DE10310255A1 - Electric heating with a contact hot plate - Google Patents

Abstract

In one embodiment of the invention, an electrical heater (11) can be created with a contact hotplate (12) on which wire-shaped heaters (14) are arranged. At least one of the heaters (14b) consists of a material with a PTC effect, for example an iron-based alloy. This maintains a cubic crystal structure centered on the inside during all operating states of the heater and at the same time exhibits the PTC effect. In addition to its heating function, such a heater (14) has independent temperature control to avoid excess temperature, which means that separate temperature limiters with switching contacts can be saved.

Description

The invention relates to an electrical Heating with a contact hotplate with the features of the generic term of claim 1.

It is known, for example cast iron hot plates to be provided on the underside with heaters, in which of insulating material Work surrounding heating coils as heating. Against overheating these cast iron hot plates have a built-in temperature limiter on. This is usually embedded in the middle of the bottom and interrupts the energy supply to the cast hotplate when exceeded a certain fixed temperature, separating it has electrical contacts. It is considered disadvantageous Always a temperature limiter as a separate component on the hotplate to arrange.

The invention is based on the object to create a heater mentioned with the disadvantages of the prior art can be avoided and in particular on a additional Temperature limiter with opening electrical contacts can be dispensed with.

This task is solved by a heater with the features of claim 1. Advantageous and preferred Embodiments of the invention are specified in the further claims and will be closer in the following explained. The wording of the claims is by express Reference made to the content of the description.

According to the heater with the Contact hotplate at least one wire-shaped or wire-like heater on to heat the contact hotplate. The heater is there made of a metallic material with PTC properties.

The advantage of a heater made of PTC material for one Contact hotplate is that above a certain one Temperature, which corresponds approximately to a limit temperature, the in intended operation actually not exceeded the electrical resistance of the heater increases sharply. This reduces the current flow accordingly and the Heating output drops. The lowering of the heating power in turn causes the return in the operating temperature range. The use of a heater from one such PTC material has the advantage that it is not just a pure securing element is available. The heater is self-limiting in addition to its property as a heating element, so it combines two properties.

Thus, by providing a PTC heater on usual Temperature limiters are dispensed with, which are primarily mechanical actuated and electrically opening Have contacts. Such temperature limiters are expensive and complex in assembly and difficult to place on some contact hot plates, especially if their outer surfaces, the available for attachment stand, are not level.

The heater can either consist of a so-called iron-based alloy, which maintains a cubic, inner-centered crystal structure during all operating states of the heater. All operating states of the heater are understood to mean different temperatures with which the heater is operated as intended. In the case of contact hotplates, these are usually temperatures up to a few hundred degrees, for example up to 300 ° C or 400 ° C. Maintaining the special crystal structure has the advantage that a phase transition from α-iron to γ-iron is avoided during normal operation. This way the heater has a better durability. Above all, however, it is possible to obtain PTC properties in addition to raising a Curie temperature of the material, which can be set relatively precisely in a desired range. Such a material is for example in the EP 1213540 A2 to which express reference is made here.

Furthermore, the heater can be made from one Cobalt-iron alloy as a metallic material with PTC properties exist, which contains, for example, about 70% cobalt iron. On such material is for example under the designation CF25 from available from Vacuumschmelze.

The heater can also be made of a tungsten alloy or a molybdenum alloy consist. Such alloys are known for use in halogen lamps or light bulbs.

A temperature resistance coefficient is preferred larger than 6 provided so that the temperature rise from the desired Temperature limit is relatively strong. Larger values can also be above chosen from 8 chosen for example 10.

Furthermore, an iron-based alloy characterized by the fact that it contains vanadium. The vanadium content can 1 to 3 or 4% by weight, for example approximately 1.5% by weight. Other alloys can a small amount of molybdenum contain, for example up to 4% by weight. Likewise, 1 to 4% by weight of titanium may be included.

The arrangement of the heater on the Contact hotplate can be such that it is on its underside is arranged for their heating. A contact should be made if possible flat be or for one if possible good heat transfer to care.

For example, it is possible to use one heater according to the invention to be arranged in a tube, for example in a coiled form. there if it is covered with insulation, for example quartz sand, Ceramics or the like. This tube in turn can go to the bottom the contact hotplate can be created, which is why it is advantageously flattened is to increase the contact area.

The heater can alternatively be airtight Be surrounded by insulation. Such isolation can be based on glass or ceramic and should be accordingly be temperature stable. It can too Be polymer ceramics. The insulation is here with the heater on arranged on the underside of the contact hotplate. It is also conceivable provide evacuated insulation within which the heater runs.

A heater or a contact hotplate can be multiple wire-shaped Have heaters, for example two or three. At least one the heater, in particular exactly one heater, consists of one the above-mentioned iron-based alloy with advantage. That's part of it the available standing heat output self-regulating, which is also either the other heaters can affect directly or over the entire heat input on the contact hotplate.

In an exemplary embodiment of the invention, it is possible to connect a heater made of the aforementioned iron-based alloy with at least one conventional heater, which can also consist of wire made of conventional resistance material, in series. The overheating of the heater with the PTC effect then causes the entire branch to be throttled or switched off and thus also the conventional heater. A conventional heater, for example, or the like a FeCrAl alloy. exhibit. Such a heating conductor is in the EP 542128 A1 to which express reference is made here.

Another exemplary execution the invention can be a parallel circuit from a heater with a aforementioned iron base alloy and a conventional heater available. In this case the heater regulates itself or just off his own branch. Furthermore, it is of course possible, too To provide mixtures of parallel and serial circuits.

The heater, especially in the aforementioned potential Shading, can be provided in several separate branches. In at least one The branches should be a heater made of an iron-based alloy be provided. It is possible all To apply electrical energy to branches together. The individual branches can in so-called defined areas run, which can surround and adjoin each other, in the but do not substantially overlap or penetrate. The surface areas are advantageously adapted to the shape of the contact hotplate, for example round and concentric to each other.

To connect such branches with multiple heaters can be one any switching device used which can produce different circuits. these can be serial and / or parallel, with several branches in completely free Circuitry interconnected and supplied with electrical energy can be. You can different branches interconnected in different ways become. Depending on the type of wiring, they are different greater Energy can be applied. This is especially true if such Contact plate according to the invention similar to a conventional hotplate with several heating circuits connected to one voltage source and is either clocked or the power by the type of wiring is adjustable.

On the one hand, a contact hotplate a so-called cast iron hotplate, as it is common. The or the heater can run between ribs, which are molded onto the underside of the cast-iron hotplate are. You can the heater, as previously described, in insulation and under Circumstances too a kind of tubular heater run.

As an alternative to a cast hotplate Is it possible, a heater mentioned above, advantageously together with other conventional ones Heaters to be used on a ceramic contact plate and attach. Here it is advantageous if the heater is flat wire has and with good heat conduction, preferably flat, is attached to the bottom of the ceramic. A flat wire shape can range up to a broad film shape to achieve a balanced relationship between heater surface, To create the heater cross section and heater power.

These and other features of preferred Further developments of the invention go beyond the claims from the description and the drawings, the individual Characteristics for each alone or in several in the form of sub-combinations one embodiment of the invention and in other fields be realized and advantageous also for yourself protectable versions can represent for the protection is claimed here

Embodiments of the invention are shown schematically in the drawings and are described below explained in more detail. In the drawings show:

1 a side view of a ceramic plate with flat heaters,

2 a top view of the underside of the ceramic plate 1 .

3 a section through a cast hotplate with embedded heaters on the underside,

4 another section through a further hotplate with heaters which run in a tube which is pressed onto the underside of a hotplate,

5 and 6 Connection examples for several heating conductors.

detailed Description of the embodiments

In 1 is a heater from the side 11 shown. This has a contact hotplate 12 , which can consist of a ceramic material, for example. Such a ceramic hotplate is for example from the EP 069 298 A1 known.

At the bottom of the hotplate 12 are heating conductors 14 arranged. The course of the heating conductor 14 will mostly come out of 2 clear. Here you can see how connections 15 starting a heating conductor 14 as a continuous band in double spiral shape on the underside of the hotplate 12 is applied.

The heating conductor 14 is divided into several different areas, which can basically be divided into two types. The heating conductor areas 14a are made of so-called normal or conventional heating conductor material, for example a FeCrAl alloy. The heating conductor areas 14b which as short pieces together with the heating conductor areas 14a the entire, continuous heating conductor 14 form, are formed from the metallic material according to the invention with PTC properties, for example an iron-based alloy, as described above. The connection of the heating conductor areas 14a and 14b with each other takes place in such a way that there are no disturbing contact resistances and no other boundary layer problems.

To apply the heating conductor 14 there are different options. On the one hand, it is possible from the heating conductor areas 14a and 14b by producing a continuous band, for example in the form of flat wire. This can be done in almost any way on the underside of the hotplate 12 are attached, for example by gluing or embedding using ceramic adhesive. Alternatively, it is conceivable to produce so-called thick-film pastes from the respective materials and onto the hotplate 12 melt.

In 3 is a heater on average 111 with a cast hotplate 112 shown on which a cooking vessel stands. The cast hotplate 112 has, according to known designs, concentric projecting ribs on the underside 118 on. Into those ribs 118 The gaps formed are each a heat conductor 114 in a spiral form by means of the insulating bedding 117 embedded.

In 4 is another heater 211 shown. There is also a heating conductor there 214 provided in a coiled form. However, here is the heating conductor 214 in a tube 216 embedded together with an insulating embedding 217 , pipe 216 and heating conductor 214 form a so-called tubular heater and are approximately triangular in cross section. This allows a larger direct contact surface for heat transfer to a hotplate above 212 to ensure. Process for deforming a pipe 216 Such a triangular shape is known to the person skilled in the art.

The pipe is advantageous 216 at the bottom of the hotplate 212 surrounded with thermal insulation or the like, which particularly preferably has a certain reflective effect upwards. This allows much of that from the pipe 216 or the heating conductor 214 generated heat up to the hotplate 212 be judged.

The pipe 216 can either by means of glue or the like, for example as described above, on the underside of the hotplate 212 be fastened, a good heat transfer being advantageously additionally created here. Alternatively, it is possible to pipe 216 and hot plate 212 to press flat against each other. Other options may include the pipe 216 to the bottom of the hotplate 212 to be soldered or bonded because of the heat conductor 214 is electrically insulated from this. Such metallic transitions would, if the hotplate had a metallic design 212 additionally improve the heat transfer.

In the 5 and 6 are possible types of wiring of heating conductors 314a and 414a made of conventional heating conductor material and heating cables 314b and 414b made of an iron-based alloy according to the invention. As can be seen, these can be connected in parallel, in series or in a mixed manner with one another in various ways.

function

The function should essentially be based on the 5 and 6 to be discribed. It is important at the outset that the heat conductors made of an iron-based alloy according to the invention have a PTC characteristic. This means that their specific electrical resistance increases sharply from a temperature of approx. 300 ° C and can more than double, for example up to 600 ° C. As described at the beginning has increased, the current flow through it decreases due to the increase in the electrical resistance of a heating conductor. This reduces the heat output, which in turn causes the resistance to drop due to the decreasing temperature.

Such self-regulating temperature, so to speak Heating conductor can can be used in different ways. In contrast to other PTC sensors or PTC elements should be the heating conductors described here be used as a heating conductor and not only as overtemperature protection.

In 5 is a parallel connection of two conventional heating conductors, so to speak 314a shown. This is a heating conductor in series 314b made of an iron-based alloy. The heating conductor 314a and 314b are attached, for example, to a contact hotplate for heating in the manner described above.

In addition to the ones shown Heating conductors, which are jointly subjected to voltage, can still additional heating conductors can be attached to the same contact hotplate. So could for example, a separately switchable two-circuit radiator become.

It can be seen that when a temperature of the hotplate to be regarded as critical is reached because of the relatively uniform temperature distribution in the hotplate, the heating conductor 314b heated. As a result, its electrical resistance increases due to the properties of the iron-based alloy used, which in turn causes a reduction in the current flowing through it. This means that less electricity flows through the conventional heating conductors 314a , This reduces their heating output and overall the heating of the hotplate. The temperature drops until the current flows through the heating conductor 314b increases again and thus also the heating output. The heating conductor thus regulates 314b not just yourself, but also the conventional heating elements 314a , Furthermore, he himself contributes to heating the hotplate.

With appropriate design of the heating conductor, especially the heating conductor 314b , it is possible to achieve a so-called steady state. The temperature and electrical resistance and thus the heating output remain approximately the same.

The 6 shows a parallel connection of a conventional heating conductor 414a and a heating conductor according to the invention 414b with an iron base alloy. Both heating conductors 414 are connected to the supply voltage. In the event of overtemperature, as can be easily seen, only the heating conductor reduces 414b his performance. The heating conductor 414a remains unaffected and continues to heat normally.

With this connection the heating conductor 414 regulates the heat conductor according to the invention 414b with the iron-based alloy only yourself. Thus, with a hotplate on which the two heating conductors 414a and 414b are arranged, the entire heat generation is reduced, but only by reducing the heat generation by the iron-based alloy heat conductor according to the invention 414b ,

There are several on one hotplate heating conductor according to the invention arranged from an iron-based alloy, which in particular together are energized with a voltage, so it is beneficial to this flat spread over to arrange a hotplate. Such is a discovery of local overheating and consideration this overheating better possible. Furthermore, it is possible with certain types of hotplates, usually especially hot Areas of conventional heating conductors by such heating conductors according to the invention made of an iron-based alloy.

The different heaters or heating conductors can have approximately the same performance.

An embodiment in numbers

The functioning of a previously described Cooking plate with a series connection of a normal heating wire and a PTC resistor will use a numerical example below be made clear.

The hotplate has a normal one Heating conductor on, its resistance of about 15 ohms at room temperature to approx. 20 ohms at 800 ° C increases. additionally the hotplate has a PTC heating conductor whose resistance increases from 5 ohms at room temperature to approx. 32 ohms at 800 ° C. The two heating conductors should be connected in series and with one Voltage of 230 V can be operated.

The temperature curve should be illustrated using the following table:

A pot with good heat extraction is at 300 ° C heated with an output of almost 2kW. Is the pot empty and if the temperature of the hotplate rises sharply, the output will decrease significantly reduced and you get at highest Temperatures of almost 800 ° C only about half the heating output of just over 1 kW. Thereby is a cooling off again or less intense heating possible.

Claims (17)

Electric heating ( 11 . 111 . 211 ) with a contact hotplate ( 12 . 112 . 212 ) and with at least one wire-shaped heater ( 14 . 114 . 214 . 314 . 414 ) on the contact hotplate, characterized in that the heater ( 14 . 114 . 214 . 314 . 414 ) consists of a metallic material with PTC properties. Heating according to claim 1, characterized in that the heater ( 14 . 114 . 214 . 314 . 414 ) consists of an iron-based alloy as a metallic material with PTC properties, which maintains a cubic inner-centered crystal structure during all operating states of the heater. Heating according to claim 1, characterized in that the heater ( 14 . 114 . 214 . 314 . 414 ) consists of a cobalt-iron alloy as a metallic material with PTC properties. Heating according to claim 1, characterized in that the heater ( 14 . 114 . 214 . 314 . 414 ) consists of a tungsten alloy or molybdenum alloy as a metallic material with PTC properties. Heating according to claim 1 or 2, characterized in that the metallic material, especially the iron-based alloy, has a resistance coefficient greater than 6, preferably greater than 8 to 10, whereby a PTC effect rises sharply from a temperature of approx. 300 ° C. Heating according to claim 2, characterized in that the iron base alloy contains vanadium, molybdenum or titanium, whereby preferably the proportion is between 1% and 4%. Heating according to one of the preceding claims, characterized in that the heater ( 14 . 114 . 214 . 314 . 414 ) on the underside of the contact hotplate ( 12 . 112 . 212 ) is arranged and applied, preferably with flat contact, it being in particular insulated ( 117 . 217 ) encased in a tube ( 216 ) is arranged and the tube is placed on the underside. Heating according to one of the preceding claims, characterized in that the heater ( 14 . 114 . 214 . 314 . 414 ) is surrounded by an insulation in an airtight manner, in particular insulation based on glass or ceramic, preferably polymer ceramic, the insulation with the heater in particular on the underside of the contact hotplate ( 12 . 112 . 212 ) is arranged. Heating according to one of the preceding claims, characterized in that it comprises a plurality of wire-shaped heaters ( 14 . 114 . 214 . 314 . 414 ) for a single contact hotplate ( 12 . 112 . 212 ), preferably 2 or 3, where in particular exactly one heater consists of an iron-based alloy mentioned above. Heating according to claim 9, characterized in that the heater ( 14b . 314b . 414b ) of the above-mentioned metallic material with PTC properties is connected in series with at least one heater ( 14a . 314a . 414a ) made of conventional resistance material. Heating according to claim 9 or 10, characterized in that the heater ( 14b . 314b . 414b ) out the above-mentioned metallic material with PTC properties is connected in parallel with at least one heater ( 14a . 314a . 414a ) made of conventional resistance material. Heating according to one of the preceding claims, characterized in that the contact hotplate ( 12 . 112 . 212 ) Heater ( 14 . 114 . 214 . 314 . 414 ) in several separate branches, at least one heater in at least one branch ( 14b . 314b . 414b ) is connected from an above-mentioned metallic material with PTC properties and the branches can be acted upon together with electrical energy. Heating according to claim 12, characterized in that the heater ( 14 . 114 . 214 . 314 . 414 ) of the individual branches run in defined surface areas, preferably with a round or rounded shape and in particular concentrically to one another. Heating according to claim 12 or 13, characterized in that branches with heaters ( 14 . 114 . 214 . 314 . 414 ) Can be interconnected with a switching device in different circuits, in series and / or in parallel, and can be supplied with electrical energy, preferably depending on the circuitry with different amounts of energy. Heating according to one of the preceding claims, characterized in that the contact hotplate ( 12 . 112 . 212 ) without overtemperature protection means, which have separating electrical contacts. Heating according to one of the preceding claims, characterized in that the contact hotplate is a cast hotplate ( 112 . 212 ), the heater or heaters ( 114 . 214 . 314 . 414 ) run between molded-on ribs on the underside of the cast-iron hotplate. Heating according to one of claims 1 to 15, characterized in that the contact hotplate ( 12 ) is made of ceramic, preferably the heater ( 14 . 314 . 414 ) in flat wire form, especially flat, are attached directly to the underside of the ceramic.