Abstract:
A cell for quenching a charge under an atmosphere of gas comprises a centrifugal or helicon-centrifugal impeller comprising a gas intake opening and gas discharge openings. The impeller is rotated by a motor to cause a flow of the gas between the charge and a heat exchanger. The quenching cell comprises first and second mobile half-volutes. In a first position, the first half-volute guides the gas discharged by a first part of the discharge openings and the second half-volute closes off a first portion of the intake opening. In a second position, the second half-volute guides the gas discharged by a second part, different from the first part, of the discharge openings and the first half-volute shuts off a second portion of the intake opening.

Description:
BACKGROUND 
     The present disclosure relates to a cell for quenching pieces, for example, steel pieces. 
     DISCUSSION OF THE RELATED ART 
     Quenching corresponds to an abrupt cooling of a piece, also called load, which has been heated beyond a temperature at which the piece has its structure modified, to obtain a specific phase which is normally stable at high temperature only. For certain materials, particularly certain metals, quenching enables to maintain at ambient temperature the specific phase which has advantageous physical properties. For other materials, particularly certain steels, a quenching can enable to transform the specific phase into a metastable phase which has advantageous physical properties. In this case, the specific hot phase is austenite, obtained by heating the steel pieces between 750° C. and 1,000° C. and the metastable phase is martensite. The quenching operation must be relatively fast and uniform so that the entire austenite turns into martensite with no forming of perlite or bainite, which have lower hardness properties than martensite. 
     In the case of a liquid quenching, the previously-heated piece is for example placed in a quench tank filled with a quenching liquid, for example, oil, stirred during the cooling. 
     The quenching may also be performed by the flowing of a quenching gas around the piece to be cooled. Gas quenching is generally performed by arranging pieces to be quenched in a quenching cell comprising a tightly closed enclosure and by circulating a quenching gas in the enclosure. Gas quenching methods have many advantages over liquid quenching methods, and especially the fact that the treated pieces come out dry and clean. 
     The gas quenching of steel pieces which have been previously submitted to a thermal treatment (heating before quenching, anneal, tempering . . . ) or to a thermochemical treatment (cementation, carbonitriding . . . ) is generally performed with a gas under pressure, generally between 4 and 20 bars. The quenching gas is, for example, nitrogen, argon, helium, carbon dioxide, or a mixture of these gases. 
     A quenching cell generally comprises at least one motor, generally an electric or hydraulic motor, rotating a stirring element, for example, a propeller, capable of circulating the quenching gas in the quenching cell. To obtain a fast cooling of the pieces introduced into the quenching cell, the quenching gas is usually circulated at the level of the pieces to be cooled at a high speed for the entire quenching operation. 
     For certain types of pieces, for example, when the pieces are solid, it may be difficult to obtain a uniform cooling of the pieces if the quenching gas flows in the quenching cell in the same direction during the entire quenching operation, and thus always reaches the pieces to be processed in the same way. In this case, it is desirable to be able to rapidly reverse the quenching gas flow direction at the level of the pieces to be cooled to improve the uniformity of the cooling. 
     A possibility to reverse the quenching gas flow direction is to use a stirring element having its rotation direction imposing the quenching gas flow direction. The quenching gas flow direction is then reversed by reversing the rotation direction of the stirring element. To achieve this, an electric or hydraulic motor, having a rotation direction capable of being reversed, may be used to rotate the stirring element. Another possibility is to provide a transmission system between the motor and the stirring element, which enables to reverse the rotation direction of the stirring element. It may however be difficult to reverse the rotation direction of an electric or hydraulic motor or to operate a transmission within a short time. The reversal of the quenching gas flow direction at the level of the pieces to be cooled can last for more than ten seconds. 
     Document US 2003/0175130 describes a quenching cell where the stirring element comprises centrifugal impellers which always rotate in the same direction. The cell further comprises a system for reversing the quenching gas flow direction at the level of the pieces to be cooled by using mobile flaps. 
     A disadvantage of such a gas quenching cell is that, in order to enable to reverse the quenching gas flow direction at the level of the pieces to be cooled, the quenching gas is radially expelled on the entire periphery of the impellers, directly into the enclosure. Whatever the quenching gas flow direction, part of the quenching gas expelled by the impellers is blocked by the flaps and loses a significant part of its kinetic energy before being recovered in the general quenching gas flow. The power efficiency of the quenching cell, for example corresponding to the ratio of the power introduced to drive the impellers for a given time period to the thermal power taken from the load by the quenching gas for this same time period, may thus be low. 
     SUMMARY 
     An object of an embodiment of the present invention is to obtain a quenching cell which has an improved power efficiency while enabling to rapidly reverse the quenching gas flow direction at the level of the pieces to be cooled. 
     Another object of an embodiment of the present invention is to obtain a quenching cell having a decreased bulk. 
     Thus, an embodiment of the present invention provides a gas quenching cell for a load. The cell comprises a centrifugal or mixed-flow impeller comprising a gas intake opening and gas discharge openings. The impeller is rotated by a motor to cause a gas flow between the load and a heat exchanger. The quenching cell comprises first and second mobile half-volutes. In a first position, the first half-volute guides the gas discharged by a first portion of the discharge openings and the second half-volute shuts off a first portion of the intake opening. In a second position, one of the first or second half-volute guides the gas discharged by a second portion, different from the first portion, of the discharge openings, and the other one of the first or second half-volute shuts off a second portion of the intake opening. 
     According to an embodiment of the present invention, the quenching cell comprises an actuator laterally shifting the first and second half-volutes with respect to the impeller. 
     According to an embodiment of the present invention, the quenching cell comprises an actuator rotating the first and second half-volutes with respect to the axis of the impeller. 
     According to an embodiment of the present invention, the quenching cell further comprises an enclosure containing the impeller, the load, and the heat exchanger; a panel located between the impeller and the load; and a plate connecting the enclosure to the panel and surrounding the impeller, the first and second half-volutes being arranged on either side of the plate. 
     According to an embodiment of the present invention, the quenching cell comprises a cylindrical portion in contact with the panel and, in the first position, the second half-volute extends between the impeller and the cylindrical wall and, in the second position, the first half-volute extends between the impeller and the cylindrical wall. 
     According to an embodiment of the present invention, the actuator comprises a worm and a nut fastened to the first half-volute and cooperating with the worm. 
     According to an embodiment of the present invention, the quenching cell comprises an additional centrifugal or mixed-flow impeller, the impeller and the additional impeller being arranged on either side of the load, the cell further comprising third and fourth additional mobile half-volutes. In the first position, the third half-volute guides the gas discharged by a first portion of the discharge openings of the additional impeller and the fourth half-volute shuts off a first portion of the intake opening of the additional impeller. In the second position, one of the third or fourth half-volute guides the gas discharged by a second portion of the discharge openings of the additional impeller, different from the first portion of the discharge openings of the additional impeller, and the other one of the first or fourth half-volute shuts off a second portion of the intake opening of the additional impeller. 
     According to an embodiment of the present invention, the impeller is a mixed-flow impeller. 
     Another embodiment of the present invention provides a method of gas quenching of a load in a quenching cell such as previously described. The method comprises the steps of:
         displacing the first and second half-volutes to the first position, the gas flowing at the load level in a first flow direction; and   displacing the first and second half-volutes to the second position, the gas flowing at the load level in a second flow direction opposite to the first flow direction.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
         FIGS. 1 and 2  are simplified lateral views of an embodiment of a quenching cell with two operating steps; 
         FIG. 3  is a perspective view of an embodiment of a mixed-flow impeller; 
         FIG. 4  is a simplified cross-section view of certain elements of the quenching cell of  FIG. 1 ; and 
         FIGS. 5 and 6  are more detailed perspective views of certain elements of the quenching cell of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The same elements have been designated with the same reference numerals in the different drawings. Further, only those steps and elements which are necessary to the understanding of the embodiment of the quenching cell and of the quenching method have been shown and described. Further, adjectives “lower”, “upper”, “above”, and “under” and nouns “bottom” and “top” are used with respect to a reference direction which, in the quenching cell embodiment described hereafter, is the vertical direction. However, the reference direction may be inclined with respect to the vertical direction and may for example be horizontal. 
       FIGS. 1 and 2  show simplified lateral views of an embodiment of a quenching cell according to the invention at two operating steps of a quenching method. 
     Cell  5  comprises an enclosure  10  for example having the general shape of a cylinder of horizontal axis D. As an example, the internal diameter of enclosure  10  may be in the order of 1 meter. As a variation, enclosure  10  may have a generally parallelepipedal shape. Enclosure  10  rests on a support  12 . Cell  5  is closed at one end while the other end comprises a door system, not shown in  FIGS. 1 and 2 , providing access to cell  5  to introduce into it or extract therefrom a load  14  to be cooled. It may be a door sliding along a horizontal direction or a guillotine door. The door enables to substantially tightly close quenching cell  5 . As a variation, cell  5  may comprise a door at each of its ends. 
     Load  14 , schematically shown in  FIGS. 1 and 2  as a rectangle, comprises a single piece or a plurality of pieces, for example, a large number of pieces arranged on an appropriate support. These may be steel pieces, for example, toothed wheels. Load  14  is maintained substantially at the center of cell  5  on rails  16 . 
     A quenching gas may be introduced into enclosure  10  or extracted from enclosure  10  via valves  18 ,  20 . The quenching gas for example is nitrogen, argon, helium, carbon dioxide, or a mixture of these gases. The quenching gas is circulated in enclosure  10  by impellers  22 A,  22 B having axes Δ A  and Δ B . Impellers  22 A,  22 B are for example, arranged on either side of load  14 . Each impeller  22 A,  22 B may be a centrifugal or mixed-flow impeller. A centrifugal impeller is an impeller which sucks in a gas in a substantially axial direction and which discharges the gas in a substantially radial direction. An axial flow impeller is an impeller which sucks in a gas in a substantially axial direction and which discharges the gas in a substantially axial direction. A mixed-flow impeller is an impeller having an intermediate operation between the operation of an axial flow impeller and the operation of a centrifugal impeller, that is, the mixed-flow impeller sucks in a gas in a substantially axial direction and discharges the gas on its periphery along directions inclined with respect to the impeller axis with a pitch greater than zero and smaller than 90°. 
     As an example, axes Δ A  and Δ B  are horizontal, confounded, and located in the median horizontal plane of enclosure  10 . A vacuum pump, not shown, may be connected to enclosure  10  and enable to create a partial vacuum in enclosure  10 . 
     Each impeller  22 A,  22 B is rotated by a motor  24 A,  24 B. Motors  24 A,  24 B may be electric motors or hydraulic motors. They may be motors  24 A,  24 B which can only operate in one rotation direction. The axis of drive shaft  26 A of motor  24 A is confounded with axis Δ A  of impeller  22 A. Drive shaft  26 A is attached at one end to impeller  22 A. The axis of drive shaft  26 B of motor  24 B is confounded with axis Δ B  of impeller  22 B. Drive shaft  26 B is attached at one end to impeller  22 B. Motors  24 A,  24 B are arranged outside of enclosure  10  and on either side of enclosure  10  in tight casings, only drive shafts  26 A,  26 B being partly arranged in enclosure  10 . 
     Cell  5  comprises, on either side of load  14 , vertical panels  28 A,  28 B which extend substantially along the entire length of enclosure  10  along axis D. Each panel  28 A,  28 B rests on legs  30 A,  30 B fastened to enclosure  10 . Rails  16  may be fastened to panels  28 A,  28 B. The quenching gas cannot flow through panels  28 A,  28 B, but can flow under panels  28 A,  28 B between legs  30 A,  30 B, and above panels  28 A,  28 B, the top of panels  28 A,  28 B having no contact with enclosure  10 . 
     A first heat exchanger  32  is held between panels  28 A,  28 B above load  14 . A second heat exchanger  34  is held between panels  28 A,  28 B above load  14 . Exchangers  32 ,  34  are schematically shown as rectangles in  FIGS. 1 and 2 . In operation, the quenching gas is cooled by flowing through heat exchangers  32 ,  34 . As an example, each heat exchanger  32 ,  34  comprises parallel tubes having a cooling liquid flowing therethrough. 
     Quenching cell  5  comprises a planar horizontal separation plate  36 A,  36 B, for each impeller  22 A,  22 B. The median plane of separation plates  36 A,  36 B contains axes Δ A  and Δ B . Each plate  36 A,  36 B connects enclosure  10  to the associated vertical panel  28 A,  28 B, substantially along the entire length of enclosure  10  along axis D. Each plate  36 A,  36 B comprises an opening, only opening  39 A being shown in  FIGS. 4 and 6 , especially providing a passage for impeller  22 A,  22 B and drive shaft  26 A,  26 B. Each plate  36 A,  36 B separates the internal volume of cell  5 , located between enclosure  10  and panel  28 A,  28 B, into an upper area  37 A,  37 B located above plate  36 A,  36 B and a lower area  38 A,  38 B located above plate  36 A,  36 B. 
     Cell  5  comprises, for each impeller  22 A,  22 B, an upper half-volute  40 A,  40 B, located above separation plate  36 A,  36 B, and a lower half-volute  42 A,  42 B, located under separation plate  36 A,  36 B. 
     Each upper half-volute  40 A,  40 B comprises a lateral wall  43 A,  43 B, a planar inner wall  44 A,  44 B, and a planar outer wall  45 A,  45 B. Planar walls  44 A,  44 B,  45 A,  45 B are perpendicular to axes Δ A  and Δ B  and comprise an inner edge corresponding to a circle portion having a diameter slightly greater than the maximum external diameter of impeller  22 A,  22 B. Each lower half-volute  42 A,  42 B comprises a lateral wall  46 A,  46 B, a planar inner wall  47 A,  47 B, and a planar outer wall  48 A,  48 B. Planar walls  47 A,  47 B,  48 A,  48 B are perpendicular to axes Δ A  and Δ B  and comprise an inner edge corresponding to a circle portion having a diameter slightly greater than the maximum external diameter of impeller  22 A,  22 B. Planar inner wall  44 A,  44 B,  47 A,  47 B is the planar wall closest to panels  28 A,  28 B and planar outer wall  45 A,  45 B,  48 A,  48 B is the wall most remote from panels  28 A,  28 B. 
     Cell  5  comprises, for each impeller  22 A,  22 B, a cylindrical wall  50 A,  50 B of axis Δ A  and Δ B  respectively. The inner diameter of cylindrical wall  50 A,  50 B is substantially equal to the maximum external diameter of impeller  22 A,  22 B. Cylindrical wall  50 A,  50 B is in contact with panel  28 A,  28 B. 
     Each half-volute  40 A,  40 B,  42 A,  42 B can be shifted along axis Δ A  (respectively Δ B ) between a first position, called guiding position, where the half-volute is close to enclosure  10 , and a second position, called screening position, where the half-volute is close to panel  28 A,  28 B. The system for displacing half-volutes  40 A,  40 B,  42 A,  42 B is not shown in  FIGS. 1 and 2 . 
       FIG. 3  shows a perspective view of impeller  22 A. It is a closed mixed-flow impeller. Impeller  22 B may be identical to impeller  22 A. Impeller  22 A comprises blades  51 A maintained between a base flange  52 A and a cover ring  54 A. Each blade  51 A has a front edge  56 A, a rear edge  58 A, and lateral edges  60 A,  62 A. Base flange  52 A comprises a central support portion  64 A and a planar portion  66 A extending around support portion  64 A. Planar portion  66 A has, seen along axis Δ A , the shape of a ring of axis Δ A  and comprises a circular outer ring  68 A. Support portion  64 A is crossed by an opening  70 A for the passage of drive shaft  26 A, not shown in  FIG. 3 . Lateral edge  62 A of each blade  51 A is attached to planar portion  66 A and extends from outer edge  68 A of planar portion  66 A to support portion  64 A. 
     Cover ring  54 A is a piece having a symmetry of revolution around axis Δ A  and comprises an internal wall  71 A, a lateral wall  72 A, and a front wall  73 A. Lateral wall  72 A is a cylindrical wall of axis Δ A  having the same diameter as circular outer edge  68 A of base flange  52 A. Front wall  73 A is a planar wall having, seen along axis Δ A , the shape of a ring of axis Δ A  having its outer edge in contact with lateral wall  72 A and comprising a circular inner edge  74 A having a diameter smaller than the diameter of lateral wall  72 A. Internal wall  71 A connects circular inner edge  74 A to lateral wall  72 A. Lateral wall  72 A comprises a circular edge  75 A in contact with blades  51 A. Internal wall  71 A connects circular inner edge  74 A to circular edge  75 A. 
     Lateral edge  60 A of each blade  51 A is attached to internal wall  71 A and extends from circular edge  75 A to circular inner edge  74 A. Circular inner edge  74 A delimits intake opening  76 A of impeller  22 A. Rear edges  58 A of blades  51 A and circular edges  68 A,  75 A delimit discharge openings  78 A of impeller  22 A. 
     In operation, impeller  22 A is rotated around axis Δ A  along arrow  79 . The quenching gas is sucked in through intake opening  76 A of impeller  22 A and is expelled through discharge openings  78 A along the entire periphery of impeller  22 A radially and towards the back. 
     For each half-volute  40 A,  40 B,  42 A,  42 B, in the guiding position, planar external wall  45 A,  45 B,  48 A,  48 B of half-volute  40 A,  40 B,  42 A,  42 B substantially prolong base flange  52 A,  52 B of the associated impeller  22 A,  22 B. Further, planar inner wall  44 A,  44 B,  47 A,  47 B of half-volute  40 A,  40 B,  42 A,  42 B extends in line with cylindrical wall  50 A,  50 B. Lateral wall  43 A,  43 B,  46 A,  46 B of half-volute  40 A,  40 B,  42 A,  42 B covers discharge openings  78 A,  78 B of the associated impeller  22 A,  22 B on one half of the periphery of impeller  22 A,  22 B. 
     For each half-volute  40 A,  40 B,  42 A,  42 B, in the screening position, external planar wall  45 A,  45 B,  48 A,  48 B of half-volute  40 A,  40 B,  42 A,  42 B is in line with cylindrical lateral wall  72 A,  72 B and inner planar wall  44 A,  44 B,  47 A,  47 B is in line with cylindrical wall  50 A,  50 B. Lateral wall  43 A,  43 B,  46 A,  46 B of half-volute  40 A,  40 B,  42 A,  42 B extends between cylindrical wall  72 A,  72 B and cylindrical wall  50 A,  50 B. Half-volute  40 A,  40 B,  42 A,  42 B, cylindrical wall  72 A,  72 B, separation plate  36 A,  36 B, and cylindrical wall  50 A,  50 B then form a screen which prevents or strongly decreases the quenching gas flow. 
     Half-volutes  40 A,  40 B,  42 A,  42 B are displaced so that, when upper half-volutes  40 A,  40 B are in the guiding position, as shown in  FIG. 1 , lower half-volutes  42 A,  42 B are in the screening position and that, when upper half-volutes  40 A,  40 B are in the screening position, as shown in  FIG. 2 , lower half-volutes  42 A,  42 B are in the guiding position. 
     In the configuration shown in  FIG. 1 , when impellers  22 A,  22 B are rotated, the quenching gas substantially flows along arrows  80  and, in particular, from bottom to top at the level of load  14 . Indeed, each lower half-volute  42 A,  42 B, in screening position, prevents or strongly decreases the quenching gas intake by the associated impeller  22 A,  22 B from lower area  38 A,  38 B. Thereby, most of the quenching gas sucked in by impeller  22 A,  22 B originates from upper area  37 A,  37 B. Further, each upper half-volute  40 A,  40 B, in guiding position, guides the flow expelled by the associated mixed-flow impeller  22 A,  22 B towards lower area  38 A,  38 B. 
     In the configuration shown in  FIG. 2 , when impellers  22 A,  22 B are rotated, the quenching gas substantially flows along arrows  81  and, in particular, from top to bottom at the level of load  14 . Indeed, each upper half-volute  40 A,  40 B, in screening position, prevents or strongly decreases the quenching gas intake by the associated impeller  22 A,  22 B from upper area  37 A,  37 B. Thereby, most of the quenching gas sucked in by impeller  22 A,  22 B originates from lower area  38 A,  38 B. Further, each lower half-volute  42 A,  42 B, in guiding position, guides the flow expelled by the associated mixed-flow impeller  22 A,  22 B towards upper area  37 A,  37 B. 
     As an example, in operation, impellers  22 A,  22 B circulate the quenching gas at the level of load  14  with a flow rate of a few cubic meters per second. 
     The quenching gas flow direction at the level of load  14  can thus be reversed by passing from the configuration shown in  FIG. 1  to the configuration shown in  FIG. 2  and conversely, impellers  22 A,  22 B always rotating in the same direction. A quenching method may comprise one or a plurality of reversals of the quenching gas flow direction at the level of load  14 . 
       FIG. 4  is a partial simplified cross-section view of  FIG. 1  along plane IV-IV and shows impeller  22 A, half-volute  40 A (in full lines), half-volute  42 A (in dotted lines) and separation plate  36 A. Half-volutes  40 B and  42 B may have a structure similar to that of half-volutes  40 A,  42 A. Half-volute  40 A comprises bearing portions  82 A,  84 A which prolong lateral wall  43 A and rest on the upper surface of separation wall  36 A. Half-volute  40 A, in guiding position, directs the gas expelled on the upper half of impeller  22 A towards lower area  38 A. Half-volute  42 A, shown in dotted lines in guiding position, comprises bearing portions  86 A,  88 A which prolong lateral wall  46 A and rest on the lower surface of separation wall  36 A. Half-volute  42 A, in guiding position, directs the gas expelled on the lower half of impeller  22 A towards upper area  37 A. 
       FIGS. 5 and 6  are perspective views of certain elements of quenching cell  5  of  FIG. 1 . These drawings only show vertical panel  28 A, impeller  22 A, half-volute  40 A in guiding position, separation plate  36 A, and motor  24 A. Further, the actuation system of half-volute  40 A is shown in  FIGS. 5 and 6 . Further,  FIG. 5  shows legs  30 A and heat exchangers  32 ,  34 . 
     Only the actuation system of half-volute  40 A is described in detail. The actuation systems of the other half-volutes may have a structure similar to the actuation system of half-volute  40 A. The actuation system of half-volute  40 A comprises an actuator  90 A which comprises two guide rods  94 A,  96 A having their axes parallel to axis Δ A . Guiding rods  94 A,  96 A are arranged on either side of half-volute  40 A and are attached at their ends to separation plate  36 A by supports  98 A. A carriage  100 A, attached to half-volute  40 A, may slide on rod  94 A. A carriage  102 A, attached to half-volute  40 A, may slide on rod  96 A. Actuator  90 A comprises an electric motor  104 A rotating, by a transmission system  106 A, a worm  108 A. The axis of worm  108 A is parallel to axis Δ A . Carriage  100 A comprises a portion  110 A forming a nut assembled on worm  108 A. 
     In operation, a rotation of endless screw  108 A results in a shifting of portion  110 A forming a nut along the axis of worm  108 A, that is, parallel to axis Δ A . This results in a shifting of half-volute  40 A along axis Δ A . According to the rotation direction of worm  108 A, half-volute  40 A is displaced from the guiding position to the screening position or from the screening position to the guiding position. 
     Motors  22 A,  22 B may be associated with speed variation devices to modify the quenching gas flow speed at the level of load  14  during a quenching operation. For this purpose, frequency variators may be used when drive motors  24 A,  24 B are electric motors. In the case where motors  24 A,  24 B are hydraulic motors, a system for varying the flow rate of the oil supplying such motors may be provided. 
     According to another embodiment of the present invention, half-volutes  40 A,  40 B,  42 A,  42 B cannot be shifted parallel to axes Δ A  and Δ B  but are rotatably mobile around axes Δ A  and Δ B . Based on the configuration shown in  FIG. 1 , each half-volute  40 A,  40 B,  42 A,  42 B may be pivoted by one half-turn around the associated axis Δ A  and Δ B . Based on the configuration shown in  FIG. 1 , half-volute  40 A, after one half-turn, covers the lower half of the periphery of impeller  22 A and half-volute  42 A, after one half-turn, extends between cylindrical walls  72 A and  50 A in upper area  37 A. Based on the configuration shown in  FIG. 1 , half-volute  40 B, after one half-turn, covers the lower half of the periphery of impeller  22 B and half-volute  42 B, after one half-turn, extends between cylindrical walls  72 B and  50 B in upper area  37 B. 
     Quenching cell  5  has several advantages: 
     Whatever the positions of the half-volutes, all the quenching gas is discharged by the impeller in the proper direction relative to the desired quenching gas flow direction at the load level. For example, in the configuration shown in  FIG. 1 , the gas expelled on the upper half of the impeller is guided by each upper half-volute towards the lower area of the cell and the gas expelled on the lower half of the impeller is directly expelled into the lower area of the cell. Thereby, the provided flow reversal system enables to improve by approximately 20% the efficiency of the quenching cell, according to tests performed by the inventors, as compared with a flow reversal system with a free impeller (with no volute). This is due to the fact that, in the present embodiment of the invention, the output flow is either directed in the proper direction for the impeller half which is free (without any volute), or channeled in the proper direction for the impeller half comprising a volute. 
     The modification of the quenching gas flow direction at the load level is obtained by displacing the half-volutes with no reversal of the impeller rotation direction. Thereby, the reversal of the flow direction of the quenching gas driven by the impellers may be performed rapidly, for example, within less than five seconds. 
     Further, the reversal of the quenching gas flow direction at the load level is obtained by a system having a decreased bulk. 
     Of course, the present invention is likely to have various alterations and modifications, which will occur to those skilled in the art. In particular, the quenching cell may be different from the previously-described cell. In particular, the axes of the centrifugal or mixed-flow impellers may be vertically arranged so that the quenching gas flows at the load level along a horizontal direction. Further, the drive shafts may be inclined with respect to the impeller axes, the drive shafts being then connected to the impellers by transmission systems, for example, comprising toothed wheels. Further, the quenching cell may comprise a single impeller for circulating the quenching gas at the load level.