This invention relates generally to self-pressurizing containers for cryogenic fluids and more particularly concerns a self-pressurizing container for the storage and distribution of liquid helium.
Helium is present in the atmosphere at a level of approximately one part per two hundred thousand. It is an inert by-product of radioactive decay, and because of its high molecular velocity, it continually escapes from the earth's atmosphere into space. Due to its relative scarcity in the atmosphere, its cost prior to 1915 was $2500 per cubic foot as a result of the high cost of separation. In 1915 helium was discovered in the natural gas wells in the southwestern United States. The world's helium supply for all practical purposes is distilled from these wells by large, on site plants and shipped worldwide from those sources in various sized liquid and high pressure tanks.
As previously stated the present invention relates to the handling of liquid helium. Liquid helium has the lowest boiling point of any element. Its boiling point of -450.degree. F. is only approximately 7.degree. F. above absolute zero. It takes very little heat (8.8 BTU per pound) to cause helium to boil at this temperature. Furthermore, it takes very little heat to increase the temperature of liquid helium (approximately 1.5 BTU per pound degree F.). Liquid helium is also very light in weight (0.94 pounds per gallon).
In transferring liquid helium from one container into another, it is necessary to build a pressure head in the helium vapor above the liquid helium without increasing the liquid's temperature substantially above its boiling point. If the liquid helium temperature is increased along with the increased pressure, high product losses will occur during the transfer process. This high loss is termed a flash-loss within the cyrogenic industry. Flash-loss is the extremely rapid boiling of the super heated liquid helium when it is depressurized. The same type of loss occurs when the lid is removed from a hot pressure cooker and all the water turns to steam. Because of the cost associated with obtaining liquid helium, it is important that flash-loss be kept to a minimum.
In the cryogenic industry, the standard procedure for building head pressure above liquid helium in a container is to connect a pressurized tank of helium gas to the liquid container. While this procedure provides sufficient head pressure and works adequately without substantial flash-loss, it requires the presence of a high-pressure cylinder of helium available on site and the necessary lines to make the connections for transfer.
In recent years, the demand for liquid helium has increased, particularly for use in super cooling magnets used in connection with magnetic resonance scanners. Such magnetic resonance scanners are used to provide images of the internal structure of the human body. In order to provide the proper resolution in such magnetic resonance scanners, it is necessary to have large magnets which are super cooled by the liquid helium. The presence of such large magnets make it impractical to have steel high-pressure cylinders available on site adjacent to the magnets because of the high magnetic fields that are involved.
While self-pressurizing containers for cryogenic fluids are known in the industry for fluids such as liquid nitrogen and liquid oxygen, such containers generally will not work when used in connection with helium. When liquid helium is used in such standard self-pressurizing containers, it is subject to a phenomenon known as the second sound oscillation which is in essence a thermal acoustical oscillation within the pressurized system.