Patent Description:
Electric heaters used for heating liquids in heating circuits of residential buildings, such as instantaneous water heaters, back-up electric heaters for heating the heat transfer medium in heat pump systems, and other similar devices, have heating elements generally constructed of resistance wires insulated with a magnesium oxide layer and housed in a stainless steel casing. Their basic feature is that the heating elements provide the power proportional only to the supply voltage and the structure of such a heater is simple.

On the other hand, the actual use of such simple heating elements for heating the heat transfer medium requires a sophisticated safety concept involving control of the heating based on outputs from temperature sensors, flow sensors, safety temperature limiters and other necessary components, since the temperature of the heating elements can reach temperatures of up to <NUM>.

Such devices are described, for example, in <CIT> or <CIT>.

Newer electric liquid heaters use other suitable elements to heat the heat transfer medium, preferably so-called PTC heating elements (PTC = positive temperature coefficient).

The PTC elements are elements that are electrically conductive at low temperatures, but after a certain temperature barrier is reached, the electrical conductivity drops sharply. Thus, when the temperature of the PTC element increases, its resistance increases. Negative feedback is used, acting on the actual heating of the PTC element due to the passing current. When powered from a constant voltage source, this will cause a drop in power on the PTC element, counteracting further temperature rise. The result is simple heating with stabilization of the temperature of the heated space.

Heating with the PTC elements is therefore safe due to the automatic temperature control and the fact that the maximum temperature of the heated element is below <NUM>. Independent power regulation does not require thermostat control.

The PTC elements can be made of ceramics or organic polymers, they are mainly produced as plates with two electrically conductive electrodes to connect the supply voltage, heating assemblies made of the PTC elements can contain insulating film and other components, such as electrically conductive adhesives.

<CIT> discloses an electric water heater for a closed-loop heating water system comprising a water tank connected to a flange, at least one heating element extending from the flange to the water tank, and a concentric wall arranged in the water tank, with the water tank being connected to a cold water inlet and a hot water outlet. The water tank housing comprises a cylindrical part attached to the flange at the top and a conical part, which is connected to the cylindrical part of the water tank housing at the bottom, with the shape of the concentric wall corresponding to the cylindrical part of the water tank. The water tank housing is placed inside the water tank so that an external circulation channel is defined between the water tank housing and the concentric wall. The electric water heater integrates a filter function for solid impurities, which are mainly formed on the heating elements during electric heating, but can also be formed elsewhere in the heating system, by separating the solid particles from the liquid flow when they hit the concentric wall and slowing them down to fall into the conical part of the water tank where they can be settled. The sediments are removed by a drain valve.

The concentric wall can be made, together with the cylindrical part of the tank housing as an integral part thereof, by moulding from plastic.

Resistance coils and a safety temperature limiter are used and heating control by an electronic control unit is assumed in the disclosed document.

The task of the invention is to make an electric heater of similar dimensions and power using PTC elements, so that the function of the integrated filter for solid impurities is preserved.

In the state of the art, liquid heaters with the PTC heating elements embedded in the tank designed to heat the liquid are known. The existing PTC liquid heating devices generally contain a PTC heating element with a heat transfer structure that is typically very complex and therefore has disadvantages such as a low heat transfer rate and uneven heat transfer.

The solution published in <CIT> improves the uniformity of heat transfer, while the particular components of the structure are bonded with conductive adhesive and require placement in a sleeve. However, the heater does not provide the function of a filter integrated in the heated liquid tank, and the manufacturing of the PTC heating assembly is complicated and expensive.

Another invention with simple integration of a PTC heating assembly is published in <CIT>. The heating element is made of a hollow body with a passage for liquid, the body has a cross-section of a regular hexagon or square. The body is heated by surfaces equipped with PTC elements. The surfaces are connected with the installation cavities for inserting the PTC elements, during assembly, the PTC heating component is firstly inserted into the installation cavity successively, and then a press for the PTC deformation pressing is used. The outer surface of the heating element is pressed together to form the final product.

The disadvantage of this solution is that the pressing can damage the PTC heating component. The arrangement of the continuous cavity with reinforcing ribs throughout the entire volume of the liquid passage causes significant pressure losses as well.

The <CIT> discloses a PTC liquid heater comprising a cylindrical heating core assembled from at least two electrothermal parts comprising a PTC ceramic combination and an axile-like aluminum profile in which heating holes are provided. Using a circular arc aluminum structure, the distance between the PTC ceramic and the outer metal tube is reduced to allow rapid transfer of heat from the PTC ceramic to the outside. However, assembly of such as PTC heater and ensuring of good insulation properties is difficult.

The present invention addresses the shortcomings of the known state of the art. It is based on the design of an electric heater with an integrated filter function published in <CIT>, and features thereof are disclosed in the preamble of the Independent Claim, with the concentric wall being conveniently used as an electrical insulator. The electric heater uses a simple PTC heating assembly with an innovative flexible wire arrangement to heat the liquid.

According to the present invention, the electric heater comprises a plastic liquid tank comprising an upper cylindrical part and a lower conical part, which are watertightly connected to each other, e.g. by a screwed or welded joint; inside the plastic tank there is basically cylindrical plastic concentric wall arranged, defining the internal circulation channel for supplying the liquid to be heated and the external circulation channel for discharging the heated liquid, which is cleared of impurities at the same time. The external circulation channel is arranged between the concentric wall and the cylindrical part of the housing, with the external circulation channel being connected to the heated liquid outlet. In the lower conical part of the tank there is a space for sedimentation of solid impurities, which are separated from the liquid flow by hitting barriers and falling into the area with the lowest flow rate. The conical part has a connection to connect a drain valve to remove the impurities.

The electric heater includes also a hollow heating module arranged in the internal circulation channel (along the axis of the upper cylindrical part of the water tank). The heating module is equipped with a connection piece for the supply of the liquid to be heated to the heater and a continuous cavity for the liquid routing and heating.

The hollow heating module is made of metal material, conducting heat and electric current well. Of the suitable materials, extruded aluminium is preferably used for the module due to its light weight, but other materials are also possible. The hollow heating module has surfaces bordered by projections arranged on its outer surface. On its inner surface (in the continuous cavity), it has ribs arranged in the preferable design to increase the heating surface and thus the heating efficiency.

After the hollow heating module is inserted into the internal circulation channel, installation gaps are created between the plastic concentric wall and the outer surfaces of the hollow heating module for insertion of the PTC heating assembly.

The concentric plastic wall has pairs of projections arranged on its inner surface, facing the internal circulation channel, which are shaped so that the projections arranged on the outer surface of the hollow heating module shape-fit between these projections.

The PTC heating assembly has a layered structure known in the prior state of the art. The PTC plates are arranged in long strips, with the length and number of the strips used determining the power of the heater (e.g. to use a PTC heater as a backup electric heater in heat pump systems, power from <NUM> to <NUM> kW is required.

Each PTC heating assembly in the present invention includes two electrical conductors (the first electrical conductor and the second electrical conductor) that are connected to two opposing surfaces of the PTC layer. The PTC layer is made of PTC plates, with a graphite pad sandwiched between the PTC layer and each electrical conductor to improve electrical and thermal conductivity. The pad can be glued with electrically conductive adhesive, its role is to ensure the best possible contact between the PTC plate and the conductor, eliminating any unevenness on both materials at the point of contact.

It is an improvement of the prior state of the art that instead of a conventional electrical conductor or conductive strip, a spring, for example a shaped electrically conductive flexible strip, is used as the first electrical conductor in the present solution. The first electrical conductor touches the concentric wall with its flexible part.

Electrically insulating film (e.g., Kapton film) is applied between the second electrical conductor and the hollow heating module to provide insulation for the second electrical conductor while not preventing heat transfer from the PTC heating assembly to the hollow heating module.

At least one PTC heating assembly generating heat is arranged on the outer surfaces of the hollow heating module; each PTC heating assembly includes a flexible metal electrical conductor for supplying electrical voltage and for pressing the PTC assembly against the concentric wall and also against the heating module.

The flexible conductor ensures tight contacts between the layers of the PTC heating assembly. Using a flexible electrical conductor, the entire PTC heating assembly is pressed against the hollow heating module on one side and the plastic concentric wall on the other side. Basically, there is no need for pressing or gluing to ensure good contact of all layers of the PTC heating assembly with each other and to ensure good contact with the heating module and the plastic concentric wall.

Heating of liquid is accomplished by applying voltage (of appropriate height and polarity) to the two conductors of the PTC heating assembly to generate heat in the PTC plates, which is transferred through the hollow heating module to the liquid.

In the preferred design, an electrically conductive body of the hollow heating module is used as the second electrical conductor.

The invention is further explained using the figures below, without limitation thereto.

<FIG> shows an electric heater according to the present invention. This includes a plastic liquid tank comprising an upper cylindrical part <NUM> and a lower conical part <NUM>, which are watertightly connected to each other, with a screwed joint of flanges <NUM> and <NUM> and a gasket <NUM>. Inside the plastic tank there is basically cylindrical plastic concentric wall <NUM> arranged, defining the internal circulation channel <NUM> for routing the liquid to be heated and the external circulation channel <NUM> for discharging the heated liquid, which is cleared of solid impurities at the same time. The external circulation channel <NUM> is arranged between the concentric wall <NUM> and the cylindrical part of the housing <NUM>, with the external circulation channel <NUM> being connected, in use, to an outlet <NUM> for heated liquid. In the lower conical part <NUM> of the tank there is a space for sedimentation of solid impurities, which are separated from the liquid flow by hitting barriers and falling into the area with the lowest flow rate. The conical part <NUM> of the tank has a connection for connecting a drain valve <NUM>, which is used to remove the settled impurities from the heater.

The electric heater includes also a hollow heating module <NUM> (shown in <FIG> and <FIG>) arranged in the internal circulation channel <NUM> (along the axis of the upper cylindrical part of the water tank). The heating module <NUM> is provided with a connection piece <NUM> with an external thread for the inlet of the liquid to be heated into the heater, a continuous cavity for routing and heating the liquid, and a connection piece <NUM> with an internal thread for the outlet of the heated water.

The hollow heating module <NUM> is watertightly connected to the heater tank by a coupling nut <NUM>, a connection piece <NUM> and O-rings <NUM>.

The hollow heating module <NUM> and its cross-section are shown in <FIG>. It has arranged surfaces <NUM> bordered by projections <NUM> on its outer surface. On its inner surface, the continuous cavity <NUM> has conveniently arranged ribs <NUM> to increase the heating surface area.

The concentric plastic wall <NUM> has pairs of projections <NUM> arranged on its inner surface, facing the internal circulation channel <NUM>, which are shaped so that the projections <NUM> arranged on the outer surface of the hollow heating module <NUM> shape-fit between these projections <NUM>. A detail of the arrangement is shown in <FIG>. The pairs of projections <NUM> are arranged for shape connection of the projections <NUM>, which are arranged on the outer surface of the hollow heating module <NUM>.

After the hollow heating module <NUM> is inserted into the internal circulation channel <NUM>, installation gaps <NUM> are created between the plastic concentric wall <NUM> and the surfaces <NUM> of the hollow heating module <NUM> for insertion of at least one PTC heating assembly PTC1, PTC2, PTC3, PTC4, PTC5, PTC6. The above example allows the use of up to six PTC heating assemblies PTC1, PTC2, PTC3, PTC4, PTC5, PTC6, the number of assemblies is not limited to this number (in other designs).

Each PTC heating assembly PTC1, PTC2, PTC3, PTC4, PTC5, PTC6 in the present invention includes two electrical conductors (<NUM>, <NUM>), the first electrical conductor <NUM> and the second electrical conductor <NUM>, that are connected to two opposing surfaces of the PTC layer <NUM>. The PTC layer <NUM> is made of PTC plates, with a graphite pad <NUM>, <NUM> sandwiched between the PTC layer <NUM> and each electrical conductor <NUM>, <NUM> to improve electrical and thermal conductivity. The pad <NUM>, <NUM> can be glued with electrically conductive adhesive, its role is to ensure the best possible contact between the PTC layer <NUM> and the electrical conductor <NUM>, <NUM>; the pad <NUM>, <NUM> eliminates any unevenness on both materials at the point of contact.

A spring, e.g. a shaped electrically conductive flexible strip, is used as the first electrical conductor <NUM> in the present solution. The first electrical conductor <NUM> with its flexible part touches the concentric wall <NUM>.

Electrically insulating film <NUM> (e.g., Kapton film) is applied between the second electrical conductor <NUM> and the hollow heating module <NUM> to provide insulation for the second electrical conductor <NUM> while not preventing heat transfer from the PTC heating assembly to the hollow heating module <NUM>.

At least one PTC heating assembly PTC1, PTC2, PTC3, PTC4, PTC5, PTC6 generating heat is arranged on the outer surfaces <NUM> of the hollow heating module; each PTC heating assembly PTC1, PTC2, PTC3, PTC4, PTC5, PTC6 includes a flexible metal electrical conductor <NUM> for supplying electrical voltage and for pressing the PTC assembly PTC1, PTC2, PTC3, PTC4, PTC5, PTC6 against the concentric wall <NUM> and also against the heating module <NUM>.

Basically, there is no need for pressing or gluing to ensure good contact of all layers of the PTC heating assembly PTC1, PTC2, PTC3, PTC4, PTC5, PTC6 with each other and to ensure good contact with the heating module <NUM> and the plastic concentric wall <NUM>.

The pair of projections <NUM> is also shape connected to the projection <NUM> in order to insulate the electrically conductive parts of the PTC heating assembly PTC1, PTC2, PTC3, PTC4, PTC5, PTC6.

After insertion of the PTC heating assembly into the installation gap <NUM>, an unfilled, free space, generally filled with air, remains in it. In the preferred solution, the free space between the first electrical conductor <NUM> and the concentric wall <NUM> and in the installation gap <NUM> is filled with a thermally conductive and at the same time electrically insulating filler material.

Heating of the liquid is accomplished by applying voltage (of appropriate height and polarity) to two electrical contacts <NUM>, <NUM> of conductors <NUM>, <NUM> of the PTC heating assembly PTC1, PTC2, PTC3, PTC4, PTC5, PTC6 to generate heat in the PTC layer and the heat is transferred through the hollow heating module <NUM> to the liquid in the internal circulation channel <NUM>. The ribs <NUM> are arranged to increase the heating area.

The plastic concentric wall <NUM> is also partially heated, and the heat is transferred to the liquid in the external circulation channel <NUM>. Thus, the generated heat is used in the present solution as efficiently as possible and the heated liquid is cleared of solid impurities before leaving the heater.

The electric heater is preferably used as an electric back-up heater in heat pump systems, for heating less pure liquids, even with glycol content, where applicable.

<FIG> shows an example schematic of the Y- wiring of three-phase voltage to the connection contacts <NUM>,<NUM> of the electrical conductors <NUM> and <NUM> of the PTC heating assemblies PTC1, PTC2, PTC3, PTC4, PTC5, PTC6. The phase voltage L2 is connected to the contacts <NUM> of the first electrical conductor <NUM> of the heating assemblies PTC1 and PTC2, the phase voltage L3 is connected to the contacts <NUM> of the first electrical conductor <NUM> of the heating assemblies PTC3 and PTC <NUM>, and the phase voltage L1 is connected to the contacts <NUM> of the first electrical conductor <NUM> of the heating assemblies PTC5 and PTC <NUM>. The contacts <NUM> of the second electrical conductor <NUM> on all heating assemblies PTC1, PTC2, PTC3, PTC4, PTC5, PTC6 are connected to zero potential.

The electrical wiring may be different, depending on the requirements for heating power.

The electric PTC heater integrates a solid impurity filter function, the solid impurities are separated from the liquid stream by hitting the ribs <NUM> arranged in the continuous cavity <NUM> of the hollow heating module <NUM> and settle in the conical part <NUM> of the tank. A drain valve <NUM> is connected to the tank to drain impurities.

In the other design of the invention, the electrically conductive body of a hollow heating module <NUM> is used as the second electrical conductor <NUM>.

In this case, the PTC heating assembly PTC1, PTC2, PTC3, PTC4, PTC5, PTC6 includes the first electrical conductor <NUM>, the PTC layer <NUM> and the graphite pads <NUM>,<NUM>, and the conductive surface of the hollow heating module <NUM> as the second electrical conductor <NUM>.

Such an arrangement is suitable for use in a pure water instantaneous heater, as a module for measuring the conductivity of the water as well as other safety measures are the mandatory parts of instantaneous heaters.

Claim 1:
An electric liquid heater, comprising
a plastic liquid tank, wherein the tank comprises an upper cylindrical part (<NUM>) and a lower conical part (<NUM>), watertightly connected to each other and wherein a basically cylindrical plastic concentric wall (<NUM>) is firmly arranged inside the tank,
a hollow heating module (<NUM>), with outer surfaces (<NUM>) and with a continuous cavity (<NUM>) for directing the liquid to be heated, wherein the hollow heating module (<NUM>) is provided with a connection piece (<NUM>) for supplying the liquid to be heated,
and wherein the plastic concentric wall (<NUM>) defines an internal circulation channel (<NUM>) and an external circulation channel (<NUM>) connected, in use, to an outlet (<NUM>) for the heated liquid,
wherein
the hollow heating module (<NUM>) is arranged in the internal circulation channel (<NUM>), at least one PTC heating assembly (PTC1, PTC2, PTC3, PTC4, PTC5, PTC6) generating heat is arranged on the outer surfaces (<NUM>) of the hollow heating module (<NUM>),
each PTC heating assembly (PTC1, PTC2, PTC3, PTC4, PTC5, PTC6) comprises a flexible metal electrical conductor (<NUM>) for supplying electrical voltage and for pressing the PTC heating assembly against the concentric wall (<NUM>) and against the hollow heating module (<NUM>).