Ceramic heating element

A heating element comprises a ceramic body having a helical heating wire embedded therein. A short quartz tube closely surrounds the hearing wirs along part of its length, and a thermocouple has its junction embedded in the body substantially in direct contact with the outside of the tube.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a ceramic heating element. 
2. Description of Related Art 
Conventional ceramic heating elements comprise a ceramic body having a 
heating (resistance) wire embedded therein. When an electric current is 
passed through the heating wire it causes the wire to heat thereby heating 
up the ceramic body and causing the latter to emit heat by radiation. 
Conventional ceramic heating elements also usually contain an in-built 
thermocouple located near to the heating wire. A difficulty with 
conventional designs of element is the positioning of the thermocouple 
within the element. When positioning a thermocouple within the ceramic 
body the thermocouple junction must be located a consistent distance from 
the heating wire in order to give accurate readings. Also there must be no 
electrical interference between the heating wire and the thermocouple as 
this can cause electrical damage. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides a heating element comprising a 
ceramic body having a heating wire embedded therein, a heat transmissive 
dielectric tube closely surrounding the heating wire along part of its 
length, and a thermocouple with its junction embedded in the body 
substantially in direct contact with the outside of the tube. 
Heat can be transferred in three ways, by conduction, convection or 
radiation. As there is no fluid within the ceramic body, heat transfer by 
convection can be ignored within a ceramic heating element. Therefore, the 
heat is transferred by radiation and conduction from the heating wires to 
the ceramic body. The ceramic material is designed to promote heat loss 
through the front surface of the body, but a problem with conventional 
element design is that heat is also lost through the back of the element. 
Accordingly, the present invention further provides a heating element 
comprising a ceramic body having front and rear surfaces, a heating wire 
embedded within the ceramic body, and a heat shield layer of a material 
which is both heat reflecting and heat insulating embedded in the ceramic 
body between the heating wire and the rear surface.

DETAILED DESCRIPTION OF THE INVENTION 
The ceramic heating element shown in the drawings includes an elongate 
ceramic body 10 of arcuate cross-section with a concave front surface 12 
and a convex rear surface 14. The body 10 has a plurality of substantially 
parallel, evenly spaced-apart, integral ribs 16 on its front concave 
surface 12, the ribs extending in the longitudinal direction of the body 
10. The body 10, including the ribs 16, is glazed. 
A conventional heating wire, in the form of a helical resistance wire 18, 
is embedded in the body 10. Respective lengths of the heating wire 18 
extend along respective ones of the ribs 15. In particular, each rib 16 is 
substantially of semi-circular cross-section and each length of the 
heating wire 18 is located substantially at the centre of curvature of the 
respective rib 16. 
A ceramic boss 20 is cast integrally with the body 10 on its rear surface 
14. Power leads 22 enter the body 10 through the boss 20 and are connected 
internally of the body 10 to supply current to the heating wire 18 in 
known manner. A wave spring and clip 24 permit mounting the heating 
element to a reflector system, also in known manner. 
To reduce heat loss through the rear surface 14 of the body 10, the body 10 
has embedded therein, between the heating wire 18 and the rear surface 14, 
a heat shield layer 28 of material which is both heat reflecting and heat 
insulating. The material 28 will substantially prevent heat loss by 
radiation through the rear surface 14 of the body 10 as it reflects the 
heat radiation back towards the front surface 12, and the material 28 will 
also substantially prevent transfer of heat by conduction to the rear 
surface 14 of the body 10. 
The heat shield layer 28 is preferably manufactured from a sheet of a high 
purity heat insulating material made of alumina silicate refractory 
fibres. After punching to produce the required shape for embedding in the 
body 10, the sheet is impregnated with an engobe material by drawing the 
sheet through a bath of a liquid engobe mixture. The bath consists of a 
mixture of 50% by volume of a ceramic glaze with reflective qualities and 
50% by volume of a slip body. The glaze and slip body should have similar 
coefficients of thermal expansion as the body 10 to reduce the likelihood 
of failure due to stress cracks. The composite material gives the heat 
shield layer 28 its heat reflecting and heat insulating properties. 
The net result of this heat loss reduction is that more of the heat is 
forced out the front surface 12 of the body 10 and so can be focused with 
greater intensity. 
This will also give the body 10 a lower thermal inertia, i.e. the amount of 
energy a body absorbs before it begins to radiate energy, and so reduce 
the maximum demand or the heating element. Thus the heating element 
designed in this fashion will reach its operating temperature faster and 
due to the reduction of heat loss will perform much more efficiently. 
The heating element further includes an in-built thermocouple sensor which 
consists of a pair of wires 30, 32 of dissimilar metal, e.g. nickel/nickel 
chrome, embedded in the body 10. One portion of the heating wire 18 near 
the boss 20 is closely surrounded by a short length of quartz tube 34, and 
the thermocouple junction 36 is located in direct contact with the outside 
of the quartz tube 34. 
By using a quartz tube any difficulties with regard electrical interference 
between the heating wire 18 and the thermocouple are avoided as quartz is 
a dielectric material. Also by using quartz, which is transparent to all 
emitted radiation, the thermocouple can follow rapidly and accurately the 
temperature change of the heating wire. By locating the thermocouple 
junction in contact with the quartz tube, which is of known diameter, the 
distance between the thermocouple and the heating wire is constant for all 
elements. This will in turn maintain a consistency in the thermocouple 
readings of different ceramic heating elements. 
The thermocouple wires 30, 32 exit the body 10 through the boss 20, 
substantially parallel to the power leads 22 (FIG. 3). In order to avoid 
electrical interference between the thermocouple wires and the power 
leads, an insulating ceramic tube 38 is placed around the thermocouple 
wires within the boss. 
In addition, the power leads 22 and the thermocouple wires 30, 32 are 
positioned within a specialised insulating ceramic clay 40, which has a 
greater dielectric strength to ensure no induced or leakage current will 
interfere with the performance of the ungrounded thermocouple junction. 
The ceramic clay 40 comprises a low thermal response, matched engobe 
material (mixture of matched slip and glaze having similar coefficients of 
expansion). This is important where controllers may not have optical 
decoupling on the thermocouple card. The combination of these two 
features, tube 38 and clay 40, both of which are dielectric materials, 
substantially eliminates the problem of electrical interference in the 
boss. 
A ceramic heating element has been manufactured according to the principles 
described above to provide a uniform radiation output with a mass 
temperature range of 300 dearees centigrade to 750 degrees centigrade 
producing a wave length range of 6-3 microns. 
The invention is not limited to the embodiments described herein which may 
be varied without departing from the scope of the invention.