WO2007017558A1

WO2007017558A1 - Steam generator

Info

Publication number: WO2007017558A1
Application number: PCT/FI2006/050353
Authority: WO
Inventors: Veikko-Matti Kallioniemi
Original assignee: F.E.M. Limited
Priority date: 2005-08-10
Filing date: 2006-08-10
Publication date: 2007-02-15
Also published as: CN101304718A; CN101304718B; EP1915120A1; FI117665B; EP1915120A4; FI20055431A0

Abstract

A steam generator e.g. for saunas comprises a water tank (2), a heating element (5) for evaporating the present in the water tank, as well as a temperature sensor that is located outside the tank and in a heat transfer connection with the wall of the tank to monitor the water level inside the tank. The heating element (5) is connected to a thermostat (3) containing a temperature sensor in a heat-conducting manner through a heat conductor (1) separated from the outer wall of the tank (2). By means of the invention it is possible to prevent the drying of the tank by utilizing a sensor to monitor the temperature of the wall of the tank, which is dependent on the location of the water level.

Description

Steam generator

The invention relates to a steam generator comprising a water tank, a heating element for evaporating the water present in the water tank, as well as a temperature sensor that is located outside the tank and in a heat transfer connection with the wall of the tank to monitor the water level inside the tank.

The steam generator described herein is intended primarily for use in sau- nas, but it can also be used for example in households in applications where hot steam is required, for example in steam irons.

Preventing of the drying of the water tank of the steam generator has been one of the major problems for the manufacturers of steam generators. At- tempts have been made to monitor the water level by means of known mechanical and electronic level sensors positioned inside the tank, which sensors do, however, have the drawback that deposits resembling boiler scale generated inside the tank disturb the function of the sensors and complicate the operation of the mechanisms. These deposits may for example cover the electronic sensor, thus entirely preventing its function.

Another solution is to allow the steam generator to operate while the water tank is empty until the thermostat on the bottom cuts off the operation of the heating element. This however damages the heating element and shortens its lifespan, because its parts are exposed to very high temperatures. Furthermore, the dried water tank may produce unpleasant odours in the environment.

It is known to use temperature sensors to find out whether the water level in the tank has sunk below a certain level. The use of such sensors is based on the fact that when the part of the tank at the location of the sensor is heated, its temperature depends on whether the heated point is above the water level

(higher temperature) or below the water level (lower temperature), because the heat of evaporation taken by the water keeps the temperature even. So- lutions operating on this principle are disclosed for example in US patent

5881207, European patent 193863 and US patent 4881493. US patent 5881207 and corresponding European patent 772000 disclose a pipe (30) sunk inside the tank at the location of the desired water level, said pipe comprising a temperature sensor (12) and a resistor (13) to be heated with electric current. Even though the deposits are not capable of directly affecting this sensor, the heating does not work any more if the resistor happens to break, and when the water lever descends below the pipe, the temperature does not rise, and warning of the drying is not received. There is also the structural drawback that the sensor pipe must be sunk in.

European patent EP-193863, in turn, discloses a steam generator, in which heating resistors (5, 6, 7) are placed outside the wall of the tank, on a suitable height. Temperature sensors (16, 17, 18) are also placed on the same height, and they are used for monitoring the temperature of the wall, which is dependent on the location of the water level. Thus, the placement of the heating resistors is determined by the desired water level, which restricts the design of the steam generator. It is often desirable that the heating resistors can be placed below the tank.

A structure and an operating principle similar to the one described above is also disclosed in US patent 4881493, in which a heating resistor (9) is wound on the outer surface of a cylindrical tank in the form of a screw thread, and a temperature sensor (10) is wound at the location of the upper part of the resistor, in parallel with the same.

The purpose of the invention is to disclose a simpler solution that does not require separate heating next to the temperature sensor, and in which the placement of the heating elements is not dependent on the location of the level to be monitored. To attain this purpose, the steam generator according to the invention is primarily characterized in that that the heating element is connected to the temperature sensor in a heat-conducting manner through a heat conductor separated from the outer wall of the tank. In a way, this heat conductor forms a bridge, via which heat is brought at a desired height (at the location of the water level to be monitored) in the wall of the tank, at which height the temperature sensor is also located. The heating element can thus be located for example underneath the tank, attached to its bottom, which is often an advantageous location.

The solution according to the present invention is not affected by deposits in the tanks or possible malfunction of the separate heating element, because heat brought by conduction outside the tank from the same heating element by means of which the heat necessary for evaporation is brought into the tank.

The point where the heat conductor is connected to the wall of the tank determines the level to be monitored. When the water level is above the connecting point, the heat coming to the connecting point via the conductor is transmitted through the heat-conductive wall into the mass of water present inside, said mass having significant thermal capacity and heat of evaporation. When the water sinks below this point, the end of the heat conductor connected to the wall as well as the wall heat up rapidly, because there is no longer water at the same height inside the wall, which would absorb the heat. The temperature sensor at the same point detects the rapid temperature increase above a predetermined threshold value and reports this to the super- visor/user, or if the temperature sensor forms a part of a thermostat, the thermostat performs an automatic control action (feeding of supplementary water into the tank and/or cutting off the heating). Yet another advantage of the invention is that it is not necessary to provide lead-through connections or sunk portions inside the tank, but all structural and functional parts are lo- cated outside the tank.

According to a preferred embodiment, two thermostats are used, said thermostats having different operating points, wherein the thermostat operating at higher temperature functions as an emergency switch, if the main ther- mostat fails.

The invention will be described in the following with reference to the appended drawings, in which:

Fig. 1 shows a steam generator from above,

Fig. 2 shows the steam generator from the front, Fig. 3 shows the steam generator from the side (right-hand side of

Fig. 2),

Fig. 4 shows the steam generator from below,

Fig. 5 shows a cross-section of the steam generator along the line A- Al of Fig. 1 ,

Fig. 6 shows an isometric view of the steam generator, and

Fig. 7 shows the temperature at the measuring point as a function of the height of the water level.

The system according to the invention can be used in various forms, depending on the use. The following description only contains one embodiment, given as an example, without restricting the invention. The water tank disclosed in the Figures also contains a steam outlet conduit and a water supply conduit, and a cover closing the tank, which are not shown.

Fig. 1 shows four different alternatives (marked with reference numerals 1-1 , 1-2, 1-3, 1-4) for placing the heat conductor 1. The heat conductor 1 is a metal plate or strip that can be bent into a desired shape, and whose one end can be connected to the heating element 5, and the other end to the side wall of the water tank 2 in the steam generator at a desired height in such a manner that it transmits heat by conduction from the heating element 5 to the side wall of the tank. Between the ends, the heat conductor 1 is isolated from the tank 2, in the Figures in such a manner that there is an air gap between them, but it is also possible that insulations are used.

In the following, reference is made to a solution in which the heat conductor 1 is connected to the right-hand side wall of the tank 2 (alternative 1-1). At least one temperature sensor is positioned at the point where the end of the heat conductor 1 is connected to the side wall of the tank 2, said temperature sen- sor monitoring the temperature at said point of connection on the above-described principle. At the point of connection the heat conductor 1 is located against the wall of the tank within an area of certain size, and in this area the sensor is positioned on top of the heat conductor 1. As shown in the figures, the sensor belongs to a thermostat 3 that forms a switch for controlling the feeding of heat and/or water. In the embodiments of the figures, two thermostats 3, 4 are positioned in parallel at the same height at the point of con- nection, wherein the thermostat 3 operating at lower temperature constitutes a main switch and the thermostat 4 operating at higher temperature constitutes an emergency switch. Thus, the second thermostat 4 is not necessarily required.

As the figures show, the thermostat 3/thermostats 3, 4 are located in that portion of the bent plate- or strip-like heat conductor 1 that is attached against the wall of the tank 2, parallelly to the wall.

The plate or strip composed of conductive metal and serving as heat conductor 1 is designed and positioned in such a manner that as small amount of heat as possible is wasted when heat is transmitted from the heating element 5 to the connecting point, in which the sensor or thermostat is located. The plate or strip is positioned against the heating element 5 located on the bottom outside the tank 2 so that it covers a sufficiently large part of the surface of the element 5 in view of heat transfer, in contact with the different sections of the elongated element 5 within sufficient lengths as shown in Fig. 4. As the figures show, the heating element 5 can be a conventional resistor to be heated with electricity. The plate or strip is bent upward, and this up- ward directed section is located sufficiently far from the wall of the tank 2 so that heat is not capable of transferring to the wall. At the end by the connecting point the plate or strip is bent upward to form a straight section, which is located against the wall and forms a connecting point to transfer heat to the wall. The part located within a distance from the tank can have the shape of the letter C that opens up toward the tank, or as shown by the alternative in Fig. 2, the bend can be made in an acute angle so that the part is directed diagonally upward toward the connecting point. The point, where the conductor 1 touches the tank again, is positioned at such a height (level 7 in Fig. 5) in relation to the bottom so that a sufficient amount of water always re- mains in the tank, even if the water level descends below this height. The height can be for example twice the desired minimum height of water. The tank is composed of metal, wherein its wall conducts heat well.

The water tank can be equipped with a suitable casing, by means of which it is also possible to cover the heat conductor 1 and the sensor/thermostat. The heat conductor and the sensor/thermostat can be placed on that side of the tank which is most appropriate in view of the placement and use of the steam generator (alternatives shown in Fig. 1).

Fig. 7 shows the temperature as a function of the height of the water level. When the water level is above the contact point of the conductor 1 (level 7), the temperature measured by the sensor is at the boiling point of water when water is evaporated in the tank. When the water level descends below the level 7, the temperature rises rapidly, and when it reaches a predetermined switching temperature that exceeds the boiling point, the sensor gives either a message or if it constitutes a part of the thermostat 3, the thermostat cuts off the heating automatically and/or brings about feeding of additional water to the tank. The operating point of the thermostat may be for example at least at 110° C. The thermostat is reset automatically at the lower temperature.

The second thermostat 4, which is a safety thermostat, operates at higher temperature, advantageously at temperature at least 25° C higher than the main thermostat 3. It is not automatically reset, but must be reset manually, because the main thermostat 3 does not in that case function properly, and it must be replaced.

The actual control actions on the basis of the readings of the sensor/thermostat 3 can be conducted automatically or manually. It is possible that only an alarm is given when a predetermined threshold value (temperature) is exceeded, and water is thereafter added manually, or the reading of the sensor that exceeds this threshold value automatically activates the water feeding device in order to add water into the tank. In both cases the reading of the sensor advantageously also cuts off the heating, i.e. supply of energy to the heating element 5. The functions can be easily implemented by means of known thermostats.

The invention is not restricted to the embodiments described above, but it can be modified within the scope of the inventive idea presented in the claims. It is possible that there are more than one sensor or thermostat at different heights, wherein it is possible to bring a separate heat conductor 1 of its own at the location of each of them from the heating element 5 or the heat conductor 1 coming from the same heating element 5, can branch off into branches ending up at different heights. The structural solutions of the heat conductor and the placement of the sensors/thermostats can be similar to those described above.

The term side wall refers to all kinds of walls which extend in the vertical direction (direction of variation of the water level), irrespective of the shape of the tank. According to the above-described embodiment, the horizontal section of the tank 2 is rectangular, but it may also have another shape. The water tank 2 may be for example cylindrical.

The invention is especially suitable for steam generation in saunas, but it can also be applied in other applications in which steam is required, especially in steam generators for households and kitchens.

Claims

Claims:

1. A steam generator comprising a water tank (2), a heating element (5) for evaporating the water present in the water tank, as well as a temperature sensor that is located outside the tank and in a heat transfer connection with the wall of the tank to monitor the water level inside the tank, characterized in that the heating element (5) is connected to the temperature sensor in a heat-conducting manner through a heat conductor (1) separated from the outer wall of the tank (2).

2. The steam generator according to claim 1 , characterized in that the heating element (5) is positioned outside the tank (2) on the bottom of the same, and that the heat conductor (1) is brought from the bottom up to a pre- determined height on the side wall of the tank.

3. The steam generator according to claim 1 or 2, characterized in that the temperature sensor is positioned on top of the heat conductor (1) in the point where the heat conductor is connected to the outer wall of the tank (2).

4. The steam generator according to any of the preceding claims, characterized in that the heat conductor (1) is composed of a metal plate or strip, which is bent in such a manner that the section between its ends is off the outer wall of the tank.

5. The steam generator according to any of the preceding claims, characterized in that the temperature sensor forms a part of an automatic thermostat (3).

6. The steam generator according to claim 5, characterized in that there are two thermostats, a main thermostat (3) and a safety thermostat (4).

7. The use of a steam generator according to any of the preceding claims for generation of steam in a sauna.