Swirl ring and flow control process for a plasma arc torch

• US 5,170,033 A
• Filed: 04/12/1991
• Issued: 12/08/1992
• Est. Priority Date: 04/24/1990
• Status: Expired due to Term

• Current Assignee: Hypertherm Incorporated
• Original Assignee: Hypertherm Incorporated
• Inventors: Luo, Lifeng, Couch, Richard W. Jr., Sanders, Nicholas A.
• Primary Examiner(s): Paschall, Mark H.
• Application Number:
  US07/682,991
• Time in Patent Office: 606 Days
• Field of Search: 219/121.51, 219/121.50, 219/121.52, 219/121.48, 219/74, 219/75, 219/121.55, 219/121.54
• US Class Current: 219/121.51
• CPC Class Codes: B23K 10/006 Control circuits therefor circuits for plasma torches H05H1/36
• CPC Class Codes: H05H 1/341 Arrangements for providing coaxial protecting fluids
• CPC Class Codes: H05H 1/3421 Transferred arc or pilot arc mode
• CPC Class Codes: H05H 1/3442 Cathodes with inserted tip
• CPC Class Codes: H05H 1/3468 Vortex generators
• CPC Class Codes: H05H 1/3478 Geometrical details
• CPC Class Codes: H05H 1/3494 Means for controlling discharge parameters

Abstract

A process and apparatus for reducing wear of an electrode in a plasma arc torch involves providing a substantial pressure drop in a flow of plasma gas from the torch to a plasma arc chamber defined by the electrode and a nozzle. The pressure drop is immediately before the plasma arc chamber. There is also a small, localized supply of the plasma gas located between the pressure drop and the plasma chamber. The supply is sufficient to stabilize the arc when the gas flow is cut off upstream of the pressure drop and the arc current is cut off very shortly thereafter. The pressure drop is small enough to permit an adequate flow of plasma gas to the plasma arc chamber during normal operations, but large enough to isolate residual gas in the torch from the plasma arc chamber when the arc current is cut off. The apparatus is preferably a swirl ring with an annular pre-chamber fed by a set of choke holes to feed gas to the chamber and create the pressure drop. A set of swirl holes feed gas from the pre-chamber to the plasma arc chamber.

Claims

1. A process for controlling wear of an electrode in a plasma arc torch at the termination of a cycle of operation where an arc current and plasma gas flow through the torch are cut off, and where the electrode is mounted within a torch body opposite and in a spaced relationship to a nozzle having an exit orifice for a plasma arc, the electrode and nozzle defining a plasma chamber, the torch body including means for directing a flow of an ionizable gas through the torch body to the plasma chamber, and the torch body having a conductive path for an arc current to produce a plasma arc in said gas flow eminating from said electrode both as a pilot arc to the nozzle and as a transferred arc to a workpiece at a full operating arc current value, comprising

creating a pressure drop in said flow of ionizable gas immediately upstream of said plasma chamber, said pressure drop being sufficiently low that the gas flow into the plasma chamber at the full operating value produces a gas pressure and gas flow pattern in said plasma chamber that stabilizes the arc, and

said pressure drop being sufficiently large that at said arc current cut off the gas pressure and the gas flow through said plasma chamber decays rapidly in coordination with the arc current cut off.

2. The electrode wear control process of claim 1 wherein said creating of a pressure drop comprises restricting the gas flow to the plasma chamber within the torch.

3. The electrode wear control process of claim 1 further comprising the step of producing a local gas chamber in said gas flow between the point of said pressure drop in the torch and said plasma chamber.

4. The electrode wear control process of claim 3 further comprising distributing the ionizable gas flow from said local gas chamber to said plasma chamber in a flow pattern that is swirling.

5. The electrode wear control process of claim 4 wherein said gas distributing also produces a generally spatially uniform flow of said gas through said plasma chamber.

6. The electrode wear control process of claim 1 wherein said gas flow shutting off precedes the arc current cut off by a very brief time interval in which the gas held in said local gas chamber and the plasma chamber stabilize the arc until it is extinguished.

7. The electrode wear control process of claim 6 wherein said arc current cut off is ramped down.

8. A gas control swirl ring for a plasma arc torch having a torch body that mounts a nozzle and an electrode at one end of the body in a spaced relationship that defines therebetween a plasma arc chamber where an arc current conducted through the torch body ignites a plasma arc in a flow of an ionizable gas directed through the torch body to the plasma arc chamber via a gas inlet passage produces a plasma arc that transfers to a workpiece via an exit orifice in the nozzle during operation in a transferred arc mode, comprising

a generally annular body having an upstream face in fluid communication with said flow of ionizable gas in the torch body and a downstream face in fluid communication with the plasma arc chamber,

a gas chamber formed in the body,

a plurality of gas inlet passages extending from said upstream face of the body to said chamber, said passages being sized to restrict the flow of ionizable gas to the chamber and thereby produce a pressure drop in the gas flow, and

a plurality of gas distribution holes formed in said body extending between said chamber and the plasma arc chamber to produce a swirling, generally uniformly distributed flow of gas in said chamber.

9. The swirl ring of claim 8 wherein said gas chamber comprises an annular recess formed in the outer surface of said body between said upstream and downstream faces.

10. The swirl ring of claim 9 further comprising an annular first sealing recess formed in said outer surface between said upstream face and first annular recess.

11. The swirl ring of claim 9 further comprising annular second and third sealing recesses formed in the upstream and downstream faces of said body.

12. The swirl ring of claim 8 further comprising a plurality of radially directed holes formed in said body extending from said gas chamber to said an inner surface of said body extending between said upstream and downstream faces and in fluid communication with the plasma arc chamber.

13. The swirl ring of claim 8 wherein each of said gas inlet passages and said gas distribution passages are mutually equiangularly spaced and are at least four in number.

14. The swirl ring of claim 8 wherein the gas pressure in the plasma arc chamber during operation is about 40 psi and said pressure drop is about 32 psi.