EP1290381B1

EP1290381B1 - A tubular-shaped heating element

Info

Publication number: EP1290381B1
Application number: EP01945250A
Authority: EP
Inventors: Fausto Fioroni
Original assignee: Thermowatt SpA
Current assignee: Thermowatt SpA
Priority date: 2000-06-08
Filing date: 2001-06-05
Publication date: 2006-03-29
Anticipated expiration: 2021-06-05
Also published as: CN1232781C; AU2001267519A1; HK1056768A1; ATE321985T1; ES2261423T3; CN1434915A; ITMO20000122A1; DK1290381T3; WO2001094861A1; EP1290381A1; DE60118378D1; DE60118378T2; IT1315636B1

Abstract

This invention describes a water heating element which can be used for both boilers and electrical appliances in general. A printed electrical resistor (4, 4.1) is applied to a piece of piping (3) used to dispense hot water, using a printing technique such as, for example, screen printing. The whole created by the said two elements constitutes the essential part of a heating element (1, 1.1) which is tubular and can have different sizes and conformations and it can incorporate other means required in water heating devices.

Description

Technical field

The invention relates to a tubular-shaped heating element for use, in particular, with boilers for sanitary purposes

Backgrownd Art

The heating devices currently used in the aforesaid boilers are generally the so-called 'armoured electrical resistors'. These are constituted of an resistive electric filament immersed in a chemically inert, electrically insulating powder, which is, in its turn, pressed into a hermetically sealed metal pipe.
The optimal thermal power dispensable per unit of surface (normally called the 'thermal load') is usually approximately 8.5W/cm2 for boiler resistors
It is practically impossible to vary the thermal load along the resistor. For the power to be dispensed mainly in the areas required, it is necessary to bend the resistor itself, either in loops or in a spiral.
With regards to the storage apparatus in which these resistors are used, it is important to make a distinction between the different types of apparatus, i.e. electrical storage heaters, the so-called "under-the-sink" electrical storage heaters, and fast-acing storage electric boilers. Electric storage heaters are generally characterised by a flange onto which one or more electrical resistors are mounted, a thermostat sheath containing one or more temperature sensors and, possibly, an anticorrosion device constituted of a magnesium anode or a cathodic protection device electrode. The inlet pipe, which is very short, just penetrates the tank and the said pipe dispenses the cold water into the bottom of the said tank. The output pipe runs through the whole tank to draw the hot water from the highest part.
The minimum installed power in this type of boiler is approximately 1,200/1,500W.
The so-called "under-the-sink" electric storage heaters are small boilers located inside the sink unit. To make the attachment to the tap unit easier, these boilers are mounted upside down and, consequently, so that the cold water is inserted in the lower part and the hot water is drawn from the upper part, the pipe that normally acts as the inlet pipe is then attached to the output and vice-versa.
The fast-acting electric boilers are similar in construction to the storage heaters, except that they may have a smaller tank to suit the function they have to fulfil, they have a higher level of installed power and, for safety reasons, they are often of the so-called "free-discharge" kind, i.e. the water contained in the tank is at room pressure.
The aforesaid armoured resistors have some drawbacks, a first of which is represented by the complex production process.
A further drawback of this type of commonly known resistance is constituted by the distribution of the thermal power achieved with a looped or spiral-shaped resistor: all forms require costly specific equipment.
Still further drawbacks are constituted by the existence of a neutral area at the end of the resistor, which no heat can be emitted from, by the tendency for lime-scale deposit, resulting in overheating, by the water in that area boiling (noisily) and by breakage, as well as the need for large openings for inserting the unwieldy forms of the looped or spiral resistor into the boiler.
The drawbacks of the current electric storage heaters are constituted, above all, by the need for a flange, as a sealing element, and a support for at least one or more resistors, a sheath for one or more thermostat probes, and, possibly, a magnesium anode or a cathodic protection system electrode.
A further drawback of this kind of boiler is that the water must be stored at at least the usage temperature, but normally at much higher temperatures than the fast-acting boilers or even higher than the instant boilers, with a lower level of installed power but with greater heat loss and lime-scale deposits forming faster, these latter having an effect on the corrosion.
A further drawback is that it is impossible, without using a complicated-shaped resistor, to concentrate a significant amount of the thermal power in the lower part of the tank in order to guarantee more uniform temperatures with less heat loss keeping the average temperature unchanged.
A still further drawback is constituted by the fact that with the armoured electrical resistors it is impossible to heat the water a final time when it is drawn from the device.
With regards to the fast-acting electric boilers or the current instant boilers, their most important drawbacks are constituted by the need to adopt larger electrical resistors than those used in the storage heaters, which consequently require larger flanges.
Before presenting the aims of this invention and describing the preferred embodiments and some possible variants, it is worthwhile mentioning the essential characteristics of a electrical resistor construction technique, which although a known technology, is still rarely used.
The said resistors are constituted of a metal support base, generally AISI 430 steel, capable of diffusing the thermal energy emitted and guaranteeing the adhesion of the overlaying layers even if heat expansion occurs.
The following are gradually applied to this metal support base:

one or more layers of dielectric material;
a printed electric circuit produced through the application of a special resistive paste through an adequate printing technology, such as screen printing for example;
a further layer or layers of dielectric material.
Hereinafter the term "printed resistor" will be used to indicate the whole unit constituted of the actual printed circuit and the layers of dielectric material underneath and above it, whether the circuit was made using a screen-printing technology or any other circuit printing technology.
The thermal loads obtainable with the printed resistors obviously depend on the capacity for heat elimination but, while the there are no particular increases in the cost of producing relatively low thermal loads in relation to those generally used in the sector, these can still be extremely high (at least 18-20W/cm2). As well as for heating liquid, the printed resistors can also function dry because they can resist up to 700° C in the air (250°C at the welded points on the electric terminals).
The subsequent layers can also be placed on curved surfaces.
The feeder cables are connected electrically to the printed resistive circuit at the end of the printed circuit, by welding in two points which are intentionally not covered by the second group of layers of dielectric material.
The document GB-2324014 discloses a heat exchanger comprising a cylindrical metal tube and an insert to define a helical water way and the said tube has a heating element in the form of a resistive and conductive track.

Disclosure of Invention

A first aim of this invention, in a device for heating water such as an electric boiler in which it is necessary to provide electric resistive means of heating the water and, possibly, means of checking the temperature of the water and/or, possibly, electrochemical means of preventing corrosion, consists in reducing the components required to realise the aforesaid means.
A second aim consists in simplifying the construction of one or more of the aforesaid heating, temperature checking and corrosion prevention means.
A third aim consists in simplifying the mounting of one or more of the aforesaid means on a device for heating water.
A fourth aim consists in limiting the lime-scale deposits that form on the aforesaid heating means.
A fifth aim consists in the possibility of eliminating the support flange from one or more of the aforesaid means.
A further aim consists in the possibility of distributing the thermal power to be emitted by the aforesaid heating means better.
A still further aim, for a storage heater, consists in enabling the water to be heated fully at the moment it is drawn, using the same heating means that heat the stored water.
A still further and final aim consists in reducing the manufacturing costs.
Achieving these and other aims is possible thanks to an original use of the commonly known technique for the application of printed resistors to a metal support.
In fact, the subject of this invention, in particular for heating water appliances for sanitary purposes, a heating element constituted of a tubular element designed to pipe water (for sanitary purposes at least) and to whose external and/or internal surfaces are applied one or more resistors printed in the way determined above.

Figure 1 is a section drawing of a heating element according to a preferred embodiment of the invention applied to the interior of a hot water storage tank;
Figure 2 is a total view of the heating element as per the embodiment in the previous figure;
Figure 3 is a section drawing, along the main axis, of the same elements shown in the previous figure with a possible variant of the invention;
Figure 4 is an enlarged section drawing, at right angles to the main axis, of a general heating element;
Figure 5 is a schematic diagram of a possible variant of the invention shown in figures 2 and 3, showing the flow of water inside the heating element during the heating phase of the said water and during the drawing phase;
Figure 6 is a schematic diagram of a further possible variant of what is shown in figure 2;
Figure 7 shows, extremely schematically, a fast-acting storage boiler using heating elements according to a variant of this invention;

There will now follow a description of a preferred embodiment of the heating element according to the invention which can be used extremely advantageously in a storage heater.
With reference to Fig.1 a heating element 1 according to the invention is shown inside the tank 2 of a storage heater. In the case given as an example, the heating element 1 is the hot water output pipe from the tank 2. As shown in figures 1 to 3, the heating element 1 is constituted of : a piece of piping 3 made of any material compatible with the printed resistor technology and with the chemical, physical and mechanical stresses to which the said piping 3 will be subjected, e.g. AISI 430 steel; a printed resistor 4 and, lastly, a pipe coupling 5. For the sake of clarity, only the actual printed circuit part of the printed resistor 4 is highlighted, although in actual fact this is hidden by the layers of dielectric material above and below it which, together with the aforesaid printed circuit constitute the printed resistors, as defined earlier. The openings 10 and 11 in the lower part and the upper part of the piping 3 are also shown.
The pipe coupling 5 is fitted with suitable connection means 6 and 7, for example, threaded bushings, to fix respectively, the said coupling 5 to both the tank 2, by means of a ring nut 8, welded to the tank 2 itself, and also to the hot water distribution system (not shown). The coupling 5 is also fitted with suitable connectors 9 for the electricity supply to the printed resistor 4, the said connectors 9 being connected electrically to the printed resistor 4 and insulated electrically from the remaining elements in a commonly known way not shown in the figure.
In figure 3, in addition to the elements already identified, there is a opening 11.b with a deflector 12 indicated, in the upper part of the piping 3, near its inlet 3.a.
In figure 4 there are (from the inside to the outside of the heating element 1): a possible layer of material 14 on which it is difficult for lime-scale to form deposits, such as polypropylene; the piping 3 onto which the subsequent printed resistor 4 will be applied, the said printed resistor 4 being constituted of one or more layers of a dielectric material anchored to the base of the support constituted by the piping 3; the actual resistive printed circuit and one or more external layers of a dielectric material. Last to be shown is a possible sheath 15 designed to protect the printed resistor 4 against abrasion.
Figure 5, which constitutes a possible variant of the invention as indicated in figure 3, shows, with arrows, the circulation direction the water will assume during the two functioning modes of a storage heater. More precisely, on the left is the circulation triggered by convective motions during the heating phase using the heating element 1 and on the right is the direction during the phase in which the hot water is drawn. The openings 10.a fitted with deflectors 12 positioned towards the inside, constitute, a variant of the openings 10
Figure 6 shows a possible variant of figure 5, where the openings 10.b are made in the piping 3 in a position with the section enlargements 13.
Figure 7 shows, mounted on a small tank 2.1, elements already shown in the previous figures: the arrows indicate the inlet and output water flow directions; although not shown in the figure, also the tank 2.1, in the same way as for the piping 3 in the previous figures, and at least for its entire cylindrical part, can constitute the support base for a printed resistor 4.1 completely identical to the printed resistor 4. The other elements indicated are identical or equivalent to those shown in the previous figures. Therefore, the fast-acting kind of boiler shown in the figure is fitted with at least two printed resistors: the aforementioned printed resistor 4.1 and the printed resistor 4 , the base element of which is constituted of a piece of piping 3 of the hot water output kind.
There will now follow a detailed description of the characteristics of the heating element 1 according to this invention.
First of all, it is obvious that the power supply to the electrical resistor 4 leads to the heating of the water stored in the storage tank 2 in a similar way to the armoured electrical resistors already in use but there are special features and advantages that will now be highlighted.
The printed resistor 4 has been drawn in the figure indifferently, whether it is constituted of fretwork or a spiral wound around the piping 3. In actual fact, this could have any route which does not intersect itself, the route could even be irregular to vary the thermal load along the length of the heating element in the most suitable way. There could be, for example, a double spiral route with a constant pitch which begins and ends at the pair of connectors 9, or a double spiral with a variable pitch.
There may be several adjacent printed resistors 4 printed on the same insulating layer, each said resistor terminating in a respective pair of connectors 9 with a view to make it possible to modulate the thermal power emitted by inserting one or more of the said printed resistors 4. There may also be several overlaid printed resistors insulated by as many insulating layers, each said resistor terminating in a respective pair of connectors 9.
These characteristics make it possible to concentrate the distribution of the thermal power generated by the printed resistors 4 in the areas where it is most useful: for storage heaters, this is generally the lower part. Furthermore, if several printed resistors 4 are required this will not increase the overall dimensions significantly because of the aforementioned possibility of creating adjacent routes for the said printed resistors 4, or by superimposing them, as long as a layer of insulating material is placed between each said route.
An important aspect of this invention is the fact that, using the same technology, and at the same time as the printed resistors 4 are printed, electrical circuits can be printed for connecting various electrical or electronic components to the exterior of the tank electrically. These said components could be useful if mounted along the heating element, the said heating element acting as a support for the said components. The said components are welded in a commonly known way to one end of the printed electric circuits, in the most suitable position along the heating element, and insulated electrically from the surrounding environment; the other end of the said circuits terminates, with suitable electrical connectors, in position with the pipe coupling. The said components can be, for example, temperatures sensors (e.g. NTCs) to be connected to corresponding electronic thermostats; unipolar safety thermostats of the bimetallic laminar kind or those with a fuse or of another kind (which are very small in size) and in this case they can be welded on directly in series with the resistor to be monitored. Lastly, these components can be cathodic anticorrosion protection devices. The temperature sensors can also be constituted of a printed resistive track with a resistivity level dependant enough on the temperature to be measured. For this purpose, the route of the printed sensor can be constituted of a track which is long enough to increase the total resistance, thus facilitating the measuring of the resistor as a function of the temperature change.
There will now follow a description of some details or construction variants of the heating element 1 according to the invention.
During the heating phase of the water in the tank 2 of a storage heater it may be necessary to stimulate the circulation of the water inside the piping 3 to prevent the water inside this latter becoming stagnant and increasing the temperature, causing lime-scale deposits to form extremely quickly and also the water in that area to boil. If the user draws the water at that moment, this latter phenomenon could result in the user being scalded. It is possible, then, as shown in figures 1 and 3, to create openings 10 in the lower part and the piping 3. As shown in figures 5 and 6, this permits a continuous natural circulation inside the piping 3 during the heating phase. When the hot water is drawn, it may also be worthwhile preventing the eddy of cold water from the lower part of the boiler through the said openings. Although it has been demonstrated, in the most common applications of the invention, that one or more openings 10, constituted of 4mm holes, are sufficient to guarantee the recirculation without the cold water eddying when the water is drawn from the tank, it is still possible to envisage other suitable adaptations: for example, in figure 5 the openings 10a. are fitted with suitable deflectors 12 and in figure 6 the openings 10b. are in position with suitable section enlargements 13 which, because of the Venturi effect, cause a local pressure increase and prevent the cold water entering. In certain embodiments, however, the diameter of the heating element 1 can be made wide enough to permit a sufficiently active natural circulation inside it without any openings 10 being necessary. In other embodiments, on the contrary, the speed of the water when it is drawn from the tank is high enough or sufficiently frequent to ensure a constant washing and so prevent or remove lime-scale deposits. For this purpose, it may be advantageous, as shown in figure 4, to provide, a layer of non-stick material 14 for the lime-scale inside the piping 3, such as polypropylene.
Still during the heating phase of the water contained inside the tank 2, it may be suitable to ensure the water that rises through natural circulation inside the piping 3 is not discharged completely at the end 3.a, where, it is known, there is often an air bubble which can obstruct the circulation. For this reason, the water can still leave through the openings 11 in figure 1 or 11.b in figure 3.
In certain applications, it may be worthwhile providing an external anti-abrasion jacket for the most external layer of insulating material constituting the printed resistor 4. This can be created, as in figure 4, with a protective sheath 15 made of a suitable material, but it could also be useful to have the said protective sheath 15 made of a material designed to act as an electrode for the cathodic protection, as described in full in another patent filed at the same time, by the same applicant. Alternatively, the said protective sheath 15 can be constituted of a sacrificial metal tubular element to protect the apparatus against corrosion (in general a magnesium anode). With any commonly known means or the means described earlier, the said electrode or the said anode are suitably connected electrically to the metal tank to be protected.
With reference to figure 1, the heating element 1 permits the functioning to be flexible, which is impossible with the commonly known armoured electrical resistors. To have a high reserve, a boiler generally keeps the water at 70-75°C, while the usage temperature is generally 40° C. If little water is required and the user wishes to limit the heat loss, the storage temperature can be lowered to the usage temperature, but never below. With the heating element 1 the water can kept at slightly lower temperature than the usage temperature and it can be reheated during the drawing phase, activating the printed resistor 4. For this purpose it is necessary for the said electrical resistor 4 to be operated by a thermostat during the storage phase and by a flow sensor unit, such as a pressure switch or a flow switch, during the water-drawing phase.
In figure 1 the heating element 1 according to the invention is shown as a pipe for drawing the hot water from a storage heater but, alternatively, this can take the form of the input pipe, as in the case of the 'under-the-sink' boiler or both the inlet pipe suitably extended inside the tank, and the output pipe for a better distribution of the thermal power in the most suitable areas.
Figure 7, instead, shows a fast-acting boiler fitted with at least two printed resistors which are separated from each other physically. The first said resistor, indicated with the number 4.1 and supported by the tank 2.1, can keep the water stored at a determined temperature, regulated by a suitable thermostat. The second, indicated with the number 4 and supported by the output pipe 3 can also heat the water during the drawing phase, regulated by a suitable flow sensor, such as a pressure switch or a flow switch.
Naturally, still with reference to figure 7, a single heating element 1 constituting the output pipe, could perform both the pre-heating function during the storage phase and the final heating function during the drawing phase as long as the said heating element 1 has the openings 11 as shown in the figure.
If necessary, the printed resistor 4 or 4.1 can be applied to the internal surface of the heating element 1 or and also the heating element 1 or can have different shaped sections (i.e. not circular) or be curved or convex, naturally within the limits of the forms permitted by the commonly known resistor printing technology.
A first advantage of the heating element 1 is that, in a storage heater, the flange can be eliminated by simply providing the following: a threaded ring nut for fixing the heating element, a welded or screwed sheath for inserting the thermostat sensors and/or the cathodic anticorrosion protection electrode and/or a threaded ring nut for inserting a magnesium anode or an electrode. It has also been shown, though, that the heating element 1 can also support, all together, the sensor units, safety thermostats and anticorrosion protection electrodes or sacrificial anodes. Therefore it is possible, in some variants, to eliminate, not only the flange, but also the other hole in the tank 2, with the exception of the hole needed for the heating element according to the invention.
In storage heaters, this invention can also permit a significant reduction in the heat loss and lime-scale formations in the tank 2.
For the sake of an example, consider a storage heater with a power of 1500W.
A flow of 0.07 litre/sec is required for a shower. At this rate, if the resistor is activated during the drawing phase, in practice, the said resistor only yields heat to the water that runs through the element because the heat exchange is much more active than on the outside where the water is not flowing. The result is that the temperature of the water drawn increases by approximately 5°C, allowing the water to be kept at 35°C if the usage temperature required is 40°C.
If the room temperature is 20°C, this results in a heat loss of 25% (thermal head: 35-20 = 15°C, rather than 40-20 = 20°C).
When the water is kept at a lower temperature, less lime-scale deposits form.
This advantage is increased for the fast-acting boiler, i.e. those with a higher level of power, as shown in figure 7.
In general, the heating element according to the invention makes it possible to manufacture a fast-acting storage heater where the same heating element can pre-heat the water stored to the storage temperatures lower than those required and then complete the heating phase at the moment of use.
An electrical resistor of the armoured kind for boilers usually has a thermal load, as mentioned earlier, of 8.5W/cm2 and it is subject to rapidly forming lime-scale deposits, which increase in direct proportion to the thermal load. A boiler output pipe is generally ½" in diameter (i.e. 21mm): supposing the length is 600mm and the installed power is 1500W, the heating element 1 has a thermal load of less than 4W/cm2, less than half that of the armoured resistor, and it is possible to further reduce the thermal load by increasing the pipe diameter and its surface while keeping the surface of the printed resistor route unchanged without there being any significant increase in the cost.

Claims

A tubular-shaped heating element (1), in particular for heating apparatus for water to be used for sanitary purposes in a storage heater, of the kind using the technology in which a printed resistor is applied to a metal surface, the said printed resistor consisting of one or more further layers of dielectric material applied to the said metal surface, printed onto which, using for example a screen printing technique, are one or more resistive electric circuits, applied to which, in their turn, are one or more layers of dielectric material, said heating element (1) being constituted of at least one printed resistor (4, 4.1) which is applied to the either the whole or part of the external or internal surface of a piping (3), the said piping having a section which is shaped in any way to allow the water to be used for sanitary purposes at least, to pass through, characterised in that the said at least one printed resistor is connected to a thermostat during the water-storage phase in a tank (2, 2.1) and to a flow sensor unit such as a pressure switch or a flow switch during the water-drawing phase.
A heating element (1) according to the claim 1, characterised in that, in the case in which more independent resistive electric circuits are on the said heating element, said resistive electric circuits can be positioned over several layers, each layer being separated from the others by one or more layers of dielectric material.
A heating element (1) according to the claims 1 and 2, characterised in that is connected to the tank (2, 2.1) with means of connection (6) and is connected to the water mains with means of connections (7), said means of connection belonging to at least one pipe coupling (5, 5.1), said pipe coupling can having connectors (9) for the electrical power supply of the at least one printed resistor (4, 4.1).
A heating element (1) according to the claims 1 and 3, characterised in that the routes of the printed resistors' (4, 4.1) resistive electrical circuits can have any form, either regular or irregular, said any form being suitable to distribute the thermal load, as required, either uniformly or differently along the piping (3).
A heating element (1) according to the preceding claims, characterised in that it may be worthwhile providing an external anti-abrasion protective sheath (15) for the most external layer of insulating material constituting the printed resistor 4, said protective sheath can be designing to protect the tank (2, 2.1) against the phenomenon of corrosion; the protective sheath (15) can advantageously consisting in an electrode in the form of an impressed cathode current protection device made, for example, of a titanium alloy or in a sacrificial anode made, for example, of a magnesium alloy; the electrical connections of the said element designed to protect the tank against corrosion can be made, in part or in whole, using the printed resistor technology.
A heating element (1) according to the preceding claims, characterised in that one or more openings (10, 10.a, 10.b), positioned in the lower part of the piping (3), connect the external surface and the internal surface of the said piping; said openings being suitable to provide for the natural circulation of the water inside the piping (3) during the heating phases.
A heating element (1) according to the claims 1 and 6, characterised in that one or more openings (10.a) are fitted with deflectors (12); said deflectors being suitable to prevent to cold water entering in the piping (3).
A heating element (1, 1.1) according to the preceding claims, characterised in that one or more openings (11, 11.b) are in the in the upper part of the piping (3) to facilitate, through natural circulation, the overflow of water from the said piping, said overflow of the water being suitable to preventing any air bubbles there may be in a inlet (3.a) becoming an obstruction.
Method to operate the tubular-shaped heating element as in claim 1, characterised in that said thermostat operates the printed resistor (4, 4.1) during the storage phase in a tank (2, 2.1) to keep the water at slightly lower temperature than the usage temperature and said flow sensor unit operates the said printed resistor during the water-drawing phase to complete the heating of the water; said heating element being suitable to be used both to keep the temperature of the water at a first pre-set storage value and then to reheat it to a second drawing temperature value.