JPH05106425A - Circular heater - Google Patents

Abstract

PURPOSE:To provide a circular heater which can prevent a temperature deterioration of the outer peripheral part. CONSTITUTION:This circular heater is composed by winding conductive heating plates 21 and 28, and by providing a center electrode 41 and an outer casing electrode 42. The heating plates are composed to make the heating amount at the outer peripheral part larger than at the internal part when a current to heat is applied between both electrodes 41 and 42, or to make the electric resistance value at the outer peripheral part larger than at the internal part. As the method to compose such a heater, the thickness and width of the outer peripheral part are made smaller than at the internal part. Or in the other way, the materials of different electric resistances may be used in the outer and inner parts.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter for collecting particulates (fine particles) discharged from a diesel engine or the like, and an annular heating element used for a preheater or the like.

[0002]

2. Description of the Related Art A self-heating type filter has been proposed as a device for collecting and burning (regenerating) the particulates as described above (Actual No. 60-38018).
No. 57-1103111). Further, there is an exhaust purification converter as a device for purifying NO, CO, HC (hydrocarbons) in the exhaust gas of a gasoline engine.
Then, the converter is going to be provided with a preheater for heating the converter at a low temperature such as when the engine is andringing. Further, also in the ceramic filter that collects the particulates, a preheater for heating and burning the collected particulates to regenerate the filter is provided upstream of the filter (Japanese Patent Laid-Open No. 61-223215). ).

[0003]

However, in the conventional self-heating type filter and the preheater, when the heating is performed, the outer peripheral portion of the filter is lower in temperature than the inner peripheral portion, and the whole is heated to a high temperature. I can't. This is because heat is easily absorbed to the outside in the outer peripheral portion and the current density of the structure itself. FIG. 15 shows the heating plate 2
1 and the corrugated plate 31 are spirally wound (see FIGS. 3 to 5 of the embodiment of the present invention), the temperature distribution is shown. This figure shows that the high temperature portion 12 is small and the outer peripheral portion forms the low temperature portion 13.

Therefore, in the outer peripheral portion, the above particulates cannot be sufficiently burned and removed. Further, in the exhaust gas purification converter, the purification performance in the outer peripheral portion is reduced. On the other hand, if the temperature of the outer peripheral portion is increased, the temperature of the inner peripheral portion becomes too high, which promotes oxidative deterioration of the filter material. In addition, thermal stress may damage the filter and converter. In view of the above conventional problems, the present invention intends to provide an annular heating element capable of reducing the temperature drop in the outer peripheral portion.

[0005]

According to the present invention, a conductive heating plate is spirally wound, and a center electrode is provided at the center of the heating plate, while an outer cylinder electrode is provided at the outer periphery thereof. A heating element, such that when a heating current is applied between the center electrode and the outer cylinder electrode, the heat generation amount per unit area in the outer peripheral portion is larger than that in the inner peripheral portion. The annular heating element is characterized in that What is most noticeable in the present invention is that a conductive strip-shaped heat generating plate is wound, and when the heating current is applied to the heat generating plate, the above-mentioned amount of heat generation is larger in the outer peripheral portion than in the inner peripheral portion. This is because the heating plate was configured to be large.

As described above, there are, for example, the following means for constructing the heating plate so that the amount of heat generated per unit area becomes larger in the outer peripheral portion. That is, the first
In this method, the heat generating plate is formed so that the outer peripheral portion has a smaller plate thickness than the inner peripheral portion. As a result, when the heating plate is spirally wound as described above,
The plate thickness of the outer peripheral portion is small. Therefore, when a heating current is applied to the heating plate, the outer peripheral portion has a small cross-sectional area, so that the electrical resistance is large and the amount of heat generation is large.

As a second means, the heat generating plate is formed such that the outer peripheral portion has a smaller plate width than the inner peripheral portion. In this case as well, the amount of heat generated is large because the cross-sectional area of the outer peripheral portion is also small. As a third means, the material of the heating plate is changed between the outer peripheral portion and the inner peripheral portion, and the outer peripheral portion is made of a conductive material having a larger specific resistance per unit length than the inner peripheral portion. There is a means to save.
For example, when a Fe-Cr-Al-based alloy is used as the conductive heating plate, the specific resistance value can be increased by increasing the Al (aluminum) concentration.

As a fourth means, when a net-like body such as a wire net is used as the heating plate, the lines of the net-like body are cut at appropriate intervals. Then, the number of the disconnection portions per unit area is set to be larger in the outer peripheral portion than in the inner peripheral portion. As a result, the outer peripheral portion has more wire breaks, and therefore the resistance when the heating current is applied is increased, and the amount of heat generation is increased. Note that, for example, the above four means can be combined appropriately.

As another annular heating element in the present invention, a conductive heating plate is spirally wound, a central electrode is provided at the center of the heating plate, and an outer cylinder is provided at the outer periphery thereof. There is an annular heating element provided with electrodes, wherein the heating plate has an electrical resistance value of the outer peripheral portion larger than an electrical resistance value of the inner peripheral portion. In this way, as means for increasing the electric resistance value of the outer peripheral portion than that of the inner peripheral portion, for example, the plate thickness of the outer peripheral portion is made smaller as in the first means, and the outer peripheral portion is made as in the second means. There is a means to reduce the board width. In addition, like the above-mentioned third means, there is a means for changing the material of the outer peripheral portion and the inner peripheral portion to increase the electrical resistivity of the outer peripheral portion. Further, there is a means, such as the above-mentioned fourth means, in which the broken portion of the mesh body is configured to have a large outer peripheral portion.

As described above, when the electric resistance value of the outer peripheral portion is set to be larger than that of the inner peripheral portion, a heating current is applied between the center electrode and the outer cylinder electrode, and the current is applied to the outer peripheral portion. When it is configured to flow radially in the radial direction as well, it is particularly effective in preventing a temperature drop in the outer peripheral portion. That is, when the heating current flows radially, the vicinity of the center electrode (ie,
The current density in the inner peripheral portion) is high, and the current density in the vicinity of the outer cylinder electrode (that is, the outer peripheral portion) is low. This is because the circumference length increases toward the outer peripheral portion and the current passing area increases.

Therefore, by making the electric resistance value of the outer peripheral portion larger than that of the inner peripheral portion as described above, the calorific value of the outer peripheral portion becomes larger than that of a constant electric resistance value, and the temperature decrease is reduced. it can. As described above, as a configuration in which the heating current is also passed in the radial direction, there is a structure in which the spirally wound heating plates are electrically connected to each other at the adjacent portions (see FIG. 11). .. Examples of the electrical connection means include welding, brazing, and conductive adhesive.

It is also possible to make the heating current flow only in a spiral shape along the heating plate wound in a spiral shape (Example 1). In this case, an electrically insulating layer is provided between the heating plates adjacent to each other in the wound state. The electrically insulating layer is formed on the surface of the heat generating plate by Al ₂ O.
It is formed in a state of ₃ or the like (see Embodiment 1), or is provided by winding it with an insulator interposed between the heat generating plates.

The annular heating element of the present invention is formed by spirally winding a conductive belt-shaped heating plate. As such a conductive heating plate, a Fe-Cr-Al alloy,
Metal materials such as stainless alloys, metal powder on metal mesh,
One obtained by integrally firing a porous sintered body such as ceramic powder,
and so on. The shape of the heating plate may be a flat plate, a corrugated plate, a wire mesh, or the like. Further, the heat generating plate may have a flat plate shape, and a corrugated plate may be interposed between the heat generating plate and the heat generating plate to be spirally wound. Alternatively, the heating plate may be a corrugated plate, and a flat plate may be interposed between the corrugated plate and the spirally wound plate.

In any of the above cases, a cavity is formed between both plates. In this cavity, the particulates and N
Exhaust gas containing O, CO, etc. is introduced. Then, as will be described later, when the annular heating element is a self-heating type filter or in the case of an exhaust purification converter, the above particulates are collected and NO and the like are purified. Further, the annular heating element of the present invention comprises the self-heating filter,
It can be used as an exhaust purification converter or a preheater. In the former two cases, the annular heating element itself has the above-mentioned particulate collection function and exhaust gas purification function. The latter preheater is arranged, for example, on the gas upstream side of the particulate collection filter and the exhaust purification converter.

Further, in the heat generating plate, the central end portion wound in a spiral shape is joined to the central electrode, and the outer peripheral end portion is joined to the outer cylinder electrode. The outer cylinder electrode has a role of fixing the wound heating plate on the outer circumference. When heating the annular heating element, a heating current is applied between the center electrode and the outer cylinder electrode.

[0016]

In the annular heating element of the present invention, the conductive heating plate is wound in a spiral shape, and the heating plate is configured such that the outer peripheral portion thereof generates a larger amount of heat than the inner peripheral portion. Therefore, when a heating current is applied between the center electrode and the outer cylinder electrode in order to generate heat in the annular heating element, a temperature drop is likely to occur in the outer peripheral portion (see the above-mentioned prior art), but the outer peripheral portion is It has a larger amount of heat generation than the inner peripheral portion. Therefore, the temperature of the outer circumference of the annular heating element is small, and the temperature of the entire heating element is almost uniform. Therefore, the unburned portion of the particulates in the annular heating element and the partial deterioration of the exhaust gas purification performance in the preheater do not occur. Also, since the temperature is almost uniform throughout,
Oxidative deterioration does not progress rapidly at high temperatures, thermal stress is less likely to occur due to the temperature difference between the outer and inner circumferences, and the annular heating element is not damaged.

Further, as described above, when the electric resistance value of the outer peripheral portion of the wound heating plate is larger than that of the inner peripheral portion thereof, as described above, the electric resistance value between the center electrode and the outer cylinder electrode is increased. Even when a heating current is applied and the current is radially applied, the amount of heat generated in the outer peripheral portion is larger than that of a constant electric resistance value. Therefore, the temperature drop in the outer peripheral portion can be reduced, and the temperature difference with the inner peripheral portion can be reduced. Therefore, according to the present invention, it is possible to provide the annular heating element capable of reducing the temperature decrease in the outer peripheral portion.

[0018]

【Example】

Example 1 An annular heating element according to an example of the present invention will be described with reference to FIGS.
Will be explained. The ring-shaped heating element 1 of this example is a self-heating type filter for collecting the particulates, which is shown in FIG.
As shown in FIG. 6, eight strip-shaped conductive heating plates 21 to 21 are formed.
28 is wound in a spiral shape, and the heat generating plates 21 to
A central electrode 41 is provided at the central portion of 28, and an outer cylindrical electrode 42 is provided at the outer peripheral portion. In addition, when the heating current is applied between the center electrode 41 and the outer cylinder electrode 42 (FIG. 6), the heat generating plate 21 generates more heat per unit area in the outer peripheral portion than in the inner peripheral portion. It is configured to be large.

The heat generating plates 21 to 28 have a corrugated shape, and eight flat plates 31 to 38 are interposed between the heat generating plates and integrally wound in a spiral shape (FIGS. 3 to 5). 5). Then, in this example, the heat generating plate is processed by corrugating the filter medium configured as shown in FIGS. 2A and 2B so that the heat generation amount is larger in the outer peripheral portion than in the inner peripheral portion. I am configuring. The filter medium 210 (heat generating plate 21) before corrugation shown in FIG.
From 1 to the outer peripheral portion 214, the size is gradually reduced (thinned). The middle part 212,213 of both is 1
There may be stages, for example, five stages, and the intermediate portion may be omitted. Further, this change in thickness may be gradually reduced instead of stepwise (see FIG. 9).

In FIG. 4, the heat generating plates 21 and 22, 2
The reason that 3 and 24, 25 and 26, and 27 and 28 are shown at the same position is that both heat generating plates are joined at both ends in both directions (see FIG. 11). Also, the flat plates 31 to 3
8 is between the heat generating plates 21 to 28, and in FIG.
2, 34, 36 and 38 are seen every other sheet as shown by the dotted line. On the other hand, since the flat plates 31, 33, 35, 37 are sandwiched between the heat generating plates, they do not appear at the dotted line position in FIG.
It is shown in parentheses in the figure.

On the other hand, the filter medium 210 (heat generating plate 21) before corrugation shown in FIG.
It is formed to be small from the outer peripheral portion 217. This plate width change may be either stepwise or gradual. Further, the above description has been made for the heat generating plate 21, but the same applies to the other heat generating plates 22 to 28. Further, FIG. 1 (a) shows a case where the heating plates 21 to 28 of varying thickness shown in FIG. 2 (a) are wound, and the left half of the figure shows the cross section of the wound part and the right half. Indicates a temperature distribution described later.

Further, FIG. 1 (b) shows the case where the heating plates 21 to 28 having different plate widths shown in FIG. 2 (b) are wound, similarly to FIG. 1 (a). In addition, the annular heating element of this example has heating plates 21 to 28 as shown in FIGS.
The flat plates 31 to 38 are interposed between them, and the cavity 20 is formed between them. Then, the heat generating plates 21 to 28 are
A corrugated filter medium for collecting the particulates is formed. The filter medium is a mesh-like body integrally formed with a porous sintered body. As is known from this, the annular heating element 1 of the present example constitutes the self-heating filter for collecting the particulates. The exhaust gas G is
Flows as shown by arrows in FIGS. 6 and 3, and the particulates therein are collected by the heat generating plates 21 to 28 as the filter medium.

The annular heating element 1 of this example has a diameter of 80 m.
m, width 20 mm. Further, each of the heat generating plates 21 to 28 and the flat plates 31 to 38 has a length of 360 mm. Also,
The heat generating plate 21 (same for 22 to 28) is 75Fe-20C.
A metal mesh of r-5Al alloy was used, and the porous powder sintered body was integrally coated and fired so as to cover the entire surface and inside the mesh. As shown in FIG. 2A, the filter medium used for the inner peripheral portion 211 of the heat generating plate 21 has a wire mesh thickness of 0.2 mm, while the filter medium used for the outer peripheral portion 214 has a wire mesh thickness of 0.13 mm. Middle part 212,
The thickness of 213 is changed stepwise. The length of each of the steps 211 to 214 is 90 mm after the wave processing.

The material of the powder sintered body is 70Fe.
-20Cr-10Al alloy. An insulating layer of Al ₂ O ₃ is formed on the surface of the heat generating plate 21. Also,
The resistance ratio between the inner peripheral portion 211 and the outer peripheral portion 214 was about 1.5. The flat plates 31 to 38 are 75Fe-2.
0Cr-5Al alloy plate, the surface of which is Al ₂ O ₃
The insulating layer is formed. Further, in the heat generating plate 21 shown in FIG. 2B, the inner peripheral portion 216 has a width of 20 mm and the outer peripheral portion 217 has a tip portion width of 13 mm. The plate thickness is the same. Others are the same as those of the heat generating plate shown in FIG.

Next, in heating the annular heating element 1, as shown in FIG. 6, the switch 44 is turned on, and the battery 43 causes 90 amperes (A) between the center electrode 41 and the outer cylinder electrode 42. ) Heating current is applied. As a result, a heating current flows between the heating plates 21 to 28, and the annular heating element 1 generates heat. And the heating plate 21
28 and flat plates 31 to 38 have Al ₂ O on their surfaces, respectively.
_Since the electric insulation layer of ₃ is formed, the heating current is
It flows spirally in each heating plate. In addition, the heating plate 21
Since the outer peripheral portion has a small thickness and the resistance ratio is 1.5 times that of the inner peripheral portion as described above, the amount of heat generated in the outer peripheral portion is large. Therefore, even if heat is radiated from the outer peripheral portion to the outside, the outer peripheral portion can maintain substantially the same temperature as the inner peripheral portion. This is the same when any of the heat generating plates shown in FIGS. 1A and 1B is used.

The right part of FIGS. 1A and 1B shows the state of temperature distribution. Further, FIG. 7 shows a similar temperature distribution as seen from the front of the annular heating element, and shows a substantially uniform high temperature portion 1.
2 is formed up to near the outer cylinder electrode 42. The low temperature section 13 is extremely small (see the conventional FIG.
(Compare with). Next, the measured values of the above temperature distribution are shown in FIG.
Shown in (a) and (b). 8 (a) is the same as FIG. 1 (a).
It is shown that the ring-shaped heating element using the heating plate of No. 1 shows the same high temperature up to the outer peripheral portion as the inner peripheral portion. This was also the case when the heating plate of FIG. 1 (b) was used.

On the other hand, FIG. 8B shows a ring-shaped heating element of a comparative example in which the heating plates 21 to 28 have the same thickness. In this case, it can be seen that at a distance of 3 cm from the center, the temperature becomes considerably lower than 500 ° C. in this way,
According to this example, it is possible to obtain an annular heating element that can uniformly heat the outer peripheral portion to the inner peripheral portion to a high temperature. Also,
In this embodiment, the filter medium has a corrugated shape and the insulating plate has a flat plate shape. However, the filter medium may have a flat plate shape and the insulating plate may have a corrugated shape, and it is necessary to form a gas flow between the filter medium and the filter material. ..

Example 2 As shown in FIG. 9, the annular heating element of this example uses a heating plate having an outer peripheral portion having a larger specific resistance than an inner peripheral portion. That is, the heating plates 21 to 28 of this example have the same plate thickness and plate width, unlike the first embodiment. As described above, the specific resistance is different between the outer peripheral portion and the inner peripheral portion. Others are the same as in the first embodiment.

The outer peripheral portion of the heat generating plate is 75Fe-20Cr.
The -5Al alloy has a high specific resistance, while the inner peripheral portion is a 79Fe-18Cr-3Al alloy having a low specific resistance. In this example, the outer peripheral portion and the inner peripheral portion are 180 mm each out of the total length of 360 mm of the heat generating plate. Welded between the two. Further, the specific resistance during this period can be changed stepwise and gradually as shown in FIG. 9A to 9C show this state. That is, FIG. 9A shows the inner peripheral portion 21 as described above.
FIG. 9 shows a state in which 1 and the outer peripheral portion 214 are divided into two.
In (b), the inner peripheral portion 211 is divided into half, and the other half is further divided into two, which are divided into an intermediate portion 212 and an outer peripheral portion 214. As a result, the resistance value per unit length is changed.

Further, FIG. 9C shows the specific resistance of the intermediate portion 212 in FIG. 9B finely and stepwisely changed. In addition, FIG. 9D shows the heating plate 2 having the same specific resistance.
It is a comparative example using 1. Also in this example, since the specific resistance between the outer peripheral portion and the inner peripheral portion is changed as described above, the amount of heat generated in the outer peripheral portion is large, and the entire annular heating element can be heated to a uniform temperature. Further, the same effect as that of the first embodiment can be obtained. In this example, the resistance value is changed stepwise as shown in FIGS. 9A to 9C by changing the material stepwise. It can also be achieved by changing the width and thickness of the plate. This also applies to the following examples.

Example 3 In this example, as shown in FIGS.
In the net-like body 29 that constitutes No. 1, disconnection portions 290 are formed in a scattered manner. That is, the heating plate 2 shown in FIG.
In the net-like body (wire net) 29 as the skeleton that constitutes No. 1, the wire is cut in the middle to provide a disconnection portion. Thereby, the flow path of the heating current is restricted and the resistance value is changed between the outer peripheral portion and the inner peripheral portion. The inner peripheral portion is not broken, and the outer peripheral portion is shown in FIG.
It was cut as shown in (a).

In this example, the number of wire breakages increases and the resistance value increases in the outer peripheral portion. Therefore, when a heating current is applied in a spiral shape, the amount of heat generated in the outer peripheral portion increases and the entire annular heating element is heated almost uniformly.
In addition, the same effect as that of the first embodiment can be obtained. The disconnection method is not limited to the interval shown in FIG.
As shown in Fig. 10 (b), the disconnection interval can be increased, and Fig. 10
The disconnection hole may be enlarged as shown in (c). Further, the change of the resistance value does not have to be limited to two steps and may be changed as shown in FIG. Other methods of changing the resistance value of the mesh-like body include changing the thickness of the wire of the wire mesh and changing the length of the wire of the wire mesh (changing the pitch).

Fourth Embodiment In this embodiment, as shown in FIGS. 11 and 12, in the annular heating element shown in the first embodiment, the heating current flows radially also in the radial direction of the annular heating element (FIG. 12). It is composed. That is, in this example, as shown in FIG. 11, the heat generating plates 21 to 28 have flat plates 31 to 28 at both ends in the width direction.
8 are interposed and joined by welding. And
Insulating layers 315, 325 and 335 of Al ₂ O ₃ are removed from the end portions of the flat plates 31 to 38 that are in contact with the heat generating plates 21 to 28. Also, in the heat generating plates 21 to 28, the Al ₂ O ₃ insulating layer (not shown) is removed at the above-mentioned joint portion.

Therefore, the heat generating plates 21 to 28 are flat plates 31 to 31.
It is electrically connected through the end of 38. Others,
This is the same as the first embodiment. In the present example, when a heating current is applied between the center electrode 41 and the outer cylinder electrode 42, the current flows in the radial direction of the annular heating element as shown by the solid line R in FIGS. 11 and 12. Also flows radially.

That is, as described above, the current flows in the radial direction through the joint portions of the heat generating plates 21 to 28 (see Embodiment 1). Further, as shown by the dotted arrow in FIG. 12, since the inside of each of the heat generating plates 21 to 28 also flows, a spiral current flow also occurs. The dotted arrow G shown in FIG. 11 indicates the flow direction of the gas passing through the heat generating plates 21 to 28 as the filter material of the annular heat generating element. According to this example, even in the structure in which the heating current flows in the radial direction in which the temperature drop in the outer peripheral portion is remarkable,
The temperature decrease in the outer peripheral portion is suppressed, and the same effect as that of the first embodiment can be obtained. Further, as the heating plate, one shown in the second or third embodiment may be used, and the same effect can be obtained in any case.

Example 5 In this example, as shown in FIGS. 13 and 14, a large number of annular heating elements 1 shown in Example 1 are arranged in a case 5, and the inlet and outlet sides of the annular heating element 1 are arranged. It shows a device for purifying particulates of a diesel engine, which is provided with radiation shields 53, 53. The annular heating element 1 is a self-heating type filter as described above. Twenty-four annular heating elements 1 are mounted on the partition plate 55 in the case 5.
The case 5 has an exhaust gas inlet port 51 and an exhaust gas outlet port 52.

Radiation shields 53, 53 are arranged on the inlet side and the outlet side of the annular heating element 1. The radiation shield is for suppressing heat radiation from the annular heating element 1, and a gap of about 1 cm is arranged on both sides of the annular heating element 1. The radiation shields 53, 53 have a thickness of 0.05
A metal plate of ~ 2 mm is used. As the metal plate, high temperature stainless steel, Al-plated stainless steel, Fe-Cr-
An Al alloy or the like is used. In this example, exhaust gas enters the introduction chamber 510 through the introduction port 51, and then the annular heating element 1
To the exhaust chamber 520, and is discharged from the discharge port 52. During this time, particulates in the exhaust gas are collected by the heat generating plates 21 to 28 which are the filter material of the annular heating element 1.

After collecting the particulates, as shown in Example 1, a heating current is applied to the heating plates 21 to 28 to heat them. At this time, since the radiation shields 53, 53 are provided on both sides of the annular heating element 1, heat radiation from both ends of the annular heating element 1 is suppressed. Therefore, according to this example,
Not only can the annular heating element 1 be heated more uniformly, but
As shown in FIG. 4, the electric energy (kW · H) required to regenerate the annular heating element 1 can be reduced as compared with the comparative example in which the radiation shield 53 is not used. In addition, the same effect as that of the first embodiment can be obtained.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an annular heating element according to a first embodiment.

FIG. 2 is a perspective view of a heat generating plate before corrugation processing according to the first embodiment.

FIG. 3 is an enlarged front view of a main part of the annular heating element according to the first embodiment.

FIG. 4 is an explanatory diagram of an annular heating element according to the first embodiment.

FIG. 5 is a front view of the annular heating element according to the first embodiment.

FIG. 6 is an explanatory diagram of a heating circuit for the annular heating element according to the first embodiment.

FIG. 7 is an explanatory diagram of temperature distribution of the annular heating element according to the first embodiment.

8 is a diagram showing the relationship between the radial distance and the temperature distribution in the annular heating element shown in Example 1. FIG.

FIG. 9 is a diagram showing the relationship between the radial distance and the resistance value per unit length in the annular heating element shown in Example 2;

FIG. 10 is an explanatory view of a broken portion of a mesh body in the heat generating plate shown in Example 3.

FIG. 11 is a radial cross-sectional view of the annular heating element according to the fourth embodiment.

FIG. 12 is an explanatory diagram of the flow direction of the heating current of the annular heating element according to the fourth embodiment.

FIG. 13 is an explanatory diagram of the use of the annular heating element according to the fifth embodiment.

FIG. 14 is a comparative explanatory diagram of reproduction power according to the fifth embodiment.

FIG. 15 is an explanatory diagram of the temperature distribution of the annular heating element shown in the conventional example.

[Explanation of symbols]

 1. ． ． Annular heating element, 21-28. ． ． Heating plate, 211,216. ． ． Inner peripheral portion, 214, 217. ． ． Peripheral part, 29. ． ． Reticulate, 290. ． ． Disconnection part, 31-38. ． ． Flat plate, 41. ． ． Center electrode, 42. ． ． Outer cylinder electrode, 53. ． ． Radiation shield,

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukihisa Takeuchi 1-1-1, Showa-cho, Kariya city, Aichi prefecture Nihon Denso Co., Ltd.

Claims (2)

[Claims] 1. An annular heating element comprising a conductive heating plate wound in a spiral shape and having a center electrode at the center of the heating plate and an outer cylinder electrode at the outer periphery thereof. When the heating current is applied between the central electrode and the outer cylinder electrode, the heat generating plate is configured so that the amount of heat generated per unit area is larger in the outer peripheral portion than in the inner peripheral portion. Characteristic annular heating element. 2. An annular heating element comprising a conductive heating plate wound in a spiral shape and having a center electrode at the center of the heating plate and an outer cylinder electrode at the outer periphery thereof. The heat generating plate is an annular heating element characterized in that an electric resistance value of the outer peripheral portion is larger than an electric resistance value of the inner peripheral portion.