GB2282945A

GB2282945A - Flat heater for heating apparatus

Info

Publication number: GB2282945A
Application number: GB9420174A
Authority: GB
Inventors: Hidetoshi Hirata
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1993-10-18
Filing date: 1994-10-06
Publication date: 1995-04-19
Anticipated expiration: 2014-10-06
Also published as: FR2711413A1; FR2711413B1; JPH07113529A; KR0165699B1; CN1111436A; GB9420174D0; GB2282945B; CN1062707C; KR950011926A; JP3020782B2

Abstract

A flat heater for a heating apparatus includes a casing 21 formed of a metal into the shape of a flat box and having a bottom, a plate-shaped heat source 23 mounted on the casing so as to be opposite to the bottom of the casing, and a heat-shielding plate 22 formed of a metal and disposed between the heat source and the bottom of the casing. The heat-shielding plate has a plurality of raised pieces 30 formed by cutting it and raising the cut portions obliquely upwardly and a plurality of conical protrusions 33 formed to protrude downwardly by way of drawing. The heat-shielding plate is mounted on the bottom of the casing with the protrusions 33 serving as support legs. The heat source is supported by the raised pieces 30. The heat-shielding plate may take other forms (figures 7 - 11, not shown). <IMAGE>

Description

FLAT HEATER FOR HEATING APPARATUS This invention relates to a flat heater used as a heat source in heating apparatus such as a cooking range with oven and grill functions.

A cooking range with oven and grill functions is provided with an external type flat heater serving as a lower heater heating food -from the underside thereof, for example. In such a flat heater, a bottom of an inner casing defining a heating chamber of the cooking range is used as a heat radiating surface. The flat heater comprises a plateshaped heat source composed of a core mica plate on which a nichrome wire is wound and two insulating mica plates sandwiching the core mica plate therebetween for insulation of the nichrome wire on the core mica plate from the other parts. The heat source is pressed against an outer bottom surface of the inner casing defining the heating chamber of the cooking range. The heat source heats the bottom of the inner casing, and heat radiating from the bottom is applied to food, thereby heating the food.

FIGS. 12 and 13 illustrate one conventional flat heater and FIGS. 14 and 15 illustrate another conventional flat heater. The flat heater 1 shown in FIGS. 12 and 13 comprises a plate-shaped heat source 2 mounted on a rectangular receptacle-like casing 4 with a glass fiber heat insulator 3 interposed therebetween. The casing 4 is secured to the the inner casing of the cooking range so that the heat source 2 is pressed against the outer bottom of the inner casing. The flat heater 5 shown in FIGS. 14 and 15 comprises a plate-shaped heat source 2 mounted on a casing 4 with a metal heat-shielding plate 6 interposed therebetween.

The casing 4 is secured to the bottom of the inner casing so that the heat source 2 is pressed against the outer bottom of the inner casing. Each of the heat insulator 3 and the heat-shielding plate 6 interrupts heat transfer from the heat source 2 to the casing 4 and also serves to press the heat source 2 against the outer bottom of the inner casing.

Since the heat insulator 3 in the flat heater 1 shown in FIGS. 12, 13 is formed of a glass fiber, it is superior in the insulation of heat. However, since the heat insulator 3 is formed by accumulating the glass fibers, the thickness of the heat insulator 3 varies widely. The wide variation in the thickness of the heat insulator 3 results in wide variation in a pressing force acting to press the heat source 2 against the outer bottom of the inner casing.

Consequently, an amount of heat transferred from the heat source 2 to the outer bottom of the inner casing varies from product to product. Furthermore, recycling the glass fiber heat insulator 3 is difficult when the flat heater 1 is disassembled and scrapped. Moreover, since the heat insulator 3 is noncombustible, it cannot be incinerated.

The heat insulator 3 is finally disposed of as a noncombustible.

On the other hand, the thickness of the heat-shielding plate 6 formed of a metal does not vary in the flat heater 5 shown in FIGS. 14 and 15. Accordingly, since the pressing force acting to press the heat source 2 against the outer bottom of the inner casing does not vary, the amount of heat transferred from the heat source 2 to the outer bottom of the inner casing does not vary widely. Furthermore, the heat-shielding plate 6 can be easily recycled. However, the metal heat-shielding plate 6 has a centuple heat conductivity or more than the glass fiber heat insulator 3.

Accordingly, since an area of the heat shielding plate 6 in contact with the heat source 2 is large, an amount of heat transferred from the heat source 2 to the heat shielding plate 6 is increased. Consequently, the heat shielding plate 6 is inferior in the insulation of heat.

Thus, the glass fiber heat insulator 3 is finally disposed of as the noncombustible which will have a bad influence upon environment although it is superior in the insulation of heat. Accordingly, it is preferable to use the metal heat-shielding plate 6 rather than use the glass fiber heat insulator 3. However, the conventional heatshielding plate 6 is inferior in the insulation of heat.

Thus, it has been desired to provide a flat heater in which a metal heat-shielding plate superior in the insulation of heat is employed.

Therefore, an object of the present invention is provide a flat heater for a heating apparatus wherein the heat-shielding plate is superior in the insulation of heat though it is formed of a metal.

The present invention provides a flat heater for a heating apparatus having a heating chamber for accommodating food to be heated therein, the heating chamber being defined by a wall member formed of a heat conductive material and including a flat surface serving as a heating surface, the flat heater comprising a casing formed of a metal into the shape of a flat box and having a bottom, a plate-shaped heat source provided on the casing so as to be opposite to the bottom of the same, the heat source being pressed against the heating surface of. the heating apparatus when the casing is mounted on the heating surface, a heat-shielding plate formed of a metal and disposed between the heat source and the bottom of the casing, and a support protrusion protruding from the heat-shielding plate so as to abut against the heat source, thereby supporting the heat source in position.

According to the above-described flat heater, the support protrusion of the heat-shielding plate is in contact with the heat source. Since the area of the support protrusion in contact with the heat source is small, an amount of heat transferred from the heat source to the heatshielding plate is small even when the heat-shielding plate is formed of a metal having a large heat conductivity.

Consequently, the heat-shielding plate can be improved in the insulation of heat.

The support protrusion may comprise a plurality of raised pieces formed by cutting the heat-shielding plate and raising cut portions obliquely and each raised piece is elastically deformed in the direction of inclination thereof when the heat source abuts against the heating surface, so that a restoring force of each raised piece urging the piece in the direction in which the piece returns to the former raised position presses the heat source against the heating surface. In this regard, each raised piece composing the support protrusion preferably extends successively over substantially the entire width of the heat-shielding plate, so that the strength of the heat-shielding plate can be improved.

In order that an amount of heat transferred from the heat source to the heat-shielding plate by radiation is rendered as small as possible, a surface of the heatshielding plate facing the heat source is preferably plated for reflection of light. Furthermore, in order that the amount of heat transferred from the heat source to the heatshielding plate by radiation in a limited space, a space between the heat source and the heat-shielding plate is preferably set to be larger than a space between the heatshielding plate and the bottom of the casing.

The invention will be described, merely by way of example, with reference to the accompanying drawings, in which: FIG. 1 is a partially longitudinally sectional side view of a first embodiment of a flat heater in accordance with the present invention; FIG. 2 is a longitudinally sectional side view of the flat heater; FIG. 3 is a longitudinally sectional side view of the flat heater taken along line 3-3 in FIG. 2; FIG. 4 is an exploded perspective view of a plateshaped heat source; FIG. 5 is an exploded perspective view of the flat heater; FIG. 6 is a longitudinally sectional view of a cooking range with oven and grill functions, to which range the flat heater in accordance with the present invention is applied; FIG. 7 is a view similar to FIG. 1, showing a second embodiment of a flat heater in accordance with the present invention;; FIG. 8 is a view similar to FIG. 1, showing a third embodiment of a flat heater in accordance with the present invention; FIG. 9 is a view similar to FIG. 1, showing a fourth embodiment of a flat heater in accordance with the present invention; FIG. 10 is a view similar to FIG. 1, showing a fifth embodiment of a flat heater in accordance with the present invention; FIG. 11 is a view similar to FIG. 1, showing a sixth embodiment of a flat heater in accordance with the present invention; FIG. 12 is an exploded perspective view of a conventional flat heater; FIG. 13 is a partially sectional view of the conventional flat heater; FIG. 14 is a view similar to FIG. 12, showing another conventional flat heater; and FIG. 15 is a view similar to FIG. 13, showing said another conventional flat heater.

A first embodiment of the present invention will be described with reference to FIGS. 1 to 6. In the first embodiment, the invention is applied to a cooking range with oven and grill functions. Referring first to FIG. 6, the cooking range comprises an outer casing 11 and an inner casing 12 defining therein a heating chamber 13. A magnetron 15 is provided outside the inner casing 12 for supplying microwaves via a waveguide 14 into the heating chamber 13. A heater lamp 16 is mounted on the upper interior of the heating chamber 13 for a grill cooking. A flat heater 17 in accordance with the present invention is mounted on the outer bottom of the inner casing 12. The bottom of the inner casing 12 serves as. a heating surface 12a for the flat heater 17.The flat heater 17 heats the heating surface 12a, and heat radiating from the heating surface 12a is applied to food accommodated in the heating chamber 13, thereby heating it. A mounting frame 18 is mounted on the bottom of the flat heater 17 and an electric motor 19 is mounted on the mounting frame 18. A rotational shaft 19a of the motor 19 projects through the flat heater 17 into the inner casing 12 or the heating chamber 13. A turntable 20 is fixed to the upper end of the shaft 19a.

Food to be heated is placed on the turntable 20, and the turntable 20 is then rotated so that microwaves are uniformly applied to the food.

Referring now to FIGS. 2 to 5, the flat heater 17 comprises a casing 21 formed into the shape of a flat rectangular receptacle and having an upper open end, a heatshielding plate 22 disposed in the casing 21 and a plateshaped heat source 23 held by the heat-shielding plate 22 so that the heat source 23 is located at the upper open end of the casing 21. The heat source 23 comprises two core mica plates 25 on which a heat-generating wire 24 such as a nichrome wire is wound and two insulating mica plates 26 sandwiching the core mica plates 25 for the purpose of insulating the heat-generating wire 24 from the other parts, as shown in FIG. 4. Each of the casing 21 and the heatshielding plate 22 is formed of a thin metal plate and has a plated surface for reflection of light.Heat (infrared radiation) radiating from the heat source 23 is reflected on the plated surface so that the casing 21 and the heatshielding plate 23 can be prevented from being easily heated by the radiant heat.

The casing 21, the heat-shielding plate 22 and the heat source 23 are connected to one another by inserting tubular eyelets 27 through holes 21a, 22a and 23a formed in these parts respectively and collapsing both ends of each eyelet 27, as shown in FIG. 5. In this regard, the inner diameter of each of the holes 21a, 22a, 23a is larger than the outer diameter of each eyelet 27 so that the heat-shielding plate 22 and the heat source 23 are movable transversely relative to the casing 21. Consequently, the difference in an amount of thermal expansion between the heat-shielding plate 22 and the heat source 23 can be assimilated.

In mount of the flat heater 17, the bottom of the casing 21 is secured to securing protrusions or mounting eyes 28 fixed on the underside of the heating surface 12a of the inner casing 12 by screws 29. As the result of securement of the flat heater 17 to the securing protrusions 28, the heat source 23 is pressed against the heating surface 12a of the inner casing 12 so that heat generated by the heat-generating wire 24 is transferred to the heating surface 12a.

The structure for supporting the heat source 23 on the heat-shielding plate 22 will now be described. The heatshielding plate 22 has a plurality of support protrusions or raised pieces 30 formed by cutting it and raising cut portions thereof, as shown in FIG. 5. A plurality of conical protrusions 33 are formed on abridge portion 32 between each elongated hole 31 and its adjacent one by way of drawing, the elongated holes being formed as the result of provision of the raised pieces 30. Each raised piece 30 has the height H1 larger than each protrusion 33.

Each raised piece 30 extends successively over substantially the entire width of the heat-shielding plate 22 and is inclined as shown in FIG. 1. Furthermore, the upper end of each raised piece 30 is bent to be inclined downwardly obliquely, so that a bent portion 30b at which each raised piece 30 is bent for forming a bent piece 30a is the top of each raised piece 30.

The protrusions 33 are abutted against the inner bottom of the casing 21 so that the heat-shielding plate 22 is held on the inner bottom of the casing 21. The bent portions 30b of the respective raised pieces 30 are abutted against the heat source 23 to hold it. In this regard, longitudinal ends of each raised piece 30 abut against portions of the lower insulating mica plate 26 in which portions the heat generating wire 24 is not disposed, as shown in FIG. 3.

Before the casing 21 is mounted on the securing protrusions 28 welded to the bottom of the inner casing 12, the distance L1 (see FIG. 1) between the inner bottom of the casing 21 and the upper end of the heat source 23 is set to be larger than the distance L2 (see FIG. 6) between the lower end of each securing protrusion 28 and the heating surface 12a of the inner casing 12. Consequently, the flat heater 17 is elastically deformed to be contracted when having been mounted on the heating surface 12a of the inner casing 12. More specifically, each raised piece 30 assuming the position shown by solid line in FIG. 1 elastically deforms in the direction of arrow A when the casing 21 has been mounted on the securing protrusions 28. Each raised piece 30 is further inclined as the result of the elastic deformation, assuming the position shown by two-dot chain line. A restoring force urging each raised piece 30 toward the former position presses the heat source 23 against the heating surface 12a of the inner casing 12. Each raised piece 30 does not elastically deform in the direction of arrow A by itself but elastically deforms, rotating about the protrusions 33 in the direction of arrow A together with the bridge portions 33. Reference numeral 34 in FIG. 5 designates lead wires connected to both ends of the heat generating wire 24 respectively.

According to the above-described flat heater 17, the heat source 23 is supported by the raised pieces 30 formed on the heat-shielding plate 22. Accordingly, since the area of the heat-shielding plate 22 in contact with the heat source 23 is extremely small, an amount of heat transferred from the heat source 23 to the heat-shielding plate 22 by conduction is rendered small. Consequently, since an amount of heat transferred from the heat-shielding plate 22 to the casing 21 is reduced accordingly, the heat shielding performance of the heat-shielding plate 22 can be improved.

Since the heat-shielding plate 22 is supported on the protrusions 33 formed on the bottom of the casing 21, the contact of the heat-shielding plate 22 with the casing 21 is nearly a point contact. Consequently, since the area of the heat-shielding plate 22 in contact with the casing 21 is extremely small, the amount of heat transferred from the heat-shielding plate 22 to the casing 21 by conduction is further rendered small.

Furthermore, the distance H1 between the heat-shielding plate 22 and the heat source 23 is larger than the distance H2 between the heat-shielding plate 22 and the bottom of the casing 21. Accordingly, an amount of heat transferred by radiation to the heat-shielding plate 23 from the heat source whose temperature is raised in a limited space can rendered as small as possible. Additionally, an amount of heat transferred from the heat-shielding plate 22 to the casing 21 by radiation can also be rendered as small as possible. Moreover, since the heat-shielding plate 22 is plated ana the radiant heat from the heat source 23 is reflected on the plated surface of the heat-shielding plate 22, the amount of heat transferred from the heat source 23 to the heat-shielding plate 22 can be further rendered small.

Each raised piece 30 raised obliquely from the heatshielding plate 22 elastically deforms so as to be bent down when the flat heater 17 has been mounted on the heating surface 12a of the inner casing 12. Since the pressure of contact of the heat source 23 with the heating surface 12a is increased by the restoring force acting to return each raised piece 30 to the former position, the heat source 23 can be maintained in the state that it is pressed against the heating surface 12a with a suitable pressure.

Consequently, heat conductivity from the heat source 23 to the heating surface 12a can be improved or the heat transfer efficiency can be improved.

Since each raised piece 30 extends successively over substantially the entire width of the heat-shielding plate 22, the strength of the heat-shielding plate 22 can be increased. Accordingly, the heat source 23 can be pressed against the heating surface 12a by the whole longitudinal length of each raised piece 30 even though the number of the protrusions 33 per raised piece 30 is small.

The bent portion 30b of each raised piece 30, when abutted against the heat source 23, causes the portion of each raised portion 30 in contact with the heat source 23 to assume the shape of a circular arc. Differing from the case where the upper distal end of each raised piece which is not bent, the circular arc portion or the bent portion 30b of each raised piece 30 does not damage the lower insulating mica plate 26 of the heat source 23. If the thin insulating mica plate 26 whose thickness is 0.3 to 0.5 mm should be broken and the raised piece 30 should be brought into contact with the heat generating wire 24, an electric leakage would occur. However, the occurrence of such an electric leakage can be prevented in the above-described flat heater 17.

The longitudinal ends of each bent portion 30b have respective cut edges which are formed when each raised piece 30 is formed by cutting and raising the heat-shielding plate 22. However, the longitudinal ends of each raised piece 30 abut against portions of the lower insulating mica plate 26 in which portions the heat generating wire 24 is not disposed. Consequently, if the lower insulating mica plate 26 should be broken by the cut edge of the bent portion 30b, the raised piece 30 could be prevented from being brought into contact with the heat generating wire 24.

FIG. 7 illustrates a second embodiment of the invention. Referring to FIG. 7, the raised pieces 36 and 37 are formed by cutting the heat-shielding plates 35 so as to be raised perpendicularly thereto alternately downwardly and upwardly, respectively. The downward raised pieces 36 are abutted against the bottom of the casing 21 so that the heat-shielding plate 35 is held on it. The upward raised pieces 37 serve as the support protrusions to support the heat source 23.

FIG. 8 illustrates a third embodiment of the invention.

The peripheral edge of the heat-shielding plate 38 is downwardly bent, thereby providing a leg 39. A plurality of raised pieces 40 each serving as the support protrusion are formed by cutting the heat-shielding plate 38 so as to be raised upwardly perpendicularly thereto. The leg 39 is abutted against the bottom of the casing 21 so that the heat-shielding plate 38 is held on it. The heat source 23 is supported by the raised pieces 40.

FIG. 9 illustrates a fourth embodiment of the invention. The heat-shielding plate 41 has trapezoidal protrusions 42 and 43 formed by way of. drawing so as to extend alternately downwardly and upwardly. The downwardly convex protrusions 42 are abutted against the bottom of the casing 21 so that the heat-shielding plate 41 is held on the casing 21. The heat source 23 is supported by the upwardly convex protrusions 43.

FIG. 10 illustrates a fifth embodiment of the invention. The heat-shielding plate 44 has corrugated portions 45 formed by way of drawing and each serving as the support protrusion extending upwardly. The casing 21 has corrugated protrusions 46 extending upwardly on the bottom thereof. The heat-shielding plate 44 is placed on the protrusions 46 to be thereby held by the casing 21. The heat source 23 is supported by the corrugated protrusions 46.

FIG. 11 illustrates a sixth embodiment of the invention. The heat-shielding plate 47 has corrugated portions 48 and 49 formed by way of drawing so as to be convex alternately downwardly and upwardly, respectively.

Thus, the heat-shielding plate 47 has a generally corrugated section. The downwardly convex corrugated portions 48 are abutted against the bottom of the casing 21 so that the heat-shielding plate 47. is held on the casing 21. The heat source 23 is supported by the upwardly convex corrugated portions 49 serving as the support protrusion.

Since each of the heat-shielding plates 35, 38, 41, 44 and 47 in the second to sixth embodiments has small areas in contact with the heat source 23 and the casing 21 respectively, the heat shielding performance of each heatshielding plate can be improved as in the first embodiment.

In the fourth embodiment shown in FIG. 9, particularly, the heat source 23 is supported on the flat upper end face of each protrusion 43. Consequently, the lower insulating mica plate 26 can be prevented from being damaged.

In each of the second and third embodiments shown in FIGS. 7 and 8 respectively, each of the raised pieces 37, 40 may have a horizontal piece formed on the distal end thereof, and the heat source 23 may be supported on the horizontal pieces.

Although the invention has been applied to the flat heater mounted on the bottom (heating surface 12a) of the inner casing 12 in the foregoing embodiments, the invention may be applied to a type of a flat heater mounted on a ceiling of the inner casing. Furthermore, stepped shafts may be secured to the casing 21, instead of the eyelets 28, and the heat-shielding plate may be supported on steps of the stepped shafts. Furthermore, although the invention has been applied to the flat heater of the cooking range with the oven and grill functions in the foregoing embodiments, the invention may be applied to the flat heaters employed in the other types of the heating apparatus.

According to the flat heater in accordance with the present invention, the metal heat-shielding plate is employed as the heat insulator interrupting heat transfer from the heat source to the inner casing. Differing from the glass fiber heat insulator, the metal heat-shielding plate can be recycled and can be prevented from having a bad influence upon the environment. Furthermore, since the support protrusions are formed on the metal heat-shielding plate to support the heat source, the area of the heatshielding plate in contact with the heat source is rendered small. Consequently, an amount of heat transferred from the heat source to the heat-shielding plate can be reduced, which can reduce an amount of heat escaping outwardly from the heat source through the heat-shielding plate and the casing in turn.Thus, an improved heat insulating efficiency can be achieved although the metal heat-shielding plate is employed in the flat heater in accordance with the present invention.

Since the heat-shielding plate and the heat source are connected to the casing so as to be movable relative to the casing, the difference in the thermal expansion among these parts can be assimilated. Consequently, occurrence of a large thermal stress causing deformation or breakage of these parts can be prevented.

Each of the plurality of raised pieces composing the support protrusion of the heat-shielding plate is formed by cutting the heat-shielding'plate and obliquely raising the cut portion and is elastically deformed in the direction of its inclination when the heat source has been brought into contact with the heating surface of the inner casing, whereupon the resultant restoring force is utilized to press the heat source against the heating surface. The abovedescribed construction produces the result that the heat source is usually pressed against the heating surface with a suitable pressing force applied thereto, so that heat transfer from the heat source to the heating surface can be performed satisfactorily.

Since each raised piece composing the support protrusion extends successively over substantially the entire width of the heat-shielding plate, strength of the heat-shielding plate can be improved and heat source can be pressed against the heating surface satisfactorily even when the heat-shielding plate has a small thickness.

The distal end of each raised piece is bent and a proximal portion of the bent portion is abutted against the heat source. Consequently, the heat source can be prevented from being damaged by the raised pieces.

Since the heat-shielding plate is plated for reflection of light, an amount of heat transferred from the heat source to the heat-shielding plate by radiation can be reduced.

Consequently, the heat insulating efficiency of the heatshielding plate can be further improved.

The space between the heat source and the heatshielding plate is set to be larger than the space between the bottom of the casing and the heat-shielding plate.

Consequently, an amount of heat transferred from the heat source to the heat-shielding plate by radiation in a limited space can be reduced.

The longitudinal ends of the support protrusion is abutted against the heat source so as to be away from the heat generating wire of the heat source. Consequently, even when the longitudinal ends of the support protrusion have cut edges and the heat source is damaged by the cut edge, the support protrusion can be prevented from coming into contact with the heat generating wire, which can prevent occurrence of an electric leakage.

The foregoing description and drawings are merely illustrative of the principles of the present invention and are not to be construed in a limiting sense. Various changes and modifications will become apparent to those of ordinary skill in the art. All such changes and modifications are seen to fall within the true spirit and scope of the invention defined by the appended claims.

Claims

WE CLAIM:

1. A flat heater for a heating apparatus having a heating chamber for accommodating food to be heated therein, the heating chamber being defined by a wall member formed of a heat conductive material and including a flat surface serving as a heating surface, the flat heater comprising: a) a casing formed of-a metal into the shape of a flat box and having a bottom; b) a plate-shaped heat source provided on the casing so as to be opposite to the bottom of the same, the heat source being pressed against the heating surface of the heating apparatus when the casing is mounted on the heating surface; c) a heat-shielding plate formed of a metal and disposed between the heat source and the bottom of the casing; and d) a support protrusion protruding from the heatshielding plate so as to abut against the heat source, thereby supporting the heat source in position.

2. A flat heater according to claim 1, wherein the wall member includes a plurality of securing protrusions to which the bottom of the casing is secured and a gap (L2) between a portion of the bottom of the casing secured to each securing protrusion and the heating surface of the wall member is shorter than the thickness (L1) of the flat heater before mount thereof onto the heating surface so that the flat heater, when mounted on the heating surface, is compressed against the heating surface so that a contact pressure between the heat source and the heating surface is increased.

3. A flat heater according to claim 1, wherein each of the heat source and the heat-shielding plate is connected to the casing to be movable relative thereto so that the difference in thermal expansion between the heat source and the heat-shielding plate is assimilated.

4. A flat heater according to claim 1, wherein the support protrusion comprises a plurality of raised pieces formed by cutting the heat-shielding plate and raising cut portions obliquely and each raised piece is elastically deformed in the direction of inclination thereof when the heat source abuts against the heating surface, so that a restoring force of each raised piece urging the piece in the direction in which the piece returns to the former raised position presses the heat source against the heating surface.

5. A flat heater according to claim 4, wherein each of the raised pieces composing the support protrusion extends successively over substantially the entire width of the heat-shielding plate.

6. A flat heater according to claim 1, wherein the casing has a plurality of protrusions formed on the bottom thereof and abutting against the heat-shielding plate to thereby hold the same in position.

7. A flat heater according to claim 1, wherein the heat-shielding plate has a plurality of protrusions each protruding to the side opposite the support protrusion and the heat-shielding plate is held via the protrusions on the bottom of the casing.

8. A flat heater according to claim 1, wherein the heat-shielding plate has a plurality of raised pieces protruding in the directions opposite to each other, is held on the bottom of the casing via the raised pieces protruding in one direction, and abuts against the heat source with the raised pieces protruding in the other direction serving as support protrusion.

9. A flat heater according to claim 1, wherein a portion of the support protrusion abutting against the heat source is formed into a plane.

10. A flat heater according to claim 4, wherein each of the raised pieces composing the support protrusion has a bent distal end and a bent portion of the bent distal end abuts against the heat source.

11. A flat heater according to claim 1, wherein a space between the heat source and the heat-shielding plate is set to be larger than a space between the heat-shielding plate and the bottom of the casing.

12. A flat heater according to claim 1, wherein the heat-shielding plate has a generally corrugated section and wherein the heat-shielding plate is held at one side thereof on the bottom of the casing via convex portions of corrugated portions and abuts at the other side thereof against the heat source with convex portions of corrugated portions serving as the support protrusion.

13. A flat heater according to claim 1, wherein the heat source comprises a heating conductor sandwiched between insulating plates such as mica plates and both longitudinal ends of the support protrusion abut against portions of the heat source in which portions the support protrusion is away from the heating conductor.

14. A flat heater according to claim 1, wherein a surface of the heat-shielding plate facing the heat source is plated for reflection of light.

15. A flat heater substantially as herein described with reference to FIGS. 1 to 11 of the accompanying drawings.