Patent Publication Number: US-2012031881-A1

Title: Blow-Back Plasma Arc Torch With Shield Fluid-Cooled Electrode

Description:
BACKGROUND OF THE INVENTION 
     The present disclosure relates generally to plasma arc torches, and more particularly to plasma arc torches of the retract or blow-back type in which the electrode is retracted during starting by means of fluid pressure acting on a piston connected to the electrode. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     The present disclosure describes a plasma arc torch of the retract or blow-back type, in which separate supplies of plasma gas and secondary fluid are provided to the torch, and the torch&#39;s electrode is cooled by the secondary fluid. The secondary fluid can be a gas or liquid water, and is also used as a shield fluid for shielding the stream of plasma gas and the electric arc that issue from the primary nozzle of the torch. 
     In one embodiment, the plasma arc torch described herein comprises:
         a torch body assembly defining a cylindrical bore therein, at least one plasma gas supply passage for conducting a flow of a plasma gas, and at least one secondary fluid supply passage for conducting a flow of a secondary fluid that is supplied to the torch separately from the plasma gas;   an electrode assembly including an electrode at a lower end of the electrode assembly, the electrode assembly defining internal passages for receiving secondary fluid and circulating the secondary fluid within the electrode assembly for cooling the electrode;   a primary nozzle coupled to the torch body assembly adjacent the electrode and defining a plasma nozzle chamber therebetween and defining a primary orifice through which plasma gas in the plasma nozzle chamber is discharged and through which an arc from the electrode extends during a transferred-arc mode of operation of the torch;   a piston connected to the electrode and comprising a piston rod joined to a piston head assembly, the piston head assembly sealingly engaging an inner surface of the cylindrical bore in the torch body assembly such that the piston is axially slidable in the cylindrical bore;   an actuating chamber defined between a lower surface of the piston head assembly and the cylindrical bore, the torch being configured to supply one of the plasma gas and the secondary fluid into the actuating chamber, wherein sufficient pressure in the actuating chamber urges the piston upwardly from a starting position in which the electrode is in contact with the primary nozzle to an operating position in which the electrode is spaced from the primary nozzle; and   a secondary nozzle coupled to the torch body assembly and defining a secondary nozzle chamber that receives secondary fluid that has cooled the electrode, and defining one or more secondary orifices through which secondary fluid in the secondary nozzle chamber is discharged so as to generally surround the plasma gas and arc;   whereby the secondary fluid cools the electrode and shields the plasma gas and arc.       

     The torch can be configured in various ways. For example, the torch can include passages that direct secondary fluid into the actuating chamber, either before or after the secondary fluid cools the electrode, in order to move the piston and electrode, after which the secondary fluid is discharged from the secondary nozzle to shield the plasma gas and arc. Alternatively, the torch can include passages that direct plasma gas into the actuating chamber for moving the piston and electrode, after which the plasma gas is discharged from the primary nozzle, and the torch can include passages for directing secondary fluid into the electrode, after which the secondary fluid is discharged from the secondary nozzle to shield the plasma gas and arc. 
     In all of the various embodiments, the secondary fluid that cools the electrode is supplied at a greater mass flow rate than the plasma gas. This allows the electrode to be cooled without dependence on the flow rate requirement of the plasma gas. In contrast, with conventional blow-back torches that employ a single gas that is split into plasma and shield gas streams within the torch, electrode cooling is necessarily dependent on (subservient to) the flow rate requirement for the plasma gas stream, because once the plasma gas stream&#39;s flow rate is determined, that also fixes the total flow rate, and hence the flow rate of gas available for cooling the electrode. 
     In some embodiments, the torch can be configured for employing a gas as the secondary fluid. In other embodiments, the torch can be configured for employing water as the secondary fluid. When water is the secondary fluid, none of the water supplied to the torch is recirculated. 
     When the secondary fluid is a gas (e.g., air), the torch can include one or more vent holes arranged to vent some of the secondary fluid to atmosphere. In this manner, a portion of the secondary fluid supplied to the torch shields the plasma gas and arc and the remainder of the secondary fluid supplied to the torch is vented through the vent hole(s). This can allow a greater flow rate of secondary fluid for cooling the electrode, beyond the flow rate needed for shielding of the plasma gas and arc. 
     In one embodiment, the piston is moved by secondary fluid supplied to the actuating chamber, and the secondary fluid first cools the electrode before entering the actuating chamber. The electrode assembly comprises a tubular electrode holder having an upper end connected to the piston and a lower end connected to the electrode. The electrode holder contains an internal coolant tube having an upper end arranged to receive secondary fluid from an internal cavity in the piston and a lower end arranged to discharge the secondary fluid against an inner surface of the electrode to cool the electrode. A coolant return passage is defined between the coolant tube and the electrode holder for conducting the secondary fluid away from the electrode after cooling of the electrode, and the electrode holder defines one or more holes connecting the coolant return passage to the actuating chamber. 
     Various passage configurations can be used for providing secondary fluid to the electrode and to the actuating chamber. For example, the piston head assembly and cylindrical bore can define a transfer chamber that is isolated from the actuating chamber, and secondary fluid can be supplied into the transfer chamber, from which the secondary fluid passes into the internal cavity in the piston for supply to the electrode. The piston head assembly can comprise a first piston head and a second piston head axially spaced below the first piston head such that the transfer chamber is defined by the axial space between the first and second piston heads. An O-ring or other seal can be arranged between each piston head and the inner surface of the bore for sealing purposes. 
     In one embodiment, the torch is configured to conduct the secondary fluid first into the transfer chamber, then into the electrode assembly to cool the electrode, then into the actuating chamber, then into the secondary nozzle chamber, and finally out the one or more secondary orifices. 
     Alternatively, a transfer chamber need not be included, and secondary fluid can be supplied to the electrode in other ways. For example, secondary fluid can be supplied through a central passage in the piston (e.g., by a hose connected to the end of the piston) to the electrode. 
     A compression spring can be arranged to constantly bias the piston toward the starting position. Sufficient pressure in the actuating chamber overcomes the spring so as to move the piston to the operating position. 
     The torch in some embodiments can be associated with a valve that shuts off supply of plasma gas to the torch when the valve is closed and allows plasma gas to be supplied to the torch when the valve is open. The valve is structured and arranged to be opened by pressure of the secondary fluid being supplied to the torch and to be closed when the secondary fluid is not being supplied to the torch. This can allow the torch to be used with power supplies having a single gas outlet. 
     A method for operating the plasma arc torch is also disclosed herein. One method comprises the steps of:
         beginning with the torch in a starting condition in which the piston is in the starting position having the electrode in contact with the primary nozzle;   supplying a plasma gas to the at least one plasma gas supply passage of the torch;   supplying, separately from the supply of the plasma gas, a secondary fluid to the at least one secondary fluid supply passage of the torch;   the piston being moved to the operating position by pressure in the actuating chamber such that the electrode is moved out of contact with the primary nozzle, while establishing a voltage potential difference between the electrode and the primary nozzle such that an arc extends between the electrode and the primary nozzle; and   transitioning to an operating condition of the torch in which the arc attaches to a workpiece.       

     The method can also include the step of venting to atmosphere a fraction of the secondary fluid being supplied to the torch so that said fraction does not pass through the one or more secondary orifices. 
     The plasma gas can be one of air, nitrogen, oxygen, argon, and H35, and the secondary fluid can be one of air, nitrogen, and liquid water. 
     In one embodiment, the secondary fluid is supplied to the secondary fluid supply passage at a mass flow rate that exceeds that required for achieving a desired flow rate of secondary fluid out the one or more secondary orifices, wherein excess secondary fluid above the desired flow rate is vented to atmosphere, and wherein the mass flow rate of the secondary fluid is determined at least in part based on a requirement for cooling of the electrode. 
     In some embodiments a gas is supplied as the secondary fluid, and a flow rate of the secondary fluid is greater than a flow rate of the plasma gas in the operating condition of the torch. 
     In some embodiments the torch can be operatively associated with a valve that shuts off supply of plasma gas to the torch when the valve is closed and allows plasma gas to be supplied to the torch when the valve is open. The valve is structured and arranged to be opened by pressure of the secondary fluid being supplied to the torch and to be closed when the secondary fluid is not being supplied to the torch. The method includes the step of supplying the secondary fluid so as to open the valve and allow the plasma gas to flow to the torch. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       Having thus described the disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is an axial cross-sectional view, on a first plane, through a plasma arc torch in accordance with one embodiment described herein; 
         FIG. 2  is an axial cross-sectional view, on a second plane, through the plasma arc torch of  FIG. 1 ; and 
         FIG. 3  is a diagrammatic depiction of a torch in accordance with another embodiment described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings in which some but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
     A plasma arc torch  10  in accordance with one embodiment of the present invention is illustrated in  FIGS. 1 and 2 , which show cross-sections of the torch on two different planes that pass through a central longitudinal axis of the torch and are angularly displaced from each other about the longitudinal axis. Thus, some features such as fluid flow paths or other features that are located at discrete locations about the longitudinal axis may be visible on one cross-section but not the other, or may appear differently on the two cross-sections. 
     The plasma arc torch  10  includes a torch body assembly  20  that comprises an upper body member  22  and a lower body member  24 . The lower body member  24  defines a cylindrical bore  26  extending axially therethrough. The cylindrical bore  26  is substantially coaxial with the longitudinal axis of the torch. The lower body member  24  is surrounded by a body insulator  28 . The upper body member  22  includes a lower portion that is received in the cylindrical bore  26  with an O-ring disposed between an outer surface of the upper body member  22  and the inner surface of the cylindrical bore  26  so as to seal the interface therebetween. An upper portion of the upper body member  22  is received in the central opening of the body insulator  28  with an O-ring disposed between the outer surface of the upper body member  22  and the inner surface of the body insulator  28  so as to seal the interface therebetween. The upper body member  22  also defines a central bore  30  extending therethrough, aligned with the cylindrical bore  26  in the lower body member  24 . 
     The torch body assembly  20  also defines at least one plasma gas supply passage for conducting a flow of a plasma gas, and at least one secondary fluid supply passage for conducting a flow of a secondary fluid that is supplied to the torch separately from the plasma gas. More particularly, in the illustrated embodiment the upper body member  22  includes a first plasma gas supply inlet  32  and a second plasma gas supply inlet  34  that respectively receive two plasma gas supply conduits  32 ′ and  34 ′. The upper body member  22  further includes a secondary fluid supply inlet  36  that receives a secondary fluid supply conduit  36 ′. 
     The first and second plasma gas supply inlets  32  and  34  are respectively aligned with first and second plasma gas supply passages  32   a  and  34   a  defined in the lower body member  24 . The secondary fluid supply inlet  36  is aligned with a secondary fluid supply passage  36 a defined between the lower body member  24  and the body insulator  28 . 
     The torch further includes a piston  40  comprising a piston rod  42  joined to a piston head assembly  44 . The piston head assembly  44  sealingly engages the inner surface of the cylindrical bore  26  in the torch body assembly such that the piston  40  is axially slidable in the cylindrical bore  26 . A recessed region of the piston head assembly  44  and the inner surface of the cylindrical bore  26  define a transfer chamber  50  therebetween. In the illustrated embodiment, the recessed region is provided by way of the piston head assembly having a first piston head  46  and a second piston head  48  that are axially spaced apart, such that the recessed region is the axial space between the two piston heads. The piston head assembly  44  (specifically, the second piston head  48 ) isolates the transfer chamber  50  from an actuating chamber  52  defined between a lower surface of the piston head assembly  44  and the cylindrical bore  26 . 
     The lower body member  24  defines a secondary fluid flow path  54  connecting the secondary fluid supply passage  36   a  to the transfer chamber  50  for supplying secondary fluid to the transfer chamber  50 . 
     The piston  40  defines one or more passages  56  arranged to receive secondary fluid from the transfer chamber  50  and conduct the secondary fluid into an internal cavity  58  in the piston  40 . 
     An electrode assembly  60  is connected to the piston  40  and includes an electrode  62  at a lower end of the electrode assembly  60 . The electrode assembly  60  defines internal passages for receiving secondary fluid from the internal cavity  58  of the piston  40  and circulating the secondary fluid within the electrode assembly  60  for cooling the electrode  62  and then conducting the secondary fluid into the actuating chamber  52 . More particularly, in the illustrated embodiment, the electrode assembly  60  comprises a tubular electrode holder  64  having an upper end connected to the piston  40  and a lower end connected to the electrode  62 . The electrode holder  64  contains an internal coolant tube  66  having an upper end arranged to receive secondary fluid from the internal cavity  58  in the piston  40  and a lower end arranged to discharge the secondary fluid against an inner surface of the electrode  62  to cool the electrode. A coolant return passage  68  is defined between the outer surface of the coolant tube  66  and the inner surface of the tubular electrode holder  64  for conducting the secondary fluid away from the electrode  62  after cooling of the electrode. The electrode holder  64  defines one or more holes  70  connecting the coolant return passage  68  to the actuating chamber  52 . 
     The plasma arc torch  10  further includes a primary nozzle  72  coupled to the torch body assembly  20  (specifically, coupled to the lower body member  24 ) adjacent the electrode  62  and defining a plasma nozzle chamber  74  therebetween. The primary nozzle  72  defines a primary orifice  76  through which plasma gas in the plasma nozzle chamber  74  is discharged and through which an arc from the electrode  62  extends during a transferred-arc mode of operation of the torch  10 . A secondary nozzle  78  (sometimes also referred to as a shield nozzle) is coupled to the torch body assembly  20  and defines a secondary nozzle chamber  80  and one or more secondary orifices  82  through which secondary fluid in the secondary nozzle chamber  80  is discharged so as to generally surround the plasma gas and arc emanating from the primary orifice  76 . Specifically, in the illustrated embodiment the secondary nozzle  78  is threaded onto a lower end of a shield retainer  84  whose upper end is threaded onto the body insulator  28 , which in turn is coupled to the upper and lower body members  22  and  24  as previously described. The illustrated embodiment has a secondary nozzle  78  that defines a single annular secondary orifice  82  between the secondary nozzle and the primary nozzle. Alternatively, the secondary nozzle could define a series of discrete secondary orifices if that were desirable in a particular application. 
     When there is sufficient pressure of the secondary fluid in the actuating chamber  52 , the piston  40  is urged upwardly from a starting position (not shown) in which the electrode  62  is in contact with the primary nozzle  72  to an operating position (shown in  FIGS. 1 and 2 ) in which the electrode  62  is spaced from the primary nozzle  72 . Upward movement of the piston  40  is resisted by a compression spring  86  arranged in the cylindrical bore  26  and having its upper end engaged against the upper body member  22  and its lower end engaged against the first piston head  46 . Thus, the pressure in the actuating chamber  52  must overcome the sum of the spring force plus friction in order to move the piston  40  to the operating position. 
     Plasma gas supplied through the plasma gas supply inlets  32  and  34  proceeds through the plasma gas supply passages  32   a  and  34   a  defined in the lower body member  24 , then through holes  88  in an insulator  90  that is coupled to a lower end of the lower body member  24 , then through an annular passage  92  defined between a pilot arc body  94  and the insulator  90 , and then through tangentially angled swirl holes (not readily visible) in a ceramic swirl ring  96  into an annular passage  98  defined between the primary nozzle  72  and the electrode  62 . The swirl ring  96  imparts swirl to the plasma gas before it enters the plasma nozzle chamber  74 , so that the plasma gas is swirling as it exits through the primary orifice  76 . 
     With regard to the secondary fluid&#39;s progression through the torch after its passage into the actuating chamber  52 , there is a secondary fluid passage  100  (specifically, a series of circumferentially spaced passages  100 ) defined in the lower body member  24  and connecting the actuating chamber  52  with the secondary nozzle chamber  80 . More particularly, in the illustrated embodiment the secondary fluid proceeds through the secondary fluid passages  100  into an annular flow path  102  defined between the lower body member  24  and the shield retainer  84 , then through an annular passage  104  defined between the shield retainer  84  and the pilot arc body  94 , and finally into the secondary nozzle chamber  80 . A secondary swirl ring  106  is disposed between the secondary nozzle  78  and the primary nozzle  72  downstream of the secondary nozzle chamber  80 . The secondary swirl ring includes tangentially angled swirl holes (not readily visible) that impart swirl to the secondary fluid flowing from the secondary nozzle chamber  80  so that the secondary fluid is discharged from the secondary orifice  82  as a swirling flow. 
     The torch  10  can also include provisions for venting some of the secondary fluid to atmosphere so that it does not pass through the secondary orifice  82 . In the illustrated embodiment this is accomplished by providing one or more vent holes  85  in the shield retainer  84 . Thus, a fraction of the total secondary fluid supplied through the secondary fluid supply inlet  36  will be vented to atmosphere through the vent hole(s)  85  and the remainder of the secondary fluid will pass through the secondary orifice  82  for shielding the plasma arc. The main benefit of venting some of the secondary fluid is that an excess amount of secondary fluid can be supplied to the torch, beyond what is needed for the desired amount of shielding of the plasma arc, so that greater cooling of the electrode can be accomplished. Venting would be used only when the secondary fluid is a gas (and particularly when it is air) as opposed to liquid water. When operating at high arc currents and using air as the secondary fluid, a high flow rate of secondary fluid is needed in order to achieve adequate electrode cooling. Venting some of the air allows attainment of the needed flow rate for cooling, yet preserves the desired amount of shielding. Operation at lower arc currents generally would not require venting, in which case a shield retainer not have vent holes could be employed. 
     Operation of the torch  10  is now described. Beginning with the torch in a starting condition in which the piston  40  is in the starting position having the electrode  62  in contact with the primary nozzle  72 , operation proceeds by supplying a plasma gas through the plasma gas supply conduits  32 ′ and  34 ′ into the plasma gas supply inlets  32  and  34  of the torch. At roughly the same time, separately from the supply of the plasma gas, a secondary fluid is supplied through the secondary fluid supply conduit  36 ′ into the secondary fluid supply passage  36  of the torch. These gas/fluid supplies are regulated by suitable flow regulators (not shown) as understood in the art. The secondary fluid is supplied at a flow rate and pressure sufficient to move the piston  40  to the operating position such that the electrode  62  is moved out of contact with the primary nozzle  72 , while at the same time a voltage potential difference is established between the electrode  62  and the primary nozzle  72  (the electrode  62  being the cathode and the primary nozzle  72  being the anode) such that a pilot arc extends between the electrode and the primary nozzle. Once the pilot arc is established, this pilot arc is “blown out” the primary orifice  76  and attaches to the workpiece. The current is ramped up and the torch is transitioned to an operating condition wherein instead of the primary nozzle  72  being the anode, the workpiece (not shown) is the anode. The desired operation on the workpiece can then proceed. 
     The torch  10  can be used with any of various plasma gases and secondary fluids. The particular plasma gas and secondary fluid employed will generally depend on the specific operation being performed, the type of metal being operated on, and other factors that would be understood by persons skilled in the art. As non-limiting examples, the plasma gas can be selected from air, nitrogen, oxygen, argon, and H35 (a mixture of argon and hydrogen), and the secondary fluid can be selected from air, nitrogen, and liquid water. 
     Some users of plasma arc torches of the conventional blow-back type (in which there is a single gas supplied to the torch, the gas in some torches being split into plasma/actuating gas and shield gas streams within the torch) possess power supplies that have only a single-gas capability. Such power supplies are adequate for use with the conventional single-gas type torches, but would not be able to supply both plasma gas and secondary fluid to the torch  10  described herein. However, such single-gas power supplies can be used with the present torch when the torch system is modified as shown in  FIG. 3 . The system includes a plasma arc torch  10  generally as described above, and a single-gas power supply  110  that includes a suitable gas flow regulator (not shown) along with components (also not shown) for regulating the electrical power supplied to the torch. Secondary fluid is supplied via a supply line  112  to an inlet of the power supply  110  and is discharged from the power supply as a regulated stream through a supply line  114  (which generally corresponds to, or feeds, the secondary fluid supply conduit  36 ′ described above). The system includes a separate regulator  116  for regulating the flow of plasma gas. Plasma gas enters the regulator  116  via a supply line  118  and exits as a regulated stream through a supply line  120  (which generally corresponds to, or feeds, the plasma gas supply conduits  32 ′ and  34 ′ described above). 
     The system includes a fluid-actuated valve  122  interposed in the plasma gas supply line  120  that shuts off supply of plasma gas to the torch when the valve is closed and allows plasma gas to be supplied to the torch when the valve is open. The valve  122  is structured and arranged to be opened by pressure of the secondary fluid being supplied to the torch and to be closed when the secondary fluid is not being supplied to the torch. Thus, secondary fluid carried in the supply line  114  is tapped off and supplied to the valve  122  to serve in opening the valve  122  whenever the secondary fluid is being supplied at a sufficient pressure to open the valve. In this manner, plasma gas will be supplied to the torch only when secondary fluid is being supplied to the torch by the power supply  110 . 
     The system can also include a gas-actuated valve  124  interposed in the secondary fluid supply line  114  downstream of the fluid-actuated valve  122 . The gas-actuated valve  124  functions similarly to the valve  122  but is opened by pressure of the plasma gas carried in the plasma gas supply line  120 . The inclusion of the gas-actuated valve  124  has the advantage that secondary fluid is supplied to the torch only if plasma gas is also being supplied to the torch. If the valve  124  were omitted, and if for some reason only the secondary fluid were being supplied, the “parts-in-place” system that is built into many plasma arc torch systems (which ensures that pilot arc current is supplied only when secondary fluid is present and the consumables are properly installed in the torch) would not “know” that plasma gas is not present. Inclusion of the valve  124  solves this problem by preventing secondary fluid from being supplied to the torch if plasma gas is not also being supplied. 
     The system depicted in  FIG. 3  can also be used with other types of plasma arc torches that employ both plasma gas and a separate secondary fluid. It is not limited for use with blow- back torches such as described herein. 
     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. For example, as previously noted, while the illustrated torch  10  employs the secondary fluid as the fluid for actuating the piston  40 , alternatively a torch in accordance with the invention can employ the plasma gas for actuating the piston. Additionally, while the illustrated torch is configured to cool the electrode with the secondary fluid before the secondary fluid enters the actuating chamber, alternatively the secondary fluid could pass through the actuating chamber before entering the electrode to cool it. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.