Patent Application
20200008287
The present application relates to a device for cooling x-ray tubes in x-ray generators using a gaseous cooling medium as coolant. The ambient air is preferably used as coolant. Further preferably, the x-ray generators are compact x-ray generators for applications in the field of the food industry.
Conventional x-ray tubes comprise an evacuated tube in which an electric filament for generating free electrons and, spaced apart therefrom, an anode are located. The electrons emitted by the filament are accelerated by an additionally applied high voltage in the electric field and are directed onto the anode. The collision between the rapid electrons and the anode leads to the generation of x-ray radiation. The x-ray radiation generated in this way can be used for the examination or treatment of people, animals or objects.
The bombardment of the anode with electrons additionally leads to the anode heating up, as most of the kinetic energy of the incident electrons is converted into heat. The quantity of heat released in the anode is dependent on the speed and number of incident electrons. In order to prevent the anode, and thus the whole x-ray tube, from heating up too strongly during operation, the quantity of heat generated must be dissipated from the x-ray tube.
For this, various types of cooling systems are used, depending on the power of the x-ray tube. When designing cooling devices for high-voltage components such as x-ray tubes it is always to be borne in mind that the x-ray electrode is at high voltage potential and that an adequate insulation of the x-ray electrode from the surroundings must be ensured.
In order to achieve effective cooling, a liquid coolant is usually introduced between an outer housing wall of the cooling device and the outer wall of the x-ray tube. An oil with a high dielectric constant is frequently used as coolant, with the result that the coolant at the same time also serves for the electrical insulation of the x-ray tube at high voltage during operation. Such a device is described in U.S. Pat. No. 4,780,901 (A), in which a dielectric oil is used as electrically insulating coolant.
A liquid-cooled x-ray radiator is known from the German utility model DE 86 15 918.6. The x-ray radiator is arranged in a housing filled with an insulating oil. In addition, a circulation cooling system is provided which has a cooler connected to the housing by two coolant lines and a circulating pump for the insulating oil. The insulating oil circulates freely around the x-ray radiator inside the housing. Outside the housing, the insulating oil is conducted via the coolant lines to the circulating pump. The coolant lines can be guided past a fan. In order to cool the coolant as effectively as possible, the coolant lines can run spirally in the region of the fan and be provided with cooling fins. The spiral course of the coolant lines serves to increase the surface area that can be used for the cooling in order to increase the dissipation of heat from the coolant to the surroundings.
Such dielectric oils permit a very steep potential curve within the coolant between high-voltage components and components at ground potential, without there being the danger of a spark discharge. A steep potential curve permits a correspondingly compact design, as very short spatial distances between high-voltage components (outer wall of the evacuated x-ray tube) and components at ground potential, i.e. zero potential, (outer walls of the housing of the cooling device) are permitted.
Especially in the field of the food or pharmaceuticals industries, however, oil-cooled systems are often disadvantageous since, in the case of leakiness, there is the danger of contamination of food or medicinal products with the oil, which is generally harmful to health. In addition, oil-cooled systems are also generally relatively high-maintenance because of the oil changes to be carried out regularly.
In principle it would be entirely possible to use air-cooling systems for x-ray tubes. However, air has poorer insulating properties. For dry air, a dielectric strength of approximately 1 kV/mm (kilovolt per millimetre) can be assumed. In order to reliably avoid a spark discharge under real conditions, distances greater by a factor of three must be provided to counter it. For typically used 100 kV x-ray tubes, a distance to be maintained of approximately 30 cm thus results between an x-ray tube at high voltage and the housing of the x-ray generator at ground potential.
Conventional air-cooled systems must thus have correspondingly larger dimensions and are thus, particularly in the case of very high operating voltages, more unwieldy and less flexible to use.
Gaseous cooling media are usually only used for external cooling in the case of conventional x-ray tubes. For example, ambient air is guided along the outer side of the x-ray radiator which is at ground potential. These devices are suitable for use when only relatively small quantities of heat need to be transported away. As the cooling is also effected from the outside, the cooling medium also does not need to have any electrical insulating properties. Such x-ray radiators are known for example from U.S. Pat. No. 4,884,292 or U.S. Pat. No. 4,355,410.
An x-ray tube, more precisely a rotary piston x-ray radiator, in which a gaseous cooling medium is used, is known from DE 298 23 735 U1. In the device described there, the cooling gas is conducted paraxially into the interior of the housing. The cooling gas serves both for cooling and for the electrical insulation of the high-voltage components from the housing. For this reason, it is also not possible to use any desired cooling gas here, but rather the cooling gas must be a high-voltage-insulating cooling gas. Sulfur hexafluoride (SF6) is named in that document as the sole example of such a gas. Since strict safety guidelines must be met when using this gas and since this gas is one of the strongest known greenhouse gases, use of this coolant is not desired.
An object of the present disclosure is therefore to provide a cooling device for x-ray generators which requires less maintenance than oil-based cooling devices but which nevertheless enables a compact design. A further object of the present disclosure is to provide a cooling device for x-ray generators in which any desired gaseous coolant can be used.
This object is achieved in the device of the type named at the beginning by the features according to claim 1.
The cooling device comprises a housing with an inlet opening, an outlet opening and a gas-conducting channel, which extends between the inlet opening and the outlet opening. A central receiving device for receiving an x-ray tube is provided. The gas-conducting channel is designed such that it guides the gaseous cooling medium directly past the high-voltage housing of the x-ray tube during operation. The cooling medium absorbs the heat produced by the x-ray tube and dissipates it towards the outside. Nevertheless, the gaseous cooling medium does come into contact with high-voltage housing parts of the x-ray tube. In order to avoid sparking along the gas-conducting channel, the cooling gas is not guided past the x-ray tube on a direct radial route but is guided through the housing of the cooling device on a spirally running path. Due to the spiral course, the actual length of the gas-conducting channel is greatly lengthened, with the result that, in spite of a compact design, a sufficiently large effective distance between the high-voltage components of the x-ray tube and the housing parts at ground potential can be provided.
The term “spiral” as used in the present description is to be understood broadly and is intended to comprise substantially any desired routing in which the cooling gas is not guided through the cooling device on a direct radial route. For example, the “spiral routing” could also be designed such that the gaseous cooling medium is guided to the x-ray tube housing on a winding or meandering path which runs only on one side of the cooling device and that the cooling medium is then guided towards the outside on a similarly shaped path which, however, runs only in the other half of the cooling device. In principle, the term “spiral routing” can also mean any desired 3D labyrinth structure which makes it possible to obtain a sufficiently large effective distance between the high-voltage components of the x-ray tube and the housing parts at ground potential.
In the most preferred embodiment of the present disclosure, however, the spiral path actually has the shape of a geometric spiral and has a plurality of windings which extend around the x-ray tube arranged centrally during operation.
The cooling gas can be substantially any desired gaseous medium. A particularly suitable cooling gas is ambient air, as this permits particularly simple and cost-effective cooling. However, pure gases such as nitrogen, helium, argon or CO2 can also be used. In particular, the design according to the disclosure of the gas-conducting channel enables any desired cooling gases to be used, or also those cooling gases to be used which cannot be used in conventional systems because of their low dielectric strength. In particular when ambient air is used as cooling gas, no cooling gas-specific safety precautions need to be taken, with the result that in this case the cooling can be used particularly variably and cost-effectively.
X-ray tubes are usually operated at high voltages between 10 and 200 kV. The high voltage used and the cooling gas substantially determine how long the gas-conducting channel must be made. In order to be able to use the cooling device as flexibly as possible, the gas-conducting channel should be long enough that no sparking along the gas-conducting channel can occur even at the maximum applicable high voltage and at maximum humidity.
The housing of the cooling device is produced from electrically insulating material. The housing preferably consists of thermoplastic such as polycarbonate, polysulfone, PVC or polyolefins, of Plexiglas or of polyoxymethylene. Plastic composites or plastic-ceramic composites can also be used as housing material. When the generated x-ray radiation is guided through the housing, the absorption of the x-ray radiation can be influenced in a targeted manner via the choice of the housing material. For example, x-ray absorbing materials can be used in order to obtain a specific or desired cross section of the x-ray beam.
The gas-conducting channel is preferably formed of two spirally arranged inner walls of the housing of the cooling device. The inner walls define a first spiral path, on which the cooling gas is conducted into the central area of the housing in which the x-ray tube is located during operation. At the same time, the inner walls define a second spiral path, on which the cooling gas is conducted out of the housing from the central area of the housing.
The thickness of the inner walls to be used depends on the high voltage used and on the housing material used. The total wall thickness, thus the sum of all wall thicknesses in the radial direction, must be chosen sufficiently large, with the result that, in the case of the high voltage used in each case, a radial sparking is prevented through the walls of the cooling device. The dielectric strength of the typically used wall material is approximately 10 times greater than the dielectric strength of the cooling gas and lies in the range of approximately 25 to 120 kV/mm. In order to prevent arcing, total wall thicknesses of approx. 0.5 to 3 cm are therefore usually to be used, which results in a wall thickness of from 1 to 3 mm for the individual inner and outer walls of the cooling device.
In a preferred embodiment, the housing of the cooling device is designed in two parts. The two housing parts can be reversibly connected to each other. The connection can be a plug-in connection, for example. Each of the housing parts which can be connected to each other preferably comprises spiral inner walls which, in the assembled state, engage in one another and thereby define the gas-conducting channel. A two-part housing is particularly easy to maintain since access to the interior of the cooling device can be gained at any time.
Further preferably, in the case of the two-part embodiment, one housing part is connected to the x-ray tube, while the other housing part is connected to the high-voltage power supply unit, for example. Here, the x-ray tube can be permanently connected to the respective housing part. In the case of a defect in the x-ray tube, the x-ray tube can be replaced together with the respective housing part. To exchange the defective x-ray tube, only the part of the two-part cooling housing connected to the x-ray tube needs to be removed and replaced by a corresponding replacement part. In this way, the two-part cooling device likewise makes maintenance of the x-ray system easier.
In a further embodiment, the gas-conducting channel can also be realized in the form of a wound-up hose structure. Such hose structures can be produced both on the basis of rectangular basic hose shapes and on the basis of round or elliptical basic hose shapes. The hose structures can then be fixed in a suitable manner. For this purpose, the hose structures can be glued or be provided with a suitable housing.
According to a further aspect, the present disclosure also relates to an x-ray generator comprising an above-described cooling device, a high-voltage generator and an x-ray tube. The high-voltage generator generates the high voltage necessary for the operation of the x-ray tube. The x-ray tube can be mechanically and electrically connected to the high-voltage generator via a central high-voltage contact. The cooling device extends radially around the x-ray tube, with the result that the x-ray tube is cooled and at the same time is electrically shielded.
The present disclosure moreover also relates to a method for cooling an x-ray generator. A high-voltage generator for generating a high voltage is provided. An x-ray tube is mechanically and electrically connected to the high-voltage generator via a high-voltage contact. An above-described cooling device is provided, wherein the gas-conducting channel defined by the cooling device extends spirally around the x-ray tube in order to cool the x-ray tube and at the same time to shield it electrically. A gaseous cooling fluid is conducted through the cooling system for cooling the x-ray generator.
The cooling power that can be achieved using the gaseous cooling fluid is less than the cooling power that can be reached with liquid coolants and is up to 40 W, preferably between 0.5 and 25 Watts and further preferably 1 to 12 W.
As already mentioned, a large part of the energy expended in an x-ray generator is converted into heat. In order to save energy and to produce as little excess heat energy as possible, an x-ray tube can also be operated in pulsed mode, in that the x-ray radiation is generated in each case only for a short time. Much less waste heat is generated by pulsed-mode operation than in the case of a continuous-wave operation. In this way, a relatively high-powered x-ray tube can be used which, however, nevertheless generates much less waste heat than a corresponding x-ray tube actuated in continuous-wave operation. With suitable dimensioning, the cooling device according to the disclosure can therefore be used particularly advantageously in pulsed-mode operation in relatively high-powered x-ray generators.
Features which are described in connection with individual embodiments can, unless otherwise indicated, also be used in connection with other embodiments.
Embodiment examples of the disclosure are explained in the following with reference to the drawings, in which:
The cooling device 14 has a housing 18 with a gas inlet opening 20 and a gas outlet opening 22 for supplying or for discharging the gaseous coolant. In the interior of the cooling device 14, the coolant is guided past the x-ray tube 12 on a spiral path in a gas-conducting channel 24. The coolant absorbs the heat generated by the x-ray tube 12 and dissipates it to the surroundings.
The x-ray tube 12 is usually operated at a high voltage of between 20 and 150 kV. The required high voltage is provided by the high-voltage source 16 and applied to the x-ray tube 12 via a correspondingly provided contacting. In order to guarantee the operational safety of the arrangement, the accessible housing parts, in particular the housing 18 of the cooling device 14, are connected to ground.
The cooling device 14 therefore not only needs to be designed such that the heat generated by the x-ray tube 12 can be dissipated but must at the same time also insulate the x-ray tube 12 electrically with respect to the surroundings.
The housing 18 of the cooling device 14 is therefore expediently manufactured from thermoplastic, e.g. from polysulfone. In the embodiment shown in
The course of the gas-conducting channel 24 in the interior of the cooling device 14 is depicted in the cross section of
The length of the gas-conducting channel 24 must be dimensioned such that sparking between the centrally arranged x-ray tube 12 at high-voltage potential and the outside of the housing 18 of the cooling device 14 at ground potential is prevented.
The minimum length of the gas-conducting channel to be used in each case depends on the level of the operating voltage of the x-ray tube. In general it can be said that the length of the gas-conducting channel should be approximately 3 mm/kV. In the case of a 100-kV x-ray tube this means that the length of the gas-conducting channel between the centrally arranged x-ray tube and the gas inlet opening or the gas outlet opening should be approximately 30 cm.
In order to guarantee the operational safety of the arrangement 10, not only must the spiral gas-conducting channel 24 of the cooling device 14 be designed sufficiently long but it must also be ensured that no sparking can occur in the radial direction through the inner and outer walls of the housing 18 of the cooling device 14.
In order to prevent such radial sparking, the sum of the wall thicknesses of the gas-conducting channel 24 in the radial direction of the cooling device 14 must be chosen such that the resulting total wall thickness prevents such sparking. The required total thickness of the walls depends on the dielectric properties of the material which is used for the housing 18 of the cooling device 14. Typically used thermoplastics have a dielectric strength of from 10 to 20 kV/mm. For a 100-kV x-ray tube this in turn means that a total wall thickness of approximately 10 mm should be provided in order to also prevent radial sparking.
The curve of the electrostatic potential in the radial direction along the line 3-3 of
In order to prevent sparking also in this embodiment of the cooling device 14, the same criteria as in the previously described embodiment apply to the length of the gas-conducting channel 24 and to the sum of the wall thicknesses in the radial direction.
This potential route for sparking is represented in
Remaining interspaces between the free ends of the inner walls 26a and 26b and the respectively opposite end walls 28a and 28b are represented exaggerated in
However, even narrow slits would be sufficient to make sparking possible. A potential spark path is drawn in as a broken line in
Moreover, when non-hazardous cooling gases such as air or nitrogen are used it is also not absolutely necessary to ensure a completely gas-tight connection between the two housing parts 18a and 18b. Nevertheless, escaping cooling gas does mix with the ambient air, but does not lead to contamination of the components or of the products to be examined, in contrast to the dielectric oils otherwise usually used.
The above embodiments serve only to illustrate the present disclosure and are not to be interpreted as limiting. Of course, a person skilled in the art will also combine individual or all features which are described in connection with individual embodiments with other embodiments of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2017 002 210.0 | Mar 2017 | DE | national |
This application is the U.S. national phase of the International Patent Application No. PCT/EP2018/055393 filed Mar. 6, 2018, which claims the priority benefit of German Patent Application No. 10 2017 002 210.0 filed Mar. 8, 2017, the contents of all being incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/055393 | 3/6/2018 | WO | 00 |
