US3599949A

US3599949A - Manufacture of steel

Info

Publication number: US3599949A
Application number: US707623A
Authority: US
Inventors: Hugh Willmott Grenfell
Original assignee: Steel Company of Wales Ltd
Current assignee: Steel Company of Wales Ltd
Priority date: 1967-02-23
Filing date: 1968-02-23
Publication date: 1971-08-17
Anticipated expiration: 1988-08-17
Also published as: GB1166000A; FR1555169A

Abstract

A method for refining a molten metal particularly steel which includes the steps of placing a hood in the closed position over and in contact with a refining vessel, introducing a stream of oxygen/carbonaceous fuel such as fuel oil into the space below the hood and above the molten metal, igniting the oxygen/carbonaceous fuel stream with the hood located in said closed position over the refining vessel and the drawing off of resulting gases through the hood to a gas washer.

Description

United States Patent 72] Inventor Hugh Willmott Grenfell 3,232,748 2/1966 Rinesch 266/34 Glamorgan, South Wales 3,241,825 3/1966 Jilek et a1... 266/34 I21] Appl. No. 707,623 3,271,130 9/1966 Denig 266/34 [22] Filed Feb. 23, 1968 3,313,535 4/1967 Hopkins. 266/34 [45] Patented Aug. 17, 1971 3,358,983 12/1967 Wegscheider et al. 266/31 [73] Assignee The Steel Company of Wales Limited 3,377,158 4/1968 Meyer et a1 266/35 Glamorgan, South Wales FOREIGN PATENTS Pmmy 22:3 :32: 772,632 4/1957 Great Britain 266/35 8737/67 1,021,099 2/1966 Great Britain 266/34 Primary Examiner-Gerald A. Dost Attorney-Krafft and Wells [54] MANUFATURE OF STEEL 10 Claims, 4 Drawing Figs.

[52] US. Cl 266/35, ABSTRACT: A method for fi i a molten meta] pal-cw 75/60 266/34 LY 266/31 larly steel which includes the steps of placing a hood in the [5 ll."-

l d iti over d i o ta t a refining vessel in- Fleld 0 Search 27, t d i a t am of o y en/carbonaceous fuel such as fuel 35, 34-1; 75/59, 60 oil into the space below the hood and above the molten metal, igniting the oxygen/carbonaceous fuel stream with the hood [56] References cued located in said closed position over the refining vessel and the UNITED STATES PATENTS drawing off of resulting gases through the hood to a gas 3,026,102 3/1962 McFeaters 266/35 washer.

PRESSURE 5 105 mg m HINT/Milt g fl/H I l m m n l I PRESSURE MNTRULLER 'AMT INVENTOR Hugh Wi/lmoif Grenfell B Kn/ 1v 211M )1 ATTORNEYS PATENTED AUG 1 719m SHEET 2 OF 2 III INVENTOR Hugh Willmaff Grenfe/l 13 701% H 20% ATTORNEYJS MANUFACTURE OF STEEL This invention relates to the manufacture of steel and is concerned particularly with the removal of the waste gases in unburned state from a steelmaking vessel such as a converter.

In particular, the invention relates to the process of manufacturing steel in a converter in which oxygen or an oxygenenriched mixture is blown therein from the top of the converter. In such a process it is necessary to remove in some way during the process the unburned waste gases, and various gas cleaning and recovery systems have been suggested which greatly reduce the normal pollution of the air associated with oxygen blend converters which is caused by ion oxide particles issuing as reddish-brown fumes with the refining gases.

One such gas cleaning and recovery system involves and use of a vertically movable hood having a skirt which is mounted above the mouth of the converter and which, after ignition has taken place, is movable into a position in engagement with or closely adjacent the mouth of the converter. In such a known system there is at the beginning of the blow much unreacted oxygen present in the waste gases due to delay in the decarburation reaction in the molten iron. Great care must therefore be taken to ensure that the oxygen content does not increase beyond the CO nitrogen, into hazard limit and this risk is usually avoided by the injection of an inert gas such as nitrogen, into the system to dilute the waste gases at this critical time. In addition, the ingress of additional air from the atmosphere into the hood may be reduced by the maintaining of a predetermined pressure in the zone of the skirt of the hood and this is usually done by means of a pressure-sensitive control which adjusts a venturi or other flow-restricting device in the ducting of the gas removal system.

From the foregoing, it will be appreciated that known gas recovery and cleaning systems for converters are not only hazardous in operation but complicated and cumbersome in use. The present invention, therefore, has for its object the provision of a greatly simplified system which avoids the above-mentioned disadvantages of the known system.

According to the present invention there is provided a method for refining a molten metal which includes the steps of placing a hood in a closed position over and in contact with a refining vessel such as a ladle or converter containing the molten metal, introducing a stream of oxygen and carbonaceous fuel into the space below the hood and above the molten metal, igniting the oxygen/carbonaceous fuel stream with the hood located in said closed position over the refining vessel and drawing off the resulting gases through the hood to a gas washer.

The converter may be operated in accordance with the par ticular method described more particularly in our copending U.S. Pat. Applications Nos. 615,310, filed Feb. 13, 1967; 671,186, filed Sept. 25, 1967 and the application of Hugh Willmott Grenfell entitled Lance with Venturi, Oxygen Nozzle,filed Feb. 12, 1968.

The operation of a refining vessel or converter in this manner using an oxygen/carbonaceous fuel mixture enables a very simple gas removal system to be provided due primarily to the lack or considerable reduction of fume and to improve ignition. The ignition of the stream of carbonaceous fuel and oxygen mixture takes place with the hood closed the process may be viewed during the process by, for instance, a television camera to determine the end point of the blow without opening the hood.

The invention also provides apparatus for the refining of molten metals which comprises in combination a refining vessel, a hood connected to the end of a gas exhaust duct disposed over an aperture in said vessel, sealing means provided for connecting the aperture and hood to prevent substantial escape of gases passing from the vessel to the hood and one or more nozzles disposed within said vessel and adapted to provide a stream of oxygen and carbonaceous fuel, the arrangement being such that the sealing device is readily removable for the purpose of loading and/or unloading the refining vessel.

It is preferred that the flame formed on ignition of the fuel/oxygen mixture is disposed within said refining vessel so that the products of combustion but not the flame itself impinge upon the melt. The carbonaceous fuel may be aliquid such as fuel oil and a proportion of finely divided particulate solids material may be entrained in said liquid in a slurry. Typical solids materials are limestone and iron ore.

Following is a description with reference to the accompany ing drawings of a particular converter hood and lance assembly for carrying the present invention into effect.

It will be appreciated that the system to be described with reference to the accompanying drawings is by way of example only and a combination of furnace, lance and the like other than that described below may be used in the method of the present invention. In the drawings:

FIG. 1 is a part sectional view of a converter and exhaust system assembly in accordance with the present invention;

FIG. 2 is a plan view of the sealing device and converter of FIG. 1;

FIG. 3 is a fragmentary view of the converter and sealing device and hood assembly shown in FIG. 1; and

FIG. 4 is a fragmentary view on a much enlarged scale of the lance shown in FIG. 11.

A refining vessel 11 is supported in an upright position and is provided towards its lower end with a taphole 12 for removal of the refined metal therefrom and the second taphole 13 for the removal of slag at the end of a blow. The converter is open at its upper end 15 and is provided with an annular lip 14 which forms a ring concentric with and perpendicular to the vertical median line of the converter. The converter is suitably lined with a known manner.

Encircling the upper end of the converter vessel 11 is an annular sealing rig 16 having a radially extending upper face 17 which slopes slightly downwards the front portion of its width and then continues down as at 18 but more abruptly to form the upper part of ring 16 of a double frustoconical configuration. The outer and more downwardly sloping face 18 is adapted to form a seal as will hereinafter be described. The remaining lower portion of ring 16 is in the form of a depending skirt 19 which with diverging sides complementary to the contiguous outer surface of converter vessel 11 to which it is fixedly attached by rivetting or other suitable means.

Coaxial with the vessel 11 is a refractory-lined hood 20 communicating with an exhaust duct 107. The hood 20 is of frustoconical form and has a circular transverse section and converges in a direction upwardly from the converter 11. The hood 20 accommodates a pressure-sensing device 1104 extending through the wall and the refractory lining of hood 20 for the purpose hereinafter described.

The hood 20 carries about its lower peripheral edge a sealing ring 21 which is attached thereto in a manner similar to the fastening of ring 16 above the upper end of converter vessel 1 1, said ring 16 having a concentric sloping face 22 which is in symmetry with the face 18 with reference both to the vertical centerline of the vessel 11 and to the horizontal plane bisecting the space intermediate these confronting faces.

The space between the vessel 11 and the hood 20 is provided to allow the vessel to pivot about a horizontal axis without interference should this be the selected manner of charging and discharging the contents of the vessel 1 1.

In order to maintain the gases issuing from the vessel 11 substantially out of contact with the air or atmosphere, said space is arranged to be closed so as to be substantially gastight and to enable handling of such gases at high temperatures. A two-part sealing member 23 is provided each part of which detachably abuts the vessel 11 at lip 14 thereof and the surface 22 of the hood 20. Sealing member 23 has the general form of a relatively short open-ended cylinder separated into two equal semicylindrical parts on a vertical median plane, the two parts being adapted to horizontal motion and having their confronting faces 24 in contact in the closed or blowing position and being spaced apart equidistant from the vertical centerline of the vessel in the retracted position. A recess 25 has been provided in the inner wall of the sealing member for the reception of a refractory lining 26. The upper and lower edges of the sealing member 23 are bevelled to form sealing

faces

27 and 28 complementary thereto and adapted to cooperate with the

faces

22 and 18 respectively thereby forming a seal which confines the space between the opening of the vessel 11 and the entrance to the hood 20. The abutting surfaces 24 of the sections of the sealing member 23 are provided with mating tongues and

grooves

29 and 30 respectively. It will be apparent that when the two sections of the sealing member 23 are brought tightly together in clamping relationship, escape of gas generated within the furnace to the atmosphere at this point would be substantially precluded.

The sealing device 23 may be provided with interconnected cord passages 32 which are in communication with a source of water supply and discharge means (not shown) whereby cooling water may be circulated within device 23 during the blowing period to cool the device and maintain a more uniform temperature.

In the closed position of the sealing device 23, the inner surface of the refractory lining 26 is offset outwardly relative to the inner edge 25 and the opening 15 in the vessel 11. During the blowing period, the sections of the sealing member are held tightly together and in sealing and clamping contact with the vessel 11 and hood by means of pressure exerted by fluid-controlled cylinders 43 of a conventional type. The sections of the sealed member 23 are provided with flange wheels 44 adapted to roll on tracks 45 straddling vessel 11' and cooperating therewith to maintain the alignment of said sections in their respective positions. The weight of the sections, tracks and operating mechanism is supported by suitable structural members such as has been indicated by reference numeral 46.

The upper end of hood 20 is connected by a duct 107 to a spark box 101 which during the blowing period serves to remove from duct 107 any particles of molten slag and metal which splash out of the converter vessel 11. The duct 107 extends into a vertical stack 109 having water sprays 103 disposed therein so that a stream or spray of water passes downwardly of the stack, countercurrent to the flow of exhaust gases therein. A drain 102 at the lower end of the stack serves to remove the washing water therefrom. At the same time, the water sprays 103 serve to reduce the temperature of the gases extracted from the converter vessel 11.

It is, of course, desirable to maintain a predetermined pressure in the hood so as to avoid the ingress of any air in the atmosphere which may increase the oxygen present at the commencement of the blow. This pressure control is effective by means of a pressure controller 106 which is connected to pressure element 104 disposed in the surface of hood 20 as referred to above. The pressure controller acts to operate a flow control valve 105 in the water supply line to the sprays 103 in stack 109. In this way, the water supply is controlled so as to vary the buoyancy of the gases in the stack thereby controlling the pressure in the hood 20 and mouth 15 of converter vessel 11.

The stream of fuel/oxygen mixture is introduced into the converter by means of a lance 110 a detail of which is more fully illustrated in FIG. 4 of the accompanying drawings.

The lance 110 comprises an elongate body member 111 which is provided with a combined delivery and burner nozzle 112 at the lower end thereof. The interior of the body member 111 of the lance 110 is built up of a number of annular passageways or conduits by which oxygen and fuel oil are supplied to a plurality of discharge orifices 113 formed in the combined delivery and burner nozzle 112. A fuel oil supply conduit comprising a copper pipe 114 is preferably located centrally of the body member 111 of the lance 110. A plurality of supply pipes 116 are welded as at 115 to the lower end of the pipe 114 and extend downwardly therefrom, the pipe 116 corresponding in number to the number of discharge orifices 113 as will be hereinafter described.

An oxygen supply conduit comprises a pipe 117 forming an annular passageway 118 around the oil supply pipe 1 14 to permit the free flow of oxygen downwardly towards the combined delivery and burned nozzle 112 of the lance. The lower end of the oxygen supply pipe 117 extends beyond the lower end of the oil supply pipe 114, so that a portion of each of the fuel oil supply pipe 114, so that a portion of each of the fuel oil supply pipes 116 is located within the oxygen supply pipes 117. The lower end of the oxygen supply pipe 1 17 is connected, for example by welding as at 119, to a discharge head 120 from which extends a plurality of oxygen supply pipes 121 located at an angle to the main longitudinal axis of the lance, for example at an angle of about 14.

in the preferred arrangement the lance is provided with six discharge orifices 113 and the oxygen supply conduit 117 is provided with six corresponding supply pipe lines 121 for the simultaneous discharge of six streams of oxygen at an angle to the longitudinal axis of the lance. The oil supply pipes 1 16 which extend downwardly from the lower end of the main oil supply conduit 114 extend in a cluster into the discharge head of the oxygen supply pipe 117 and are then bent outwardly at 122 so that their outer ends 123 are each located in a corresponding oxygen supply pipe 121. The arrangement is such that the six streams .of oxygen pass through the corresponding supply pipes 121 to the discharge orifices 113 in the form of annular streams around the ends 123 of the oil supply pipes 116. The movement of the oxygen through the supply pipes 121 to the discharge orifices 113 ensures the entrainment of the fuel oil in the streams of oxygen when the lance 110 is being used as a burner.

Each of oxygen supply pipes 121 incorporates a venturi 133 the throat of which is disposed intermediate the point of communication of supply pipe 121 with the oxygen supply conduit 117 and the discharge orifice 113 of supply pipe 121. Each venturi has its throat disposed rearwardly of the discharge point of the fuel pipe located in the oxygen supply pipe 121 to provide maximum gas velocity passed the end of the fuel pipe 123 debouching within supply pipe 121.

The forward converging frustoconical surface of the venturi 133 is inclined at an angle of 13 to the axis of the pipe while the rearward diverging frustoconical surface of the pipe is inclined at an angle of 7.

The venturi serves to accelerate the oxygen supplied through supply pipe 121 to a sonic velocity with a result that fuel leaving the end of the pipe 123 disposed within pipes 121 is atomized by the sonic flow of oxygen gas passing the end of the pipe to produce intimate mixing of the fuel and oxygen prior to combustion externally of the lance after leaving nozzle orifices 133.

The main oil supply conduit 114 is mounted for movement within the body member 111 of the lance 110 so that it can be adjusted upwardly or downwardly along the longitudinal axis of the lance. In this way the position of the ends 123 of the oil supply pipes 116 in the oxygen supply pipes 121 can be varied and this gives a certain amount of flame adjustment for the lance 110 and when it is being used as a burner.

The outer casing which forms the body 111 of the lance extends around the oxygen supply conduit 117 and is flared outwardly at its lower end as at 124 to accommodate the outwardly directed oxygen supply pipes 121. The discharge and burner nozzle 112 is welded as at 125 to the flared outer end 126 of the outer casing of the body 111 of the lance and 111 nozzle 112 is dome shaped in construction and is provided as previously mentioned with the six spaced discharge orifices 113 to which the oxygen supply pipes 121 communicate.

The annular space 127 provided between the outer casing of the elongate body member 111 of the lance 110 and the outer surface of the oxygen supply conduit 117 is conveniently used for the supply of cooling fluid such as water. To this end the space is divided into two

annular passageways

128, 129 by a further annular conduit 130 and the cooling water flows downwardly in passageway 128 through the elongate body member 111 around the outer surface of the oxygen supply conduit 117 into the discharge burner nozzle 112 of the lance and then returns upwardly through the annular passageway 129 formed adjacent the outer casing of the lance. The lower end 131 of conduit 130 is also flared outwardly to accommodate the oxygen supply pipes 121. v

The lance 110 enters the refining apparatus through the top surface of the cranked part of duct 107, passes coaxially of the hood '20 and the sealing means 23 into the converter vessel 1 1.

Means (not shown) are provided for raising and lowering the lance 1 relative to the converter 1 1.

In operation, with the lance 110 withdrawn from the converter 11, and the sealing elements 23 in their retracted positions, the converter 11 is tilted about its axis and is charged with hot metal. The converter 11 is returned to its vertical position, the sealing elements 23 are moved to their closed or clamping positions so that the edges of the sealing elements 23 abut or make contact with the sealing ring 16 and 21 respectively. The lance 110 is then lowered into the furnace until the nozzle end 13 of the lance 110 is spaced from the upper surface of the melt. Oxygen is first passed down the appropriate conduit of the lance and the fuel oil supply is turned on. The fuel oil/oxygen mixture ignites by itself and the proportion of fuel to oxygen are adjusted to produce the desired flame. The position of the lance 110 within the vessel 11 is further adjusted so that the products of combustion but not the flame itself impinge upon the surface of the melt.

After ignition, the proportion of oxygen is rapidly increased until an excess of oxygen is passing into the refining vessel. The products of the refining reaction pass out of the mouth 15 of the converter 11 into the hood and through duct 107 to stack 109. Splashes of .metal and slag passing out of converter 11 with the gases are retained in spark box 101. The remaining gases pass to stack 109 and travel up the stack 109 against the water sprays 103 which remove dust and cool the gases which then pass out into the atmosphere.

During the blow, the lance 110 may be raised or lowered within the converter 11 always provided that the flame itself does not impinge upon the surface of the melt. During the course of the blow, a fluxing agent such as powdered limestone may be introduced into the converter 11 with the fuel oil passing down the lance 110. The slag formed during the blow floats on the top of the melt and the nozzle of the lance 110 may in fact be dipping within the slag itself. The force of the gases being emitted from the lance penetrate the slag layer and may cause a depression in the melt surface. At the end of the blow, the lance l 10 is retracted upwardly of the converter the refined metal may be tapped by a taphole 12 from the base of the converter 11 while slag is removed from taphole 13. Alternatively, the sealing devices 23 may be retracted from the mouth 15 of the converter 11 and the converter vessel 11 may be tilted about is axis to teem the molten metal.

The method and apparatus described above has the advantage that ignition of the fuel oil/oxygen mixture is certain and may be permitted to take place with the hood and sealing device in the fully closed position.

The control of the water sprays may be in accordance with preset arrangements to maintain a desired pressure in the hood and preferably this pressure is slightly below, for example a few millimeters below, atmospheric pressure.

1 claim:

1. Apparatus for the refining of molten metals comprising in combination a refining vessel, a hood connected to the end of a has exhaust duct disposed over an aperture in the vessel, sealing means for connecting the aperture and hood to prevent substantial escape of gases passing from the vessel to the hood and one or more nozzles disposed within the said vessel and adapted to provide a stream of oxygen and carbonaceous fuel, stack means in flow combination with the hood, spray means disposed to spray water into the gas flow from the hood and a pressure-responsive element mounted within the hood and operably connected to a pressure controller adapted to operate a flow conn'ol valve in the water supply to the sprays, whereby the buoyancy of the gases within the stack may be varied blyigontrolling the water splrays within the stack thereby contro g the pressure in the cod at the mouth of the refining vessel.

2. Apparatus as claimed in claim 1 wherein the hood is provided with a movable sealing skirt and arranged so as to close the gap between the lower edge of the hood and the mouth of the refining vessel.

3. Apparatus as claimed in 2, wherein the sealing skirt comprises a plurality of sealing elements each movable laterally of the refining vessel from an open to a closed position in which said element contacts the lower peripheral edge of the hood and the mouth of the refining vessel.

4. Apparatus as claimed in claim 3, wherein the sealing skirt is cylindrical and consists of two semicylindrical portions each arranged for retraction in a horizontal plane to disengage from the peripheral edges of said hood and said vessel.

5. Apparatus as claimed in claim 4, wherein a first sealing ring is provided around the aperture'of the refining vessel and a second sealing ring is provided around the hood, the sealing device being arranged so that the lower edge thereof is capable of detachably engaging the first sealing ring and the upper edge thereof is capable of detachably engaging the second sealing ring so that in the engaged position the sealing device provides a continuation of the hood and duct to conduct gases from the refining vessel via the sealing device and hood to the duct.

6. Apparatus as claimed in 1, means for introducing a stream of oxygen and carbonaceous fuel into the converter or furnace comprises one or more nozzles each having a body member provided with a fuel supply conduit centrally located therein, and an oxygen supply conduit surrounding said fuel supply conduit to provide an annular passageway for the supply of oxygen, wherein the body member is provided with a delivery nozzle at one end thereof formed with at least one discharge orifice communicating with said oxygen supply conduit through an oxygen supply pipe and incorporating means for accelerating gas flow in said oxygen supply pipe and wherein the fuel supply conduit is provided at its outlet with a fuel supply pie extending therefrom and each having its end portion situated in the oxygen supply pipe so that oxygen flowing from said supply pipe to the discharge orifice will flow in an annulus around the end of the corresponding fuel supply pipe whereby fuel will be entrained in the oxygen supply when discharged from the discharge orifices.

7. Apparatus as claimed in claim 6, wherein each nozzle comprises a plurality of discharge orifices each communicating with said oxygen supply conduit through a plurality of oxygen supply pipes disposed at an angle to the longitudinal axis of the body and wherein a plurality of fuel supply pipes extend from the fuel supply conduit each having its end portion situated in a corresponding oxygen supply pipe.

8. Apparatus as claimed in claim 7, wherein the body member is provided with cooling means in the form of annular passageways surrounding the oxygen supply conduits.

9. Apparatus as claimed in claim 8, wherein the means for accelerating gas flow in the oxygen supply pipes comprises a venturi, the area of the throat of said venturi being determined by the velocity required for oxygen leaving the nozzle.

10. Apparatus as claimed in claim 9, wherein the converging section of each venturi in the direction of gas flow is at an angle of and wherein the diverging section is at an angle of 7 each to the longitudinal axis of the oxygen supply pipe.

Claims

2. Apparatus as claimed in claim 1 wherein the hood is provided with a movable sealing skirt and arranged so as to close the gap between the lower edge of the hood and the mouth of the refining vessel.
3. Apparatus as claimed in 2, wherein the sealing skirt comprises a plurality of sealing elements each movable laterally of the refining vessel from an open to a closed position in which said element contacts the lower peripheral edge of the hood and the mouth of the refining vessel.
4. Apparatus as claimed in claim 3, wherein the sealing skirt is cylindrical and consists of two semicylindrical portions each arranged for retraction in a horizontal plane to disengage from the peripheral edges of said hood and said vessel.
5. Apparatus as claimed in claim 4, wherein a first sealing ring is provided around the aperture of the refining vessel and a second sealing ring is provided around the hood, the sealing device being arranged so that the lower edge thereof is capable of detachably engaging the first sealing ring and the upper edge thereof is capable of detachably engaging the second sealing ring so that in the engaged position the sealing device provides a continuation of the hood and duct to conduct gases from the refining vessel via the sealing device and hood to the duct.
6. Apparatus as claimed in 1, means for introducing a stream of oxygen and carbonaceous fuel into the converter or furnace comprises one or more nozzles each having a body Member provided with a fuel supply conduit centrally located therein, and an oxygen supply conduit surrounding said fuel supply conduit to provide an annular passageway for the supply of oxygen, wherein the body member is provided with a delivery nozzle at one end thereof formed with at least one discharge orifice communicating with said oxygen supply conduit through an oxygen supply pipe and incorporating means for accelerating gas flow in said oxygen supply pipe and wherein the fuel supply conduit is provided at its outlet with a fuel supply pie extending therefrom and each having its end portion situated in the oxygen supply pipe so that oxygen flowing from said supply pipe to the discharge orifice will flow in an annulus around the end of the corresponding fuel supply pipe whereby fuel will be entrained in the oxygen supply when discharged from the discharge orifices.
7. Apparatus as claimed in claim 6, wherein each nozzle comprises a plurality of discharge orifices each communicating with said oxygen supply conduit through a plurality of oxygen supply pipes disposed at an angle to the longitudinal axis of the body and wherein a plurality of fuel supply pipes extend from the fuel supply conduit each having its end portion situated in a corresponding oxygen supply pipe.
8. Apparatus as claimed in claim 7, wherein the body member is provided with cooling means in the form of annular passageways surrounding the oxygen supply conduits.
9. Apparatus as claimed in claim 8, wherein the means for accelerating gas flow in the oxygen supply pipes comprises a venturi, the area of the throat of said venturi being determined by the velocity required for oxygen leaving the nozzle.
10. Apparatus as claimed in claim 9, wherein the converging section of each venturi in the direction of gas flow is at an angle of -* and wherein the diverging section is at an angle of 7* each to the longitudinal axis of the oxygen supply pipe.