Patent Publication Number: US-7221736-B2

Title: Expansion device for fluid coolant/insulation in a x-ray apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention concerns an expansion device for the coolant/insulating fluid in an x-ray apparatus as well as an x-ray radiator with such an expansion device. 
   2. Description of the Prior Art 
   In a medical x-ray apparatus, to reduce the heat that accumulates due to the generation of x-ray radiation it is necessary to provide cooling for the x-ray tube arranged in an x-ray radiator. For cooling it is known to surround the x-ray tube with a coolant which can (as, for example, insulating oil) exhibit insulation properties. The coolant and/or insulation must exhibit a pressure that is high enough to prevent an outgassing, since otherwise the cooling capacity and/or the ability to withstand electrical voltages are reduced, but is low enough to prevent damaging of the x-ray tube due to overpressure. 
   For this purpose, expansion devices are known that ensure a defined minimum pressure for the coolant and/or insulation means and allow an expansion (caused by a temperature increase) of the coolant and/or insulation within a defined range. Particularly in the case of x-ray apparatuses with rotary piston x-ray tubes rotating in coolant and/or insulation, negative pressure can be created due to the rotation movement. Such known expansion devices are formed by a reservoir in which the coolant and/or insulation means is located and a sealed air chamber (such as, for example, an airbag or a metal bellows) is provided to ensure the minimum pressure. An expansion of the coolant and/or insulation compresses the sealed air chamber, leading to an excessive, exponential increase of the pressure. If the pressure rises too high, the x-ray apparatus is deactivated (triggered by a pressure switch). 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a simple expansion device for an x-ray apparatus that ensures a defined minimum pressure of the coolant and/or insulation and that also prevents an excessive pressure load of the x-ray apparatus given a temperature rise. 
   The above object is achieved by an expansion device for an x-ray apparatus with a coolant and/or insulation having an elastic pressure element with an essentially linear force-displacement characteristic curve that still ensures a minimum pressure while allowing a linear pressure rise in a simple manner dependent on a rising temperature of the coolant and/or insulation. Given an elevated temperature, an excessive final pressure (as with an exponential pressure rise) is prevented and thus the components of the x-ray apparatus, in particular the x-ray tube in the coolant and/or insulation, are less pressure-loaded and thus can be designed overall for a lower pressure and so can be produced with less expenditure. 
   The term “curve” is used herein in the mathematical sense of describing a graphed relationship between two parameters, and thus encompasses a relationship that is entirely or substantially linear. 
   In a manner advantageous for a simple realization of the expansion device, an arrangement of the cooling and/or insulation volume is provided in an expansion reservoir so that the expanded volume presses against the elastic pressure element given an expansion of the cooling and/or insulation means volume due to the temperature rise. 
   In an embodiment of the invention advantageous for by low-cost production, sealing of the expansion reservoir is provided by a piston that can be shifted due to the volume increase and thus can be pressed against the elastic pressure element. In an advantageous manner, the expansion device has a piston that can be displaced along the inner wall of the expansion reservoir. At one axial (in the displacement direction) side of the piston, the coolant and insulation is enclosed by the expansion reservoir and at other axial side the expansion reservoir has an air chamber open to the outside and thus free of counter-pressure. 
   A further embodiment of the expansion device that can be produced in a simple and particularly compact manner seals the expansion reservoir with a bellows (in particular a metal bellows) that is compressed due to the expansion of the coolant and/or insulation and is thereby pressed against the elastic pressure element. The bellows can be arranged so as to be compressed within the expansion reservoir. On the outside of the bellows the coolant and/or insulation is enclosed by the expansion reservoir and on the inside of the bellows an air chamber is enclosed that is open to the outside. 
   In a further embodiment of the expansion device seals the expansion reservoir with a bellows (in particular a metal bellows) that is expanded due to the expansion of the coolant and/or insulation and thereby is pressed against the elastic pressure element. The bellows can expand within the expansion reservoir. On the inside of the bellows the coolant and/or insulation is enclosed and on the outside of the bellows the expansion reservoir encloses an air chamber open to the outside. 
   In a further embodiment of the invention, the expansion device has an elastic pressure element is given the elastic element with an essentially linear elastic characteristic curve by means of a biasing element. To accommodate a slow rise of the pressure, the biasing element gives the elastic pressure element a characteristic curve having a slope between 8% and 45%. According to a further embodiment of the invention, the biasing element is in the form of a gas pressure spring with an essentially linear force-displacement characteristic curve. 
   Although the invention can be used to advantage in an x-ray radiator with an x-ray tube in a coolant and/or insulation, the invention is not limited to, this type of apparatus. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a known expansion device for an x-ray apparatus with an expansion reservoir for a closed coolant and insulation volume that presses on a closed air compression chamber given a temperature rise. 
       FIG. 2  shows an inventive expansion device for an x-ray apparatus with an expansion reservoir for a closed coolant and insulation volume that presses on a gas pressure spring with a linear elastic characteristic curve via a displaceable piston given a temperature rise. 
       FIG. 3  shows an inventive expansion device for an x-ray apparatus with an expansion reservoir for a closed coolant and insulation volume that presses on a gas pressure spring with a linear elastic characteristic curve via a bellows given a temperature rise. 
       FIG. 4  shows an inventive expansion device for an x-ray apparatus with an expansion reservoir or a closed coolant and insulation volume that is located in a bellows and presses on a gas pressure spring with a linear characteristic curve given a temperature rise. 
       FIG. 5  shows a temperature-pressure diagram with a known exponential characteristic curve and with a linear characteristic curve that is achieved with the inventive expansion device. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a known expansion reservoir  1  that is connected at its right end with an x-ray radiator  2  via an oil feed  3 , and has a closed air chamber  5  at its left end and a,piston  12  that can be displaced along its inner wall  14 . A coolant and/or insulation in the form of oil  7  is enclosed between the piston and the right end of the expansion reservoir such that the closed air chamber  5  presses against said oil  7  with a minimum pressure given a normal, non-elevated temperature. A rotary piston x-ray tube  9  to be cooled is supported in the x-ray radiator  2  filled with oil  7  such that it can rotate. Given a temperature rise, the oil volume in the right end of the expansion reservoir  1  increases and thereby presses harder on the piston  12 , such that the piston  12  is displaced and the counter-pressure in the closed air chamber (and therewith also the pressure in the oil  7 ) exponentially increases. 
   A safety pressure switch  8  deactivates the x-ray apparatus given excessive pressure, meaning pressure endangering the rotary piston x-ray tube  9  of the x-ray radiator  2 . This can lead to unwanted interruptions of examinations involving the x-ray apparatus. 
     FIG. 2  shows an inventive expansion device that differs from the prior art in that an expansion reservoir  10  has an air chamber  15  open to the outside instead of a closed air chamber, such that the formation of an excessive pressure is prevented given a temperature rise. The piston  12  is displaced along the inner wall  14  of the expansion reservoir  10  corresponding to the essentially linear force-displacement characteristic curve of a pressure element, in particular of a gas pressure spring  11 . According to one embodiment of the invention, an arrangement of the elastic pressure element (in particular of the gas pressure element  11 ) in the air chamber  15  open to the outside is provided in the expansion reservoir  10 . The linear elastic characteristic curve (i.e. the linear relationship between pressure and displacement), of the gas pressure spring preferably has a slope between 8% and 45%, particularly preferably between 8% and 15%. Due to the linear elastic characteristic curve of the inventive gas pressure spring and because air escape to the environment via the opening  13  is possible given the opened air chamber  15  (and the air is not likewise compressed, as in the prior art), the pressure of the oil  7  merely rises linearly. Endangerment of the components of the x-ray radiator  2  due to an exponential pressure increase given just a relatively slight temperature rise can thus is precluded. 
     FIG. 3  shows a further embodiment of the inventive expansion device in which the expansion reservoir  10  has a metal bellows  4  (in which the gas pressure spring  11  with a linear elastic characteristic curve is disposed) instead of a piston. If an expansion of the oil  7  occurs due to a temperature rise, the gas pressure spring  11  located in the metal bellows  7  is compressed corresponding to its elastic constant. The metal bellows  14  is likewise compressed and emits air into the environment through the openings  13 ; the pressure in the expansion reservoir  10  likewise rises linearly. 
     FIG. 4  shows a further embodiment of the inventive expansion device in which, in contrast to  FIG. 3 , the oil  7  is arranged inside the metal bellows  4 . The opened air chamber  15  is located outside of the metal bellows  4  in the expansion reservoir. If an expansion of the oil  7  occurs due to a temperature increase, the metal bellows  4  expands and compresses the gas pressure spring  11  corresponding to its elastic constant; here the pressure in the oil  7  also rises linearly. 
     FIG. 5  shows a temperature-pressure diagram that contrasts the exponential pressure rise  17  in the oil, of a typical expansion device with a closed air chamber according to the prior art with the linear pressure rise  16  of an inventive expansion device with an opened air chamber  15  and a gas pressure spring  11  with an elastic characteristic curve rise of approximately 15%. A pressure that could endanger the x-ray tube is only achieved (if at all) by the inventive expansion device at a clearly higher temperature than in a system according to the prior art. 
   The invention can be briefly summarized as follows. In an expansion device for an x-ray apparatus, in particular with a rotary piston x-ray tube with a sealed coolant and/or insulation volume under a minimum pressure and pressing with this minimum pressure against an elastic pressure element, an elastic pressure element with a linear force-displacement characteristic curve (in particular in the form of a gas pressure spring  11  with a linear elastic characteristic curve) is provided to prevent an excessive pressure load given a temperature rise. 
   Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.