US3780261A

US3780261A - Automatic control for electrode boilers

Info

Publication number: US3780261A
Authority: US
Inventors: Williams R Eaton
Original assignee: Williams R Eaton
Current assignee: Eaton Williams Group Ltd
Priority date: 1971-05-19
Filing date: 1972-05-19
Publication date: 1973-12-18
Anticipated expiration: 1990-12-18
Also published as: CA981357A; GB1381113A; NL166318C; NL7206863A; CH534328A; DE2225398A1; DE2225398B2; DE2225398C3; FR2138075B1; FR2138075A1

Abstract

An electronic controller for an electrode boiler in which the heating current passing through an electrode depends upon the water level in the boiler, in which circuitry controls feed and drain valves to carry out an automatic repetitive cycle to feed water into the boiler to a predetermined level, to sense when boiling has reduced the level to another predetermined point, to open the drain valve to allow a quantity of water to drain out to remove solid matter precipitated during boiling, then to re-open the feed valve to restore the original level so that boiling may recommence. An alternative feed, boil, feed, drain cycle is provided.

Description

United States Patent [191. f 1 1 3, 80,261

Eaton-Williams Dec. 18, 1973 1 AUTOMATIC CONTROL FOR ELECTRODE 3,408,941 11/1968 Sorensen 137/392 x ER 3,446,937 5/1969 Hugentobler.... 219/297 3,671,142 6/1972 Calabrese...., 137/392 Inventor: Raymond Herbert ms, 3,523,175 8/1970 Gygax 219/272 Beech House, Pendennis Rd., 3,269,364 8/ 1966, Higgins 219/284 X Sevenoaks, England 1 2,006,631 7/1935 Eaton 219/285 X [22] Filed May 1972 Primary Examiner-C. L. Albritton [21] Appl. No.2 255,058 Attorney-Hyman Berman et a1.

[30] Foreign Application Priority Data [57] ABSTRACT May19197l Great Britain u 1 5386/71 An electronic controller for an electrode boiler in Mar. 10, 1972- Great Britain 11,287/72 which the heating Current Passing through an electtode depends upon the water level in the boiler, in 52 us. 01 219/324, 122/382, 137/392, which circuitry eehtrels feed and drain valveS to carry 219 271 219 234 out an automatic repetitive cycle to feed water into [51 1m. F24h 1/00 the boiler to a predetermined level, to sense. h 58 Field-ofSe'arch 219/271, 272, 275, boiling has reduced the level t anether'predeter- 3,605,798 4/1970 Green et al. 137/392 219/323, 324, 327, 333, 284, 285; 122/382; mined point, to open the drain valve to allow a quan- 37/3 392; 2 1/139 142 34 1 65 tity of water to drain out to remove solid matter precipitated during boiling, then to re-open the feed valve 5 Ref Cited I to restore the original level so that boiling may.recom- UNITED STATES PATENTS mence. An alternative feed, boil, feed, drain cycle is provided. 3,209,125 9/1965 Morrissey 219/333 X I 11 Claims, 4 Drawing Figures 0/ 0/77 Valve 7 PATENTED um 8 [975 I SHEET 1 OF 3 38 m 53mm E S H mm 1 1 mx N @B 5 xi E EEG: Emu

mix smo m ES A 1 AUTOMATIC CONTROL FOR ELECTRODE BOILERS This invention relates generally to automatic controls for electrode boilers, and more particularly to a control for a boiler in which the water is boiled so that the steam given off may be used in an air conditioning systern. Such boilers are sometimes referred to as humidifiers.

The type of boiler with which the invention is concerned may be of any shape but is conveniently cylindrical, with its axis vertical, and having a height greater than its diameter. The boiler has a steam outlet pipe at the top and aconnection at the bottom, which may serve both as a water inlet and a water outlet. This has an advantage over separate inlet and outlet connections in that each inrush of supply water will clear'the passages of any lodged solids, thereby leaving these passages clear when the boiler is drained. A filter may be incorporated in the boiler, so placedthat inlet and outlet water must pass through it. The boiler contains water heating electrodes arranged vertically and extending over most of the height of the boiler. The arrangement of the electrodes may be varied in dependence upon whether the boiler is provided with electrodes for single-phase or three-phase operation.

The control valves normally comprise one electrically controled (that is, solenoid operated) feed water valve and one electrically controlled drain valve which when the boiler has a common inlet and outlet connection, may be connected to the two sides of a T-junction the central branch of which is connected to the boiler. When the feed valve is opened and the drain valve remains closed feed water enters the boiler and when the drain valve is opened and the feed valve remains closed water drains from the boiler.

The electronic controller which is the subject of the invention is arranged to detect the magnitude of the electrode current and to operate the feed valve and drain valve in relation to predetermined sequences and in dependence upon the magnitudes of changes in electrode current. The controller is also arranged to detect the point at which electrode maintenance is necessary. For this purpose an additional electrode which will be referred to as the boiler full sensing electrode is desirably installed in the boiler.

The invention consists of an electronic controller for an electrode boiler in which the heating current passing through an electrode depends upon the depth of im-' mersion of the electrode and hence upon the water level in the boiler, comprising circuitry to operate an electrically controlled feed valve to allow feed water to enter the boiler, first sensing means in the circuitry to detect when the electrode current has risen to a predetermined normal level, means responsive to the first sensing means to cause the feed valve to be closed, whereupon the water is allowed to boil, second sensing means to detect when the electrode current has fallen to another predetermined level due to loss of water boiled away, andmeans responsive to the second sensing means to initiate a desired sequence of steps includto cause an electrically controlled drain valve to be opened whereupon water is allowed to drain from the boiler, the circuitry also comprising third sensing means to detect when the electrode current falls to another predetermined level due to loss of water drained from the boiler, and means responsive to the third sensing means to cause the drain valve to be closed and the feed valve to be opened, whereupon feed water is again allowed to enter the boiler, the first sensing means and the means responsive thereto causing the feed valve to be closed when the electrode current has again risen to the predetermined normal level so that boiling may recommence, the circuitry being arranged to carry out the feed, boil, drain cycle repetitively.

In another embodiment of the invention the means responsive to the second sensing means comprises means to re-open the feed valve, the circuitry comprising also further means responsive to the first sensing means to cause the feed valve to be closed and an electrically controlled drain valve to be opened when the electrode current has again reached the predetermined normal level, third sensing means to detect when the electrode current has fallen to a further predetermined point, and means responsive to the third sensing means to cause the drain valve to be closed and the feed valve to be re-opened, the first sensing means and the means responsive thereto causing the feed valve to be closed when the electrode current has again risen to the pre-- determined normal level so that boiling may recommence, the circuitry being arranged to carry out the feed, boil, feed, drain cycle repetitively. 1

Additional circuitry may be provided to sense whe the electrode current has risen by a predetermined amount above the predetermined normal level and to open the drain valve allowing water to drain from the boiler until the electrode current has fallen to a value near to the predetermined normal level when the additional circuitry closes the drain valve and permits the continued operation of repetitive cycles of feed, boil, drain as already described.

Additional circuitry may also be provided to sense when the boiler becomes overfilled and to close the feed valve and if required to initate an alarm signal.

Means may also be provided to vary the quantity of water drained away in relation to the amount boiled away during each cycle of operations so that this may be greater, equal or lesser as required.

Commencing with an empty boiler, the electric supply to the electrodes is switched on. Initially, since the boiler is empty, no electrode current flows and the controller opens the feed valve while the drain valve remains closed. Water therefore passes into the boiler and as the water level rises the electrodes become progressively more deeply immersed so that the electrode current progressively rises. When the electrode current has reached a predetermined normal value which will hereinafter be referred to as the operating point current" the controller causes the feed valve to close. The water is then heated due to the passage of current between the electrodes. As the water temperature rises the conductivity of the water may increase and in addition the level of the water in the boiler may rise due to expansion. Consequently the electrode current increases and when it rises to a predetermined level above the operating point current, usually about of the operating point'current, the controller opens the drain valve so that the water level in the boiler falls.

When the electrode current falls to another predetermined point which is set at about or just below the normal operating point current the controller causes the drain valve to close. As the temperature of the water in the boiler progressively rises from that of the cold feed water supply to boiling point the drain valve may be operated several times in order to keep the water level at approximately the desired point. When the boiling point is reached, boiling continues and the water level is progressively reduced as water is boiled away, resulting in a progressive reduction of the electrode current as the electrodes become less deeply immersed. This continues until the water level has fallen to the point where the electrode current is reduced to a magnitude which will be referred to as the boiling period lower limit currentto which is chosen to be at a value of approximately 80 percent of the operating point current. When this point is reached the controller causes the drain valve to open and water drains from the boiler so that the electrode current falls as a result of the progressively decreasing immersion of the electrodes. When the electrode current has fallen to another predetermined level which will be referred to as the "drain period lower limit current which is normally selected at a value between 50 percent and 70 percent of'the operating point current, the controller closes the drain valve and opens the feed valve. Fresh water again enters the cylindeand the electrodes become progressively more deeply immersed until the operating point current is again reached, at which point the controller closes the feed valve and, with both feed and drain valves closed, another period of boiling takes place. The successive operating cycle of feed, boil, drain as described above is continued. The operations of feeding and draining take place in a comparatively short time depending on feed water pressures and other conditions whilst the operation of boiling may take place in a comparatively longer time. These three successive operations constitute the water purification cycle and the object of draining water away is to allow solids, previously dissolved in the water and precipitated as the water is boiled away, to be drained out. The result is that, with a water supply containing a given amount of dissolved solids, the life of the boiler and electrodes, before they become so furred up as to'require major maintenance, is very considerably lengthened. The amount of water normally drained away is equal to the amount of water which is boiled away during a boiling period but, as will appear later, the amount drained away may be considerably more or less than the amount boiled away depending upon the amount of purification which is desired.

The boiler actually used may be of a type which is constructed in a very inexpensive manner so that when it and the electrodes become thoroughly furred up the whole boiler may be thrown away and replaced by a new one.

After a long period of normal operation the electrodes gradually become furred up or'coated with scale and this reduces the conductivity of the immersed parts of the electrodes so that a greater depth of immersion becomes necessary in order to reach the desired operation point current. This will continue until it eventually becomes impossible to achieve the operating point current even with the electrodes totally immersed and this would cause the feed valve to remain open and allow the boiler to fill itself completely and overflow through the steam outlet. To overcome this difficulty the boiler is preferably provided with a boiler full sensing electrode." This electrode extends downwardly from the top of the boiler to a point which is selected as the highest permissible water level. When the water reaches this electrode the controller causes the feed valve to close and also illuminates a warning lamp to indicate that the electrodes require maintenance. If this is not carried out the boiler will continue to operate by preventing the water level from exceeding that set by the boiler full sensing electrode even if the normal operating point current is not reached. The operating cycle will then continue but with the maximum electrode current achieved becoming progressively less. The steam output will also be progressively reduced until electrode'maintenance (or replacement of the whole boiler) takes place.

Two embodiments of the automatic controller according to the invention are illustrated in the circuits shown in the accompanying drawings and these will now be described.

In the drawings:

FIG. 1 shows the circuit of a controller to operate the boiler automatically and repetitively in a feed, boil, drain cycle;

FIG. 2 shows the circuit of a controller to operate the boiler automatically and repetitively in a feed, boil, feed, drain cycle.

FIG. 3 shows a part of the top of a boiler to be operated by a controller according to the invention, illustrating a current transformer having its primary winding connected in series with the power feed to one of the electrodes; and

FIG. 4 shows an additional boiler full" sensing electrode inserted in the boiler.

' In a practical embodiment the circuits will be set out on printed circuit boards and the terminals of the boards are indicated at the left hand sides of thedrawings.

Referring first to FIG. 1, a transformer (not shown) is provided having its primary winding connected to the supply mains and having a secondary winding to provide an output of 22 volts r.m.s., the secondary winding being connected to terminals 10 and 11. The transformer secondary voltage is applied to a full wave rectitier MR1 and the output is fed through a'resistor R2 to a reservoir capacitor C2 and thence through a resistor R5 to a pair of Zener diodes ZD2 and ZD3 in series to provide stabilised, smoothed power supplies of 12 volts and 6 volts d.c. A small capacitor C4 is connected in parallel with the Zener diodes for transient suppression purposes.

FIG. 3 shows a part of the top of a boiler casing 41 having two heating electrodes, respectively 42 and 43, connected respectively to power feed lines 44 and 46. Line 44 is connected to one end of the primary winding 45 of a current transformer generally indicated at 47 the other end of the primary winding being connected to the power line feeding the electrode 42. The current transformer 47 has a secondary winding 48 which is connected to

terminals

7 and 8. A typical ratio for this current transformer would be -1, so that the secondary current passing the

terminals

7 and 8 will be approximately one one hundred fiftieth of the electrode current.

Also connected between

terminals

7 and 8, but not shown on the drawing, is a series combination of a fixed resistor and a variable resistor which provide a load for the secondary winding of the current transformer. The setting of the variable resistor, therefore, determines the level of the electrode current at which certain predetermined control voltages are developed across the secondary winding of the current transformer. For example, the arrangement may be such that when the detecting circuitry detects that the operating point current has been reached the electrode current may be about 15 amperes when the variable resistor is 'set at minimum resistance. The voltage applied to

terminals

7 and 8 is passed to a full wave bridge rectifier MR2 and the output of this rectifier is fed through a resistor R1 to a smoothing capacitor C1 having a voltage divider R3 and R4 connected in parallel therewith. Also connected in parallel with capacitor C1 is a Zener diode ZDl. Capacitor C1 and resistor R1 together with loading resistors R3 and R4 provide a smoothing circuit having a time constant sufficient to even out the short term fluctuations of the electrode current, such as may be caused by bubbling or water turbulence in the boiler. Such short term variations could result in random and erratic operation of the control circuits and consequently of the feed and drain valves. A typical output voltage from this circuitry as developed across capacitor C1, corresponding to the operating point current, would be 10 volts d.c. The voltage developed across R3 and effectively applied as input voltage to the sensing circuitry would be two-fifths of this voltage, corresponding to 4 volts d.c. The Zener diode ZDl is provided to limit the maximum voltage which-can be developed across this circuitry under fault conditions or any other condition of rapid change of electrode current, and it therefore prevents any possible damage to the control circuitry.

The control voltage as developed across R3 is applied to the inverting inputs of three differential comparator integrated circuits [C1, IC2 and 1C3. The non-inverting inputs of these three integrated circuits are set at varying points on a chain of potential divider resistors between the +l2 and -6 voltage supplies, such that the outputs of these integrated circuits will change state at different values of input signal.

In the absence of an input from the circuit associated with bridge rectifier MR2, all three of these integrated circuits will have their outputs in the negative condition. This is because their inverting inputs will be held at a positive potential in relation to their non-inverting inputs. Under these circumstances an n-p-n transistor 01 will have its base negatively biased and will thereforebe in the of condition permitting the base of an n-p-n transistor 02 to be positively biased via resistor chain R24 and R25 which is fed from a line 13 connected via line 14 to the positive pole of the smoothed d.c. power supply provided by MR1 and associated components. 02 will, therefore, be in the on" condition and the resulting collector current will be drawn through R31 through the gate of a bi-directional gated rectifier GRl. This is a type of rectifier which, when actuated by the application of a gate current, will pass current in both directions, that is to say, it may be used as an alternating current switch. One form of such rectifier is known as a triac. The flow of current from the gate GRl to the collector of 02 causes GRl to become triggered and consequently the live mains supply connected to terminal 6 is passed through OR! to terminal 2 which is connected to the feed valve (not shown) which now opens. Similarly, the output of 1C2 will be in the negative condition causing transistor n-p-n O3 to be switched off" and permitting base current to flow via resistors R39 and R40 into the base of n-p-n transistor ()4 which will become switched on". The collector current of Q4 will be drawn through the gate circuit of bi-directional gated rectifier GR2 which, when p-n-p transistor O7 is in the off" condition, would cause GR2 to be triggered and the mains supply connected to line 13 is passed through GR2 to terminal 1 which is connected to the drain valve and would cause this valve to open. However, in the condition above described transistor 02 is in the on condition, causing base current to be drawn through resistors R31 and R33 from the base of transistor Q7 which causes O7 to be switched on, effectively short circuiting the gate circuit of gated rectifier GR2 and preventing GR2 from becoming triggered. Under the circumstances above described, in the absence of output from bridge rectifier MR2 it will be seen that the circuitry causes the feed valve to be open and the drain valve to remain closed. Water now enters the boiler and as the electrode current rises due to the progressively deepening immersion of the electrodes, the voltage developed across C1 rises and, correspondingly, the input voltage developed across R3 also rises. When the input voltage applied to the inverting input of integrated circuit IC2 passes the biasing voltage applied to the non-inverting input of [C2, the output of IC2 will change to a positive state causing base current to flow into transistorQ3 which becomes switched on to permit the base of transistor O4 to become negatively biased via resistor R41, wherefore Q4 becomes switched off causing the flow of collector current through R42, and hence through the gate circuit of gated rectiflfer GR2, to cease.

As the boiler electrode current continues to rise further the input voltage applied to the inverting input of [Cl will pass the biasing voltage applied to the noninverting input of ICl, causing the output of this integrated circuit to become positive and causing base current to flow into the base of transistor 01, which becomes switched on. This permits the base of transistor O2 to become negatively biased via resistor R29 such that Q2 becomes switched off and the collector current through R31 and through the gate of gated rectififer GRl ceases to flow. Gated rectifier GRl then ceases to be triggered and the mains live feed to the feed valve via terminal 2 is switched off. Simultaneously, when 02 has become switched off its collector voltage will rise to approximately the +12 voltage and base current will cease to flow into the base of transistor 07, which also becomes switched of Under these circumstances gate current still does not flow through gated rectifier GR2 due to the fact both that gate current is no longer supplied by the collector of transistor Q4 and also because the short circuiting effect of transistor Q7 has been removed. Under this condition the feed valve and the drain valve will be in the closed position and the water in the boiler will become heated and begin to boil away. The electrode current may continue to rise above the operating point current, as may be the case with a boiler starting up from cold, due to the fact that the operating point current may be reached before the temperature of the water has reached boiling point. Underthese circumstances, both feed and drain valves will be closed but the electrode current will rise due to the increasing conductivity of the water as its temperature rises and also due to expansion of the water causing deeper immersion of the electrodes. Such rising electrode current above operating point current will cause the voltage developed across R3 to exceed that corresponding to the operating point current and typically at about 110% of the operating point current it is arranged that the voltage applied to the inverting input of integrated circuit [C3 will pass the biasing voltage applied to the noninverting input of lC3. This will cause the output of lC3 to change from the negative to the positive condition and hence base current will flow through R46 into the base of n-p-n transistor 05, causing O to be switched on. As a result, the collector current of Q5 will be drawn through the gate of gated rectifier GRZ via resistor R44 and GRZ will become triggered, thus opening the drain valve. Water will then drain from the boiler and as a result of decreasing immersion of the electrodes the electrode current, and consequently the voltage developed across R3, will also fall. As a result of integrated circuit 1C3 changing state, the biasing voltage applied to its non-inverting input will have been slightly increased due to current flowing through the resistor chain R37 and R45, thus introducing an element of hysteresis into the circuit. The values of R37 and R45 will be so chosen that as the current falls to approximately the operating point current, integrated circuit lC3 will again change state andits output will become negative, thus applying negative bias to transistor Q5. Transistor Q5 will therefore be switched off and gate current will cease to flow into the gate of gated rectifier GR2 which will cease to be triggered and permit the drain valve to close. During the heating period of cold water to boiling point several successive operations of the drain valve in this manner may take place.

When the water in the boiler has reached boiling point in the manner above described it will gradually boil away causing progressively less immersion of the electrodes and consequently progressively lower electrode current and progressively lower input voltage across R3. As the input voltage to the non-inverting input of lC2 falls in this manner, it will pass the bias voltage applied to the non-inverting input and 1C2 will again change state causing transistor Q3 to become switched off as a result transistor 4 becomes switched on causing gate current to flow into the gate circuit of gated rectififer GR2 via resistor R42 and hence again opening the drain valve.-

The hysteresis circuit is also applied to integrated circuit lCl, comprising resistors R13, RVl and R16. This hysteresis circuit will have caused the biasing voltage applied to the non-inverting input of [C1 to be changed to a value corresponding to typically between 40 percent and 70 percent of the operating point current according to the setting of potentiometer RVl. The circuit values associated with [C2 will have been selected such that the drain valve opens on falling electrode current as described above at approximately 80 percent of the operating point current.

Water will then drain from the boiler and the electrodes will become progressively less immersed and consequently the electrode current and the input voltage developed across R3 will progressively decrease. When this input voltage applied to the inverting input of [C1 passes the change bias applied to the noninverting input of 1C] as a result of the hysteresis circuit described above, lCl will again change state and its output becomes negative, causing transistor O1 to be switched off. As a further result transistor 02 will become "switched on" causing gate current to be drawn through the gate of gated rectifier GR! via resistor R31 and again opening the feed valve. A second result of 02 becoming switched on will be that base current flows into the base of transsitor 07 through resistor R33 causing O7 to be switched on and effectively short circuiting the gate circuit of gated rectifier GR2 resulting in the drain valve becoming nonenergized and closing.

As water now enters the boiler the electrodes will become progressively more deeply immersed, resulting in an increase in electrode current and a corresponding increase in voltage developed across R3. The electrode current will continue to rise until the input applied to the inverting input of integrated circuit lCl passes the bias voltage applied to the non-inverting input of lCl, whereupon ICl will again change state and by virtue of the circuitry described above the feed valve will be closed and the drain valve will remain closed. Water will then become boiled away from the boiler and the cycle of boil, drain and feed will continue repetitively. By varying the setting of potentiometer RVl the point at which the feed valve re-opens and the drain valve is closed can be varied at will between approximately 40 percent and percent of the operating point current. This varies the amount of water drained from the boiler on each cycle and with the typical values given, this may be arranged to be anything between half the quantity which is boiled away (corresponding to a 70 percent setting) or double the quantity boiled away (corresponding to a 40 percent setting).

As previously explained, an additional electrode known as the boiler full sensing electrode is provided.

FIG. 4 shows the upper part of a boiler casing 41, similar to that of FIG. 3, showing the

electrodes

42 and 43 with their

respective feed lines

44 and 46. The additional boiler full" sensing electrode is shown at 49 connected by a line 51 to terminal 12 of FIG. 1. The electrode 49 is of such length that its lower end is at a level indicated in dotted lines at 50, which is the maxi mum level to which the boiler is to be filled; when the water rises to this level it comes into contact with the electrode, which then receives a potential lying between the neutral and live mains potentials. In the three phase boiler it is convenient to arrange the electrode at a position equally spaced from the three phase electrodes and in this way its potential is close to that of the neutral line of the mains supply. In a single phase boiler it is convenient to arrange this electrode near to the neutral end of the heating electrode.

When the'boiler full sensing electrode becomes immersed current flows from terminal 12 through a capacitor C3 and resistor R6 to a Zener diode ZD4 so that half wave pulses appear across ZD4. These pulses pass via a diode D9 and charge a capacitor C7. Two

n-p-n transistors Q8 and Q9 are arranged in a circuit commonly referred to as a Schmitt trigger, together with associated resistors R21, R22, R27, R47 and R30. In the absence of input signals from C7 the Schmitt trigger is in the condition in which Q8 conducts and O9 is non-conducting. When the cylinder fullensing electrode becomes immersed a voltage is developed across capacitor C7 changing the state of the Schmitt trigger such that Q8 becomes non-conducting and 09 conducts. When Q9 conducts, gate current -'is drawn through the gate of bi-directional gated rectifier GR3, causing GR3 to become triggered and a mains live alarm signal to be passed to the boiler full output terminal 9. Additionally, current is drawn through R18 causing p-n-p transistor 06 to be switched on." Transistor Q6 then effectively short circuits the gate circuit of gated rectififer GRl, preventing GRl from being triggered and, therefore,.preventing opening of the feed valve. After the boiler full sensing electrode has become immersed the feed valve can be switched off" and boiling in the boiler continues until the water level has fallen to a point at which the boiler full sensing electrode is no longer immersed-At this point the Schmitt trigger comprising 08 and Q9 reverts to its original condition, with O8 conducting, and the feed valve is permitted to open. The boiler then refills until the boiler full sensing electrode again become immersed and this process continues repetitively.

The boiler full sensing circuit comprising C7, R14 and R15 is arranged to have an overall time constant such that when the feed valve is permitted to open slight-over-filling takes place, with the result that successive openingsof the feed valve take place at reasonable intervals of time, for example one to five minutes.

When the boiler electrodes are at the end of their useful life, or at other times when maintenance work is required, it is necessary to drain the boiler, so a manual control means is provided for this purpose. A double pole single throw switch is provided and is connected to terminals 3, 4 and 5. When this switch is closed to connect terminal to terminal 4 line 13 is connected to the gate of CR1, thereby short circuiting the gate and preventing operation of GR], so that the feed valve is prevented from opening. Line 13.is also connected via terminal 3 directly to the drain valve terminal 1 so that the drain valve is opened and the boiler is allowed to empty completely. v

FIG. 2 shows the circuit of a controller which is arranged to carry out a slightly more elaborate cycle con-. sisting in four steps in the sequence feed, boil, feed, drain. The additional feed step provides extra purification since it is then possible to drain a larger quantity of water from the boiler, thereby removing a large quantity of solids with the drained water. Referring to this Figure, a transformer, in conjunction with a rectifier MRI 1, resistor R54, capacitors C24 and C25 and Zener diode ZDl 1, provides a smoothed and stabilized dc. power supply equivalent to that described in connection with FIG. 1, except that it provides a single output at volts.

Terminals

23 and 24, equivalent to

terminals

7 and 8 of FIG. 1, receive the output of the secondary winding of the current transformer, which is rectified and smoothed by MR12, R51, R52, C22, C23 and ZD12.

Terminals

25 and 26 are for connection of a variable resistor which, when connected, is in series with R21 across

terminals

23 and 24 to provide a load as descriform of such rectifier is known as a traic."'The flow of current from the gate of GRll to the collector of Q12 causes GRll to become triggered and consequently the live mains supply connected to a terminal 27 and a line 29 is passed through GRll to a terminal 28 which is connected to the feed valve. Water now enters the boiler and as the electrode current rises, due to the proger ly deepening immersion of the electrodes, the voltage developed across C23 also rises until the voltage corresponding to the selected operating point current is reached. This voltage is applied via a diode D16 and a resistor R82 to the base of 012 so that the Schmitt trigger circuit changes to the triggered state in which Q13 conducts and 012 is non-conductive. The gate current of GRll is consequently reduced below the holding" level and GR ceases to conduct, thereby removing the energizing supply from the feed valve, so that the feed valve closes. As 013 changes into the conducting state a pulse is transmitted via a capacitor C28 to a known type of pulse steering circuit composed of a resistor R88 and diodes D17 and D18.

Two transistors Q18 and 019 are connected in a bistable multivibrator circuit which includes resistors R80, R79, R77, R78 and capacitors C30 and C31. Due to the pulse steering circuit constituted by capacitor C28 and diodes D17 and D18, this circuit changes its state each time the pulse steering circuit receives a pulse via capacitor C28. That is to say, the bistable multivibrator changes its state each time the electrode current rises to the operating point current.

When the bistable multivibrator changed to its first state in which Q19 conducts, Q19 draws its collector current through the gate of a further bi-directional gated rectifier GR12 so that GR12 is triggered and the mains supply on line 29 is transmitted to a terminal 30 which is connected to the drain valve, whereby the drain valve is energized and opened. This allows water to flow out of the boiler so that the electrode current is progressively reduced, and consequently the voltage developed across capacitor C23 is also reduced. After a certain quantity of water has been drained the voltage across C23 falls to the level corresponding to the drain period lower limit current, at which the Schmitt trigger circuit 012 and Q13 reverts to its former state with Q12 conducting Q13 non-conducting. As soon as 012 begins to conduct a gate current again flows through GR11 and the feed valve is again opened.

Since the bistable circuit is in the state in which Q19 is conductive the drain valve would also remain open and this condition is unacceptable, as the resulting water flowv into and out of the boiler would depend upon water pressures and water valve characteristics. An additional circuit is therefore provided comprising a transistor 020 which will be referred to as the drain valve inhibitor transistor. The collector of 012 is connected via R84 to the gate of CR1! and thence to the line 29. When 012 is non-conductive the collector current is negligible and the voltage at the collector of Q12 is substantially at the same level as line 29, so that the current into the base of Q20 is minute and Q30 is nonconductive. When 012 is conducting there is a voltage drop of some 9 to l3 volts across R84 and the collector of Q12 is negative with respect to line 29 by this amount. Consequently current flows through a resistor R into the base of Q20, which becomesfully conductive and the collector/emitter circuit of 020 short circuits the gate of GR12 to the line 29 so that GR12 is untriggered and therefore off. Hence, when. the bistable multivibrator is in its first condition in which 019 conducts, and at the same time the feed valve is open to allow water to enter the boiler, the drain valve is prevented from opening by the action of thedrain valve inhibiting transistor Q20. When, under this condition, the electrode current has increased to the operating point current, the pulse produced when the Schmitt trigger 012/013 changes state causes the multivibrator to trigger to its second condition in which 018 conducts. Under these circumstances both the feed valve and the drain valve remain closed and boiling of the water in the boiler continues until the water level has been reduced to the point at which the voltage across C23 is reduced to the level at which the Schmitt trigger 012/013 again reverts to the condition in which 012 conducts. 7

The circuitry so far described provides automatically the successive functions of feed, drain, feed, boil, feed, etc. which is desired. The difference between the operating point current and the boiling period lower limit current depends upon the difference in voltage level at which the Schmitt trigger 012/013 triggers and reverts to its prevous state and is determined by the component values in circuit. The difference between the operating point current and the drainperiod lower limit current is determined in the same way, the latter being approximately the same as the boiling period lower limit current. It follows that the amount of water boiled away during the boiling period is substantially equal to that drained away during the draining period. In areas where the water supply contains a comparatively large quantity of dissolved solids it may be desirable to arrange that a greater quantity of water is drained away during the draining period than is boiled away during the boiling period so that the water purification cycle is increased to prevent an unduly high build-up of precipitated solids in the boiler. A special circuit is provided for this purpose and is brought into operation when a connection is made by means of an on-off switch between two

terminals

31 and 32.

When 018 of the bistable multivibrator is conducting and 019 is non-conductive GR12 is untriggered and the drain valve is de-energized. This condition corresponds with that existing during the boiling period. When 018 is conducting it causes a further transistor 017 to conduct as the base current of 017 is derived via resistors R86 and R90. When 017 is conductive and the switch connected to

terminals

31 and 32 is closed a further transistor 016 also conducts as it receives base current through the collector of 017 via a resistor R76 and the switch, with resistor R75 connected between base and emitter. When 016 is conductive a resistor R74 is effectively placed in parallel with R73 and modifies the voltage at which the Schmitt trigger 012/013 reverts to its original state. In this manner closure of the switch between

terminals

31 and 32 enables the boiling period lower limit current to be raised while the drain period lower limit current remains the same. In a practical case the components may be selected so that the quantity of water drained away during the draining period is double that boiled away during the boiling period. Clearly it is a matter of choice to provide a variable resistor in place of R74 so that the ratio between the twowater quantities may be varied at will.

As previously explained, the electrode current may rise above the operating point current during the warming-up period, when the conductivity of the water may incrase due to increasing temperature and its level may rise due to expansion. Circuitry is therefore provided to ensure that the increase in electrode current is not excessiv'e. This comprises a further Schmitt trigger 014 and 015 with its associated components R69, R70, R66 and R56. The component values of the 014/015 Schmitt trigger are arranged so that when the electrode current is within the normal operating range 014 is conductive and this condition is secured by base current flowing through resistor R66 into the base of 014. When the voltage across C23 rises to a value corresponding to an electrode current approximately llO percent of the normal operating point current a negative feed is provided to the base of 014 via a diode D4,

and a resistor R67 and this causes the Schmitt trigger 014/015 to change state so that 015 is conductive. The collector current of 015 then flows through resistor R65 from the gate of GR12 so that GR12 is trig gered and the drain valve is opened. The water level in the boiler is thus reduced to the level at which the Schmitt trigger 014/015 reverts to its normal state, when 014 conducts and 015 is non-conductive. The GR12 trigger current is consequently cut off and the drain valve is closed. The component values associated with the Schmitt trigger 014/015 are so selected that at the point at which the drain valve is closed the voltage across C23 corresponds substantially to that at which the electrode current is equal to the normal operating point current.

As previously explained, an additional electrode konwn as the boiler full sensing electrode is provided and is so placed that it only becomes immersed when the water level in the boiler has reached the maximum permissible level. This electrode is so positioned in the boiler that when the water comes into contact with it the electrode receives a potential which lies between the neutral and the live mains potential. ln a threephase boiler it is convenient to arrange the electrode at a position equally spaced from the three-phase electrodes and in this way its potential is close to the neutral of the mains supply.

When the boiler full sensing electrode becomes immersed a current flows from a terminal 23 through a capacitor C33 and a resistor R57 to a Zener diode ZD13 so that half wave pulses appear across ZD13. These pulses pass via a diode D10 and charge a capacitor C32. A further Schmitt trigger, 021/022 and associated resistors R58, R59, R61, R62 and R64 is provided andin the absence of input signals from C32 it is in the condition in which 021 conducts, producing a base current which passes through R63 to the base of 021. When the boiler full sensing electrode becomes immersed a voltage is developed across C32 which triggers the Schmitt trigger 021/022 into the condition in which 022 is conductive. The collector current of 022 is drawn via a resistor R60 from the gate of a further gated rectifier GR13, so that GR13 is triggered and the live main line 29 is connected to a terminal 34 to energize a boiler full lamp or any other desired external warning device. As 022 becomes conductive its collector goes negative with respect to the line 29 and current flows through a resistor R92 to the base of a p-n-p transistor 011 so that 011 becomes conductive. The collector and emitter of 011 effectively short circuits the gate of GRll to the line 29, thereby preventing GRll from triggering so that opening of the feed valve is prevented. Qll is referred to as the feed valve inhibiting transistor. After the boiler full sensing electrode has become immersed the feed valve is therefore kept switched off and boiling in the boiler continues until the water level has fallen to a point at which the boiler full sensing electrode is no longer immersed. At this point the Schmitt trigger Q2l/Q22 reverts to its original condition with 021 conducting and the feed valve is permitted to open. The boiler then refills until the boiler full sensing electrode again becomes immersed and this process continues.

The boiler full sensing circuit comprising C31, R57, ZD13, C32 and R63 is arranged tohave an overall time constant such that when the feed valve is permitted to open slight overfilling takes place, with the result that successive openings of the feed valve take place at reasonable intervals of time, for example-1 to 5 minutes.

When the boiler electrodes are at the end of their useful life, or at other times as may be convenient, it is necessary to provide a manually controlled means for draining the boiler. For this purpose a double pole single throw switch is provided and is connected to terminals 35, 36 and 37. When this switch is closed line 29 is connected to the gate of GRl 1, thereby short circuiting the gate to line 29 and preventing operation of GRl 1, so that the feed valve is prevented from opening. Line 29 is also connected directly to the drain valve terminal 30 so that the drain valve is opened and the boiler is allowed to empty completely.

I claim:

1. An electronic controller for controlling, in accordance with a repetitive cycle, an electrode boiler in which the heating current passing through an electrode depends upon the depth of immersion of the electrode and hence upon the water level in the boiler, the controller comprising circuitry for initially operating an electrically controlled feed valve to allow feed water to enter the boiler, first sensing means in the circuitry to detect when the electrode current has risen to a level corresponding to a desired normal water level, means responsive to the first sensing means to cause the feed valve to be closed, whereupon the water is allowed to boil, second sensing means to detect when the electrode current falls to a level corresponding to a water level lower than the said normal water level due to loss of water boiled away, and means responsive to the second sensing means to initiate a desired sequence of steps including the opening of a drain valve to allow a quantity of water to drain from the boiler and the subsequent closure of the drain valve and the re-opening of the feed valve to allow the boiler to re-fill to the said normal level.

2. A controller as claimed in claim 1 in which the means responsive to the second sensing means comprises means to cause an electrically controlled drain valve to be opened whereupon water is allowed to drain from the boiler, the circuitry also comprising third sensing means to detect when the electrode current falls to a level corresponding to another water level due to loss of water drained from the boiler, and means responsive to the third sensing means to cause the drain valve to be closed and the feed valve to be opened, whereupon feed water is again allowed to enter the boiler, the first sensing means and the means responsive thereto causing the feed valve to be closed when the electrode current has again risen to a level corresponding to the said normal water level so that boiling may recommence, the circuitry being arranged to carry out the feed, boil, drain cycle repetitively.

3. A controller as claimed in claim I in which the means responsive to the second sensing means comprises means to re-open the feed valve, the circuitry comprising also further means responsive to the firstsensing means to cause the feed valve to be closed and an electrically controlled drain valve to be opened when the electrode current has again reached a level corresponding to the said normal water level, third sensing means to detect when the electrode current has fallen to a level corresponding to a further water level,

ahd means responsive to the third sensing means to cause the drain valve to be closed and the feed valve to be re-opened, the first sensing means and the means responsive thereto causing the feed valve to be closed when the electrode current has again risen to a level corresponding to the said normal water level so that boiling may recommence, the circuitry being arranged to carry out the feed-boil, feed, drain cycle repetitively.

,4. A controller as claimed in claim 1 comprising additional sensing means to sense when the electrode current has risen to a level corresponding to a maximum water level, and means responsive to the additional sensing means to cause the drain valve to open, thereby draining water from the boiler and bringing about a reduction in the electrode current.

5. A controller as claimed in claim 1 in which the circuitry comprises means to prevent the opening of the drain valve while the feed valve is open.

6. A controller as claimed in claim 1 comprising a current transformer having a primary winding for con nection in series with a boiler heating electrode, and a secondary winding, the controller including means to sense the voltage developed in the transformer secondary winding.

7. A controller as claimed in claim 6 in which the sensing means comprise two-state circuits which change state when the voltage of the transformer secondary winding reaches predetermined levels.

8. A controller as claimed in claim 1 a boiler full electrode for insertion in the boiler, the controller including circuit elements which cause the feed valve to be closed when the boiler full electrode becomes immersed, to prevent overfilling of the boiler.

9. A controller as claimed in claim 8 comprising circuitry to activate warning means when the boiler full electrode becomes immersed.

10. A controller as claimed in claim 1 in which the circuitry comprises means to vary the amount of water drained from the boiler in relation tothe amount dawayrivri s aqhsysls-m lli A controller as claimed in claim 1 comprising means to enable the drain valve to be opened and kept open in order to drain the boiler.

Claims

3. A controller as claimed in claim 1 in which the means responsive to the second sensing means comprises means to re-open the feed valve, the circuitry comprising also further means responsive to the first sensing means to cause the feed valve to be closed and an electrically controlled drain valve to be opened when the electrode current has again reached a level corresponding to the said normal water level, third sensing means to detect when the electrode current has fallen to a level corresponding to a further water level, and means responsive to the third sensing means to cause the drain valve to be closed and the feed valve to be re-opened, the first sensing means and the means responsive thereto causing the feed valve to be closed when the electrode current has again risen to a level corresponding to the said normal water level so that boiling may recommence, the circuitry being arranged to carry out the feed-boil, feed, drain cycle repetitively.

4. A controller as claimed in claim 1 comprising additional sensing means to sense when the electrode current has risen to a level corresponding to a maximum water level, and means responsive to the additional sensing means to cause the drain valve to open, thereby draining water from the boiler and bringing about a reduction in the electrode current.

6. A controller as claimed in claim 1 comprising a current transformer having a primary winding for connection in series with a boiler heating electrode, and a secondary winding, the controller including means to sense the voltage developed in the transformer secondary winding.

8. A controller as claimed in claim 1 a ''''boiler full'''' electrode for insertion in the boiler, the controller including circuit elements which cause the feed valve to be closed when the boiler full electrode becomes immersed, to prevent overfilling of the boiler.

10. A controller as claimed in claim 1 in which the circuitry comprises means to vary the amount of water drained from the boiler in relation to the amount boiled away during each cycle.

11. A controller as claimed in claim 1 comprising means to enable the drain valve to be opened and kept open in order to drain the boiler.