Heater device and throughflow boiler

Abstract

To multi-locate a load factor of a throughflow boiler so as not to become an unbalanced load.SOLUTION: A heater device includes: a first electric heater 71 having a V-phase heater element 16V as a first-phase heater element, a U-phase heater element 16U as a second-phase heater element, and a W-phase heater element 16W as a third-phase heater element, which are connected to a three-phase AC power supply and connected to each other; a second electric heater 72 having the V-phase heater element, the U-phase heater element, and the W-phase heater element connected to each other; a third electric heater 73 having the V-phase heater element, the U-phase heater element, and the W-phase heater element connected to each other; a switch unit for switching a supply state of an AC power from the three-phase AC power source to the first electric heater, the second electric heater, and the third electric heater; and a controller.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to a heater device and a once-through boiler. [Background technology]

[0002] In the technical field related to boilers, there is known an electric boiler as disclosed in Patent Document 1. The electric boiler disclosed in Patent Document 1 is a once-through boiler in which an electric heater is disposed in each of a plurality of water tubes. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-169356 Summary of the Invention [Problem to be solved by the invention]

[0004] When multi-positioning the load factor of a once-through boiler in which an electric heater is provided for each of multiple water tubes, it is necessary to change the heat generation amount while all of the electric heaters are driven.When the electric heaters are controlled on and off, it is difficult to change the heat generation amount while all of the electric heaters are driven, making it difficult to multi-positioning the load factor of a once-through boiler.

[0005] Furthermore, if the electric heater is a three-phase heater with three heater elements, the three heater elements are mutually star-connected or delta-connected. When three-phase AC power is supplied to a three-phase heater from a three-phase AC power source, the output of the three-phase heater becomes 100%. When single-phase AC power is supplied to a three-phase heater from a three-phase AC power source, the output of the three-phase heater is reduced. In other words, by switching the AC power supplied to a three-phase heater between three-phase and single-phase, the load factor of a once-through boiler can be multi-positioned. However, when single-phase AC power is supplied to a three-phase heater from a three-phase AC power source, an unbalanced load is generated, with the load being biased toward a specific phase of the heater element, which may violate standards such as wiring regulations.

[0006] The technology disclosed in this specification aims to multi-position the load factor of a once-through boiler so as to prevent unbalanced loads. [Means for solving the problem]

[0007] This specification discloses a heater device including: a first electric heater connected to a three-phase AC power supply and having a first-phase heater element, a second-phase heater element, and a third-phase heater element wired to each other; a second electric heater connected to the three-phase AC power supply and having the first-phase heater element, the second-phase heater element, and the third-phase heater element wired to each other; a third electric heater connected to the three-phase AC power supply and having the first-phase heater element, the second-phase heater element, and the third-phase heater element wired to each other; a switch unit that switches the supply state of AC power from the three-phase AC power supply to the first electric heater, the second electric heater, and the third electric heater; and a controller. The switch unit includes a first specific switch that is turned on or off so as to connect or disconnect the three-phase AC power supply to a third-phase heater element of the first electric heater, to connect or disconnect the three-phase AC power supply to a first-phase heater element of the second electric heater, and to connect or disconnect the three-phase AC power supply to a second-phase heater element of the third electric heater. The controller turns on the first specific switch when operating the first electric heater, the second electric heater, and the third electric heater at a first output, and turns off the first specific switch when operating them at a second output lower than the first output. [Effects of the Invention]

[0008] According to the technology disclosed in this specification, the load factor of a once-through boiler can be multi-positioned so as to prevent unbalanced load. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a front view schematically showing a once-through boiler according to a first embodiment. [Figure 2] FIG. 2 is a side view schematically showing the once-through boiler according to the first embodiment. [Figure 3] FIG. 3 is a side view showing the electric heater according to the first embodiment. [Figure 4] FIG. 4 is a bottom view showing the electric heater according to the first embodiment. [Figure 5]FIG. 5 is a diagram showing an example of wiring of the heater element according to the first embodiment. [Figure 6] FIG. 6 is a diagram showing an example of wiring of the heater element according to the first embodiment. [Figure 7] FIG. 7 is a wiring diagram showing a three-phase AC power supply, a switch unit, and an electric heater according to the first embodiment. [Figure 8] FIG. 8 is a diagram illustrating the operation of the electric heater according to the first embodiment. [Figure 9] FIG. 9 is a diagram illustrating the operation of the electric heater according to the first embodiment. [Figure 10] FIG. 10 is a diagram illustrating the operation of the electric heater according to the first embodiment. [Figure 11] FIG. 11 is a diagram illustrating the operation of the first electric heater according to the second embodiment. [Figure 12] FIG. 12 is a diagram illustrating the operation of the first electric heater according to the third embodiment. [Figure 13] FIG. 13 is a diagram illustrating the operation of the electric heater according to the fourth embodiment. [Figure 14] FIG. 14 is a diagram illustrating the operation of the electric heater according to the fifth embodiment. [Figure 15] FIG. 15 is a diagram illustrating the operation of the electric heater according to the sixth embodiment. [Figure 16] FIG. 16 is a diagram illustrating the operation of the electric heater according to the seventh embodiment. [Figure 17] FIG. 17 is a diagram illustrating the operation of the electric heater according to the eighth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments will be described with reference to the drawings, but the technology disclosed in this specification is not limited to the embodiments. The components of the embodiments described below can be combined as appropriate. In addition, some components may not be used.

[0011] In the embodiment, a three-dimensional Cartesian coordinate system is defined for the electric boiler, and the positional relationship of each part will be described with reference to the three-dimensional Cartesian coordinate system. The direction parallel to the X-axis in a horizontal plane is defined as the X-axis direction. The direction parallel to the Y-axis in a horizontal plane perpendicular to the X-axis is defined as the Y-axis direction. The direction parallel to the Z-axis perpendicular to the horizontal plane is defined as the Z-axis direction. The Z-axis direction is the up-down direction. The +Z side is the upper side, and the -Z side is the lower side.

[0012] [First embodiment] A first embodiment will be described.

[0013] <Once-through boiler> FIG. 1 is a front view schematically showing a once-through boiler 1 according to this embodiment. FIG. 2 is a side view schematically showing the once-through boiler 1 according to this embodiment. Each of FIGS. 1 and 2 shows a cross section of a portion of the once-through boiler 1. In this embodiment, the once-through boiler 1 is an electric boiler that uses an electric heater 7 as a heat source. The electric heater 7 is a three-phase heater.

[0014] As shown in Figures 1 and 2, the once-through boiler 1 comprises a water supply device 2, a lower header 3 connected to the water supply device 2, a plurality of water pipes 4 connected to the lower header 3, a plurality of riser pipes 5 connected to each of the plurality of water pipes 4, an upper header 6 connected to the riser pipes 5, an electric heater 7 for heating water in the water pipes 4, a water level detection device 8 for detecting the water level in the water pipes 4, a switch unit 30 for switching the supply state of AC power from a three-phase AC power source 9 to the electric heater 7, and a controller 10 for controlling the once-through boiler 1.

[0015] The water supply device 2 supplies water to the water pipes 4 via the lower header 3. The water supply device 2 has a water supply pipe 11 connected to the lower header 3, a water supply pump 12 arranged on the water supply pipe 11, and a water supply valve 13 arranged on the water supply pipe 11. When the water supply pump 12 is driven with the water supply valve 13 open, water is supplied to the lower header 3 via the water supply pipe 11. When water is supplied to the lower header 3, water is supplied to the water pipes 4.

[0016] The lower header 3 contains water supplied from the water supply device 2. The lower header 3 is connected to the lower ends of each of the multiple water pipes 4. The lower header 3 distributes the water supplied from the water supply device 2 to each of the multiple water pipes 4. The water supplied to the lower header 3 from the water supply device 2 is supplied to each of the multiple water pipes 4. A drain pipe 14 is connected to the bottom of the lower header 3. A drain valve 15 is arranged on the drain pipe 14. When the drain valve 15 is opened, at least a portion of the water contained in the lower header 3 is discharged through the drain pipe 14.

[0017] The water pipes 4 circulate water from the lower header 3. The water pipes 4 are cylindrical members with water flow paths. The water pipes 4 are arranged so that their central axes are parallel to the Z axis. The lower ends of the water pipes 4 are connected to the upper part of the lower header 3. Multiple water pipes 4 are arranged at intervals in the X axis direction. Each of the multiple water pipes 4 is connected to the lower header 3.

[0018] The electric heater 7 heats the water in the water pipes 4. The electric heater 7 is a sheathed heater. The electric heater 7 is disposed in each of the multiple water pipes 4. The electric heater 7 is detachable from the water pipes 4. The electric heater 7 has a heater element 16 disposed inside the water pipe 4, a flange 17 fixed to the upper end of the heater element 16, and a connector 18 disposed on top of the flange 17.

[0019] The heater element 16 is immersed in water inside the water pipe 4. The surface of the heater element 16 comes into contact with the water supplied inside the water pipe 4. The heater element 16 faces the inner surface of the water pipe 4 with a gap between them. The electric heater 7 is inserted into the water pipe 4 from the opening at the top end of the water pipe 4. A flange 17 is fixed to the top end of the water pipe 4. A connector 18 is connected to a three-phase AC power supply 9. AC power from the three-phase AC power supply 9 is supplied to the heater element 16 via the connector 18. When AC power is supplied to the heater element 16 and the heater element 16 heats up, the water in the water pipe 4 is heated and steam is generated.

[0020] The riser pipe 5 circulates the steam generated in the water pipes 4. The riser pipe 5 is a cylindrical member having a steam flow path. The riser pipe 5 connects the water pipes 4 and the upper header 6. The steam generated in the water pipes 4 is supplied to the upper header 6 through the riser pipe 5. The lower end of the riser pipe 5 is connected to the upper end of the water pipe 4. The upper end of the riser pipe 5 is connected to the lower part of the upper header 6. A plurality of riser pipes 5 are provided so as to be connected to each of the plurality of water pipes 4. Each of the plurality of riser pipes 5 is connected to the upper header 6.

[0021] As shown in Figure 2, in the Y-axis direction, the positions of the lower header 3 and water pipes 4 are different from the position of the upper header 6. A portion of the riser pipe 5 is bent. The lower end of the riser pipe 5 is connected to the -Y side surface of the upper part of the water pipe 4. The upper end of the riser pipe 5 is connected to the lower part of the upper header 6.

[0022] The upper header 6 receives the steam supplied from the riser pipes 5. The upper header 6 communicates with the upper ends of the multiple water pipes 4 via the riser pipes 5. The upper header 6 collects the steam supplied from the multiple riser pipes 5. A steam pipe 19 is connected to the top of the upper header 6. A steam valve 20 is disposed on the steam pipe 19. When the steam valve 20 opens, at least a portion of the steam received in the upper header 6 is discharged through the steam pipe 19.

[0023] The water level detection device 8 detects the water level of the water pipe 4. The water level refers to the position of the water surface in the Z-axis direction. The water level detection device 8 includes a detection vessel 21, multiple electrode rods 22 arranged inside the detection vessel 21, a lower connecting pipe 23 connecting the detection vessel 21 to the lower header 3, and an upper connecting pipe 24 connecting the detection vessel 21 to the upper header 6. In this embodiment, three electrode rods 22 are provided. However, for example, four or more electrode rods 22 may be provided. The upper ends of the electrode rods 22 are held on the top lid of the detection vessel 21 via an insulating material. The lower ends of the electrode rods 22 are spaced apart from the detection vessel 21. The positions of the lower ends of the multiple electrode rods 22 in the Z-axis direction are different from one another. The detection vessel 21 is made of a conductive material. The detection vessel 21 functions as a common electrode for the multiple electrode rods 22. Water is contained in the detection vessel 21. The water level in the detection vessel 21 and the water level in the water pipe 4 are substantially equal. When the water contained in the detection container 21 comes into contact with the lower ends of the electrodes 22, electricity is conducted between the detection container 21 and the electrodes 22 via the water. When a current flows through the electrodes 22, the water level detection device 8 determines that the electrodes 22 have come into contact with the water. The water level detection device 8 detects the water level in the detection container 21 based on the presence or absence of a current flowing through each of the multiple electrodes 22. The water level detection device 8 can detect the water level in the water tube 4 by detecting the water level in the detection container 21.

[0024] One electric heater 7 is provided for each water tube 4. If m is a natural number, the number of water tubes 4 that the once-through boiler 1 has is [m x 3]. The number of electric heaters 7 that the once-through boiler 1 has is [m x 3]. In other words, the number of water tubes 4 that the once-through boiler 1 has is a multiple of three. The number of electric heaters 7 that the once-through boiler 1 has is a multiple of three.

[0025] In this embodiment, the once-through boiler 1 has three electric heaters 7. In the following description, the three electric heaters 7 will be referred to as a first electric heater 71, a second electric heater 72, and a third electric heater 73, respectively, as appropriate.

[0026] <Electric heater> Fig. 3 is a side view showing the electric heater 7 according to this embodiment, and Fig. 4 is a bottom view showing the electric heater 7 according to this embodiment.

[0027] The electric heater 7 heats the water in the water pipe 4 to generate steam. The electric heater 7 is a flange heater. The electric heater 7 has multiple heater elements 16 arranged inside the water pipe 4, a flange 17 connected to the upper ends of the heater elements 16, and a connector 18 connected to the top of the flange 17.

[0028] The heater element 16 has a pair of straight portions and a bent portion connecting the lower ends of the pair of straight portions. A flange 17 is fixed to the upper end of the heater element 16. The flange 17 is fixed to the upper end of the water pipe 4. A connector 18 is disposed on the upper part of the flange 17. The connector 18 is connected to a three-phase AC power source 9. AC power from the three-phase AC power source 9 is supplied to the heater element 16 via the connector 18.

[0029] The electric heater 7 is driven by AC power supplied from a three-phase AC power supply 9. One electric heater 7 has at least three heater elements 16. If n is a natural number, the number of heater elements 16 that one electric heater 7 has is [n × 3]. In other words, the number of heater elements 16 that one electric heater 7 has is a multiple of three.

[0030] In this embodiment, one electric heater 7 has three heater elements 16. The three heater elements 16 are assigned to the U phase, V phase, and W phase. In the following description, the three heater elements 16 will be appropriately referred to as the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W, respectively.

[0031] 5 and 6 are diagrams showing examples of wiring of the heater element 16 according to this embodiment. The U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W are connected to each other. As shown in FIG. 5, the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W may be star-connected. As shown in FIG. 6, the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W may be delta-connected.

[0032] <Switch unit> 7 is a wiring diagram showing a three-phase AC power supply 9, a switch unit 30, and an electric heater 7 according to the embodiment. The three-phase AC power supply 9 has an R-phase power supply 9R, an S-phase power supply 9S, and a T-phase power supply 9T.

[0033] 7, the R-phase power supply 9R, the S-phase power supply 9S, and the T-phase power supply 9T are delta-connected. Alternatively, the R-phase power supply 9R, the S-phase power supply 9S, and the T-phase power supply 9T may be star-connected.

[0034] 7, U-phase heater element 16U, V-phase heater element 16V, and W-phase heater element 16W are star-connected. Note that U-phase heater element 16U, V-phase heater element 16V, and W-phase heater element 16W may also be delta-connected.

[0035] The switch unit 30 is disposed between the three-phase AC power supply 9 and the electric heater 7. The switch unit 30 switches the supply state of AC power from the three-phase AC power supply 9 to the electric heater 7. The switch unit 30 is controlled by the controller 10.

[0036] The first electric heater 71, the second electric heater 72, and the third electric heater 73 are connected in parallel to one another to the three-phase AC power supply 9. The switch unit 30 includes a first switch unit 31 that switches the supply state of AC power from the three-phase AC power supply 9 to the first electric heater 71, a second switch unit 32 that switches the supply state of AC power from the three-phase AC power supply 9 to the second electric heater 72, and a third switch unit 33 that switches the supply state of AC power from the three-phase AC power supply 9 to the second electric heater 72. The first electric heater 71 and the three-phase AC power supply 9 are connected via the first switch unit 31. The second electric heater 72 and the three-phase AC power supply 9 are connected via the second switch unit 32. The third electric heater 73 and the three-phase AC power supply 9 are connected via the third switch unit 33.

[0037] The first switch unit 31 has a switch 31U connected to the U-phase heater element 16U of the first electric heater 71, a switch 31V connected to the V-phase heater element 16V of the first electric heater 71, and a switch 31W connected to the W-phase heater element 16W of the first electric heater 71.

[0038] The second switch unit 32 has a switch 32U connected to the U-phase heater element 16U of the second electric heater 72, a switch 32V connected to the V-phase heater element 16V of the second electric heater 72, and a switch 32W connected to the W-phase heater element 16W of the second electric heater 72.

[0039] The third switch unit 33 has a switch 33U connected to the U-phase heater element 16U of the third electric heater 73, a switch 33V connected to the V-phase heater element 16V of the third electric heater 73, and a switch 33W connected to the W-phase heater element 16W of the third electric heater 73.

[0040] <Operation of electric heater> 8, 9, and 10 are diagrams illustrating the operation of the electric heater 7 according to this embodiment. In the examples shown in Fig. 8, 9, and 10, the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W are star-connected.

[0041] Fig. 8 is a diagram showing the state of the switch unit 30 when the output of the electric heater 7 is 100%. Fig. 9 is a diagram showing the state of the switch unit 30 when the output of the electric heater 7 is lower than 100%. Fig. 10 is a diagram showing the state of the switch unit 30 when the output of the electric heater 7 is 0%. In the example shown in Fig. 9, the output of the electric heater 7 is 50%.

[0042] Turning on switches 31U, 31V, 31W, 32U, 32V, 32W, 33U, 33V, and 33W means connecting the three-phase AC power supply 9 and heater element 16, and turning off switches 31U, 31V, 31W, 32U, 32V, 32W, 33U, 33V, and 33W means disconnecting the three-phase AC power supply 9 and heater element 16. In other words, turning on switches 31U, 31V, 31W, 32U, 32V, 32W, 33U, 33V, and 33W means supplying AC power from the three-phase AC power supply 9 to heater element 16, and turning off switches 31U, 31V, 31W, 32U, 32V, 32W, 33U, 33V, and 33W means not supplying AC power from the three-phase AC power supply 9 to heater element 16.

[0043] As shown in FIG. 8, when the electric heater 7 is driven at 100% output, the controller 10 turns on the switches 31U, 31V, and 31W of the first switch unit 31, turns on the switches 32U, 32V, and 32W of the second switch unit 32, and turns on the switches 33U, 33V, and 33W of the third switch unit 33.

[0044] In the state shown in FIG. 8 , AC power is supplied to the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W of the first electric heater 71. AC power is supplied to the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W of the second electric heater 72. AC power is supplied to the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W of the third electric heater 73. Three-phase AC power is supplied to the first electric heater 71, the second electric heater 72, and the third electric heater 73. By supplying three-phase AC power to the first electric heater 71, the second electric heater 72, and the third electric heater 73, the once-through boiler 1 operates at full load. The load factor of the once-through boiler 1 is 100%.

[0045] As shown in FIG. 9, when the electric heater 7 is driven at an output of 50%, which is lower than 100%, the controller 10 turns on switches 31U and 31V of the first switch unit 31 and turns off switch 31W, turns on switches 32V and 32W of the second switch unit 32 and turns off switch 32U, turns on switches 33W and 33U of the third switch unit 33 and turns off switch 33V.

[0046] In the state shown in FIG. 9 , AC power is supplied to the U-phase heater element 16U and the V-phase heater element 16V of the first electric heater 71, but not to the W-phase heater element 16W. AC power is supplied to the V-phase heater element 16V and the W-phase heater element 16W of the second electric heater 72, but not to the U-phase heater element 16U. AC power is supplied to the W-phase heater element 16W and the U-phase heater element 16U of the third electric heater 73, but not to the V-phase heater element 16V. Single-phase AC power is supplied to each of the first electric heater 71, the second electric heater 72, and the third electric heater 73. By supplying single-phase AC power to each of the first electric heater 71, the second electric heater 72, and the third electric heater 73, the once-through boiler 1 operates at partial load. The load factor of the once-through boiler 1 is 50%.

[0047] 10, when the output of the electric heater 7 is to be set to 0%, the controller 10 turns off the switches 31U, 31V, and 31W of the first switch unit 31, turns off the switches 32U, 32V, and 32W of the second switch unit 32, and turns off the switches 33U, 33V, and 33W of the third switch unit 33. Since no AC power is supplied to the first electric heater 71, the second electric heater 72, and the third electric heater 73, the once-through boiler 1 stops operating. The load factor of the once-through boiler 1 is 0%.

[0048] <Effects> As described above, in this embodiment, the switch unit 30 has the switch 31W of the first switch unit 31, the switch 32U of the second switch unit 32, and the switch 33V of the third switch unit 33 that are turned on or off so that the three-phase AC power supply 9 is connected or disconnected to the W-phase heater element 16W of the first electric heater 71, the three-phase AC power supply 9 is connected or disconnected to the U-phase heater element 16U of the second electric heater 72, and the three-phase AC power supply 9 is connected or disconnected to the V-phase heater element 16V of the third electric heater 73. The switch 31W of the first switch unit 31, the switch 32U of the second switch unit 32, and the switch 33V of the third switch unit 33 are examples of first specific switches.

[0049] When driving the first electric heater 71, the second electric heater 72, and the third electric heater 73 at 100% output, the controller 10 turns on the switch 31W of the first switch unit 31, the switch 32U of the second switch unit 32, and the switch 33V of the third switch unit 33. When driving the first electric heater 71, the second electric heater 72, and the third electric heater 73 at 50% output, the controller 10 turns off the switch 31W of the first switch unit 31, the switch 32U of the second switch unit 32, and the switch 33V of the third switch unit 33. The 100% output is an example of a first output. The 50% output is an example of a second output that is lower than the first output.

[0050] According to this embodiment, by turning the first specific switch on or off, the first electric heater 71, the second electric heater 72, and the third electric heater 73 are driven, and the heat generation amount of each of the first electric heater 71, the second electric heater 72, and the third electric heater 73 can be changed. When the once-through boiler 1 is operated at a partial load, AC power is not supplied to the W-phase heater element 16W of the first electric heater 71, AC power is not supplied to the U-phase heater element 16U of the second electric heater 72, and AC power is not supplied to the V-phase heater element 16V of the third electric heater 73. This prevents the once-through boiler 1 from becoming unbalanced, in which the load is biased toward the heater elements 16 of a specific phase.

[0051] As described above, according to this embodiment, the load factor of the once-through boiler 1 can be multi-positioned so as to prevent unbalanced load.

[0052] In the present embodiment, the switch unit 30 includes switches 31U and 31V of the first switch unit 31, switches 32V and 32W of the second switch unit 32, and switches 33W and 33U of the third switch unit 33 that are turned on or off so as to connect or disconnect the three-phase AC power supply 9 to or from the U-phase heater element 16U and the V-phase heater element 16V of the first electric heater 71, to or from the V-phase heater element 16V and the W-phase heater element 16W of the second electric heater 72, and to or from the W-phase heater element 16W and the U-phase heater element 16U of the third electric heater 73. The switches 31U and 31V of the first switch unit 31, the switches 32V and 32W of the second switch unit 32, and the switches 33W and 33U of the third switch unit 33 are examples of second specific switches.

[0053] When the first electric heater 71, the second electric heater 72, and the third electric heater 73 are operated at 100% output, the controller 10 turns on the switches 31U and 31V of the first switch unit 31, the switches 32V and 32W of the second switch unit 32, and the switches 33W and 33U of the third switch unit 33. When the first electric heater 71, the second electric heater 72, and the third electric heater 73 are operated at 50% output, the controller 10 turns on the switches 31U and 31V of the first switch unit 31, the switches 32V and 32W of the second switch unit 32, and the switches 33W and 33U of the third switch unit 33. When the controller 10 stops operation of the first electric heater 71, the second electric heater 72, and the third electric heater 73, it turns off the switch 31W of the first switch unit 31, the switch 32U of the second switch unit 32, and the switch 33V of the third switch unit 33, and also turns off the switches 31U and 31V of the first switch unit 31, the switches 32V and 32W of the second switch unit 32, and the switches 33W and 33U of the third switch unit 33.

[0054] The controller 10 can switch the output of the electric heater 7 between 50% and 0% by turning on or off the second specific switch while the first specific switch is in the off state.

[0055] [Second embodiment] A second embodiment will be described below. In the following description, the same or equivalent components as those in the above-described embodiment are denoted by the same reference numerals, and the description of these components will be simplified or omitted.

[0056] In the first embodiment described above, one electric heater 7 has three heater elements 16. In the present embodiment, an example in which one electric heater 7 has six heater elements 16 will be described.

[0057] FIG. 11 is a diagram illustrating the operation of the first electric heater 71 according to this embodiment. The first electric heater 71 has six heater elements 16. The first electric heater 71 has a first group of elements consisting of three heater elements and a second group of elements consisting of three heater elements. The first group of elements includes a U-phase heater element 16U1, a V-phase heater element 16V1, and a W-phase heater element 16W1, which are interconnected. The second group of elements includes a U-phase heater element 16U2, a V-phase heater element 16V2, and a W-phase heater element 16W2, which are interconnected. The upper ends of the first group of elements and the second group of elements are fixed to the flange 17 described in the first embodiment. The first group of elements and the second group of elements are connected in parallel to the three-phase AC power supply 9. The first group of elements and the second group of elements are disposed inside one water pipe 4.

[0058] 11, U-phase heater element 16U1, V-phase heater element 16V1, and W-phase heater element 16W1 of the first group of elements are mutually star-connected, and U-phase heater element 16U2, V-phase heater element 16V2, and W-phase heater element 16W2 of the second group of elements are mutually star-connected.

[0059] The first switch unit 31 has six switches 31U1, 31V1, 31W1, 31U2, 31V2, and 31W2 connected to the first and second group elements, respectively.

[0060] The switch 31U1 is an example of a first switch that is turned on or off so as to connect or disconnect the three-phase AC power supply 9 and the U-phase heater element 16U1 of the first group elements.

[0061] The switch 31V1 is an example of a second switch that is turned on or off so as to connect or disconnect the three-phase AC power supply 9 and the V-phase heater element 16V1 of the first group elements.

[0062] The switch 31W1 is an example of a third switch that is turned on or off so as to connect or disconnect the three-phase AC power supply 9 and the W-phase heater element 16W1 of the first group elements.

[0063] The switch 31U2 is an example of a fourth switch that is turned on or off so as to connect or disconnect the three-phase AC power supply 9 and the U-phase heater element 16U2 of the second group elements.

[0064] The switch 31V2 is an example of a fifth switch that is turned on or off so as to connect or disconnect the three-phase AC power supply 9 and the V-phase heater element 16V2 of the second group elements.

[0065] The switch 31W2 is an example of a sixth switch that is turned on or off so as to connect or disconnect the three-phase AC power supply 9 and the W-phase heater element 16W2 of the second group elements.

[0066] The controller 10 can change the output of the first electric heater 71 by changing the combination of on and off of the switches 31U1, 31V1, 31W1, 31U2, 31V2, and 31W2.

[0067] The controller 10 can set the output of the first electric heater 71 to 100% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, on, on, on, on).

[0068] The controller 10 can set the output of the first electric heater 71 to 75% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, off, on, on, on).

[0069] The controller 10 can set the output of the first electric heater 71 to 50% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, on, off, off, off). The controller 10 can also set the output of the first electric heater 71 to 50% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, off, on, on, off).

[0070] The controller 10 can set the output of the first electric heater 71 to 25% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, off, off, off, off).

[0071] The controller 10 can set the output of the first electric heater 71 to 0% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (off, off, off, off, off, off).

[0072] 11 shows an example of a first electric heater 71 and a first switch unit 31 connected to the first electric heater 71 according to this embodiment. The structure and operation of the second electric heater 72 and a second switch unit 32 connected to the second electric heater 72 according to this embodiment, and the structure and operation of the third electric heater 73 and a third switch unit 33 connected to the third electric heater 73 are similar to the structure and operation of the first electric heater 71 and the first switch unit 31 connected to the first electric heater 71 according to this embodiment, and therefore description thereof will be omitted.

[0073] As described above, in this embodiment as well, the load factor of the once-through boiler 1 can be set to multiple positions so as to prevent unbalanced load. In this embodiment, the load factor of the once-through boiler 1 is set to five positions.

[0074] [Third embodiment] A third embodiment will be described below. In the following description, the same or equivalent components as those in the above-described embodiment are denoted by the same reference numerals, and the description of these components will be simplified or omitted.

[0075] 12 is a diagram illustrating the operation of a first electric heater 71 according to the present embodiment. The first electric heater 71 and first switch unit 31 according to the present embodiment are modified versions of the first electric heater 71 and first switch unit 31 described in the second embodiment described above. This embodiment differs from the second embodiment described above in that U-phase heater element 16U1, V-phase heater element 16V1, and W-phase heater element 16W1 of the first group elements are mutually delta-connected, and U-phase heater element 16U2, V-phase heater element 16V2, and W-phase heater element 16W2 of the second group elements are mutually delta-connected.

[0076] As in the second embodiment described above, the first switch unit 31 has six switches 31U1, 31V1, 31W1, 31U2, 31V2, and 31W2 connected to the first and second group elements, respectively.

[0077] The controller 10 can change the output of the first electric heater 71 by changing the combination of on and off of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2).

[0078] The controller 10 can set the output of the first electric heater 71 to 100% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, on, on, on, on).

[0079] The controller 10 can set the output of the first electric heater 71 to 66.6% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, on, on, on, off).

[0080] The controller 10 can set the output of the first electric heater 71 to 50% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, on, off, off, off).

[0081] The controller 10 can set the output of the first electric heater 71 to 33.0% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, off, on, on, off).

[0082] The controller 10 can set the output of the first electric heater 71 to 16.6% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (on, on, off, off, off, off).

[0083] The controller 10 can set the output of the first electric heater 71 to 0% by setting the on / off combinations of the six switches (31U1, 31V1, 31W1, 31U2, 31V2, 31W2) to (off, off, off, off, off, off).

[0084] 12 shows an example of a first electric heater 71 and a first switch unit 31 connected to the first electric heater 71 according to this embodiment. The structure and operation of the second electric heater 72 and the second switch unit 32 connected to the second electric heater 72 according to this embodiment, and the structure and operation of the third electric heater 73 and the third switch unit 33 connected to the third electric heater 73 are similar to the structure and operation of the first electric heater 71 and the first switch unit 31 connected to the first electric heater 71 according to this embodiment, and therefore description thereof will be omitted.

[0085] As described above, in this embodiment as well, the load factor of the once-through boiler 1 can be set to multiple positions so as to prevent unbalanced load. In this embodiment, the load factor of the once-through boiler 1 is set to six positions.

[0086] [Fourth embodiment] A fourth embodiment will be described below. In the following description, the same or equivalent components as those in the above-described embodiments are denoted by the same reference numerals, and the description of these components will be simplified or omitted.

[0087] FIG. 13 is a diagram illustrating the operation of the electric heater 7 according to this embodiment. The electric heater 7 and switch unit 30 according to this embodiment are modified versions of the electric heater 7 and switch unit 30 described in the first embodiment. In the first embodiment, the first switch unit 31 has three switches (31U, 31V, 31W), the second switch unit 32 has three switches (32U, 32V, 32W), and the third switch unit 33 has three switches (33U, 33V, 33W). This embodiment differs from the first embodiment in that the first switch unit 31 is composed of one switch, the second switch unit 32 is composed of one switch, and the third switch unit 33 is composed of one switch.

[0088] The first switch unit 31 is an example of a second specific switch that is turned on or off so that the three-phase AC power supply 9 and the U-phase heater element 16U of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the W-phase heater element 16W of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the V-phase heater element 16V of the third electric heater 73 are connected or disconnected.

[0089] The second switch unit 32 is an example of a third specific switch that is turned on or off so that the three-phase AC power supply 9 and the V-phase heater element 16V of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the U-phase heater element 16U of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the W-phase heater element 16W of the third electric heater 73 are connected or disconnected.

[0090] The third switch unit 33 is an example of a first specific switch that is turned on or off so that the three-phase AC power supply 9 and the W-phase heater element 16W of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the V-phase heater element 16V of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the U-phase heater element 16U of the third electric heater 73 are connected or disconnected.

[0091] When the electric heater 7 is driven at 100% output, the controller 10 turns on the first switch unit 31, the second switch unit 32, and the third switch unit 33.

[0092] As shown in FIG. 13, when the electric heater 7 is driven at an output of 50%, which is lower than 100%, the controller 10 turns on the first switch unit 31, turns on the second switch unit 32, and turns off the third switch unit 33.

[0093] When stopping the driving of the electric heater 7, the controller 10 turns off the first switch unit 31, the second switch unit 32, and the third switch unit 33.

[0094] As explained above, in this embodiment as well, the load factor of the once-through boiler 1 can be multi-positioned so as to prevent unbalanced load.

[0095] [Fifth embodiment] A fifth embodiment will be described below. In the following description, the same or equivalent components as those in the above-described embodiments are denoted by the same reference numerals, and the description of these components will be simplified or omitted.

[0096] 14 is a diagram illustrating the operation of the electric heater 7 according to this embodiment. The electric heater 7 and the switch unit 30 according to this embodiment are modified versions of the electric heater 7 and the switch unit 30 described in the above-described fourth embodiment. In the above-described fourth embodiment, the switch unit 30 has a first switch unit 31, a second switch unit 32, and a third switch unit 33. This embodiment differs from the above-described fourth embodiment in that the second switch unit 32 is omitted.

[0097] The first switch unit 31 is an example of a second specific switch that is turned on or off so that the three-phase AC power supply 9 is connected or disconnected to the U-phase heater element 16U and the V-phase heater element 16V of the first electric heater 71, the three-phase AC power supply 9 is connected or disconnected to the W-phase heater element 16W and the U-phase heater element 16U of the second electric heater 72, and the three-phase AC power supply 9 is connected or disconnected to the V-phase heater element 16V and the W-phase heater element 16W of the third electric heater 73.

[0098] The third switch unit 33 is an example of a first specific switch that is turned on or off so that the three-phase AC power supply 9 and the W-phase heater element 16W of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the V-phase heater element 16V of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the U-phase heater element 16U of the third electric heater 73 are connected or disconnected.

[0099] When the electric heater 7 is driven at 100% output, the controller 10 turns on the first switch unit 31 and turns on the third switch unit 33.

[0100] As shown in FIG. 14, when the electric heater 7 is driven at an output of 50%, which is lower than 100%, the controller 10 turns on the first switch unit 31 and turns off the third switch unit 33.

[0101] When stopping the driving of the electric heater 7, the controller 10 turns off the first switch unit 31 and the third switch unit 33.

[0102] As explained above, in this embodiment as well, the load factor of the once-through boiler 1 can be multi-positioned so as to prevent unbalanced load.

[0103] [Sixth embodiment] A sixth embodiment will be described below. In the following description, the same or equivalent components as those in the above-described embodiments are denoted by the same reference numerals, and the description of these components will be simplified or omitted.

[0104] FIG. 15 is a diagram illustrating the operation of the electric heater 7 according to this embodiment. The electric heater 7 and the switch unit 30 according to this embodiment are modified versions of the electric heater 7 and the switch unit 30 described in the fourth embodiment. This embodiment differs from the fourth embodiment in that the once-through boiler 1 has six water tubes 4 and six electric heaters 7. In this embodiment, the electric heaters 7 include a first electric heater 71 arranged on the first water tube 4, a second electric heater 72 arranged on the second water tube 4, a third electric heater 73 arranged on the third water tube 4, a fourth electric heater 74 arranged on the fourth water tube 4, a fifth electric heater 75 arranged on the fifth water tube 4, and a sixth electric heater 76 arranged on the sixth water tube 4.

[0105] In each of the first electric heater 71, the second electric heater 72, the third electric heater 73, the fourth electric heater 74, the fifth electric heater 75, and the sixth electric heater 76, the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W are interconnected.

[0106] As in the above-described fourth embodiment, the first switch unit 31 is composed of one switch, the second switch unit 32 is composed of one switch, and the third switch unit 33 is composed of one switch.

[0107] The first switch unit 31 is an example of a second specific switch that is turned on or off so that the three-phase AC power supply 9 is connected or disconnected to the U-phase heater element 16U of the first electric heater 71 and the U-phase heater element 16U of the fourth electric heater 74, the three-phase AC power supply 9 is connected or disconnected to the W-phase heater element 16W of the second electric heater 72 and the W-phase heater element 16W of the fifth electric heater 75, and the three-phase AC power supply 9 is connected or disconnected to the V-phase heater element 16V of the third electric heater 73 and the V-phase heater element 16V of the sixth electric heater 76.

[0108] The second switch unit 32 is an example of a third specific switch that is turned on or off so that the three-phase AC power supply 9 is connected or disconnected to the V-phase heater element 16V of the first electric heater 71 and the V-phase heater element 16V of the fourth electric heater 74, the three-phase AC power supply 9 is connected or disconnected to the U-phase heater element 16U of the second electric heater 72 and the U-phase heater element 16U of the fifth electric heater 75, and the three-phase AC power supply 9 is connected or disconnected to the W-phase heater element 16W of the third electric heater 73 and the W-phase heater element 16W of the sixth electric heater 76.

[0109] The third switch unit 33 is an example of a first specific switch that is turned on or off so that the three-phase AC power supply 9 is connected or disconnected to the W-phase heater element 16W of the first electric heater 71 and the W-phase heater element 16W of the fourth electric heater 74, the three-phase AC power supply 9 is connected or disconnected to the V-phase heater element 16V of the second electric heater 72 and the V-phase heater element 16V of the fifth electric heater 75, and the three-phase AC power supply 9 is connected or disconnected to the U-phase heater element 16U of the third electric heater 73 and the U-phase heater element 16U of the sixth electric heater 76.

[0110] When the electric heater 7 is driven at 100% output, the controller 10 turns on the first switch unit 31, the second switch unit 32, and the third switch unit 33.

[0111] As shown in FIG. 13, when the electric heater 7 is driven at an output of 50%, which is lower than 100%, the controller 10 turns on the first switch unit 31, turns on the second switch unit 32, and turns off the third switch unit 33.

[0112] When stopping the driving of the electric heater 7, the controller 10 turns off the first switch unit 31, the second switch unit 32, and the third switch unit 33.

[0113] As explained above, in this embodiment as well, the load factor of the once-through boiler 1 can be multi-positioned so as to prevent unbalanced load.

[0114] [Seventh embodiment] A seventh embodiment will be described below. In the following description, the same or equivalent components as those in the above-described embodiments are denoted by the same reference numerals, and the description of these components will be simplified or omitted.

[0115] 16 is a diagram illustrating the operation of the electric heater 7 according to this embodiment. In this embodiment, a first electric heater 71 and a fourth electric heater 74 are arranged in the first water tube 4, a second electric heater 72 and a fifth electric heater 75 are arranged in the second water tube 4, and a third electric heater 73 and a sixth electric heater 76 are arranged in the third water tube 4. In other words, the once-through boiler 1 has three water tubes 4 and six electric heaters 7.

[0116] The switch unit 30 includes a first switch unit 31, a second switch unit 32, a third switch unit 33, a fourth switch unit 34, a fifth switch unit 35, and a sixth switch unit 36. The first switch unit 31 is configured with one switch, the second switch unit 32 is configured with one switch, the third switch unit 33 is configured with one switch, the fourth switch unit 34 is configured with one switch, the fifth switch unit 35 is configured with one switch, and the sixth switch unit 36 ​​is configured with one switch.

[0117] The electric heater 7 includes a first electric heater 71 and a fourth electric heater 74 arranged in the first water tube 4, a second electric heater 72 and a fifth electric heater 75 arranged in the second water tube 4, and a third electric heater 73 and a sixth electric heater 76 arranged in the third water tube 4.

[0118] The relationship between the first, second, and third switch units 31, 32, and 33 and the first, second, and third electric heaters 71, 72, and 73 is the same as that in the fourth embodiment described above with reference to Fig. 13. The fourth switch unit 34 is connected to the fourth electric heater 74. The fifth switch unit 35 is connected to the fifth electric heater 75. The sixth switch unit 36 ​​is connected to the sixth electric heater 76.

[0119] The first switch unit 31 is turned on or off so that the three-phase AC power supply 9 and the U-phase heater element 16U of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the W-phase heater element 16W of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the V-phase heater element 16V of the third electric heater 73 are connected or disconnected.

[0120] The second switch unit 32 is turned on or off so that the three-phase AC power supply 9 and the V-phase heater element 16V of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the U-phase heater element 16U of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the W-phase heater element 16W of the third electric heater 73 are connected or disconnected.

[0121] The third switch unit 33 is turned on or off so that the three-phase AC power supply 9 and the W-phase heater element 16W of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the V-phase heater element 16V of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the U-phase heater element 16U of the third electric heater 73 are connected or disconnected.

[0122] The fourth switch unit 34 is turned on or off to connect or disconnect the three-phase AC power supply 9 to the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W of the fourth electric heater 74.

[0123] The fifth switch unit 35 is turned on or off so as to connect or disconnect the three-phase AC power supply 9 to the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W of the fifth electric heater 75.

[0124] The sixth switch unit 36 ​​is turned on or off so as to connect or disconnect the three-phase AC power supply 9 to the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W of the sixth electric heater 76.

[0125] When the first, second, third, fourth, fifth and sixth switch units 31, 32, 33, 34, 35 and 36 are turned on, the electric heater 7 is operated at 200% output.

[0126] The load factor of the once-through boiler 1 is changed by changing the switch combination in one water tube 4. For example, in the third water tube 4 in which the third electric heater 73 and the sixth electric heater 76 are arranged, by changing the on / off combination of the third switch unit 33 and the sixth switch unit 36, the controller 10 can change the output of the electric heater 7 in the third water tube 4 to 0%, 50%, 100%, 150%, or 200%.

[0127] [Eighth embodiment] An eighth embodiment will be described below. In the following description, the same or equivalent components as those in the above-described embodiments will be denoted by the same reference numerals, and the description of those components will be simplified or omitted.

[0128] 17 is a diagram illustrating the operation of the electric heater 7 according to the present embodiment. The electric heater 7 and switch unit 30 according to the present embodiment are modified versions of the electric heater 7 and switch unit 30 described in the seventh embodiment. This embodiment differs from the seventh embodiment in that the relationship between the first, second, and third switch units 31, 32, and 33 and the first, second, and third electric heaters 71, 72, and 73 is the same as in the fourth embodiment described with reference to FIG. 13, and the relationship between the fourth, fifth, and sixth switch units 34, 35, and 36 and the fourth, fifth, and sixth electric heaters 74, 75, and 76 is also the same as in the fourth embodiment described with reference to FIG. 13.

[0129] Similar to the seventh embodiment described above, the switch unit 30 includes a first switch unit 31, a second switch unit 32, a third switch unit 33, a fourth switch unit 34, a fifth switch unit 35, and a sixth switch unit 36. The first switch unit 31 is configured with one switch, the second switch unit 32 is configured with one switch, the third switch unit 33 is configured with one switch, the fourth switch unit 34 is configured with one switch, the fifth switch unit 35 is configured with one switch, and the sixth switch unit 36 ​​is configured with one switch.

[0130] As in the seventh embodiment described above, the electric heater 7 includes a first electric heater 71 and a fourth electric heater 74 arranged in the first water tube 4, a second electric heater 72 and a fifth electric heater 75 arranged in the second water tube 4, and a third electric heater 73 and a sixth electric heater 76 arranged in the third water tube 4.

[0131] The first switch unit 31 is turned on or off so that the three-phase AC power supply 9 and the U-phase heater element 16U of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the W-phase heater element 16W of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the V-phase heater element 16V of the third electric heater 73 are connected or disconnected.

[0132] The second switch unit 32 is turned on or off so that the three-phase AC power supply 9 and the V-phase heater element 16V of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the U-phase heater element 16U of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the W-phase heater element 16W of the third electric heater 73 are connected or disconnected.

[0133] The third switch unit 33 is turned on or off so that the three-phase AC power supply 9 and the W-phase heater element 16W of the first electric heater 71 are connected or disconnected, the three-phase AC power supply 9 and the V-phase heater element 16V of the second electric heater 72 are connected or disconnected, and the three-phase AC power supply 9 and the U-phase heater element 16U of the third electric heater 73 are connected or disconnected.

[0134] The fourth switch unit 34 is turned on or off so that the three-phase AC power supply 9 and the U-phase heater element 16U of the fourth electric heater 74 are connected or disconnected, the three-phase AC power supply 9 and the W-phase heater element 16W of the fifth electric heater 75 are connected or disconnected, and the three-phase AC power supply 9 and the V-phase heater element 16V of the sixth electric heater 76 are connected or disconnected.

[0135] The fifth switch unit 35 is turned on or off so that the three-phase AC power supply 9 and the V-phase heater element 16V of the fourth electric heater 74 are connected or disconnected, the three-phase AC power supply 9 and the U-phase heater element 16U of the fifth electric heater 75 are connected or disconnected, and the three-phase AC power supply 9 and the W-phase heater element 16W of the sixth electric heater 76 are connected or disconnected.

[0136] The sixth switch unit 36 ​​is turned on or off so that the three-phase AC power supply 9 and the W-phase heater element 16W of the fourth electric heater 74 are connected or disconnected, the three-phase AC power supply 9 and the V-phase heater element 16V of the fifth electric heater 75 are connected or disconnected, and the three-phase AC power supply 9 and the U-phase heater element 16U of the sixth electric heater 76 are connected or disconnected.

[0137] When the first, second, third, fourth, fifth and sixth switch units 31, 32, 33, 34, 35 and 36 are turned on, the electric heater 7 is operated at 200% output.

[0138] The load factor of the once-through boiler 1 is changed by changing the switch combination in one water tube 4. For example, in the third water tube 4 in which the third electric heater 73 and the sixth electric heater 76 are arranged, by changing the on / off combination of the third switch unit 33 and the sixth switch unit 36, the controller 10 can change the output of the electric heater 7 in the third water tube 4 to 0%, 50%, 100%, 150%, or 200%.

[0139] 17, the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W are star-connected. When the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W are delta-connected, the controller 10 can change the output of the electric heater 7 in one water pipe 4 to 0%, 33%, 66%, 100%, 133%, or 200%. [Explanation of symbols]

[0140] 1...Once-through boiler, 2...feedwater device, 3...lower header, 4...water pipe, 5...riser pipe, 6...upper header, 7...electric heater, 8...water level detection device, 9...three-phase AC power supply, 9R...R-phase power supply, 9S...S-phase power supply, 9T...T-phase power supply, 10...controller, 11...feedwater pipe, 12...feedwater pump, 13...feedwater valve, 14...drain pipe, 15...drain valve, 16...heater element, 16U...U-phase heater element, 16U1...U-phase heater element, 16U2...U-phase heater element, 16V...V-phase heater element, 16V1...V-phase heater element, 16V2...V-phase heater element, 16W...W-phase heater element, 16W1...W-phase heater element, 16W2...W-phase heater element, 17...flange, 18...connector, 19...steam pipe, 20...steam valve, 21...detection vessel , 22...electrode rod, 23...lower communicating pipe, 24...upper communicating pipe, 30...switch unit, 31...first switch unit, 31...switch, 31V...switch, 31W...switch, 31U1...switch, 31V1...switch, 31W1...switch, 31U2...switch, 31V2...switch, 31W2...switch, 32...second switch unit, 32U...switch, 32V...switch, 32W...switch, 33...third switch unit, 33U...switch, 33V...switch, 33W...switch, 34...fourth switch unit, 35...fifth switch unit, 36...sixth switch unit, 71...first electric heater, 72...second electric heater, 73...third electric heater, 74...fourth electric heater, 75...fifth electric heater, 76...sixth electric heater.

Claims

1. a first electric heater connected to a three-phase AC power source and having a first phase heater element, a second phase heater element, and a third phase heater element wired together; a second electric heater connected to the three-phase AC power supply and having a first phase heater element, a second phase heater element, and a third phase heater element wired together; a third electric heater connected to the three-phase AC power supply and having a first phase heater element, a second phase heater element, and a third phase heater element wired together; a switch unit that switches a supply state of AC power from the three-phase AC power source to the first electric heater, the second electric heater, and the third electric heater; a controller; the switch unit includes a first specific switch that is turned on or off so that the three-phase AC power supply and a third-phase heater element of the first electric heater are connected or disconnected, the three-phase AC power supply and a first-phase heater element of the second electric heater are connected or disconnected, and the three-phase AC power supply and a second-phase heater element of the third electric heater are connected or disconnected, the controller turns on the first specific switch when the first electric heater, the second electric heater, and the third electric heater are operated at a first output, and turns off the first specific switch when the first electric heater, the second electric heater, and the third electric heater are operated at a second output lower than the first output. Heating device.

2. the switch unit includes a second specific switch that is turned on or off so that the three-phase AC power supply and a first-phase heater element and a second-phase heater element of the first electric heater are connected or disconnected, the three-phase AC power supply and a second-phase heater element and a third-phase heater element of the second electric heater are connected or disconnected, and the three-phase AC power supply and a third-phase heater element and a first-phase heater element of the third electric heater are connected or disconnected, the controller turns on the first specific switch and the second specific switch when operating the first electric heater, the second electric heater, and the third electric heater at a first output, turns off the first specific switch and turns on the second specific switch when operating them at a second output lower than the first output, and turns off the first specific switch and the second specific switch when stopping operation. The heater device according to claim 1 .

3. each of the first electric heater, the second electric heater, and the third electric heater includes a first group element having a first phase heater element, a second phase heater element, and a third phase heater element wired together, and a second group element having a first phase heater element, a second phase heater element, and a third phase heater element wired together; The heater device according to claim 1 .

4. The switch unit a first switch that is turned on or off to connect or disconnect the three-phase AC power source and the first-phase heater element of the first group element; a second switch that is turned on or off to connect or disconnect the three-phase AC power supply and the second-phase heater element of the first group element; a third switch that is turned on or off so as to connect or disconnect the three-phase AC power supply and the third-phase heater element of the first group element; a fourth switch that is turned on or off so as to connect or disconnect the three-phase AC power supply and the first-phase heater element of the second group element; a fifth switch that is turned on or off so as to connect or disconnect the three-phase AC power supply and the second-phase heater element of the second group element; a sixth switch that is turned on or off so as to connect or disconnect the three-phase AC power supply and the third-phase heater element of the second group element, the controller changes the combination of on and off of the first switch, the second switch, the third switch, the fourth switch, the fifth switch, and the sixth switch to change the outputs of the first electric heater, the second electric heater, and the third electric heater. The heater device according to claim 3 .

5. the first specific switch is configured by one switch, The switch unit a second specific switch that is turned on or off so that the three-phase AC power supply and a second-phase heater element of the first electric heater are connected or disconnected, the three-phase AC power supply and a third-phase heater element of the second electric heater are connected or disconnected, and the three-phase AC power supply and a first-phase heater element of the third electric heater are connected or disconnected; a third specific switch that is turned on or off so that the three-phase AC power supply and a first-phase heater element of the first electric heater are connected or disconnected, the three-phase AC power supply and a second-phase heater element of the second electric heater are connected or disconnected, and the three-phase AC power supply and a third-phase heater element of the third electric heater are connected or disconnected, the controller turns on the first specified switch, the second specified switch, and the third specified switch when operating the first electric heater, the second electric heater, and the third electric heater at a first output, turns off the first specified switch and turns on the second specified switch and the third specified switch when operating them at a second output lower than the first output, and turns off the first specified switch, the second specified switch, and the third specified switch when stopping operation. The heater device according to claim 1 .

6. the first specific switch is configured by one switch, The switch unit a second specific switch that is turned on or off so that the three-phase AC power source and the second-phase heater element and the first-phase heater element of the first electric heater are connected or disconnected, the three-phase AC power source and the third-phase heater element and the second-phase heater element of the second electric heater are connected or disconnected, and the three-phase AC power source and the first-phase heater element and the third-phase heater element of the third electric heater are connected or disconnected, the controller turns on the first specific switch and the second specific switch when operating the first electric heater, the second electric heater, and the third electric heater at a first output, turns off the first specific switch and turns on the second specific switch when operating them at a second output lower than the first output, and turns off the first specific switch and the second specific switch when stopping operation. The heater device according to claim 1 .

7. a fourth electric heater connected to the three-phase AC power supply and having a first phase heater element, a second phase heater element, and a third phase heater element wired together; a fifth electric heater connected to the three-phase AC power supply and having a first phase heater element, a second phase heater element, and a third phase heater element wired together; a sixth electric heater connected to the three-phase AC power supply and having a first phase heater element, a second phase heater element, and a third phase heater element wired together; the first specific switch is configured by one switch, the first specific switch is turned on or off such that the three-phase AC power supply is connected or disconnected to a third-phase heater element of the first electric heater and a third-phase heater element of the fourth electric heater, the three-phase AC power supply is connected or disconnected to a first-phase heater element of the second electric heater and a first-phase heater element of the fifth electric heater, and the three-phase AC power supply is connected or disconnected to a second-phase heater element of the third electric heater and a second-phase heater element of the sixth electric heater; The switch unit a second specific switch that is turned on or off so that the three-phase AC power supply is connected or disconnected to a second-phase heater element of the first electric heater and a second-phase heater element of the fourth electric heater, the three-phase AC power supply is connected or disconnected to a third-phase heater element of the second electric heater and a third-phase heater element of the fifth electric heater, and the three-phase AC power supply is connected or disconnected to a first-phase heater element of the third electric heater and a first-phase heater element of the sixth electric heater; a third specific switch that is turned on or off so that the three-phase AC power supply is connected or disconnected to a first-phase heater element of the first electric heater and a first-phase heater element of the fourth electric heater, the three-phase AC power supply is connected or disconnected to a second-phase heater element of the second electric heater and a second-phase heater element of the fifth electric heater, and the three-phase AC power supply is connected or disconnected to a third-phase heater element of the third electric heater and a third-phase heater element of the sixth electric heater, the controller turns on the first specified switch, the second specified switch, and the third specified switch when operating the first electric heater, the second electric heater, the third electric heater, the fourth electric heater, the fifth electric heater, and the sixth electric heater at a first output, turns off the first specified switch and turns on the second specified switch and the third specified switch when operating them at a second output lower than the first output, and turns off the first specified switch, the second specified switch, and the third specified switch when stopping operation. The heater device according to claim 1 .

8. A plurality of water pipes; a lower header communicating with lower ends of the plurality of water pipes; an upper header communicating with upper ends of the plurality of water pipes; The heater device according to claim 1, a first electric heater is disposed in the first water pipe; a second electric heater is disposed on the second water pipe; a third electric heater is disposed on the third water pipe; Once-through boiler.

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