NL1010813C2 - A microcontroller controlled device that generates steam in a controlled manner by means of electricity without the use of separate sensors. - Google Patents

Description

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A microcontroller controlled device that generates steam in a controlled manner by means of electricity without the use of separate sensors.

The invention relates to an electronic, microcontroller-controlled device with which a certain amount of steam is produced per unit time. Depending on the design of the device, it is possible with this invention to control the pressure and humidity of the leaving steam.

The current state of the art in producing steam using electricity for steam cleaners or saunas, for example, is described below. An electric instantaneous water heater is brought to approximately a desired temperature by means of mechanical thermostats mounted on the outside or by temperature sensors connected to an electronic circuit. When the desired temperature is reached, the device is ready for use.

10 Using a manual switch or switching an electronic output, a pump or valve is activated through which water is pumped into the instantaneous water heater. The inflowing water is heated to the boiling point in the first part of the passage by the flow-through heater, after which the water is converted into steam over most of the passage by means of the energy supplied. If the passing water is not completely evaporated, the non-evaporated water in the water vapor is entrained to the outlet of the steam generator. The water / steam ratio determines the humidity. There are also steam generators consisting of a boiler with a volume of half to a few liters. There is an electric heating element at the bottom of the boiler. With these steam generators, the energy supply to the heating element is switched off by means of a pressure sensor when the pressure of the boiler exceeds a certain value or when the temperature becomes too high. Most versions have an open connection to the outside air.

The aforementioned devices according to the current state of the art present the following drawbacks: 1. In modern appliances and systems in which steam has to be generated, use is often made of rather bulky electric instantaneous water heaters which, especially if they are designed for service, with a power supply from a 230 volt mains connection, suffer from a long warm-up time. This applies to a great extent to the aforementioned kettle versions, in which all the water must first be brought to a boil.

1010813¾ -2- 2. In addition, these steam generators use thermostats mounted on the outside of the instantaneous water heater. These thermostats have the disadvantage that they often react slowly because of the mass and specific heat of the thermostat itself and the heat resistance between the flow heater and the thermostat, which largely determine the response time.

3. Furthermore, there are large tolerances in the measurement accuracy and a large hysteresis of these thermostats.

4. In addition, switching these thermostats is accompanied by sparks, which limits the life of these parts.

10 5. Due to the relatively slow heating of the heating elements, layers of scale settle, which can lead to poor heat transfer to the water. This can also lead to blockages. With the current steam generators, this effect is delayed by the use of filters in the water supply. The service life can be extended by periodically descaling.

The invention solves the aforementioned drawbacks by using a flow heater with a small volume and a large electrical power density to transfer the required energy in the form of heat to the water.

The mass of the flow-through heater is small, so that little energy is required to bring the flow-through heater itself up to temperature. The resistance in which the supplied electric energy 20 is converted into heat is arranged such that the available heat can be transferred very directly to the water. To obtain a very fast and accurate control, the positive temperature coefficient of said resistance is used to determine the temperature. The principles of the steam generator are explained with the help of the figures and applications are explained.

Figure 1 shows the principle of an electric instantaneous water heater based on a heating element using a dissipating resistor made in thick film technology, whereby the generated heat is transferred very directly to the rest of the element. Figure 2 shows the block diagram of an electronic circuit that will explain the principle of temperature measurement and the operation of the steam generator control.

Figure 3 shows the design of a steam generator that draws the water to be evaporated from a pressureless reservoir.

Figure 4 shows a circuit in which the power supply to the heating element is switched off in the event of a malfunction of the microcontroller or in which the electrical control of the heating element is interrupted if an error occurs in the said circuit.

Figure 5 shows the side of a heating element on which a resistance track is arranged between thin insulating and protective layers.

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Figure 6 shows a spiral labyrinth in which the water flows as it is heated by the underlying heating element.

Figure 7 shows the side of a heating element on which a resistance track as in Figure 5 is arranged between thin insulating and protective layers, but here the resistance track is spiral-shaped.

Figure 8 shows a spiral-shaped labyrinth in which the water flows while it is heated by the underlying heating element, which, in contrast to that in Figure 6, is spiral-shaped.

Figure 9 shows graphs of the temperature distribution of the water in the flow-through heater at four different pumping speeds as a function of the position in the spiral labyrinth.

Figure 10 shows a graph of the resistance increase with respect to the resistance of a certain heating element at 0 ° C as a function of the 'optimal flow' (F), in which all the pumped water at the outlet of the flow heater has all evaporated.

In order to realize a steam generator where an electric heating track can be used as a temperature sensor, it is necessary that the lowest possible thermal resistance and thermal capacity is present between the heating track and the water to be heated, so that a short response time is present. Figure 1 shows a heating element (2) based on thick film technology. With this type of heating element it is currently possible to dissipate powers up to approximately 60 watts per cm2.

The thick film resistor (23) (fig. 5) is thermally and mechanically coupled by a thin electrical insulator to a slightly convex carrier (2) (fig. 1) which can be made of stainless steel (stainless steel). The thickness of the stainless steel is approximately 1 mm. The electrical connection to the heating element is made with the aid of an adapter (6) (fig. 1), which establishes the connection to the ends of the heating track (24) (fig. 5) with spring contacts. The supply voltage is connected to terminals (7) (fig. 1) 25. The flow-through heater (figure 1) is created by a convex cover (3), which is equipped with a water inlet (1) and a steam outlet (5), using a welded connection (4) on the heating element, the lower spherical segment ( 2), to pair. Depending on the application, the mentioned bulb segments (2 + 3) can also be joined by a round V-shaped clamp with a layer of elastic plastic or rubber that is suitable for higher temperatures. Due to the rapid heating of the stainless steel, any limescale layers on the water side break down into small pieces. This breaking off is caused by the creation of mechanical stresses between the stainless steel and the limescale, because the heating element expands earlier and faster by heating up. The use of a water inlet filter is therefore not necessary.

In order to improve the operation of the flow-through heater shown in figure 1, a spiral object with a high melting point and a high thermal resistance and a low specific -4 heat can be placed in this space, which ensures that at (1) (see also 26 (fig. 6) or 28 (fig. 8)) water supplied along the outside of this spiral flows in a spiral labyrinth to the center and in the form of steam and possibly together with unevaporated water the outlet (5) (see also 27 (Fig. 6) or 29 (Fig. 8)). An advantage of the use of a spiral is that the flow-through heater can also be used at an inclination angle of more than 45 ° without creating hot spots on the heating element. This is thanks to the spiral that ensures that the water cannot fall back to one side of the flow heater. The said spiral labyrinth (25) (Fig. 6) may be an integral part of a plastic cover for forming the flow-through heater (Fig. 1) together with the heating element (2). In Figure 2, the current 10 through the heating element (15) is measured by converting the current through the low-impedance measuring resistor (14) into a voltage. This measured AC voltage is applied to the electronic circuit (12) which slightly amplifies the measured voltage and then converts it into a DC voltage proportional to the AC current through the measuring resistor (14). At a constant supply voltage (11), the current through the measuring resistor (14) is influenced by the temperature variation over the heating track of the heating element (15), which is the result of the positive temperature coefficient of the resistance material of the heating track.

Calculations show that a voltage change of approximately 5 millivolts per degree Kelvin across the measuring resistor (14) can occur with a 1500 watt heating element. Since the measuring resistance (14) has a resistance value which is a factor of 100 to 200 less than the track resistance of the heating element (15), only a measuring resistance suitable for a low power is required here.

Since in practice the supply voltage (11) can vary considerably, an electronic and software error correction is provided. In order to be able to carry out the said correction in the microcontroller (8), a direct voltage is applied to the microcontroller (8) via the electronic circuit (9), which is proportional to the prevailing voltage on the heating element. Circuit (10) provides the power for the electronic circuits of said control. With the help of the potentiometer (13) (or by means of a value stored in the microcontroller), which is controlled by a reference voltage (Vref), the desired temperature can be set and maintained by the control. With the help of a user interface, consisting of some buttons and a display, which are connected to the microcontroller (not shown), certain settings (such as pressure, steam humidity and flow) can be made and the status can be checked via the display. the device is.

If the desired calculated temperature drops undesirably, this indicates the supply of too much water. However, the microcontroller (8) can maintain the temperature by adjusting the pump power to a lower value. Due to the mentioned control properties of the invention, the system recognizes within a few tenths of a second that the water supply stagnates due to a defective pump or pump control, or simply because the water has run out.

10108.3 * 5-

When the temperature thus ascertained, the microcontroller will supply the power by means of Stop triac Tri to the heating element (15). This gives a very quick dry boil protection. In the control system described so far, the temperature cannot be measured after switching off the heating element (15). By heating again after a short pause, while the pump remains activated, it can be established whether the temperature has dropped.

If this is not the case, the breaks will have to be extended. Depending on the application, it is always possible to connect an external temperature sensor to the flow heater, with which the microcontroller can measure the temperature development (with a longer response time) without the need for energy supply to the heating track. Using, for example, an optical triac Tr2 (preferably with zero-crossing detection against mains contamination), the microcontroller (8) can quickly switch the water pump (16) on and off to control the water flow at the inlet (1) (fig. 1) . Triac Tri (and also triac Tr2) is controlled by the software by the microcontroller (8) in such a way that after a control interruption a positive half period of the sine voltage (0 -] 80 °) is followed by a negative half period (180 - 360 °), and vice versa, so that network pollution 15 is prevented. During the said regulation of the electric power supplied to the heating element, the measured voltage on the measuring resistor (14) will not be a continuous sine shape. However, since circuit (12) uses the amplitude of the voltage, the temperature measurement can be continued normally, it being understood that due to the time constant for smoothing the signal from the rectifier of circuit (12) this output voltage will drop, which must be corrected in the software 20. The software therefore takes into account a measurement error that arises from changing the measured mains voltage, due to a changed load, which is passed on by circuit (9) to the microcontroller (8). Figure 3 shows schematically a possible application of the invention, which could be applied in a steam cleaner, sauna or any other device which requires a stable supply of steam for a good operation. The device provides a pump (18) controlled by the electronics (21), which can build up sufficient pressure to supply water even when the steam pressure rises, the flow heater (19) and, if this is desired in the device, an electronically adjustable pressure relief valve (20), thereby achieving that the pressure in the flow-through heater (19) will rise to a value equal to the set pressure value of the pressure relief valve by further temperature rise, as a result of which the pressure relief valve will open and steam will escape with an increased pressure. Due to the spread in the resistance values of the different heating elements produced, the present invention provides a system for automatically calibrating the temperature during the start-up of the device, for example each time at start-up. This uses the boiling point of water under atmospheric conditions. To this end, if present in the device, the pressure relief valve (20) 35 is opened electronically completely.

1 U,.; S.

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The pump (18) is put into operation by the electronics (21) until the flow heater of figure 1 is filled with more than 50% with water, so that the hollow heating element is completely filled. Then electric energy is sent to the flow heater (19) at maximum power. By the described method of measuring temperature, it will be found that the temperature of the heating element (2) (fig. 1) will rise.

This rise will continue until the water in the flow-through heater reaches the boiling point. The temperature of the water will then remain stable (100 ° C) until all the water has evaporated. The boiling point may then be slightly higher due to the rise in pressure due to the resistance of the steam through the passage through the outlet (5) and the connected hoses. The temperature of the heating track (23) (fig. 5) on the heating element (2) (fig. 1) will be slightly higher because of the heat resistance between the heating track and the water to be evaporated. It is then possible to calibrate very precisely by regulating the power to the heating element so far that the temperature difference mentioned becomes minimal and the water still boils. The derived measured value is stored in a non-volatile memory and can then be used as a reference point to also be able to measure other temperatures by calculation, since the properties of the heating element are then known.

Since the calibration mentioned above was also carried out when the device was tested in the factory, a temperature determination can already be carried out within a second after the heating element has been switched on. The result of this measurement is a good approximation for the water temperature in the reservoir (17) (fig. 3). Since the energy required to heat 1 gram of water from 20 ° C to 100 ° C ^ 20 is only 334 Joules (-12.9%), while the said heating and evaporation (at 100 ° C) of this amount is 2590 Joule of energy costs, an error of 10 ° C in the determination of water temperature in the reservoir (17) (fig. 3) would lead to a percentage error, in the determination of the amount of water that is supplied with a certain amount of energy will evaporate, from maximum

25 J34W - lorei. | Ooa =, | OT

2590 [7] (100 - 20) [C]

This calibration makes it possible to calculate the pressure in the flow heater (19) (fig. 3) when using the electronically controlled pressure relief valve, or when closing the solenoid outlet either intentionally or unintentionally. As the pressure increases, the boiling point will also rise. At a boiling point of 100 ° C, the absolute pressure is 101.3.103 Pa. For example, if the maximum pressure may be three times higher (304.103 Pa), the new cooking temperature may not exceed 133.8 ° C. Depending on the speed at which the temperature rises, it can be determined whether there is a lack of water or whether there is too high a pressure. In the latter case, the electronics will be able to guarantee safety through the current to the heating element using triac tri switch off.

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The said temperature measurement / pressure determination is in principle also possible with conventional heating elements in a boiler (see current state of the art), taking into account the longer response times and therefore possible occurrence of dangerous situations.

As mentioned, with said flow-through heater and electronics steam with a predetermined mass per unit time, a predetermined humidity and a predetermined pressure can be generated. When controlling the quantities mentioned, the pump (16 (fig. 2) and 18 (fig. 3)) will pump a certain amount of water per minute into the flow-through heater. This water will be led to the center of the heating element via the spiral labyrinth (figure 6 and figure 8). At a certain set power to the heating element, the microcontroller 10 calculates how much water can be converted into steam per minute at a certain pressure. The desired pressure is set (if present in the device) with the electronic pressure reducing valve. Figure 5 and Figure 7 show two embodiments of the thick film resistors on the heating element. The advantage of the heating element in figure 7 and figure 8, because the heating track runs exactly under the spiral labyrinth, is that the temperature course in the water 15 from the inlet (28) to the outlet (29) increases with the temperature of the resistance track, so that this track does not have large temperature transitions, while in figure 5 and figure 6 the resistance track (23) runs along places where the temperature of the water can be very different. The trace of the heating element of figure 7 therefore has a longer life. If the pump (16 (fig. 2) and 18 (fig. 3)) pumps too little water at a certain power into the instantaneous water heater, all the water 20 will already reach the output (see 5 (fig. 1)). , 27 (Fig. 6) and 29 (Fig. 8)) have been converted to steam. Since the heat transfer to steam is much worse than to water, the temperature of the heating element will rise sharply in those places. The resistance of the heating track is in fact determined by three temperature ranges (Figure 9): 1. The first part of the spiral labyrinth, the water is heated from the entry temperature T | N to 25 at the boiling point.

2. The section where the energy available there is used to evaporate the boiling water.

3. The last part of the spiral labyrinth where the temperature of the heating element can be considerably higher than the boiling temperature at the prevailing pressure.

Figure 9 shows the temperature trend graphically.

Graph B shows the graph in which the amount of water pumped into the flow heater (Figure 1) has just evaporated when the outlet (5) is reached. At an inflow temperature of 20 ° C of the water at the inlet of the flow heater (1) and a boiling point of 100 ° C, heating to this boiling point takes 12.9% of the available power. This power is used in the first 13% of the passage through the spiral labyrinth. In Graph A in Figure 9, the flow is reduced by a factor of 2, reducing the trajectory in the labyrinth in which the water boils to half the trajectory in Graph B.

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The result is that just after half (about 56% of the trajectory through the spiral) all the water has evaporated. As a result, the temperature rises sharply in this area. Graphs C and D respectively give a flow that is twice and about five times as great as for graph B. For graphs B to D, the steam humidity is increasing. For example, in graph D, only half of the water has evaporated.

5 The purpose of Graph E is to show that at higher pressures, more time is required to bring the water to the higher boiling point, forcing the water to flow further into the labyrinth to absorb the required energy. However, the flat part (where the water boils and is converted into slow) of graph E is shorter than the flat part of graph A because the evaporation value at a higher pressure is lower.

The resistance of the heating element is due to the sum of the resistances of the three mentioned temperature ranges. Figure 10 shows how the resistance to the resistance at 0 ° C, represented by the vertical axis, depends on the flow and the prevailing pressure (at atmospheric pressure or when using an electronically controlled pressure relief valve (20) (fig. .3)), at constant power. When starting from a heating element according to figure 7 and figure 8, the heating track at a certain position is at approximately the same temperature as the water that flows past on the other side of the heating element. For each print a graph can be derived as shown in figure 10. On the horizontal axis, the term 'optimal flow (F)' refers to the flow where all the water supplied has completely evaporated during the complete run through of the spiral labyrinth. The break point in the graph in Figure 10, at F = 1, is a reference point for the control. In the case of an open connection (atmospheric pressure) to the outside air, the resistance calculated by the microcontroller can be related to a steam humidity of 0% at a boiling point corresponding to the prevailing atmospheric pressure. With this method, by determining the resistance at a 'kink', at the prevailing pressure, a fast method is available for determining the aforementioned 'optimal flow (F)'. From the determination of this break point in the graph, the microcontroller can increase the steam humidity by increasing the pump power, or by reducing the power to the heating element. When the power is reduced, less water can be converted into steam, so that the control is no longer at a point of 'optimal flow' on a graph. For example, halving the power input to the heating element will shift the operating point of the control to the point 2x the 'optimum flow' (F).

30 Deviations from the theory described here in a particular version of a device, caused by the differences in resistance that occur in the hoses and fittings connected to the output (5) (fig. 1), are corrected per software type by means of software. of measurements of the properties of the respective device type.

I 3 3 3 J

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Since in figure 2 the triac Tri has a certain chance of malfunctioning, which would keep it constantly conducting, an external safety thermostat is normally required that switches off the voltage when a maximum temperature is exceeded. With devices using the invention described here and an extension of the electronics according to figure 4 described below, it is not necessary to install a safety thermostat.

Relay RY1 (figure 4) has a make contact ryl with which the heating element (15) (fig. 2) can be switched off in the event of a fault.

The relay will drop out if the program in the microcontroller is not or is no longer properly executed.

In that case, the lower connecting pin of capacitor C1 will no longer be provided with a square signal 10. Transistor T3 will thereby turn off, so that C2 will then charge to such an extent that the base current of transistor T1 becomes so low that this transistor will also turn off. This will cause the relay RY1 to drop and open contact ry 1.

This means that the voltage supply to the heating element is broken. The safety circuit of Figure 4 is tested very regularly by the microcontroller for proper operation. The voltages on the 15 collectors of all transistors are controlled by the microcontroller interface (22) on all kinds of combinations of the two control signals from the microcontroller interface to the transistors T2 and T3. Any detected deviation will cause the relay RY1 and triac Tri to switch off. In addition, as already mentioned, the total system is designed in such a way that, even at a certain limit, increasing pressure, at too high a temperature (thus also with boiling dry), will very quickly interrupt the power supply to the heating element. The proper functioning of triac Tri is tested after every switch-off by checking that no current flows through the measuring resistor (14) (fig. 2).

By using a microcontroller and suitable software, it has become possible to generate steam in a controlled and integrally safe manner by means of the above-mentioned method.

25! w;

Claims (16)

1. A microcontroller-controlled steam generator, characterized in that the measured current through the heating element and the measured voltage across the heating element are used to control the properties of the steam, so that the use of separate sensors is not necessary. 5 An electronic system according to claim 1, characterized in that the influence on the measurement result, caused by fluctuations in the voltage, is compensated by the program executed in the microcontroller using the measured voltage across the heating element. An electronic system according to one or more of the preceding claims, characterized in that measuring errors caused by mutual differences in the resistance of produced heating elements can be prevented by having a short automatic calibration periodically carried out, by water under atmospheric pressure in the bring the flow-through heater to the boil, measuring the flow through the heating element. An electronic system according to one or more of the preceding claims, characterized in that the temperature of the inflowing water can be approximated by filling the flow-through heater with a sufficient amount of water and when the voltage across the heating element is switched on. take a measurement immediately. An electronic system according to one or more of the preceding claims, characterized in that a control system 25 is realized by using a flow heater with a low mass and low specific heat and a high power per unit area and a relatively small volume. is that it can react quickly to fluctuations, so that the set desired values can be efficiently maintained. 6. An electronic system according to one or more of the preceding claims, characterized in that the water is led from the water inlet to the steam outlet via a spiral or otherwise shaped labyrinth, through which the water and steam are fed through the flow-through heater via a defined route. flows, so that at a certain power and flow, the temperature can be theoretically derived at any point, which temperature development determines the total resistance of the heating track of the flow heater. -11 - An electronic system according to one or more of the preceding claims, characterized in that the temperature of the water flowing past can be determined at any position when the measurements are performed by the microcontroller. An electronic system according to one or more of the preceding claims, characterized in that the microcontroller can determine a point by changing the flow at a certain input power, at which all the water after the passage through the flow heater has been converted into steam, called the 'optimum flow' (F), which is detected by a more pronounced decrease in flow than normal, due to the temperature rise in the region of the steam output of the flow heater, as it decreases further the flow. 9. An electronic system according to one or more of the preceding claims, characterized in that, due to the property that the microcontroller can influence the power supplied to the through-flow heater, the system can adjust the steam humidity to a desired value at a given unchanging water flow. arrange for. 10. An electronic system according to one or more of the preceding claims, characterized in that, through the property that the microcontroller can influence the pump power, at a certain unchanging electric power, the system can regulate the steam humidity to a desired value. An electronic system according to one or more of the preceding claims, characterized in that, due to the use of a labyrinth in the flow-through heater, due to the fact that the water cannot flow away to one side of the flow-through heater, the steam generator is inclined at approximately 25 inclines. 45 ° can be used. An electronic system according to one or more of the preceding claims, characterized in that when the boiling-dry or when the flow-through heater is tilted too far, the temperature will rise rapidly as a result of the rapid rise, and the electronics will immediately rise. ensure that the voltage of the heating element is switched off. % - 12- An electronic system according to one or more of the preceding claims, characterized in that when the steam outlet is shut off the rise in pressure and the consequent rise in the resistance of the heating element will be signaled by the microcontroller, which will lead to disconnection of the power supply to the heating element. An electronic system according to one or more of the preceding claims, characterized in that the life of the heating element can be extended by making the heating track uniform to the labyrinth so that there are no large temperature differences, and mechanical stresses in the heating track thereby caused originate. An electronic system according to one or more of the preceding claims, characterized in that by using said heating elements based on thick film technology, the use of a supply filter is not necessary, since the stainless steel is heated quickly, and the rapid local expansion of the stainless steel, breaking off any limescale layers present on the water side into small pieces, whereby these small pieces are carried out with the steam. 16. An electronic system according to any one or more of the preceding claims, characterized in that, in principle, in order to guarantee safety, no separate pressure sensor or overtemperature sensor is required, since the electronics described already monitor these matters during normal operation, while in the event of a malfunction in the electronics itself, an error in the program run in the microcontroller will be detected by the rest of the electronics, while an error in the latter part of the electronics will be detected by periodic readings performed by the microcontroller, which will lead to disconnection of power to the instantaneous water heater. 0 0 0 g / 3 i