MIXING DEVICE FOR MIXING TWO MEDIA WITH GREATLY DIFFERENT TEMPERATURES
United States Patent 3698430
A mixing device for mixing two media of greatly different temperatures, especially for mixing liquid metals such as sodium, comprising a cylindrical mixing chamber having an axial inlet conduit and an inlet conduit connected transversely to the wall of the mixing chamber for the media to be mixed, and also an axial outlet conduit, wherein a perforated cylinder is positioned concentrically inside the mixing chamber, this cylinder forming the wall of the mixing part proper, and also wherein the axial inlet conduit is conically contracted, at the place where it is connected to the mixing chamber, to a diameter which is smaller than the diameter of the perforated cylinder and the diameter of the perforated cylinder is at least equal to the diameter of the axial inlet conduit before its contraction, and wherein furthermore there are annular spaces around the axial inlet and outlet conduits where they are connected with the mixing chamber, which communicate via relatively small ports with the inlet conduit and outlet conduit at one side and with the mixing chamber at the other side.
US Patent References:
Slide valve for controlling the flow of suspended solids
Hoopes - March 1953 - 2631759
APPARATUS FOR FLOWING PRESSURE-FLOWABLE FOOD PRODUCTS INTO A CONTAINER
Mueller - January 1969 - 3420268
Mixing device
Depotie - September 1924 - 1509718
Slide valve for controlling the flow of suspended solids
Hoopes - March 1953 - 2631759
APPARATUS FOR FLOWING PRESSURE-FLOWABLE FOOD PRODUCTS INTO A CONTAINER
Mueller - January 1969 - 3420268
Mixing device
Depotie - September 1924 - 1509718
Inventors:
Van Gasselt, Max L. G. (Apeldoorn, NL)
Van Rijs, Pieter A. (Apeldoorn, NL)
Basting, Willem J. (Apeldoorn, NL)
Van Rijs, Pieter A. (Apeldoorn, NL)
Basting, Willem J. (Apeldoorn, NL)
Application Number:
04/843634
Publication Date:
10/17/1972
Filing Date:
07/22/1969
View Patent Images:
Export Citation:
Assignee:
N.V. Neratoom (The Hague, NL)
Primary Class:
Other Classes:
976/DIG.011, 376/352
International Classes:
B01F5/04; G21C1/02; G21C1/00; F17D1/16; E03B7/09
Field of Search:
176/44,50,51,62,87 137/604
Primary Examiner:
Quarforth, Carl D.
Assistant Examiner:
Gaither, Roger S.
Claims:
We claim
1. A mixing device for mixing two fluid media with greatly different temperatures comprising a mixing chamber with inlet conduits for the two media to be mixed and an outlet conduit, said mixing chamber consisting of a substantially cylindrical housing with inlet conduits connected to one of the axial ends of this housing and transversely to the wall of this housing respectively, the outlet conduit being at the other axial end and a circumferentially perforated cylinder being positioned concentrically at some distance inside this housing in open communication with the two inlet conduits and the outlet conduit and means freely and movably supporting said perforated cylinder in said housing, the said axial inlet conduit at the location of the junction with the mixing chamber having a throat portion which decreases in the direction of the perforated cylinder to a diameter less than the diameter of the perforated cylinder, the diameter of the perforated cylinder being at least equal to the diameter of the throat portion at its entrant end.
2. A mixing device as claimed in claim 1 wherein around the axial inlet conduit where the said inlet has a throat portion decreasing in diameter towards the junction with the mixing chamber there is an annular space communicating via relatively small ports with the throat portion of this inlet conduit at one side and with the mixing chamber at the other.
3. A mixing device as claimed in claim 1, wherein around the axial outlet conduit where it connects with the mixing chamber there is likewise an annular space which communicates via relatively small ports with this outlet conduit at one side and with the mixing chamber at the other.
4. A mixing device as claimed in claim 2, wherein the outer wall of the annular spaces is formed by gradual contractions at the axial ends of the cylindrical housing which connect with the walls of the axial inlet and outlet conduits.
5. A mixing device as claimed in claim 1, wherein the perforated cylinder is positioned so that it is supported in the cylindrical housing with a certain amount of play.
6. A mixing device as claimed in claim 3, wherein the outer wall of the annular spaces is formed by gradual contractions at the axial ends of the cylindrical housing which connect with the walls of the axial inlet and outlet conduits.
1. A mixing device for mixing two fluid media with greatly different temperatures comprising a mixing chamber with inlet conduits for the two media to be mixed and an outlet conduit, said mixing chamber consisting of a substantially cylindrical housing with inlet conduits connected to one of the axial ends of this housing and transversely to the wall of this housing respectively, the outlet conduit being at the other axial end and a circumferentially perforated cylinder being positioned concentrically at some distance inside this housing in open communication with the two inlet conduits and the outlet conduit and means freely and movably supporting said perforated cylinder in said housing, the said axial inlet conduit at the location of the junction with the mixing chamber having a throat portion which decreases in the direction of the perforated cylinder to a diameter less than the diameter of the perforated cylinder, the diameter of the perforated cylinder being at least equal to the diameter of the throat portion at its entrant end.
2. A mixing device as claimed in claim 1 wherein around the axial inlet conduit where the said inlet has a throat portion decreasing in diameter towards the junction with the mixing chamber there is an annular space communicating via relatively small ports with the throat portion of this inlet conduit at one side and with the mixing chamber at the other.
3. A mixing device as claimed in claim 1, wherein around the axial outlet conduit where it connects with the mixing chamber there is likewise an annular space which communicates via relatively small ports with this outlet conduit at one side and with the mixing chamber at the other.
4. A mixing device as claimed in claim 2, wherein the outer wall of the annular spaces is formed by gradual contractions at the axial ends of the cylindrical housing which connect with the walls of the axial inlet and outlet conduits.
5. A mixing device as claimed in claim 1, wherein the perforated cylinder is positioned so that it is supported in the cylindrical housing with a certain amount of play.
6. A mixing device as claimed in claim 3, wherein the outer wall of the annular spaces is formed by gradual contractions at the axial ends of the cylindrical housing which connect with the walls of the axial inlet and outlet conduits.
Description:
The invention relates to a mixing device for mixing two media with greatly different temperatures, especially for mixing liquid metals such as sodium in loops of nuclear reactor circuits, comprising a mixing chamber with inlet conduits for the two media to be mixed and an outlet conduit, this mixing chamber consisting of a substantially cylindrical housing with inlet conduits connected to one of the axial ends of this housing and transversely to the wall of this housing respectively, the outlet conduit being at the other axial end and a circumferentially perforated cylinder being positioned concentrically at some distance inside this housing in open communication with the two inlet conduits and the outlet conduit.
Such a mixing device is known for mixing two media with different pressures.
This mixing device is, however, unsuitable for various reasons for mixing media with greatly different temperatures, such as for instance sodium flows in loops of nuclear reactor circuits.
In mixing such media, during which the temperature difference may be 200° C and over, the material and the construction of the mixing device have to satisfy particularly high standards.
The construction must then be such that the walls are safeguarded against thermal shocks.
In the event of rapid temperature variations high thermal stresses may occur in the component parts, which may lead to rupture.
Besides good mixing of the media, a low pressure loss and a stable flow after mixing are desirable.
In addition, good mixing must also be obtained within a wide range of mixture ratios of both media.
In the known mixing device, in the mixing zone inside the perforated cylinder positioned in the cylindrical mixing device there is another perforated tube positioned concentrically at a distance and in the direction of the axial inlet conduit bending towards the mixing chamber, it being closed at the front and in open communication at the back with the annular space formed between the wall of the mixing chamber and the perforated cylinder.
Such an embodiment gives an unfavorable flow field in the mixing part proper with an unstable flow during mixture of the media.
During mixing, therefore, large pressure losses will occur which, in view of the purpose of that mixing device, may have a favorable effect on mixing, but which must in fact be avoided with the mixing device according to the invention.
Furthermore, when mixing media with greatly different temperatures, this known mixing device will be subject to a considerable extent to thermal shocks resulting from excessive temperature variations at various places at the walls and at the location of the common junctions of these walls of the mixing chamber, which makes this mixing device totally unsuitable for the above-mentioned application.
It is the object of the invention to provide a mixing device which does not have the above-mentioned disadvantages.
With the mixing device according to the invention the occurrence of thermal stresses resulting from temperature differences and especially the occurrence of rapid temperature variations will remain within certain limits, while the mixing device is moreover stable from the aspect of flow engineering and the pressure loss occurring in the mixer is slight.
With the mixing device according to the invention, moreover, great variation is possible in the mixture ratios of the media to be mixed, while mixing is good.
For this purpose the mixing device according to the invention has the feature that the axial inlet conduit at the location of the junction with the mixing chamber conically contracts to a diameter smaller than the diameter of the perforated cylinder and the diameter of the perforated cylinder is at least equal to the diameter of the axial inlet conduit before its conical contraction.
In this way, in the mixing part from the axial inlet conduit to the opposite axial outlet conduit, a continuing flow is obtained of, for instance, the hot medium, and a flow fed transversely to the mixing part of the other, for instance, cold medium, which latter flow in the annular space formed between the wall of the mixing chamber and the perforated cylinder forms primarily a flowing jacket around the throughflow of hot medium in the axial direction. The colder medium will then gradually penetrate through the perforated cylinder and mix with the throughflow of hot medium, a temperature gradient then being built up. The conical contraction of the axial inlet conduit at the location of the junction with the mixing chamber and with the mixing part proper gives a favorable flow pattern for mixture proceeding in accordance with the desired temperature gradient.
The contraction of the axial inlet conduit is now such that macro-turbulence occurs in the main flow and the static pressure for the most unfavorable mixture ratio (i.e., when main flow and transverse flow are equal) will hardly change in the axial direction.
By making the diameter of the perforated cylinder of the mixing device according to the invention at least equal to the diameter of the axial inlet conduit before its conical contraction, a regular influx over the entire surface of the perforated cylinder is brought about so that the requisite mixing exists along the entire length of the mixing part. The hot liquid flow will be kept by the cold transverse flow within an imaginary cylinder owing to the damming effect of the transverse flow. Owing to a very strong turbulence within this imaginary cylinder the cold medium is transported further inwards and mixed with the main flow. This prevents the walls of the mixing device alternately coming into contact during mixing with the colder and then the hot medium, and therefore these walls are not subject to thermal shocks resulting from a varying Δ T across the walls.
Preferably, moreover, in the mixing device according to the invention the axial inlet conduit to the mixing part will be connected before this conical contraction by a continuous wall to the outer jacket outside the periphery of the perforated cylinder. In this way, an annular space is formed around this conical contraction, which is bounded by the conical contraction, the continuous wall and the front wall of the mixing part. By providing relatively small ports at the conical contraction of the axial inlet conduit and the front wall of the mixing part, the said annular space will at one side be in open communication with the contracting part of the inlet conduit and at the other side with the mixing chamber. The annular space then functions as a thermal shield. In the case of very great temperature differences between the two media, a correct temperature gradient will furthermore still be obtainable in this annular space by building in one or more baffles. The built up temperature gradient does not give rise to high thermal stresses and furthermore in case of a rapid change in the temperature of the axial supply conduit, known as temperature shock, this change, then retarded, will be dispersed in the outer jacket, and welds etc. will not therefore undergo a thermal shock. A slight flow through this space is necessary to prevent contamination.
In the same way an annular space can be formed also around the axial outlet conduit where it connects with the mixing chamber, and may communicate via relatively small ports with this outlet conduit at one side and with the mixing chamber at the other. Preferably, moreover, the outer wall of these annular spaces will be formed by contractions at the axial ends of the outer wall which connect with the outer wall of the axial inlet and outlet conduits respectively.
This embodiment has in the first place advantages from the flow engineering aspect and in addition the flow and mixing respectively of the two media from axial inlet conduit to outlet conduit will then proceed according to a specific temperature gradient. Structurally, the mixing device according to the invention thereby presents the advantage that the entire outer wall can be streamlined and formed without any sharp deflections or corners and the component walls can be welded together, and therefore no joins, bolted connections and the like vulnerable to thermal stresses need be applied. The perforated cylinder positioned concentrically in the mixing chamber housing is in fact the only constructional element located in a direct contact zone between the hot and colder media, and hence is subject to thermal stresses.
With the mixing device according to the invention the perforated cylinder is to this end preferably positioned so that it is supported in the mixing chamber housing with a certain amount of play.
The length of the mixing part and perforated cylinder respectively, and also the diameter, can be determined so that the resulting pressure distribution guarantees good mixing in the mixing device according to the invention within a wide range of mixture ratios.
The mixing device according to the invention presents a solution for the problem of mixing two media with greatly different temperatures and is particularly suitable for application in sodium technology for mixing liquid sodium in loops of nuclear reactor circuits.
The invention will be illustrated and elucidated below with reference to a drawing showing an embodiment of a mixing device for mixing two streams of liquid sodium with greatly different temperatures.
The accompanying FIGURE shows a longitudinal section of the mixing device.
In this, 1 is the mixing part proper of the mixing device with an axial inlet conduit 2 for a hot sodium stream, transversely a tangential inlet conduit 3 for the cold sodium stream and an outlet conduit 4 for the mixed sodium stream. The tangential inlet conduit 3 leads into an annular space 5 formed between wall 6 of the mixing part and a double-walled perforated cylinder 7 concentrically positioned therein. The axial inlet conduit 2 is provided at the location of its connection with the mixing part with a conical contraction 8 and leads into the mixing chamber 1, with a diameter smaller than the diameter of the perforated cylinder.
The hot sodium fed in via inlet conduit 2 will flow axially through the mixing part in the direction of the outlet conduit 4 and within the periphery of the perforated cylinder. If a cold sodium stream is fed in simultaneously via the tangential inlet conduit 3, this sodium will primarily fill the annular space 5 in the mixing part and forms as it were a jecket of cold sodium around the hot axially flowing sodium. This cold sodium will next penetrate through the perforations of the inner cylinder and gradually mix with the hot sodium, a temperature gradient thereby being built up radially in this cylinder, which prevents thermal shock stresses occurring in the outer wall owing to rapid temperature variations through alternate contact with hot sodium and then with cold sodium. The wall of the axial inlet conduit 2 at the place where this changes into the conical contraction 8 is continued to the mixing chamber for connection to the cylindrical outer wall 6 of the mixing part at the front side, and thereby forms a cylindrical widening wall section 9, which is joined by means of welded seams 10 and 11 at one side to the axial inlet conduit and at the other to the front side of the outer jacket.
The fore front-plate 12 of the mixing part to which the wall of the conical contraction 8 is fixed with a welded seam 13 does not continue to the outer jacket 6, as a result of which an annular gap 14 is formed. This fore front-plate 12 has no mechanical connection with the outer jacket 6. This fore front-plate is also provided with ports 15.
The wall of the conical contraction 8 of the axial inlet conduit is furthermore provided with ports 16. The conical contraction 8 is in this way enclosed by an annular space 17 bounded by the wall of the conical contraction 8, the wall section 9, and the fore front-plate 12; this space 17 may thereby communicate via ports 14, 15 and 16 at one side with the axial inlet conduit and at the other with the mixing part. In the transitional zone from mixing part to outlet conduit 4 an annular space 18 is also formed for the same reasons, this being bounded by the rear front-plate 19, the wall of the outlet conduit 4, and a contracting wall section 20, which is joined by means of welded seams 21 and 22 to the outer jacket and to the wall of outlet conduit 4 respectively.
This annular space also communicates via an annular gap 23 and ports 24 made in the wall of the outlet conduit with the mixing part at one side and with the outlet conduit at the other.
These ports (15, 16 and 24) are not essential conditions for the functioning of the mixing device. The annular gaps 14 and 23 may themselves suffice for the necessary replenishment of this space. The cylinder 7 is perforated in both walls and is enclosed with a certain amount of play and freely supported between the flanged edges 25 and 26 welded to the fore and rear front-plates 12 and 19 respectively of the mixing part.
Such a mixing device is known for mixing two media with different pressures.
This mixing device is, however, unsuitable for various reasons for mixing media with greatly different temperatures, such as for instance sodium flows in loops of nuclear reactor circuits.
In mixing such media, during which the temperature difference may be 200° C and over, the material and the construction of the mixing device have to satisfy particularly high standards.
The construction must then be such that the walls are safeguarded against thermal shocks.
In the event of rapid temperature variations high thermal stresses may occur in the component parts, which may lead to rupture.
Besides good mixing of the media, a low pressure loss and a stable flow after mixing are desirable.
In addition, good mixing must also be obtained within a wide range of mixture ratios of both media.
In the known mixing device, in the mixing zone inside the perforated cylinder positioned in the cylindrical mixing device there is another perforated tube positioned concentrically at a distance and in the direction of the axial inlet conduit bending towards the mixing chamber, it being closed at the front and in open communication at the back with the annular space formed between the wall of the mixing chamber and the perforated cylinder.
Such an embodiment gives an unfavorable flow field in the mixing part proper with an unstable flow during mixture of the media.
During mixing, therefore, large pressure losses will occur which, in view of the purpose of that mixing device, may have a favorable effect on mixing, but which must in fact be avoided with the mixing device according to the invention.
Furthermore, when mixing media with greatly different temperatures, this known mixing device will be subject to a considerable extent to thermal shocks resulting from excessive temperature variations at various places at the walls and at the location of the common junctions of these walls of the mixing chamber, which makes this mixing device totally unsuitable for the above-mentioned application.
It is the object of the invention to provide a mixing device which does not have the above-mentioned disadvantages.
With the mixing device according to the invention the occurrence of thermal stresses resulting from temperature differences and especially the occurrence of rapid temperature variations will remain within certain limits, while the mixing device is moreover stable from the aspect of flow engineering and the pressure loss occurring in the mixer is slight.
With the mixing device according to the invention, moreover, great variation is possible in the mixture ratios of the media to be mixed, while mixing is good.
For this purpose the mixing device according to the invention has the feature that the axial inlet conduit at the location of the junction with the mixing chamber conically contracts to a diameter smaller than the diameter of the perforated cylinder and the diameter of the perforated cylinder is at least equal to the diameter of the axial inlet conduit before its conical contraction.
In this way, in the mixing part from the axial inlet conduit to the opposite axial outlet conduit, a continuing flow is obtained of, for instance, the hot medium, and a flow fed transversely to the mixing part of the other, for instance, cold medium, which latter flow in the annular space formed between the wall of the mixing chamber and the perforated cylinder forms primarily a flowing jacket around the throughflow of hot medium in the axial direction. The colder medium will then gradually penetrate through the perforated cylinder and mix with the throughflow of hot medium, a temperature gradient then being built up. The conical contraction of the axial inlet conduit at the location of the junction with the mixing chamber and with the mixing part proper gives a favorable flow pattern for mixture proceeding in accordance with the desired temperature gradient.
The contraction of the axial inlet conduit is now such that macro-turbulence occurs in the main flow and the static pressure for the most unfavorable mixture ratio (i.e., when main flow and transverse flow are equal) will hardly change in the axial direction.
By making the diameter of the perforated cylinder of the mixing device according to the invention at least equal to the diameter of the axial inlet conduit before its conical contraction, a regular influx over the entire surface of the perforated cylinder is brought about so that the requisite mixing exists along the entire length of the mixing part. The hot liquid flow will be kept by the cold transverse flow within an imaginary cylinder owing to the damming effect of the transverse flow. Owing to a very strong turbulence within this imaginary cylinder the cold medium is transported further inwards and mixed with the main flow. This prevents the walls of the mixing device alternately coming into contact during mixing with the colder and then the hot medium, and therefore these walls are not subject to thermal shocks resulting from a varying Δ T across the walls.
Preferably, moreover, in the mixing device according to the invention the axial inlet conduit to the mixing part will be connected before this conical contraction by a continuous wall to the outer jacket outside the periphery of the perforated cylinder. In this way, an annular space is formed around this conical contraction, which is bounded by the conical contraction, the continuous wall and the front wall of the mixing part. By providing relatively small ports at the conical contraction of the axial inlet conduit and the front wall of the mixing part, the said annular space will at one side be in open communication with the contracting part of the inlet conduit and at the other side with the mixing chamber. The annular space then functions as a thermal shield. In the case of very great temperature differences between the two media, a correct temperature gradient will furthermore still be obtainable in this annular space by building in one or more baffles. The built up temperature gradient does not give rise to high thermal stresses and furthermore in case of a rapid change in the temperature of the axial supply conduit, known as temperature shock, this change, then retarded, will be dispersed in the outer jacket, and welds etc. will not therefore undergo a thermal shock. A slight flow through this space is necessary to prevent contamination.
In the same way an annular space can be formed also around the axial outlet conduit where it connects with the mixing chamber, and may communicate via relatively small ports with this outlet conduit at one side and with the mixing chamber at the other. Preferably, moreover, the outer wall of these annular spaces will be formed by contractions at the axial ends of the outer wall which connect with the outer wall of the axial inlet and outlet conduits respectively.
This embodiment has in the first place advantages from the flow engineering aspect and in addition the flow and mixing respectively of the two media from axial inlet conduit to outlet conduit will then proceed according to a specific temperature gradient. Structurally, the mixing device according to the invention thereby presents the advantage that the entire outer wall can be streamlined and formed without any sharp deflections or corners and the component walls can be welded together, and therefore no joins, bolted connections and the like vulnerable to thermal stresses need be applied. The perforated cylinder positioned concentrically in the mixing chamber housing is in fact the only constructional element located in a direct contact zone between the hot and colder media, and hence is subject to thermal stresses.
With the mixing device according to the invention the perforated cylinder is to this end preferably positioned so that it is supported in the mixing chamber housing with a certain amount of play.
The length of the mixing part and perforated cylinder respectively, and also the diameter, can be determined so that the resulting pressure distribution guarantees good mixing in the mixing device according to the invention within a wide range of mixture ratios.
The mixing device according to the invention presents a solution for the problem of mixing two media with greatly different temperatures and is particularly suitable for application in sodium technology for mixing liquid sodium in loops of nuclear reactor circuits.
The invention will be illustrated and elucidated below with reference to a drawing showing an embodiment of a mixing device for mixing two streams of liquid sodium with greatly different temperatures.
The accompanying FIGURE shows a longitudinal section of the mixing device.
In this, 1 is the mixing part proper of the mixing device with an axial inlet conduit 2 for a hot sodium stream, transversely a tangential inlet conduit 3 for the cold sodium stream and an outlet conduit 4 for the mixed sodium stream. The tangential inlet conduit 3 leads into an annular space 5 formed between wall 6 of the mixing part and a double-walled perforated cylinder 7 concentrically positioned therein. The axial inlet conduit 2 is provided at the location of its connection with the mixing part with a conical contraction 8 and leads into the mixing chamber 1, with a diameter smaller than the diameter of the perforated cylinder.
The hot sodium fed in via inlet conduit 2 will flow axially through the mixing part in the direction of the outlet conduit 4 and within the periphery of the perforated cylinder. If a cold sodium stream is fed in simultaneously via the tangential inlet conduit 3, this sodium will primarily fill the annular space 5 in the mixing part and forms as it were a jecket of cold sodium around the hot axially flowing sodium. This cold sodium will next penetrate through the perforations of the inner cylinder and gradually mix with the hot sodium, a temperature gradient thereby being built up radially in this cylinder, which prevents thermal shock stresses occurring in the outer wall owing to rapid temperature variations through alternate contact with hot sodium and then with cold sodium. The wall of the axial inlet conduit 2 at the place where this changes into the conical contraction 8 is continued to the mixing chamber for connection to the cylindrical outer wall 6 of the mixing part at the front side, and thereby forms a cylindrical widening wall section 9, which is joined by means of welded seams 10 and 11 at one side to the axial inlet conduit and at the other to the front side of the outer jacket.
The fore front-plate 12 of the mixing part to which the wall of the conical contraction 8 is fixed with a welded seam 13 does not continue to the outer jacket 6, as a result of which an annular gap 14 is formed. This fore front-plate 12 has no mechanical connection with the outer jacket 6. This fore front-plate is also provided with ports 15.
The wall of the conical contraction 8 of the axial inlet conduit is furthermore provided with ports 16. The conical contraction 8 is in this way enclosed by an annular space 17 bounded by the wall of the conical contraction 8, the wall section 9, and the fore front-plate 12; this space 17 may thereby communicate via ports 14, 15 and 16 at one side with the axial inlet conduit and at the other with the mixing part. In the transitional zone from mixing part to outlet conduit 4 an annular space 18 is also formed for the same reasons, this being bounded by the rear front-plate 19, the wall of the outlet conduit 4, and a contracting wall section 20, which is joined by means of welded seams 21 and 22 to the outer jacket and to the wall of outlet conduit 4 respectively.
This annular space also communicates via an annular gap 23 and ports 24 made in the wall of the outlet conduit with the mixing part at one side and with the outlet conduit at the other.
These ports (15, 16 and 24) are not essential conditions for the functioning of the mixing device. The annular gaps 14 and 23 may themselves suffice for the necessary replenishment of this space. The cylinder 7 is perforated in both walls and is enclosed with a certain amount of play and freely supported between the flanged edges 25 and 26 welded to the fore and rear front-plates 12 and 19 respectively of the mixing part.