NL1010064C2 - Regulating and checking for safety aspects of heating systems for liquids using electric heating elements as a sensor. - Google Patents

Description

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Regulating and checking for safety aspects of heating systems for liquids using electric heating elements as a sensor.

The invention relates to an electronic microcontroller controlled system with which the temperature of liquids (usually water) and solids, the heat content of a certain volume of liquid, the limescale on electric heating elements, defects of heating elements and incorrect commissioning of heating elements can be determined, whereby use is made of the heating elements used in the appliance or system as temperature sensors. By using a microcontroller within an electronic circuit specially designed for this purpose, the control is integrally safe.

Modern devices and systems in which liquids (usually water) must be heated use temperature sensors specially made for that purpose.

10 Appliances can include electric boilers, kettles, coffee makers, washing machines, dishwashers and the like.

Many versions of these devices use the cubic expansion coefficient of liquids. With this system, a small reservoir is mounted in a tube which is located in the environment to be heated. The reservoir is connected via a capillary tube to a second reservoir with an easily movable membrane on one side. By heating the first-mentioned reservoir, the volume of the second reservoir increases due to expansion. When the membrane has undergone a certain preset displacement, a mechanically coupled electrical switch goes from the conductive (closed contacts) to the non-conductive state. This switching takes place at a temperature determined by a rotary knob which is mechanically coupled to the said switch. When the desired temperature is reached, the current to the heating elements is therefore interrupted. After the temperature has dropped, the said thermostat switch reaches the point that it switches back to the conductive state. In this way, the temperature can be maintained within certain limits.

Most of the said appliances have an over-temperature protection, often in combination with the aforementioned thermostat controller, which functions in substantially the same way. This overtemperature protection has its own independent liquid reservoir, capillary and switching system. The desired switching temperature is fixed at a high factory setting. yet just acceptable value. If the temperature becomes too high due to the failure of the thermostat switch, the over-temperature protection will interrupt the voltage supply. The power supply can only be restored by pressing a reset button on the overtemperature protection after the temperature has dropped sufficiently.

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Nowadays, electronic control systems are often built into the said types of devices. The electronics in these devices obtain information about the prevailing temperature using sensors such as PTCs and NTCs. (resistors with a certain positive or negative temperature coefficient as a function of temperature). The outputs of the electronics are equipped with 5 relays or triacs to control the heating elements. An electronic control is able to achieve and maintain the desired temperature more accurately. Furthermore, electronic controls offer more comfort to the user. These controls often use a microprocessor that communicates with the user via display and pushbuttons. Despite the electronic control mentioned, in connection with safety requirements (EN 60335-2-21), use is usually made of the said 10 mechanical over-temperature protection, which in the event of said fault interrupts used phases of the grid.

Electric water heaters usually use the cheaper night power tariff. The desired final temperature of the water in the boiler has been set by the user or the installer. A commonly used temperature is 60 ° C. In practice, a boiler can be set at a temperature between 35 ° C and 85 ° C, which can be reached in one night. When hot water is drawn, which has reached a certain temperature during the night-time heating phase, this water will at the top of the barrel through a pipe and at the bottom of the barrel are replaced by cold inflowing water. There will then be hot water at the top of the vessel (with properly insulated boilers, a temperature drop of up to 1 Kelvin per 24 hours will occur), while at the bottom of the boiler there will be 20 relatively cold water. Due to the cold water inflow, a sharp transition (1-2 cm) from cold to warm water will be maintained.

Today's more luxurious water heaters are fitted on the outside of the vessel with a heat content sensor, usually consisting of a number of temperature-dependent resistors, with which it is possible to approximately determine the height at which the transition from hot to cold water is located. Since the temperature of the water above the said boundary layer is also known, the heat content of the boiler can be determined by the electronics. Usually the number of LEDs or other indication on a display indicates how many showers or baths can still be taken when using a mixer tap set at 38 ° C.

The aforementioned devices according to the current state of the art have the following drawbacks: 1. A separate temperature sensor and (in the case of boilers) a heat content sensor must always be installed. The objection is higher costs due to additional components, cabling and extra production costs.

2. The heating elements must be periodically checked by a service technician for the degree of calcification in order to guarantee proper operation in the long term.

3. The failure of a heating element in a multi-element system is often detected indirectly, as the desired temperature is reached later due to the reduced capacity. This reduces comfort.

0 M010064 -3- 4. Failure to connect an appliance or system to the mains voltage, forgetting to supply water, may result in faulty or damaged heating elements.

5. A cumbersome and relatively expensive mechanical over-temperature protection is usually required, which also causes additional production costs.

The invention solves the aforementioned drawbacks by using the heating elements themselves as a temperature sensor. In addition to being able to carry out temperature and heat content determinations with the new temperature measuring system, the use of heating elements as a sensor can also be used to detect the degree of calcification, the defect of a heating element and the absence of water.

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The principle of temperature measurement with heating elements is explained on the basis of the figures and applications invented are explained.

Figure 1 shows the measuring bridge and the supply voltage at which the heating element can be switched as desired with the help of two AC relays.

Figure 2 shows the basic circuit with which the changeover contacts in Figure 1 are switched by the two controllers through the microcontroller (4) and checked for proper operation.

Figure 3 shows a drawing of four heating elements as they can be arranged at the bottom of a boiler.

Figure 4 shows the schematic construction of a boiler boiler in which the heating elements 20 are housed at the bottom. Just above half the height, the transition layer between cold water (below) and warm water is shown.

In order to use the heating elements (figure 3) as a sensor, an electronic circuit has been designed that makes this possible. The basic circuit (figure 1 and figure 2) for one heating element is based on two independent relays with one changeover contact each. In the rest position of each of these relays, the heating element is electrically placed in a measuring bridge by the rest position of the contacts. With the help of a differential amplifier, a voltage is formed that is a function of the temperatures prevailing in the heating element. The output signal of the differential amplifier 2 is applied to an analog / digital converter of the microcontroller. The electronic circuit which controls the two relays mentioned in Figure 2 comprises a microcontroller 4 with which, per transistor, each of which controls a relay, it is constantly checked via two inputs on the microcontroller whether the relays RY1 and RY2 are in the desired state in terms of control . In addition, the electronic circuit includes a transistor T4 with which the power supply V2 to the relay is cut off when a control signal (a square wave) on an output of the microcontroller 4 enters a static state. This can happen, for example, when the microcontroller 4 enters a fault state. In this circuit 3 it is also checked at short intervals via an input on the microcontroller whether the supply voltage for the relays corresponds to the desired state. The correct function of the latter safety shut-off function is checked at least once a day by the square wave signal. which is supplied by the microcontroller 4 to capacitor C2.

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To significantly extend the life of the relays, the rise and fall of the two relays is not done simultaneously, but in such a way that the order of rise and fall is constantly changed. As a result, there is always a different relay that switches the full power, while the other switches practically without load. Said control of the two relays occurs in a particular application with the same number as the number of heating elements used. The safety switch-off function via transistor T4 can be carried out in a single manner if the number of heating elements to be controlled is not too large. If a controlled function is found to be a fault, all relays are put into the idle state by the microcontroller and an alarm is triggered.

The invention makes use of the realized possibility to carry out temperature measurements with normal heating elements.

The invention allows the following new measurement methods:

Determining the heat content of a certain volume of a liquid in a vessel.

With the aid of two principles, which complement each other, the determination of the heat content of, among other things, boilers can be carried out without the said heat content sensor. The two principles 15 for determining the heat content can be used if there are at least two heating elements (figure 3) available which must be mounted at the bottom of the boiler (figure 4). Starting from Figure 3, a number of embodiments are possible, the operation of which are explained below.

Principle 1: 20 With a boiler as in figure 4, the water will heat all elements A, B, C and D to 60 ° C with a water quantity that has been fully heated up at night (for example to a temperature of 60 ° C). By always using all elements as a sensor, it is possible to determine to which height cold cold water has flowed in during hot water tapping at 60 ° C. For example, if the cold water has passed element D 3, then the resistance of this element will have decreased to a value corresponding to the temperature of this cold water. The temperature of the cold water can therefore be determined by measuring the resistance of element D. The resistance of elements A and B will also decrease because they are therefore approximately 20% in this cold water. When the cold water rises to half of element C with further drawing of hot water, the resistance of element C will have approximately a resistance matching the average temperature of the cold and the hot water. Elements A and B are then approximately 50% in the cold water. Both the value given by element C and the resistance of elements A and B allow the heat content to be determined. If the cold water rises above the element C, and lower than the height of the elements A and B, then only by measuring the resistance of A or B can the height of the transition between cold and hot water (and therefore the heat content).

35 Principle 2:

If, after using an amount of hot water, cold water exceeds elements A and B, the heat content can be determined by controlling elements A, B and possibly C for some time (a few tens of seconds). By dissipating electrical energy, the water that abuts the elements is heated and rises as a result.

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When this heated water reaches the boundary layer from cold to warm water, some disturbance and turbulence will occur on the boundary layer. Due to the supply of more and more water from the underlying elements, this water will move down the wall of the boiler in Figure 4. Element D now functions as a temperature sensor with which it is determined how long it takes for an increase in temperature to be signaled. The measured time between the start of the heating phase and the signaling of the temperature increase of element D is a variable for calculating the heat content of the boiler. With simple boilers, where only elements A and B are present, the heat content can also be determined by heating with element A and measuring with clement B. Versions with element A as heat source and element D as sensor are also very suitable. For each type of boiler, calculations and / or measurements can be used to determine how high the boundary layer is, with which the heat content can then be calculated.

If measurement is to be carried out in accordance with principle 2, then the amount of the heat content must be determined either after each hot water draw or periodically according to this principle 2. Each heat demand can be signaled by natural temperature fluctuations of the inflowing cold water through each of the heating elements connected as a temperature sensor. Since the described system must always be controlled by a microprocessor or microcontroller, it is possible to register the usage pattern over time, which makes it possible to carry out a heat content determination when water is not detected or not used. This method can prevent the use of a flow sensor.

20 The method of protection of the heating elements when the heating elements are incorrectly switched on when an appliance or system is not filled with water.

In current embodiments of temperature controls of hot water appliances, if electrical energy is supplied to the elements while the said appliance is not yet filled with water, then these elements may fail as the temperature will rise very quickly to unacceptable values. The temperature sensor located elsewhere in the device would not detect this high temperature, or would not detect it in time.

As stated, this invention makes use of the said possibility of using a heating element as a temperature sensor. Before the long-term supply of electrical energy to a heating element (figure 4), the resistance of the element is first measured to determine the current (water or air) temperature. Then energy is briefly (approximately 1 - 5 seconds) sent to the element.

Immediately afterwards, a resistance measurement of the element is performed. In case the device is filled with water, the measured resistance of a certain maximum will quickly fall back to the temperature of the water. If the appliance is not, or is not sufficiently filled with water, the heating element will not be able to lose the heat as quickly. The measured temperature will therefore decrease much more slowly. In such a situation, the microcontroller will ensure that an alarm (eg lighting of an "Error" LED) is issued. The appliance can only restart after a certain period of time and after the heating element has cooled sufficiently * 1010064 - 6-

Determining the degree of calcification of a heating element.

During the first hours of operation of a heating element (figure 3), there is no question of limescale that deteriorates the transmission of heat to the water. During the first day, the microcontroller will check how quickly the resistance of each 5 element drops after a power cut at a number of temperatures during the warm-up phase. The measurement is carried out in the same way as described above under "The protection of the heating elements when the heating elements are incorrectly switched on when the water heater is not filled with water." This measurement is done at certain resistance intervals after a few minutes without heating. so that the temperature of the water is virtually the same everywhere in the boiler. A condition for a good determination is that the transition layer from cold to hot water is above the elements or that all the water in the appliance has the same temperature and therefore no transition layer is present in the appliance. The measurement results at predetermined resistance values of the elements are stored in a non-volatile memory such as an E2Prom. Interpolation allows the microcontroller to determine the rate of temperature drop, and thus the time constant, for each resistance value.

"15 The measurement is repeated periodically, for example once a month, and compared with the measured values stored in the first hours. If the temperature drop-off time specified by the manufacturer is exceeded, the microcontroller can give an alarm. This can be done, for example, by: having an LED light up, indicating that maintenance is required. If the periodic measurement indicates that a maximum value (to be set by the manufacturer) has been exceeded, then the device can be switched off permanently. This prevents an element from burning out. Here too, the microcontroller can provide an alarm and / or indication.

Immediate signaling of a defective heating element.

When a heating element (figure 3) fails (electrical insulation), this is immediately noticed by measuring too high a voltage. A closure with the jacket of the * - 25 heating element, if not previously detected by an external earth leakage circuit breaker, is signaled by the decrease of the resistance of the heating element in the measuring bridge. In both cases the microcomputer can give an alarm.

Protection against overheating.

By using two independently controllable relays RY1 and RY2 and being able to switch off both relays via transistor T4, and by the fact that the said functions are continuously tested by the microcontroller 4, and because in the event of a failure of the microcontroller, the square wave signal is sent to current transistor T4 is lost, the current control is very reliable. This design eliminates the need for a separate over-temperature protection.

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Claims (9)

1. An electronic system for determining the temperature of liquids and solids, characterized in that the temperature coefficient of the resistance wire in heating elements is used. An electronic system according to claim 1, characterized in that the heating element can optionally be connected via the changeover contacts of two independently controlled relays to a supply voltage or to a resistance measuring bridge circuit. An electronic system according to one or more of the preceding claims, characterized in that during heating of the solid or liquid only temperature measurements are taken in order to obtain data so that it can be calculated how much time it takes at the set power to achieve the desired final temperature, so that only at the end of the heating cycle some measurements need to be taken to set the final temperature within the desired tolerance, with the aim of extending the life of the contacts of the two relays. An electronic system according to one or more of the preceding claims, characterized in that the two relays are switched on and off in a different order, with the aim of significantly increasing the electrical life of the relays by reducing each relay by half of the total 20 number of switching actions under full load during the life of the electronic system. 5. An electronic system according to any one or more of the preceding claims, characterized in that the microcontroller continuously monitors the relay switching actions, and because the microcontroller may fail if the other electronics fail beforehand, by the other electronics. microcontroller has been checked for proper operation, all relays fall into the rest position so that all heating elements are switched off from the supply voltage by means of two independent contacts, so that a certain safety shutdown function is guaranteed. 30 An electronic system according to one or more of the preceding claims, characterized in that the thickness of limescale on a heating element is determined by the time constant of the cooling of the resistance wire in the heating element logged in the first operating hours of the heating element in, for example, E2Prom. at different temperatures - * 5 periodically compared with the last measured value, so that when certain values are set by the manufacturer, a maintenance request is made or the appliance or system is switched off completely when an absolute maximum is exceeded. 0 10 0 6 4 -8- An electronic system according to one or more of the preceding claims, characterized in that the heat content of a boiler is determined by, in the case that the cold / warm transition layer is still in the region of the heating elements, which can be determined. by constantly decreasing the resistance of the heating element which occurs during 5 tapping activities, the resistance resulting from the addition of the two distinguishable parts of the heating element which are in the cold part and in the warm part of the water are measured and together with the known temperature of the water at the top of the boiler to be used for the calculation, which calculation can be performed more accurately when the water temperature can be measured by a low-lying heating element. 10 An electronic system according to one or more of the preceding claims, characterized in that the heat content of a boiler is determined by, in case the cold / warm transition layer is above the heating elements, which can be determined after tapping activities which are determined by fluctuating the resistance due to natural fluctuations in the temperature of the water flowing in during tapping, dissipating electric energy in a high-lying spiral element or in a near-center vertical element (such as A in figure 3) measuring how long it takes before the temperature on another heating element that is connected as a temperature sensor starts to rise, which time, depending on the type of boiler, indicates the height of the transition layer. 9. An electronic system according to claims 7 and 8, characterized in that the accuracy of the heat content determination can be improved by applying statistical data collected during the use of the device, »1010064