EP0268516B1 - Cooling device for an x ray tube - Google Patents

Description

The invention relates to a device for cooling an X-ray source, of the type in which an X-ray tube is cooled using a fluid which is put into forced circulation (see document: PATENT ABSTRACTS OF JAPAN, vol. 8, no.44 (E-229) [1481], 25 February 1984; & JP-A-58 198 899 (HITACHI SEISAKUSHO KK) 18-11-1983).

An X-ray source consists of an X-ray tube contained in an equipped sheath. The sheath provides protection against X-rays, electrical and mechanical shocks. More and more often, the X-ray source further comprises a system for cooling the X-ray tube and the equipped sheath; this cooling being imposed by the fact that the electrical energy used to produce X-rays is transformed into X-rays with a yield of the order of 1%, that is to say that 99% of this energy is transformed into heat inside the source.

In very lightly loaded radiological applications, that is to say where the frame rates are low and correspond to an average dissipation power of the order of 200 Watts, natural convection phenomena are sufficient to ensure cooling. ; a small fan close to the duct making it possible to further increase the convection around the latter so as to reach approximately 400 Watts.

With modern radiodiagnostic techniques, such as vascular examinations, radiocinema, or the scanner, the frame rates are very high and can correspond to an average dissipation power of several thousand watts. For such radiological applications, the cooling systems are much more important, and their efficiency conditions the operation and the performance of these radiodiagnostic installations where the thermal load of the radiogenic source is very high.

The most common cooling method, when the thermal load is high, consists in cooling the X-ray tube using a fluid already contained in the sheath to provide electrical insulation; this fluid being oil for example. The fluid or oil is forced into circulation around the X-ray tube, and outside the sheath where it passes through a cooling circuit which includes a heat exchanger: the fluid or oil, having received the heat produced by the X-ray tube , is cooled in turn when it passes through the heat exchanger; the heat exchanger being for example of the oil exchanger type or else of the type comprising a second circuit in which a second cooling fluid circulates, water for example.

Thus the X-ray tube and the sheath are cooled by the oil which circulates in the sheath, the oil itself being cooled thanks to the heat exchanger whose dimensions must allow the heat produced by the power dissipated during a operating cycle of the X-ray source.

In radiology, and especially in the uses of scanner, an operating cycle is formed of two consecutive periods of which the first corresponds to an intensive operation of the radiogenic source called time of examination, and whose second is called time of rest and corresponds when the operation of the latter stops. With the scanner, the examination of a patient requires a large number of linked sectional views, so that during the examination time, the thermal load regime is very high; then between two examinations of patients, that is to say during the rest time no heating is brought to the radiogenic source. As a result, the heat exchangers used in the prior art are oversized with respect to the power dissipated during an operating cycle. As a result, these heat exchangers have the disadvantages, in addition to a high cost, a large size and weight which lead to the use of heavy and complex mechanical means to make the sheath mobile.

,   (1);

où µ est la capacité calorifique équivalente de l'ensemble.With this method, the oil passing through the sheath receives, during the examination time, a quantity of heat Q which is divided into two parts Q1, Q2: the first part Q1 is removed by the heat exchanger during the time d 'exam ; the second part Q2 raises the temperature of the sheath-cooling circuit assembly by a value ϑ such that:

ϑ = $\frac{\text{Q2}}{\text{µ}}$

, (1);

where µ is the equivalent heat capacity of the set.

It should be noted that in the prior art, in order not to lead to completely inadmissible dimensions of the heat exchanger, the solution consists in increasing the heat capacity µ of the assembly formed by the sheath and the cooling circuit. , by increasing the volume of the fluid or oil which is used to cool the sheath and the X-ray tube. This solution, apart from increasing the volume and the weight of the assembly, has the drawback of reducing the efficiency of the heat exchanger.

Indeed, assuming for the example that the heat exchanger is of the external oil-air type, as a first approximation, the quantity of heat which it makes it possible to dissipate is proportional to the temperature difference between the fluid or the oil which passes through the heat exchanger and the outside air.

où Pe est la puissance moyenne pendant un examen ; T1 est le temps correspondant à l'examen ; α est la coefficient d'échange de l'échangeur thermique; µ est la capacité calorifique de l'ensemble.On the other hand, the temperature rise of the assembly from a flow temperature ϑ 1, to a maximum admissible temperature ϑ m is written:

where Pe is the average power during an examination; T1 is the time corresponding to the examination; α is the heat transfer coefficient of the heat exchanger; µ is the heat capacity of the assembly.

It appears from the first relation (1) above that the exchange coefficient α must be as large as possible to limit the rise in temperature of the assembly, the coefficient α being limited on the one hand by the dimension of l 'heat exchanger, and further limited by the maximum allowable temperature of the assembly.

It also appears from the second relation (2) above, that the higher the heat capacity µ, the slower the rise in temperature. Also, in the prior art, the increase in the heat capacity µ which is carried out to limit the dimensions of the heat exchanger, is such that the maximum admissible temperature is reached at the end of the examination time T1. It follows that it is only at the end of the examination time T1 that the exchange coefficient α is the greatest: this leads to increasing the dimensions of the exchanger so that part of the benefit provided by l The increase in oil volume is lost.

The present invention relates to a device for cooling an X-ray source, making it possible to obtain efficient cooling of the sheath and the X-ray tube with a small volume of fluid or oil used to cool the sheath and the X-ray tube, while using a heat exchanger of small dimensions compared to the prior art. This is obtained by a new arrangement of means which makes it possible, in particular, to carry out a storage of heat during the examination, then a restitution of this heat to the heat exchanger between examinations, so that the heat exchanger functions, in the best conditions, at a regime close to continuous.

According to the invention, a cooling device for an X-ray source, comprising a sheath containing an X-ray tube operating with a higher thermal load during an examination time than during a rest time which follows the examination time, the sheath further containing a fluid to which the X-ray tube gives up its heat, the fluid being forced into circulation in a given direction in the sheath and in a cooling circuit comprising a heat exchanger, the heat stored by the fluid being partially removed the heat exchanger according to a first amount of heat, a second amount of the heat stored by the fluid tending to raise the temperature of the sheath-cooling circuit assembly, is characterized in that the cooling circuit comprises means for, on the one hand, storing a third quantity of heat stored by the fluid when the latter reaches a predetermined temperature, and on the other hand, for restoring this third quantity of heat to the heat exchanger via the fluid during the rest time which follows the examination time.

• la figure 1 représente de manière schématique un dispositif de refroidissement selon l'invention ;
• la figure 2 est un diagramme qui illustre le fonctionnement du dispositif de refroidissement de l'invention.

The invention will be better understood from the description which follows, given by way of nonlimiting example, and with reference to the two appended figures among which:

• Figure 1 schematically shows a cooling device according to the invention;
• FIG. 2 is a diagram which illustrates the operation of the cooling device of the invention.

FIG. 1 shows a cooling device 1 intended to cool an X-ray source 2. The X-ray source 2 comprises a sheath 3 containing in a conventional manner an X-ray tube 4. The X-ray tube 4 is of a conventional type, and comprises a vacuum-tight envelope 5, a first end 6 of which carries a cathode 7 disposed opposite an anode disc 8, the anode disc constituting in the nonlimiting example described, a rotating anode. The anode disc 8 is secured along its axis of symmetry 9. to a rotor 10 which is itself carried by the second end 11 of the casing 5, via a support axis 12; the rotation of the rotor 10 and the anode disc 8 around the axis of symmetry 9 is ensured by a stator 13 disposed outside the casing 5. The cathode 7 and the anode 8 are electrically connected respectively on the side of the first end 6 and the second end 11 of the casing 5, to a first and to a second high-voltage end 14,15 mounted conventionally on the sheath 3 of way out of the latter without compromising its tightness. The high voltage ends 14,15 are intended to be connected in a known manner to one or more electrical sources (not shown); the other electrical connections necessary for the operation of the X-ray tube 4 being also known, and not participating in the invention, they are not shown in the figure.

When the X-ray tube 4 is in operation, the cathode 7 generates an electron beam 19 which bombards the anode 8 at a point where it forms a focal point 16 from which X-rays are emitted; these X-rays form a beam 17 which leaves the sheath 3 through an outlet window 18. The heat generated in the anode 8 by the bombardment of the electron beam 19 is transferred to the envelope 5, conventionally, mainly by thermal radiation from the anode 8. The sheath 3 contains a fluid 20 in which the envelope of the X-ray tube bathes. The fluid 20 is conventionally constituted by oil, the primary function of which is to provide electrical insulation in the sheath 3, which in the non-limiting example described, has the second function of cooling the X-ray tube 4; the fluid 20 or oil being called oil in the following description to simplify the latter. To this end, the sheath 3 has at each of these two opposite ends 21, 22 an orifice 23, 24 by which it communicates with a cooling circuit 25 in which the oil 20 is forced into circulation, so as to be cooled outside the sheath 3 after receiving heat produced by the X-ray tube 4.

The cooling circuit 25 comprises on the one hand a pump 26 of a conventional type, intended to force the circulation of the oil 20, and on the other hand comprises a heat exchanger 27 of a type in itself conventional also . In the nonlimiting example described, the heat exchanger 27 is of the oil-air type, that is to say that the oil 20 which passes through the heat exchanger transfers its heat to the ambient air. The heat exchanger 27 can for example be constituted by a radiator comprising a coil (not shown) provided with fins, and the oil 20 passing through this coil gives off heat to the ambient air by convection; this convection can be favored by a fan (not shown).

According to a characteristic of the invention, the cooling circuit further comprises means 28 for storing heat stored by the oil 20 before it passes through the heat exchanger 27.

The first orifice 23 of the sheath 2, located on the side of the second end 11 of the X-ray tube 4, that is to say on the side of the anode 8, communicates with a pipe 30 in which the oil 20 is conducted in the means 28 for storing heat. In the nonlimiting example described, an expansion device 31 is disposed between the sheath 3 and the means 28 intended to store the heat. The expansion device 31 makes it possible to compensate for expansions of the oil 20 by a modification of its volume; this expansion device 31 being of a type known per se. A second pipe 32 connects the means 28 for storing heat to an inlet 33 of the heat exchanger 27, the outlet 34 of which is connected by a third pipe 35 to the inlet 36 of the pump 26; an outlet 37 of the pump 26 being connected by a fourth pipe 38 to the inlet of the sheath 3, that is to say to the second orifice 24 which is disposed on the side of the cathode 7. The pump 26 determines at the oil 20 a direction of circulation, represented in FIG. 1 by the arrows 40, such as the oil 20 which enters the sheath 3 through the second orifice 24, crosses the sheath 3 in the direction of the first orifice 23 through which it exits of the sheath 3 to pass into the dilation device 31, then to pass through the means 28 intended to store the heat, and then pass through the heat exchanger 27 then the pump 26 before returning to the sheath 3.

The means 28 for storing heat have the function of storing heat stored by the oil 20 in contact with the X-ray tube 4. But, according to another characteristic of the invention, this function is only ensured from time when the oil temperature 20 a reaches a predetermined value when the oil 20 passes through the means 28 for storing the heat; that is to say that the means 28 for storing heat plays the role of a thermal flywheel whose action is controlled with a temperature threshold.

To this end, in the nonlimiting example described, the means 28 for storing heat comprise a second heat exchanger 41. The second heat exchanger 41 delimits an enclosed volume in which the fusion of a solid body C under the effect of the heat transferred to the second heat exchanger 41 by the oil 20. The second heat exchanger 41 comprises a coil 42 in which the oil 20 to be cooled circulates. The coil 42 is provided with fins 43 which form partitions, the interlacing of which constitutes cells 44 which are filled with the body C, symbolized in the figure by a cloud of points.

The nature of the body C is chosen so that its melting point is close to the predetermined temperature which in the nonlimiting example described corresponds to the maximum temperature which is desired for the oil 20 in the sheath 3.

The body C is also chosen to have a sufficiently high latent heat of fusion, of at least 10 calories per gram for example, so as to allow, from its fusion, to store a lot of heat in a small volume. Thus for example, if it is desired that the oil 20 does not exceed a temperature of 80 ° C in the sheath 3, the body C can be constituted for example by stearic acid which melts at 70 ° C and has latent heat of fusion of the order of 50 calories per gram. The fusion of the body C also makes it possible on the one hand to store a large amount of heat during the examination time T1 where the X-ray tube 4 operates with a high thermal load, and on the other hand allows this heat to be restored to the first heat exchanger 27, that is to say the oil 20, during the rest time T2 when the X-ray tube 4 operates with a reduced or zero thermal load, by the fact that the body C resolidifies and restores the heat that 'it accumulated during its merger.

In operation during the examination time T1, the heat produced by the X-ray tube 4 is transferred to the oil during this examination time, and this heat is divided into two quantities Q1, Q2: the first quantity Q1 is removed by the first heat exchanger 27 during the exam time T1; the second quantity of heat Q2 raises the temperature of the assembly formed by the sheath 3 and the entire cooling circuit 25, as long as the melting of the body C has not occurred. When the melting of the body C occurs, the latter absorbs calories as a function of its latent heat of fusion, so that the temperature of the oil 20 at the inlet 33 of the first heat exchanger is substantially stabilized at the same temperature as the body melting temperature C. This makes it possible to significantly reduce, compared to the prior art, the heat capacity μ of the sheath-cooling circuit assembly 3.25 by reducing the volume of the oil 20 so that the maximum temperature ϑ m of the oil 20 in the sheath 3 is quickly reached, and that the heat exchange at the level of the first heat exchanger 27 takes place at a high temperature; this has the consequence of increasing the efficiency of the first heat exchanger 27 compared to the prior art, and thus makes it possible to reduce its dimensions.

FIG. 2 illustrates this operation by a diagram which shows the variations in the temperature ϑ of the oil 20 at the inlet 33 of the first heat exchanger, as a function of time T, by a first curve 50.

Assuming that the X-ray source 2 has already been heated and cooled, the oil temperature 20 at instant to where the examination time T1 begins at a high thermal load regime, is at a starting temperature ϑ 1 between the temperature ϑ o of the ambient area and the maximum temperature ϑ m of the oil 20 in the sheath 3. With the cooling device according to the invention, the temperature of the oil 20 increases from instant to at a second instant t1 where it reaches the melting temperature ϑ f. The fusion of the body C having started at the second instant t1, this fusion absorbs a third quantity of heat Q3 which corresponds to the calories that the X-ray tube 4 gives up to the oil 20 when the latter has reached the maximum temperature ϑ m; so that the temperature of the oil 20 at the inlet of the first heat exchanger 27 is kept substantially constant at the same value as the melting temperature ϑ f; the melting temperature ϑ f may be a few degrees C lower than the maximum temperature ϑ m of the oil 20 in the sheath 3.

The oil 20 maintains a temperature close to the melting temperature ϑ f after the end, at a third instant t2 of the examination time T1. Indeed, from the third instant t2 which also corresponds to the start of the rest time T2, the X-ray tube 4 no longer produces heat, and the temperature of the oil 20 at the outlet of the sheath 3 decreases. As a result, the body C tends to re-solidify and gives back to the oil 20 the heat which it has stored during the examination time T1. It follows that the temperature of the oil 20 at the inlet of the first heat exchanger 27, retains substantially the same temperature as the melting temperature ϑ f until a fourth instant t3 when the body C is fully resolidified. From this instant t4, the temperature of the oil 20 at the inlet of the first heat exchanger 27 decreases to reach the starting temperature ϑ 1 at a fifth instant t4.

This allows during a third time T3, to obtain an operation of the first heat exchanger 27 at a temperature close to the melting temperature ϑ f which is itself close to the maximum temperature ϑ m of the oil 20 in the sheath 3 Consequently, the first heat exchanger 27 works at a high temperature, which improves the exchange coefficient α, during the third time T3 which can be greater than the examination time T1.

A second curve 51 shown in dotted lines in FIG. 2 illustrates the operation of a cooling device according to the prior art. In the prior art, the oil which has received heat from an X-ray tube must be cooled by a cooler, for example of the same type as the first heat exchanger 27. When it is put into operation at a high thermal load regime, from the instant to, the temperature of this oil increases, from the starting temperature ϑ 1, up to to reach the maximum temperature ϑ m admissible in the sheath at the third instant t2, then decreases until finding the starting temperature ϑ 1. It is observed in this case that, unlike the case of the invention, a temperature close to the maximum temperature ϑ m is only retained during a fourth time T4 much less than the examination time T1; that is to say that a heat exchanger, in an installation of the prior art, has a mediocre exchange coefficient α during the examination time T1. As explained in the preamble, a solution to the mediocre exchange coefficient α in the prior art consists in increasing the volume of the oil to increase the heat capacity of the assembly.

est supérieure au produit de la capacité calorifique µ par la différence Δ ϑ entre la température de départ ϑ 1 et la température maximum ϑ m, soit:

$\text{µ.Δϑ < Q2 + Q3.}$

On the other hand, with the present invention, thanks to the means 28 for storing the heat stored in the oil 20, the heat capacity equivalent to α of the sheath-cooling circuit assembly 3-25 is much lower than in the prior art , from which it results, on the one hand, a significant reduction in weight and size, and from where it results, on the other hand, that the maximum temperature ϑ m is reached very largely before the end of the time of T1 exam. This means that the sum of the second and third amounts of heat $\text{Q2 + Q3}$

is greater than the product of the heat capacity µ by the difference Δ ϑ between the flow temperature ϑ 1 and the maximum temperature ϑ m, that is:

$\text{µ.Δϑ <Q2 + Q3.}$

Another advantage provided by the invention resides in the fact that it is not necessary to wait until the oil 20 has returned to the starting temperature ϑ 1 to start a new examination time T1, it is that is to say to put the X-ray tube 4 back into operation with a high thermal load. On the contrary, in the prior art, it is necessary that the oil is at the temperature of departure to start a new examination, since the maximum temperature ϑ m would be reached more quickly, and that in order not to exceed this maximum temperature ϑ m it would be necessary to shorten the duration of the examination time.

Yet another advantage provided by the invention is that by maintaining an almost constant oil temperature, the effects of thermal expansion are minimized.

This description constitutes a nonlimiting example, which shows that a cooling device for an X-ray source in accordance with the invention makes it possible to obtain, compared to the prior art, much more efficient and safer cooling, while reducing significantly both the size and the weight of the sheath 3, of the heat exchanger 27 and of the oil 20.

The products capable of constituting the body C are numerous, and are chosen in particular according to the power dissipated during the examination time, the duration of the examination time and the maximum desired temperature of the oil in the sheath 3 So, for example, if it is accepted that the temperature of the oil 20 in the sheath 3 can rise to around 110 ° C., the body C can be methyl fumarate, the density of which is 1.37. and the melting point is at 102 ° C, with a latent heat of fusion of 60 calories per gram: if we consider a volume of seven liters of this body, these seven liters can store once reached the temperature of 102 ° C, approximately 2,353,000 Joules. One liter of oil can store around 400 calories per degree and, for a temperature rise from 50 ° C to 100 ° C, one liter of oil can store 20,000 calories, i.e. 84,000 Joules. Thus, we observe that to store 2,350,000 Joules with oil, it would be necessary to use approximately 27 liters of additional oil, that is to say a volume approximately four times greater than that of the body C without, however, obtaining the effect of limiting the maximum temperature ϑ m, or improving the exchange coefficient α of the first heat exchanger 27.

Claims (10)

1. A cooling device for a radiogenic source, comprising a sheath (3), said sheath (3) containing a radiogenic tube (4) functioning with a higher thermal load during an examination time (T1) than during a quiescent time (T2), which follows the examination time (T1), said sheath (3) containing furthermore a fluid (20) to which the radiogenic tube (4) yields its heat, said fluid (20) being subjected to a forced circulation in a given direction (40) in the sheath (3) and in a cooling circuit (25) comprising a heat exchanger (27), the heat stored by the fluid (20) during the time of examination (T1) being partly removed by the heat exchanger (27) in accordance with a first quantity (Q1) of heat, a second quantity (Q2) of heat stored by the fluid (20) tending to raise the temperature of the sheath-circuit cooling arrangement (3 and 25), characterized in that the cooling circuit (25) comprises means (28) in order on the one hand to hold a third quantity (Q3) of heat stored by the fluid (20), when the latter reaches a predetermined temperature (ϑm), and on the other hand to restore this third quantity (Q3) of heat to the heat exchanger (27) by way of the fluid (20) during the quiescent time (T2).
2. The cooling device as claimed in claim 1, characterized in that the means (28) for holding the third quantity (Q3) of heat comprise a second heat exchanger (41) in which the fluid circulates and in which there is contained a solid body (C) which fuses absorbing the third quantity (Q3) of heat owing to its latent heat of fusion when the fluid (20) substantially reaches the fusion temperature (ϑf) of the body (C).
3. The cooling device as claimed in claim 2, characterized in that the body (C) which has fused during the examination time (T1) resolidifies during the quiescent time (T2) while yielding the third quantity (Q3) of heat to the first heat exchanger (27) by way of the fluid (20).
4. The cooling device as claimed in any one of the preceding claims characterized in that the means (28) for holding the heat are arranged upstream from the first heat exchanger (27) in terms of the direction (40) of the circulation of the fluid (20).
5. The cooling device as claimed in any one of the preceding claims, characterized in that the arrangement constituted by the sheath (2), the cooling circuit (25) and the fluid (20) has a thermal capacity (µ) such that the product of multiplication of the thermal capacity (µ) with a temperature difference (Δϑ) between a starting temperature (ϑ1) and the maximum temperature (ϑm) is less than the sum ( $\text{Q2 + Q3}$

   

   ) of the second and third quantities of heat $\text{(µ·Δϑ < Q2 + Q3)}$

   

   .
6. The cooling device as claimed in claim 2 or claim 3, characterized in that the fusion temperature of the body (C) is comprised between 50° C and 120° C.
7. The cooling device as claimed in claim 2, claim 3 or claim 5, characterized in that the body (C) has a latent heat of fusion equal to or larger than 10 calories per gram.
8. The cooling device as claimed in claim 2 or claim 3, characterized in that the body (C) is made up of methyl fumarate.
9. The cooling device as claimed in claim 2 or claim 3, characterized in that the body (C) is made up of stearic acid.
10. The cooling device as claimed in claim 2 or claim 3, characterized in that the body (C) is made up of naphthalene.

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