ELECTRIC FLUID HEATER

Abstract: An electric fluid heater includes the secondary coil of a transformer which secondary coil consists of a heating element having a multiple-turned flow path which however is single-turned electrically. When the primary coil of the transformer is supplied with electric power, the secondary coil heats fluid which flows in the flow path.

Description: FIELD OF THE INVENTION This invention relates to an electric fluid heater used to heat city water, liquid chemicals, gases or other fluid which, in particular, reaches a high temperature of several hundred degrees or a high pressure of several decade times the standard atmospheric pressure. BACKGROUND OF THE INVENTION Japanese Post-examination Publication JP-Y2-40-3353 of a utility model application discloses a water heater which is supplied with electric power from the primary of a transformer and uses the secondary coil as a tubular conductive heating element to heat water flowing therein. The heating water-inlet tube, i.e. the secondary coil, as well as the primary coil (wire coil) of the transformer is insulated throughout its entire length and entire surface area. That is, multiple turns of the secondary coil are insulated from each other to prevent short-circuit between respective turns thereof. Opposite ends of the water inlet tube, i.e. the entrance and exit of water, are electrically connected by an externally bridging short-circuit to prevent electrical leakage to the exterior of the water inlet tube. This system certainly operates well when it heats water up to a modest temperature about 100°C in the standard atmospheric pressure. However, if it is used as a large-scaled heater subject to a high temperature, high voltage and high pressure, various difficulties arise in its mechanical structure, and the efficiency of the transformer decreases. Further, this system fails to effectively use heat of the primary coil. French Patent Specification No. 911,767 describes an apparatus for electrically heating fluids comprising a primary coil for receiving electric power wound on the same core of a transformer as a secondary coil. The secondary coil is in the form of a tube which is multiply wound around the core. The windings of the tube are electrically connected to one another. In a three phase arrangement, the primary coils may be arranged spaced apart but on the same core legs of the transformer as the secondary coils. Fluid flowing within the secondary coil is heated by means of electric currents induced therein by the electric power supply to the primary coil. Swiss Patent Specification 216899 discloses an electric fluid heater which operates according to the same principles as the one disclosed in the above mentioned French patent specification. In this case, both the primary and secondary coils are wound around the same core leg of the transformer, the secondary coil being wound around the primary coil. In the heaters described in the above mentioned French and Swiss patent specifications, it is necessary to electrically connect the inlet and outlet tubes of the secondary coil by means of a good electrical conductor. United States Patent Specification 2,181,274 relates to induction heating construction comprising primary and secondary coils in which the secondary coil is composed of a magnetic and a non-magnetic portion. US Patent No 4,256,945 discloses a heater using the skin effect. This patent utilizes such a nature that a conductor behaving as a heat generator varies in magnetic permeability with temperature to vary the resistance of the conductor and to use it for temperature control of the conductor. This does not serve to overcome the disadvantage of the above-mentioned publication JP-Y2-40-3353. OBJECT OF THE INVENTION It is therefore an object of the invention to overcome the problem involved in the prior art system and to provide a system reliably, effectively operative in high-temperature, high-pressure fluid heating and preferably using heat produced in the primary coil effectively. SUMMARY OF THE INVENTION According to the present invention, there is provided an electric fluid heater as defined in either one of Claims 1 and 2 below. Optional but preferred features of the invention are defined in the dependent claims. Embodiments of the present invention permit omission of insulation among multiple turns of the secondary coil, an increase of the window occupation ratio (the rate of the cross-sectional area occupied by the primary and secondary coil conductors in the window area of the transformer core), material saving and an increase in the system efficiency. Further, embodiments of the invention arrangement make it possible to average unbalances in the temperature of the secondary coil conductor used as a heating element to decrease the heat transfer area of the heating element, i.e. to decrease the required material of the system. In the preferred embodiment, if the temperature of the fluid at the entrance of the heater is lower than the maximum heat-resistant temperature of the insulator of the primary coil, the fluid is used as a coolant of the primary coil. More specifically, the primary coil is configured as a tube made from copper or other high-conductive material as it is normally made from copper, aluminum, silver or other high-conductive material, and the fluid to be heated is used in the tube as a coolant of the primary coil. The fluid is heated at the exit of the primary coil up to a modest degree which is about 10% of the total heating power, for example, and is subsequently fed to the true heater, secondary coil, via a pressure pump provided at the position, if necessary. Electrical insulation is normally required between the primary coil and the pump. However, it may be omitted if a point between the primary coil and the pump or the pump itself can be connected to ground. The secondary coil, although multiple-turned as the flow path is single-turned electrically. Therefore, insulation is not required normally between the pump and the secondary coil. Particularly, when the skin effect of the secondary coil is large, and the current is concentrated to the external superficial portion of the innermost circumferential wall of the secondary coil, insulation between the pump and the secondary coil is not required also in absence of ground connection of the interpoint or of the pump. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic transversal cross-sectional view for explanation of an electric fluid heater embodying the invention; Figures 2A and 2B are two front elevations of a secondary coil conductor for explanation of an electric fluid heater embodying the invention heater; Figures 3A and 3B are views showing relationships between the heating element temperature and the fluid temperature in the invention system at 3A and in the known utility model (JP-Y2-40-3353) at 3B; Figure 4 is a front elevation of an electric fluid heater of single-phase core-type according to the invention; Figure 5 is a front elevation of an electric fluid heater of three-phase core-type according to the invention; Figure 6 is a schematic transversal cross-sectional view of the heater having a double-turned flow path 7; Figure 7 is a schematic transversal cross-sectional view of a further embodiment of the invention system; Figures 8 and 9 are schematic views of a circuit and a flow path in a still further embodiment of the invention system; Figures 10A and 10B are schematic views of a circuit and a flow path in a yet further embodiment of the invention system of single-phase core type; Figures 11A and 11B are schematic views of a circuit and a flow path in a yet further embodiment of the invention system of three-phase core type (delta-type). Figures 12A and 12B are schematic views of a circuit and a flow path in a yet further embodiment of the invention system of three-phase core-type (star-type); and Figure 13 is a front elevation of a water heater according to Japanese Post-examination Publication JP-Y2-40-33533 of a utility model application. DETAILED DESCRIPTION The invention is described below in comparison with the prior art technology, referring to the drawings. However, the invention must never be construed as being limited to the illustrated embodiments. Figure 13 is a plan view of a hot water system disclosed by Japanese Post-examination Publication JP-Y2-40-33533 of a utility model application. In this drawing, reference numeral 61 refers to a primary coil, 62 to a secondary coil (water inlet tube), 63 to a core, 65 to an entrance tube for inletting water to be heated to the secondary coil 62, 66 to an exit tube of hot water from the secondary coil, and 64 to an electrical connection between tubes 65 and 66. The inlet tube 62 is made from aluminum, and its inner and outer surfaces are coated by aluminum oxide. An arrow shows the flowing direction of the fluid to be heated. Figure 1 is a schematic cross-sectional view for explaining an embodiment of the invention, taking a single-phase core type as an example. In the drawing, reference numeral 1 denotes a primary coil, and 2 designates a secondary coil which serves as a heating element. A core 3 is common to the coils 1 and 2, and the system is supplied with power from a power source 4. Reference mark XY denotes a core axis. Fluid to be heated, entering through a metal tube 5, is heated by a flow path 7 having continuous double or more turns while it flows therein, and exits from a metal tube 6. The illustrated flow path 7 is four-turned about the core 3. The positional relationship between the entrance and exit tubes 5 and 6 may be opposite to the illustration. Figure 2A schematically shows the secondary coil conductor portion. An electric field e produced in the secondary coil conductor 2 as shown by arrows in the drawing is vertical to the core axis XY having the primary coil wound thereon. Therefore, any different points on the secondary coil conductor on any line parallel to the core axis XY are identical in electric potential, so that if the fluid entrance tube 5 and the fluid exit tube 6 are disposed on or near the line, no electrical arc is produced on possible metallic contact between the tubes 5 and 6. Therefore, the system safety is complete, no ground current is produced also when both tubes are connected to ground, and no electric shock occurs. 19 is a ground connection. It should be noted here that the insulation between the electrical connection 64 and the coil in the aforegoing publication shown in Figure 13 is not necessary because the secondary coil conductor (heating element) is single-turned. However, the primary coil has an electrical, thermal insulation corresponding to the temperature of the heater, and electrical thermal insulation on the surface of the secondary coil against the core 3 is required. However, since the invention system merely requires electrical insulation for a low voltage corresponding to a single turn as compared to the prior art utility mode, the occupation ratio of the transformer core window can be increased. Figure 2B shows an arrangement different from that of Figure 2A in which welding 20 is provided throughout the entire length or at some points of an secondary tube coil 2′ so that multiple turns (four turns in the illustration) of the fluid passing secondary tube coil 2′ are electrically united into a single turn. In lieu of the welding, the secondary tube coil 2′ may be casted into a single body with conductive material in a fashion similar to Figure 1. A1so in Figure 2B, the electric field e produced in the secondary tube coil 2′ is substantially vertical to the core axis XY, which means that any different points on a line parallel to the core axis XY are identical in electric potential. Therefore, if the fluid entrance metal tube 5 and the fluid exit tube 6 are provided on or near the line, the same result as that of Figure 2A is obtained upon metallic contact between the tubes 5 and 6. The aforegoing explanation related to Figures 2A and 2B is based on an assumption that the skin effect of an alternating current within the secondary coil conductor may be disregarded, and on a specific arrangement in which the metal tubes 5 and 6 are disposed on identical potential points a and b on the secondary coil conductor. In case that the skin effect is significantly large, according to the invention, in the secondary coil conductor 2 or in the secondary tube coil 2′, and if the primary coil 1 and the secondary coil conductor 2 are concentrical as in Figure 1, the secondary alternating current represents a concentrated flow to the vicinity 8 of the secondary coil surface opposed to the primary coil, provided that the following relationship is established between the thickness t (cm) of the flow path wall in the radial direction of the secondary coil and the skin depth S (cm) of the alternating current: t > 2S (1) Therefore, the electric field e shown in Figures 2A and 2B is never produced in location other than the vicinity of the secondary coil surface opposed to the primary coil, and electrical arc upon metallic contact between the tubes 5 and 6 and electric shock against human beings or animals can be prevented by positioning the metal tubes 5 and 6 in the location having no electric field. Additionally, a limited portion of the secondary coil conductor, i.e. the portion opposed to the primary coil, may be made from ferromagnetic material different from the material of the remainder portion of the secondary coil conductor so that the alternating current flowing in the secondary coil conductor concentrates to the limited portion opposed to the primary coil. The skin depth S (cm) in expression (1), as well known, can be expressed by: where ρ (ohm.cm) is the resistivity of the secondary coil conductor, µ is the specific permeability, and f (Hz) is the power source frequency. The above-described skin effect increases with ℓ/d where ℓ (cm) is the height of the secondary coil conductor 2, and d (cm) is its inner radius. The substantially same result is obtained by an oval or rectangular turn of the secondary coil other than an accurate cylindrical turn. The skin depth S will be about 1mm with a steel conductor and about 1cm with a copper conductor at a commercial frequency (50 to 60 Hz). Therefore, when the secondary coil conductor is made from steel, the skin effect may be regarded to be large. If the secondary coil conductor is made from copper or non-magnetic steel, locations of the tubes 5 and 6 are preferably selected as shown above, disregarding the skin effect of the secondary coil conductor. In case that the entrance and exit tubes 5 and 6 of the secondary coil conductor are disposed on or near a line parallel to the core axis XY, or alternatively if the skin effect inside the secondary coil conductor is large, no insulation flange is required for the entrance tube nor for the exit tube of the secondary coil conductor. However, in case that the skin effect inside the secondary coil conductor cannot be disregarded and that the entrance and exit tubes are disposed at positions substantially isolated from a line parallel to the core axis XY, insulation flanges are sometimes required for the entrance and exit tubes. The electrical single turn of the secondary coil conductor unlike the prior art utility model system of Figure 13 gives a further advantage that the temperature of the secondary coil conductor, i.e. the heating tube, can be uniformed in the length direction of the flow path 7. More specifically, the temperature of the fluid entering through the tube 5 in Figure 1 gradually increases in the length direction of the flow path 7. The temperature tendency of the secondary coil heating element is low near the entrance tube 5 and high near the exit tube 6. However, since the single-turn secondary coil heating element is thermally unitary, the heat flows in and along the conductor from the tube 6 to the tube 5 and increases the temperature near the tube 5 while decreasing the temperature near the tube 6. This configuration is shown in Figure 3A in which no large change occurs in the temperature ϑh of the secondary coil conductor in the flowing direction D, but the temperature ϑf of the fluid gradually increases. In the prior art utility model (JP-Y2-40-3353) of Figure 13, the secondary tube coil 62 must be electrically insulated throughout its entire surface. Since such an electrical insulator is a thermal insulator, too, no great thermal transmission is expected in the secondary tube coil wall in the direction opposite to the fluid flow direction. That is, the temperature ϑh of the heating element, i.e. the secondary tube coil 62, linearly increases, maintaining a substantially constant temperature difference with respect to the fluid temperature ϑf as shown in Figure 3B. These fluid temperature ϑf and the heating element temperature ϑh reach their maximum degrees at the fluid exit. Since the fluid temperature ϑf and the heating element temperature ϑh has their allowable maximum temperatures ϑfm and ϑhm, the secondary coil conductor 2 of Figure 1 according to the invention is obviously superior for its less thermal transmitting surface if the thermal transmission coefficients between the heating element surface and the fluid are identical between them. This is described in detail in pages 75 through 84 of "Kogyo Dennetsu Sekkei (Industrial Electric Heating Design)" by Masao Andoh (Nikkan Kogyo Shinbunsha). The reduction in the heat transfer area contributes to a reduction of material not only of the heating element portion but also of the entire heater system including the core. The aforegoing description exclusively refers to an invention transformer of single-phase shell-type. However, since no potential difference exists between the metal tubes 5 and 6, in either a core-type or three-phase transformer, it is established by connecting equivalents of the tubes 5 and 6 in series or in parallel in two core legs in case of core type and in three core legs in case of three-phase type. Their examples are shown in Figures 4 and 5. Figure 4 is a front elevation of a single-phase core-type electric fluid heater, and Figure 5 is a front elevation of a three-phase core-type electric fluid heater. In these drawings, reference numerals 2a, 2b and 2c denote secondary coil conductors including fluid flow paths, reference numerals 5 and 6 designate fluid entrance and exit, and numerals 5″ and 6′ shown fluid conduits connecting the flow paths inside the secondary coil conductors. Inside the secondary coil conductors 2a, 2b and 2c exists the primary coil. Illustration of the power source and the wiring therefrom to the primary coil is omitted in Figures 4 and 5. Figures 1 and 2 shows the flow path 7 of single layer and four turns as an example. However, the flow path 7 may be of two or more layers and multiple turns as an example shown in Figure 6 in which reference numerals 1, 2, XY, 5, 6 and 7 designate the same members or parts as those in Figure 1. Obviously, two or more layers and multiple turns may be employed also in Figures 4 and 5. As described above, the invention arrangement, as compared to the prior art system, simplifies its construction, decreases the required material, increases the heating efficiency and reliability, and establishes a high-temperature, high-pressure fluid heater which the prior art technology could not provide. Further description follows about a further embodiment of the invention in which the primary coil is entirely or partly made in the form of a metal tube which includes an entrance for inletting fluid to be heated, and means for compressing the fluid from the metal tube when desired and subsequently feeding it to the secondary coil used as the heating element. Figure 7 is a schematic cross-sectional view for explanation of an arrangement of single-phase shell-type according to the embodiment. Reference numeral 1 designates a wire coil of the primary coil, 1′ denotes a tube portion of the primary coil, 2 refers to the secondary coil conductor used as the heating element. The core 3 is common to the wire coil 1, tube portion 1′ and secondary coil conductor 2, and the power source 4 supplies the primary coil 1 and 1′ with electric power. Reference mark XY shows the core axis. Arrows show the flowing direction of fluid to be heated which also serves as a coolant. The fluid to be heated enters through the inlet tube 5′ to the primary coil and flows along the tube 1′, cooling the primary coil 1 and 1′ and thereby increasing its own temperature. The fluid is compressed by the pump 9, and flows in the flow path 7 of the secondary coil through the entrance tube 5. The fluid is heated while flowing along the flow path 7, and subsequently exits through the exit tube 6. Reference numerals 10 and 11 are insulation flanges. If the tube 5′ is an insulative hose, the member at 10 need not be an insulation flange. In Figure 7, the secondary coil conductor 2 is a unitary cylindrical member throughout its entire length, and a spiral flow path is provided inside the cylinder wall. Although the secondary coil conductor 2 is single-turned electrically, the flow path is shown in multiple turns. In this case, the potential difference between the entrance tube 5 and the exit tube 6 is significantly small, and no insulation flange is required in these tubes in most cases. Figure 8 is a schematic view of the circuit and flow path of an embodiment of the invention system. In this drawing, reference numeral 1′ denotes a tubular primary coil which is entirely used as a flow path of fluid to be heated and used as a coolant. Reference numerals 2, 3, 5, 5′, 6, 9, 10 and 11 show the same members or parts as those in Figure 7. Arrows show the flowing direction of the fluid, 14 denotes the fluid entrance tube of the primary coil, 15 designates the fluid exit tube of the primary coil, and numerals 17 and 18 denote power source terminals. In this system, the primary coil is entirely tubular to allow the fluid to flow therethrough. Figure 9 is a schematic view of the circuit and flow path of a further embodiment of the invention system. In this drawing, reference numerals 1 and 1′ denote the wire coil and the tube portion of the primary coil as in figure 7. The other reference numerals show the same members or parts as those in Figure 8. Arrows show the flowing direction of the fluid. In the system of Figure 9, a limited portion of the primary coil is configured as a tube of copper or other material. If the primary coil is multiple-layered as shown in Figure 7, for example, a limited portion of its outermost layer opposed to the secondary coil is configured as a tube so that the fluid to be heated and used as a coolant flows therein. This arrangement is demanded to drop the allowable voltage of the insulation flanges 10 and 11 when the voltage between primary coil terminals 17 and 18 is high. In Figures 8 and 9, when the primary coil shares 10% of the heating amount of the fluid, maintaining 0°C for the fluid temperature at the entrance and maintaining 500°C for same at the exit, the temperature at the exit tube 15 of the primary coil is 50°C. This modest temperature is acceptable, when using a normal construction of the pump 9. The aforegoing description refers, to a single-phase shell-type heater. However, the invention may be used for single-phase core-type heating and three-phase type heating. Examples of single-phase core-type are shown in Figures 10A and 10B, examples of three-phase delta-type are shown in Figures 11A and 11B, and examples of three-phase star-type are shown in Figures 12A and 12B. In these drawings, reference numerals 3, 5, 5′, 5″, 6, 6′, 9, 10, 11, 17 and 18 show the same members or parts as those in the aforegoing description. Reference numeral 1a, 1b and 1c show wire coils of the primary coil, and 1a′, 1b′ and 1c′ denote tube portions of the primary coil. Reference numerals 2a, 2b and 2c refer to the secondary coil conductor used as the heating element, 12, 13, 51, 52, 55, 56, 57, 58 and 59 refer to insulation flanges, 22, 26, 27, 28, 29, 30, 31, 33 and 34 refer to electrical connections, and arrows show the flowing direction of the fluid to be heated and used as a coolant. Figure 10A is a schematic view of a circuit and a flow path in an arrangement of a single-phase core-type invention system. In this system, each half 1a′ (1b′) of the metal tube primary coil 1′ of Figure 8 is wound inside the secondary coil 2a (2b), and both halves 1a′ and 1b′ are connected in series to each other, electrically and phisically (in the sense of the flow path). Therefore, the system of Figure 10A is identical to the system of Figure 8 electrically and in the flow path arrangement. Figure 10B is a schematic view of the circuit and a flow path in an arrangement of single-phase core-type in which the arrangement of Figure 9 is used. In Figure 10B, primary coil wire coils 1a and 1b and metal tube primary coils 1a′ and 1b′ series-connected to the wire coils 1a and 1b are all wound on the core 3, respectively. One end of the tube 1b′ to be connected to the wire coil 1b is connected to the fluid inlet tube 5′ whereas the other end of the tube lb′ is connected via the insulation flange 55 to one end of the tube 1a′ to be connected to the wire coil 1a. The said other end of the tube 1b′ or a conduit communicating therewith is connected by the electrical connection 28 at a position before the insulation flange 55 to one end of the primary coil wire portion 1a remote from the connection with the tube 1a′. The secondary coils 2a and 2b are wound outside the primary coils 1a, 1a′, 1b and 1b′ respectively. Figures 11A and 12A are schematic views each showing a circuit and a flow path of an embodiment of a three-phase core-type invention system in which the arrangement of Figure 10A is applied. Figure 11A shows a three-phase delta-type system, whereas Figure 12A shows a three-phase star-type system. Figures 11B and 12B are schematic views each showing a circuit and a flow path of an embodiment of a three-phase core-type invention system in which the arrangement of Figure 10B is applied. Figure 11B shows a three-phase delta-type system whereas Figure 12B shows a three-phase star-type system. In the systems of Figures 7, 8, 9, 10A, 10B, 11A, 11B, 12A and 12B, since the primary coil is cooled by the fluid to be heated, it does not invite much difficult problem in the insulation material nor in the mechanical construction. Beside this, the heater efficiency is improved because the primary coil loss in the original sense can be used effectively. Particularly, when diminishing the cross-sectional area of the current path of the primary coil, and elevating the current density by several times up to 10A/mm² in case of copper tube, for example, it never invites any loss increase and rather decreases the dimension and weight of the primary coil. Further, when the secondary coil (heater) is wound on its outer circumference, its dimension and weight are decreased, and this leads to reduction in dimension and weight of the entire invention system.

Claims: An electric fluid heater comprising in combination: a primary coil (1) wound around a core leg of a transformer core (3) for receiving an electric current; a secondary coil (2) wound around the primary coil (1), the secondary coil (2) comprising a multiple of turns of a tube defining a fluid flow path (7), adjacent turns of the tube being electrically connected to one another so that the secondary coil (2) has a single turn electrically but multiple turns as a flow path; and a fluid entrance tube (5) of metal and a fluid exit tube (6) of metal connected to the secondary coil (2) for connecting the fluid flow path (7) to a fluid supply; wherein when an alternating electric current flows through the primary coil (1), an alternating electric current is generated by induction in the secondary coil (2) thereby causing the secondary coil (2) to heat whereby fluid passing through its fluid flow path (7) can be heated; characterized in that the secondary coil (2) is made from a material whose skin effect with respect to the alternating current flowing therein is sufficiently great so as to concentrate the alternating current to a limited portion (8) of the secondary coil (2) opposed to the primary coil (1) thereby preventing occurrence of any electrical arc upon metallic contact between the entrance and exit tubes (5, 6). An electric fluid heater comprising in combination; a primary coil (1) wound around a core leg of a transformer core (3) for receiving an electric current; a secondary coil (2) wound around the primary coil (1), the secondary coil (2) comprising a multiple of turns of a tube defining a fluid flow path (7), adjacent turns of the tube being electrically connected to one another so that the secondary coil (2) has a single turn electrically but multiple turns as a flow path; and a fluid entrance tube (5) of metal and a fluid exit tube (6) of metal connected to the secondary coil (2) for connecting the fluid flow path (7) to a fluid supply; wherein when an alternating current flows through the primary coil (1), an alternating electric current is generated by induction in the secondary coil (2) thereby causing the secondary coil (2) to heat whereby fluid passing through its fluid flow path (7) can be heated; characterized in that the portion of the secondary coil (2) opposed to the primary coil (1) is made from a ferromagnetic material different from the material of the remainder of the secondary coil (2), whereby alternating current flowing in the secondary coil can be concentrated in the portion of the secondary coil (2) facing the primary coil (1). An electric fluid heater according to claim 1 or 2, wherein the primary coil (1) is entirely or partly configured as a metal tube (1') which is provided with a fluid entrance tube (5') for inletting fluid to be heated therethrough and means (9) for urging the fluid from said metal tube (1') and subsequently feeding it to said secondary coil (2) used as a heating element. An electric fluid heater according to claim 3, wherein the urging means is in the form of a pump (9).

Inventors: ANDO, MASAO, NANRI, TAKESHI, SHO, MIKIO
Applicants: CHISSO ENGINEERING CO. LTD.
Application year: 1987
Main classification: H05B6
IPC codes: H05B6, H05B3, F24H1