removes a large quantity of heat. Also, cooling helium gas from about 300xe2x80x2 K down to about 40xe2x80x2 K requires an amount of energy much less compared with that when producing liquid helium of about 4xe2x80x2 K by liquefying helium gas of about 40xe2x80x2 K. Therefore, compared with conventional systems liquefying the entire volume of helium gas recovered, this system offers outstanding economic benefit by lowering remarkably the amount of energy consumed in liquefying helium gas by shortening the running time of the refrigerator, etc.
Also, this system recovers and liquefies low-temperature helium gas in the vicinity of the surface of liquid helium in the liquid helium reservoir, which greatly helps save the amount of energy needed in the process of liquefying helium gas, leading to a large reduction in operating cost.
Moreover, this system adapts a method for refrigerated helium gas or low-temperature helium gas to flow around the line supplying liquid helium liquefied by the refrigerator. This isolates the line from surrounding high-temperature parts and protects the liquid helium from evaporating as it flows though the line, which minimizes the loss of energy in a helium gas liquefying process and makes this system a more efficient liquid helium circulation system.