Patent Publication Number: US-8119954-B2

Title: Convective heating system for industrial applications

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 10/703,497, filed Nov. 10, 2003 which claimed the benefit of U.S. Provisional Patent Application Ser. No. 60/438,321 filed Jan. 7, 2003, each of which is hereby incorporated by reference in its entirety. This also claims the benefit of U.S. Provisional Patent Application Ser. No. 60/832,608, filed Jul. 24, 2006 and also hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Heating of gases can be carried out by a variety of techniques including conduction, radiation and convection. A wide variety of thermal processing applications are found throughout industry including materials processing and chemical applications. The industrial process of heat-treating, joining, curing and drying are carried out in many different types of systems, furnaces and ovens. The heating method of choice for such applications is normally a radiative technique with radiant electric heating elements placed along the walls of the furnace. Although such a method is efficient for very high temperature applications, the use of convection as the heat transfer mechanism often proves to be efficient in the lower temperature ranges. The following prior art patents all pertain to various methods of heating gases; namely, U.S. Pat. Nos. 5,766,458; 5,655,212 and 5,963,709. Discussions on convective heating are available from (1) M. Fu, Kandy Staples and Vijay Sarvepalli. A High Capacity Melt Furnace for Reduced Energy Consumption and Enhanced Performance. Journal of Metals (JOM), May 1998, pg 42 and (2)  ADVANCE MATERIALS  &amp;  PROCESSES  magazine (pages 213 to 215, October, 1999). 
     The proper selection of thermal heating for industrial applications such as processing ovens and furnaces is a critical decision to meet the needs of almost all engineering products during their manufacture. The considerations of heating devices and techniques are much different for such industrial applications compared to residential or consumer applications such as hair dryers, hot air popcorn poppers and the like, examples of which are disclosed in U.S. Pat. Nos. 4,350,872; 4,794,255 and 4,149,104. The differences are largely due to the vastly divergent temperature, pressure and airflow requirements. Oven and furnace design for industrial applications must take into consideration heat transfer methods, the temperature uniformity, movement of the product, atmosphere, construction and the heat generation method. Heat processing equipment is usually classified as ovens operating to 1000° C. and as furnaces above this temperature. Batch and continuous designs are the common choices depending on the flexibility and productivity requirements. The source of heat is normally provided by oil, gas or electricity. 
     Gas heating techniques include convection, forced convection and radiation. Natural convection is slow and not very uniform. Forced convection on the other hand is easily controllable and can be directed for odd shapes. Radiant heat transfer at higher temperatures may be faster for some products, but may contribute other problems to the process like non-uniformity and distortion, to mention a few. Forced convection offers advantages over radiant heating for a number of industrial applications. Forced hot convection is also used for fuel cells, automobile test beds and product qualifications. 
     SUMMARY OF THE INVENTION 
     These and other problems in the prior art have been addressed by this invention which, in one embodiment, is an industrial gas heater having a tubular enclosure with a gas entry port spaced from a gas exit port. The industrial gas heater, in various embodiments, includes an inner helical coil contained within the tubular enclosure and an outer helical coil also contained within the tubular enclosure and surrounding the inner coil to define a substantially unobstructed annular space between the coils. Each coil is electrically heated to convectively heat a gas entering the tubular enclosure via the gas entry port, passing through the annular space between the coils and exiting the tubular enclosure via the gas exit port. 
     In various other embodiments according to this invention, the inner and outer coils are each right circular helical coils and are arranged concentrically. The inner and outer coils may be wound in opposite directions from each other or in the same direction. The individual coils may be formed from a generally continuous wire concentrically wound into a right circular helical coil. In other embodiments of this invention, the inner and outer coils may have different configurations from one another. A spacer may be positioned within the tubular enclosure and proximate the gas exit port and adjacent distal ends of the inner and outer coils to minimize deformation of the coils. 
     The tubular enclosure may be a housing in the form of a right circular cylinder having an open end proximate the gas entry port and an end cap closes the open end of the housing. In various embodiments of this invention, the outer coil is positioned in close proximity to or in contact with an inner surface of the tubular enclosure to minimize gas flow between the outer coil and the inner surface of the tubular enclosure and to maximize heat transfer to the gas. 
     Since the present invention is intended for industrial applications, the inner and outer coils are adapted to heat the gas flowing through the annular space and exiting the gas exit port to a temperature in the range of 500° C. to about 1500° C. and at a rate in the range of about 1 cubic foot per minute (CFM) to about 1000 CFM. 
     In another embodiment of this invention, multiple of the industrial gas heaters are arranged and mounted in a sealed gas flow chamber. In a further modification, each of the wires utilized for the coils in the gas heaters are themselves configured as coils. Moreover, the industrial gas heater of this invention may be utilized to generate super-saturated steam. 
     This invention also includes a method for heating a gas for industrial applications including the steps of introducing the gas into a tubular enclosure through an entry port and then flowing the gas through a substantially unobstructed annular space within the tubular enclosure and between inner and outer helical coils. The helical coils are electrically heated to heat the gas flowing there through. The gas is then expelled out of the tubular enclosure through an exit port at a temperature in the range of 500° C. to about 1500° C. and at a rate in the range of about 1 CFM to about 1000 CFM. In various other embodiments of this method, the gas is rifled or spiraled between adjacent turns of the inner and outer coils to increase the heat transfer to the gas. The inner and outer coils may be oppositely wound from one another so that the gas spiraling between the adjacent turns of the inner coil is in the direction opposite the gas spiraling between the adjacent turns of the outer coil to thereby increase the heat transfer to the gas. 
     As a result, a convective heating system and associated method for heating a gas for industrial applications are provided that overcome many of the shortcomings associated with known systems and techniques in the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of an exemplary embodiment of an industrial heating system according to this invention; 
         FIG. 2  is a disassembled side elevational view of the heating system of  FIG. 1 ; 
         FIG. 3  is an assembled side elevational view of the heating system of  FIG. 2 ; 
         FIG. 4  is an enlarged perspective view of a spacer utilized in the heating systems of  FIG. 1 ; 
         FIG. 5  is a cross-sectional view showing an annular space between inner and outer heating coils and the bare and uniform wires comprising the coils of the system of  FIGS. 1-3 ; 
         FIG. 6  is a perspective schematic view of the rifling airflow through the inner and outer heating coils as well as a cross sectional view of the bare and uniform composition of the wires comprising the inner and outer coils; 
         FIG. 7  is a perspective view of another embodiment of an industrial heating system according to this invention adapted to convert liquid to high temperature gas, e.g., generate supersaturated steam; 
         FIG. 8  is a perspective view of a further embodiment of an industrial heating system according to this invention; 
         FIG. 9  is a partially disassembled perspective view of the system of  FIG. 8 ; 
         FIG. 10  is a perspective view of an alternative embodiment of heating coils to be utilized in an industrial heating system according to this invention; and 
         FIG. 11  is a graphical illustration of how to adjust the system of  FIG. 7  for different levels of specific humidity. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention provides a new technique for very low cost convective heat generation. One aspect of the invention is to heat the air or gas through a concentric energized heating coil system. We have found that the concentric design heats the gas to a more consistent temperature in an energy efficient manner. 
     Referring to  FIGS. 1-3 , an exemplary embodiment of an industrial gas heater  10  according to this invention is shown. The heater  10  includes a generally right circular cylindrical tubular housing  12  having a gas entry port  14  at a first end of the housing  12  spaced from a gas exit port  16  at an opposite end of the housing  14 . The housing  14  may be a monolithic ceramic tube or other material such as a metallic enclosure. However, we have found that the temperature of the gas heated within the assembly is increased anywhere from 25-200° C. when a ceramic housing is utilized. 
     The gas entry port  14  is proximate an open end  18  of the housing  14  and is selectively closed by an end cap  20  mounted on the open end  18  of the housing  14 . The end cap  20  may be made from a ceramic of approximately 90 percent aluminum oxide. The cap  20  includes an annular sidewall  22  and an end wall  24 . The end cap  20  is a partially open end cap and according to various embodiments of this invention, the end cap  20  can be fully or partially open with additional openings for electrical feed-throughs and thermocouple feed-throughs. A stepped passage  26  is formed on the interior of the sidewall  22  and the gas entry port  14  is on the end wall  24 . The opening diameter of the gas entry port  14  to the gas exit port  16  may be at a ratio of about 2:1. 
     The gas heater  10  includes an inner helical coil  28  and an outer helical coil  30  contained within the tubular housing  12 . The inner and outer coils  28 ,  30  are coaxially aligned and concentrically arranged as right circular helical coils within the housing  12  to define a substantially unobstructed annular space  32  for passage of gas through the housing  12  from the entry port  14  to the exit port  16 . In one embodiment, each coil  28 ,  30  is formed from a generally continuous wire  28   a ,  30   a , respectively, concentrically wound into right circular helical coils. The wires  28   a ,  30   a  have cross sections  28   f ,  30   f  respectively which indicate a solid, unsheathed and bare composition for wires  28   a  and  30   a . In this embodiment the wires  28   a  and  30   a  have no coating, insulation, cladding or sheathing of any kind, but are solid pieces of uniform material across their diameters. A diameter of the wire  28   a ,  30   a  for each coil may range from about 0.1 mm to about 6 mm. A gap  28   b ,  30   b  between the adjacent turns  28   c ,  30   c  of each coil  28 ,  30  may 8 range from about 0.01 mm to about 85 mm. The gap or pitch of each coil  28 ,  30  may increase adjacent to the entry port  14  and terminal lead wires  28   d ,  30   d . 
     In a further embodiment as shown in  FIG. 10 , the wires  28   b ,  30   b  of either or both of the coils  28 ,  30  are themselves right circular helical coils to increase the heat transfer from the coils  28 ,  30  to the gas. The diameter of the coiled-coil configuration of  FIG. 1o  may range from about 0.5 mm to about 10 mm. 
     We have found that where the outer coil  30  is in close proximity to and/or in contact with the inside face of the tubular housing  12 , the gas processed in the heater lo is heated approximately 25° to 200° C. higher than if the outer coil  30  is not in such a configuration relative to the housing  12 . Additionally, a spacer  34  which may be ceramic is positioned at the distal end of the coils  28 ,  30  proximate the gas exit port  16 . The spacer  34  increases the useful life of the coils  28 ,  30  and minimizes coil deformation over extended periods of use. 
     One embodiment of the spacer  34  is shown in  FIG. 4  and includes a central, annular circular ring  35  that is adapted to be mounted on a central rod  40 . The rod  40  may be ceramic or another material. The spacer  34  has a number, three of which are shown in  FIG. 4 , vanes  37  radiating outwardly from the ring  35 . The vanes  37  are equally spaced around the circumference of the ring  35  and each have an outwardly tapered or flared configuration. 
     Terminal lead wires  28   d ,  30   d  extend from the proximal end of the respective coils  28 ,  30  and through the end wall  24  of the end cap  20  to be electrically coupled to a power cord  36  and a power source (not shown) for heating the coils  28 ,  30 . Any power requirement may be appropriate for the coils  28 ,  30 , but typically 110-volt (approximately 1 kilowatt) modules are utilized. 
     A thermocouple lead  38  is positioned coaxially and longitudinally within the coils  28 ,  30  for reading the gas temperature adjacent the gas exit port  16 . The thermocouple  38  is mounted on the central rod  40  positioned coaxially relative to the inner and outer coils  28 ,  30  in the housing  12 . The arrangement and juxtaposition of the coils, thermocouple, central rod and housing are among the features of the present invention that provide for a very compact, space-saving design for the gas heater. 
     Among the advantages provided by a gas heater  10  according to this invention is the increased contact between the gas flowing from the entry port  14  to the exit port  16  with the coils  28 ,  30 . For example, the coils  28 ,  30  may be similarly wound or wound in opposite directions as shown in  FIG. 6 . Gas flowing through the housing  12  passes through the annular space  32  between the coils  28 ,  30  as shown in  FIG. 5 . The annular space  32  and flow path of the gas in this area is generally unobstructed to provide for appropriate thermal exchange from the coils  28 ,  30  to the gas. Additionally, gas flowing between the adjacent turns  28   c ,  30   c  of the respective coils  28 ,  30  flows in a riffling or spiraling configuration as schematically shown in  FIG. 6  with flow paths  28   e ,  30   e . With the windings of the respective coils  28 ,  30  being in opposite direction, increased mixing of the gas with the coils  28 ,  30  is provided to obtain a more turbulent gas flow. The thermal exchange may be further enhanced with the coil  28 ,  30  configuration shown in  FIG. 10 . Each of these arrangements provides for increased thermal transfer from the heated coils  28 ,  30  to the gas relative to prior art industrial gas heating systems. 
     Radial dimensions of the annular spacing  32  ( FIG. 5 ) may range from about 1.5 mm to about 20 mm with a presently preferred annular spacing  32  being about 2 mm. The range of gap spacing between the adjacent turns  28   c ,  30   c  of the wires  28   a ,  30   a  in the coils  28 ,  30  is between about 35 mm and about 85 mm with the presently preferred being about 40 mm for the inner coil  28  and about 65 mm for the outer coil  30 . The cross sectional area of the annular spacing  32  ranges between about 15 mm 2  to about 6000 mm 2  with the presently preferred being derived from the above-identified gap spacing ranges. 
     An alternative embodiment of an industrial heating assembly  100  according to this invention is shown in  FIGS. 8-9  with components of the heating assembly  100  that are the same or similar to corresponding components of the heater  10  being labeled in a similar manner. The heating assembly  100  according to this embodiment of the invention utilizes a heating cartridge  102  with multiple gas heaters  10  of the type disclosed in  FIGS. 1-3  mounted in a generally parallel orientation relative to each other between a pair of generally circular spaced end plates  104 . The end plates  104  are maintained in a spaced configuration by a series of spaced threaded rods or bolts  106  positioned around the periphery of the plates  104  and secured to the plates  104  by mechanical fasteners such as nuts  108  or the like. The cartridge  102  is shown in one configuration and those of ordinary skill in the art will readily appreciate that the number of gas heaters  10 , their arrangement and configuration is available in a wide variety of different embodiments according to this invention. 
     The cartridge  102  is mounted within a sealed chamber  110  which is formed by a pair of mating dome-shaped enclosures  112   a ,  112   b . The enclosure  112   a  proximate a gas entry port  114  of the heating assembly  100  includes a gas entry conduit  116  having a flange  118  adapted to mate with a gas feed supply (not shown). The enclosure  112   b  at a gas exit port  120  of the heating assembly  100  likewise includes a conduit  122  and compatible flange  124  for mating with downstream equipment to provide a sealed heating assembly  100 . 
     Each of the dome-shaped enclosures  112   a ,  112   b  includes a peripheral flange  126   a ,  126   b  which is adapted to mate with the corresponding flange of the other enclosure  112   a ,  112   b  as shown in  FIG. 9 . The flanges  126   a ,  126   b  each include a number of through holes  128  which, when aligned with a corresponding through hole in the opposite flange, allow a threaded bolt  130  to pass there through so that a nut  132  can be threadaby mounted on the bolt  130  to secure the flanges  126   a ,  126   b  and dome-shaped enclosures  112   a ,  112   b  together to provide the sealed chamber  110 . A gasket or other seal (not shown) may be provided and sandwiched between the flanges  126   a ,  126   b  as appropriate. The appropriate valves, gauges and instrumentation  134  may be mounted in communication with the interior of the chamber  110  for monitoring the gas heating therein. Various embodiments of the industrial gas heating assembly  100  shown in  FIGS. 8-9  may be provided in 12 kW, 24 kW and 36 kW, 48 kW, 60 kW or other designs. 
     A further embodiment of an industrial heater  100  according to this invention is shown in  FIG. 7  and is adapted to generate super heated steam. Traditionally, boiling water at high pressure and then heating the steam at high pressure have produced super heated steam. The embodiment of  FIG. 7  provides a device where the flow of hot air over an orifice causes a super saturated steam jet. Components of the industrial heater and steam generator  200  shown in  FIG. 7  that are the same or similar to corresponding components of the heater lo as shown in  FIGS. 1-5  are labeled in a similar manner. The words “superheated”, “supersaturated” and variations thereof are interchangeable. Superheated steam for the purposes of this specification is steam at less than 100° C. at 1 atmosphere or at high pressures greater than 1 atmosphere. It also encompasses H 2 O in the form of gas at any temperature. Although we use the word steam to illustrate making H 2 O gas or vapor we anticipate with this word any embodiment for the conversion of any fluid to a gaseous state with our apparatus and method. the word supersaturated steam is used to indicate H 2 O or other materials in the form of gas at temperatures above 100° C. at pressures of about 1 atmosphere (see  FIG. 7 ) and/or higher (see  FIG. 9 ). By supersaturated steam we also infer H 2 O in the form of vapor. One objective of this aspect of this invention is to make supersaturated steam at 1 atmosphere; whereas, it normally takes high pressure to make supersaturated steam. Although we use the word steam to illustrate making H 2 O gas or vapor we anticipate with this word any embodiment for the conversion of any fluid to a gaseous state with our apparatus and method. We also intend to use the words superheated and supersaturated interchangeably. 
     The heater and steam generator  200  includes a gas inlet source  202 , which may be pressurized or unpressurized, and a power cord grip  204  proximate a gas inlet  206  of the device. A manifold housing  208  is mounted on the gas entry end of a casing  210  that is generally a right circular tube. An industrial gas heater lo according to a variety of embodiments according to this invention such as those shown in  FIGS. 1-3  is mounted within the casing  210 . 
     Proximate the gas exit port  16  of the industrial gas heater  10 , a delivery tube  212  is mounted to an end plate  214  of the casing  210 . The delivery tube  212  is in communication with a fluid reservoir or cup  216  which may be a polycarbonate reservoir. The delivery tube  212  advantageously includes a venturi assembly therein. A supply or feed line  218  from the reservoir  216  is regulated by a needle valve  220 , the operation of which is well know by those of ordinary skill in the art. The valve  220  may be either mechanical, electromechanical, semiconductor, nano valve, needle valve, self regulation condition by water level or any other commonly understood regulating device with or without feedback. The feed line  218  is coupled to the delivery tube  212  as shown in  FIG. 7 . The supply feed line  218  may be stainless steel piping or other appropriate material. The delivery tube  212  feeds into a reactor vessel  222  having a generally bulbous configuration. Contained within the reactor vessel  222  is a porous medium  224  such as steel wool or other generally non-dissolvable media; however, a dissolvable media may be utilized within the reactor vessel  222 , if appropriate. The porous medium  224  may be made of metallic, ceramic, polymer, intermetallic, nano-materials, or composite materials or combinations and mixtures thereof. The porosity may be reticulated or well defined. The porosity may be even or uneven and may vary from nanometer-size to centimeter sized pores. An exit nozzle  226  is provided on the reactor vessel  222  and may include a diffuser  228 . 
     The liquid to be heated into super saturated steam is contained within the reservoir  216  and fed to the venturi tube through the inlet pipe as regulated by the needle valve. The gas heated by the gas heater passes into the delivery or venturi tube  212  that is connected to the liquid reservoir  216 . As the hot gas passes through the venturi tube  212 , it draws the liquid from the reservoir  216 . The liquid flow as previously stated is controlled by the needle valve  220 . The liquid is atomized in the venturi tube  212  and the liquid/gas mixture enters the reactor vessel  222  where the liquid is vaporized. The unique design of the reactor vessel  222  provides for total vaporization of the liquid. The vaporized fluid exiting the reactor vessel  222  may be re-circulated through the system  200  and introduced into the gas inlet  202 . For example, this may be achieved through a recirculation loop  230 . Furthermore, the apparatus and method of this invention may produce steam by the addition of H 2 O through one or both of the coils in the gas heater  10 . This introduction of the H 2 O may be at the inlet, outlet or in-between the gas passage and the H 2 O may be added in the form of a liquid, gas or mist. 
     We have noted that the position of the valve  220  influences the air steam mixture. For example, at 100 ml of water in 462 seconds, a high 40% specific humidity value at 375° C. at about 1.3 cfm of hot air is generated. The relative humidity is estimated to be about 40% at this temperature assuming full compositional scale ideal gas mixing with no mixing enthalpy. Further, at 375° C., a pressure of 22 MPa (i.e., approximately 220 times atmospheric pressure) is needed to initiate condensation of the mixture. Alternatively, cooling the gas to about 110° C. at one atmosphere is required to initiate condensation. Specific humidity is defined as the mass of H 2 O divided by the mass of air. 
     Steam temperature depends on the water valve  220  setting and air inflow setting. Typical settings at a full power of 1 kW for the gas heater  1 o are as follows: gas at 1.45 CFM and water at 200 ml in 45 minutes yields steam air temperature of approximately 350° C. Gas at 1.4 CFM and water at 200 ml in 20 minutes yields steam air temperature of about 250° C. Further, gas at 1.8 CFM and water at 200 ml in 20 minutes yields steam air temperature of about 150° C. The above examples utilize a gas inlet temperature at approximately 30° C. and the water inlet temperature at approximately 30° C. 
     Possible applications for the industrial heating assembly and steam super saturated generator  200  of  FIG. 7  include high temperature super-heated steam-air or steam-gas generation. This could be utilized for layering, epoxy drying and other film uses where super-heated steam is required at one atmospheric pressure. Applications for formica polymeric materials, drying, degreasing, wood conditioners etc. are contemplated. This application is ideal for steam drying or steam oxidation as well as for spray deposition and spray cooling. Nano-crystal and larger crystal-sized production is possible by dissolving, gasification (i.e., steaming) and precipitation on cooling the gas. Silicon purification may be possible also for use in thermo-electrics and solar cell applications. Other applications for the system of  FIG. 7  include fogging, gas moisturizing, hot coating, steam generation, vapor deposition, cooking, rice making, cleaning, drying and epoxy hardening. Applications in energy devices such as fuel cells are anticipated. 
     The graph shown in  FIG. 11  provides exemplary data of how to adjust the system  200  of  FIG. 7  for different levels of specific humidity. Note as the specific humidity increases, there is a corresponding decrease in overall temperature as total energy is conserved. For the graph in  FIG. 11 , the steam gas thermocouple is positioned at the gas exit port. Variations of the data shown in the graph of  FIG. 11  may be expected to be varied upon replacement of the thermocouple, restrictions on gas and water flow and other random errors normally present in multi-variant measurements. As one of ordinary skill in the art will appreciate, specific applications would require optimization of all valve settings for optimum results. Standard water steam temperature, pressure diagrams and saturated steam and super-heated steam pressure and temperature tables may be utilized for such optimization. 
     Various embodiments of the heaters  10 ,  100 ,  200  according to this invention were tested and the results are summarized and presented herein. The following tests were done with (1) metallic wire and (2) with molybdenum disilicide wire and the following results were obtained. 
     Metallic Wire. Commonly available metallic heating wire  28   a ,  30   a  made of Nickel Chromium alloy or Fe—Al—Cr or Fe—Al, Ni—Cr alloy was used. Generally, such metallic wires can be heated in air to about 1200° C. Wire diameters from 0.1 mm to a 1.2 mm were tried for the experiments. We conducted the following experiments with the Fe—Al—Cr alloy. Alloys made of Fe—Al—Cr—Nb or Fe—Al—Cr—Mo—Nb were expected to perform similarly as are other metallic &amp; intermetallic systems. 
     In one experiment, the gas was heated to 850° C. at a 3.5 scfm (standard cubic feet per minute, standard conditions are normally 25° C. and 1.0 astrosphere) flow rate with the following design features of the heater. Other experiments were also conducted where gas was heated to close to 1000° C. The experiment utilized a wire coil with a wire diameter of 1.2 mm for the inner and outer coils  28 ,  30 . The outer coil wire  30   a  separation (pitch) was 0.285 mm and the inner coil wire  28   a  separation (pitch) was 0.285 mm. The wires  28   a ,  30   a  of the inner and outer coils  28 ,  30  were wound in opposite directions. A thermocouple  38  was located at about 3 mm from the gas exit port  16 . When located at this location, the thermocouple read up to 980° C. It is expected that the upper range with metallic elements will be about 1000° C. for ambient air. Other gases, depending on their thermal properties, will have a different exit temperature. Metallic elements made of Mo, W or other such higher temperature metals provide higher gas exit temperatures up to 3000° C. 
     We contemplate that the wire sizes for the inner and outer coils  28 ,  30  could be different for different industrial applications. Similarly the pitch can be different for each coil  28 ,  30  and different at different locations in the same coil according to this invention. For example, the coil pitch proximate to the incoming power leads  28   d ,  30   d  could be larger than at the main heating sections of the coils  28 ,  30  to keep the contacts relatively cooler. Spacers and other inserts between the coils  28 ,  30  are contemplated, if required, according to this invention. 
     It is thought that the presence of the inner coil  28  serves to overcome the surface or conda effect and thus improves contact with the gas flowing through the tubular housing  12 . 
     Some further experiments were conducted. Coil design was adjusted with the appropriate physics in mind. 
     Experiment 1: The outer coil  30  provides rifling of the gas that increases heat transfer from the coil to the gas. A helical coil wire  30   a  of 240 mm long and 13.2 mm mean diameter, working out for 8.2 Ohms (18 SWG A1 commercial wire) was used for testing. The coil was inserted in an open-ended ceramic tube  12 . The exit end of the coil was brought back to the inlet side through a ceramic insulating tube. The coil was operated at 110V, at a power rating of 1.47 kW. The airflow was maintained at 5 SCFM@ 0.4 Kgs/cm 2  working pressure. The exit temperature of the air stabilized at 560° C. 
     Experiment 2: The inner coil  28  over comes the conda surface effect, and provides for annular area heating of the gas, which provides for the highest heat transfer to the gas. The exit end of the coil  28  was wound on its return on the ceramic insulating tubular housing  12 . The resulting coil resistance was 10.8 Ohms. The coil  28  was operated with the same airflow, air pressure and operating voltage of 110V as in Experiment  1 . The coil now operated at 1.1 kW, and the exit temperature stabilized at 806° C. 
     Experiment 3: The inner coil  28  was wound in the opposite direction of the outer coil  30  to provide opposite rifling to the gas with respect to the outer coil. This causes a turbulence effect on the airflow, which increases heat transfer to the gas. All other parameters were the same as Experiment 2. The exit temperature stabilized at 845° C. Therefore, the opposite winding configuration gave a nearly 50° C. higher temperature. Table 1 below gives further experimental details and exit temperatures. 
     Experiment 4: An experiment was conducted with an inner coiled-coil  28  and an outer coiled-coil  30  ( FIG. 10 ). The gap was between 6 to 10 mm (i.e. the outer diameter (OD) of the inner coiled-coil, was 40 mm and the inner diameter (ID) of the outer coiled-coil was about 60 mm). The wire  28   a ,  30   a  itself was 0.8 mm in diameter and the diameter of the coiled-coil was about 8 mm. The material of the wire was Fe—Cr—Al alloy. At about 1.6 SCFM we found a temperature of 650° C. was reached in a few minutes at the exit for air. When water was introduced as a mist, at the inlet point a final steam gas temperature of 230° C. was obtained. 
     Experiment 5: Several modules as described in Experiments 3 and 4 were arranged in parallel and superheated steam was generated both by mist injection before the coil and ahead of the coil. This air-supersaturated steam was continuously recirculated through the assembly in order to increase the H 2 O content in the gas. Experiments are continuing in order to get more quantitative readings of the specific humidity. The modules and method of heating were found to be suitable for recirculation. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Coil 
                   
                   
                 Airflow cross 
                   
                   
                   
                 Exit 
               
               
                 Experiment 
                 resistance 
                 Voltage 
                 Current 
                 section area 
                 Power 
                 Air Flow 
                 Air Pressure 
                 temperature 
               
               
                 Number 
                 (Ohms) 
                 (Volts) 
                 (Amps) 
                 (mm2) 
                 (kW) 
                 (SCFM) 
                 (Kg/cm 2 ) 
                 of air (° C.) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Experiment 1 
                 8.2 
                 110 
                 13.4 
                 25.1 
                 1.47 
                 5 
                 7 
                 560 
               
               
                 Experiment 2 
                 10.8 
                 110 
                 10 
                 17.2 
                 1.1 
                 5 
                 7 
                 806 
               
               
                 Experiment 3 
                 10.8 
                 110 
                 10 
                 17.2 
                 1.1 
                 5 
                 7 
                 845 
               
               
                 Experiment 4 
                 11.0 
                 110 
                 10 
                 55.2 
                 l.l 
                 3.5 
                 0.4 
                 850 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Typical Results of the Present Invention 
               
               
                 UAT5 Ref: p83(4) HIPAN Primary: 208 Volts, Secondary: 40 Volts tap. 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Temperature, C. 
                 Flow, 
                 Secondary 
                 Primary 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Time 
                 Set point 
                 Process 
                 SCFM 
                 Current 
                 Volts 
                 Current 
                 Volts 
                 Comments 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 10:00 
                 0 
                 RT 
                 2.0 
                 0 
                 0 
                 0 
                 0 
                 Started 
               
               
                 10:03 
                 1400 
                  542 
                 2.0 
                 93 
                 14 
                 16 
               
               
                 10:05 
                 1400 
                 1167 
                 2.0 
                 103 
                 21 
                 18 
               
               
                 10:07 
                 1400 
                 1371 
                 2.0 
                 95 
                 21 
                 18 
               
               
                 10:08 
                 1400 
                 1400 
                 2.0 
                 106 
                 18 
                 15 
               
               
                 10:20 
                 1400 
                 1402 
                 2.0 
                 105 
                 18 
                 18 
               
               
                 10:30 
                 1400 
                 1400 
                 2.0 
                 79 
                 16 
                 14 
               
               
                 10:38 
                 1400 
                 1400 
                 2.0 
                 77 
                 16 
                 13 
               
               
                 10:38:50 
                 1400 
                 1400 
                 3.0 
                 86 
                 18 
                 14 
               
               
                 10:48 
                 1400 
                 1400 
                 3.0 
                 86 
                 17 
                 14 
               
               
                 10:58 
                 1400 
                 1400 
                 3.0 
                 81 
                 16 
                 14 
               
               
                 11:08 
                 1400 
                 1400 
                 3.0 
                 81 
                 16 
                 15 
               
               
                 11:08:50 
                 1400 
                 1400 
                 4.0 
                 89 
                 18 
                 16 
                 81 
               
               
                 11:20 
                 1400 
                 1400 
                 4.0 
                 96 
                 19 
                 17 
                   
                 End 
               
               
                   
               
               
                 RT: Room temperature 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Typical Results of the Present Invention 
               
               
                 UAT5 Ref: p95(4) HIPAN Primary: 240 Volts, Secondary: 40 Volts tap. 
               
            
           
           
               
               
               
            
               
                   
                 Temperature, C. 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Set 
                   
                 Flow, 
                 Secondary 
                 Primary 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Time 
                 point 
                 Process 
                 In-situ 
                 SCFM 
                 Current 
                 Volts 
                 Current 
                 Volts 
                 Comments 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                  9:35 
                 0 
                 RT 
                 RT 
                 3.0 
                 0 
                 0 
                 0 
                 0 
                 Started 
               
               
                  9:39 
                 1050 
                 1046 
                 621 
                 3.0 
                 89 
                 13 
                 15 
               
               
                  9:42 
                 1372 
                 1334 
                 942 
                 3.0 
                 102 
                 19.6 
                 18 
               
               
                  9:43 
                 1372 
                 1372 
                 1032 
                 3.0 
                 95 
                 18.5 
                 17 
               
               
                  9:47 
                 1372 
                 1372 
                 1055 
                 3.0 
                 123 
                 22 
                 19 
                   
                 End 
               
               
                 10:47 
                 1400 
                 392 
                 432 
                 3.0 
                 0 
                 0 
                 0 
                 0 
                 Re- 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 started 
               
               
                 10:49 
                 1400 
                 1042 
                 702 
                 3.0 
                 124 
                 19.7 
                 22 
               
               
                 10:50 
                 1400 
                 1375 
                 954 
                 3.0 
                 98 
                 18.8 
                 17 
               
               
                 10:51 
                 1400 
                 1397 
                 1022 
                 3.0 
                 95 
                   
                 16 
               
               
                 10:52 
                 1400 
                 1400 
                 1074 
                 3.0 
                 89 
                 17 
                 16 
               
               
                 11:00 
                 1400 
                 1400 
                 1165 
                 3.0 
                 81 
                   
                 15 
               
               
                 11:10 
                 1500 
                 1500 
                 1279 
                 1.0 
                 70 
                   
                 12 
               
               
                 11:13 
                 1500 
                 1500 
                 1301 
                 1.0 
                 67 
                 14 
                 12 
                 81 
               
               
                 11:18 
                 1500 
                 1500 
                 1314 
                 1.0 
                 66 
                 12 
                 12 
               
               
                 11:26 
                 1500 
                 1500 
                 1316 
                 0.5 
                 56 
                 11 
                 10 
               
               
                 11:28 
                 1500 
                 1500 
                 1315 
                 1.0 
                 60 
                 12 
                 10 
               
               
                 11:39 
                 1500 
                 1500 
                 1316 
                 1.0 
                 58 
                 11 
                 10 
                 88 
               
               
                 11:53 
                 1500 
                 1500 
                 1322 
                 1.0 
                 57 
                 11 
                 10 
                 69 
               
               
                 12:05 
                 1500 
                 1500 
                 1322 
                 1.0 
                 56 
                 11 
                 10 
                 69 
               
               
                 12:55 
                 1500 
                 1500 
                 1324 
                 1.0 
                 55 
                 11 
                 10 
               
               
                  1:31 
                 1500 
                 1500 
                 1324 
                 1.0 
                 55 
                 11 
                 10 
               
               
                  2:05 
                 1500 
                 1500 
                 1328 
                 1.0 
                 55 
                 11 
                 10 
               
               
                  3:30 
                 1500 
                 1500 
                 1332 
                 1.0 
                 55 
                 11 
                 10 
               
               
                  5:00 
                 1500 
                 1500 
                 1332 
                 1.0 
                 55 
                 11 
                 10 
                 70 
                 End 
               
               
                   
               
            
           
         
       
     
     It is contemplated that molybdenum disilicide wires  28   a ,  30   a  can be heated in air to 1900° C. for this invention. However, such wires are more brittle than metallic wire. The molybdenum disilicide coils were obtained from Micropyretics Heaters International, Inc. of Cincinnati, Ohio (www.MHI-INC.COM). 
     Wire  28   a    30   a  diameters of 3 mm, 4 mm or 5 mm may be used with this invention. An experiment was conducted with outer coil wire  30   a  separation (pitch) at 12.7 mm and inner coil wire  28   a  separation (pitch) at 12.7 mm. The gap between the coils  28 ,  30  tested was varied from 4 mm to 15 mm. Best results were obtained with the 5 mm wire. 
     The best test results of Table 2 show a temperature of 1165° C. to 1400° C. at different measurement positions with 1400° C. as set point on the controller and airflow set to 1 scfm. 
     The best test results of Table 3 show a temperature of 1332° C. to 1500° C. at different measurement positions with 1500° C. as set point on the controller and airflow set to 1 scfm. In an experiment with the inner coil  28  at about 40 mm and the outer coil at about 65 mm, a wire thickness of about 0.8 mm and coil of about 1 mm diameter Fe—Cr—Al alloy, barely separated for the coiled wire embodiment, the exit temperature with air was 650° C. with a flow rate of about 1.6 scfm (estimated approximate). The pitch separation of the coils may be smaller for metallic coil materials and larger for ceramic materials. We were also able to introduce a water mist into these coil arrangements and obtain a high quality steam output (see  FIG. 7 ). 
     As a result of this invention, as yet unavailable very high temperatures in gases for industrial applications are obtainable because of the new coil in coil design with the proper spacing and gaps with the two coils  28 ,  30  electrically coupled. It is also found that opposite winding in the inner and outer coils  28 ,  30  gives rise to very high temperatures of the gas at the exit port  16 . 
     The typical industrial applications for this invention involve low cost heating. Three different types of industrial applications are considered without limiting the invention from other industrial applications: 
     1. Heating of any gas, including steam, directed into chamber such as an oven or furnace that may or may not have other heating systems in it. 
     2. Heating of any gas, including steam, passing though the coils. 
     3. Heating any gas, including steam, directed at a surface for applications such as coatings, hardening, debinding, glowing, etc. 
     The coils  28 ,  30  may be electrically heated or heated by a combination of electric and other thermal methods. The coils  28 ,  30  can be metallic, molybdenum disilicide, silicon carbide, intermetallic, ceramic or other materials. 
     From the above disclosure of the general principles of the present invention and the preceding detailed description of various embodiments, those skilled in the art will readily comprehend the various modifications to which this invention is susceptible. Therefore, we desire to be limited only by the scope of the following claims and equivalents thereof.