GB2074828A - Electric heater - Google Patents

Abstract

An electric heater for use in an electric fire or the like, includes an insulating tube 4 such as a quartz tube, and a coiled resistance wire 5 enclosed in the insulating tube, in which the individual turns of the coiled wire 5 have a non-circular, e.g. elliptical, shape and are radially displaced from adjacent turns by spirally twisting the coil about the axis of the heater so as to decrease the time required for heating the wire to a specified temperature and improve the efficiency of heat radiation. The resistance wire 5 is made of iron- chrome-aluminium alloy so as to lengthen the life of the wire. <IMAGE>

Description

SPECIFICATION Electric heater The present invention relates to an electric heater and, more particularly, to an electric radiant heater assembly for use in a room heater, a cooking heater and the like.

Generally, the electric heater, which is disposed in front of a reflector, includes a quartz tube and an ohmic resistive element sheathed in the quartz tube.

For the ohmic resistive element, a wire made of nickel-chrome alloy, generally referred to as nichrome wire, has been widely used. Although nichrome wire is considerably easy to obtain and work with, there have been such disadvantages that the wire becomes thinner as the oxidation of the material composing the wire proceeds, and therefore, may be broken when a large current is sup plied. In other words, nichromewire has a consider ably short lifetime. Particularly when the nichrome wire wound in a shape of coil is employed, a short circuit accidentally formed between neighboring two turns in the coil often results in burning out of the wire due to the increase of the current.Furthermore, since a large portion of the periphery of the coil is held in contact with an inner surface of a nichrom wire supporting member, heat generated by the wire is transmitted to the supporting member. This heat transmission adversely delays a rise in the tempera tureofthe resistance wire to the required level after the power supply.

In order to prevent the oxidation of the wire, there has been proposed an improved heating unit including a hermetically sealed quartz tube filled with inert gas, such as argon, and ohmic resistive element, usually tungsten wire, sheathed in the hermetically sealed quartz tube. Since the tungsten wire reaches, during its heat generation, about 2400"C which is much higher than the melting point (about 1500 C) of the quartz, the improved heating unit required means for supporting the tungsten wire in a position separated from the inner wall of the quartz, which means is formed, e.g., by tantalum. Therefore, the above described improved heating unit requires additional members, such as inert gas and support ing means, and thus, resulting in increase in manufacturing steps and manufacturing cost.Furth ermore, since the tungsten wire emits much light as in a light bulb, it is inappropriate to be used in room heaters and cooking heaters.

In order to suppress the light emission from the ohmic resistive element, there has been proposed a heating unit employing molybdenum as an ohmic resistive element. Although this heating unit suppresses the light emission, it generates heat about 1400-1600 Cwhich is still higher than the upper limit temperature fqr use in safety of the quartz. Therefore, this heating unit also requires the supporting means for supporting the wire. Furthermore, the employment of molybdenum has such a disadvantage that, when the quartz tube is broken by accident during the heat generation, the molybdenum turns into molybdenum oxide by the air, resulting in sublimation to scatter white molybdenum oxide powder therearound.

In consideration of the above disadvantages, it has been concluded that the heating unit should preferably be formed without filling the quartz tube with inert gas. Figures 5 and 6 show a typical prior art heater including an insulating tube, such as a quartz tube, 11 and ohmic resistive wire 12 inserted in the quartz tube 11 for generating heat when electric current flows therethrough.Since tha wire 12 is formed in a shape of coil with each turn forming a complete circle when viewed from one end of the coil, as shown in Figure 7a, there is such a disadvantage that the contacting area between the wire 12 and the inner surface of the insulating tube 11 is considerably large, and therefore, the heat generated by the wire 12 is adversely transmitted to the insulating tube 11, resulting in long period of time to reach the desired heat radiation level after the power supply, as shown by a dotted line in Figure 8.

Furthermore, since the turns in the coil are aligned with each other, as shown in Figure 7b, interference of heat radiation takes place between neighboring two turns of the coil so as to undesirably prevent the free heat radiation and thus, the efficiency of heat radiation is considerably low as shown by a dotted line in Figure 9.

One prior art device is disclosed in U.S. Patent No.

3551643, in which the periphery of the individual turns of the coiled resistance wire forms a cyclical, spiral shape about the axis of the heater so as to produce a turbulent flow of a fluid by obstructing the axial flow of the fluid through the heater and thus, heat transfer from the resistance wire to the fluid promoted so as to increase the temperature of the fluid flowing between two cylinders. The abovedescribed U.S. Patent discloses a method of utilizing the conduction heat efficiently with the temperature of the resistance wire exceeding 1000"C. Although the U.S.Patent as described above can be effectively applied to heat exchange by means of a fluid such as a gas flowing through a quartz tube, it discloses neither efficient heat radiation by disposing the electric heater in front of a reflector nor an effective and economical heater through a full understanding of the life in terms of the oxidation, and the relations among the wire material, wire diameter and coil pitch.

According to the present invention, there is provided an improved electric heater element including, an insulating tube such as a quartz tube, and a coiled resistance wire enclosed in the insulating tube, in which the individual turns of the coil have a noncircular shape and are radially displaced from adjacent turns by twisting e.g. spirally twisting the coil about the axis of the heater. The wire is preferably made of iron-chrome-aluminium alloy. In preferred embodiments to be described hereafter, the time required for heating the resistance wire to a specified temperature is decreased, the efficiency of heat radiation is improved, and the life of the electric heater is lengthened remarkably.

The present invention will be illustrated by the following description of a preferred embodiment thereof with reference to the accompanying drawings, in which; Figure 1 is a perspective view of an electric stove to which an electric heater according to the present invention may be applied, Figure 2 is a fragmentary cross sectional view of the electric heater and a reflector employed in the stove of the Figure 1, Figure 3 is a longitudinal sectional view of the electric heater of Figure 1, Figures 4(a) and 4(b) are a partial perspective view of a resistance wire of the electric heater of Figure 3 and a side elevational view of the resistance wire, respectively, Figure 5 is a fragmentary cross sectional view of a conventional electric heater and a reflector employed in an electric stove.

Figure 6 is a fragmentary side sectional view showing, on an enlarged scale, the construction of the conventional electric heater Figure 5, Figures 76a) and 7fbJ are a side elevational view of a resistance wire of the conventional electric heater of Figure 6 and a front elevational view of the resistance wire, respectively, Figure 8 is a characteristic diagram showing the time required for heating the resistance wire to a specified temperature, according to the present invention and that of a conventional electric heater, and Figure 9 is a characteristic diagram showing the efficiency of heat radiation according to the present invention and that of a conventional electric heater.

Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the views of the accompanying drawings.

Referring now to the drawings, there is shown in Figure 1 an electric heater 3 according to one preferred embodiment of the present invention which is generally disposed in front of a reflector 2 provided on a frame 1 of an electric stove with the electric heater 3 being supported at opposite ends thereof by two opposing side walls of the frame 1 and includes an insulating tube 4 such as a quartz tube and a resistance wire 5 enclosed in the insulating tube 4.

Since the resistance wire 5 is made of ironchrome-aluminium alloy, aluminium contained therein is deposited on the surface of the wire 5 so as to form an film of aluminium oxide. Accordingly, iron and chrome contained therein is secluded from the atmosphere and therefore, they are less likely to be subjected to oxidation. Thus, the life of the electric heater 3 is improved and the surface of the resistance wire 5 is electrically insulated. Meanwhile, as shown in Figures 4(a) and (b), the resistance wire 5 is wound in a shape of coil, while individual turns of the coil have a non-circular shape and are radially displaced from adjacent turns by spirally twisting the coil about the axis of the heater.When the resistance wire 5 formed as described above is inserted into the insulating tube 4, some turns of the non-circular coiled wire 5 come into contact, at their peripheral portions or tips, with the inner surface of the insulating tube 4, while other turns thereof do not come into contact, at their peripheral portions or tips, with said inner surface of the insulating tube 4.

Since the resistance wire 5 has been sequentially twisted about the axis of the heater, the contact portions between the wire 5 and the inner surface of the insulating tube 4 are disposed slantly with respect to the axis of the insulating tube 4. By sequentially twisting a non-circular coiled wire as described above, it is not all turns of the resistance wire 5 that contact the inner surface of the insulating tube 4, but some turns thereof are spaced therefrom by certain distances as most clearly seen in Figure 3 in contrast with a conventional circular coiled wire, with a consequent decrease in the contact area between the wire 5 and the inner surface of the insulating tube 4.

Meanwhile, due to its non-circular profile, an apparent outer diameter D of the coil of the resistance wire 5 can be made, for example, 1.5 times as large as an outer diameter a (Figure 3) of a conventional circular coil as shown in Figure 3, so that the time required for heating the resistance wire 5 to a specified temperature, according to the present invention can be decreased to approximately half of that of a conventional circular coiled resistance wire, as shown by a solid line in Figure 8.

Meanwhile, since adjacent turns of the coil of the spirally twisted resistance wire 5 are disposed slantly with respect to the axis of the insulating tube 4, a portion of adjacent turns of the coil is not disposed to confront one another, such a phenomenon that interference of heat energy emitted from individual turns of the coil of the resistance wire 5 results in an abnormal rise in the temperature of the wire 5 as is found in a conventional wire is desirably eliminated. Furthermore, since radiant heat emitted from the resistance wire 5 is diffused at a wide angle, partly due to an apparent increase in the coil diameter as described above, the radiant heat energy of the present invention, shown in a solid line in Figure 9 is improved by 10 to 15 % as compared with that of a conventional wire shown in a dotted line in Figure 9.

Especially, an elliptical coiled resistance wire 5 provides smoother bending portions as compared with a rectangular coil disclosed in the U.S. Patent No. 3551643 described earlier and thus, it is less likely to be broken due to internal stress during manufacture of the resistance wire, thereby improving the life of the wire considerably.

Meanwhile, inventors of the present invention prepared various samples and conducted through life tests and power supply tests thereof. The results of the tests have shown that a resistance wire manufactured under the following conditions is most suitable for an electric heater for use in electric stoves, cooking ovens and the like. Ten iron-chromealuminium alloy resistance wires 5 of 0.4 to 0.6 mm in diameter, with a power consumption of 800 W were used for the above-described tests. The relation between the wire diameter d and coil diameter D of the resistance wire 5 as shown in Figure 4 provides longest life thereof and most efficient heat radiation when expressed by: 10d < D < 15d.

Namely, when the coil diameter D exceeds 1 sod, each turn of the coil is so tilted in the axial direction of the heater as to contact either of corresponding adjacent turns, so that a short circuit occurs between the corresponding adjacent turns, thereby resulting in breaking of the wire. Meanwhile, when the coil diameter D is less than lod, large heat interference takes place between corresponding adjacent turns of the coil and thus, heat is accumulated in the vicinity of the resistance wire, so that radiant heat to be emitted out of the electric heater is reduced while manufacture of such a coil is difficult.

In an electric stove in general, when the resistance wire is red heated so as to utilize the radiant heat efficiently, the wire temperature of 800 to 9500C is most suitably used.

Meanwhile, the relation between the coil diameter D and a twisting pitch P of the spirally twisted resistance wire 5 provides longest life thereof and most rapid heating of the stove when expressed by: 5D < P < 7D. Namely, when the twisting pitch P exceeds 7D, the large pitch Pslackensthe coil of the resistance wire 5 between each pitch and thus, the contact area between the wire 5 and the inner surface of the insulating tube 4 is undesirably increased, so that part of radiant heat emitted from the wire 5 is transferred to the insulating tube 4 at the start of power supply with consequent deterioration in rapid heating which is required by electric stoves.Meanwhile, when the twisting pitch P is less than 5D, the contact area between the wire 5 and the inner surface of the insulating tube 4 is increased as in the case above, so that rapid heating is deteriorated.

Meanwhile, the relation between the wire diameter d and a pitch P' between corresponding adjacent turns of the coil of the resistance wire 5 is most suitable when expressed by: P' = 2 to 3d.

Namely, since an apparent diameter of the noncircular coil of the present invention can be made larger than that of a conventional circular coil, an overall axial length of the electric heater of the present invention can be made approximately half of that of a conventional one, provided that both heaters have the same calorific value. When both heaters have the same overall axial length thereof, the pitch P' between corresponding adjacent turns of the coil of the present invention can be increased by 50 % as compared with a conventional coil, thus providing a fairly economical electric heater.

In accordance with the present invention as described above, the resistance wire composing the electric heater, which is wound in a non-circular shape and then, twisted spirally, is enclosed in the insulating tube such as a quartz tube, whereby the time required for heating the resistance wire to a specified temperature is decreased with the improved efficiency of heat radiation and thus, the electric heater of the present invention is suitable for use in electric stoves, cooking ovens, and the like, and in the form described above has been found to be highly efficient and reliable in actual use.

Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as included therein.

The use of iron-chrome-aluminium alloy for the wire has been found to length the life of the element.

Claims (10)

1. An electric heater element comprising an insulating tube; and a coil of resistance wire enclosed in said insulating tube; said coil including individual turns which have a non-circular shape and are radially displaced from adjacent turns by spirally twisting said coil about an axis of said electric heater.

2. An electric heater as claimed in claim 1, wherein said resistance wire is wound in an elliptical shape.

3. An electric heater element as claimed in claim 1 or claim 2, wherein a coil diameter of said resistance wire is 10 to 15 times a wire diameter of said resistance wire and said resistance wire is red heated to temperatures ranging from approximately 800 to 10000C during normal use thereof.

4. An electric heater element as claimed in claim 1,wherein a twisting pitch of said resistance wire is 5 to 7 times said coil diameter of said resistance wire.

5. An electric heater element as claimed in claim 1,wherein a pitch between corresponding adjacent turns of said coil of said resistance wire is 2 to 3 times said wire diameter of said resistance wire.

6. An electric heater element as claimed in any preceding claim wherein said resistance wire is made of iron-chrome-aluminium alloy.

7. An electric heater element substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 4 of the accompanying drawings.

8. An electric heater, stove or oven having an element as claimed in any one of claims 1 to 7.

9. An electric heater, stove or oven as claimed in claim 8 wherein the element is disposed in front of a reflector provided on a frame and is supported at opposite ends by said frame.

10. An electric heater, stove or oven substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.