Abstract:
The present invention relates to an arrangement for treatment of articles by hot pressing and preferably by hot isostatic pressing. In particular, the present invention relates to such an arrangement capable of obtaining a rapid rate without the need of special purpose valves for the cooling. A furnace chamber is provided inside the pressure vessel of the arrangement and a heat insulated casing arranged to surround the furnace chamber. A bottom insulating portion is arranged beneath the furnace chamber. Further, a fan having a controllable number of revolutions for circulating the pressure medium within the furnace chamber is arranged in the pressure vessel, and preferable within the furnace chamber. At least one feeding passage is arranged to allow feeding of pressure medium from a region being colder than a region within the furnace chamber towards an inlet of the fan, wherein an amount of pressure medium being fed to the inlet of the fan can be controlled by adjusting operational parameters of the fan.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to an arrangement for treatment of articles by hot pressing and preferably 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 without the need of any special purpose valves for the cooling. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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 states, such as a pressing state, a heating state, and a cooling state. 
         [0004]    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, preferably from 800 to 2000 bars and from 300° C. to 3000° C., preferably from 800° C. to 2000° C., respectively. 
         [0005]    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. Many presses known in the art suffer from slow cooling of the articles and efforts have therefore been made to reduce the cooling time of the articles. Notwithstanding the fact that a reduced cooling time is an important factor to take into account, high temperature uniformity during, for example, a steady-state state and a pressing state is also considerable importance. Thus, in addition to the capability of rapid cooling, a capability of achieving a high temperature uniformity during, for example, the steady-state state is desired. 
         [0006]    Mechanical means for forcing a convective circulation may be applied to obtain an enhanced cooling rate. This is a way to achieve rapid cooling of treated articles although these are contained in a well insulated furnace chamber. Co-pending application PCT/EP2007/10997 discloses a hot isostatic pressing arrangement with these characteristics. Devices for forcing convection in high-pressure and/or high-temperature systems are however often subject to early wear or frequent machinery breakdown. This is in particular true of mechanical fans or ventilators with moving parts. Accordingly, such pressing arrangements may require relative frequent maintenance resulting in undesired production disruptions and stops. 
         [0007]    Furthermore, the available space for loading articles in pressing arrangement is often limited and mechanical means such as fans or ventilators decrease this available space, a problem that is even more pronounced in smaller pressing arrangements. 
         [0008]    The use of mechanical means also entails a relatively complex and expensive construction of the pressing arrangement. 
         [0009]    Hence, there is still a need within the art of improved pressing arrangements capable of controlled, rapid cooling during a cooling state and of high temperature uniformity during steady-state and pressing. 
       SUMMARY OF THE INVENTION 
       [0010]    A general object of the present invention is to provide an improved pressing arrangement for hot pressing, and preferably hot isostatic pressing, capable of an accurate control of the temperature and a rate of change of the temperature of the pressure medium and of articles being treated within the pressing arrangement. 
         [0011]    A more specific object of the present invention is to provide an improved pressing arrangement for hot pressing, and preferably hot isostatic pressing, capable of controlled, rapid cooling rate during a cooling state and of high temperature uniformity during e.g. steady-state and a pressing state. 
         [0012]    Another object of the present invention is to provide an improved pressing arrangement for hot pressing, and preferably hot isostatic pressing, having a construction with reduced constructional complexity and reduced maintenance requirements. 
         [0013]    A further object of the present invention is to provide an improved pressing arrangement for hot pressing, and preferably hot isostatic pressing, capable of achieving a circulation of pressure medium within the furnace chamber without cooling to create high temperature uniformity during e.g. steady-state and a pressing state. 
         [0014]    Yet another object of the present invention is to provide an improved pressing arrangement for hot pressing, and preferably hot isostatic pressing, capable of achieving a controlled and rapid cooling rate of articles and/or pressure medium without the need of any special purpose valves for the cooling equipment. 
         [0015]    These and other objects of the present invention are achieved by means of a pressure vessel and method for such vessel having the features defined in the independent claims. Embodiments of the present invention are characterized in the dependent claims. 
         [0016]    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. 
         [0017]    According to a first aspect of the invention, there is provided a pressing arrangement for hot pressing comprising pressure vessel including a furnace chamber adapted to hold articles. A heat insulated casing is arranged to surround the furnace chamber. Further, a bottom insulating portion is arranged beneath the furnace chamber. A fan having an adjustable number of revolutions (rpm) is arranged to provide a flow of pressure medium into the furnace chamber and a circulation of the pressure medium within the furnace chamber when operated. At least one feeding passage is arranged to provide a connection between a region being colder than a region within the furnace chamber and an inlet of the fan so as to enable a flow of pressure medium from the cold region to the inlet for mixing the cold flow with a flow of warm pressure medium into the furnace chamber, wherein the amount of the flow of cold pressure medium being fed to the inlet of the fan can be controlled by adjusting the number of revolutions of the fan. 
         [0018]    According to a second aspect of the present invention, there is provided a method for a pressing arrangement for hot pressing. The pressing arrangement comprises a pressure vessel including a furnace chamber adapted to hold articles, which furnace chamber is provided inside the pressure vessel, a heat insulated casing arranged to surround the furnace chamber, a bottom insulating portion arranged beneath the furnace chamber, and a fan having an adjustable number of revolutions, the fan being arranged to provide a flow of pressure medium into the furnace chamber and a circulation of the pressure medium within the furnace chamber when operated. At least one feeding passage is arranged to provide a connection between a region being colder than a region within the furnace chamber and an inlet of the fan. The method comprises adjusting the number of revolutions of the fan to control a flow of pressure medium from the cold region to the inlet of the fan where the cold flow is mixed with a flow of warm pressure medium and which mixed flow is then fed into the furnace chamber. 
         [0019]    Thus, the capability of the fan to create a circulation effect with warm pressure medium or to create a cooling effect within the furnace chamber can be adjusted. By regulating or controlling the fan, the flow of cold pressure medium through the feeding passage from the cold region beneath the bottom insulation portion towards the inlet of the fan can be controlled, a suction effect at the inlet of the fan and a flow of pressure medium into the furnace chamber can be controlled. Consequently, a cooling state can be controlled and, if desired, the flow of pressure medium can be substantially impeded to achieve a steady-state state, wherein a circulation of pressure medium is maintained within the furnace chamber to obtain a uniform temperature in the hot zone. 
         [0020]    The present invention is hence based on the insight that the large and significant density differences of the pressure medium that occur within a pressing vessel for hot pressing and, in particular, for hot isostatic pressing, can be utilized to obtain an accurate control of the cooling rate of articles being pressed. These large density differences are created by the high pressures and temperatures differences of the pressure vessel in such an arrangement. Often, such a pressing arrangement operates at pressures ranging from 200 to 5000 bars, and preferably between 800 and 2000 bars and at temperatures ranging from 300° C. to 3000° C., and preferably between 800 and 2000° C. 
         [0021]    Further, the present invention is based on the insight that a fan can be used to control the cooling rate accurately in order to, for example, obtain a desired cooling rate by utilizing this large density difference. The present invention can also be used for state control, i.e. whether steady-state or cooling is applied, by utilizing the density differences while running the fan. This can be achieved without using any special purpose valves for the cooling. More specifically, at least one feeding passage is arranged to enable a feeding of pressure medium from a cold region beneath the bottom insulation portion towards an inlet of the fan, which fan preferably is arranged in a lower end of the furnace chamber where the pressure medium is significantly warmer than the cold region. Often the temperature difference between the cold region beneath the bottom insulation portion and the furnace chamber may be 1000° C. or even more. Thus, there exists a large density difference between these two regions, which, according to the present invention, is utilized to achieve a pressing arrangement capable of an accurately regulated or controlled cooling state whereby a desired cooling rate or maintaining a steady-state condition can be obtained by controlling the number of revolutions of the fan. At steady-state, the fan is operated at a number of revolutions such that a flow of warm pressure medium is circulated in the furnace chamber without the addition of a flow of cold pressure medium via the feeding passage from the cold region below the bottom insulating portion. At a certain number of revolutions of the fan, cold pressure medium starts to flow out from the feeding passage under given temperature and pressure conditions within the pressure chamber, e.g. at a given temperature difference between the warm region in furnace chamber and the cold region below the bottom insulating portion and at a given pressure within the pressure vessel. The cold pressure medium is mixed with the flow of warm pressure medium and is fed into the furnace chamber by the fan, and thereby a controlled and variable cooling can be achieved. 
         [0022]    If the fan is operated at a number of revolutions being lower than this certain limit number, the feeding passage is substantially closed for feeding and accordingly the flow of cold pressure medium from the cold region below the bottom insulating portion is shut off. That is, the pressure difference between the region beneath the bottom insulation portion and the passage above the bottom insulation portion is too low to create a sufficient suction effect that would draw pressure medium through the feeding passage. Under these conditions, a steady-state is maintained and a flow of warm pressure medium is circulated through the furnace chamber without the addition of the flow of cold pressure medium. 
         [0023]    On the other hand, if the fan is operated above this certain limit number of revolutions, a higher pressure difference is created, which, in turn, forces pressure medium to flow through the feeding passage. Accordingly, a flow of cold pressure medium is created in addition to the flow of the warm pressure medium. Consequently, by operating the fan at different number of revolutions above the certain limit number of revolutions, it is possible to control the amount of cold pressure medium being fed to the inlet of the fan during a cooling state and thereby control the cooling rate. Further, if steady state is desired, it is possible to shut-off or close the feeding of pressure medium, which is achieved by operating the fan at lower number of revolutions below the certain limit number of revolutions. 
         [0024]    The certain limit number of revolutions where a steady-state state turns into a cooling state is determined by a number of parameters. A non-exhaustive list includes:
       The density difference between the pressure medium above the bottom insulation portion and the pressure medium in the region beneath the bottom insulation portion.   The arrangement of the feeding passage and its outlet relative, radially and vertically, to
           the fan;   the passage between the bottom insulation portion and furnace chamber;   an inlet of a guiding passage between a heat insulating portion and a housing of the heat insulating casing; and/or   an upper surface of the bottom insulation portion.   
           The dimensions of the feeding passage, in particular, the diameter of the feeding passage.   The number of feeding passages.       
 
         [0033]    According to embodiments of the present invention, which may combined with one or more of other embodiments described herein, the fan is thus configured such that operation at a number of revolutions below the certain limit number of revolutions results in that the flow of cold pressure medium is shut off. 
         [0034]    According to embodiments of the present invention, which may combined with one or more of other embodiments described herein, the fan is hence configured such that operation at a variable number of revolutions above the certain limit number of revolutions results in a variable flow of cold pressure medium towards the inlet of the fan. 
         [0035]    In embodiments of the present invention, which may combined with one or more of other embodiments described herein, the at least one feeding passage is thus arranged with dimensions such that the fan and the at least one feeding passage co-operate to achieve a substantial cut off of the flow of pressure medium and the variable flow of cold pressure medium. 
         [0036]    According to embodiments of the present invention, which may combined with one or more of other embodiments described herein, the at least one feeding passage is hence arranged with an outlet located at a radial and vertical distance relative to the fan such that the fan and the at least one feeding passage co-operate to achieve the substantial cut off of the flow of cold pressure medium and the variable flow of cold pressure medium. 
         [0037]    In embodiments of the present invention, which may be combined with one or more of other embodiments described herein, the outlet of the at least one feeding passage is located at a radial and vertical distance from an inlet of a guiding passage in the heat insulated casing such that the at least one feeding passage and the inlet of the guiding passage co-operate to achieve the substantial cut off of the flow of cold pressure medium and the variable flow of cold pressure medium. 
         [0038]    According to embodiments of the present invention, which can be combined with one or more of other embodiments described herein, the at least one feeding passage is at least one conduit is arranged in the bottom insulating portion at a radial distance from a the fan and wherein an outlet of the conduit is located in connection to a passage above the bottom insulating portion. 
         [0039]    In embodiments of the present invention, which may be combined with one or more of other embodiments described herein, the at least one conduit is arranged to extend into the passage such that the outlet is located at a distance from the bottom insulation portion. 
         [0040]    According to embodiments of the present invention, which may be combined with one or more of other embodiments described herein, the at least one feeding passage is arranged such that an outlet is located in connection to a central passage for supplying pressure medium to the fan. 
         [0041]    In embodiments of the present invention, which may combined with one or more of other embodiments described herein, the support means for supporting a load compartment within the furnace chamber is arranged such that pressure medium is allowed to flow into the passage above the bottom insulation portion. 
         [0042]    According to embodiments of the present invention, which may be combined with one or more of other embodiments described herein, the support means is provided with through holes arranged to allow pressure medium is allowed to flow into the passage above the bottom insulation portion. 
         [0043]    In embodiments of the present invention, which may be combined with one or more of other embodiments described herein, the feeding passage is provided with a valve. 
         [0044]    Other objectives, features and advantages of the present invention will appear from the following detailed description, the attached dependent claims, and from the appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0045]    The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings. In the following Figures, like reference numerals denote like elements or features of embodiments of the present invention throughout. Further, reference numerals for symmetrically located items, elements or feature indicators are only denoted once in the Figures. On the drawings: 
           [0046]      FIG. 1  shows a schematical side view of a pressing arrangement according to an embodiment of the present invention; 
           [0047]      FIG. 2  shows a schematical side view of the pressing arrangement of  FIG. 1  during steady-state; 
           [0048]      FIG. 3  shows a schematical side view of the pressing arrangement of  FIG. 1  during a cooling state; 
           [0049]      FIG. 4  schematically illustrates a detail view of a pressing arrangement according to the present invention; 
           [0050]      FIG. 5  shows a schematical side view of a pressing arrangement according to another embodiment of the present invention; 
           [0051]      FIG. 6  shows a schematical side view of a pressing arrangement according to yet another embodiment of the present invention; 
           [0052]      FIG. 7  shows a schematical side view of a pressing arrangement according to still another embodiment of the present invention; and 
           [0053]      FIG. 8  shows a schematical side view of a pressing arrangement according to a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0054]    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 may comprise features and elements that are, for the sake of simplicity, not indicated in the drawings. 
         [0055]    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. 
         [0056]      FIG. 1  shows a pressing arrangement  100  according to an embodiment of the invention. The pressing arrangement  100 , which is intended to be used for pressing of articles, comprises a pressure vessel  1  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. The pressure vessel  1  includes a furnace chamber  18 , which comprises a furnace (or heater)  36 , or heating elements, for heating of the pressure medium during the pressing state of the treatment cycle. The furnace  36  may, as shown in for example  FIG. 1 , be located at the lower portion of the furnace chamber  18 , or may be located at the sides of the furnace chamber  18 . The person skilled in the art realises that it is also 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. The furnace chamber  18  does not occupy the entire pressure vessel  1 , but leaves guiding passage  10  around it. During normal operation of the pressing arrangement  100 , the guiding passage  10  is typically cooler than the furnace chamber  18  but is at equal pressure. 
         [0057]    The furnace chamber  18  further includes a load compartment  19  for receiving and holding articles  5  to be treated. The load compartment  19  rests upon support means  44 , which support means may be, for example, a number of pillar-like elements or a ring shaped element provided with through holes  45  for allowing passage of warm pressure medium for circulation into the furnace chamber  18 . 
         [0058]    The furnace chamber  18  is surrounded by a heat insulated casing  3 . The heat insulated casing  3  comprises a heat insulating portion  7  and a housing  2  arranged to surround the heat insulating portion  7 , which thermally seals off the interior of the pressure vessel  1  in order to reduce heat loss. A first guiding passage  10  is formed between the inside of the outer walls of the pressure vessel  1  and the housing  2 . The first guiding passage  10  is used to guide the pressure medium from the top of the pressure vessel  1  to the bottom thereof. 
         [0059]    Moreover, a second guiding passage  11  is formed between the housing  2  of the furnace chamber  18  and the heat insulating portion  7  of the furnace chamber  18 . The second guiding passage  11  is used to guide the pressure medium towards the top of the pressure vessel. The second guiding passage  11  is provided with one or more inlets  14  for supplying pressure medium thereto, as well as an opening  13  at the top of the pressure vessel for allowing flow of the pressure medium into the first guiding passage  10 . 
         [0060]    The inlets  14  are preferably located below the upper edge of the lower heat insulating portion  6 . An outer convection loop is thereby formed by the first and second guiding passages  10 ,  11  as well as in a lower portion, below the bottom insulating portion  6 , of the pressure vessel  1 . A fan  30  having a controllable number of revolutions is arranged at a lower end of the furnace chamber  18  for providing a circulation the pressure medium within the furnace chamber  18 . By operating the fan  30  an inner convection loop can be enhanced, in which inner convection loop pressure medium has a flow through a passage  16 , an upward flow through the load compartment  19  and a downward flow along a peripheral portion  12  of the furnace chamber  18 . As will be explained in detail below, an additional flow of cold pressure medium into the furnace chamber  18  from a region below the bottom of the casing  3  can be achieved by operating the fan  30  above a certain limit number of revolutions. 
         [0061]    The bottom of the casing  3  comprises a bottom insulating portion  6 . The bottom insulating portion  6  may be provided with a central passage  37  for supplying pressure medium to the fan  30  and further into the furnace chamber  18 . 
         [0062]    Moreover, the outer wall of the pressure vessel  1  may be provided with channels or tubes (not shown), in which a coolant for cooling may be provided. In this manner, the vessel wall may be cooled in order to protect it from detrimental heat. The coolant is preferably water, but other coolants are also contemplated. The flow of coolant is indicated in  FIG. 1  by the arrows on the outside of the pressure vessel. 
         [0063]    Even though it is not shown in the figures, the pressure vessel  1  may be opened, such that the articles within the pressure vessel  1  can be removed. This may be realized in a number of different manners, all of which being apparent to a man skilled in the art. 
         [0064]    Furthermore, at least one feeding conduit or passage  40  is arranged for allowing a flow of cold pressure medium towards an inlet or intake  39  of the fan  30  from the cold region  42  beneath the bottom insulating portion  6  utilizing the density difference between the pressure medium in the cold region  42  and the pressure medium of the passage  16 . As the amount of cold pressure medium being fed from the cold region  42  to the fan  30  can be controlled by means of adjusting the number of revolutions of the fan  30 . At a low number of revolutions below a certain limit number of revolutions, the flow of cold pressure medium through the feeding passage  40  is shut off and thus no cold pressure medium will be fed to the fan  30  from the cold region  42  via the passage  40 . This is because of the relatively low underpressure at the outlet  41  of the conduit  40 . That is, the underpressure is too low to create a sufficient suction effect which would force or draw pressure medium from the cold region  42  upwards through the conduit  40  and further towards the fan  30 . At a certain number of revolutions of the fan  30 , pressure medium from the cold region  42  commences to flow out from the conduit towards to inlet  39  of the fan  30 . This certain number of revolutions depends, inter alia, on a density difference between the pressure medium flowing in the passage  16  and the pressure medium in the region  42 , the specific radial location of the conduit  40  relative to the fan  30  (which fan  30  preferably is placed at a central axis CA of the pressure vessel  1 ) when the conduit  40  is arranged in the bottom insulating portion  6 , and the design of the conduit  40  including e.g. diameter of the conduit and the position of the outlet  41  of the conduit  40  relative to the bottom insulation portion  6  and relative to the outlet  14 . This certain number of revolutions is defined as a limit number of revolutions of the fan  30 . Consequently, if the fan  30  is operated at or above the certain limit number of revolutions, a cooling can be achieved by means of the additional flow of cold pressure medium that is feed or drawn through the conduit  40  and further to the fan  30  from the cold region  42 , which, in turn, achieves a mixing of the cold pressure medium and the flow of warm pressure medium. The mixed flow is fed into the furnace chamber  18  (this is described below with reference to  FIG. 3 ) and provides a cooling effect. 
         [0065]    In the embodiment shown in  FIG. 1 , the feeding conduit  40  is arranged in the bottom insulating portion  6  so as to feed or guide cold pressure medium from the cold region  42 , where the pressure medium may have an approximate temperature of 60-180° C., towards the inlet or intake fan  30 , which is arranged in the furnace chamber  18  where the pressure medium may have an approximate temperature of 1200° C. In  FIGS. 5 -8 , further embodiments of the present invention having different locations of and arrangements of the feeding conduit are illustrated and with reference to these figures the embodiments will be discussed below. 
         [0066]    Operation of an exemplary pressing arrangement in accordance with the present invention will now be described generally with reference to  FIGS. 2 and 3 . 
         [0067]    As will be described in the following, a treatment cycle may comprise several states, such as loading state, pressing and/or heating state, cooling state, in which, according to the present invention, a cooling rate can be controlled by the adjusting a number of revolutions of the fan  30  to vary a flow of cold pressure medium into the furnace chamber  18 , and unloading state. 
         [0068]    First, the pressure vessel  1  is opened such that the furnace chamber  18 , and the load compartment  19  thereof, may be accessed. This can be accomplished in a number of different manners known in the art and no further description thereof is required for understanding the principles of the invention. 
         [0069]    Then, the articles to be pressed are positioned in the load compartment  19  and the pressure vessel  1  is closed. 
         [0070]    When the articles have been positioned in the load compartment  19  of the pressure vessel  1 , pressure medium is fed into the pressure vessel  1 , for instance by means of a compressor, a pressurized storage tank (a pressure supply), a cryogenic pump, or the like. The feeding of pressure medium into the pressure vessel  1  continues until a desired pressure is obtained inside the pressure vessel  1 . 
         [0071]    While, or after, feeding pressure medium into the pressure vessel  1 , the furnace (the heating elements)  36  of the furnace chamber  18  is (are) activated and the temperature inside the load compartment is increased. If needed, the feeding of pressure medium continues and the pressure is increased until a pressure level has been obtained that is below the desired pressure for the pressing process, and at a temperature below the desired pressing temperature. Then, the pressure is increased the final amount by increasing the temperature in the furnace chamber  18 , such that the desired level of the pressure is reached. Alternatively, the desired temperature and level of pressure is reached simultaneously or the desired pressure is reached after the desired temperature has been reached. A man skilled in the art realizes that any suitable method known in the art may be utilized to reach the desired pressing pressure and temperature. For instance, it is possible to equalize the pressure in the pressure vessel and a high pressure supply, and to then further pressurize the pressure vessel, by means of compressors, and further heat the pressure medium at the same time. An inner convention loop may be activated by the fan  30  included in the furnace chamber  18  in order to achieve an even temperature distribution. 
         [0072]    In accordance with the embodiments described herein, the desired pressure is above approximately 200 bars, and the desired temperature is above approximately 400° C., for example, about 1200° C. 
         [0073]    After a selected time period at which the temperature and pressure is maintained, i.e. the actual pressing state, the temperature of the pressure medium is to be decreased, i.e. cooling is initiated, as will be described below. 
         [0074]    The pressure medium used during the pressing state can, when the temperature has been decreased enough in the cooling state, be discharged from the pressure vessel  1 . For some pressure mediums, it may be convenient to discharge the pressure medium into a tank or the like for recycling. 
         [0075]    After decompression, the pressure vessel  1  is opened such that the pressed articles  5  may be unloaded from the load compartment  19 . 
         [0076]    With reference now to  FIGS. 2 and 3 , a steady-state state and a cooling state, will be explained in more detail. The discussion below is related to the embodiment of the present invention illustrated in  FIG. 1 . Again the terms “hot” or “warm” and “cold” are to be interpreted in relation to an average temperature of the pressure medium within the pressure vessel. Further, the arrows indicate the flow direction of the pressure medium. 
         [0077]    First, turning to  FIG. 2 , the flow directions of the pressure medium during steady-state are illustrated by means of arrows. As can be seen, pressure medium that has passed downwards through the peripheral portion  12  of the furnace chamber  18  and through the passage  10  enters into the passage  16  above the bottom insulation portion  6  and is further circulated into the furnace chamber  18  by means of the fan  30  or flows through the inlet  14  into the passage  11 . The pressure medium in the region  42  is cold and may have an approximate temperature of 60-180° C., while the pressure medium flowing in the passage  16  is warm and may have an approximate temperature of 1200° C., hence entailing a significant density difference between pressure medium in these two regions. During a steady-state state, the fan  30  is operated with a number of revolutions below the limit number of revolutions discussed above and, accordingly, no additional flow of cold pressure medium is fed to the fan  30  via the feeding passage  40  from the cold region  42 . Thereby, high temperature uniformity can be achieved within the furnace  18 . 
         [0078]    In  FIG. 3 , a cooling state where cold pressure medium is fed from the cold region  42  to mix with the flow of warm pressure medium so as to obtain a cooling is illustrated. By controlling the number of revolutions of the fan  30 , or the number of revolutions, above the certain limit number of revolutions, the feeding of cold pressure medium can be controlled in accurate way. Thereby, it is possible to achieve a desired cooling rate of, for example, the article  5 . Given a certain set of parameters, as discussed above, that influences the certain limit number of revolutions, a cooling rate can be accurately controlled by varying the number of revolutions of the fan and thereby, in turn, the amount of cold pressure medium being fed into the furnace chamber  18 . 
         [0079]    As illustrated in  FIG. 3 , pressure medium that has passed downwards through the peripheral portion  12  of the furnace chamber  18  and through the passage  10  enters into the passage  16  above the bottom insulation portion  6  and is further circulated into the furnace chamber  18  by means of the fan  30  or flows through the inlet  14  into the passage  11 . The pressure medium in the region  42  has a low temperature, an approximate temperature of 60-180° C., while the pressure medium flowing in the passage  16  is warm, e.g. an approximate temperature of 1200° C. The pressure medium in the region  42  beneath the bottom insulation portion  6  has a significantly higher density than the pressure medium in the passage  16  above the bottom insulation portion  6 , e.g. in the magnitude of about 3 times higher. A sufficient pressure difference between the region  42  and the passage  16  at the outlet  41  is required in order to create a flow of pressure medium through the conduit  40 , into the passage  16  and further to the fan  30 . Below, an example of a required underpressure at the outlet  41  will be discussed with reference to  FIG. 4 , which is a detailed view of the pressing arrangement  100  shown in  FIGS. 1-3 . If the static pressure within the furnace chamber is at about 1000 bar and the pressure medium of the passage  16  has a temperature of about 1100° C., the flowing pressure medium in the passage  16  will have a density of about 282 kg/m 3 . Further, if the pressure medium in the cold region  42  has a temperature of about 150°, the density will be about 742 kg/m 3 . In this example embodiment, the distance, x, between the outlet  41  of the conduit  40  and the inlet  14  is 250 mm. Therefore, the pressure difference required to initiate a flow of pressure medium through the conduit  40  will be about 11 mbar or 1128 Pa. The pressure difference can be influenced by increasing the distance between the outlet  41  of the conduit  40  and the inlet  16 , i.e. by increasing the distance x. That is, the certain limit number of revolutions will be higher, which entails that the fan  30  can be operated at higher number of revolutions without initiating the enhanced cooling effect provided by the cold pressure medium flowing through the conduit  40  and mixes with the warm pressure medium flowing in the passage  16 . 
         [0080]    With reference to  FIGS. 5-8 , further embodiments of the present invention will be discussed. The same reference numerals are used in  FIGS. 5-8  for corresponding features or portions of the pressure vessel shown in  FIG. 1 . Further, the description of these features or portions will be omitted below. 
         [0081]    First, with reference to  FIG. 5 , an embodiment of a pressing arrangement  110  where a feeding conduit  50  is arranged in wall of a central passage  37 ′ instead of the bottom insulation portion  116  will be discussed. The feeding conduit  50  is arranged in the central passage  37 ′ in a section beneath the bottom insulation portion  116  such that an outlet  51  of the feeding conduit  50  is located in the central passage  37 ′. Cold pressure medium can thereby be fed from the cold region  42  to the fan  30  for mixing with flow of pressure medium from the passage  16  via the central passage  37 ′ by adjusting the number of revolutions of the fan  30  in a corresponding manner as has been described above. Similarly, as has been described above, by operating the fan  30  at a number of revolutions being lower the certain limit number of revolutions for this particular embodiment under given temperature and pressure conditions a steady-state state can be maintained. 
         [0082]    Referring now to  FIG. 6 , yet another embodiment of the present invention will be discussed. In this embodiment of a pressing arrangement  120 , the pressure vessel  1  includes a bottom insulation portion provided with two feeding conduits  60   a  and  60   b . The feeding conduits  60   a  and  60   b  are arranged in the bottom insulation portion  126  such that their respective outlet  61   a  and  61   b  is located in the passage  16 . By adjusting the number of revolutions of the fan  30 , the flow of cold pressure medium from the cold region  42  into the passage  16  via conduits  60   a  and  60   b  and further to the fan  30  for mixing with the flow of warm pressure medium from the passage  16  can be controlled in a corresponding manner as has been described above. 
         [0083]    Referring now to  FIG. 7 , a further embodiment of the present invention will be discussed. In this embodiment of a pressing arrangement  130 , the pressure vessel  1  includes a bottom insulation portion  136  provided one feeding conduit  70 . The feeding conduit  70  is arranged in the bottom insulation portion  136  such an outlet  71  is located in the passage  16 . However, in this embodiment, the conduit  70  is extended into the passage  16  and the outlet  71  will hence be located at a distance from the bottom insulation portion  136 . By adjusting the number of revolutions of the fan  30 , the flow of cold pressure medium from the cold region  42  into the passage  16  via conduits  60   a  and  60   b  and further to the fan  30  for mixing with the flow of warm pressure medium from the passage  16  can be controlled in a corresponding manner as has been described above. 
         [0084]    With reference now to  FIG. 8 , still another embodiment of the present invention will be discussed. In this embodiment of a pressing arrangement  140 , the pressure vessel  1  includes a bottom insulation portion  146  provided one feeding conduit  80  arranged in the bottom insulation portion  146  such an outlet  81  is located in the passage  16 . The feeding conduit  80  is provided with a valve  85  at an inlet  84 . When the valve  85  is open, this embodiment of the present invention will function as the embodiment described with reference to  FIG. 1 . However, the valve  85  enables an instant throttling of the feeding conduit  80  during a cooling state, i.e. during a state where fan  30  is operated at a number of revolutions above the certain limit number of revolutions such that cold pressure medium flows through the feeding conduit  80  and mixes with pressure medium passing through the passage  16  above the bottom insulation portion  146 . Thereby, it is possible to e.g. control the mixing ration between cold and warm pressure medium very accurately by opening/closing the valve  85 . 
         [0085]    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.