Abstract:
Systems and methods for heating a fluid are disclosed. In certain embodiments, plasma is generated and passed through a conduit. A fluid to be heated can be passed over the conduit, thereby inducing a heat transfer from the plasma to the fluid. In certain embodiments, multiple plasma generators and corresponding conduits are provided and the conduits are positioned within a housing, facilitating a more effective heat transfer. In some embodiments, the plasma generator includes an outer shell, an anode, a cathode, and insulating elements, and generates plasma by passing a gas through an electric arc created between the anode and the cathode.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 13/671,460, filed Nov. 7, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/558,949, filed Nov. 11, 2011, both of which applications are hereby incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure concerns embodiments of a heating assembly that incorporates one or more plasma generators for heating a fluid. 
       BACKGROUND 
       [0003]    A heat exchanger is a device designed to transfer heat from a first substance to a second, thereby decreasing the heat content of the first substance and increasing the heat content of the second. Heat exchangers have various industrial and commercial applications, including use in power plants, refrigerators, automobile radiators, etc., and various configurations of heat exchangers are known in the art. Methods of heating fluids have various specific applications which include heating cleaning fluids for treating a well bore or pipeline, and heating gases or liquids for use in fracking operations. In at least some of these applications, fluid-heating devices may need to be used in remote and/or numerous locations in a short time span. While many configurations of heat exchangers and devices for heating fluids are known, there is always a need for improvements in efficiency, capacity, portability, and other relevant characteristics of these devices. 
         [0004]    Plasma is a state of matter distinct from the traditionally known liquid, gas, and solid states. Generally speaking, it is a gas whose particles have been ionized. Plasma can be created by various natural and artificial methods, including by the exposure of a gas to extreme heat and/or magnetic fields. Methods of generating and using plasma include, as examples, plasma globes, plasma television screens, fluorescent lamps, neon signs, and arc welding. In arc welding, an electric current is passed through the air between two spaced apart pieces of conductive material, thereby creating an electric arc (a very high temperature plasma) between them. Thus, in arc welding, an electric current is used to create a high temperature plasma which can heat and melt the materials to be welded. 
         [0005]    Accordingly, it would be desirable to provide improved methods of generating high temperature plasma. Additionally, it would be advantageous to provide improved methods and devices for heating fluids utilizing the heat of high temperature plasma. Improvements in efficiency, capacity, and portability of such methods and devices would all be valuable. 
       SUMMARY 
       [0006]    Disclosed herein are embodiments of an invention allowing the generation of high-temperature plasma and its use for heating a fluid by heat exchange. In some embodiments, a plasma generator comprises an anode and a cathode between which an electrical potential difference can be established. A gas, such as air, is passed between the anode and the cathode, and an electric arc (a high temperature plasma) is created between the electrodes and through the gas. The high temperature plasma and/or high temperature exhaust gases can extend through a conduit over which a fluid to be heated flows, thereby allowing a heat exchange between the plasma and the fluid. Certain embodiments provide a coolant to flow within the anode and/or the cathode to protect against overheating. Certain embodiments utilize a plurality of plasma generators and a plurality of conduits. Certain embodiments utilize supplementary heat exchangers which use engine coolant, engine exhaust, or plasma exhaust to pre-heat the fluid to be heated before it flows over the conduit. 
         [0007]    In one embodiment, a heating apparatus includes plural plasma generators and plural conduits, each conduit extending from a plasma generator and configured to receive plasma and/or plasma exhaust therefrom. Each conduit can comprise a burn chamber and a coil, with each burn chamber extending from a respective plasma generator and each coil extending from a respective burn chamber. A conduit housing can be provided which surrounds the conduits, and through which a fluid to be heated can flow. In some embodiments, an insert extends through the coils within the conduit housing such that a smaller volume of water passes through the conduit housing. 
         [0008]    In another embodiment, a method comprises generating plasma within a burn chamber that is surrounded by a housing. A fluid is allowed to flow through the housing and over the burn chamber, thereby receiving heat from the plasma. The generation of plasma may be cyclical or periodic, such that the plasma generator is not constantly generating plasma. If multiple plasma generators are utilized, their cycles may be coordinated such that plasma is constantly generated by at least one of the generators. 
         [0009]    In yet another embodiment, a plasma generator comprises a casing, an outer insulator positioned coaxially within the casing, a cathode positioned coaxially within the outer insulator, an inner insulator positioned coaxially within the cathode, and an anode positioned coaxially within the inner insulator. A difference in electrical potential can be established between the anode and the cathode, and thus an electric arc can be generated when a gas is passed between them. The inner insulator can have air channels extending along its length to allow a gas to be provided to the gap between the electrodes. The cathode and the anode can be provided with ducts or channels for allowing a coolant fluid (e.g., water) to flow through, in order to protect against overheating of the various components. Materials, components, and configurations can additionally be selected to increase the transfer of heat from the electrodes to the coolant fluid to further protect against overheating. 
         [0010]    The disclosed embodiments should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and nonobvious features and aspects of the various disclosed embodiments, alone or in various combinations and sub-combinations with one another. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic view of a heating assembly for heating a fluid, according to one embodiment. 
           [0012]      FIG. 2  is perspective view of a heating assembly for heating a fluid, according to one embodiment. 
           [0013]      FIG. 3  is a rear elevation view of the heating assembly of  FIG. 2 . 
           [0014]      FIG. 4  is front elevation view of the heating assembly of  FIG. 2 . 
           [0015]      FIG. 5  is a right side elevation view of the heating assembly of  FIG. 2 . 
           [0016]      FIG. 6  is a left side elevation view of the heating assembly of  FIG. 2 . 
           [0017]      FIG. 7  is a top plan view of the heating assembly of  FIG. 2 . 
           [0018]      FIG. 8  is an exploded, perspective view of the plasma heat exchanger incorporated in the heating assembly of  FIG. 2 . 
           [0019]      FIG. 9  is a cross-sectional view of the plasma heat exchanger of  FIG. 8 . 
           [0020]      FIG. 9A  is an enlarged view of the forward end portion of the heat exchanger section shown in  FIG. 9 . 
           [0021]      FIG. 10  is a cross-sectional view of a plasma generator, according to one embodiment. 
           [0022]      FIG. 11  is a perspective view of the plasma generator shown in  FIG. 10 . 
           [0023]      FIG. 12  is a side elevation view of the plasma generator shown in  FIG. 10 . 
           [0024]      FIG. 13  is a front elevation view of the plasma generator shown in  FIG. 10 . 
           [0025]      FIG. 14  is an enlarged, perspective view of the air injection cap of the plasma generator shown in  FIG. 10 . 
           [0026]      FIG. 15  is a cross-sectional view of the air injection cap shown in  FIG. 14 . 
           [0027]      FIG. 16  is a front elevation view of the air injection cap shown in  FIG. 14 . 
           [0028]      FIG. 17  is a front elevation view of the inner insulator of the plasma generator shown in  FIG. 10 . 
           [0029]      FIG. 18  is a side elevation view of the inner insulator shown in  FIG. 17 . 
           [0030]      FIG. 19  is a cross-sectional view of the inner insulator taken along line  19 - 19  of  FIG. 17 . 
           [0031]      FIG. 20  is a front elevation view of the outer insulator of the plasma generator shown in  FIG. 10 . 
           [0032]      FIG. 21  is a side elevation view of the outer insulator shown in  FIG. 20 . 
           [0033]      FIG. 22  is a cross-sectional view of the outer insulator taken along line  22 - 22  of  FIG. 20 . 
           [0034]      FIG. 23  is a perspective view of the nozzle of the plasma generator shown in  FIG. 10 . 
           [0035]      FIG. 24  is a cross-sectional view of the nozzle shown in  FIG. 23 . 
           [0036]      FIG. 25  is a front elevation view of one of the heat sinks of the plasma heat exchanger shown in  FIG. 8 . 
           [0037]      FIG. 26  is a cross-sectional view of the heat sink taken along line  26 - 26  of  FIG. 25 . 
           [0038]      FIGS. 27 and 28  are cross-sectional views of an alternative plasma generator, according to another embodiment. 
           [0039]      FIGS. 29 and 30  are cross-sectional views of the cathode of the plasma generator shown in  FIGS. 27 and 28 . 
           [0040]      FIG. 31  is a perspective view of the cathode of the plasma generator shown in  FIGS. 27 and 28 . 
           [0041]      FIGS. 32 and 33  are cross-sectional views of one embodiment of the anode of the plasma generator shown in  FIGS. 27 and 28 . 
           [0042]      FIG. 34  is a cross-sectional view of another embodiment of the anode of the plasma generator shown in  FIGS. 27 and 28 . 
           [0043]      FIG. 35  is a cross sectional view of the inner insulator of the plasma generator shown in  FIGS. 27 and 28 . 
       
    
    
     DETAILED DESCRIPTION 
       [0044]      FIG. 1  is a schematic view of a heating assembly  10 , according to one embodiment. The heating assembly  10  in the illustrated embodiment generally includes a plasma heat exchanger  12 , an engine driven electrical generator  14  (e.g., a generator with a diesel engine) that supplies electrical current to the plasma heat exchanger, an engine exhaust heat exchanger  16 , an engine coolant heat exchanger  18 , and one or more plasma exhaust heat exchangers  20 . The plasma exhaust heat exchangers  20  receive heated exhaust gases from the plasma heat exchanger  12  for preheating a fluid flowing into the plasma heat exchanger. The engine exhaust heat exchanger  16  receives exhaust gases from the generator&#39;s engine for preheating the fluid flowing into the plasma heat exchanger. The engine coolant heat exchanger  18  receives the coolant liquid from the generator&#39;s engine and the fluid flowing into the plasma heat exchanger. The inlet fluid to the plasma heat exchanger  12  cools the engine coolant liquid in the engine coolant heat exchanger  18 . 
         [0045]    The heating assembly  10  can be used to heat any type of fluid, including without limitation, liquids, such as water, diesel fuel, or kerosene, and gases, such as nitrogen, to name a few. For purposes of description, the heating assembly  10  will be described in the context of heating water, although the assembly can be used to heat other fluids. 
         [0046]    In use, water to be heated in the plasma heat exchanger  12  enters the assembly via an inlet conduit  22  (e.g., pipe). A portion of the inlet water can be directed to flow through respective conduits  24 , respective plasma exhaust heat exchangers  20 , and respective conduits  26 , and then into the plasma heat exchanger  12 . Hot exhaust gases from the plasma heat exchanger  12  flow through respective conduits  32 , respective plasma exhaust heat exchangers  20 , and then through an exhaust manifold  34  that exhausts the gases to atmosphere. Inlet water flowing through plasma exhaust heat exchangers  20  therefore is pre-heated by the hot exhaust gas from the plasma heat exchanger. 
         [0047]    A portion of the inlet water also can be directed to flow through a conduit  28 , the engine exhaust heat exchanger  16 , a conduit  30 , and then into the plasma heat exchanger  12 . Hot exhaust gases from the generator&#39;s engine flows through conduit  36 , the engine exhaust heat exchanger  16 , and then an exhaust conduit  38 , which vents the exhaust gases to atmosphere. Inlet water flowing through the engine exhaust heat exchanger  16  therefore is preheated by the hot exhaust gases from the generator&#39;s engine. 
         [0048]    A portion of the inlet water also can be directed to flow through a conduit  40 , the engine coolant heat exchanger  18 , a conduit  42 , and then into the plasma heat exchanger  12 . The engine coolant from the generator&#39;s engine (e.g., water or a water/antifreeze mixture) circulates through the engine coolant heat exchanger  18  via conduits  44 ,  46  to be cooled by the inlet water flowing into the plasma heat exchanger. Inlet water directed into the plasma heat exchanger via conduits  26 ,  30 , and  42  is heated by plasma inside the plasma heat exchanger  12 , as described in detail below. Heated water exits the plasma heat exchanger through an outlet conduit  48 , from which the heated water can be directed to one or more users or processes requiring heated water. 
         [0049]      FIGS. 2-7  are various views of a specific implementation of the heating assembly  10  shown schematically in  FIG. 1 . The components of the heating assembly of  FIGS. 2-7  that are the same as the components in  FIG. 1  are given the same respective reference numerals and therefore are not repeated here. As best shown in  FIG. 7 , the electrical generator  14  includes an engine  50  (e.g., a diesel, natural gas, or gasoline engine) that powers the generator. The generator  14  functions to provide electrical current to the plasma heat exchanger for generating plasma and to power other components of the assembly as needed. As can be appreciated, the use of an engine-driven generator allows the heating assembly  10  to be portable and/or used in applications where an electrical power supply is not readily available. If an electrical power supply is readily available, the generator  14  would not be needed. It also should be noted that any other source of electrical current can be used in place of the generator  14 , such as fuel cells, batteries, etc. 
         [0050]    The heating assembly  10  can also include an air compressor  52  (e.g., a rotary screw compressor or reciprocating compressor) that serves as a source of gas supplied to the plasma heat exchanger  12  for generating plasma. The compressed air from compressor  52  can flow through a conventional air/water separator  56 , and into a compressed air storage tank  54 . As best shown in  FIGS. 2 and 4 , compressed air in the tank  54  is supplied to the plasma heat exchanger via compressed air conduits  64 , as further described below. The compressor  52  can be powered by electrical current from the generator  14  or another convenient power source. The air compressor  52  can also be replaced by any convenient source of a compressed gas that can be used in the generation of plasma. For example, the plasma heat exchanger can be supplied with an inert gas (e.g., helium, argon) from an inert gas source (e.g., a storage tank) if one is readily available. 
         [0051]    In an alternative embodiment not shown in  FIGS. 2-7 , an air dryer can be fluidly connected to the separator  56  and the tank  54 . In this alternative embodiment, compressed air from the compressor  52  can flow first through the separator  56 , then through the dryer, which removes all or substantially all water vapor from the compressed air. After passing through both the separator  56  and the dryer, the compressed air can then flow into the tank  54 . While many commercially available air dryers may be used, one that has been found to be suitable is the Ingersoll Rand HL400 Series desiccant air dryer. 
         [0052]    The heating assembly  10  can also include water pumps  58  placed in the inlet water conduits  22 . As best shown in  FIGS. 3 and 7 , pressurized water from pumps  58  flow through conduits  22 , a manifold  60 , where it is distributed to conduits  24 ,  28 , and  40 . In the embodiment illustrated in  FIGS. 2-7 , the components of the heating assembly  10  are arranged together on a frame. In an alternative embodiment, however, the components are not all arranged together in such a fashion and at least one of the components (e.g., the generator  14  or the air compressor  52 ) is provided in a location remote from the remainder of the assembly. In this alternative embodiment, wires, tubes, or other appropriate connecting elements are used to connect each of the remote components to the remainder of the assembly. 
         [0053]      FIG. 8  shows an exploded view of the plasma heat exchanger  12 . The plasma heat exchanger  12 , in the illustrated embodiment, comprises a nozzle plate  100 , a burner housing  102 , a coil housing  104 , a diverter  106 , an exit plate  108 , an exit flange  110 , an outlet manifold  112 , one or more plasma generators  114  (also referred to as plasma torches or plasma nozzle assemblies), one or more gaskets  116 , one or more heat sinks  118 , one or more seals  120 , one or more burn chambers  122  disposed in the burner housing  102 , one or more coils  124  disposed in the coil housing, and a support ring  126  that supports the diverter  106  within the coil housing  104 . 
         [0054]    The nozzle plate  100  includes one or more apertures  128 , each of which is sized to receive and support a respective plasma generator  114 . As best shown in  FIGS. 9 and 9A , each plasma generator  114  extends through a corresponding aperture  128  and partially into a respective burn chamber  122 . The inflow end of each burn chamber  122  (the end closest to the nozzle plate  100 ) is connected to the nozzle plate  100  with a heat sink  118 . A gasket  116  (or equivalent sealing element) can be positioned between each heat sink  118  and the inside surface of the nozzle plate  100 . Another gasket  120  (or equivalent sealing element) can be positioned between each heat sink  118  and an end flange  144  of an adjacent burn chamber  122 . Each plasma generator  114  can be secured to the nozzle plate  100  and a burn chamber  122  by a plurality of bolts  142  that extend through the plasma generator  114 , the nozzle plate  100 , a respective gasket  116 , a respective heat sink  118 , and an end flange  144  of the respective burn chamber  122 . 
         [0055]    Each plasma generator  114  receives compressed air from the compressor  52  (or compressed gas from another source) and electrical current from the generator  14  (or another current source) to generate plasma, which is directed into respective burn chambers  122 . Each burn chamber  122  is in fluid communication with a respective coil  124  that receives plasma and/or heated exhaust gases from the burn chamber. Each coil  124  can have an end portion  138  that extends through a corresponding aperture  140  in end plate  108  and is fluidly connected to a respective conduit  32  ( FIG. 5 ) that directs heated exhaust to flow into respective plasma exhaust heat exchangers  20  ( FIG. 5 ). Each burn chamber  122  and respective coil  124  collectively form a conduit that receives plasma and/or hot exhaust gases used to heat a liquid in the plasma heat exchanger  12 . In an alternative embodiment, the coil  124  or a portion thereof can be a straight, non-coiled conduit. 
         [0056]    The burner housing  102  includes one or more inlet openings  130  (three in the illustrated embodiment) spaced in the circumferential direction around the outer surface of the housing. Each opening  130  is fluidly connected to a respective conduit  26  ( FIG. 1 ). Thus, the fluid to be heated (e.g., water) flows through conduits  26  and into the housing  102  via openings  130 . The housing  102  can further include secondary openings  132  that receive fluid to be heated from conduits  30  and  42 . Fluid entering the heat exchanger via openings  130 ,  132  flows through the burner housing and over the burner chambers  122 , and then upon entering the coil housing  104 , the diverter  106  causes the fluid to flow radially toward the inner surface of the coil housing so as to flow over the coils  124  (as indicated by arrows  136 ). At the rear end of the coil housing, the fluid flows outwardly through outlet conduits  134  and into outlet manifold  112 . 
         [0057]    Referring to  FIGS. 10 and 11 , the plasma generator  114  will now be described in greater detail. The plasma generator  114  in the illustrated embodiment comprises a nozzle housing  160 , an air injection cap  162 , an end plate  164 , a nozzle  166  disposed partially in the housing  160 , an electrode  168  centrally positioned within the nozzle  166 , an outer insulator  170  disposed between the housing  160  and the nozzle  166 , and an inner insulator  172  disposed between the electrode  168  and the nozzle  166 . The electrode  168  serves as the anode of the plasma generator and the nozzle  166  serves as the cathode of the plasma generator. In use, the two sides of an electrical potential source are electrically connected to these components to establish an electric arc. 
         [0058]    The air injection cap  162  can be secured to the nozzle  166  by a plurality of bolts  174  that extend through corresponding openings in the cap  162  and are tightened into corresponding openings in an end flange  178  of the nozzle  166 . The electrode  168  can be secured to air injection cap  162  by a central bolt  176  that extends through an opening in the cap  162  and is tightened in a central opening in the electrode  168 . As best shown in  FIGS. 11 and 13 , the air injection cap  162  can be secured to the nozzle housing  160  by a plurality of bolts  184  that extend through corresponding openings in the cap  162  and are tightened in corresponding openings in the nozzle housing  160 . 
         [0059]    The air injection cap  162  includes an inlet conduit  180  that is fluidly connected to a source of compressed gas (e.g., compressed air). In the illustrated embodiment, for example, the inlet conduit  180  is connected to a compressed air line  64  that supplies compressed air from tank  54  to the plasma generator  114 . As best shown in  FIGS. 14-16 , the air injection cap  162  includes a side opening  182  that extends from the outer surface of the cap to an internal space  186  of the cap. The inlet conduit  180  extends into the side opening  182  so that compressed gas flows through the opening  182  and into the internal space  186  of the air injection cap  162 . 
         [0060]    The air injection cap  162  can further include a slot  194  that extends all the way through the side wall of the air injection cap. A conductor bar  196  ( FIGS. 12 and 13 ) is inserted into and through the slot  194  so as to physically and electrically contact the end surface of the electrode  168  ( FIG. 10 ). The air injection cap  162  can also be formed with a recessed portion  198  that receives the head of a bolt  200  ( FIG. 13 ). The bolt  200  extends through the air injection cap  162  and is tightened into a corresponding opening  202  ( FIG. 23 ) in the flange  178  of the nozzle  166 . A first cable or other electrical conductor (not shown) electrically connected to the positive side of the generator  14  is connected to the conductor bar  196  and a second cable or other electrical conductor (not shown) electrically connected to the negative side of the generator  14  is connected to the bolt  200 . In this manner, the electrode  168  can be placed in electrical contact with the positive side of the generator and the nozzle  166  can be placed in electrical contact with the negative side of the generator. 
         [0061]    As best shown in  FIGS. 17-19 , the inner insulator  172  comprises a central opening  188  that receives the electrode  168  and a plurality of longitudinally extending, outer openings  190  that are angularly spaced about the central opening  188 . As shown in  FIG. 10 , the openings  190  are aligned with internal space  186  of the air injection cap  162  and allow compressed gas to flow through the insulator  172 . As best shown in  FIGS. 20-22 , the outer insulator  170  comprises a central opening  192  sized to fit around the nozzle  166 . The insulators  170 ,  172  help insulate the nozzle housing and adjacent components of the heat exchanger  12  from the heat generated inside the plasma generator  114 . The insulators  170 ,  172  can be made of alumina or any of various other suitable materials. In one example, the insulators are made of 99% alumina. 
         [0062]    As best shown in  FIG. 9A , the nozzle generators  114  are mounted to the nozzle plate  100  such that the nozzle housing  160  and the nozzle  166  extend partially into the burner housing  102 . A heat sink  118  is co-axially mounted around the portion of each nozzle housing extending into the burner housing. As best shown in  FIGS. 25 and 26 , the heat sink  118  can comprise an annular ring shaped structure comprising a central opening  206  adapted to receive a nozzle housing  160  and a plurality of axial spaced, annular fins  208 . The heat sinks  118  assist is transferring heat from the plasma generators  114  to the surrounding fluid. Thus, the heat sinks  118  help promote heating of the fluid in the burner housing  102  and help cool the plasma generators  114  to keep them below the desired operating temperature. 
         [0063]    In one specific embodiment, the various components of the heat exchanger  12  and the nozzle generator  114  are made of the following materials. The air injection cap  162  and the end plate  164  are made of polytetrafluoroethylene (PTFE). The nozzle  166  and the electrode  168  are made of a copper-tungsten alloy. The inner and outer insulators  172 ,  170 , respectively, are made of 99% alumina. The housing  160  is made of 316L stainless steel. The conductor bar  194  is made of copper. The burner housing  102 , the coil housing  104 , the diverter  106 , the burn chambers  122 , the coils  124 , the outlet pipe  112 , and the heat sinks  118  are made of stainless steel, such as 316L or 310L stainless steel. 
         [0064]    Referring to  FIGS. 27-35 , an alternative plasma generator  300  will now be described. Multiple plasma generators  300  can be used in place of the plasma generators  114  within the heat exchanger  12 . The plasma generator  300  in the illustrated embodiment comprises a housing  302  and an air and water injection cap  304 . The housing  302  houses several nested cylindrical components including an outer insulator  306  in contact with the inner surface of the housing  302 , a cathode  308  in contact with the inner surface of the outer insulator  306 , an inner insulator  310  in contact with the inner surface of a cathode  308 , and an anode  312  in contact with the inner surface of the inner insulator  310 . An electrical potential difference is established between the cathode  308  and the anode  312  when connected to a source of electricity, and thus an electric arc can be generated in the air passing between them. 
         [0065]    The outer insulator  306  is generally cylindrically shaped and comprises an insulating material. As best seen in  FIGS. 29-31 , the cathode  308  is generally cylindrically shaped and includes a system of ducts or channels to allow a coolant fluid to flow through its structure. In the illustrated embodiment, the cathode  308  includes four ducts or channels, each projecting axially through the interior of the cathode  308 . As illustrated, two inflow ducts  316  carry water (or another coolant fluid) into the cathode from a water source, while two outflow ducts  318  receive water from the inflow ducts  316  via channels  320  and carry the water out of the cathode  308 . Each channel  320  extends between and fluidly connects an inflow duct  316  to a respective outflow duct  318 . As best shown in  FIG. 35 , the inner insulator  310  is generally cylindrically shaped and, as illustrated, includes six air channels  314  for carrying air through the plasma generator  300 . 
         [0066]    As best illustrated in  FIGS. 32-34 , the anode  312  is generally cylindrically shaped and includes a larger diameter cylindrical portion  322 , a transition portion  324 , a smaller diameter cylindrical portion  326 , a water inlet extension  328  and a water outlet extension  330 . The anode  312  further comprises an inlet duct or channel  332  and an outlet duct or channel  334 , each extending through the larger cylindrical portion, one transfer duct or channel  336  extending through the transition portion  324 , and one distal channel  338  in the smaller cylindrical portion  326 . The water inlet extension  328 , the inlet duct  332 , the transfer duct  336 , the outlet duct  334 , and the water outlet extension  330  are in fluid communication such that a pressurized fluid introduced into the water inlet extension  328  will flow through the inlet duct  332  along the length of the larger diameter portion  322 , through the transfer duct  336 , back through the outlet duct  334  along the length of the larger diameter portion  322 , and exit through the water outlet extension  330 . The anode  312  can be fabricated either by machining from a solid piece of material ( FIG. 34 ), or by casting ( FIGS. 32-33 ). A cylindrical slug  340  may be positioned in the distal channel  338 . The slug  340  can comprise, as one specific example, halfnium coated in silver, and may aid in transferring heat energy from plasma generation from the smaller cylindrical portion  326  to the water or other coolant fluid carried through the transfer duct  336 . As shown, the slug  340  can be positioned such that an end portion of the slug extends into the transfer duct. 
         [0067]    In the illustrated configuration, pressurized water can be provided to and withdrawn from the various ducts in the anode and the cathode via conduits through the injection cap  304 . The provision of flowing water helps insulate and protects against overheating of the anode  312  and cathode  308 , which carry electric current for the generation of plasma. Also in this configuration, air for generating plasma is provided via conduits through the injection cap  304  to the air channels  314 , which carry the air through the plasma generator. 
         [0068]    In one specific embodiment, the components of the plasma generator  300  are made of the following materials. The injection cap  304  is made of PTFE. The cathode  308  and anode  312  are made of a copper-chromium alloy. The inner insulator  310  and the outer insulator  306  are made of 99% alumina, and the housing  302  is made of stainless steel such as grade  303  stainless steel. 
         [0069]    Referring again to  FIG. 10 , to generate plasma, an electrical potential difference is established between the electrode  168  and the nozzle  166 , which causes an electric arc to be established across the radial gap  214  between the end portion of the electrode  168  and the surrounding portion of the nozzle  166 . Compressed air (e.g., compressed air at 20 psig) supplied to the air injection cap  162  flows through the nozzle  166  as indicated by arrows  210 . As the compressed air crosses the electric arc, the air is ionized, creating plasma, or a plasma arc, which is discharged outwardly through the outlet opening  212  of the nozzle and into the respective burner chamber  122 . The fluid to be heated in the heat exchanger  12  (e.g., water) flows over the burner chambers  122  and the coils  124  and therefore is heated by the heat of plasma and exhaust gases in the burner chambers and the coils. 
         [0070]    The frequency of the power supply to the plasma generators can be adjusted to vary the electric arc between the electrode  168  and the nozzle  166 . In particular, increasing the frequency above 60 Hz, to about 80-85 Hz or greater, can increase the frequency of sparks across the gap  214  to form a substantially annular electric arc extending between the electrode  168  and the nozzle  166 , which promotes the generation of plasma from the air crossing the electric arc. The frequency of the power supply can be increased in some embodiments to at least 100 kHz, and in some embodiments up to  50  GHz. 
         [0071]    The assembly  10  can further include a controller to control the operation of the various components of the assembly, including the generator  14 , the air compressor  52 , the pumps  58 , and the plasma generators  114 . The controller can be programmed (such as by user input) to set various operating parameters, such as the voltage, current and frequency of power supplied to each plasma generator and the operating sequence of each plasma generator. For example, each plasma generator  114  can be cycled on and off in a predetermined sequence with the other plasma generators to avoid overheating of the generators. In a specific implementation, for example, only one plasma generator is cycled on while the other two are cycled off. Initially, each plasma generator is cycled on for a period of about 5-7 seconds and then for a period of about 3 seconds for each subsequent cycle. It should be noted that the operating parameters of the generators  114  (including the operating sequence and frequency) can be varied depending on the specific application. 
         [0072]    In a specific application, the heating assembly  10  is used to heat a cleaning fluid for treating a well bore or pipeline used in the transfer of hydrocarbon fluids, such as oil and gas. In the transfer and production of hydrocarbon fluids, well bores, pipelines and other conduits become clogged and/or fouled from accumulation of various compounds. A known technique for cleaning well bores and pipelines involves heating a solution and injecting the solution into the well bore and/or pipeline. A known heating system used for this purpose utilizes friction heating to heat about 4,800 gallons of water per hour to about 250 degrees F. The assembly  10  of the present disclosure can be used to heat about 18,000 gallons of water per hour from ambient (about 68 degrees F.) to about 290 degrees F. The heating assembly  10  can also be used to heat any of various other fluids, such as diesel fuel and kerosene, for cleaning well bores and pipelines. The heated fluid can also be used for fracking in which the fluid is injected into a well bore under pressure to create fractures in underground rock formations, such as shale rock and coal beds. 
         [0073]    In another application, the heating assembly can be used to heat nitrogen for use in fracking. In such an application, liquid nitrogen stored in a tank (which can be on or adjacent the heating assembly) is supplied to an expansion chamber, which allows the nitrogen to expand into a gas. From the expansion chamber, the nitrogen flows into the plasma heat exchanger and is heated to at least about 85 degrees F. The heated nitrogen exiting the heat exchanger can be pressurized and injected into a well bore for fracking, as known in the art. In another embodiment, the nitrogen can be fed into the plasma generators  114  (instead of the compressed air) to create high temperature plasma from the nitrogen. The nitrogen cools to an appropriate working temperature and then can be pressurized and injected into a well bore. 
         [0074]    The heating assembly  10  can also be used in a variety of other applications. For example, the heating assembly can be used in a variety of different industrial processes requiring a relatively large supply of a heated fluid, for heating a building, or for rapidly boiling water. In alternative embodiments, a plasma generator  114  can be used apart from the heat exchanger  12  for a variety of applications where heat from plasma can be utilized. For example, the plasma generator  114  can be used as a plasma torch for cutting metal, burning or incinerating material, such as trash or waste, or for various other uses. 
         [0075]    In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. I therefore claim as my invention all that comes within the scope and spirit of these claims.