EP0508942A2 - Method of and means for controlling a steam generator - Google Patents

Abstract

The steam generating capacity of a steam generator, used in particular for air humidification and in which the water to be heated is used as heating resistance, is controlled directly following a request signal (y), the heating current being continuously switched on and off by a timing signal with variable pulse duty factor. The possibility of deriving the water level values and electric conductivity values, required for controlling the water economy, from the temporal pattern (48) of this current, which possibility most importantly is lost with direct control of the heating current, is reestablished by the calculation of a current pattern (52) standardised on uncontrolled operation. With the invention, the reactivity and controlling accuracy of the steam generating capacity control of steam generators of the above type can be considerably improved. <IMAGE>

Description

The invention relates to a method for controlling a steam generator used in particular for air humidification according to the preamble of patent claim 1, and to a device for carrying out the method according to the preamble of patent claim 7.

In this type of evaporator, also called an electrode evaporator, a single or multi-phase alternating current is passed through the water via two or more electrodes and the water itself is used as a heating resistor to generate the heat required for the evaporation. The evaporated amount of water is automatically replaced by fresh water at certain intervals.

In practice, it is important to be able to control the steam output, on the one hand to ensure continuous steam delivery, on the other hand to change the amount of steam emitted in a targeted manner so that, for example, the air humidity in a room can be kept at a certain value or changed according to a specified program .

The steam output is determined in normal operation, i.e. apart from starting processes and other malfunctions and special cases, and for a given geometry of the steam generator by the heating current. This depends primarily on two parameters: the immersion length of the electrodes and the electrical conductivity of the water. The known electrode evaporators use an intermittent cycle of filling and draining Water to regulate these two quantities that determine the current strength. During the evaporation process, the water level drops and the current strength decreases according to a certain characteristic. The immersion length of the electrodes can be brought to a desired value in a simple manner by filling in fresh water at certain time intervals and, if appropriate, by draining water. Mastering the conductivity of the water is more difficult. This must be adjustable for two reasons. First, the water composition, especially the mineral salt content, and thus the conductivity, varies greatly from place to place. Second, the constituents determining conductivity are not also evaporated, so that their concentration increases continuously in the course of the evaporation process, which continuously increases the conductivity. This increase is desirable if the conductivity of the fresh water is too low, but it also leads to undesirably high conductivity. To reset the conductivity, part of the water enriched with conductivity-determining constituents is therefore also drained off at certain time intervals and replaced with fresh water. In continuous operation, this results in a repeated sequence of filling phase, steam phase and, if necessary, drain phase, the devices being designed in such a way that steam is also generated during the filling and drain phases.

There are various methods for determining the time and duration of the different phases, all of which in some way derive the current water level and the conductivity of the water from the course of the heating current. For example, the Swiss patent specification No. 663 458 describes a method in which the current strength is measured over a predetermined period of time during the vapor phase and the decrease thereof is compared with an amount corresponding to the optimal conductivity. If the current intensity decreases by less than this amount in this period, i.e. the conductivity is less than optimal, no water is drained off at the end of the vapor phase. If, on the other hand, the current intensity decreases by the amount mentioned before the end of the time period, that is to say the conductivity is greater than optimal, a drainage period is calculated from the remaining time period while the water is being drained off. Another method can be found in Swiss Patent Specification No. 672 015. Here, the fact is taken advantage of that the heating current strength increases a little further after filling, before it drops again, and that the shape of the "current peak" thus formed depends on the conductivity of the water. To reset the conductivity, water is drained off when a characteristic parameter, preferably the duration of the current peak, exceeds a predetermined limit value.

Since in all of these known methods the steam output is regulated solely via a level control of the water level, the regulation is not very precise and relatively slow. This is disadvantageous even with the usual use of the steam generator. In particular, however, this circumstance prevents the use of such steam generators in all those cases in which high demands are placed on the moisture control, such as in laboratories or in special production rooms in the semiconductor industry.

It is therefore a main object of the present invention to provide a method and a device which improves the responsiveness and the control fidelity, that is to say the accuracy, stability etc., of the steam output control.

The solution to this problem is defined by the features specified in independent patent claims 1 and 7. Refinements and preferred embodiments of the invention result from the dependent patent claims.

The clocked direct control of the heating current intensity used according to the invention achieves a significantly faster and more precise control of the steam output than is possible with controls via the level control of the water level according to the prior art. The response time is determined by the period of the clock signal, which will generally be from about 100 milliseconds to one second. The possibility that a direct control of the heating current strength is lost, the possibility of deriving the water level and the electrical conductivity from the time profile of this heating current strength is restored by the calculation according to the invention of a current profile standardized to uncontrolled operation.

Furthermore, in the known methods, apart from the comparatively slow level control of the water level, the control dynamics are additionally blocked for a period of time required to determine the conductivity, usually in the order of magnitude of 10 to 100 seconds, while in contrast to this method according to the invention, the control can be carried out continuously.

• Fig. 1 eine schaubildliche Darstellung eines Dampferzeugers mit einem Blockschema einer erfindungsgemässen Steuer- und Regelungseinrichtung,
• Fig. 2 zwei Diagramme zur Veranschaulichung der Steuerung des in den Elektroden fliessenden Wechselstroms durch ein Taktsignal mit veränderlichem Tastverhältnis,
• Fig. 3 ein Diagramm mit einem zeitlichen Stromverlauf (Durchschnittsstrom pro Taktperiode) bei erfindungsgemässer Steuerung nach einem angenommenen Anforderungssignal von 100% sowie mit einem Stromverlauf, wie er sich bei Vollaussteuerung ergäbe,
• Fig. 4 einen schematischen Prozessverlauf der Grössen Wasserpegel, elektrische Leitfähigkeit und Tastverhältnis, und
• Fig. 5 ein Diagramm mit einem Stromverlauf bei erfindungsgemässer Steuerung nach einem dynamisch variierenden Anforderungssignal sowie mit einem zugehörigen erfindungsgemäss normierten Stromverlauf.

The invention is explained in more detail below on the basis of exemplary embodiments and with reference to the accompanying drawings. Show it:

• 1 is a diagrammatic representation of a steam generator with a block diagram of a control and regulating device according to the invention,
• 2 shows two diagrams to illustrate the control of the alternating current flowing in the electrodes by means of a clock signal with a variable pulse duty factor,
• 3 shows a diagram with a current curve over time (average current per cycle period) in the control according to the invention after an assumed request signal of 100% and with a current curve as it would result with full control,
• Fig. 4 shows a schematic process flow of the quantities water level, electrical conductivity and duty cycle, and
• 5 shows a diagram with a current profile in the control according to the invention according to a dynamically varying request signal and with an associated current profile standardized according to the invention.

In Fig. 1, a steam generator is shown schematically, together with a block diagram of a control and regulation device according to the invention. The Container 2 is filled to an indicated level with water. A pair of electrodes 4 are partially immersed in the water. Depending on the design of the steam generator, there may also be more than two electrodes. The electrodes can be connected via lines 6 to an electrical AC voltage network. The connection is usually made via a switching element (not shown), for example via a contactor. At the bottom of the container 2 there is a combined inlet / outlet 8. This comprises a feed line 10 and a discharge line 12. The feed line can be connected, for example, to a water supply network, and the discharge line, for example, to a sewage line that leads into the public sewage system or a any wastewater reservoir opens out. Both Zuals and derivation are each provided with an electrically controllable valve 14 and 16. In the ceiling of the container 2 there is a steam outlet 18, via which the generated steam exits directly or via a distribution system or via an air conditioning system into a room to be humidified, such as a common room for people, a highly air-conditioned production system, a laboratory or an air-conditioned equipment cabinet can.

According to the invention, the electrodes 6 are connected to the AC voltage via a power controller 20 which can be controlled by a clock signal and with which the current in the electrodes can be controlled in a clocked manner. The power controller 20 is preferably a power semiconductor component, for example a triac. This type of power control is known in principle, the use according to the invention is discussed in more detail below. The current flowing in the electrodes is included a current sensor 22, for example a current transformer.

A request signal y, which represents a steam output setpoint and is generated, for example, by a moisture measuring and regulating device or according to a fixed program, serves as a reference variable for the current control.

An electronic current control device 24 has a control logic part 26, a device adaptation part 28 and a clock generator 30. The clock generator 30 is electrically connected to the power controller 20 and the current sensor 22 to the control logic part 26. The request signal y is introduced into the device adaptation part 28. An electronic water control device 32 has a current normalization part 34, a demand standardization part 36 and a drain / fill control 38. The current normalization part 34 is electrically connected to the control logic part 26. The controllable valves 14 and 16 of the supply and discharge for the water are electrically connected to the drain / fill control 38. Furthermore, the current control and water regulation device are provided with a parameter input device 40 and a display device 42. The function of the current control device 24 and the water control device 32 is explained below. These devices can be implemented electronically in a variety of configurations known to those skilled in the art. Digital components are preferably used, in particular an appropriately programmed microprocessor.

The operation of the control of the current will now be discussed in more detail with reference to FIG. 2. In the figure, two diagrams a) and b) can be seen, both of which show the profile of an alternating current i and an associated clock signal s. In the exemplary embodiment shown, the clock signal is a square-wave signal with the constant pulse period T, the pulse duration t _e and the pulse duty factor V = t _e / T defined thereby. The above-mentioned power controller 20 is actuated with the clock signal, so that it passes the current during the pulse duration and blocks for the remaining time. An effective average current I _d per pulse period is therefore dependent on the duty cycle and can be controlled by changing this duty cycle. Diagram a) shows a longer duty cycle and diagram b) shows a shorter one. To make it clear that the peak value of the alternating current is not influenced in this type of control, the peak values are drawn differently in the two diagrams. In order to keep the disturbing transient elements that inevitably arise with each switching as small as possible, semiconductor components are preferably used as the power controller 20, which only switch after the clock signal has been switched on or off close to the subsequent zero crossing of the alternating current. In this way - with the so-called half-wave packet control - a single current packet always consists of a number of whole half-waves (cf. FIG. 2).

Since the above-mentioned transient elements can also have DC components, the use of this type of control in the electrode evaporators affected here can lead to electrolytic effects, which on the one hand lead to material migration and on the other hand to fission gas separation to lead. For this reason, in the present invention the clock signal s is preferably controlled such that always two successive half-wave packets form a common control packet, in which both half-wave packets have the same number of half-waves, but the second half-wave packet is antisymmetric with respect to the first axis with respect to the zero axis. 2, the first two half-wave packets each form such a control packet. With this procedure, the direct current components are formed into low-frequency alternating current components that cannot have any adverse effects. On the contrary, there are indications that such low-frequency alternating currents can delay or even prevent the formation of limescale in tap water systems, which would advantageously extend the operating life and lifespan of steam generators according to the invention.

Returning to FIG. 1, the flow of the current control can now be described. The required steam power, that is to say the required effective average current I _d in the electrodes, is determined in percent with respect to an available nominal power by the request signal y. The request signal y is processed in the device adaptation part 28, taking into account the system boundary conditions (mains voltage, device type, any limitations, etc.) and then fed to the control logic part 26. The control variable, the alternating current in the electrodes, is determined by the current sensor 22 as real current, rectified and also supplied to the control logic part 26 as an electrical signal. The control logic part calculates the effective average current by integrating this signal over each pulse period T, prepares it for one the value comparable to the request signal y and calculates a newly adapted duty cycle V for the clock signal s from the deviation. The clock 30 is operated with the new duty cycle V, and this controls the power controller 20 with a correspondingly adapted clock signal s. In a preferred embodiment, the control logic part 26 also ensures that the control packets mentioned above are each generated with two antisymmetric half-wave packets.

The control of the current according to the invention has two goals compared to the prior art. Firstly, the current change occurring during the filling phase-vapor phase-drain phase by changing the water level should be compensated for and secondly the current should be controlled practically continuously according to the request signal y. FIG. 3 serves to explain the compensation of the current change caused by the changing water level. This represents a schematic current-time diagram over a cycle filling phase of a duration T _E , vapor phase of a duration T _D , discharge phase of a duration T _A , in which as An example of a possible process profile is an uncontrolled current profile 44 and a current profile 46 controlled according to a constant request signal of 100%. The uncontrolled current profile 44 basically follows the water level, that is to say it rises during the filling phase, then decreases as evaporation progresses during the vapor phase and continues to decrease during the draining phase until it rises again during the subsequent filling phase. In the case of the current curve 46 controlled according to the invention, these changes are compensated for by appropriate adaptation of the pulse duty factor of the clock signal, so that the average current to for small deviations consistently 100%. The circled slots in the figure show the associated clock signal and the pulse duty factor (in percent) for characteristic points. In order for the full compensation scope to be covered, the duty cycle corresponding to a requirement of 100% must be less than 100%; it is therefore chosen, for example, as shown in FIG. 3, that 100% requirement corresponds to 80% duty cycle.

For further illustration of the process, the quantities water level h, electrical conductivity k and pulse duty factor t _e / T corresponding to the process in FIG. 3 are shown schematically.

The situation becomes even more complicated if the request signal is not kept constant, but is variable, as intended by the invention. FIG. 5 shows a current curve 48 controlled according to a variable request signal in a current-time diagram similar to FIG. 3. Obviously, such a controlled current flow cannot be used directly to regulate the water balance, i.e. to regulate the filling and draining phases, as is the case with the methods according to the prior art, since it is precisely the influences of the quantities to be regulated and the water level and electrical conductivity can be balanced.

The invention therefore uses its own procedure to calculate a current profile normalized to uncontrolled operation from the controlled current profile. For this purpose, a normalized current I _{n is} calculated, which corresponds to the effective value of the corresponds to an alternating current acting during a respective pulse duration t _e by dividing the measured acting average current I _d by the respective duty cycle V. 5 shows the curve of I _n over the actual time t calculated from the controlled current curve 48 as curve 50. Since this standardized current only acts during the switch-on time or pulse duration of the clock signal, an associated standardized time t _{n must also} be calculated in order to achieve the standardized current profile. This can be done by adding up the pulse durations t _e from a certain point in time, the fictitious start of the standardized current profile. If this calculation of the normalized current curve is expressed in formulas, the following therefore applies:

Curve 52 in FIG. 5 represents such a normalized current profile with a start at the beginning of the vapor phase T _D. The normalized current profile corresponds to a fictitious uncontrolled current profile and can therefore be used to determine the water level and electrical conductivity, for example by one of the known methods . The start of the standardized current curve does not necessarily have to be at the beginning of the vapor phase, but can be freely selected depending on the method used.

In the device according to the invention (FIG. 1), the calculation of the normalized current profile takes place in the current normalization part 34, which receives the required information about the measured average current and the clock signal from the control logic part 26.

In addition to the possibility of using one of the known methods for regulating the water balance after the normalized current profile has been calculated, the invention proposes in a preferred embodiment a new method for determining the suitable length of the steam and discharge phase. This method is based on the idea of comparing the measured sequence represented by the normalized current profile with a theoretical sequence calculated from the profile of the request signal. The theoretical sequence describes how the evaporation process should take place under ideal conditions, i.e. with ideal electrical conductivity of the water and without disturbing influences such as limescale deposits on the electrodes, blistering in the heated water, etc. For the theoretical sequence, an ideal average pulse duration t _{eid of} the clock signal, which is tuned to a request of 100%, is weighted with the respective value y _{i of} the request signal y, that is, an ideal time t _{id is} calculated by adding up the the ideal pulse duration multiplied values y _i . Written as a formula, this means:

The ideal pulse duration is a theoretical mean value over the entire dynamic range related to the water balance and can be determined for each specific steam generator type. 3, for example, the ideal pulse duration would be 80% of the clock period.

und eine ideale Zeitdauer

With these requirements, the regulation of the water balance can proceed, for example, as follows. During the filling phase, the (increasing) normalized current is monitored, and as soon as it reaches a first predetermined value I ₁ , the filling is ended so that the pure vapor phase begins. At the same time, the calculation of the standardized time t _n and the ideal time t _{id is} started. In the course of the vapor phase, the normalized current drops in a manner dependent on the current control and the electrical conductivity of the water. As soon as the normalized current reaches a second predetermined value I ₂ , the calculation of the normalized and the ideal time is stopped. A certain number of m clock periods have elapsed between the start and the stop of the calculation. This results in two time periods, namely a standardized time period

and an ideal length of time

The difference between these two time periods can be used to calculate the length of the drain phase necessary to reset the electrical conductivity of the water, based on a function optimized for the type of steam generator. If the standardized time period corresponded to the ideal time period, if the evaporation process had run ideally, no water would have to be drained off and the next filling phase could be started directly.

As an alternative to stopping the calculation of the normalized and ideal time period when a certain second value of the normalized current has been reached, it would also be possible to specify a fixed value for the ideal time period or the normalized time period and to stop the calculation when the predetermined value has been reached .

In the device according to the invention shown in FIG. 1, the method just described is implemented by the water control device 32. As already described above, the current normalization part 34 takes care of the calculation of the normalized current and the normalized time. The calculation of the ideal time from the respective value of the request signal is carried out by the request normalization part 36. The drain / fill control 38 controls the valves 14 and 16. It receives the respectively calculated value of the normalized current from the current normalization part 34. As soon as this reaches the value I ₁ during the filling phase, the drain / filling control 32 closes the valve 14 and starts the calculation of the standardized and ideal time periods T _n and T _id . If the normalized current reaches the value I ₂ , or alternatively if the ideal time period reaches a predetermined value, the drain / fill control stops the calculation of the normalized and ideal time period, determines a drain time period from the difference between these time periods, opens during this drain period the valve 16 and then starts a next filling phase with the opening of the valve 14, with which the next water balance cycle begins.

Claims (7)

Method for controlling a steam generator used in particular for air humidification, in which an electrical alternating current is generated in the water located in the region of the electrodes by means of at least two electrodes which at least partially protrude into the water to be evaporated and the water is thus heated, - fresh water is added at certain intervals to replace the evaporated water during a filling phase, - from time to time to reset the conductivity of the water in the steam generator during a drain phase, a part of this water is drained and replaced by fresh water, and the current strength of an alternating current flowing in the electrodes is measured,
characterized in that - The alternating current is controlled following a request signal (y) representing a steam power setpoint by continuously switching the alternating current on and off by means of a clock signal (s), the pulse duty factor (V) of the clock signal being changed for control purposes, and that - To determine variables that can be used to regulate the filling phases and the drain phases, a temporal current curve standardized to uncontrolled operation is calculated. Method according to claim 1, characterized in that by dividing an average current (I _d ) measured per pulse period by the respective duty cycle (V) a fictitious normalized current (I _n ) and by summing up the pulse durations (t _e ) of successive pulses of the clock signal (s ) a fictitious normalized time (t _n ) is calculated, the normalized current (I _n ) as a function of the normalized time (t _n ) giving the normalized current curve over time. Method according to claim 1 or 2, characterized in that during a certain number (m) of successive clock periods of the clock signal (s), a standardized time period (T _n ) characteristic of the normalized current profile and an ideal time period (T _id. ) Characteristic of the profile of the request signal ) is calculated, and the duration of the next drain phase is calculated as a function of the difference between the normalized and ideal duration. Method according to claim 3, characterized in that the normalized time period (T _n ) by summing up the pulse durations (t _e ) of the specific number (m) of successive clock periods and the ideal time period (T _id ) by summing up the specific number in the respective clock period (m) successive clock periods required values (y _i ) of the request signal (y) multiplied by a predetermined ideal pulse duration (t _eid ) is formed. Method according to one of Claims 1 to 4, characterized in that the alternating current is switched on and off in half-wave packets, that is to say for the duration of a certain number of half-waves, with switching in each case in the zero-crossing range. A method according to claim 5, characterized in that to eliminate DC components generated by switching on and off, the clock signal is checked so that two successive half-wave packets of the controlled alternating current form a common control packet in which both half-wave packets have the same number of half-waves, but with respect to the zero axis are antisymmetric to each other. Device for carrying out the method according to claim 1, with a container (2) for holding water, at least two electrodes (4) located at least partially in the container (2), which can be connected to an electrical AC voltage network, means (14, 16) for inlet and outlet of water and a current sensor (22) for measuring the current of an alternating current flowing in the electrode, characterized in that it has a controllable power controller (20), a current control device (24) and a water excitation device (32) , With the current control device (24), the power controller (20) can be controlled by means of a clock signal (s) with a duty cycle (V) that can be changed by the current control device (24) following a request signal (y), and the water control device (32) is designed in such a way that that from the current control device (24) transmitted information about the duty cycle (V) u nd the measured Current strength can be calculated based on an uncontrolled operation, temporal current curve, depending on which the means (14, 16) for the inlet and outlet of water can be controlled.

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