Plasma Torches

Abstract

A method of cooling the nozzle of a plasma torch by forcing water through the porous wall of the constrictor tube of the nozzle. This method results in a number of advantages over the use of a transpiring gas, including, no radiation shield being required to protect the constrictor tube; the energy density of the plasma arc is higher, the boundary of the plasma arc is better defined; and there is less acoustic and ultra-violet radiation from the plasma arc. A plasma torch is described having a porous constrictor tube and means for supplying water for transpiration-cooling.

Description

This invention relates to plasma torches and to a method of cooling the nozzles of a plasma torch.

Plasma torches are known in which the nozzle is cooled by the flow of a cooling fluid (usually water) through an annular chamber surrounding the bore of the nozzle. The same chamber may serve to cool the throat of the nozzle, or a separate chamber may be used. The material of the nozzle is impervious to the cooling fluid, and an arc produced by a torch having such an arrangement is known as a "water-cooled arc."

The nozzle of a plasma torch is formed to provide a throat leading to a bore. That part of the nozzle in which the bore is formed is known as, a constrictor tube.

It is also known, for example from the following articles, "Transpiration cooling of a constricted Electric-arc Heater" by J. E. Anderson and E. R. G. Eckert, AIAA Journal, April 1967; "Experimental Investigation of a Transpiration-Cooled, Constricted Arc" by E. Pfender, G. Gruber and E. R. G. Eckert, Proceedings of the Third International Symposium on High Temperature Technology, held September 1967;

"Study of a Transpiration-Cooled, Constricted Arc" by J. Heberlein, E. Pfender, and E. R. G. Eckert, ARL Technical Documentrary Report 70-0007, January 1970 published Aerospace Research Laboratories, Office of Aerospace Research, United States Air Force; and

"Transpiration-Cooling of the Constrictor Walls of an Electric High-Intensity Arc," by J. Heberlein and E. Pfender, Transactions of the ASME, May 1971, to form the constrictor tube of a plasma torch of a porous material and to feed a transpiring gas, usually argon or nitrogen, through the walls of the tube while the torch is in use. The arc produced by a torch having this arrangement is known as a "transpiration-cooled arc."

SUMMARY

According to one aspect of the present invention, a method of cooling the nozzle of a plasma torch, which nozzle has a constrictor tube formed of a porous material, comprises the step of forcing water through the porous material into the bore of the nozzle while the plasma torch is in use.

According to another aspect of the present invention, a plasma torch, provided with a nozzle having a porous constrictor tube, comprises means for supplying water to the outside of the constrictor tube, and pumping means for raising the pressure of the water so supplied to a predetermined value.

The pore size of the material is preferably in the range of 1.5 to 1500 microns and more preferably is 15 microns. The constrictor tube may be formed from a ceramic material.

The throat portion may be removably sealed within a sleeve of the torch and have a peripheral recess which forms, with the inside surface of the sleeve, a cavity for the flow therethrough of cooling water. By this means cooling water may pass through the wall of the sleeve via inlet and outlet pipes for circulation around the peripheral recess, and the throat portion may be removed from the sleeve without any disturbance of the cooling system.

Preferably the constrictor tube is removable and the throat portion is formed having a first part and a second part, the second part being disposed between the first part and the constrictor tube and being separable from the first part and removable from the torch, and the first part being formed with the peripheral recess and being removably sealed within the sleeve. Thus if it is required to change the bore size of the nozzle, the constrictor tube can be replaced by a tube of the required bore size, and the second part of the throat portion can be replaced by a corresponding part having an appropriate configuration for matching the first part to the replacement constrictor tube.

When the plasma torch is in use transpiring water is forced through the wall of the porous constrictor tube and becomes partially vaporized as it emerges from the pores on the inside surface of the tube. This use of water as the transpiring medium results in a number of advantages over the use of a gas as the transpiring medium. Such advantages include:

a. More of the heat energy radiated from the plasma arc is absorbed by the partially vaporized water than would be absorbed by a transpiring gas and therefore when using water it may not be necessary to use an auxiliary means for reducing the amount of radiant heat energy incident upon the wall of the constrictor tube.

b. The large change of volume which occurs when the water vaporizes results in a greater constriction of the arc, and thus a higher energy density, than can be achieved using a transpiring gas.

c. The boundary between the plasma arc and surrounding layer of partially vaporized water is more sharply defined than when using a transpiring gas.

d. There is less acoustic radiation from the plasma arc external to the constrictor tube. This is thought to be due to less turbulence in the partially vaporized water surrounding the plasma arc than is experienced using a transpiring gas, the flow being more streamlined because of the higher density of the vapour and water droplets.

e. The partially vaporized water absorbes ultra-violet radiation from the plasma arc more strongly than a transpiring gas.

The increased power density of the arc resulting from the constriction by the transpiring water (partially vaporized) enables the torch to be used to obtain an improved quality of cut compared with conventional water-cooled arcs and gas transpiration-cooled arcs. Also, the maximum linear rate of cutting at which acceptable cut quality is achieved is greater than that for such arcs.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly cut-away schematic representation of a plasma torch;

FIG. 2 is a partly schematic longitudinal section in the region of the nozzle assembly of the torch of FIG. 1; and

FIG. 3 is a corresponding section showing a modification of the nozzle of FIG. 2.

DETAILED DESCRIPTION

In FIG. 1 a plasma torch, indicated generally at 10, comprises a torch body 11 within which is mounted a water-cooled cathode 12. The cooling of the cathode is by conventional water-cooling, water being supplied via inlet pipe 13 and being returned via outlet pipe 14, disposed at the top of the plasma torch. The cathode has a conical portion 15 ending in a radiussed tip 16. The conical portion 15 is disposed within a conical throat portion 17 of a nozzle assembly of the plasma torch. The conical throat leads into a cylindrical bore in a constrictor tube portion 18 which is coaxial with the throat portion 17.

The throat portion 17 is formed of copper and includes an internal annular cavity 19 for the flow of water to cool the cathode in a conventional manner.

The constrictor tube portion 18 is formed from a tube of porous ceramic material around which is an annular cavity 20 connected via an inlet pipe 21 to a pump 22 for controlling the supply of water at a pressure in the region of 300 lbs/sq. in for transpiration cooling of the constrictor tube portion.

In FIG. 2, which shows the nozzle assembly and surrounding components more clearly, the throat portion 17 basically comprises an upper horizontal disc having a large central hole therein, a lower horizontal disc of the same radius as the upper disc and having a small central hole therein, and a conical wall which joins the periphery of the large central hole to the periphery of the small central hole.

The throat portion 17 is mounted within a sleeve 23 being sealed to the inside surface thereof by means of an upper O-ring 24 fitted in a peripheral groove in the rim of the upper disc, and by means of a lower O-ring 25 fitted in a peripheral groove in the rim of the lower disc. The upper and lower discs and the conical wall form a peripheral recess which becomes the cavity 19 when bounded by the inside surface of the sleeve 23. An inlet pipe 26 and an outlet pipe 27 sealed to the sleeve 23 communicate with the cavity 19 for the flow and return of cooling water. By this means the throat portion 17 can be removed from the sleeve while leaving the pipes 26 and 27 in place.

The body 11 has a short downwardly extending annular wall 28 fitting within the sleeve 23 and forming an abutment for defining the uppermost position of the throat portion 17. The throat portion 17 is retained within sleeve 23 by means of a retaining ring 29 fitting in a recess extending around the inside surface of the sleeve 23.

An insulating sleeve 30 fits around the outside of sleeve 23 and has at its lower edge an inwardly directed flange 31 with a central circular aperture of slightly less diameter than the inside diameter of sleeve 23. A cap 32 is screwed to the insulating sleeve 30. A retaining cap 33 is screwed into cap 32 and maintains the tube of porous ceramic material, forming the constrictor tube 18, pressed upwardly against the lower surface of the throat portion 17 which is in turn pressed upwardly against wall 28. A seal 34 is provided at the upper surface of the tube 18, and also between the lower surface and the retaining cap 33. The diameter of the small central hole in the lower horizontal disc of the throat portion 17 is the same as the diameter of the bore of the constrictor tube 18, and the retaining cap 33 has an exit hole 35 coaxial with, and of the same diameter as the bore of the constrictor tube 18, and having its lower peripheral edge flared.

Inlet pipe 21 communicates with the top of a vertical channel 37 in the wall of sleeve 23. The top of channel 37 is otherwise blind and its bottom opens into a shallow annular recess 38 in the upper surface of the flange 31. A series of small bores 39 lead from the recess 38 into the annular cavity 20 between the constrictor tube 18 and the flange 31. The bores 39 are angled at 60.degree. to the longitudinal axis of the plasma torch.

FIG. 3 shows a modification of the arrangement of FIG. 2 in which the throat portion 17 is formed of upper and lower parts 17a and 17b respectively, and has a different throat configuration. The upper throat portion 17a is formed basically as the throat portion 17 shown in FIG. 2 but differs in that the aperture at the lower end is larger and is threaded to receive the lower throat portion 17b and in that the wall joining the upper and lower discs extends axially and turns inwardly at its lower end to form with a corresponding portion of the lower throat portion 17b, an inwardly directed shoulder 40 which leads into the conical throat 41 of the lower throat portion 17b. The conical throat 41 corresponds to the lower part of the throat portion 17. An O-ring seal 42 is provided between throat portions 17a and 17b.

By utilizing the above arrangement of FIG. 3, the bore size of a plasma torch may readily be changed by the replacement of the lower throat portion 17b, the constrictor tube 18 and the retaining cap 33, by the equivalent items for the different bore size.

The porosity of the ceramic material is selected not so small as to require too high a supply pressure for the transpiring water, yet not so large as to introduce too much water into the constrictor bore. A preferred range of pore size is from 1.5 to 1500 microns. The most preferred pore size is 15 microns at which a suitable flow rate for the transpiring water is 100 ml/min for a bore diameter of 5 mm, and under these conditions the constriction ratio is in the region of 3 to 1, in other words the arc becomes constricted by the partially vaporized water and its effective diameter is approximately one-third of the diameter of the bore of the constrictor tube.

By forming the constrictor tube from a ceramic material, any tendency of the plasma torch to produce a double arc is greatly reduced. However, if it is not required to take advantage of this feature of the ceramic constrictor tube, then the tube may be formed from another porous material, for example, a sintered metal.

The bore of the constrictor tube may be profiled in order to obtain a degree of focussing such that the arc diameter is reduced in the region at, or adjacent, the exit of the bore.

Claims

I claim:

1. A method of cooling the nozzle of a plasma torch, comprising the steps of providing a plasma torch having a constrictor tube formed of porous material, providing a supply of working medium for the plasma torch, operating the plasma torch in a manner to produce a constricted plasma arc jet, and forcing water through the porous constrictor tube into the bore of the tube whereby the plasma arc column in the constrictor tube becomes highly constricted and of increased energy density.

2. In a plasma torch including a constrictor tube formed of a porous material and a means for providing a supply of working fluid to the torch to produce a plasma arc jet, the improvement comprising means for providing a supply of water to the outside of said constrictor tube and pumping means for raising the pressure of the water so supplied to a predetermined value to force the water through the walls of said porous constrictor tube into the bore thereof whereby the plasma arc column in the constrictor tube becomes further constricted and of increased energy density.

3. A plasma torch as claimed in claim 2 wherein the porous material is a ceramic material.

4. A plasma torch as claimed in claim 2 wherein the pore size of the porous material lies in the range from 1.5 to 1500 microns.

5. A plasma torch as claimed in claim 4 wherein the pore size is 15 microns.

6. A plasma torch provided with a nozzle having a constrictor tube formed of a porous material, and comprising means for supplying water to the outside of the constrictor tube, pumping means for raising the pressure of the water so supplied to a predetermined value, and having a sleeve fitting coaxially around the nozzle and wherein the nozzle has a throat portion removably sealed within the sleeve and having a peripheral recess which forms, with the inside surface of the sleeve, a cavity for the flow therethrough of cooling water.

7. A plasma torch as claimed in claim 6 wherein the constrictor tube is removable, and wherein the throat portion is formed having a first part and a second part, the second part being disposed between the first part and the constrictor tube and being removeably sealed to the first part and removable from the torch, and the first part being formed with the peripheral recess and being removably sealed within the sleeve.

8. A plasma torch provided with a nozzle having a throat portion and a porous ceramic constrictor tube, and including: a sleeve fitting around the nozzle, the throat portion having an O-ring seal at each of two axially-spaced positions on the peripheral surface thereof, the O-ring seals engaging the inside surface of the sleeve, the throat portion also having a peripheral recess disposed between the O-ring seals; an inlet tube and an outlet tube communicating through the sleeve with the peripheral recess for flow and return of a coolant thereto; an annular groove in the inside surface of the sleeve disposed below the throat portion, a retaining ring fitting within the annular groove and retaining the throat portion within the sleeve; a stop within the sleeve above the throat portion for limiting the upward movement thereof, a screw cap mounted for axial movement relative to the sleeve and having a coaxial central aperture corresponding to the bore of the constrictor tube; a seal between the screw cap and the lower end of the constrictor tube; a seal between the upper end of the constrictor tube and the lower end of the throat portion; an annular cavity around the constrictor tube, a passageway communicating with said annular cavity, an inlet pipe for introducing transpiring water into the passageway, and means for controlling the pressure of water supplied to the passageway.

9. A plasma torch as claimed in claim 8 wherein the throat portion is formed having a first part and a second part, the second part being disposed between the first part and the constrictor tube and being removeably sealed to the first part, and the first part being formed with the peripheral recess and carrying the O-ring seals.