Abstract:
The present invention relates to an arrangement for treatment of articles by hot pressing and, in particular, by hot isostatic pressing. In particular, the present invention relates to a pressing arrangement for treatment of articles by hot pressing, and preferably hot isostatic pressing, capable of providing a controlled, rapid cooling rate. The pressing arrangement comprises a pressure vessel including a furnace chamber comprising a heat insulated casing and a furnace adapted to hold the articles. At least one cooling circuit is arranged on an outside of the pressure vessel, the cooling circuit comprising a coolant and being arranged to enable the coolant to flow along a central portion of an outer wall of the pressure vessel. The pressure vessel is arranged with a non-uniform wall thickness, where the vessel wall is thickest at upper and lower end portions of the pressure vessel.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to an arrangement for treatment of articles by hot pressing and, in particular, by hot isostatic pressing. In particular, the present invention relates to a pressing arrangement for treatment of articles by hot pressing, and preferably hot isostatic pressing, capable of providing a controlled, rapid cooling rate. 
     BACKGROUND OF THE INVENTION 
     Hot isostatic pressing (HIP) is a technology that finds more and more widespread use. Hot isostatic pressing is for instance used in achieving elimination of porosity in castings, such as for instance turbine blades, in order to substantially increase their service life and strength, in particular the fatigue strength. Another field of application is the manufacture of products, which are required to be fully dense and to have pore-free surfaces, by means of compressing powder. 
     In hot isostatic pressing, an article to be subjected to treatment by pressing is positioned in a load compartment of an insulated pressure vessel. A cycle, or treatment cycle, comprises the steps of: loading, treatment and unloading of articles, and the overall duration of the cycle is herein referred to as the cycle time. The treatment may, in turn, be divided into several portions, or phases, such as a pressing phase, a heating phase, and a cooling phase. 
     After loading, the vessel is sealed off and a pressure medium is introduced into the pressure vessel and the load compartment thereof. The pressure and temperature of the pressure medium is then increased, such that the article is subjected to an increased pressure and an increased temperature during a selected period of time. The temperature increase of the pressure medium, and thereby of the articles, is provided by means of a heating element or furnace arranged in a furnace chamber of the pressure vessel. The pressures, temperatures and treatment times are of course dependent on many factors, such as the material properties of the treated article, the field of application, and required quality of the treated article. The pressures and temperatures in hot isostatic pressing may typically range from 200 to 5000 bars, and preferably from 800 to 2000 bars and from 300° C. to 3000° C., and preferably from 800° C. to 2000° C. respectively. 
     When the pressing of the articles is finished, the articles often need to be cooled before being removed, or unloaded, from the pressure vessel. In many kinds of metallurgical treatment, the cooling rate will affect the metallurgical properties. For example, thermal stress (or temperature stress) and grain growth should be minimized in order to obtain a high quality material. Thus, it is desired to cool the material homogeneously and, if possible, to control the cooling rate. However, it is also of importance not to increase the total manufacturing costs of a pressing arrangement and/or the costs associated with operating the pressing arrangement in too large extent in attempt to satisfy the requirements with regard to desired cooling rate and homogenous cooling. 
     Prior art hot isostatic pressing arrangements are often manufactured with uniform cylinder vessel walls and an outer cooling circuit in which a cooling liquid is circulated. Thereby, a transmission of heat or thermal energy through the vessel walls can be achieved. A traditional prior art pressure vessel cylinder is shown in  FIG. 1   a . The pressure vessel cylinder  1  is closed at the respective ends by means of upper and lower lids  2  and  3 , respectively. Radial pre-stressing means  4   a  are provided around the envelope surface of the pressure vessel cylinder for accommodate radial forces exerted on the pressure vessel walls and axial pre-stressing means  4   b  are provided for accommodating axial forces exerted on the lids  2 ,  3 . The radial pre-stressing means can be provided around the entire envelope surface of the pressure vessel cylinder. Due to the pre-stressing means  4   a ,  4   b , the lids  2 ,  3  are capable of closing the pressure vessel  1  without any threading means or similar to attach the lids. Moreover, the outer wall of the pressure vessel  1  is provided with channels, or tubes,  5  in which a coolant for cooling may be provided. The coolant is preferably water, but other coolants are also contemplated. The flow of coolant is indicated in  FIG. 1  by the arrows in the channels  5 . During cooling, thermal energy is transferred from the warm pressure medium through the pressure vessel wall to the circulating cooling liquid. Furthermore, in order to be used in a pressing arrangement, the pressure vessel  1  is normally provided with means such a furnace, load compartment, heat isolation means etc., which not are shown in  FIG. 1   a  for purposes of clarity. 
     In  FIG. 1   b , another prior art pressure vessel is shown. The pressure vessel  10  has a so called “dog-bone” design. This pressure vessel  10  is not provided with any pre-stressing means in this solution. In the illustrated configuration, the lids  12 ,  13  are attached to the pressure vessel  10  by means of threaded sections  14   a  and corresponding threaded sections  14   b  of the pressure vessel  10 . Because there is no pre-stressing means for accommodating radial and axial forces exerted on the pressure vessel  10  and on the lids  12 ,  13 , the pressure vessel  10  has to be made stronger, in particular, at the end portions where the lids are attached. To absorb the significant axial load exerted primarily from the lids, the pressure vessel  10  is provided with thick walls at the portions at the upper and lower lid. Thereby, the pre-stressing means can be omitted in this design. As can be seen in  FIG. 1   b , the upper and lower end portions  16 ,  17 , respectively, of the pressure vessel wall are significantly thicker than the central portion  18  of the pressure vessel  10 , which has a reduced thickness to save weight. A relation between outer diameter, od, and inner diameter, id, (od/id) is at least 1.2 (and often up to 1.3-1.4) at the central portion  18  where the vessel  10  has its thinnest wall thickness. At the thicker portion of the pressure vessel wall  16 , the relation between outer diameter, od, and inner diameter, id, (od/id) is about 1.4-1.9. The significant radial and axial forces that have to be absorbed by the pressure vessel  10  require such high diameter relation od/id. 
     To provide an enhanced cooling capability, cooling elements are arranged in connection to the outer wall of the pressure vessel  10  in which a coolant is circulated. The coolant is preferably water, but other coolants are also contemplated. During cooling, thermal energy is transferred from the warm pressure medium through the pressure vessel wall to the circulating cooling liquid. 
     However, these prior art pressure vessels are impaired with drawbacks. The traditional uniform pressure vessel provided with axial and radial pre-stressing means may not provide a sufficiently rapid cooling without additional means for achieving such enhanced cooling. For example, heat exchangers have been suggested for that purpose. A heat exchanger arranged inside the pressure vessel do on the other hand add complexity in that, for example, pipes for supplying cooling medium has to be arranged in though holes of the pressure vessel. This may also entail increased maintenance needs. 
     The “dog-bone” solution, on the other hand, is very heavy due to the wall thickness despite the reduced wall thickness at the central portion. 
     To conclude, there is therefore a need within the art of improved pressure vessels for pressing arrangements capable of controlled, rapid and homogenous and cooling of articles and pressure medium. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide an improved pressure vessel for a pressing arrangement, which enables a controlled, rapid and homogenous cooling. 
     A further object of the present invention is to provide an improved pressure vessel for a pressing arrangement that can be manufactured with a high degree of tolerance. 
     These and other objects of the present invention are achieved by means of a pressing arrangement having the features defined in the independent claims. Embodiments of the present invention are characterized in the dependent claims. 
     In the context of the present invention, the terms “cold” and “hot” or “warm” (e.g. cold and warm or hot pressure medium or cold and warm or hot temperature) should be interpreted in a sense of average temperature within the pressure vessel. Similarly, the term “low” and high” temperature should also be interpreted in a sense of average temperature within the pressure vessel. 
     According to an aspect of the present invention there is provided a pressure vessel for isostatic pressing comprising radially pre-stressing means arranged for exerting radial compressive forces on the pressure vessel. The pressure vessel is arranged with a non-uniform wall thickness, wherein the has portions with a thicker wall thickness at an upper and a lower end portion of said pressure vessel and having portions with a thinner wall thickness at a central portion of the pressure vessel where a substantial amount of the heat transmission from the pressure medium to a cooling circuit comprising a coolant occurs. 
     The pressure vessel according to the present invention is advantageously used in a pressing arrangement for hot isostatic pressing in connection of treatment of articles. 
     Generally, to achieve cooling within the pressure vessel and cooling of the articles being treated within the pressure vessel, pressure medium is circulated through the furnace chamber and a cooler region of the pressure vessel, such as the intermediate space outside the furnace chamber. Thus, while the amount of pressure medium contained in the furnace chamber is approximately constant, there is a positive net flow of heat away from the article in the furnace chamber. 
     The present invention is on an overall level concerned with enhancing and speeding up the cooling in a controlled manner. More specifically, the present invention is based on the insight that a controlled and rapid cooling of, for example, articles to a desired temperature can be achieved (i.e. the cooling rate can be significantly increased) by making the pressure vessel wall thinner at portions or parts where a significant amount of the heat is transferred to the cooling circuit in comparison to the end portions of the pressure vessel wall. 
     The pressures and temperatures in hot pressing, and hot isostatic pressing, may typically range from 200 to 5000 bars, and preferably, from 800 to 2000 bars and from 300° C. to 3000° C., and preferably from 800° C. to 2000° C., respectively. By arranging the pressure vessel with thin walls at selected parts or portions, the heat removal or heat transmission through the vessel wall to the coolant flowing through the cooling circuit arranged outside the pressure vessel wall can be greatly enhanced. However, the inventor has realized that providing the pressure vessel with a thinner vessel wall may entail a number of problems. For example, it may be difficult to process a pressure vessel having such thinner vessel wall to achieve or obtain the required tolerances. Another problem that might arise is that it may be difficult to attach necessary construction parts, such as e.g. end plates for holding a wire winding, at the upper and lower ends and of the pressure vessel if the pressure vessel wall is made thinner. Starting from this, the inventor has reached the further insight that the increased cooling rate can be achieved at the same time as the above-mentioned problems are eliminated if the pressure vessel is made with a non-uniform wall thickness where the upper and lower end of the pressure vessel has the thickest wall thickness and portions where a significant amount of the heat is transferred is made thinner. That is, the part or portion of the vessel having a thinner wall thickness is located where the significant heat transmission to the coolant occurs. In preferred embodiments, the pressure vessel has a diameter relation, od/id, of less than 1.2, typically 1.1 or less and preferably below 1.07 at a central portion of the pressure vessel. 
     The present invention provides a number of advantages in comparison to the prior art. For example, the high degree of tolerance required within the art can be maintained by making the end portions at the upper and lower end of the pressure vessel adjacent to upper and lower lids thicker than other portions of the vessel. Thereby, the end parts will be rigid and can be processed to obtain a desired and required tolerance. Further, by making portions of the pressure vessel wall thinner than the end portions where a significant amount of the heat transfer occur, the cooling rate can be increased significantly by the improved heat transfer between the hot pressure medium and the coolant flowing in the cooling circuit. Accordingly, an increased cooling rate can be achieved. 
     However, the present invention can be combined with, for example, a heat exchanger or heat sink arranged within the pressure vessel to even further increase and speed up the cooling rate. 
     A further advantage of the present invention, is that the control of the cooling procedure can be improved, which, in turn, leads to better quality of the articles processed by the hot pressing arrangement. For example, the articles are often tension free after the hot pressing procedure. 
     The present invention is also suitable in very large hot isostatic pressing arrangements. The larger a hot isostatic pressing arrangement is made, problems related to the cooling process due to, for example, will be more and more pronounced. For example, the large amounts of articles treated during a pressing procedure may lead to a cooling process that is more difficult to control. Another problem with large pressing arrangements is that the heat transfer or heat transmission often is poorer than in more regular sized pressing arrangements due to a larger pressure medium volume in relative to a vessel wall surface in comparison to a smaller pressure vessel arrangement. 
     By implementing the concept of the present invention, these problems can be overcome with or at least significantly reduced. 
     According to an embodiment of the present invention, the pressure vessel is cylinder shaped and arranged with a wall thickness being thinner at the cooling portion than a wall thickness at the lower and the upper end of the pressure vessel. Consequently, the heat transfer via the pressure vessel wall to the coolant flowing in the cooling circuit can be made very efficient. 
     According to preferred embodiments of the present invention, the wall thickness relationship between the end portions and the central portion is between about 1.1-2.5 and typically between 1.3-1.7. 
     According to embodiments of the present invention, the wall thickness of the pressure vessel is gradually reduced along tapered portions from the upper and lower end portions, respectively, to the central portion. 
     In embodiments of the present invention, an outer and/or inner wall of the pressure vessel is provided with steps resulting in a reduced thickness at said central portion. 
     According to embodiment of the present invention, the pressure vessel is arranged with an inner wall and/or an outer wall shaped so as to form a recess. 
     According to embodiments of the present invention, the radially pre-stressing means is provided around the envelope surface of the pressure vessel cylinder. 
     According to embodiments of the present invention, the radially pre-stressing means is arranged around the envelope surface of said recess formed in said outer wall. 
     Features from two or more embodiments outlined above can be combined, unless they are clearly complementary, in further embodiments. Likewise, the fact that two features are recited in different claim does not preclude that they can be combined to advantage. 
     The different embodiments of the present invention described herein can be combined, alone or in different combinations, with embodiments in different combinations described in the patent applications “Improved outer cooling loop” and “Pressing arrangement” filed on the same day as the present application by the same applicant. The content of the patent applications “Non-uniform cylinder” and “Pressing arrangement”, respectively, are included herein by reference. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Embodiments of the present invention will now be described with reference to the accompanying drawings, on which: 
         FIG. 1   a  is a schematical side view of a pressure vessel according to prior art; 
         FIG. 1   b  is a schematical side view of another pressure vessel according to prior art; 
         FIG. 2  is a schematical side view of a pressure vessel according to an embodiment of the present invention; 
         FIG. 3  is schematical side view of a pressure vessel according to a further embodiment of the present invention; 
         FIG. 4  is a schematical side view of a pressure vessel according to another embodiment of the present invention; 
         FIG. 5  is a schematical side view of a pressure vessel according to still another embodiment of the present invention; 
         FIG. 6  is a schematical side view of a pressure vessel according to a further embodiment of the present invention; and 
         FIG. 7  is a schematical side view of a pressure vessel according to a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The following is a description of exemplifying embodiments of the present invention. This description is intended for the purpose of explanation only and is not to be taken in a limiting sense. It should be noted that the drawings are schematic and that the pressing arrangements of the described embodiments comprise features and elements that are, for the sake of simplicity, not indicated in the drawings. 
     Embodiments of the pressing arrangement according to the present invention may be used to treat articles made from a number of different possible materials by pressing, in particular by hot isostatic pressing. 
       FIGS. 1   a  and  1   b  show pressure vessels according to prior art. Below, a number of embodiments of pressure vessels according to the present invention will be discussed with reference to  FIGS. 2-7 , which embodiments may be used in a pressing arrangement for hot isostatic pressing. 
     A pressure vessel usually is provided with means (not shown), such as one or more ports, inlets and outlets, for supplying and discharging a pressure medium. The pressure medium may be a liquid or gaseous medium with low chemical affinity in relation to the articles to be treated. When arranged in a pressing arrangement for hot isostatic pressing, the pressure vessel includes a furnace chamber (not shown), which comprises a furnace (or heater) (not shown), or heating elements, for heating of the pressure medium during the pressing phase of the treatment cycle. The person skilled in the art realises that it is possible to combine heating elements at the sides with heating elements at the bottom so as to achieve a furnace which is located at the sides and at the bottom of the furnace chamber. Clearly, any implementation of the furnace regarding placement of heating elements, known in the art, may be applied to the embodiments shown herein. It is to be noted that the term “furnace” refers to the means for heating, while the term “furnace chamber” refers to the volume in which load and furnace are located. 
     Furthermore, the outer wall of the pressure vessel may be provided with one or more cooling circuits  39  (see e.g.  FIG. 2 ) including channels or tubes, in which a coolant for cooling may be provided. In this manner, the pressure vessel wall may be cooled in order to protect it from detrimental heat. The flow of coolant is indicated in the figures by the arrows. The use of an external cooling circuit  39  enables efficient cooling even though the pressure vessel can be carefully heat insulated for energy-economical operation. Additional benefit from external cooling of the pressure vessel can be achieved by providing flow guiding means, such as baffles, plates, flanges and channels, for guiding pressure medium from the relative vicinity of the outer wall of the pressure vessel towards an upstream side of the pump. Preferably, the guiding means are arranged in such manner that the pump forces a convective circulation loop of which a substantive portion is proximate to the externally cooled outer wall of the pressure vessel. This causes heat transfer away from the hot articles and out of the pressure vessel. As will be discussed below, the heat transfer can be significantly improved by the present invention. 
     In  FIGS. 2-7 , a number of different embodiments of the present invention are schematically illustrated and will hereinafter be discussed. In the following, only parts and elements related to the present invention will be discussed and described. Hence fittings inside the pressure vessel—including e.g. load compartment of the furnace chamber, casing, heat insulating portions, any apertures between a furnace chamber and an intermediate space will not be discussed in the following and are not shown in  FIGS. 2-7 . 
     With reference to  FIG. 2 , a first embodiment of the present invention will be discussed. The pressure vessel  40  is arranged with a varying or non-uniform vessel wall thickness over its length. In preferred embodiments of the present invention, the pressure vessel  40  has a vertically elongated and cylindrical shape. At portions of the vessel at an upper end  41  and at a lower end  42  of the pressure vessel  40 , where upper removable lid  22  and lower removable lid  24  are arranged, the pressure vessel wall  46  has its largest thickness. According to this embodiment, the pressure vessel  40  has tapered portions  43  and  44 , hence entailing that the vessel wall thickness gradually decreases compared to the thickness of the end portions  41 ,  42  of the vessel  40 . Further, the pressure vessel  40  has central portion  45  having a thinner wall thickness compared to the end portions  41 ,  42  and the tapered portions  43  and  44 , where heat is primarily transferred to the coolant flowing in the cooling circuit  39 . Thereby, the heat transfer between the pressure medium within the pressure vessel  40  and the coolant of the cooling circuit  39  can be significantly improved. According to this embodiment of the present invention, an outer wall  46  of the pressure vessel  40  is thus partly inclined (at the tapered portions  43 ,  44  of the pressure vessel  40 ) from the end portions  41  and  42 , respectively, to the cooling portion  45  to form a recess  49 . Radially pre-stressing means  38  is arranged around the envelope surface of the pressure vessel wall  46  for exerting radial compressive forces on the pressure vessel  1 . In an embodiment of the present invention, the pre-stressing means  38  is band-shaped and wound around the envelope surface and arranged in the recess  49 . The diameter relation, od/id, at a central portion  45  of the pressure vessel  10 , is less than 1.2, typically 1.1 or less and preferably below 1.07. 
     With reference now to  FIG. 3 , a second embodiment of the present invention will be discussed. The pressing arrangement  200  includes a pressure vessel  50  arranged with a varying or non-uniform vessel wall thickness over its length. In preferred embodiments of the present invention, the pressure vessel  50  has a vertically elongated and cylindrical shape. At portions of the vessel an upper end  51  and at a lower end  52  of the pressure vessel  50 , where upper removable lid  22  and lower removable lid  24  are arranged, the pressure vessel wall thickness has its largest thickness. The outer pressure vessel wall  56  is provided with steps  57  and  58 , which reduces the pressure vessel wall thickness abruptly between the upper end portion  51  and the lower end portion  52  on one hand compared to a central portion or cooling portion  55  so as to form a recess  59  in the outer pressure vessel wall  56 . Hence, the pressure vessel  50  is provided with a thinner wall thickness over the cooling portion  55  of the vessel, where heat is primarily transferred to the coolant flowing in the cooling circuit  39 . This entails that the heat transfer between the pressure medium within the pressure vessel  50  and the coolant of the cooling circuit can be greatly enhanced. According to this embodiment of the present invention, the outer wall  56  of the pressure vessel  50  is thus provided with steps  57 ,  58  between thicker wall portions  51 ,  52  and thinner wall portion  55 . Radially pre-stressing means  38  is arranged around the envelope surface of the pressure vessel wall  56  for exerting radial compressive forces on the pressure vessel  1 . In an embodiment of the present invention, the pre-stressing means  38  is band-shaped and wound around the envelope surface and arranged in the recess  59 . The diameter relation, od/id, at a central portion  55  of the pressure vessel  50 , is less than 1.2, typically 1.1 or less and preferably below 1.07. 
     Turning now to  FIG. 4 , yet another embodiment of the present invention will be discussed. The pressing arrangement  300  includes a pressure vessel  60  arranged with a varying or non-uniform vessel wall thickness over its length. In preferred embodiments of the present invention, the pressure vessel  60  has a vertically elongated and cylindrical shape. At portions of the vessel an upper end  61  and at a lower end  62  of the pressure vessel  60 , where upper removable lid  22  and lower removable lid  24  are arranged, the pressure vessel wall thickness has its largest thickness. The outer pressure vessel wall  66  is inclined from the upper end portion  61  to the lower end portion  62  so as to form a wide u-shaped recess  69  in the pressure vessel and, thereby, a central wall portion  65  having thinner wall thickness compared to the end portions  61  and  62 . Hence, the pressure vessel  60  has a thinner wall thickness over the central portion or cooling portion  65  of the vessel, where heat is primarily transferred to the coolant flowing in the cooling circuit  39 . This entails that the heat transfer between the pressure medium within the pressure vessel  60  and the coolant of the cooling circuit can be greatly enhanced. Radially pre-stressing means  38  is arranged around the envelope surface of the pressure vessel wall  46  for exerting radial compressive forces on the pressure vessel  1 . In an embodiment of the present invention, the pre-stressing means  38  is band-shaped and wound around the envelope surface. The diameter relation, od/id, at a central portion  65  of the pressure vessel  60 , is less than 1.2, typically 1.1 or less and preferably below 1.07. 
     With reference to  FIG. 5 , another embodiment of the present invention will be discussed. The pressing arrangement  400  includes a pressure vessel  70  arranged with a varying or non-uniform vessel wall thickness over its length. In preferred embodiments of the present invention, the pressure vessel  70  has a vertically elongated and cylindrical shape. At portions of the vessel an upper end  71  and at a lower end  72  of the pressure vessel  70 , where upper removable lid  22  and lower removable lid  24  are arranged, the pressure vessel wall thickness has its largest thickness. The pressure vessel  70  comprises step-down portions  73  and  74  where the pressure vessel wall thickness in a step-wise manner is reduced from the end portions  71  and  72  to central or cooling portion  75 . The outer pressure vessel wall  76  is provided with a number of steps to form a recess  79  such that the central wall portion  75  has a thinner wall thickness compared to the end portions  71  and  72 . Hence, the pressure vessel  70  has a thinner wall thickness over the central portion or cooling portion  75  of the vessel, where heat is primarily transferred to the coolant flowing in the cooling circuit  39 . This entails that the heat transfer between the pressure medium within the pressure vessel  70  and the coolant of the cooling circuit can be greatly enhanced. Radially pre-stressing means  38  is arranged around the envelope surface of the pressure vessel wall  76  for exerting radial compressive forces on the pressure vessel  1 . In an embodiment of the present invention, the pre-stressing means  38  is band-shaped and wound around the envelope surface and arranged in the recess  79 . The diameter relation, od/id, at a central portion  75  of the pressure vessel  70 , is less than 1.2, typically 1.1 or less and preferably below 1.07. 
     With reference to  FIG. 6 , another embodiment of the present invention will be discussed. The pressing arrangement  500  includes a pressure vessel  80  arranged with a varying or non-uniform vessel wall thickness over its length. In preferred embodiments of the present invention, the pressure vessel  80  has a vertically elongated and cylindrical shape. At portions of the vessel an upper end  81  and at a lower end  82  of the pressure vessel  80 , where upper removable lid  22  and lower removable lid  24  are arranged, the pressure vessel wall thickness has its largest thickness. According to this embodiment, the pressure vessel  80  has tapered portions  83  and  84 , hence entailing that the vessel wall thickness gradually decreases compared to the thickness of the end portions  81 ,  82  of the vessel  80 . Further, the pressure vessel  80  has central portion or cooling portion  85  having a thinner wall thickness compared to the end portions  81 ,  82  and the tapered portions  83  and  84 . The wall of the vessel  80  has a thinner wall thickness over the cooling portion  85  of the vessel, where heat is primarily transferred to the coolant flowing in the cooling circuit  39 . Thereby, the heat transfer between the pressure medium within the pressure vessel  80  and the coolant of the cooling circuit  39  can be significantly improved. According to this embodiment of the present invention, an inner wall  88  of the pressure vessel  80  is thus partly inclined (at the tapered portions  83 ,  84  of the pressure vessel  80 ) from the end portions  81  and  82  to the cooling portion  85  in comparison to the outer pressure vessel wall  86  and forms a wide recess  89  between the end portions  81  and  82 . Radially pre-stressing means  38  is arranged around the envelope surface of the pressure vessel wall  86  for exerting radial compressive forces on the pressure vessel  1 . In an embodiment of the present invention, the pre-stressing means  38  is band-shaped and wound around the envelope surface. The diameter relation, od/id, at a central portion  85  of the pressure vessel  80 , is less than 1.2, typically 1.1 or less and preferably below 1.07. 
     With reference now to  FIG. 7 , another embodiment of the present invention will be discussed. The pressing arrangement  600  includes a pressure vessel  90  arranged with a varying or non-uniform vessel wall thickness over its length. In preferred embodiments of the present invention, the pressure vessel  90  has a vertically elongated and cylindrical shape. At portions of the vessel at an upper end  91  and at a lower end  92  of the pressure vessel  90 , where upper removable lid  22  and lower removable lid  24  are arranged, the pressure vessel wall  96 ,  98  has its largest thickness. According to this embodiment, the pressure vessel  90  has tapered portions  93  and  94 , hence entailing that the vessel wall thickness gradually decreases compared to the thickness of the end portions  91 ,  92  of the vessel  90 . Further, the pressure vessel  90  has central portion  95  having a thinner wall thickness compared to the end portions  91 ,  92  and the tapered portions  93  and  94 . The wall of the vessel  40  has a thinner wall thickness over the central portion or cooling portion  95  of the vessel, where heat is primarily transferred to the coolant flowing in the cooling circuit  39 . Thereby, the heat transfer between the pressure medium within the pressure vessel  90  and the coolant of the cooling circuit  39  can be significantly improved. According to this embodiment of the present invention, an outer wall  96  of the pressure vessel  90  is thus partly inclined (at the tapered portions  93 ,  94  of the pressure vessel  90 ) from the end portions  91  and  92  to the cooling portion  95  to form a recess  99   b . Further to this embodiment of the present invention, an inner wall  98  of the pressure vessel  90  is partly inclined (at the tapered portions  93 ,  94  of the pressure vessel  90 ) from the end portions  91  and  92 , respectively, to the cooling portion  95  to form a recess  99   a . Radially pre-stressing means  38  is arranged around the envelope surface of the pressure vessel wall  96  for exerting radial compressive forces on the pressure vessel  1 . In an embodiment of the present invention, the pre-stressing means  38  is band-shaped and wound around the envelope surface and arranged in the recess  99   b . The diameter relation, od/id, at a central portion  95  of the pressure vessel  90 , is less than 1.2, typically 1.1 or less and preferably below 1.07. 
     Even though the present description and drawings disclose embodiments and examples, including selections of components, materials, temperature ranges, pressure ranges, etc., the invention is not restricted to these specific examples. Numerous modifications and variations can be made without departing from the scope of the present invention, which is defined by the accompanying claims. 
     EXAMPLE 1 
     According to an example pressing arrangement according to the present invention, an outer diameter, od, is 1590 mm and an inner diameter, id, is 1450 mm at the end portions of the pressure vessel. The diameter relation at the end portions is hence about 1.1. The vessel wall thickness at the end portions is 70 mm. At the central portion of the pressure vessel, the wall thickness is between 40-60 mm. Accordingly, the diameter relation is about 1.06-1.08 at the central portion of the pressure vessel. A pressing arrangement having the above dimension is produced by the applicant under a model name QIH232. A wall thickness of 50 mm at the central portion entails an improved transfer of thermal energy of about 40% compared to a pressure vessel having a uniform wall thickness.