Once-through boiler

Abstract

To increase a steam generation amount of a once-through boiler utilizing an electric heater and to increase the number of positions of a load factor.SOLUTION: A once-through boiler includes a water pipe, a lower header, an upper header, an electric heater arranged in the water pipe, and a switch unit 30 for switching a supply state of AC power from a three phase AC power supply 9 to the electric heater. The electric heater includes a first group element 711, 721, 731 having a first U-phase heater element 16U, a V-phase heater element 16V, and a W-phase heater element 16W, which are mutually connected, and a second group element 712, 722, 732 having a second U-phase heater element 16U, a V-phase heater element 16V, and a W-phase heater element 16W, which are mutually connected. The switch unit includes a first switch that operates to connect or disconnect the three phase AC power supply and the first group element, and a second switch that operates to connect or disconnect the three phase AC power supply and the second group element.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to 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 arranged in a water tube. [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] In once-through boilers in which electric heaters are installed in the water tubes, there is a demand for technology that can increase the amount of steam generated and enable multiple load factors.

[0005] The technology disclosed in this specification aims to increase the amount of steam generated by a once-through boiler that uses an electric heater and to enable the load factor to be varied in multiple positions. [Means for solving the problem]

[0006] This specification discloses a once-through boiler. The once-through boiler includes water tubes, a lower header communicating with the lower ends of the water tubes, an upper header communicating with the upper ends of the water tubes, an electric heater disposed on the water tubes, and a switch unit for switching the supply state of AC power from a three-phase AC power source to the electric heater. The electric heater includes a first group of elements having a first U-phase heater element, a first V-phase heater element, and a first W-phase heater element wired together, and a second group of elements having a second U-phase heater element, a second V-phase heater element, and a second W-phase heater element wired together. The switch unit includes a first switch operable to connect or disconnect the first group of elements to or from the three-phase AC power source, and a second switch operable to connect or disconnect the second group of elements to or from the three-phase AC power source. [Effects of the Invention]

[0007] According to the technology disclosed in this specification, it is possible to increase the amount of steam generated by a once-through boiler that uses an electric heater and to achieve multi-position load factors. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a front view schematically showing a once-through boiler according to an embodiment. [Figure 2] FIG. 2 is a side view that schematically shows a once-through boiler according to an embodiment. [Figure 3] FIG. 3 is a diagram showing an example of wiring of the heater element according to the embodiment. [Figure 4] FIG. 4 is a diagram showing an example of wiring of the heater element according to the embodiment. [Figure 5] FIG. 5 is a wiring diagram showing a three-phase AC power supply, a switch unit, and an electric heater according to the embodiment. [Figure 6] FIG. 6 is a wiring diagram showing a switch unit 30 and an electric heater 7 according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] 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.

[0010] 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.

[0011] [Once-through boiler] FIG. 1 is a front view schematically showing a once-through boiler 1 according to an embodiment. FIG. 2 is a side view schematically showing the once-through boiler 1 according to an embodiment. Each of FIG. 1 and FIG. 2 shows a cross section of a portion of the once-through boiler 1. In the 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] The electric heater 7 heats the water in the water pipes 4 to generate steam. The electric heater 7 is a flange 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] The water level detection device 8 detects the water level of the water tube 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 container 21, multiple electrode rods 22 arranged inside the detection container 21, a lower connecting pipe 23 connecting the detection container 21 to the lower header 3, and an upper connecting pipe 24 connecting the detection container 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 upper lid of the detection container 21 via an insulating material. The lower ends of the electrode rods 22 are spaced apart from the detection container 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 container 21 is made of a conductive material. The detection container 21 functions as a common electrode for the multiple electrode rods 22. Water is contained in the detection container 21. The water level in the detection container 21 and the water level in the water tube 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.

[0022] One electric heater 7 is disposed for each water tube 4. In the embodiment, the once-through boiler 1 has three water tubes 4 and three electric heaters 7. In the following description, the three water tubes 4 will be appropriately referred to as the first water tube 41, the second water tube 42, and the third water tube 43, and the three electric heaters 7 will be appropriately referred to as the first electric heater 71, the second electric heater 72, and the third electric heater 73.

[0023] The first electric heater 71 is disposed in the first water pipe 41. The second electric heater 72 is disposed in the second water pipe 42. The third electric heater 73 is disposed in the third water pipe 43.

[0024] [Electric heater] The electric heater 7 is driven by AC power supplied from a three-phase AC power supply 9. One electric heater 7 has six heater elements 16. Of the six heater elements 16, two heater elements 16 are assigned to the U phase, two heater elements 16 are assigned to the V phase, and two heater elements 16 are assigned to the W phase. In the following description, the heater elements 16 assigned to the U phase will be referred to as U-phase heater elements 16U, the heater elements 16 assigned to the V phase will be referred to as V-phase heater elements 16V, and the heater elements 16 assigned to the W phase will be referred to as W-phase heater elements 16W.

[0025] 3 and 4 are diagrams showing examples of wiring of the heater element 16 according to the 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. 3, 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. 4, the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W may be delta-connected.

[0026] 5 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.

[0027] 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.

[0028] The first electric heater 71 has six heater elements 16. The first electric heater 71 has a first group element 711 consisting of three heater elements 16 and a second group element 712 consisting of three heater elements 16. The first group element 711 has a first U-phase heater element 16U, a first V-phase heater element 16V, and a first W-phase heater element 16W, which are connected to each other. The second group element 712 has a second U-phase heater element 16U, a second V-phase heater element 16V, and a second W-phase heater element 16W, which are connected to each other. The first group element 711 and the second group element 712 are connected to each other in parallel with the three-phase AC power supply 9. The first group element 711 and the second group element 712 are arranged inside the first water pipe 41.

[0029] The second electric heater 72 has six heater elements 16. The second electric heater 72 has a third group element 721 consisting of three heater elements 16 and a fourth group element 722 consisting of three heater elements 16. The third group element 721 has a third U-phase heater element 16U, a third V-phase heater element 16V, and a third W-phase heater element 16W, which are connected to each other. The fourth group element 722 has a fourth U-phase heater element 16U, a fourth V-phase heater element 16V, and a fourth W-phase heater element 16W, which are connected to each other. The third group element 721 and the fourth group element 722 are connected to each other in parallel with the three-phase AC power supply 9. The third group element 721 and the fourth group element 722 are arranged inside the second water pipe 42.

[0030] The third electric heater 73 has six heater elements 16. The third electric heater 73 has a fifth group element 731 consisting of three heater elements 16 and a sixth group element 732 consisting of three heater elements 16. The fifth group element 731 has a fifth U-phase heater element 16U, a fifth V-phase heater element 16V, and a fifth W-phase heater element 16W, which are connected to each other. The sixth group element 732 has a sixth U-phase heater element 16U, a sixth V-phase heater element 16V, and a sixth W-phase heater element 16W, which are connected to each other. The fifth group element 731 and the sixth group element 732 are connected to each other in parallel with the three-phase AC power supply 9. The fifth group element 731 and the sixth group element 732 are arranged inside one water tube 4.

[0031] In addition, in each of the first group element 711, the second group element 712, the third group element 721, the fourth group element 722, the fifth group element 731, and the sixth group element 732, the U-phase heater element 16U, the V-phase heater element 16V, and the W-phase heater element 16W may be delta-connected.

[0032] 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.

[0033] The first electric heater 71, the second electric heater 72, and the third electric heater 73 are connected in parallel with one another to the three-phase AC power supply 9. The switch unit 30 has a first switch 311 arranged between the three-phase AC power supply 9 and a first group element 711 of the first electric heater 71, a second switch 312 arranged between the three-phase AC power supply 9 and a second group element 712 of the first electric heater 71, a third switch 321 arranged between the three-phase AC power supply 9 and a third group element 721 of the second electric heater 72, a fourth switch 322 arranged between the three-phase AC power supply 9 and a fourth group element 722 of the second electric heater 72, a fifth switch 331 arranged between the three-phase AC power supply 9 and a fifth group element 731 of the third electric heater 73, and a sixth switch 332 arranged between the three-phase AC power supply 9 and a sixth group element 732 of the third electric heater 73.

[0034] The first switch 311 operates to connect or disconnect the three-phase AC power supply 9 and the first group element 711. The second switch 312 operates to connect or disconnect the three-phase AC power supply 9 and the second group element 712. The third switch 321 operates to connect or disconnect the three-phase AC power supply 9 and the third group element 721. The fourth switch 322 operates to connect or disconnect the three-phase AC power supply 9 and the fourth group element 722. The fifth switch 331 operates to connect or disconnect the three-phase AC power supply 9 and the fifth group element 731. The sixth switch 332 operates to connect or disconnect the three-phase AC power supply 9 and the sixth group element 732.

[0035] Turning on the switches (311, 312, 321, 322, 331, 332) means connecting the three-phase AC power supply 9 and the heater element 16, and turning off the switches means disconnecting the three-phase AC power supply 9 and the heater element 16. In other words, turning on the switches means supplying AC power from the three-phase AC power supply 9 to the heater element 16, and turning off the switches means not supplying AC power from the three-phase AC power supply 9 to the heater element 16.

[0036] [Electric heater operation] Next, the operation of the electric heater 7 will be described. When operating the first electric heater 71 at 100% output, the controller 10 turns on both the first switch 311 and the second switch 312. When operating the first electric heater 71 at 50% output, the controller 10 turns on one of the first switch 311 and the second switch 312 and turns off the other. When operating the first electric heater 71 at 0% output, the controller 10 turns off both the first switch 311 and the second switch 312.

[0037] When operating the second electric heater 72 at 100% output, the controller 10 turns on each of the third switch 321 and the fourth switch 322. When operating the second electric heater 72 at 50% output, the controller 10 turns on one of the third switch 321 and the fourth switch 322 and turns off the other. When operating the second electric heater 72 at 0% output, the controller 10 turns off each of the third switch 321 and the fourth switch 322.

[0038] When the controller 10 operates the third electric heater 73 at 100% output, it turns on both the fifth switch 331 and the sixth switch 332. When the controller 10 operates the third electric heater 73 at 50% output, it turns on one of the fifth switch 331 and the sixth switch 332 and turns off the other. When the controller 10 sets the third electric heater 73 to 0% output, it turns off both the fifth switch 331 and the sixth switch 332.

[0039] 100% output is an example of a first output. 50% output is an example of a second output that is lower than the first output. 0% output is an example of a third output that is lower than the second output.

[0040] The controller 10 controls the switch unit 30 so that the outputs of the multiple electric heaters 7 (71, 72, 73) are the same. The controller 10 operates each of the multiple electric heaters 7 (71, 72, 73) simultaneously at 100% output. When the controller 10 operates the first electric heater 71 at 100% output, the controller 10 also operates the second electric heater 72 and the third electric heater 73 at 100% output. The controller 10 operates each of the multiple electric heaters 7 (71, 72, 73) simultaneously at 50% output. When the controller 10 operates the first electric heater 71 at 50% output, the controller 10 also operates the second electric heater 72 and the third electric heater 73 at 50% output. The controller 10 simultaneously sets each of the multiple electric heaters 7 (71, 72, 73) to 0% output. When the controller 10 sets the first electric heater 71 to 0% output, it also sets the second electric heater 72 and the third electric heater 73 to 0% output.

[0041] When the electric heaters 7 (71, 72, 73) operate at 100% output, the load factor of the once-through boiler 1 is 100%. The once-through boiler 1 operates at full load.

[0042] When the electric heaters 7 (71, 72, 73) operate at 50% output, the load factor of the once-through boiler 1 is 50%. The once-through boiler 1 operates at partial load.

[0043] When the electric heaters 7 (71, 72, 73) are at 0% output, the load factor of the once-through boiler 1 is 0%, and the once-through boiler 1 stops operating.

[0044] [effect] As described above, in the embodiment, one electric heater 7 (71, 72, 73) has a first group of elements (711, 721, 731) having a first U-phase heater element 16U, a first V-phase heater element 16V, and a first W-phase heater element 16W that are interconnected, and a second group of elements (712, 722, 732) having a second U-phase heater element 16U, a second V-phase heater element 16V, and a second W-phase heater element 16W that are interconnected. The switch unit 30 has first switches (311, 321, 331) that operate to connect or disconnect the three-phase AC power supply 9 and the first group elements (711, 721, 731), and second switches (312, 322, 332) that operate to connect or disconnect the three-phase AC power supply 9 and the second group elements (712, 722, 732).

[0045] According to the embodiment, six heater elements 16 are arranged on one water tube 4. When both the first switch (311, 321, 331) and the second switch (312, 322, 332) are turned on and AC power is supplied to both the first group elements (711, 721, 731) and the second group elements (712, 722, 732), the six heater elements 16 generate heat, thereby increasing the output of the electric heater 7 compared to when three heater elements 16 are arranged on one water tube 4. This increases the amount of steam generated by the once-through boiler 1. Furthermore, when one of the first switch (311, 321, 331) or the second switch (312, 322, 332) is turned on and the other is turned off, the output of the electric heater 7 is reduced to 50%. When both the first switch (311, 321, 331) and the second switch (312, 322, 332) are turned off, the output of the electric heater 7 becomes 0%. When the switch unit 30 is controlled, the load factor of the once-through boiler 1 is made multi-positionable.

[0046] The once-through boiler 1 has a plurality of water tubes 4 (41, 42, 43) and a plurality of electric heaters 7 (71, 72, 73). The controller 10 controls the switch unit 30 so that the outputs of the plurality of electric heaters 7 (71, 72, 73) are the same. 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.

[0047] [Other embodiments] In the above-described embodiment, one electric heater 7 has two group elements. For example, the first electric heater 71 has a first group element 711 and a second group element 712. One electric heater 7 may have three group elements, four group elements, or any number of group elements equal to or greater than five. One group element has a U-phase heater element 16U, a V-phase heater element 16V, and a W-phase heater element 16W, which are wired together.

[0048] FIG. 6 is a wiring diagram showing a switch unit 30 and an electric heater 7 according to another embodiment. As shown in FIG. 6, for example, a first electric heater 71 may have a first group element 711, a second group element 712, and a third group element 713. The switch unit 30 has a first switch 311 arranged between the three-phase AC power source 9 and the first group element 711, a second switch 312 arranged between the three-phase AC power source 9 and the second group element 712, and a third switch 313 arranged between the three-phase AC power source 9 and the third group element 713. The first switch 311 operates to connect or disconnect the three-phase AC power source 9 and the first group element 711. The second switch 312 operates to connect or disconnect the three-phase AC power source 9 and the second group element 712. The third switch 313 operates to connect or disconnect the three-phase AC power source 9 and the third group element 713. Similar to the first electric heater 71, the second electric heater 72 and the third electric heater 73 may also have at least three group elements.

[0049] In the above-described embodiment, one group element is a three-phase heater having a U-phase heater element 16U, a V-phase heater element 16V, and a W-phase heater element 16W that are wired together. That is, the electric heater 7 has a plurality of three-phase heaters. The electric heater 7 may also have a single-phase heater. One electric heater 7 may also have a three-phase heater and a single-phase heater. That is, one electric heater 7 may be provided with a plurality of three-phase heaters, or may also be provided with a three-phase heater and a single-phase heater. [Explanation of symbols]

[0050] 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, 16V...V-phase heater element, 16W...W-phase heater element, 17...flange, 18...connector, 19...steam pipe, 20...steam valve, 21...detection vessel, 22 ...electrode rod, 23...lower connecting pipe, 24...upper connecting pipe, 30...switch unit, 41...first water pipe, 42...second water pipe, 43...third water pipe, 71...first electric heater, 72...second electric heater, 73...third electric heater, 311...first switch, 312...second switch, 321...third switch, 322...fourth switch, 331...fifth switch, 332...sixth switch, 711...first group element, 712...second group element, 721...third group element, 722...fourth group element, 731...fifth group element, 732...sixth group element.

Claims

1. Water pipes and a lower header communicating with a lower end of the water pipe; an upper header communicating with an upper end of the water pipe; an electric heater disposed in the water pipe; a switch unit that switches the supply state of AC power from a three-phase AC power source to the electric heater, The electric heater is a first group of elements including a first U-phase heater element, a first V-phase heater element, and a first W-phase heater element that are wired together; a second group of elements including a second U-phase heater element, a second V-phase heater element, and a second W-phase heater element that are wired together; The switch unit a first switch that operates to connect or disconnect the three-phase AC power supply and the first group elements; a second switch that operates to connect or disconnect the three-phase AC power supply and the second group elements; Once-through boiler.

2. Equipped with a controller, The controller turning on each of the first switch and the second switch when operating the electric heater at a first output; When the inverter is operated at a second output lower than the first output, one of the first switch and the second switch is turned on and the other is turned off.

2. The once-through boiler according to claim 1.

3. The water pipe is provided in plurality, the first group elements and the second group elements are arranged in each of the plurality of water tubes, The controller simultaneously operating each of the plurality of electric heaters at a first output; operating each of the plurality of electric heaters at a second output simultaneously; 3. The once-through boiler according to claim 2.

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