Patent Number: 039873064
Section: summary

BACKGROUND OF THE INVENTION This invention relates to the production of ultra-violet (U.V.) radiation, and relates particularly, but not exclusively, to a method of, and an apparatus for, treatment of a material by irradiation with U.V. radiation. It is known to produce U.V. radiation by the discharge of electrical energy through a gas, and a device operating on such a principle is called a gas discharge lamp. SUMMARY According to one aspect of this invention there is provided a method of producing U.V. radiation comprising the steps of effecting, in a discharge chamber, an electrical discharge between two electrodes, and arranging for at least a part of the electrical discharge between the two electrodes to be surrounded by a preselected liquid. The pressence of the surrounding liquid prevents the hot plasma of the discharge region between the electrodes from expanding freely, in contrast to the operation of the conventional gas discharge lamp, and as a consequence the plasma can attain a high temperature and pressure with the resultant emission of U.V. radiation. It will be appreciated that the preselected liquid must have appropriate electrical characteristics to permit the electrical discharge therethrough, and also that the liquid molecules when ionised by the energy of the electrical discharge must emit radiation of a desired wavelength at which the preselected liquid must be substantially transparent. For convenience this discharge will be referred to as a main discharge. Where one of the electrodes is formed by a preselected conductive liquid immiscible with said preselected liquid, the method may include the step of generating an initiating electrical discharge in the vicinity of the discharge region between said two electrodes just prior to effecting said electrical discharge. This initiating discharge may be generated between a third electrode and either of said two electrodes, and is for the purpose of providing in the discharge region of the main discharge a mixture of the molecules, or even ions, of said preselected liquid and of the liquid electrode. According to another aspect of this invention there is provided a method of treating a material by irradiation with U.V. radiation, comprising the steps of producing U.V. radiation by a method as described above, transmitting U.V. radiation thus produced through a U.V. radiation-transparent portion of the chamber and disposing the material in a treatment region where it is irradiated by the U.V. radiation transmitted from the chamber. The material may be treated in batches, and thus may be stationary in the treatment region, or may be treated in a continuous process by transporting the material continuously through the treatment region and continuously effecting electrical discharge at a rate appropriate to the speed of the material to obtain a required treatment. A required treatment may be each elemental portion of material receiving U.V. radiation from two discharges, or three discharges, and a minimum treatment may be each elemental portion being irradiated once and once only. The material may be solid, e.g. bandage to be sterilized, or liquid. In the latter case the treatment region will be constituted by a chamber through which the liquid flows. The discharge chamber may conveniently be a sealed chamber, and this avoids splashing-out of drops of the preselected liquid ejected from the discharge region. Where it is not required for the chamber to be sealed, it may have an open top portion which constitutes the or a part of the U.V. transparent portion of the chamber. Where the material to be treated is a liquid, the transporting step preferably comprises feeding the liquid to be treated through a treatment chamber which constitutes the treatment region. The treatment chamber may be outside or inside the discharge chamber and have inlet and outlet passages for the flow therethrough of the liquid to be treated. By the term `inside` is meant that the discharge chamber encircles the treatment chamber, and by the term `ouside` is meant that the discharge chamber does not encircle the treatment chamber. In the latter case it will be understood that the treatment chamber may encircle the discharge chamber. It will be appreciated that the thickness or depth of the treatment chamber will depend on the degree of absorption of U.V. radiation by the liquid to be treated and will be such that the extreme regions of this liquid will still receive an adequate amount of U.V. radiation appropriate to the particular treatment. There may be included the step of automatically adjusting the length of the part of the electrical discharge which is surrounded by the preselected liquid such as to maintain predetermined electrical characteristics of the electrical discharge. Where the whole of the electrical discharge is so surrounded, the spacing between the electrodes will be altered, either or both of the electrodes being moved. In one form of this invention the main discharge occurs from the tip of an electrode submerged in said predetermined liquid, through the liquid direct to the surface thereof, and then along the surface to the wall of the chamber which is conductive and forms the other electrode for the discharge. In the operation of such a form, the length of the submerged part of the discharge path can be altered by causing the preselected liquid to have a circulating motion which results in a raising of the level of the peripheral surface regions and a depression of the central surface region. The submerged electrode is preferably arranged directly under the region where greatest depression occurs to obtain maximum sensitivity i.e. the maximum change in submerged length of the discharge path for a given circulatory speed of the preselected liquid. According to a further aspect of this invention there is provided an apparatus for producing U.V. radiation comprising two electrodes spaced apart to form a discharge region therebetween, a discharge chamber containing the discharge region, first means arranged to apply high voltage between the electrodes, and means arranged to supply a preselected liquid to the discharge region such that, on application of the high voltage an electrical discharge occurs between the electrodes, at least a part of this discharge being along a submerged path in the preselected liquid, and at least a portion of the discharge chamber being transparent to the U.V. radiation whereby U.V. radiation produced may be transmitted from the discharge chamber. One of the electrodes may be in the form of a rod mounted in the wall of the chamber, and the other electrode may be in one of several alternative forms. Firstly the other electrode may be in the form of a rod similar to the said one electrode and similarly mounted in the wall of the chamber; secondly the other electrode may be formed by the chamber itself provided that it is formed of a conductive material, or if not, then formed with a conductive coating on its inner surface; thirdly the other electrode may be in the form of a layer of a preselected conductive liquid, preferably mercury, which is immiscible with said preselected liquid and which is disposed at the bottom of the discharge chamber, it will be appreciated that in this last case said one electrode would be disposed in the chamber above the surface of the conductive liquid. In this specification the term rod includes an electrode having a channel for liquid flow. The channel may be in the form of an axial bore or a hollow cylindrical annulus. One preferred form of discharge chamber is formed as a hollow cylinder having two end portions having flat inner surfaces and a hollow cylindrical intermediate portion having a cylindrical inner surface. In this case the two electrodes may be in the form of rod electrodes, each axially mounted in a respective end portion such that the discharge region between the electrodes is approximately at the centre of the chamber. Either an end plate forming an end portion of the chamber, or a tube forming the intermediate portion of the chamber, or both of these, may be formed of a material, preferably quartz, transparent to U.V. radiation. An apparatus for producing U.V. radiation may be used in the treatment of materials by irradiation with U.V. radiation. Where such a material is a liquid, a treatment chamber having inlet and outlet passages for the flow therethrough of a liquid to be treated may be provided either externally or internally of the discharge chamber. A treatment chamber internally of the discharge chamber may be formed by a hollow tube of U.V. transparent material, preferably quartz, passing through the discharge chamber. In this case it will be appreciated that the U.V. transparent wall is common to the treatment chamber and the discharge chamber and constitutes the aforementioned portion transparent to U.V. radiation. Also, the end walls and the intermediate portion of the chamber need not be U.V. transparent unless specifically required. The treatment chamber may be constituted by a plurality of the hollow tubes which may be symmetrically arranged around the axis of discharge chamber. The electrodes may be mounted in the end portions or in the intermediate portion. The inner surfaces of the chamber may be made highly reflective by mechanical treatment, e.g. polishing, to make maximum use of the generated U.V. radiation. If required, the intermediate portion may have an elliptical cross-sectional shape as alternative to the cylindrical shape. The electrodes may be arranged such that the electrical discharge is at one of the foci, and the treatment chamber, e.g. one or more hollow tubes, would then be arranged at the other focus. A form of treatment chamber which can be utilised with a U.V. transparent cylindrical intermediate portion is one which is a cylindrical annulus formed between the outer cylindrical surface of the intermediate portion, and the inner cylindrical surface of a sleeve mounted between the end portions. The mounting of the end portions to the intermediate portion and to the sleeve will be such as to seal the discharge chamber from the treatment chamber. One end portion of the discharge chamber may be made U.V. transparent and means may be provided for producing a thin film flow of liquid to be treated across the outer surface of this end portion. One form of such means for producing a thin film flow may be a plate spaced from the end portion such as to form a thin disc-like chamber. The inlet and outlet passages are arranged such that the thin film flow is substantially radial within the treatment chamber. Preferably the inlet passage is aligned with the axis of the chamber, and the outlet passage communicates with the peripheral regions of the treatment chamber via an annular collecting chamber therearound. In one form of an apparatus the axis of the intermediate portion is arranged vertically and an electrode is mounted axially in the bottom end portion, being insulated therefrom. Liquid inlet and outlet passages are arranged to provide a constant head of liquid in the chamber such that the top of the electrode is submerged in the liquid. The inlet passage is arranged such that a rotational movement is imparted to the liquid thereby to cause the level of liquid above the electrode tip to decrease. The electricl discharge is from the electrode tip to the surface of the liquid, and then along the surface to the inner surface of the intermediate portion which is conductive or has a conductive coating thereon and acts as an electrode. Means may be provided for controlling the rate of flow of liquid into the discharge chamber in dependence upon the electrical parameters of the discharge in order to maintain automatically constant discharge conditions. The inner surface of the discharge chamber may in such an apparatus be shaped to enhance the irradiation of a treatment chamber at the top of the chamber, and may be for example a hollow hemisphere or parabolloid, polished or treated if required to increase the reflectivity of the material. A cheap and convenient form of the preselected liquid through which the electrical energy is discharged is water, either commercial or distilled. The electrodes should be formed of a rigid material resistant to corrosion and electrically conductive. A convenient material is stainless steel, and if required the discharge chamber may also be formed of stainless steel. It is possible to use, say, water, as the liquid to surround the hot plasma of the discharge region and yet to introduce selected molecules of another material into the discharge region for excitation by the electrical discharge in order to obtain an emitted radiation characteristic of this other material. A preferred form of this material is mercury which when excited emits U.V. radiation at 254 nm wavelength. A preferred form of an apparatus for achieving such excitation of the molecules of the material is arranged with the axis of its cylindrical intermediate portion vertical. There is an inlet at the bottom of the discharge chamber for connection to a supply of mercury for establishing a predetermind static height of mercury in the chamber. Further inlet and outlet means permit a swirling flow of water on top of the mercury. A first electrode is mounted in the wall of the discharge chamber and extends through the water to a point just above the surface of the mercury, which forms a second electrode, and on or near the axis of the chamber. Another electrode is similarly mounted and also extends through the water to another point adjacent the first point. Means are provided for applying a high voltage between the other electrode and either the first electrode or the mercury to cause an initiating discharge which results in mercury molecules being interspersed with water molecules in the region between the first electrode and the mercury. A main electrical discharge between the first electrode and the mercury may be synchronised with the initiating discharge. In order to maintain the electrical parameters of the discharge the rate of inflow of the water may be controlled; the swirling water induces a corresponding swirling movement of the mercury by friction. Alternatively, the water need not be swirling and the height of the mercury in the discharge chamber may be adjusted by control of the head of the mercury supply. It will be appreciated that electrodes must be electrically insulated from each other if there is to be a high voltage applied between them. Preferably the electrodes, other than liquid-type electrodes or those formed by the discharge chamber walls, are mounted in an insulating sleeve which is sealingly secured in a wall of the chamber. Instead of introducing water into the chamber via an inlet in the wall, the water may be introduced (and removed if required) via a passage or passages through the electrode, or between the electrode and the inner surface of its sleeve. In order to absorb selectively an unwanted wavelength or range of wavelengths, or reflect selectively a wanted wavelength or range of wavelengths, an appropriate coating may be applied to a surface of the discharge chamber or of the treatment chamber. Selective absorption may also be achieved by the use of an appropriate material as the U.V. transparent portion of the discharge chamber. Conveniently this material may be quartz doped with a metallic ion for example, tungsten.