There are many processes that require the use of a fluid that is a gas at normal temperature and pressure but, for ease of handling and storage prior to use, is supplied in liquified form to be gasified when required. One example is shown in UK Pat. No. 1,289,551, which describes a method of storing natural gas as a liquid by pressurising and cooling it, the user pipeline being supplied with the gasified product obtained by expansion of the stored liquid. The supply is continuously maintained and is stabilised in accordance with the user demand by allowing the liquid to be vaporised in a heat exchanger. Specifically, the heat exchanger employs a medium that undergoes a phase change at a temperature between the boiling points of the liquified gas at the minimum and maximum supply rates of the gas through the pipeline.
In another known apparatus described in UK Pat. No. 1,281,613 liquid nitrogen is used to provide a heated pressurised gas for fluid power purposes by being vaporised in a heat exchanger containing heated alumina pellets as the heat exchange medium through which the nitrogen percolates.
The fluids in these two examples cannot be liquified at normal (i.e. ambient) temperatures, and must be considerably cooled also. There are many fluids that can be liquified by pressure without substantially lowering their temperature below normal, and when the liquid is subsequently vaporised it is possible that the absorption of energy that occurs will result in cooling of the fluid sufficient to transform some of the fluid directly into the solid phase. The unwanted change to the solid phase is undesirable as it can considerably delay the rate at which a quantity of the fluid is vaporised. This possibility exists for carbon dioxide, as one example of a commonly used gas, and similarly nitrous oxide.
One use of pressurised gas, and in particular carbon dioxide, is for the gasification of liquids for consumption. Apparatus is known in which an integral chamber in the apparatus is filled with liquid and a sealed bulb or capsule of liquid carbon dioxide is inserted and pierced to provide the discrete quantity of fluid required to gasify the liquid. The fluid is discharged directly from the bulb into the top of the container above the liquid surface so that it vaporises without difficulty, but the process is extremely inconvenient because the liquid must then be agitated under pressure to accelerate its gasification. In addition, this known apparatus operates as a syphon using the gas pressure to eject the gasified liquid, so that a substantial part of the gas charge remains in the container and cannot be used. The apparatus has a further major disadvantage that the chamber must be fully discharged of liquid before a further quantity of liquid can be gasified, so that its usefulness is limited.
Other apparatus is known from UK Pat. No. 1,453,363 in which the liquid in a removable container is gasified. It is therefore possible to produce larger quantities of gasified liquid over a period of time, but the apparatus is capable of operating only with one particular size and form of container to which it is adapted. Moreover this apparatus relies on the use of a large pressurised cylinder and gas is drawn off from the top of the cylinder to provide the gasifying medium that is discharged through the liquid to gasify it quickly. This apparatus is necessarily rather complex and because it requires a large capacity cylinder of the gasifying medium it is inherently more dangerous in the event of accident or misuse. Because of the large amount of pressurised gas provided by the cylinder contents the size of the container of liquid to be gasified is restricted to limit the effects of its possible rupture at the pressures that might be attained in this apparatus.
It is an important factor in all apparatus in which high gas pressures can be generated, such as for producing gasified drinks, particularly apparatus intended for general use and in situations where there may be little or no preventive maintenance, that the apparatus is safe to operate having regard to the maximum gas pressures that may be generated. It is therefore customary to provide some form of pressure relief valve, but that may not be sufficient in itself because over an extended period the valve may become defective without it necessarily becoming apparent to the user.
In UK Pat. No. 1,453,363 mentioned above, there is a cam-operated pressure relief mechanism that is operated and freed every time the lip of a bottle of liquid to be aerated is brought into sealing engagement with the pressure gas supply means, so that it is ensured that the mechanism is still operative before pressure is applied and the pressure relief valve will be released if it has previously become stuck through a long period of non-use. This system requires a relatively complex mechanism that is expensive to produce, and moreover, although it ensures the valve mechanism operates, it cannot ensure that pressure relief gas path is not blocked. There may well be solid or gummy residues left in the fluid flow passages of the apparatus from previous use, and if these were to impede or block the escaping gas there still could be a dangerous over-pressure built up even though the valve mechanism itself operates perfectly.