CA1291785C - Water heating apparatus - Google Patents

Abstract

A B S T R A C T

Water heating apparatus by flow of electric current through the water from a set, for example, of eight electrodes whose surface areas for current flow are proportional to the value of two raised to the eight exponents zero to seven, that is 1 to 128. This gives an area range of 1 : 256 which can be switched in binary digital manner automatically by solid state circuitry in response to a temperature window, electric current flow or other indicators. This gives automatic compensation for wide water conductivity ranges and flow rates without any moving parts to give a practical mass-produced industrial or domestic water heater.

Description

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9~785 , 1. Field of the Invention ~his invention ooncerns improvements in and relating to water heating aFearatus ror use in any type of application, domestic, in~us~rial or other. More particularly the invention is concerned with such apparatus which uses the medium of the water for the heating effect of the electric current, for example, by means of uninsulated electrodes immersed in the water.

2. Prior Art A large amount of water heating is done by means of thermostat controlled electric heating of the water in a drum, certainly in domestic applications a vast majority of cases. This allows the water to be heated 810wly by a relatively low capacity heater and stored at the required temperature in the drum from which it can be drawn from time to time. The serious disadvantage is very poor efficiency of this system due to heat loss from the drum since the water is held at a high temperature largely continuously. Despite this disadvantage this system is adopted widely probably kecause it is technically simple and inexpensive and the electrical FDwer demand is relatively low albeit for long periods.

Demand water heater~ are in use on a smaller scale where the elements heat only when a 10w of water through the apparatus is initiated , .... . . .

requiring relatively high electrical p~wer supply levels if a strcng flow of very hot water is to be supplied. The performance of such heaters is moreover vitally affected by the conductivity of the water which varies to a remarkable degree from place to place and from time S to time. So much so that fixed design demand heaters are likely to give unsatisfactory performance in many instances. Reference is made to Table A which shDws the extreme range, for example, from as low as 1 millisiemes at a Swartruggens minimum to 993 millisiemens at a Britstown maximum. The table shows that not only is the range extreme from place to place but also in many individual places.

Table B show8 that categorising result~ on a regional basis does not introduce any uniformity which would permit ~upplying apparatus specially adapted to suit each region.

Some designs have been proposed which include electrodes moveable in respect of the water to allow adjustment to compensate for variations in conductivity. It would seem that these designs, however, have not been successful probably because of high cost and low reliability as well as the difficulty that the average user especially, for èxample, in the domestic context cannot be relied cn to make suitable adjustments especially when the oonductivity of water in a single area swings between wide limits as Table A shows. Automated controls for servo motor actuated adjustment have not been seriously proposed certainly not in the domestic context, for example.

The "Electrical Engineer's Handbook" 1973 Ed., publ. Butterworth, at 9178~5 page 14-55 says: "...variation in water resistivity (which can be allowed for in large installations) makes mass production or domestic use very difficult." The "Electrical Engineer's Reference Book"
publ. G. Newnes Ltd., at page 15-24 says:"...the great variation that one may expect in the resistivity of water suwlies which while it can be easily allowed for in designing specific high-loading installations, presents an almost insuperable difficulty to mass productions for general domestic use." ~o solution for these problems is offere --TABLE A

Min. Max. Av. Mean ~edian 1541 Kommandb Drift, EP10,9 8S,9 55,3 33 1620 King Williams Town, EP 2~,0 83,6 52 50 1730 Queenstown, EP 2 26 10 8 1850 Umzinkulu, Natal 22 46 32 38 L920 P~burg, Natal 5 20 7 7 1182 Uniondale, SW Cape14 58 22 18 1210 Uitenhage, EP 15 23 19 20 1323 ~Jaterford, EP 5 550 158 154 1430 Grahamstown, EP 43 136 90 100 1513 Stennsburg, EP 21 81 32 23 0911 Laingsburg, Karoo 35 130 70 65 0925 Calitzdorp, Kaloo 9 ;3 30 42 ~,91785 1010 Mossel Bay, SW Cape72 227 139 176 1090 H~u~u~dorp, EP 18 28 23 24 1130 Willowmore, EP 25 850 298 273 2010 Bergville, ~atal 4 17 8 - 7 2030 Newcastle, Natalo 7 39 17 13 2113 Eshowe, ~at~l 15 26 20 22 2144 Pongola, Natal 6 114 40 95 2154 Ermelo, E.lvl 4 10 5 4 0121 Pretoria, Tvl 15 69 45 52 0130 Swaartruggers 1 49 19 9 0140 Nylstrocm, N.Tvl 5 8 6 5 0150 Potgietersrus, N.qvl 3 37 11 15 0160 Pietersrburg, N.Tvl 5 21 8 7 0180 Louis Trichardt, ~.Tvl 10 44 22 15 0190 Parfuri, ~.Tvl 5 55 21 17 0232 Groblersdaal, N.Tvl 6 38 21 16 0280 Duiwelskloof, N.Tvl 3 13 5 6 0312 Villiers, OFS 10 31 18 15 0390 Christiana, Tvl 11 127 52 77 0323 Potchefstroom, Tvl 17 79 51 59 0352 Bloemfontein, OFS 16 52 29 37 0420 Weppenaar, OFS 10 38 19 23 0434 Colesburg, Karo~ 10 28 16 17 0460 Britstown, Cape 58 993 352 329 0510 ClanWilliam, W.Cape 1 14 8 9 0710 Malmsbury, W.Cape 2 52 13 11 0740 Caledon, W.Cape 8 12 9 9 0810 W~rcester, W.Cape 4 15 7 6 0840 Robertson, W.Cape 6 17 10 7 S Read's ~oreh~le 17 L~west 1 8 5 4 Highest 58 993 . 352 329 1~3LE B

E.P. Natal S.W. Karoo E. N. W. OFS N. W.
Cape Tvl. Tvl. Tvl.Cape Cape Av. 46 76 30 97 41 10 16 43 24 109 8 High 273 95 176 65 16 52 77 37 329 11 Jt~R.S

Low 8 7 18 17 4 5 9 15 6 It is an object of this invention to provide water heating means by the heating effect of electric current flow in the water which is capable of compensating for wide water conductivity ranges, for example of the extent indicated above.

It is a further object to do this in a practical solution, thus without moving parts which can be subject to wear, sticking or other unreliability or poor durability.

SUMM~RY OF THE INVENTION
In accordance with this invention there is provided a water heating apparatus for water having an electrical conductivity, comprising:
(a) a plurality of electrode pairs immersed in water (b) ewitching means for selectively connecting one or more of the electrode pairs to a source of electrical power whereby the water is heated by the passage of electrical current therethrough each of the electrode pairs being configured for transmitting a different magnitude of electrical power into the water relative to the other electrode pairs; and (c) transducer means operatively responsive to the electrical current for controllina the switching means.

Preferably the plurality of heating elements are of different sizes one from another so that different selections of the ~?.9~'7~S

elements can provide a full choice of different heating effects over ther full range required.

Preferably the levels of electrical power associated with the electrode pairs are related substantially in proportion to bit values in a multiple bit binary sequence wherein the control of the switching means by the transducer means is effected by digital control circuitry having a multiple bit binary control sequence, the bits of the control sequence corresponding, respectively, to the bit values of the electrode pairs. For example, the combination of the elements can vary substantially in accordance with a binary sequence involving multiple bits.
Such a sequence can allow for the selection of any total heating effect with unit steps. For example, one could provide a range of over 256:1 in as many steps up to 256 as are desired. This can accommodate substantially the extreme range of conductivities which have been detected in waters as shown in Table A. The binary system is chosen because it can be executed directly in a digital control circuitry whether the elements are switched on or off to provide the binary sequence. The response to the detected characteristic will thus be to control the switching of the elements in a required manner.

Preferably the transducer means comprises means for measuring an output temperature of the water.

Thus a "window~ of output temperatures, for example, a range say between 60C and 70C or, for example 45C to 50C for a ~ ,~

~?~9~7~3~5 practical bathing temperature, is specified for control of the response. When the temperature is detected to exceed this range the response is to reduce the total value of the elements switched in and conversely.

Alternatively the detected characteristic may be the total heating current, however, preferably this characteristic is used to influence the response only to control an upper limit of heating current, for example, such as will be convenient in any particular electrical installation such as, for example, 40 amps in a multiple or single phase application. Thus the system will be inhibited from delivering more than 40 amps even if the output temperature falls below the defined window. Making the response dependant only on the detected heating current is not preferred because at very low liquid flow rates of the demand type system and in a heat in tank type sy~tem this could lead to boiling of the water at the elements.

The invention is preferably applied in the context of a demand heater where the heating elements switch on only when water flow through the apparatus commences, in which the switching means comprises suitable solid state circuitry and the apparatus has the advantage of fixed structure with no moving parts. In such apparatus at least of of the elements may be sub-divided into sub-elements electrically connected in parallel to facilitate convenient overall dimensions for the apparatus, the sub-divided element being configured for transmitting a greater magnitude of electrical power than at least one of the other elements.

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Preferably the switching in of the heating elements is executed by a suitable solid state circuitry and it will thus be appreciated that the apparatus has the advantage of fixed structure with no moving parts. Nevertheless exhibiting the extreme flexibility necessary to cope with the conductivity ranges described and with wide variations in flow rates through the apparatus in the case of a demand heating application.

Preferably the lay out of heating elements which are immersed in the water is adapted to provide for water flow past all areas of the electrodes so as to avoid local overheating and preferably earth screens are used at the inlets and outlets to the apparatus for protection against earth leakages and/or neutral connection failures.

-9a-~;?;9~t785 BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully described by way of an example with reference to the accompanying drawings in which:

Figure 1 is a schematic general arrangement of the apparatus, Figure 2 is a schematic elevation of the interior showing the heating elements of the apparatus between top and bottom panels, Figure 3 shows elevation of a S7 de closure wall, Figure 4 is a plan view of the heating element arrangement, that is a schematic view of the electrodes, Figure 5 is an electrical diagram of the heating circuitry showing the ~ignificant bit arrangement in an 8 bit example, Figure 6 is an electrical diagram showing a typical switching control circuitry using an 8 bit example; and Figure 7 is a pictorial schematic diagram showing a simplified configuration of the apparatus of Fig. 1, including a portion of the control circuitry of Fig. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in Figure 1 the apparatus 1 comprises a heating unit 2 having a water inlet 3 and an outlet 4 both connected by about 1 metre of electrically non-conductive conduit, e.g. rubber hose to allow for 1~91785 earth leakage safety protection to the extern21 water supply at inlet coupling 5 and outlet coupling 6. The inlet stretch of rubber hose 7 has an earth screen 8 and the outlet stretch of rubber hose 9 has a earth screen 10. The earth (E) conductors are connected to both external oouplin~s 5 and 6, or the piping that both inlet and outlet 3 and 4 are connected to should be piping of metal ~electrically conductive) construction, the neutral ~) conductors are connected to the neutral heating electrodes inside the unit 2 and the live (L) conductors to the live heating electrodes inside the unit 2. This is designed to provide protection against earth leakages and neutral connection failure. If the neutral disconnects then even in the case of hi~h water oonductivities the end user i5 ~till preserved at electrical earth potential or an earth leakage protection unit in the supply will trip. I the earth is working and no trip occurs then, of course, any current present is insufficient to be hazardous.
If the earth leakage is not wor~ing and the neutral is disconnected then the end user is still at electrical earth and thus not exposed.
The earth screens are in between electrified water and the electrically earthed external couplings 5 and 6. In addition a sufficient, e.g. around 1 metre length of rubber hose between the heating unit 2 and the neutral screens provide protection.

Figures 2 to 4 show the layout of the heating electrodes within the heating unit 2. The unit 2 i5 a ~lat box and al} the electrodes 11, 12 are mounted in sealing contact between the base 20 and lid 21 ~ ~9~785 of the box. The live electrodes 11 are sealed to the right hand wall 22 of the box but terminate short of the left hand wall 23 while all of the neutral electrodes 12 are closed to the left hand wall 23 of the box and terminate short of the right hand wall 22. This defines a zig zag path for the water to follow through the unit being heated by the electrical current which flows through it from the live electrodes 11 to the neutral electrodes 12 as it passes down the passages. Non-conductive inserts 24 are provided. The electrode lengths are shown, and reference is made to Table C. The water then exits from the exit 4. The conductors all have the same height so that their effective conducting areas are determined by their lengths and these lengths correspond to the bit values of the, for example 8 bit, binary code. It is this automated variation of total ~conductive area~ exposed to the water for electric current ~low, controlled by digital electronics or other state-of-the-art solid state switching means, that allows for the variance in the conductivity of the water.

The electrodes shown represent in some cases sub-elements of the multiple heating elements which can be switched in in any required combination. Table C indicates an example of the relationships in an 8 bit binary system.

~BLE C

Binary Length Arrangement Bit Digit of electrode of sub elements value - -Significan oe element 7 5400 mm 10 x 540 mm 128 `~`
.. , ., . . . . . . ~ .
6 1450 mm 5 x 290 mm 64 - 5 580 mm 4 x 145 mm 32 4 230 mm 2 x 115 mm . 16 3 101 mm 1 x 101 mm 8 2 . 58 mm 1 x 58 mm 4 - -1 29 mm 1 x 29 mm 2 ` l~
, ~ 12 mm 1 x 12 mm As will be seen the 8 elements provide in effect 8 binary bits of an 8 bit byte for a binæ y digital electronic control system. The column under "Bit value" shows that from the 8 digits a total of 256 steps are eossible. The colu~n under "length" shows the effective tot 1 length of the electrodes corresponding to each bit and the column under "arrangement" shows how the longer electrodes are sub divided into sub elements in the manner indicated. ~eference to figure 2 in comparison with Table C will show the layout thereof. Within a window of control, that is a range of values of output temperature this selection of lengths allow3 a combination which will satisy the control w ndow over the very wide range of different conductivities and flow rates to be catered for. As stated an overriding control ~?~91785 is p2ovided to ensure that the total current supplied does not exceed a specified limit appropriate to the capacity of the installation.~

Figure 5 shows the electrical connections and here the same reference numerals are used for the electrodes as were used in figure 2. This connection diagram shows how each of the 8 bits are made up by Y appropriate ccnnections of the electrodes in the heating unit 2. As -. . , . ~ , . . . .
-~ will be seen all neutral conductors are inter-connected in parallel to "
: the sueQly. The various livé conductors are çonnected according to - their groupings of binary bits to thyristors or triacs to control , " .. , , ., ,,, , . , . . ,: , , . : , , , - , 1~ switching the elements as required. m e numbers in the circles are the signiicant binary bits and the number zero, of course, represents the least sig~iicant bit and 7 thé mcst ~ig~ificant bit. During all switching ~equences the least ~igniicant bit is switched first on or off so that under a control actuation the system climbs or drops to the required total conducting area for any set of conditions through the least significant bit steps of the binary number system. In this arrangement thi~ corresponds to a 12 mm length of the electrode.
Standard control theory may be implemented to ensure avoiding current peaks or instabilities in current control.

Figuxe 6 shows, for example, in the case of the bit 0 how the live conductors are connected by means a of triac or th,vristor, each triac or thyristor having a sufficient current carrying capacity for the sub element it feeds but all of them being controlled in parallel by a single control wire and all of them being supplied with power in ~ ,?..~9~.t7R5 parallel. Here the bits 0 through 7 are similarly switched.

The system is designed to monitor in the first place temperature, cut off being at 55C. The main advantage of this is that during slow flow rates the current drawn can be substantially reduced yet still achieving the required temperature rise from typical ambient temperatures of about 1~ -20C. This also will not allow the actual boiling of water should this system switch on while the tap is closed. Current, however, is in the second place monitored and the influence of this protection is merely to limit the maximum current supply to the system as a whole to 40 amps per phase which is a convenient maximum, for example, for a domestic installation. If a multiple phase system is in use it is moreover designed to simultaneously switch in all three phases ~egments so as to provide a balanced ioad at all times, also a matter of considerable importance which is favoured by the electricity supply authorities.

If it is preferred to have a lower maximum current, say 20 amps, the design parameters of the apparatus may be adjusted to achieve this.

It will be observed that in this arrangement the structure is permanent without moving parts and the electrodes are at a constant distance from each other designated by design.

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Fi~ure 7 shows a simplified configuration of the unit 2 having four bits of binary control. There are four of the live electrodes ll, designated lla, llb, llc, and lld in Fig. 7, the electrode lla having a length , the electrode llb having a length 2 , and the electrode llc having a length 4 . The electrode lld is segmented, each of the two segments having the length 4 . Thus the total length of the electrodes ll is 15 .
The zig-zag path of the water, from the water inlet 3 to the outlet 4 passes on opposite sides of each of the live electrodes ll as indicated by the arrows in Fig. 7, five of the neutral conductors 12 forming partitions that are equally spaced from the live electrodes ll as discussed above. Thus, by virtue of the uniform width of the live electrodes ll and the neutral electrodes 12, the heating power associated with each of the live electrodes 11 (at a given conductivity ~f the water) is proportional to its length.

As further shown in Fig. 7, each of the live electrodes 11 is selectively connected to the line power (L) by a counterpart of the triac/zero crossover circuitry of Fig. 6, the counterparts being designated drivers 31a, 31b, 31c, and 31d in Fig. 7. The drivers 31 are connected, respectively, to the counter outputs 0-3 of a 4-bit binary up-down counter 32, the counter 32 corresponding to one of the ~up/down count~ elements of Fig. 6.
The counter 32 is driven from a pair of ~-input NAND gate~ 33, designated 33a and 33b in Fig. 7, for incrementing or decrementing the counter 32 in re~ponse to the temperat~re and/or current sense circuitry as described above in connection -15a-~917~.5 with Fig. 6. In particular, a counterpart of the oscillator of Fig. 6, designated oscillator 34, is connected to respective inputs of the gates 33a and 33b for setting a rate at which the counter 32 is to be incremented or decremented. The other input of the gate 33a, labeled "Increase" is responsive to a state of the comparator circuitry of Fig. 6, indicating a need for increased heating power. Accordingly, the counter is incremented at intervals such as every half second while the nIncrease~ signal is active. Conversely, the counter 32 is decremented at the same rate while the "Decrease~ signal is active (when there is a need for decreased heating power). Thus the unit 2, in response to successive states of the counter 32 causes the water to be heated at rates proportional to successive ones of the integers 0~15, multiplied by the conductivity of the water.

The heating unit 2 should, of course, be water tight and should not allow leakage over, under or around the elements in any circumstances.

-15b-~;~91785 ThP system must be able to withstand a pressure of 120 psi at 65~C.

The system also has a flow restriction at the outlet; this is to limit the flow to a dèsired rate and to limit aeratin~ to a minimum within the system. This flow restriction may be made adjustable to allow adjustment on installation to suit local available water -`
' ' .' ~
pressure.

The electrodes may be of carbon. ' - . .: , . .. .
If exceptionally low conductivity ~pure) water gives problems the apparatus may ke given a "~alting" device at the water inlet to add a substance which will ionise in the water to increase the free ions and thus conductivity of the water.

Claims (10)

1. A water heating apparatus for water having an electrical conductivity, comprising:
a plurality of electrode pairs immersed in water;
switching means for selectively connecting one or more of the electrode pairs to a source of electrical power whereby the water is heated by the passage of electrical current therethrough each of the electrode pairs being configured for transmitting a different magnitude of electrical power into the water relative to the other electrode pairs; and transducer means operatively responsive to the electrical current for controlling the switching means.

2. A water heating apparatus as claimed in claim 1, in which the levels of electrical power associated with the electrode pairs are related substantially in proportion to bit values in a multiple bit binary sequence wherein the control of the switching means by the transducer means is effected by digital control circuitry having a multiple bit binary control sequence, the bits of the control sequence corresponding, respectively, to the bit values of the electrode pairs.

3. A water heating apparatus as claimed in claim 1, in which the transducer means comprises means for measuring an output temperature of the water.

4. A water heating apparatus as claimed in claim 1, applied in the context of a demand heater where the heating elements switch on only when water flow through the apparatus commences, in which the switching means comprises suitable solid state circuitry and the apparatus has the advantage of fixed structure with no moving parts.

5. A water heating apparatus as claimed in claim 4, in which at least one of the elements is sub-divided into sub-elements electrically connected in parallel to facilitate convenient overall dimensions for the apparatus, the sub-divided element being configured for transmitting a greater magnitude of electrical power than at least one of the other elements.

6. A water heating apparatus as claimed in claim 5, in which the apparatus includes a flat box with all the electrodes mounted in sealing contact between a base and a lid of the box with one electrode of each pair sealed to a first side wall of the box but terminating short of an opposite second side wall and the other electrode of each pair sealed to the second side wall and terminating short of the first side wall so defining a zig-zag path for flow of water to follow through the box from inlet to outlet.

7. A water heating apparatus as claimed in claim 6, in which the electrodes are uniformly spaced, having uniform width, and lengths approximately in the ratio of the binary bit values 128, 64, 32, 16, 8, 4, 2, and 1, enumerated from most significant bit to least significant bit, respectively.

8. A water heating apparatus as claimed in claim 1, in which earth screens are used at the inlets and outlets to the apparatus for protection against earth leakages and/or neutral connection failures and in which lengths of non-conducting hose connect between the inlet and outlet of the apparatus to external couplings.

9. A water heating apparatus as claimed in claim 3, in which the transducer means comprises means for measuring a total electrode current through the water.

10. A water heating apparatus as claimed in claim 9, in which a window of the measured output temperature is used for control, and in which only an upper limit of heating current is effected by the detection of total electrode current.