Field of the invention

The present invention relates to an electrode boiler for providing hot water or steam and a method for operating the electrode boiler. Background

An electrode boiler includes a number of electrodes, one for each phase, immersed in a water bath. Heat is generated by passing an AC electrical current between the electrodes or to a counter electrode using the water as conductor. The boilers are designed for low voltage ((230 - 690 V) or high voltage (1 - 36 kV) supply and with heat ratings from about 0 - 100 MW. Electrode boilers have many advantages, such as an efficiency approaching 100%, easy control of output range and a fast startup.

The output of the boiler may be controlled with the water level covering more or less of the electrodes. The output may be shut off either with switches or by lower- ing the water level out of reach of the electrodes. Each method has its merits. The switches are subject to wear as they have to handle large currents. By lowering the water level below the electrodes, the boiler may be shut off avoiding switches alto gether. However, when water is drained and gets below the electrode tips, an arc may be created. This arc may reach another electrode directly or via metal parts of the boiler. Such a situation may lead to a full short circuit.

From GB 472,479 there is known a steam producing electrode boiler wherein the bottom of the electrodes terminate in an "arc deflecting pot". In case the level of water in the boiler drum should drop below the electrodes, the pot secures that the tips of the electrodes is still surrounded by water preventing the current from being broken causing the formation of arcs.

Summary of the invention

It is an object of the present invention to provide an electrode boiler that may be shut off by lowering the water level without the risk of creating an arc and thereby short circuit the power supply. This is achieved in a boiler as defined in the appended claim 1. In particular, the inventive electrode boiler includes a container holding water, a number of elec trodes, each electrode including a first upper end connected to a source of electric power and a second lower end submerged in the water, and means for controlling the water level in said container. Said means include a circulation pump supplying water to the container through a supply line, and a throttle valve. Furthermore, the supply line is connected to an arc stopper located beneath the second lower end of each electrode, the arc stopper comprising a receptacle made of an electrically in sulating material, wherein an upper rim of said side wall is positioned level with or just above the second lower end of the associated electrode.

The electrode boiler is operated using he method according to claim 2 and 3. In particular, when shutting off the boiler by dropping the water level inside the con tainer, the presence of the insulating arc stopper will effectively prevent the for mation of arcs. Brief description of the drawings

The advantages of the invention will become apparent by reading the following de tailed description in reference to the appended drawings, in which

Fig. 1 is a general overview of a boiler installation showing its main components,

Fig. 2 shows a detail of a boiler according to the present invention. Detailed description

The invention relates to an electrode boiler, and the embodiment of the invention described in the following paragraphs is intended to supply hot water. As shown in Fig. 1, the boiler 1 is connected in a closed water loop to a circulation pump 2 and a heat exchanger 3. The pump circulates water continuously through the boiler, in which it is heated, the produced heat being removed in the heat exchanger 3 whereupon the now colder water is sent back to the boiler for reheating. In a corre sponding embodiment for steam generation, the heat exchanger will be omitted and the steam delivered directly to a pipe network.

The boiler includes a boiler drum 4. Inside the drum 4 there is mounted an inner container 5 on electrically insulated brackets. A number of electrodes 7 are sus- pended from the roof of the drum, in conductive rods passing through insulated bushings. The rods are connected to a source of AC electric power by means of conductors 10.

Furthermore, the boiler system includes a 3-way throttle valve 6 controlling the flow in the closed water loop including the circulation pump 2 and the supply line 16 supplying water to the container 5, a level safety gauge 8 and level control system 9, a pressure safety valve 11, a throttle valve 14 for draining water from the inner container 5, a processed feed water inlet 12 and a blow down outlet line 13.

The inner container 5 holds water at a predefined level. The water level is con- trolled by the throttle valve 14, from readings from a level gauge. The production from the boiler may be controlled by changing the water level. The water is sup plied through the supply line 16 which branches out into a number of small tubes entering the bottom of the inner container 5.

Fig. 2 is a schematic illustration of the invention. Below each electrode 7 there are mounted arc stoppers 15. Each arc stopper comprises a receptacle of an electrically insulating material, such a ceramic material (e.g. porcelain), glass or PFTE. The arch stoppers may be positioned at the bottom of the container 5. The arc stoppers are mounted to ensure that the top rim of the receptacle is positioned on level with, or preferably a bit above the lower end of the associated electrode 7. In the bottom of each arc stopper there is an opening connecting the receptacle to the water sup ply line 16.

When it is desired to stop production, the water in the inner container 5 is drained via the throttle valve 14. The water outside the arc stopper 15 will drain away com pletely, while water inside the arc stopper will remain at level with the upper rim of the receptacle and maintain in contact with the electrode 7. Then, the circulation pump 2 supplying water through the supply line 16 is stopped, whereupon the wa ter level will drop also inside the arc stoppers 15. At some point the water will lose contact with the electrode 7. This is the moment when an arc could occur. Howev er, the presence of the arc stopper means that the arc will have to climb an insulat- ing barrier to reach another electrode or the metal of the surrounding container 4. This will effectively prevent the formation of an arc.

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